Measurement of capillary pressure properties of gas diffusion materials used in proton exchange membrane fuel cells

Abstract

A capillary pressure curve is used to account for the mass transport limitation in the porous gas diffusion layer (GDL) in modeling two-phase transport phenomena in a fuel cell. Various assumptions and approximations on capillary pressure curve have been made to develop models due to the scarcity of the experimental data of gas diffusion layers (GDL). The lack of experimental data of capillary pressure versus liquid water saturation level hinders progress in model validation. Capillary pressure versus saturation curves for imbibition (liquid water displaces air) and drainage of a nonwetting phase (water) were measured for SGL SIGRACET® and Toray TGPH060 gas diffusion materials. The volume displacement method and neutron imaging technique were employed to measure capillary pressure curves for gas diffusion materials. The experimental results showed that 30wt% wet-proofed carbon paper displayed lower water saturation at the same capillary pressure compared to that of 10wt% and 20wt% wet-proofed carbon papers. No significant capillary pressure hysteresis was observed due to the hydrophobic material coated on the gas diffusion material. The capillary pressure curve of microporous layer showed higher water saturation level which could be attributed to water penetrated through the cracks on the surface of the microporous layer. Due to the similar structure of the gas diffusion material, capillary pressure curves could be correlated to a single Leverett function. It is believed that the liquid water only transports through the hydrophilic pores generating air/water/carbon system. Neutron imaging technique is used to measure the capillary pressure curve in in-plane direction due to the limited resolution of the current detector system. The results showed that water does not imbibe spontaneously into 0wt% wet-proofed carbon paper because of the pore structure on the surface. The catalyst layer imbibes water spontaneously which shows that when a fuel cell is operated at flooding condition catalyst layer can be readily flooded due to its hydrophilic property.