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 en e r g y 3 6 ( 2 0 1 1 ) 1 3 0 8 9e1 3 0 9 5

Avai lab le a t www.sc iencedi rec t .com

journa l homepage : www.e lsev ier . com/ loca te /he

A bi-functional cathode structure for alkaline-acid directethanol fuel cells

L. An, T.S. Zhao*, J.B. Xu

Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,

Hong Kong SAR, China

a r t i c l e i n f o

Article history:

Received 11 May 2011

Received in revised form

1 July 2011

Accepted 11 July 2011

Available online 19 August 2011

Keywords:

Fuel cell

Alkaline-acid direct ethanol fuel cell

Bi-functional structure

Impregnation-reduction method

Hydrogen peroxide decomposition

Nickelechromium foam

* Corresponding author. Tel.: þ852 2358 8647E-mail address: metzhao@ust.hk (T.S. Zh

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.07.025

a b s t r a c t

An issue associated with the use of hydrogen peroxide as the oxidant in the so-called

alkaline-acid direct ethanol fuel cell (AA-DEFC) is the problem of H2O2 decomposition,

which causes a significant decrease in the cathode potential. The present work addresses

this issue by developing a bi-functional cathode structure that is composed of the

nickel-chromium (NieCr) foam (functioning as the diffusion layer) with a highly dispersed

gold particles (functioning as the catalyst layer) deposited onto the skeleton of the foam.

This integrated cathode structure allows not only a reduction in H2O2 decomposition, but

also an enhancement in the species transport in the cathode of the AA-DEFC. The fuel cell

performance characterization shows that the use of the bi-functional cathode structure in

the AA-DEFC enables the peak power density to be increased to 200 mW cm�2 from

135 mW cm�2 resulting from the use of the conventional cathode.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction of PEM-DEFCs. Moreover, this problem is rather difficult to be

Ethanol is a sustainable, carbon-neutral transportation fuel

source. It is an ideal fuel for direct oxidation fuel cells for

portable and mobile applications, as it offers multiple

advantages over hydrogen and methanol, including higher

energy density and ease of transportation, storage and

handling. Hence, direct ethanol fuel cells (DEFCs) have

recently received ever-increasing attention [1e4]. DEFCs can

be divided into two types in terms of the membrane

employed: proton exchange membrane DEFCs (PEM-DEFCs)

and anion exchange membrane DEFCs (AEM-DEFCs). In the

past, efforts have been mainly made to the development of

PEM-DEFCs and significant progress has been made [5e7].

However, the sluggish ethanol oxidation reaction (EOR)

kinetics is still a main barrier that limits the cell performance

.ao).2011, Hydrogen Energy P

solved based on the acid electrolyte, even with the Pt-based

catalysts. Unlike in acid media, the EOR kinetics in alkaline

media becomes much faster than that in the acid medium. It

has been recently demonstrated that when the acid electro-

lyte was changed to alkaline one, i.e. AEM, the cell perfor-

mance could be substantially improved [8e13]. However,

a barrier that limits the performance of AEM-DEFCs is that

state-of-the-art AEMs do not allow the fuel cell to operate at

high temperatures (<60 �C) [14]. Another important parameter

that limits the performance of DEFCs operating under both

acid and alkaline media is that thermodynamically, their

theoretical voltage is low (1.14 V).

Recently, we proposed a new type of DEFC, termed as

alkaline-acid DEFC (AA-DEFC) shown in Fig. 1a, which is

composed of an alkaline anode, a membrane and an acid

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e Schematic of the alkaline-acid direct ethanol fuel

cell (AA-DEFC).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 3 0 8 9e1 3 0 9 513090

cathode [15]. The anolyte is an aqueous solution of ethanol

and sodium hydroxide, while the catholyte is an aqueous

solution of hydrogen peroxide and sulfuric acid. In addition,

a cation exchange membrane (CEM) is employed to conduct

sodium ions. On the anode, ethanol reacts with OH� provided

by NaOH according to [16]:

CH3CH2OHþ 5NaOH/CH3COONaþ 4Naþ þ 4e� þ 4H2O (1)

The produced electrons pass through an external electrical

load and arrive at the cathode. In the meantime, Naþ ions as

the charge carrier migrate from the anode to the cathode to

close the internal circuit. On the cathode, hydrogen peroxide

reacts with Hþ provided by H2SO4 and electrons to produce

water according to:

2H2O2 þ 2H2SO4 þ 4e�/2SO2�4 þ 4H2O (2)

Hence, Na2SO4 will be produced by combining Naþ and in

the cathode, which can be taken away by the cathode solution

flow. The overall reaction is obtained by combining the EOR

given by Eq. (1) and the hydrogen peroxide reduction reaction

(HPRR) given by Eq. (2), i.e.:

CH3CH2OHþ 5NaOHþ 2H2O2 þ 2H2SO4/CH3COONa

þ 2Na2SO4 þ 8H2O ð3Þ

The theoretical voltage of the above described AA-DEFC is

2.52 V, which is much higher than that of the conventional

DEFCs (1.14 V) [17]. It was demonstrated that the peak power

density of the above described AA-DEFC with a 175-mm thick

CEMwas found to be as high as 240mWcm�2 at 60 �C,which is

about 4 times higher than the performance of conventional

DEFCs reported in the literature [15]. However, an issue with

the use of the oxidant in the fuel cell is the problem of H2O2

decomposition, causing a drastic drop in the cathode potential

as a result of the following. First, H2O2 decomposition to H2O

andO2 results in both the HPRR and oxygen reduction reaction

(ORR) simultaneously taking place at the cathode, but the

potential associated with the ORR is much lower than that of

the HPRR, leading to a drastic drop in the cathode potential

[18]. Second, its decomposition will lower the H2O2 concen-

tration in the cathode catalyst layer (CL), causing the large

concentration loss. Third, the presence of gaseous O2 in the

cathode leads to an increase in the gas void fraction, lowering

the effective active area [19]. In general, H2O2 can decompose

into water and oxygen homogeneously and heterogeneously

[20]. Homogeneous decomposition represents the sponta-

neous decomposition of H2O2 due to its intrinsic instability,

and the rate of decomposition is dependent on the tempera-

ture and concentration of hydrogen peroxide, as well as the

pH value, while the heterogeneous decomposition takes place

on the surface of the catalysts, e.g.: Pt and Pd, and its

decomposition rate depends on the specific catalyst [20].

Therefore, in order to alleviate H2O2 decomposition, on one

hand, the operating temperature, H2O2 concentration, and pH

value should be optimized, lowering the homogeneous

decomposition rate [17,21]; on the other hand, the appropriate

cathode catalysts need to be developed that can promote the

direct electro-reduction of H2O2, eliminating the heteroge-

neous decomposition. Recently, Gu et al. used Pourbaix

diagrams to guide an experimental testing, and found that

gold is an effective catalyst, which canminimize gas evolution

of hydrogen peroxide [22].

In addition, due to its liquid phase, hydrogen peroxide has

much lowerdiffusivity (DH2O2 ; water ¼ 1.37� 10�9m2 s�1) [23] than

the gaseous oxidant, i.e.: oxygen (DO2 ; air ¼ 1.775 � 10�5 m2 s�1)

[24], indicating that themass transfer rate of hydrogenperoxide

is much smaller than oxygen. For this reason, the mass trans-

port resistance of liquid-phase H2O2 in the cathode electrode is

much larger than the gaseous O2. Conventionally, the electrode

for O2-based fuel cells is composed of a dense CL and a backing

layer (BL), as shown in Fig. 1a, and typically their porosities are

30% and 73%, respectively. It should be mentioned that the

porosity of the electrode is one of the key factors affecting the

mass transport [25]. Hence, a highly porous electrode is needed

for the AA-DEFC to lower the mass transport resistance. Also,

the high-porosity electrode can facilitate the produced oxygen

removal, thereby alleviating the side reaction (ORR). Based on

the understanding of the above-mentioned hydrogen peroxide

transportation and the conventional electrode structure, cata-

lyst depositiononaporoussubstrate isanattractiveapproach to

reduce the electrode polarization [26]. This approach not only

can decrease the mass transport resistance due to its high

porosity,butalsocan increase theactiveareaascomparedto the

conventional method [22]. For this reason, a variety of electro-

catalytic cathodes for the hydrogen peroxide reduction reaction

have been investigated in recent years [21,27,28]. Cao et al.

prepared an Au/Ni foam electrode with a three dimensional

networkstructureby thespontaneousdeposition for analkaline

direct NaBH4eH2O2 fuel cell, exhibiting an open-circuit voltage

of about 1.07V anda peak power density of 75mWcm�2 at 40 �C[21]. Yang et al. proposed a nanostructured Ag catalyzed nickel

cathode for an aluminum-hydrogen peroxide using an electro-

depositing technique, yielding a maximum power density of

420 mW cm�2 at 45 �C [27]. They also prepared a novel nano-

structuredPdeAgcatalyzedporouscathodefor themagnesium-

hydrogen peroxide fuel cell, resulting in a maximum power

density of 140 mW cm�2 at 50 �C [28].

In this work, we report a bi-functional cathode structure

using the impregnation-reductionmethod for an alkaline-acid

direct ethanol fuel cell (AA-DEFC). This bi-functional structure

is composed of a nickel-chromium (NieCr) foam layer with

a highly dispersed gold layer deposited onto the skeleton of

the foam. The motivation of applying the bi-functional

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 en e r g y 3 6 ( 2 0 1 1 ) 1 3 0 8 9e1 3 0 9 5 13091

structure to the cathode of the AA-DEFC is to increase the

catalytic activity, lower the H2O2 decomposition rate, and

lower the mass transport resistance. We demonstrated that

the AA-DEFC with the bi-functional electrode can yield

a much higher power density of 200 mW cm�2 than that with

the conventional one (135 mW cm�2).

2. Experimental

2.1. Preparation of the bi-functional electrode

A simple impregnation-reduction method described else-

where [29,30] was used in the present work to prepare the

bi-functional electrode. Au particles were deposited directly

onto the surface of the NieCr foam by chemical reduction of

Au precursor salt using NaBH4 as the reductant. The NieCr

foam supplied by the RECEMAT� International offers over 95%

porosity and the estimated average pore diameter of 0.4 mm.

To clean the substrate surface, the NieCr foamwas immersed

in 0.5 M sulfuric acid for 20 min to remove the oxide layer at

room temperature. After that, the NieCr foam substrate was

rinsed with deionized (DI) water to remove chemicals prior to

further use.

The bi-functional electrode was prepared by following the

steps: 1) the NieCr foam substrate with the pre-treatment was

immersed in 2.0 mM sodium citrate (Na3C6H5O7) aqueous

solution with a 10-min magnetic stirring; 2) the precursor,

2.0 mM HAuCl4 aqueous solution, was injected into the

sodium citrate aqueous solution for a 10-min magnetic stir-

ring. It should bementioned that themolar ratio of Na3C6H5O7

to HAuCl4 is equal to one in the aqueous solution; 3) the pre-

prepared 5.0 mM NaBH4 aqueous solution as the reductant

was added into the Na3C6H5O7-HAuCl4 mixed solution for

a 24-h magnetic stirring; 4) the NieCr foam substrate was

taken out and dried at 70 �C for 3 h; 5) the substrate was

weighed and then the Au loading was calculated (this proce-

dure was repeated until the Au loading was enough); 6) the

Au/NieCr was rinsed in the DI water to remove the chemicals

for 24 h and the accurate Au loading was weighed

(1.2 mg cm�2 in this work) after 3-h drying at 70 �C prior to

further use.

2.2. Membrane electrode assembly

Amembrane electrode assembly (MEA), with an active area of

1.0 cm � 1.0 cm, was comprised of a CEM between an anode

electrode and a cathode electrode. The CEM was a Nafion 117

membrane, (175 mm thick), which was treated as a cation

conductor. The procedures of treating the Nafion membrane

included [31]: i) immersing it in 10 wt.% NaOH solution; ii)

heating it to 80 �C for 1 h; and iii) washing it by the DI water

several times. The anode electrode was formed by following

the steps: i) a catalyst ink was prepared by mixing a home-

made PdNi/C with a loading of 1.0 mg cm�2, ethanol as the

solvent and 5 wt.% Nafion solution as the binder; ii) the anode

catalyst ink was stirred continuously in an ultrasonic bath for

20 min such that it was well dispersed and iii) the anode

catalyst ink was brushed onto a piece of nickel foam (Hohsen

Corp., Japan) that served as the backing layer. In this work,

a conventional cathode electrode was prepared for compar-

ison by following the steps: i) the cathode catalyst ink was

prepared by mixing a commercial 30 wt.% Au/C (Fuel Cell

Store) with a loading of 1.2 mg cm�2, ethanol as the solvent

and 5 wt.% Nafion solution as the binder; ii) the cathode

catalyst ink was stirred continuously in an ultrasonic bath for

20 min such that it was well dispersed and iii) the cathode

catalyst ink was brushed onto a piece of carbon cloth (type A,

ETEK) as the backing layer to form a cathode electrode.

2.3. Fuel-cell setup and instrumentation

As shown in Fig. 1, each MEA was fixed between an anode and

a cathode flow field. Both flow fields were made of 316L

stainless steel plate, in which a single serpentine flow

channel, 1.0 mm wide and 0.5 mm deep, was grooved by the

wire-cut technique. An alkaline solution containing 3.0 M

ethanol and 5.0 M NaOH was fed into the anode flow channel

by a peristaltic pump at a flow rate of 2.0 mL min�1; while an

acid solution containing 4.0 M H2O2 and 1.0 M sulfuric acid

(H2SO4) was fed into the cathode flow channel by another

peristaltic pump at a flow rate of 2.0 mL min�1 [17]. Addi-

tionally, the cell temperature was measured with a thermo-

couple located at the anode current collector, and two

electrical heating rods were installed in the cell fixtures to

control the operating temperature. An Arbin BT2000 (Arbin

Instrument Inc.) was employed to measure the polarization

curves. Anode polarization data for this present fuel cell were

obtained employing an Hg/HgO (MMO) reference electrode.

Cathode polarization data were derived by subtracting anode

polarization values from the respective cell polarization data.

The internal resistance of the cell was measured by the built-

in function of the Arbin BT2000. The morphologies of the

NieCr metal foam and bi-functional electrode were examined

with a scanning electron microscope (JEOL-6700F). The X-ray

diffraction (XRD) patterns were obtained with a Philips

powder diffraction system (model PW 1830).

3. Results and discussion

3.1. Characterization of the bi-functional electrode

Fig. 2 shows the SEM images of the morphology and structure

of the NieCr foam. It can be seen that the NieCr foam shows

the highly porous structure, suggesting that the mass trans-

port resistance is much lower than the conventional elec-

trode. After deposition, it is seen from Fig. 3 that the skeleton

of the NieCr foam is coated by a uniform Au layer, which can

be demonstrated by the composition analysis using XRD

shown in Fig. 4. At the higher magnification, the coated layer

has a 3-D structure and consists of fine particles, as shown in

Fig. 3b.

3.2. Cell performance comparison

Fig. 5 compares the cell performance of the AA-DEFC with

three different MEAs consisting of the same anode, the same

CEM, but different cathode electrodes, i.e.: MEA-1: PdNi/

C þ CEM þ the conventional cathode (Au/C brushed on the

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 3 0 8 9e1 3 0 9 513092

carbon cloth), MEA-2: PdNi/C þ CEM þ the substrate (NieCr

foamwithout the Au catalyst), andMEA-3: PdNi/CþCEMþ the

bi-functional cathode (Au deposited on the NieCr foam). It is

seen from Fig. 5a that the cell with the bi-functional electrode

can yield a peak power density of 200 mW cm�2, which is 48%

higher than that with the conventional one (135 mW cm�2).

Also, the cell with the bi-functional electrode yields

a maximum current density of 525 mA cm�2, which is 50%

higher than the conventional design (350 mA cm�2). The

substantially increased performance is mainly attributed to

the higher catalytic activity and the enhanced species trans-

port in the cathode. It is worth mentioning that during the

experiments, it was observed that in the cathode outlet that

the H2O2 decompositionwas indeed alleviated in the AA-DEFC

with the Au-based electrode as compared to the AA-DEFCwith

the Pt-based electrode reported earlier [16].

3.2.1. Discussion of polarization curvesAs shown in Fig. 5a, the AA-DEFC with the bi-functional

electrode can yield the higher cell performance than that

with the conventional one in the whole current density range.

It can also be seen from Fig. 5b that the improved cell

performance mainly results from the cathode contribution,

i.e.: the bi-functional structure. The reasons for this behavior

are explained as follows. Generally, in the low current density

region, the overpotential is predominated by the activation

Fig. 3 e Surface morphologies: (a) and (b) Bi-functional

electrode.

Fig. 2 e Surface morphologies: (a) and (b) NieCr foam.

polarization. Hence, it can be seen from Fig. 5c that the

cathode overpotential with the bi-functional electrode is

smaller than that with the conventional design, indicating

that the activation loss of the bi-functional electrode is lower

than the conventional one at the low current densities. This

30 40 50 60 70 80 90

Inte

nsity

2θ (degree)

NiCr

Ni

Ni

Au

NiCr

Ni

Au NiAuAu

Fig. 4 e XRD pattern of the bi-functional electrode.

Table 1e The open-circuit voltage and internal resistancewith various cathode electrodes.

MEA-1 MEA-2 MEA-3

OCV (V) 1.469 1.376 1.465

IR (U) 0.676 1.250 0.938

Fig. 5 e Cell performance comparison among various

cathode electrodes. (a) Polarization curves. (b) Anode and

cathode potentials. (c) Anode and cathode overpotentials.

0 1 2 3 40.0

0.5

1.0

1.5

Temperature: 60oCAnode: 3M EtOH+5M NaOH,2mL min-1

Cathode: 4M H2O2+1M H2SO4,2mL min-1

Z'',O

hm

Z',Ohm

MEA-1 MEA-3

Fig. 6 e Nyquist plots of the AA-DEFC impedance spectra.

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 en e r g y 3 6 ( 2 0 1 1 ) 1 3 0 8 9e1 3 0 9 5 13093

fact shows that the catalytic activity toward HPRR of the

bi-functional electrode is higher than the conventional one. At

moderate current densities, although the bi-functional elec-

trode has the higher internal resistance (IR) than the

conventional one (Table 1), the overall cathode performance

with the bi-functional electrode is higher than that with the

conventional one, as evidenced by Fig. 5b. The higher IR with

the bi-functional electrode is mainly attributed to the

increased contact resistance between the porous electrode

and the stainless steel current collector, due to that the

bi-functional electrode has an extremely high porosity (over

95%), whereas the porosity of the carbon cloth is only 73% [32].

However, its higher catalytic activity and lower mass trans-

port resistance due to the extremely high porosity not only

can compensate the increased ohmic loss, but also can

improve the cathode performance. Therefore, at moderate

current densities, the cell with the bi-functional electrode can

also yield the better performance. As for the high current

density region, it can be seen from Fig. 5c that the conven-

tional electrode has much higher overpotential as compared

to the present one, where the main overpotential is the

concentration loss. This behavior can be explained as follows.

With regard to the conventional electrode, the low porosity

and small pore size are formed in the dense CL, resulting in

a low permeability resisting the mass transport of reactants

[33]. On the other hand, as illustrated in Fig. 3, the bi-

functional electrode made of the NieCr foam has much

higher porosity and larger open pores, resulting in a higher

permeability that facilitates the species transport. In

summary, the improved cell performance is mainly attributed

to both the higher catalytic activity and the enhanced mass

transfer in the cathode.

3.2.2. EIS analysisAs mentioned above, the improved performance of the AA-

DEFC with the bi-functional electrode can be mainly attrib-

uted to the higher catalytic activity and the enhanced species

transport in the cathode. In order to further prove this point,

the electrochemical impedance spectra (EIS) for the AA-DEFC

with the bi-functional and conventional electrodes were

measured and the results are shown in Fig. 6. It is seen from

this figure that the cell with the bi-functional electrode shows

a larger impedance spectra than does the conventional one at

the high frequency, whereas the bi-functional electrode

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 3 0 8 9e1 3 0 9 513094

exhibits the smaller impedance spectra in the moderate and

low frequency regions. The arc at high frequency is related to

the internal cell resistance [34,35]. The increased contact

resistance as a result of an extremely high porosity (over 95%)

results in the increased internal cell resistance for the cell with

the bi-functional electrode. Hence, the bi-functional electrode

shows larger impedance spectra at the high frequency. At

moderate and low frequencies, the arc is mainly attributed to

the catalytic activity andmass transfer resistance. The smaller

impedance spectra at the moderate and low frequencies for

the cell with the bi-functional electrode imply that the acti-

vation loss and concentration loss are smaller than the

conventional one, meaning that both the HPRR and species

transport are enhanced with the bi-functional electrode.

Therefore, the above EIS spectra confirmed that the improved

performance with the bi-functional electrode is primarily

attributed to the higher catalytic activity and the enhanced

species transport in the cathode.

3.3. Effect of H2O2 concentrations

Fig. 7 shows the polarization curves with different hydrogen

peroxide concentrations ranging from 2.0 M to 8.0 Mwhen the

sulfuric acid concentration was fixed at 1.0 M and the anode

solution was composed of 3.0 M ethanol and 5.0 M NaOH. It

can be seen that the cell voltage first increased with the

hydrogen peroxide concentration and then decreased over the

whole current density range; the 4.0-M hydrogen peroxide

concentration exhibited the highest performance. The reason

leading to this phenomenon is explained as follows.When the

hydrogen peroxide concentration is increased from 2.0 M to

4.0 M, the cell voltage is increased. In general, increasing the

H2O2 concentration can lower the anode performance as

a result of the H2O2 crossover; in the meantime, the H2O2

transport at the cathode is also enhanced, thereby reducing

the concentration polarization loss. Moreover, it can be seen

from Fig. 7 that the reduced concentration loss not only

compensates the mixed potential in the anode resulting from

the H2O2 crossover but also improves the cell performance.

Therefore, the cell voltage improved with increasing the

hydrogen peroxide concentration from 2.0 M to 4.0 M. When

0 100 200 300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.0 M 4.0 M 8.0 M

Current Density,mA cm-2

Cel

l Vol

tage

,V

0

50

100

150

200

250Temperature: 60oCAnode: 3M EtOH+5M NaOH, 2mL min-1

Cathode: H2O2+1M H2SO4, 2mL min-1

MEA-3 Power D

ensity,mW

cm-2

Fig. 7 e Effect of H2O2 concentration on the cell

performance.

the hydrogen peroxide concentration was further increased to

8.0 M, the cell voltage is decreased. This is because when the

hydrogen peroxide concentration exceeds 4.0M, it is sufficient

to ensure the H2O2 transport in the cathode [17]. On the other

hand, the high hydrogen peroxide concentration causes

rather serious H2O2 crossover, leading to the large anode

overpotential and thus decreasing the cell performance, as

shown in Fig. 7. Therefore, the cell voltagewas degradedwhen

the hydrogen peroxide concentration was increased to 8.0 M,

resulting in an optimal hydrogen peroxide concentration that

can yield the highest performance.

4. Concluding remarks

In this work, we have developed a bi-functional cathode

structure for the alkaline-acid direct ethanol fuel cell. The bi-

functional structure is composed of the NieCr foam with

a uniform Au layer deposited onto its skeleton. The experi-

mental results indicate that the use of the bi-functional elec-

trode as the cathode of the AA-DEFC can significantly improve

the cell performance as compared with the use of the conven-

tional one. The AA-DEFC with the bi-functional electrode can

yield a peak power density of 200 mW cm�2, which is 48%

higher than thatwith theconventional one (135mWcm�2). The

improved performance with the bi-functional electrode can

mainly be attributed to the facts: i) the bi-functional structure

reducesH2O2decompositionand fast removes theproducedO2;

ii) thebi-functional structurehas awell depositedAu layeronto

the skeleton of the highly porous NieCr foam, resulting in the

higher catalytic activity and lower the species transport

resistance.

Acknowledgments

The work described in this paper was fully supported by

a grant from the Research Grants Council of the Hong Kong

Special Administrative Region, China (Project No. 623010).

r e f e r e n c e s

[1] Antolini E, Gonzalez ER. Alkaline direct alcohol fuel cells.J Power Sources 2010;195:3431e50.

[2] Zhao TS, Li YS, Shen SY. Anion-exchange membrane directethanol fuel cells: Status and Perspectives. Front EnergyPower Eng China 2010;4:443e58.

[3] Bianchini C, Shen PK. Palladium-based electrocatalysts foralcohol oxidation in half cells and in direct alcohol fuel cells.Chem Rev 2009;109:4183e206.

[4] Song S, Tsiakaras P. Recent progress in direct ethanol protonexchange membrane fuel cells (DE-PEMFCs). Appl Catal BEnviron 2006;63:187e93.

[5] Li G, Pickup PG. Analysis of performance losses of directethanol fuel cells with the aid of a reference electrode. J PowerSources 2006;161:256e63.

[6] Rousseau S, Coutanceau C, Lamy C, Leger JM. Direct ethanolfuel cell (DEFC): electrical performances and reactionproductsdistribution under operating conditions with differentplatinum-based anodes. J Power Sources 2006;158:18e24.

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 en e r g y 3 6 ( 2 0 1 1 ) 1 3 0 8 9e1 3 0 9 5 13095

[7] Zhou WJ, Song SQ, Li WZ, Zhou ZH, Sun GQ, Xin Q, et al.Direct ethanol fuel cells based on PtSn anodes: the effect ofSn content on the fuel cell performance. J Power Sources2005;140:50e8.

[8] Modestov AD, Tarasevich MR, Leykin AY, Filimonov VY. MEAfor alkaline direct ethanol fuel cell with alkali doped PBImembrane and non-platinum electrodes. J Power Sources2009;188:502e6.

[9] An L, Zhao TS, Shen SY, Wu QX, Chen R. Performance ofa direct ethylene glycol fuel cell with an anion-exchangemembrane. Int J Hydrogen Energy 2010;35:4329e35.

[10] Fujiwara N, Siroma Z, Yamazaki S, Ioroi T, Senoh H,Yasuda K. Direct ethanol fuel cells using an anion exchangemembrane. J Power Sources 2008;185:621e6.

[11] An L, Zhao TS, Shen SY, Wu QX, Chen R. An alkaline directoxidation fuel cell with non-platinum catalysts capable ofconverting glucose to electricity at high power output. J PowerSources 2011;196:186e90.

[12] Varcoe JR, Slade RCT. Prospects for alkaline anion-exchangemembranes in low temperature fuel cells. Fuel Cells 2005;5:187e200.

[13] Li YS, Zhao TS, Liang ZX. Performance of alkaline electrolyte-membrane-based direct ethanol fuel cells. J Power Sources2009;187:387e92.

[14] An L, Zhao TS, Chen R,WuQX. A novel direct ethanol fuel cellwith high power density. J Power Sources 2011;196:6219e22.

[15] An L, Zhao TS. An alkaline direct ethanol fuel cell witha cation exchange membrane. Energy Environ Sci 2011;4:2213e7.

[16] Bianchini C, Bambagioni V, Filippi J, Marchionni A, Vizza F,Bert P, et al. Selective oxidation of ethanol to acetic acid inhighly efficient polymer electrolyte membrane-directethanol fuel cells. Electrochem Commun 2009;11:1077e80.

[17] An L, Zhao TS. Performance of an alkaline-acid directethanol fuel cell. Int J Hydrogen Energy 2011;36:9994e9.

[18] Raman RK, Choudhury NA, Shukla AK. A high output voltagedirect borohydride fuel cell. Electrochem Solid-State Lett2004;7:A488e91.

[19] Cao D, Chen D, Lan J, Wang G. An alkaline directNaBH4eH2O2 fuel cell with high power density. J PowerSources 2009;190:346e50.

[20] Dow EG, Bessette RR, Seeback GL, Marsh-Orndorff C,Meunier H, VanZee J, et al. Enhanced electrochemicalperformance in the development of the aluminum/hydrogenperoxide semi-fuel cell. J Power Sources 1997;65:207e12.

[21] Cao D, Gao Y, Wang G, Miao R, Liu Y. A direct NaBH4eH2O2fuel cell using Ni foam supported Au nanoparticles aselectrodes. Int J Hydrogen Energy 2010;35:807e13.

[22] Gu L, Luo N, Miley GH. Cathode electrocatalyst selection anddeposition for a direct borohydride/hydrogen peroxide fuelcell. J Power Sources 2007;173:77e85.

[23] Miley GH, Hawkins G, Englander J. Development of a coupled2De3D fuel cell model for flow field analysis, excerpt from theproceedings of the COMSOL users conference 2006 Boston.

[24] Yang WW, Zhao TS, Xu C. Three-dimensional two-phasemass transport model for direct methanol fuel cells.Electrochim Acta 2007;53:853e62.

[25] Zhao TS, Xu C, Chen R, Yang WW. Mass transportphenomena in direct methanol fuel cells. Prog EnergyCombust Sci 2009;35:275e92.

[26] Zhang JJ. PEM fuel cell electrocatalysts and catalyst layers:fundamentals and applications. 1st ed. Guildford, Surrey, U.K.: Springer-Verlag London Ltd; 2008.

[27] Yang W, Yang S, Sun W, Sun G, Xin Q. Nanostructured silvercatalyzed nickel foam cathode for an aluminumehydrogenperoxide fuel cell. J Power Sources 2006;160:1420e4.

[28] Yang WQ, Yang SH, Sun W, Sun GQ, Xin Q. Nanostructuredpalladium-silver coated nickel foam cathode formagnesiumehydrogen peroxide fuel cells. Electrochim Acta2006;52:9e14.

[29] Fedkiw PS, Her WH. An impregnation-reduction method toprepare electrodes on Nafion SPE. J Electrochem Soc 1989;136:899e900.

[30] Liu R, Her WH, Fedkiw PS. In situ electrode formation ona nation membrane by chemical platinization. J ElectrochemSoc 1992;139:15e23.

[31] Li ZP, Liu BH, Arai K, Suda S. A fuel cell development forusing borohydrides as the fuel. J Electrochem Soc 2003;150:A868e72.

[32] Chen R, Zhao TS. Porous current collectors for passive directmethanol fuel cells. Electrochim Acta 2007;52:4317e24.

[33] Li YS, Zhao TS. A high-performance integrated electrodefor anion-exchange membrane direct ethanol fuel cells.Int J Hydrogen Energy 2011;36:7707e13.

[34] Mueller JT, Urban PM. Characterization of direct methanolfuel cells by ac impedance spectroscopy. J Power Sources1998;75:139e43.

[35] Mueller JT, Urban PM, Holderich WF. Impedance studies ondirect methanol fuel cell anodes. J Power Sources 1999;84:157e60.