The electrocatalysis of the oxygen reduction reaction (ORR) on five binary Pi alloys (PtCr/C, PtMn/C, PtFe/C, PtCo/C, and PtNi/C) supported on high surface area carbon in a proton exchange membrane fuel cell was investigated. All the alloy electrocatalysts exhibited a high degree of crystallinity with the primary phase of the type Pt3M (LI2 structure with fcc type lattice) and a secondary phase (only minor contribution from this phase) being of the type PtM (LIo structure with tetragonal lattice) as evidenced from x-ray powder diffraction (XRD) analysis. The electrode kinetic studies on the Pt alloys at 95~ and 5 atm pressure showed a two-to threefold increase in the exchange current densities and the current density at 900 mV as well as a decrease in the overvoltage at i0 mA em -2 relative to Pt/C eleetrocatalyst. The PtCr/C alloy exhibited the best performance. In situ EXAFS and XANES analysis at potentials in the double-layer region [0.54 V vs. reversible hydrogen electrode (RHE)] revealed (i) all the alloys possess higher Pt d-band vacancies per atom (with the exception of PtMn/C alloy) relative to Pt/C electrocatalyst and (it) contractions in the Pt-Pt bond distances which confirmed the results from ex situ XRD analysis. A potential excursion to 0.84 V vs. RHE showed that, in contrast to the Pt alloys, the Pt/C electrocatalyst exhibits a significant increase in the Pt d-band vacancies per atom. This increase, in Pt/C has been rationalized as being due to adsorption of OH species from the electrolyte following a Temkin isotherm behavior, which does not occur on the Pt alloys. Correlation of the electronic (Pt d-band vacancies) and geometric (Pt-Pt bond distance) with the electrochemical performance characteristics exhibits a volcano type behavior with the PtCr/C alloy being at the top of the curve. The enhanced electrocatalysis by the alloys therefore can be rationalized on the basis of the interplay between the electronic and geometric factors on one hand and their effect on the chemisorption behavior of OH species from the electrolyte.The role of Pt/C and Pt alloys on the mechanism of the oxygen reduction reaction (ORR) has been investigated previously, 1-4 however the mechanism still remains elusive. One of the first investigations I of the ORR on Pt alloy electrocatalysts was in phosphoric acid; the effect of changes in the Pt-Pt interatomic distances, caused by alloying, was examined. The strength of the [M-HO2]aas bond, the intermediate formed in the rate-determining step of the molecular dioxygen reduction, was shown to depend on the Pt-Pt bond distance in the alloys. A plot of the electrocatalytic activity vs. adsorbate bond strength exhibited a volcano type behavior. 5 It was shown that the lattice contractions due to alloying resulted in a more favorable Pt-Pt distance (while maintaining the favorable Pt electronic properties) for dissociative adsorption of 02. This view was disputed by Glass et al. ~ in their investigation on bulk alloys of PtCr (the binary alloy at the top of the volcano ...

This review paper describes recent developments in both the fundamental and technological aspects of direct methanol fuel cells (DMFCs). Most previous studies in this field have dealt with fundamental aspects, whereas in recent years, the technology of these devices has become the object of significant interest. This is mainly due to the fact that a probable application of DMFCs in portable power sources and in hybrid electrical vehicles has only recently been envisaged. The section on fundamentals is particularly focused on the electrocatalysis of the methanol oxidation reaction and oxygen electroreduction. In this regard, particular relevance is given to the interpretation of the promoting effect on Pt of additional elements and some aspects of the electrocatalysis of oxygen reduction in the presence of methanol crossover have been treated. The technology section deals with the development of both components and devices. Particular emphasis is given to the development of high surface area electrocatalysts and alternative electrolyte membranes to Nafion, also the fabrication methodologies for the M&E assembly have been discussed. The last part of the paper describes the recent efforts in developing DMFC stacks for both portable and electro‐traction applications. The current status of the technology in this field is presented and some important technical and economical challenges are been discussed.

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The effect of different alloying conditions (alloying temperature, annealing period) on the electrocatalytic activities for the oxygen reduction reaction (ORR) by three carbon-supported Pt alloy electrocatalysts (WCr, WCo, m i ) was investigated and correlated with electronic and structural parameters determined by in-situ XAS. The results indicate that all the Pt alloys show enhanced ORR activities relative to a W C electrocatalyst. However, the electrocatalytic activity and activation energy for ORR in the case of Pt/Ni and Pt/Co alloys show marked effect due to different alloying conditions. This was in contrast to W C r alloy, where both parameters remained unchanged over the range of alloying conditions. Those electrochemical results were correlated with those obtained from in-situ X-ray absorption spectroscopic (XAS) investigations, which provided information on the electronic (Pt 5d-orbital vacancy, from the X-ray absorption near-edge structure) and geometric (Pt-Pt bond distances, from the extended X-ray absorption fine structure) factors. In-situ XAS results indicate that the supported alloys possess higher P t Sd-orbital vacancies and shorter Pt-Pt bond distances.In addition, the XAS results showed that alloying inhibited chemisorption of oxygenated species (OH) on the Pt at potentials above 0.8 V vs RHE. Correlation of electrocatalytic activities and activation energies for ORR with parameters obtained from in-situ XAS studies indicates that, in the case of m i and Pt/Co alloys, higher alloying temperature and longer annealing periods result in higher Pt 5d-orbital vacancies with the geometric parameters remaining unchanged. The W C r alloy on the other hand revealed no dependence of either the Pt d-orbital vacancies or the geometric parameters on alloying temperature. These observations indicate that the dependence of electrocatalytic activities and activation energy for Pt/Co and Pt/Ni alloys on the thermal history and the absence of such an effect in the W C r alloy could be related to the differences in the Pt Sd-orbital vacancies. IntroductionEnhancement of electrocatalytic activities for an oxygen reduction reaction (ORR) by alloying Pt with first-row transition elements was first reported in phosphoric acid'&nd more recently in proton exchange membrane fuel cells (PEMFCS).~.~ Based on early findings of activity enhancements in phosphoric acid fuel cells (PAFCs), several investigations were conducted to ascertain the role of alloying on the electrocatalytic activity for ORR, detailed reviews of which have been presented e l~e w h e r e .~.~ Recent investigations on five binary carbonsupported Pt alloy electrocatalysts in a PEMFC environment (WCr, Pt/Mn, Pt/Fe, WCo, Pt/Ni) involving both ex-situ XRD and in-situ XAS spectroscopy have revealed interesting correlations between the electronic, geometric, and electrocatalytic activities for ORR by Pt and Pt alloys.' Of special relevance was the application of in-situ XAS spectroscopy, which consists of the near-edge part, X-ray absorption near-edge st...

A knowledge of the temperature dependence of the electrode-kinetic parameters for oxygen reduction at the platinum/ proton exchange membrane (PEM) interface and of mass-transport parameters of oxygen in the PEM is of vital importance in analyzing the performance of proton exchange-membrane fuel cells. The microelectrode technique which was previously developed to determine these parameters at the platinum/Nation | interface at 25~ was used in the present investigation at the same electrode/electrolyte interface. This study was carried out in the temperature range of 30-80~ and at 5 atm of oxygen pressure. The results showed a linear increase of the Tafel slope with temperature in the low current density region, but the Tafel slope was found to be independent of temperature in the high current density region. The values of the | activation energy for oxygen reduction at the platinum/Nation interface are nearly the same as those obtained at the platinum/trifluoromethane sulfonic acid (TFMSA) interface but less than values obtained at the Pt/H3PO4 and Pt/HC104 interfaces. The diffusion coefficient of oxygen in Nation increases with temperature while its solubility decreases with temperature. These parameters also depend on the water content of the membrane. The conductivity of the membrane increases with temperature until it reaches a plateau at a temperature of 80~ the dependence of the conductivity on temperature was correlated with the variation of water content of Nation with temperature.

Water loss and the coincident increase in membrane resistance to proton conduction are significant barriers to high performance operation of traditional proton exchange membrane fuel cells at elevated temperatures where the relative humidity may be reduced. We report here approaches to the development of high temperature membranes for proton exchange membrane fuel cells; composite perfluorinated sulfonic acid membranes were prepared to improve water retention, and non-aqueous proton conducting membranes were prepared to circumvent the loss of water. Experimental results of composite membranes of Nafion and zirconium phosphate show improved operation at elevated temperatures. Imidazole impregnated membranes poisoned the electrocatalysts. Cesium hydrogen sulfate membranes were not able to produce appreciable current. A brief analysis of temperature requirements for CO tolerance and a framework for understanding water loss from fuel cell membranes are presented. #

Silicon oxide/Nafion composite membranes were studied for operation in hydrogen/oxygen proton-exchange membrane fuel cells ͑PEMFCs͒ from 80 to 140°C. The composite membranes were prepared either by an impregnation of Nafion 115 via sol-gel processing of tetraethoxysilane or by preparing a recast film, using solubilized Nafion 115 and a silicon oxide polymer/gel. Tetraethoxysilane, when reacted with water in an acidic medium, undergoes polymerization to form a mixture of SiO 2 and siloxane polymer with product hydroxide and ethoxide groups. This material is referred to as SiO s /-OH/-OEt. When Nafion is used as the acidic medium, the SiO 2 /siloxane polymer forms within the membrane. All composite membranes had a silicon oxide content of less than or equal to 10 wt %. The silicon oxide improved the water retention of the composite membranes, increasing proton conductivity at elevated temperatures. Attenuated total reflectance-Fourier transform infrared spectroscopy and scanning electron microscopy experiments indicated an evenly distributed siloxane polymer of SiO 2 /-OH/-OEt in the composite membranes. At a potential of 0.4 V, silicon oxide/Nafion 115 composite membranes delivered four times the current density obtained with unmodified Nafion 115 in a H 2 /O 2 PEMFC at 130°C and a pressure of 3 atm. Furthermore, silicon oxide-modified membranes were more robust than the control membranes ͑unmodified Nafion 115 and recast Nafion͒, which degraded after high operation temperature and thermal cycling.