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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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