Effects of porosity and water saturation on the effective diffusivity of a cathode catalyst layer

The performance of a proton exchange membrane fuel cell, PEMFC, is significantly affected by the rate of oxygen diffusion through the cathode catalyst layer, CCL. Continuum-scale modelling of PEMFCs requires knowledge of the effective oxygen diffusivity as a function of CCL porosity and its water saturation. To provide this functionality, we used three-dimensional pore-scale modelling to simulate the diffusion of oxygen under different liquid water saturations in CCLs having different porosity values. Solving the governing equations for immiscible two-phase flow, fluid distributions at different saturation levels were obtained. We show that the presence of liquid water initiates a hindering effect by decreasing the diffusive transport of oxygen. Oxygen diffusion, including dissolution of oxygen into the liquid water phase, was taken into account to calculate effective diffusivity values of the entire domain. The resulting effective diffusivity values showed good agreement with values reported in the literature, which are often based on quasi-empirical relationships. Utilizing a large number of simulation results, a correlation equation was developed for the effective diffusivity of oxygen as a function of porosity and liquid water saturation, which is appropriate to be used for macroscopic modelling of flow and transport in CCLs.