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

Flooding within a Polymer Electrolyte Membrane (PEM) fuel cell occurs during operation as a result of product water vapor condensing near the surface of the cathode; this can be detrimental to fuel cell performance due to its role in reducing oxygen transport throughout the GDL. Previous Gas Diffusion Layer (GDL) transport models have made use of a zero-saturation boundary condition at the GDL/Oxygen (O₂) gas channel (GC) interface. However, the physical correctness of this saturation boundary condition is still unclear. Further investigation of the saturation boundary condition could lead to a more robust model of the GDL saturation distribution and cathodic flooding. This exploration provides a one-dimensional two-phase transport model for saturation as well as liquid water and gaseous oxygen pressure distributions throughout the cathode-side gas diffusion layer (GDL) within a PEM fuel cell. The focus of this investigation is on the impact of non-zero saturation boundary conditions at the GDL / GC interface, and its impact on two-phase transport within the porous medium, with regard to fuel cell performance. Saturation boundary conditions at this location are determined based on GDL interfacial liquid coverage of water droplets that diffuse through the porous medium during operation and block oxygen transport paths. The results of this investigation suggest that non-zero saturation boundary conditions at the GDL/GC interface are evident when analyzing two-phase phenomena, which affect the overall saturation distribution throughout the GDL, and consequent performance of the fuel cell. In addition, GDL membranes with large porosities were found to improve gas and liquid transport by lowering the saturation at the GDL / Cathode interface that would otherwise impede oxygen transport.