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The dissolution of Fe and Ni from Pt 1Ϫx M x ͑M ϭ Fe, Ni; 0 Ͻ x Ͻ 1͒ oxygen reduction electrocatalysts was studied under simulated operating conditions ͑low pH, 80°C͒ of proton exchange membrane ͑PEM͒ fuel cells. The alloys were prepared combinatorially by sputtering Pt and M ͑M ϭ Fe, Ni͒ onto thin films of nanostructured whisker-like supports, and mapped over the entire composition range of the binary systems. For 0 Ͻ x Ͻ 1.0, we observe the formation of randomly ordered substitutional solid solutions of Pt 1Ϫx Fe x and Pt 1Ϫx Ni x alloys. Electron microprobe measurements show that transition metals are removed from all compositions during acid treatment, but that the percentage removed increases with x, acid strength, and temperature. For small values of x (x Ͻ 0.6) no substantial changes in the lattice size are observed upon dissolution of Fe or Ni suggesting that the dissolved transition metals originate from the surface. However, for electrocatalysts with x Ͼ 0.6, the lattice constant expands indicating that transition metals dissolve also from the bulk. X-ray photoelectron spectroscopy results show complete removal of surface Ni ͑Fe͒ after acid treatment at 80°C for all compositions. The results of the acid treatments compare well to the composition changes that occur when a Pt 1Ϫx Fe x or Pt 1Ϫx Ni x combinatorial catalyst library is used in an operating PEM fuel cell.

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Various durability properties of a nanostructured thin film based catalyst system (3M's NSTF) which does not contain carbon or additional ionomer, are compared with conventional carbon supported dispersed Pt catalysts. The NSTF catalysts are shown to have mass losses by TGA at 170 o C that are two orders of magnitude less. The NSTF whisker support material in combination with the large-grained polycrystalline thin film catalysts that encapsulate the whiskers give significantly greater durability under normal fuel cell operating conditions and also under accelerated high voltage conditions, whether scanning between 0.6 and 1.2 volts at 20mV/sec under H 2 /N 2 , or under steady state conditions of 1.5 volts. At 1.5 volts, NSTF catalysts lose no surface area, specific activity or fuel cell performance over periods as long as 3 hours, whereas the Pt/C based electrodes lose both large amounts of surface area, activity and performance in just 30 minutes at 1.5 volts.

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normalPt1−xnormalMx (M=Ru,Mo,Co,Ta,Au,Sn) random alloy samples, covering most of the binary composition range, have been prepared via magnetron sputtering. The alloys were deposited through shadow masks onto 3M nanostructured thin-film catalyst support for testing in a 64-electrode polymer electron membrane fuel cell (PEMFC). CO stripping voltammograms and hydrogen oxidation polarization curves with pure hydrogen and with reformate containing up to 50ppm CO were measured on all the samples. In agreement with reports in the literature, Ru, Mo, and Sn were found to improve the CO tolerance of Pt, although the intrinsic hydrogen oxidation activity of Pt decreased significantly as the Sn content increased. The addition of Co to Pt had no impact on CO tolerance, possibly because of loss of surface Co through dissolution in the fuel cell. The addition of Au to Pt led to an increase in hydrogen oxidation overpotential when CO was present. Small amounts of Ta gave a small reduction in hydrogen oxidation overpotential in the presence of CO, but the overpotentials were still too high for practical application in a reformate-fed fuel cell.

The stability of Pt-based or other metallic alloys under the corrosive working conditions of proton exchange membrane fuel cells ͑PEMFCs͒ is an important factor to consider in the search for improved PEM electrocatalysts. The dissolution of transition metals ͑Co, Ni, Mn, Fe͒ from Pt 1−x−y M x M y Ј electrocatalysts after operation in PEM hydrogen fuel cells or after treatment in 1 M H 2 SO 4 at 80°C is reported. Catalyst libraries were prepared by combinational sputtering on nanostructured thin-film supports that were used directly in fuel cells and for acid testing. In each library the Pt mass loading was kept fixed at 0.1 mg cm −2 while the amounts of M and MЈ varied with position. The transition metal content of all compositions of all libraries, determined by electron microprobe, was significantly reduced in the same way after fuel cell or acid testing. Samples with x + y ജ 0.25 before fuel cell or acid testing were found with x + y Ϸ 0.25 after exposure due to the dissolution of transition metals, independent of the choice of M and MЈ for the elements studied here. We suggest that the composition Pt 1−x−y M x M y Ј with x + y Ϸ 0.25 is near the percolation limit for diffusion of transition metals from the interior of the alloy. The acid treatment described here mimics fuel cell testing from a corrosion standpoint and hence can be used as a simpler, ex situ test for PEM electrocatalysts.At present, polymer electrolyte membrane fuel cells ͑PEMFCs͒ are under intense research because they provide one avenue for "green" electricity production. However, prior to the successful commercialization of hydrogen fuel cells, besides fuel-related issues, numerous other shortcomings need to be addressed. 1-3 Materials challenges for the fuel cell include low-cost proton conducting membranes, as well as economical high-performance catalysts for both fuel and oxidant electrodes. For the cathode catalyst, where the oxygen reduction reaction ͑ORR͒ takes place, improvement of the kinetic activity is highly desired because an overvoltage of about 400 mV, or about 70% of the total losses in a typical PEMFC, occurs at this electrode. Although the effects of numerous factors like catalyst composition, catalyst grain size, crystalline facets, support structures, etc., on the ORR have been studied, details of the ORR mechanism still remain elusive. [4][5][6][7][8] Current ORR catalysts consist of nanometer-size Pt particles supported on carbon black. When the dispersion of Pt particles on highsurface-area carbon supports reaches its limits, further improvements in activity, durability, and cost reduction can be achieved by new catalyst formulations such as Pt alloys, non-Pt-based catalysts, and/or the development of new support structures. 9-12 Other current ideas that are actively explored in improving the catalytic activity and lowering the Pt content involve Pt monolayers on active alloy cores, metal oxides and strong metal-support interactions, and additives such as heteropolyacids. [13][14][15][16][17] Binary and ternary Pt tran...

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