An efficient bifunctional electrocatalyst derived from layer-by-layer self-assembly of a three-dimensional porous Co-N-C@graphene

Three-dimensional(3D)porous carbon-based materials with tunable composition and microstructure are of great interest for the development of oxygen involved electrocatalytic reactions.Here,we report the synthesis of 3D porous carbon-based electrocatalyst by self-assembling Co-metal organic frameworks(MOF)building blocks on graphene via a layer-by-layer technique.Precise control of the structure and morphology is achieved by varying the MOF layer to tune the electrocatalytic properties.The as-produced electrocatalyst exhibits an excellent catalytic activity for the oxygen reduction reaction in 0.1 mol LKOH,showing a high onset potential of 0.963 V vs.reversible hydrogen electrode(RHE)and a low tafel slope of 54 mV dec,compared to Pt/C(0.934 V and 52 mV dec,respectively).Additionally,it shows a slightly lower potential vs.RHE(1.72 V)than RuO(1.75 V)at 10 mA cmin an alkaline electrolyte.A rechargeable Zn-air battery based on the as-produced 3D porous catalyst demonstrates a high peak power density of 119 mW cmat a cell voltage of 0.578 V while retaining an excellent stability over 250 charge-discharge cycles.