Degradation of perfluorinated ionomer membranes for PEM fuel cells during processing with H2O2

As a typical degradation of the proton exchange membranes (PEMs) for fuel cells, formation of hydrogen peroxide (H2O2) on the cathode surface has been presented to be a key issue which leads to the decomposition of PEM. Using perflurosulfonated ionomeric membranes with different equivalent weights (EW = 900, 1000, and 1100) as test samples, degradation of PEM was investigated systematically in practical fuel cell usage conditions (e.g., 80 degrees C) during the progress of H2O2 treatment. Membranes were characterized for proton conductivity by ac impedance technique, pulsed-field-gradient spin-echo NMR, Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA), and extensile experimentation. Durability studies over a period of 1 month operation revealed evident membrane degradation ascribed to the decomposition of sulfonic acid groups in pendant side chains. The products of cross-linked S-O-S (condensation sulfonates) were strongly demonstrated by IR spectroscopy as a result of long H2O2 treatment times, which suggests oxidation provoked by H2O2. Proton conductivity and the water self-diffusion coefficient decreased significantly due to the loss of water inside the membranes. TGA revealed further changes in the membrane morphology, where the onset and decomposition temperatures of the membranes changed upon exposure to H2O2. Membranes with high EW showed a faster decomposition rate than the other ones, whereas the mass loss step showed the reverse case. Although the membranes still retained their bulk physical properties in that they remained flexible and plastic, the tensile analysis showed decreased tensile strength and increased elongation-to-break accompanied by an increased Young's modulus, which suggests a mechanically weaker membrane after exposure to H2O2. (C) 2006 The Electrochemical Society.