Enhancing the stability and capacitance of vanadium oxide nanoribbons/3D-graphene binder-free electrode by using VOSO4 as redox-active electrolyte

Abstract

The composites consisting of vanadium oxide nanoribbons and three-dimensional graphene (VOx-3DG) are synthesized using the hydrothermal method and they are employed as binder-free cathode for supercapacitors. Although VOx-3DG exhibits the high capacitance of 325 mF cm−2 at 1 mA cm−2 in 0.5 M KCl electrolyte, its cycling stability is very poor. Therefore, an innovative and effective method is proposed to improve the stability and capacitance of VOx-3DG by adding VOSO4 into KCl electrolyte. The function of the VOSO4 additive is twofold: (i) it facilitates electrodeposition of vanadium oxide nanoneedles over the electrode surface to keep the nanoribbon structure intact during charge-discharge process, and (ii) it serves as a redox-active electrolyte to provide the additional capacitance contribution via its specific V4+/V5+ conversion. Due to the aforementioned synergistic effects, VOx-3DG electrode after 500 cycles shows the capacitance of 8072 mF cm−2 at 2 mA cm−2, being 7.2 times higher than the initial value. A series of studies are performed to optimize the overall electrode performance. For example, the loading amount of electrodeposited vanadium oxide and electrode capacitance are profoundly modulated by several parameters such as the concentration of VOSO4, the concentration and pH value of the buffer solution. Finally, the asymmetric supercapacitor with VOSO4/KCl shows the increase in energy density by 33% compared with the one with pure KCl electrolyte. Furthermore, the former delivers a maximum energy density of 16 Wh kg−1 at a power density of 200 W kg−1 and exhibits 88% capacitance retention after 10,000 cycles.