UBC Theses and Dissertations
Novel microporous layers with improved interfacial characteristics for PEM fuel cells Janse van Vuuren, Magrieta Jeanette Leeuwner
High efficiencies and reduced greenhouse gas emissions promote proton exchange membrane fuel cells (PEMFCs) as a promising energy conversion technology. However, its widespread commercial application is hampered by certain cost, performance and durability limitations. The interface between the MPL (microporous layer) and the cathode CL (catalyst layer) plays an important role in a PEMFC’s overall performance, since it houses the reaction sites for the oxygen reduction reaction. The interface may furthermore significantly affect mass transport behavior, ohmic contributions and the hydration state of the membrane at different humidities. The main objective of the study was therefore to advance PEMFC research through the development of alternative MPLs offering dual-functional improvements: enhanced interfacial characteristics and improved operational flexibility via suitability for low cathode humidity applications. Alternative MPLs were evaluated based on extensive material characterization and single cell performance testing. Graphene was demonstrated to be a promising alternative. The material displays beneficial interfacial characteristics (a stacked planar morphology, superior conductivity, adhesive behaviour, and improved electrical connectivity with the CL) and furthermore results in improvements in the kinetic and ohmic polarization regions, compared to the conventional CB (carbon black) MPL. Although graphene MPLs also suffer from mass transport limitations, the problem can be addressed through the addition of CB. The addition increases the MPL’s water permeability, which helps to establish a balance between water removal (for the prevention of flooding) and water retention (for membrane hydration) at high and low RH (relative humidity). For graphene tested under one-dimensional control, this results in synergistic performance enhancements, showing a 30% and 80% increase in the maximum power density at 100% and 20% cathode RH. In addition to increased water permeability, other common effects resulting from the creation of CB composites (also observed for reduced graphene oxide and graphite) include decreased surface wettability and through-plane resistance. For the application to low loaded CCMs (0.1 mg cm-²), the potential to improve performance with graphene-based MPLs appears restricted by the catalyst loading itself. Nevertheless, graphene helps to improve performance preservation of low loaded CCMs at low humidity conditions, as also demonstrated for conventionally loaded CCMs (0.4 mg cm-²).
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