Ultrafine Iridium‐Oxide Solid Solutions for Efficient Acidic Water Oxidation

ABSTRACT The high cost and inadequate catalytic performance of IrO 2 are primary factors hindering the development of proton exchange membrane water electrolyzer (PEMWE). Herein, we propose dual criteria of cohesive energy ( ΔCE ) and atomic radius ( ΔR ) discrepancies between Ir and heteroatoms for guiding the synthesis of ultrafine IrO 2 ‐based solid solutions (M x Ir 1−x O 2 ). Elements of Mo and W with lowest ΔCE and ΔR exhibit the most facilitated alloying with Ir and minimized particle coarsening, enabling the fabrication of sub‐3 nm M x Ir 1−x O 2 (x = 33.3%−50%). The extensive Mo substitution in Mo 1/3 Ir 2/3 O 2 effectively modulates Ir─O covalency and generates abundant Brønsted acid sites as proton acceptors. These factors synergistically accelerate the deprotonation of *OH and *OOH and promote rapid proton transfer, thereby significantly reducing the energy barrier of oxygen evolution reaction (OER) and alleviating the proton accumulation‐induced site poisoning. Resultantly, Mo 1/3 Ir 2/3 O 2 delivers a low overpotential of 540 mV at 1 A cm −2 and an excellent stability of 700 h. PEMWE using Mo 1/3 Ir 2/3 O 2 achieves a high current density of 3 A cm −2 at 2.26 V and steadily operates at 1 A cm −2 for 625 h. This work transcends traditional hetero‐element doping paradigm for IrO 2 modification, offering a new perspective for developing high‐performance and low‐iridium OER catalysts.