This work addresses issues of long-term durability of hydrogen-air polymer electrolyte fuel cells ͑PEFCs͒. The chromium in a Pt 3 Cr binary alloy catalyst has been found to migrate from cathode to anode during the course of life testing when operating within the oversaturated, or high-humidity, gas feed regime ͑one or both inlet feeds with a dew point equal to or higher than cell operating temperature͒ above 1 A/cm 2 current density. Other major factors such as membrane degradation, dissolution of catalyst-layer recast ionomer, catalyst oxidation, and catalyst agglomeration/growth have been identified as simultaneous, gradual processes that can lead to long-term PEFC failure. In situ cyclic voltammetry measurement of electrochemically active catalyst surface area shows a continuous decrease, revealing that catalyst agglomeration and/or growth may be a major cause of membrane electrode assembly degradation during middle-term life tests ͑i.e., operation times up to about 2000 h͒ under high-humidity conditions. Membrane and/or recast ionomer degradation was confirmed by the presence of fluoride and sulfate anions in the cathode outlet water. Scanning and transmission electron microscopy observation of a tested MEA suggest the loss of carbon-supported catalyst clusters and possible dissolution of recast Nafion ionomer.

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