Pore network modeling of a microporous layer for polymer electrolyte fuel cells under wet conditions

The gas diffusion layers (GDLs) of polymer electrolyte fuel cells have been developed with applying microporous layers (MPLs) in their catalyst layer (CL) side to alleviate the accumulation of liquid water in the CL for oxygen transport to the cathode CL. A three-dimensional porous structure of our in-house hydrophobic MPL is numerically modeled with a pore network model (PNM). The convective air permeability and oxygen diffusivity, which depend on liquid saturation, are evaluated. To construct the PNM, focused ion beam scanning electron microscopy (FIB-SEM) is used to derive the pore size distribution (PSD). The model is ex-situ validated through air permeability and oxygen diffusivity tests with controlled saturation of non-volatile wetting liquid that is stable in the hydrophobic MPL. Oxygen diffusivity of the MPL is obtained by identifying the diffusion resistances of the concentration boundary layers and GDL substrate in the tests. The model predicts the effects of liquid water saturation in the MPL on the air and liquid water permeations, and the oxygen diffusion, and thus can be used to design optimal PSDs for practical cells.