To understand the difference in degradation characteristics between carbon-supported platinum (Pt/C) and platinum-nickel alloy (Pt1Ni1/C) cathode catalysts in membrane electrode assemblies (MEAs) of a polymer electrolyte membrane fuel cell (PEMFC), constant current operation of MEA in a single cell was conducted for 1100 h. A significant change in cell potential for the Pt1Ni1/C MEA was observed throughout the test. High-resolution transmission electron microscopy showed that sintering and detachment of metal particles in the Pt1Ni1/C catalyst occurred more sparingly than in the Pt/C catalyst. Instead. X-ray photoelectron spectroscopy element mapping revealed dissolution of Ni atoms in the Pt1Ni1 catalysts even when the Pt1Ni1/C catalyst used in the MEA was well synthesized. (C) 2011 Elsevier Ltd. All rights reserved.