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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 ) 2 0 9 7 – 2 1 0 4

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The oxidation of NaBH4 on electrochemicaly treatedsilver electrodes

Elif Sanlia, Bekir Zuhtu Uysala, Mehmet Levent Aksub,�

aFaculty of Engineering, Department of Chemical Engineering, Gazi University, Clean Energy Research Center, Ankara, TurkeybFaculty of Education, Department of Chemistry Education, Gazi University, Clean Energy Research Center, Ankara, Turkey

a r t i c l e i n f o

Article history:

Received 19 October 2007

Received in revised form

16 January 2008

Accepted 16 January 2008

Available online 28 March 2008

Keywords:

Fuel cells

Sodium borohydride

Ag electrodes

nt matter & 2008 Internane.2008.01.049

thor. Tel.: +90 312 2126470maksu@gazi.edu.tr (M.L.

a b s t r a c t

This study is related to the development of suitable anode material for the direct

borohyride fuel cells (DBFCs). The effect of electroactive Ag oxides upon the oxidation of

NaBH4 was investigated. The treatment of Ag surface with H2O2 gave a granulated compact

layer which was observed to give a very high current density and good reversibility at lower

concentration compared with the electrode treated with NaOH which resulted a fibrous

layer with a lower electrocatalytic effect. The mode of surface oxidation was found to have

a profound effect upon the behavior of the electrode. It was also observed that there is an

optimum NaBH4 concentration to obtain the maximum current density.

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

reserved.

1. Introduction

Fuel cells are alternative energy conversion devices for clean

and efficient power generation. Some fuel cells, such as polymer

electrolyte fuel cell (PEMFC), alkaline fuel cell (AFC), and

phosphoric acid fuel cell (PAFC) require gaseous hydrogen as

the fuel. At present, most fuel reforming systems and hydrogen

storage methods are still not convenient for small applications.

Recently, the DBFC using the BH�4 aqueous solution as the fuel

has been developed based on the following equation:

Anode reaction: BH�4 þ 8OH� ¼ BO�2 þ 6H2Oþ 8e; E ¼ �1:24 V,

Cathode reaction: O2 þ 4H2Oþ 8e ¼ 8OH�; E ¼ 0:4 V,

Overall reaction: BH�4 þ 2O2 ¼ BO�2 þ 2H2O; E ¼ 1:64 V.

Electrochemical reaction of direct borohydride fuel cell

(DBFC) takes place in an alkaline medium and theoretically

generates 8e per one ion of BH�4 . Borohydride has been proposed

as an alternative fuel for hydrocarbons, because of its high

oxidation capacity and energy density. Compared to other

tional Association for Hy

.Aksu).

organic fuels, which have problems such as low activity,

low capacity, toxicity, and low oxidation efficiency, BH�4is easily stored and distributed, chemically stable, and non-

combustible.

The biggest disadvantage for the wide spread use NaBH4 is

the recycle and the cost issues. There are studies for the

regeneration of the resulting metaborate [1]. The cost

effective generation of sodium borohydride has also been

widely investigated [2].

DBFC application, using H2O2 as an oxidant is based upon

the direct electro-reduction of H2O2 [3]:

Cathode reaction: 4H2O2 þ 8e ¼ 8OH�; E ¼ 0:87 V,

Overall reaction: BH�4 þ 4H2O2 ¼ BO�2 þ 6H2O; E ¼ 2:11 V.

The direct anodic oxidation of BH�4 takes place at more

negative potentials and provides a high power density than

that of hydrogen and methanol [4].

In DBFC system, hydrogen is generated via hydrolysis of

borohydride which is not a desirable reaction:

BH�4 þ 2H2O ¼ BO�2 þ 2H2; E ¼ 1:24 V.

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

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AB5-type and AB2-type hydrogen storage alloys used as the

anode materials instead of the noble metals showed high

catalytic activities in the electrochemical oxidation borohy-

dride [2,5]. Suda reported that Ag electrode deppresses the

hydrolysis reaction [6].

Different type of DBFC have been reported in the literature:

�

direct borohydride/air fuel cell [6–8,10],

�

direct borohydride/peroxide fuel cell [9,11,12], and

�

membraneless direct borohydride fuel cell [13].

This study mainly deals with the catalytic effect of Ag2O.

Oxide films on metals play an important role in many fields of

applied electrochemistry. This makes the metals with cataly-

tic surface oxides such as Ag important in many electro-

chemical applications.

Silver oxides can exist in several phases, with silver (l) oxide

being the most thermodynamically stable. Other phases can

exist at low temperatures and high oxygen partial pressures.

The anodic oxidation of Ag metal in alkaline solution

proceeds via a two step mechanisms. The first step is the

oxidation of Ag to Ag2O, which is supposed to be a diffusion

controlled reaction. Second oxidation step is the oxidation of

Ag2O to Ag2O2 which proceed via nucleation and crystal

growth process [14]:

Ag þ 2OH� ¼ AgðOHÞ�2 þ e�,

2Agþ 2OH� ¼ Ag2OþH2Oþ 2e�,

Ag2Oþ 2OH� ¼ Ag2O2 þH2Oþ 2e�.

In order to evaluate the measured impedance spectra and

to obtain kinetic data of the BH4- oxidation reaction the

reaction steps have to be translated in to an appropriate

equivalent circuits which contain various impedance ele-

ments representing the reaction steps involved [15,16]. The

double layer capacity (Cdl) of electrode surface and charge

transfer resistance (Rct) change due to the formation of

Ag/Ag2O and Ag2O/AgO couples. Ag2O layer which acts as a

catalyst is highly resistive compared to the Ag and AgO

since it gives much larger impedance loops. The increase

in Warburg impedance indicates the diffusion of reacting

species are hindered.

In our previous papers, anodic behavior of Ag electrode and

various metal electrodes in alkaline NaBH4 solution has been

studied [17–19]. It was concluded that NaBH4 was oxidized on

silver (l) oxide (Ag2O) layer. The number of the electron

transferred was calculated as 6 by using coulometric method.

Concerning the DBFC application the use of Ag electrode gives

a 6e� compared to theoretical 8e reaction. According to the

literature, Au gives 7e, Pd gives 6e and Ni gives 4e reactions

[20]. Another study reported that the number of electrons

transferred was 7 with the use of Ag anode material [21].

Fig. 1 – The cyclic voltammetric curves of clean Ag surface in

6 M NaOH blank and 6 M NaOHþ 0:1 M NaBH4 solutions.

2. Experimental

The electrochemical experiments were performed with the

use of three electrode system using Ag disc working, Pt wire

counter and Ag/AgCl reference electrodes (BAS) with CHI

660 B potentiostat–galvanostat. The surface oxidation process

of the silver electrode surface was carried with the use of

1.16 M H2O2 and 6 M NaOH solutions using CV technique. The

direct oxidation of NaBH4 was studied in 6 M NaOH (pH 13)

solution in order to prevent the hydrolysis reaction.

Electrodes were also characterized by physical methods

such as scanning electron microscopy (SEM) and X-ray

diffraction (XRD). The SEM studies were performed with the

Joel SEM and the XRD characterizations of the electrodes were

carried out in RIGAKU, D/MAX-2200 ULULTIMENT/PC model.

Electrochemical impedance spectra were obtained in the

frequency range of 10 mHz–100 kHz at open circuit voltage

using PCI4-750 Gamry Instrument. An Ag disc electrode was

used as the working electrode. Results were represented in

Nyquist plots.

3. Results and discussion

3.1. The oxidation of the NaBH4 upon the unoxidized bareAg surface

3.1.1. The effect of surface treatmentFig. 1 compares the cyclic voltammetric behavior of clean Ag

surface in 6 M NaOH blank and 6 M NaOHþ 0:1 M NaBH4

solutions.

The cyclic voltammogram obtained with clean mechani-

cally polished Ag electrode with 0.1 M NaBH4 þ 6 M NaOH

solution revealed that the oxide formation peak (peak I) of

Ag electrode shifts towards 200 mV more negative potentials

(Fig. 1). The peak observed at 0.68 V belongs to the oxidation

of NaBH4. It is seems that the oxidation of NaBH4 takes place

via catalytic Ag2O.

It was observed that the peak current values gave a linear

increase proportional to the concentration of NaBH4. The

increase in concentration caused the oxidation potential to

shift towards more positive potentials as depicted in Fig. 2.

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Fig. 2 – The change of peak currents with NaBH4 concentration on clean Ag electrode.

0.00V

-200.0 mV

-400.0 mV

0.00 s 2.00 ks 4.00 ks 6.00 ks

t (s)

E/V

(Ag/

AgC

I)

OCP clean AgOCP Ag 0.45 V 0.49 mCOCP Ag 0.45 V 7.12 mC

Fig. 3 – The OCP curves in 10�3 M NaBH4: (a) clean Ag surface, (b) the surface oxidized at 0.45 V with a charge of 7.12 mC, and

(c) the surface oxidized at 0.45 V with a charge of 0.49 mC.

30.00 kohm

20.00 kohm

10.00 kohm

0.00 kohm

-Zim

age

(ohm

)

20.00 ohm 40.00 ohm 60.00 ohm0.00 ohmZreal (ohm)

ZCURVE (EIS CLEAN Ag)ZCURVE (EIS 7.12mC 0.5 M NaOH)

II

I

Fig. 4 – The Nyquist diagrams of the clean and oxidized Ag surfaces in 10�3 M NaBH4 solution.

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3.2. The effect of surface oxidation

The next step was the determination of open circuit

potentials (OCP) of Ag surfaces. The Ag electrodes were

loaded with charges of 0.49 and 7.12 mC at 0.45 V for the

formation of Ag2O in NaBH4 containing media. The OCP

values of the clean Ag surface and the surfaces loaded with

0.49 and 7.12 mC were �138, �380 and �508 mV, respectively.

The OCP values show a shift towards negative potentials with

the charge (Fig. 3). The plateau region observed at 100 mV

showed that the oxide layer obtained with 7.12 mC was much

more stabile.

Fig. 4 shows the impedance loops related to clean and

oxidized Ag surfaces. The impedance analysis of the clean

and oxidized Ag surfaces revealed that there formed a thick

multilayer Ag2O oxide as a result of the oxidation process.

The loop obtained for the oxidized surface was much larger

than the one obtained with the clean surface in 0.001 M

NaBH4 þ 0:1 M NaOH solution. The non-closing nature of the

loops was the indication of a diffusion control and dissolution

of the oxide formed at higher frequencies. This indicates that

Fig. 5 – XRD patterns and SEM micrograph

the oxide formed upon the surface is non–compact in nature.

The equivalent circuit contains Warburg impedance in

parallel with the charge transfer resistance due to kinetic

and diffusion complications.

3.3. The effect of particle size

The Ag surface was coulometrically oxidized in 6 M NaOH and

1.16 M H2O2 for 1.5 h at 0.45 V in order to examine the effect of

the particle size of catalytic Ag2O on the oxidation of NaBH4.

Fig. 5 shows the XRD patterns and SEM micrograph of the

surface film formed upon the Ag surface after being treated in

6 M NaOH.

The XRD data showed the formation of Ag2O film on the Ag

surface treated in 6 M NaOH. In addition there is evidence of

the formation of higher Ag oxides (NaAg2O3). The SEM

micrographs show that there was a fibrous structure upon

the surface at magnifications as small as� 100 (Fig. 5).

The Ag surface was treated with H2O2 in order to eliminate

the effect of higher Ag oxides. The XRD data revealed the

formation of thick Ag2O layer and the peaks corresponding to

of the Ag surface oxidized in 6 M NaOH.

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Fig. 6 – XRD patterns and SEM micrograph of the Ag surface oxidized with H2O2.

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higher Ag oxides were disappeared (Fig. 6). The SEM micro-

graphs showed that the surface is covered with a much more

homogenous and granulated layer in complete contrast with

the fibrous structure formed in 6 M NaOH at magnifications as

big as� 5000.

Fig. 7 shows the effect of Ag surfaces treated with NaOH

and H2O2 upon the oxidation of 0.1 M NaBH4. It is clear that

the surface oxidized in with H2O2 showed a much greater

electrocatalytic activity on the oxidation of NaBH4.

However, this catalytic effect seemed to have diminished at

higher NaBH4 concentrations and both surfaces oxidized in

NaOH and H2O2 gave similar catalytic behavior (Fig. 8). This is

due to the reaction which becomes kinetically rather than

diffusion controlled at higher concentrations.

It is obvious from Fig. 9 that the highest reversibility in

current density is obtained with an NaBH4 concentration of

0.2 M. The current density gradually decreases and the quasi-

reversible cathodic peak gradually disappears at higher con-

centration. One interesting phenomenon is that the oxidation

potential shifts towards more anodic potentials at higher

concentrations. One possible explanation for that is the

increased concentration overvoltage imposed at higher con-

centrations. This shows that there is an optimum concentration

of NaBH4 to obtain the highest current density and reversibility.

The Nyguist diagrams of the oxidation of 0.1 and 1 M NaBH4

are entirely different. At lower concentrations a non-closing

loop with Warburg impedance appears indicating diffusion

controlled process (Fig. 10a). On the other hand, the Nyquist

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Fig. 7 – Oxidation of 0.1 M NaBH4 in 6 M NaOH on Ag surface oxidized with H2O2 and NaOH.

Fig. 8 – Oxidation of 1 M NaBH4 in 6 M NaOH on Ag surface oxidized with H2O2 (—) and NaOH (- - - -).

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diagram obtained with higher NaBH4 concentration (1 M,

Fig. 10b) appears to have a closed loop with inductive element.

The most quoted explanation for this low frequency inductive

behavior is an adsorption process at the electrode surface [22].

4. Conclusions

The coulometric studies revealed that the oxidation NaBH4

follows a 6e route on Ag2O surface. The reaction taking place

upon the surface is therefore:

Ag2Oþ BH�4 þ 6OH� ! 2Ag þ BO�2 þ 5H2Oþ 6e�.

The conclusions drawn from this study are as follows:

1.

The treatment of Ag surface with 6 M NaOH and 1.16 M

H2O2 results in the formation of Ag oxide layers with

different structures. The layer obtained with NaOH has a

fibrous structure while the layer obtained with H2O2 has a

highly granulated uniform appearance. This structural

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Fig. 9 – Oxidation of NaBH4 at different concentrations on Ag electrode oxidized in H2O2.

Fig. 10 – The Nyquist diagrams of: (a) 0.1 M and (b) 1 M NaBH4 oxidized on Ag surface treated in H2O2.

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difference is also reflected as a negative shift in the oxide

formation potentials of the latter electrode.

2.

The oxidation of NaBH4 appears to take place via catalytic

Ag2O oxide layer. The oxidation takes place in a much

more efficient manner especially at lower concentrations

on granulated layer obtained as a result of the treatment of

Ag surface with H2O2.

3.

There is an optimum concentration to obtain the highest

current density and the quasi-reversible behavior. The

current density seems to be reduced at higher concentra-

tions as a result of kinetically controlled reaction and

poisoning of the surface by the adsorbed products.

4.

The coulometric studies indicate that NaBH4 follows 6e

electron transfer oxidation reaction.

Acknowledgments

We are grateful to Technical Education Faculty for taking

the SEM micrographs. Thanks are also due to State Planning

Organizing (DPT) (Project : 2003K120470-33 and Project:

2001K120590) and Tubitak (Project: 106T667).

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