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, RuB, 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.