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

www.elsevier.com/locate/apcatb

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, Italy
b
Instituto de Quı́mica de São Carlos, USP, C.P. 780, São 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 can donate OH species for desorption of the adsorbed methanol

residues [9]. Methanol oxidation has been extensively
The use of methanol as energy carrier and its direct investigated since the early 1970’s with two main topics:
electrochemical oxidation in direct methanol fuel cells (DMFCs) identification of the reaction intermediates, poisoning species
represents an important challenge for the polymer electrolyte and products, and modification of Pt surface in order to achieve
fuel cell technology, since the complete system would be simpler higher activity at lower potentials and better resistance to
without a reformer and reactant treatment steps. The use of poisoning. The results have been reviewed by several authors
methanol as fuel has several advantages in comparison to [10–12]. The main reaction product is CO2 [13], although
hydrogen: it is a cheap liquid fuel, easily handled, transported, significant amounts of formaldehyde [14,15], formic acid [13]
and stored, and with a high theoretical energy density [1–3]. and methyl formate [15,16] were also detected. Most studies
Although a lot of progress has been made in the development of conclude that the reaction can proceed according to multiple
DMFC, its performance is still limited by the poor kinetics of the mechanisms. However, it is widely accepted that the most
anode reaction [3–5] and the crossover of methanol from the anode significant reactions are the adsorption of methanol and the
to the cathode side through the proton exchange membrane [6–8]. oxidation of CO, according to this simplified reaction
Methanol oxidation is a slow reaction that requires active mechanism:
multiple sites for the adsorption of methanol and the sites that
CH3 OH ! ðCH3 OHÞads (1)

ðCH3 OHÞads ! ðCOÞads þ 4Hþ þ 4e
(2)

* Corresponding author. Tel.: +39 0109162880; fax: +39 0109182368.
1
Present address: Instituto de Quı́mica, UnB, C.P. 4478, Brasilia, DF 70919-
970, Brazil. ðCOÞads þ H2 O ! CO2 þ 2Hþ þ 2e
(3)

0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2005.09.014
138 E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149

Platinum is the most active metal for dissociative adsorption of Fe in the sublayers) and strong modifications of their
methanol, but, as it is well-known, at room or moderate chemisorptive properties and electrocatalytic performances
temperatures it is readily poisoned by carbon monoxide, a [48]. This behaviour was attributed to the electronic effect of
by product of methanol oxidation. To date, the remedy has intermetallic bonding of the alloying component-rich second
been to use binary or ternary eletrocatalysts based on platinum, layer with the top-most Pt atoms. The electrocatalytic behaviour
all containing ruthenium as the activity promoting component of Pt alloys with increasing contents of the second element can be
[17–22]. According to the bifunctional mechanism [23,24], the explained by the model of Toda et al. [49], based on an increase of
CO-poisoned platinum is regenerated via a surface reaction d-electron vacancies of the thin Pt surface layer caused by the
between CO- and O-type species associated with ruthenium to underlying alloy.
yield CO2. According to the ligand model [12,23,25], instead, The ensemble effects where the dilution of the active
the change in Pt electronic properties induced by the presence component with the catalytically inert metal changes the
of Ru rends Pt atoms more susceptible for OH adsorption [23] distribution of active sites, open different reaction pathways
or even for dissociative adsorption of methanol [12]. But also [50]. The dissociative chemisorption of methanol requires the
when Pt–Ru is used as anode electrocatalyst the power density existence of several adjacent Pt ensembles [51,52] and the
of a DMFC is about a factor of 10 lower than that of a proton presence of atoms of the second metal around Pt active sites
exchange membrane fuel cell operated on hydrogen if the same could block methanol adsorption on Pt sites due to the dilution
Pt loading is used. Therefore, a number of Ru-alternative effect. Consequently, methanol oxidation on the binary-
elements, showing a co-catalytic activity for the anodic oxida- component electrocatalyst is suppressed. On the other hand,
tion of methanol, if used either as platinum alloys or as oxygen adsorption, which usually can be regarded as
adsorbate layers on platinum, have been investigated [26–33]. dissociative chemisorption, requires only two adjacent sites
The problem of methanol crossover in DMFCs has been and is not affected by the presence of the second metal.
extensively studied [6–8,34,35]: methanol adsorbs on Pt sites in Pt–Ni and –Co alloy catalysts have been proposed both as
the cathode for the direct reaction between methanol and oxygen. methanol-tolerant cathode material and anode material with
The mixed potential, which results from the oxygen reduction improved MOR for DMFCs. The choice of Co and Ni to modify
reaction and the methanol oxidation occurring simultaneously, Pt electrocatalyst to improve the MOR is due to the lowering of
reduces the cell voltage, generates additional water and increases the electronic binding energy in Pt by alloying with these metals,
the required oxygen stoichiometric ratio. This problem could be promoting the C–H cleavage reaction at low potential. Moreover,
solved either by using electrolytes with lower methanol the presence of cobalt or nickel oxides provides an oxygen source
permeability or by developing new cathode electrocatalysts for CO oxidation at lower potentials. On the other hand, a higher
with both higher methanol-tolerance and higher activity for the methanol-tolerance is expected on Pt–Co and –Ni alloy catalysts
oxygen reduction reaction (ORR) than Pt. Higher methanol- than on Pt, ascribed to the dilution effect of Pt, hindering the
tolerance is reported in the literature for non-noble metal methanol adsorption. Furthermore, these alloys present an imp-
electrocatalysts based on chalcogenides [35–38] and macro- roved activity for the oxygen reduction than Pt alone. On the basis
cycles of transition metals [39,40]. These electrocatalysts have of this discrepancy, we will attempt to outline the electro-
shown nearly the same activity for the ORR in the absence as well chemical activity for the MOR of Pt–Ni and –Co alloy catalysts.
as in the presence of methanol. However in methanol-free
electrolytes, these materials did not reach the catalytic activity of 2. Structural characterization of Pt–Co and –Ni alloys
dispersed platinum. Developing a sufficiently selective and
active electrocatalyst for the DMFC cathode remains one of the In the composition range from 0 to 50 at.% Co(Ni), Pt and
key tasks for further progress of this technology. The current Co(Ni) form a substitutional continuous solid solution and two
direction is to test the activity for the oxygen reduction reaction in ordered phases [53–56]. The dependence of the lattice
the presence of methanol of some Pt alloys with the first row parameter of the Pt–Co and –Ni bulk alloys on the alloy
transition metals which present a higher activity for the ORR than composition is reported in Fig. 1. In the region of 75 at.% Pt
platinum in low temperature fuel cells operated on hydrogen, and there are face-centered cubic (fcc) superlattices Pt3Co and
use them as DMFC cathode electrocatalysts [41–45]. The Pt3Ni of the Cu3Au (LI2) type. Regular termination of the bulk
improvement in the ORR electrocatalysis has been ascribed to LI2 structure normal to the three major zone axes produces a
different factors such as changes in the Pt–Pt interatomic distance variety of surface compositions, from the pure Pt ((2 0 0) and
[46] and the surface area [47]. But the behaviour of binary alloys (2 2 0) planes), 25 at.% Co(Ni) ((1 1 1) plane) to 50 at.%
with respect to electrocatalysis can be better understood in terms Co(Ni) ((1 0 0) and (1 1 0) planes) [57].
of the electronic ‘‘ligand effect’’ and/or the geometric ‘‘ensemble To better understand the relationship between the surface
effect’’. To rationalise these effects it is necessary to know composition and the catalytic activity, it is very important to
precisely the local concentration and arrangement of both determine if surface segregation, i.e. enrichment of one element
components at the very surface (in contact with the reactants), at the surface relative to the bulk, takes place during the
and also in the sublayers which influence electronically the outer preparation of these alloy catalysts. The details of segregation
atoms [48]. The electronic effect of elements present in the are still not completely understood, especially in the case of
sublayers is illustrated on PtNi (1 1 1) and Pt3Fe (1 1 1), which segregation in nanoparticles in which the characteristics may
present a quasi-complete Pt surface layer (with more or less Ni or differ from those of the bulk. This is not surprising, considering
 E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149 139

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
Fig. 1. Dependence of the Pt–Co and –Ni lattice parameters [53–56] on the oxide content in the carbon-supported Pt–Co alloy electro-
Co(Ni) content in the alloy. catalyst is lower than that in Pt and Pt–Ni [45]. Finally, X-ray
adsorption near-edge structure (XANES) analysis revealed that
that nanoclusters represent a finite quantity of material, so there Pt3Co and Pt3Ni possess higher Pt d-band vacancies per atom
is no infinite source/sink of constituent atoms and hence relative to Pt [66].
material balance constraints become important [58]. Conflict-
ing results regarding the surface segregation on Pt–Co and –Ni 3. Preparation of Pt–Co and –Ni alloy catalysts
alloys are reported in literature. XPS data by Shukla et al. [45]
indicated some surface enrichment of base metal in Pt–Co/C Generally, the starting materials used in the preparation of
(atomic ratio 0.84 versus 0.72 in the bulk) and Pt–Ni/C (0.86 unsupported Pt–Co(Ni) alloys are Pt and Co(Ni) metals. Pure Pt
versus 0.64) prepared by alloying at high-temperature. Paulus and Co(Ni) metals can be alloyed by melting in an arc furnace
et al. [59] found 70 at.% Pt on the surface of the Pt3Ni particles under an inert atmosphere (Pt–Ni) [67] or by sputtering (Pt–Co
and 58 at.% Pt on the surface of the Pt3Co particles for the and –Ni) [49]. Unsupported nanosized Pt–Ni catalysts were
Pt3Ni(Co) catalyst by E-TEK. The value for Pt3Ni is very close synthesized at room temperature by Park et al. [60] using a
to the bulk composition of 75 at.% Pt and indicates that no conventional reduction method of Pt and Ni precursors (H2PtCl6
segregation has taken place, whereas in case of the Pt3Co a and NiCl2) with NaBH4. XRD data suggested a good alloy
slight segregation of Co to the surface is observed. For the Pt– formation. The size of the alloy nanoparticles was approximately
Ni (1:1) catalyst by E-TEK, however, they found only about 3–4 nm. Zhang et al. [68] obtained Pt–Co nanoparticles by a two-
20 at.% Pt on the surface Pt and for the Pt–Co (1:1) catalyst by microemulsion technique. The microemulsion system was
E-TEK 35 at.% Pt on the surface, indicating Ni(Co) segregation composed of Triton X-100 as the surfactant, propanol-2 as a
to the surface. Park et al. [60] observed enrichment of Pt at the co-surfactant, and either the Pt–Co precursor solution or a
surface of Pt–Ni nanoparticles relative to the bulk. For instance, hydrazine solution dispersed in a continuous oil phase of
Pt–Ni (1:1) had 53.4 at.% of Pt and 46.6 at.% Ni. It is known cyclohexane. Platinum–cobalt nanoparticles were formed upon
that, given a similar size, the metal having the lower heat of contact between the precursor containing microemulsion
sublimation tends to surface segregate in binary alloys. The droplets and the hydrazine containing microemulsion droplets.
heats of vaporization of Pt and Ni are 509.6 and 370.3 kJ/mol, For all Pt–Co compositions, a narrow distribution of the particle
respectively [61]. Therefore, an enrichment of Ni at the surface size centered around 2 nm was observed.
is expected. However, a strong surface enrichment in Pt was Martz et al. [69] prepared different Pt/M/Pc and Pt/M/
found by low-energy ion scattering (LEIS) in Pt–Ni alloys complex catalysts (with M = Co, Ni, and Pc = phthalocyanine)
[62,63]. This shows that the thermodynamic explanation fails to in the composition Pt:M = 80:20 by an impregnation method. A
predict the enrichment behaviours. Further, Mukerjee and commercially available platinum catalyst was impregnated
Moran-Lopez [64] used the electronic theory of d-band density with solutions of cobalt phthalocyanine (CoPc) and nickel
of states of pure components to demonstrate that surface phthalocyanine tetrasulphonic salt (NiPc). After the reaction,
enrichment of Pt in Pt–Ni should occur. Finally, to produce a part of the catalyst was heat treated at 700 8C under a nitrogen
different surface composition, Stamenkovic et al. [58] either atmosphere. The resulting catalysts were structurally and
annealed at 727 8C or mildly sputtered with a 0.5 keV beam of electrochemically characterized before (Pt/M/Pc) and after heat
Ar+ ions clean Pt3Co and Pt3Ni samples. The surface treatment (Pt/M/Complex). The Pt/M/Pc had an average
composition of alloy samples was determined by LEIS particle size of about 3 nm, while the average size after heat
spectroscopy. The LEIS spectra taken after mild sputtering treatment increased to about 7 nm.
unambiguously showed that Ni(Co) are present in the outer- For that regarding carbon-supported alloys, the method of
most layer of the clean sputtered surface. Surface composition preparation of Pt–Co/C and Pt–Ni/C commonly used consists
was estimated to be 75 at.% of Pt and 25 at.% of Ni(Co), equal of the formation of carbon-supported platinum followed by the
140 E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149

deposition of the second metal on Pt/C and alloying at high- Recently, methods to synthesize carbon-supported Pt–Co
temperatures. This thermal treatment at high-temperatures and –Ni catalysts at low temperature, to avoid metal particle
gives rise to an undesired metal particle growth, by sintering of sintering, have been developed. Xiong et al. [76] prepared Pt–
platinum particles [70]. Using this method, Beard and Ross [71] Co(Ni) alloy catalysts on a high surface area carbon support by
prepared Pt–Co/C catalysts in the atomic ratio 3:1 starting from reducing a mixture of chloroplatinic acid and the respective
commercial Pt/C in two ways. One way (series A) consisted in metal salt solution with sodium formate in aqueous medium.
the preparation of an acidic (pH 2) Co(OH)2 solution, followed Typically, the reduction reaction was carried out at 70 8C. In the
by Pt/C addition into this solution. In the other way (series B), case of Co, the reduction was also carried out by adding first a
Pt/C was added into a basic (pH 11) solution of the cobalt few drops of sodium borohydride followed by further reduction
precursor. Thermal treatments at 700, 900 and 1200 8C under with sodium formate. The particle size was 3.6 and 4.5 nm,
inert atmosphere were performed on each catalyst. Following without and with sodium borohydride, respectively. Xiong and
thermal treatment in series A the lattice parameter decreased Manthiram [77] synthesised a highly dispersed Pt–Co alloy
with increasing heating temperature, indicative of alloy catalyst on a carbon support in the nominal Pt:Co atomic ratio
formation. In series B the lattice parameter decreased after 80:20 by the microemulsion method, using sodium bis(2-
heating, but to a lesser extent than in series A. The particle size ethylhexyl)sulphosuccinate as the surfactant, heptane as the oil
for series A at each thermal treatment temperature was larger phase and NaBH4 as the reducing agent. The synthesis occurred
than the corresponding size in series B. The final particle size of at room temperature. By XRD analysis the samples prepared by
the series A material treated at 1200 8C (12 nm) was about four the microemulsion method showed broad reflections compared
times larger than that of the starting Pt catalyst. Shukla et al. to those obtained by the high-temperature route, indicating a
[45] prepared Pt–Co/C and Pt–Ni/C with a Pt:Co(Ni) atomic smaller particle size for the former. The reflections of the Pt–Co
ratio 1:1 nominal composition starting from 16 wt.% Pt/C, samples shifted to higher angles compared to that of Pt,
dispersed in distilled water. The pH of the solution was raised to indicating a contraction of the lattice and alloy formation.
8 with dilute ammonium hydroxide. The required amount of However, the shift was more significant for the samples
Co[(NO3)]2 or Ni[(NO3)]2 salt solution was added to this prepared by the high-temperature route compared to those
solution. This was followed by the addition of dilute HCl until a prepared by the microemulsion method, suggesting a greater
pH of 5.5 was attained. The resulting powder was heat-treated extend of alloy formation in the former case. Deivaraj et al. [78]
at 900 8C in a nitrogen atmosphere for 1 h. Min et al. [72] synthesised carbon-supported Pt–Ni by hydrazine reduction of
prepared carbon-supported Pt–Co and Pt–Ni alloy catalysts Pt and Ni precursors under different conditions, namely by
starting from commercial Pt/C (10%) catalyst. Appropriate heating at 60 8C, by prolonged reaction (12 h) at room
amounts of CoCl2 and NiCl2 solutions were added to Pt/C. The temperature and by microwave-assisted reduction. The particle
atomic ratio of Pt to Co(Ni) was 3:1. These catalysts were size of Pt–Ni prepared by microwave-assisted reduction was the
subjected to thermal treatment at 700, 900 or 1100 8C in a lowest, in the range 2.9–5.6 nm, while the particle size of Pt–Ni
reducing atmosphere. XRD measurements indicated a decrease prepared by thermal treatment at 60 8C and by prolonged
of lattice parameter, i.e. an increase in the degree of alloying, reaction at room temperature were in the ranges 12.5–50 and
with increasing heating temperature. The particle size, obtained 13–25 nm, respectively. Yang et al. [79] used the carbonyl
from both XRD and TEM measurements, increased with chemical route to prepare carbon-supported Pt–Ni. Pt and Ni
increasing thermal treatment temperature. Oliveira Neto et al. carbonyl complexes were synthesized simultaneously using
[73] prepared Pt–Co/C with various Pt:Co atomic ratios in the methanol as solvent through the reaction of Pt and Ni salts with
range 9:1–1:9 by the following procedure: CoSO4 was CO at about 55 8C for 24 h. After the synthesis of Pt–Ni
dissolved in a methanol/water solution containing a small carbonyl complexes, Vulcan XC-72 carbon was added to the
amount of NH4OH. A commercial 20% Pt/C catalyst was added mixture under a N2 gas flow and stirred for more than 6 h at
and the suspension was thermally treated at 1000 8C in a about 55 8C. Subsequently, the solvent was removed and the
reducing atmosphere. The amount of CoSO4 and Pt/C in the catalyst powder was subjected to heat treatment at different
mixture were those corresponding to the desired final temperatures under nitrogen and hydrogen, respectively. The
composition of Pt–Co/C. Cyclic voltammetry was used to alloying temperature under hydrogen ranged from 200 to
evaluate the platinum active area which decreased with 500 8C. According to the authors, the nearly linear relationship
increasing Co contents in the samples following an exponen- between the lattice parameter and the EDX composition again
tially decay. This behaviour was interpreted as due to the attests that Ni is completely alloyed with Pt. Furthermore, the
covering of the active Pt sites by cobalt. Using the same method metal particle size decreases with increasing the content of non-
as described by Shukla et al. [45], Salgado et al. [74] prepared precious metal in the alloy. Finally, carbon-supported Pt–Co
carbon-supported Pt–Co alloy catalysts with Pt:Co atomic [80,81] and Pt–Ni [82] alloy electrocatalysts were prepared by
ratios 90:10, 85:15, 80:20 and 75:25. The degree of alloying impregnating high surface area carbon with Pt and Co(Ni)
increased with increasing Co content in the catalyst. precursors, followed by reduction of the precursors with NaBH4
Conversely, the metal particle size decreased with increasing at room temperature. The metal particle size was in the range
Co content in the catalyst. Finally, Sirk et al. [75] synthesized 3.8–4.8 nm. It has to be remarked that, independently of the
carbon-supported Pt–Co by mixing a Co oxide sol precursor EDX composition, the actual composition of the alloy was
with Pt/C, followed by heat treatment at 700 or 900 8C. 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 structure is more active than both pure Pt and Pt3Co, suggesting
degree of alloying was low for the catalyst with high content that a uniform monatomic layer of Pt surface atoms, with Pt
(30 at.%) of the non-precious metal. depletion and Co enrichment in the second layer, has unique
catalytic properties. According to the authors, these results show
4. Oxygen reduction reaction and stability of Pt–Co and that the kinetics of the ORR is dependent not only on the nature of
–Ni in PAFC and PEMFC environment alloying component (Pt < Pt3Ni < Pt3Co) but also on the exact
arrangement of the alloying element in the surface region
The search for catalysts for the oxygen reduction reaction (Ptbulk < Pt3Co < ‘‘Pt-skin’’ on Pt3Co). They proposed, in
(ORR) that are more active, less expensive and with greater agreement with Toda et al. [49], that the catalytic improvement
stability than Pt has resulted in the development of Pt alloys. It on the ‘‘Pt-skin’’ is caused by electronically modified Pt atoms on
has been reported that alloying platinum with transition metals top of the Co-enriched layer. The enhancement of the catalytic
enhances the electrocatalytic activity for the ORR. This activity for the ORR on Pt3Ni and Pt3Co alloy surface was
enhancement has been ascribed to different factors such as ascribed to the inhibition of Pt–OHad formation on Pt sites
geometric factors (decrease of the Pt–Pt bond distance) [83], surrounded by ‘‘oxide’’-covered Ni and Co atoms.
dissolution of the more oxidisable alloying component [84], In a study on the ORR activity of carbon-supported Pt–Co
change in surface structure [71] or electronic factors (increase alloy with Pt:Co atomic ratio 55:45 under phosphoric acid fuel
of Pt d-electron vacancy) [49]. cell (PAFC) conditions, Watanabe et al. [87] observed higher
Considering the use of Pt–Co and –Ni as methanol resistant activity on the alloys than on Pt. They found that the ordered
cathode materials in low temperature fuel cells, the ORR Pt–Co structure presents 1.35 times higher mass activity
activity of these catalysts will be briefly discussed. compared to the disordered alloy. Moreover, they demonstrated
Mukerjee and Srinivasan [66] investigated the electrocata- that both Pt and Co dissolve out from small-size alloy particle
lysis of the ORR on five carbon-supported binary Pt alloys and Pt redeposits on the surface of large-size ones in hot H3PO4.
(PtCr/C, PtMn/C, PtFe/C, PtCo/C and PtNi/C) in proton The observed decay in the performance of the alloy catalysts
exchange membrane fuel cells (PEMFC). All five binary alloy was then explained by the leaching of the alloying non-precious
catalyst showed a two–three folds activity enhancement in metal to the electrolyte. The alloy with a disordered crystallite
terms of the electrode kinetic parameters obtained from half- structure, which is more corrosion-resistant than an ordered
cell data, as compared to that on Pt. According to the authors, one, maintains higher electrocatalytic activity for a longer time.
the enhanced ORR activity by the alloys was rationalised on the Regarding the stability of Pt–Co alloy catalysts in PAFC
basis of the interplay between the electronic and geometric conditions, it has to be pointed out that Beard and Ross [71], as
factors on one hand and their effect on the chemisorption previously reported, found an opposite result.
behaviour of OH species from the electrolyte. Xiong and Manthiram [88] investigated the electrocatalytic
Toda et al. [49] studied the ORR activity in perchloric acid activity of carbon-supported Pt–Co in PEMFCs in a wide range
solution of bulk Pt alloys with Ni, Co and Fe at room of compositions (27–77 at.%). They found that alloys with
temperature. Maximum activity was observed at ca. 30, 40 and ordered Pt3Co or PtCo structures have higher ORR activity than
50% content of Ni, Co and Fe, respectively, observing 10, 15 Pt or disordered Pt–Co alloys. The same authors studied the
and 20 times larger kinetic current densities than that on pure effect of atomic ordering on the ORR activity of carbon-
Pt. By X-ray photoelectron spectroscopy (XPS) measurements supported Pt–M (M = Fe, Co, Ni and Cu, Pt:M 80:20 wt.%, ca.
they found that Ni, Co or Fe disappeared from all the alloy 55:45 at.%) [77]. Evaluation of the Pt–M alloy catalysts for
surface layers and the active surfaces were covered by a Pt-skin oxygen reduction in proton exchange membrane fuel cells
of a few monolayers. The authors proposed the modification of indicates that the alloys with the ordered structures have higher
the electronic structure of the Pt-skin layer originating from that catalytic activity with lower polarization losses than Pt and the
of the bulk alloys. More recently, the temperature dependence disordered Pt–M alloys. According to the authors, the enhanced
of the ORR activity on the same bulk alloy catalysts in 0.1 catalytic activity is explained on the basis of optimal structural
HClO4 solution in the temperature range 20–90 8C was and electronic features, like the number of Pt and M nearest
investigated by the same research group [85]. They found that neighbors, d-electron density in Pt, atomic configuration on the
from 20 to 50 8C the apparent rate constants kapp for the ORR on surface, and Pt–Pt distance.
Pt–M electrodes were 2.4–4 times larger than that on a pure Pt Recently many studies were performed on carbon-supported
electrode. The kapp values at the alloy electrodes decreased by Pt–Co electrocatalysts in a wide range of Pt:Co compositions
elevating the temperature above 60 8C, and settled to almost the prepared with different methods. Salgado et al. [74] investi-
same values observed on the Pt electrode. gated in PEMFC carbon-supported Pt–Co alloys prepared by
Stamenkovic et al. [58,86] studied the intrinsic catalytic alloying at 900 8C, with Co atomic ratio 10, 15, 20 and 25 at.%.
activity of Pt3Ni and Pt3Co bulk alloy catalysts for the ORR with Pt75Co25/C showed the best kinetic parameters for the ORR,
particular emphasis on the description of alloy surface prepara- ascribed to the optimal Pt–Pt bond distance. Xiong et al. [76]
tion. They demonstrated that the ability to make a controlled and investigated carbon-supported Pt–M (M = Fe, Co, Ni and Cu)
well-characterized arrangement of two elements in the electrode synthesized at low temperature by reduction with sodium
surface region is essential to interpreting the kinetic results. They formate, in H2SO4 solutions and in PEMFCs. The Pt–M alloy
observed that in 0.1 mol L
1 HClO4 at 60 8C, the ‘‘Pt-skin’’ catalysts showed improved catalytic activity for the ORR in
142 E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149

comparison to Pt. Among the various alloy catalysts observed. Moreover, the carbon-supported Pt3Ni alloy showed
investigated, the Pt–Co catalysts presented the best perfor- better resistance to sintering than a pure platinum catalyst.
mance, with the maximum catalytic activity for a Pt:Co atomic According to the authors, the mobility of platinum on a carbon
ratio around 1:7. surface is hindered when Ni is present; thus, the sintering effect
Paulus et al. [59,89] investigated the oxygen reduction of platinum atoms is suppressed.
kinetics on carbon-supported Pt–Ni and Pt–Co alloy catalysts in On the other hand, Park et al. [60] observed no dissolution of
the atomic ratio Pt:M 3:1 and 1:1 using the thin film RDE method Ni in the bulk Pt–Ni (1:1) alloy nanoparticle catalyst in
in 0.1 mol L
1 HClO4 in the temperature range between room 2.0 mol L
1 CH3OH + 0.5 mol L
1 H2SO4 in the potential range
temperature and 60 8C. Kinetic analysis revealed a small activity 0–1.6 V versus NHE. Although some dissolution of Ni could take
enhancement (per Pt surface atom) of ca. 1.5 for the 25 at.% Ni place, the amount dissolved from the Pt lattice was apparently
and Co catalysts, and a more significant factor of 2–3 for the very small. According to the authors, this indeed implies that the
50 at.% Co in comparison to pure Pt. The 50 at.% Ni catalyst was metallic state of nickel is either passivated by Ni hydroxides or
less active than Pt and unstable at oxygen electrode potentials at exists as a stable phase within the platinum lattice.
60 8C. Yang et al. investigated the effect of the composition on Salgado et al. [74] evaluated the stability of the Pt75Co25/C
ORR activity of Pt–Ni [79] alloy catalysts prepared by a Pt- catalyst following 24 h of PEMFC operation. The Pt:Co atomic
carbonyl route. The maximum activity of the Pt-based catalysts ratio increased from the nominal composition to 82:18. On the
was found with ca. 30–40 at.% Ni content in the alloys, basis to XRD analysis, the amount of cobalt lost was ascribed to
corresponding to Pt–Pt mean interatomic distances of ca. the loss of non-alloyed cobalt. A better stability of the cell with the
0.2704–0.2724 nm. Thus, the authors concluded that the high cobalt-containing catalyst upon several cycles between 0.05 and
activity of these catalysts for the ORR comes from the favorable 0.78 V versus RHE than that of the cell with Pt/C was observed.
Pt–Pt mean interatomic distance caused by nickel alloying and Yu et al. [91] evaluated the durability of Pt–Co cathode
the disordered surface structures induced by the particle size. catalysts in a dynamic fuel cell environment with continuous
As previously reported, there is evidence of dissolution of water fluxing on the cathode. The results indicated that cobalt
the transition metal from the Pt alloy in hot H3PO4. However, dissolution neither detrimentally reduces the cell voltage nor
the operating environment of the polymer electrolyte fuel cells dramatically affects the membrane conductance. The overall
is not nearly as severe as in phosphoric acid fuel cells then a performance loss of the PtCo/C membrane electrode assem-
better stability of these alloy catalysts in the PEMFC blies (MEAs) was less than that of the Pt/C MEA.
environment would be expected. Gasteiger et al. [92] proposed a pre-leaching of the alloy to
Mukerjee and Srinivasan [65] investigated durability and minimize the contamination of the membrane electrode
stability of carbon-supported Pt3Cr, Pt3Co and Pt3Ni alloy assembly (MEA) during operation owing to Co dissolution.
catalysts in PEMFCs. The lifetime studies on these catalysts under They tested leached and unleached catalysts in small 50 cm2
PEMFC operational conditions showed only negligible losses in single cells under oxygen to evaluate catalyst activity. A
performance over periods of 400–1200 h. In this time range a high multiply leached Pt–Co/C catalyst shows the highest activity
stability of the ratio between the amount of the alloying (with a gain of about 25 mV over Pt/C) over the entire range of
component and the amount of Pt in the catalyst was observed. current densities as compared to Pt/C under identical
As previously reported, by XPS measurements, Toda et al. conditions.
[49] found that most of the Ni, Co or Fe easily disappeared from Finally, Bonakdarpour et al. [93] studied the dissolution of
all the Pt alloy surface layers, probably by dissolution, by Fe and Ni from Pt1
xMx (M = Fe, Ni) catalyst under simulated
submitting the surface to an anodic potential of 1.1 V, even in operating conditions of PEMFCs. Electron microprobe
diluted acid solution. However, the alloy compositions measurements showed that transition metals are removed from
determined with EDX analysis did not show apparent all compositions during acid treatment, but that the amount of
differences before and after the electrochemical experiments. metal removed increases with x, acid strength and temperature.
Also, it was observed negligible differences in the XRD For low M content (x < 0.6) the dissolved transition metals
patterns before and after electrochemical tests. These results originated from the surface, while for x > 0.6 the transition
indicate that the loss of the base metal only occurs within few metals dissolved also from the bulk. XPS results indicated
monolayers of the alloy surface. The modification of the complete removal of surface Ni(Fe) after acid treatment at
electronic structure of this Pt layer with respect to that of the 80 8C for all compositions.
bulk alloys gives rise to an enhancement of the ORR.
Colon-Mercado et al. [90] evaluated the catalytic, corrosion 5. The methanol oxidation reaction on Pt, Pt–Ni and
and sintering properties of commercial Pt/C and Pt3Ni/C –Co electrocatalysts
catalysts using an accelerated durability test. The degree of
alloying of the Pt3Ni catalyst was not indicated. They found that 5.1. Improved activity for the MOR on Pt–Ni and –Co
the total amount of Ni dissolved depends on the applied electrocatalysts
potential, and increases from 8.3 to 12% when the potential is
increased from 0.4 to 0.9 V versus the standard hydrogen Pt–Ni, –Co and other transition metal alloys were
electrode. A strong correlation between the amount of Ni investigated by Page et al. [30] as low cost alternative catalysts
dissolved and the oxygen reduction activity of the catalyst was for the direct oxidation of methanol and compared them with Pt
 E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149 143

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
Fig. 2. Cyclic voltammetries at 75 8C in 1 mol L
1 CH3OH of the carbon transient CV results, the 1:0.5 atomic ratio of Pt:Co alloy
membrane electrodes with Pt, Pt–Ru, –Co and –Ni. A saturated calomel shows the best performance with the highest catalytic activity
electrode (SCE) was used as the reference electrode. Reprinted from Ref. of all the composition investigated. The order of activity
[30] with permission from Elsevier.
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
and Pt–Ru using cyclic voltammetry. Commercial carbon- addition to the possible enhancement of formaldehyde oxi-
supported Pt–Ni and –Co in the atomic ratio 1:1 were used. The dation by cobalt, the alloying of Co atoms to Pt lowers the
alloy catalyst Pt–Co/C was found to be a better catalyst for electronic binding energy in Pt and favours the C–H cleavage
methanol oxidation in acid solution compared with Pt and other reaction at low potential. Moreover, the presence of cobalt
transition metal alloys. Fig. 2 shows the cyclic voltammetries at oxides provides an oxygen source for CO oxidation at lower
75 8C in 1 mol L
1 CH3OH of the carbon membrane electrodes potentials. The two considerations combined determine the
with Pt, Pt–Ru, –Co and –Ni. The highest oxidation current was optimum Pt:Co ratio to be about 1:0.5.
obtained with the Pt–Co electrocatalyst. Compared with Pt–Ru/ Zeng and Lee [96] prepared carbon-supported Pt and Pt–Co
C, Pt–Co/C is less costly and has better electrochemical catalysts by reduction of metal precursors with NaBH4.
performance. The onset potential of methanol oxidation in Electrochemical measurements by cyclic voltammetry and
0.5 mol L
1 H2SO4 + 1 mol L
1 CH3OH at 25, 50 and 75 8C chronoamperometry demonstrated consistently high catalytic
was evaluated and found to shift negatively at high-temperatures. activity and improved resistance to carbon monoxide for the
The apparent activation energy for CH3OHads formation was Pt–Co catalysts, particularly for that prepared in unbuffered
overcome at lower potentials as the temperature was increased. solution (smaller particle size).
The different potentials for the onset of the CH3OH oxidation on Park et al. [60] studied the electro-oxidation of methanol in
carbon-supported catalysts are given in Table 1. At 75 8C (DMFC sulphuric acid solution using unsupported Pt, Pt–Ni (1:1 and 3:1),
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 Onset potential Onset potential at
at 25 8C at 50 8C 75 8C
(mV vs. RHE) (mV vs. RHE) (mV vs. RHE) Fig. 3. Chronopotentiograms of methanol oxidation at 20 mA cm
2 in
Pt–Co (1:1) 395 345 270 1 mol L
1 CH3OH in 1 mol L
1 KOH at room temperature using carbon papers
Pt–Ni (1:1) 370 335 280 with nanoparticles of different ratios of Pt to Co and the same Pt loading,
Pt–Ru (1:1) 300 280 250 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
Pt 385 345 320 1:0.25; (f) Pt–Co 1:0.75; (g) Pt–Co 1:0.5. Reprinted from Ref. [68] with
permission from Elsevier.
144 E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149

and Pt–Ru (1:1) alloy nanoparticle catalysts. The methanol with that of Pt, Pt–Ru and –Ni. The onset potential for methanol
oxidation current measured on the Pt–Ni based catalysts in oxidation was in the order Pt–Ru–Ni (5:4:1) < Pt–Ru
2.0 mol L
1 CH3OH + 0.5 mol L
1 H2SO4 at room temperature (1:1) < Pt–Ni (1:1) < Pt–Ni (3:1) < pure Pt. Pt–Ru–Ni had a
exceeded that obtained with pure Pt. The comparison of the onset larger current density, a larger turnover number and a smaller
potentials for methanol oxidation on Pt–Ni electrocatalysts activation energy for methanol oxidation than Pt–Ru (1:1).
(320 mV for Pt:Ni atomic ratio = 3:1, and 290 for Pt:Ni = 1:1) Polarization and power density data in single DMFC tests were
and on Pt (350 mV) indicated that the Pt–Ni nanoparticles show in good agreement with the voltammetry and chronoampero-
relatively good electrocatalytic activity. Using Pt alloy nano- metry data, for which Pt–Ru–Ni showed a higher catalytic
particles, Park et al. [60] measured plots of oxidation current activity than Pt–Ru (1:1). According to the authors, one way to
versus time (chronoamperometry, CA) in 2.0 mol L
1 interpret this result it is that the shift of d electron density from
CH3OH + 0.5 mol L
1 H2SO4 at 0.42 V, for 3600 s. For each Ni to Pt would reduce the Pt–CO bond energy. Furthermore, Ni
catalyst, the decay in the methanol oxidation was different; for (hydro)oxides on the Pt–Ru–Ni nanoparticles could promote
instance, pure platinum nanoparticles required 10 min to reach methanol oxidation via a surface redox process.
70% of the initial current and the oxidation current is reduced
steeply. After 1 h, the current decreased below 40% of the initial 5.2. No effect of Co(Ni) presence on the MOR activity on
value. In contrast, Pt–Ni (1:1) and Pt–Ru (1:1) supported higher Pt–Co and –Ni electrocatalysts
currents, and it may be concluded that they have higher activity
than pure Pt. After 1 h, the order of surface activity for the Goikovic [99] investigated the electrochemical oxidation of
methanol oxidation was Pt–Ni (1:1) > Pt–Ru (1:1) > Pt. By methanol on a Pt3Co bulk alloy in acid solutions. Contrary to
combining voltammetry and CA, the authors concluded that Pt– the previous results, she found that cobalt does not show a
Ni (1:1) represent the best alternative candidate for the DMFC promoting effect on the rate of methanol oxidation on the Pt3Co
anode catalysts with respect to Pt–Ru, even if it has to be remarked bulk alloy with respect to a pure Pt surface.
that, from the results depicted in the second paper of this series Drillet et al. [67] prepared an unsupported Pt70Ni30 catalyst by
[97], is difficult to asses such improvement. In the Pt–Ni alloy melting together Pt and Ni pellets in a vacuum arc and studied the
nanoparticles, the Ni species included metallic Ni, NiO, Ni(OH)2, methanol oxidation and the electrochemical oxygen reduction
and NiOOH, and the ratio between the three oxides was similar for reaction at Pt and Pt70Ni30 in 1 mol L
1 H2SO4/0.5 mol L
1
the different Pt–Ni alloys. XPS Pt4f peak values for Pt–Ni and Pt– CH3OH. By cyclic voltammetry they found no significant
Ru alloy nanoparticles were compared to the value obtained from difference in the methanol oxidation on Pt and Pt70Ni30,
pure Pt. The peaks were shifted from
0.09 for Pt–Ru to
0.35 particularly regarding the onset potential for methanol oxidation.
and
0.36 eVat Pt–Ni; that is, they moved toward the lower Pt4f On the other hand, by means of a rotating disc electrode they
binding energy. The binding energy shift for Pt in the Pt–Ni found that in a methanol containing electrolyte solution the onset
nanoparticles was interpreted to result from the modification of potential for oxygen reduction at Pt–Ni is shifted to more positive
the electronic structure of platinum by electron transfer from Ni to potentials and the alloy catalyst has an 11 times higher limiting
Pt. According to the authors, the electron transfer may contribute current density for oxygen reduction than Pt. Thus, they
to the enhanced CO oxidation (CO generated from methanol concluded that Pt–Ni as cathode catalyst should have a higher
oxidation), that is, to the CO tolerance on the Ni-containing methanol-tolerance for fuel cell applications.
composites, in comparison to pure Pt samples. However, the
competing effect in the Ni enhancement may be due to the surface 5.3. Decreased activity for the MOR on Pt–Ni and –Co
redox activity of Ni oxides toward the CO. electrocatalysts
Mathiyarasu et al. [98] investigated the electrocatalytic
activity of electrodeposited Pt–Ni alloy layers on an inert Salgado et al. [100] found that the onset potential for
substrate electrode for methanol oxidation reaction. By solid- methanol oxidation at room temperature on Pt–Co/C electro-
state polarization measurements in 0.5 mol L
1 CH3OH/ catalysts with Pt:Co atomic ratio 85:15 and 75:25 is shifted to
0.5 mol L
1 H2SO4 solutions they observed that the onset of more positive potentials than Pt. According to the authors, the
the electro-oxidation shifts to less anodic potential values, carbon-supported Pt–Co/C alloy electrocatalysts possess
while also exhibiting current enhancements up to about 15 enhanced oxygen reduction activity compared to Pt/C in the
times the currents obtained for the pure Pt electrodeposit. A presence of methanol in a sulphuric acid electrolyte. The higher
critical composition of Pt92Ni8 was found to exhibit the methanol-tolerance of Co-containing catalysts with respect to
maximum electrocatalytic activity, beyond which the activity that of Pt alone can be clearly seen in Fig. 4, where the
drops. According to the authors, while the promotion of the potentials at 0.1 mA cm
2 ðE0:1mAcm
2 Þ are plotted against
electro-oxidation is understood to be largely due to the alloy methanol concentration. The decrease of E0:1mAcm
2 on the Pt/C
catalyst, surface redox species of Ni oxide formed during the electrocatalyst with increasing methanol concentration is much
electro-oxidation process may also contribute to the oxygena- higher than that on the alloys, showing that the Pt–Co/C
tion of COads, thereby enhancing the oxidation current. electrocatalysts have a better tolerance to the presence of
Park et al. [60,97] also investigated the effect of Ni insertion methanol than Pt/C in sulphuric acid solution.
on PtRu catalysts in the methanol oxidation. The activity of Antolini et al. [101] prepared carbon-supported Pt70Ni30 by
Pt–Ru–Ni in the atomic ratio 5:4:1 for the MOR was compared NaBH4 reduction of the precursors and investigated the activity
 E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149 145

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
60 mV molCH3 OH
1 LÞ
than for PtNi ð
31 mV molCH3 OH
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.
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
Yang et al. [103] prepared carbon-supported Pt–Ni alloy
electrocatalysts. Reprinted from Ref. [100] with permission from Elsevier. catalysts with 40 wt.% total metal loading via the carbonyl
complex route, and studied the activity for the MOR and for
for the methanol oxidation and the oxygen reduction reactions ORR in the presence of methanol in H2SO4. By linear sweep
in sulphuric acid. They found that the current densities for the voltammetry measurements, as can be seen in Fig. 6, they found
methanol oxidation reaction on the Pt–Ni/C alloy electro- that the methanol oxidation current densities on Pt–Ni alloy
catalyst were lower than that on the Pt/C electrocatalyst and the catalysts are lower than that on a Pt/C catalyst and that the
onset potential for methanol oxidation at room temperature on methanol oxidation peaks on Pt–Ni alloy catalysts shift slightly
the Pt–Ni/C (440 mV) shifted to more positive potentials as to more positive potentials as compared to the Pt/C catalyst,
compared to Pt/C (375 mV), indicating that the alloy indicating that the oxidation of methanol on the alloy catalysts
electrocatalyst is less active for methanol oxidation than the is less active than that on a Pt/C catalyst. They observed
Pt/C electrocatalyst. The experimental results regarding the significantly enhanced electrocatalytic activities for ORR in
ORR in H2SO4 solution in the presence of methanol are methanol-containing electrolyte than pure Pt. Among the
summarized in Fig. 5, where the mixed potential is plotted cathode catalysts used, a maximum activity was found with a
versus the methanol concentration at 0.05 and 0.1 mA cm
2 Pt:Ni atomic ratio of 2:1. According to the authors, the high
(specific activity). The polynomial regression for the Pt and Pt– methanol-tolerance of Pt–Ni alloy catalysts during oxygen
Ni data was the following: reduction could be ascribed to a lowered activity for methanol
oxidation, originated from the composition effect and the
Pt : E ¼ E0
73 ½CH3 OH þ 6:8 ½CH3 OH2 (4)
disordered structure of the alloy catalysts.
Pt
Ni : E ¼ E0
44 ½CH3 OH þ 6:6 ½CH3 OH2 (5)

Fig. 5. Electrode potential vs. methanol concentration at 0.05 and 0.1 mA cm
2 Fig. 6. Linear sweep voltammetries of methanol oxidation on nanosized Pt/C and
(current expressed as specific activity) for Pt/C and Pt70Ni30/C electrocatalysts. Pt–Ni alloy catalysts in nitrogen saturated 0.5 mol L
1 H2SO4 + 0.5 mol L
1
Circles: Pt/C; triangles: Pt70Ni30/C. Solid symbols: j = 0.05 mA cm
2; open CH3OH solution at a scan rate of 5 mV s
1 and a rotation speed of 2000 rpm. Solid
symbols: j = 0.1 mA cm
2. Reprinted from Ref. [101] with permission from line: Pt/C; dashed line: Pt2Ni/C; dotted line: Pt3Ni2/C; dashed dotted line: PtNi/C.
Elsevier. Reprinted from Ref. [103] with permission from Elsevier.
146 E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149

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 Fig. 8. Dependence of the onset potential for the MOR on the atomic
potential for the MOR of Pt-based alloy was always lower than percentage of alloyed Co(Ni).
that of Pt, almost independent of the Co(Ni) content for low
Co(Ni) content, and decreasing with increasing amounts of the that it is possible to substantially improve the performance of
non-precious metal. The different behaviour between supported DMFCs by employing carbon-supported Pt–Co binary alloy as
and bulk alloy catalysts may depend on the higher degree of oxygen reduction catalysts on the cathode side in place of carbon-
alloying of unsupported alloys than that of carbon-supported supported platinum. They found for the cell operating at 90 8C
alloy catalysts [70]. Low Co(Ni) contents reduce the methanol with 2 bar oxygen pressure a maximum power density of 125 and
oxidation by the ensemble effect where the dilution of the 160 mW cm
2 for Pt/C and Pt–Co/C, respectively.
active component with the catalytically inert metal reduces the Salgado et al. [100] investigated the polarization curves in
methanol adsorption, while high Co(Ni) contents improve the single DMFC with Pt/C and Pt–Co/C as cathode electrocatalysts
MOR by electronic effects and by the presence of higher and Pt80Ru20/C as anode materials operating with 2 mol L
1
amounts of Co(Ni) oxide species, both enhancing CO methanol solution at 90 8C and a cathode pressure of 3 atm. On
oxidation. The linear dependence of the onset potential for the basis of the specific activity, a larger improvement of the cell
the MOR on the amount of alloyed Co(Ni), as shown in Fig. 8, performance was observed with Co-containing electrocatalysts
seems to confirm the decrease of the rate of methanol oxidation with respect to Pt/C, as shown in Fig. 9.
for low non-precious metal contents. Yang et al. [103] tested carbon-supported Pt–Ni with
40 wt.% metal loading in DMFCs. Fig. 10 presents a
5.5. Tests in DMFC comparison of power density against current density in DMFCs
with different cathode catalysts. The maximum power density
Neergat et al. [44] carried out tests in DMFC at 70 and 90 8C was found with a Pt:Ni atomic ratio of 2:1, similarly to results
operating at 2 bar and ambient oxygen pressure using Pt/C and from previous half-cell tests. For example, the maximum power
Pt–Co/C (Pt:Co = 1:1) as cathode materials. Their study showed

Fig. 9. Polarization curves in single DMFC with Pt–Co/C and Pt/C electro-
Fig. 7. Ratio of the onset potential for the MOR for Pt–Co(Ni) and Pt at room catalysts for oxygen reduction at 90 8C and 3 atm O2 pressure using a 2 mol L
1
temperature vs. the nominal Co(Ni) content in the catalyst. Plots for carbon- methanol solution. Anode Pt80Ru20/C. Current densities normalized with respect
supported (&) and bulk (*) alloy catalysts. 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

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

the value of DEc
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
summarized as follows: (i) the cells with Pt75Co25/C and Pt75Ni25/
C electrocatalysts as cathode materials performed better than
Fig. 10. Power density against current density curves recorded in a single
DMFC using different catalysts at 100 8C. Reprinted from Ref. [103] with those with the same alloy catalyst as anode material; (ii) the
permission from Elsevier. performance of the cell with Pt75Ni25/C as cathode material was
better than that of the cells with Pt/C and Pt75Co25/C both in terms
density for a Pt2Ni/C cathode catalyst is 101.5 mW cm
2 as of mass activity and specific activity; (iii) the performance of the
compared to 80.3 mW cm
2 for a Pt/C catalyst. cell with Pt75Co25/C as anode material was slightly better than
Antolini et al. investigated the behaviour in DMFC of that of the cell with Pt75Ni25/C in terms of thegeometric area of the
carbon-supported Pt–Ni with Pt:Ni prepared by reduction of electrode; (iv) the performance of cells with Pt75Co25/C and
precursors with NaBH4 [82] and of commercial carbon- Pt75Ni25/C as anode catalysts were slightly worse in terms of mass
supported Pt75Ni25 and Pt75Co25 [104], both as anode and as activity and almost the same in terms of specific activity than that
cathode materials. The results of DMFC tests using Pt–Ni by of the cell with Pt/C; (v) the enhanced performance of the cell with
NaBH4 indicated that (i) Pt–Ni electrocatalysts perform better Pt75Ni25/C as cathode material over that with Pt75Co25/C can be
as cathode than as anode materials; (ii) the performance of the rationalised on the basis of a higher methanol-tolerance, ascribed
cell with Pt–Ni electrocatalysts as cathode materials depends to electronic effects. On the basis of these results, the Pt75Ni25/C
on the amount of effectively alloyed Ni and does not depend on electrocatalyst appears to be a promising cathode material for
the total amount of Ni in the material; (iii) the performance of direct methanol fuel cells. In this nominal composition (Pt:Co(Ni)
the cell with Pt–Ni electrocatalysts as anode material increases 3:1), the binary electrocatalysts used as anode material, instead,
with increasing amounts of NiO species. Fig. 11 shows the didn’t show a substantial improvement over pure Pt.
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 6. Conclusions
anode electrocatalyst (Ea) from Refs. [74,75], as a function of
the amount of unalloyed nickel. DEc
a is indicative of the 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
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- ORR and higher methanol-tolerance than Pt.
catalyst (Ea) from Refs. [82] and [104], as a function of the atomic percentage of When used as anode materials, instead, notwithstanding the
unalloyed nickel. results of electrochemical tests in half-cell showing that Pt–Co
148 E. Antolini et al. / Applied Catalysis B: Environmental 63 (2006) 137–149

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