Elucidating the Role of Catalyst layer Resistance in AEM Water Electrolysis

Enhancing catalyst-layer efficiency and durability at high current densities is essential for scalable electrolyzer deployment. Unlike proton exchange membrane systems that are dominated by anodic losses, anion exchange membrane (AEM) systems can exhibit catalyst layer resistances at both electrodes, warranting the use of reference electrodes for kinetic decoupling. We present a methodology to quantify catalyst layer resistance, utilization heterogeneity, and transport limitations (ionic or electronic) by means of impedance diagnostics with dual reference electrodes integrated in the flow channels of each half-cell. Under nominal catalyst loading, we observed elevated local current densities near the membrane anode catalyst interface, implicating ionic conductivity as the primary cause. For the cathode, electronic resistance seems to be limiting. Combined, they manifest as an additional 50–80 mV overpotential attributable to the catalyst layer, a non-negligible fraction of the total 1.9 V of cell voltage at 2 A cm−2. Reducing the catalyst loading improved the reaction homogeneity at the anode; however, for the cathode, it resulted in a marked decrease in effective electronic conductivity, which exacerbated the spatial heterogeneity and further elevated the cathodic kinetic overpotential. These findings highlight the need for designing and evaluating catalyst layer morphologies for balanced electrochemical activity and efficiency.