▎ 摘　　要

Heteroatom-doped graphene have encouraged intensive research as promising metal-free oxygen reduction reaction (ORR) electrocatalysts but the correlation between the precursor material and final ORR activity remains unclear. In this work a serial of nitrogen/sulfur (N/S)-doped graphene catalysts were synthesized by modifying graphene oxide (GO) with cysteine as a NS source and sequential thermal annealing. It is disclosed that the cysteine-GO reaction time shows a significant influence on the ORR activity of N/S-doped graphene. A unique process of oxidation-induced in situ disulfide formation is further found to be involved in the synthesis of optimal N/S-doped graphene, which displays ORR activity superior to commercial Pt/C in alkaline media. This work suggests that the heteroatom source itself and careful optimization of reaction conditions are critical to obtain high performance doped-graphene electrocatalyst. (C) 2017 Elsevier Inc. All rights reserved.