Fuel cell membranes undergo simultaneous or individual chemical and mechanical degradation under dynamic fuel cell operating conditions. This combined stress development effect compromises the functionality of the membrane and ultimately, the overall durability of the fuel cell system. Therefore, it is critical to understand the underlying degradation mechanisms and failure modes under operational conditions. In this thesis, an extensive research methodology including accelerated stress tests, visualization techniques, and finite element modeling is adopted in order to understand and mitigate membrane degradation. The membrane characterization is facilitated using a non-invasive laboratory-based X-ray computed tomography (XCT) system for 3D visualization of membrane damage progression over the lifetime of the fuel cell. The 3D XCT approach is first applied to understand the degradation mechanism responsible for combined chemical and mechanical membrane degradation. The XCT approach is further expanded to 4D in situ visualisation through periodic same location tracking within a miniature operational fuel cell. Fuel cell membranes with mechanical reinforcements and chemical additives are tested as existing mitigation strategies for the isolated degradation stressors. Under pure chemical degradation, the chemically and mechanically reinforced membrane does not show membrane thinning or shorting sites and exceeds the lifetime of the non-reinforced membrane by 2x. The reinforced membrane also mitigated/delayed the crack development during pure mechanical degradation as compared to the non-reinforced membrane. However, significant membrane degradation is still observed and attributed to buckling and delamination mechanisms within the membrane electrode assembly (MEA). Mitigation of these mechanisms is demonstrated through two novel approaches proposed in this thesis: i) reduced surface roughness gas diffusion layers (GDLs); and ii) bonded MEAs. Both mitigation strategies are tested using the same experimental workflow and shown to provide substantial mitigation against fatigue driven mechanical membrane degradation via reduced membrane buckling, resulting in a doubling of the test lifetime in each case. Complementary finite element simulations corroborate the experimental findings and further estimate the critical GDL void sizes to prevent membrane buckling and the required interfacial MEA adhesion quality to stabilize the MEA for improved membrane durability.
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Thesis advisor: Kjeang, Erik
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