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Applied Catalysis B: Environmental 63 (2006) 137–149

The methanol oxidation reaction on platinum alloys

with the first row transition metals

The case of Pt–Co and –Ni alloy electrocatalysts

for DMFCs: A short review

Ermete Antolini a,*, Jose R.C. Salgado b,1, Ernesto R. Gonzalez b

a Scuola di Scienza dei Materiali, Via 25 Aprile 22, 16016 Cogoleto, Genova, Italyb Instituto de Quımica de Sao Carlos, USP, C.P. 780, Sao Carlos, SP 13560-970, Brazil

Received 13 July 2005; received in revised form 15 September 2005; accepted 22 September 2005

Available online 8 November 2005

Abstract

In recent years there has been much activity in examining Pt alloys with first row transition metals as catalysts materials for DMFCs. In this

work, the electrochemical oxidation of methanol on Pt–Co and –Ni alloy electrocatalysts is reviewed. The effect of the transition metal on the

electrocatalytic activity of Pt–Co and –Ni for the methanol oxidation reaction (MOR) has been investigated both in half-cell and in direct methanol

fuel cells. Conflicting results regarding the effect of the presence of Co(Ni) on the MOR are examined and the primary importance of the amount of

non-precious metal in the catalyst is remarked. For low base metal contents, an enhancement of the onset potential for the MOR with increasing

Co(Ni) amount in the catalyst is observed, whereas for high contents of the base metal, a drop of the MOR onset potential with increasing Co(Ni) is

found. As well as the base metal content, an important role on the MOR activity of these catalysts has to be ascribed to the degree of alloying.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Methanol oxidation; Platinum alloy catalysts; Nickel; Cobalt; Direct methanol fuel cell

1. Introduction

The use of methanol as energy carrier and its direct

electrochemical oxidation in directmethanol fuel cells (DMFCs)

represents an important challenge for the polymer electrolyte

fuel cell technology, since the complete systemwould be simpler

without a reformer and reactant treatment steps. The use of

methanol as fuel has several advantages in comparison to

hydrogen: it is a cheap liquid fuel, easily handled, transported,

and stored, and with a high theoretical energy density [1–3].

Althougha lotofprogresshasbeenmade in thedevelopmentof

DMFC, its performance is still limited by the poor kinetics of the

anodereaction[3–5]and thecrossoverofmethanolfromtheanode

to the cathode side through the proton exchangemembrane [6–8].

Methanol oxidation is a slow reaction that requires active

multiple sites for the adsorption of methanol and the sites that

* Corresponding author. Tel.: +39 0109162880; fax: +39 0109182368.1 Present address: Instituto de Quımica, UnB, C.P. 4478, Brasilia, DF 70919-

970, Brazil.

0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2005.09.014

can donate OH species for desorption of the adsorbed methanol

residues [9]. Methanol oxidation has been extensively

investigated since the early 1970’s with two main topics:

identification of the reaction intermediates, poisoning species

and products, and modification of Pt surface in order to achieve

higher activity at lower potentials and better resistance to

poisoning. The results have been reviewed by several authors

[10–12]. The main reaction product is CO2 [13], although

significant amounts of formaldehyde [14,15], formic acid [13]

and methyl formate [15,16] were also detected. Most studies

conclude that the reaction can proceed according to multiple

mechanisms. However, it is widely accepted that the most

significant reactions are the adsorption of methanol and the

oxidation of CO, according to this simplified reaction

mechanism:

CH3OH ! ðCH3OHÞads (1)

ðCH3OHÞads ! ðCOÞads þ 4Hþ þ 4e� (2)

ðCOÞads þH2O ! CO2 þ 2Hþ þ 2e� (3)

E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149138

Platinum is the most active metal for dissociative adsorption of

methanol, but, as it is well-known, at room or moderate

temperatures it is readily poisoned by carbon monoxide, a

by product of methanol oxidation. To date, the remedy has

been to use binary or ternary eletrocatalysts based on platinum,

all containing ruthenium as the activity promoting component

[17–22]. According to the bifunctional mechanism [23,24], the

CO-poisoned platinum is regenerated via a surface reaction

between CO- and O-type species associated with ruthenium to

yield CO2. According to the ligand model [12,23,25], instead,

the change in Pt electronic properties induced by the presence

of Ru rends Pt atoms more susceptible for OH adsorption [23]

or even for dissociative adsorption of methanol [12]. But also

when Pt–Ru is used as anode electrocatalyst the power density

of a DMFC is about a factor of 10 lower than that of a proton

exchange membrane fuel cell operated on hydrogen if the same

Pt loading is used. Therefore, a number of Ru-alternative

elements, showing a co-catalytic activity for the anodic oxida-

tion of methanol, if used either as platinum alloys or as

adsorbate layers on platinum, have been investigated [26–33].

The problem of methanol crossover in DMFCs has been

extensively studied [6–8,34,35]: methanol adsorbs on Pt sites in

the cathode for the direct reaction betweenmethanol andoxygen.

The mixed potential, which results from the oxygen reduction

reaction and the methanol oxidation occurring simultaneously,

reduces the cell voltage, generates additionalwater and increases

the required oxygen stoichiometric ratio. This problem could be

solved either by using electrolytes with lower methanol

permeability or by developing new cathode electrocatalysts

with both higher methanol-tolerance and higher activity for the

oxygen reduction reaction (ORR) than Pt. Higher methanol-

tolerance is reported in the literature for non-noble metal

electrocatalysts based on chalcogenides [35–38] and macro-

cycles of transition metals [39,40]. These electrocatalysts have

shownnearly the same activity for theORR in the absence aswell

as in the presence of methanol. However in methanol-free

electrolytes, thesematerials did not reach the catalytic activity of

dispersed platinum. Developing a sufficiently selective and

active electrocatalyst for the DMFC cathode remains one of the

key tasks for further progress of this technology. The current

direction is to test the activity for the oxygen reduction reaction in

the presence of methanol of some Pt alloys with the first row

transitionmetalswhich present a higher activity for theORR than

platinum in low temperature fuel cells operated on hydrogen, and

use them as DMFC cathode electrocatalysts [41–45]. The

improvement in the ORR electrocatalysis has been ascribed to

different factors such as changes in the Pt–Pt interatomicdistance

[46] and the surface area [47]. But the behaviour of binary alloys

with respect to electrocatalysis can be better understood in terms

of the electronic ‘‘ligand effect’’ and/or the geometric ‘‘ensemble

effect’’. To rationalise these effects it is necessary to know

precisely the local concentration and arrangement of both

components at the very surface (in contact with the reactants),

and also in the sublayers which influence electronically the outer

atoms [48]. The electronic effect of elements present in the

sublayers is illustrated on PtNi (1 1 1) and Pt3Fe (1 1 1), which

present a quasi-complete Pt surface layer (withmore or less Ni or

Fe in the sublayers) and strong modifications of their

chemisorptive properties and electrocatalytic performances

[48]. This behaviour was attributed to the electronic effect of

intermetallic bonding of the alloying component-rich second

layer with the top-most Pt atoms. The electrocatalytic behaviour

of Pt alloyswith increasing contents of the second element can be

explainedby themodel ofToda et al. [49], based on an increase of

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

underlying alloy.

The ensemble effects where the dilution of the active

component with the catalytically inert metal changes the

distribution of active sites, open different reaction pathways

[50]. The dissociative chemisorption of methanol requires the

existence of several adjacent Pt ensembles [51,52] and the

presence of atoms of the second metal around Pt active sites

could block methanol adsorption on Pt sites due to the dilution

effect. Consequently, methanol oxidation on the binary-

component electrocatalyst is suppressed. On the other hand,

oxygen adsorption, which usually can be regarded as

dissociative chemisorption, requires only two adjacent sites

and is not affected by the presence of the second metal.

Pt–Ni and –Co alloy catalysts have been proposed both as

methanol-tolerant cathode material and anode material with

improved MOR for DMFCs. The choice of Co and Ni to modify

Pt electrocatalyst to improve the MOR is due to the lowering of

the electronic binding energy in Pt by alloying with thesemetals,

promoting theC–H cleavage reaction at lowpotential.Moreover,

the presence of cobalt or nickel oxides provides an oxygen source

for CO oxidation at lower potentials. On the other hand, a higher

methanol-tolerance is expected on Pt–Co and –Ni alloy catalysts

than on Pt, ascribed to the dilution effect of Pt, hindering the

methanol adsorption. Furthermore, these alloys present an imp-

roved activity for the oxygen reduction thanPt alone.On thebasis

of this discrepancy, we will attempt to outline the electro-

chemical activity for the MOR of Pt–Ni and –Co alloy catalysts.

2. Structural characterization of Pt–Co and –Ni alloys

In the composition range from 0 to 50 at.% Co(Ni), Pt and

Co(Ni) form a substitutional continuous solid solution and two

ordered phases [53–56]. The dependence of the lattice

parameter of the Pt–Co and –Ni bulk alloys on the alloy

composition is reported in Fig. 1. In the region of 75 at.% Pt

there are face-centered cubic (fcc) superlattices Pt3Co and

Pt3Ni of the Cu3Au (LI2) type. Regular termination of the bulk

LI2 structure normal to the three major zone axes produces a

variety of surface compositions, from the pure Pt ((2 0 0) and

(2 2 0) planes), 25 at.% Co(Ni) ((1 1 1) plane) to 50 at.%

Co(Ni) ((1 0 0) and (1 1 0) planes) [57].

To better understand the relationship between the surface

composition and the catalytic activity, it is very important to

determine if surface segregation, i.e. enrichment of one element

at the surface relative to the bulk, takes place during the

preparation of these alloy catalysts. The details of segregation

are still not completely understood, especially in the case of

segregation in nanoparticles in which the characteristics may

differ from those of the bulk. This is not surprising, considering

E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149 139

Fig. 1. Dependence of the Pt–Co and –Ni lattice parameters [53–56] on the

Co(Ni) content in the alloy.

that nanoclusters represent a finite quantity of material, so there

is no infinite source/sink of constituent atoms and hence

material balance constraints become important [58]. Conflict-

ing results regarding the surface segregation on Pt–Co and –Ni

alloys are reported in literature. XPS data by Shukla et al. [45]

indicated some surface enrichment of base metal in Pt–Co/C

(atomic ratio 0.84 versus 0.72 in the bulk) and Pt–Ni/C (0.86

versus 0.64) prepared by alloying at high-temperature. Paulus

et al. [59] found 70 at.% Pt on the surface of the Pt3Ni particles

and 58 at.% Pt on the surface of the Pt3Co particles for the

Pt3Ni(Co) catalyst by E-TEK. The value for Pt3Ni is very close

to the bulk composition of 75 at.% Pt and indicates that no

segregation has taken place, whereas in case of the Pt3Co a

slight segregation of Co to the surface is observed. For the Pt–

Ni (1:1) catalyst by E-TEK, however, they found only about

20 at.% Pt on the surface Pt and for the Pt–Co (1:1) catalyst by

E-TEK 35 at.% Pt on the surface, indicating Ni(Co) segregation

to the surface. Park et al. [60] observed enrichment of Pt at the

surface of Pt–Ni nanoparticles relative to the bulk. For instance,

Pt–Ni (1:1) had 53.4 at.% of Pt and 46.6 at.% Ni. It is known

that, given a similar size, the metal having the lower heat of

sublimation tends to surface segregate in binary alloys. The

heats of vaporization of Pt and Ni are 509.6 and 370.3 kJ/mol,

respectively [61]. Therefore, an enrichment of Ni at the surface

is expected. However, a strong surface enrichment in Pt was

found by low-energy ion scattering (LEIS) in Pt–Ni alloys

[62,63]. This shows that the thermodynamic explanation fails to

predict the enrichment behaviours. Further, Mukerjee and

Moran-Lopez [64] used the electronic theory of d-band density

of states of pure components to demonstrate that surface

enrichment of Pt in Pt–Ni should occur. Finally, to produce a

different surface composition, Stamenkovic et al. [58] either

annealed at 727 8C or mildly sputtered with a 0.5 keV beam of

Ar+ ions clean Pt3Co and Pt3Ni samples. The surface

composition of alloy samples was determined by LEIS

spectroscopy. The LEIS spectra taken after mild sputtering

unambiguously showed that Ni(Co) are present in the outer-

most layer of the clean sputtered surface. Surface composition

was estimated to be 75 at.% of Pt and 25 at.% of Ni(Co), equal

to the bulk concentration of Pt3Co and Pt3Ni alloys. Conversely,

The LEIS data showed that the first layer of a clean annealed

Pt3Ni(Co) surface contains only Pt atoms, implying that the

‘‘Pt-skin’’ structure can also be created on a polycrystalline

Pt3Ni(Co) alloy.

The actual Pt–Co and –Ni alloy catalysts, particularly the

carbon-supported catalysts, are formed by alloyed and non-

alloyed Co(Ni) species. The degree of alloying depends on the

preparation method of the catalyst. X-ray photoelectron

spectroscopy (XPS) analysis on commercial carbon-supported

Pt3Co and Pt3Ni electrocatalysts indicated the presence of PtO,

CoO and NiO on the surface [65]. In the same way, XPS

analysis on unsupported Pt–Ni alloy nanoparticles indicated the

presence of metallic Ni, NiO, Ni(OH)2 and NiOOH [60].

Moreover, XPS data suggested that the amount of platinum

oxide content in the carbon-supported Pt–Co alloy electro-

catalyst is lower than that in Pt and Pt–Ni [45]. Finally, X-ray

adsorption near-edge structure (XANES) analysis revealed that

Pt3Co and Pt3Ni possess higher Pt d-band vacancies per atom

relative to Pt [66].

3. Preparation of Pt–Co and –Ni alloy catalysts

Generally, the starting materials used in the preparation of

unsupported Pt–Co(Ni) alloys are Pt and Co(Ni) metals. Pure Pt

and Co(Ni) metals can be alloyed by melting in an arc furnace

under an inert atmosphere (Pt–Ni) [67] or by sputtering (Pt–Co

and –Ni) [49]. Unsupported nanosized Pt–Ni catalysts were

synthesized at room temperature by Park et al. [60] using a

conventional reduction method of Pt and Ni precursors (H2PtCl6and NiCl2) with NaBH4. XRD data suggested a good alloy

formation. The size of the alloy nanoparticleswas approximately

3–4 nm. Zhang et al. [68] obtained Pt–Co nanoparticles by a two-

microemulsion technique. The microemulsion system was

composed of Triton X-100 as the surfactant, propanol-2 as a

co-surfactant, and either the Pt–Co precursor solution or a

hydrazine solution dispersed in a continuous oil phase of

cyclohexane. Platinum–cobalt nanoparticles were formed upon

contact between the precursor containing microemulsion

droplets and the hydrazine containing microemulsion droplets.

For all Pt–Co compositions, a narrow distribution of the particle

size centered around 2 nm was observed.

Martz et al. [69] prepared different Pt/M/Pc and Pt/M/

complex catalysts (with M = Co, Ni, and Pc = phthalocyanine)

in the composition Pt:M = 80:20 by an impregnation method. A

commercially available platinum catalyst was impregnated

with solutions of cobalt phthalocyanine (CoPc) and nickel

phthalocyanine tetrasulphonic salt (NiPc). After the reaction,

part of the catalyst was heat treated at 700 8C under a nitrogen

atmosphere. The resulting catalysts were structurally and

electrochemically characterized before (Pt/M/Pc) and after heat

treatment (Pt/M/Complex). The Pt/M/Pc had an average

particle size of about 3 nm, while the average size after heat

treatment increased to about 7 nm.

For that regarding carbon-supported alloys, the method of

preparation of Pt–Co/C and Pt–Ni/C commonly used consists

of the formation of carbon-supported platinum followed by the

E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149140

deposition of the second metal on Pt/C and alloying at high-

temperatures. This thermal treatment at high-temperatures

gives rise to an undesired metal particle growth, by sintering of

platinum particles [70]. Using this method, Beard and Ross [71]

prepared Pt–Co/C catalysts in the atomic ratio 3:1 starting from

commercial Pt/C in two ways. One way (series A) consisted in

the preparation of an acidic (pH 2) Co(OH)2 solution, followed

by Pt/C addition into this solution. In the other way (series B),

Pt/C was added into a basic (pH 11) solution of the cobalt

precursor. Thermal treatments at 700, 900 and 1200 8C under

inert atmosphere were performed on each catalyst. Following

thermal treatment in series A the lattice parameter decreased

with increasing heating temperature, indicative of alloy

formation. In series B the lattice parameter decreased after

heating, but to a lesser extent than in series A. The particle size

for series A at each thermal treatment temperature was larger

than the corresponding size in series B. The final particle size of

the series A material treated at 1200 8C (12 nm) was about four

times larger than that of the starting Pt catalyst. Shukla et al.

[45] prepared Pt–Co/C and Pt–Ni/C with a Pt:Co(Ni) atomic

ratio 1:1 nominal composition starting from 16 wt.% Pt/C,

dispersed in distilled water. The pH of the solution was raised to

8 with dilute ammonium hydroxide. The required amount of

Co[(NO3)]2 or Ni[(NO3)]2 salt solution was added to this

solution. This was followed by the addition of dilute HCl until a

pH of 5.5 was attained. The resulting powder was heat-treated

at 900 8C in a nitrogen atmosphere for 1 h. Min et al. [72]

prepared carbon-supported Pt–Co and Pt–Ni alloy catalysts

starting from commercial Pt/C (10%) catalyst. Appropriate

amounts of CoCl2 and NiCl2 solutions were added to Pt/C. The

atomic ratio of Pt to Co(Ni) was 3:1. These catalysts were

subjected to thermal treatment at 700, 900 or 1100 8C in a

reducing atmosphere. XRD measurements indicated a decrease

of lattice parameter, i.e. an increase in the degree of alloying,

with increasing heating temperature. The particle size, obtained

from both XRD and TEM measurements, increased with

increasing thermal treatment temperature. Oliveira Neto et al.

[73] prepared Pt–Co/C with various Pt:Co atomic ratios in the

range 9:1–1:9 by the following procedure: CoSO4 was

dissolved in a methanol/water solution containing a small

amount of NH4OH. A commercial 20% Pt/C catalyst was added

and the suspension was thermally treated at 1000 8C in a

reducing atmosphere. The amount of CoSO4 and Pt/C in the

mixture were those corresponding to the desired final

composition of Pt–Co/C. Cyclic voltammetry was used to

evaluate the platinum active area which decreased with

increasing Co contents in the samples following an exponen-

tially decay. This behaviour was interpreted as due to the

covering of the active Pt sites by cobalt. Using the same method

as described by Shukla et al. [45], Salgado et al. [74] prepared

carbon-supported Pt–Co alloy catalysts with Pt:Co atomic

ratios 90:10, 85:15, 80:20 and 75:25. The degree of alloying

increased with increasing Co content in the catalyst.

Conversely, the metal particle size decreased with increasing

Co content in the catalyst. Finally, Sirk et al. [75] synthesized

carbon-supported Pt–Co by mixing a Co oxide sol precursor

with Pt/C, followed by heat treatment at 700 or 900 8C.

Recently, methods to synthesize carbon-supported Pt–Co

and –Ni catalysts at low temperature, to avoid metal particle

sintering, have been developed. Xiong et al. [76] prepared Pt–

Co(Ni) alloy catalysts on a high surface area carbon support by

reducing a mixture of chloroplatinic acid and the respective

metal salt solution with sodium formate in aqueous medium.

Typically, the reduction reaction was carried out at 70 8C. In thecase of Co, the reduction was also carried out by adding first a

few drops of sodium borohydride followed by further reduction

with sodium formate. The particle size was 3.6 and 4.5 nm,

without and with sodium borohydride, respectively. Xiong and

Manthiram [77] synthesised a highly dispersed Pt–Co alloy

catalyst on a carbon support in the nominal Pt:Co atomic ratio

80:20 by the microemulsion method, using sodium bis(2-

ethylhexyl)sulphosuccinate as the surfactant, heptane as the oil

phase and NaBH4 as the reducing agent. The synthesis occurred

at room temperature. By XRD analysis the samples prepared by

the microemulsion method showed broad reflections compared

to those obtained by the high-temperature route, indicating a

smaller particle size for the former. The reflections of the Pt–Co

samples shifted to higher angles compared to that of Pt,

indicating a contraction of the lattice and alloy formation.

However, the shift was more significant for the samples

prepared by the high-temperature route compared to those

prepared by the microemulsion method, suggesting a greater

extend of alloy formation in the former case. Deivaraj et al. [78]

synthesised carbon-supported Pt–Ni by hydrazine reduction of

Pt and Ni precursors under different conditions, namely by

heating at 60 8C, by prolonged reaction (12 h) at room

temperature and by microwave-assisted reduction. The particle

size of Pt–Ni prepared bymicrowave-assisted reduction was the

lowest, in the range 2.9–5.6 nm, while the particle size of Pt–Ni

prepared by thermal treatment at 60 8C and by prolonged

reaction at room temperature were in the ranges 12.5–50 and

13–25 nm, respectively. Yang et al. [79] used the carbonyl

chemical route to prepare carbon-supported Pt–Ni. Pt and Ni

carbonyl complexes were synthesized simultaneously using

methanol as solvent through the reaction of Pt and Ni salts with

CO at about 55 8C for 24 h. After the synthesis of Pt–Ni

carbonyl complexes, Vulcan XC-72 carbon was added to the

mixture under a N2 gas flow and stirred for more than 6 h at

about 55 8C. Subsequently, the solvent was removed and the

catalyst powder was subjected to heat treatment at different

temperatures under nitrogen and hydrogen, respectively. The

alloying temperature under hydrogen ranged from 200 to

500 8C. According to the authors, the nearly linear relationshipbetween the lattice parameter and the EDX composition again

attests that Ni is completely alloyed with Pt. Furthermore, the

metal particle size decreases with increasing the content of non-

precious metal in the alloy. Finally, carbon-supported Pt–Co

[80,81] and Pt–Ni [82] alloy electrocatalysts were prepared by

impregnating high surface area carbon with Pt and Co(Ni)

precursors, followed by reduction of the precursors with NaBH4

at room temperature. The metal particle size was in the range

3.8–4.8 nm. It has to be remarked that, independently of the

EDX composition, the actual composition of the alloy was

around 92:8. As a consequence, for low Co(Ni) content

E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149 141

(10–15 at.%) a high degree of alloying was attained, while the

degree of alloying was low for the catalyst with high content

(30 at.%) of the non-precious metal.

4. Oxygen reduction reaction and stability of Pt–Co and

–Ni in PAFC and PEMFC environment

The search for catalysts for the oxygen reduction reaction

(ORR) that are more active, less expensive and with greater

stability than Pt has resulted in the development of Pt alloys. It

has been reported that alloying platinum with transition metals

enhances the electrocatalytic activity for the ORR. This

enhancement has been ascribed to different factors such as

geometric factors (decrease of the Pt–Pt bond distance) [83],

dissolution of the more oxidisable alloying component [84],

change in surface structure [71] or electronic factors (increase

of Pt d-electron vacancy) [49].

Considering the use of Pt–Co and –Ni as methanol resistant

cathode materials in low temperature fuel cells, the ORR

activity of these catalysts will be briefly discussed.

Mukerjee and Srinivasan [66] investigated the electrocata-

lysis of the ORR on five carbon-supported binary Pt alloys

(PtCr/C, PtMn/C, PtFe/C, PtCo/C and PtNi/C) in proton

exchange membrane fuel cells (PEMFC). All five binary alloy

catalyst showed a two–three folds activity enhancement in

terms of the electrode kinetic parameters obtained from half-

cell data, as compared to that on Pt. According to the authors,

the enhanced ORR activity by the alloys was rationalised on the

basis of the interplay between the electronic and geometric

factors on one hand and their effect on the chemisorption

behaviour of OH species from the electrolyte.

Toda et al. [49] studied the ORR activity in perchloric acid

solution of bulk Pt alloys with Ni, Co and Fe at room

temperature. Maximum activity was observed at ca. 30, 40 and

50% content of Ni, Co and Fe, respectively, observing 10, 15

and 20 times larger kinetic current densities than that on pure

Pt. By X-ray photoelectron spectroscopy (XPS) measurements

they found that Ni, Co or Fe disappeared from all the alloy

surface layers and the active surfaces were covered by a Pt-skin

of a few monolayers. The authors proposed the modification of

the electronic structure of the Pt-skin layer originating from that

of the bulk alloys. More recently, the temperature dependence

of the ORR activity on the same bulk alloy catalysts in 0.1

HClO4 solution in the temperature range 20–90 8C was

investigated by the same research group [85]. They found that

from 20 to 50 8C the apparent rate constants kapp for the ORR on

Pt–M electrodes were 2.4–4 times larger than that on a pure Pt

electrode. The kapp values at the alloy electrodes decreased by

elevating the temperature above 60 8C, and settled to almost the

same values observed on the Pt electrode.

Stamenkovic et al. [58,86] studied the intrinsic catalytic

activity of Pt3Ni and Pt3Co bulk alloy catalysts for the ORRwith

particular emphasis on the description of alloy surface prepara-

tion. They demonstrated that the ability to make a controlled and

well-characterized arrangement of two elements in the electrode

surface region is essential to interpreting the kinetic results. They

observed that in 0.1 mol L�1 HClO4 at 60 8C, the ‘‘Pt-skin’’

structure is more active than both pure Pt and Pt3Co, suggesting

that a uniform monatomic layer of Pt surface atoms, with Pt

depletion and Co enrichment in the second layer, has unique

catalytic properties. According to the authors, these results show

that the kinetics of theORR is dependent not only on the nature of

alloying component (Pt < Pt3Ni < Pt3Co) but also on the exact

arrangement of the alloying element in the surface region

(Ptbulk < Pt3Co < ‘‘Pt-skin’’ on Pt3Co). They proposed, in

agreement with Toda et al. [49], that the catalytic improvement

on the ‘‘Pt-skin’’ is caused by electronicallymodifiedPt atomson

top of the Co-enriched layer. The enhancement of the catalytic

activity for the ORR on Pt3Ni and Pt3Co alloy surface was

ascribed to the inhibition of Pt–OHad formation on Pt sites

surrounded by ‘‘oxide’’-covered Ni and Co atoms.

In a study on the ORR activity of carbon-supported Pt–Co

alloy with Pt:Co atomic ratio 55:45 under phosphoric acid fuel

cell (PAFC) conditions, Watanabe et al. [87] observed higher

activity on the alloys than on Pt. They found that the ordered

Pt–Co structure presents 1.35 times higher mass activity

compared to the disordered alloy. Moreover, they demonstrated

that both Pt and Co dissolve out from small-size alloy particle

and Pt redeposits on the surface of large-size ones in hot H3PO4.

The observed decay in the performance of the alloy catalysts

was then explained by the leaching of the alloying non-precious

metal to the electrolyte. The alloy with a disordered crystallite

structure, which is more corrosion-resistant than an ordered

one, maintains higher electrocatalytic activity for a longer time.

Regarding the stability of Pt–Co alloy catalysts in PAFC

conditions, it has to be pointed out that Beard and Ross [71], as

previously reported, found an opposite result.

Xiong and Manthiram [88] investigated the electrocatalytic

activity of carbon-supported Pt–Co in PEMFCs in a wide range

of compositions (27–77 at.%). They found that alloys with

ordered Pt3Co or PtCo structures have higher ORR activity than

Pt or disordered Pt–Co alloys. The same authors studied the

effect of atomic ordering on the ORR activity of carbon-

supported Pt–M (M = Fe, Co, Ni and Cu, Pt:M 80:20 wt.%, ca.

55:45 at.%) [77]. Evaluation of the Pt–M alloy catalysts for

oxygen reduction in proton exchange membrane fuel cells

indicates that the alloys with the ordered structures have higher

catalytic activity with lower polarization losses than Pt and the

disordered Pt–M alloys. According to the authors, the enhanced

catalytic activity is explained on the basis of optimal structural

and electronic features, like the number of Pt and M nearest

neighbors, d-electron density in Pt, atomic configuration on the

surface, and Pt–Pt distance.

Recently many studies were performed on carbon-supported

Pt–Co electrocatalysts in a wide range of Pt:Co compositions

prepared with different methods. Salgado et al. [74] investi-

gated in PEMFC carbon-supported Pt–Co alloys prepared by

alloying at 900 8C, with Co atomic ratio 10, 15, 20 and 25 at.%.

Pt75Co25/C showed the best kinetic parameters for the ORR,

ascribed to the optimal Pt–Pt bond distance. Xiong et al. [76]

investigated carbon-supported Pt–M (M = Fe, Co, Ni and Cu)

synthesized at low temperature by reduction with sodium

formate, in H2SO4 solutions and in PEMFCs. The Pt–M alloy

catalysts showed improved catalytic activity for the ORR in

E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149142

comparison to Pt. Among the various alloy catalysts

investigated, the Pt–Co catalysts presented the best perfor-

mance, with the maximum catalytic activity for a Pt:Co atomic

ratio around 1:7.

Paulus et al. [59,89] investigated the oxygen reduction

kinetics on carbon-supported Pt–Ni and Pt–Co alloy catalysts in

the atomic ratio Pt:M 3:1 and 1:1 using the thin filmRDEmethod

in 0.1 mol L�1 HClO4 in the temperature range between room

temperature and 60 8C. Kinetic analysis revealed a small activity

enhancement (per Pt surface atom) of ca. 1.5 for the 25 at.% Ni

and Co catalysts, and a more significant factor of 2–3 for the

50 at.%Co in comparison to pure Pt. The 50 at.%Ni catalyst was

less active than Pt and unstable at oxygen electrode potentials at

60 8C. Yang et al. investigated the effect of the composition on

ORR activity of Pt–Ni [79] alloy catalysts prepared by a Pt-

carbonyl route. The maximum activity of the Pt-based catalysts

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

corresponding to Pt–Pt mean interatomic distances of ca.

0.2704–0.2724 nm. Thus, the authors concluded that the high

activity of these catalysts for the ORR comes from the favorable

Pt–Pt mean interatomic distance caused by nickel alloying and

the disordered surface structures induced by the particle size.

As previously reported, there is evidence of dissolution of

the transition metal from the Pt alloy in hot H3PO4. However,

the operating environment of the polymer electrolyte fuel cells

is not nearly as severe as in phosphoric acid fuel cells then a

better stability of these alloy catalysts in the PEMFC

environment would be expected.

Mukerjee and Srinivasan [65] investigated durability and

stability of carbon-supported Pt3Cr, Pt3Co and Pt3Ni alloy

catalysts inPEMFCs.The lifetimestudieson thesecatalysts under

PEMFC operational conditions showed only negligible losses in

performance over periods of400–1200 h. In this time range a high

stability of the ratio between the amount of the alloying

component and the amount of Pt in the catalyst was observed.

As previously reported, by XPS measurements, Toda et al.

[49] found that most of the Ni, Co or Fe easily disappeared from

all the Pt alloy surface layers, probably by dissolution, by

submitting the surface to an anodic potential of 1.1 V, even in

diluted acid solution. However, the alloy compositions

determined with EDX analysis did not show apparent

differences before and after the electrochemical experiments.

Also, it was observed negligible differences in the XRD

patterns before and after electrochemical tests. These results

indicate that the loss of the base metal only occurs within few

monolayers of the alloy surface. The modification of the

electronic structure of this Pt layer with respect to that of the

bulk alloys gives rise to an enhancement of the ORR.

Colon-Mercado et al. [90] evaluated the catalytic, corrosion

and sintering properties of commercial Pt/C and Pt3Ni/C

catalysts using an accelerated durability test. The degree of

alloying of the Pt3Ni catalyst was not indicated. They found that

the total amount of Ni dissolved depends on the applied

potential, and increases from 8.3 to 12% when the potential is

increased from 0.4 to 0.9 V versus the standard hydrogen

electrode. A strong correlation between the amount of Ni

dissolved and the oxygen reduction activity of the catalyst was

observed. Moreover, the carbon-supported Pt3Ni alloy showed

better resistance to sintering than a pure platinum catalyst.

According to the authors, the mobility of platinum on a carbon

surface is hindered when Ni is present; thus, the sintering effect

of platinum atoms is suppressed.

On the other hand, Park et al. [60] observed no dissolution of

Ni in the bulk Pt–Ni (1:1) alloy nanoparticle catalyst in

2.0 mol L�1CH3OH + 0.5 mol L�1H2SO4 in thepotential range

0–1.6 VversusNHE.Although somedissolution ofNi could take

place, the amount dissolved from the Pt lattice was apparently

very small. According to the authors, this indeed implies that the

metallic state of nickel is either passivated by Ni hydroxides or

exists as a stable phase within the platinum lattice.

Salgado et al. [74] evaluated the stability of the Pt75Co25/C

catalyst following 24 h of PEMFC operation. The Pt:Co atomic

ratio increased from the nominal composition to 82:18. On the

basis to XRD analysis, the amount of cobalt lost was ascribed to

the lossofnon-alloyedcobalt.Abetter stabilityof thecellwith the

cobalt-containing catalyst upon several cycles between 0.05 and

0.78 V versus RHE than that of the cell with Pt/C was observed.

Yu et al. [91] evaluated the durability of Pt–Co cathode

catalysts in a dynamic fuel cell environment with continuous

water fluxing on the cathode. The results indicated that cobalt

dissolution neither detrimentally reduces the cell voltage nor

dramatically affects the membrane conductance. The overall

performance loss of the PtCo/C membrane electrode assem-

blies (MEAs) was less than that of the Pt/C MEA.

Gasteiger et al. [92] proposed a pre-leaching of the alloy to

minimize the contamination of the membrane electrode

assembly (MEA) during operation owing to Co dissolution.

They tested leached and unleached catalysts in small 50 cm2

single cells under oxygen to evaluate catalyst activity. A

multiply leached Pt–Co/C catalyst shows the highest activity

(with a gain of about 25 mVover Pt/C) over the entire range of

current densities as compared to Pt/C under identical

conditions.

Finally, Bonakdarpour et al. [93] studied the dissolution of

Fe and Ni from Pt1�xMx (M = Fe, Ni) catalyst under simulated

operating conditions of PEMFCs. Electron microprobe

measurements showed that transition metals are removed from

all compositions during acid treatment, but that the amount of

metal removed increases with x, acid strength and temperature.

For low M content (x < 0.6) the dissolved transition metals

originated from the surface, while for x > 0.6 the transition

metals dissolved also from the bulk. XPS results indicated

complete removal of surface Ni(Fe) after acid treatment at

80 8C for all compositions.

5. The methanol oxidation reaction on Pt, Pt–Ni and

–Co electrocatalysts

5.1. Improved activity for the MOR on Pt–Ni and –Co

electrocatalysts

Pt–Ni, –Co and other transition metal alloys were

investigated by Page et al. [30] as low cost alternative catalysts

for the direct oxidation of methanol and compared them with Pt

E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149 143

Fig. 2. Cyclic voltammetries at 75 8C in 1 mol L�1 CH3OH of the carbon

membrane electrodes with Pt, Pt–Ru, –Co and –Ni. A saturated calomel

electrode (SCE) was used as the reference electrode. Reprinted from Ref.

[30] with permission from Elsevier.

and Pt–Ru using cyclic voltammetry. Commercial carbon-

supported Pt–Ni and –Co in the atomic ratio 1:1 were used. The

alloy catalyst Pt–Co/C was found to be a better catalyst for

methanol oxidation in acid solution compared with Pt and other

transition metal alloys. Fig. 2 shows the cyclic voltammetries at

75 8C in 1 mol L�1 CH3OH of the carbon membrane electrodes

with Pt, Pt–Ru, –Co and –Ni. The highest oxidation current was

obtained with the Pt–Co electrocatalyst. Compared with Pt–Ru/

C, Pt–Co/C is less costly and has better electrochemical

performance. The onset potential of methanol oxidation in

0.5 mol L�1 H2SO4 + 1 mol L�1 CH3OH at 25, 50 and 75 8Cwas evaluated and found to shift negatively at high-temperatures.

The apparent activation energy for CH3OHads formation was

overcome at lower potentials as the temperature was increased.

The different potentials for the onset of the CH3OH oxidation on

carbon-supported catalysts aregiven inTable 1.At 75 8C(DMFC

operation temperature) the onset potentials for the MOR on Pt–

Co and –Ni were lower than that on pure Pt.

Chi et al. [94] prepared nanoparticles of different atomic

ratios of Pt–Co and measured the peak currents in cyclic

voltammetry of formic acid oxidation (as previously reported

formic acid may be a reaction product of the oxidation of

methanol). The maximum activity of Pt–Co catalysts was about

one order of magnitude higher than that of pure Pt

nanoparticles. The optimum Pt:Co atomic ratio was between

Table 1

Potential for the onset of CH3OH oxidation at various temperatures on carbon-

supported alloy catalysts [30]

Catalyst Onset potential

at 25 8C(mV vs. RHE)

Onset potential

at 50 8C(mV vs. RHE)

Onset potential at

75 8C(mV vs. RHE)

Pt–Co (1:1) 395 345 270

Pt–Ni (1:1) 370 335 280

Pt–Ru (1:1) 300 280 250

Pt 385 345 320

1:1.1 and 1:3.5. The presence of Co appears to significantly

enhance the electro-oxidation of formic acid.

Zhang et al. [68,95] prepared unsupported Pt–Co nanopar-

ticles using a water-in-oil reverse microemulsion of water/

Triton X-100/propan-2-ol/cycloexane with hydrazine solution

as the reducing agent. Electrodes with different Pt–Co

compositions were tested for methanol oxidation in an alkaline

electrolyte. From the cyclic voltammograms, the electrodes

with low Co content (Pt:Co < 1:1) outperformed those with

high Co content (Pt:Co > 1:1), while all Pt/Co electrodes better

performed than the pure Pt electrode. The authors also made a

comparison of steady-state currents obtained by current step

experiments. Fig. 3 shows the chronopotentiograms at

20 mA cm�2 for methanol oxidation at room temperature on

electrodes with different Pt:Co ratios. In agreement with the

transient CV results, the 1:0.5 atomic ratio of Pt:Co alloy

shows the best performance with the highest catalytic activity

of all the composition investigated. The order of activity

for Pt:Co with different compositions is 1:0.5 > 1:0.75 >1:0.25 > 1:3 > 1:2 > 1:1 > 1:0. According to the authors, in

addition to the possible enhancement of formaldehyde oxi-

dation by cobalt, the alloying of Co atoms to Pt lowers the

electronic binding energy in Pt and favours the C–H cleavage

reaction at low potential. Moreover, the presence of cobalt

oxides provides an oxygen source for CO oxidation at lower

potentials. The two considerations combined determine the

optimum Pt:Co ratio to be about 1:0.5.

Zeng and Lee [96] prepared carbon-supported Pt and Pt–Co

catalysts by reduction of metal precursors with NaBH4.

Electrochemical measurements by cyclic voltammetry and

chronoamperometry demonstrated consistently high catalytic

activity and improved resistance to carbon monoxide for the

Pt–Co catalysts, particularly for that prepared in unbuffered

solution (smaller particle size).

Park et al. [60] studied the electro-oxidation of methanol in

sulphuric acid solutionusingunsupportedPt, Pt–Ni (1:1 and3:1),

Fig. 3. Chronopotentiograms of methanol oxidation at 20 mA cm�2 in

1 mol L�1 CH3OH in 1 mol L�1 KOH at room temperature using carbon papers

with nanoparticles of different ratios of Pt to Co and the same Pt loading,

0.495 mg cm�2. (a) Pt; (b) Pt–Co 1:3; (c) Pt–Co 1:2; (d) Pt–Co 1:1; (e) Pt–Co

1:0.25; (f) Pt–Co 1:0.75; (g) Pt–Co 1:0.5. Reprinted from Ref. [68] with

permission from Elsevier.

E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149144

and Pt–Ru (1:1) alloy nanoparticle catalysts. The methanol

oxidation current measured on the Pt–Ni based catalysts in

2.0 mol L�1 CH3OH + 0.5 mol L�1 H2SO4 at room temperature

exceeded that obtainedwith pure Pt. The comparison of the onset

potentials for methanol oxidation on Pt–Ni electrocatalysts

(320 mV for Pt:Ni atomic ratio = 3:1, and 290 for Pt:Ni = 1:1)

and on Pt (350 mV) indicated that the Pt–Ni nanoparticles show

relatively good electrocatalytic activity. Using Pt alloy nano-

particles, Park et al. [60] measured plots of oxidation current

versus time (chronoamperometry, CA) in 2.0 mol L�1

CH3OH + 0.5 mol L�1 H2SO4 at 0.42 V, for 3600 s. For each

catalyst, the decay in the methanol oxidation was different; for

instance, pure platinum nanoparticles required 10 min to reach

70% of the initial current and the oxidation current is reduced

steeply. After 1 h, the current decreased below 40% of the initial

value. In contrast, Pt–Ni (1:1) and Pt–Ru (1:1) supported higher

currents, and it may be concluded that they have higher activity

than pure Pt. After 1 h, the order of surface activity for the

methanol oxidation was Pt–Ni (1:1) > Pt–Ru (1:1) > Pt. By

combining voltammetry and CA, the authors concluded that Pt–

Ni (1:1) represent the best alternative candidate for the DMFC

anodecatalystswith respect toPt–Ru, even if it has tobe remarked

that, from the results depicted in the second paper of this series

[97], is difficult to asses such improvement. In the Pt–Ni alloy

nanoparticles, theNi species includedmetallicNi,NiO,Ni(OH)2,

andNiOOH,andthe ratiobetween the threeoxideswassimilar for

thedifferentPt–Ni alloys.XPSPt4f peakvalues for Pt–Ni andPt–

Ru alloy nanoparticleswere compared to thevalue obtained from

pure Pt. The peaks were shifted from �0.09 for Pt–Ru to�0.35

and�0.36 eVat Pt–Ni; that is, theymoved toward the lower Pt4f

binding energy. The binding energy shift for Pt in the Pt–Ni

nanoparticles was interpreted to result from the modification of

theelectronic structureofplatinumbyelectron transfer fromNi to

Pt. According to the authors, the electron transfer may contribute

to the enhanced CO oxidation (CO generated from methanol

oxidation), that is, to the CO tolerance on the Ni-containing

composites, in comparison to pure Pt samples. However, the

competing effect in theNi enhancementmaybedue to the surface

redox activity of Ni oxides toward the CO.

Mathiyarasu et al. [98] investigated the electrocatalytic

activity of electrodeposited Pt–Ni alloy layers on an inert

substrate electrode for methanol oxidation reaction. By solid-

state polarization measurements in 0.5 mol L�1 CH3OH/

0.5 mol L�1 H2SO4 solutions they observed that the onset of

the electro-oxidation shifts to less anodic potential values,

while also exhibiting current enhancements up to about 15

times the currents obtained for the pure Pt electrodeposit. A

critical composition of Pt92Ni8 was found to exhibit the

maximum electrocatalytic activity, beyond which the activity

drops. According to the authors, while the promotion of the

electro-oxidation is understood to be largely due to the alloy

catalyst, surface redox species of Ni oxide formed during the

electro-oxidation process may also contribute to the oxygena-

tion of COads, thereby enhancing the oxidation current.

Park et al. [60,97] also investigated the effect of Ni insertion

on PtRu catalysts in the methanol oxidation. The activity of

Pt–Ru–Ni in the atomic ratio 5:4:1 for the MOR was compared

with that of Pt, Pt–Ru and –Ni. The onset potential for methanol

oxidation was in the order Pt–Ru–Ni (5:4:1) < Pt–Ru

(1:1) < Pt–Ni (1:1) < Pt–Ni (3:1) < pure Pt. Pt–Ru–Ni had a

larger current density, a larger turnover number and a smaller

activation energy for methanol oxidation than Pt–Ru (1:1).

Polarization and power density data in single DMFC tests were

in good agreement with the voltammetry and chronoampero-

metry data, for which Pt–Ru–Ni showed a higher catalytic

activity than Pt–Ru (1:1). According to the authors, one way to

interpret this result it is that the shift of d electron density from

Ni to Pt would reduce the Pt–CO bond energy. Furthermore, Ni

(hydro)oxides on the Pt–Ru–Ni nanoparticles could promote

methanol oxidation via a surface redox process.

5.2. No effect of Co(Ni) presence on the MOR activity on

Pt–Co and –Ni electrocatalysts

Goikovic [99] investigated the electrochemical oxidation of

methanol on a Pt3Co bulk alloy in acid solutions. Contrary to

the previous results, she found that cobalt does not show a

promoting effect on the rate of methanol oxidation on the Pt3Co

bulk alloy with respect to a pure Pt surface.

Drillet et al. [67] prepared an unsupportedPt70Ni30 catalyst by

melting together Pt andNi pellets in a vacuumarc and studied the

methanol oxidation and the electrochemical oxygen reduction

reaction at Pt and Pt70Ni30 in 1 mol L�1 H2SO4/0.5 mol L�1

CH3OH. By cyclic voltammetry they found no significant

difference in the methanol oxidation on Pt and Pt70Ni30,

particularly regarding the onset potential formethanol oxidation.

On the other hand, by means of a rotating disc electrode they

found that in amethanol containing electrolyte solution the onset

potential for oxygen reduction at Pt–Ni is shifted tomore positive

potentials and the alloy catalyst has an 11 times higher limiting

current density for oxygen reduction than Pt. Thus, they

concluded that Pt–Ni as cathode catalyst should have a higher

methanol-tolerance for fuel cell applications.

5.3. Decreased activity for the MOR on Pt–Ni and –Co

electrocatalysts

Salgado et al. [100] found that the onset potential for

methanol oxidation at room temperature on Pt–Co/C electro-

catalysts with Pt:Co atomic ratio 85:15 and 75:25 is shifted to

more positive potentials than Pt. According to the authors, the

carbon-supported Pt–Co/C alloy electrocatalysts possess

enhanced oxygen reduction activity compared to Pt/C in the

presence of methanol in a sulphuric acid electrolyte. The higher

methanol-tolerance of Co-containing catalysts with respect to

that of Pt alone can be clearly seen in Fig. 4, where the

potentials at 0.1 mA cm�2 ðE0:1mAcm�2Þ are plotted against

methanol concentration. The decrease of E0:1mAcm�2 on the Pt/C

electrocatalyst with increasing methanol concentration is much

higher than that on the alloys, showing that the Pt–Co/C

electrocatalysts have a better tolerance to the presence of

methanol than Pt/C in sulphuric acid solution.

Antolini et al. [101] prepared carbon-supported Pt70Ni30 by

NaBH4 reduction of the precursors and investigated the activity

E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149 145

Fig. 4. Dependence of the potential at 0.1 mA cm�2 on methanol concentration

during O2 reduction in 0.5 mol L�1 H2SO4 for carbon-supported Pt and Pt–Co

electrocatalysts. Reprinted from Ref. [100] with permission from Elsevier.

for the methanol oxidation and the oxygen reduction reactions

in sulphuric acid. They found that the current densities for the

methanol oxidation reaction on the Pt–Ni/C alloy electro-

catalyst were lower than that on the Pt/C electrocatalyst and the

onset potential for methanol oxidation at room temperature on

the Pt–Ni/C (440 mV) shifted to more positive potentials as

compared to Pt/C (375 mV), indicating that the alloy

electrocatalyst is less active for methanol oxidation than the

Pt/C electrocatalyst. The experimental results regarding the

ORR in H2SO4 solution in the presence of methanol are

summarized in Fig. 5, where the mixed potential is plotted

versus the methanol concentration at 0.05 and 0.1 mA cm�2

(specific activity). The polynomial regression for the Pt and Pt–

Ni data was the following:

Pt : E¼E0 � 73 ½CH3OH� þ 6:8 ½CH3OH�2 (4)

Pt�Ni : E¼E0 � 44 ½CH3OH� þ 6:6 ½CH3OH�2 (5)

Fig. 5. Electrode potential vs. methanol concentration at 0.05 and 0.1 mA cm�2

(current expressed as specific activity) for Pt/C and Pt70Ni30/C electrocatalysts.

Circles: Pt/C; triangles: Pt70Ni30/C. Solid symbols: j = 0.05 mA cm�2; open

symbols: j = 0.1 mA cm�2. Reprinted from Ref. [101] with permission from

Elsevier.

In a first approximation up to 2 mol L�1 CH3OH the depen-

dence of E on [CH3OH] is linear for both Pt and Pt–Ni, and dE/

d[CH3OH] is about twice that for Pt ð�60mV molCH3OH�1 LÞ

than for PtNi ð�31mV molCH3OH�1 LÞ both at 0.05 and

0.1 mA cm�2. Then, it seems that Pt–Ni is more methanol-

tolerant than pure Pt.

Inuikai and Itaya [102] prepared bimetallic Pt–Ni catalysts

by vapour deposition of alternate layers of Pt and Ni on

Pt(1 1 1) at room temperature. The methanol oxidation reaction

was carried out in an electrochemical chamber. The onset

potential and that of the maximum current for the MOR were

shifted to higher potentials. No Ni was detected on the surface

by Auger electron spectroscopy (AES). Then, in agreement to

Toda et al. [49], the electrochemical behaviour of Pt–Ni could

be ascribed to the modification of the electronic structure of Pt

atoms on top of the Ni layer.

Yang et al. [103] prepared carbon-supported Pt–Ni alloy

catalysts with 40 wt.% total metal loading via the carbonyl

complex route, and studied the activity for the MOR and for

ORR in the presence of methanol in H2SO4. By linear sweep

voltammetry measurements, as can be seen in Fig. 6, they found

that the methanol oxidation current densities on Pt–Ni alloy

catalysts 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 methanol on the alloy catalysts

is less active than that on a Pt/C catalyst. They observed

significantly enhanced electrocatalytic activities for ORR in

methanol-containing electrolyte than pure Pt. Among the

cathode catalysts used, a maximum activity was found with a

Pt:Ni atomic ratio of 2:1. According to the authors, the high

methanol-tolerance of Pt–Ni alloy catalysts during oxygen

reduction could be ascribed to a lowered activity for methanol

oxidation, originated from the composition effect and the

disordered structure of the alloy catalysts.

Fig. 6. Linear sweep voltammetries ofmethanol oxidation on nanosizedPt/C and

Pt–Ni alloy catalysts in nitrogen saturated 0.5 mol L�1 H2SO4 + 0.5 mol L�1

CH3OHsolutionat a scan rateof 5 mV s�1 and a rotation speedof2000 rpm.Solid

line: Pt/C; dashed line: Pt2Ni/C; dotted line: Pt3Ni2/C; dashed dotted line: PtNi/C.

Reprinted from Ref. [103] with permission from Elsevier.

E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149146

Fig. 8. Dependence of the onset potential for the MOR on the atomic

percentage of alloyed Co(Ni).

5.4. Effect of Co(Ni) content on the MOR activity of Pt–Co

and –Ni catalysts

As previously reported, the comparison of the onset

potential for the methanol oxidation on Pt–Co and –Ni alloy

catalysts with that on Pt presented conflicting results. The

disagreement of the results reported in the literature may

depend on the Co(Ni) content in the catalyst. To evaluate the

effect of the content of the non-precious metal on the onset

potential, we have plotted the Pt–Co(Ni)/Pt onset potential ratio

for the MOR at room temperature versus the nominal Co(Ni)

content in the catalyst for carbon-supported and bulk alloy

catalysts. As can be seen in Fig. 7, in the case of carbon-

supported catalysts, the onset potential for the methanol

oxidation went through a maximum at near 30 at.% Co(Ni). For

that regarding the bulk alloy catalysts, instead, the onset

potential for the MOR of Pt-based alloy was always lower than

that of Pt, almost independent of the Co(Ni) content for low

Co(Ni) content, and decreasing with increasing amounts of the

non-precious metal. The different behaviour between supported

and bulk alloy catalysts may depend on the higher degree of

alloying of unsupported alloys than that of carbon-supported

alloy catalysts [70]. Low Co(Ni) contents reduce the methanol

oxidation by the ensemble effect where the dilution of the

active component with the catalytically inert metal reduces the

methanol adsorption, while high Co(Ni) contents improve the

MOR by electronic effects and by the presence of higher

amounts of Co(Ni) oxide species, both enhancing CO

oxidation. The linear dependence of the onset potential for

the MOR on the amount of alloyed Co(Ni), as shown in Fig. 8,

seems to confirm the decrease of the rate of methanol oxidation

for low non-precious metal contents.

5.5. Tests in DMFC

Neergat et al. [44] carried out tests in DMFC at 70 and 90 8Coperating at 2 bar and ambient oxygen pressure using Pt/C and

Pt–Co/C (Pt:Co = 1:1) as cathode materials. Their study showed

Fig. 7. Ratio of the onset potential for the MOR for Pt–Co(Ni) and Pt at room

temperature vs. the nominal Co(Ni) content in the catalyst. Plots for carbon-

supported (&) and bulk (*) alloy catalysts.

that it is possible to substantially improve the performance of

DMFCs by employing carbon-supported Pt–Co binary alloy as

oxygen reduction catalysts on the cathode side in placeof carbon-

supported platinum. They found for the cell operating at 90 8Cwith 2 bar oxygen pressure amaximumpower density of 125 and

160 mW cm�2 for Pt/C and Pt–Co/C, respectively.

Salgado et al. [100] investigated the polarization curves in

single DMFCwith Pt/C and Pt–Co/C as cathode electrocatalysts

and Pt80Ru20/C as anode materials operating with 2 mol L�1

methanol solution at 90 8C and a cathode pressure of 3 atm. On

the basis of the specific activity, a larger improvement of the cell

performance was observed with Co-containing electrocatalysts

with respect to Pt/C, as shown in Fig. 9.

Yang et al. [103] tested carbon-supported Pt–Ni with

40 wt.% metal loading in DMFCs. Fig. 10 presents a

comparison of power density against current density in DMFCs

with different cathode catalysts. The maximum power density

was found with a Pt:Ni atomic ratio of 2:1, similarly to results

from previous half-cell tests. For example, the maximum power

Fig. 9. Polarization curves in single DMFC with Pt–Co/C and Pt/C electro-

catalysts for oxygen reduction at 90 8C and 3 atm O2 pressure using a 2 mol L�1

methanol solution. Anode Pt80Ru20/C. Current densities normalized with respect

to the Pt surface area. Reprinted from Ref. [100] with permission from Elsevier.

E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149 147

Fig. 10. Power density against current density curves recorded in a single

DMFC using different catalysts at 100 8C. Reprinted from Ref. [103] with

permission from Elsevier.

density for a Pt2Ni/C cathode catalyst is 101.5 mW cm�2 as

compared to 80.3 mW cm�2 for a Pt/C catalyst.

Antolini et al. investigated the behaviour in DMFC of

carbon-supported Pt–Ni with Pt:Ni prepared by reduction of

precursors with NaBH4 [82] and of commercial carbon-

supported Pt75Ni25 and Pt75Co25 [104], both as anode and as

cathode materials. The results of DMFC tests using Pt–Ni by

NaBH4 indicated that (i) Pt–Ni electrocatalysts perform better

as cathode than as anode materials; (ii) the performance of the

cell with Pt–Ni electrocatalysts as cathode materials depends

on the amount of effectively alloyed Ni and does not depend on

the total amount of Ni in the material; (iii) the performance of

the cell with Pt–Ni electrocatalysts as anode material increases

with increasing amounts of NiO species. Fig. 11 shows the

difference (DEc�a = Ec � Ea) between the potential of the cell

with Pt–Ni as cathode (Ec) and that of the cell with Pt–Ni as

anode electrocatalyst (Ea) from Refs. [74,75], as a function of

the amount of unalloyed nickel. DEc�a is indicative of the

Fig. 11. The difference (DEc�a = Ec � Ea) between the potential of the cell

with Pt–Ni as cathode (Ec) and that of the cell with Pt–Ni as anode electro-

catalyst (Ea) fromRefs. [82] and [104], as a function of the atomic percentage of

unalloyed nickel.

‘‘cathode quality factor’’ of the electrocatalyst. The higher is

the value ofDEc�a, the higher is the probability that the material

is an effective methanol-resistant oxygen reduction electro-

catalyst, and the lower is the feasibility that it can be used in

both the sides of the cell. As can be deduced from Fig. 11, the

presence of NiO species in the electrocatalyst decreases DEc�a,

i.e. increases the characteristics of the bimetallic alloy as anode

material, confirming the results observed by half-cell measure-

ments. This result, together with the result shown in Fig. 8

(onset potential versus amount of alloyed Co(Ni)), clearly

indicates the importance of the degree of alloying on the

electrochemical properties of the catalyst.

Regarding the DMFC tests with commercial Pt75Co25/C and

Pt75Ni25/C alloy catalysts, the experimental findings can be

summarizedas follows: (i) thecellswithPt75Co25/CandPt75Ni25/

C electrocatalysts as cathode materials performed better than

those with the same alloy catalyst as anode material; (ii) the

performance of the cell with Pt75Ni25/C as cathode material was

better than that of the cellswith Pt/C andPt75Co25/Cboth in terms

of mass activity and specific activity; (iii) the performance of the

cell with Pt75Co25/C as anode material was slightly better than

thatof thecellwithPt75Ni25/Cin termsof thegeometricareaof the

electrode; (iv) the performance of cells with Pt75Co25/C and

Pt75Ni25/Casanodecatalystswere slightlyworse in termsofmass

activity and almost the same in terms of specific activity than that

of thecellwithPt/C; (v) theenhancedperformanceof thecellwith

Pt75Ni25/C as cathode material over that with Pt75Co25/C can be

rationalised on the basis of a highermethanol-tolerance, ascribed

to electronic effects. On the basis of these results, the Pt75Ni25/C

electrocatalyst appears to be a promising cathode material for

directmethanol fuelcells. In thisnominalcomposition (Pt:Co(Ni)

3:1), the binary electrocatalysts used as anode material, instead,

didn’t show a substantial improvement over pure Pt.

6. Conclusions

Conflicting results regarding the methanol oxidation over Pt

electrocatalyst modified with either Ni or Co have been

observed in electrochemical tests in half-cell. From an analysis

of literature data the main effect of Co(Ni) content in Pt-based

alloys can be identified. It was observed an opposite effect of

the Co(Ni) presence in going from low Co(Ni) content

(negative effect on the MOR) to high Co(Ni) content (positive

effect on the MOR). The decreased activity for the MOR in the

presence of low Co(Ni) contents was ascribed especially to the

dilution effect of Pt, hindering the methanol adsorption. The

positive effect on the MOR for high Co(Ni) contents is related

to both electronic effects, enhancing CO oxidation, and to the

presence of oxide species, particularly nickel oxides. There-

fore, it has to be emphasized the importance of the degree of

alloying on the electrochemical properties of the catalyst.

Tests in DMFCs using Pt–Co (Pt–Ni) as cathode materials

indicated that these alloys are good methanol-tolerant cathodic

catalysts. Generally, they present both higher activity for the

ORR and higher methanol-tolerance than Pt.

When used as anode materials, instead, notwithstanding the

results of electrochemical tests in half-cell showing that Pt–Co

E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149148

[30] and Pt–Ni [60] are better catalysts for the MOR than Pt–

Ru, tests in DMFC [82,104] were not encouraging. Indeed, the

performance of cells with Pt–Co/C and Pt–Ni/C as anode

catalysts were slightly worse in terms of mass activity and

almost the same in terms of specific activity than that of the cell

with Pt/C. Further tests using Pt–Ni and –Co catalysts with high

Co(Ni) content and different degree of alloying have to be

carried out to better clarify the behaviour of these catalysts in

direct methanol fuel cells.

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