▎ 摘　　要

Developing highly active bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the oxygen electrocatalysis based energy conversion or storage remains challenges. Herein, an efficient catalyst of cobalt oxide (Co3O4) nanocrystals grown on nitrogen and sulfur co-doped graphene foam (NSGF) is prepared via a hydrothermal method. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) measurements demonstrate Co3O4 is of regular single crystal morphology in the Co3O4/NSGF composite with 1:1 mass ratio of cobalt to graphene foam. For ORR, the hybrid favors a direct 4e- pathway, similar to that of Pt/C, but outperforms Pt/C with a long-term durability. Besides, it achieves a small overpotential of 0.48 V vs. SCE at the current density of 10 mA cm(-2) and there is only 0.018 V vs. SCE overpotential increasing after 200 cycles for OER testing. The good electrochemical activities of our fabricated bifunctional catalyst are attributed to electrochemical contributions of the loaded Co3O4 nanocrystals, the integral structure of three-dimensional graphene foam, the heteroatoms doping effects and the synergistic effects between Co3O4 and NSGF. (C) 2016 Elsevier Ltd. All rights reserved.