Effects of Gas Diffusion Layer Compression on Electromechanical Properties and Polymer Electrolyte Fuel Cell Performance

Abstract

The Gas diffusion layer (GDL) is an essential functional component of the Polymer electrolyte fuel cells (PEFC) as it enables the efficient transport of reactants and offers mechanical stability. The influence of compressive loads on the performance of GDL has been the subject of extensive research. In this thesis, a numerical method is explored to investigate interface properties in the bipolar plate (BPP)|GDL and GDL|Polymer electrolyte membrane (PEM) under material and geometrical heterogeneities. Observations indicate that the results are sensitive to GDL material models and endplate designs. This implies that endplates designed to improve the electrical contact resistance and contact pressure at the BPP|GDL interface may not necessarily guarantee an improvement in bulk properties due to a localised, nonintuitive relationship between the electrical interface contact resistance (ICR) and bulk properties. The combined influence of non-uniform ICR and inlet relative humidity (RH), on a single flow channel, along with the heterogeneous flow properties of the GDL, is considered for the PEFC performance evaluation. The results indicate that heterogeneous GDL with non-uniform ICR distribution leads to a ~4.4% decrease in current density at 0.3V compared to homogeneous GDL under full humidification. However, the current density increases by ~19% under fully humidified anode and a partially humidified cathode. Furthermore, the GDL heterogeneity caused by the two clamping designs is simulated to predict the transport characteristics and performance of a 25cm2 active area PEFC. Compared to the conventional endplate design, the proposed endplate configuration offers increased cell performance, which may result from the uniform GDL properties. In addition, the experimental cyclic response of commercially available GDLs with/without MPL (microporous layer) is envisioned for mechanical response at various temperatures and hotpress conditions. The GDL with MPL has a substantial strain response with low force resistance, but GDL w/o MPL has a higher stress-to-strain ratio. The significance of pre- and post-hotpress conditions demonstrated that mechanical response increased by more than 120% in post-hotpress conditions. The thesis concludes with a newly developed phenomenological material model to predict cyclic electrical conductivity in GDLs.