Predictors of Membrane Electrode Assembly Activity for Iridium Oxide Catalysts in Proton Exchange Membrane Water Electrolysis

A primary barrier to the widespread deployment of proton exchange membrane water electrolyzers (PEMWEs) is the high usage of iridium for the oxygen evolution reaction (OER) catalyst. Reducing iridium loadings from current values of 1–3 mg Ir ·cm −2 to ≤0.10 mg Ir ·cm −2 is critical, and achieving this target requires substantial improvements in catalyst activity and durability. Despite ongoing progress, optimization remains constrained by a limited fundamental understanding of how specific catalyst properties influence performance in membrane electrode assembly (MEA) environments. Studies of catalyst activity have largely relied on purely empirical approaches, leaving foundational knowledge for rational catalyst design—such as understanding how crystallinity and oxidation state impact performance—not well established. This work bridges this gap in knowledge by systematically correlating structural and chemical properties of six commercial IrO x catalysts with their electrochemical performance in MEAs. Each catalyst was characterized for BET surface area, chemical composition, and local electronic configuration prior to incorporation into MEAs for testing. Among the ten descriptors examined, BET surface area showed the weakest correlation with OER activity (R 2 = 0.02), whereas Ir 3+ coverage and μ 1 coverage showed the strongest correlation (R 2 = 0.99). Density functional theory calculations reveal the molecular origins of these correlations, resolving μ 1 sites as the most electrophilic species and thus the most probable active centers for the OER. Overall, this integrated experimental–theoretical approach establishes robust descriptors for IrO x performance and lays the groundwork for the design of next-generation PEMWE catalysts.