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.