We present a durability test of polymer electrolyte membrane fuel cell ͑PEMFC͒ in open circuit condition. Such a condition enhances the deterioration of the membrane electrode assembly. ac impedance spectroscopy measurements and scanning electron microscopy observation suggest that the degradation occurred at the cathode. Direct gas mass spectroscopy of the cathode outlet gas indicated the formation of HF, H 2 O 2 , CO 2 , SO, SO 2 , H 2 SO 2 , and H 2 SO 3 . A kinetic model is presented assuming that the H 2 gas cross leakage from the anode caused the cathode degradation. The model determines the rate of degradation using the permeability across the electrolyte membrane measured for crossover H 2 gas.Future demand for ecologically benign power generation and transportation has motivated studies on polymer electrolyte membrane fuel cell ͑PEMFC͒. A number of studies have recently focused on the durability characteristics of the membrane electrode assembly ͑MEA͒ and the impact of operational conditions. 1-6 It has been reported that the open circuit operation without electric loading enhances the degradation of the MEA with H 2 gas and air supply. [7][8][9] The gradual decrease of the open circuit ͑OC͒ voltage V OC over 100 h is followed by a sudden increase of H 2 gas leakage across the electrolyte membrane, which has been ascribed to the formation of radical hydroperoxyl HO 2 ͑Ref. 7, 8, and 10-12͒ or thermal decomposition. 9 This study aims to provide a model for the degradation mechanism under OC operation, which is thus to be used as a means for lifetime assessment.Gas chromatogram ͑GC͒ can be used to analyze the outlet exhaust gas to identify the gas components resulting from degradation processes. GC requires a column suitable to separate gases. For example, MS-5A column often used for H 2 gas determination cannot be applied for CO 2 gas determination because the latter is too adsorptive. Therefore, we used direct gas mass spectroscopy ͑DGMS͒ which can separate gases without using a column. ExperimentalCarbon papers 200 m thick, coated with particulate Pt catalysts of 0.5 mg/cm 2 ͑Electrochem Inc., MA͒ over 5 cm 2 active area, were hot-pressed for 10 min at 120°C and pressure of 0.7 N/cm 2 onto free-acid form of Nafion 112 which had been precleaned with 3 wt % aq. H 2 O 2 and 1 N aq. H 2 NO 3 at 60°C in order to remove organic and cationic impurities. The resulting MEA was enclosed in a serpentine cell ͑Electrochem Inc.͒.Durability testing was carried out with an electric load PLZ152WA ͑Kikusui, Japan͒. H 2 and O 2 gases were supplied whose temperatures were kept at 70°C in the bubble-through humidifier. The flow rate was set at 40 mL/min while we confirmed that it had no significant effect on the MEA degradation. Before and after a 24 h OC operation, impedance analysis was done with an LCR tester 3522LCR ͑Hioki, Japan͒. The specimens for scanning electron microscopy ͑JEOL͒ were dried and then collected from the cell using a single-edge razor blade ͑Kaijirushi, Japan͒.DGMS ͑Bruker AXS͒ was conducted to chara...

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