Electronic tuning of RuO₂ polarizes metal-oxygen redox for proton exchange membrane water electrolysis.

The metal-oxygen redox behavior governs the performance of transition metal oxides in many electrochemical reactions, especially for RuO₂ with the activity-stability paradox in the anode oxygen evolution reaction of proton exchange membrane water electrolyzers. Herein, we modulate the electronic structure of RuO₂ near the Fermi level to promote reversible Ru redox while suppressing the oxidative release of lattice oxygen. As a result, the RuO₂ integrated with electron-rich p-block metals Sb achieves an overpotential of 220 mV and long-term operational stability of 1200 h at 10 mA cm^-2. The assembled proton exchange membrane water electrolyzers can operate steadily over 100 h at 100, 500, and 1000 mA cm^-2. Further advanced in-situ characterizations reveal the more reversible and milder Ru redox and the passivated lattice oxygen reactivity, which suppresses drastic structural changes of RuO₂ during the oxygen evolution reaction. This work highlights the importance of engineering metal-oxygen redox behavior and provides insights for designing high-performance catalysts for energy conversion and storage devices.