A physics-based theoretical model that predicts the chemical degradation of the perfluorosulfonic acid polymer electrolyte membrane during fuel cell operation is developed. The model includes the transport and reaction of crossover gases, hydrogen and oxygen, to produce radicals in the membrane that subsequently react with the polymer to release hydrogen fluoride. The model assumes that a uniform distribution of nanometer-sized platinum deposits in the membrane (as a model input) originating from cathode dissolution provides the sites for radical generation. The degradation rate, measured by the release of hydrogen fluoride, depends on the net radical generation sites in the membrane, the concentration of the crossover gases, the hydration level of the membrane, the operating temperature, the operating voltage, and the thickness of the membrane. The model-predicted trends agree well with those reported and with our experimental results reported in the first article of this series by Madden et al. [ J. Electrochem. Soc. , 156 , B657 (2009)] . Furthermore, the model provides insight to the factors that affect radical generation vs radical quenching, which aids in explaining the experimentally observed nonlinear trends of fluoride emission with reactant concentration and membrane thickness.

Microscopy of polymer electrolyte membranes that have undergone operation under fuel cell conditions, have revealed a well defined band of platinum in the membrane. Here, we propose a physics based model that captures the mechanism of platinum precipitation in the polymer electrolyte membrane. While platinum is observed throughout the membrane, the preferential growth of platinum at the band of platinum is dependent on the electrochemical potential distribution in the membrane. In this paper, the location of the platinum band is calculated as a function of the gas concentration at the cathode and anode, gas diffusion coefficients and solubility constants of the gases in the membrane, which are functions of relative humidity. Under H2/N2 conditions the platinum band is located near the cathode-membrane interface, as the oxygen concentration in the cathode gas stream increases and/or the hydrogen concentration in the anode gas stream decreases, the band moves towards the anode. The model developed in this paper agrees with the set of experimental data on the platinum band location and the platinum particle distribution and size

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