Graphene/VO₂ hybrid material for high performance electrochemical capacitor

Graphical abstract: Graphene/VO₂ hybrid materials are prepared by one-step simultaneous hydrothermal reduction technology. The prepared graphene (1.0)/VO₂ hybrid material shows a specific capacitances of 225 F g⁻¹ in 0.5 mol L⁻¹ K₂SO₄ solution. Furthermore, an asymmetric electrochemical capacitor with graphene (1.0)/VO₂ as a positive electrode and graphene as a negative electrode is assembled, and it can work in a cell voltage of 1.7 V and show excellent capacitive property. - Highlights: • Graphene/VO₂ hybrid material has been prepared by one-step hydrothermal reduction. • Graphene/VO₂ hybrid material exhibits high specific capacitance. • An asymmetric capacitor working at 1.7 V in aqueous solution is assembled based on graphene/VO₂ electrode. • The asymmetric capacitor exhibits high energy density. - Abstract: Vanadium oxides have attracted significant attention for electrochemical capacitor because of their extensive multifunctional properties. In the present work, graphene/VO₂ (RG/VO₂) hybrid materials with different RG amounts are prepared in a mixture of ammonium vanadate, formic acid and graphite oxide (GO) nanosheets by one-step simultaneous hydrothermal reduction technology. The hydrothermal treatment makes the reduction of GO into RG and the formation of VO₂ particles with starfruit morphology. The starfruit-like VO₂ particles are uniformly embedded in the hole constructed by RG nanosheets, which makes the electrode–electrolyte contact better. A high specific capacitance of 225 F g⁻¹ has been achieved for RG(1.0)/VO₂ electrode with RG content of 26 wt% in 0.5 mol L⁻¹ K₂SO₄ electrolyte. An asymmetrical electrochemical capacitor is assembled by using RG(1.0)/VO₂ as positive electrode and RG as negative electrode, and it can be reversibly charged–discharged at a cell voltage of 1.7 V in 0.5 mol L⁻¹ K₂SO₄ electrolyte. The asymmetrical capacitor can deliver an energy density of 22.8 Wh kg⁻¹ at a power density of 425 W kg⁻¹, much higher than those of the symmetrical electrochemical capacitor based on the RG and RG(1.0)/VO₂ electrodes. Moreover, the asymmetrical capacitor preserves 81% of its initial capacitance over 1000 cycles at a current density of 5 A g⁻¹