(33a) Gas Diffusion Layer Fiber Size Distribution and Diffusion Regime Effects on PEM Fuel Cell Performance

The effective diffusivity of fuel cell gas diffusion layers (GDL) of bimodal (two-diameter) fiber size distribution was computed through random walks in the bulk, transition and Knudsen diffusion regime, for both in-plane and transverse flow. The bulk regime effective diffusivity was used to derive also the formation factor (hence thermal and electrical conductivity) and viscous permeability of the gas diffusion media. All transport properties were applied into a polymer electrolyte membrane (PEM) fuel cell model to better describe transport of the feed gases through bimodal fibrous gas diffusion layers. The model was used to evaluate the effects of key GDL and process parameters on fuel cell efficiency. The effective diffusivities computed for the bimodal GDL media in all diffusion regimes practically coincide with those for unimodal (one-diameter) media, for porosities above 0.5. This is not the case at lower porosities, where bimodal GDL structures exhibit higher diffusivities than the unimodal ones, due to a significantly lower percolation threshold (0.045 vs. 0.11). This difference in the percolation porosity gives rise to a significant difference in the viscous permeability as well, with the bimodal fiber structures found to be much more permeable to viscous flow than the unimodal ones in the low porosity range. The PEM fuel cell model predictions were evaluated at three GDL porosity levels, and the best results were achieved with the intermediate porosity of 0.4. The high GDL porosity tested, 0.7, led to a slightly lower prediction for the power density, but the low porosity of 0.12 resulted in a dramatic reduction in the value of that key fuel cell performance parameter. Bulk diffusion was found to be the dominant mass transfer mechanism in the fuel cell GDL. Transition or Knudsen diffusion can be established when using an impractically low pressure or fiber size, both having a detrimental effect on fuel cell performance. The use of bimodal fiber structures as gas diffusion layers in the fuel cell, instead of the typically used unimodal ones, made no difference at an average or high porosity, but proved to be very beneficial at low porosity, i.e., for highly compressed fuel cell diffusion media.