Local Degradation at Membrane Defects in Polymer Electrolyte Fuel Cells

Membrane defects, such as pinholes and cracks, are common failure modes in polymer electrolyte membranes. The elevated gas crossover at these defect sites is expected to accelerate membrane degradation locally. The defect site will expand, leading to premature cell failures. In order to understand the degradation pathway, degradation processes were analyzed at membrane defects using mass spectrometry, synchrotron assisted X-ray tomographic microscopy and FTIR spectro-microscopy, and thermochromic pigments. MEAs with artificially implemented membrane defects, 10 μm in diameter, were operated in fuel cells under accelerated stress test conditions. The gas crossover was analyzed online using a tracer gas concept that allows for monitoring the evolution of the defect size. Chemical, mechanical and thermal degradation processes have been identified and investigated. A synergetic effect that results from the combination of the processes accelerates degradation, leading to an exponential increase of the gas crossover. Polymer fracturing, polymer melting, the formation of COOH groups, catalyst sintering, carbon corrosion and local temperatures of up to 140°C were observed at membrane defects. Degradation processes are affected by the degradation side, gas crossover, gas composition, mechanical stress and the local inhomogeneities of the gas diffusion electrode. From the results, an overall degradation mechanism at membrane defects is formulated.