Understanding the Instability Origin for Design of Robust Pure RuO2 Anode Catalyst in PEM Water Electrolysis

Abstract Ruthenium dioxide (RuO2) shows great promise as an acidic oxygen evolution reaction (OER) catalyst in proton exchange membrane water electrolysis (PEMWE), yet its insufficient stability─often attributed to Ru overoxidation─actually has a hidden cause. Herein, we show that the true culprit is instead residual chloride impurities inherited from the most commonly used RuCl3 precursor. Owing to strong Ru–Cl coordination, substantial chlorine residues are trapped in the RuO2 lattice during conventional synthesis; during electrolysis, they gradually leach out, dramatically accelerating Ru dissolution and lattice oxygen loss and triggering structural collapse. Guided by this insight, we developed a small-molecule-assisted synthesis that yields Cl-free RuO2. The resulting catalyst preserves exceptional structural integrity, sustains its initial activity through 80,000 accelerated degradation cycles, and delivers stable PEMWE operation at 500 mA cm–2 for 500 h, outperforming all reported single RuO2 catalysts and rivaling even the best doped or composite Ru-based catalysts. Our findings resolve a long-standing paradox in RuO2 catalysis and establish chloride elimination as a critical design principle for durable oxide electrocatalysts.