Oxygen-Deficient Ruthenium Oxide for Selective Oxygen Evolution in Additive-Free Brine Electrolysis

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Here, low-crystalline ruthenium oxide (S-RuO x ) with abundant oxygen vacancies was synthesized, after which its activity and selectivity toward oxygen evolution reaction (OER) in additive-free brine solution were compared with those of commercial ruthenium(IV) dioxide (C-RuO2), a benchmark catalyst for OER in an alkaline electrolyte. S-RuO x delivered a current density of 10 mA cm−2 at a significantly low overpotential (465 mV) in a 0.5 M NaCl solution without requiring an alkali. The estimated Faradaic efficiency toward chloride oxidation reaction (COR), FE(COR), was 2%, and exceptional OER was achieved without generating chlorine oxide species. This sharply contrasts the fact that C-RuO2 required an overpotential of 525 mV to generate 10 mA cm−2, where the FE(COR) was 59%. The activity and selectivity toward OER decreased after reducing the oxygen vacancies by sintering S-RuO x at different temperatures. S-RuO x continued to generate 10 mA cm−2 in 0.5 M NaCl solution for ≥60 h while maintaining the increasing potential at <30 mV. However, FE(COR) increased from a few percent for 20 h to 34% probably because of an irreversible decrease in vacancies. Notably, the addition of an alkali or a buffer could only enhance OER.