New insight of tailor-made graphene oxide for the formation of atomic Co-N sites toward hydrogen evolution reaction

The designed synthesis of graphene oxide (GO) for the formation of nitrogen-coordinated atomic metal sites (M-N-C) is crucial for developing efficient M-N-C catalysts. Here, the effect of basal plane oxygen functional groups of GO on the adsorption of cobalt ions and their Co-N transition is investigated by tailoring the oxygen content and composition of oxygen functional groups on the basal plane of GO sheets. The tailor-made GO shows that the specific oxygen functional groups, such as hydroxyl, epoxy, and carbonyl groups, can be efficiently incorporated with formation of defected structures on the basal plane of GO sheets rather than their edge. The adsorption of cobalt ions on the GO sheets and their transition to the Co-N state are systematically investigated by XPS and XAFS analyses. Annealing temperature-controlled experiments reveal that the incorporation of nitrogen around cobalt atoms is facilitated by the increased number of oxygen functional groups on the basal plane rather than those on the edge of GO sheets, which provide energetically favorable environments for Co-N formation. The hydrogen evolution reaction (HER) activities of the resulting cobalt atom-immobilized and annealed GO (Co/GOx-T) catalysts correlate to the degree of Co-N saturation, while also demonstrating excellent HER catalytic durability.