Cation‐Vacancy Induced Compressive Strain Localization in RuO ₂ Catalyst for High‐Performance Acidic Oxygen Evolution

Designing acid-stable RuO₂ catalysts capable of overcoming the activity-stability trade-off remains pivotal for advancing proton exchange membrane water electrolyzers (PEMWEs). Here, we introduce a cation-vacancy engineering strategy to generate localized compressive strain in RuO₂ by electrochemically leaching Cd from a pre-doped lattice. This strain modulation simultaneously elevates the Ru valence state (+4.35) and strengthens Ru─O covalent bonds, optimizing *OH/*O/*OOH adsorption energetics while suppressing over-oxidation. The resulting V_Cd-RuO₂ catalyst achieves an overpotential of 203 mV at 10 mA cm^-2 in 0.1 M HClO₄. Integrated into a PEMWE, it sustains >600 h operation at 200 mA cm^-2 with a voltage degradation rate of 0.1 mV h^-1. The MEA based on V_Cd-RuO₂ required cell voltages outperformed commercial RuO₂ by 120-180 mV at industrially relevant current densities (0.5-1.5 A cm^-2), thereby demonstrating significant energy efficiency. Multiscale analyses confirm that compressive strain stabilizes high-valence Ru sites through enhanced orbital overlap, reconciling catalytic activity with structural durability. This work establishes vacancy-driven strain engineering as a universal approach for designing robust, Ir-free OER electrocatalysts.