Constructing a breakthrough pressure and blockage based PEMFC model for elucidating water and gas transport in gas diffusion layers

The performance of proton exchange membrane fuel cells (PEMFCs) is strongly relied on the transport of water and gas in a gas diffusion layer (GDL). In this study, it introduces a novel concept of water blockage as an alternative to the traditional definition of water saturation. Concurrently, breakthrough pressure drainage mechanism is proposed. Building on these concepts, it develops a new model for the water and gas transport elucidation. The breakthrough pressure is defined as the point at which the hydraulic pressure at the entrance of the pores exceeds a critical threshold, enabling liquid water to penetrate into the pores of MPLs. At the same time, it is considered that the pore area filled with water in MPLs cannot be transported by gas, and the ratio of this area to the total pore area of the cross section parallel to the surface of the MPL is defined as water blockage. The water blockage will be more conducive to evaluate the water and gas transport performance of MPLs than water saturation.

Based on the breakthrough pressure and water blockage, a new model of water and gas transport in PEMFCs is constructed. In-depth study on the water and gas transport process with the new model, it is found that: (1) The water blockage in the MPL increases with the increase of back pressure. It is showed that the increase of back pressure is a double-edged sword. In the low back pressure range, increasing the back pressure can increase oxygen partial pressure, contributing an enhanced exchange current density and cell performance. However, when the back pressure is high enough, the increased water blockage would reduce the gas transport path, resulting in a serious concentration polarization and a decline of the performance at high current density. (2) The water blockage is also strongly related to the pore size in MPLs. Macropores are more likely to be filled by water and used as liquid water transport channels, while pores with small size that are not easy to be blocked which is applied as gas transport paths. Therefore, the MPL with bimodal pore distribution is more conducive to achieving excellent water and gas transport performance. (3) On the premise of maintaining or improving the porosity of the MPL, increasing the proportion of small pores can significantly reduce the water blockage at high current density and enhance the cell performance.