▎ 摘　　要

The rational design of nonprecious-metal electrocatalysts with activities comparable to or greater than that of platinum is extremely valuable to the development of high energy density metal-air batteries. Herein, the two-step preparation of a highly active oxygen electrocatalyst based on ultrasmall cobalt nanoparticles stabilized in a nitrogen-doped graphene matrix is reported. The catalyst performs as well as the commercial Pt/C catalyst in the oxygen reduction reaction, and better than the Pt/C catalyst in the oxygen evolution reaction. This particular electrocatalyst could significantly lower the overpotentials of oxygen electrochemical reactions in aqueous lithium-air batteries to attain a round-trip efficiency of about 79.0% at a current density of 0.1 mA cm(-2), thereby surpassing the performance of the commercial Pt/C catalyst. The good performance may be attributed to strong metal-support interactions, maximized by a high dispersion of ultrasmall cobalt nanocrystals in a nitrogen-doped graphene matrix, which yields electrocatalytic properties greater than the sum of its parts.