Electrochemical water splitting by a robust electrocatalyst plays a key role in hydrogen production for clean and sustainable energy. In this work, a kind of nanohybrid of graphene supported atomic Co and nanocrystalline Co3O4 was synthesized through electrochemical synthesis of atomic cobalt onto graphene using 1-Butyl-3-methylimidzolium tetrachlorocobalt [(BMIm)(2)CoCl4] as an electrolyte. By virtue of the synergistic effect of single-cobalt atoms and Co3O4 nanoparticles, the nanohybrid presents enhanced electrocatalytic performance toward the oxygen evolution reaction. The low onset potential is 0.49 V (vs. Ag/AgCl) and the overpotential of the current density of 10 mA cm(-2) is 0.30 V in aqueous solution, which is comparable to commercial RuO2. Moreover, the nanohybrid exhibits remarkably long stability for over 45 hat a fixed potential of 0.6 V. (C) 2018 Elsevier Ltd. All rights reserved.