Modeling Two-Phase Water Transport in Hydrophobic Diffusion Media for PEM Fuel Cells

Abstract

The prevailing method for modeling water transport in hydrophobic fuel cell diffusion media employs the same two-phase generalization of Darcy’s law widely used for analyzing the movement of oil and water through soil and porous rock. Recent experiments, however, have identified certain problems when the same approach is applied to the nonwoven carbon papers used in fuel cell diffusion media. These problems could be addressed by generalizing the Darcy formulation still more, but significant challenges, such as measuring the relevant material properties, still remain. Instead, this paper proposes a simpler model adapted specifically for fuel cells. Based on analysis of published experiments, the proposed one-dimensional model for heat and two-phase water transport assumes a uniform liquid pore saturation wherever the diffusion medium is wet and neglects the capillary pressure gradients required to transport water in liquid form. In addition to the properties for dry transport, the two-phase water transport model requires just one additional material parameter—the ratio of dry to wet effective gas diffusion coefficients—which can be measured by straightforward limiting current methods. Example half-cell solutions for a Toray diffusion medium show how the proposed model can reproduce changes in oxygen transport resistance observed in limiting current experiments.