Effects of diameter ratio between fibers and grains for gas diffusion layer on transport properties via the Shan-Chen method

This study characterizes dual porous layers with fibrous and granular structures with hydrophobic agents, resembling gas diffusion substrate (GDS) and microporous layer (MPL) in polymer electrolyte fuel cell (PEFC). The ratio of characteristic lengths between fiber (GDS) and grain (MPL) diameters was varied from 0.04 to 1.00, creating 150 models that were analyzed in this study. The fiber diameter was fixed at 7.00 µm, yielding grain diameters from 0.31 to 7.00 µm. Effective conduction and diffusion coefficients, absolute permeability, corresponding homogeneity of temperature, concentration, and fluid density distribution were examined in both dry and saturated conditions using the Shan-Chen method based on the lattice Boltzmann equation. Results show that smaller diameter ratios (0.04 and 0.10) lead to decreased permeability and diffusivity at lower saturation ($S_{\text{nw}}<0.7)$. However, these values converge at higher saturation regardless of the diameter ratio. Due to the fine pore network and hydrophobicity at lower diameter ratios, the homogeneity index (Geary’s coefficient) at highly saturated conditions ($S_{\text{nw}}>0.9)$ was boosted significantly—up to 22.2-fold for permeability and 0.079 from almost 0 (strong clustering) for diffusivity—compared to the coarsest structure (diameter ratio of 1.00). These findings indicate that employing a finer MPL network near reaction sites with catalyst and a coarser network near flow channels may enhance PEFC performance, offering a theoretical basis for designing gas diffusion layers with graded pore or porosity structures.