Fluorine Mediates the Deprotonation Pathway over Ruthenium Oxide for Stable Water Electrolysis

Ruthenium oxide (RuO₂) is a promising alternative anode catalyst to iridium oxide in proton exchange membrane water electrolysis (PEMWE). However, the unsatisfactory stability of RuO₂ and the sluggish proton-coupled electron transfer kinetics of the oxygen evolution reaction (OER) hinder practical application. We report here a fluorine-tuned RuO₂ with terminal fluorine (F_ter) and bridging fluorine (F_bri) sites that successfully tackles the activity-stability paradox faced by Ru-based catalysts. Comprehensive experimental and theoretical studies reveal an unconventional fluorine-assisted deprotonation mechanism by which the proton transfer is decoupled from the electron transfer during the OER. The F_ter acts as a proton relay that accelerates deprotonation of intermediates, while F_bri contributes to suppressing the lattice oxygen oxidation route. The catalyst exhibited a low overpotential of 191 millivolts at 10 mA per square centimeter and maintained this current density over 2000 h. A practical PEMWE based on this catalyst delivered a current density of 1000 mA per square centimeter at a mere 1.72 V and operated stably over 300 h with a voltage degradation rate of 84 microvolts per hour.