Cobalt/nitrogen co-doped carbon nanosheets from coal-based graphene quantum dots boosted oxygen reduction

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We report on cobalt/nitrogen co-doped carbon nanosheets (Co/NCNs) as a highly efficient electrocatalyst for the oxygen reduction reaction (ORR). The Co/NCNs were synthesized through pyrolysis at 900 °C using coal-based graphene quantum dots that contain nitrogen as a source of both carbon and nitrogen, NaCl as a template, and trace amounts of cobalt salt as a metal source. The Co/NCNs-0.1 exhibited ORR activity that was equivalent to that of Pt/C, with a half-wave potential of 0.85 V and a limiting current density of 4.9 mA cm−2. Furthermore, the zinc-air battery incorporating Co/NCNs-0.1 demonstrated an open circuit voltage of 1.423 V, a maximum power density of 149 mW cm−2, and a specific capacity of 806.5 mAh g−1 at a current density of 20 mA cm−2. The results of density functional theory calculations indicate that metal Co, when protected by graphitic-N-doped carbon, holds a greater negative charge and exhibits a lower energy barrier compared to other types of nitrogen species. This property contributes to the catalytic activity of the ORR.