Pore network modeling of liquid water and oxygen transport through the porosity-graded bilayer gas diffusion layer of polymer electrolyte membrane fuel cells

In polymer electrolyte membrane fuel cells (PEMFCs), understanding the effect of graded porosity on liquid water distribution and oxygen diffusion inside the porous components is essential for the improvement of water management strategy. In this work, a regular three-dimensional pore network model is constructed to represent the bilayer gas diffusion layer (GDL) consisting of a porosity-graded micro-porous layer (GMPL) and a gas diffusion backing layer (GDBL). Based on this model, the liquid water distribution at breakthrough, relative oxygen effective diffusivity and limiting current density as a function of water saturation are obtained using invasion percolation algorithm and resistor-network theory, respectively. Parametric studies are also conducted to elucidate the effects of several structural factors, such as the graded porosity value of the GMPL, the GMPL thickness and the number of sub-layers in GMPL. The simulated results illustrate that the introduction of graded porosity can relieve the extent of the flooding phenomenon to a certain extent, and the underlying mechanism can be explained from the perspective of the pore scale. Furthermore, an optimum graded porosity value where the material has the preferred oxygen transfer capacity and high limit current density under partially saturated condition (water saturation is between 0.025 and 0.5) can be found. (c) 2019 Published by Elsevier Ltd.