A free-standing graphene/MnO2 composite material has been prepared through a simple hydrothermal method, followed by freeze-dried in vacuum treatment. The structure and morphology of the composite were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of mass ratio of reactants and electrolyte on the electrochemical properties of the materials were investigated. The results show that the specific capacitance of the composite is the largest in KOH electrolyte. In addition, the specific capacitance of the composite material can reach 224 F/g (the current density is 1 A/g) when the mass ratio of reactants is 1: 4.
