▎ 摘　　要

A great deal of research effort has been dedicated for designing an efficient bifunctional metal-free carbon catalyst for electrochemical water splitting, however, it remains challenging to introduce multiactive sites into a single catalyst. Herein, a series of bifunctional catalysts is designed that exploit intramolecular reorientation of nitrogen atom within the hollow nanoball from polyacrylonitrile on graphene through thermal treatment to achieve covalent doping. The as synthesized hollow carbon nanoballs on graphene (HCNB@G) catalysts have huge defective edges and microporous channels and contain a high density of electroactive graphitic-N and pyridinic-N as dual-active sites. As a result, the optimized HCNB@G-700 enhances electrocatalytic activities for both oxygen evolution and hydrogen evolution reactions (OER and HER) in 1.0 m KOH by generating 217 and 108 mV overpotential to gain a current density of 10 mA cm(-2), respectively. This remarkable electrocatalytic activity mainly originates from the appropriate combination of suitable nitrogen active sites and edge defects with microchannels that not only facilitate the penetration of electrolyte but also improve the stability of the HCNB@G. In addition, the superior conductivity and high surface area of HCNB@G boost electrocatalytic activity by supporting efficient charge transport and enabling rapid mass diffusion with gas release, respectively.