Journal of

www.elsevier.com/locate/jelechem

Journal of Electroanalytical Chemistry 576 (2005) 305–313

ElectroanalyticalChemistry

Methanol tolerant oxygen reduction on carbon-supportedPt–Ni alloy nanoparticles

Hui Yang a,*, Christophe Coutanceau b, Jean-Michel Leger b, Nicolas Alonso-Vante b,Claude Lamy b

a College of Chemistry and Environmental Science, Nanjing Normal University, Nanjing 210097, PR Chinab Laboratory of Electrocatalysis, UMR CNRS 6503, University of Poitiers, Poitiers 86022, France

Received 7 August 2004; received in revised form 17 October 2004; accepted 20 October 2004

Available online 24 December 2004

Abstract

The preparation of carbon-supported Pt–Ni alloy catalysts at a 40 wt% total metal loading and with high Ni content within the

alloys and their electrocatalysis for the oxygen reduction reaction has been studied. Emphasis is placed on the methanol-tolerant

oxygen reduction on as-prepared alloy catalysts and their application in direct methanol fuel cells. It was found that as-prepared

alloy catalysts have single-phase disordered structures and small particle sizes with a relatively narrow size distribution even at

40 wt% loading. As compared to pure Pt/C catalyst for oxygen reduction, such alloy catalysts exhibited enhanced electrocatalytic

activities in pure acidic electrolyte and significantly enhanced electrocatalytic activities in methanol-containing electrolyte. The high

methanol tolerance of Pt–Ni alloy catalysts during oxygen reduction could be ascribed to a lowered activity of methanol oxidation,

which may originate from the composition effect and the disordered structure of the alloy catalysts. Fuel cell tests confirmed that as-

prepared Pt–Ni alloy catalysts for oxygen reduction are more active than a commercial Pt/C catalyst with the same metal loading

and that the maximum activity was found with a Pt/Ni atomic ratio of 2:1, which is similar to results in half-cell tests.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Oxygen reduction reaction; Pt–Ni alloy; Nanoparticle; Methanol tolerance; Direct methanol fuel cell

1. Introduction

The direct methanol fuel cell (DMFC) is a good can-

didate as a power source for applications in transporta-

tion and in portable electronic devices [1,2] because

methanol is an abundant, inexpensive liquid fuel, and

it is easy to store and transport [3]. Although good pro-

gress has been made in the development of DMFCs re-cently, there are still some problems that need to be

addressed in terms of efficiency and power density,

including the poor kinetics of both the anode [3] and

0022-0728/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2004.10.026

* Corresponding author. Present address: CASI and Department of

Chemistry, The City College of New York, J-1121, 138st and Convent

Ave., NY 10031, USA. Tel.: +1 212 6506078; fax: +1 212 6506848.

E-mail addresses: huiyang65@hotmail.com, hyang@ccny.cuny.edu

(H. Yang).

cathode reactions [4] and the cross-over of methanol

from the anode to the cathode through the proton ex-

change membranes [5,6]. Methanol crossover results in

a significant loss in coulombic efficiency and voltage effi-

ciency of a DMFC because the Pt/C is commonly used

as the cathodic catalyst, and methanol would be oxi-

dized at the cathode [6,7]. To avoid this problem, one

strategy is the development of novel membranes, whichare less methanol permeable, or the modification of

existing membranes; another route is the use of oxygen

reduction catalysts, which are inactive towards metha-

nol oxidation or have a high methanol tolerance.

To avoid platinum cathode depolarization, various

transition metal macrocycles [8,9] and ruthenium-based

chalcogenides [10,11] have been tested for methanol-tol-

erant oxygen reduction because these compounds are

306 H. Yang et al. / Journal of Electroanalytical Chemistry 576 (2005) 305–313

inactive towards the oxidation of methanol. However,

the intrinsic catalytic activities of these catalysts towards

the reduction of oxygen are lower than that of the Pt

based catalysts and the long term stability under fuel cell

operating conditions at high potentials is not as good as

for Pt based catalysts. Thus, it is necessary to developnovel Pt based electrocatalysts that can catalyze the oxy-

gen reduction but limit the oxidation of methanol. Pre-

vious studies have proved that the use of some Pt alloy

catalysts could be an efficient route to meet this require-

ment because the carbon-supported Pt–Cr alloy catalyst

[12,13] and bulk Pt70Ni30 alloy catalyst [14] have been

found to exhibit a high methanol tolerance during oxy-

gen reduction in comparison to pure Pt.Aiming to increase the catalytic activity of the oxygen

reduction reaction (orr) and to lower the cost of the cat-

alysts, many investigations have shown that some Pt

based alloy catalysts, such as Pt–M, (where M = Co,

Ni, Fe and Cr) [12–28], exhibited an enhanced electro-

catalytic activity for the orr with respective to Pt alone.

Such an activity enhancement was explained by the in-

creased Pt d-band vacancy (electronic factor) [15–17]and by the favorable Pt–Pt interatomic distance (geo-

metric effect) [15,26]. Recent studies by quantum

mechanical calculations also showed that the enhanced

electrocatalytic activity of some Pt based alloy catalysts

can be rationalized by the shifting of the d-band center

increasingly away from the Fermi level [29] or by the

favorable Pt–Pt distance for the dissociative adsorption

of O2 on such catalysts [30]. Beyond just the search forPt-based alloys and quantification of their catalytic

activities for the oxygen reduction is research to exploit

the effect of the nanosized dimensions and dispersion of

the catalyst nanoparticles on the orr electrocatalytic

properties. This is particularly important for long-term

DMFC operation. It is known that Pt based alloy cath-

ode electrocatalysts are available only at low metal load-

ing, which is not quite suitable for a DMFC. Forexample, E-Tek Inc., which is one of the famous catalyst

companies in the world, only has Pt based alloy cathode

catalysts with 10 and 20 wt% metal loadings [31]. The

particle size of their alloy catalysts is much larger than

that of pure Pt catalyst with the same metal loading.

Challenges for the preparation of such Pt alloy catalysts

with high metal loading include the need for synthesis

procedures resulting in catalysts with desirable composi-tion, controlled nanoparticle size and a narrow distribu-

tion. In fact, many investigators are still searching for

efficient methods even for the preparation of pure Pt/C

catalyst. The syntheses of catalysts through steps involv-

ing carbonyl complexes, however, have been shown to

provide nanosized Pt-based alloy catalysts with a nar-

row size distribution [12,13,28,32–34].

In our previous paper [28], we have reported thepreparation of 20 wt% Pt–Ni alloy nanoparticles with

different Pt/Ni atomic ratios via a carbonyl complex

route and the effects of structure and composition of

the alloy catalysts on oxygen reduction activity in pure

acid solution. The as-prepared bimetallic catalysts with

different Pt/Ni atomic ratios showed an enhancement

by a factor of ca. 1.5–3 in the mass activity and by a fac-

tor of ca. 1.5–4 in the specific activity for the orr in pureacid solution. The maximum activity of the Pt based cat-

alysts was found with ca. 30–40 at.% Ni content in the

alloys. However, for the practical application in a

DMFC, Pt-based electrocatalysts with high metal load-

ing are highly desirable in order to reduce the inner

resistance of the catalytic layer. With the aim of improv-

ing the catalyst activity and increasing the Pt utilization

within the catalysts, in the present paper, we focused onthe preparation of Pt–Ni alloy catalysts with high total

loading (40 wt%) and high Ni contents (30–50 at.% Ni

content) and their electrocatalytic activity for metha-

nol-tolerant oxygen reduction. A comparison based on

model DMFC tests for Pt and Pt–Ni alloy nanoparticle

catalysts is also reported.

2. Experimental

2.1. Preparation of carbon supported nanosized Pt–Ni

alloy catalysts

The Pt–Ni alloy nanoparticle catalysts with 40 wt%

metal loading were prepared via a carbonyl complex

route, followed by H2 reduction at 300 �C [28]. Briefly,Pt and Ni carbonyl complexes were synthesized simulta-

neously using methanol as the solvent through the reac-

tion of Pt and Ni salts with CO at about 55 �C for 24 h

with constant mechanical stirring until the solution

turned green. The amount of sodium acetate added to

the mixture was adjusted to a sodium acetate/Pt molar

ratio of 6:1. The resultant carbonyl complexes are a mix-

ture of Pt and Ni carbonyl complexes or a Pt–Ni molec-ular complex, identified by infrared spectra [28]. After

the synthesis of Pt–Ni carbonyl complexes, Vulcan

XC-72 carbon was added to the mixture under N2 gas

flow and stirred at about 55 �C for more than 6 h. When

Vulcan XC-72 carbon was added to the Pt–Ni carbonyl

complexes, the green color of carbonyl mixture com-

pletely disappeared in a few minutes. This could result

in the bonding of Pt–Ni species onto the surface of car-bon via a surface reaction and may favor a stronger

interaction with the carbon support. After carbon was

added to the carbonyl solution, we found that the mix-

ture obtained was stable in air, conversely to the case

for pure Pt. This implies that the carbonyl route is more

feasible to prepare Pt–Ni alloy catalysts. Subsequently,

the solvent was removed and the catalyst powder was

subjected to heat treatment at different temperatures un-der nitrogen and hydrogen, respectively. We ascertained

that the alloying temperature under hydrogen was cru-

H. Yang et al. / Journal of Electroanalytical Chemistry 576 (2005) 305–313 307

cial for the formation of an alloy. When the alloying

temperature was lower than 200 �C, a mixture of pure

Pt phase and an alloy phase was found from XRD

determinations. With the increase in alloying tempera-

ture, we found a slight increase in particle size and a

slight decrease in lattice parameter. A suitable alloyingtemperature was ca. 300 �C. Such an alloying tempera-

ture is significantly lower than that given in previously

published results [35,36], which could be beneficial to

the formation of small alloy particles with a narrow par-

ticle size distribution. After heat treatment, the sample

was washed with water until no chlorine ions were de-

tected and then dried under nitrogen at about 130 �C.

2.2. Physical characterization of the nanosized Pt–Ni

alloy catalysts

X-ray diffraction (XRD) measurements of Pt-basedcarbon-supported catalysts were carried out on a Sie-

mens D5005 X-ray diffractometer (h–h) using Cu Karadiation (k = 0.15406 nm). The sample containing ca.

20 mg of catalysts was deposited on a Si wafer cut along

the (511) plane. The XRD spectra were obtained using

high resolution in the step-scanning mode with a narrow

receiving slit (0.5�) with a counting time of 15 s per 0.1�.Scans were recorded in the 2h range of 15–90�. The iden-tification of the phases was made by referring to the

Joint Committee on Powder diffraction Standards Inter-

national Center for Diffraction Data (JCPDS-ICDD)

database. The lattice parameters were simulated and re-

fined by using FullProf 98 software.

2.3. Electrode preparation and electrochemical

measurements

The performance of as-prepared Pt–Ni alloy catalysts

and a commercial Pt/C (40 wt%, E-Tek) catalyst for the

orr was evaluated firstly with a half-cell configuration

based on the linear scan voltammogram (LSV) measure-

ments. Porous electrodes were prepared as described

previously [12,13]. Ten milligram of catalysts, 0.5 mL

of Nafion� solution (5 wt%, Aldrich) and 2.5 mL ofwater were mixed ultrasonically. A measured volume

(3 lL) of this ink was transferred via a syringe onto a

Table 1

Structural parameters of the Pt/C and Pt–Ni alloy catalysts

Catalysts Metal loading

(wt%)

Pt real surface area

(m2 g�1)

Lattice par

(nm)

Pt/Ca 40 33.8 0.3923

Pt:Ni (2:1) 40 44.3 0.3837

Pt:Ni (3:2) 40 38.1 0.3821

Pt:Ni (1:1) 40 36.4 0.3804

Pt:Ni (1:1)a 10 0.3811

a From E-Tek Inc. [31].

freshly polished glassy carbon disk (3 mm in diameter).

After the solvents were evaporated overnight at room

temperature, the prepared electrode served as the work-

ing electrode. Each electrode contained about

56 lg cm�2 of the metal.

All chemicals used were of analytical grade. All thesolutions were prepared with ultra-pure water (MilliQ,

Millipore). Electrochemical measurements were per-

formed using an Autolab potentiostat/galvanostat and

a conventional three-electrode electrochemical cell. The

counter electrode was a glassy carbon plate and a mer-

cury/mercury sulfate electrode served as the reference

electrode, being connected to the working electrode

compartment by a Luggin capillary. However, all poten-tials are quoted with respect to the reversible hydrogen

electrode (RHE). The electrolyte used for half-cell mea-

surements was 0.5 M H2SO4 or 0.5 M H2SO4 + 0.5 M

CH3OH. Due to a slight contamination from the Naf-

ion� solution, the porous electrodes were cycled at 50

mV s�1 between 0.05 and ca. 1.0 V until reproducible

cyclic voltammograms were obtained, prior to any

LSV measurements. The upper potential was set to ca.1.0 V vs. RHE so that changes in the particle size and

surface composition of the catalysts could be avoided.

Note that no marked changes in the shape and size of

the CVs were observed throughout the electrochemical

measurements and that the Pt real surface area of all

the catalysts remained almost constant, indicating that

the catalysts are stable under these experimental condi-

tions. The Pt real surface areas of all the catalysts weredetermined by CO stripping voltammetry at the scan

rate of 20 mV s�1. The commonly accepted value is

484 mC cm�2 (Pt) [28]. The real surface areas of Pt

based catalysts obtained are given in Table 1. The elec-

trochemical activity for the orr was measured with the

rotating disk electrode (RDE) technique (Autolab speed

control). High purity nitrogen and oxygen were used for

deaeration of the solutions. During the measurements, agentle nitrogen or oxygen flow was maintained above

the electrolyte surface. Unless stated otherwise, all

half-cell tests were performed at a temperature of

20 ± 1 �C.The fuel cell tests in a single DMFC with a 5 cm2 geo-

metric area were carried out with a Globe Tech test

bench. Cathodes were prepared from the catalytic ink

ameter Average particle size

(nm)

Estimated Pt/Ni atomic ratio

(at.%)

4.7 (100)

3.1 67.1:32.9

3.2 61.0:39.0

3.0 54.6:45.4

3.1 57.3:42.7

308 H. Yang et al. / Journal of Electroanalytical Chemistry 576 (2005) 305–313

consisting of a mixture of Nafion� solution, isopropanol

and catalytic powder, pasted on a carbon gas diffusion

electrode according to a technique commonly used and

described previously [37]. Prior to the preparation of

the membrane electrode assembly (MEA), the electrodes

were heated at 150 �C to recast the Nafion� film. Theanode catalyst used was E-TEK 40 wt% Pt–Ru (1:1)/

C. The metal loading on each electrode was close to

2 mg cm�2. The MEAs were prepared by hot pressing

a pretreated Nafion� 117 film with an anode on one side

and a cathode on the other side at 130 �C for 90 s under

a pressure of 35 kg cm�2.

3. Results and discussion

In our previous study, we reported that the catalyst

preparation procedure via a carbonyl chemical route

seems to be an efficient method to obtain Pt–Ni alloy

nanoparticles with small particle size and a narrow par-

Fig. 1. X-ray diffraction patterns of the carbon-supported 40 wt% Pt–

Ni alloy catalysts with: (a) wide scanning; (b) fine scanning of the (111)

peak.

ticle distribution. With the aim of improving the catalyst

activity and decreasing the cost of the catalysts, we fo-

cused on the preparation of Pt–Ni alloy catalysts with

high total loading (40 wt%) and high Ni contents (30–

50 at.% Ni content). Fig. 1 shows the XRD patterns

of the carbon-supported Pt–Ni alloy catalysts with a me-tal loading of 40 wt% and different Pt/Ni atomic ratios,

heat-treated at 300 �C. The first peak located at ca. 24.8�in all the XRD patterns is associated with the Vulcan

XC-72 carbon support. The other four peaks are charac-

teristic of face-centered-cubic (fcc) crystalline Pt

(JCPDS-ICDD, Card No. 04-802), corresponding to

the planes (111), (200), (220) and (311) at 2h values

of ca. 39.8�, 46.5�, 67.8� and 81.2�, respectively; indicat-ing that all the alloy catalysts are principally single-

phase disordered structures (i.e., solid solutions). Rela-

tive to the same reflections in bulk Pt (cf. the reference

lines – solid vertical bars of Pt in Fig. 1 referring to

the Joint Committee on Powder Diffraction Data

(JCPDS-ICDD) database), the diffraction peaks for

the Pt–Ni alloy catalysts are shifted slightly to higher

2h values. The higher angle shifts of the Pt diffractionpeaks, as can be clearly seen in Fig. 1(b), reveal the for-

mation of an alloy involving the incorporation of Ni

into the fcc structure of Pt. Since XRD is mass sensitive,

a small fraction of very much larger particles within the

samples could produce the narrower diffraction peaks.

Therefore, the broad diffraction peaks, as shown in

Fig. 1, suggests that as-prepared Pt–Ni alloys exist in

small particle sizes with a relatively narrow particle sizedistribution and in a disordered form. No peak for pure

Ni or its oxides was found.

The lattice parameters of Pt–Ni alloy catalysts, which

reflect the formation of the solid solution, calculated

using the (111) crystal faces, are provided in Table 1.

The lattice parameters obtained for all the Pt–Ni alloy

catalysts are smaller than those for Pt/C and are close

to that of E-Tek Pt–Ni (1:1, 10 wt%). In fact, the de-crease in the lattice parameters within the alloy catalysts

reflects the progressive increase in the conversion of Ni

into the alloyed state. Using our previous linear relation-

ship between the lattice parameter and EDX composi-

tion (Vegard�s law behavior for solid solution) [28], the

practical composition of three Pt–Ni alloy catalysts with

40 wt% loading can be estimated and is shown in

Table 1. It was found that the practical compositionwas very close to the nominal value, confirming that

Ni is completely alloyed with Pt within all the as-pre-

pared Pt–Ni catalysts.

The average size of the Pt–Ni alloy nanoparticles was

estimated by using Scherrer�s equation [38]

d ¼ 0:94kKa1=Bð2hÞ cos hB;

where d is the average particle diameter, kKa1 is the

wavelength of the X-ray radiation (0.154056 nm), hB is

the angle of the (220) peak, and B(2h) is the width in

H. Yang et al. / Journal of Electroanalytical Chemistry 576 (2005) 305–313 309

radians of the diffraction peak at half height. The aver-

age particle sizes obtained for all the catalysts are given

in Table 1. For the sake of comparison, the data ob-

tained from E-Tek Pt/C (40 wt%) and Pt–Ni (1:1, 10

wt%) catalysts are also given. From the table, it can be

seen that the mean particle size of as-prepared Pt–Ni al-loy catalysts is much lower than that of commercial Pt/C

with the same loading and is very close to that of a E-

Tek Pt–Ni (1:1) catalyst with only 10 wt% loading. Such

average particle sizes are also smaller than that of E-Tek

Pt–Ni alloy catalysts with 20 wt% loading [18,31]. Com-

pared with the mean particle size of the Pt–Ni alloy cat-

alysts with 20 wt% loading [28], the mean particle size

obtained shows only a slight increase. The obviousadvantages of the carbonyl complex route include a lower

alloying temperature and an efficient and scalable pro-

duction of the alloy catalysts with small nanoparticles

even at a high metal loading.

Fig. 2 illustrates the orr on the Pt/C and carbon-sup-

ported Pt–Ni catalysts with 40 wt% total metal loading

under the similar experiment conditions. From the fig-

ure, the orr on all the catalysts is diffusion-controlledwhen the potential is less than 0.6 V vs. RHE and is un-

der mixed diffusion-kinetic control in the potential re-

gion between 0.6 and 0.85 V. In the Tafel region (the

potential is higher than 0.85 V) and the mixed potential

region, it was found that the orr activities on all the Pt–

Ni alloy catalysts investigated here are higher than that

on a pure Pt catalyst. From the figure, it was found

that the orr activity increases in the sequence Pt–Ni(2:1)/C > Pt–Ni (3:2)/C > Pt–Ni (1:1)/C > Pt/C when

the potential is higher than 0.80 V. The decrease in the

overpotential of the orr on a Pt–Ni (2:1)/C catalyst at

the same current density is ca. 50 mV in contrast with

that on a commercial Pt/C catalyst, which is in good

agreement with previously reported results with 20

wt% metal loading. Clearly, the compositions for the

Fig. 2. LSVs of a commercial E-Tek Pt/C and as-prepared nanosized

Pt–Ni alloy catalysts in 0.5 M H2SO4 saturated with pure oxygen at a

scan rate of 5 mV/s and a rotation speed of 2000 rpm. Current densities

are normalized to the geometric surface area.

Pt–Ni (2:1)/ and Pt–Ni (3:2)/C catalysts are in the range

of ca. 30–40 at.% Ni content, in which the maximum

activity for the orr was found previously and explained

by the more favorable Pt–Pt interatomic distance for

the dissociative adsorption of O2 [28]. The diffusion-lim-

ited current densities of the orr for the different catalystsinvestigated here are rather similar, which is different

from findings in our previous paper [28], since the diffu-

sion-limited current densities of the orr are related to the

roughness of the electrodes, the transport properties of

O2 within the different electrodes and/or the difference

in electrolytes. The different diffusion-limited currents

in our previous paper could be ascribed to the difference

in the dispersion or roughness of these catalytic elec-trodes (particle size effect). Moreover, the changes in

specific activity based on the Pt real surface area of these

Pt based catalysts for the orr are in the sequence Pt–Ni

(2:1)/C > Pt–Ni (3:2)/C > Pt–Ni (1:1)/C > Pt/C and the

kinetic enhancement on the Pt–Ni alloy catalysts is a

factor of ca. 2–6 in comparison to pure Pt/C, which de-

pends on the polarization potential. Note that the

enhancement factor for the orr on alloy catalysts with40 wt% metal loading is higher than that with 20 wt%

loading. This difference may be ascribed to the particle

size effects. In addition, we also found the open circuit

potential of the Pt–Ni alloy catalysts in oxygen-

saturated solution is about 30–80 mV higher than that

of a Pt/C catalyst, suggesting that the oxygen adsorption

on the alloy surface is more favored than on pure Pt

surface.It is known that the crossover of methanol from the

anode to the Pt based cathode can lead to a further

reduction of the cell voltage by ca. 200–300 mV, partic-

ularly when practical air flows are used in a DMFC.

Thus, it is highly desirable to develop orr electrocata-

lysts, which have a high methanol tolerance for DMFC

applications. To evaluate the orr activity in methanol-

containing electrolyte, the change in the orr activity onthe Pt/C and homemade nanosized Pt–Ni alloy catalysts

in the presence of 0.5 M CH3OH is shown in Fig. 3. As

compared to the orr in pure H2SO4 solution (cf. Fig. 2),

all the catalysts for the orr showed an increase in over-

potential in the presence of methanol. For the orr on a

pure Pt catalyst in methanol-containing solution, the

overpotential in the Tafel region increases by ca. 160–

200 mV, and depends on the current density; the onsetpotential increases by ca. 300 mV. The significant in-

crease in overpotential of the orr on a pure Pt catalyst

is definitely due to simultaneous oxygen reduction and

methanol oxidation. Obviously, the overall electrochem-

ical process is a combination of the orr and the metha-

nol oxidation reaction (mor), which leads to the

formation of a mixed potential [39,40]. By using the

Pt–Ni alloy catalysts, there is also an activity decreaseof the orr in methanol-containing electrolyte. However,

the potential loss on all the alloy catalysts is only ca.

Fig. 3. LSVs of the Pt/C and carbon-supported nanosized Pt–Ni alloy

catalysts in 0.5 M H2SO4 + 0.5 M CH3OH solution saturated with

pure oxygen at a scan rate of 5 mV/s and a rotation speed of 2000 rpm.

Fig. 4. LSVs of methanol oxidation on Pt/C and nanosized Pt–Ni

alloy catalysts in nitrogen saturated 0.5 M H2SO4 + 0.5 M CH3OH

solution at a scan rate of 5 mV/s and a rotation speed of 2000 rpm.

310 H. Yang et al. / Journal of Electroanalytical Chemistry 576 (2005) 305–313

30–60 mV in comparison to that in pure acid solution. It

is very clear that the orr activity on the Pt–Ni alloy cat-

alysts in methanol-containing solution is much higher

than that on pure Pt, indicating that the Pt–Ni alloy cat-

alysts exhibit a high methanol tolerance during the orr

in contrast to pure Pt catalyst. Meanwhile, the currentdensity of methanol oxidation in oxygen saturated solu-

tion on the alloy catalysts at high potentials (>0.85 V) is

lower than that on a pure Pt catalyst, and it decreases in

the order of Pt/C > Pt–Ni(1:1)/C > Pt–Ni(3:2)/C > Pt–

Ni(2:1)/C. In the diffusion-controlled region, it was

found that the diffusion-controlled current densities of

the orr in the presence of methanol are slightly lower

than those in Fig. 2, which could be ascribed to the factthat some of the catalytically active sites for the orr have

been blocked or covered by adsorbed intermediates

from methanol or by methanol molecules because the

methanol oxidation and oxygen reduction are occurring

in parallel at different sites of Pt based catalysts. A com-

parison of curves in Fig. 3 clearly shows that the maxi-

mum activity for the orr in methanol-containing

solution was also found with a Pt/Ni atomic ratio of2:1. The results obtained for the methanol-tolerant orr

are comparable with those obtained on Pt–Cr alloy cat-

alysts [12,13]. Furthermore, it can be seen from a com-

parison with previously published results that the orr

activity on the Pt–Ni alloy catalysts is higher than that

on the transition metal macrocycles [8,9] and on the

ruthenium based transition metal chalcogenide catalysts

[10,11], which are inactive for the oxidation of metha-nol. Recently, Li et al. [27] found that a Pt–Fe alloy cat-

alyst is very active for the orr in pure acid electrolyte but

not more active in methanol-containing electrolyte as

compared to a Pt/C catalyst. However, Shukla et al.

[41] did find that an ordered Pt–Fe alloy catalyst exhib-

ited significantly high oxygen-reduction activity in the

presence of methanol. The difference in the methanol-

tolerant orr could be ascribed to the different structures

of their catalysts.

To understand the origin of the high methanol toler-

ance of Pt–Ni alloy catalysts during the orr, methanol

oxidation in N2 saturated solution was studied under

similar experimental conditions. Fig. 4 shows the LSVsof methanol oxidation on a Pt/C and carbon-supported

nanosized Pt–Ni alloy catalyst in nitrogen saturated 0.5

M CH3OH + 0.5 M H2SO4 solution. It is found that the

methanol oxidation current densities on Pt–Ni alloy cat-

alysts are lower than that on a Pt/C catalyst and that the

methanol oxidation peaks on Pt–Ni alloy catalysts shift

slightly to more positive potentials as compared to the

Pt/C catalyst, indicating that the oxidation of methanolon the alloy catalysts is less active than that on a Pt/C

catalyst. In fact, a comparison of Figs. 3 and 4 shows

that a higher reactivity of the mor corresponds to a lower

orr activity on all the catalysts at different polarization

potentials. This fact could explain the high methanol

tolerance of the Pt–Ni alloy catalysts during the orr. It

is well established for methanol oxidation that at least

three neighboring Pt atoms in the proper crystallo-graphic arrangement are necessary to activate the

chemisorption of methanol [42,43]. For the Pt–Ni alloy

catalysts investigated here, the possibility of finding

three neighboring Pt atoms on the surface is very low

if no Pt surface enrichment takes place. This can explain

the fact that all the Pt–Ni alloy catalysts showed a low

reactivity for methanol oxidation, and thus a high meth-

anol tolerance during the orr. Therefore, the high activ-ity for methanol-tolerant oxygen reduction could be

ascribed to the composition effect and the disordered

structure of Pt–Ni alloy catalysts. Of course, a good dis-

persion of the Pt–Ni alloy catalysts in our study is also

very important to determine a high activity for metha-

nol-tolerant oxygen reduction. However, some authors

reported that the Pt–Ni alloy catalysts are more active

for methanol oxidation than a pure Pt catalyst [44].

Fig. 5. Cell voltage against current density curves recorded in a single

DMFC using different catalysts at 100 �C ðPCH3OH ¼ 1:9 bar;

PO2¼ 2:5 barÞ: (a) whole curves and (b) zoom between 0 and 200

mA cm�2.

Fig. 6. Power density against current density curves recorded in a

single DMFC using different catalysts at 100 �C ðPCH3OH ¼ 1:9 bar;

PO2¼ 2:5 barÞ.

H. Yang et al. / Journal of Electroanalytical Chemistry 576 (2005) 305–313 311

We believe that the controversy between our results and

those published is due to the difference in the pretreated

potential ranges. In the paper by Park et al. [44], the Pt–

Ni alloy electrodes were first pretreated in H2SO4 solu-

tion over a wide potential range of 0–1.6 V vs. RHE,

which may lead to the formation of surface redox Nispecies and/or the Pt surface enrichment and thus an en-

hanced methanol oxidation activity. Toda et al. [16] also

reported a Pt enrichment after the dissolution of Ni

from Pt–Ni alloy film at <1.1 V, whereas, in our experi-

ments, the potential has an upper limit of 1.0 V so that

Pt surface enrichment and/or the formation of surface

redox Ni species could be avoided. In fact, we did find

a Pt enrichment on the surface and an enhanced electro-catalytic activity for methanol oxidation on Pt–Ni alloy

catalysts if the catalytic electrodes were cycled between

0.05 and >1.2 V vs. RHE. We also found that the lower

onset potential and peak potential for methanol oxida-

tion and for CO oxidation on nanosized Pt–Ni alloy cat-

alysts could be correlated to the changes in lattice

parameters of the Pt and Pt–Ni alloy catalysts. That

is, the pretreated potential range or working potentialis crucial to obtain a high methanol tolerance during

the orr.

From the above half-cell tests, it is clear that as-

prepared Pt–Ni alloy catalysts exhibited an enhanced

catalytic activity for the orr both in the absence and

presence of methanol as compared to a Pt/C catalyst.

Here, we present a comparison of the orr activity on dif-

ferent cathode catalysts based on the performance eval-uation of a model DMFC. Note that our model DMFC

is not optimized and its performance is not as good as

some published results; however, our data should be

comparable because our fuel cell tests with different

cathode catalysts were conducted under the same exper-

imental conditions. Figs. 5 and 6 show a comparison of

the DMFC polarization curves obtained with different

cathode catalysts and the same anode catalyst at 100�C. From Fig. 5, it can be seen that the open circuit volt-

age (ocv) for Pt–Ni alloy catalysts is higher than the ocv

for pure Pt. The highest ocv was found with a home-

made Pt–Ni (2:1)/C catalyst. For a DMFC using a cat-

alyst with a Pt/Ni atomic ratio of 3:2 or 2:1, it appears

that its activity is better than that of a pure Pt catalyst

in all current ranges measured. For a DMFC using a

Pt–Ni (1:1)/C catalyst, its performance is better thanthat using E-Tek Pt/C when the current density is less

than 200 mA cm�2 (cf. Fig. 5(b)), but it becomes worse

at high current densities. Such results on Pt–Ni (1:1)/C

are somewhat inconsistent with previous half-cell mea-

surements, which may be due to some possible changes

in the surface composition and/or particle sizes of such a

catalyst occurring at high current density. Among the

cathode catalysts used, a maximum activity was foundwith a Pt–Ni (2:1)/C catalyst. The decrease in the over-

voltage with such a cathode at the same current density

is ca. 50 mV as compared to that with a Pt/C catalyst,

which is consistent with the previous half-cell measure-

ments. Fig. 6 presents a comparison of power density

against current density in DMFCs with different cath-ode catalysts. It is clear that the maximum power den-

sity was found with a Pt/Ni atomic ratio of 2:1. For

example, the maximum power density for a Pt–Ni

(2:1)/C cathode catalyst is 101.5 mW cm�2 compared

to 80.3 mW cm�2 for a Pt/C catalyst.

Fig. 7. Cell voltage (a) and power density (b) against current density

curves recorded in a single DMFC using a 40 wt% Pt–Ni (2:1)/C at

different temperatures ðPCH3OH ¼ 1:9 bar; PO2¼ 2:5 barÞ.

312 H. Yang et al. / Journal of Electroanalytical Chemistry 576 (2005) 305–313

Fig. 7 shows the effect of the working temperature on

DMFC performances using a Pt–Ni (2:1)/C catalyst.When the working temperature is increased from 50 to

100 �C, the ocv increases from 0.67 to 0.80 V, and the

maximum power density increases from 20.5 to 101.5

mW cm�2.

4. Conclusions

The oxygen reduction reaction in the absence and

presence of methanol on Pt–Ni alloy catalysts with 40

wt% metal loading and their application in direct meth-

anol fuel cell has been investigated. The as-prepared al-

loy catalysts have single-phase disordered structures and

small particle sizes with a narrow size distribution even

at 40 wt% loading. Such catalysts exhibited significantly

enhanced electrocatalytic activity for the orr in metha-nol-containing electrolytes compared to a pure Pt/C cat-

alyst. The high methanol tolerance of Pt–Ni alloy

catalysts during the orr could be explained by the low-

ered reactivity of methanol oxidation, which may origi-

nate from the composition effect and the disordered

structure of the alloy catalysts. Fuel cell tests also con-

firmed that as-prepared Pt–Ni alloy catalysts for the

oxygen reduction are more active than a Pt/C catalyst

with the same metal loading and that the best perfor-

mance was found with a Pt/Ni atomic ratio of 2:1, sim-

ilarly to results from previous half-cell tests.

Additionally, we have found a catalyst preparation pro-

cedure via the carbonyl complex route that seems to be

efficient for synthesis of Pt–Ni alloy catalysts with smallnanoparticles even at high metal loadings.

Acknowledgements

H.Y. thanks the National Natural Science Founda-

tion of China (20003005), the Natural Science Founda-

tion of Jiangsu Province, China (BQ2000009) andNational ‘‘211’’ Key Project, China, for support of this

work. The authors gratefully acknowledge the French

Fuel Cell Network, and the Ministry of Research for

their support in the framework of the OPTIMET pro-

gram (research Grant No. 00S0060).

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