An overview of platinum-based catalysts as methanol-resistant oxygen reduction materials for direct methanol fuel cells

Journal of Alloys and Compounds 461 (2008) 253–262

Review

An overview of platinum-based catalysts as methanol-resistant oxygen reduction materials for direct methanol fuel cells E. Antolini a,b,∗ , T. Lopes b , E.R. Gonzalez b a

b

Scuola di Scienza dei Materiali, Via 25 aprile 22, 16016 Cogoleto, Genova, Italy Instituto de Qu´ımica de S˜ao Carlos, USP, C. P. 780, S˜ao Carlos 13560-970, SP, Brazil

Received 14 June 2007; received in revised form 25 June 2007; accepted 25 June 2007 Available online 30 June 2007

Abstract Low-temperature fuel cells, with either hydrogen or methanol as the fuel, represent an environmentally friendly technology and are attracting considerable interest as a means of producing electricity by direct electrochemical conversion of hydrogen/methanol and oxygen into water/water and carbon dioxide. Platinum has the highest catalytic activity for oxygen reduction of any of the pure metals and when supported on a conductive carbon serves as state of the art cathode material in low-temperature fuel cells. Regarding the direct methanol fuel cells (DMFCs), one of the major problems is the methanol crossover through the polymer electrolyte. The mixed potential, which results from the oxygen reduction reaction and the methanol oxidation occurring simultaneously, reduces the cell voltage, generates additional water 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 than Pt. Pt alloyed with first-row transition elements is proposed as cathode material with improved methanol tolerance for direct methanol fuel cells. In the light of the latest advances on this field, this paper presents an overview of platinum-based catalysts as methanol-resistant oxygen reduction materials for direct methanol fuel cells. © 2007 Published by Elsevier B.V. Keywords: Fuel cells; Electrode materials; Metals and alloys; Nanostructured materials

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methanol tolerant oxygen reduction catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Catalysts with lower MOR activity than Pt (reducing the mixed potential) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Catalysts with reduced methanol adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Catalysts with reduced CO oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Catalysts with higher MOR activity than Pt (reducing the CO poisoning) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Catalysts with same MOR but higher ORR than Pt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

253 255 255 255 258 259 259 261 261 261

1. Introduction

∗

Corresponding author at: Scuola di Scienza dei Materiali, Via 25 aprile 22, 16016 Cogoleto, Genova, Italy. E-mail address: [email protected] (E. Antolini). 0925-8388/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.jallcom.2007.06.077

The use of methanol as energy carrier and its direct electrochemical oxidation in direct methanol fuel cells (DMFCs) represents an important challenge for the polymer electrolyte fuel cell technology, since the complete system would be simpler without a reformer and reactant treatment steps. The use

254

E. Antolini et al. / Journal of Alloys and Compounds 461 (2008) 253–262

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]. In addition to the poor kinetics of the anode reaction [3–5], the major problem in the development of DMFC is the crossover of methanol from the anode to the cathode side through the protonexchange membrane [4–6], which decreases fuel efficiency and introduces parasitic currents due to methanol oxidation at Pt/C commonly used as the cathodic catalyst, resulting in a potential loss. In the case of DMFC the electrode reactions at the cathode are + 6(H+ + e− ) → 3H2 O

(1)

CH3 OH + 23 O2 → CO2 + 2H2 O

(2)

3 2 O2

Platinum has the highest catalytic activity for oxygen reduction of any of the pure metals and when supported on a conductive carbon serves as state of the art electrocatalysts in DMFC air cathodes [7]. However, due to kinetic limitations of the oxygen reduction reaction (ORR), a considerable loss in efficiency occurs in the cathode electrode, as the result of cathode overpotential [7]. Recently Norskov et al. [8], using density functional theory calculations, showed that the overpotential of the reaction (1) can be linked directly to the proton and electron transfer to adsorbed oxygen or hydroxide being strongly bonded to the surface at the electrode potential where the overall cathode reaction is at equilibrium. Their model predicts a volcano-shaped relationship between the rate of the cathode reaction and the oxygen adsorption energy. The model explains why Pt is the best elemental cathode material and why alloying can be used to improve its performance. Platinum is also 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. Methanol oxidation is a slow reaction that requires active multiple sites for the adsorption of methanol and the sites that can donate OH species for the desorption of the adsorbed methanol residues [9]. Methanol oxidation has been extensively investigated and 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 concluded 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: CH3 OH → (CH3 OH)ads

(3)

(CH3 OH)ads → (CO)ads + 4H+ + 4e−

(4)

(CO)ads + H2 O → CO2 + 2H+ + 2e−

(5)

The problem of methanol crossover in DMFCs has been extensively studied [4–6,17,18]: methanol adsorbs on Pt sites in the cathode for the direct reaction between methanol and oxygen.

The mixed potential, which results from the oxygen reduction reaction and the methanol oxidation occurring simultaneously, reduces the cell voltage, generates additional water and increases the required oxygen stoichiometric ratio. Moreover, the Pt surface is poisoned by CO which is a by-product of methanol oxidation. This poisoning effect ultimately results in instability as well as a reduction in cell performance. These problems 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 than Pt. Higher methanol tolerance is reported in the literature for non-noble metal electrocatalysts based on ruthenium chalcogenide catalysts [19–21] and macrocycles of transition metals [22,23]. These electrocatalysts have shown nearly the same activity for the ORR in the absence as well as in the presence of methanol. However in methanol free electrolytes, these materials 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. Catalysts with lower activity for the methanol oxidation reaction (MOR) than Pt to decrease the mixed potential and, on the other hand, catalysts with higher MOR activity to decrease CO poisoning have been investigated. 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 transition metals which present a higher activity for the ORR than platinum in low-temperature fuel cells operated on hydrogen, and use them as DMFC cathode electrocatalysts [24–27]. The improvement in the ORR electrocatalysis has been ascribed to different factors such as changes in the Pt–Pt interatomic distance [28] and the surface area [29], but the behaviour of binary alloys with respect to electrocatalysis can be better understood in terms of the electronic ”ligand 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 [30]. The electronic effect of elements present in the sublayers is illustrated on PtNi(1 1 1) and Pt3 Fe(1 1 1), which present a quasicomplete Pt surface layer (with more or less Ni or Fe in the sublayers) and strong modifications of their chemisorptive properties and electrocatalytic performances [30]. 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 alloys with increasing contents of the second element can be explained by the model of Toda et al. [31], based on an increase of d-electron vacancies of the thin Pt surface layer caused by the underlying alloy. Also, the increased catalytic activity for the ORR with increasing Pt particle size was ascribed to the weaker adsorption of OH on large particles, because there is a change in d-band occupancy related to a change in particle size. In the light of the latest advances on this field, this paper presents an overview of platinum-based catalysts as methanolresistant oxygen reduction materials for direct methanol fuel cells. The activity for the ORR of these catalysts is also evaluated.

E. Antolini et al. / Journal of Alloys and Compounds 461 (2008) 253–262

255

dation reaction than Pt and catalysts with higher MOR activity than Pt. Finally, a short section regarding catalysts with same methanol tolerance but higher ORR than Pt has been inserted in this work. 2.1. Catalysts with lower MOR activity than Pt (reducing the mixed potential) These catalysts have been divided in catalysts with lower MOR by reduced CH3 OH dissociative adsorption and catalyst with lower MOR activity by reduced CO oxidation.

Fig. 1. DMFC polarization data obtained at 85 ◦ C with a Pt–Ru/C catalyst as anode material and Pt/C (circles) or Pt–Fe/C (triangles) catalysts as cathode materials. Full symbols: cell voltage; open symbols: power density. Reprinted from Ref. [25], Copyright 2004, with permission from Elsevier.

2. Methanol tolerant oxygen reduction catalysts Both alloyed and non-alloyed Pt–M catalysts presented higher methanol tolerance than Pt. Shukla et al. [25] and Xiong and Manthiram [32] reported higher methanol tolerance for alloyed Pt–Fe/C (Pt:Fe atomic ratio = 46:54) and non-alloyed Pt–TiOx /C (Pt:Ti = 1:1), respectively, than Pt/C during the ORR in the presence of methanol. The performances of DMFCs with Pt–Fe/C and Pt–TiOx /C as cathode materials are reported in Figs. 1 and 2, respectively. In the case of Pt–Fe the activity for the ORR was lower than that of Pt/C, whereas for that regarding Pt–TiOx /C the ORR activity was higher than that of Pt/C. In both these work the activity for methanol oxidation of these catalysts was not measured, and the reason of the higher methanol tolerance of the binary catalysts than that of Pt was not well clarified. From now on, we have divided the methanol tolerant catalysts in two parts: catalyst which lower activity for the methanol oxi-

Fig. 2. Comparison of the electrochemical performances in DMFC single cell of Pt/TiOx /C catalysts and Pt/C. Anode: Pt–Ru/C with 1 mg cm−2 metal loading; cathode: Pt/TiOx /C or Pt/C with 1 mg cm−2 Pt loading; methanol (3 M) flow rate: 2.5 mL/min; O2 pressure: 40 psi; cell temperature: 70 ◦ C. Reprinted from Ref. [32], Copyright 2004, with permission from Elsevier.

2.1.1. Catalysts with reduced methanol adsorption The main way to reduce methanol adsorption is the so-called “ensemble effect”. The ensemble effect, where the dilution of the active component with catalytically inert metals by alloying changes the distribution of active sites, open different reaction pathways [33]. The dissociative chemisorption of methanol requires the existence of several adjacent Pt ensembles [1,34] 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 binarycomponent 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. The reduced methanol adsorption can be also related to the decreased Pt–Pt bond distance and to electronic effects by alloying. It has to be promptly pointed out that the dilution effects, the geometric effects and the electronic effect are present at the same time. Generally, these carbon supported methanol tolerant oxygen reduction alloy catalysts are prepared using low-temperature preparation methods, to avoid an undesired metal particle growth occurring by Pt and M alloying at high temperatures (>700 ◦ C) [35]. Yang et al. investigated the effects of both Cr content (Pt:Cr atomic ratio 3:1, 2:1and 1:1), at fixed metal loading of 20 wt.% [36], and the metal loading (20 and 40 wt.%), at fixed Pt:Cr atomic ratio 1:1 [37], in the carbon supported Pt–Cr alloy catalysts, prepared through the carbonyl chemical route, on the ORR and MOR, using linear scan voltammetry (LSV). A high degree of alloying of Cr with Pt occurred in all the catalyst. The particle size was in the range 3.1–3.4 nm. The specific activity of the Pt–Cr alloys catalysts for the ORR in the absence of methanol is higher than that of the Pt catalyst and increases with increasing the Cr content. As compared to the ORR in pure HClO4 solution, all the catalysts for the ORR showed an increase in overpotential in the presence of methanol. They ascribed the change in the apparent number of electrons transferred per oxygen molecule in methanol-containing electrolyte to the methanol oxidation. The lower reactivity of oxygen reduction on Pt-based catalysts is related to the fact that some of the catalytic active sites for the ORR have been blocked or covered by adsorbed intermediates from methanol or by methanol molecules. They observed a significant increase in overpotential, ca. ∼200 mV, of the ORR on pure Pt, while the potential loss on Pt–Cr/C alloy catalysts was only ca. 70 mV in comparison to that in pure acid

256

E. Antolini et al. / Journal of Alloys and Compounds 461 (2008) 253–262

solution. The ORR activity, both in mass activity and specific activity in the kinetic and mixed regions on the Pt–Cr alloy catalysts was much higher than that on pure Pt, indicating that the Pt–Cr alloy catalysts exhibited a high methanol tolerance than pure Pt catalyst. That is, the alloy catalysts can catalyse the oxygen reduction but effectively limit the oxidation of methanol. The current density of methanol oxidation in oxygen saturated solution on alloy catalysts at high potentials was lower than on pure Pt and decreased with increasing Cr content in the catalyst. Moreover, the ORR activity of Pt–Cr in the presence of methanol was nearly independent of the metal content. LSV measurements in 0.5 M HClO4 + 0.5 M CH3 OH indicated that the current densities of the MOR on Pt–Cr/C catalysts are much lower than that on Pt/C. The high methanol tolerance of Pt–Cr/C during the ORR was explained by the weak competition reaction of methanol oxidation, which could be induced by the presence of Cr atoms in the alloys (ensemble effect). Yang et. al. [38] prepared carbon supported Pt–Ni alloy catalysts with various Ni content and 40 wt.% total metal loading via the carbonyl complex route, and studied the activity for the MOR and for the ORR in the absence and in the presence of methanol in H2 SO4 . A high degree of alloying between Ni and Pt was observed. The particle size was ca. 3.0 nm. The changes in specific activity for the ORR based on the Pt real surface area of these Pt-based catalysts are in the sequence Pt–Ni(2:1)/C > Pt–Ni (3:2)/C > Pt–Ni (1:1)/C > Pt/C and the kinetic enhancement on the Pt–Ni alloy catalysts is a factor of ca. 2–6 in comparison to pure Pt/C. The maximum activity for the ORR in methanol-containing solution was also found with a Pt:Ni atomic ratio of 2:1. The methanol oxidation on Pt/C and Pt–Ni/C alloy catalysts in nitrogen saturated 0.5 M CH3 OH + 0.5 M H2 SO4 solution was investigated by LSV. They found that the methanol oxidation current densities on Pt–Ni/C alloy catalysts are lower than that on a Pt/C catalyst and that the methanol oxidation peaks on Pt–Ni/C catalysts shift slightly to more positive potentials as compared to the Pt/C catalyst. They observed that a higher reactivity of the MOR corresponds to a lower ORR activity in the presence of methanol on all the catalysts. According to the authors, this fact could explain the high methanol tolerance of the Pt–Ni alloy catalysts during the ORR. The low reactivity for methanol oxidation of Pt–Ni catalysts, and thus a high methanol tolerance during the ORR was ascribed to the composition effect and the disordered structure (ensemble effect) of Pt–Ni alloy catalysts. They also suggested that the lower onset potential and peak potential for methanol oxidation and for CO oxidation on nanosized Pt–Ni alloy catalysts could be correlated to the changes in lattice parameters of the Pt and Pt–Ni alloy catalysts. They also tested the Pt–Ni/C catalysts in DMFCs. Fig. 3 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 density for a Pt–Ni/C (2:1) cathode catalyst is 101.5 mW cm−2 as compared to 80.3 mW cm−2 for a Pt/C catalyst. Salgado et al. [39] investigated the methanol tolerance of Pt–Co/C alloy catalysts with Pt:Co atomic ratio = 85:15 and

Fig. 3. Power density against current density curves recorded in a single DMFC using different catalysts at 100 ◦ C. Reprinted from Ref. [38], Copyright 2005, with permission from Elsevier.

75:25 in H2 SO4 solution and in DMFC. XRD analysis indicated the formation of PtCo alloys. The particle size was ca. 4.0 nm. The ORR activity of the Co-containing catalysts in methanol-free H2 SO4 solution was higher than that of pure Pt. LSV measurements of methanol oxidation in 1 M H2 SO4 + 3 M CH3 OH solution showed that the current densities for the MOR on the Pt–Co/C catalysts are much lower than that on Pt/C and that the onset potential for methanol oxidation on Pt–Co /C catalysts shifts to more positive potentials than that on Pt/C. LSV measurements of the ORR activity in O2 -saturated H2 SO4 solutions in the presence of methanol and tests in DMFCs indicated that carbon supported Pt–Co/C alloy electrocatalysts possess enhanced oxygen-reduction activity compared to Pt/C in the presence of methanol. A cobalt atomic fraction of 0.15 seems to be enough to improve the methanol tolerance of these binary electrocatalysts. The high methanol tolerance of Pt–Co/C electrocatalyts during the ORR was ascribed to the low activity of the binary electrocatalysts for methanol oxidation by the ensemble effect. Antolini et al. [40] studied the ORR in the absence and presence of methanol on carbon-supported Pt–Cr alloy catalysts with Pt:Cr atomic ratio 9:1, 3:1 and 1:1, and their activity was compared with that of Pt. The Pt–Cr catalysts were in the alloyed form. The particle size was in the range 3.1–4.8 nm. The experimental findings can be summarized as follows: (i) in pure 1 M H2 SO4 the Pt–Cr/C (3:1) alloy catalyst showed the best activity for ORR, while the ORR activity of Pt–Cr/C (9:1) and (1:1) was lower than that of Pt/C; (ii) in O2 -free 1 M H2 SO4 /3 M CH3 OH the lowest activity for MOR was observed at the Pt–Cr/C (9:1) catalyst; (iii) the best ORR activity in 1 M H2 SO4 /1–3 M CH3 OH was showed by the Pt–Cr/C (9:1) catalyst. The decrease in MOR activity passing from pure Pt to Pt–Cr (9:1) was explained on the basis of the above-mentioned “ensemble effect”. The presence of Cr atoms, however, reduces the Pt–CO bond strength substantially, enhancing the oxidation of CO. Indeed, it is known that the strong adsorption of OH and CO on small particle (<5 nm) slows the methanol oxidation, as

E. Antolini et al. / Journal of Alloys and Compounds 461 (2008) 253–262

a result of a significant increase in the d-band vacancy at potentials higher than 0.54 V [41]. But such an increase in Pt d-band vacancy is reduced by increasing the content of the non-precious metal in the alloy [42]. Then, the electronic effect of alloying explains the increase of MOR activity with increasing Cr content in the alloy. Thus, 9:1 seems to be an optimum value of the Pt:Cr ratio that maximises the ensemble effect and minimises the removal of CO. Carbon supported Pt–Ni electrocatalysts in the Pt:Ni atomic ratio 90:10 and 70:30 were prepared by Antolini et al. [43] by the reduction at room temperature of Pt and Ni salts with sodium borohydride and tested in direct methanol fuel cells both as anode and as cathode materials. These Pt–Ni electrocatalysts contain about the same amount of Ni alloyed (6–8 at.%), but have a markedly different amount of non-alloyed Ni. The particle sizes were 4.1 and 4.8 nm for the Pt–Ni (90:10) and Pt–Ni (70:30), respectively. The performance in DMFC of Pt–Ni as cathode was higher than that of Pt–Ni as anode. Pt–Ni as cathode showed better performance than Pt both in terms of mass activity and specific activity. The enhanced methanol tolerance of Pt–Ni was ascribed mainly to the alloyed nickel, while the presence of unalloyed Ni and/or NiO species seems to have no effect on the performance of direct methanol fuel cells with Pt–Ni as cathode electrocatalyst. The activity for the oxygen reduction reaction of Pt–Ni/C electrocatalyst in the Pt:Ni atomic ratio 70:30 was investigated in sulphuric acid both in the absence and in the presence of methanol [44]. In methanol-free sulphuric acid the Pt–Ni/C alloy catalyst showed a lower specific activity towards the oxygen reduction compared to pure platinum. In O2 -free H2 SO4 the onset potential for methanol oxidation at Pt–Ni/C was shifted to more positive potential, indicative of a lower activity for the methanol oxidation than platinum. The higher ORR activity in the methanol containing electrolyte of Pt–Ni/C electrocatalyst was ascribed to the low activity of the binary electrocatalyst for methanol oxidation, arising from a composition effect. As reported by Antolini et al. [45], 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 of the alloyed metal on Pt 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. 4, seems to confirm the decrease of the rate of methanol oxidation for low non-precious metal contents. Scott et al. [46] prepared three Pt–Fe/C catalysts with atomic ratios of 3.8:1, 1.2:1 and 1:2.7, respectively, by the impregnation method. The decrease of the lattice parameter of all three samples indicated the formation of PtFe alloys. They compared the performance of the Pt–Fe/C (1:2.7) and the Pt/C gas diffusion electrodes in 0.5 M H2 SO4 + 1 M CH3 OH solution, where current density is normalised to the Pt loading. Methanol was oxidised on both electrodes, which resulted in two peaks with current densities of 32.2 and 6.9 mA cm−2 for the Pt/C and Pt–Fe/C (1:2.7) electrode, respectively, suggesting higher methanol tolerance of the latter. The current densities by steadystate polarisation data obtained at −0.4 V in O2 -saturated 0.5 M

257

Fig. 4. Dependence of the onset potential for the MOR on the atomic percentage of alloyed Co (Ni). Reprinted from Ref. [45], Copyright 2006, with permission from Elsevier.

H2 SO4 solutions using Pt–Fe/C were higher than those using Pt/C, both in the absence and in the presence of methanol. DMFC tests with the Pt–Fe/C (1.2:1) cathode showed better performance than that with the Pt/C cathode, e.g. an increase of 10% in power density. As is well-known, iron alone does not form active sites for ORR, but alloying it with Pt can lead to performance enhancement due to previously described effects. Better methanol tolerance of the PtFe alloys than of Pt can be attributed to the role of Fe addition. Iron itself is not active for methanol oxidation and its addition will partly block contact between Pt particles and methanol molecules, reducing the adsorption of methanol. Quantum calculations show that the strong reactivity of Pt with organic compounds is depressed by alloying with Fe [47], resulting in higher methanol tolerance of Pt–Fe cathodes, compared to Pt. A hindering in the CH3 OH adsorption on Pt by the presence of copper oxide was invoked by Lee et al. [48] to explain the lower

Fig. 5. Linear sweep voltammogram of Pt and Pt–CuO in 0.5 M CH3 OH/0.5 M H2 SO4 . Scan rate = 20 mV/s. Reprinted from Ref. [48], Copyright 2007, with permission from Elsevier.

258

E. Antolini et al. / Journal of Alloys and Compounds 461 (2008) 253–262

methanol oxidation on a CuO-modified Pt than unmodified Pt cathode. As shown in Fig. 5, the linear sweep voltammograms of Pt and Pt–CuO in 0.5M CH3 OH/0.5M H2 SO4 indicated that methanol oxidation onset potentials on both electrodes are similar, but the difference of oxidation current between an unmodified and a modified Pt electrode becomes larger on the anodic scan and it reaches about 30 mA at 900 mV. Moreover, copper oxide modified Pt cathode produced about 20% higher DMFC power density than that of unmodified Pt cathode applying a methanol of 1.0 M and it was ascribed to the blocking effect of copper oxide to the electrooxidation of crossovered methanol on a cathode surface. Furthermore, a little bit higher oxygen partial pressure resulted from oxygen supplying in the reduction of CuO could explain the different performance between modified and unmodified Pt cathode. According to the authors, the higher performance of Pt–CuO than Pt cathode could be understood by selective adsorption of oxygen. 2.1.2. Catalysts with reduced CO oxidation Another way to slow the methanol oxidation is to reduce CO oxidation, by decreasing the formation of oxygen-containing species required for CO oxidation, and/or by increasing the strength of CO and OH adsorption on Pt. Xia et al. [49] synthesized carbon supported ordered intermetallic phase PtBi2 nanoparticles with an average particle size of 4.1 nm. Cyclic voltammetry (CV) measurements showed that, after the oxygen was introduced into the 0.5 M H2 SO4 solution, a strong oxygen reduction wave starting at ca. 100 mV can be observed, demonstrating that PtBi2 has catalytic activity toward oxygen reduction. In order to test the methanol tolerance of this catalyst, they added 5.0 M methanol to the solution. Such cyclic voltammogram curves did not change with the increase in cyclic times; it is obvious that no methanol oxidation could be observed. This result clearly indicates that the PtBi2 catalyst has an electrochemical resistance to methanol oxidation, and the stable cyclic voltammogram curve shows that the methanol tolerant property of PtBi2 /C did not change with time. The mechanism for methanol tolerance of the PtBi2 /C catalyst is not clear at this stage. One of the possible explanations is that the formation of the PtBi2 phase may result in a charge redistribution, which in turn may lead to difficulties in the formation of oxygen-containing species from water dissociation. This is one of the key steps for methanol oxidation. Another fact is that the Pt–Pt distance in PtBi2 is larger than that in other Pt-based cat-

alysts, which may prohibit the formation of oxygen-containing species. Stassi et al. [50] prepared 60 wt% Pt–Fe/C and Pt–Cu/C catalysts with Fe and Cu content of 5 wt%. The slight decrease of the lattice parameter indicated a moderate degree of alloying of Fe with Pt, whereas, a significant lattice contraction was found for the Pt–Cu catalysts. Polarization curves for ORR for Pt, Pt–Cu and Pt–Fe, in an oxygen saturated H2 SO4 solution at 60 ◦ C showed similar electrocatalytic behaviour for Pt and Pt–Cu catalysts, whereas the onset potential for the ORR on Pt–Fe catalyst is shifted towards higher potential, indicating better catalytic characteristics of this alloy for ORR compared to the previous ones. The polarization curve on the Pt–Fe catalyst is less negatively shifted in the presence of methanol than that on Pt. According to the authors, this clearly indicates a promoting effect of the bimetallic catalyst in enhancing the ORR and a better tolerance to methanol. According to the authors, the smaller particle size of the Pt–Fe catalyst (2.4 nm) with respect to the Pt catalyst (2.8 nm) used for comparison could be responsible of the enhanced activity. Indeed, Maillard et al. [24] showed enhanced methanol tolerance for ORR of Pt-based catalysts as a function of particle size decrease. As previously reported, the strong adsorption of OH and CO on small particles slows the methanol oxidation, as a result of a significant increase in the d-band vacancy. Such an effect, however, is not observed for Pt–Cu/C catalyst, which shows a similar activity to Pt/C despite of its particle size is quite small compared to carbon supported pure Pt catalyst (2.1 nm versus 2.8 nm). Koffi et al. [51] synthesized non-alloyed Pt–Cr/C electrocatalysts with Pt:Cr in the range 90:10 to 70:30 by a colloidal route method. The non-alloyed character of the catalysts was showed by XRD analysis. These catalysts displayed better activity than Pt/C in methanol-free as well as in low concentration methanol containing solutions in potentials range interesting for DMFC application. As can be seen in Table 1, the Pt–Cr electrocatalyst having an nominal atomic ratio, close to (80:20) showed higher activity for ORR in methanol-free oxygen saturated electrolyte, whereas the catalyst having an atomic ratio of (70:30) displayed higher activity for ORR at low overpotentials in saturated oxygen electrolyte containing 0.1 M methanol. The catalytic activity towards the methanol electrooxidation reaction (MOR) of all Cr-containing catalysts, measured in 0.1 M CH3 OH solution, was lower than that of Pt. Regarding the effect of Cr content in Pt–Cr/C catalysts, The Pt–Cr/C (90:10) catalyst displayed

Table 1 Surface activity (SA) and mass activity (MA) in O2 saturated electrolytes with or without 0.1 M CH3 OH, and Tafel slopes for different Pt1−x Crx /C catalysts Catalyst

Pt/C Pt–Cr/C (90:10) Pt–Cr/C (85:15) Pt–Cr/C (80:20) Pt–Cr/C (75:25) Pt–Cr/C (70:30)

SA (␮A cm−2 ; E = 0.8 V vs. RHE)

MA (mA mg−1 Pt ; E = 0.8 vs. RHE)

Tafel slopes

O2

O2

O2 + 0.1 M CH3 OH

E > 0.74 vs. RHE

E < 0.74 vs. RHE

7.6 12.1 12.3 13.3 9.32 7.2

4.4 5.5

57 80

130 140

5.6

70

131

6.2

73

132

11.3 18.5 19.8 21.9 16.7 13.3

O2 + 0.1 M CH3 OH 7.8 8.5 9.2 11.4

Reprinted from Ref. [51], Copyright 2005, with permission from Elsevier.

E. Antolini et al. / Journal of Alloys and Compounds 461 (2008) 253–262

259

the best activity for this reaction, whereas the Pt–Cr/C (80:20) and (70:30) catalysts displayed an activity towards MOR almost three times lower. In the potential range from 0.0 to 0.5 V versus RHE platinum is known to adsorb dissociatively the methanol molecule to form adsorbed CO species [52,53]. According to the authors, it is likely that the interaction between adsorbed CO, oxygen and catalysts undergoes some changes as a function of chromium, lowering the activity for the methanol oxidation. But, the alloyed Pt–Cr catalysts prepared using the carbonyl route by Yang et al. [36] display a better activity towards ORR than the non-alloyed catalysts by Koffi et al. [50], in the absence as well as in the presence of methanol. 2.2. Catalysts with higher MOR activity than Pt (reducing the CO poisoning) An alternate approach to prevent the loss of unit cell performance by methanol crossover and to assure the long-term stability of cell performance is to prevent the poisoning of Pt by removing the CO adsorbed on the Pt. According to bifunctional and electronic effects well-known in designing of alloy catalysts for methanol oxidation at anode in DMFC [12,54-56], Pt-based alloy catalysts can be prepared for oxygen reduction so that the Pt is not poisoned. During alloy formation the presence of a second metal should be able to remove CO poisoning of Pt catalysts and, at the same time, not affect the catalytic activity for oxygen reduction to any extent. Park et al. [57] synthesized unsupported PtRh alloy nanoparticles in the Pt:Rh atomic ratio 3:1, 2:1 and 1:1 by the reduction of Pt and Rh precursors at room temperature with NaBH4 , and investigated their ORR activity and methanol tolerance by LSV measurements and tests in DMFCs. By XRD measurements alloy formation between Pt and Rh was observed. The average particle size of Pt–Rh (x:1) particles was about 4–5 nm. Linear sweep voltammetry of Pt, Pt–Ru, and Pt–Rh for methanol oxidation in 0.5 M H2 SO4 + 2 M CH3 OH indicated that Pt–Ru nanoparticles possess the optimal catalytic activity for methanol oxidation. However, the MOR activity of the Pt–Rh (2:1) alloy catalyst was superior to that of Pt and the other Pt–Rh catalysts, showing a higher oxidation current and lower onset potential than pure Pt and Pt–Rh (3:1) and (1:1). Even if further studies are needed to clarify the mechanism of the enhanced activity of Pt–Rh alloy, the authors concluded that the alloy may contribute to enhanced CO oxidation. By LSV measurements in an O2 saturated H2 SO4 solution it resulted that pure Pt catalyst has the best performance for the ORR. On the other hand, Pt–Rh (1:1) and Pt–Rh (3:1) alloys were not functional as oxygen reduction catalysts. The activity of Pt–Ru was also much lower than pure Pt in oxygen reduction. However, Pt–Rh (2:1) showed considerable catalytic activity for oxygen reduction. From the electrochemical results of oxygen reduction and methanol oxidation, Pt–Rh (2:1) could be an optimum catalyst for the elimination of CO poisoning during the oxidation of crossover methanol as well as for the oxygen reduction. Fig. 6 shows a comparison of DMFC performance at 30 ◦ C using Pt–Rh (2:1), Pt–Ru (1:1) and Pt as the cathode catalyst. It is known that the performance of a DMFC at low-temperature, such as 30 ◦ C, is strongly affected by methanol

Fig. 6. Comparison of DMFC performance at 30 ◦ C using Pt–Rh (2:1), Pt–Ru (1:1) and Pt as the cathode catalysts. Full symbols: cell voltage; open symbols: power density. Reprinted from ref. [57], Copyright 2006, with permission from Elsevier.

crossover to the cathode through a Nafion membrane [58]. As can be seen in Fig. 6, cell performance with Pt–Rh (2:1) as cathode material is superior to that for Pt–Ru (1:1) and Pt as cathode catalysts. The open circuit voltages were 0.73, 0.70, and 0.69 V for Pt–Rh (2:1), Pt–Ru (1:1) and Pt, respectively. Cell performances were 60, 40 and 45 mW cm−2 at Pt–Rh (2:1), Pt–Ru (1:1) and Pt, respectively. According to the authors, it is probable that this higher catalytic activity is the result of the removal of CO or COH from the Pt during crossover methanol oxidation by alloy formation with the Rh metal and thus increases the number of available sites for oxygen reduction. 2.3. Catalysts with same MOR but higher ORR than Pt Li et al. [59] investigated the ORR activity of carbon supported Pt–Fe catalysts, prepared by a polyol synthesis method and thermally treated at 300 ◦ C (Pt–Fe/C300) or 900 ◦ C (Pt–Fe/C900), both in methanol-free and in methanol-containing perchloric acid electrolyte (1.0 M HClO4 + 0 1 M CH3 OH) and in direct methanol single cell. As comparison, the same measurements were performed on a Pt–Fe/C alloy catalyst, prepared by a two-step method (Pt–Fe/C900B). They found that the ORR activity of Pt–Fe/C300 is higher than that of Pt/C, Pt–Fe/C900 and Pt–Fe/C900B catalysts. The Pt–Fe/C300 catalyst consists of most Pt and little Fe (Pt:Fe atomic ratio = 93:7), and has a mean particle size of 2.8 nm. CV and rotating disk electrode (RDE) experiments are also used to test the methanol tolerant properties of Pt–Fe/C300 and Pt/C catalysts in a 1.0 M HClO4 + 0.1 M CH3 OH electrolyte. Although the onset potential of methanol oxidation for the two catalysts do not shift, the hydrogen desorption peak of Pt–Fe/C300 is higher than that of the Pt/C sample while the methanol oxidation peak of Pt–Fe/C300 is smaller than that of Pt/C catalyst, due to the smaller electrochemical surface area of Pt–Fe/C300 than that of Pt/C. From RDE measurements it resulted that nearly no methanol oxidizing peak shift between these two catalysts occurs and the limiting current densities are almost same. Pt–Fe/C300 exhibited better performance than other Pt–Fe/C or Pt/C catalysts when employed as

260

Table 2 Comparison of MOR activity and ORR activity in the absence and in the presence of methanol of different Pt-based binary catalysts for DMFCs with those of Pt alone Structural characteristics

ORR activity with respect to Pt

MOR activity with respect to Pt

ORR activity in the presence of CH3 OH with respect to Pt.

Reference

Pt–Fe/C (Pt/Fe = 0.85).

Formation of a fct PtFe alloy. Particle size 11 nm Non-alloyed Pt–Ti catalyst. Presence of TiOx . Particle size 3.8 nm Formation of PtCr alloys. Particle size 3.1–3.4 nm Formation of PtNi alloys. Particle size ca. 3.0 nm Formation of PtCo alloys. Particle size ca. 4.0 nm Formation of PtCr alloys. Particle size in the range 3.1–4.8 nm Ni partially alloyed with Pt. Particle size 4.1 and 4.8 nm Formation of PtFe alloys

Lower

n.d.

Higher

[25]

Higher

n.d.

Higher

[32]

Higher

Lower

Higher

[36,37]

Higher

Lower

Higher

[38]

Higher

Lower

Higher

[39]

Higher for PtCr/C (3:1); lower for PtCr/C (9:1) and (1:1) Lower

Lower

[40]

Lower

Higher for PtCr/C (3:1); lower for PtCr/C (9:1) and (1:1) Higher

Higher

Lower

Higher

[46]

Presence of fcc Pt metal and copper oxide Moderate alloying of Pt and Fe. Particle size 2.4 nm Formation of PtCu alloy. Particle size 2.1 nm Cr non-alloyed with Pt Alloy formation between Pt and Rh. Particle size about 4–5 nm

Higher

Lower

Higher

[48]

Higher

Lower

Higher

[50]

Same

Same

Same

[50]

Higher Lower

Higher Higher activity of the Pt–Rh (2:1) alloy catalyst

[51] [57]

Low degree of alloying. Particle size 2.8 nm n.d.

Higher

Lower Higher activity of Pt–Rh (2:1). Same activity of Pt–Rh (3:1) and (1:1) Same

Higher

[59]

Higher

Same

Higher

[60]

Pt–TiOx /C (Pt/Ti = 1).

Pt–Cr/C with Pt/Cr atomic ratio: 1, 2 and 3 Pt–Ni/C with Pt/Ni atomic ratio: 1, 1.5 and 2 Pt–Co/C with Pt/Co atomic ratio: 3 and 5.7 Pt–Cr/C with Pt/Cr atomic ratio: 1, 3 and 9 Pt–Ni/C with Pt/Ni atomic ratio: 2.3 and 9 Pt–Fe/C with Pt/Fe atomic ratio of 0.37, 1.2 and 3.8 Pt–CuO Pt–Fe/C with Fe content of 5 wt.% Pt–Cu/C with Cu content of 5 wt.% Pt–Cr/C with Pt/Cr in the range: 2.3–9 Unsupported Pt–Rh alloy nanoparticles with Pt/Rh atomic ratio: 1, 2 and 3 Pt–Fe/C with Pt/Fe atomic ratio: 13.2 Unsupported Pt–Ni nanoparticles with atomic ratio: 2.3

[43]

E. Antolini et al. / Journal of Alloys and Compounds 461 (2008) 253–262

Catalyst

E. Antolini et al. / Journal of Alloys and Compounds 461 (2008) 253–262

261

cathode material in direct methanol single cell test. The authors ascribed the enhancement of the cell performance to its higher ORR activity, which is probably attributed to more Pt0 species existing and Fe ion corrosion from the catalyst. Drillet et al. [60] prepared an unsupported Pt–Ni (70:30) catalyst by melting together Pt and Ni pellets in a vacuum arc and studied the methanol oxidation and the electrochemical oxygen reduction reaction at Pt and Pt–Ni in 1 M H2 SO4 + 0.5 M CH3 OH. By cyclic voltammetry they found no significant difference in the methanol oxidation on Pt and Pt–Ni (70:30), particularly regarding the onset potential for methanol oxidation. They found that in pure sulphuric acid, the overpotential of ORR at 1 mA cm−2 is about 80 mV lower at Pt–Ni than at pure Pt. On the other hand, by means of a rotating disc electrode they found that in a methanol containing electrolyte solution the onset potential for oxygen reduction at Pt–Ni is shifted to more 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. The comparison of the oxygen reduction activity in methanolfree solutions as well as in the presence of methanol, and of the methanol oxidation activity of different Pt-based binary catalysts with those of Pt alone are summarized in Table 2.

role regarding the MOR activity and the ORR activity. So, the preparation method, leading to differences in alloying, particle size, morphology and surface area has an important influence on the catalytic activity of these catalysts. Summarizing, higher methanol tolerance can be obtained by the following ways:

3. Discussion and conclusions

The authors thank CAPES/Brazil, Progr. PVE 2007, and the Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico (CNPq, Proc. 142097/2005-5), for financial assistance to the project.

Generally, the higher ORR activity of Pt-based binary catalysts in the presence of methanol than that of pure Pt was related both to a higher ORR activity and mainly to a higher methanol tolerance of the binary catalysts. In some cases, however, lower ORR activity and higher methanol tolerance or higher ORR and same methanol tolerance of Pt–M catalysts than Pt was observed. As reported by Mukerjee et al. [42], the dependence of ORR activity of Pt–M catalysts on the M content goes through a maximum. For that regarding the MOR activity, the formation of a Pt–M alloy gives rise to two counteracting effects: a decrease of the MOR activity by the dilution effect, and an increase of the MOR activity due to M presence that reduces the Pt–CO bond strength substantially, enhancing the oxidation of CO. The former effect prevails at low M contents, while the latter is predominant for high M contents. As a consequence, the dependence of the MOR activity on M content goes through a minimum. For alloyed Pt–Ni/C [38] and Pt–Rh/C [57] catalysts and for non-alloyed Pt–Cr/C [51] the best composition for the ORR activity was almost the same than that for methanol tolerance, whereas for the alloyed Pt–Cr/C catalyst [40] the optimum Pt:Cr atomic ratio for the ORR activity was lower (3:1) than that for methanol tolerance (9:1). In this case, it is needed to achieve a catalyst composition with the best mixing of the ORR and MOR activities. As can be seen in Table 2, controversial results regarding the ORR and MOR activities of Pt–Fe [25,50,59] and Pt–Ni [38,43,60] catalysts have been reported. These conflicting results are related to the different Pt:M composition, degree of alloying, Pt and M oxide content, surface composition and the particle size of these catalysts. Indeed, all theses factors play an important

(1) Methanol adsorption reduction (ensemble effect). This method is the more successful, decreasing both the mixed potential and CO poisoning. (2) CO oxidation lowering. This method reduces the mixed potential but does not reduce CO poisoning. (3) CO oxidation enhancement. This method reduces CO poisoning but increases the mixed potential. The better performance of catalysts with lower methanol adsorption than those with lower CO oxidation is evident by comparing the methanol tolerance of alloyed [36] and nonalloyed [51] Pt–Cr/C: both these two catalysts possess higher methanol tolerance than Pt, but the alloyed Pt–Cr (decrease adsorption) was more methanol tolerant than non-alloyed Pt–Cr (decrease CO oxidation). Acknowledgements

References [1] C. Lamy, A. Lima, V. Le Rhun, C. Coutanceau, J.M. Leger, J. Power Sources 105 (2002) 283. [2] A. Hamnett, Catal. Today 38 (1997) 445. [3] E. Reddington, A. Sapienza, B. Gurau, R. Viswanathan, S. Sarangapani, E.S. Smotkin, T.E. Mallouk, Science 280 (1998) 1735. [4] A. Heinzel, V.M. Barragan, J. Power Sources 84 (1999) 70. [5] J. Cruickshank, K. Scott, J. Power Sources 70 (1998) 40. [6] K. Ramya, K.S. Dhathathreyan, J. Electroanal. Chem. 542 (2003) 109. [7] S. Gottesfeld, T.A. Zawodzinski, in: R.C. Alkire, H. Gerischer, D.M. Kolb, C.W. Tobias (Eds.), Polymer Electrolyte Fuel Cells. In Advances in Electrochemical Science and Engineering, vol. 5, 1st ed., Wiley–VCH, Weinheim, 1997, p. 195. [8] J.K. Norskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J.R. Kitchin, T. Bligaard, H. Jonsson, J. Phys. Chem. B 108 (2004) 17886. [9] V.S. Bagotzsky, Y.B. Vassiliev, O.A. Khazova, J. Electroanal. Chem. 81 (1977) 229. [10] R. Parsons, T. VanderNoot, J. Electroanal. Chem. 257 (1988) 9. [11] S. Wasmus, A. Kuver, J. Electroanal. Chem. 461 (1999) 14. [12] T. Iwasita, Electrochim. Acta 47 (2002) 3663. [13] T. Iwasita, W. Vielstich, J. Electroanal. Chem. 201 (1986) 403. [14] C. Korzeniewski, C. Childers, J. Phys. Chem. B 102 (1998) 489. [15] K. Ota, Y. Nakagava, M. Takahashi, J. Electroanal. Chem. 179 (1984) 179. [16] Y. Jusys, J. Kaiser, R.J. Behm, Electrochim. Acta 47 (2002) 3693. [17] B. Gurau, E.S. Smotkin, J. Power Sources 112 (2002) 3339. [18] P.M. Urban, A. Funke, J.T. Muller, M. Himmen, A. Docter, Appl. Catal. A: Gen. 221 (2001) 459. [19] M. Bron, P. Bogdanoff, S. Fiechter, I. Dorbant, M. Hilgendorff, H. Schulenburg, H. Tributsch, J. Electroanal. Chem. 500 (2001) 510. [20] R.W. Reeve, P.A. Christensen, A.J. Dickinson, A. Hamnett, K. Scott, Electrochim. Acta 45 (2000) 4237.

262

E. Antolini et al. / Journal of Alloys and Compounds 461 (2008) 253–262

[21] T.J. Schmidt, U.A. Paulus, H.A. Gasteiger, N. Alonso-Vante, R.J. Behm, J. Electrochem. Soc. 147 (2000) 2620. [22] R. Jiang, D. Chu, J. Electrochem. Soc. 147 (2000) 4605. [23] P. Convert, C. Coutanceau, P. Crouigneau, F. Gloaguen, C. Lamy, J. Appl. Electrochem. 31 (2001) 945. [24] F. Maillard, M. Martin, F. Gloaguen, J.M. Leger, Electrochim. Acta 47 (2002) 3431. [25] A.K. Shukla, R.K. Raman, N.A. Choudhury, K.R. Priolkar, P.R. Sarode, S. Emura, R. Kumashiro, J. Electroanal. Chem. 563 (2004) 181. [26] M. Neergat, A.K. Shukla, K.S. Gandhi, J. Appl. Electrochem. 31 (2001) 373. [27] A.K. Shukla, M. Neergat, P. Bera, V. Jayaram, M.S. Hegde, J. Electroanal. Chem. 504 (2001) 111. [28] V. Jalan, E.J.J. Taylor, J. Electrochem. Soc. 130 (1983) 2299. [29] M.T. Paffett, G.J. Berry, S. Gottesfeld, J. Electrochem. Soc. 135 (1988) 1431. [30] J.C. Bertolini, Surf. Rev. Lett. 3 (1996) 1857. [31] T. Toda, H. Igarashi, H. Uchida, M. Watanabe, J. Electrochem. Soc. 146 (1999) 3750. [32] L. Xiong, A. Manthiram, Electrochim. Acta 49 (2004) 4163. [33] N.M. Markovic, P.N. Ross, Surf. Sci. Rep. 45 (2002) 121. [34] H.A. Gasteiger, N.M. Markovic, P.N. Ross, E.J. Cairns, Electrochim. Acta 39 (1994) 1825. [35] E. Antolini, Mater. Chem. Phys. 78 (2003) 563. [36] H. Yang, N. Alonso-Vante, J.-M. Leger, C. Lamy, J. Phys. Chem. B 108 (2004) 1938. [37] H. Yang, N. Alonso-Vante, C. Lamy, D.L. Akins, J. Electrochem. Soc. 152 (2005) A704. [38] H. Yang, C. Coutanceau, J.-M. Leger, N. Alonso-Vante, C. Lamy, J. Electroanal. Chem. 576 (2005) 305. [39] J.R.C. Salgado, E. Antolini, E.R. Gonzalez, Appl. Catal. B 57 (2005) 283. [40] E. Antolini, J.R.C. Salgado, L.G.R.A. Santos, G. Garcia, E. Pastor, E.A. Ticianelli, E.R. Gonzalez, J. Appl. Electrochem. 36 (2006) 355. [41] S. Mukerjee, J. McBreen, J. Electrochem. Soc. 143 (1996) 2285.

[42] S. Mukerjee, S. Srinivasan, M.P. Soriaga, J. McBreen, J. Electrochem. Soc. 142 (1995) 1409. [43] E. Antolini, J.R.C. Salgado, A.M. dos Santos, E.R. Gonzalez, Electrochem. Solid State Lett. 8 (2005) A226. [44] E. Antolini, J.R.C. Salgado, E.R. Gonzalez, J. Power Sources 155 (2006) 161. [45] E. Antolini, J.R.C. Salgado, E.R. Gonzalez, Appl. Catal. B 63 (2006) 137. [46] K. Scott, W. Yuan, H. Cheng, J. Appl. Electrochem. 37 (2007) 21. [47] A. Fortunelli, A.M. Velasco, J. Mol. Struct.: THEOCHEM 586 (2002) 17. [48] J.K. Lee, J. Choi, S.J. Kang, J.M. Lee, Y. Tak, J. Lee, Electrochim. Acta 52 (2007) 2272. [49] D. Xia, G. Chen, Z. Wang, J. Zhang, S. Hui, D. Ghosh, H. Wang, Chem. Mater. 18 (2006) 5746. [50] A. Stassi, C. D’Urso, V. Baglio, A. Di Blasi, V. Antonucci, A.S. Arico, A.M. Castro Luna, A. Bonesi, W.E. Triaca, J. Appl. Electrochem. 36 (2006) 1143. [51] R.C. Koffi, C. Coutanceau, E. Garnier, J.-M. Leger, C. Lamy, Electrochim. Acta 50 (2005) 4117. [52] A. Kabbabi, R. Faure, R. Durand, B. Beden, F. Hahn, J.-M. Leger, C. Lamy, J. Electroanal. Chem. 444 (1998) 41. [53] P. Waszczuk, A. Wieckowski, P. Zenelay, S. Gottesfeld, C. Coutanceau, J.-M. Leger, C. Lamy, J. Electroanal. Chem. 511 (2001) 55. [54] N.M. Markovic, H.A. Gasteiger, P.N. Ross, X. Jiang, I. Villegas, M.J. Weaver, Electrochim. Acta 40 (1995) 91. [55] S.L. Goikovic, T.R. Vidakovic, D.R. Durovic, Electrochim. Acta 48 (2003) 3607. [56] P.A. Christensen, A. Hamnett, G.L. Troughton, J. Electroanal. Chem. 362 (1993) 207. [57] K. Park, D. Han, Y. Sung, J. Power Sources 163 (2006) 82. [58] S.-A. Lee, K.-W. Park, J.-H. Choi, B.-K. Kwon, Y.-E. Sung, J. Electrochem. Soc. 106 (2002) 1869. [59] W. Li, W. Zhou, H. Li, Z. Zhou, B. Zhou, G. Sun, Q. Xin, Electrochim. Acta 49 (2004) 1045. [60] J.F. Drillet, A. Ee, J. Friedemann, R. Kotz, B. Schnyder, V.M. Schmidt, Electrochim. Acta 47 (2002) 1983.