The lifetime of polymer electrolyte membrane (PEM) fuel cells is governed by the operational stability of the membrane electrode assembly (MEA). Under dynamic automotive operating conditions and duty cycles, the membrane is subjected to chemical and mechanical degradation, which cause hydrogen leaks and ultimate cell failure. Chemical degradation is generally accepted as the key factor in the membrane decay process where membrane stability is affected by the formation of reactive radicals [1] and their attack on the ionomer molecular structure in the membrane [2]. H₂O₂ formed at the electrodes during fuel cell operation may diffuse into the membrane and decompose into hydroxyl radical (^.OH) via the Fenton’s reaction mechanism in presence of Fe²⁺ [3], commonly present as traces in membranes due to fabrication and/or operation-induced contamination. Low relative humidity (RH), and high temperature, reactant gas pressures and cell voltages accelerate the chemical degradation [1,4]. Steady-state open circuit voltage (SOCV) conditions are widely used in accelerated stress tests (AST) to intensify chemical stressors. In SOCV conditions, an Fe-ion redox cycle is generated in the MEA to preserve a relatively high Fe²⁺ concentration in the membrane, which leads to the most severe chemical membrane degradation through the Fenton mechanism [5,6].

Previously, the effects of isolated chemical degradation were exhibited at relatively mild stress levels at 90% RH (lifetime: 497 hours) and 100% RH (lifetime: 643 hours) where end-of-life (EOL) CCMs fractured at low strains right after passing their yield point during ex situ tensile tests [9]. In this work, an in situ SOCV-based AST with high stress levels and moderate humidity conditions were applied to induce pure chemical membrane degradation and establish its induced gradual decay in mechanical properties. Fluoride loss, an indication of global chemical degradation, increased steadily for the entire duration of the AST upto 140 hours (EOL). SEM investigations revealed gradual thinning of the membrane; however, no cracks were observed in the membrane. Previously, membranes subjected to pure mechanical degradation [7] and combined chemical and mechanical degradation [8-9] depicted localized damage (cracks and holes) that were incorporated due to RH cycling. Such features were not evident in the present work under isolated chemical stress.

Thereafter, ex situ tensile experiments were performed with periodically extracted, partially AST-degraded CCM samples under both room (25^oC, 50% RH) and fuel cell conditions (70^oC, 90% RH). A dynamic mechanical analyzer (TA Instruments Q800 DMA) equipped with an environmental chamber was used. Reductions in ultimate tensile strength and fracture strain were observed as a function of AST operation time. Hygrothermal expansion test results revealed an overall decay in hygral expansion at 70^oC of 33%, whereas the decay in thermal expansion at 90% RH was 40%. This was comparable to the results of CCMs subjected to pure mechanical degradation [7], where the hygral expansion decay from BOL upto 20,000 RH cycles was 33-50%, and a 50%-decay occurred in thermal expansion. For CCMs subjected to combined chemical and mechanical degradation, the decay in hygral expansion was only 25-30%, whereas 80% decay in thermal expansion was observed [8]. In summary, the observed microstructure-property relationship revealed the crucial role of chemical degradation by means of membrane thinning.

Acknowledgements

This research was supported by Mitacs through the Accelerate program, Ballard Power Systems, Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada and Simon Fraser University. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program.

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