Enhanced electrochemical oxygen reduction reaction by restacking of N-doped single graphene layers

Graphene, which is a two-dimensional layer structure of sp(2)-hybridized carbon, has been highlighted recently as a promising material for energy conversion. Herein, graphene-derived catalysts are developed for application in oxygen reduction reactions (ORRs) in acidic media via heat-treatment with dicyandiamide and a small amount (<1 wt.%) of transition metal. In ORRs, bare graphene exhibits 0.58 V (vs. RHE) of the onset potential; however, it increases to similar to 0.9 V through modification steps and records a mass activity of 1.28 mA mg(-1) at 0.75 V. Through the correlation curve between the ORR activities and the number of restacked graphene layers, it is proposed that the stacking of a few layers is desirable in the ORRs rather than a single layer catalyst. The graphene-derived catalysts exhibit graphite properties of facile electron transfer as the restacking of graphene layers increases, without degradation of the pyridinic-N on the graphene edge.