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Morphological changes occurring in membrane electrode assemblies ͑MEAs͒ were monitored using transmission electron microscopy ͑TEM͒ during the course of life testing of H 2 /air polymer electrolyte fuel cells ͑PEFCs͒. In the fresh catalyst layers, anode Pt particles were found to have smaller particle sizes, better dispersion, and less agglomeration on the carbon-support surfaces than did the cathode Pt 3 Cr alloy particles. The operation-induced agglomeration of catalyst particles was evaluated for both the anode and cathode after defined life testing periods. Agglomeration of catalyst particles occurred primarily during the first 500 h of testing, which was confirmed by both TEM analysis and electrocatalytic surface area measurement. After 500 h, degradation of the recast Nafion ionomer network within the catalyst layers likely contributes more significantly to MEA performance degradation. Migration of metal catalyst particles toward the interface between the catalyst layer and membrane was observed at both electrodes. The Pt anode catalyst was less stable than the Pt 3 Cr cathode catalyst under high current density and high humidity conditions, which was confirmed by the higher extent of migration observed for the pure Pt than for the Pt 3 Cr. Some Pt particles ͑from both electrodes͒ were found to migrate into the membrane during the testing period.

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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