Direct and environmentally benign synthesis of manganese oxide/graphene composites from graphite for electrochemical capacitors

• The Mn3O4/graphene composites were directly synthesized from graphite.
• This method does not require the use of strong acids and toxic reducing agents.
• The Mn3O4/graphene composite had a specific capacitance of 367 F/g.
• The cycling and electrochemical stabilities of the Mn3O4/graphene were good.

We develop a direct and environmentally benign method to prepare manganese oxide (Mn3O4)/graphene composites via one-step hydrothermal synthesis from graphite without using strong acids and toxic reducing agents. Structural and morphological analyses reveals that the irregularly shaped Mn3O4 nanoparticles are well-dispersed on the graphene flakes. Cyclic voltammetry and galvanostatic charge–discharge tests indicate that the charge-storing mechanism of the Mn3O4/graphene composites is pseudocapacitive. The Mn3O4/graphene composite exhibits a specific capacitance of 367 F/g at a current density of 5 A/g. After 3000 charge–discharge cycles, the Mn3O4/graphene electrode retains 91.8% of its initial specific capacitance. From electrochemical impedance spectra, it is evident that the changes in both the equivalent series resistance and charge-transfer resistance of the Mn3O4/graphene electrode before and after 3000 charge–discharge cycles are small, indicating good cycling and electrochemical stability of the Mn3O4/graphene electrode.