Solvothermal synthesis of molybdenum oxide on liquid-phase exfoliated graphene composite electrodes for aqueous supercapacitor application

MoO₃–graphene nanocomposites have been prepared through a green and facile synthesis method and serve as the main electrode materials for electrochemical capacitor in mild aqueous electrolyte. First, graphene sheets were synthesized via mild sonication treatment of graphite flakes in an optimum ratio of ethanol and de-ionized water, followed by deposition of MoO₃ onto graphene sheets via low-temperature solvothermal treatment using ethylene glycol- de-ionized water as mixed solvent. The composites were characterised using X-ray diffraction, Raman spectroscopy and electron diffraction and the results revealed that α-MoO₃ particles were successfully synthesized and anchored homogeneously onto graphene sheets. The electrochemical capacitance properties of MoO₃–graphene nanocomposites were measured by cyclic voltammetry, galvanostatic charge discharge method and electrochemical impedance spectroscopy. The results showed that the specific capacitance of MoO₃–graphene nanocomposites is 148 F/g, which is much higher than that of pure MoO₃ electrodes (84 F/g) in neutral Na₂SO₃ electrolyte. The specific capacitance is superior to those reported in the literatures using neutral aqueous electrolytes. Moreover, over 81% of the original capacitance was retained after 1600 cycles, indicating a good cycle stability of composite materials. The high specific capacitance and excellent cyclic stability of the electrode is believed originated from the synergistic effect of the highly conductive graphene material and the pseudocapacitive behavior of the MoO₃ nanoparticles in neutral electrolyte.