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Nafion membrane degradation was studied in a polymer electrolyte membrane fuel cell ͑PEMFC͒ 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 H 2 O 2 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.

Nafion membrane degradation was studied in a polymer electrolyte membrane fuel cell ͑PEMFC͒ under accelerated decay conditions. Fluoride emission rate ͑FER͒ determined by fuel cell effluent water analysis was used to quantify the membrane degradation. Membrane degradation is most likely caused either directly or indirectly by the species formed as a result of the H 2 and O 2 reaction on the catalyst. To further understand the mechanism, the effects of the catalyst location, type, its interaction with O 2 and H 2 O, and cell current density on the FER were investigated and their implications on the underlying membrane degradation mechanism are discussed.

The involvement of H 2 O 2 in the membrane degradation mechanism in a polymer electrolyte membrane fuel cell ͑PEMFC͒ was investigated. Measurement of fluoride concentration in the effluent water was used as an indicator of the membrane degradation rate. It was found that H 2 O 2 is formed in the fuel cell in small concentrations but is not the main source of harmful species, which degrade the membrane. H 2 O 2 decomposition due to impurities or the catalyst leading to the possible formation of radical species would only account for a small fraction of the membrane degradation rate in a fuel cell.Nafion and Nafion-based membranes are the most commonly used membranes in polymer electrolyte membrane fuel cells ͑PEM-FCs͒. At present, the lifetime of these membranes in state-of-the-art membrane electrode assemblies at steady-state operation is below the target lifetime required for the use of PEMFCs in automotive and stationary applications. The lifetime is likely to decrease during actual operation in a fuel cell stack due to the noncontinuous nature of operation. Increasing the membrane life or developing a new membrane with improved life requires understanding of the membrane degradation mechanism during operation in a PEMFC.The oxygen reduction reaction is considered to proceed by the following pathways in acid solution 1The above two pathways have been quantitatively studied using rotating-ring disk electrodes ͑RRDE͒. 2 Results have shown that the yield of H 2 O 2 increases with a decrease in disk potential reaching the maximum in the potential range of H 2 adsorption, i.e., near the anode potential in an operating fuel cell. One of the commonly used ex situ beaker tests for studying membrane degradation is soaking a piece of membrane in Fenton's reagent. Radical species are formed by the reaction of H 2 O 2 with Fe 2+ 3The radical species attack the polymer to degrade the membrane. The presence of radicals has been detected by electron spin resonance spectroscopy. 4 After actual cell operation for a considerable amount of time post-test analysis of the membrane electrode assembly shows that membrane degradation mainly occurs near the anode-membrane interface. 5,6Based on the above-mentioned results ͑RRDE studies, membrane degradation in Fenton's reagent, and preferential membrane decay near the anode side͒, membrane degradation was thought to be because of the H 2 O 2 generation at the anode as an intermediate in the oxygen reduction reaction. ͑The presence of O 2 at the anode is from O 2 crossover from the cathode through the membrane.͒ The H 2 O 2 at the anode then diffuses into the membrane and reacts with bivalent metal cations ͑M 2+ ͒, present as impurities in the membrane to form active oxygen species, which can then attack the polymer and degrade the membrane. 6,7 However, this proposed mechanism is not consistent with recent reports on membrane degradation at the highly accelerated decay conditions of open-circuit voltage ͑OCV͒. 8,9 Studies show that the cathode is also involved in the degradation mechan...