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Nanostructured platinum catalyst layer prepared by pulsedelectrodeposition for use in PEM fuel cells

N. Rajalakshmi*, K.S. Dhathathreyan

Centre for Fuel cell Technology, ARC-International, 120, Mambakkam Main Road, Medavakkam, Chennai 600 100, India

a r t i c l e i n f o

Article history:

Received 8 June 2007

Received in revised form

5 May 2008

Accepted 5 May 2008

Available online 20 September 2008

Keywords:

PEMFC

Pulsed electrodeposition

Platinum

Fuel cells

Catalyst layer

* Corresponding author.E-mail address: lakshmiraja2003@yahoo.

0360-3199/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.05.100

a b s t r a c t

Nanostructured thin catalyst layer with uniform distribution of platinum particles on

a GDL useful for PEM fuel cell was obtained by preferential pulsed electrodeposition (PED)

from a dilute solution of chloroplatinic acid. A low platinum loading on the electrode was

obtained by PED method, without any loss in fuel cell performance compared with elec-

trodes prepared by conventional brush coating method. The electrodeposition was opti-

mized by varying the duty cycle and current density. The fuel cell performance was found

to be 350 mA/cm2 at an operating voltage of 0.6 V at 60 �C with hydrogen and air as reac-

tants at ambient pressure. The nanostructured thin catalyst layer showed a very less

ohmic resistance of 0.00076 mU/cm2.

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction protons and electrons [3,4]. In the conventional method,

Fuel cells are receiving considerable attention as power

sources due to their high efficiency, modular nature and

environmental acceptability [1]. Polymer electrolyte

membrane fuel cells (PEMFC), being a low temperature fuel

cells have great potential for use in, stationary, transport and

portable applications. One of the challenges facing PEMFC

commercialization is to improve the utilization of platinum

within the catalyst layer that should ultimately reduce the

platinum loading in the electrodes [2]. Ideally all the platinum

in the catalyst layer should be active for the hydrogen oxida-

tion and oxygen reduction reactions in the fuel cell. For this to

happen the fuel and oxidant must react with the catalyst at

the electrolyte–catalyst interfacial region, which is a three-

phase reaction zone. Hence the electrode needs to be engi-

neered to allow fast access of the reactants into this zone and

the electrolyte/catalyst interface must be able to transfer both

com (N. Rajalakshmi).ational Association for H

platinum catalyst supported on carbon is mixed with ion-

omers and the resulting colloidal solution is pasted or sprayed

onto a porous carbon support layer like carbon paper or cloth.

In such cases most of the platinum particles are not accessible

to the reactants because they are deposited within the porous

structure of the carbon support and hence they do not take

part in the electrochemical reaction effectively [5]. Develop-

ment of improved electrodes with high utility of platinum is in

progress in many laboratories by way of improving the cata-

lyst ink composition, extending the wet area of catalytic

region by adding a semi hydrophobic carbon powder layer

between the substrate layer and the catalyst layer, sputter

deposition of the catalyst directly onto the surface of Nafion

bonded carbon paper, introducing novel electrodeposition

methods, etc. [6–10]. Among all the methods, electrodeposi-

tion of catalysts on the carbon substrate is promising in

increasing the utility of catalysts as it can control the growth

ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

0 10 20 30 40 50 60-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

0.010100mV 200 mV 300 mV 400 mV

Cu

rren

t (A

)

Time (secs)

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 5 6 7 2 – 5 6 7 7 5673

rate of particles and uniform deposition. Electrodeposition of

platinum by DC deposition, pulse deposition and by cyclic

voltammetry onto the carbon substrate of anode and cathode

of a PEMFC have been reported [11–13]. However, the ionomer

content, which is responsible for the proton network in the

catalyst layer, needs to be optimized along with the loading of

platinum, in order to enhance the performance. In this paper

we report our results on pulsed electrodeposition method of

preparing the polymer electrolyte membrane fuel cell elec-

trodes by optimizing the ionomer content. The duty cycle of

the applied pulse current also varied to get nanostructured

thin films of catalyst layers at the interface of electrolyte and

electrode. The fuel cell performance of the electrodes

prepared by pulsed electrodeposition and by conventional

method has also been presented in these studies.

Fig. 2 – Electrodeposition at various voltages.

2. Experimental

2.1. Preparation of carbon substrate layer forelectrodeposition

The catalyst layer of the electrodes for fuel cell is prepared by

pulsed electrodeposition on a carbon substrate paper from

Ballard Advanced Materials. The substrate layer of 30 cm2 area

was pretreated for electrodeposition consisting of three steps.

In the first step, the substrate layer was treated with hydro-

phobic agent and sintered at 350 �C for about 3 h. In the

second step, it was then coated with carbon powder and teflon

solution and sintered at 350 �C for about 3 h. In the third step,

the electrodes were coated with 5% Nafion solution. This

method paves way for reducing the platinum loading as well

to create a thin film of nanostructured platinum layer on the

treated carbon substrate layer.

2.2. Electrodeposition of nanostructured thin filmcatalyst layer

Pulsed electrodeposition has been selected to prepare the

catalyst layer for PEMFC electrodes due to its advantages

compared to DC electrodeposition in terms of controlled

particle size, adhesion with the substrate, uniformity of

deposition, etc. The pulsed electrodeposition method has

three independent variables to control the deposition namely

ton

V

toff

tcycle

t (sec)

Fig. 1 – Schematic of pulse electrodeposition.

ON time, OFF time and current density. The duty cycle is the

rate determining step for pulsed electrodeposition, in addition

to mass transport, because the properties of metal deposits

can be influenced by both ON time, during which the forma-

tion of nuclei and growth of existing crystal occur and OFF

time during which deposition of ions takes place. Hence in the

pulsed electrodeposition, the rate of nuclei formation

increases and the metal deposits become smaller. A typical

pulse electrodeposition schematic is shown in Fig. 1.

For pulsed electrodeposition of platinum, chloroplatinic

acid solution of 0.1 M mixed with dilute hydrochloric acid of

0.5 M is used as plating bath. The dilute solution of metal salts

is used based on the consideration of easily cleaning up the

residues of metal salts after deposition. A three electrode

system from Autolab Potentiostat/Galvanostal Model 30 has

been used as current generator for electrodeposition in

various steps of current. Pulses of current density ranging

from 1 mA cm�2 to 4 mA cm�2 current density, over a period of

pulse time ON ranging from 20 s to 50 s with OFF time ranging

from 50 s to 60 s, The duty cycle was found to lie in the range

50–20% for the electrodes of interest. The total amount of

charge is equal to 0.2–0.5 C cm�1 for various cases depending

on the current density and charging time. Several electrode-

position procedures like pulsed waveform, reverse waveform

and square waveform were tried to deposit platinum on the

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0 60 mA40 mA20 mA

Vo

ltag

e (V

)

Time (secs)

Fig. 3 – Effect of platinisation with respect to current.

0 50 100 150 200 250 300-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8 20mA30mA

Vo

latg

e (V

)

Time (secs)

Fig. 4 – Effect of square pulse for platinisation.

0 40 80 120 160 200 240 280 320-0.2

0.0

0.2

0.4

0.6

0.8

1.010s,50s20s,50s30s,50s50s,50s

Vo

ltag

e (V

)

Time (secs)

Fig. 6 – Platinisation with respect to duty cycle for a reverse

pulse.

0.10

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 5 6 7 2 – 5 6 7 75674

substrate layer. After each deposition, the electrode was

voltammetrically cycled in 0.5 M H2SO4 from �0.2 V to 1.2 V at

a scan rate of 50 mV/s in order to remove the chlorine ions

hidden in the electrodes, to activate the electrode electro-

chemically and to evaluate the electrochemical deposition.

From the hydrogen absorption peak at 0.05–0.1 V, the elec-

trochemical deposition can be estimated and the incom-

pletely platinised electrode was moved directly into the

deposition bath for further deposition. The catalyst layers of

the electrodes were subjected to structural microanalysis

using SEM and EDAX.

2.3. Electrode membrane assembly (EMA) fabricationand fuel cell testing

The EMAs were made by hot pressing the electrodes at 130 �C

for 2 min using a 1135 Nafion membrane and inserted

between two graphite blocks having serpentine flow field

channels for hydrogen and air. The assembly was tightened

between end plates and copper current collector with the help

of bolts and nuts and pinch implementation for uniform tor-

que. The polarization curves were obtained for the electro-

deposited fuel cell electrode using a custom built test bench

facility having provision for flow control of reactants using

0 50 100 150 200 250 300

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2 20mA 30mA

Vo

ltag

e (V

)

Time (secs)

Fig. 5 – Effect of reverse pulse for platinisation.

mass flow controller, calibrated bubble humidifiers with

temperature controllers, and a DC electronic load box.

3. Results and discussion

The deposition of Pt from chloroplatinic acid is described by

the following equations [14].

PtCl2�6 þ 2e / PtCl2�

4 þ 2Cl� (1)

PtCl2�4 þ 2e / Pt þ 4Cl� (2)

The formation of platinum particles on the electrodes

during the potential sweeps catalyses the reduction of

hydrogen ions to hydrogen atoms. The behaviour of current

versus potential is shown in Fig. 2 for the platinised electrode.

Another evidence for the platinisation of the electrode can be

seen from Fig. 3, where the potential of the electrode varies

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

20mA40mANaf coatedsubstrate layer30mA

cu

rren

t (A

)

voltage(V)

Fig. 7 – Half cell cyclic voltammogram for various

electrodes, Scanrate 50 mV/s.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 5 6 7 2 – 5 6 7 7 5675

from 0.52 Vto 0.93 V, showing the increase of deposition with

various pulsing current of 20–60 mA. The duty cycle was found

to be 50%.

Figs. 4 and 5 show how the pulse current can be activated

to get a higher voltage for deposition by reversing the potential

for a certain period at the same current density. The voltage

was found to be 0.75 V for a current of 30 mA, which increased

to 1.1 V, when the pulse is reversed for 50 s at 20 mA. It was

found that both peak current density and duty cycle control

the nucleation rate and the crystal growth. The optimum

conditions of pulse electrodeposition were found to be 100 ms

Fig. 8 – (a–c) SEM, EDAX of variou

of ON time and 400 ms of OFF time for a duty cycle of 20% in

the millisecond range. By the optimization of Nafion content

in the gas diffusion layer, the catalyst coating can be

controlled and a nanostructured catalyst layer can be

obtained, in order to obtain a required loading of catalyst with

nanoparticle size, as higher Nafion content leads to higher

potential for electrodeposition. Electrodeposition occurs only

when there is electrical contact leading to high utilization. If

deposit through Nafion, where there is ionic contact, thin

layers of deposition at high loadings and high dispersions can

be obtained. In addition, more Nafion content at the GDL

s pulse deposited electrodes.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 5 6 7 2 – 5 6 7 75676

increases the potential for electrodeposition, leading to higher

loading and higher particle size. Similarly at a particular

current pulse, the ON and OFF time has been varied to opti-

mize the duty cycle as shown in Fig. 6. It was found that at the

same current, the voltage increases to 0.82 V from 0.75 V,

when the pulse is switched off for 50 s.

Fig. 7 shows the cyclic voltammogram (CV) of the half cell

experiments, conducted with the help of electrodeposited

layer as working electrode in a 0.5 M H2SO4 acid. From the CV

we can see that the unplatinised diffusion layer and the

Nafion treated unplatinised layer do not show any hydrogen

absorption and only the platinised layer shows the hydrogen

absorption, which increases with pulse current and invariably

the voltage, and the deposition of platinum is more as seen by

the increase in limiting current to 0.08 A. The pulsed electro-

deposited catalyst layers were structurally analysed by SEM as

shown in Fig. 8a–c, and found that the platinum loading on the

carbon substrate varied from 36 wt% to 71 wt% depending on

the pulse current density and duty cycle. However, the atomic

percent shows the loading of platinum in the catalyst layer,

which varied between 3.5% and 13.5%. Fig. 8b shows the

uniform distribution of platinum particles (where the atomic

percent loading is 3.5%) compared to Fig. 8a and c, where the

platinum particles are seen as agglomerates. The particle size

was found to vary between 20 nm and 40 nm. The fuel cell

polarization curve at 60 �C (with humidified hydrogen and air)

is shown in Fig. 9, and it is compared with conventional

electrodes. Although the fuel cell performance in the activa-

tion region is almost the same compared to conventional

electrodes, the electrodes prepared by pulsed electrodeposi-

tion give a better performance in the ohmic region, which can

be attributed to the structure of the catalyst layer prepared by

the pulsed electrodeposition method compared to conven-

tional methods [15].

The polarization curves were fitted to the following equa-

tion

E ¼ Eo � b logðiÞ � cðRiÞ (3)

Where Eo, b and R are open circuit voltage, tafel slope and

resistance, respectively. It was found that the tafel slope was

0 100 200 300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0 Pt/CPED

Vo

ltag

e(V

)

Current density (mA/cm2)

Fig. 9 – Fuel cell performance for H2/Air system at 60 8C,

ambient pressure, humidified reactants for conventional

and PED electrode.

18 and 47 mV/decade for the conventional and electro-

deposited electrode, respectively and the ohmic resistance

was found to be 0.0010 mU/cm2 and 0.00076 mU/cm2, respec-

tively. From the kinetic parameters obtained, one can see that

the ohmic resistance is very small compared to conventional

electrode, as the catalyst layer is very thin and having plat-

inum particles of nanostructure dimension. In conventional

electrodes although platinum particle size ranging from 3 nm

to 10 nm is being used, the real particle size in the electrode is

not the same as it gets agglomerated and the size increases

during catalyst ink preparation. But in the case of electro-

deposited catalyst layer, the particle size in the catalyst layer

is around 20 nm and the platinum loading is reduced by about

six times. In the conventional electrode 20% Pt/C is being used,

while for the electrodeposited catalyst layer 3.5% Pt is

observed, about six times reduction in catalyst requirement

with a better performance in the ohmic region due to their

low ohmic resistance. Further work is in progress to increase

the electrode area to 150 cm2 and optimize the pulse

conditions.

4. Conclusion

Nanostructured thin platinum catalyst layer for PEMFC elec-

trodes were obtained by preferential pulsed electrodeposition

with an optimized Nafion content, duty cycle and current

density. The nanostructured thin catalyst layer showed a very

less ohmic resistance of 0.00076 mU/cm2. The platinum

loading also was found to decrease by about six times

compared to conventional loading of platinum due to prefer-

ential pulsed electrodeposition during the optimization of

Nafion content. The fuel cell performance was found to be

350 mA/sq cm at 0.6 V.

Acknowledgement

The authors would like to thank Dr. G. Sundararajan, Director

(ARCI) for his support and encouragement and DST for

financial support. The authors would like to acknowledge

Dr. K. Ravichandra of ARCI, for his help in taking the SEM

pictures.

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