Performance and Degradation\nAnalysis of Low and High\nEquivalent Weight Short Side Chain PFSA Membranes in PEMFCs

Widespread commercialization of proton exchange membrane\nfuel cells\n(PEMFCs) still requires optimization of various components for extended\nlifetime and high performance, even under harsh operating conditions.\nIn this work, the PEM was studied in detail, using different perfluorinated\nsulfonic acid (PFSA) ionomers with short and long side chains, low-\nand high-equivalent weight (EW), and varying relative humidity. The\nPEMs were carefully characterized electrochemically in fuel cell tests.\nThe performance and ionic conductivity were determined at three different\nhumidities. A superior performance was found for the low-EW PEM due\nto the higher proton conductivity at low humidities. The low-EW ionomer\nelectrodes were analyzed for the evolution of their properties (catalyst\nutilization and protonic sheet resistance) with humidity. Next, the\nPEMs were subjected to open circuit voltage (OCV)-holds at elevated\ntemperature and low humidity to evaluate the EW-dependent chemical\ndegradation rate in this accelerated stress test. The degree of degradation\nwas spatially resolved with confocal Raman microscopy. A clear correlation\nbetween EW and degradation was found for the short-side chain ionomer,\nwith low-EW degrading substantially less than high-EW. In summary,\nwe provide evidence of improved performance and degradation resistance\nof low-EW PFSA membranes over high-EW membranes in PEMFCs.