Theoretical Insights into the Oxygen Evolution Reaction Activity of Monoclinic, Orthorhombic, and Tetragonal Iridium Dioxide Polymorphs

IrO 2 is a benchmark oxygen evolution reaction (OER) electrocatalyst, and recent studies have revealed that its OER activity can be markedly improved by the coexistence of multiple crystallographic phases. Hence, elucidating the influence of crystal phase and surface structure on OER activity is crucial for the rational design of catalysts with improved performance and reduced overpotential. In this study, first-principles calculations were performed to accurately simulate the overpotential of iridium dioxide catalysts and to clarify the surface electronic-structure factors governing the overpotential. The activity of IrO 2 electrocatalysts depends on the surface index and space group. Using three density functional methods (PBE, RPBE, and optPBE-vdw), we compared the activity of stable IrO 2 surfaces across the P 4 2 / mnm (tetragonal), C 2/ m (monoclinic), and Pbcn (orthorhombic) phases. The catalytic activity followed the order C 2/ m > Pbcn > P 4 2 / mnm . We explain the differences in catalytic performance among the three space groups in terms of free energy changes induced by variations in the surface-layer atomic structure. Accordingly, projected Crystal Orbital Hamilton Population analysis was employed to investigate how O–Ir–O angle variations modulate the bonding interactions of HO*, O*, and HOO* groups. The O–Ir–O angle acts as a key geometric descriptor that directly reflects the adsorption strength of these groups on the IrO 2 surface. This structure–bonding interaction relationship offers valuable guidance for the rational design of highly active IrO 2 OER catalysts via crystal symmetry control.