Crystal CoxB (x = 1–3) Synthesized by a Ball-Milling Method as High-Performance Electrocatalysts for the Oxygen Evolution Reaction

Development of noble-metal-free and active electrocatalysts is crucial for the oxygen evolution reaction (OER) in the water-splitting process. Herein, crystal CoxB catalysts (x = 1–3) of the OER are fabricated by a ball-milling method. Among these CoxB catalysts, Co2B exhibits the best OER activity, with a current density of 10 mA cm–2 at an overpotential of 287 mV in 1 M KOH solution. Such OER activity of Co2B is favorably comparable to that of the commercial IrO2 and most recently reported OER catalysts. Furthermore, the Co2B catalyst exhibits excellent stability with a stable current density of 50 mA cm–2 over 12 h of continuous electrolysis operation. X-ray photoelectron spectroscopy and cyclic voltammetry demonstrate that the B in CoxB makes oxidation easier, leading to their enhanced OER activities in comparison to metal Co. In addition, the Co2B electrocatalyst also exhibits high activity in the hydrogen evolution reaction; thus, the catalyst can be used as a bifunctional catalyst for full water splitting.