Ultrafine Iridium-Oxide Solid Solutions for Efficient Acidic Water Oxidation

The high cost and inadequate catalytic performance of IrO₂ 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 IrO₂-based solid solutions (M_xIr_1-xO₂). 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_xIr_1-xO₂ (x = 33.3%-50%). The extensive Mo substitution in Mo_1/3Ir_2/3O₂ 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_1/3Ir_2/3O₂ delivers a low overpotential of 540 mV at 1 A cm^-2 and an excellent stability of 700 h. PEMWE using Mo_1/3Ir_2/3O₂ 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 IrO₂ modification, offering a new perspective for developing high-performance and low-iridium OER catalysts.