In this study, the effective oxygen diffusivity in the dry or partially-saturated gas diffusion layer (GDL) is numerically investigated by an oxygen diffusion model in GDLs reconstructed by a stochastic method. The predicted effective diffusivity in dry GDLs is compared with various diffusivity models from literatures. Reasonable agreements with other models were obtained. The effect of the PTFE loading in the dry Toray carbon paper is also investigated and compared with recent experimental data. It is found that the effective diffusivity becomes lower under higher PTFE loading due to the decreased pore volume, as expected. The relative effective oxygen diffusivity in partially-saturated GDLs is calculated using the two-phase volume of fluid (VOF) model and an oxygen diffusion model. The effects of different local water profiles and porosity distribution on the effective oxygen diffusivity in both the through-plane (TP) and in-plane (IP) directions are investigated and compared with a lattice Boltzmann model and experimental data. The present results are in good agreement with other studies. It is found that local water profile has significant impacts on the effective diffusivity in partially-saturated GDLs and the diffusivity in the TP direction is more sensitive to the water distribution than the IP direction
