Proton exchange membrane (PEM) electrolysis is a promising route for renewable hydrogen production. However, to enable widespread, low-cost hydrogen generation, PEM electrolyzers must make advances in performance and durability with greatly decreased loadings of iridium as the anode oxygen evolution catalyst. The ionic and electronic resistance of the anode catalyst layer is an important consideration, as high internal resistance lowers catalyst utilization and cell performance while accelerating degradation. Catalyst layer resistance (CLR) is relatively well-understood in fuel cells and other porous electrode systems. However, characterization of CLR is not routinely used in the PEM electrolysis community, and the impacts of CLR for PEM electrolysis are not widely understood.

Here we will present in-situ methods for measuring CLR in electrolysis cells using a non-faradaic H₂/H₂O condition as well as methods for calculating the associated voltage losses. These methods are applied to anode catalyst layers based on IrO₂ nanoparticles as well as dispersed nano-structured thin film (NSTF) Ir catalysts. Trends of CLR, performance, and durability with electrode properties such as loading and interactions between the porous transport layer and catalyst layer will be discussed. We will also present investigation of characteristic uneven degradation of the catalyst layer caused by CLR and strategies for mitigating this degradation mechanism.