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Platinum and platinum-based electrocatalysts for the methanol oxidation reaction (MOR) are commonly used as the anode material in direct methanol fuel cells (DMFCs). Photo-oxidation promoted by ultraviolet and visible light is a promising method to increase the catalytic activity of DMFC anode electrocatalysts. Photocatalytic and electrocatalytic methanol oxidation can be coupled by addition of TiO2, a semiconductor photocatalyst, to Pt. In the presence of TiO2, an increase of the MOR activity of Pt-based electrocatalysts takes place also in dark conditions. This review deals with the methanol photo-oxidation on Pt/TiO2 catalysts, highlighting the effect of TiO2 morphology, nanoparticles, or 1D nanostructures, on the MOR activity under illumination. Comparison of reaction mechanisms in the presence and the absence of light are presented, and the roles of Pt and TiO2 during electrochemical and photochemical reactions are discussed.

Pt electrocatalysts supported on pristine graphene nanosheets (GNS) and nitrogen-doped graphene nanoplatelets (N-GNP) were prepared through the ethylene glycol process, and a comparison of their CO tolerance and stability as anode materials in polymer electrolyte membrane fuel cells (PEMFCs) with those of the conventional carbon (C)-supported Pt was made. Repetitive potential cycling in a half cell showed that Pt/GNS catalysts have the highest stability, in terms of the highest sintering resistance (lowest particle growth) and the lowest electrochemically active surface area loss. By tests in PEMFCs, the Pt/N-GNP catalyst showed the highest CO tolerance, while the poisoning resistance of Pt/GNS was lower than that of Pt/C. The higher CO tolerance of Pt/N-GNP than that of Pt/GNS was ascribed to the presence of a defect in graphene, generated by N-doping, decreasing CO adsorption energy.

Carbon supported platinum is commonly used as anode and cathode catalyst in low-temperature fuel cells. The need of modify the chemical characteristics of the supported catalyst has emerged due to several factors, such as reducing the price of the active catalyst and increasing its activity, selectivity, and long-term stability. Thus, pure Pt is now rapidly being replaced by oxide promoted Pt and Pd or Pt- and Pd-based alloy catalysts in low-temperature fuel cells. In addition to the chemical characteristics of the catalysts, many studies on nanocatalysts have been addressed to correlating the catalytic activity with some physical characteristics of supported fuel cell catalysts. This review article examines the role played by metal particle size, inter-particle distance and metal loading on the support in determining the catalytic activity of supported catalysts.

Among noble metal electrocatalysts, only iridium presents high activity for both the oxygen reduction reaction (ORR) in acid medium, in the oxide form, and the oxygen evolution reaction (OER) in acid medium, alloyed with first row transition metals. Indeed, platinum, the best catalyst for the ORR, has poor activity for the OER in any form, and ruthenium, the best catalyst for the OER, in the oxide form, possess poor activity for the ORR in any form. In this work, an overview of the application of Ir and Ir-containing catalysts for the OER in proton-exchange membrane water electrolyzer anodes, for the ORR in proton exchange membrane fuel cell cathodes, and for both OER and ORR in unit regenerative fuel cell oxygen electrodes is presented.

Combinatorial chemistry and high-throughput screening represent an innovative and rapid tool to prepare and evaluate a large number of new materials, saving time and expense for research and development. Considering that the activity and selectivity of catalysts depend on complex kinetic phenomena, making their development largely empirical in practice, they are prime candidates for combinatorial discovery and optimization. This review presents an overview of recent results of combinatorial screening of low-temperature fuel cell electrocatalysts for methanol oxidation. Optimum catalyst compositions obtained by combinatorial screening were compared with those of bulk catalysts, and the effect of the library geometry on the screening of catalyst composition is highlighted.

In view of their possible use as anode materials in acid direct ethanol fuel cells, the electrocatalytic activity of Pt-Ru and Pt-Ru-M catalysts for ethanol oxidation has been investigated. This minireview examines the effects of the structural characteristics of Pt-Ru, such as the degree of alloying and Ru oxidation state, on the electrocatalytic activity for ethanol oxidation.

The effect of alloying degree on the ethanol oxidation activity of a PtRu/C catalyst with a Pt/Ru atomic ratio of 1:1 was investigated by measurements in a half-cell and in a single direct ethanol fuel cell. The increase of the amount of Ru alloyed from one third to two thirds of the total Ru content in the catalyst clearly resulted in a decrease of the ethanol oxidation activity. As the amount of the highly active hydrous ruthenium oxide was near the same, the lower activity of the PtRu/C catalyst with higher alloying degree was mainly ascribed to the presence of an excessive number of Ru atoms around Pt active sites, hindering ethanol adsorption on Pt sites. The reduced ethanol adsorption could be also related to the decreased Pt–Pt bond distance and to the electronic effects by alloying.

Highly dispersed platinum or platinum-based catalysts on a conductive support are commonly used as electrode materials in low-temperature fuel cells. The performance and, in particular, the stability of these catalysts strongly depend on the characteristics of the support. Being the use of plain carbon, ceramic or polymer materials not completely satisfactory, in the last years hybrid polymer–carbon, ceramic–carbon and polymer–ceramic materials have been proposed as fuel cell catalyst supports. These hybrid materials, possessing the properties of each component, or even with a synergistic effect, would present improved characteristics with respect to the bare components. In this paper we present an overview of these hybrid materials as low-temperature fuel cell catalyst supports. The improved characteristics of the mixed supports with respect to the individual component and their effect on the electrochemical activity are highlighted.

Carbon supported Pt and Pt–Ni (1:1) nanoparticles were prepared by reduction of metal precursors with NaBH 4 . XRD analysis indicated that only a small amount of Ni alloyed with Pt (Ni atomic fraction in the alloy about 0.05). The as-prepared catalysts were submitted to chronoamperometry (CA) measurements to evaluate their activity for the oxygen reduction reaction (ORR). CA measurements showed that the ORR activity of the as-prepared Ni-containing catalyst was higher than that of pure Pt. Then, their stability was studied by submitting these catalysts to durability tests involving either 30 h of constant potential (CP, 0.8 V vs. RHE) operation or repetitive potential cycling (RPC, 1000 cycles) between 0.5 and 1.0 V vs. RHE at 20 mV s −1 . After 30 h of CP operation at 0.8 V vs. RHE, loss of all non-alloyed Ni, partial dissolution of the Pt–Ni alloy and an increase of the crystallite size was observed for the Pt–Ni/C catalyst. The ORR activity of the Pt–Ni/C catalyst was almost stable, whereas the ORR activity of Pt/C slightly decreased with respect to the as-prepared catalyst. Loss of all non-alloyed and part of alloyed Ni was observed for the Pt–Ni/C catalyst following repetitive potential cycling. Conversely to the results of 30 h of CP operation at 0.8 V vs. RHE, after RPC the ORR activity of Pt–Ni/C was lower than that of both as-prepared Pt–Ni/C and cycled Pt/C. This result was explained in terms of Pt surface enrichment and crystallite size increase for the Pt–Ni/C catalyst.

A carbon supported Pt–Pd catalyst with a Pt:Pd atomic ratio 77:23 was prepared by reduction of metal precursors with formic acid and characterized by EDX, XRD and XPS techniques. A decrease of the lattice parameter compared with that of pure Pt was observed, indicating the formation of a Pt–Pd alloy. Tests in H 2 SO 4 solution in the absence of ethanol showed that the Pd-containing is slightly more active than pure Pt for the oxygen reduction reaction (ORR). In the presence of ethanol a larger increase in overpotential of the ORR on pure Pt than that on Pt–Pd was found, indicating a higher ethanol tolerance of the binary catalyst. The enhanced performance at 90 °C of the direct ethanol fuel cell with Pt–Pd/C as cathode material confirmed the results of half cell tests, and was essentially ascribed to a reduced ethanol adsorption on Pt–Pd.

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.

The development of high performance electrode materials is currently one of the main activities in the field of the low temperature fuel cells, fuelled with H 2 /CO or low molecular weight alcohols. A promising way to attain higher catalytic performance is to add a third element to the best binary catalysts actually used as anode and cathode materials. In Part I of this review an overview of the preparation and structural characteristics of Pt-based ternary catalysts was presented. This part of the review deals with the electrochemical properties of these catalysts regarding their CO tolerance and electrocatalytic activity for methanol and ethanol oxidation in the case of anode materials, and their activity for oxygen reduction and stability in fuel cell conditions when used as cathode materials.

Carbon supported Pt alloyed with first row transition elements (Pt–M/C) is being used as improved cathode catalyst for low temperature fuel cells. These catalysts have been usually prepared by deposition of the non-precious metal onto pre-formed carbon supported platinum, followed by alloying at temperatures of the order or above 700 °C. As the thermal treatment at high temperature gives rise to an undesired metal particle growth, synthetic methods based on the simultaneous deposition of Pt and M on the carbon substrate, followed by thermal treatment at lower temperature have been developed. In this paper the formation of Pt–M/C by low/intermediate temperature methods is reviewed. Moreover, to investigate the effect of the conditions used in the synthesis on the Pt:M atomic ratio, the degree of alloying and the particle size, carbon supported Pt–Co electrocatalysts with nominal Pt:Co atomic ratio 75:25 were prepared by a low temperature chemical reduction of the precursors with sodium borohydride at two different temperatures and NaBH 4 concentrations. The physical characterization of these electrocatalysts was performed by energy dispersive X-ray analysis and X-ray diffraction.

In direct alcohol fuel cells, ethanol crossover causes a less serious effect compared to that of methanol because of both its smaller permeability through the Nafion® membrane and its slower electrochemical oxidation kinetics on a Pt/C cathode. The main interest in direct ethanol fuel cells (DEFCs) is to find an anode catalyst with high activity for the oxidation of ethanol. However, due to the low activity of pure platinum for the oxygen reduction reaction (ORR), research on cathode electrocatalysts with improved ORR and the same or improved ethanol tolerance than that of Pt are also in progress. In this work, a commercial carbon supported Pt–Co (3:1) electrocatalyst (E-TEK) was investigated as cathode material in DEFCs and the activity compared to that of Pt. In the cathodic potential region (0.7–0.9 V versus RHE) Pt/C and Pt–Co/C showed the same activity for the oxidation of crossover ethanol. But the performance of Pt–Co/C as cathode material in DEFCs in the temperature range 60–100 °C is better than that of Pt/C both in terms of mass activity and specific activity, due to an improved activity of the alloy for oxygen reduction.

The electrochemical oxidation of ethanol on carbon supported Pt, PtRu and Pt3Sn catalysts was studied in acid solutions at room temperature and in direct ethanol fuel cells (DEFC) in the temperature range 70–100 °C. In all the experiments, an enhancement of the activity for the ethanol oxidation was observed on the binary catalysts. In acid solution the improvement at low current densities was higher on PtRu than on Pt3Sn. In DEFC tests, at 70 °C the cells with PtRu and Pt3Sn showed about the same performance, while for T > 70 °C the cells with Pt3Sn as anode material performed better than those with PtRu as anode material. The apparent activation energy for ethanol oxidation on PtRu catalyst was lower than on Pt3Sn, particularly at high cell potentials, i.e. at low current densities. At low temperatures and/or low current densities, the positive effect of Ru oxides on the bifunctional mechanism determined the enhancement of activity for the ethanol oxidation reaction, while at high temperatures the positive effect of Sn alloying (enlarged lattice parameter) on CH3CH2OH adsorption and C–C cleavage prevails.

The activity for the oxygen reduction reaction (ORR) on carbon supported Pt–Ni electrocatalysts prepared by reduction of Pt and Ni precursors with NaHB 4 was investigated in sulphuric acid both in the absence and in the presence of methanol and compared with that of a commercial Pt/C electrocatalyst. In methanol-free sulphuric acid solution the Pt 70 Ni 30 /C alloy electrocatalyst showed a lower specific activity towards oxygen reduction compared to Pt/C. In O 2 -free H 2 SO 4 the onset potential for methanol oxidation on Pt 70 Ni 30 /C was shifted to more positive potentials, which indicates a lower activity for methanol oxidation than platinum. In the methanol containing electrolyte the higher methanol tolerance of the Pt 70 Ni 30 /C electrocatalyst for the ORR was ascribed to the lower activity of the binary electrocatalyst for methanol oxidation, arising from a composition effect.

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.

The effects of both geometric (Pt–Pt bond distance) and electronic (Pt d-band vacancy) factors on the electrocatalytic activity for the oxygen reduction reaction (ORR) of carbon supported Pt and Pt–Co alloy catalysts were investigated by X-ray diffraction and X-ray absorption spectroscopies in conjunction with electrochemical measurements in proton exchange membrane (PEM) fuel cells. In the presence of cobalt, a decrease in the Pt–Pt bond distance and an increase in the metal particle size were observed. In PEM fuel cells the ORR activity on all the Pt–Co/C catalysts was higher than that on Pt/C. A poor fit between the ORR activity and the Pt–Pt bond distance was found. Conversely, the ORR activity presented a linear relationship with the integrated intensity of the Pt L 3 edge at 1.1 V, related to the Pt d-band vacancy. This is because this parameter takes into account both alloying (Pt–Pt bond distance) and particle size effects.

The electrocatalysis of the oxygen reduction reaction on carbon supported Pt and Pt–Co (Pt/C and Pt–Co/C) alloy electrocatalysts was investigated in sulphuric acid (both in the absence and in the presence of methanol) and in direct methanol fuel cells (DMFCs). In pure sulphuric acid Pt–Co/C alloys showed improved specific activity towards the oxygen reduction compared to pure platinum. In the methanol containing electrolyte a higher methanol tolerance of the binary electrocatalysts than Pt/C was observed. The onset potential for methanol oxidation at Pt–Co/C was shifted to more positive potentials. Accordingly, Pt–Co/C electrocatalyts showed an improved performance as cathode materials in DMFCs.

Carbon supported Pt and Pt 70 Co 30 electrocatalysts for the oxygen reduction reaction (ORR) were prepared by reduction with formic acid and tested in polymer electrolyte fuel cells. In the presence of Co an increase of the Pt particle size was observed in the as-prepared electrocatalyst and no evidence of Pt–Co alloy formation was detected from XRD measurements. Following thermal treatment (TT) at 900 °C of the Pt 70 Co 30 /C electrocatalyst, the presence in the XRD pattern of secondary Pt reflexions shifted to higher angles indicated partial alloy formation. The fuel cell performance with the as-prepared Pt 70 Co 30 /C electrocatalyst was inferior than that with Pt/C. The electrocatalytic activity increased with a TT of the binary electrocatalyst, and the value of the mass activity of the Pt 70 Co 30 /C electrocatalyst thermally treated at 900 °C was only slightly lower than that of Pt/C, notwithstanding the larger metal particle size, about five times that of pure Pt. On the other hand, there was a remarkable increase of the specific activity for the ORR of the Co-containing catalyst after TT at 900 °C with respect to Pt alone, which was ascribed to both the increased metal particle size and alloy formation. At high current densities the performance of PEMFC electrodes decreased with increasing Pt particle size.

Carbon-supported Pt−Co electrocatalysts in the Pt:Co atomic ratio 85:15, mainly for application in polymer electrolyte fuel cells, have been prepared by different methods. The materials were tested in single cells with respect to the oxygen reduction reaction and their performances were compared. Of the several methods considered, the preparation of the electrocatalyst via the deposition and reduction of a Co precursor on previously formed carbon-supported platinum gave the best results. By increasing the Co content, a decrease of metal particle size and an improvement in the activity for the ORR of these catalysts was observed. For the electrocatalysts with a Pt:Co atomic ratio of 75:25, a good stability upon cycling was also found.

Since the 1980s there has been a significant lowering of the platinum loading of polymer electrolyte fuel cell electrodes from about 4–10 mg cm−2(platinum black) to about 0.4 mg cm−2 or even less (carbon supported platinum), by the introduction of ionomer (liquid Nafion®) impregnated gas diffusion electrodes, extending the three-dimensional reaction zone. From the 1990s to the present studies have been carried out to decrease the loss of performance during cell operation due both to the presence of liquid water causing flooding of the catalyst layer and mass transport limitations and to the poisoning of platinum by the use of reformed fuels. This review deals with the developments in electrode configuration going from dual layer to three layer electrodes. The preparation methods, the characteristics and the optimal composition of both diffusion and reactive layers of these electrodes are described. The improvement in the performance of both CO tolerant anodes and cathodes with enhanced oxygen reduction by Pt alloying is also discussed.

Supported platinum electrocatalysts are generally used in low temperature fuel cells to enhance the rates of the hydrogen oxidation and oxygen reduction reactions. In such catalysts, the high surface to volume ratios of the platinum particles maximize the area of the surfaces available for reaction. It is the structure and proper dispersal of these platinum particles that make low-loading catalysts feasible for fuel cell operation, lowering the cost of the system. If the platinum particles cannot maintain their structure over the lifetime of the fuel cell, change in the morphology of the catalyst layer from the initial state will result in a loss of electrochemical activity. This loss of activity in the platinum/carbon catalysts due to the agglomeration of platinum particles is considered to be a major cause of the decrease in cell performance, especially in the case of the cathode. In the light of the latest advances on this field, this paper reviews the preparation methods of these catalysts, their microstructural characteristic and their effect on both thermal and in cell conditions stability.

The electrocatalysis of the oxygen reduction reaction (ORR) on carbon supported Pt:V 1:1 catalyst in polymer electrolyte fuel cells (PEFC) was investigated. At an oxygen pressure of 1 atm results indicate a lower electrocatalytic activity for the ORR in the presence of vanadium. However, at an O2 pressure ≥2 atm an enhanced electrocatalytic property of PtV/C compared with Pt/C is revealed. This result indicates the occurrence of a different electrocatalytic mechanism for the ORR on Pt/C and PtV/C. An increase of mass transport overpotentials is observed for the PtV/C catalyst, and this was related to the presence of vanadium oxide. Indeed, XRD analysis revealed that only about 30% of V present in the catalyst is alloyed with Pt, forming a face centred cubic (fcc) Pt3V solid solution. A thermal treatment at 850 °C under reducing atmosphere leads to the formation of an ordered fcc Pt2V phase. After this, the ORR activity of PtV/C at O2 pressure 1 atm is higher than that of Pt/C.

The formation mechanism of Pt/C catalysts using non-oxidized active carbon support and the weak reducing agent Na2S2O4 was investigated. Platinum on carbon catalysts were fabricated by an impregnation/reduction process of the Pt-precursor H2PtCl6 on carbon support. The effect of thermal treatment in argon up to 700°C on the structural characteristics of these catalysts was studied by XRD and TEM analyses. The importance of carbon support properties on Pt/C formation was recognized. Before thermal treatment a very weak internal organization (a very small particle size and amorphous structure) in the metal was obtained. Thermal treatment at relatively low temperatures leads to the growth and then to the crystallization of platinum particles in the well-known face centered cubic structure. The sintering of Pt particles occurs through the migration of Pt atoms on the carbon support, likely by a bridge-bonding mechanism on sulfur atoms. A fast growth of Pt particles occurred in the temperature range 300—400°C. Thermal crystallization, instead, occurred mostly going from 400 to 550°C. Following annealing at 550°C, the formation of platinum sulfide was revealed. The sample thermally treated at 700°C showed an anomalous XRD pattern with Pt reflexions shifted towards high angles and an increase of Pt[111]/Pt[220] peak intensity ratio.

Carbon supported PtRu alloys were prepared by impregnation of Pt and Ru precursors on a porous carbon support, followed by reduction of the metals with Na2S2O4. After reduction, the samples were thermal treated in argon up to 700°C. The samples were characterized by atomic absorption (AAS) and X-ray diffraction (XRD) measurements. Before thermal treatment only carbon reflexions were visible in XRD pattern. The reflexions of face centered cubic (f.c.c.) PtRu alloy were revealed in XRD pattern starting from thermal treatment at 300°C. No hexagonal close packed (h.c.p.) RuPt reflexions were detected. During thermal treatment, part of Ru reacted with sulphur forming RuS2. Ru content in the alloy increased with increasing thermal treatment temperature. The results indicated that, first, f.c.c. Pt with few Ru alloyed was formed, then, with increasing thermal treatment temperature, part of Ru atoms present in the sample in an amorphous form entered in the crystal structure of the platinum by a diffusion-controlled mechanism. Also after thermal treatment at high temperatures there was a large part of unalloyed Ru, with only about 49% of the Ru alloyed with the Pt. PtRu particle size was in the range 15–20 nm.

The historical development and current status of gas diffusion electrodes for polymer electrolyte fuel cells ( PEFCs) are presented. Two and three-layer electrodes are compared. The effects of both the characteristics and the amount of the materials composing the electrode on the cell performance are presented. These include: the characteristics of the porous substrate; the wettability of the diffusion layeer; the characteristics of the catalyst, which is commonly platinum supported on carbon (Pt loading, Pt particle size, Pt/C ratio, Pt deposition method); the presence of perfluorosulfonate ionomer; and the way that the catalytic layer is filled. Attention is also given to the fabrication process of the electrodes and thermal treatments.

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.

Carbon supported platinum is commonly used as anode and cathode electrocatalyst in low-temperature fuel cells fuelled with hydrogen or low molecular weight alcohols. The cost of Pt and the limited world supply are significant barriers to the widespread use of these types of fuel cells. Moreover, platinum used as anode material is readily poisoned by carbon monoxide, present in the reformate gas used as H2 carrier in the case of polymer electrolyte fuel cells, and a byproduct of alcohol oxidation in the case of direct alcohol fuel cells. In addition, Pt alone does not present satisfactory activity for the oxygen reduction reaction when used as cathode material. For all these reasons, binary and ternary platinum-based catalysts and non-platinum-based catalysts have been tested as electrode materials for low temperature fuel cells. Palladium and platinum have very similar properties because they belong to the same group in the periodic table. The activity for the oxygen reduction reaction (ORR) of Pd is only slightly lower than that of Pt, and by addition of a suitable metal, such as Co or Fe, the ORR activity of Pd can overcome that of Pt. Conversely, the activity for the hydrogen oxidation reaction (HOR) of Pd is considerably lower than that of Pt, but by adding of a very small amount (5 at%) of Pt, the HOR activity of Pd attains that of pure Pt. This paper presents an overview of Pd and Pd-containing catalysts, tested both as anode and cathode materials for low-temperature fuel cells.

To decrease the Pt content, a polymer electrolyte membrane fuel cell (PEMFC) was formed using a carbon supported Pd96Pt4 catalyst as the anode material, and a carbon supported Pd49Pt47Co4 catalyst as the cathode material. The as-obtained Pd-based PEMFC with an overall Pd:Pt:Co atomic composition of electrodes (anode + cathode) = 72:26:2 exhibited a performance not too far from that of the fuel cell with the conventional 100% Pt electrodes. With a Pt content of 35 wt% of that of the cell with full Pt electrodes, at a current density of 1 A cm−2 the performance loss of the cell with the Pd-based catalysts was only 11%, with 6% ascribed to the anode catalyst and 5% to the cathode catalyst. The maximum power density of the Pd-based cell was 76% of that of the cell with Pt catalysts.

The particle size effect for ethanol oxidation on carbon supported platinum catalysts in the Pt particle size range from 2.2 to 3.6 nm was investigated both in half-cell and in a single direct ethanol fuel cells (DECF). The specific activity for ethanol oxidation presented a maximum at a Pt particle size of 2.5 nm, ascribed to the best compromise between structural effects and/or oxophilicity effects of the Pt surface. In DEFCs, the particle size effect at the anode side was less significant than that observed in half-cell measurements, likely due to concomitant effects of ethanol oxidation products on the oxygen reduction at the cathode side.

Platinum-ruthenium catalysts supported on carbon (PtRu/C) have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), specific surface area analysis (BET), X-ray photoelectron spectroscopy (XPS) and in proton exchange membrane (PEM) fuel cell tests. The results indicate the presence of strong metal-carbon interactions, which hinder the formation of a single-phase face-centered cubic (fcc) PtRu alloy. The particle size of the PtRu/C catalysts was smaller than both carbon-supported platinum (Pt/C) and ruthenium (Ru/C) catalysts. In the bimetallic electrocatalysts the intercrystallite distance decreased with respect to pure Pt and Ru metals. PEM fuel cell tests in H2/air operation mode revealed a decrease of performance with increasing carbon content of the catalyst, at a fixed Pt loading. In H2 + 100 ppm CO/air operation mode the maximum performance of the PEM fuel cell was attained at 0.63 atomic fraction Ru.

The effect of Nafion loading in the catalyst layer of cathodes for polymer electrolyte fuel cells (PEFCs) was investigated. Steady-state galvanostatic polarisation (GP), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were carried out at 25°C in 1.0 M H2SO4 on electrodes with different Nafion loadings. EIS supplied information on the polarization resistance and CV on the electrochemical active area for the oxygen reduction reaction (ORR). These parameters, together with those derived from GP, allowed a better identification of the features governing the ORR. An optimum Nafion loading was identified. This loading was found to correspond to a minimum in the polarization resistance and in the oxygen reduction overpotential and a maximum in the electrochemical active area.