CARBON-PLASTIC COMPOSITE ELECTRODE ... - UNSWorks

CARBON-PLASTIC COMPOSITE ELECTRODE FOR VANADIUM

REDOX FLOW BATTERY APPLICATIONS

A thesis submitted as part of the requirements for the degree of

Doctor of Philosophy (Ph.D.)

by

SHillUANG ZHONG (M.E.)

School of Chemical Engineering and Industrial Chemistry

Th~ University of New South Wales

AUSTRALIA

March, 1992

ill-,'lYERSITY OF NEW SOlJTH WALES Thesis/Project Report Sheet

. ZHoN, SurnameorFum1lyname: ............................................................................................................................................................................................. .. SH t HU P.,.1'-iC#' First name: ...................... .....• : ....... ................................. , ... ... .. ... . .. . .. Other name/s ................................................. , ................................................... ..

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Abstract 350 words maximum: (PLEASE TYPE)

Carbon-plastic composite electrodes (CPCE) for the vanadium redox flow battery (Vbattery) have been developed and investigated. Electrical, mechanical and electrochemical propenies a,; well as the solution permeability of the carbon-plastic composite (CPC) materials were evaluated. The electrode kinetics of vanadium redox couples at graphite electrodes, and the activation treatment on the surface of the CPC was also studied. The surface physical and chemical pmpcnies of carbon/graphite felts were investigated intensively with SEM and XPS techniques to establish their suitability and properties as electrode active layers on the CPCE. Electrocatalysis and cell performance of the V-battery employing the CPCE were also evaluated.

CPC material modified with SEBS rubber has been shown to have excellent electrical and mechanical properties, as well as being solution impermeable. An overall energy efficiency of 88% has been achieved for a V-battery employing the CPCE at a charge/discharge current density of 21.7 mA/cm2

• The electrodes were tested for more than 5780 hours (1240 cycles) and shown excellent stability.

The redox reactions of vanadium redox couples are electrochemically irreversible at the flat graphite electrode. The io for V(V)N(IV) couple was determined to be 2.47 x 10

4 A/cm2• The k0

for V(III)N(II) couple was calculated to be 3.63 x 104 cm/sec. The diffusivity of V(IV) and V(III)

species ·was found independent of vanadium concentration with diffusion coefficient values of 2.14 x IO"' cni2/scc and 1.25 x. 1� cm2/sec respectively.

The PAN based GFD 2 graphite felt was found more stable in air and anodic oxidation than that of rayon based FMI graphite felt. XPS analysis revealed that anodic oxidation of the CPCE in the V-battery results in a formation of four types of carbon-oxygen groups on the graphite felt surf ace. The overcharged samples show a high surface concentration of -CO3• groups which is believed to contribute to electrode deterioration.

Excellent cell performance has been achieved by using silver as the electrocatalyst for the negative electrode and a thermally treated GFD 2 felt as the positive electrode. A cell resistance value of 1.5 n.cm2 and an overall energy efficiency of 80% at 100 mA/cm2 were observed with a V-battcry using these catalysed electrodes.

Ded.aral!on relating to dlspo�ltlon of project reportltbesls

I am fully •wm of the policy of the university relating to the retention and use ofhigherde�,projectreports and theses, namelythatthe University retains thecopicuubmitt«l for examination 11.nd is free to allow them to be con.sultedprb9rrowed. Subject to the provisions oft.be CopyrightAct 1968, the Universitymay issue aprojectreportortheais in whole orln part, in photostate orrnictofilm or other copying medium. l also authorise the publication by University Microfilms of a 350word abstract in Dissenation Abstracts International (applicable to doctorates only).... ................ 11.. .•. 3 .. 0�;1, .................... .

The Univc1tity recognises that there maybe exceptional circumstance�, requiring �t,rlction.s on oqpying or conditions on ust, Reques,"1�trestrlction for a period ofupto 2 years must he made in writing to the Registrar. Requests for a lcingerperiod of restrictio�.r,a)1'1pecoia�ineitcepti9nal cWumstances ifaccompanied bya letter of support from the Supervisor or Head of School. Such requests must he submitted' with the'thesis/proJ�,report. FOR OFFICE USE ONLY Date of completion of requirements for Award:

V Tms SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF nm TI:mSIS

I hereby declare that this submission is my own work and that, to the best of my

knowledge and belief, it contains no material previously published or written by

another person nor material which to a substantial extent has been accepted for the

award of any other degree or diploma of a university or other institute of higher

learning, except where due acknowledgment is made in the text.

PUBLICATIONS ARISING FROM THIS STUDY

1. S. Zhong, R. Butford and M. Skyllas-Kazacos, "Composite Conducting Polyethylene Electrode for Electrochemical Application", Proceedings of Workshop on New Developments in Electrode Materials and their Applications, Dept. of Ind., Tech. and Commerce, Australia (Ed. Leo Wood and Alan J. Jones), Wollongong, New South Wales, Feb., (1990) 276-284.

2. S. Zhong, M. Kazacos, R.P. Burford and M. Skyllas-Kazacos, "Fabrication and Activation Studies of Conducting Plastic Composite Electrodes for Redox Cell Applications", J. Power Sources, 36 (1991) 29-43.

3. S. Zhong and M. Skyllas-Kazacos, "The Electrochemical Behaviour of V(V)N(IV) Redox Couple at Graphite Electrode", J. Power Sources, 39 (1992) 1-9.

4. S. Zhong and M. Skyllas-Kazacos, "Activation Studies of Conducting Carbon-plastic Electrodes for Vanadium Redox Battery Applications", Electrochemistry in Australia: From Natural Resources to Value Added Products, Royal Australian Chemical Institute Electrochemistry Division, New Castle, July, (1991).

5. S. Zhong, M. Kazacos and M. Skyllas-Kazacos, "Carbon-plastic Electrode Material", Australian Patent (applied), Aug. (1991).

6. S. Zhong, M. Kazacos and M. Skyllas-Kazacos, "Fabrication and Evaluation of Carbon-plastic Electrodes for Vanadium Battery application", to be submitted.

7. S. Zhong, C. Padeste, M. Kazacos and M. Skyllas-Kazacos, "Surface Physical and Electrochemical Characteristics of Carbon Fibre Electrodes", J. Power Sources, submitted.

8. S. Zhong, M. Kazacos and M. Skyllas-Kazacos, "Electrocatalysts for Vanadium Redox Flow cell System", to be submitted.

ACKNOWLEDGMENTS

I wish to thank Associate Professor Maria Skyllas-Kazacos for the help,

supervision and guidance she has given throughout the study of this project. I also

wish to thank Professor Robert P. Burford for his help and advice in polymer

processing and properties measurement and Dr. Celestino Pedeste for his help in

XPS analysis.

The help and encouragement of Mr. Michael Kazacos during my study is most

appreciated. The assistance of Mr. Youzhen Cheng, Dr. Djen Keshermen, Messrs.

Dun Rui Hong, S. C. Chieng, H. Chau, Mrs Kate Nasev and Mr. Steve Jacenyick,

as well as the staff of vanadium development team and fellow postgraduate

students of the electrochemistry group, are gratefully acknowledged.

I would also like to thank the NSW Department of Minerals and Energy, Australia

and the Australian Research Council for their financial support.

Especially, I wish to thank my wife, Nianshan Hu, for her love, encouragement

and full support throughout the duration of my studies. Also, thanks to my

parents, sisters and brothers, for their love and support during the whole of my

life.

i

ABSTRACT

Carbon-plastic composite electrodes for the vanadium redox flow battery have

been developed and investigated. Electrical, mechanical and electrochemical

properties as well as the solution permeability of the carbon-plastic composite

materials were evaluated. The electrode kinetics of vanadium redox couples at

graphite electrodes, and the activation treatment on the surface of the carbon

plastic composite were also studied. The surface physical and chemical properties

of carbon/graphite felts were studied intensively with SEM and XPS techniques to

establish their suitability and properties as electrode active layer on the conducting

plastic composite electrodes. Electrocatalysis and cell performance of the

vanadium redox flow battery employing the composite electrode was also

evaluated.

Carbon-HDPE composite material (composition of 60% HDPE, 20% graphite

fibre, 20% carbon black) modified with SEBS thermo-plastic rubber has been

shown to have excellent electrical and mechanical properties, as well as being

solution impermeable. Chemical treatment of graphite fibre based composite

results in a surface area enhancement and an improved reactivity for the vanadium

ion redox reactions.

An overall energy efficiency of 88% can be achieved with the graphite

felt/carbon-HDPE composite electrodes at a cell charge/discharge current density

of 21.7 mA.cm-2• The composite electrodes were tested for more than 5780 hours

(1240 cycles) in the vanadium redox battery and have shown excellent stability.

ii

The kinetics of the V(V)N(IV) and V(ill)N(TI) redox couple reactions have been

found to be electrochemically irreversible at the flat graphite electrodes. The

exchange current density, i0, for V(V)N(IV) couple at this electrode was

determined to be 2.47 x 104 A.cm·2• The electron transfer rate constant for

V(ITI)N(II) couple was calculated to be 3.63 x 104 cm.s·1• The diffusivity of

V(IV) and V(III) species was found independent of vanadium concentration with

diffusion coefficient values of 2.14 x 10·6 cm2.s·1 and 2.60 x 10"6 cm2.s·1

respectively. The equilibrium exchange current density values at graphite felt

electrodes increased by a factor 102, however, showing that acceptable charge

transfer rate are possible with these porous flow-through electrodes.

The PAN based GFD 2 graphite felt was found more stable in air and anodic

oxidation than that of rayon based FMI graphite felt. XPS analysis revealed that

when operated as positive electrodes under normal cell charge/discharge

conditions, four types of carbon-oxygen groups formed on the graphite felt

surface. The overcharged samples show a high surface concentration of -Co3•

groups which is believed to contribute to electrode deterioration.

In the electrocatalysis and activation studies, excellent cell performance was

achieved by using silver as the electrocatalyst for the negative electrode and a

thermally treated GFD 2 felt as positive electrode. A cell resistance of 1.5 Q.cm2

and an overall energy efficiency of 80% at 100 mA.cm2 were observed with a

vanadium cell employing these catalysed electrodes, compared with 3.0 Q.cm2 for

the untreated electrodes.

iii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS

ABSTRACT TABLES OF CONTENTS

LIST OF FIGURES LIST OF TABLES

LIST OF SYMBOLS

CHAPTER I INTRODUCTION

1.1 Background

1.2 Early Research Work on Electrodes for the Vanadium Redox Flow Battery

1.3 The Aim of the Present Project and Brief Description

of the Thesis

CHAPTER II LITERATURE REVIEW

2.1 General Requirements for Electrodes Used in the Redox Flow Battery

2.2 Electrodes Used in Redox Flow Cells

2.2.1 Electrode Materials

2.2.2 Structural Characteristics of Electrodes for Redox

Flow Cells

2.3 Conducting Carbon-Plastic Composite Electrodes

2.3.1 The Concept of Conducting Carbon-Plastic Composite Electrodes

2.3.2 Advances in Carbon-Plastic Composite Electrodes

2.3.2.1 Carbon-PTFE Composite Electrodes

2.3.2.2 Polyolefm Based Carbon-Plastic Composite

lV

i

ii

iv xi

XXI

xxiv

1

1

4

6

9

9

12

12

16

25

25

27 27

Electrodes

2.4 Carbon-Plastic Composite Materials

2.4.1 Mechanism of Electrical Conducting in Composite Materials

2.4.2 The Effect of Conducting Fillers Content on Electrical Conductivity of Composites

2.4.3 The Influence of Particle Sizes and Surface Area of Conducting Fillers on Electrical Conductivity

2.4.4 The Influence of Polymer Phase on Electrical Conductivity of Composites

2.5 Carbon/Graphite as Electroactive Layer

2.5.1 Surface Microstructure of Carbon/Graphite Materials

29

34

35

37

38

39

40

and Its Relation to Interaction with Oxygen 42

2.5.2 Carbon-Oxygen Surface Complexes and Surface Treatment 48 2.5.2.1 Formation of Carbon-Oxygen Surface Complexes 48 2.5.2.2 Specific Carbon-Oxygen Surface Complex and Their

Influence on Electrochemical Behaviour of Carbon Electrodes 54

2.5.3 Electrocatalysts 59

2.6 Theoretical Background 61

2.6.1 Electrochemical Techniques 61 2.6.1.1 Cyclic Voltammetry 62

2.6.1.2 Rotating Disc Voltammetry 66

2.6.2 Performance Characteristics of Redox Flow Cell 68 2.6.2.1 Polarisation Curve and Cell Resistance 68 2.6.2.2 Cell Efficiencies 70

v

CHAPTERITI EXPERTIWENTAL

3.1 Preparation and Evaluation of Carbon-Plastic Composite Sheets and Electrodes

3.1.1 Preparation 3 .1.1.1 Materials

3.1.1.2 Equipment 3.1.1.3 Preparation Procedure

3.1.2 Evaluation 3.1.2.1 Evaluation of Electrical and Physical Properties

for Composite Materials 3.1.2.2 Evaluating of Electrical and Electrochemical

Properties of Composite Electrodes

3.2 Experimental Procedures for Electrode Kinetics and Electrode Activation Study

3.2.1 Electrode Kinetics Study

3.2.1.1 Electrodes 3.2.1.2 Electrolyte and Electrochemical Cell

3.2.1.3 Electrochemical Equipment

3.2.2 Surface Chemical Treatment f Carbon-Plastic Composite Materials

3.2.2.1 Preparation of Carbon-Plastic Composite Materials

3.2.2.2 Preparation of Carbon-Plastic Composite Electrodes for Chemical Treatment and Cyclic Voltammetry

3.2.2.3 Procedure for Surface Chemical Treatment

3.2.2.4 Evaluation

3.3 Graphite Felts Studies

3.3.1 Felt Materials

3.3.2 Physical Property Measurements

3.3.3 Evaluation of Electrochemical properties

3.3.3.1 Preparation of Carbon-Plastic Electrodes for

vi

72

72

72

73

74 77

79

81

84

97

97 97 99

100

102 102

103

105

105

106

106

106

107

Cyclic Voltammetry

3.3.3.2 Design of the Felt-Electrode Support

3.3.3.3 Measurement Conditions

3.3.4 Oxidation of the Felt Samples and Surface Analysis

3.3.4.1 Air-Oxidation 3.3.4.2 Electrochemical Oxidation

3.3.4.3 Oxidation in Positive Half-Cell of the Vanadium Redox Flow Battery

3.3.5 Felt Activation Treatment and Evaluating 3.3.5.1 Activation Treatment

3.3.5.2 Evaluations

-~

CHAPTER IV FABRICATION AND EVALUATION OF CARBONPLASTIC COMPOSITE MATERIALS AS ELECTRODE MATRIX LAYERS

4.1 Low Density Polyethylene Based Carbon-Plastic

Composites

4.1.1 Electrical Conductivity of Composites

4.1.1.1 The Effect of Content and Type of Carbon Fillers 4.1.1.2 The Effect of Processing on Resistivity

4.1.1.3 The Effect of Carbon Black on Resistivity

4.1.1.4 The Effect of Composition on Resistivity

4.1.2 Mechanical properties

4.1.3 Permeation

4.1.4 Electrochemical Stability and Activity of the

Carbon-LDPE Composite Electrodes

4.1.4.1 Electrochemical Stability

4.1.4.2 Cell Performance of Composite Electrodes

4.2 High Density Polyethylene Based Carbon-Plastic

107 108

108

110

111

111

1}'~·

113

113

113

116

117

117 118

124

127 129

130

135

137

137

141

Composites and Electrodes 142

4.2.1 Evaluation of SEBS Modified Carbon-HDPE Composites 143

Vll

4.2.1.1 Effect of Composition on Electrical and

Mechanical Properties

4.2.1.2 Solution Permeability of the SEBS Modified

Carbon-HDPE Composites

4.2.1.3 Microstructure of SEBS Modified Carbon-HDPE

Composites

4.2.2 Evaluation of SEBS Modified Carbon-HDPE Composite

Electrodes

4.2.2.1 Electrical Resistance and Cell Resistance

4.2.2.2 Cell Performance Testing

Constant Current Charge/Discharge Behaviour

The Effect of Current Density on Cell Efficiency

Short-Term Cell Performance Test

Long-Term Cell Stability Test

Electrode Deterioration Test

4.3 Summary

143

148

151

153

153

160

161

161

164

164

167

174

CHAPTER V ELECTRODE KINETICS AND ACTIVATION STUDIES 176

5.1 Electrode Kinetics of V(V)N(IV) and V(III)N(II)

Couples at Graphite Electrode

5.1.1 The Electrode Kinetics of V(V)N(IV) Couple

5.1.1.1 Reproducibility of Electrode Surface Preparation

5.1.1.2 Cyclic Voltammetry

5.1.1.3 Rotating Disc Voltammetry

5.1.1.4 Diffusion Coefficient

5.1.1.5 Kinetics Parameters

5.1.2 The Electrochemical Behaviour of V(III)N(II) Couple

5.1.2.1 The Effect of Electrochemical Oxidation on

Electrode Behaviour

5.1.2.2 Determination of Kinetics Parameters

5.2 Chemical Activation of the Carbon-Plastic Composite

Substrates

5.2.1 Influence of Treatment Time

vm

176

177

179

179

182

186

196

200

201

203

208

210

5.2.2 The Effect of Treatment Temperature 219

5.2.3 Cyclic Stability 222

5.2.4 Mechanism of Chemical treatment 225

5.3 Summary 225

CHAPTER VI GRAPHITE FELT AS ELECTRODE ACTIVE LAYER 229

6.1 Comparison of Characteristics of Some Graphite Felts

6.1.1 Electrical Conductivity of Graphite Felt

6.1.2 Surface Area and Pore Size of Graphite Felt Samples

6.1.3 Electrochemical Activity of Commercial Graphite

Felts

6.2 Surface Microstructure and Oxidation Characteristics

of FMI and GFD Graphite Felts

6.2.1 The Effect of Anodic Oxidation on Electroactivity

of Felt Electrodes in Vanadium Solution

6.2.2 The Effect of Gas Oxidation on the Formation of

Surface Carbon-Oxygen Complex

6.2.3 Surface Microstructure of Fibres in FMI and GFD 2

Felts

6.2.4 The Effect of Charging and Overcharging on the

Surface Characteristics of Graphite Felt Electrodes

6.2.4.1 General Analysis

6.2.4.2 Samples Used as Electrodes at Normal and

Overcharging Cell Operating Conditions

lX

231

231

232

237

244

246

250

263

264

267

269

6.3 Electrocatalysis Studies

6.3.1 Activation Evaluation of Treated Felt Samples With

Cyclic Voltammetry

6.3.2 The Effect of Concentration of AgN03 Solution on

Characteristics of Treated GFD 2 Felt

6.3.2.1 Effect of AgN03 Concentration on Weight Increase

6.3.2.2 The Effect of Deposited Silver on Felt Electrical

Resistivity

6.3.2.3 Effect of Deposited Silver on Cell Resistance and

polarisation behaviour

6.3.3 Cell Performance Test

6.3.3.1 The Influence of Treatment on Cell Efficiencies

6.3.3.2 The Effect of Current Density

6.3.3.3 The Cell Discharge Behaviour

6.3.3.4 Long-Term Stability Test

6.4 Summary

CHAPTER VII CONCLUSIONS

REFERENCES

X

280

281

290

290

292

292

295

296

298

300

302

304

306

311

LIST OF FIGURES

Figure Page

1.1 Two-tank electrically rechargeable redox flow cell. (Ref 3.) 2

2.1 Configuration of NASA laboratory redox flow cell. (Ref 4.) 19

2.2 Components of a single redox flow cell and full-function

stack. (Ref 183) 20

2.3 Configuration of carbon-plastic composite electrode,

(a) end-electrode; (b) bipolar electrode. 23

2.4 Transition-like phenomena of composite material (Ref 87.). 36

2.5 Schematic diagram of the carbonisation process for a

graphitizable organic material (Ref 102.). 43

2.6 Structural comparison of graphite and carbon, (a) graphite;

(b) carbon (Ref 101.). 45

2.7 Proposed functional groups on carbon (Ref 101.). 55

2.8 Cyclic Voltammograms of three typical electrode process,

(a) reversible, diffusion controlled; (b) Totally irreversible;

(c) quasi-reversible (Ref 183.). 63

2.9 Typical cell polarisation behaviour of redox flow cell system

(Ref 44.). 64

2.10 Constant-current charge and discharge performance of the

vanadium redox flow cell (Ref 44.). 69

X1

3.1 Equipment for preparing carbon-plastic composite sheet and

electrode (a) internal mixer; (b) hot-pressure. 76

3.2 Configuration of moulds used for preparing carbon-plastic

composite sheet and electrode: (a) mould for making sheet;

(b) window-mould for bonding graphite felt onto composite sheet. 78

3.3 Flow-chart of the process for preparing carbon-plastic

composite and electrodes. 80

3.4 The set-up for resistivity measurement (ASTM D-991). 82

3.5 Diagram of testing apparatus for evaluating permeability of

carbon-plastic composite. 85

3.6 Four-probe method for measuring the resistance of

carbon-plastic composite electrodes, (a) for end-electrode;

(b) for bipolar. 87

3.7 Expanded diagram of a single-cell vanadium redox flow battery. 89

3.8 Expanded diagram of a two-cell multi-cell vanadium redox flow

battery. 90

3.9 Electrical circuit used for charge-discharge cycling. 96

3.10 Construction of graphite rod electrode used for cyclic

voltammetry and rotating disc voltammetry. 98

3.11 Electrochemical cell for cyclic voltammetry and rotating disc

voltammetry. 101

3.12 Schematics of carbon-plastic composite sheet and electrode for

chemical treatment studies: (a) composite sheet; (b) electrode

construction. 104

xu

3.13 Construction of carbon-plastic/graphite-felt composite electrode

for cyclic voltammetry. 109

3.14 Modified NASA cavity fill-in single cell used as test cell

for activated felt electrodes for vanadium redox flow system. 115

4.1 Effect of carbon content on resistivity of carbon-LDPE

composite. 119

4.2 The development of graphite fibre network in carbon-LOPE

composite. Magnification: 100x. Fibre content: a) 5%;

b)10%; c) 20%. 122

4.3 Effect of blending time on data scattering. 125

4.4 Effect of carbon black (black pearl) to graphite fibre

ratio on resistivity. Proportion of polymers: sample 1,

65% LDPE + 5% SEBS; sample 2, 60% LDPE + 10% SEBS;

sample 3, 55% LDPE + 15% SEBS. 128

4.5 Effect of carbon black (black pearl) to graphite fibre

ratio on mechanical properties. Solid line: tensile strength;

dashed line: elongation. Proportion of polymers same as that

for Figure 4.4. 132

4.6 Effect of rubber content on mechanical properties. Solid

line: tensile strength; dashed line: elongation. Carbon

fillers: sample 1, 5% CB + 25% GF; 2, 10% CB + 20% GF;

3, 20% CB + 10 GF% (CB =black pearl, GF =graphite fibre). 134

4.7 Permeation behaviour of carbon-LOPE composite of composition

70% LDPE, 15% CB (black pearl), 15% GF (graphite fibre). 136

4.8 Cyclic voltammogram of graphite felt bonded to (1)

carbon-LOPE composite and (2) to graphite rod electrode,

sweep rate: 100 mv.s·1, vs SCE. Composition of composite

given as that for Figure 4.7. 138

xm

4.9 Cyclic voltammetry stability testing for carbon-LOPE

composite electrode. Sweep rate: 100 mv.s·I, vs SCE,

100 cycles. Composition of composite same as that for

Fig. 4.7. 140

4.10 Effect of SEBS content on electrical and mechanical

properties of carbon-HDPE composites. Composition given

as sample 1,2,3 and 4 in Table 4.11. 147

4.11 Permeation behaviour of SEBS modified carbon-HDPE

composite of composition 40% HDPE, 20% SEBS, 10% CB

(Degussa XE 2), 10% CB (Degussa FW 200), 20% GF. 150

4.12 Cross section of a SEBS modified carbon-HDPE composite

(Cu-mesh backed), magnification: a) 40x: b):440x.

Composition same as that for Fig. 4.11. 153

4.13 Reproducibility Evaluation of four-probe method for

measuring resistance of composite electrode 155

4.14 Effect of processing on resistance of composite electrodes

(Composition of electrode matrix same as that for Fig. 4.11). 156

4.15 Morphology of composite electrode, a) cross section of

boundary area, b) electrode matrix layer surface (felt

has been removed). Composition of electrode matrix same

as Fig. 4.11, Graphite felt: GFD 2. 158

4.16 Typical charge/discharge curve of vanadium redox cell

employing SEBS modified carbon-HDPE composite electrode

(matrix layer: same as that for Fig. 4.11; active layer:

Toray felt). ich. = idis. = 21.7 rnA cm·2; cell voltage

up limit= 1.75 V, lower limit= 0.80 V. 162

4.17 Effect of current density on the cell efficiencies of a

vanadium redox flow cell employing SEBS modified

carbon-HDPE composite electrodes. Electrodes and other

xiv

conditions same as that for Fig. 4.16. 163

4.18 Short-term single cell performance of the composite

electrodes of Fig. 4.16. Other conditions same as that

for Fig. 4.16. 165

4.19 Short-term performance of a two-cell vanadium redox

battery with the composite electrodes of Fig. 4.16.

Curves 1: coulombic, 2: voltage; 3, energy efficiencies.

ich. = idis. = 30 mA.cm-2• Other conditions same as that

for Fig. 4.16. 166

4.20 Long-term single cell stability test for the composite

electrodes of Fig. 4.16, a) efficiency variation,

l:coulombic, 2: voltage, 3: energy; b) voltage efficiency

variation. Other conditions same as that for Fig. 4.16. 168

4.21 Effect of overcharging time on cell resistance of

vanadium redox flow cell with the (Toray felt)/(SEBS

modified carbon-HOPE) composite electrodes. 170

4.22 Effect of overcharging on cell performance with the

composite electrodes of Fig. 4.21. 171

4.23 Cross section of Japanese Toray composite end-electrode.

Magnification: 20x. a) normal electrode; b) after seriously

overcharged. 173

5.1 Cyclic voltammograms for graphite electrode in 0.1 M

VOSOJ3 M H2S04 solution, sweep rate: 10 mv.s-1•

(a) anodic voltammograms for testing reproducibility of

surface preparation; (b) cyclic voltammograms for testing

the cyclic stability, 1st to lOth cycles. 181

5.2 Cyclic voltammograms of graphite electrode in (a) 0.05 M V(V)

+ 0.05 M V(IV) in 3M H2S04; (b) 3M H2S04• sweep rates: 1,

10, 20, 40, 60 mv.s-1 for curves 1 to 5 respectively. 184

XV

5.3 Cyclic voltammograms of graphite electrode in the potential

range 0 to 0.9 V, other conditions are same as those for

Figure 5.2(a). 185

5.4 Cathodic and anodic voltarnrnograms of a rotating disc graphite

electrode in electrolyte of Figure 5.2(a); sweep rate: 1 m V.s-1;

(a) rotation speeds: zero, 60 rpm, 120 rpm, 240 rpm, 600 rpm,

1200 rpm for curves 1 to 6 respectively. (b) rotation speeds:

1200 rpm, 1800 rpm, 2400 rpm for curves 1 to 3 respectively. 188

5.5 Plot of limiting current density(iJ vs square root of angular

velocity for anodic voltarnmograms of Figure 5.4. 190

5.6 Anodic rotating disc voltammograms at a graphite electrode in

0.5 M V(IV)/3 M H2S04 solution. rotation speeds: 120 rpm,

360 rpm, 720 rpm, 1080 rpm for curves 1 to 4 respectively. 192

5.7 (a) Plot of limiting current density vs square root of angular

velocity at various vanadium concentrations; (b) Plot of

limiting current density vs vanadium concentration at various

electrode rotation speeds. 194

5.8 Plot of iL jconc. vs square root of angular velocity for

anodic voltammograms of Figure 5.6 at various concentrations. 195

5.9 Plot of anodic overpotential vs log(iL.a - i)/i from

voltammograms of Figure 5.4. 197

5.10 Cathodic and anodic polarisation curves of graphite felt and

reticulated vitreous carbon (RVC) electrodes in 1.0 M V(V)

+ 1.0 M V(IV) in 3 M H2S04• 199

5.11 Cyclic voltammograms of graphite electrode in 0.1 M V3+/3 M

H2S04 solution, scan rate: 40 mv.s·1• 1, normally polished;

2, 3 min anodicly oxidised (dash line for 6 and 9 min anodic

oxidation). 202

XVI

5.12 Cyclic voltammograms of 9 mins anodicly oxidised graphite

electrode in 0.1 M V3+/3 M H2S04• Numbers on curves

corresponding to scan rates in mV.s-1 205

5.13 Peak current vs. square root of scan rate for voltammograms

in Fig. 5.12. 205

5.14 Ln(ipc) vs. (~ -E0') for voltammograms in Fig. 5.12. 209

5.15 Cyclic voltammograms of PE2525 composite electrode in 1 M

V(Ill) + 1 M V(IV)/3 M H2S04 vanadium solution, 1) untreated;

2) 5 mins treated; 3) graphite (scan rate = 100 m V.s-1). 211

5.16 Cyclic voltammograms of PE75 composite electrode in vanadium

solution (same as mentioned in Figure 5.15), 1) untreated;

2) 5 mins treated; 3) graphite( scan rate = 100 m V.s-1). 212

5.17 Cyclic voltammograms of graphite electrode in vanadium solution

(same as mentioned in Fig.5.15), 1) untreated; 2) 5 mins treated

(scan rate= 100 mV.s"1). 214

5.18 The effect of treatment time on peak current for PE2525

composite electrode. 216

5.19 The effect of treatment time on peak current for PE75

composite electrode. 218

5.20 The effect of treatment temperature on peak current for PE7 5 and

PE2525 composite electrode. 221

5.21 The cyclic stability test of treated PE 2525 composite electrode

(30 mins, 65°C), scan rate= 100 mV.s-1• 223

5.22 The cyclic stability test of treatment composite electrode: 1) 1 hr;

2) 2 hrs; 3) 4 hrs; all treated at 65°C (scan rate= 100 mV.s"1). 224

5.23 Cyclic voltammograms of PE composite bonded with graphite felt:

xvii

1) untreated; 2) 5 mins at 65°C; 3) 40 mins at 65°C (scan

rate = 100 mv.s-1). 226

5.24 SEM pictures of treated (4 hrs) PE75 composite material,

a) surface direction; b) cross-section. 227

6.1 Pore and surface area distribution of GFD 2 graphite felt,

(a) pore intrusion volume; (b) cumulative surface area. 235

6.2 Cyclic voltammograms of various graphite felt electrodes in

0.05 M V(lll) + 0.05 M V(IV) in 3M H2S04 solution, sweep

rate: 60 mv.s-1; (a) FMI; (b) GFA 2; (c) GFD 2 and (d) RVG 1000. 241

6.3 Cyclic voltammogram of graphite rod electrode in vanadium

solution (same as that in Figure 6.2); sweep rate: 60 mv.s-1• 243

6.4 Cyclic voltammograms of graphite felt electrodes in 3 M H2S04

solution, (1) before anodic oxidation; (2) after holding at

+1.5 V for 15 minutes, (a) FMI; (b) GFD 2. Sweep rate: 60 mV.s-1• 249

6.5 Cyclic voltammograms of graphite felt electrodes in 0.1 M

VOSOJ3 M H2S04 solution, (1) before anodic oxidation;

(2) after holding for 15 minutes at + 1.5 V in 3 M H2S04,

(a) FMI; (b) GFD2. 252

6.6 Overall XPS spectra of GFD 2 graphite felt, (N) untreated;

(T) treated in air at 400 °C for 30 hours. 254

6.7 C(ls) region XPS spectra of graphite felts treated in air and

in N2 at 400 °C for 30 hours, (a) GFD 2; (b) FMI. 255

6.8 0(1s) XPS region spectra of graphite felts treated in air and

in N2 at 400 °C for 30 hours, (a) GFD 2; (b) FMI. 258

6.9 Fitted 0(1s) XPS spectra of FMI graphite felts treated at 400

°C for 30 hours, (a) in N2; (b) in air. 260

XVlll

6.10 Fitted 0(1s) XPS spectra of GFD 2 graphite felts treated at 400

°C for 30 hours, (a) in N2; (b) in air. 262

6.11 Side-view of graphite felt fibres, (a) FMI; (b) GFD 2. 265

6.12 Cross-section of graphite felt fibres, (a) FMI; (b) GFD 2. 266

6.13 Fitted C(ls) spectra of FMI graphite felt, (a) treated in

N2 at 400 °C for 30 hours; (b) after normal use as electrode;

(c) after overcharge for 50 minutes at 21.7 mA.cm·2• 272

6.14 Fitted C(ls) spectra of GFD 2 graphite felt, (a) treated in



6.15 C(ls) region spectra of FMI graphite felt, (a) treated in



6.16 C(1s) region spectra of GFD 2 graphite felt, (a) treated in


(c) after overcharge for 50 minutes at 21.7 mA.cm-2• 279

6.17 Cyclic voltammogram of thermally-treated (in air at 400 °C for 30

hours) GFD 2 graphite felt electrode in vanadium solution (same as

for Figure 6.2), sweep rate: 60 mv.s·1• 282

6.18 Cyclic voltammogram of GFD 2 graphite felt electrode treated with

0.1 M NiS04 solution (conditions same as that for Figure 6.17). 283

6.19 Cyclic voltammogram of GFD 2 graphite felt electrode treated with

0.1 M MnS04 solution (conditions same as that for Figure 6.17). 284

6.20 Cyclic voltammograms of GFD 2 Graphite felt electrode treated

with 0.1 M AgN03 solution, (a) in the same vanadium solution as

for Figure 6.17; (b) in 3M H2S04; (c) after 50 cycles within the

potential range of -0.7 V to +0.5 V. 288

XIX

6.21 The effect of AgN03 concentration on the increased weight of

treated GFD 2 graphite felt. 291

6.22 Polarisation behaviour of a vanadium redox flow cell

employing GFD 2 graphite felt electrode treated under

various conditions. Electrolyte: 2 M vanadium sulphate

in 2.5 M H2S04; Membrane: CMV. Solid line: charging;

dashed line: discharging. 294

6.23 A typical charge/discharge curve of a vanadium redox flow

cell employing treated GFD 2 graphite electrode (positive:

thermally treated; negative: treated with 0.05 M AgN03).

Current density: 40 mA.cm·2• Other conditions same as for

Figure 6.22. 297

6.24 The effect of current density on cell efficiencies of a

vanadium redox flow cell with thermally and AgN03 treated

GFD 2 graphite felt electrodes. Other conditions same as

for Figure 6.22. 299

6.25 Cell discharge behaviour of the vanadium redox flow cell

with thermally and AgN03 treated GFD 2 graphite felt

electrodes. Other conditions same as for Figure 6.22 301

6.26 Long~term stability test on the vanadium redox flow cell

with thermal and AgN03 treated GFD 2 graphite felt

electrodes. Charge/discharge current density: 100 rnA cm-2;

upper and lower voltage limits are 1.75 V and 0.8 V

respectively. Other conditions same as for Figure 6.22. 303

XX

LIST OF TABLES

Page

2.1 Characteristics comparison of graphite and carbon 47

3.1 Composition and resistivity of composite PE used in

chemical treatment studies (CB = carbon black,

GF = graphite fibre) 102

4.1 Resistivity comparison of carbon-LDPE composites 120

4.2 Resistivity comparison of different composition

(CB =Cabot Black Pearl2000) 123

4.3 Composition of a typical carbon-LDPE composite 123

4.4 The effect of blending time on resistivity of

carbon-LDPE composites 126

4.5 Composition of carbon-LDPE composite materials 129

4.6 Comparison of the electrical resistivity of

carbon-LDPE composites 130

4.7 Composition and mechanical properties of rubber modified carbon-LDPE composites 133

4.8 Cell performance of carbon-LDPE composite electrodes 141

4.9 Composition of carbon-plastic materials 144

4.10 Comparison of the electrical and mechanical properties

of the prepared carbon-plastics with commercial available conductive materials 145

4.11 Optimisation study on the SEBS modified carbon-HDPE composite 146

xxi

4.12 Resistivity and mechanical properties of SEBS modified

carbon-HOPE composite materials (thickness of samples

= 0.75-0.85 mm) 148

4.13 Electrical resistance comparison for some SEBS modified

(carbon-HDPE)/(graphite-felt) Electrodes 157

4.14 Cell resistance comparison for some SEBS

modified (carbon-HDPE)/(graphite-felt) electrodes 160

5.1 Summary of limiting current density versus angular velocity

for the V (IV) oxidation reaction 189

5.2 Diffusivity of V(IV) species 191

5.3 The kinetic parameters of V(V)N(IV) couple 198

5.4 Experimental and calculated data for V(ill) + e = V(ll) reaction 207

5.5 The influence of treatment time on the electrode behaviour

of PE2525 ("e" electrode) in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04 215


of PE2525 ("s" electrode) in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04 215


of PE75 ("s" electrode) in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04 217


of PE75 ("e" electrode) in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04 217

5.9 The influence of treatment time on the electrode behaviour of

graphite in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04 219

5.10 The effect of treatment temperature on the electrode

behaviour of PE2525("e") electrode in

[1 M V(IV) + 1 M V(V)]/2.5 M H2S04 220

xxii

5.11 The effect of treatment temperature on the electrode behaviour

of PE75("e") Electrode in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04 220

6.1 Comparison of electrical resistivity of commercially

available graphite felts 232

6.2 Surface area comparison of some commercial graphite felts 236

6.3 Electrochemical activity comparison of some felt electrodes 242

6.4 XPS results comparison on FMI and GFD 2 felts 263

6.5 O:C ratios for felt samples with various treatments 268

6.6 Results from C 1 s peak curve fitting 270

6.7 Comparison of FMI and GFD 2 graphite felts 280

6.8 The effect of surface treatment on the electrochemical

activity of GFD 2 felt 289

6.9 The effect of AgN03 concentration on weight change of the felt 290

6.10 The effect of deposited silver on resistivity 292

6.11 Cell resistance of GFD 2 felt in 25 cm2 cell (CMV membrane) 293

6.12 Cell performance of GFD 2 felt in 25 cm2 cell (CMV membrane) 298

6.13 The effect of discharging current density on the capacity of the

vanadium redox flow cell with activated electrode 302

xxiii

Symbol

A

c

d

El%

I

i

LIST OF SYMBOLS

Definition

Electrode surface area (cm2)

Concentration (mol.cm-3)

Bulk concentration of reactive species (mol.cm-3)

Bulk concentration of oxidant (mol.cm-3)

Thickness of carbon-plastic composite (mm)

Diffusion coefficient of electroactive species 0 and R, respectively (cm2.s-1)

elongation at break (%)

Formal potential of an electrode (volts)

Peak potential (volts)

Anodic peak potential (volts)

Cathodic peak potential (volts)

Peak potential separation (volts)

Current (amperes, A and milliamperes, rnA)

Peak current (amperes, A and milliamperes, rnA)

Current density (A.cm-2)

Limiting current density (A.cm-2)

Anodic limiting current density (A.cm-2)

xxiv

IL,c Cathodic limiting current density (A.cm-2)

4 Peak current density (A.cm-2)

ipa Anodic peak current density (A.cm-2)

(ipa)o Uncorrected anodic peak current density (A.cm-2)

4c Cathodic peak current density (A.cm-2)

(isp)o Current density at switch potential (A.cm-2)

k0 Standard heterogeneous rate constant (cm.s-1)

kb Heterogeneous rate constant of the backward reaction (cm.s-1)

kr Heterogeneous rate constant of the forward reaction (cm.s-1)

M Concentration of solutions (moles.litre-1)

n Number of electrons participating in the electrode reaction

0 Oxidised form of a standard system 0 + ne ,.. R

R (i) Reduced form of a standard system 0 + ne ,.. R

(ii) Resistance (Q)

(iii) Cell resistance (Q.cm2)

SCE Saturated Calomel Electrode

T (i) Temperature (°C)

(ii) Absolute temperature (K)

t Time (hours, minutes)

v Voltage (volts)

v Scan rate (mv.s-1)

XXV

Tla

Tlc

TlE

'Tlv

n

p

v

O'u

Constants

F

R

Transfer coefficient

Electrode overpotential (volts)

Anodic overpotential (volts)

(i) Cathodic overpotential (volts)

(ii) Cell coulombic efficiency (%)

Cell energy efficiency (%)

Cell voltage efficiency (%)

Resistance (ohm)

Volume resistivity (ohm.cm)

Angular velocity of rotation (s-1)

Viscosity of electrolyte (cm2.s-1)

Tensile strength at break (N.mm-2)

Faraday constant, 96487 coulombs.mot-1

Universal gas constant, 8.314 J.mol-I.K1

xxvi

1.1 Background

CHAPTER I

INTRODUCTION

Over the past two decades, a new electrochemical energy storage system, the

redox flow battery, has been investigated widely and developed intensively [1].

Originally disclosed by Morrill in 1971[2], the concept of a redox flow battery

was defined as an electrochemical energy storage device which had electrolytes

containing electroactive species flowing past the inert electrodes and remaining

dissolved throughout cycling (Figure 1.1). This new system differs from the usual

energy storage batteries in two major aspects: the electrolyte and electrode. Instead

of being stored within the battery container, the energy-bearing chemicals of the

redox battery, or electrolytes, are stored in separate liquid reservoirs; differing

from the other batteries, in which the electrodes are involved in electrochemical

reactions as reactants or products and usually leading to the limitation of life time,

the electrodes (inert electrical conducting materials) of the redox flow battery act

as a source or sink for electrons, and in both halves of the cell, the components of

the half reaction are the two oxidation states of a constituent of the electrolytic

phase. These two major differences give this new battery system some of the most

attractive features: infinite power or energy storage capability and cycle life.

Extensive pioneering research work on the development of a promising redox flow

battery was conducted by the Lewis Research Centre which was founded by

1

Figure 1.1

Anode fluid

Inert electrode

Cathode fluid

membrane

Two-tank electrically rechargeable redox flow cell. (Ref 3 .) •

2

NASA (National Aeronautics and Space Administration, U.S.A.) from the early

1970s to the early 1980s. The outcome from this decade of extensive research,

was the Fe-Cr redox flow battery system, considered by many at that time to be

the most promising of its kind[3-ll]. Although there are a number of other redox

flow battery systems being studied and developed throughout the world, most of

the effective research has been based on systems which contain more than one

redox species when fully discharged, leading to a few disadvantages for redox

flow batteries, such as electrolyte contamination and battery capacity losses

whenever the anolyte and the catholyte cross over or leak through the separator.

Efforts to establish a common redox species battery system and a few possible

systems like Fe3+/Fe2+//Fe2+/Fe0 and Cf'+/C~+//Cr3+/cr+ [12] have been studied, but

no effective design was reported.

On the other hand, vanadium has the specific electronic configuration, i.e., 3d34s2,

which results in the oxidation states of +5, +4, +3 and +2 in aqueous systems[13-

23]. This property attracted some interest in utilising vanadium redox couples in

either fuel cell or redox flow cell[4, 24], however, there were no real efforts in

this area until the mid 1980's when Skyllas-Kazacos and co-workers, carried out

an electrochemical study of vanadium redox couples in various electrolytes and at

a range of electrodes[25-27]. This work led to the invention of a new all

vanadium redox flow battery system[25].

In this system, V(V)N(IV) and V(III)N(II) redox couples with sulphuric acid

media are employed as catholyte and anolyte respectively. An open circuit voltage

3

of higher than 1.5 V is obtained when the cell is fully charged. The redox

reactions in both halves of the cell can be illustrated as:

positive half-cell:

( 1-1)

negative half-cell:

(1-2)

Because of its unique advantages[25] over other conventional energy generation

and storage systems, a research and development project for the vanadium redox

flow battery was therefore established, and the investigation has covered electrode,

electrolyte, membrane and battery design. As one of the most important

components of the battery, the electrode and its related characteristics doubtlessly

became one of the key aspects of the research and development which led to the

present study.

1.2 Early Research Work on Electrodes for the Vanadium Redox Flow

Battery

The initial studies were conducted by Sum and Skyllas-Kazacos on glassy carbon

4

and graphite electrodes[27]. Using cyclic voltammetry and rotating disc

voltammetry, they concluded that the electron transfer rate for V(V)N(IV) couple

at glassy carbon electrode is rather slow with a value of k0 = 7.5 x 104•

Rychcik and Skyllas-Kazacos subsequently investigated the suitability of various

electrode materials for the V(V)N(IV) reactions and showed that on gold and

glassy carbon electrodes, the redox reactions were be irreversible, while on the

lead and titanium electrodes, passivating phenomena were observed in the

potential range of interest[26]. According to their report, platinised titanium and

iridium oxide-coated DSA (dimensional stable anode) showed better

electrochemical reversibility and cell performance, but the high cost of these

materials would be prohibitive for large-scale applications.

The possibility of using a conducting polymer, like polypyrrole, as the electrode

material for the vanadium battery was also investigated. Unfortunately, the results

were not satisfactory even though high electrical conductivity polypyrrole samples

were already fabricated[28]. In the mean time, Japanese carbon-plastic/graphite

felt composite electrodes were introduced to the vanadium battery system and

were found to be most promising electrodes leading to overall energy efficiencies

of up to 90% in 3 KW prototype batteries. However, the high cost and the

unsatisfactory mechanical properties of these electrodes were identified as

requiring further optimisation before commercialisation of the vanadium redox

flow battery.

5

1.3 The aim of the present project and brief description of the thesis

The aim of this project was to develop a carbon-plastic composite electrode and

study the relevant characteristics for the redox cell applications. Since a carbon

plastic composite electrode has a multi-layer structure, each layer being designed

for a different function to achieve satisfactory cell performance for the overall

electrode[29], some specific aspects for each layer should be emphasised. It is

therefore necessary to separate the project into the following sections: electrode

matrix layer studies, electrode kinetics and activation studies and electrode surface

layer studies.

In Chapter II, a literature review on carbon-plastic composite electrodes is

presented. Chapter ill deals with the experimental aspects involved in this project.

Chapter IV concentrates on the development and studies of carbon-plastic

composite materials used as the matrix layer of the composite electrode. The study

was initiated with low density polyethylene (LDPE) plastic which is the most

common and economical polymer material. Conducting fillers based on

carbon/graphite products were screened to determine a suitable composition for

obtaining satisfactory electrical conductivity. In order to enhance the mechanical

properties of the polyethylene based carbon-plastic composite material, however,

further work was directed at thermo-plastic rubber modification and the

preparation of rubber modified carbon-plastic composite material and electrodes.

The most important part of the chapter is the evaluation of the composite

6

materials and electrodes in a laboratory scale vanadium battery. Short-term

performance testing, long-term variation and stability testing as well deterioration

tests on the electrodes, are included in this section.

As a graphite felt based composite material is employed as the electrode for the

vanadium battery, it is desirable to establish the electrode kinetics at the graphite

electrode. Graphite felt was selected as the electrode material since it has a high

surface area for the reactions and earlier studies[27] have shown that graphite is

stable over a wide range of voltages in the vanadium redox system. The ftrst part

of Chapter V deals with the electrode kinetics of V(V)N(IV) and V(III)N(II)

couples at a graphite rod electrode. On the other hand, even though the thick

graphite felt used as the active layer of the composite electrode offers the benefit

of a large surface area and minimum polarisation losses, it results in high

hydraulic pressure when the electrolyte is passing through. Minimising the

thickness or totally eliminating the felt layer may solve this problem. This was

thus a further objective of the second which concentrates on activation studies

conducted directly on the surface of the carbon-plastic composite material.

Chapter VI concentrates on a comparison of some of the physical properties of

several kinds of thin graphite felts which were evaluated as active layers for the

electrodes. The measurements cover surface area, pore size and distribution,

electrical conductivity and electrochemical activity of felt samples from different

sources. The focus then shifts to the surface characteristics of the carbon/graphite

felts and their relationship to the electrochemical activity. Carbon-oxygen surface

7

functional groups on the felt surface and the conditions leading to their formation

are discussed. Intensive investigation on this aspect is carried out by XPS method

(X-ray photoelectron spectroscopy) and scanning electron microscopy (SEM) to

compare some surface physical and chemical properties of two different types of

graphite felts as well as their effect on the electrochemical reactivity and

electrooxidation stability. The third part of the chapter describes the

electrocatalysis of the selected graphite felt for the vanadium redox reactions in

both halves of the cell, and also presents the results from the cell performance

testing of the successfully catalysed electrodes.

The thesis ends with conclusions of the work carried out, and a number of

suggestions for further research on the carbon-plastic composite electrodes for the

vanadium redox battery applications.

8

CHAPTER II

LITERATURE REVIEW

2.1 General Requirements for Electrodes used in the Redox Flow Battery

The general requirements of the electrode material for a redox flow battery

system, can be roughly divided into three major aspects: physical, chemical and

electrochemical[31]. The physical requirements include high electrical conductivity

and mechanical strength. High electrical conductivity enables the cell to have

minimum ohmic losses during operation. Satisfactory mechanical strength of the

material is required to withstand the hydraulic pressure from the electrolyte

pumping without dimensional change during battery operation and to physically

withstand bending during assembly. Secondly, the electrode materials should be

chemically and electrochemically stable. Therefore, it should not be attacked by

the electrolyte while immersed or when current is passing through the system.

Finally, the electrochemical requirements for an electrode material are the most

important and hardest to satisfy and we need to consider some theoretical aspects

to grasp them more fully. The basic function of a cell or battery is to provide the

highest possible cell voltage and current so that the maximum power or energy

can be obtained from this electrochemical system. For example, the cell voltage of

any electrochemical cell can be simply described as[30]:

(2-1)

9

where Vcen is cell voltage, AEeq is the equilibrium potential difference between the

two redox couples selected; 'Tlc and 'Tla are the anodic and cathodic overpotentials

in positive and negative half cells respectively; and IR is the ohmic voltage drop

due to the electrode material, the electrolyte, the membrane and the contact

resistances. For an ideal electrochemical cell, the cell voltage would be the same

as the equilibrium potential difference value of the two redox couples selected, but

for any practical cell, the cell voltage, as shown above, deviates from that due to

the polarisation losses in terms of overpotentials 11 •• 'Tlc and ohmic losses.

Assuming the ohmic losses can be minimised by utilising high electrical

conductivity material, enhancing the conductivity of the electrolyte and eliminating

the contacting resistance, the cell voltage is still lower than the equilibrium

potential difference of the redox couples, due to the polarisation losses taking

place at the interface of the electrode and the electrolyte in both half cells.

Generally, electrode polarisation consists of both concentration and activation

polarisation[31]. In the redox flow battery system, concentration polarisation can

be partly reduced when the flow rate of the electrolyte is reasonable high[32,183].

Under these conditions, the activation polarisation becomes the main factor

affecting the cell voltage. It is well-known that the activation overpotential in an

electrode obeys the Tafel equation:

f1=2.3 RT log(i/i0 ) a.nF

10

(2-2)

where i is the current density passing through the working electrode, i0, the

exchange current density and 2.3RT /a.nF is the Tafel slope. According to the

equation, there are two ways to minimise the activation overpotential: by

enhancing the exchange current density or decreasing the applied current density.

The former is normally linked to the nature of the electrode material, however,

electrocatalysts are also widely used to enhance the exchange current density of

certain redox reactions[30]. The best example of this is given by Bowden and

Rideal [33]: The i0 value for hydrogen evolution was found to be w-to A.cm-2 on

mercury while 10-3A.cm-2 for the same reaction on platinum. Further increases in

i0 can be achieved by employing porous materials and the advantages of which are

briefly summarised by Bennion[34] as:

i. It reduces the true current density for a given apparent or overall current thus

minimising the activation overpotential;

ii. It increases the mass transfer rate of the reactants by reducing the distance

between the storage site and the reaction site.

Hence, from these electrochemical considerations, a suitable electrode for a redox

flow cell should meet the following basic requirements:

1. high electron transfer rate, i.e., high exchange current density for the expected

redox reactions;

2. large surface area which reduces the current density and maximises the total

current;

3. suitable surface structure for the electrolyte to access the electrode surface

readily therefore minimising the concentration polarisation.

11

Beside these technical requirements, careful consideration must also be given to

the cost of the electrode materials. Inexpensive materials are always welcomed

and considered whenever a new system is to be used in an industrial and therefore

large-scale setting.

2.2 Electrodes used in Redox Flow Cells

2.2.1 Electrode materials

In the initial research on the redox flow cell conducted by NASA-Lewis research

centre, a range of electrode materials were screened[35]. Carbon, being readily

available and inexpensive, was the first candidate material tested for several redox

couples. The evaluation was carried out using cyclic voltammetry and the results

indicated that only the Fe(ill)/Fe(II) redox reaction is electrochemically reversible

on the carbon electrode. The other redox couples like Cr(lll)/Cr(ll), V(V)N(IV),

· V(Ill)N(ll), Br-/Br3• and Ti02+/I'i3+ were found to be irreversible. In particular,

for the two promising redox couples, Cr(ill)/Cr(II) and Ti02+/fi3+, the carbon was

found to be a poor electrode material. It was observed that in the chromium

system, the coevolution of hydrogen limited the rate of charge(reduction of

chromic ion) to only a few milliamperes per cm2(io = 0.1 mA.cm·2 on carbon),

while with the titanium redox couple, a surface poisoning reaction took place

during charging.

Further studies indicated that on the B4C electrode, the bromine redox reaction

12

showed fair reversibility, while Cr(III)/Cr(ll) and V(Ill)N(II) showed improved

electrochemical behaviour on this same electrode. More extensive tests were

conducted using a laboratory scale single cell with porous carbon and graphite

materials in different forms such as cloth, felt, foam, carbon chips and reticulated

vitreous carbon (RVC)[35]. It was found that the capacity loss was unacceptably

high for the Fe-Cr redox cell. Therefore, the screening was expanded to utilise

metals like Ag, Ta, Pb, Bi, Ti and Nb, and alloys such as Hg-Ag, Hg-Pb and W

Re in an attempt to find a more reactive surface for the Cr(ID)/Cr(ID redox

reactions. Better cell efficiencies and power outputs were obtained with some non

graphite surfaces such as Pb, but an undesirable lead dissolution during charge

was observed. For the titanium couple, it was found that an oxidised screen of an

alloy of 97 percent tungsten-3 percent rhenium gave much better reversibility than

the graphite electrode.

As a result of the early research work on electrode materials, a further project was

establish to develop an electrocatalytic surface on the graphite felt substrate[36-

38]. The Gold-Lead catalyst system was found to be successful for enhancing the

Cr(III)/Cr(ID redox reaction rate and reducing hydrogen evolution. This was

believed to be due to the catalysing ability of gold for the redox reactions and the

high overpotential for hydrogen evolution on the lead surface. The catalyst system

was prepared either by electrodepositing gold and lead[36] or by gas

reduction[37]. In both case, encouraging results were achieved from the cell

performance testing with the gold-lead catalysed graphite felt electrode.

13

To avoid using the expensive metal catalyst, further efforts were made with

surface ion-etching onto the planer carbon surface and with using silver-lead and

bismuth-lead catalysts as an alternative to the gold-lead system[37]. The former

one produced a negative effect, i.e., hydrogen evolution was more active than the

chromium reaction, while the latter showed some benefit but not as good as the

gold-lead catalyst.

A Japanese group at ETL (Electrotechnical Laboratory) which has also been very

active in the development of the redox flow battery, also carried out electrode

materials screening[39-41]. Their interest in electrode materials has concentrated

on various graphite fibres from all the sources throughout Japan, and over one

hundred carbon/graphite fibres obtained by pyrolysis of various hydrocarbon

substrates were evaluated. The results obtained indicated that some of the PAN

(polyacrylonitrile) based graphite fibre electrodes showed good cell voltage

efficiencies and are suitable as electrode materials.

Another valuable research effort on electrode materials for redox flow cell was

conducted by Ashimura and Miyake[32]. In their studies, a porous flow-through

carbon electrode was employed. The cathodic polarisation behaviour of the

Fe(IIT)/Fe(m and BrJ.Br· redox reactions with the electrode was evaluated and

compared with that obtained with a smooth platinum electrode. The results

revealed that the electrode kinetics on this electrode were controlled by mass

transfer, and the electrode polarisation was reduced as the flow rate of the

electrolyte increased. The effect of electrode thickness and surface area were also

14

evaluated, and it was found that increasing both the surface area and electrode

thickness resulted in a decreased polarisation.

Electrode materials evaluation and development were also carried out in the all

vanadium redox battery system[26,27,42-45]. A number of materials including

metals like gold, lead, platinised titanium, iridium oxide dimensional stable anode

(DSA) and carbon graphite products, such as graphite plate and rod, glassy carbon,

carbon/graphite felts, carbon cloth and graphite paper were tested. It was reported

that on gold and glassy carbon electrodes the V(V)N(IV) redox reactions are

irreversible, while on the lead and titanium electrodes, passivating phenomena

were observed in the potential range of interest. However, platinised titanium and

iridium oxide DSA electrode showed better electrochemical reversibility and cell

performance than the other materials[26]. The glassy carbon electrode showed a

slow electrode reaction rate for both V(V)N(IV) and V(III)N(II) couples and was

also very sensitive to the electrode surface preparation procedure[27 ,42].

Other efforts were concentrated on using carbon/graphite felt based electrodes[43-

45], and promising results were obtained from both the laboratory small scale

cells[ 43,44] and a 1 KW prototype battery[ 45]. In the former case, one cell with a

carbon felt electrode was tested over 2,000 hours and still retained good

charging/discharging behaviour, while a 90% overall energy efficiency was

obtained from the 1 KW prototype battery employing graphite felt/carbon-plastic

composite electrodes.

In summary, it appears that the preferred electrode materials for a redox flow cell

15

are porous carbon/graphite felts, cloths and foams, even though the electrode

kinetics for most redox couples on these surface are relatively slow.

2.2.2 Structural Characteristics of Electrodes for Redox Flow Cells

A suitable structure of the electrodes is always desirable for any kind of

electrochemical cell or battery. For example, in order to maximise the power

output in the hydrogen-oxygen fuel cell, metal materials, such as palladium,

platinum, silver and nickel, which possess high activity for the hydrogen and

oxygen reactions, always have high porosity to assist the electrochemical reactions

in the gas-solution-solid interface[46]. For a redox flow cell, even though only two

phases are involved, the importance of a porous structure still applies. It is

generally agreed that, most redox reactions at a carbon electrode are slow, i.e.,

they exhibit low exchange current density values. According to a simple

mathematical model of a porous electrode, i0, the exchange current density, act as

the product ai0 where "a" is the active electrode area per unit volume. If the value

of io for a desired reaction is low, it might still be made to operate at an

acceptable apparent current density and low overpotential by using a porous

electrode with a large "a". The other benefit of using a porous electrode, as

mentioned previously, is the increase in the mass transport rate by reducing the

distances between storage sites and reaction sites[34]. This is probably the reason

why in almost all redox flow cell systems, porous electrodes are commonly

employed.

16

Even though the principle is the same for each, there are still a number of types of

porous electrodes that exist for redox flow cell application, each with its

advantage and disadvantages. The discussion which follows looks at four types of

electrode:

1). Bulk porous electrode

Bulk porous electrode means that the bulk electrode material has a porous

structure and the electrode functions as current collector and electrochemical

reactor. This kind of electrode is widely used in fuel cells. The "Raney" electrode, ,,

which is made of a electroactive metal for the hydrogen or oxygen reactions, is an

example. By forming a metal mixture of an active metal with a undesirable metal

like aluminium, and then dissolving the unwanted metal in strong alkaline

solution, a porous metal electrode, the Raney electrode can be obtained[ 46]. The

method for making a porous carbon electrode is also reported in the same book.

As another example, bulk porous carbon electrodes were employed as positive and

negative electrodes for Fe-Cr and Fe-Br redox flow cells by Ashimura et al[32,

47-50]. In their research, the pore size and surface area of the porous carbon

electrode and their effects on electrolyte flow rate, electrode polarisation behaviour

and cell performance characteristics were evaluated. The regeneration of the redox

couple was also one of their main interests.

The advantages of this kind electrode is the ease of operation and assembly. Its

main disadvantage is the lack of design flexibility.

17

2). NASA cavity fill-in electrode

The configuration of a typical NASA laboratory scale redox cell is illustrated by

Figure 2.1[4] and 2.2[183]. Among the components shown, two pieces of graphite

cloth are the inert electrodes for both the positive and negative half cell. Two

graphite plates in the cell terminal are used as current collectors by contacting the

graphite cloth by compression. Because the cell cavity is determined by the

thickness of flow frame and the carbon/graphite felt or cloth are compressed to the

same thickness, it is called a cavity fill-in electrode. This electrode design was

copied by other groups who studied redox flow cell, like the Japanese group at

ETL[38-40]. In the early research work on the all-vanadium redox flow cell

reported in 1987[43], this concept was also employed and the cell performance

was tested with Le carbonne graphite felt electrodes.

It can be seen that in this design, the electrode consists of two zones: an

electrically conducting zone and an electrochemical reaction zone. The graphite

plate in each terminal of the cell functions as the electrically conducting zone, or

in general, as the current collector, while the graphite felt or cloth acts as the

active layer for redox reactions. The separation of these two layers gives the cell

the benefits of being easy to seal and easy to replace the active electrode

materials. The problem it raises is that the thickness of the active layer should be

18

Lead

,_. \0

Inlet

L Graphite current

collector

Figure 2.1

' ' ' ' ' ' ' '

Graphite

Graphite cloth electrode-, I J I I

/ L Anion-permeable

membrane

'-Rubber gasket

30-32 mils thick

(0. 80 mm)

-./

/ /

/

/ /

/

current collector \ \ \ \

Configuration of NASA laboratory redox flow cell. (Ref 4.).

Trim cells

Insulator

pi<Jte

End plate

llll!

Bipolor p\ote

Outlet

Hydrogen inlet

Repeating/ ): cell units '''"

Reb<Jiance cell Open-circuit voltage cell

Figure 2.2 Components of a single redox flow cell and full- function stack.

(Ref 183).

20

higher than the flow frame otherwise high ohmic losses will be introduced by the

poor contact between the active layer and current collector. A high compression,

however, leads to a high pressure drop through the cell, which must be avoided if

pumping energy losses are to be minimised. The other disadvantage of this

electrode arrangement is that the poor flexibility of carbon/graphite materials

requires the carbon/graphite plates to have a minimum thickness (e.g. 0.5 em),

resulting in a high space occupation of the cell stack.

3). Multi-layer bonded structure- conducting carbon-plastic composite electrode

It is not a new idea that a better electrode structure for the redox flow cell could

be achieved with a multi-layer bonded structure. In their paper on the technology

of PTFE-bonded carbon-plastic composite electrodes for fuel cell application in

the past few decades, Kordesh et al[29] pointed out that to optimise the properties

of a fuel cell, only a multi-layered, composite electrode structure can be

successful. The principle layers considered for the fuel cell electrodes are as

follow:

(i). A highly catalysed carbon layer is required. It should not be too hydrophobic,

otherwise it will not achieve a good interfacial contact with the electrolyte.

(ii). The second layer, often called the"diffusion layer", has the purpose of

transporting the gas with a minimum of gas-pressure drop (absolute or partial

pressure drop) to the wet catalysed carbon layer.

(iii). The third component is the current collector.

21

Even though the functions of each layer are not exactly the same for the redox

flow cell, the principle is almost the same. In the redox flow cell, layer (i) and (ii)

would merge into one layer and instead of the requirement of possessing low gas

pressure drop, it should allow rapid access by electrolyte so that the hydraulic

pressure can be minimised.

This idea has already been successfully applied not only to a fuel cell but also to a

redox flow cell system. Fujii et al, of the Japan's NEDO (New Energy

Development Organisation) used carbon-plastic composite electrodes for 80, 400

and 4000 KW zinc/bromine battery stacks and reported the advantages of low cost

and large capacity by using conductive carbon-plastic composite electrodes[51-

53]. Kazacos and Skyllas-Kazacos also presented encouraging cell performance

results, e.g. 88% overall energy efficiency, in both laboratory and 1 KW prototype

vanadium redox battery with the same type of electrodes[43,44].

The structure of carbon-plastic composite electrode is shown in Figure 2.3.

Differing from the NASA type electrode, the electrically conducting layer, the

graphite plate, is replaced by a conducting carbon-plastic layer with a metal foil or

mesh which increases the electrical conductivity of the composite materials (in the

case of end-electrode) and the principle layers are bonded to form a complete

electrode. Depending only on the electrical conductivity of the material and the

bonding techniques, and not on the compression of the electrode active layer to

the graphite plate, the ohmic losses from contact can thus be minimised. The other

main benefits of using conducting carbon-plastic composite electrodes are their

22

graphite felt ~ ------

!IIJ\ ~llll 1 1\1111

II l)l\11\'ll\\li1

1ji'I!\l\\. \\ '\ 1~\1!1: tl: II I .J.Itl 1.\r , 1.,

1~!!1!11 Jlllj ~~~~~~ ~ jml!jlll jll\1 I \!IIlli 1

11111:! IIi i[,Jiq lit lill·i ill ·I, IH.! carbon-plastic plate

(a)

graphite felt ~

carbon-plastic

plate (with Cu foil)

(b)

Figure 2.3 Configuration of carbon-plastic composite electrode, (a) bipolar

electrode; (b) end-electrode.

23

low weight and low cost compared with the graphite or carbon plate, and the ease

with which they can be moulded into any size and shape. This means that a thin,

impermeable and conducting carbon plastic sheet can be used as an alternative to

a graphite plate leading to a significant reduction in cell thickness.

4). Multi-function Electrodes

The term multi-function electrodes refers to electrodes which have functions in

addition to those of a normal electrode. One example is a flow frame attached

bipolar carbon-plastic electrode created by the Exxon Research and Engineering

Company[54]. In order to overcome a number of problems encountered in the

Zinc-Bromine redox cell, such as cell leakage caused by electrode thickness

variation, the necessity for inner tie rod bolts and slow processing, resulting from

the separation of bipolar electrode and flow frame, an insert-injection moulding

technique was developed. By replacing the former separate structures with the

insert-injection moulded electrodes (one bipolar electrode and two flow frames

being combined together), the problems mentioned were greatly diminished. The

labour per unit cell was also decreased.

The other example was given by DeCasperis et al, who invented an electrode with

the thickness and porosity of the edge part being different from that of the central

part to prevent gas leakage in the fuel cell[55]. By using a thermosetting resin

carbon fibre composite material, a highly effective edge sealed electrode was

completed, and gas leakage was minimised.

24

As the third type of electrode structure described above was the main interest of

the present project, further detailed discussion is presented in the following

section.

2.3. Conducting Carbon-Plastic Composite Electrode

2.3.1 The concept of conducting carbon-plastic composite electrode

Nearly all plastics are poor conductors of electricity, and their resistivity is in the

order of 1012-1016 n.cm. In the past few decades, the demand for plastics with

certain electrical conductivity is increasing. Depending on the application, the

requirements for the electrical conductivity vary. For example, in the case of

preventing static charges for safety reasons, a material with a resistivity value of

1 OS n.cm might be adequate, while in the case of electrode materials for energy

storage and conversion, as described previously, the higher the conductivity, the

better the materials for battery application. As an example, a conductive plastic

with a resistivity value less than 5 n.cm was considered as a candidate for

developing an electrode material for the zinc-bromine battery[54]. The

conductivity of conducting plastics required for the electrochemical cell

application should be at least close to those of metal conductors.

There are two different methods for producing conductive plastics:

25

(i). By synthesising special polymers that have conjugated double bonds that can

be made highly conductive by complexing or doping[56-58], i.e., intrinsically

conductive polymer.

(ii). By incorporating conductive fillers in insulating plastic matrices which can be

defined as composite materials.

As reviewed by Chandler and Pletcher[59], there is a large number of intrinsically

conducting polymers. Among them, polypyrrole, polyacetylene, polyaniline and

polyfuran are characterised by electronic conductivities up to 104 n-1.cm-t, which is

certainly high enough for them to act as their own current collector and hence

possible to be used as electrodes for battery systems. Unfortunately, most of these

are unstable when exposed to ambient conditions. For example, the conductivity of

polyacetylene decreases with time even under inert condition. Polypyrrole which is

considered the most stable conducting polymer, was found to be attacked by the

acidic and oxidative positive electrolyte of the vanadium battery[28]. Therefore,

intrinsically conducting polymers are unsuitable for this project.

Composite materials are formed by incorporating conductive fillers in insulating

plastic matrices. There are a few kinds of conducting fillers commonly employed

in fabricating conducting composite plastics. They are:

1) carbon black, carbon/graphite fibres and powders;

2) metals in the form of powders, fibres and ferrites;

3) metal-coated fillers, e.g. aluminised glass fibres, nickel-plated mica, etc.

26

Since the metal filler would be attacked by the acidic electrolyte, the interest of

the present study is further narrowed onto the field of composite materials based

on carbon and graphite fillers.

In general, the composite sheet itself has a low surface area and it is always

necessary to attach an active layer to form a complete electrode. These active

layers are also made of carbon or graphite. Therefore. the concept of carbon

plastic composite materials are those made by mixing conductive carbon fillers

with nonconductive plastic resins. Carbon-plastic composite electrodes refer to

those consisting of carbon-plastic composite matrix layers and electroactive

surface layers based on carbon or graphite.

2.3.2 Advances in Carbon-plastic Composite Electrodes

2.3.2.1 Carbon-PTFE composite electrodes

The early applications of carbon-plastic composite electrodes in electrochemical

cell research was based on carbon-polytetrafluoroethylene (PTFE) composites. A

detailed overview in this area was given by Kordecsh et al[29], and as described

in their paper, The Energy Research Corporation (ERC) made a great deal of

effort to develop carbon-PfFE composite electrodes for fuel cell applications. For

example, in order to improve the electrode hydrophilicity of the normal carbon

PTFE composite electrode for fuel cell and other electrochemical cell applications,

Baker et al[60] invented a new composite electrode material using a low

27

percentage of PTFE and a high proportion of electroactive material. The

composite electrodes were then tested in a few different batteries like nickel-zinc,

silver oxide-zinc, and nickel-cadmium cells. Higher cell open-circuit voltages and

amps-hours were obtained from this novel carbon-PTFE composite electrode.

Again Baker et al described PTFE-bonded substrates and PTFE-bonded fuel cell

electrodes in great detai1[61]. The substrate manufacturing starts with graphite and

dry powder PTFE (7.5%), mixed into a lubricant (mineral spirit); the lubricant is

filtered off and the remaining "dough" is rolled in repeated milling operations to a

sheet with a thickness of 0.25 mm. In the next step the dried material was pressed

at about 25 kg/cm2 pressure. The sheets have a porosity of 55% and a resistivity

of 0.05 Q.cm in the plane and 0.07 Q.cm perpendicular to it.

Other examples can be seen in a series of U.S. Patents [62-65]. The former

patents[62-64] propose the use of non-conductive plastic binders which are

rendered conductive by the presence of conductive carbon. Such conductive

plastic, e.g., PTFE, is admixed with active material and the mixture is then

fabricated into sheet form by moulding under pressure at the melting temperature.

The latter patent[65], PTFE is used to provide a degree of hydrophobicity in a

battery electrode, analogous to the fuel cell electrode use of PTFE, to increase the

gas recombination capability of the battery and hence diminish the rate of battery

internal pressure development.

More details about the carbon-PTFE composite electrode was given by Kordesch

et al in [29], while the effects of physical structure on the cell performance of

28

· carbon-PTFE composite electrode was studied by Liu[66].

2.3.2.2 Polyolefin Based Carbon-Plastic Composite Electrodes

Apart from PTFE plastic which is expensive and difficult to process, polyolefin is

low cost and easily moulded. Furthermore, polyolefin plastics are chemically

stable in most aqueous systems and organic solvents. They are mechanically rigid

and non-brittle, making them the most common plastic resins employed in

fabricating carbon-plastic composite materials and electrodes for electrochemical

cell applications.

The development of polyolefin based carbon-plastic composite electrodes started

in 1959 at Union Carbide Corporation (UCC). The early research work developed

an electrode for an oxygen sodium-amalgam cell. Polyethylene bonded active

carbon was used as the electrode active layer[67]. In the years between 1960 and

1970 the development of multi-layered plastic-bonded "composite" electrodes was

aimed at alkaline hydrogen-air fuel cells. The plastic materials were polyethylene

and PTFE[29].

A great deal of research work in this field was encouraged after the redox flow

cell concept was introduced. Bellows et al[54], who was developing a circulating

zinc-bromine battery found that previous zinc-bromine batteries had been plagued

by bromine corrosion of metallic electrodes. Even titanium-based electrodes

showed evidence of pitting corrosion, resulting from bipolar shunt currents and

29

from cell reversal. By using a conductive carbon-plastic composite electrode, this

particular problem has been solved. The carbon-plastic composite electrode is used

for both the zinc and bromine electrodes. An extra layer of porous carbon is added

at the Br2 electrode to increase the reactive surface area. According to their report,

the conductivity of the carbon-plastic composite electrode they developed was

sufficiently high (about 1 (O.cm)"1 ) that there is virtually no ohmic loss in the

case of the bipolar electrodes. However, it is not high enough for monopolar, i.e.,

end-electrodes, but a metallic current collector can be moulded into the carbon

plastic to increase lateral conductivity. There is no observable contact resistance

between the metal screen and carbon-plastic because of the high contact area and

high compressive stresses being used during moulding operations.

As the electrode component, an electrically conductive carbon-polyolefin

composite material was invented by Tsien[68], member of the mentioned zinc

bromine battery research and development group of Exxon Research and

Engineering Company. In his invention, a balanced mixture of fillers was

employed to increase the extrudability of the composite materials. The

composition of the composite comprised a polypropylene-ethylene copolymer; a

silica filler; a fibre-reinforcing agent and a high surface area carbon black. A

composite having 100 parts of copolymer, 15 to 30 parts of carbon black, 0.25 to

1.0 parts fumed silicon dioxide and 1 to 10 parts of a fibre reinforcing agent

selected from carbon fibres and glass fibres gives rise to an electrical resistivity

of about 2 Q.cm and excellent flexural strength, suitable for electrode fabrication.

30

Additional inventions in polyolefm based carbon-plastic composite and electrodes

can be seen in U.S. Patents[69-72]. For example, Meyer[69] invented a high

density polyethylene based carbon-plastic material with stabilising agents to

prevent deterioration of the electrical conductance characteristics, while

Kawashima et al[70] described their invention of a temperature sensitive carbon

polyethylene composite material to allow this material only to work within a

certain temperature range. A great deal of research and development of carbon

plastic electrodes with different composition and structure were reported in recent

decades. Among them, Ogawa and Kishi[73] , of the Toho Rayon Co. Ltd.,

reported a bipolar electrode for redox flow cells. The electrodes are prepared from

mixtures of carbon fibres and carbon black blended with a resin. Thus, a mixture

of 35% conductive carbon black and 65% powdered polyethylene was hot pressed

to form plates having 30 mm diameter manifolds and slit, and areas around the

manifolds and the slits were coated with a polyethylene based electrical insulator

to obtain bipolar electrodes. A 6-cell Fe-Cr redox flow battery using these bipolar

electrodes was found to give high coulombic efficiencies and a low cell resistance,

e.g. 94.2% and 2.3 O.cm2 respectively. Ogawa et al also invented an activation

technique to active a carbon felt electrode made of acrylic-based carbon fibres[74].

The felt was carbonised at 1300 °C, in nitrogen gas for 5 minutes, activated at 880

°C in steam for 30 minutes, and electrolysed in a 20% hypochlorite solution with

3 V d.c. for 5 minutes to obtain a felt with surface area of 20 m2/g and containing

3.9% N and 28 ppm Cl. An Fe-Cr redox flow cell using this electrodes had

coulombic efficiencies over 98%. Jinnai[75] developed a carbon-plastic electrode

which consists of composite materials having around 50% carbon black and

31

graphite powder and two electroactive layers, a porous carbon-plastic layer and a

carbon cloth. Long life time, light weight and low cost was reported for this

electrode. To improve the cell performance of carbon-plastic electrodes in a large

capacity redox flow cell, Kitamura[76] of Mitsubishi Plastics Industries Ltd.

invented a non-fraying and long lifetime electrode with an edge-sealed structure.

Instead of using the conventional blend techniques for preparing carbon-plastic

composite materials, a new method was created by some Japanese researchers[77-

81]. Kitanak:a et al invented a polyolefin based carbon-plastic composite electrode

with very low electrical resistance through the thickness direction and suitable for

redox flow and metal-halogen batteries application[77,78]. According to their

report, 2 carbon fibre mats (base wt. 30 g/m2) and three pieces of 100 micron

conductive polypropylene sheets were stacked alternately, pressed at 250 °C, and

15 kg/cm2 for 5 min, cooled to 100 °C, the pressure was released, the composite

was cooled to room temperature, sandwiched between a pair of PAN-base fibre

cloth (fibre diameter 7 micron, 6000 fibres/strand, and base wt. 600g/m2), pressed

at 240 °C, and 10 kg/cm2 for 5 minutes, cooled to 100 °C, the pressure was

released, and cooled to room temperature to obtain an electrode having a

resistance of 0.045 n.cm2 in its thickness direction and impermeable to liquid.

These electrodes have been employed in the vanadium redox flow battery and

promising results were obtained under the normal cell performance

conditions[44,45]. However, electrode deterioration phenomena was observed

when the cell was operated under overcharge condition for a short period.

Furthermore, small pinhole in the conducting plastic layer led to eventual

32

penetration of the electrolyte to the copper current collector at the end-electrode

resulting in corrosion of the copper and contamination of the vanadium

electrolytes[192].

Fushimi[79], of The Meidensha Electric Mfg. Co. Ltd., also invented a carbon

polyethylene composite electrode, which has a polyethylene-impregnated carbon

fibre sheet sandwiched between a pair of carbon-plastic layers and then pressed to

form the electrode matrix layer. Electrode of this structure were reported to have a

high resistance to warping, and Zn-Br batteries using these electrodes have long

cycle life.

Employing the similar principle, but using epoxy resin instead of polyolefin

plastic, Ogawa and Kishi[80,81] reported a successful bipolar electrode for redox

flow batteries application. This electrode was fabricated by hot-pressing 3 pieces

of carbon-fibre cloth impregnated with a blend of carbon-resin to form a 0.1-lmm

thick bipolar matrix plate and then bonding two carbon-fibre sheets on both sides

to complete the manufacturing. A Fe-Cr redox battery using this bipolar electrodes

had 1.8 n.cm2 cell resistance and 81.4% energy efficiency.

It is obvious that the latter technique, i.e. the "sandwich method", of fabricating

carbon-plastic composite electrodes has some advantages such as uniform

electrical conductivity and mechanical properties through the plane. However, the

thickness of the blended carbon-plastic layer is a critical point, because when too

thick it will result in the electrical resistance increasing through the thickness

33

direction, while if too thin it could increase the electrolyte permeation through the

formed electrode matrix plate, and also increase the difficulty of the subsequent

active layer bonding procedure.

There are still a large number of carbon-plastic composite electrodes based on

thermoplastic rather than PTFE and polyolefin, like polyvinylidene difluoride

based carbon-plastic composite[82] and cross-linked poly(styrene)-co

poly(vinylpyridine) carbon composite electrodes[83]. The former requires longterm

(more than 170 hours) to prepare and the latter shows unsatisfactory mechanical

properties. They are therefore not considered appropriate for redox cell

applications and are therefore not included in the present project.

2.4. Carbon-plastic Composite Materials

As described previously, a carbon-plastic composite electrode is a combination of

a composite matrix and a surface active layer. Even though the general

requirements for an entire electrode have been discussed, further analysis of the

individual layers is desirable, since each layer has its specific functions, i.e., the

former acts as electrically conducting and mechanically supporting layer while the

latter functions as the electrochemical reaction zone, and in fact, the requirements

for each layer are different. It is therefore necessary to separate the review into

two sections: carbon-plastic composite materials and surface active layers.

In this section, the factors influencing the properties of composite materials are

34

reviewed, especially, as the most important factor for electrode materials

applications, electrical conductivity of carbon-plastic composite materials are

emphasised. In order to understand more about the electrical conductivity

behaviour of composite materials, composite materials filled with metallic particles

are also included.

2.4.1 Mechanism of Electrical Conducting in Composite Materials

Being a mixture of insulating polymers and conducting carbon fillers, carbon

plastic composite materials are the typical two-phase systems. A transition-like

phenomenon (i.e., the conductivity of the composite discontinuously increases at

some content of the conductive phase) is always observed in conductive composite

materials (see Figure 2.4). The electrical conduction mechanism of two-phase

systems, in particularly, thermoplastics and elastomer have been well studied[84-

94], and a few models have been established to explain and predict the electrical

conduction behaviour for existing or new composite materials. Grekila and

Tien[84] studied the electrical conductivity of Zr02-Ca0 composite and explained

the electrical conduction phenomena with a "contact" theory. This means that

when the content of conducting phase reaches a certain value, the electrical paths

formed by contacting are completed, therefore, the sharp increase in conductivity

can be obtained after that point. Aharoni[85] also utilised a concept of an average

number of contacts to discuss the possibility of the electrical network formation of

thermoplastic composites. Some other researchers[89,90] who studied the electrical

conduction behaviour of elastomer composites have a different point of view.

They believe that the jump of the electrical conductivity is due to the "tunnel

effect" in the thin layers of insulating phase sandwiched by conducting

35

3 10

Resistivity (ohm- em)

- o-O-o 0 '

v 1

' 0 '

o o-

~ ~------_.--------~--------~---0 5 10 15

Volume Percent Block

Figure 2.4 Transition-like phenomena of composite material (Ref 87.).

36

particles, and not to a "passing through network condition". Some different

interpretations were given by Bueche[86,87] and Miyasaka et al[88]. The former

applied Ploy's "gelation" theory to predict the sharp increase of the electrical

conductivity at the particular carbon contents, while the latter raised a "particle

coagulation" theory based on their experimental result observed, i.e., gas

absorptivity of carbon powder in the carbon-natural rubber composites markedly

decreases near the break point of the electrical conductivity, and this reduction

must be due to the decrease in the sorption sites on the surface of carbon particles,

which is caused by the coagulation of the particles.

2.4.2. The Effect of Conducting Filler Content on Electrical Conductivity of

Composites

Regardless of the different explanations of the sharp change of conductivity, one

common point that can be seen is that a certain critical content of conducting

fillers should be added to the composite so that the composite behaves as an

electrical conductor and that the conductivity of the composite will be determined

by the number and geometry of continuous paths of the conducting phase[84].

Depending on the nature and the particle size of the conducting filler, the critical

content and trends of conductivity against content after the critical point are

different. For the Zr02-Ca0 system, the critical content of CaO was 0.39 mole

fraction[84], while for PVC-Ni system, it was found to be 10% (volume percent)

Ni of the composite[93]. In a PMMA-Cu composite, the sharp change in

conductivity was also found around 10 volume percent[92], at which the resistivity

value of the composite changed from 1012 .Q.cm to 1~ .Q.cm, while in a ~6H74

wax-carbon black system, the critical increase in conductivity was observed at 7

volume percent black[87], where resistivity drops from 105 down to 101 .Q.cm.

37

Mter the critical point, the trend in electrical conductivity versus content of

conducting filler is moderate. For example, in the iron/polymide-amide composite,

"square" resistivity changed from 4x1011 Q to 3.6xl(f Q at 19.5 V% iron and only

a very small decline was observed when the iron content increases up to 50 V%

[85]. However, depending on the type and particle size of the fillers, some

difference can be observed[84,85,87,92,93].

2.4.3 The Influence of Particle Sizes and Surface Area of Conducting Fillers

on Electrical Conductivity

According to the two-phase electrical conducting theories[84-88], in particular, the

"contacting" theory discussed in [84,85], the electrical conducting paths are

formed by contacts between conducting filler particles. Therefore, increasing the

possibility and the area of contact should lead to an increase of conductivity. The

surface area of conducting fillers increases with decreasing particle size, so that

this will result in a large contact area. By applying small particles and increasing

the content of total particles, the probability of forming current paths increases.

Based on various experimental results, Scarisbrick[95] pointed out that at a

distance of less than 10 nm between the particles, a transfer of electrons through

the polymer becomes probable. Malliaris et al[93] studied the influence of particle

size on the electrical conductivity of a HDPE-Ni compacted mixture system and

found that, at the same loading, increasing the ratio of ~ (radius of polymer

particles) to ~ (radius of metal particles) resulted in an increase in the electrical

conductivity of the composite. The critical content at which conductivity suddenly

jumps, also decreases. A few mathematical models [86,88,93] have been

established and used to predict the relationship between resistivity of composites

and the characteristics of conducting fillers (particle size and volume). For

38

example, Malliaris and Turner [93] related the volume of conducting fillers to the

radius ratio of polymer to metal particles:

VB = 100[l+(<l>/4)(R/Rm)J1

where V u is the volume at which the formation of the electrical conduction path

completed, and ~ is the radius of polymer and ~ the radius of metal, and <I>, a

factor which depends in the mode of packing of the metallic particles. This model

has been found to be fitted by experimental data.

The influence of carbon black particle size on the electrical conductivity of the

carbon-plastic composites has been extensively and intensively investigated[96-

99]. Even though the electrical conductivity of carbon black is lower than that of

metal particles, the trends in the relationship between conductivity and particle

size are the same.

2.4.4 The Influence of Polymer Phase On Electrical Conductivity of

Composites

The previous few sections on composite materials, focussed only on the

conducting phase. As described by Grekila et al[84], however, the properties of

two-phase system are dependent on the microstructure as well as on the properties

of each phase. The polymer phase, although insulating to electrical current, acts as

a matrix for the composite, influencing the conductivity of composite materials in

several ways as a result of the different polymer characteristics. Essentially, these

are[96]:

a) different rheological behaviour in the processing stages;

b) different wettablity of polymer for conducting fillers, especially, for carbon

39

black;

c) different crystallinity;

d) multi-phase polymer construction(e.g. ABS);

e) the specific resistance of the polymer itself.

One of the most important factors, the influence of crystallinity of the polymer on

the electrical conductivity, was examined and comparison was given for several

carbon black-thermoplastic composites. It was found that the conductivity of the

composites at the same carbon black loading obeyed the sequence: PP

(polypropylene) >HDPE (high density polyethylene) >LDPE (low density

polyethylene) which corresponds to the crystallinity order[96].

It should also be noted that the crystallinity of the polymer phase is affected

greatly by a number of factors, such as process history and characteristics of

fillers. The former includes cooling speed, rheological conditions, presssure

conditions and availability of a crystal-nucleus promoting substance, while the

latter relates to type and particle size of conducting fillers[96].

Miyasaka et al[88] also concentrated their studies on the influence of the polymer

phase on the electrical behaviour of the composites and concluded that the critical

carbon content corresponding to the break point varies depending on the polymer

species and tends to increase with the increase in the surface tension of the

polymer species.

There are still other factors, like processing conditions, influencing the electrical

behaviour of the composite materials. These are not covered in the present review,

however, detailed discussion can be obtained from references [96-99].

2.5. Carbon/Graphite as Electroactive Layer

40

Carbon/graphite materials are widely used as electrodes for electrochemical cell

applications because they are inert to most chemicals and solvents, and also

because of their reasonable cost. There are a great deal of commercially available

carbon and graphite products, each composed almost exclusively of carbon atoms

and yet having distinctive properties[100]. In the field of redox flow cell,

carbon/graphite materials in high surface area, high porosity forms are the most

preferred[34]. The utility of carbon cloth in NASA Fe-Cr redox flow cell is one of

examples[35-38]. But again there are several hundred grades of carbon cloth and

felts available from different sources and it was found that the variation in

electrochemical activity from felt to felt is considerable [39-41]. An understanding

of the physical and chemical properties of carbon/graphite materials employed as

electroactive layer and their relationship to the electrochemical reactivity would

thus assist in the optimisation of efficiency for a certain electrochemical system.

On the other hand, oxygen complexes are easily formed on the carbon and

graphite surface[101] and are known to influence the electrochemical behaviour of

the electrodes. Suitable surface treatment would enhance the electroactivity while

severe oxidation would result in deterioration of the carbon/graphite electrodes.

Furthermore, compared with noble metal electrodes, the electron transfer rate of

many redox reactions at carbon/graphite electrodes is much slower. It was realised

that trace amounts of some noble metal on carbon substrate give much better

electrode performances[36-38]. A consideration of electrocatalysts is therefore

desirable.

41

2.5.1 Surface Microstructure of Carbon/Graphite Materials and Its Relation

to Interaction with Oxygen

Thrower[lOO] has stated that the properties of carbon/graphite are always

determined by the microstructure of the materials, which are in turn determined by

the starting materials, or precursors, and the processing conditions including

forming method, heating rate, fmal temperature, etc. The details of the

carbonisation process and the formation of possible compounds and structures

according to each temperature step were clearly described by Singer and his co

worker[102,103]. As an example, Figure 2.5 illustrates the carbonisation process

for a graphitizable organic material. Depending on the processing temperature, the

materials will behave like carbon (below 2500 °C) or graphite (above 2500 °C) or

in many cases, have both characteristics. From the aspect of microstructure,

Golden et al[lOl] defmed graphite as crystalline carbon, which exhibits well

ordered parallel stacks of carbon layers, and carbon black as amorphous

microcrystalline carbon which is composed of more disordered structures. A

schematic diagram explaining the structural difference between these two typical

carbon materials is shown in Figure 2.6[101].

Besides the processing conditions, the microstructure of carbon/graphite materials

are dependent on the precursors, or raw materials used. Again Thrower[lOO]

studied the effect of precursors including needle coke, fluid coke and Santa Maria

coke, on the microstructure of the final graphite products by transmission electron

microscopy (TEM) and concluded that different precursors can be used to produce

42

-H

CC0~-5000C AROMATIC HYDROCARBON

l COKE

J CARBON

" GRAPHITE

Figure 2.5 Schematic diagram of the carbonisation process for a

graphitizable organic material (Ref 102.).

43

graphite materials with quite different microstructures.

In contrast to the bulk carbon/graphite materials considered above, carbon fibres

are relatively new materials. New types of fibres from various sources are

constantly being introduced and the influence of processing on the microstructure

of the fibres is still not well understood. However, the raw materials for

fabricating carbon fibres are well recognised. The most common precursors used

for carbon fibre manufacture are rayon, polyacylonitrile (PAN) and pitch[lOO],

among them, the latter two are of major current importance. The effect of

precursors on the microstructure of carbon fibres has been reviewed by

Johnson[104], and it is generally agreed that PAN and pitch based fibres have

their own specific microstructure. For high modulus (type I) PAN fibres, it is well

recognised that they have an outer sheath in which the turbostratic layer planes

and crystallites are more highly oriented than in the core, and are also

considerably larger, while the structure of the central core region is very

complicated and considered much more random[105].

In pitch fibres which have an almost perfectly circular cross section, the crystallite

size is larger and is considered a consequence of the liquid crystal origin[106].

Thrower and Jones studied pitch fibres (VSB-32) and found that they have a

similar microstructure to the high modulus PAN fibres, which are composed of a

microfibrillar core surrounded by an oriented sheath[107].

Many other factors which are not included here are also very important in

44

(a)

Stt·ucture of Microcrystalline Carbon

(b)

Figure 2.6 Structural comparison of graphite and carbon, (a) graphite; (b)

carbon (Ref 101.).

45

influencing the microstructure of carbon/graphite materials. It is therefore not

surprising that the remarkable differences in properties, especially chemical and

electrochemical activity are always observed for carbon products. It would be of

great value therefore, to relate the microstructure and the activity of carbon so that

a maximum efficiency would be obtained by using the proper carbon materials or

by modifying the surface to suite the selected system requirements.

Golden et al studied the relation between crystalline nature and the oxygen

chemisorption ability of carbon materials and concluded that the chemisorption of

oxygen on carbon is associated with unsaturated carbon atoms either at the edge

of basal planes or present as defects in the basal planes. Both edge area and defect

concentration are greater in microcrystalline carbon than in graphitic materials

[101]. Based on their interpretation, a summary can be made in Table 2.1.

In fact, the oxygen chemisorption on carbon/graphite surface at room temperature

has been recognised for over 100 years. The early reports were given by a few

researcher who studied the gas adsorption behaviour on charcoal surfaces. For

example, Smith, in 1863, reported that oxygen had irreversible adsorption on

charcoal[l08]. His observation was proved later by Lowry and Hulett[109].

Further evidence for the oxygen-carbon interaction was supported by Ward and

Rideal[110], who found that even at 0 °C, the heat of 0 2 adsorption on carbon was

40 Kcal/mole, indicating the strong adsorption tendency.

46

Table 2.1 Characteristics Comparison of Graphite and Carbon

Characteristics Graphite Carbon

structure crystalline microcrystalline

edge area small large

concentration of surface lower higher

defect sites

unpaired electrons lower higher

concentration

capacity for chemisorption of weak: strong

02

formation of carbon-oxygen less extensive more extensive

surface complex

More ,recently, one group of scientists intensively studied the oxygen

chemisorption behaviour on carbon surfaces[lll-114] . As one example, they

studied the oxygen chemisorption behaviour on a cleaned carbon surface in the

temperature range 25 °C to 400 °C with an oxygen pressure of 500 millitor, and

found out that within the temperature range 25-250 °C, oxygen was adsorbed in

the form of lactone groups, while above 300 °C, it appeared as carbonyl

groups[lll]. They also studied oxygen chemisorption on a highly graphitised

carbon black which was previously oxidised in oxygen at 625 °C, in the

temperature range 300 to 600 °C at low pressures up to 0.5 mm Hg, and reported

that a peak: adsorption was obtained at a temperature around 400 °C[112-114].

47

2.5.2 Carbon-Oxygen Surface Complexes and Surface Treatment

The strong carbon-oxygen interaction resulting from the microcrystalline structure

of carbon/graphite materials leads to the existence of various carbon-oxygen

functional groups, or in other words, carbon-oxygen surface complexes[101].

2.5.2.1 Formation of Carbon-Oxygen Surface Complexes

The formation of carbon-oxygen surface complexes has been widely investigated.

The use of oxidising gases is one of the effective ways to obtain various surface

functional groups. Ogawa studied the effects of oxidation conditions on the

formation of carbon-oxygen surface complexes and reported that when carbon is

heat treated and cooled in a high vacuum, some functional groups which show

basic behaviour existed, while on exposure to oxygen at 400 °C, the functional

groups turned acidic[l15]. The temperature dependence of surface functional

group formation was studied by Kruyt et al[l16], and by Boehm[l17]. According

to their reports, when carbon materials were treated in oxygen gas between 200 °C

and 500 °C, acidic surface groups are formed, while basic surface complexes were

observed when the carbon was treated in a vacuum or in an inert gases

atmosphere, and then contact with oxygen only after cooling to low temperature.

King[ 118] reported that the acidic surface was obtained when carbon was exposed

to oxygen near its ignition point temperature and the maximum amount of acidic

surface functional groups were formed at 420 °C.

48

Again Boehm and co-workers[ll9], studied the reaction of oxygen with

microcrystalline carbon and found that in the temperature range 400-450 °C, four

kinds of carbon-oxygen surface complexes with different acidities were formed.

The acidity of the four groups is in the order of: carboxyl group > hydroxy

lactone > phenolic hydroxyl group > carbonyl group.

Oxygen is not the only gas which can react with carbon to form surface

complexes, many oxidising gases are also effective for this purpose. Smith et a1

found that treatment of activated carbon samples with water at 100 and 200 °C for

various periods of time resulted in the formation of two different carbon-oxygen

surface functional groups[120]. It has been reported also that C02[121] and oxides

of nitrogen[122] react with carbon, forming carbon-oxygen complexes as

intermediates.

Oxidising solutions are also used to form carbon-oxygen surface complexes. A

20% increase in oxygen content was observed when a charcoal sample was treated

with concentrated nitric acid[123]. Behrman and Gustafson[124] reported that after

reacting repeatedly with chlorine water at room temperature, a sugar charcoal

sample showed oxygen content up to 25% when the treated sample was outgassed

at 1200 °C. Similar research was carried out by some other scientists and

supporting results were reported[l25,126].

Some more intensive research was conducted on the treatment of carbon with

concentrated nitric acid. Using either charcoal or carbon black, some workers have

49

suggested that such treatment leads to the formation of carboxylic, phenolic and

quinonic groups[l27]. In addition, Boehm et al also reported that reactions of

carbon with aqueous sodium hypochlorite lead to the formation of phenolic and

carboxylic groups[l19].

As the oxygen-carbon interactions are very extensive, electro-oxidising, or anodic

polarisation can also result in the formation of surface functional groups. The

existence of surface functional groups formed by anodic electrolysis is confirmed

by the work of Laser and Ariel[128] who studied the behaviour of glassy carbon

on anodic polarisation and subsequent reduction in acid medium. From current

voltage and reflectance-voltage curves they concluded that the overall process on

anodic polarisation is the results of three processes: formation of a redox couple

caused by oxygen chemisorption, irreversible redox reactions of existing surface

groups and, at sufficiently positive potentials, the evolution of oxygen. Possible

surface groups formed on oxidation are, for example, carbonyl groups that

subsequently can be reduced to hydroxyl groups at more negative electrode

potentials.

The quinone/hydroquinone couple is also a possible surface group, as found on

oxidised/reduced pyrolytic graphite[129]. Dunsch and Naumann also reported

evidence for the existence of oxygen-containing groups and chemisorbed oxygen

at the glassy carbon surface[l30]. Further evidence for the existence of carbon

oxygen surface complexes was given by Cabaniss et al[l31] who studied the

effect of anodic polarisation on the electroactivity of a glassy carbon electrode

50

(GCE) to ruthenium complex redox reactions. The electrode was anodically

oxidised in nondegassed 0.1 M H2S04 at an applied potential of +1.8 V vs SCE

for 30 minutes. X-ray photoelectron spectroscopy was employed to analyse the

surface groups of the oxidised and oxidised-reduced GCE surface. Results

obtained proved that the oxidation results in an increase in the concentration of

phenol, alcohol, ether ketone, quinone and aldehyde surface groups but a decrease

in the carboxylate and acid anhydride groups.

Although a relatively new material, carbon/graphite fibres and their surface

properties have been investigated extensively and intensively[131-161]. The most

representative research work on the formation and existence of carbon-oxygen

functional groups on carbon fibres would be that conducted by the research team

directed by Sherwood[151-161]. Using X-ray photoelectron spectroscopy as the

main technique to analyse the surface groups, a continuing and systematic research

has been carried out since 1982. Proctor and Sherwood, in 1982, discussed the

ability and power of XPS as a surface analysis technique for identifying the small

changes in the chemical properties of the carbon fibre surfaces caused by heating,

and also used the technique to study the effect of heat treatment on the carbon

oxygen functional groups of PAN based carbon fibres[151]. The comparison was

made between a sample without any treatment and a sample treated at 1400 °C.

Results showed that the heat treatment causes some losses of oxygen (01jC15

intensity ratio = 0.058 and 0.022 for untreated and heat treated sample

respectively), but increases the number of carbon-oxygen surface groups.

51

Proctor and Sherwood later studied the effect of chemical treatment on the

formation of surface groups on carbon fibres[152]. The research was carried out in

sulphuric acid and ammonium bicarbonate solution. According to their report, the

anodic potential and electrolyte both play an important role in the surface group

formation, since C=O groups were detected when the fibre electrode was

anodically polarised at a lower potential and C-0 groups appeared at rather high

anodic polarisation. C-N groups were detected only in the sample which had been

anodically electrolysed in ammonium bicarbonate solution.

Further studies of electrochemical treatment have involved different solutions and

different pH's. Kozlowski and Sherwood[153] treated PAN based carbon fibre

electrodes electrochemically in nitric acid solution and concluded that the amount

of surface carbon-oxygen groups is dependent on the anodic potential applied, the

oxidation time and also the concentration of nitric acid. The main carbon-oxygen

functional group formed in this case was carbonyl. It was also found that 12

minutes is the time needed to obtain the maximum surface oxide formation when

the fibre electrode was treated at +2.0 V vs SCE in 0.18 mol/L HN03• A

substantial topographical change was also observed for the sample treated under

severe oxidation conditions, e.g. + 3.0 V vs SCE for 20 minutes. The effects of

pH on the surface properties of carbon fibres were concluded as: the amount and

type of surface oxides varies considerably depending on the pH of the electrolyte.

C=O groups increase with pH, while C-OH decrease with pH, and carboxylic

acid/ester (-C02R) groups were present and seen to increase with pH[154]. Some

more details about the effect of electrochemical treatment on the surface chemistry

52

of carbon fibres are described in [155-157].

More recently, Xie and Sherwood reported their research on the surface functional

groups of carbon fibres from different precursors[158]. According to XPS analysis,

differences were found between PAN and pitch based carbon fibres: In the core

and valence-band region, a highly graphitic and an almost unoxidised nature was

observed for pitch based carbon fibres. They also conducted a new study using

microwave plasma to treat PAN and pitch based carbon fibres. In PAN fibres, an

air plasma treated sample showed an increase in C=O groups. This was also

observed in Pitch fibres. Differences were reported between these two fibres: for

PAN fibre, a longer treatment time caused fibre damage, while pitch fibres

showed a treatment time independent behaviour up to 20 minutes, indicating that

it is harder to be oxidised [159-161].

Sun also investigated the treatment of a rayon based graphite felt (FMI graphite

felt) for redox flow cell applications[28]. Oxidising solutions and gases, and

anodic oxidation methods were employed. A considerable increase in surface

oxygen ratio (to carbon) was reported for the samples subjected to various

treatments. It was identified by XPS analysis that the amount of C-0 and C=O

groups increased after treatment.

53

2.5.2.2. Specific Carbon-Oxygen Surface Complexes and Their Influence on

Electrochemical Behaviour of Carbon Electrodes

As discussed in the previous section, a number of carbon-oxygen surface

complexes are believed to be present on carbon surfaces. As summarised by

Yeager et al, the main surface groups are carboxyl, phenol, quinone, lactone and

anhydride and cyclic peroxide[162], although hydroquinones, aldehydes,

fluorescene-type lactones, normal lactones and ethers have also been suggested.

Figure 2.7 shows the structure of the proposed functional groups.

The chemical nature of surface groups on carbon can be characterised by their

acidity, i.e. the ability to adsorb acids or bases. Much effort has been made to

determine the chemical nature of various carbon-oxygen surface complexes(see

references in [101]). As a general agreement, carboxylic, lactone and phenolic

groups are acidic surface complexes, and carbon with these functional groups on

the surface is considered to have an "acidic" nature. Even though some "basic"

character of carbon has been observed, there is still no satisfactory interpretation

for the experimental phenomena[lOl].

However, the influence of surface functional groups on the electrochemical

reactions is well recognised. For instance, Yeager et al reported that the oxygen

reduction reaction is inhibited on a basal plane graphite surface, but is relatively

easy on other types (e.g. that include edge plane) carbon electrode[162]. Kamau et

al studied the relation of electrochemical activity to the surface functional groups

54

CARBOXYL PHE~~OL

~0 OH R-C @

~OH

LAC TOt~ E

CH2 - 0

I \ C=O

CH2 /

~ CHR

.~r·mYDR I DE

\ c = 0 \

0 I

c = 0 I

QurNmJE

0

0 0

Figure 2.7 Proposed functional groups on carbon (Ref 101.).

55

on the glassy carbon electrode surface and concluded that the existence of

phenolic-like groups, introduced by high speed polishing, enhances the

electroactivity of GCE for the anodic oxidation of ferrocyanide and

ascorbate[l63]. Sundberg et al also reported that carbonyl surface groups were

found on an electrochemically treated glassy carbon electrode, and the treated

electrode showed better reproducibility and electrocatalytic performance for the

oxygen reduction reaction in chloride media[164]. Studying the activation of

highly ordered pyrolytic graphite electrodes for heterogeneous electron transfer,

Bowling et al tried to link the electrochemical performance with carbon

microstructure[l65]. The electrochemical and vibrational spectroscopic properties

of a highly ordered pyrolytic graphite electrode were determined before and after

modification by electrooxidation or pulsed laser irradiation. Both treatments

greatly accelerated the heterogeneous electron transfer rate constants for the

Fe(CN)t'4- and dopamine redox systems by approximately six orders of

magnitude. This enhancement was believed to be due to the increase of surface

active sites.

The relation of surface groups to electroactivity of carbon fibre electrodes has also

been widely investigated. Wightman et al studied the electrochemical reversibility

of carbon fibre electrodes in ascorbic acid and ferricyanide solution and found out

that the reversibility on a carbon fibre electrode treated with high current density

increased significantly[166]. Jannakoudakis used an electrooxidation method to

modify carbon fibre electrodes[167]. The fibre electrodes were treated in 1 N

N~S04 solution by potentiostatic double-pulse application for 6 minutes. The

56

applied potential range extended from +2.3 V to -0.3 V versus SCE and the

duration of oxidative pulse was six times longer than that of the reductive pulse. It

was believed that after treatment, a large number of surface acidic groups is

obtained, since the subsequent palladium electrodeposition was successfully

carried out due to the Pd ions exchanging with the hydrogen ions of the acidic

groups formed on the fibre surface.

Sun also successfully explained the relationship between the enhancement of

vanadium redox cell efficiencies and the surface functional groups formed in the

graphite fibre electrode surface[28]. He treated the graphite fibre electrode

thermally with oxidising solution and electrochemical methods, and found that the

surface oxygen content changed, resulting in the enhancement of the

electroactivity for vanadium redox reactions[28].

In contrast with the opinion that the carbon-oxygen surface complexes enhance the

electrode activity, a few researchers have a different point of view. Poon et

al[l68] studied the effects of a laser pulse on the electroactivity of a glassy carbon

electrode for the ferri-/ferrocyanide and ascorbic system, and concluded that, fast

electron transfer for ferri-/ferrocyanide, dopamine and ascorbic acid is unrelated to

background current, adsorption, ablation, surface oxygen content, and microscopic

surface area. They concluded that the most likely laser effects that promote

electron transfer are desorption of impurities and formation or exposure of active

region on the glassy carbon surface[168].

57

Lausevic and Jenkins also reported the effects of anodic oxidation on the

electroactivity of a glassy carbon electrode and suggested that if glassy carbon is

used as an anode there is a danger of irreversible change of surface morphology

and chemistry, resulting eventually in severe spalling erosion by the formation of

a non-adherent oxidised layer leading to cases of catastrophic exfoliation at high

voltage or high current density[l69]. An oxide film formed on the surface of

pyrolytic graphite electrode during anodic electrolysis was also observed, and it

was considered that the ftlm interferes with the electroactivity of the

electrode[170]. Lipka et al also pointed out that graphite fibres can be oxidised

when maintained for an extended length of time at high anodic potentials where

substantial oxygen evolution occurs leading to loss of mechanical stability and

increased resistivity[171]. Further evidence was given by Neffe who studied the

effects of electrochemical oxidation on the PAN based carbon fibre

electrodes[l72]. He found that anodic oxidation of the fibres at 2.7 V vs SCE in 1

M sulphuric acid with 2000 C/g charge passing, produces cracking of the fibre

skin perpendicular, parallel or askew to the fibre axis.

A major conclusion that can be drawn therefore is that carbon and oxygen have

strong interactions and that there are several ways to produce carbon-oxygen

surface complexes which can somewhat enhance electroactivity of certain redox

reactions. However, over-oxidation of carbon will degrade the electrochemical

properties.

58

2.5.3 Electrocatalysts

As pointed out by Pletcher[30], the need for electrocatalytic materials is most

obvious in large energy intensive electrolytic processes as well as in large battery

systems for load levelling or energy storage. Carbon and graphite, especially,

carbon and graphite felts and cloth are widely used as electrode materials for

redox flow systems. Unfortunately, electron transfer rates for most redox couples

on carbon surface are slow, even though some surface treatments have been found

to enhance the reactivity. In addition, for redox flow cell systems, oxygen

evolution in the positive half cell and hydrogen in the negative half cell during

charging, result in coulombic losses of the cell. Catalysts which can inhibit these

side reactions and enhance the rate of main reactions are therefore desirable. The

importance of using electrocatalysts was also overviewed by Bockris

recently[173].

Depending on the nature of electrochemical system, many materials can be used

as electrocatalysts. For example, metal oxides were employed as electrode

catalysts for ascorbic acid oxidation. Dispersed onto a glassy carbon surface, 37

metal oxides (MOx's) were evaluated[l74]. It was found that the highest degree of

activation was exhibited by those MOx's with a metal oxidation state of +3 or

higher, such as zirconium oxide, cobaltic oxide, chromic oxide, etc.

In many cases, noble metals are utilised as electrocatalysts. As described

previously, the NASA research group successfully developed a number of

59

electrocatalytic systems, such as Au-Pb, Ag-Pb and Bi-Pb to catalyse the

chromium reaction and depress hydrogen evolution in the Fe-Cr redox cell[36-38].

Jannak:oudakis electrodeposited palladium onto a carbon fibre electrode surface

and found that the catalysed electrode showed high activity in the electrooxidation

of formic acid, the electroreduction of nitrobenzene and in the hydrogenation of

benzoyl chloride and oleic acid[167]. Cheng and Hollax studied the electrode

kinetics and electrocatalysis of Cr(ill)/Cr(II) reactions with cyclic voltammetry

and found that at a graphite electrode with small amounts of Au, the addition of

thallium-I-chloride not only accelerates the Cr(ill)/Cr(II) reaction in HCl

electrolyte catalytically, but also raises the hydrogen overpotential more than lead

and bismuth, which were the heavy metal catalysts already tested in the practical

Fe-Cr redox flow cel1[175].

Ashimura et al studied the performance of a iron-titanium redox fuel cell system,

and utilised platinum black loaded on carbon black as catalyst to regenerate the

reactants[48]. This catalyst was also used in some other redox cell

system[ 47 ,49,50].

Silver and its compounds are also used as electrocatalysts. Dekanski et al reported

an electroactivity improvement of glassy carbon electrode by immersing the

electrode into AgN03 solution for about 30 days. A deposition of metallic silver

has been observed on the treated samples[176]. Another example was given by

Takeuchi and Thiebolt[177].

60

Loading electrocatalysts onto a solid electrode substrate is a common way to

obtain a catalysed electrode. The electrocatalysing ability of an electrode can also

be achieved by simply adding a solution containing the catalyst elements into the

electrolyte of the electrochemical system. Gahn et al invented the bismuth-lead

catalyst system for a mixed reactant iron-chromium redox flow cell. By adding

bismuth nitrate into both half-cell electrolytes, a significant cell resistance decrease

was observed. Lead chloride was added into the negative half-cell to prevent

hydrogen evolution. It is believed that 50 micrograms of bismuth per square

centimetre of electrode projected area is deposited during cyclic

charging/discharging, and the lead layer was deposited on the bismuth layer[l78].

The vanadium redox flow battery is a new energy storage system, and is

promising for scale-up to industrial applications. Carbon fibre based electrodes

have been utilised successfully in this system. However, to maximise the power

output and broaden the application range, the search for suitable electrocatalysts is

therefore desirable and is one of the interests of the present project.

2.6. Theoretical Background

2.6.1 Electrochemical Techniques

Many steady-state and impulse electroanalytical techniques are available to

determine electrochemical parameters and assist in both improving existing

battery systems and evaluating couples as candidates for new batteries[l79].

61

Cyclic voltarnmetry (CV) and rotating disc electrode (RDE) voltarnmetry are the

two techniques employed in the current studies.


Of the electroanalytical techniques, cyclic voltammetry (or linear sweep

voltarnmetry as it is sometimes known) is one of the most common techniques

for initial electrochemical studies of new system and has proved useful in

obtaining information about complicated electrode reactions. Its principles and

practise have been described in some books[l79,180] and reviews[181, 182]. Here,

only the applicable solutions are listed.

All redox reactions can be broadly divided into three classes[179]:

1) Nemstian or reversible systems

2) totally irreversible systems

3) quasi-reversible systems.

with the general equation:

(2-3)

where kr and kb are the forward and backward heterogeneous rate constants of the

electron transfer. In reversible system, kc = ~; in totally irreversible processes, ~

is negligible; and in quasi-reversible system, kc > ~.

62

(b)

Potential E

(c)

~

~r-----~======~~==~~~3:--------::J u

Figure 2.8 Cyclic Voltammograms of three typical electrode process, (a)

reversible, diffusion controlled; (b) Totally irreversible; (c) quasi

reversible (Ref 183.).

63

~E so~----~~--~----~------~--~ u <t E

""' 60 >-!-l/)

z ~ 40 1-z ~ 20 cc: ::J u

0 1.1 1.3 1.5 1.7 CELL VOLTAGE /V

I •

1.9

Figure 2.9 Typical cell polarisation behaviour of redox flow cell system

(Ref 44.)..

64

According to the nature of the electrochemical reactions, the cyclic voltammogrms

have different shapes. Figure 2.8(a), (b) and (c) represent three typical

voltammograms of reversible, irreversible and quasi-reversible process

respectively[l83]. A reversible, diffusion-controlled reaction exhibits an

approximately symmetrical pair of current peaks as shown in Figure 2.8(a). The

peak potential separation .1Ep can be determined by the equation:

llE = 2. 3RT P nF

(2-4)

and the value is independent of potential sweep rate. A completely irreversible

electrode process produces a single peak as shown in Figure 2.8(b). The peak

potential is sweep-rate dependent.

The corresponding peak current for reversible and irreversible redox reactions

can be described as:

(2-5)

• 0 an F , ~p=O. 227 nFAC0 exp [- ( a ) (E.,...-E0 )

RT "' (2-6)

65

where: 4 - peak current (amperes, A)

A- area of electrode (cm2)

Co 0 - bulk concentration of species 0 (mol.cm -3)

v - potential sweep rate (V.s-1)

Do - diffusion coefficient of 0 (cm2.s-1)

For quasi-reversible processes the current peaks are separated more and the

shape of the peak is less sharp at its summit and is generally more rounded as

shown in Figure 2.8(c). Again the peak potential is dependent on the sweep rate

and the separation is much greater than that given by equation 2-4. The

mathematical model and data analysis for this electrode process is more

complicated[ 179].

2.6.1.2 Rotating Disc Electrode Method

For an electrode process which is limited by mass transport, rotating disc

voltammetry is often employed to investigate the mechanisms of redox reactions

and determine the diffusion coefficient. The Levich equation is the most important

principle for this electroanalytical technique[179]:

(2-7)

where iL is limiting current density (A.cm-2); n, the number of electrons

participating in the reaction; D, the diffusivity of reactive species (cm2.s-1); v, the

viscosity of electrolyte (cm2.s-1), m, the angular velocity of rotation (s-1) and Cb,

66

the concentration of reactive species in the bulk electrolyte (mol.cm-3).

Rotating disc voltammetry also can be used for determining the basic

electrochemical parameters io and a. of an electrode process limited by mass

transfer[179]. As described by Bard, for an electrode reaction which is

activation controlled, the exchange current density i 0 and transfer coefficient a.

can be determined by the well-known Tafel equation. For an electrode process

affected by mass transport, the following equation applies[179]:

RT io RT <i1. c-i) 11 =--ln-.-+ --ln--=.:._.:::--a.nF ~ 1 , c a.nF i

( 2-8)

where T) is cathodic overpotential; R, the normal gas constant; T, the temperature;

a., the transfer coefficient; n, the number of electrons participating in the reaction;

F, the faraday's constant; J.o, the exchange current density; i1.c, the cathodic limiting

current density and i, the measured cathodic current density.

Equation (2-8) indicates that cathodic overpotential should be linearly related to

ln(i1,c-i)/i, and io and a. can be determined by the intercept and the slope of the

straight line.

2.6.2 Performance Characteristics of Redox Flow Cell

67

As a new energy storage system, a number of characteristics for redox flow cell

should be taken into account. Among them, cell polarisation behaviour, cell

resistance and efficiencies are the basic factors for evaluating an electrochemical

cell[l83].

2.6.2.1 Polarisation Curve and Cell Resistance

In redox flow cell systems, the reactant is flowing through the electrode, as

mentioned previously. The flow rate therefore influences the polarisation

behaviour of the cell. Hence, an assumption of a constant electrolyte flow rate is

necessary when the polarisation behaviour of the cell is evaluated. In this way

conventional polarisation curves can be drawn for each of several different state-of

charge (SOC) levels. Figure 2.9 shows typical polarisation curves obtained for a

vanadium redox flow cell[44]. The numbers near the curves indicated the SOC.

The linear voltage-current relationship indicates that the ohmic polarisation is the

principle cause for the polarisation[183].

The slope of the linear voltage-current curve is defined as the cell resistance with

a unit of n.cm2• It includes the faradaic resistance, electrolyte resistance,

membrane resistance and resistance of electrode materials and electrical contact.

Cell resistance is usually utilised to evaluate the performance of an

electrochemical cell, especially, the voltage efficiency of the cell, however, it is

difficult to determine the resistance of the individual components.

68

2: 2.0 w ~ 1.5 r--: _J

§; 1.0 0\

_J '-0

uj 0.5 L)

00 1 2 3 4 TIME/h

Figure 2.10 Constant-current charge and discharge performance of the

vanadium redox flow cell (Ref 44.).

2.6.2.2 Cell Efficiencies

Typical charge/discharge curves for a redox flow cell can be illustrated by Figure

2.10, which was obtained in the vanadium redox flow cell with constant

charge/discharge current[ 44].

The ability of a redox flow cell to accept charge or be (re-)charge is measured in

terms of the coulombic and energy efficiencies of the charge/discharge cycles.

The coulombic efficiency of a redox flow cell cycled under stated conditions is

defined as[183]:

(2-9)

where idis and ich refer to the currents flowing during discharge and charge

respectively. Side reactions like oxygen anQ. hydrogen evolution during charging

at high states-of-charge, as well as self-discharge are the factors affecting the

coulombic efficiencies. The coulombic efficiency is also expressed as the

amperehour (Ah) efficiency.

70

The overall cycle energy efficiencies is given by

(2-10}

~here vdis and vch are the cell voltages during discharge and charge respectively.

The overall cycle energy efficiency is also called the watt-hour (Whr) efficiency.

The Energy efficiency is influenced by both the coulombic efficiency and cell

polarisation losses. A quantitative measure of the effects of the cell polarisation

during the charge-discharge cycle are revealed in the voltage efficiency given by:

(2-11}

The voltage efficiency is alway related to the cell resistance. The higher the

resistance the lower the voltage efficiency. As mentioned previously in this

section, cell resistance consists of faradaic resistance, electrolyte resistance,

membrane resistance, electrode material and contact resistance, therefore, voltage

efficiency is easily affected by each of the cell components.

71

CHAPTER ill

EXPERIMENTAL

As the main body of the thesis is separated into three chapters, each emphasising

specific aspect of the carbon-plastic composite electrode, it is desirable to describe

the experimental procedures separately and correspondently to the relevant

chapter. Hence, the first part of this chapter deals with the experimental aspects of

preparation and evaluation of carbon-plastic composite electrodes. The second part

focuses on the illustration of some electrochemical techniques which were

employed to study the electrode kinetics and evaluate the effects of electrode

surface activation. Relevant to the surface characterisation of the graphite felt

electrodes, the third part of this chapter concentrates on some surface analytical

techniques such as scanning electron microscopy (SEM) and X-ray photoelectron

spectroscopy (XPS).

3.1 Preparation and Evaluation of Carbon-Plastic Composite Sheets and

Electrodes

3.1.1 Preparation

The materials, apparatus and procedures to prepare the carbon-plastic composite

sheets and electrodes are detailed below.

72

3.1.1.1 Materials

Carbon and graphite products

carbon black:

CABOT "black pearl 2000" (Cabot Corporation, Billerica Technical Center, USA)

Degussa FW 200 carbon black (fine powder, 13-35 nm, Degussa Australia Pty.

Ltd.)

Degussa XE-2 carbon black (coarse powder,> 35 nm)

graphite powder: LONZA KS-2.5 graphite (LONZA Inc. Fairlawn, USA)

graphite fibre: KUREHA C-203s (Kureha Chemical Industry Co., Ltd., Chuo-ku,

Tokyo, Japan).

Polymers

low density polyethylene (LDPE, MPI 7(17/1200), Manis Pty. Ltd., Australia)

high density polyethylene (HDPE, Hostalen GM 7655, Hoechst Australia Ltd.,

Melbourne, Australia)

polypropylene (PP, Hostalen PPT 1070, Hoechst Australia Ltd., Melbourne)

styrene-ethylene-butylene-styrene copolymer (SEBS, National Starch and Chemical

Pty. Ltd., Australia)

Graphite Felt

FMI graphite felt, 6 mm, Fibre Materials, Inc., Maine, USA

Toray Graphite felt, 6 mm, Toray Industries, Inc., Tokyo, Japan.

Le Carbonne Graphite felt, 6 mm, Le Carbone-Lorraine Australia Pty. Ltd.

73

Metal mesh

Swiss Screen, 100 mesh (150 micron) brass mesh (Swiss Screen Pty. Ltd.,

Australia)

3.1.1.2 Equipment

An internal mixer (RHEOMIX-600, HAAKE INC., U.S.A) was employed to blend

the carbon-plastic composite materials. A heating and temperature controlling

system was designed to adjust and control the temperature of the mixing head of

the machine from ambient to 250 °C. The capacity of the chamber was designed

for producing up to 70 grams polymer mixture, however, depending on the density

of the ingredients, the weight of mixtures varies. A pressing machine (STACY

HYDRAULICS 40 tonnes hot press, A.E.I. Engineering Pty. Ltd., Australia) was

required for moulding the composite sheets and fabricating the composite

electrodes. It also has heating and temperature controlling system in the

temperature range up to 300 °C. The two heating plates were designed with

dimensions 65x50 em. A cooling water system was also installed through the

heating plates, enabling a quench process to be applied whenever necessary.

Figure 3.1(a) and 3.1(b) show the photos of these machines.

Two types of moulds are needed for fabricating the composite materials and

electrodes. Type 1 is for casting the mixed carbon-plastic into thin sheets. These

74

(a)

75

Figure 3.1

(b)

Equipment for preparing carbon-plastic composite sheet and

electrode, (a) internal mixer; (b) hot-pressure.

76

moulds are made of moulding steel and two sizes, 152x152x(0.3-2.0) mm and

210x210x(0.3-1.0) mm were employed. One of the moulds used was shown in

Figure 3.2(a). Type 2 mould is for bonding the graphite felt onto the composite

sheet to form the composite electrodes. Depending on the thickness of the felts

and the shapes of the electrodes, the dimension of the mould varies. For example,

for the 6 mm graphite felts, the window thickness was 4 mm, while for a 3 mm

graphite felt, a window thickness of 2 mm was employed. The Window thickness

being two-thirds that of the felt is designed for obtaining enough pressure in felt

bonding process. A typical window mould for bonding the graphite felt on to the

composite was illustrated in Figure 3.2(b).

3.1.1.3 Preparation Procedure:

Depending on the polymer used, the processing temperature and time varies.

However, the sequences are the same. The following is an example for making a

rubber modified carbon-plastic composite and relevant composite electrodes. The

procedure is also illustrated in Figure 3.3.

The ingredients, except for the graphite fibre, were dry mixed properly and then

blended at the blade speed of 50 rpm in the internal mixer at 195 °C for 10 min

utes, followed by the addition of the graphite fibre slowly and then blending at

least for another 20 minutes. The mixture was pressure-moulded with 250 kg/cm2

pressure at 220 °C for at least 30 minutes. The moulded composite material was

77

Figure 3.2

cover

mould / ~-=-:Z __ ~/7 ~/========--~· 7

(a)

(b)

Configuration of moulds used for preparing carbon-plastic

composite sheet and electrode: (a) mould for making sheet; (b)

window-mould for bonding graphite felt onto composite sheet.

78

cooled down rapidly to obtain the thin, smooth, conductive and flexible carbon

plastic sheet.

To prepare an end-electrode, only two more steps are needed. Firstly, a metal

mesh (copper or brass mesh is preferred) is placed in the bottom of the mould.

The carbon-plastic sheet is placed on top and the mould heated up to 220 °C. The

temperature is maintained for 20 minutes before applying the same pressure of

250 kg/cm2 for 15 minutes. Secondly, while keeping the hot carbon-plastic sheet

with the metal mesh backing in the mould, a window is placed on the top of the

hot sheet and a graphite felt is placed in the window. A pressure of a quarter of

the original pressure is then applied to the mould with the window, and this is

maintained for 10 minutes. The mould is then rapidly cooled down to obtain the

carbon-plastic and graphite felt composite end-electrode.

To manufacture a bipolar electrode, two windows are required. Placing the two

windows on both sides of the prepared composite carbon-plastic sheet, two pieces

of electrochemically active layers are placed into the two windows. By following

the same procedure of bonding felt for the end-electrode, a bipolar electrode can

be obtained.

3.1.2 Evaluation

The important properties governing the use of materials as electrodes, include:

electrical conductivity, mechanical integrity, permeability, electrochemical activity,

79

HDPE + SEBS + Carbon Black Internal Mixing at Graphite fibre added,

Dry mixing ~ 195 OC, 10 min. I----?~~ blending for 20 min.

blade speed 50 rpm

Cool down and obtain the Pressure moulding Initiator added (if

composite

Figure 3.3

1F-(-~~L..20 OC, 250 kg/cm21't~:-----t have any), blending

30 min. for 10 min.

Felt-bonding on both

sides at 220 °C, 60

kg/cm2, < 10 min.

Cool down and get

Bipolar Electrode

l Metal mesh backing at

220 °C, 250 kg/cm2,

30 min.

Felt-bonding at 220 °C

60 kg/cm2, < 10 min.

Cool down and get

End-Electrode

Flow-chart of the process for preparing carbon-plastic composite

and electrodes 1

80

stability in electrolyte and cycle life. These properties have been evaluated in the

present study as a function of the composition of the carbon-plastic composite

materials and electrodes.

3.1.2.1 Evaluation of Electrical and Physical Properties for Composite Materials

(1) Electrical Resistivity

The ASTM D-991 method was employed for evaluating the electrical resistivity of

various carbon-plastic materials prepared. Figure 3.4 shows the set-up for

resistivity measurement. The test specimens were cut into rectangular shapes with

dimensions of 60x40 mm, and normally three specimens from different part of the

same sheet were tested and averaged. According to this standard method, the

resistivity of the composite can be determined by the following equation:

p=O. 001 Vwd/ Il

where:

p: volume resistivity, n.m;

V: potential difference (Volts) across electrodes;

I: current (Amps) through the current electrodes;

w: width of specimen, mm;

d: thickness of specimen, mm;

1: distance between potential measuring electrodes, mm.

81

( 3-1)

Constant "eight

Heavy object

Current probes

Figure 3.4 Set-up for resistivity measurement (ASTM D-991).

82

(2) Mechanical Properties

The mechanical properties of the composite materials were characterised by the

values of tensile strength at break and percentage elongation at break and were

determined by ASTM D 638 method. The tests were performed on an Instron M

1115 Universal Testing machine within the Polymer Science Department at

UNSW. Again, at least three specimens were cut from different parts of the same

composite sheet using a 4 em standard specimen puncher. The specimens were

then tested with an intergrip distance of 2 em and a cross head speed of 0.1

em/min and an accompanying chart speed of 5 em/min. The low cross head speed

was selected since the highly loaded carbon-plastic composites is very brittle. The

tensile strength of the specimens can be therefore determined by the following

equations:

(3-2)

where cru is tensile strength at break with units of N/mm2, W is the load at break,

in Newtons, and A0 is the original cross-section area in mm2•

%El =IlL/ L 0x100 (3-3)

(3-4)

where %EL is the elongation at break, Lll.. = L-L0, the difference in the length of

83

the specimen at break and at origin; Sx-head• the speed of the cross head, em/min;

schart• the chart paper speed, em/min, and d, the distance of chart travelled, em.

(3) Permeability

To evaluate the permeation rate of the material, a small round cell fitted with

solution pumping system was employed, the sheet being used as a separator

between the two electrolyte compartments. The thickness of the composite sheet

employed was 0.35 mm (half the thickness of those used for electrode fabrication).

On one side was 2M V(IV) in 3M H2S04 solution(the positive electrolyte of

vanadium battery) and on the other side was distilled water(see Figure 3.5). Both

solutions were pumped for 15 days, and 5 ml of sample was withdrawn from the

water side periodically. The concentration of vanadium and sulphur(as SOt) in

the blank solution was determined using Inductively Coupled Plasma (ICP)

analysis.

3.1.2.2 Evaluation of Electrical and Electrochemical Properties of

Composite Electrodes

(1) Electrical Resistance Determination for composite electrodes

To determine the electrical resistance of the carbon-plastic/graphite-felt composite

electrodes, the four-probe method was employed. Figure 3.6(a) illustrates the

principle of the testing.

84

RESERVOIR

DISTILLED WATER

Figure 3.5

CARBON-PLASTIC SHEET

2 ~1 V( IV )/3

11 H 2 so ..

Diagram of testing apparatus for evaluating permeability of carbon

plastic composite .

85

The resistance of end-electrode is determined by measuring the current-voltage

curves and then working out the slope of the I-V curve by the linear regression

method. For the end-electrodes, two end-electrodes are laid tete-a-tete and set

between two copper plates. These copper plates are compressed with a pressure of

50 g/cm2• A series DC current is then applied to the testing electrodes through the

copper plates. Potential drop values are obtained by a multimeter which is

connected to the other two corners of the copper plates. Plotting the potential drop

in Volts versus the current density in A.cm-2 and working out the value of the

slope, the resistance of two end-electrodes can be determined with units of Q.cm2•

The resistance of an individual end-electrode is taken as half of the value

determined above.

The resistance of a bipolar electrode can also be determined by the same method.

However, only one bipolar electrode is needed and is also set between two copper

plates (see Figure 3.6(b)).

(2) Evaluation of Electrochemical Properties

i) Test Cell Construction

In contrast to electrical resistance testing of the electrodes which does not involve

any electrochemical reaction, the electrochemical properties of the electrodes are

measured using a complete vanadium redox flow cell.

86

Figure 3.6

DC current

DC current

constant pressure

50 g/cm2

(a)

constant pressure

50 g/cm2

(b)

copper plate

copper plate

Four-probe method for measuring the resistance of carbon-plastic

composite electrode, (a) for end-electrode; (b) for bipolar .

87

The expanded diagram of a lab-scale single cell used for evaluating the

electrochemical properties of the composite electrodes is shown in Figure 3.7. The

components for a half cell consist of end-electrode, separator, flow frame, current

collector, insulating plate and end-plate. The prepared composite end-electrodes

were for both half cells. The graphite felt projected area of the composite

electrodes tested was 138 cm2• A cation exchange membrane (Selemion CMV

Asaki Glass Co., Japan) was employed as the separator, while a window-cut PVC

plate with solution outlet and inlet was utilised as flow frame. Two copper plates

of the same size as the electrodes were used as the current collectors. Before the

aluminium end-plate, a PVC plate was employed as insulating layer to prevent the

two halves of the cell from short-circuiting. Both halves have the same

components and arrangement and the two halves are assembled by bolts and nuts

between two end-plates.

To test the electrochemical properties of the composite electrode in a multi-cell

battery, a unit-cell which could be inserted into the battery is required. A unit-cell

consists of a bipolar electrode, two flow frames and one membrane. By adding a

number of unit-cells to the single cell, a multi-cell battery can be obtained. Figure

3.8 illustrates the components of a two-cell battery.

ii) Preparation of the Acidic Vanadium Solutions

The acidic vanadium solution used in redox flow cell tests was produced by

electrolysis of vanadium pentoxide (V20 5) powder in sulphuric acid. Since V(V)

88

Figure 3.7 Expanded diagram of a single-cell vanadium redox flow battery.

89

Figure 3.8

BATTERY COMPONENTS

Expanded diagram of a two-cell vanadium redox flow battery.

90

CELL COMPONENTS

can not be obtained directly from the insoluble V20 5 and as V(ll) and V(III)

sulphate are not commercially available, these also had to be specially prepared as

described below:

a) Preparation of 2M V3·5+/2.5 M H2S04 solution

Solution of composition 1 M V(III) + 1 M V(IV) sulphate (2 M V3·5+) in 2.5 M

H2S04 solution was obtained by electrolysis of 1 mole per litre vanadium

pentoxide suspension in 5 molar sulphuric acid. The electrolytic cell employed

was made of perspex, with two lead electrodes and a Selemion CMV membrane

which was used to separate the two half-cell compartments. The vanadium

pentoxide suspension was placed into the negative half-cell, and the positive half

cell was filled with the same H2S04 supporting electrolyte as that of the negative

half-cell. The cathodic electrolyte was agitated by bubbling nitrogen in order to

prevent V 20 5 solid from settling to the bottom of the cell. This also aided in the

mass transport of vanadium ions and vanadium pentoxide in the vicinity of the

cathodic surface where reduction occurs. A DC power supply and two multimeters

were employed. During the electrolysis process, the following reactions occur at

the cathode:

(3-5)

(3-6)

91

(3-7)

(3-8)

The V2+ ion obtained also reacts with V20 5 solid directly in the solution,

(3-9)

and with V02+ to form V3+. Assuming 100% current efficiency in the production

of V(III) and V(IV), the theoretical time required to form a 50:50 mole% mixture

was calculated for a current density of 20 mNcm2• After electrolysis was

completed at the theoretical time, 2M V35+/2.5 M H2S04 solution (i.e. isovolumic

mixture of V3+ and V02+ solution) was emptied into a measuring cylinder and

distilled water added to reach the original mark, making up for water loss during

electro! ysis.

b) Preparation of V(II), V(V), V2·5+ and V4·5+ sulphate solution

Equal volumes of 2 M V3.5+/2.5 M H2S04 solution were placed in each half-cell of

the vanadium redox flow cell and then a charging current applied. During

charging, the following reactions take place:

at the positive half cell:

92

(3-10)

(3-11)

at the negative half cell:

(3-12)

(3-13)

When the solutions were fully charged as indicated by the bright violet of the

V(II) solution and the bright yellow of the V(V) solution, they were removed from

the electrolysis cell. As shown by Equations (3-10), (3-11) and (3-13), the

charging reactions used to produce V(II) and V(V) involve a change in pH so that

the fully charged solutions actually consist of 2 M V(ll) in 2 M H2S04 and 2 M

V(V) in 4 M H2S04•

The 1 M V(II) + 1 M V(ITI) and 1 M V(IV) + 1 M V(V) solutions were obtained

by mixing V(ll) and V(V) solutions in the appropriate ratios. These solutions

represent the 50% state-of-charge negative and positive half cell electrolytes and

were used for cell resistance measurements.

93

iii) Cell resistance measurement

Cell resistance is one of the basic parameters widely used for evaluating the

performance of an entire electrochemical cell. It is also used to study the

properties of individual cell components, such as electrodes, electrolytes and

separators. In the last section of Chapter II, the concept and determination of cell

resistance have been described. However, the details of testing are described as

follow:

A complete vanadium redox flow cell, a DC power supply, two multimeters, a

resistor of 0.1 n, two Iwaki MD-10 magnet pumps, two glass reservoirs with PVC

tubes connected to the cell and pumps were employed for the cell resistance tests.

This equipment was also used for evaluating the cell performance of the

composite electrodes. The electrolytes used in each half cell corresponded to 50%

state-of-charge (SOC) vanadium solutions, i.e. 1 M V(II) + 1 M V(III) in 2 M

H2S04 solution as the negative electrolyte, and 1 M V(N) + 1 M V(V) in 3.5 M

H2S04 solution as the positive electrolyte. The cell was charged for 1 minute at a

constant current density within the range from 5 to 40 mA.cm-2• After a 15

seconds rest interval, the cell was discharged for 1 minute at the same current

density as that used for charging. The above procedure was repeated at various

current densities to obtain plots of current versus voltage for the charge and

discharge cycle. From the slopes of these I-V plots, values of cell resistance could

be calculated for the charge and discharge processes at 50% SOC.

94

iv) Cell Performance Testing

In addition to the apparatus used for cell resistance tests, an Automatic Battery

Cycling Controller and a YOKOGA W A 3057 Portable Chart Recorder were

required for recording the cell charge/discharge behaviour (see Figure 2.10). The

cell charge and discharge cycles were controlled automatically and continuously

by the controller between set upper and lower cell voltage limits. The electrical

circuit used for the charge/discharge cycling is shown in Figure 3.9.

For single cell performance tests, two carbon-plastic composite end-electrodes

were needed, while in the case of a multi-cell battery, a number of bipolar

electrodes plus two end-electrodes were required. For example, in a two cell

multi-cell battery, two end-electrodes and one bipolar electrode are utilised.

Equal volumes of 2 M V3.5+/2.5 M H2S04 solution were used initially in each half

cell to generate the positive and negative fully charged electrolytes from an initial

charging cycle. It is necessary to balance the electrolytes in the initial cycle if the

electrolyte in both sides of the cell are not charged to 100% SOC at the same

time.

After balancing the solution and making sure non-leakage in the system is leak

proof, cell voltage, current and time for each charge/discharge cycle are recorded.

Cell efficiencies can be calculated from the recorded curves by using the equations

listed in Section 2.6.

95

DC power supply I I

~-- -+! --"-'-0----, I

automatic cycling controller

chart recorder

-\ \ :

;

\ i

flow cell

!+-Tl ' I I

L___ i ~~II

Figure 3.9 Electrical circuit used for charge-discharge cycling.

96

3.2 Experimental Procedures for Electrode Kinetics and Electrode

Activation Study

3.2.1 Electrode Kinetics Study

Cyclic voltammetry and rotating disc voltammetry were employed for studying the

electrode kinetics of V(V)N(IV) redox couple. The following apparatus was

utilised.

3.2.1.1 Electrodes

A graphite rod working electrode was employed for studying the electrode kinetics

of the V(V)N(IV) redox couple. A graphite rod of 3.5 mm in diameter (cross

sectional area = 0.096 cm2, CMG, Carbon Brush MFG Pty Ltd, Rosebery,

Australia) was cut 2.5 em long and then electrically connected to a stainless steel

rod by inserting the graphite rod into the same diameter hole drilled in one end of

the stainless steel rod. The graphite rod and lower part of the metal rod were

sealed in epoxy resin. After the resin cured, a machining process was applied to

obtained a graphite electrode for cyclic voltammetry and rotating disc

voltammetry. Figure 3.10 shows the construction of the graphite rod electrode.

The graphite rod electrode was polished with 600 grit (= P1200) silicon carbide

polishing paper, followed by 800 grit Flexboc polishing paper and 0.3 micron

alumina powder (Buehler Ltd, Lake Bluff, IL, USA) on a polishing cloth(LECO

97

stainless steel rod

graphite rod

epoxy resin

Figure 3.10 Construction of graphite rod electrode used for cyclic voltammetry

and rotating disc voltammetry .

98

Corporation, Michigan, USA). The electrode was then rinsed thoroughly with

distilled water, followed by ultrasonification for 15 minutes using an Ultrasonic

cleaner (LECO Corporation, Michigan, USA).

In addition to the graphite rod electrode, graphite felt and Reticulated Vitreous

Carbon (RVC) electrodes were also employed for steady-state voltammetry

studies. These electrodes were prepared by heat-pressing a piece of graphite felt

and RVC onto one side of a conducting plastic sheet with copper foil backing on

the other side (see section 3.1.1). The geometric surface area for these electrodes

was 1 cm2• A wire was soldered onto the copper foil for electrical contact. The

back including the soldered area of these electrodes was sealed by PVC glue

painting to prevent copper from being attacked by the solution.

A pure platinum wire was used as counter electrode, while a type K -601 (with

saturated K2S04 solution) Hg/Hg2S04 electrode (Radio Meter Co.) was employed

as reference electrode.

3.2.1.2 Electrolyte and Electrochemical Cell

Two kinds of electrolytes were employed. The frrst one was VOSOiMerck, BDH

Chemical Australia Pty Ltd) of various concentrations in 3 M H2S04• The second

electrolyte was a 50:50 mixture of V(IV) and V(V) corresponding to a 50% state

of charge (SOC) positive vanadium redox cell solution. The latter was prepared by

mixing equal volumes of V(N) solution (0% SOC) with V(V) solution (100%

99

SOC, which had been prepared by fully charging the V(IV) solution to V(V) in

the positive half-cell of a vanadium redox cell).

The electrolysis cell used for this study is shown in Figure 3.11.

3.2.1.3 Electrochemical equipment

The cathodic and anodic linear sweep voltammograms and the cyclic voltammo

grams were obtained with a Pine RD3 potentiostat(Pine Instrument Co., Grove

City, Pennsylvania, USA) and a model-D Riken Denshi X-Y recorder. The elec

trode rotation speed was controlled by a MSR speed controller(Pine Instrument

Co.) combined with a Type 0824-16 Bi-Mode Torque Unit(Servo-Tek Production

Co., Hawthorn, N.J., USA). All potentials were measured against a Hg/Hg2S04

reference electrode and are reported with respect to the Hg/Hg2S04 reference in

this study. Cathodic or anodic linear sweep voltammograms were always started

from the static potential and the cathodic voltammogram was obtained before the

anodic one. In the case of cyclic voltammetry, the initial scan direction was

always positive.

100

6 em

N 2

electrolyte

working electrode

Tenon cover

\,

7 em

reference electrode

counter electrode /

1

,/

(Pt wire)

I I

I ..;

I I L. -i

2em

Figure 3.11 Electrochemical cell for cyclic voltammetry and rotating disc

voltammetry .

101

3.2.2 Surface Chemical Treatment of Carbon-Plastic Composite Materials

3.2.2.1 Preparation of Carbon-plastic Composite Materials for Chemical

Treatment

In order to eliminate the use of the graphite felt layer on the composite, direct

chemical treatment of the carbon-plastic composite surface was conducted.

Considering that a highly conductive surface is always required for an electrode

material, two specific composite materials with higher loading of conducting

fillers were prepared. Low density polyethylene (LDPE) was employed because of

its ease of processing for a high filler loading. The preparation procedure was the

same as that described in Section 3.1.1.3, however, the processing temperature

was 140 °C. The composition and resistivity of the two composites (named

PE2525 and PE75 respectively) are listed in Table 3.1.

Table 3.1 Composition and Resistivity of Composite PE used in Chemical

Treatment Studies (CB =carbon black, GF =graphite fibre)

samples composition(%) Resistivity (O.cm)

PE2525 LDPE 50

C.B. 25 0.13

G.F. 25

PE75 LDPE 25

G.F. 75 0.17

102

3.2.2.2 Preparation of Carbon-plastic Composite Electrodes for Chemical

Treatment and Cyclic Voltammetry

Small rectangular (7xl.6x10 mm) specimens were cut from each of the composites

PE2525 and PE75 in Table 3.1, firmly contacted to a graphite rod, and then sealed

in a glass tube with epoxy resin. The surface area of the prepared electrodes was

0.11 cm2• Two kinds of electrodes were prepared for each composite, one

designated the "s" electrode and the other called the "e" electrode. For the "s"

electrode the surface exposed to the electrolyte is the as-prepared surface of the

composite, while for the "e" electrode, the cross section of the composite is

exposed to the solution (see Figure 3.12). For comparison, a graphite plate

electrode with the same surface area was fabricated in the same way.

For the purpose of simulating the electrode working conditions during battery

operation, a small electrode was also fabricated for the cyclic voltametric tests as

follows. A piece of graphite felt with a diameter of 1 em and a piece of copper

mesh of the same size were heat-pressed onto either side of the composite

polyethylene sheet; the felt side was used as the working surface and the copper

mesh as the current collector. The copper mesh side with electrical connection was

then sealed with epoxy resin. The electrode projected surface area was 0.785 cm2•

103

WORKING SURFACE OF •a• ELECTRODES

(a)

epoxy resin

(b) (b)

COI-1PRESS DIRECTION

COMPOSITE PE SHEET

WORKING SURFACE OF 'e' ELECTRODES

graphite rod

glees tube

electrode ~eteriel

~glass tube

~epoxy resin

working surface SECTION A -A

Figure 3.12 Schematics of carbon-plastic composite sheet and electrode for

chemical treatment studies, (a) composite sheet; (b) electrode

construction.

104

3.2.2.3 Procedure for Surface Chemical Treatment of Conducting Plastic

The chemical treatment solution employed had the following composition:

H2S04 (97%) A.R.

K2Cr20 7 A.R.

H20

800 g/1

30 g/1

The prepared electrodes were immersed into the solution for various times and at

various temperatures.

3.2.2.4 Evaluation of Surface Treatment of Conducting Plastic

The effectiveness of surface treatment was evaluated using cyclic voltammetry and

scanning electron microscopy (SEM).

The prepared electrodes were placed into the electrolyte [(1 M V3+ + 1 M

V02+)/(3M H2S04)] together with a SCE reference electrode and a graphite

plate(3x4 em) counter electrode. A PAR 174 potentiostat/galvanostat and its

associated equipment were employed for the cyclic voltammetric experiments.

The treated surfaces of the carbon-plastic composite materials were also examined

under the scanning electron microscope.

105

3.3 Graphite Felts Studies

3.3.1 Felt Materials

The following felts samples were employed for this study:

FMI: 3 mm in thickness, FMI graphite felt (Fibre Materials, Inc., Maine, USA)

GFD 2: 2.5 mm in thickness, Sigri Electrographit GMBH, Germany

GFA 2: 3.0 mm in thickness, same as above

RVG 1000: 2.0 mm in thickness, Le Carbone-Lorraine Australia Pty. Ltd.

Toray: 6 mm in thickness, Toray Industries, Inc., Tokyo, Japan.

3.3.2 Physical Property Measurements

(1) Electrical Conductivity

The electrical conductivity of different felt samples was tested with ASTM D-991

method which was described in Section 3.1.2.1.

(2) Surface Area and Pore Size

The mercury intrusion method was employed to measure the pore size and surface

area of the different felts. An AutoPoro-9200 instrument (Micromeritics, USA)

was employed, and the results were analysed and printed out by the computer

106

system (Malvern Instruments, England).

3.3.3 Evaluation of Electrochemical Properties

The cyclic voltammetry method was employed to evaluate the electrochemical

activity of different felts. The apparatus used are the same with those described in

Section 3.2.1.3.

3.3.3.1 Preparation of Carbon-Plastic Composite Electrodes for Cyclic

Voltammetry

The felt samples were punched with a 6 mm diameter puncher and then hot

pressed to the SEBS modified carbon-HDPE composite sheet which had a copper

mesh backing to complete the fabrication of a small graphite felt composite·

electrode. To prevent the felt from being crushed during pressing, a metal plate

with holes of 6 mm diameter, was employed during the felt-bonding procedure. A

release film with holes of the same diameter was also used between the metal

plate and the composite sheet to prevent them from sticking together. After hot

bonding the felt samples onto the carbon-plastic composite sheets, a 12 mm

diameter puncher was employed to punch out the entire small composite electrode,

i.e. a 6 mm diameter felt layer with a 12 mm diameter carbon-plastic matrix plate.

The extra 3 mm around the edge was required for placing an '0' ring to stop

solution leakage to the metal current collector.

107

3.3.3.2 Design of the Felt-Electrode Support

The small carbon-plastic composite electrode was then placed into a specially

designed electrode support which is described in Figure 3.12. The electrode

support consisted of a current collector, pressing tube, housing tube and '0' ring.

The current collector was a copper rod with copper plate in the bottom to make

electrical contact to the felt electrode when the pressing tube was screwed in. The

housing tube and the '0' ring combined with the pressing tube to enable the felt

part of the electrode to be exposed to the electrolyte while preventing the

electrolyte from getting into the housing tube through the composite sheet. This

special design of the electrode support made the replacement of the felt electrode

very easy and convenient.

3.3.3.3 Measurement Conditions

The felt electrode placed in the electrode support was then used as a working

electrode. A platinum wire was employed as counter electrode and a Hg/HgS04

was used as reference electrode. The sweep was started from the electrode's static

potential and the initial direction was always positive. The sweep rate employed

was 60 mV/sec and the testing was carried out in a solution of 0.05 M V(ID) +

0.05 M V(IV) in 3M H2S04•

108

HDU9!~~G

Figure 3.13 Construction of carbon-plastic/graphite-felt composite electrode

for cyclic voltammetry .

109

3.3.4 Oxidation of Felt Samples and Surface Analysis

To compare the oxidation sensitivity and stability, FMI and GFD graphite felts,

which are currently used as electrodes in the vanadium redox battery, were

oxidised under various conditions and then analysed by Scanning Electron

Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).

The SEM analysis was conducted with a S 360 Scanning Electron Microscope unit

set in University of New South Wales (manufactured by Laica Cambridge Ltd.,

England), while XPS measurement was carried out by Dr. Celestino Padeste using

a X-ray Photoelectron Spectroscope at the Surface Analysis Centre, School of

Chemistry, UNSW. Details are as follows:

The XPS spectra were recorded on a KRA TOS XSAM 800 spectrometer equipped

with a hemispherical analyser. Mg Ka. radiation was used at a maximum power of

180 Watt. The base pressure in the analyser chamber was less 10-9 torr from each

sample a piece of about 4x2x10 mm was cut and mounted on a stainless steel

sample holder overlapping the edge of the holder in a way that the sample could

be measured without significant intensity from the steel holder. The analyser was

run in the fixed transmission mode at pass energy 80 e V /1 e V step size for wide

scan spectra and pass energy 40 eV/0.1 eV step size for high resolution spectra of

the Cls and 01s regions. Typical acquisition times were about 5 minutes for wide

scans, and 7-10 minutes for the region spectra. The spectrometer was calibrated on

the Cu 2p312 line (932.7 eV) and the Ag 3d512 line (386.2 eV). Instrument control

110

as well as data collection and processing were performed using KRATOS DS800

software on a PDP 11 computer.

3.3.4.1 Air-oxidation

Both FMI and GFD graphite felt samples were cut into strips with a dimensions of

5x2 em. The samples were placed in a furnace and heated up to 400 °C. After

exposure to air at this temperature for 30 hours, the samples were taken out from

the furnace, cooled down and stored in glass sample tubes for surface analysis.

For comparison, another two samples were treated with nitrogen gas at the same

temperature and time period.

3.3.4.2 Electrochemical Oxidation

FMI and GFD felt electrodes were fabricated with the method described in Section

3.3.3.1 and then placed in the electrode support (see Section 3.3.3.2). Using the

electrochemical equipment and electrolysis cell shown in Section 3.2.1.3, the two

felt electrodes were held at + 1.5 V versus Hg/Hg2S04 reference electrode for 15

minutes respectively. Cyclic voltammetry was used to evaluate the effect of

anodic-oxidation on the electrochemical activity of the felt electrodes for the

vanadium redox reactions.

111

3.3.4.3 Oxidation in Positive Half-Cell of the Vanadium Redox Flow Battery

In the positive half-cell of the vanadium redox battery, during charging at

relatively high SOC, and after the reaction of V(IV) ~ V(V) is completed, oxygen

evolution can occur if the charging current is still applied to the battery. The

generated oxygen might attack the felt electrode and result in degradation of the

electrode. An experiment was thus conducted to compare the oxidation stability of

the FMI and GFD felts under normal and overcharging cell operating conditions.

Two pairs of FMI and GFD felt end-electrodes were fabricated with the procedure

described in Section 3.1.1.3. The felt projected area was 138 cm2• The electrodes

were tested in a single cell battery. A charging/discharging current of 3 amperes

was applied to the cell, and retained for 50 minutes after the electrolytes in both

halves were fully charged. The positive electrode was then removed from the cell,

washed thoroughly with tap water, and then immersed in distilled water over

night. After draining the water from the felts, the cleaned electrode was dried in

an oven at 60 °C for 8 hours. Felt samples were taken from these electrodes and

analysed with X-ray photoelectron spectroscopy (XPS). For comparison, FMI and

GFD felt samples were also taken from positive electrodes operated in the

vanadium redox battery under normal charging/discharging conditions, i.e. in the

potential range where the vanadium redox reactions are predominant. After

handling in the same way, these samples were also analysed with XPS.

112

3.3.5 Felt Activation Treatment and Evaluation

3.3.5.1 Activation Treatments

The GFD 2 graphite felt was used for the activation treatments. The treatments

included thermal treatment and electrocatalysts. For the thermal treatment, the felt

sample was simply placed in the furnace at a temperature of 400 °C for 30 hrs.

For the electrocatalysis treatment, the felt samples were firstly treated under the

same conditions as the thermal treatment and then placed into solutions containing

the catalyst ions to be investigated. Three kinds of solutions were prepared for this

study: 0.1 M NiS04, 0.1 M MnS04 and 0.1 M AgN03• After the felt was saturated

with the catalyst solution, it was placed in the furnace and dried at 100 °C for 48

hrs. The amount of loaded catalyst was determined by a Mettler AK 160

electronic balance (Cyrulla's Instruments).

3.3.5.2 Evaluations

Using the procedure described in Section 3.3.3.1, the treated felt samples were

hot-bonded to form the small felt electrodes. The effect of the activation

treatments on the electroactivity of the felt electrodes were evaluated by cyclic

voltammetry described in Sections 3.3.3.2 and 3.3.3.3.

The successfully treated felt samples were also evaluated in a vanadium redox

flow cell. Because of the limitations imposed by the felt samples, the tests were

113

carried out with a small-scale test cell (see Figure 3.14) which had a modified

NASA cavity fill-in cell structure [28]. Cell resistance, polarisation behaviour and

long-term stability tests were conducted.

114

P.V.C. plate membrane Eraphitc plate

copper rod

neoprene rubber gasket

Figure 3.14 Modified NASA cavity fill-in single cell used as test cell for

activated felt electrode for the vanadium redox flow system.

115

CHAPTER IV

FABRICATION AND EVALUATION OF CARBON-PLASTIC COMPOSITE

MATERIALS AS ELECTRODE MATRIX LAYERS

As discussed in the previous · sections, carbon-plastic composites are promising

electrode materials for a large number of industrial electrochemical applications

since they offer a low cost, low weight alternative to the commonly used graphite

or carbon electrodes and can also be easily moulded into any shape and size.

Japanese made carbon-plastic composite electrodes (Toray industries) have been

successfully used in the vanadium redox flow battery, and energy efficiencies of

up to 90% have been achieved with these electrodes in 1 kW (normal batteries)

[44,45]. Unfortunately, some disadvantages, such as mechanical brittleness,

solution permeability (particularly for the end-electrodes) and high cost, were

noted in recent studies.

In order to scale-up the vanadium redox flow battery for commercialisation, an

electrode material with better long-term cell performance and lower cost is

desirable. The objective of the present study is to develop formulations and

processes to give such a product. Because of its chemical stability and cost

effectiveness, polyolefin thermo-plastics were employed as the basic materials for

the composites. In particular, low density polyethylene (LDPE) was used for the

initial study because of its cost and processing effectiveness, while high density

polyethylene (HDPE) was utilised for the more intensive investigation, since

116

further requirements in mechanical properties such as toughness and rigidness are

desirable for the materials to be used as electrodes.

The important properties governing the use of materials as electrode matrix layers,

were described earlier, but include: conductivity, mechanical integrity,

permeability, electrochemical activity and stability during cell operation. These

properties have been evaluated in this chapter as a function of the composition of

the carbon-plastic composites. However, for the low density polyethylene (LDPE)

based composites, the study was limited to the effect of formulation on the

electrical properties, while for the high density polyethylene (HDPE) based

composites, the study focussed on the cell performance characteristics of the

composites as the matrix layer of the composite electrodes in the vanadium redox

cell.

4.1 LOW DENSITY POLYETHYLENE BASED CARBON-PLASTIC

COMPOSITE

4.1.1 Electrical Conductivity of Composites

According to the electrical conduction theory of two-phase systems[84-88], the

conductivity of a composite is dependent greatly on the content, the type and the

shape of the conducting fillers. Because of its chemical inertness, carbon and

graphite products were employed as conducting fillers. There are a great deal of

commercially available carbon and graphite products. As the common types used

117

in composite plastic manufacture, carbon powders, graphite powder and chopped

graphite fibre were utilised for the initial studies.

4.1.1.1 The Effect of Content and Type of Carbon Fillers

Figure 4.1 and Table 4.1 show the electrical resistivity behaviour of three carbon

materials used in the same proportion in the conducting composite mixture. From

the Figure and the Table, it is clear that for each composite, the resistivity

decreases with an increase in the carbon content, indicating that the average

electrical contact area of the filler particles increases when the total amount of

filler increases[84,85]. However, the dependence of resistivity on the carbon

contents for each composite is different. For the composites with graphite powder

filler (GP), the resistivity decreases only slightly with carbon content up to 20%,

while for the composite containing carbon black (CB, Black Pearl 2000), it is seen

to have a less marked dependence on filler. A dramatic decrease in resistivity (7

orders of magnitude) when the graphite fibre (GF) content was increased from

10% to 20% is observed, however. It is believed that an electrical network for the

passage of current through the composite is formed when the content reaches a

certain value. In order to prove this, samples with different graphite fibre content

were examined under the optical microscope. The samples were prepared as thin

sheets (0.3 mm in thickness) with the procedure described in section 3.1.1.3 and

examined with an Vanox Olympus Universal Research microscope (Olympus

Optical Co. Ltd., Tokyo, Japan), the relevant photos were being taken with the

attached camera.

118

15

o GP b. GF o CB

.--. 12 ~ '""' E 0 .

~~ E ...c 0 9 0 '-'

::J) 4-'

> 4-' B en ·-en ID L ...__.

0) 3 0 a_________o .&

0 0 5 10 15 20 25

content (7.)

Figure 4.1 Effect of carbon content on resistivity of carbon-LDPE composite.

119

Figure 4.2 shows the microstructure of the composites. It can be seen from the

photos that the contact between fibres become more sufficient when the fibre

content increases, especially, for the composite containing 20 % graphite fibres.

Comparing the resistivity values of the three composites at the same loading, the

use of graphite powder gives rise to a less conductive composite than the other

materials. Within the content range studied (5% to 20%), the composite with

carbon black (black pearl 2000) shows the best electrical conductivity. This might

be due to the high surface area of black pearl which results in high value of

average contact numbers inside the composite[85]. However, at a content of 20%,

the conductivity of composite with chopped graphite fibre is close to that achieved

with carbon black. Based on this experimental result, carbon black and chopped

graphite fibre were considered as conducting fillers for the further studies.

Table 4.1 Resistivity comparison of carbon-LDPE composites

Content Materials Resistivity

(%) (O.cm)

graphite powder > 1012

5% graphite fibre 1 X 1010

carbon black (Black Pearl) 3.13 X 102

graphite powder 3.3 X 1010

10% graphite fibre 2.4 X 108

carbon black (Black Pearl) 3.26 X 10

graphite powder 3.3 X 109

20% graphite fibre 4.9

carbon black (Black Pearl) 8.2

120

(a)

(b)

121

Figure 4.2

(c)

The development of graphite fibre network in carbon-LDPE

composite. Magnification: lOOx. Fibre content: a) 5%; b) 10%

c) 20%.

122

Further experiments showed that the combination of graphite fibre and carbon

black gave better electrical properties (Table 4.2) than graphite fibre alone. This

might be due to the fact that the combination of chopped fibre and carbon black

particles results in a more effective contact and leads to an enhancement in

electrical conductivity.

Table 4.2 Resistivity comparison of different composition (CB = Cabot Black

Pearl 2000)

composition resistivity (Q.cm)

80% PE + 20% CB 8.2

80% PE + 20% GF 4.9

80% PE + 10% CB + 10% GF 1.6

Additional formulations were thus tested and samples were prepared with the aim

of reducing resistivity to less than 1 Q.cm. The following formulation was found

to have a resistivity of around 1 Q.cm:

Table 4.3 Composition of a Typical Carbon-LDPE Composite

LDPE 70%

CB (CABOT pearl 2000) 15%

GF (KUREHA c-203s) 15%

123

4.1.1.2 The Effect of Processing on Resistivity

As an electrode matrix layer, the electrical homogeneity throughout the composite

sheet is considered as one of the most important characteristics. The combination

of graphite fibre and carbon black as conducting fillers for carbon-plastic

composites gave rise to a better electrical conductivity, it also, however, might

cause an electrical non-homogeneity, since it affects the mixing and moulding

process during preparation. It is therefore necessary to evaluate the effect of

processing conditions, in particular the internal blending time, on the electrical

homogeneity of the carbon-LDPE composites.

Since the melting point of the selected LDPE is 114 °C, and the viscosity of the

highly loaded carbon-plastic mixture at Tm is very high, the interal mixing and

moulding temperature was set at 150 °C to ease the processing. The composition

used in this experiment is shown in Table 4.3. Keeping the other processing

conditions constant (see section 3.1.1.3), the samples were blended at a blade

speed of 50 rpm for various time periods and then moulded into thin sheets with

dimensions 150 x 150 mm. Three specimens sized 60 x 40 mm were cut from

different parts of each sheet and measured with the ASTM D-991 method.

Table 4.4 shows the experimental results. Column p gives the resistivity value of

each specimen corresponded to blending times of 5, 10, 20 and 40 minutes. The

last column of the Table shows the maximum and the relative deviation values of

the specimens. This is also illustrated in Figure 4.3.

124

0.7

0.6 o experimental 1+< average

0

r-.. n o.5 . E 0 ..c 0 .._., 0.4 . ::D 0 .....,

·;: 0.3 0 0 0

-j-1

en ~ 0.2 L

0.1

0.0 0 10 20 30 40 50

blending time. tcmin)

Figure 4.3 Effect of blending time on data scattering .

125

From the table and figure, it can be seen that the average resistivity values of the

four samples corresponding to different blending times are close, however, a

fluctuation in the data is observed for the samples prepared with the shorter

blending times of 5 and 10 minutes. A significant improvement is thus obtained

by increasing the blending time. For instance, the relative deviation decrease to

6.6% for the sample blended for 40 minutes, compared with 40% relative

deviation in the resistivity of that prepared with 5 minutes blending.

Table 4.4 The Effect of Blending Time on Resistivity

sample mixing thickness, resistivity, average maximum &

time, d(mm) p(Q.cm) resistivity, relative

t(min) p (Q.cm) deviation,

p,Q.cm (%)

0.60 0.34

1 5 0.57 0.56

0.40 0.16(40%)

0.52 0.31

0.51 0.31

2 10 0.52 0.29

0.34 0.10(29.4%)

0.48 0.44

0.59 0.37

3 20 0.55 0.38

0.36 0.03(8.3%)

0.49 0.33

0.53 0.29

4 40 0.58 0.32

0.30 0.02(6.6%)

0.48 0.29

126

It therefore can be concluded that for preparing an electrically homogeneous

carbon-plastic composite containing chopped graphite fibre, more than 20 minutes

internal blending is necessary. However, the moulding process is also an important

factor influencing the properties of the composite.

4.1.1.3 The Effect of Carbon Black on Resistivity

As described above, a combination of carbon black (CB) and graphite fibre (GF)

in the ratio 1:1 (total carbon loading 30 wt% of the composite) produced a

composite with a resistivity less than 1 n.cm. Further experiments showed that the

resistivity of the composites varies significantly as a function of the ratio of CB to

GF. Figure 4.4 illustrates the effect of carbon black on electrical resistivity for

three composites. The total loading of carbon black and graphite fibres was 30%,

only the ratio of black to graphite fibre varied. From the Figure, it can be seen

that in all cases, the electrical conductivity of the composites increases with the

carbon black content. This might be due to the high surface area and high

electrical conductivity of the carbon black particles[96-99].

It was also found that the composition of polymer affects the electrical resistivity

at the lower carbon black ratio. This can be seen from Figure 4.4. The samples

with the lower proportion of SEBS block copolymer, have a higher resistivity

value, while the composites with higher SEBS ratio show better conductivity. This

might be due to the lower surface tension of SEBS to the conducting fillers, so

that the higher SEBS content leads to a better contact for the fillers, thus showing

127

15 I 0

12 o sarrple 1 E \ u . E

A sarrple 2 ..c g 0

• :J)

o sarrple 3 -jJ

·-> 8

-jJ \ CD ·-

CD Q) L

3 A

8~ :8==--0

0.0 0.5 1 .0 1.5 2.0 2.5 ratio of CB to GF

Figure 4.4 Effect of carbon black (black pearl) to graphite fibre ratio on

resistivity. Proportion of polymers: sample 1, 65% LDPE + 5%

SEBS; sample 2, 60% LDPE + 10% SEBS; sample 3, 55% LDPE + 15% SEBS.

128

high conductivity[88,96]. However, this effect becomes negligible for higher ratios

of carbon black, as shown in Figure 4.4, since in this case, the large volume of

carbon black makes the electrical contact within the composite the dominant

factor.

4.1.1.4 The Effect of Composition on Resistivity

In order to improve the electrical conductivity of composites with the same

loading of conducting fillers, two kinds of Degussa carbon blacks were used and

composites with different composition were prepared and tested. Table 4.5 gives

the composition of several samples, while Table 4.6 shows the resistivity

comparison of the composites.

Table 4.5 Composition of Carbon-LDPE Composite Materials

Sample number and

material composition

(%, w/w)

1 2 3 4 5

LDPE(MIP 7) 70 70 70 70 60

C.B.(Cabot Pearl 2000) 20

C.B.(Deggussa XE-2) 10 20 10

C.B.(Degussa FW 200) 10 20 10

G.F.(c-203s) 10 10 10 10 20

129

Table 4.6 Comparison of the Electrical Resistivity of Carbon-LDPE Composites

Sample Thickness (mm) Resistivity (O.cm)

(by ASTM D-991)

1 0.34 0.42

2 0.34 0.24

3 0.32 0.15

4 0.25 0.43

5 0.32 0.21

It can be seen from Table 4.6 that for the same carbon black loading, the

composite with Cabot Pearl 2000 and Degussa FW 200 have similar resistivity

values, The lowest resistivity value is obtained by using Degussa XE 2 carbon

black, however, this sample was more physically brittle. Sample 2 was thus

prepared with a combination of the two kinds of Degussa carbon blacks, and the

resulting composite gave good electrical conductivity with better mechanical

properties. Even lower resistivity is observed for sample 5, which also contained

both Degussa XE-2 and FW 200, but in which the content of chopped graphite

fibre was higher.

4.1.2 Mechanical properties

Increasing the ratio of carbon black to graphite fibre resulted in an enhancement

of electrical conductivity, but also caused a deterioration in the mechanical

130

properties of the composite. Keeping the total percentage of carbon black and

graphite fibre in the composite at 30% and varying the ratio of carbon black to

graphite fibre, the mechanical properties of the composites were examined. Figure

4.5 shows the trends in tensile strength and elongation of the three kinds of

composites given in Figure 4.4. It can be seen that in all cases, the elongation

value decreases with increasing proportion of carbon black, indicating that the

larger volume of carbon black causes a degradation, or even a discontinuity in the

plastic phase[84,85], leading to a brittle appearance of the composite. The effect

of carbon black on the tensile strength of the composites seems to be less marked,

even though a slight increase was observed with increasing ratio of carbon black.

In order to improve the flexibility of the composite therefore, a number of

formulations have been tried, such as: silicon rubber + 20% graphite powder, 40%

LDPE + 60% graphite fibres, 40% LDPE + 30% graphite powder+ 25% graphite

fibre + 5% kevlar fibres (recommended by A/Prof. R.P. Burford), 70% PP + 20%

graphite fibres + 10% CB, 70% Nylon 6 + 20% graphite fibre + 10% CB.

Unfortunately, these formulations were unsucessful in getting mechanically

flexible composites.

The study was therefore redirected to the rubber modification techniques. A

thermo-plastic rubber, styrene-ethylene-butylene-styrene (SEBS) block

copolymer[193] was employed. Table 4.7 lists the composition of various

modified carbon-LDPE composite and the mechanical properties of these

composites. The effect of rubber content on the tensile strength and elongation of

the composites is also illustrated in Figure 4.6.

131

~ 18·---------------~--------~----------~···-----·--·-~--~ :2:

.. -a m L

..0

+-' a ..c +-' rn c (!) L

+-' (I)

m

(I) c m +'

o sarrpie 1 A sarrpie 2 o sample 3

16

8

12

'

g

... ... ...... .... A..._ c.._

..... --'!:a.---... ---------------0--.._

.._- --- --- ----"-·A--4

61...----~·

--- --- -----o--·-----~z

0.0 0.5 1.0 1.5 2.0 2.6

ratio of CB to GF

... -a

CD L

...a

.......... 0

c 0

-+-' 0 0) c 0

m

Figure 4.5 Effect of carbon black (black pearl) to graphite fibre ratio on

mechanical properties. Solid line: tensile strength; dashed line:

elongation. Proportion of polymers same as that for Figure 4.4 .

132

Table 4. 7 Composition and Mechanical Properties of Rubber Modified Carbon

LOPE Composites

Sample number and composition

Material and properties (%, w/w)

1 2 3 4 5 6 7 8 9

LDPE(MIP 7) 65 60 55 65 60 55 65 60 55

C.B.(Cabot Pearl 2000) 5 10 20

G.F.(c-203s) 25 20 10

SEBS (K 1652) 5 10 15 5 10 15 5 10 15

tensile strength, MPa 13.6 13.2 12.5 14.8 14.4 13.8 14.3 14 13.6

elongation,% 3.6 5.0 5.9 3.3 5.4 5.8 2.2 3.2 4.3

From Table 4.7 and Figure 4.6, it can be concluded that for all samples, at the

same carbon black proportion, increasing the ratio of SEBS results in an

enhancement of the flexibility of the carbon-LDPE composites, indicating that

thermo-plastic rubber is suitable for modifying the carbon-plastic composites to

improve their mechanical properties, in particular, the flexibility of the materials.

However, within the range of rubber content examined above, the flexibility of the

composites was not greatly improved. Further investigation was therefore desirable

and this will be discussed latter.

The effect of carbon black on the flexibility of the composite can also be observed

from this experiment, i.e. at the same rubber content, the composite with higher

carbon content shows a lower elongation value.

133

a a_ :E

• 15 -a

CD 1... .n ..,.._; 12 a

..c

..,.._;

~ g co 1...

.4-J 00

OJ 6

o sarple 1

--.... --·0

8

6

-- -c -

.. -a co 1...

..0

.4-J a c 0

....j.J

a CD c a CD -

3 L_l. ---~....c=----..1..,--_ -- ..........___ ___ 2 0 HJ 20 3J

Figure 4.6

content of SEBS to total polymer, Z

Effect of rubber content on mechanical properties. Solid line: tensile

strength; dashed line: elongation. Carbon fillers: sample 1, 5% CB +

25% GF; 2, 10% CB + 20% GF; 3, 20% CB + 10 GF% (CB = black pearl, GF = graphite fibre).

134

4.1.3 Permeation

When the composites are employed in electrode fabrication for redox cell

applications, an electrochemically active layer is bonded to the top and a current

collector (normally a metal foil or mesh) to the back of the composite sheets. If

the thin composite polyethylene sheets have a high permeability, the electrolyte

(which in the present system is vanadium sulphate in sulphuric acid) will pass

through the sheet and attack the current collector, and also possibly leak from the

battery during operation. This phenomenon has been observed frequently in the

laboratory for the carbon-plastic composite electrodes purchased from the Japanese

company Toray. As these materials are to be employed as the matrix layer of the

composite electrodes, their permeation behaviour is therefore one of the most

important characteristics.

A carbon-LDPE composite sheet with a thickness of 0.35 mm was cut and

installed in the apparatus shown in Figure 3.4. The experimental conditions for

this measurement were slightly different from those described in Section 3.1.2.1.

in that 3M H2S04, rather than distilled water, was used as the blank electrolyte.

The results obtained are illustrated in Figure 4. 7. The vanadium ions concentration

of the first reading is seen to jump up sharply from the initial (blank solution), but

levels off for the following 15 days testing, showing only very low levels, less

than 1.6 ppm vanadium. It appears therefore that the permeation rate of vanadium

ions is negligible for this material of composition 70% LDPE, 15% CB (black

pearl 2000), 15% GF, therefore. The sudden increase in the first data point is

135

E §: 1.2 .

c ~ 1.0 4-1 a L

~ 0.8 m 0 c 8 0.6 r.

+ 5 0.4 >

0.2

l ______ o---- I o----6

0 0

o.ooL-----~3------~e-------gL------,~2------~,5------~,s

t.i me( dOt:J)

Figure 4.7 Permeation behaviour of carbon-LDPE composite of composition

70% LDPE, 15% CB (black pearl), 15% GF (graphite fibre).

136

however, thought to come from the solution pumping system which had

previously been operated with a vanadium redox cell before this test. The actual

vanadium levels resulting from permeation through the composite layer is

therefore probably lower than shown in Figure 4.7.

4.1.4 Electrochemical Stability and Activity of the Carbon-LDPE Composite

Electrodes

4.1.4.1 Electrochemical Stability

In the vanadium redox battery, the following reactions occur during charging and

discharging:

At the positive electrode:

charging

V02+ + H20 - e ~ VO/ + 2H+ discharging

At the negative electrode:

charging y3++e ~ y2+

discharging

In addition, oxygen and hydrogen can evolve at the positive and negative

electrodes respectively during charging at high SOC's, or if the battery is

overcharged.

137

-c: ClJ

300.....-----------------~

-300 L __ _J1.:_6 --!:--~--:_0f-.4-:---!;-7,:--~;---;:;--;1. 6

Potential IV vs SCE

Figure 4.8 Cyclic voltarnmograrn of graphite felt bonded to (1) carbon-LDPE

composite and (2) to graphite rod electrode, sweep rate: 100

rnV/sec, \ts SCE. Composition of composite given as that for Figure 4.7.

138

A good electrode for the vanadium redox flow battery should have a high electro

chemical activity for the vanadium redox reactions, and also be stable to the

oxidative V(V) ions and oxygen which may be formed during charging. These

characteristics were thus evaluated using cyclic voltammetry. Figure 4.8 shows

cyclic voltammograms obtained for a conducting plastic electrode (composition

70% PE,l5% CB, 15% OF) onto which a piece of FMI graphite felt was bonded.

The four peaks corresponding to the above electrode reactions of the four

vanadium species can be observed (peaks A and B corresponding to V(IV)

oxidation and V(V) reduction respectively and peaks C and D to V(III) reduction

and V(II) oxidation respectively), thus indicating that the graphite felt bonded

composite polyethylene electrode structure has good electrochemical activity in the

electrolyte. Curve 2 illustrates the electrochemical behaviour of a similar size

graphite rod electrode with the same graphite felt bonded (with silicon glue) to the

surface. A broader current peak and larger peak potential separation are observed

for the latter electrode, which is probably due to a higher contact resistance

leading to high IR drops with the latter electrode. From the two curves, it appears

that the composite polyethylene is a better electrode substrate material, probably

because a lower contact resistance between the graphite felt and carbon-plastic

composite substrate can be achieved by heat pressing. Figure 4.9 gives further

cyclic voltammetry results which show the stability of the electrode during

operation. After 100 cycles, a negligible change in the electrode behaviour was

recorded, showing that these materials have excellent long-term characteristics for

the vanadium battery applications.

139

<{ E

300

0

-200

-1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 Potential/V vs SCE

Figure 4.9 Cyclic voltammetry stability testing for carbon-LDPE composite

electrode. Sweep rate: 100 mV-si,'., vs SCE, 100 cycles.

Composition of composite same as that for Fig. 4.7.

140 .

4.1.4.2 Cell Performance of Composite Electrodes

Further to the cyclic voltammetry study which indicated the electroactivity and

cycling stability of the composite electrodes, additional carbon-LDPE composite

electrodes were prepared for cell performance testing. Two types of laboratory

scale redox flow cells were employed: a 23 cm2 flow-through square cell and a 95

cm2 flow-by round cell. With the preparation procedure described previously, end-

electrodes corresponding to the cavity of each cell design were prepared and tested

Equal volume of 2 M V3.5+/2.5 M H2S04 solution (see Section 3.1.2.2) were

employed as electrolytes for both sides of the vanadium battery for the initial

cycle and the following test. A cation exchange membrane (CMV) was used as

separator for all tests. A constant charge/discharge current of 40 mA.cm·2 was

applied for both flow-through and flow-by test cells. Using the apparatus

illustrated in Figure 3.8, the cell charge/discharge curves were recorded. Average

cell efficiencies for 5 consecutive cycles were determined and these are listed in

Table 4.8.

Table 4.8 Cell Performance of Carbon-LDPE Composite Electrodes

efficiency, % flow-through cell flow-by cell

(23 cm2) 95 cm2

FMI felt FMI felt Toray felt

columbic, Tlc 92 96 91

voltage, Tlv 70 73 71

energy, Tle 65 70 64

141

It can be seen from Table 4.8 that the carbon-LDPE composite electrode with the

Toray graphite felt as the electrode active layer shows the highest cell efficiencies.

This might be due to the better electroactivity of the felt layer. For the electrode

employing the FMI felt, the cell efficiencies obtained from both the flow-through

and flow-by test cells were similar. The cell efficiencies obtained here indicate

that the carbon-LDPE composite electrode is a promising candidate for application

in the vanadium redox flow cell. However, further study is necessary for

enhancing electrochemical activity of the composite electrode, although improved

cell design should lead to significantly higher energy efficiencies in the vanadium

redox battery.

4.2 HDPE Based Carbon-Plastic Composites and Electrodes

From all the results discussed above, it can be concluded that the LDPE based

composite has good electrical conductivity, and promising cell efficiencies can be

obtained with the electrodes which were made by bonding a piece of graphite felt

onto the composite sheet. However, this composite material was found to be very

brittle and easily broken during assembly. It was also found that the thermo-plastic

rubber, SEBS block copolymer, can improve the flexibility of the LDPE based

composites, but at the proportion range investigated, the enhancement in

elongation value of the composites was not significant. Further studies were

therefore desirable. On the other hand, for use as an electrode substrate, especially,

142

for a larger electrode, rigidness of the composite sheet is required so that the

electrodes assembled in the battery, in particular, the bipolar electrodes can

withstand the hydraulic pressure without shifting. Thus, more intensive studies

were focussed on optimising mechanical properties of the HDPE based composites

by thermo-plastic rubber modification while maintaining the electrical conductivity

and impermeable properties. Furthermore, with the successful composite materials

used as the electrode matrix layer, cell performance tests including polarisation

behaviour, cell resistance, efficiencies and long-term stability of the composite

electrodes with various active layers are emphasised. Electrode deterioration test

under overcharge condition is also discussed.

4.2.1 Evaluation of SEBS Modified Carbon-HDPE Composites

4.2.1.1 Effect of Composition on Electrical and Mechanical Properties

In this section, the composition of the Rubber Modified Carbon-Plastic Composite

has been studied in relation to the electrical and mechanical properties of the

materials prepared. Based on the results obtained from the initial studies, two

kinds of Degussa carbon black powders and Kuraha graphite fibre chops were

used as electrical conducting fillers with a proportion of 10%, 10% and 20%

respectively. Considering the requirement of mechanical rigidity, two of the most

common thermo-plastics, high density polyethylene (HDPE) and polypropylene

(PP) were selected as the basic polymers. Styrene-ethylene-butylene-styrene

(SEBS) was selected as the modifying polymer. For comparison, LDPE based

143

carbon-plastic composite was also prepared. The compositions of the samples were

listed in Table 4.9, and a comparison of the electrical and mechanical properties of

the samples prepared, as well as of some commercially available conducting

plastics, is summarised in Table 4.10.

Table 4.9 Composition of Carbon-plastic Materials

Sample number and composition (%, w/w)

material 1 2 3 4 5

HDPE(GM7655) 60 30

LDPE(MIP 7) 60

PP(PPT 1070) 60 30

SEBS(G1652) 30 30

C.B.(Deggussa XE-2) 10 10 10 10 10

C.B.(Degussa FW 200) 10 10 10 10 10

G.F.(c-203s) 20 20 20 20 20

In Table 4.10, Toray-E refers to the composite plastic used as the matrix layer of

a Toray end-electrode while Toray-B refers to that used as the substrate of a

Toray bipolar electrode. The data listed in this Table were obtained from the

average values of at least three specimens taken from different parts of each of the

moulded composite sheets. Resistance values listed in column 4 were obtained by

the four-probe method. The principle of this test method was described in Section

3.1.2.2 and Figure 3.5. However, the electrical contact area of the test samples

144

was 1 cm2 and a constant pressure of 1 kg.cm·2 was applied to make a firm

contact between the sample and the copper plates during testing.

Table 4.10 Comparison of the Electrical and Mechanical Properties of the

Prepared Carbon-Plastics with Those of the Commercially Available

Conductive Materials.

Sample Thickness Resistivity Resistance Tensile Elongation

(Q.cm) (Q.cm~ strength (%)

(mm) ASTM D-991 cross-section (MPa)

1 0.65 0.17 0.77 29.4 4.2

2 0.51 0.22 1.26 16.1 10.7

3 0.36 0.13 0.68 brittle brittle

4 0.47 0.24 0.84 23.4 4.9

5 0.32 0.22 0.70 12.5 4.0

Toray- 0.26 - 19.70 32.3 1.5

E

Toray- 0.30 0.04 8.01 38.7 2.4

B

From Table 4.10, a few conclusions can be drawn:

1) The addition of thermo-plastic rubber results in a slight decrease in electrical

conductivity and tensile strength, but leads to an increase in the elongation which

means the flexibility of the composite increases after modified with SEBS rubber.

2) The polypropylene based carbon-plastic composite is too brittle for an electrode

material, since the elongation value is low even after SEBS modification.

145

3) comparing the electrical and mechanical properties of the SEBS modified

carbon-HDPE composite and the Japanese Toray sample, although the electrical

conductivity along the surface direction (column 3 in Table 4.10) is not as high,

the conductivity of the modified composite in the cross-sectional direction and the

flexibility are better than the Toray one.

Even though the results obtained show that carbon-HDPE modified with SEBS in

the ratio of 1:1 (SEBS : HDPE) has good mechanical properties as well as

electrical conductivity, further optimisation of the ratio of rubber to plastic is

needed, since a high ratio of rubber will result in a high amorphous phase inside

the composite and a decrease in the electrical conductivity and the rigidness of the

composite. Table 4.11 shows the composition of the various SEBS modified

samples prepared. Table 4.12 gives the comparison of the mechanical and

electrical properties for the different compositions. The variation in the properties

of the composite as a function of the SEBS content is illustrated in Figure 4.10.

Table 4.11 Optimisation study on the SEBS Modified Carbon-HDPE Composite

Materials Sample number and content(%, w/w)

1 2 3 4

HDPE(GM 7665) 60 50 40 30

SEBS(G 1652) 10 20 30

C.B.(Degussa XE-2) 10 10 10 10

C.B.(Degussa FW200) 10 10 10 10

G.F.(Kuraha c-203s) 20 20 20 20

146

1.0 40

o res i s t i v I t!d A tens I I e strength o e I ongat.l on

0.8 \ 15 3) ,...., ,...., A a

E (L 0 :L ,...., . .._,

~ E .c ._, -6 0.6 -rJ

c Ol 0

._, c :;:; 10

:J) 20

Q) -rJ L

a -rJ 0) > CD c :;:; 0.4 A 0 (f)

Q)

-Q) (f) CD

Q) c 5 L

0----------10 Q)

0.2 D -rJ

/ 0--..._ /0

0 0.0 0 0 10 20 30 40

SEBS contentC7.)

Figure 4.10 Effect of SEBS content on electrical and mechanical properties of

carbon-HDPE composites. Composition given as sample 1,2,3 and 4

in Table 4.11 .

147

Table 4.12 Resistivity and Mechanical Properties of SEBS Modified Carbon

HDPE Composite Materials (thickness of samples= 0.75-0.85 mm)

Sample Resistivity Tensile strength Elongation

No. (Q.cm) at break,(MPa) at break(%)

1 0.14 29.4 4.2

2 0.28 18.8 6.9

3 0.24 21.8 10.1

4 0.17 16.1 10.7

From both Table 4.12 and Figure 4.10, it can be concluded that the SEBS content

has only a minor effect on the electrical conductivity of the composites, however,

the tensile strength and elongation varies significantly with increasing rubber ratio.

The elongation value levels off when the rubber content reaches 20%, and the

decrease in tensile strength levels off in the range 10% to 20% SEBS. A SEBS

content of 20% is thus seen as the optimum level for the modified carbon-HDPE

composite material.

4.2.1.2 Solution Permeability of the SEBS Modified Carbon-HDPE Composites

As discussed in 4.1.3, solution permeability of the composites used as electrode

matrix layers is considered as one of the important characteristics of the carbon-

plastic composites. For LDPE based composite, it was found that solution

permeation through a 0.35 mm carbon-LOPE composite sheet was negligible for a

18 days testing period. For the successful carbon-HDPE composite, i.e. sample

148

number 3 in Table 4.11, solution impermeability is also expected. Therefore, a

similar test as described below, was conducted.

A small round cell fitted with solution pumping system was employed (see

Section 3.1.2 and Figure 3.4), the sheet being used as a separator between the two

electrolyte compartments. The thickness of the composite sheet employed was

0.35 mm (half the thickness of those used for electrode fabrication). On one side

was 2 M V(IV) in 3 M H2S04 solution (the positive electrolyte of vanadium

battery) and on the other side was distilled water (see Figure 3.4). Both solutions

were pumped for 15 days, and 5 ml samples were withdrawn from the water side

periodically. The concentration of vanadium and sulphur (as SO/) in the blank

solution was determined using Inductively Coupled Plasma (ICP) analysis, and the

results are illustrated in Figure 4.11.

The non-zero intercepts in Figure 4.11 is again probably due to impurities

introduced from the pumps, the containers and other cell components. A higher

sulphur concentration was also detected throughout the testing, which might be

due to the difference in bulk concentration as well as the difference in ion

permeability between the sulphate and vanadium ions. However, the detected

values of both vanadium and sulphur are very low during the 15 day period,

showing that the composite material prepared is suitable for redox cell

applications.

149

35~--------------------------------------------~

30 1:!. suI phur

r-.

E 25 0.. 0.. .......

~ 20~ a

.!::; 15 c Q) (.) c 8 10

5cl-o-o-o--

0 0 3

o vanadium

0 0

6 9 12 15 18 time. Cda!::J)

Figure 4.11 Permeation behaviour of SEBS modified carbon-HDPE composite

of composition 40% HDPE, 20% SEBS, 10% CB (Degussa XE 2),

10% CB (Degussa FW 200), 20% GF.

150

4.2.1.3 Microstructure of SEBS Modified Carbon-HDPE composites

The properties of a material are believed to be related to its microstructure. SEBS

modified carbon-HDPE composite has shown excellent electrical conductivity,

mechanical properties and solution impermeability. In order to interpret the

observed phenomena, a moulded and copper-mesh backed sample with the

composition of sample number 3 in Table 4.11 was examined under the scanning

electron microscope (SEM). A true cross section of the sample was obtained by

the cryogenic fracture method. A sample strip with dimensions 5 x 1 x 0.07 em

was cut and then immersed into liquid nitrogen for 2 minutes. After this treatment,

the sample was taken out and quickly broken to get a true cross section with an

area of 10 x 0.7 mm. The specimen prepared was then bonded (with the cross

section facing-up) to an aluminium specimen support with silver glue and

examined with a S-360 SEM unit. The surface structure of the sample is

illustrated in Figures 4.12 (a) and (b).

In Figure 4.12 (a), fibres can be observed through the cross section, indicating that

the processing procedure gives rise to a good distribution of fibres in the mixture.

A graphite fibre orientation perpendicular to the thickness direction which is the

direction of pressure for moulding also appears, showing that the electrical

network formed by the graphite fibres is mainly two-dimensional. This is

probably the reason why the composite has solution impermeable behaviour.

Figure 4.12 (b) shows a photograph of the cross-section under higher

151

a)

b)

Figure 4.12 Cross section of a SEBS modified carbon-HDPE composite (Cu

mesh backed), magnification: a) 40x: b):440x. Composition same as

that for Fig. 4.11 .

152

magnification. It can be seen that the graphite fibres are "sandwiched" by the

continuous phase of polymer-carbon black mixture, explaining the good electrical

and mechanical properties of the composites. However, carbon black particles can

not be identified under this testing condition.

4.2.2. Evaluation of SEBS Modified Carbon-HDPE Composite Electrode

4.2.2.1 Electrical Resistance and Cell Resistance

The composite sheet of composition number 3 in Table 4.11 has thus been found

to be suitable for use as an electrode matrix layer for the redox cell. A number of

end-electrodes were thus fabricated with different active layers by hot-bonding

(see Section 3.1.1.3 and Figure 3.3) onto the sheet, and the electrical resistance

values of the composite electrodes determined using the four-probe method, for

which the measuring procedure and method for determining resistance values were

described in Section 3.1.2.2 and Figure 3.5.

The reliability of the testing method was first evaluated. A pair of commercially

available Toray end-electrodes was used for this evaluation. Three independent

measurements were carried out on the same electrodes to establish any deviation

caused by the test procedure. Figure 4.13 shows the results obtained. Only a slight

variation was observed, indicating that reproducible result can be achieved with

this method.

153

70

r-,/ 60 0 isi:. o 3rd 0

D.

50 D. 2nd r--.

> E

"---'

> 40 .

Q) 0) d 30 __,_..

0 >

20

10 0/0

0 0 5 10 15 20 25 30 35 40 45

oppl ied current dens it'd. i CmR/cm 2 )

Figure 4.13 Reproducibility Evaluation of four-probe method for measuring

resistance of composite electrode .

154

To evaluate the electrical resistance variation caused by processing, four pairs of

end-electrodes prepared by the procedure described in Section 3.1.1.3 and Figure

3.3 were tested. All the electrodes were made by hot bonding Toray graphite felt

onto the SEBS modified, copper mesh backed carbon-plastic composites. A good

reproducible result can also be seen in Figure 4.14, showing that the variation

introduced from the preparation procedure is minor.

Thus, the data obtained with four-probe method is believed to be reliable. The

electrical resistance of various composite electrodes prepared in this laboratory

were therefore evaluated with this method and the results are listed in Table 4.13.

For comparison purpose, the resistance of commercial Toray electrodes was also

tested.

Table 4.13 shows that resistance of various composite electrodes varies

corresponding to the different active layer used. The lowest resistance value was

obtained for the electrode fabricated with Toray graphite felt, while the highest

was observed from that with the FMI felt. This could be due to either the different

electrical properties and surface area of the different graphite felt materials, but is

most likely due to the variations in the contact resistance between the graphite felt

and composite sheet during hot-bonding. The different felt materials probably

require slightly different bonding condition to achieve optimum contact. Table

4.13 also shows that the resistance value of the composite end-electrode prepared

with Toray felt is close to that of the Japanese Toray electrode, even though the

thickness of composite sheet is more than two times greater than that of latter one.

155

30~----------------------------------------~

o No. i L-. No.2 o No.3 *No.4

r--.. 20 0 > E * "--'

> . 0 Q)

o/*L-. ()) d _....,

0 10 c,D >

OD /*

/''' :0

0 0 3 6 g 12 15 18

applied current densit\j. CmA/cm 2 )

Figure 4.14 Effect of processing on resistance of composite electrodes

(Composition of electrode matrix same as that for Fig. 4.11).

156

This indicates that the electrical properties of SEBS modified carbon-HDPE

composite are superior to those of Japanese Toray carbon-plastic.

Table 4.13 Electrical Resistance Comparison for Some SEBS Modified Carbon-

HDPE/Graphite-felt Electrodes

Electrode Thickness (from Resistance through

Cu to plastic), mm thickness direction

(ohm.cm2)

1. end electrode

PRJFMI felt(6 mm) 0.9 2.35

PR/Le carbon felt (6 mm) 0.9 1.51

PR!foray Felt (6 mm) 1.00 0.68

purchased Toray end- 0.28 0.7

electrode

2. bipolar

purchased Toray bipolar 0.30 1.11

electrode

PR!foray felt 0.70 1.74

Figure 4.15 (a) and (b) show photographs of the cross section and the matrix layer

surface (the bonded felt has been removed) of a typical SEBS modified composite

electrode prepared in this study. Again, cryogenic fracture method was employed

to obtain the true cross section and surface. From Figure 4.15 (a), a firm bonding

between felt and composite matrix layer can be seen, indicating that the

157

a)

b)

Figure 4.15 Morphology of composite electrode, a) cross section of boundary

area, b) electrode matrix layer surface (felt has been removed).

Composition of electrode matrix same as Fig. 4.11, Graphite felt:

GFD2.

158

effectiveness of the electrical contact between surface active layer and composite

matrix layer can be obtained by the processing procedure designed. In order to

obtain further information about the electrical contact between the graphite felt

and the composite materials, the bonded felt was removed by cryogenic fracture

method: put the composite electrode into liquid nitrogen for 2 minutes and then

break along the interface. The exposed matrix layer surface (top view) is shown in

Figure 4.15 (b). The large amount of the fibres seen from the photograph, i.e.

Figure 4.15 (b) explains the efficient electrical conductivity of the composite

electrode.

Cell resistance values of the vanadium redox flow cell employing the composite

electrodes were also tested using a complete single cell and associated apparatus

(see Figure 3.8) with 50% state of charge (SOC) electrolyte in both positive and

negative half cell. Cell resistances for various carbon-plastic composite electrodes

were determined by the principle described in section 2.6.2.1 and are shown in

Table 4.14. From this table, it can be seen that the composite electrodes

employing the Toray graphite felt have the lowest cell resistance, compared with

the other types of felt and the Japanese electrode. This may be due to the high

real surface area and good electrical conductivity of the felt material. Further

evaluation and comparison on some physical properties of felt materials will be

discussed in Chapter VI.

The Toray graphite felt was thus employed to fabricate composite electrode for a

series of cell performance tests in the vanadium redox flow cell.

159

Table 4.14 Cell Resistance Comparison for Some SEBS Modified Carbon-

HDPE/Graphite-felt Electrodes

Graphite felt Thickness Membrane SOC(%) Cell resistance

(mm) (ohm.cm2)

Toray 6 CMV 50 3.03

Le Carbon 6 CMV 50 4.26

FMI (non) 6 CMV 50 3.90

Japanese Toray 6 Nafion 117 50 3.55

electrode

4.2.2.2 Cell Performance Testing

To evaluate the cell performance of any component in a vanadium redox flow cell

or battery, the cell charge/discharge behaviour at constant current[39,51-53,184],

cell efficiencies[39-41, 51-53, 184-187] and long-term cycling stability[51-53,

185-187] are evaluated. The following cell performance tests were thus performed

with the SEBS modified carbon-HDPE composite electrodes:

- constant current charging/discharging behaviour,

- cell efficiencies vs current density;

- single cell short-term charge/discharge testing;

- multi-cell short-term charge/discharge testing;

- single-cell long-term charge/discharge testing.

160

- electrode deterioration under overcharge conditions.

Constant Current Charging/Discharging Behaviour

Figure 4.16 shows the typical charge/discharge behaviour of the vanadium redox

cell with the SEBS modified carbon-plastic electrode at a charge/discharge current

density of 21.7 mA.cm-2• A piece of CMV membrane was employed as separator.

Average efficiency values were calculated from the recorded curves of the first

seven cycles. A coulombic efficiency of 96.5%, voltage efficiency of 89.4% and

energy efficiency of 86.0% were obtained respectively, indicating that the

composite electrode has good electrical conductivity and electrochemical activity.

The Effect of Current Density on Cell Efficiency

Figure 4.17 illustrates the effect of increasing charge/discharge current density on

the cell efficiencies for the composite electrodes. At the lower current densities,

the cell voltage and energy efficiency are very high with a columbic efficiency of

more than 95%. As expected, the cell voltage and energy efficiencies decrease

with increasing current density, due to increased IR losses, however, around 80%

energy efficiencies can still be obtained at a current density of 40 mNcm2• Further

optimisation of the electrode resistance and electroactivity of the felt, could

however reduce the cell resistance and further improve cell performance.

161

> 1.6 .. Cl)

0)

cd ......, 1.2 0

>

Cl)

0.8 0

Time,min

Figure 4.16 Typical charge/discharge curve of vanadium redox cell employing

SEBS modified carbon-HDPE composite electrode (matrix layer:

same as that for Fig. 4.11; active layer: Toray felt). ich. = idis. = 21.7

mNcm2; cell voltage up limit= 1.75 V, lower limit= 0.80 V.

162

110

o coulomblc b. vol tags o energ!d 105

,-., 100 N 0 ..._,.

-o-(/) 0 Q) 95 --

0 c Q)

0 90 ...,_ ...,_ Q) 85

Q)

80 A 0 0

75

70 10 20 30 40

current densith)CrrA/cm2)

Figure 4.17 Effect of current density on the cell efficiencies of a vanadium

redox flow cell employing SEBS modified carbon-HDPE composite

electrodes. Electrodes and other conditions same as that for Fig. 4.16.

163

Short-Term Cell Performance Test

Figures 4.18 and 4.19 show the short-term cell charge/discharge behaviour for the

SEBS modified carbon-HDPE composite electrode in both a single cell and in a

two-cell vanadium battery respectively. A piece of CMV membrane was used as

the separator, and a current density of 15 mA.cm-2 was applied for the single cell,

while two pieces of DMV membrane (Asahi Glass Co., Japan) were employed and

a current density of 30 mA.cm-2 was used for the two-cell battery. In both case,

fairly constant voltage efficiencies with slightly fluctuating coulombic and energy

efficiencies versus cycle number were observed. This indicates that the

electrochemical activity of the electrode and other cell components are very stable

in the vanadium redox flow battery.

It should be noted that the sharp drop in coulombic efficiency observed at cycle

number 62 in Figure 4.19 is due to the electrolytes crossing over through a

degraded membrane. This was corrected by inserting a new piece of membrane

which resulted in an increase in coulombic efficiency to around 90%.

Long-Term Stability Test

Longer-term testing of the electrochemical activity of the electrode was carried out

in a single cell by continuous charge/discharge cycling over 5780 hrs (1240

cycles) at a current density of 21.7 mA.cm-2• The results are shown in Figures

4.20(a) and (b). The voltage efficiency is relatively constant, decreasing by only

164

105~--------------------------------------------,

100

:J) (.) c Ol95 (.)

'+--'+--

Q)

-90 m (.)

85

o coulomblc t:.. vol tags o energy

0

o--o-_o--o--o--o--~ ~~0~ 0

8DL-----~----~------L-----~----~------~----~

30 40 50 60 70 80 90 100 qJC I e number

Figure 4.18 Short-term single cell performance of the composite electrodes of

Fig. 4.16. Other conditions same as that for Fig. 4.16.

165

100~------------------------------------------~

95 1:11

90 1

- 85 ~ 0 ......,

>- 80 (.) z w (.) 75 L!. L!. w 70

65

60

55 i

0 10 20 30 40 50 60 70 80 90 No. OF CYCLE

Figure 4.19 Short-tenn perfonnance of a two-cell vanadium redox battery with

the composite electrodes of Fig. 4.16. Curves 1: coulombic, 2:

voltage; 3, energy efficiencies. ich. = idis. = 30 mNcm2• Other

conditions same as that for Fig. 4.16.

166

5% over the 5780 hours of testing, showing the electrode is promising for

application in vanadium redox battery. The coulombic efficiency is seen to

fluctuate slightly, this being due to variation in the state-of-charge from cycle to

cycle caused by fluctuations in flow rate, and probably due to the temperatures

difference during day and night. A gradual decrease in the coulombic and energy

efficiencies versus cycle number is also observed and shown as region A and B in

Figure 4.20(a). This was found to be due to the degradation of the CMV

membrane used. Region A shows the first run of a piece of CMV membrane. At

the 178th cycle, a coulombic efficiency of lower than 75% was recorded,

indicating that the self-discharge of the cell was significant. After inserting a new

piece of CMV membrane, the coulombic and energy efficiencies recovered to their

original values, however, a similar trend appeared as recorded in region B. A

piece of modified Daramic membrane[l88,189] was therefore employed to replace

the CMV membrane for the subsequent cycling. Constant coulombic and energy

efficiencies were obtained with this membrane for the remaining duration of the

test. Thus, after removing the interfering effects of the membrane on the all

stability test, the Toray felt-SEBS modified composite electrode is seen to exhibit

excellent long-term activity in the vanadium redox cell.

Electrode Deterioration Test

Earlier tests with graphite positive electrodes in the vanadium redox cell have

shown that if oxygen evolution takes place in the positive half cell during

charging, mechanical degradation of the carbon can occur due to C02 formation.

167

105.------------------------,

a)

65

60 0 200 400 600 800 1000 1200 1400

CYCLE NUMBER 110

105

iOO _...._ ,o o'- 95 "-"

>- 90 u b) z 85 UJ u 80 u. !.L 75 w

70

65 --~--- voltage

60 0 200 400 600 800 1000 1200 1400

CYCLE NUMBER

Figure 4.20 Long-term single cell stability test for the composite electrodes of

Fig. 4.16, a) efficiency variation, 1 :coulombic, 2: voltage, 3: energy;

b) voltage efficiency variation. Other conditions same as that for

Fig. 4.16.

168

Furthermore, it has been observed that the commercial Toray electrode is damaged

when the electrodes are operated under overcharging conditions for more than 10

minutes. It is therefore necessary to evaluate the life of the composite electrodes

[(Toray graphite-felt)/(SEBS modified carbon-HDPE)] under overcharging

conditions. This was performed by applying a constant charge current to a fully

charged vanadium redox flow cell (single cell) for 10 minutes, followed by a

series of measurements in cell resistance, cell efficiencies and cell charge-up

potential in 5 consecutive cycles. This performance was repeated 5 times, i.e. the

total overcharge time was 50 minutes. Figure 4.21 shows the trend in cell

resistance versus overcharge time for a cell that had been subjected to an

overcharge current of 3 A (21.7 mA.cm-2). It can be seen that the cell resistance

increases slowly within 40 minutes and then increases dramatically at 50 minutes,

indicating that the electrode had been seriously damaged. Figure 4.22 summaries

the cell performance behaviour versus overcharge time. It can be concluded that

overcharging results in an increase in cell resistance and cell charge-up voltage

and a decrease in cell voltage efficiencies. Thus, when the cell resistance increases

above a certain value, the cell charging voltage would be very high. At this stage,

the battery can not be operated due to the poor voltage efficiency. This can be

interpreted as follow:

According to Equation 2.1, cell voltage is determined by the equilibrium potential

difference of the redox couples, the polarisation losses and ohmic losses.

Combining the last two component into one gives the total polarisation losses

which is written as V pot.(=IRce11). The cell voltage during charging and discharging

169

80

o charging D. discharging r-. 70 ru

E 0 . 60 E ..c 0 ..._.,

50 . rn 0

a 40 -1,-1

(f)

(f) 30 rn L

rn 20 0

10 0 6- o- ~'"-'

0 0 10 20 30 40 50 60

overcharging time. Cmin)

Figure 4.21 Effect of overcharging time on cell resistance of vanadium redox

flow cell with the (Toray felt)/(SEBS modified carbon-HDPE)

composite electrodes .

170

6 12 120

A coulombic D voltage liE ener9tJ ,--.. ,..... > (\J

...._,5 E10 A A 100 ,--.. 0 A A A N . . ..........

CD E 0) ..c D D- . 0 0 liE lit -o

~==----rn ...... ......... Q) -4 8 D 80 ·-0 . D > (]) ... o c

0 Q)

§- a _,. .. .o-- tJ io3 -+Jfl .. .. 60 (f) .. .. '+-0) .. + '+-L (f) _..0"" / Q) 0 (]) / .. / ..c L

o----- --0""'- /

02 /

-4 / 40 -+'" Q) +- - - - - _.._. - - - - - +- - - - - D

(]) Q) 0

charge-up 0 0 res is. + 2 20

0 10 20 30 40 50

over charge time. Cmin)

Figure 4.22 Effect of overcharging on cell performance with the composite

electrodes of Fig. 4.21.

171

can be written as:

During charging: V =V +V cell,ch. eq. pol.,ch. (a)

During discharging: V 11 dis = V - V 1 dis ce ' . eq. po ., . (b)

Therefore, the cell charging voltage at 100% SOC, which is referred to here as the

cell charge-up voltage, will be a function of cell resistance. For example, after 50

minutes, the cell resistance increased to 72 n.cm2 for the charging process (see

Figure 4.21), and a cell charge-up voltage of 2.95 V was recorded, indicating the

cell has polarisation losses of 1.35 V. Considering that at 100% SOC, Veq.= 1.60

V, and assuming V poi.,ch. = V poi.,dis.• the cell voltage efficiency at the beginning of

the discharge process can be readily determined from Equation (a) and (b), and is

found to be very low and reduces further with decreasing SOC during discharging.

The reason for the increased cell resistance with cell overcharge was studied in

seriously overcharged Japanese Toray end-electrode and as shown in Figure 4.23.

It was found to be due to the formation of a gap between the felt and composite

sheet when the electrode is operated under serious overcharging conditions. This is

due to the decomposition of the graphite felt at the interface with the composite

sheet which destroys the electrical contact between the felt and substrate. Further

studies revealed that the oxidation of the graphite felt may also contribute to the

increase in cell resistance and this will be discussed in Chapter VI.

172

conducting ,boundary plastic layer

of

a)

b)

Figure 4.23 Cross section of Japanese Toray composite end-electrode.

Magnification: 20x. a) normal electrode; b) after seriously

overcharged

173

This problem can be readily solved by a suitable controller in which an

appropriate upper limit in cell voltage can be set and the battery system can be

operated within the safe range to eliminate any risk of electrode degradation.

The composite electrodes fabricated in this project have however been shown to

be more stable than the Japanese Toray electrodes under overcharge conditions in

the vanadium cell since the overcharge time which can be tolerated by these

electrodes is much higher than that which leads to a destruction of the Japanese

ones. These electrodes are thus superior to the commercial electrodes for

application in the vanadium redox battery.

4.3 SUMMARY

1. LDPE based carbon-plastic composite materials show promising electrical and

electrochemical properties, however, the mechanical properties are not satisfactory

as electrode matrix layer for redox flow cell applications.

2. Carbon-HDPE composite material modified with SEBS thermo-plastic rubber,

has been shown to have good electrical and mechanical properties, as well as

being impermeable to the acidic vanadium electrolyte.

3. By hot pressing a graphite felt and a metal mesh on either side, conducting

plastic electrodes can be fabricated for use in the vanadium redox cell. The

174

electrical and cell resistances values measured for these electrodes is comparable

to those of the Japanese conductive plastic felt electrodes.

4. An overall energy efficiency of 88% can be achieved with this electrode in cell

charge/discharge testing. Long-term charge/discharge testing was conducted over

more than 5780 hours (1240 cycles) and the SEBS modified carbon-HDPE

composite was shown to be stable and is thus a reliable electrode matrix material

for the vanadium redox battery.

175

CHAPTERV

ELECTRODE KINETICS AND ACTIVATION STUDIES

5.1 Electrode Kinetics of V(V)/V(IV) and V(III)/V(ll) Couples at Graphite

Electrode

Graphite felt bonded onto a carbon-plastic composite substrate has been shown to

be an excellent electrode for the vanadium redox flow battery application. In

Chapter 4, the electrochemical behaviour of this electrode was characterised by the

cell polarisation behaviour, cell resistance, cell efficiency and cycling stability.

Even though these parameters indicate the operational characteristics of the

electrode in the vanadium redox cell, they are not able to give kinetic information

on the electrode and redox couples. A better understanding of the electrochemical

behaviour of the redox system at a selected electrode would lead to a more

effective strategy for improving the electron transfer rate of the system. It is

therefore necessary to undertake further electrochemical studies of an individual

electrode rather than with the entire battery. As described previously, the carbon

plastic composite material was used as the electrode matrix layer and graphite

fibre based felt materials as the surface active layer at which the redox reactions

take place. Graphite is therefore considered as the electrode material. For the all

vanadium battery, the redox couple for the positive half cell is V(V)N(IV) while

the negative is V(ill)N(ID. The present investigation thus concentrated on the

electrode behaviour of the vanadium redox couples at graphite electrodes. In

particular, a more intensive study was first carried out on establishing the

176

electrode kinetics of the V(V)N(IV) couple at the graphite rod electrode.

5.1.1 The Electrode Kinetics of V(V)/V(IV) Couple

The electrochemical properties of vanadium on a dropping mercury electrode

(DME) in aqueous systems have been extensively studied[13-19], but studies on

normal solid electrodes are limited. During their initial screening of redox couples

for redox cell applications, the NASA group[4] studied both vanadium couples

with cyclic voltammetry at solid electrodes and reported that the V(V)N(IV)

couple exhibits irreversible behaviour, although the reversibility of V(Ill)N(II) on

a B4C electrode was better than that of Cr(III)/Cr(II).

Sum et al[27] also studied the kinetics of the V(V)N(IV) couple on glassy carbon

and gold electrodes with cyclic voltammetry and rotating disc voltammetry and

concluded that the system is electrochemically irreversible with a value of k0 = 7.5

x 10-4 cm.s-1 and a diffusion coefficient of 1.4 x 10-6 cm2.s-1 for V(V). They also

found that the electrochemical behaviour of V(V)N(IV) couple on a glassy carbon

electrode is affected greatly by the surface preparation procedure.

A number of workers[20-23] have investigated the electrochemical behaviour of

the V (V)N (IV) couple at noble metal electrodes such as platinum and gold, and

have found that in most cases, an oxidation film which forms on the electrode

surface influences the electrochemical reactions. Rychick and Skyllas-Kazacos[26]

evaluated the suitability of various electrode materials for the V (V)/(IV) reactions

177

and indicated that on gold and glassy carbon electrodes, the redox reactions are

irreversible, while on the lead and titanium electrodes, passivating phenomena

were observed in the potential range of interest. Although platinised titanium and

DSA (Dimensional Stable Anode) electrodes have no such problem and showed

better reversibility, the high cost of these materials would be prohibitive for large

scale applications in the vanadium battery.

As pointed out by Miller and Zittle[23], at a pyrolytic graphite electrode, cathodic

voltammograms were reproducible and there was no evidence that the surface of

the electrode was oxidised by contact with V(V), which means this material is

suitable for studying the electrochemical behaviour of the V(V)N(N) redox

couple.

Since graphite felt is employed as the surface active layer of the electrode in both

the positive and negative sides of the vanadium redox battery, it is desirable to

establish the electrode kinetics of V(V)N(N) redox reactions at the graphite

electrode. Kinetic studies were thus performed here using both cyclic voltammetry

and rotating disc voltammetry. The diffusion coefficient for V(IV) and the elec

trode kinetic parameters were calculated and the equilibrium exchange current

density values for the V(V)N(N) couple were determined for graphite felt and

reticulated vitreous carbon electrodes. The experimental procedure for the

electrode kinetic study was described in detail in 3.2.1.

178

5.1.1.1 Reproducibility of Electrode Surface Preparation

Sum et al[27] previously reported that the electrode surface preparation procedure

has a critical effect on the electrode kinetics of the vanadium redox couples at a

glassy carbon electrode. The reproducibility of the surface preparation procedure

on the behaviour of the graphite electrode was thus f'rrst tested. Figure 5.l(a)

shows three anodic voltammograms obtained after successively repolishing the

graphite electrode surface using the procedure described in section 3.2.1.1. All the

curves recorded represent the f'rrst sweep after polishing and show that the

reproducibility obtained at the graphite electrode surface is much better than that

of the glassy carbon surface[27,42]. Figure 5.l(b) shows the cyclic stability of the

graphite electrode from the 1st to the lOth cycle over the potential range of 0 to

+ 1.10 V. Apart from the f'rrst cycle, only a slight variation can be observed among

the remaining voltammograms showing that the graphite electrode is a suitable

working electrode for studying the electrochemical behaviour of the V (V)N (IV)

redox couple.


A series of cyclic voltammograms corresponding to the V(V)N(IV) redox couple

at the graphite electrode at various sweep rates in 0.05 M V(IV) + 0.05 M V(V)

sulphate in 3 M H2S04 solution is shown in Figure 5.2(a) for the potential range

-0.504 V to +0.90 V. Four peaks (marked A,B,C and D) are observed in each

cyclic voltammogram indicating that besides the expected V(V)N(IV) redox

179

0.8

<(

E 0.6

. 0 0.25 0.5 0.75 1.0 1.25 Electrode potential vs Hg/Hg

2S04, V

(a)

180

.. -

0.6

0 0.25 0.5 075 1.0 Electrode potential vs Hg/Hgz.S04 , V

(b)

Figure 5.1 Cyclic voltammograms for graphite electrode in 0.1 M

VOSOJ3 M H2S04 solution, sweep rate: 10 mV s4 . (a) anodic

voltammograms for testing reproducibility of surface preparation;

(b) cyclic voltammograms for testing the cyclic stability, 1 st to 10 th cycles.

181

reactions, there are some other electrochemical processes taking place during

scanning. Comparing the cyclic voltammograms obtained at the same graphite

electrode in 3 M H2S04 under the same experimental conditions shown in Figure

5.2(b), it can be seen that over this potential range in the dilute vanadium solution,

several peaks associated with the carbon-oxygen surface reactions are interfering

with the vanadium reactions. When the potential is limited between 0 and 0.9 V

however, the voltammograms of Figure 5.3 were obtained, showing that

interference from carbon-oxygen surface reactions is minimised.

5.1.1.3 Rotating Disc Voltammetry

For further details of the electrode kinetics of V(V)N(IV) on the graphite disc

electrode, cathodic and anodic voltametric experiments were also performed with a

rotating graphite disc electrode in the same solution used for Figure 5.2(a) and

5.3. A potential sweep rate of 1 mV.s-1 was employed and the electrode rotation

speed was varied from 60 rpm to 2400 rpm. The current versus overpotential

curves recorded in Figure 5.4(a) and 5.4(b) show that at each rotation speed,

cathodic and anodic limiting currents are observed, the magnitude increasing with

electrode rotation speed, indicating that under these conditions, the electrode

processes become mass transfer controlled[l79].

The 11-i curves also show that the cathodic and anodic electrode processes are

quite different. Thus, in the anodic voltammograms, there is only one wave corres

ponding to V(IV) being oxidised to V(V), while the cathodic voltammograms are

182

... -......... c 0 (1) lolo-

::J o-0.2

-0.4

' .D

A

~5

-0.2 0.1 0.4 0.7 1.0 Electrode potenflal vs Hg/Hg2S04-, V

(a)

183

<(

E --

0.3r---------------1

0.1 0.4 0.7 1.0 Electrode potential vs Hg/Hg2S04 ,V

(b)

Figure 5.2 Cyclic voltammograms of graphite electrode in (a) 0.05 M

V(V) + 0.05 M V(IV) in 3 M H2S04; (b) 3 M H2S04• sweep rates:

1, 10, 20, 40, 60 mV. s·1 for curves 1 to 5 respectively.

184

0.4

<( 0.2 E

- 0.4 L_ _ __J __ __!_ __ ---'-----J

-0.2 0.1 0.4 0.7 1.0

Electrode potential vs Hg/Hg2 804 , V

Figure 5.3 Cyclic voltammograms of graphite electrode in the potential range 0

to 0.9 V, other conditions are same as those for Figure 5.2(a).

185

more drawn out, suggesting at least two reactions are involved. This indicates that

besides the main V(V) species, other species such as V(V)-SOl- complexes[190]

or carbon-oxygen surface complexes[lOl] (see Figure 5.2(b)) are reduced within

this potential range during cathodic scanning.

Figure 5.4(b) shows the cathodic and anodic voltammograms obtained at electrode

rotation speeds ranging from 1200 rpm to 2400 rpm. Unlike the curves obtained at

the lower rotation speeds, the 3 pairs of rt-i curves are very close, particularly, in

the anodic branch, the curve obtained at 2400 rpm is almost the same as the one

at 1800 rpm, indicating that at sufficiently high rotation speeds, the electrode

process is no longer diffusion controlled but becomes kinetically complicated.

5 .1.1.4 Diffusion Coefficient

The diffusion coefficient for the V (IV) reactive species was determined from the

Levich equation:

(5-1)

where iL is limiting current density (A.cm-2); n, the number of electrons

participating in the reaction; D, the diffusivity of reactive species (cm2.s-1); v, the

186

0:>

< en :c (Q

..........

:c (Q

r\J (f)

p <

cathodic

(a)

187

I ~

a-

I ~

0 "-' < <D ""'1

'"0 0 co ....... <D :J ....... 0:>

< (j)

I {Q ........ I

{Q 1\) (j)

~0 ~

< N

Figure 5.4

0 0:> ....... ::J"' 0 c.. 0

Current/!, rnA

LU Vl

0:> :J 0 c.. 0

(b)

Cathodic and anodic voltammograms of a rotating disc graphite

electrode in electrolyte of Figure5.2(a); sweep rate: 1 mV s:-1 ; (a)

rotation speeds: zero, 60 rpm, 120 rpm, 240 rpm, 600 rpm, 1200

rpm for curves 1 to 6 respectively. (b) rotation speeds: 1200 rpm,

1800 rpm, 2400 rpm for curves 1 to 3 respectively.

188

viscosity of electrolyte (cm2.s-1), ro, the angular velocity of rotation (s-1

) and Cb,

the concentration of reactive species in the bulk electrolyte (mol.cm-3).

A summary of limiting currents and the corresponding angular velocity obtained

directly from Figure 5.4(a) and 5.4(b) is given in Table 5.1. To avoid introducing

errors caused from the interference of other cathodic reactions discussed above,

only the anodic curves were analysed. Plotting the square root of angular speed

versus anodic limiting current density, yields the straight lines of Figure 5.5,

indicating that up to an electrode rotation speed of 1800 rpm, the V (IV) oxidation

reaction obeys the Levich equation. From the slope of the straight lines, the

diffusion coefficient for V(IV) was calculated as 2.00 x 10·6 cm2.s·1•

Table 5.1, Summary of Limiting Current Density versus Angular Velocity for the

V (IV) Oxidation Reaction

(1)112[ (s·l ))112 i1 c(A.cm-2)x 103 i18(A.cm·2)xl03

2.51 3.44 3.44

3.54 5.00 4.69

5.01 6.88 6.46

7.93 10.83 10.21

11.21 15.10 14.06

13.73 18.23 17.19

15.85 20.83 18.23

189

2S~x~1~0_3 __________________________ ~

>-. 4-

tn c QJ

w

c 10 QJ L.. L.. ::J

LJ 5

Angular velocity, w~ sec-112

Figure 5.5 Plot of limiting current density(iL) vs square root of angular velocity

for anodic voltammograms of Figure 5.4.

190

Rotating disc voltammograms were also obtained for a range of vanadium

concentrations. The concentration of VOS04 was varied from 0.01 to 0.5 M, and

electrode rotation speeds of 120, 360, 720 and 1080 rpm were employed. Figure

5.6 shows typical anodic voltammograms obtained in the 0.5 M VOS04 solution.

The anodic limiting current density was plotted versus the square root of angular

speed at each concentration and the results are shown in Figure 5.7(a). The anodic

limiting current density versus vanadium concentration was also plotted to yield

Figure 5.7(b). The linear relationship in both Figure 5.7(a) and 5.7(b) illustrates

that the V(IV) oxidation reaction at these concentrations obeys the Levich

equation. Plotting limiting current density over vanadium concentration versus

square root of angular velocity yields Figure 5.8. The diffusion coefficients were

determined from the slopes of the straight lines in Figure 5.8 and are shown in

Table 5.2. From the values, it can be concluded that in the concentration range

studied, the diffusion coefficient of V(IV) species is essentially constant with an

average value of D = 2.14 x 10·6 cm2.s·1•

Table 5.2, Diffusivity of V(IV) Species

Conc.of V(IV), mol.cm·3 Diffusivity, D(cm2.s-1)

0.01 X 10"3 2.46 X 10"6

0.05 X 10"3 2.00 X 10-6

0.25 X 10-3 2.25 X 10"6

0.5 X 10"3 1.86 X 10"6

191

10 4

8 <(

E 6 ...

2 -.......... ~ (}.) 4 '!... '!... ::J 1 ()

2

Electrode potential vs Hg/Hg2SO~, V

Figure 5.6 Anodic rotating disc voltammograms at a graphite electrode in 0.5

M V(IV)/3 M H2S04 solution. rotation speeds: 120 rpm, 360 rpm,

720 rpm, 1080 rpm for curves 1 to 4 respectively.

192

x10-3

1 00.07/Vf 6 0. 05 /Vf N I <> 0.25 /Vf 00-5/0 E u <( ..

_l

8 >..

-+-t/1 c:: OJ

"0

-+- 4 c OJ '-'-:=J

LJ

0 0 3 6 9 12 15

Angular velocity, w~ se(Y2

(a)

193

('l I

E u

<(._ ...1

>-. 4-

U")

c <lJ

-o 4-c <lJ '-'-:::J

L)

Figure 5.7

x10-3

12 rpm

6360 0 720 0 120

0 7080

8

4

0~--L---~------~------~~~ 0 0.2 0.4 0.6

Concentration V4 .. {M)

(b)

(a) Plot of limiting current density vs square root of angular

velocity at various vanadium concentrations; (b) Plot of limiting

current density vs vanadium concentration at various electrode rotation speeds.

194

- 0.4 .----------------:;;;r------, I ~

E \)

<C E 0.3 u c: 0 u

- 0.2 _]

0.1

0 0.07/vf

<> 0.2510

6 0.0510 _/)

0 0.5/0 /

/ 0

o~~--L-----~----~----~----~ 0 3 6 9 12

Angular velocity w 112 (sec)1f2 15

Figure 5.8 Plot of iL.Jconc. vs square root of angular velocity for anodic

voltammograms of Figure 5.6 at various concentrations.

195

5.1.1.5 Kinetics Parameters

For an electrode reaction which is activation controlled, the exchanging current

density i0 and transfer coefficient a can be determined by the well-known Tafel

equation. For an electrode process affected by mass transport, the following

equation applies [ 179]:

T}=- RT ln .io - RT ln iL,a-i ( 1-a;) nF ~ L, a ( 1-a;) nF i

(5-2)

where 11 is anodic overpotential; R, the normal gas constant; T, the temperature; a,

the transfer coefficient; n, the number of electrons participating in the reaction; F,

the Faraday constant; i0, the exchange current density; iL,a• the anodic limiting

current density and i, the measured anodic current density. Equation (2) indicates

that the anodic overpotential should be linearly related to ln(iLa-i)1i, and i0 and a

can be determined from the intercept and the slope of the straight line.

Taking the anodic current within the range 0.1 iL,a< i <0.9 iL.a• which is normally

considered the combined mass transport and activation region, from the anodic

voltammograms shown in Figure 5.4(a) and 5.4(b), the anodic overpotential was

plotted against log(iL,a - i)/i at each rotation speed and the results are shown in

Figure 5.9. From the intercepts and the slopes of the straight lines, i0 and a

values were calculated and are summarised in Table 5.3, the average i0 and a

196

1.0

-> 0.8 d

4-c 0.6 OJ

4-0 Cl. C-

g;: 0.4 0 u

"'0 0 0.2 c

<(

0 -3

rpm * 600 + 7200

0 7800 0 2400

-2 -1 0 log(iL_a-i)/i

rpm 6 720

0240

1 2

Figure 5.9 Plot of anodic overpotential vs log(iL a - i)/i from voltammograms of

Figure 5.4

197

Table 5.3, The Kinetic Parameters of V(V)N(IV) Couple

electrode rotation transfer coefficient, exchange current

speed, rpm a density ,io(A.cm "2)

60 0.73 1.18 X 10-4

120 0.70 2.09 x 10-4

240 0.70 2.67 x 10-4

600 0.72 2.71 X 10-4

1200 0.71 2.84 x 10-4

1800 0.72 4.18 x 10-4

2400 0.71 1.61 x 10-4

values being 2.47 x 10-4 A.cm-2 and 0.71 respectively. The low value of the

exchange current density shows that the electron transfer rate of V(V)N(IV)

couple at the graphite electrode is slow, while the a value indicates the asymmetry

in the activation energy peak for this electrochemical system. The low value of i0

could also be associated with a low effective surface area for the electrode

reactions at the graphite electrode. In the practical cells and batteries, these

problems are overcome by using a high surface area graphite felt[44] to replace

the flat graphite plate.

Figure 5.10 shows the cathodic and anodic voltammograms at graphite felt and

reticulated vitreous carbon electrodes in the 1.0 M V(IV) + 1.0 M V(V) (50%

198

0.6

0.4 > E -0.2 d

o PEIF!vfl + PE!RIC

~ 0~~~-------------------------------QJ

""5 0..

~ 0.2 >

0

0.4

0.6 0 40 80 120 160 Current density (mA/c.m 2

)

200

Figure 5.10 Cathodic and anodic polarisation curves of graphite felt and

reticulated vitreous carbon (RVC) electrodes in 1.0 M V(V) + 1.0 M V(IV) in 3 M H2S04•

199

SOC) vanadium sulphate solution obtained under steady-state galvanostatic condi

tions. The I-V curves were recorded manually. A linear relationship is observed

between electrode overpotential and current density on the high surface area felt

and RVC electrodes. Plotting the overpotential versus current density (based on

geometric surface area) in the range of 50 mv and using the Linear Polarisation

equation[179], the i0 values for these two electrodes were determined as 0.92

X w-2 A.cm-2 for graphite felt and 0.80 X w-2 A.cm-2 for RVC electrode

respectively. These values are significantly higher than those obtained at the

graphite disc electrode and are due to the higher surface area of these electrodes.

In spite of the high surface area, however, the kinetics of the V(V)N(IV)

reactions are limited by the number of active sites on the carbon and graphite

surfaces. Further improvements in the effective surface area have been achieved

with various surface activation treatment methods which have been successfully

used to increase the number of active sites for the V(V)N(IV) redox couple

reaction on graphite felt electrodes so that energy efficiencies of up to 90% have

been achieved with these electrodes in the vanadium redox cel1[28].

5.1.2 The Electrochemical Behaviour of V(ID)/V(ll) Couple

As mentioned previously, the electrochemical behaviour of the V(III)N(II) couple

was reported by NASA group, and it was concluded that the redox reaction is

electrochemically irreversible at a carbon electrode[ 4]. Sum and Skyllas

Kazacos[ 42] studied the electrode kinetics of this redox couple at gold and glassy

200

carbon electrode and found out that at the gold electrode, the low hydrogen

overpotential masks the V(Ill) reduction peak so that it can not be observed. At

the glassy carbon electrode, the kinetics were found to be greatly influenced by

the electrode surface preparation procedure. The electrode kinetics were thus

established at a mechanically polished glassy carbon electrode surface which gave

the best reproducibility for this substrate.

The suitability of graphite for use as an electrode material to study the electrode

kinetics of vanadium species has already been discussed previously. In the present

study, the electrochemical behaviour of the V(lll)N(II) couple at the graphite

electrode was investigated using cyclic voltammetry. A 0.1 M V3+ in 3 M H2S04

solution was used as electrolyte. This solution was prepared by mixing equal

volume of 0.1 M V(IV)/3 M H2S04 solution with 0.1 M V2+/3 M H2S04 solution

(the latter was obtained from a fully charged negative electrolyte).

5.1.2.1 The Effect of Electrochemical Oxidation on Electrode Behaviour

Figure 5.11 shows the cyclic voltammograms of the V(Ill)N(II) couple at

different conditions. Curve 1 is the initial sweep obtained for a graphite electrode

surface prepared by the "standard" polishing procedure (section 3.2.1.1). The

V(Ill) reduction peak B is observed, but is close to the hydrogen evolution curve,

indicating a relatively high overpotential for V(Ill) reduction at this surface. After

scanning the electrode to a positive potential of + 1.10 V and holding at this

potential for 3 minutes, a cyclic voltammogram with a lower peak potential

201

0.4

A

0.2

<{

E 0 ........ c Q) ~ ~

:::::J 0 -0.2

-0.4

-1.45 -1.05 -.65 -.25 0 .15 Potential, V (vs Hg/Hg2 804 )

Figure 5.11 Cyclic voltammograms of graphite electrode in 0.1 M V3+/3 M

H2S04 solution, scan rate: 40 mV s·' ,. 1, normally polished; 2, 3

min anodicly oxidised (dash line for 6 and 9 min anodic oxidation)

202

separation value, a more clear V(Ill) reduction peak (peak B ') and a higher peak

current was obtained (Curve 2), suggesting that the electrochemical oxidation of

the graphite surface at the positive potential has led to an activation of the surface

and an enhancement of the kinetics of the V (III)N (II) redox coupe. Further

treatments, (for example, 6 and 9 minutes oxidation at 1.0 V), did not lead to

further improvement, indicating that no further increase in the number of active

sites was created by this treatment (dash line refers to 6 and 9 minutes oxidation).

5.1.2.2 Determination of Kinetics Parameters

It is generally agreed that the V(Ill)N(II) redox couple is electrochemically

irreversible[ 4,42]. For such an electrochemical system at 25 °C, the peak current,

iP' is given by[ 179]:

(5-3)

where 4 is in amperes; A, the electrode area (cm2); C0°, the bulk concentration of

oxidant (mol.cm-3); v, the potential sweep rate (V.s-1

); D0

, the diffusion coefficient

of the oxidant(cm2.s-1), a., the electron transfer coefficient and na, the number of

electrons involved in the rate-determining step. A plot of 4 vs. v112 should

therefore give a straight line with slope proportional to D0

•

Furthermore, for a totally irreversible process, the peak potential EP' and the

203

difference between ~ and the formal potential, E0', is a function of scan rate, and

is related to the heterogeneous rate constant, k0• The peak current of an

irreversible process may also be expressed as equation 2-6 (see Section 2.6.1.1).

According to the equation, a plot of 1n(~) vs. (EP - E0') determined at different

scan rates, should thus have a slope of -(<Xn8F/RT) and an intercept proportional to

Based on this electrochemical theory, a series of cyclic voltammograms at

different potential scan rates was obtained at the anodically oxidised graphite

electrode and is illustrated in Figure 5.12. Plotting the cathodic peak current vs.

the square root of scan rate, yields a straight line given in Figure 5.13. Assuming

a value of ana equal to 0.5, the value of diffusion coefficient calculated from the

slope of the straight line was 1.25 x 10·6 cm2.s·\ which is very close to that

obtained from the glassy carbon electrode[42].

The formal potential of the electrode, E0', was estimated from the cyclic

voltammograms by taking the mean of the average of the anodic and cathodic

peak potentials, Epa and Epc, i.e.,

m

L (Epai + Epci)/2 i=l

E0 '(estimate) = ------------- (5-4) m

where m is the total number of scans.

204

<!: E ~

0 c 0,) 1... 1... ::::l ()

-0.2

-0.4

-0.6

Potential,V (vs Hg/Hg2S04 )

Figure 5.12 Cyclic voltammograms of 9 mins anodicly oxidised graphite

electrode in 0.1 M V3+/3 M H2S04 • Numbers on curves

corresponding to scan rates in m V s~1 ~.

205

1.0~--------------------------------------~

,.......

~ 0.8 0/ '-'

0 Q_

0/ . 0/

-iJ 0.5 c

Q) L L :J 0

0 -a 0.4 Q) 0. 0 0 0/

l:J 0

0.2 / ..c -iJ a 0

0.0 0.0 0.1 0.2 0.3 0.4

square root of scan rate. v 1/2 c v,n)

Figure 5.13 Peak current vs. square root of scan rate for voltammograms in Fig. 5.12

206

The ratio of the peak: current V4c was calculated from the expression:

(5-5)

where (~)0 is the uncorrected anodic peak current with respect to the zero current

baseline and (isp)o is the current at the switching potential[l79].

Table 5.4 shows the values of V4c and E0' calculated from the cyclic

voltammograms of Figure 5.12. The mean value of 4J4c is 1.07, closed to the

unity expected. However, deviations are observed, suggesting that there be some

kinetic complications for the irreversible system[179].

Table 5.4 Experimental and Calculated Data for V(III) + e ,... V(II) Reaction

v (isp)o C~o ~c Epa I;,., ~ ~J~c EO'*

(V/S) (10'3A) oo-3A) (10'3A) (V) (V) (V) (V)

0.01 0.21 0.10 0.25 -0.89 -1.15 0.26 0.893 -1.02

0.02 0.28 0.16 0.33 -0.86 -1.18 0.32 0.982 -1.02

0.04 0.39 0.27 0.44 -0.84 -1.21 0.37 1.130 -1.03

0.06 0.49 0.34 0.56 -0.81 -1.23 0.42 1.117 -1.02

0.08 0.58 0.40 0.65 -0.79 -1.25 0.46 1.134 -1.02

0.10 0.66 0.46 0.73 -0.77 -1.27 0.50 1.154 -1.02

*Estimated

207

The average value of -1.02 V for the formal potential of the electrode process

shown in Table 5.4, was used to plot In(~) versus (Ep -E0') for the relationship

given by equation 2.10. A straight line was obtained from the cyclic

voltammograms of Figure 5.12 and is shown in Figure 5.14, indicating that the

electrode process obeys this equation. From the slope of the straight line, the value

of a was found to be 0.24, indicating asymmetry anodic and cathodic kinetics.

Using this value, the diffusion coefficient of V(III) was determined as 2.60 x 10-6

cm2.s-1• From the intercept, the heterogeneous rate constant, k0

, was calculated as

3.63 x 104 em/sec, one order of magnitude larger than that at glassy carbon

electrode[42], showing that a faster electron transfer rate can be obtained by

varying the electrode material.

5.2 Chemical Activation of the Carbon-Plastic Composite Substrates

As described in Chapter 4, the carbon-plastic composite electrode consists of a

piece of graphite felt as the surface layer to offer a high surface area for redox

reactions. High hydraulic pressures and high costs are also introduced by this felt

layer however. On the other hand, as shown in the first section of this chapter, a

flat graphite surface will not allow a high current to flow through the electrode so

as to obtain a desirable power output for the redox flow cell system. An electrode

surface having a high surface area and a high concentration of active sites with

low space occupation is therefore desirable. The present investigation examined

the possibility of obtaining such a surface from the prepared carbon-plastic

composite materials. Carbon-LDPE composite was selected for the present study.

208

0 a_

'-"

-Q.O I

I -8.5 f

I

-8.0

~ 0~

0

~ 0~

c -7.5 r

-7.0 ~

0~ 0" I

I I

J -6.5L_ ____ ~L_ ____ ~L_ ____ _J ______ ~------~--

-0.10 -0.13 -0. 16 -0. 1 g -0.22 -0.25 -0.28

CEp- E 0')

Figure 5.14 Ln(4c) vs. (Epc -E0') for voltammograms in Fig. 5.12.

209

The details of preparation and the following experimental procedure were

described in Section 3.2.2.

The surface of the conducting composite polyethylene, although electrically

conductive, is however covered by the bonding plastic which reduces the surface

hydrophilicity and reaction area. The purpose of chemical treatment is to increase

the surface hydrophilicity, the reactive surface area and, if possible, to increase the

active sites or functional groups which can catalyse the electrochemical reactions.

Polyethylene is very stable to most chemicals and solvents and only strongly

oxidative and acidic solutions can attack its surface. The chromate-sulphuric acid

system was thus employed for the surface treatment.

5.2.1 Influence of Treatment Time

The prepared electrodes were polished with sand paper in the order of grit 120,

320, 400 (for the "e" and graphite electrodes only), cleaned with acetone and

water and then etched in the hot (65 °C) chemical treatment solution for various

time periods. A description of the electrode configuration ("e" and "s") was given

in Figure 3.11 of Section 3.2.2.2. The cyclic voltammograms for the two kinds of

composite polyethylene electrodes and for the graphite electrodes are shown in

Figures 5.15 to 5.17 respectively. Further details are given in Tables 5.5 to 5.8

and Figures 5.18 and 5.19.

In the Tables, ~1 and ~2 refer, respectively, to the peak current of the two

210

3 15

10

5 <! E

1 +- 0 c QJ

'-- '

'--c -5

-10

-15 1 I L_J_ ____ L_ __ _L ____ ~--~~--~

-1.6 -0.8 0 0.8 1.6 2.4 Potentiai/V vs SCE

Figure 5.15 Cyclic voltammograms of PE2525 composite electrode in 1 M

V(III) + 1 M V(IV)/3 M H2S04 vanadium solution, 1) untreated; 2)

5 mins treated; 3) graphite (scan rate = 100 mV s:'1 ).

211

15

10

5

<{

E

+- 0 c QJ

'-'-::::,

LJ -5

-10

-15

-1.6 -0.8 0 0.8 1.6 2.4 Potentiai/Vvs SCE

Figure 5.16 Cyclic voltammograms of PE75 composite electrode in vanadium

solution (same as mentioned in Figure 5.15), 1) untreated; 2) 5 mins

treated; 3) graphite(scan rate= 100 mV {' ).

212

electrode reactions: V(IV) - e --7 V(V) (peak A in Figure 4.8), and V(III) + e --7

V(II) (peak C in Figure 4.8), while AEP1' AEP2 refer to the peak potential

separations of the two redox couples, respectively.

The results in Figures 5.15 and 5.16 and Tables 5.5 to 5.8, show that both compo

site electrodes exhibit an increase in peak current with increasing treatment time.

This is probably due to an increase in the surface hydrophilicity as well as to

increased exposure of the conductive material, which was initially covered by

bonding polyethylene. Both effects would enhance the vanadium ion reactions.

Comparing the data in the Figures 5.15 to 5.19 and Tables 5.5 to 5.8, it is clear

that the effect of the short time treatment on the peak current is more pronounced

in the case of PE 2525 than for the PE 75 electrodes. This is probably due to the

fact that carbon black is more conductive than graphite fibre. With longer

treatment times, however, the effect on peak current is greater with the PE 75

electrodes (cf. Figures 5.18 and 5.19). An explanation for this reversal in

behaviour may lie in the fact that more of the graphite fibres are exposed so that

the surface area of the PE75 electrode becomes greater than that of the PE2525

composite. For the same composite, however, the "e" electrodes are seen to

produce a higher reacting current. This can be attributed to the graphite fibres

having a higher electrochemical activity in the cross-sectional direction compared

with their surface.

213

20 2

15 1

10

-<I: 5 E

-+-c 0 OJ ~

~

:J w

-5

-10

-15

-1.6 2.4

Potent iai/V vsSCE

Figure 5.17 Cyclic voltammograms of graphite electrode in vanadium solution

(same as mentioned in Fig.5.15), 1) untreated; 2) 5 mins

treated(scan rate = 100 mV s;-' ).

214

Table 5.5 The influence of treatment time on the electrode behaviour

of PE2525 ("e" electrode) in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04

Time (min) ~1 (rnA) ~2 (rnA) ~1 (V) Lllipz (V)

0 6.40

5 17.00 12.60 1.00 0.85

30 19.00 17.00 0.90 0.64

60 21.00 16.50 0.60 0.52


of PE2525 ("s" electrode) in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04

Time (min) ~1 (rnA) ~2 (rnA) ~1 (V) Lllipz (V)

0 4.00

5 14.60 12.40 0.60 0.52

30 16.40 13.00 1.32 0.52

60 18.40 14.60 0.84 0.52

215

<{

E

..:r::: d QJ

(]_

o PE2525{"e ") Ip1 o PE2525 (" e ")lp2

0 PE2525 {II s ")Ip1 x PE2525{ "s ")lp2

Time (min)

Figure 5.18 The effect of treatment time on peak current for PE2525 composite

electrode.

216


of PE75 ("s" electrode) in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04

Time (min) ~1 (rnA) ~2 (rnA) ~1 (V) LlliP2 (V)

0 0.60

5 7.00 7.00 0.52 0.72

30 9.60 9.40 0.68 0.68

60 12.40 12.40 0.68 0.68

120 16.00 15.60 0.68 0.68

240 22.00 22.00 0.88 0.88

480 32.00 30.00 1.20 1.08


of PE75 ("e" electrode) in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04

Time (min) ~1 (rnA) ~2 (rnA) ~1 (V) LlliP2 (V)

0 1.60

5 10.80 10.80 0.48 0.80

30 17.00 17.00 0.60 0.68

60 22.00 23.80 0.60 0.64

120 30.00 29.20 0.60 0.68

240 38.00 37.00 0.72 0.84

480 49.60 48.80 1.00 1.00

217

<( E

..._ c Q) L-

'-:::J u

_:.:::: d Q)

o_

40

30

10

0

0 1

o PE75 ("s")Ip1 o PE75 (''s")l p2

2 3 4 Time {h)

<> PE75 ("e" }I p 1 x PE75 ("e")lp 2

Figure 5.19 The effect of treatment time on peak current for PE75 composite

electrode.

218

Table 5.9 The influence of treatment time on the electrode behaviour of graphite

in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04

time (min) ~1 (rnA) ~2 (rnA) dEpt (V) dEP2 (V)

0 15.00 13.00 0.32 0.40

5 22.00 17.00 0.40 0.40

30 22.00 18.00 0.64 0.48

60 26.00 20.80 0.64 0.48

90 24.00 19.20 0.64 0.48

In the case of the graphite electrode (see Figure 5.17 and Table 5.9), a 5 minute

etch caused the peak current to increase but longer treatment yielded no further

improvement. Since the graphite electrode has a constant surface area, only the

first etch would be expected to increase the surface hydrophilicity.

The general trends in peak current versus treatment time for the two kinds of

composite plastic electrodes are summarised in Figure 5.18 and 5.19.

5.2.2 The Effect of Treatment Temperature

In order to optimise the treatment conditions, the effect of temperature on the

electrochemical behaviour of the carbon-plastic electrode was also studied. The

temperature of the treatment solution was controlled by a thermo-bath. The

prepared electrode was treated for a 30 minute time period at various

219

temperatures, and then cycled in the vanadium solution. Table 5.10 and 5.11

summarise the results, while Figure 5.20 illustrates the trend of the peak current

versus treatment temperature.

Table 5.10 The Effect of Treatment Temperature on the Electrode Behaviour of

PE2525("e") Electrode in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04

temp.(lC) 4l(mA) iPimA) Lillpl(V) .!illPz(V)

30 16.4 14.0 0.60 0.80

50 20.8 16.4 0.60 0.60

65 19 17.0 0.90 0.64

80 13.2 11.2 0.80 0.72

Table 5.11 The Effect of Treatment Temperature on the Electrode Behaviour of

PE75("e") Electrode in [1 M V(IV) + 1 M V(V)]/2.5 M H2S04

temp.(°C) 4l(mA) iPz{mA) ~~(V) Lillp2(V)

30 12.0 12.0 0.40 0.72

50 15.2 14.8 0.40 0.60

65 22.0 18.0 0.64 0.48

80 15.2 14.8 0.72 0.48

220

30.0.------------------------------------------------,

l o PE2525C"e 8 )Ip1

25 _0 ~ PE2525C"e•)Ip2

20.0

r- 15.0 z UJ 0::: 0::: :J u 10.0 ~ a: w Q_

5.0

0

o PE75C "e"') Ip1 j

* PE75C "e") Ip21 I

I

0.0~----~------~----~------~------~----~----~

20 30 40 50 60 70 80 90 TEMP C°C)

Figure 5.20 The effect of treatment temperature on peak current for PE75 and

PE2525 composite electrode.

221

It can be seen from Tables 5.10, 5.11 and Figure 5.20 that the treatment

temperature is important for achieving an active electrode surface. The optimum

treatment temperature is also dependent on the composition of the composite. For

PE2525, the most suitable treatment temperature range is around 50 °C, while for

PE75, around 65 °C was suggested. Higher treatment temperatures result in a

decrease in electrochemical activity in both composite electrodes. This may

probably be due to the plastic being soften at higher temperature, causing a

decrease of surface hydrophilicity, and because of its higher plastic proportion, the

PE2525 composite shows more sensitivity to this temperature effect.

5.2.3 Cyclic Stability

The PE2525 and PE75 electrodes were continuously cycled between their positive

and negative potentiallimits(+2.0V and -1.2V respectively) to study their stability.

The composite with 25% graphite fibre and 25% carbon black is not

electrochemically stable, since the peak current decreases with cycle number

(Figure 5.21). This is probably due to the carbon black particles being removed

from the surface, leading to a reduction in the reactive surface area. The

composite with 75% graphite fibre (and the graphite plate electrode) exhibited

very good stability during cycling, however (Figure 5.22), and would therefore be

more suitable for use as electrode materials.

222

15

10

5 <t E

0 -+-c Q)'

c... c... ::J -5 u

-10

-15

-1.6 -0.8 0 0.8 1.6 2.4

Potentiai/V vs SCE

Figure 5.21 The cyclic stability test of treated PE 2525 composite electrode (30

mins, 65°C), scan rate = 100 mV s-1 ,.

223

15

10

5 <l: E

0 -4--

c CJ' '--'--:::J -5 u

-10

-15

-1.6 -0.8 0 0.8 1.6 2.4

Potentia!IV vs SCE

Figure 5.21 The cyclic stability test of treated PE 2525 composite electrode (30

mins, 65°C), scan rate = 100 mV s:"'·.

223

40r-----------------~----~

30

20

<! 10 E -

+-c 0 a.J t..... t..... :J'

u

-10

-20

-30

P o t en t i o l/ V vs S C E

Figure 5.22 The cyclic stability test of treatment composite electrode: 1) 1 hr; 2)

2 hrs; 3) 4 hrs; all treated at 65°C(scan rate = 100 mV s~~ ).

224

5.2.4 Mechanism of Chemical treatment

The treatment solution employed in this study is a highly oxidisingacidic solution.

Its function could be to simply increase surface area and hydrophilicity or

alternatively create functional groups or active sites for the vanadium reactions. To

find out which is the more important effect of the treatment, a graphite felt

electrode(bonded onto a 70% PE, 15% CB, 15% GF conducting polyethylene

sheet) was subjected to the treatment process and then tested by cyclic

voltammetry. Because the felt electrode has a high and constant surface area, if

the treatment creates functional groups or active sites, the peak current should

increase significantly. The results in Figure 5.23, however, show that there is little

difference in the peak current before and after treatment. We can thus conclude

that the present treatment does not increase the number of active sites on the

electrode surface, but rather increases the surface area and surface hydrophilicity

of the composite polyethylene electrodes. Further evidence is given by the SEM

photographs in Figures 5.24, which show that after treatment, more of the graphite

fibres are exposed on the surface, only a small part of surface still being covered

by the polyethylene. This causes the peak current to increase significantly after

treatment.

5.3 Summary

The kinetics of the V(V)N(IV) redox couple reaction have been found to be elec

trochemically irreversible at the flat graphite electrode. Rotating disc voltammetry

225

300~-----------------------------,

<I: E

-4-c aJ '-'-:::J

200

100

0

L.J -100

-1.6 -0.8 0 0.8 1.6 2.4 3.2 PotentiGl/V vs SCE

Figure 5.23 Cyclic voltammograms of PE composite bonded with graphite felt:

1) untreated; 2) 5 mins at 65°C; 3) 40 mins at 65°C(scan rate = 100 mV s~' ~).

226

(a)

(b)

Figure 5.24 SEM pictures of treated (4 hrs) PE75 composite material, a) surface

direction; b) cross-section.

227

studies reveal that the diffusivity of V(IV) species is independent of vanadium

concentration and that the value of the diffusion coefficient is 2.14 x 10-6 cm2.s-1•

Although the exchange current density at a flat graphite electrode is low, it

increased by two orders of magnitude at the graphite felt electrode showing that

this material is suitable for use in the vanadium redox cell.

The electrochemical behaviour of the V(IIT)N(II) couple at the graphite electrode

was characterised by a diffusion coefficient of 2.60 x 10-6 cm2.s-1 and an electron

transfer rate constant of 3.63 x 104 cm.s-1, showing slow electrode kinetics.

Chemical treatment of the graphite fibre-based composite polyethylene substrate

can also result in a surface area enhancement and improved reactivity for the

vanadium ion redox reactions. Whilst carbon-black composite polyethylene is not

an electrochemically stable material after treatment, graphite fibre-based

conducting polyethylene is. Although significant improvements in the effective

surface area have been achieved with surface treatment of these materials, further

research on the activation of the exposed graphite fibres is required to achieve

better electrochemical activity for the vanadium reactions. Direct activation of the

conducting plastic surface will then eliminate the need for the graphite felt and

dramatically reduce the cost of the vanadium redox battery electrodes, while main

taining a high efficiency.

228

CHAPTER VI

GRAPHITE FELT AS ELECTRODE ACTIVE LAYER

As mentioned previously, the composite electrode has a matrix layer and active

layer, each performing different functions. Since the electrochemical reactions of

the redox couples in the battery take place at the active layer, particularly at the

interface between the electrolyte and the surface of the active layer material, its

physical, chemical and electrochemical properties are important factors in battery

application. Normally, the active layer of an electrode should have:

a) high electrical conductivity (to minimise the IR losses)

b) high surface area but low space occupation (to enhance power density of

battery)

c) high electrochemical activity (to minimise the polarisation overpotential'll)

d) good stability during operation

e) low cost

To meet all the requirements, the most suitable materials are carbon or graphite

felts. The results in Chapter 4 showed that the commercial Toray graphite felt is

a good electrode active layer of the composite electrodes for the vanadium redox

flow battery. Unfortunately, however, it has a high space occupation (6 mm in

thickness), and in relation to its surface area, it has a low concentration of active

sites on the surface and its cost is also high. To increase the power density of a

battery, it is necessary to increase the number of the cells per unit volume. On the

other hand, totally eliminating the felt layer, would of course reduce the space

229

occupation of the electrode, but would also lead to difficulties in obtaining a high

surface area to allow high enough current to pass through. As shown in Chapter 5,

chemical etching leads to an increased surface area and therefore peak current for

the non-felt composite electrode, however, the high proportion of graphite fibre in

the composite material results in a difficulty of processing and a high cost. The

surface treatment step would also limit the large scale application of the electrode

because of the high acidity, oxidising nature and toxicity of the solution.

Possessing a high surface area but less space occupation and less complication for

processing, thinner graphite felts were therefore considered as the alternative

surface active layer for the carbon-plastic composite electrode. With the aim of

enhancing the electrochemical activity while reducing the space occupation, the

present study included the screening of felt samples, the oxidation sensitivity of

selected felt samples and the application of catalysts onto the felt to increase the

electroactivity of the electrode for the vanadium redox reaction. In the first part of

this chapter, the electrical, structural and electrochemical properties of a few

commercially available carbon and graphite felts are evaluated. Using scanning

electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), the

second part of this chapter compares the oxidation sensitivity of two typical

graphite felt samples treated under various oxidative conditions. Focused on

enhancing the electrochemical activity of the graphite felt, the third part of the

Chapter concentrates on the felt activation and electrocatalysis study. Cell

efficiencies and the stability of the catalysed felt electrode were also evaluated.

230

6.1 Comparison of Characteristics of Some Graphite Felts

It is well-known that depending on the precursors and the processing temperature

and techniques, the properties of graphite felt vary considerably. As the electrode

active layer material, the most important characteristics concerned are electrical

conductivity, the surface area and pore size distribution property and the

electrochemical activity.

6.1.1 Electrical Conductivity of Graphite Felt

One of the most important characteristics of an electrode material is electrical

conductivity, since a highly conductive felt can minimise the IR losses during

battery operation. The resistivity of the four kinds of commercially available thin

felts were thus tested with ASTM D-991 method. For comparison, the Toray 6

mm graphite felt was also tested, and the results are listed in Table 6.1.

Apart from the GFD 2, the resistivity values of the other 3 graphite felts are very

close, but much higher than GFD 2 and Toray felt. This might be due to the

different stage of graphitisation or in other words, the different surface

microstructure of the fibres resulting from the processing procedure and precursors

used in manufacturing the various felt samples. This would lead to a different

capability of interaction with oxygen which in turn would affect the electrical

properties of the felt. As evidence, Golden et al pointed out that the electrical

resistance of carbon increases when oxygen chemisorption on its surface takes

231

place[l01]. As reported by the manufacturers, the precursor for GFD 2 felt is

polyacrylonitrile (PAN), while FMI and RVG 1000 felts are based on rayon. The

significant difference in electrical resistivity between these two types of felt

samples are thus probably associated with the different precursors used in their

manufacture.

Table 6.1 Comparison of electrical resistivity of commercially

available graphite felts

Felt Precursor !l.V/I Thickness Resistivity

(ohm) (mm) (ohm.cm)

FMI rayon 0.192 3 0.023

Sigri GFA 2 - 0.220 3 0.026

Sigri GFD 2 PAN 0.038 2.5 0.0038

Le Carbonne rayon 0.341 1.5 0.020

RVG 1000

Toray - 0.038 6 0.0091

6.1.2. Surface Area and Pore Size of Graphite Felt Samples

Since the graphite felt would be employed as the electrode active layer for the

vanadium redox flow battery, and the electrolyte will flow through the felt during

operation, the porosity and structure of the felt will affect the flow rate of the

electrolyte and therefore influence the polarisation behaviour of the redox flow

battery as well as pumping energy loss[32]. On the other hand, the active surface

232

area of the electrode material will affect the electrochemical behaviour of the

electrode, including the current distribution and electrode overpotential[34]. It is

therefore important to know the structure and the surface area of the felt samples.

Using the mercury intrusion method, Figure 6.1 shows typical surface area

analysis curves which illustrate the structural characteristic of GFD 2 graphite

felt. The cumulative curve in Figure 6.1(a) levels off when the pore diameter is

smaller than 10 micrometer, while the incremental curve shows a dramatic pore

volume increase in the pore diameter range from 10 to 100 micrometer. This

indicates that the pore volume of the felt is attributed to the macro pores, or more

precisely, to the space between all the fibres of the felt. This type of pore is

commonly defined as the "main pore"[32]. Figure 6.1(b) shows the cumulative

surface area as a function of pore diameter. It can be seen that the high surface

area of the felt is mainly attributed to the micro pores which are around 0.01 to

0.001 micromete in diameter.

Each of the felts, including the Toray felt has similar structural properties. The

total measured specific surface area and pore volume values are listed in Table

6.2.

Table 6.2 shows the relation between the main pore volume and specific surface

area of each of the five felt samples. It can be seen that the main pore diameter

for all the felt samples ranges from 101 to 1(j J.lffi, and the main pore volume is

more than 90% of the total measured pore volume, indicating that the structure of

felt

233

0) ..........

E 16 o Incremental o Cumulative ...

(])

{a) § 12 0 > c 8 0 (j) :::s "'"" 4 ......., c ·- %

(])

0 "'"" 0 a.. -3 -2 -1 0 1 2 3

Log pore diameter , J.lffi

234

200 (b)

-2 -1 0 1 2 3 Log pore diameter, J.Jm

Figure 6.1 Pore and surface area distribution of GFD 2 graphite felt, (a) pore

intrusion volume; (b) cumulative surface area

235

Table 6.2 Smface Area Comparison of commercial available Graphite Felts

Felt Thick- Main pore (Main pore Specific Specific surface

ness diameter volume)/ (Total surface area area for total

(mm) (p) measured pore for main pores measured pores

volume),% (mz/g) (mz/g)

FMI 3 188.5- 93.2 0.329 238.27

36.5

GFA2 3 187.4- 93.3 0.767 374.44

20.3

GFD2 2.5 188.5- 92.8 0.390 266.86

13.0

RVG 1.5 181.5- 91.5 0.485 388.38

1000 15.1

Toray 6 188.5- 91.2 0.575 508.264

13.0

samples is rather similar. However, differences in the smface area values can be

observed, showing that a different microstructure for the various felt samples. A

significant difference in specific smface area between main pores and total

measured pores for each felt sample is also noted, illustrating the unique structural

characteristics of the graphite felts, i.e., the co-existence of macro-pores (space

between fibres of the felt) and micro-pores. This phenomena was also reported by

Ashimura et al[32].

The value of main pore volume and specific smface area are of most concern in

battery applications. This is based on the following consideration: During battery

operation, the bulk electrolyte can not flow into the tiny pores of the felt so that

236

the electroactive species will become depleted within the pores leading to

concentration polarisation. Since these tiny pores will not contribute to the

effective electrode area therefore, the surface area fraction from these tiny pores

should be deducted from the total surface area for electrochemical reaction

considerations. A similar treatment to the measured data can be seen in ref.[32].

6.1.3 Electrochemical Activity of Commercial Graphite Felts

The electrochemical activity of some graphite felts was tested with cyclic

voltammetry. The electrode potential scan rate was 60 mV/sec and the testing was

carried out in a solution of 0.05 M V(III) + 0.05 M V(IV) sulphate in 3 M H2S04•

The preparation and the construction of the felt electrodes were described in

Section 3.3.3, while the electrochemical cell used for cyclic voltammetry was

illustrated in Figure 3.10. Figures 6.2 (a), (b), (c) and (d) show the cyclic

voltammograms of the felt composite electrodes. For comparison, a cyclic

voltammogram obtained in the same solution with the graphite rod electrode

(illustrated in Figure 3.9) is given in Figure 6.3. In Figure 6.3, peaks A and B

correspond to the V (II) ,.... V (III) redox reactions while peaks C and D to

V(IV) ,.... V(V). Region E is related to oxygen evolution, and F to hydrogen.

Comparing Figure 6.2 and 6.3, it can be seen that in contrast to the graphite rod

electrode, for each of the felt electrodes, the V (III) ----? V (II) reduction reaction is

masked by hydrogen evolution (region F), indicating that the overpotential for the

V(III) reduction reaction is greater at the felt electrodes compared with the

graphite rod.

237

80 c

<! 40 E ..

of-)

c: 0 (1,)

'-'-:::;,

0 -40

-80

-100

-2.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 Potentiaf,V vsHg/Hg2S04

(a)

238

150

100

<( 50 E .. ...., ~ 0 '':::7

0 -50

-100

-150 \.-.-:-.L----L------L-----...I--L-----L.----1.-~-------2.0 ·1.5 -1.0-0.5 0 0.5 1.0 1.5 2.0

Potentiai,V vs Hg/Hg2S04

(b)

239

100

<t 50 E ..

-100

-1.0-0.5 0 0.5 1.0 1.5 Potent i a I , V v s H g I H g2S 0 4

(c)

240

<{

E .. ~ c: Cl,)

'-'-::l ()

120~----------------------~

c

40

0

-40

-80

F -120 -~~--....__---L---L---1.--_j_......L

-2.0 -1.5 -lO -0.5 0 0.5 1.0 1.5 Potentia I, V v s Hg/Hg2S04

(d)

Figure 6.2 Cyclic voltammograms of various graphite felt electrodes in 0.05 M

V(III) + 0.05 M V(IV) in 3 M H2S04 solution, sweep rate: 60

mY s:'/; (a) FMI; (b) GFA 2: (c) GFD 2 and (d) RVG 1000.

241

To compare the electrochemical activity of the felt electrodes, the peak current

and peak potential separation values are employed. For the peak current

comparison, peaks A and C were selected, while the V (IV)N (V) redox couple

was selected for peak potential separation evaluation. Results obtained from the

cyclic voltammograms in Figure 6.2 are listed in Table 6.3. It can be seen from

Table 6.3 that the GFA 2 felt electrode has the highest peak current values for

both peak A and C, corresponding to the highest surface area value, but also has

the highest peak potential separation, indicating a high electrode overpotential for

the V(IV)N(V) redox reactions or a high electrode resistance. Among the

remaining three, the GFD 2 and RVG 1000 felt composite electrodes have a

higher peak current value and lower peak potential separation, which means these

two kinds of felt have better electrochemical activity. Considering that GFD 2 felt

has a better electrical conductivity, it was selected as the electrode active layer for

further studies.

Table 6.3 Electrochemical Activity Comparison of Some Felt Electrodes

Felt Felt weight Real sur- ~1 ~2 Lffip

electrodes face area V(ID-? V(IV)--7 V(IV)/

V(III) V(V) V(V)

(g) (cm2) (rnA) (rnA) (V)

FMI 0.0082 26.98 32 80 1.34

GFA2 0.0071 54.46 75 165 1.95

GFD2 0.0092 35.88 45 115 1.14

RVG1000 0.0052 25.22 52 105 1.14

242

<( E ..

.f-)

c::: Q) c... c...

1.2

0.8

0.4

0-0.4

-O.B

-1.2

A

0

-1.5 -0.5 0 0.5 1.0

Figure 6.3 Cyclic voltammogram of graphite rod electrode in vanadium

solution (same as that in Figure 6.2); sweep rate: 60 mV s-' ,,

243

6.2 Surface Microstructure and Oxidation Characteristics of FMI and GFD

Graphite Felts

Possessing inertness to most of chemicals and solvents, and having reasonable

cost, carbon or graphite felts are widely used as electrode active layers on carbon

plastic composite electrodes for redox flow cell applications[35-41]. There are

several hundred grades of carbon/graphite felts available from different sources.

Researchers have found that the variation in electrochemical activity from felt to

felt is considerable[39-41]. From the experimental results described previously, it

can be seen that the electrical and the electrochemical properties of graphite felts

employed for the applications in the vanadium redox flow cell are, indeed, quite

different. It was also observed in our laboratory that some felts can be easily

activated for the vanadium redox reactions by some oxidation methods[28] while

other can not. Understanding the reasons for this behaviour will help us to select a

more suitable felt as the electrode active layer or to choose an effective activation

method for a particular type of felt, so that the efficiency of the vanadium redox

flow battery can be further improved.

The difference in characteristics of various felts is believed to be partly due to the

microstructure of the materials, which is in turn determined by precursors and

processing conditions including forming method, heating rate, final temperature

etc.[l00-103]. As pointed out by Thrower[lOO], the most common precursors used

for carbon felts manufacture are rayon, polyacrylonitrile (PAN) and pitch. The

effect of precursor on the microstructure of carbon fibres has been reviewed by

244

Johnson[l04], and it was revealed that fibre made from different precursors has its

own specific microstructure. For example, high modulus (type I) PAN fibre

exhibits an outer sheath with centre core structure. It was also found that the

turbostratic layer planes and crystallites are more highly oriented than that in the

core, and also considerably larger[105]. Differing from the PAN based fibres, a

specific structural characteristic, i.e., an almost perfectly circular cross section,

was observed and reported for pitch based fibres[106].

Though the microstructure of the fibre affects a number of characteristics of fibre

or felt materials, its effect on the formation of carbon-oxygen surface functional

groups, which links directly to the electroactivity of the felt electrode, is probably

the most important aspect for redox cell applications. The strong interaction of

graphite felt materials with oxygen could lead to a enhancement in electroactivity

obtained by some suitable activation treatments[28,166,167]. However, it might

also result in a high possibility of electrode deterioration caused by surface

oxidation during charging[l68-172]. Thus the positive graphite felt electrode of

the vanadium redox flow battery operated under chemically and electrochemically

oxidising conditions during the charging process because of the anodic potentials

applied the existence of the oxidising V(V) ions in the charged electrolyte.

Furthermore, during cell over-charge, the electrode is operated under severe

oxidising conditions since oxygen evolution takes place and the electrode is in

contact with the highly oxidising fully charged electrolyte. The behaviour of the

graphite felt electrodes under these conditions is therefore a key issue, as a long

cycle life performance of the vanadium redox flow battery is dependent greatly on

245

the oxidation stability of the graphite felt.

In the present study, the oxidation behaviour of two type of graphite felts, FMI

and GFD 2 which are based on rayon and PAN respectively, were evaluated with

various oxidation treatments. Firstly, the two felts were anodically oxidised at a

positive electrode potential to compare the effect of anodic oxidation on the

electroactivity of the felt in the vanadium solution. Cyclic voltammogram was

used for the electroactivity evaluation. A gas oxidation experiment was also

performed on the two felt samples, and the change in the concentration of surface

carbon-oxygen functional groups was analysed using X-ray photoelectron

spectroscopy (XPS). Scanning electron microscopy (SEM) was then used to

compare the microstructure and orientation of the two felts. Finally, the felt

samples were fabricated into composite electrodes by bonding onto conducting

plastic substrate and tested in a vanadium redox cell under normal cell operation

and overcharge conditions. The surface characteristics of these two felts after

anodic oxidation in the vanadium redox flow cell were analysed and compared by

XPS analysis.

6.2.1 The Effect of Anodic Oxidation on Electroactivity of Felt Electrodes in

Vanadium Solution

The small FMI and GFD 2 felt composite electrodes (geometric surface area =

0.028 cm2) for cyclic voltammetry were prepared by the procedure described in

Section 3.3.3. The experimental apparatus and method were also shown in the

246

same section. The supporting electrolyte of the vanadium redox flow cell, 3 M

H2S04• was used for the anodic oxidation experiments, while a solution of 0.1 M

VOS04 in 3 M H2S04 was employed to evaluate the electroactivity of the

composite felt electrode before and after anodic oxidation. The anodic oxidation

was performed by holding the electrode at a positive potential of + 1.5 V versus

Hg!Hg2S04 reference electrode for 15 minutes. In order to avoid the accumulation

of gas bubbles inside the felt, the electrode was rotated with a rotation speed of

500 rpm during anodic oxidation.

Figure 6.4 shows the cyclic voltammograms for the FMI and GFD felt composite

electrodes in 3 M H2S04• The potential range was from -1.6 V to + 2.05 V. In

both Figures 6.4(a) and (b), curves 1 and 2 refer to the voltammograms obtained

before and after anodic oxidation respectively. Similar cyclic voltammograms for

both FMI and GFD 2 felt electrodes were observed before anodic oxidation: in

addition to the hydrogen and oxygen evolution, a reducing peak around -0.5 V

(peak B in both Figures) appears, indicating that some carbon-oxygen functional

groups exist and reduced at that potential. After anodic oxidation, a more clear

oxidation peak around + 1.3 V (peak A in both cases) is observed, showing that a

change of surface carbon-oxygen complexes takes place at the electrode surface. It

was also observed that for GFD 2 felt electrode, an extra reduction peak, peak C,

appeared at the electrode potential of -1.10 V, exhibiting a more complicated

electrode process after electro-oxidation.

Figure 6.5 shows the effect of anodic oxidation on the electroactivity of the felt

247

A

<( ~ E 25 ~~

c: (l) I-I-:::3 ()

0

i 8

-so~~~~--~--~--~--~~--~--~~ -1.5 -1.0 -05 0 0.5 1.0 15. 2.0

(a)

248

75

50 A

<( 25 l E -~ c Q) lo....

0 lo.... :::J ()

-25

-50 c B

-1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0

(b)

Figure 6.4 Cyclic voltammograms of graphite felt electrodes in 3 M H2S04

solution, (1) before anodic oxidation; (2) after holding at +1.5 V for

15 minutes, (a) FMI; (b) GFD 2. Sweep rate: 60 mVs"''~.

249

electrode in the vanadium solution. Mter holding for 15 minutes at a potential of

+ 1.5 V at which oxygen evolution starts to take place, both FMI and GFD 2 felt

electrodes showed considerable electrode deterioration, indicating that anodic

electrolysis of the felt in sulphuric acid at this condition results in a serious

oxidation of the felt electrodes. This result is consistent with that observed by

other authors [ 168-172], however, others have claimed that anodic oxidation could

enhance the electroactivity of graphite fibre electrodes for some electrochemical

systems[28,166,167]. A difference between the two felt electrodes can also be

observed. At the recording sensitivity, the GFD 2 felt electrode seems to maintain

some electroactivity for the vanadium redox reaction after anodic oxidation (curve

2 in Figure 6.5(b)), while for the FMI felt electrode (Figure 6.5(a)), a flat cyclic

voltammogram was obtained. This could be due to a superior oxidation stability

for the PAN based fibre compared with the rayon based fibre.

6.2.2 The Effect of Gas Oxidation on the Formation of Surface Carbon

Oxygen Complex

To understand more about the surface oxygen-interaction characteristics of the

FMI and GFD 2 felt samples which are currently used in the vanadium redox cell

development, samples were thermal treated in both nitrogen and air environments.

As reported by Golden et al[l01] and supported by experimental results obtained

previously in this lab[28], thermal treatment at a temperature of 400 °C and a

period of 30 hours produced the maximum carbon-oxygen interaction. The surface

250

50

-0.5 0 05 1.0 1.5 2.0 Potentiai,V (vs Hg/Hg2 S04 )

(a)

251

<(

E

100

50

~ 25

0

-50

-1.0 -0.5 0 0.5 1.0 1.5 2.0 Potentiai,V (vs Hg/Hg2 804)

(b)

Figure 6.5 Cyclic voltammograms of graphite felt electrodes in 0.1 M

VOSOJ3 M H2S04 solution, (1) before anodic oxidation; (2) after

holding for 15 minutes at +1.5 V in 3M H2S04, (a) FMI; (b) GFD2.

252

of the samples treated under these conditions was analysed with XPS (X-ray

photoelectron spectroscopy) and the results are shown in Figures 6.6 to 6.10 and

summarised in Table 6.4.

Figure 6.6 shows the overall XPS spectra for the thermally treated (labelled as

"T") and untreated (labelled as "N") GFD 2 felt. For both felt samples, an intense

Cls peak, an Ols peak and a weak O(KLL) Auger peak are observed. Signals

from other elements were not detected, indicating that their contents are very low.

Similar overall spectra were obtained in a previous study[28]. Figure 6.7 shows

the Cls region spectra for both the FMI and GFD 2 felt samples treated in N2 and

air environments respectively. No visible differences can be seen neither between

different samples nor between different treatment conditions, and both samples

show very narrow Cls "graphitic" peaks, indicating that both of the felts are

probably graphitised.

Since the samples treated in N2 and in air all show very similar Cls spectra, only

the Ols spectra, which show a relatively poor signal to noise ratio due to the low

amount of oxygen in the samples, could be used to work out changes in the

surface functional groups.

Some differences in the O(ls) region spectra between the two felt samples can be

seen in Figure 6.8. For the GFD 2 felt (Figure 6.8(a)), the O(ls) peak does not

shift under different oxidation treatment conditions, while for the FMI felt (Figure

6.8(b)), the sample treated in air has its peak shifted about 0.5 eV compared with

253

Title: C-felt GFD (T

~

""0 c 0 (J QJ lA

' lA .. c :::s 0 (J

:n .. .... lA c QJ .. c .....

1000

Figure 6.6

c: "" "" .. _.,_._c. <·i.:< '· : ... :::'5:::::· "T Scan: 1 Base Cps:297.486 Scale: 2.47905 N

800 600 400 200 Binding Ene~g~ (eU)

Overall XPS spectra of GFD 2 graphite felt, (N) untreated; (T)

treated in air at 400 °C for 30 hours.

254

<:I c 0 () <II Ill

' Ill ... c ;:, 0 ()

j) ... ... Ill c <II ... c

<:I c 0 () <II Ill

' Ill ... § 0 ()

j) ... ... Ill c <II ... c ....

Figure 6.7

Cls

(a)

295 290 285 280 275 Bindin9 Ener9~ (eVl

(b)

295 290 285 280 275

C(ls) region XPS spectra of graphite felts treated in air and in N2 at

400 °C for 30 hours, (a) GFD 2; (b) FMI.

255

the one in N2• More detailed analysis revealed that these two felts samples show

different behaviour towards oxidation. Figure 6.9 and 6.10 illustrate the O(ls)

curve fitting spectra for the FMI and GFD 2 felt samples with different treatments.

It can be seen that for both of the felt samples, curve fitting reveals three peaks.

Peak 1 (ca. 535.4 eV for FMI and 535.6 eV for GFD respectively), has a low

intensity, and it probably due to the adsorbed water and some chemisorbed oxygen

[159]. Peak 2 (ca. 533.0 Ev for both FMI and GFD) and peak 3 (ca. 531.1 Ev for

both FMI and GFD) are the 0(1s) signals from C-OH and C=O (and/or C-0-C)

functional groups, respectively[159]. The most significant difference between these

two felts is the sensitivity to oxidation. Treatment of GFD 2 felt under oxidising

conditions (in air) results in a slight decrease in the intensity and relative peak

area of peak 3 which corresponds to the C=O functional groups decreased with no

significant effect on peak 2 (see Figure 6.9). A similar phenomenon was also

observed and reported by Proctor and Sherwood who studied the effect of heat

treatment on the surface characteristics of PAN based carbon fibres[151]. For the

FMI felt, however, the intensity and relative peak area of peak 2 decreased and

that of peak 3 increased significantly, indicating that when treated under identical

conditions in air, the C-OH surface functional groups on the FMI felt are oxidised

or converted to C=O or C-0-C functional groups.

It was also observed that for the FMI felt, the surface oxygen atomic

concentration is much higher for the sample treated in air than for that treated in

N2, but for the GFD 2 felt, not much difference is observed. Therefore, we can

conclude that the GFD 2 felt is more graphitic than the FMI, and would be less

256

Title: C-felt GFD (Tl Run:CPCC06 Re~: 1 (01s CoA:cpcc08 Re~: 1 (01s

540

Scan: 1 Base Cps: 8221 Max Cps: 10914 Scan: 1 Base Cps:11418.6 Scale: 2.95588

535 530 525 Bindin~ Ener~~ (eV)

(a)

257

Title: carbon-felt FMI (T) Run:CPCC12 Reg: 1 (01s l Scan: CoM:cpcc10 Reg: 1 (01s ) Scan:

~ s:: 0 (J Q.l 11'1

' 11'1 ...,. § 0 (J

~ ...,. ... 11'1 c Q.l ...,. s::

540

1 Base Cps: 9032 Max Cps: 15879 1 Base Cps:13035.2 Scale: 2.20973

535 530 Binding Energ~ (eVl

(b)

525

Figure 6.8 O(ls) XPS region spectra of graphite felts treated in air and in N2 at

400 °C for 30 hours, (a) GFD 2; (b) FMI .

258

Run: CPCC10 Reg: Scan: 1 Chans:

1 107

Start eV: 538.45 End eV: 527.80

Fit: 1.2

100% lntensit!::t: 100% Area

Line Ell'lt. Energ!::t

GAUSS 01s 531.08 GAUSS 01s 533.02 GAUSS 01::. 535 .4~:

3274. 133241.

Int.

50.03 87.88 13.2:::

538 536 534 532 530 528

FWHM Area

2.64 34.08 2.47 56.17 .-. ...,,-, C.( 0 '? .47

538 536 534 532 530 528 Binding Energ!::t

(a)

259


1 101

Start eV: 537.95 End eV: 527.95

Fit: 1.5

100% Intensit~: 100% Area

Line ElMt. Energ~

GAUSS Ols 531.18 GAUSS Ols 533.02 GAUSS Ois 534.88

7480. 312240.

Int.

76.72 73.88 15.02

538 536 534 532 530 528

FWHM Area

2.48 48.07 2.18 40.71 2.9·~ 11 .25

538 536 534 532 530 528 Binding Energ~

(b)

Figure 6.9 Fitted O(ls) XPS spectra of FMI graphite felts treated at 400 °C for

30 hours, (a) in N2; (b) in air.

260


1 106

Start eV: 539.20 End eV: 528.70

Fit: 3.0

100% Intensit~: 100% Area

Line Ell"'t. En erg~

GAUSS 01s 531.12 GAUSS 01s 532.95 C:iAU:::S 01::. 535.5:3

1001. 42707.

Int.

51.89 84.29 2B.22

538 536 534 532 530

FWHM Area

2.15 27.43 2.47 51.49 3.00 2[1.85

538 536 534 532 530 Binding Energ~

(a)

2 61

Run: CPCC06 Reg: Sc:an: 1 Chans:

1 107

Sta~t eV: 538.45 End eV: 527.80

Fit: 2.2

100% Intensit\::1: 100% A~ea

Line Ell"lt. Ene~g\::1

GAUSS 01s 531.24 GAUSS 01s 533.00 GAUSS Ois 535.62

2970. 119148.

Int.

36.39 89.09 18.00

538 536 534 532 530 528

FWHM A~ea

2.56 24.44 2.66 62.27 ... , -,.-, c. • ' c. 12.77

538 536 534 532 530 528 Binding Ene~g\::1

(b)

Figure 6.10 Fitted O(ls) XPS spectra of GFD 2 graphite felts treated at 400 °C

for 30 hours, (a) in N2; (b) in air.

262

prone to form carbon-oxygen surface functional groups, such as C=O or C-0-C.

It should be noted that the treatment of the FMI felt in air at 400 °C for 30 hours

was found to produce an enhancement in the electroactivity for the vanadium

redox flow cell applications[28]. This was believed to be due to the

electrocatalysing effect of increased C=O and C-OH functional groups[28].

Table 6.4 XPS Results Comparison on FMI and GFD 2 Felt

GFD FMI Characteristics

N2, 400 °C, Air, 400 °C, N2, 400 °C, Air, 400 °C, 30 hrs 30 hrs 30 hrs 30 hrs

ch peak, b.e., e v 284.3 284.3 284.3 284.3

ols peak 1, b.e., 535.58 535.62 535.43 (9.47) 534.88 (11.25) eV (area,%) (20.85) (12.77)

ols peak 2, b.e., 532.95 533.00 533.02 533.02 (40.71) eV (area,%) (51.49) (62.27) (56.17)

ols peak 3, b.e., 531.12 531.24 531.08 531.18 (48.07) eV (area,%) (27.43) (24.44) (34.08)

C=O/C-0 area 0.533 0.392 0.607 1.181 ratio

Atomic cone. of 2.9 3.6 5.7 10.1 oxygen(%)

6.2.3 Surface Microstructure of Fibres in FMI and GFD 2 Felts

As explained by Singer[102], depending on the precursors and processing

conditions, the behaviour of carbon products can vary from that of carbon which

has a microcrystalline structure and more readily reacts with oxygen, to that of

263

graphite which exhibits a highly ordered crystalline orientation and less

susceptible to oxygen. The experimental results shown above indicate that the FMI

and GFD 2 felts exhibit a considerable difference in interaction with oxygen. This

must be attributed to their differences in surface microstructure.

To prove this, the two types of felt were examined under the scanning electron

microscope (SEM). The morphology of fibres from FMI and GFD 2 felt are

shown in Figure 6.11 and 6.12. A totally different appearance of the individual

fibres between the two felts can be observed. The fibre from the FMI felt, shown

in Figure 6.11(a), has an uneven and dusty surface, while Figure 6.11(b) for the

GFD 2 fibre shows a clean and flat surface. Figure 6.12(a) and (b) give a

comparison in the microstructure of the cross-section for the FMI and GFD 2

fibres respectively. It is clear that the construction for an individual GFD 2 fibre is

uniform and solid, while in the case of FMI sample, the individual fibre looks like

a bundle of a several fibres. As the FMI fibre is less graphitic, its structural

characteristics should give rise to more edge planes and a high defect

concentration which can more readily react with oxygen[100]. This explains why

the FMI felt is more sensitive to air oxidation and anodic electrolysis than the

GFD 2 felt.

6.2.4. The Effect of Charging and Overcharging on the Surface

Characteristics of Graphite Felt Electrodes

It was shown in Sections 6.2.1 and 6.2.2 that rayon based and PAN based graphite

264

(a)

(b)

Figure 6.11 Side-view of graphite felt fibres, (a) FMI; (b) GFD 2.

265

(a)

(b)

Figure 6.12 Cross-section of graphite felt fibres, (a) FMI; (b) GFD 2.

266

felts display different interactions with oxygen under anodic and gas oxidation

conditions. In the vanadium redox flow battery, the felt active layer on the

positive electrode of the battery works as an anode and contacts an oxidising

electrolyte during cell charging. Furthermore, at high states-of-charge, oxygen

evolution takes place. If the cell is overcharged, oxygen evolution becomes the

predominant reaction in the positive half of the vanadium redox flow battery. The

change in the surface chemical characteristics of the graphite felt electrodes under

these operating conditions is important, since it will directly affect the

electroactivity and cycle life of the electrode in the vanadium redox flow cell.

FMI and GFD 2 felt samples were bonded onto the carbon-plastic composite sheet

to form the composite electrodes for the laboratory scale test cell with the

procedure described in Section 3.2.2.2. The electrodes were subjected to normal

charge/discharge and overcharging conditions. After operation in the test cell, the

felt specimens were prepared with the procedure described in section 3.3.4.3 for

XPS analysis.

6.2.4.1 General Analysis

XPS analysis of the carbon felt samples were performed and interpreted by Dr.

Celestino Padeste and the results obtained are presented in Tables 6.5 and 6.6.

In the survey spectra of all samples only carbon and oxygen peaks were found,

indicating that the surface concentrations of any other contaminants were below

267

1%.

From each sample detailed Cls and Ols region scans were recorded.

Quantification of the Cls and Ols peak areas using the quantification factors of

the DS800 system and linear background subtraction yielded O:C ratios as given

in Table 6.5. For comparison purposes, some data from Table 6.4 is also included.

The high oxygen concentration of the untreated GFD 2 felt must be interpreted as

a contamination by medium volatile carbon/oxygen substances, since they are

easily removed by a treatment in N2 at 400 °C. This sample is therefore not

considered in the further interpretations.

Table 6.5 O:C ratios for felt samples with various treatment

Sample Treatment FMI GFD2

ID %0 %C %0 %C

NON non treated - - 10.4 89.6

N2 N2, 400 °C, 30 hrs 4.0 96.0 2.2 97.8

AIR air, 400 °C, 30 hrs 7.4 92.6 2.8 97.2

AU after used as electrode 23.0 77.0 18.3 81.7

oc used at overcharging cond. 29.3 70.7 28.3 71.7

268

Treatment of the felts in air causes a slight oxidation whereas the oxidation under

normal cell working conditions and especially under overcharging conditions is

quite pronounced.

Detailed peak analysis was performed on Cis peaks of the felts after they had

been used as positive electrodes in the vanadium redox flow battery under normal

and overcharging conditions.

6.2.4.2 Samples Used as Electrodes at Normal and Overcharging Cell Operating

Conditions

In the aforementioned series of papers [151,154,155,159-161], the C1s line is

fitted by 5 peaks: the main peak ("graphitic carbon") and four peaks in the higher

binding energy region arising from carbon bound to oxygen ("oxides 1,2,3 and

4"). Oxide 1 (b.e. shift ""' 1.5 e V) was assigned to carbon with bounds to one

oxygen atom such as alcohols (C-OH) or ethers (C-0-C). Oxide 2 (b.e. shift ""' 3

eV) is the peak of carbonyl (C=O) type groups, and oxide 3 (b.e. shift ""' 4.5 eV)

corresponds to carboxyl (COOH) or ester (COOR) groups. Oxide 4 (b.e. shift ""' 6

eV) is most probably a superimposition of -C03- type groups and the n-n* shake

up satellite typical for carbon compounds with extended aromatic systems. This

assignment of the peaks and their relative positions is in agreement with data

published by other authors[191].

The curve fittings for both felts at various oxidation conditions are shown m

269

Figures 6.13 and 6.14, and the calculated data for all samples are tabulated in

Table 6.6.

The results show that for both types of felts working at normal condition leads to

the formation of C-OH groups and small amounts of carbon in higher oxidation

states, while overcharging conditions favour the higher oxidised groups as C=O

and -COOH and -COOR. This second effect is more pronounced on the GFD

samples.

Table 6.6 Results from C1s peak curve fitting

Sample Main peak Oxide 1 Oxide 2 Oxide 3

position area position area position area position area

(eV) (%) (eV) (%) (eV) (%) (eV) (%)

AU 284.5 81.3 285.9 13.0 287.6 2.2 288.9 3.3

FMI oc 284.5 71.2 285.9 15.2 287.7 7.4 288.9 5.2

AU 284.5 85.5 285.9 11.2 287.7 1.7 289.0 1.6

GFD oc 284.6 71.2 286.1 14.9 287.4 9.5 288.9 4.6

No attempt was made to quantify the "oxide 4" peak, which arises from -C03-

groups and probably the 1t-1t* shake up. A qualitative comparison (Figure 6.15) of

the FMI (N2), FMI (AU) and FMI (OC) spectra shows a reduction of the intensity

in the binding energy range of the "oxide 4" peak (analogous for GFD 2 felts, e.g.

Figure 6.16). This is most likely to be interpreted by a loss of a part of the

aromaticity of the felt during the use as electrode. It was also observed that with

270

Run: CPCC10 Seem:

Start eV: 290.98 End eV: 282.08

Fit: 33.1

2 90

100% lntensit~: 49376. 100% Area 817106.

Line Elmt. Energ~ Int. FWHM Area

CCi0 Cis 284.6i 0.00 GAUSS Cis 284.96 0.00 GAUS:':; Cls 2:::6.40 0.00 GAUSS (:1:;;. 288 .. 7'3 0.(1(1

(a)

Run: CPCCi6 Scan: 1

Start eV: 290.98 End eV: 282.08

Fit: 1.5

100% lntensit\:1: 100% Area

2 90

i5288. 3582i0.

i .25 1.25 1 .23 1 .2(1

Line Elmt. Energy Int. FWHM

CCifJ Cis 284.54 99.94 i.53 GAUSS Cls 285.9(1 18.42 1.58 GAU~:;::; Cl:=. 2:::7 .. t.~: 4.14 1.19 GAUS~:; Ci:=. 2::::::.92 4.90 1.51

(b)

0.00 0.00 0.(1(1 0.(1(1

Area

8i.32 13.05 2.19 :3.29

271

1~:.: :~-1 290 288 286

•'

/' I

/ l

.~·--__ , ~------------

290 288 286

290 288 286

290 288

284

'\

284

284

Run: CPCC14 Reg: 2 Scan: 1 Chans: 90

Start eV: 290.98 End eV: 282.08

Fit: 1.7

100% Intensi t~: 100% Area

Line Elmt. Ener9~

CC10 C1s 284.51 GAUSS Cls 285.90 GAUS:3 C1:s. 287.67 GAU:::::: Cl:s. 2SE:.90

(c)

7156. 187516.

Int.

94.84 21.69 10.7:::

t: .26

FWHM

1.58 1.75 1 • 7::: 1 "'"" • • _1(

290 288 286 284

Area

71.42 15.23 7 .4:1 5.17

290 288 286 284 Binding Energ~

Figure 6.13 Fitted C(ls) spectra of FMI graphite felt, (a) treated in N2 at 400 °C

for 30 hours; (b) after normal use as electrode; (c) after overcharge

for 50 minutes at 21.7 mNcm2.

272

Run: CPCC08 Scan: 1

Start eV: 290.98 End eV: 282.08

Fit: 33.1

2 90

100% Intensit~: 29296. 100% Area 487477.

Line Elmt. Ener~~ Int. FWHM Area

CC10 C1s 284.61 GAUSS C1s 284.96 GAU::;:;:; Cl::. 2:::6.40 GAUSS Cl::. 2::::::.79

(a)

Run: CPCC20 Scan:

Start e~J: 290.98 End ev: 282.08

Fit: 1.6

100% Intensit~:

100% Area

Line Elmt. Ener~~

CC10 Cis 284.54 GAUSS Cls 285.90 GAUSS CL 287 . 7:~: GAU::;:::; Cis. 2::::9.00

(b)

0.00 1.25 0.00 1.25 o .o~:::1 1 .2:::: 0.00 1 .2~:1

2 90

12568. 273365.

Int.

99.45 15.31

~: .. 05 2.56

FWHM

1.50 1.51 1.20 1 .::::o

0.00 0.00 0.00 0.00

Area

85.56 11.15

1.73 1.5:::

290 288 286 284

290 288 286 284 Binding Energl:f

290 288 286 284

290 288 286 284

273

Run: CPCC18 Reg: 2 Seem: 1 Chcms: 90

Start eU: 290.98 End eU: 282.08

Fit: 1.6

100% Intensit!;l: 100% Area

Line Elmt. Energ!:J

CC10 C1s 284.58 GAUSS Cis 286.09 GAUS::O: CL;. 2:::7.40 GAUSS Cls 2:::::: .'j2

(c)

5096. 148950.

In~._

95.78 23.66 17.20 9.15

FWHM

1.75 1.75 1.54 1 .:C:9

Area

71 .21 14.90 9.51 4.56

290 288 286 284

290 288 286 284 Binding Energ!:J

Figure 6.14 Fitted C(ls) spectra of GFD 2 graphite felt, (a) treated in N2

at 400

°C for 30 hours; (b) after normal use as electrode; (c) after

overcharge for 50 minutes at 21.7 mNcm2 •

274

the oxidation condition of the felt electrode being more severe, a more asymmetric

Cls spectra on high binding energy side is recorded (Figure 6.15(c) and 6.16(c)).

Combining this with the experimental results described in section 4.2.2.5, which

shows a dramatically increased cell resistance after overcharging the Toray felt

composite electrode for 50 minutes, it can be concluded that the overcharging of

graphite felt composite electrode for a period time would lead to a loss of

electroactivity due to the formation of high oxygen containing (e.g.-C03- type)

carbon-oxygen surface functional groups.

It should be also noted that the XPS results for GFD 2 felt composite electrode

used at normal and overcharge conditions do not show a lower oxidation

sensitivity than the FMI felt as described in Sections 6.2.1 and 6.2.2. This can be

explained in terms of the electrode current density. As mentioned previously, both

felt composite electrodes were tested with a current of 3 A. Because of the

differences in felt thickness and specific surface area, the actual current density

values for each felt electrode during charging/discharging would be quite different.

Table 6.7 gives the comparison.

It can be seen from the table that the real current density through GFD 2 felt

composite electrode is about 60% higher than that of FMI felt electrode.

According to this calculation, if the actual current densities applied to both

electrodes were equal, then less oxidation sensitivity for the GFD 2 felt composite

electrode would be expected.

275

"t '-<f~

Title:-~ (N2J Rtm:CPCC10 Re:;,: 2 (C1s

'U s:: 0 u Ql oil

' oil ... s:: ~ 0 u

j) ... ... oil s:: Ql ... s:: ....

Scan: Base Cps: 2512 ~l•:1x Cps: 55889

/ __,/

~-~---_ .... _______ .-... -----------295

Title: FMI after reaction Run:CPCC16 Re:;,: 2 (Cls ) Scan:

j) ... .... oil s:: Ql ... s::

290

295 290

285 Binding Energ~ (eVJ

Base Cps: 1132 11ax Cps: 12070

285

276

(a)

(b)

Title: FMI overchar~ed~ top Run:CPCC14 Re~: 2 (C1s ) Scan: 1 Base Cps: 834 Max Cps: 6041

(c)

~--295 290 285

Bindin~ Ener~~ (eV)

Figure 6.15 C(ls) region spectra of FMI graphite felt, (a) treated in N2 at 400 °C

for 30 hours; (b) after normal use as electrode; (c) after overcharge

for 50 minutes at 21.7 mNcm2.

277

Title:-GFD (N2J Run:CPCC08 Re::_1: 2 (Cls ) Scan: Base Cps: 1544 Max Cps: 33115

"'0 c 0 u Ill

3000

~ 2000 JJl ~

c :::; 0 u

295

Title: GFD II after use Run:CPCC20 Re::_1: 2 (Cis

"'0 c 0 u Ill JJl

" oJI ~

c :::; 0 u

295

290 285 Bindin9 Ener9~ (eVl

Seem: Base Cps: 731 Nax Cps: 9811

290 285 Bindin9 Ener9~ (eVl

278

(a)

280

(b)

Title:-GFD overcharged~ top Run :CPCC18 Re3: 2 (Cls ) Scan: 1 Base Cps: 561 t1ax Cps: 4284

::n .... ... rJl c q, .... c

295 290 285

Figure 6.16 C(ls) region spectra of GFD 2 graphite felt, (a) treated in N2 at 400

°C for 30 hours; (b) after normal use as electrode; (c) after

overcharge for 50 minutes at 21.7 mA/cm2.

279

Table 6.7 Comparison of FMI and GFD 2 Graphite Felts

Felt Geometric Thick- Specific Weight Real Current Geometric Real

area ness surface of felt surface applied current current

(cm2) (nun) area used area (A) density density

(cm2/g) (g) (cm2) (mA/cm2) (mA/cm2)

FMI 138 6 3290 7.24 23820 3.0 21.74 0.13

GFD2 138 2.5 3900 3.676 14336 3.0 21.74 0.21

6.3 Electrocatalysis Studies

Combining the results obtained from the experiments described previously, it

seems that the GFD 2 graphite felt is the most suitable felt for use as the active

layer of the composite electrode. However, from the peak current value obtained

from cyclic voltammetry and the data listed in Table 6.7, the current density in

terms of real surface area for the GFD 2 felt electrode is still seen to be very low,

indicating that the effective surface area of the electrode is relatively low and

needs to be further improved, if greater power densities are to be achieved for the

vanadium redox cell. It has been reported that some oxidation treatments [28] can

be used to increase the electrochemical activity of felt electrodes, but the

enhancement in cell efficiencies was still limited. It was also reported that the

electroactivity of a graphite fibre electrode for the vanadium redox reactions,

based on the cyclic voltammograms, increases after scanning the electrode in

electrolytes containing noble metal ions like In3+ and Ir+ [28]. However, this has

not been verified by further investigation. On the other hand, the surface area

measurements revealed that the felt has a micro pore structure which would be

280

suitable as an electrocatalyst support. As evidence, the NASA group has already

reported the successful application of a gold-lead electrocatalyst system on the

graphite felt electrode surface to increase the cell efficiencies of the iron

chromium redox flow cell battery[36-38]. In order to obtain higher efficiencies for

the new all vanadium redox flow cell system, it is appropriate to investigate a new

activation method employing electrocatalysts and study its effect on the

electrochemical activity.

6.3.1 Activation Evaluation of Treated Felt Samples With Cyclic

Voltammetry

Figures 6.17 to 6.20 show cyclic voltammograms of the GFD 2 graphite felt

electrodes with different treatments, and the effect of the treatments is summarised

in Table 6.8. As suggested by Pletcher[30], a few transition metal elements were

selected as electrocatalysts. Solutions containing Ag+, Nf+ and Mn2+ were prepared

and the felt samples were treated with the procedure described in Section 3.3.5.1.

The treated felts were fabricated into small felt composite electrodes for cyclic

voltammetry and tested in the vanadium solution. Details for this test are

described in Section 3.3.3.

For the felt treated with Ni2+ and Mn2+, no significant improvement is observed

from the cyclic voltammograms (Fig. 6.18 and 6.19 respectively), indicating that

the existence of these ions neither increases the electron transfer rate nor enhances

the concentration of active sites for the vanadium redox reactions.

281

50 <(

E .. ......, 0 c: G)

'-'-:;:, -50 ()

-100

-20 -15 -10 -0.5 0 0.5 1.0 1.5 Potentiai,V vs Hg/Hg2S04

Figure 6.17 Cyclic voltammogram of thermally-treated (in air at 400 °C for 30

hours) GFD 2 graphite felt electrode in vanadium solution (same as

for Figure 6.2), sweep rate: 60 m V s·' ~ .

282

<! E

150------------------------~

100

50

0

-50

-100

F 150~~~~--~~--~~--~

-2.0 -1.5 -1.0-0.5 0 0.5 1.0 1.5 Potential, V vs Hg/Hg2S04

Figure 6.18 Cyclic voltammogram of GFD 2 graphite felt electrode treated with

0.1 M NiS04 solution (conditions same as that for Figure 6.17).

283

<(

E

150

100

.. ..f-)

~ 0 ''::J

() -50

-100

-150

-2.0 -1.5 -1.0-0.5 0 0,5 tO 1.5 Potential, V vs Hg/Hg2S04

Figure 6.19 Cyclic voltammogram of GFD 2 graphite felt electrode treated with

0.1 M MnS04 solution (conditions same as that for Figure 6.17).

284

An improvement in the electrochemical activity is however observed in Figure

6.17 for the felt which had been subjected to thermal treatment, as evidenced by

the increase in the peak current for peaks A and C. This is consistent with that

reported in reference [28]. An interesting cyclic voltammogram was obtained, with

the electrode treated with AgN03 solution as shown in Figure 6.20(a): peaks A

and B are sharper and more clear and the two peaks are closer than for the other

treatment. Furthermore, the peak currents for peaks A and C increase, indicating

that the existence of silver can catalyse the vanadium reactions, particularly, for

the V(II)N(III) redox couple. In addition, two extra peaks are observed in the

cyclic voltammogram, probably, due to the redox reactions of silver on the

electrode surface. To confrrm this, the electrode was removed from the vanadium

solution, rinsed with distilled water several times, and then immersed in a solution

which only contained 3 M H2S04 and scanned under the same conditions. The

cyclic voltammogram in Figure 6.20(b) shows that only two peaks exist within the

potential range for oxygen and hydrogen evolution, the peak potentials of which

confrrm that the peaks correspond to the Ag/ Ag(l) redox couple reactions, since

the recorded peak potential of +0.08 V (versus saturated Hg/Hg2S04 reference

electrode) for silver oxidation reaction is very close the standard potential value,

(i.e. +0.03 V versus the same reference electrode). Narrowing the sweep potential

range and continuing to sweep the electrode for 50 cycles, the clear and

reproducible cyclic voltammograms of Figure 6.20(c) were obtained, indicating

that the silver catalyst has in fact been loaded on the felt electrode and is not

being leached out during cycling. This result is in agreement with that reported by

Dekansk:i et al [176], who found that after immersion in AgN03 solution for a

285

<:( E

150

100

50

0

-150

0 0.5 1.0 1. 5 Potentiar,v vs Hg/Hg2S04

a)

286

<t E ..

100~--------------------.

50

...., 0 c:: Q)

''-

~ -50

-100

-150 F

-20 -1.5 -1.0-0.5 0 0.5 1.0 1.5 Potentiai,V vs Hg/Hg2S04

b)

287

<( E

20

10

..

-20

-30

m

n -1.0 -0.5 0 0.5 1.0

Potentiai,V vsHg/Hg2S04

c)

Figure 6.20 Cyclic voltammograms of GFD 2 Graphite felt electrode treated

with 0.1 M AgN03 solution, (a) in the same vanadium solution as

for Figure 6.17; (b) in 3 M H2S04; (c) after 50 cycles within the

potential range of -0.7 V to +0.5 V.

288

period of time, the electroactivity of a glassy carbon electrode increases

significantly. They also proved, with XPS and AES (Auger electron spectroscopy)

techniques, that the deposited silver is in the metallic form, and that a pre-oxidised

surface will allow a higher amount of silver deposition during immersing than one

which has not been preoxidised. It can thus be concluded that by pre-oxidising

and then immersing the graphite felt into a AgN03 solution, metallic silver is

formed on the graphite felt surface. This deposited silver appears to be a

successful electrocatalyst for the vanadium species redox reactions, especially, for

the V(III)N(ll) redox reactions as shown in the cyclic voltammograms of Figure

6.20(a).

Table 6.8 The Effect of Surface Treatment on the Electrochemical

Activity of GFD 2 Felt

Treatment Weight ~I ~2 t.Ep peak for

changed V(ll)~ V(IV)~ V(IV)/ V(III)~

AW,(rng) V(III) V(V) V(V) V(II)

(rnA) (rnA) (V)

None - 45 115 1.14 not clear

Thermal - 85 170 1.40 " "

0.1 M NiS04* 0.5 25 140 1.70 " "

0.1 M 1.5 70 160 1.40 " "

MnSO/

0.1 M 1.3 60 160 1.34 clear

AgN03*

* The graphite felt samples were thermally pretreated.

289

6.3.2 The Effect of Concentration of AgN03 Solution on Characteristics of

Treated GFD 2 Felt

6.3.2.1 Effect of AgN03 Concentration on Weight Increase

Four pieces of GFD 2 felt with dimensions 5 x 5 em were prepared. Silver nitrate

solutions with concentrations of 0.002 M, 0.01 M, 0.05 M and 0.1 M were also

made by dissolving the required amount of AgN03 in distilled water. After heating

in air at 400 °C for 30 hours, the felt samples were immersed into the different

solutions for 30 minutes, followed by a 48 hours drying step (details described in

Section 3.3.5.1). The weight change for each sample was measured with an

analytical balance and the values are summarised in Table 6.9. The effect of

AgN03 concentration on weight increase in per gram felt is also illustrated in

Figure 6.21.

Table 6.9 The Effect of AgN03 Concentration on Weight Change of the Felt

Sample Weight Concentration Weight after Increased (Weight

before of AgN03, treatment, weight increase )/(per

treatment, (g) (M) (g) llW (mg) gram felt)

1 0.6475 0.002 0.6485 1.0 1.544

2 0.6544 0.01 0.6631 8.7 13.295

3 0.7022 0.05 0.7500 47.8 68.072

4 0.6600 0.1 0.7533 93.3 141.364

The data listed in Table 6.9 and the straight line shown in Figure 6.21 indicate

290

,-., i50 r------- ·------••. J '

. 'U ID mOO 0 ID L 0 c

30~ '

0.02 0.04 0.06 0.08 Cone. of si iver nitrate, U~)

0.10 o. !2

Figure 6.21 The effect of AgN03 concentration on the increased weight of

treated GFD 2 graphite felt

291

that within the AgN03 concentration range studied, the amount of deposited silver

is linearly related to the concentration of the AgN03 solution used.

6.3.2.2 The Effect of Deposited Silver on Felt Electrical Resistivity

The resistivity values of the treated felts were determined by the ASTM D-991

method and Table 6.10 gives the results. It can be seen that the treatment does not

lead to a considerable change in electrical conductivity, although a slight decrease

in resistivity with increasing amount of loaded silver is observed.

Table 6.10 The Effect of Deposited Silver on Resistivity

Sample Treatment 1:1 W /(gram felt), Resistivity,

mg/g p(.Q.cm)

0 thermal, 400 °C, - 0.038

30 hours (in air)

1 0.002 M AgN03 1.544 0.038

2 0.01 M AgN03 13.295 0.036

3 0.05 M AgN03 68.072 0.036

4 0.1 M AgN03 141.364 0.035

6.3.2.3 Effect of Deposited Silver on Cell Resistance and polarisation behaviour

The effect of deposited silver catalyst on the electrochemical properties of the

292

graphite felt electrode was also evaluated in terms of cell resistance. This test was

carried out with a modified NASA cavity-fill-in single cell (see Section 3.3.5.2)

In this test, the AgN03 treated GFD 2 graphite felt was used as the negative

electrode. As has already been proved that thermal treatment enhances the

electroactivity of graphite felt for the V(V)N(IV) redox reaction, GFD 2 felt was

subjected to treatment at 400 °C for 30 hours in air before being employed as a

positive electrode for the vanadium redox flow cell. A piece of CMV membrane

was employed as separator and 50% SOC vanadium redox electrolytes were

utilised. Cell resistance values for electrodes treated under various conditions are

summarised in Table 6.11, while cell polarisation curves corresponding to different

electrode treatments are illustrated in Figure 6.22.

Table 6.11 Cell Resistance of GFD 2 Felt in 25 cm2 Cell (CMV membrane)

Cell resistance (Q.cm2)

Electrode Reb. R.n.. Raver.

Untreated on both sides 2.99 3.07 3.03

Positive thermally treated 2.34 2.65 2.50

Both sides thermally treated 2.26 2.17 2.22

0.002 M 2.03 2.04 2.04

0.01 M 1.89 1.91 1.90 Positive thermal,

negative AgN03 0.05 M 1.47 1.47 1.47

0.1 M 1.48 1.37 1.43

Untreated FMI (3 mm)[28] 4.09 4.22 4.16

293

1.60 ..... 1 --

l i o non-treat. A post. treated o both treated I

" !.ref ! ~ I o-~1.00~ ~g~-~ ~~--- -~~ "---"-ai 1.466~ -----~~ rno ~---- A---- -o------c __ ~ ~0--0 1.40~ -----A--- --o--> I ~~ ~~

I --1:!.-- -..a.. __ ...._........ ---.. --A -I --- --o-.._

1.35r ---A-_ I

1.:rl L--__ .__ __ ...__ __ ..__ __ ..__ __ ..___ __ ~ _ ____.

o.o 0.1 0.2 0.3 0.4 0.6 0.6 0.7 a..&rrent. I (A)

Figure 6.22 Polarisation behaviour of a vanadium redox flow cell employing

GFD 2 graphite felt electrode treated under various conditions.

Electrolyte: 2 M vanadium sulphate in 2.5 M H2S04; Membrane:

CMV. Solid line: charging; dashed line: discharging.

294

Figure 6.22 shows three pairs of cell polarisation curves corresponding to the

untreated, positive thermally treated and both sides treated (positive thermal,

negative treated with 0.1 M AgS04) electrodes. In all cases, linear relationship

between cell current and cell voltage was recorded for both charge (solid line) and

discharge (dashed line) processes. Comparing the slope of I-V curves which

related directly to the electrode polarisation behaviour, it is clear that the treatment

of felt electrode, in particular, treatment with AgN03 solution, leads to a

considerable decrease in cell polarisation rate.

It can be seen from Table 6.11 that both thermal and AgN03 treatments improved

the electroactivity of the GFD 2 felt significantly, since a considerable decrease in

cell resistance is recorded for the treated electrodes. It was also seen that a lower

cell resistance can be obtained by increasing the concentration of AgN03 solution,

which probably in turn increases the amount of silver deposited on the surface of

the graphite fibre. However, the influence of AgN03 concentration on the cell

resistance becomes less marked when the concentration is higher than 0.05 M.

Considering that the higher silver deposition would result in a higher hydrogen

evolution rate, and also less cost effectiveness, 0.05 M AgN03 solution was

selected as the GFD 2 felt activation treatment solution for further study.

6.3.3 Cell Performance Test

The GFD 2 felt subjected to treated with 0.05 M AgN03 solution was used as the

negative electrode while the thermally treated felt was employed as the positive

295

electrode in a vanadium redox flow cell and the effect of treatment on cell

efficiencies, the influence of current density on cell efficiency, and the cell

discharging behaviour was evaluated. Long-term stability tests were also

performed with the treated felt electrodes. ·

6.3.3.1 The Influence of Treatment on Cell Efficiencies

Table 6.12 gives the comparison of cell efficiencies for the vanadium redox cell

employing electrodes subjected to various treatments. With the untreated felt

electrode, a 80.9% voltage efficiency at the charge/discharge current density of 40

mA.cm·2 indicates that the polarisation losses associated with the inactivated

electrode material are high. Replacing the positive electrode with a thermally

· treated sample, resulted in an increase in cell voltage efficiency, showing an

improvement in the electroactivity of the felt electrode for the vanadium redox

reactions. Further enhancement in cell voltage and energy efficiencies were

obtained when a silver loaded GFD 2 felt was substituted for the untreated

negative felt electrode, confirming that the electron transfer rate for the

V(III)N(II) couple is catalysed by the deposited silver catalyst.

Figure 6.23 shows a typical charge/discharge curve of a vanadium redox cell

employing treated felt electrode at a constant charge/discharge current density of

40 mNcm2• The nearly symmetrical charge/discharge curve indicates that highly

electroactive electrodes are obtained and the cell polarisation losses are minimised.

296

2.01

> l6t (J) Q) 1.2 ell -0 > 0.8 Q)

()

0.4 Charge Discharge

0 0 40 80 120 160 200 240

Time, min

Figure 6.23 A typical charge/discharge curve of a vanadium redox flow cell

employing treated GFD 2 graphite electrode (positive: thermally

treated; negative: treated with 0.05 M AgN03). Current density: 40

mNcm2• Other conditions same as for Figure 6.22 .

297

Table 6.12 Cell performance of GFD 2 felt in 25 cm2 cell (CMV membrane)

electrode cell efficiency at 40 mA.cm-2, (%)

coulombic voltage energy

non-treat. on both sides 93.94 80.92 76.02

positive thermal treated 92.75 86.49 80.22

positive thermal 94.03 93.10 87.51

negative AgN03 (0.05 M)

6.3.3.2 The Effect of Current Density

Figure 6.24 illustrates the effect of constant charge/discharge current density on

the cell efficiencies for the treated GFD 2 electrode. Comparing the cell

efficiencies obtained using the electrodes with the 6 mm thick commercial Toray

felt (see Section 4.2.2.2 and Figure 4.17), similar trends in cell efficiencies versus

current density are observed. However, a few considerable improvements can be

noted. Firstly, the charge/discharge current density for obtaining an 80% overall

energy efficiency was enhanced from 40 mA.cm-2 (Figure 4.17) to 100 mA.cm-2,

again showing the electroactivity of the treated felt electrode is much higher than

that without treatment. Secondly, at the lower current density of 20 mA.cm-2, more

than 96% voltage efficiency can be achieved, at least 6% higher than that with the

6 mm Toray felt, indicating that electrode polarisation losses are minimised by the

298

'-'

:Jj 0

~ 10

Eoo 0 .__

\t--

ID 00 }-

o coulombic h. voltage

oo~------L------~------~------~----___j-

0 ~ 40 60 so !00 cJJr rent de'ls i ty. ( rrfl/ ern c )

120

Figure 6.24 The effect of current density on cell efficiencies of a vanadium

redox flow cell with thermally and AgN03 treated GFD 2 graphite

felt electrodes. Other conditions same as for Figure 6.22 -

299

graphite felt electrodes activated with thermal treatment and silver catalyst.

Another significant improvement in cell performance which is also observed is

that at a charge/discharge current density as high as 100 mA.cm-2, 82% overall

energy efficiency can still be obtained. It should be also noted that the felt is less

than half the thickness and the weight of the 6 mm Toray felt, which means half

of the space and the material can be saved.

6.3.3.3 The Cell Discharge Behaviour

Charging the cell at a current density of 40 mA.cm-2, and discharging with various

current density, 20, 40, 60, 80 and 100 mA.cm-2, yields Figure 6.25 which shows

the discharging behaviour of the cell with the activated electrodes. The cell

capacity at various discharging current density is also summarised in Table 6.13.

From Figure 6.25 and Table 6.13, it is clear that the cell with the activated

electrode can be operated over a wide range of current densities, although the cell

capacity seems to increase with increasing discharge current density up to 100

mA.cm-2, due to lower self-discharge rate across the membrane at the short

discharge times.

300

2.0 Charge: 40 mA/cmZ Discharge:

1 2 3 4 5 curve 16 20, 40,60,80, 700, mA!cmZ

> (l)

1.2 OJ ~

::::: 0 > = 0.8 5 4 3 2 Q)

()

0.4 Charge Discharge

0 0 40 80 120 160 200 240 280 320 360

Time, min

Figure 6.25 Cell discharge behaviour of the vanadium redox flow cell with

thermally and AgN03 treated GFD 2 graphite felt electrodes. Other

conditions same as for Figure 6.22 .

301

Table 6.13 The Effect of discharging Current Density On the Capacity of the

Vanadium Redox Flow Cell with Activated Electrode

Current Density, Current, I(A) Discharge time, t(hr) Capacity (A.hr)

i(mA.cm·~

20 0.5 3.65 1.83

40 1.0 1.94 1.94

60 1.5 1.33 2.00

80 2.0 1.01 2.02

100 2.5 0.81 2.03

6.3.3.4 Long-Term Stability Test

A long-term stability test was also conducted to evaluate the cycle life of the

activated electrode. After being run within safe operating conditions, i.e. within

the cell voltage range which the vanadium redox reactions are dominant, for 50

cycles, the cell was further tested at a constant charge/discharge current density of

100 mA.cm-2• A piece of CMV membrane was employed as separator. The cell

efficiencies versus cycle number are illustrated in Figure 6.26. A constant voltage

efficiency of around 84% throughout the test indicates that the electroactivity of

the treated electrode is very stable. The slight decrease and fluctuation in

coulombic and energy efficiencies are associated with degradation of the CMV

membrane from cycle to cycle. The test was stopped at 230th cycle where the

membrane was totally damaged. A longer-term test with a more stable membrane

302

110

105

100

- 95 t. - 90 ~ () c 85 <1)

'(3 80 ;: '+-w 75

70 ---- Coulombic -+- Voltage __,..,._ Energy

65

60 40 60 80 100 120 140 160 180 200 220 240

Cycle number

Figure 6.26 Long-term stability test on the vanadium redox flow cell with

thermal and AgN03 treated GFD 2 graphite felt electrodes.

Charge/discharge current density: 100 mNcm2; upper and lower

voltage limits are 1.75 V and 0.8 V respectively. Other conditions

same as for Figure 6.22 .

303

would give further information on the stability of the electrode, however, the

results obtained do indicate that the silver catalyst is a successful electrocatalyst

for the vanadium redox cell applications. A more stable membrane which has

shown excellent performance in the vanadium redox cell has been developed in

the laboratory[188,189], however, more extensive long-tern tests on the catalysed

electrode are beyond the scope of the present project.

6.4 Summary

Depending on precursors and processing conditions, the characteristics of different

graphite felts vary considerably. PAN based felt exhibited better electrical

conductivity and electrochemical activity compared with rayon based graphite felt.

The PAN based GFD 2 graphite fibres have clean and flat surfaces, and also have

a uniform structure in the cross-section of the individual fibre, while the rayon

based FMI felt shows a dusty and uneven surface. In addition, the cross-section of

individual FMI felt fibres exhibited a bundle structure. The latter is believed to be

less graphitised and thus have more edge planes and higher defect sites which

result in a higher capability to interact with oxygen.

It was discovered from cyclic voltammetric studies that the FMI graphite felt is

more readily oxidised during anodic electrolysis than the GFD 2 felt, resulting in

deactivation of the felt for vanadium redox cell. XPS analysis revealed that the

304

FMI felt readily forms carbonyl (C=O) functional groups during thermal treatment

in air while the GFD 2 felt favoured the formation of alcohol (C-OH) groups.

When operated as positive electrodes under normal cell charge/discharge

conditions, both felts exhibited a significant increase in oxygen content on surface.

Cls curve fitting indicates that there are four types of carbon-oxygen groups

formed on the graphite felt surface. The overcharged samples show a high surface

concentration of -CQ3• groups which is believed to be the reason for electrode

deterioration.

Excellent cell performance can be achieved by using silver as the electrocatalyst

for the negative electrode and a thermal treated GFD 2 felt as the positive

electrode. The cell resistance decrease from 3.0 Q.cm2 for the untreated electrodes

to less than 1.5 Q.cm2 for the activated electrodes. The cell voltage efficiency at

40 mA.cm·2 increased by 12% after the successful treatment. A significant

enhancement in the maximum charge/discharge current density which will allow at

least 80% overall energy efficiency was also recorded (40 mA.cm·2 for untreated

compared with 100 mA.cm·2 for activated electrodes). Long-term test showed that

the silver electrocatalyst and thermally treated electrodes are very stable for the

vanadium redox flow battery application.

305

CHAPTER VII

CONCLUSIONS

With the aims of developing a novel carbon-plastic composite electrode for redox

flow cell applications, the present project covers the study of electrode fabrication

and evaluation, electrode kinetics and activation, and intensive investigations in

the physical and chemical properties as well as the electrocatalysis of the electrode

surfaces.

In the section of electrode fabrication and evaluation, the following conclusions

are drawn:

According to two-phase conduction theory, the electrical conductivity of a carbon

plastic composite material depends on the nature, content and particle size of the

conducting fillers. Extensive investigations have shown that a composite with a

combination of carbon black and graphite fibre gives rise to a satisfactory

electrical conductivity (around 0.5 n.cm) for electrode matrix layer applications.

LDPE based carbon-plastic composite materials with a composition of 60% LDPE,

20% graphite fibre, 20% carbon black exhibit promising electrical and

electrochemical properties, however, the mechanical properties are not satisfactory

for an electrode matrix layer for redox flow cell applications.

Carbon-HDPE composite material modified with SEBS thermo-plastic rubber, at

the same loading of conducting fillers, has been shown to have good electrical and

306

mechanical properties, as well as being impermeable.

By hot pressing a graphite felt and a metal mesh on either side, conducting plastic

electrodes can be fabricated for use in the vanadium redox cell. The electrical and

cell resistances values measured for these electrodes are comparable to those of

the commercially acquired conductive plastic/felt electrodes, i.e., Japanese Toray

electrode. Unlike the Japanese electrodes, however, these materials possess

excellent mechanical properties and are impermeable to the electrolyte.

An overall energy efficiency of 88% can be achieved with these electrodes in cell

charge/discharge testing at a charge/discharge current density of 21.7 mA.cm-2•

Long-term cyclic charge/discharge testing has been conducted over more than

5780 hours (1240 cycles) and the SEBS modified carbon-HDPE composite was

shown to be stable and is thus a reliable electrode matrix material for the

vanadium redox battery.

In the section of electrode kinetics and activation studies, it can be summarised

that:

The kinetics of the V (V)N (IV) redox couple reaction have been found to be elec

trochemically irreversible at the flat graphite electrode. Rotating disc voltammetric

studies revealed that the diffusivity of V(IV) species is independent of vanadium

concentration and the value of the diffusion coefficient is 2.14 x 10-6 cm2.s-1•

Although the exchange current density at the flat graphite electrode is low (i0 =

307

2.47 x 10·4 A.cm-2), it increased by two orders of magnitude at a graphite felt

electrode with a geometric area of 1 cm2, showing that carbon/graphite is a

suitable material for use in the vanadium redox cell.

The electrochemical behaviour of the V(III)N(II) couple at graphite electrode was

also found to be an electrochemically irreversible process and characterised by a

diffusion coefficient of 2.60 x 10·6 cm2.s-1 and a electron transfer rate constant of

3.63 x 10·4 cm.s·I, showing slow electrode kinetics at the flat graphite electrode.

Chemical treatment of graphite fibre-based composite polyethylene can result in a

surface area enhancement and improved reactivity for the vanadium ion redox

reactions. This is believed to be due to the enhancement in electrode surface area

and surface hydrophilicity.

Whilst carbon-black composite polyethylene is not an electrochemically stable

material after treatment, graphite fibre-based conducting polyethylene is. Although

significant improvements in the effective surface area have been achieved with

surface treatment of these materials, further research in seeking an alternative

activation method to obtain better electrochemical activity for the vanadium

reactions is required, since the treated carbon-plastic surface does not show

sufficient surface area for redox flow cell application.

The intensive investigation to the graphite felt as a electroactive layer for the

carbon-plastic composite electrode gives rise to the conclusions as follows:

308

Depending on precursors and processing conditions, the characteristics of different

graphite felts vary considerably. PAN based felt exhibited better electrical

conductivity and electrochemical activity compared with rayon based graphite felt.

The PAN based GFD 2 graphite fibres have clean and flat surfaces, and also have

a uniform structure in the cross-section of the individual fibre, while the rayon

based FMI felt shows a dusty and uneven surface. In addition, the cross-section of

individual FMI felt fibres exhibited a bundle structure. The latter is believed to be

less graphitised and thus have more edge planes and higher defect sites which

result in a higher capability to interact with oxygen.

It was discovered from cyclic voltammetric studies that the FMI graphite felt is

more readily oxidised during anodic electrolysis than the GFD 2 felt, resulting in

deactivation of the felt for vanadium redox cell. XPS analysis revealed that the

FMI felt readily forms carbonyl (C=O) functional groups during thermal treatment

in air while the GFD 2 felt favoured the formation of alcohol (C-OH) groups.

When operated as positive electrodes under normal cell charge/discharge

conditions, both felts exhibited a significant increase in oxygen content on surface.

Cls curve fitting indicates that there are four types of carbon-oxygen groups

formed on the graphite felt surface. The overcharged samples show a high surface

concentration of -C03- groups which is believed to be the reason for electrode

deterioration.

Excellent cell performance can be achieved by using silver as the electrocatalyst

309

for the negative electrode and a thermally treated GFD 2 felt as the positive

electrode. The cell resistance decrease from 3.0 n.cm2 for the untreatment

electrodes to less than 1.5 n.cm2 for the activated electrodes. The cell voltage

efficiency at 40 mA.cm-2 increased by 12% after the successful treatment. A

significant enhancement in the maximum charge/discharge current density which

allow at least 80% overall energy efficiency was also recorded (40 mA.cm-2 for

untreated compared with 100 mA.cm-2 for activated electrodes). Long-term test

showed that the silver electrocatalyst and thermally treated electrodes are very

stable for the vanadium redox flow battery application.

Further research work is recommended on the following aspects:

i) A study of plastic additives such as lubricants for the SEBS modified carbon

HOPE composite materials to facilitate the processing;

ii) Electrode kinetics study for porous graphite felt electrodes used in the redox

flow battery

iii) Improvement in the silver electrocatalyst to reduce the hydrogen evolution at

the negative side of the vanadium battery during the charging process.

iv) Electrocatalysis to inhibit graphite felt electrode deactivation processes during

overcharge.

310

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