Investigating the Effect of Non-Uniform Microporous Layer Intrusion on Oxygen Transport in Dry and Partially Saturated Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers

Abstract

State-of-the-art gas diffusion layers (GDL) used in polymer electrolyte membrane (PEM) fuel cells have a bi-layered structure comprising of a carbon fiber substrate and a microporous layer (MPL). Addition of the MPL has been shown to improve the fuel cell performance [1-2]. Earlier studies have also shown that the configuration of the MPL relative to the GDL has an effect on gas transport through the combined diffusion medium [3-4]. This study investigates the effect of the MPL intrusion and morphology on the oxygen transport in PEM fuel cell GDLs. In particular, we investigate the effect of varying MPL layer characteristics, both interior and exterior to the carbon fiber substrate in dry and saturated conditions, on the oxygen diffusivity and liquid water saturation at breakthrough conditions. Stochastic generation methods were utilized to create GDLs with varying MPL intrusion depths and morphologies. The generated morphologies of the non-uniform MPL layers were validated using reconstructed microscale X-ray-computed tomography. Pore network modelling was applied to simulate the oxygen transport behavior within these stochastically generated materials. The liquid water saturation was calculated using an invasion percolation algorithm. Three distinct scenarios were studied: varied MPL intrusion depth, varied MPL outer layer thickness, and constant MPL thickness with varied intrusion depth. In controlling the MPL intrusion, it was found that ~50-60 µm is the critical MPL thickness beyond which the saturated oxygen effective diffusion coefficient rapidly increases and the substrate saturation rapidly decreases (Figure 1). This study suggests that when the GDL is invaded by liquid water through cracks in the MPL, preferential MPL configurations exist for improving oxygen transport and minimizing substrate saturation. The results of this study can be used to guide the continued improvement of MPL fabrication.