Tuning metal-oxygen covalency by lanthanides governs stability and reaction pathways in acidic oxygen evolution

The durability of RuO₂ anodes in acidic oxygen evolution reaction (OER) is governed by the coupling between lattice geometry and valence electronic structure of the RuO bond. Here, we develop a lanthanide-driven design strategy in which substitutional lanthanide ions (Ln³⁺) regulate RuO₆ octahedral distortion through ionic radius, thereby tuning O 2p-Ru 4d hybridization and covalency-related descriptors. Density functional theory (DFT) calculations reveal that controlled RuO bond elongation weakens orbital overlap and increases the formation energies of oxygen and ruthenium vacancies, suppressing lattice‑oxygen participation (often associated with the lattice oxygen mechanism, LOM) and favoring the adsorbate evolution mechanism (AEM). Among the series, gadolinium substitution achieves an optimal balance, with an average RuO bond length of 1.99 Å resolved by extended X-ray absorption fine structure (EXAFS). In 0.1 M perchloric acid, the optimized catalyst delivers an overpotential of 216 mV at 10 mA cm^-2 with a Tafel slope of 56 mV dec^-1 and shows only ∼50 mV degradation over 800 h. In a proton exchange membrane water electrolyzer (PEMWE), it sustains 100 mA cm^-2 for 120 h with negligible voltage decay. This work establishes lanthanide-regulated covalency modulation as a predictive, bond-level strategy for stabilizing ruthenium-based catalysts in acidic OER.