A green strategy for the synthesis of graphene supported Mn3O4 nanocomposites from graphitized coal and their supercapacitor application

A by-product free strategy based on modified Hummers method was proposed to synthesize graphene/Mn3O4 composites without any additional manganese source. Coal-derived graphite (CDG) was used as carbon source instead of conventional natural graphite flakes and MnSO4 produced from the modified Hummers was in situ transformed into Mn3O4 by precipitation in air. After reduction with hydrazine, the reduced coal-derived graphene oxide/Mn3O4 (RCDGO/Mn3O4) was obtained and employed as the electrode material for the supercapacitors. In addition, K2SO4 produced from the modified Hummers was used as electrolyte, as a result, residual-free was achieved during the whole process, and the atom utilization was calculated as high as about 97%. A maximum specific capacitance of 260 F g−1 was achieved for RCDGO/Mn3O4 composite with 86% Mn3O4 in saturated K2SO4 electrolyte solution based on the synergetic effects between coal-derived graphene and attached Mn3O4 nanoparticles. Its specific energy density reached 8.7 Wh kg−1 at a current density of 50 mA g−1 when used as a symmetrical supercapacitor. The good capacitance retention (92–94%) was also observed after 1000 continuous cycles of galvanostatic charge–discharge.