Increase in Oxygen Transport Resistance Caused By Degradation in the Porous Layers in a Proton Exchange Membrane Fuel Cell

Abstract

The increase in mass transport loss accounts for important performance loss when various degradation mechanisms occur in the proton exchange membrane fuel cell (PEMFC)[1-5]. The deterioration of mass transport could be ascribed to two aspects: the increase of liquid water saturation, due to the worse water transport, and increased oxygen diffusion resistance in the pore space. The loss of hydrophobicity has been identified in catalyst layer (CL) [1, 6], gas diffusion layer (GDL) [7, 8], and micro porous layer (MPL) [9, 10], which leads to worse water management. Microscopic characterizations also confirm severe damages to the microstructure, which may hinder the oxygen diffusion in the multiscale porous components[4, 11]. In this study, the oxygen transport resistance at the cathode side of a PEMFC is measured to unravel the increase in oxygen transport resistance associated with microstructure damage. Limiting current measurements are conducted under different dry oxygen fraction conditions before and after an accelerated stress testing (AST). Both GDLs with and without MPL are employed at the cathode side. The results show the presence of MPL increase the total oxygen transport resistance due to the additional layer and its lower porosity. After the aging testing, the oxygen transport resistances in both cells increases significantly. These agree with the severe pore microstructure damage caused by aging testing. The cell with MPL has even larger increases. Further decouple of the total resistance indicates that the cell with MPL has smaller increases in molecular diffusion resistance, but presents significantly greater increases in the other resistance. This implies that pore structure damage occurs in MPL itself and results in a severe increase in Knudsen diffusion resistance. Fig.1 (a) Oxygen transport resistance under different pressure; (b) Decouple of oxygen transport resistance without and with MPL at the cathode. Figure 1