▎ 摘　　要

It is highly desirable but remains challenging to develop efficient bifunctional electrocatalysts for both the oxygen reduction and oxygen evolution reactions (ORR/OER) in rechargeable metal-air batteries and unitized regenerative fuel cells. Herein, we developed a facile and cost-effective strategy to prepare a cobalt and nitrogen codoped three-dimensional (3D) graphene catalyst through inserting carbon nanospheres into the interlayers of graphene sheets. The catalyst exhibited not only excellent ORR performance but also excellent OER performance, and both were greatly enhanced by the insertion of carbon nanospheres. Its activities for the ORR/OER ranked among the best for doped carbon catalysts thus far reported. Its overall oxygen electrode activity parameter (Delta E) was as low as 0.807 V, which was much lower than that of Pt/C, and made it comparable with the best nonprecious metal based catalysts to date. Furthermore, the catalyst exhibited excellent stability toward ORR and OER, making it a new noble-metal-free bifunctional catalyst for future applications in the fields of alternative energy conversion and storage systems.