Oxygen transport resistance correlated to liquid water saturation in the gas diffusion layer of PEM fuel cells

Abstract

Under typical proton exchange membrane fuel cell (PEMFC) operating conditions, temperature gradients through the porous gas diffusion layer (GDL) can result in product water condensation. As a result, non-uniform partial saturation of the GDL changes the local effective porosity and tortuosity encountered by oxygen diffusing to the catalyst layer. This additional transport resistance reduces the partial pressure of oxygen at the catalyst surface of an air-fed cathode. In the current work, this phenomenon is investigated in two-dimensions using limiting current experiments that define GDL boundary conditions along with simultaneous neutron imaging to measure the local water content relative to the flow field geometry. The subsequent effective diffusion coefficient vs. saturation relationship derived from this method is reported for two common GDL carbon fiber substrates. It is also shown that the land vs. channel distribution of liquid water must be accounted for to accurately predict diffusion resistance. These results represent the first time that the effect of water saturation on effective diffusion coefficient has been directly measured in situ, thus enabling accurate determination of the exponent “n” in the modified Bruggeman relationship for two commercially available gas diffusion layer materials.