Ionothermal Route to Phase-Pure RuB₂ Catalysts for Efficient Oxygen Evolution and Water Splitting in Acidic Media

Developing high-efficiency and stable oxygen evolution reaction (OER) materials remains a great challenge, especially in an acidic medium. In this study, we report the first synthesis of phase-pure RuB2 by using a salt melt of potassium chloride and lithium chloride as the solvent. When used as an electrocatalyst in 0.5 M sulfuric acid, the as-prepared RuB2 needs an overpotential of only 223 mV for the OER to produce an operationally relevant current density of 10 mA cm–2. This is almost the smallest overpotential value for Ir/Ru-based acidic OER catalysts reported to date. More importantly, in the acidic full water splitting, RuB2 exhibits outstanding activity and stability, which merely needs a cell voltage of 1.525 V to reach a current density of 10 mA cm–2, lower than those of the most bifunctional electrocatalysts reported in acid electrolytes. Theoretical calculation results further identify the formation of *OOH as the rate-determining step and the lower energy barrier for the RuB2 catalyst, improving the OER activity. This work represents a significant addition to exploring a new class of transition metal borides with outstanding activity in acidic water electrolysis and beyond.