▎ 摘　　要

Nitrogen (N) doping in graphene-based materials has been demonstrated as an effective strategy in constructing active sites of metal-free catalysts for oxygen reduction reaction (ORR). The practical applications of metal-free electrocatalysts in metal-air batteries or fuel cells, nevertheless, have been hampered by their unsatisfactory catalytic performance due to insufficient catalytic active sites. In this work, a novel N-rich graphene nanospheres (NGNs) have been achieved by adopting an edge engineering strategy through annealing the mixture of graphitic carbon nitride (g-C3N4) and edge-rich graphene nanospheres (GNs) composed of graphene nanoflakes. Benefiting from the exposure of edge defects in the GNs, the N loading surprisingly achieved as high as 14.01 at%. The half-wave potential and limiting current density of the synthesized NGNs-900 catalyst can reach 0.872 V and 4.25 mA cm(-2), respectively, which are superior to that of the commercial Pt/C. Based on the experimental and theoretical results, the synergistic effect of graphitic-N and pyridinic-N in NGNs catalysts has been distinguished as the origin of the boosted ORR performance. This work proposes a facile synthesis strategy to optimize the N-doped carbon-based catalysts for ORR, which have great potential to replace noble-metal catalysts.