The Effects of Catalyst Layer Microstructure and Water Saturation on the Effective Diffusivity in PEMFC

The effective diffusivity of oxygen in a cathode catalyst layer of PEMFC depends on both the catalyst layer microstructure and the changes in oxygen pathways due to liquid water. In this paper, various aspects of the effective diffusivity are investigated using pore-scale simulation, with emphasis on the effects of Knudsen diffusion. Based on microstructures reconstructed by the sphere-based simulated annealing method, both the higher-order lattice Boltzmann method (LBM) and the continuum diffusion equation with the Bosanquet approximation are used to evaluate the effective diffusivity considering Knudsen diffusion. Results show that the continuum diffusion equation reproduces the results from the higher-order LBM that simulates the effects of Knudsen diffusion based on the kinetic theory, when the mean pore size is evaluated using the erosion-dilation method or the chord length distribution. It is also shown that the change of the pore size with water saturation levels should be considered to predict the effects of Knudsen diffusion accurately. The dependence of the effective diffusivity on the agglomerate size is explained by the effects of Knudsen diffusion. Derjaguin's correction for Knudsen diffusion is found to have negligible effects on the effective diffusivity for the present cases. The empirical correlations of various parameters are presented.