Capillary Transport Analysis in Macro-Homogeneous Diffusion Media of PEM Fuel Cells

The interplay of electrochemistry, two-phase flow, and heat transfer generates complex transport phenomena within the porous materials of fuel cells that are not yet fully understood. This lack of comprehensive understanding complicates the modeling of liquid water transport, which is critical because the hydration of the polymer electrolyte membrane significantly impacts the cell performance. The liquid water transport mechanisms in porous media can be explained by capillary force, hydraulic permeation and gravity effects, as well as water condensation and evaporation. In general, the liquid water transport is mainly driven by the capillary force, while body forces, such as gravity, do not significantly affect its momentum. Due to limited experimental data on capillary pressure and saturation in gas diffusion media, the Leverett approach has been widely used for modeling liquid water transport in PEMFCs. The Leverett approach is a polynomial fitting of capillary pressure data for water imbibition in unconsolidated sand packs. Due to its nature, this approach may not accurately predict capillary pressure in gas diffusion media. Fuel cell GDM materials, naturally hydrophilic, are typically coated with a nonwetting polymer like polytetrafluoroethylene to create hydrophobic surfaces and pores. The resulting nature of GDM materials, with intermediate wettability due to the coexistence of hydrophilic and hydrophobic pore spaces, complicates transport phenomena. Consequently, the applicability of the traditional Leverett approach is questionable. This work focuses on capillary transport within PEMFCs, highlighting key experimental and modeling approaches for predicting the capillary pressure-saturation relationship. Starting from the Leverett function, improved models have been proposed and are here implemented in a 3D-CFD model. This research provides an overview of key experimental and theoretical developments in understanding capillarity in PEMFCs. Furthermore, it implements selected capillary pressure correlations in a 3D-CFD model to evaluate their performance in simulating water transport within the porous media, providing guidelines for their use in large-scale models.