Determination of the Relationship Between Capillary Pressure and Saturation in PEMFC Gas Diffusion Media

Abstract

This paper describes a method of measuring the relationship between capillary pressure and porous media saturation in the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC). Such a relationship is commonly used to model the liquid water flow in the GDL. The method utilized to characterize the GDL behavior mimics the actual transport of liquid water within the GDL by utilizing the actual fluids of interest in a PEMFC cathode (water and air), and by introducing all water from a single face to simulate the water production at the catalyst layer. Other porosimetry methods rely on totally non-wetting or totally wetting fluids to achieve saturation and consequently the resulting capillary pressure measurements must be scaled to the emulate the situation in the PEMFC GDL. Capillary pressure versus saturation curves for two different GDL materials (one paper, one cloth), each with four different bulk loadings of PTFE (0, 10, 20 and 30 wt%), were measured. Results show that the PTFE loading has a relatively small effect on the capillary pressure within the pressure range normally associated with PEMFC water transport. The results also show that carbon cloth based GDL materials require greater capillary pressures than paper materials to achieve significant saturation and that compression has a homogenizing effect on the pore structure and the slope of the capillary pressure – saturation Pc(S) behavior of both materials. Representative curves for the derivative of the Pc(S) function are developed for each type of diffusion media within the appropriate saturation range.