Balancing Iridium Loading and Catalyst Layer Thickness in TiO _x -Based Anodes for Polymer Electrolyte Water Electrolysis

Polymer electrolyte water electrolysis (PEWE) is a promising route for sustainable hydrogen production, but its deployment is constrained by the cost and scarcity of iridium-based oxygen evolution reaction (OER) catalysts. Reducing the anode Ir loading while maintaining performance requires careful optimization of the catalyst layer (CL) architecture. In this work, commercial TiO 2 -supported iridium oxide (IrO x ) catalysts with Ir contents ranging from 10 to 75 wt% were systematically evaluated at fixed Ir loadings of 0.5 and 0.1 mg Ir cm −2 . By combining structural characterization, intrinsic electronic conductivity measurements, and electrochemical analysis, correlations between catalyst composition, CL thickness, and electrolysis performance were established. The catalysts with low Ir contents (10 wt%) formed thick, TiO 2 -rich layers associated with limited electronic connectivity, whereas those with high-Ir contents (75 wt%) produced ultrathin layers that, despite improved electronic conductivity, showed reduced effective catalyst utilization and increased sensitivity to structural inhomogeneity. These findings demonstrate that efficient low-Ir anodes require joint optimization of Ir content, catalyst layer thickness, and microstructural connectivity to balance conductive network formation and effective transport pathways. Intermediate-Ir catalysts (30–45 wt%) represent the most practical pathway toward scalable, low-Ir PEMWE electrodes.