Impact of Compression on Effective Thermal Conductivity and Diffusion Coefficient of Woven Gas Diffusion Layers in Polymer Electrolyte Fuel Cells

The contrasting effect of compression on the ability of gas diffusion layer (GDL) in polymer electrolyte membrane fuel cell to conduct fluid, heat and electron implies that there is an optimal clamping force for cell performance. For a given GDL, understanding its associated optimal compression needs to know how its conductive ability changes with compressive pressure. In this paper we investigated the impact of compression on the effective diffusion coefficient and thermal conductivity of a carbon-cloth GDL. The interior microstructures of the GDL under different compressions were acquired using X-ray tomography; microscopic models were then developed to simulate gas diffusion and heat transfer in the microstructures in both in-plane and through-plane directions. The effective diffusion coefficient and thermal conductivity were calculated by volumetrically averaging the simulated gas diffusive and thermal flux rates at micron scale. The results show that both effective diffusion coefficient and thermal conductivity were anisotropic and their values in the in-plane direction were higher than in the through-plane direction. With porosity decreasing under the compression, the effective diffusion coefficient decreased faster in the through-plane direction than in the in-plane direction; the formula derived by Nam and Kaviany was capable of describing the change of the effective diffusion coefficient with porosity in the in-plane direction but not in the through-plane direction. For heat transfer, as the porosity decreased, the thermal conductivity increased faster in the through-plane direction than in the in-plane direction, and the increase in both directions could fit to the formula of Das et al.