Recent movement toward zero-emission mobility has propelled significant technological advancements in commercialization of polymer electrolyte membrane fuel cells (PEMFCs). PEMFCs provide electricity by reacting hydrogen with oxygen through two half-reactions occurring inside two respective anodic/cathodic microporous catalyst layers (CLs) with thicknesses of ~2-8 µm. Other products of the overall reaction include water and waste heat. All the electricity generation/consumption and most of the heat generation modes occur inside the CLs through a set of highly coupled multi-physics phenomena (a coupling between the electrochemical reactions, transport of species, electron conduction, and heat conduction). This necessitates knowing thermal and electronic conductivities of CLs for optimizing the fuel cell performance in various operating conditions. In this thesis, novel procedures are developed to measure thermal and electronic conductivities of CLs at low error rates. The procedures are based on novel methods to increase the amount of catalyst in the testbeds for enhancing the signal to noise ratio while ensuring complete deconvolution of the CL bulk signal. Further, a comprehensive platform is developed to characterize microstructure of CLs from different aspects, including a complete scheme for characterizing cracks for the first time. Separate measurements of in-plane and through-plane electronic conductivities, for the first time, uncovers anisotropic microstructure of CLs. CL designs with various compositions and structures are made and characterized. Observed trends in the conductivity data are linked to various structural properties of the CLs to understand structure-property correlations. A complete set of closed-form multi-scale structural models are developed for the conductivities in different directions to understand the underlying physics and provide tools for development of CLs with desired conductivities. The developed models agree well with the experimental data and precisely predict the structural trends. The models also explain and predict effects of different operating conditions. Using the developed tools, design guidelines are proposed for fabricating CLs with desired thermal and electronic conductivities, whose proof of concepts were made and successfully tested in the experimental phase of this research. Order of magnitude analyses show significant potentials for enhancing the fuel cell performance by tuning the conductivities through engineering the microstructure.
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Thesis advisor: Bahrami, Majid
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