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Carbon supported Pt–Pd alloy as an ethanol tolerant oxygenreduction electrocatalyst for direct ethanol fuel cells

T. Lopesa, E. Antolinia,b,*, E.R. Gonzaleza

aInstituto de Quımica de Sao Carlos, USP, C.P. 780, Sao Carlos, SP 13560-970, BrazilbScuola di Scienza dei Materiali, Chemistry, Via 25 aprile 22, 16016, Cogoleto, Genova, Italy

a r t i c l e i n f o

Article history:

Received 5 December 2007

Received in revised form

22 March 2008

Accepted 2 May 2008

Available online 16 September 2008

Keywords:

Direct ethanol fuel cells (DEFC)

Electrocatalyst

Platinum

Palladium

Oxygen reduction

* Corresponding author. Scuola di Scienza deE-mail address: ermantol@libero.it (E. An

0360-3199/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.05.030

a b s t r a c t

A carbon supported Pt–Pd catalyst with a Pt:Pd atomic ratio 77:23 was prepared by

reduction of metal precursors with formic acid and characterized by EDX, XRD and XPS

techniques. A decrease of the lattice parameter compared with that of pure Pt was

observed, indicating the formation of a Pt–Pd alloy. Tests in H2SO4 solution in the absence

of ethanol showed that the Pd-containing is slightly more active than pure Pt for the

oxygen reduction reaction (ORR). In the presence of ethanol a larger increase in over-

potential of the ORR on pure Pt than that on Pt–Pd was found, indicating a higher ethanol

tolerance of the binary catalyst. The enhanced performance at 90 �C of the direct ethanol

fuel cell with Pt–Pd/C as cathode material confirmed the results of half cell tests, and was

essentially ascribed to a reduced ethanol adsorption on Pt–Pd.

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

reserved.

1. Introduction electrochemical oxidation kinetics on the Pt/C cathode. The

Ethanol is an attractive liquid fuel for direct alcohol fuelled

systems. It is the major renewable biofuel obtained from the

fermentation of biomass, and ethanol is less toxic than

methanol. Wang et al. [1] compared the performance of fuel

cells operating on various methanol-alternative fuels. They

found that ethanol is a promising alternative fuel with an

electrochemical activity comparable to that of methanol.

In the direct ethanol fuel cell (DEFC), the ethanol fed to

the anode compartment can permeate through the electro-

lyte to the cathode, similar to what happens in the direct

methanol fuel cell (DMFC), i.e. methanol crossover. Song

et al. [2] found that the ethanol permeated to the cathode

exhibited a less serious effect on the cell performance

compared to methanol because of both its smaller perme-

ability through the Nafion� membrane and its slower

i Materiali, Via 25 apriletolini).ational Association for H

influence of ethanol crossover on the DEFC performance,

however, is not negligible [3,4]. The effect of the ethanol

concentration and the operating temperature on the ethanol

crossover rate was investigated by Andreadis and Tsiakaras

[3]. They found that the ethanol crossover rate dependence

on the ethanol feed concentration is an almost linear func-

tion presenting a maximum at about [CH3CH2OH]¼ 10 M.

They observed also that the ethanol crossover rate increases

as the temperature increases and the apparent activation

energy is about 4 kcal/mol indicating the physical nature of

the crossover process. As reported by Song et al. [4], the

method of preparation of the membrane electrode assembly

(MEA) also affects the ethanol crossover. Rousseau et al. [5]

evaluated the crossover of ethanol during the DEFC opera-

tion from the amount of reaction products of ethanol

oxidation. For Nafion� 115 and 112 membranes, the evaluated

22, 16016, Cogoleto, Genova, Italy.

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

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crossover rates at 20 �C and [CH3CH2OH]¼ 1 M were

8.66� 10�8 and 13� 10�8 mol cm�2 s�1, respectively.

Another main objective in the development of DEFCs is the

achievement of anode catalysts with high activity for ethanol

oxidation [6] and, due to the low activity of Pt for the oxygen

reduction reaction (ORR), research on cathode catalysts

alternative to pure Pt are also in progress. The requirements of

a suitable cathode material for the DEFC are an improved ORR

activity and an ethanol tolerance higher than pure Pt. The

alloys of transition metals, such as V, Cr, Co, Ti and Ni, with

platinum have been found to exhibit significantly higher

electrocatalytic activities towards the oxygen reduction reac-

tion than platinum alone in low temperature fuel cells [7–16].

These Pt–M alloy electrocatalysts improve both the perfor-

mance and the resistance to sintering and coalescence of the

nanoparticles under the operating conditions of the phos-

phoric acid and proton exchange membrane fuel cells.

Many papers were devoted to the research of platinum

based methanol tolerant cathodes for DMFCs, as reported by

Antolini et al. in a recent review [17]. The negative effect of

methanol crossover on cell performance is mitigated by using

Pt-based binary catalysts [17]. Higher methanol tolerance is

reported in the literature for non-noble metal electrocatalysts

based on chalcogenides [18–20] and macrocycles of transition

metals [21,22]. These electrocatalysts have shown nearly

the same activity for the ORR in the absence as well as in

the presence of methanol. However in methanol free elec-

trolytes, these materials did not reach the catalytic activity of

dispersed platinum.

Conversely, few works were addressed to the development

of ethanol tolerant oxygen reduction catalysts for DEFCs.

Savadogo and Rodriguez-Varela studied the catalytic activity

of carbon supported Ru [23] and unsupported Pd and Pd–Co

[24] catalysts for the ORR in an acid medium with and without

ethanol. They found that these catalysts have a high tolerance

to ethanol; their ORR activity in the absence of ethanol,

however, was considerably lower than that of Pt. In our

previous work [25], a single direct ethanol fuel cell with Pt–Co/

C as cathode catalyst performed better than the cell with Pt/C.

Recent works showed that the addition of Pd to Pt increases

the ORR activity of platinum [26–28] and that the depen-

dence of the ORR activity on the Pd content goes through

a maximum. Li et al. [26] prepared Pt–Pd/C (Pt:Pd¼ 3:1 and 1:1)

and Pt/C catalysts by a modified polyol method. They found

that the catalytic activity for the ORR of Pt–Pd/C in the Pt:Pd

atomic ratio 3:1 is improved compared with that of Pt/C or Pt–

Pd/C (1:1). They found that O2 is more readily adsorbed and

easily dissociated on the Pd-modified Pt surface. According to

the authors, this result mainly originates from the weakening

of the O–O bond on Pd-modified Pt clusters. Ye and Crooks

[27] prepared Pt–Pd bimetallic nanoparticles containing an

average of 180 atoms and composed of seven different

Pt:Pd ratios within sixth-generation, hydroxyl-terminated,

poly(amidoamine) dendrimers. Cyclic voltammetry and

rotating disk voltammetry measurements showed that the

Pt:Pd ratio of the nanoparticles determines their efficiency for

the oxygen reduction reaction (ORR). The maximum activity

for the ORR occurs at a Pt:Pd ratio of 5:1, which corresponds to

a relative mass activity enhancement of 2.4 compared to

otherwise identical monometallic Pt nanoparticles. Finally, Xu

and Lin [28] found that an electrodeposited Pt–Pd (9:1) catalyst

presents significantly higher stability and catalytic activity for

both the methanol oxidation reaction (MOR) and the ORR than

the corresponding electrodeposited Pt.

On this basis, a carbon supported Pt–Pd catalyst was

prepared by reduction of metal precursors with formic acid.

This synthesis method was successfully used to prepare

carbon supported Pt–Sn alloy catalysts [29]. In a previous work

carried out in our laboratory on a Pt–Pd/C catalyst with a Pt:Pd

atomic ratio 9:1 [30], it was found that this Pd-containing

catalyst has higher ethanol tolerance than Pt/C under ORR

operation. Moreover, a single DEFC with Pt–Pd (9:1) catalyst as

cathode material presented better performance than that

with Pt. Preliminary results indicated that a DEFC with a

Pt–Pd/C catalyst with Pt:Pd atomic ratio 3:1 as cathode mate-

rial performs better than that with Pt–Pd/C (9:1). On this basis

we selected to focus this work on the properties of Pt–Pd/C

(3:1) as oxygen reduction, ethanol tolerant catalyst for DEFCs.

2. Experimental

2.1. Catalyst preparation

A carbon supported Pt–Pd catalyst with nominal Pt:Pd atomic

ratio 75:25 was prepared by reduction of metal precursors

with formic acid. An appropriate amount of carbon powder

(Vulcan XC-72, Cabot, 240 m2 g�1) was suspended in 2 M for-

mic acid solution and the suspension heated to 80 �C. Chlor-

oplatinic acid (H2PtCl6$6H2O, Johnson Matthey) solution and

a palladium chloride (PdCl2$2H2O, MERCK) solution were

slowly added to the carbon suspension. The suspension was

left to cool at room temperature and the solid filtered and

dried in an oven at 80 �C for 1 h. The material was 20 wt.%

metal (Ptþ Pd) on carbon.

2.2. Characterization of the catalyst

The atomic ratio of the Pt–Pd/C catalyst was determined by

the EDX technique coupled to a scanning electron microscope

LEO Mod. 440 with a silicon detector with Be window and

applying 20 keV.

X-ray diffractograms of the catalysts were obtained at the

D12A-XRD1 beam line of the Brazilian Synchrotron Light

Laboratory. Scans were done for 2q values between 20 and

100�. The lattice parameters were obtained by refining the unit

cell dimensions by the least squares method [31].

X-ray photoelectron spectroscopy (XPS) experiments were

carried out using a conventional Al-Ka radiation (1486.6 eV),

in an ultrahigh vacuum system at a base pressure of

1� 10�10 mbar, using a VSW HA 100 analyzer operated in the

fixed analyzer transmission mode with a pass energy of

58.7 eV.

2.3. Electrochemical measurements

In order to test the electrochemical behavior in sulphuric acid

solution (with and without ethanol), the electrocatalysts were

used to make gas diffusion electrodes (GDE). A diffusion layer

was made with carbon powder (Vulcan XC-72) and 15 wt.%

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polytetrafluoroethylene (PTFE) and applied over a carbon cloth

(PWB-3, Stackpole). On top of this layer, the electrocatalyst

was applied in the form of a homogeneous dispersion of

Pt–Pd/C or Pt/C, Nafion� solution (5 wt.%, Aldrich) and iso-

propanol (Merck). All electrodes were made to contain 1 mg

Pt cm�2.

The oxidation of ethanol on Pt–Pd/C and Pt/C was tested in

a direct ethanol fuel cell system fed with a 1 M ethanol solu-

tion at the anode. Hydrogen was supplied to the cathode,

which operated simultaneously as auxiliary and reference

electrode. The experiments were done at room temperature,

40 and 90 �C with a 1285 A Solartron Potentiostat connected to

a personal computer and using the software CorrWare for

Windows (Scribner).

For the DEFC studies, the electrodes were hot pressed on

both sides of a Nafion� 115 membrane at 125 �C and

50 kg cm�2 for 2 min. The Nafion� membranes were pre-

treated with a 3 wt.% solution of H2O2, washed and then

treated with a 0.5 M solution of H2SO4. The geometric area of

the electrodes was 4.6 cm2, and the anode materials were

20 wt.% Pt/C and PtRu/C (1:1) from E-TEK. The cell polarization

data at 60 �C/1 atm and 90 �C/3 atm O2 pressure were obtained

by circulating a 1 M aqueous ethanol solution at the anode.

Fig. 1 – (a) XRD patterns of the in-house prepared Pt–Pd/C

and of Pt/C from E-TEK catalysts, (b) Detail of the fcc (311)

peak.

Fig. 2 – Pt 4f XPS spectra of Pt–Pd/C and Pt/C catalysts.

3. Results and discussion

In this work the characteristics and the catalytic activity of the

Pt–Pd/C catalyst were compared with those of a commercial

Pt/C catalyst by E-TEK.

3.1. Characterization of the Pt–Pd catalyst

The Pt:Pd atomic ratio, determined by EDX measurements on

several different regions of the carbon supported Pt–Pd parti-

cles, was 77:23, near the same as the nominal composition.

The metal loading was 20 wt.%.

The XRD patterns of the carbon supported Pt–Pd and the

commercial Pt/C from E-TEK are shown in Fig. 1a. The XRD

patterns of both catalysts show the characteristic peaks of the

face-centered cubic (fcc) crystalline Pt. These diffraction peaks

are slightly shifted to higher 2q values in the Pt–Pd catalyst

with respect to the corresponding peaks in the pure Pt cata-

lyst. Detailed Pt and Pt–Pd (311) peaks are shown in Fig. 1b. The

shift of the peaks to higher angles reveals the alloy formation

between Pt and Pd, which is caused by the incorporation of Pd

in the fcc structure of Pt. The lattice parameter of the Pt–Pd

alloy, calculated from the (311) peak was 0.3908 nm, between

those of pure fcc Pt (a0¼ 0.3923 nm) and pure fcc Pd

(a0¼ 0.3890 nm). No peaks of metallic Pd or Pd oxides were

detected in the Pt–Pd catalyst, but their presence cannot be

discarded because they may be present in a very small particle

size or even in an amorphous form.

The average size of the Pt and Pt–Pd alloy crystallites was

estimated using Scherrer’s equation d¼ 0.94 k1/B(2) cos q,

where d is the average crystallite size, k1 the wavelength of the

X-ray radiation (0.15406 nm), q the angle of the (220) peak, and

B(2) is the width in radians of the diffraction peak at half

height. The calculated average crystallite sizes were 2.8 and

3.4 nm for Pt and Pt–Pd, respectively.

Fig. 2 shows the XPS Pt 4f spectra of the Pt/C and Pt–Pd/C

catalysts. Unlike the result of Xu and Lin [28], which observed

a slight shift of the Pt 4f peak towards lower angles, a slight

shift of the Pt 4f 7/2 peak towards higher angles by 0.12 eV was

observed for the Pt–Pd/C catalyst. This shift could be related to

different oxidation states of platinum, to metal–metal inter-

action, to platinum–carbon interactions, or to small cluster-

size effects. The Pt 4f spectra of Pt–Pd/C and Pt/C catalysts

were deconvoluted into three doublets of the same binding

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energy (same components) and intensity (same amount),

indicating a substantially absence of different Pt oxidation

states. Indeed, the XPS analysis of the Pt–Pd catalyst revealed

the presence of 57% Pt0, 31% PtO and 12% PtO2, while in the Pt

catalyst were present 59% Pt0, 29% PtO and 12% PtO2. As

previously reported, the particle size of the Pt–Pd/C catalyst

was larger than that of Pt/C. Being improbable that the

presence of Pd influences the platinum–carbon interactions,

the shift could be ascribed to metal–metal interactions.

The presence of metal–metal interactions should indicate

a change in the electronic structure of Pt when it is alloyed

with Pd.

3.2. The catalytic performance of Pt and Pt–Pd catalysts

3.2.1. Oxygen reductionThe experimental results regarding the ORR in H2SO4 solution

at room temperature are shown in Fig. 3. The current density

for the ORR ( jORR) is expressed in terms of mass activity which,

being the Pt loading 1 mg cm�2 for all the electrodes, is

equivalent to activity in terms of the geometric surface area.

The onset potential for the ORR and the slope of the current

density–potential plot, (dj/dE ), are slightly larger for the Pt–Pd/

C electrocatalyst in comparison to the Pt/C electrocatalysts.

This means that the activity for the ORR of the Pt–Pd/C catalyst

is slightly higher than that of Pt/C. The overpotentials of

Pt–Pd/C at a current density of 0.1 A mg�1 Pt is about 65 mV

lower than that of Pt/C. The activity enhancement observed

when using the Pt–Pd alloy electrocatalyst can be ascribed to

different factors such as changes in the Pt–Pt interatomic

distance [7] and, particularly, in the Pt electronic configuration

[13]. Toda et al. [13] proposed a new mechanism for the

enhancement of the ORR on Pt–M alloys, based on an increase

of d-electron vacancies of the thin Pt surface layer caused by

the underlying alloy. On the basis of their model, such an

increase of 5d vacancies led to an increased 2p electron

donation from O2 to the surface Pt, resulting in an increased

O2 adsorption and a weakening of the O–O bond, which

enhances the ORR. Working with X-ray absorption spectros-

copy (XAS), Mukerjee et al. [32] explained the enhanced elec-

trocatalysis of Pt-based alloys on the basis of the interplay

Fig. 3 – Oxygen reduction at room temperature in 0.5 M

H2SO4 on Pt–Pd/C and Pt/C electrocatalysts. Sweep rate

1 mV sL1. Solid line Pt/C; dashed line Pt–Pd/C.

between the electronic (Pt d-vacancy) and geometric factors

(Pt coordination number) and their effect on the chemisorp-

tion behavior of OH species from the electrolyte. As reported

by Antolini et al. [33], the activity of Pt–Co for the ORR can be

rationalized in terms of the OH adsorption on Pt, which

reduces the performance of the catalyst. It was found that the

increase of both the metal particle size and the Co content in

the alloy reduces the OH adsorption on Pt, increasing the

activity of the catalyst for the ORR.

3.2.2. Ethanol oxidationFig. 4 shows the linear sweep voltammograms for ethanol

oxidation at room temperature (Fig. 4a), 60 (Fig. 4b) and 90 �C

(Fig. 4c) on Pt–Pd/C and Pt/C catalysts. For fuel cell

Fig. 4 – Slow scan voltammograms for ethanol oxidation on

Pt–Pd/C and Pt/C electrocatalysts in 1.0 M ethanol solution

at (a) room temperature, (b) 60 and (c) 90 8C. Sweep rate

1 mV sL1.

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applications, the working potentials of interest for the cathode

are located between 0.7 and 0.9 V versus RHE. In this potential

region at room temperature and 60 �C the activity for the

ethanol oxidation reaction (EOR) of Pt–Pd/C is higher than that

of Pt/C, whereas at 90 �C the pure Pt presents a higher current

density. The different dependence of the EOR activity on

temperature of Pt–Pd and Pt is indicative of differences in the

activation energy for the reaction. The current density for the

EOR ( jEOR) can be expressed in terms of the Arrhenius equa-

tion:

jEOR ¼ A expð � Ea=RTÞ (1)

where A is a pre-exponential factor, Ea is the activation

energy, R is the gas constant and T is the absolute tempera-

ture. In a simplified model, A is related to ethanol adsorption

and Ea is the activation energy for the oxidation of ethanol

intermediates adsorbed on Pt. The Arrhenius plots of the

Pt–Pd and Pt catalysts obtained from the values of jEOR at 0.9 V

are shown in Fig. 5. The resulting A and Ea values were

200 A cm�2 and 9.4 kJ mol�1 for Pt–Pd/C, and 1007 A cm�2 and

11.3 kJ mol�1 for Pt/C, respectively. The pre-exponential factor

of Pt was about five times higher than that of Pt–Pd, while the

activation energy of Pt was 20% higher than that of Pt–Pd. The

decrease of the pre-exponential factor for the Pt–Pd/C catalyst

can be ascribed to a lower ethanol adsorption. Indeed, the

ensemble effects, where the dilution of the active component

with the catalytically inert metal changes the distribution of

active sites, open the possibility of different reaction path-

ways [34]. There are not references on the activity for ethanol

electrooxidation of Pd in acid solution, but it is known that

palladium is completely inactive for methanol electro-

oxidation in acid solution [35,36]. So it can be assumed that Pd

is inactive also for ethanol electrooxidation in acid solution.

The dissociative chemisorption of ethanol requires the exis-

tence of several adjacent Pt ensembles [37,38] and the pres-

ence of atoms of the second metal around the Pt active sites

could block ethanol adsorption on Pt sites due to the dilution

effect. Consequently, the oxidation of ethanol on the binary

electrocatalyst is more difficult. On the other hand, oxygen

adsorption, which usually can be regarded as dissociative

Fig. 5 – Arrhenius plots of the current density at 0.9 V for

Pt–Pd/C and Pt/C.

chemisorption, requires only two adjacent sites and is not

affected by the presence of the second metal.

The decrease of the activation energy for ethanol oxidation

by the presence of Pd in the catalyst can be attributed to

alloying effects. Pd acts as a catalytically enhancing agent,

modifying the electronic properties of the Pt.

3.2.3. Oxygen reduction in the presence of ethanolFig. 6 shows the ORR activity of the prepared Pt–Pd/C alloy

electrocatalyst (Fig. 6a) and the commercial Pt/C electro-

catalyst (Fig. 6b) in 0.5 M H2SO4 at room temperature in the

absence and in the presence of ethanol concentrations from

0.5 to 2 M. The value of the current density is positive when it

is due to the ethanol oxidation, while it is negative when

related to the oxygen reduction. As compared to the ORR in

pure ethanol-free H2SO4 solution, in the presence of ethanol

both Pt/C and Pt–Pd/C showed an increase in overpotential for

the same current density. Fig. 7 shows the current density for

the EOR versus ethanol concentration at 1 V, where the

current density for the ORR is near zero. For comparison, the

current density for ethanol oxidation at [CH3CH2OH]¼ 1 M in

the absence of oxygen is also reported. Unlike that in O2-free

environment, in the presence of oxygen the current density

for ethanol oxidation is always higher on Pt than on Pt–Pd.

Fig. 6 – Oxygen reduction at room temperature in 0.5 M

H2SO4 containing different amounts of ethanol. (a) Pt–Pd/C,

(b) Pt/C. Sweep rate 1 mV sL1. Ethanol concentration: solid

line 0.0 M, dashed line 0.1 M, dotted line 0.5 M, dashed

dotted line 1 M, dashed dotted line 2 M.

Fig. 7 – Slow scan voltammograms for ethanol oxidation in

the presence of O2 at 1 V in 0.5 M H2SO4 containing

different amounts of ethanol. Sweep rate 1 mV sL1.

Fig. 9 – Polarization curves in a single DEFC with Pt–Pd/C

and Pt/C electrocatalysts as cathode materials for oxygen

reduction at 60 8C/1 atm and 90 8C/3 atm O2 pressure using

a 1 M ethanol solution. Cathode Pt loading 1 mg cmL2. (a)

Pt/C E-TEK as anode material, (b) Pt–Ru/C (1:1) E-TEK as

anode material. Anode Pt loading 1 mg cmL2.

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Indeed, in the presence of O2 the most part of the adsorbed

ethanol intermediates is oxidized and, as a consequence,

being the amount of adsorbed ethanol intermediates on pure

Pt higher than that on Pt–Pd, as previously reported, the

current density due to ethanol oxidation on Pt is higher than

that on PtPd.

The dependence of the current density j on ethanol

concentration at 0.8 V, where j¼ jORRþ jEOR, is shown in Fig. 8.

The increase of the current density on the Pt/C electrocatalyst

with increasing ethanol concentrations is higher than that on

the alloy, showing that the Pt–Pd/C electrocatalyst has a better

tolerance to the presence of ethanol than Pt/C under ORR

operating conditions.

3.2.4. Direct ethanol fuel cell testEven if voltammetry experiments are very useful to test the

activity of an electrocatalyst, it is necessary to determine the

performance of the same catalyst in a complete fuel cell.

Working conditions such as temperature, pressure and fuel

flow are crucial to determine the real performance of

a system. The DEFC polarization curves at 60 and 90 �C are

shown in Fig. 9 with Pt/C (Fig. 9a) and Pt–Ru/C (Fig. 9b) as anode

Fig. 8 – Dependence of the current density at 0.8 V on

ethanol concentration under O2 reduction in 0.5 M H2SO4.

materials. At 60 �C the performance of the cell with Pt–Pd/C is

about the same than that of the cell with Pt/C as cathode

material. At 60 �C the ethanol crossover is negligible, so an

eventual difference in cell performance has to be ascribed to

differences in ORR activity. Therefore, it can be deduced that

the effect of the Pd presence on the ORR activity of platinum is

negligible in DEFC operation. This difference between

measurements taken in the fuel cell environment and in half-

cell experiment can be related to the experimental conditions,

mainly the working temperature. Otherwise, at 90 �C an

enhancement in the performance of the cells with Pt–Pd/C

with respect to the cells with Pt/C was observed. As ethanol

crossover increases with increasing temperature [3], the

better performance of the cell with Pt–Pd/C can be ascribed to

the higher ethanol tolerance of the binary catalyst than Pt/C.

The difference in the performance of the cells with Pt–Pd/C

(3:1) and Pt/C decreases with increasing the current density

(see Fig. 9b), in agreement with the results of Andreadis and

Tsiakaras, which found that the ethanol crossover decreases

for increasing current densities [3]. The values of the

maximum power density (MPD) of cells with different anodes

and cathodes at 90 �C/3 atm are reported in Fig. 10. The

maximum power density increases 250% in going from Pt to

Fig. 10 – Histograms of the maximum power density of

direct ethanol fuel cells operating at 90 8C with Pt/C and

Pt–Pd/C as cathode materials, and Pt/C and Pt–Ru/C as

anode materials.

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Pt–Ru as anode material, and 70% in going from Pt to Pt–Pd as

cathode material. The gain in cell performance in going from

a cell using pure Pt both as anode and cathode material to

a cell with Pt–Ru and Pt–Pd as anode and cathode materials,

respectively (DMPDTot¼ 15.3 mW cm�2) is about the sum of

the gains in the performance of the cell with Pt–Ru as

anode and Pt as cathode (DMPD¼ 10.8 mW cm�2), and the

cell with Pt as anode and Pt–Pd as cathode material

(DMPD¼ 3.3 mW cm�2). As reported by Lopes et al. [25], tests

in a DEFC at various temperatures showed an enhancement of

the cell performance when Pt–Co/C (3:1) was used as cathode

material with respect to the cell with Pt/C, both in terms of

mass activity and in terms of specific activity. Considering

that Pt–Co/C and Pt/C have the same activity for the EOR in the

cathodic potential region, the improvement was ascribed to

the higher ORR activity of the binary alloy catalyst. The gain in

the performance of the cell operating at 90 �C/3 atm with

Pt as anode and Pt–Co/C as cathode material with respect to

the cell with Pt/C both as anode and cathode material

(DMPD¼ 2.8 mW cm�2) was slightly lower than that observed

in this work comparing the performance of DEFCs with Pt/C

and Pt–Pd/C as cathode materials. In the former case,

however, the enhanced performance of the cell with Pt–Co/C

as cathode material with respect to that with Pt/C was

ascribed to the higher ORR activity of the Co-containing

catalyst.

On the basis of the experiments in H2SO4 solution

described above, the poorer performance of the cell operating

at 90 �C with Pt/C catalyst with respect to that with Pt–Pd/C as

cathode material can be essentially ascribed to Pt poisoning

owing to ethanol crossover. As shown in this work, Pt

poisoning and, as a consequence, the decrease in DEFC

performance due to ethanol crossover can be reduced using

an ethanol tolerant catalyst.

4. Conclusions

The activity for the oxygen reduction reaction on carbon

supported Pt–Pd electrocatalysts prepared by reduction of

metal precursors with formic acid was investigated in

sulphuric acid both in the absence and in the presence of

ethanol and compared with a commercial Pt/C catalyst. In

ethanol-free sulphuric acid the Pt–Pd/C alloy catalyst showed

a slightly higher activity towards the oxygen reduction

compared to pure platinum. In the presence of ethanol

a higher increase in overpotential of the ORR on pure Pt than

that on Pt–Pd was found, indicating a higher ethanol tolerance

of the binary catalyst. Tests in DEFC at 60 �C indicated that the

performance of the cell with Pt–Pd/C was about the same than

that of the cell with Pt/C as cathode material, while at 90 �C an

enhancement of the cell performance when Pt–Pd/C was used

as cathode material was observed with respect to the cell with

Pt/C. Considering that at 60 �C the ethanol crossover is negli-

gible, the improvement of DEFC performance at 90 �C was

ascribed to the higher ethanol tolerance of Pt–Pd/C.

Acknowledgements

The authors thank CAPES/Brazil, Progr. PVE 2007, and the

Conselho Nacional de Desenvolvimento Cientıfico e Tecnolo-

gico (CNPq, Proc. 142097/2005-5), for financial assistance to the

project. Thanks are also due to the Brazilian Synchrotron Light

Laboratory, LNLS, for helping with the physical characteriza-

tion of the catalysts.

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