Modeling the Effects of the Microporous Layer on the Net Water Transport Rate Across the Membrane in a PEM Fuel Cell

In this study, a model was developed to evaluate the effects of the microporous layer (MPL) on the net liquid water transport rate across the membrane. The results support the hypothesis that the improvement in fuel cell performance observed when an MPL is used in the cathode side is related to its effect on the water transport process in the electrode and membrane. Due to its high hydrophobicity, the MPL increases the liquid water pressure in the cathode to levels much higher than that in the anode, resulting in an increased back-transport rate of liquid water from the cathode to the anode. This reduces the amount of water that is transported out of the cathode through the gas diffusion layer (GDL) to the cathode flow channels resulting in a lower saturation level in the GDL, and consequently, faster oxygen transport to the catalyst sites. This model showed that the state of zero-net-water-transport-across-the-membrane could be achieved with the appropriate capillary properties of the porous media. Two capillary properties of the MPL identified to have the greatest impact on the proton-exchange-membrane (PEM) fuel cell performance are the liquid water saturation level at p(c)=0 (p(g)=p(l)) and the slope of the capillary curve in the hydrophobic region.