Equilibrium concentrations of dissolved platinum species from a Pt/C electrocatalyst sample in 0.5 M H 2 SO 4 at 80°C were found to increase with applied potential from 0.9 to 1.1 V vs reversible hydrogen electrode. In addition, platinum surface area loss for a short-stack of proton exchange membrane fuel cells ͑PEMFCs͒ operated at open-circuit voltage ͑ϳ0.95 V͒ was shown to be higher than another operated under load ͑ϳ0.75 V͒. Both findings suggest that the formation of soluble platinum species ͑such as Pt 2+ ͒ plays an important role in platinum surface loss in PEMFC electrodes. As accelerated platinum surface area loss in the cathode ͑from 63 to 23 m 2 /g Pt in ϳ100 h͒ was observed upon potential cycling, a cycled membrane electrode assembly ͑MEA͒ cathode was examined in detail by incidence angle X-ray diffraction and transmission electron microscopy ͑TEM͒ to reveal processes responsible for observed platinum loss. In this study, TEM data and analyses of Pt/C catalyst and cross-sectional MEA cathode samples unambiguously confirmed that coarsening of platinum particles occurred via two different processes: ͑i͒ Ostwald ripening on carbon at the nanometer scale, which is responsible for platinum particle coarsening from ϳ3 to ϳ6 nm on carbon, and ͑ii͒ migration of soluble platinum species in the ionomer phase at the micrometer scale, chemical reduction of these species by crossover H 2 molecules, and precipitation of platinum particles in the cathode ionomer phase, which reduces the weight of platinum on carbon. It was estimated that each process contributed to ϳ50% of the overall platinum area loss of the potential cycled electrode.
This paper discusses the mechanisms of surface area loss of supported platinum (Pt) electrocatalysts in lowtemperature fuel cells. It is argued that submonolayer dissolution of Pt nanoparticles governs the surface area loss at high voltages by increasing the loss of Pt from carbon and coarsening of Pt nanoparticles on carbon.
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