Oxygen Diffusion Characteristics of Gas Diffusion Layers With Moisture

Abstract

The mass transfer characteristics of the gas diffusion layer (GDL) in a polymer electrolyte fuel cell (PEFC) are closely related to the performance. In this study, the oxygen diffusivity of paper and cloth type porous media, which are generally used as GDLs, were measured with respect to liquid water content, using experimental apparatus consisting of an oxygen sensor based on a galvanic battery. Paper type porous media, both non treated and hydrophilic treated, and the cloth type porous media with non treated surface were used as GDL specimens. The porosity of both specimens was almost the same, but the representative pore diameter of the cloth type GDL was approximately three times larger than that of paper type GDL. Two methods were utilized to impregnate liquid water into the porous GDL media to realize different water distributions in the specimens at the initial state; vacuum impregnation and moist air condensation impregnation. The oxygen diffusivities of the specimens were measured to clarify the influence of the two impregnation methods on the oxygen diffusion characteristics. Moreover, the relation between the measurement of oxygen diffusivity and the visualization of the liquid water distribution by using Neutron Radiography [Tasaki et al. (2007)] was investigated for the paper and cloth type GDLs. The oxygen diffusivity in the paper type porous media decreased precipitously with increasing water saturation by the vacuum impregnation method, whereas the diffusivity decrease was relatively small when impregnated by the moist air condensation method. For the cloth type porous media with weaving threads, oxygen diffusion characteristics were independent of the water impregnation method. Thus, the porous medium’s microstructure plays an important role in determining diffusion characteristics, especially in the presence of liquid water.