Chloride Residues in RuO₂ Catalysts Enhance Its Stability and Efficiency for Acidic Oxygen Evolution Reaction

Abstract . Ruthenium dioxide (RuO 2 ) is a benchmark electrocatalyst for proton exchange membrane water electrolyzers (PEMWE), but its stability during the oxygen evolution reaction (OER) is often compromised by lattice oxygen involvement and metal dissolution. Despite that the typical synthesis of RuO 2 produces chloride residues, the underlying function of chloride have not well investigated. In this study, we synthesized chlorine‐containing RuO 2 (RuO 2 ‐Cl) and pure RuO 2 catalysts with similar morphology and crystallinity. RuO 2 ‐Cl demonstrated superior stability, three times greater than that of pure RuO 2 , and a lower overpotential of 176 mV at 10 mA cm −2 . Furthermore, the RuO 2 ‐Cl catalysts that were in situ synthesized on a platinum‐coated titanium felt could maintain high performance for up to 1200 hours at 100 mA cm −2 . Computational and experimental analyses show that chloride stabilizes RuO 2 by substituting the bridging oxygen atoms, which subsequently inhibits lattice oxygen evolution and Ru demetallation. Notably, this substitution also lowers the energy barrier of the rate‐determining step by strengthening the binding of *OOH intermediates. These findings offer new insights into the previously unknown role of chloride residues and how to improve RuO 2 stability.