Identifying Active Sites of IrO_x Catalysts for OER: A Combined Operando XAS, SEIRAS, and Theoretical Study

Iridium oxide is a benchmark catalyst for anodic oxygen evolution reactions due to its high activity and durability. However, debates persist regarding the short lifespan of reaction intermediates and whether amorphous phases exhibit higher activity in comparison to crystalline ones. Herein, we examined IrO_x catalysts with different degrees of crystallinity (SA3.5, SA58, SA103, and SA14.6, named after their BET surface area) and revealed the relation between their structures and OER activities. The atomic pair distribution function analyses show that the amorphous IrO_x (SA3.5, SA103, and SA14.6) possesses monoclinic and orthorhombic-like structural phases or motifs, which differ from the typical tetragonal symmetry of crystalline IrO₂. Operando X-ray absorption spectroscopy (XAS) and operando surface-enhanced infrared absorption spectroscopy (SEIRAS) revealed that the SA3.5 sample, with higher monoclinic content, exhibits a structure containing corner- and edge-sharing octahedra with highly undercoordinated sites, leading to structural defects. These defects generate active electrophilic O^I- species, facilitating the generation of *OOH intermediates and enhancing OER activity. Theoretical studies reveal that they also lower the energy barrier for the monoclinic crystal structure. These findings offer a fundamental understanding of amorphous iridium oxides and provide inspiration for developing new OER catalysts.