A Novel Flexible Hybrid Battery-Supercapacitor Based on a Self-Assembled Vanadium-Graphene Hydrogel

A novel flexible hybrid battery-supercapacitor device is proposed consisting of high specific surface area electrodes paired with an electrolyte, which contains a redox species that can exist in more than two oxidation states. The two initially equal half-cells of the device consist of a reduced graphene oxide hydrogel which encapsulates vanadium ions, synthesized with a single-step method. During charge, the oxidation state of the vanadium ions changes, resulting in two half-cells with different potentials which considerably increases the energy density. The achieved maximum capacity of more than 225 mAh g(-1) is roughly eight times higher than that of comparable graphene hydrogel supercapacitors without vanadium content, but the potentiostatic charging time is only double. Operated as a supercapacitor, it retains 95% of the initial capacitance over 1000 cycles. As battery, the losses are more significant, retaining around 50% of the initial capacity. However, these losses during battery operation can be almost entirely restored by electric measures. The vanadium ion addition also improves the self-discharge characteristics of the device. Moreover, the self-discharge does not permanently damage the hybrid device since both half-cells initially consist of the same vanadium graphene hydrogel and discharging resets it to initial conditions.