Effective diffusivity in partially-saturated carbon-fiber gas diffusion layers: effect of through-plane saturation distribution

Click on the DOI link to access the article (may not be free).The effective diffusivity of gaseous species in partially-saturated finite-size porous media is a valuable parameter for mathematical modeling of many processes, but it is difficult to measure experimentally. In this work, the effective diffusivity of carbon-fiber gas diffusion layers (GDLs) used in polymer electrolyte fuel cells (PEFCs) was determined by performing lattice Boltzmann (LB) simulations on X-ray tomographic reconstructions of invading water configurations. Calculations on dry GDLs were in close agreement with previous experimental data; the effective diffusivity was reduced by the addition of PTFE due to the loss of pore volume and the higher tortuosity of transport paths. The effect of water saturation was significantly larger. It was found that the resistance of water to gas transport was extremely dependent on the saturation distribution through the porous medium, particularly the peak saturation, and not just the average saturation as is typically considered in the literature. Through-plane diffusion was dramatically limited in materials with high-peak local saturations, even at low average saturation levels. No significant limitations were observed for diffusion in the material plane. The computed results demonstrate the strong sensitivity of finite-size porous media to local conditions, highlighting the difficulties of applying volume-averaged continuum-scale modeling techniques to micro-scale materials.This work was supported by the Natural Science and Engineering Research Council of Canada's Discovery Grant program, the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Program, of the U. S. Department of Energy under contract DE-AC02-05CH11231, and Project ENE2011-24574 of the Spanish Ministerio de Economia y Competitividad (MEC). XCT experiments were performed in the beamline 8.3.2 at ALS, Lawrence Berkeley National Laboratory, which is a national user facility funded by the Department of Energy, Office of Basic Energy Sciences under contract DE-AC02-05CH11231. Numerical calculations were conducted in the supercomputing clusters Guillimin and Colosse managed by Calcul Quebec and Compute Canada [71]. The operation of these supercomputers is funded by the Canada Foundation for Innovation (CFI), Ministere de l'Economie, de l'Innovation et des Exportations du Quebec (MEIE), RMGA and the Fonds de recherche du Quebec - Nature et technologies (FRQ-NT)