Membrane degradation mechanisms in PEMFCs

Nafion membrane degradation was studied in a polymer electrolyte membrane fuel cell (PEMFC) under accelerated decay conditions. Fuel cell effluent water was analyzed to determine the fluoride emission rate. Experimental findings show that formation of active oxygen species from H2O2 decomposition or the direct formation of active oxygen species in the oxygen reduction reaction are not the dominating membrane degradation mechanisms in PEMFCs. Instead, membrane degradation occurs because molecular H-2 and O-2 react on the surface of the Pt catalyst to form the membrane-degrading species. The source of H-2 or O-2 is from reactant crossover through the membrane. The reaction mechanism is chemical in nature and depends upon the catalyst surface properties and the relative concentrations of H-2 and O-2 at the catalyst. The membrane degradation rate also depends on the residence time of active oxygen species in the membrane and volume of the membrane. The sulfonic acid groups in the Nafion side chain are key to the mechanism by which radical species attack the polymer. (C) 2007 The Electrochemical Society.