Surface Coordination‐Dependent Oxygen Evolution Pathways in Rutile IrO 2

Rutile iridium dioxide (IrO 2 ) is renowned as an effective catalyst for the oxygen evolution reaction (OER), yet how its atomic structure governs orientation‐dependent activity remains unclear. Here, we investigate epitaxial rutile IrO 2 thin films with crystallographically well‐defined (110), (101), and (001) orientations and show that their OER activity follows the order: (110) > (101) > (001). Atomic‐resolution scanning transmission electron microscopy (STEM) analysis identifies distinct surface terminations and Ir‐O coordination motifs that differ from commonly assumed idealized surface models. Experimentally informed electronic‐structure calculations reveal that the (110) and (101) surfaces exhibit enhanced O 2 p states near the Fermi level, favoring oxygen‐involved charge transfer, whereas the (001) surface shows a more Ir‐centered electronic character. Reaction‐barrier calculations further indicate an energetic preference for the quadruple oxygen coupling mechanism (QOCM) on the (110) and (101) surfaces, whereas the (001) surface is more consistent with an Ir‐centered adsorbate evolution mechanism (AEM)‐like pathway. These results establish surface coordination as a key descriptor linking atomic termination, electronic structure, reaction pathway, and OER activity, providing a basis for designing high‐performance IrO 2 catalysts through surface‐structure engineering.