Liquid Water Saturation and Oxygen Transport Resistance in Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers

Abstract Liquid water accumulation in the gas diffusion layers (GDLs) of polymer electrolyte membrane (PEM) fuel cells is governed by a complex interplay of factors, the full scope of which is not yet fully established in literature. This study presents the combined effects of relative humidity (RH) and current density on liquid water accumulation and oxygen mass transport resistance in the cathode GDLs of a PEM fuel cell. Through-plane liquid water saturation distributions were measured in situ using synchrotron X-ray radiography while simultaneously performing limiting current-based characterizations of oxygen transport resistance. At low current densities ( −2 ), lower cathode RH levels resulted in the largest oxygen transport resistances due to reductions in ionomer hydration in the catalyst layer. In the intermediate current density range (1.5–2.1 A · cm − 2 ), high RH levels resulted in the largest oxygen transport resistances due to the observed significant sensitivity of liquid water accumulation to cathode RH. At high current densities (>3.0 A · cm − 2 ), cathode GDL liquid saturation levels were high regardless of cathode inlet RH, and the oxygen transport resistance was therefore less sensitive to RH. Furthermore, it was established that liquid water tends to preferentially accumulate in regions of higher local porosity within the GDL, identified by combining measured liquid water saturations with micro-computed tomography (μCT) characterizations of the through-plane porosity profile. Finally, the strong relationship between GDL oxygen transport resistance and liquid water-free (effective) pore space of the GDL was examined in order to consider the feasibility of predicting oxygen transport resistance based on overall liquid saturation.