Environment-benign synthesis of rGO/MnOx nanocomposites with superior electrochemical performance for supercapacitors

Chemical oxidation synthesis of graphene oxide (GO) through modified Hummers methods has been widely employed for producing graphene-or reduced GO (rGO)-based advanced functional materials such as rGO/MnOx nanocomposites. However, the manganese species in GO colloids are usually washed out during GO synthesis through modified Hummers methods, causing manganese waste and environmental risk. In this paper, we report preparation of rGO/MnOx nanocomposites, in which MnOx is composed of Mn2O3, Mn3O4, and MnO2 components, through anneal treatment of the precursor counterparts obtained by simple pH tuning of GO colloids. The rGO/MnOx nanocomposites exhibit superior electrochemical performance for supercapacitors. rGO/MnOx-5, derived from GO colloids with pH 5, exhibits a high gravimetric discharge capacitance (C-dis) of 191 F g(-1) at 20 A g(-1) and a high capacitance retention (82.9%) relative to C-dis at 1 A g(-1). Furthermore, typical symmetric supercapacitor cells made from rGO/MnOx-5 show a high areal capacitance (172 mF cm(-2)) and excellent capacitance retention (96.6%) at 2 A g(-1) (10 mA cm(-2)) for 20,000 cycles, holding great potential for practical applications. The superior electrochemical performance of rGO/MnOx nanocomposites is attributed to multiple charge storage mechanisms in association with the coexistence of mixed-valent manganese oxides. (C) 2017 Elsevier B.V. All rights reserved.