Mason, TJ; (2013) Advanced diagnostic techniques to study the electrochemical and mechanical properties of polymer electrolyte fuel cells. Doctoral thesis , UCL (University College London).

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Abstract

Polymer electrolyte fuel cells (PEFCs) are a key technology as the world strives for a low carbon future. The main obstacles facing mass-market uptake are the high cost and the longevity of the units; as such, research is needed to enhance performance and understand the degradation mechanisms. In this study, dynamic compression is applied using a cell compression unit (CCU) to study the effect on performance of a membrane electrode assembly (MEA) and its individual components with dimension change. Electrochemical impedance spectroscopy (EIS) is used to delineate the effect of compression on contact resistance, membrane resistance and mass transfer losses. Derived parameters such as the ‘displacement factor’ are used to characterise a representative range of commercial gas diffusion layers (GDLs). Increasing compaction pressure leads to a non-linear decrease in resistance for all GDLs. Different GDLs have different intrinsic resistance; however, all GDLs of the same class share a common compaction profile (change in resistance with pressure). Cyclic compression of Toray GDL leads to progressive improvement in resistance and reduction in thickness that stabilises after ~10 cycles. During initial hydration of Nafion membranes there is a direct relationship between membrane conductivity and dimensional change (swelling) of MEAs. Electrode flooding is found to result in membrane hydration and an increase in stress or strain, depending on the compression mode of the fuel cell. Results suggest that hydration cycles and flooding events can lead to cell degradation due to the stresses imposed. With increasing compression, a significant reduction in net performance is observed, with the most significant differences occurring in the mass transport regions of the performance curves. As the compression increases, the high-frequency resistance reduces with the improvement in contact resistance between the GDL and bipolar plate material, concurrently the low frequency resistance increases with increasing compression.

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