Performance and Degradation Analysis of Low and High Equivalent Weight Short Side Chain PFSA Membranes in PEMFCs

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