Suppress Ru Over‐Oxidation Through Oxygen Vacancy Engineering – Showcase With Ni‐Modified RuO ₂ via Molten‐Salt Synthesis

ABSTRACT Activity and durability are critical parameters for competent anode electrocatalysts in proton exchange membrane (PEM) electrolyzers. To move beyond Ir‐based catalysts, we present an Ir‐free Ni‐incorporated RuO ₂ prepared via solid‐state, molten‐salt synthesis (MSS) as an effective and scalable electrocatalyst for acidic oxygen evolution reaction (OER). This work demonstrates that moderate Ni incorporation optimizes the Ru oxidation state and favors the adsorbate‐evolving mechanism (AEM), which improves activity. In contrast, higher Ni content promotes lattice oxygen oxidation mechanism (LOM) pathways, which lead to instability. XRD and DFT analyses reveal that Ni incorporation induces lattice contraction via Ru─O bond compression, consistent with Ni ²⁺ substitutional doping at Ru ⁴⁺ sites. A systematic increase in Ru 3p binding energy with increasing Ni loading, suggesting the presence of more active Ru sites. Meanwhile, Ni 2p shifts to lower binding energy, confirming an electronic interaction between Ru and Ni. High‐resolution microscopy reveals a uniform distribution of Ru and Ni, indicating the formation of a solid solution. Electrochemical studies demonstrate that Ru _0.75 Ni _0.25 O _x retains 56% of its initial activity after 6000 cycles and exhibits excellent stability at 10 mA cm ⁻² , with only a 4% potential increase after 50 h of continuous OER.