Impact of Microchannel Boundary Conditions and Porosity Variation on Diffusion Layer Saturation and Transport in Fuel Cells

Abstract

Polymer electrolyte membrane (PEM) fuel cell flooding can be detrimental to energy generation performance due to its role in reducing gas diffusion layer (GDL) oxygen transport. Previous transport models have made use of a zero-saturation boundary condition at the GDL/oxygen gas channel (GC) interface. However, the physical accuracy of this boundary condition is still unclear and further investigation is needed to lead to a more robust model of the GDL saturation distribution. This work provides a half-cell two-phase transport model for saturation as well as liquid water and gaseous oxygen pressure distributions. This work focuses on the impact of nonzero saturation boundary conditions at the GDL/GC interface, and its impact on GDL two-phase transport. Saturation boundary conditions at this location are determined based on GDL interfacial liquid coverage of water droplets that form as a result of product water that diffuses through the porous medium and blocks oxygen transport paths. The results indicate that nonzero saturation boundary conditions, which are a consequence of GDL/GC liquid droplet coverage, increase cathode saturation by as much as 4% and 12% for low and high current density conditions respectively. It is also shown that cathode saturation dependence on the interfacial liquid coverage fraction α is reduced with an increase in porosity ɛ. As ɛ increases from 0.3 to 0.5, the cathode saturation difference is reduced by 22%. This investigation also evaluated the inclusion and optimization of a microporous layer (MPL) within the half-cell system. It was found that cathode saturation reductions were more significant for increasing MPL porosity than for GDL porosity. The results suggest that its inclusion was able to reduce cathode saturation by up to 90% at the GDL/MPL interface for near zero α values.