Pore-scale analysis of effective electrical conductivity of unsupported catalyst layers for polymer electrolyte membrane water electrolyzers

Reducing iridium (Ir) loading in polymer electrolyte membrane water electrolyzers (PEMWEs) is essential for lowering system cost, but it also strongly affects catalyst-layer (CL) connectivity and electronic transport. In this work, a three-dimensional pore-scale model is developed to investigate percolation-governed electrical conductivity in unsupported IrOx CLs by explicitly resolving agglomerates, ionomer, and pore space. The analysis systematically examines the effects of Ir loading, agglomerate size, agglomerate volume fraction, ionomer content, and PTL/CL interfacial pore size on in-plane (IP) and through-plane (TP) conductivity. The results reveal a critical percolation transition controlling IP transport, with conductivity rapidly collapsing below approximately 0.1–0.2 mg Ir cm −2 due to conductive-network fragmentation. In contrast, TP conductivity remains comparatively preserved because of geometric confinement, leading to pronounced transport anisotropy at low loadings. Smaller agglomerates, higher agglomerate fractions, moderate ionomer contents, and reduced PTL/CL interfacial pore sizes are shown to enhance conductive-network robustness and reduce sheet resistance. Percolation-inspired correlations and experimentally actionable design guidelines are further derived from the pore-scale simulations. Overall, the study provides new mechanistic insight into percolation-controlled electronic transport in unsupported PEMWE CLs and establishes practical structure–property relationships for the design of low-Ir electrodes with improved electrical conductivity.