Analysis of Water Transport inside Hydrophilic Carbon Fiber Micro-Porous Layers with High-Performance Operation in PEFC

Abstract

Polymer electrolyte membrane fuel cells using a hydrophilic micro-porous layer (MPL) consisting of carbon fiber (CF) and ionomer show better performance than those using conventional hydrophobic MPL with carbon black (CB) under a wide range of humidity conditions. This study investigates the effects of the wettability and structure of the MPL on cell performance and discusses the mechanism for improving cell performance by the hydrophilic CF-MPL at high humidity conditions. The water distribution inside the MPL is evaluated by a freezing method and cryo-SEM observations, and a comparison of the distribution and the cell performance of hydrophilic and hydrophobic MPL cells with various structures is made. The results show that the performance improvement is caused by preventing increases in concentration overvoltage at high current densities; where the fiber structure and thicker layers are important with hydrophilic MPL. In hydrophilic CF-MPL with better performance, liquid water accumulates in pores at the catalyst layer (CL) side, while there is no liquid water at the gas diffusion layer side. These results indicate that the hydrophilic CF-MPL enhances the liquid water transport from the CL due to the hydrophilic properties, and promotes water evaporation due to the large pores in the thick fiber structure.