Understanding the influence of iridium oxide catalyst state on the performance in oxygen evolution reaction

Proton-exchange membrane (PEM) water electrolysis is a critical technology for hydrogen production. The oxygen evolution reaction (OER) kinetics at the anode significantly determines the electrolysis performance, requiring the development of active and stable catalysts for high conversion rates. Despite extensive experimental studies, it is still difficult to fully understand how the catalyst state, i.e., the structure, morphology and oxidation state - which vary by synthesis conditions - affect the OER kinetics and free energies. In this study, we delve into the influence of catalyst calcination on catalyst state and its relationship with OER kinetics by a combination of experimental analysis and microkinetic modeling. Our results show that the increasing degree of crystallinity upon calcination, and thus, the reduced number of active sites is the main reason for the decreasing performance of Ir-oxide nanoparticles. Additionally, water adsorption step becomes thermodynamically more favorable, CUS-mediated PCET and O₂ release are modestly hindered, and the bridge-site redox contribution declines with increasing crystallinity. These subtle, systematic shifts help explain the non-linear structure–activity relationships reported in the literature. This understanding of the interplay between catalyst synthesis conditions and OER performance facilitates the tailored design and optimization of high-performance catalysts for more efficient electrocatalytic water electrolysis.