Graphene electrode functionalization for high performance hybrid energy storage with vanadyl sulfate redox electrolytes

Abstract

Hybrid energy storage devices face the challenges of pairing suitable redox chemistries with stable electrodes to simultaneously realize the energy storage mechanisms of batteries and supercapacitors. Functionalization of electrodes enables improved catalytic activity and long-term cycling stability by introducing desired functional groups. Herein, we demonstrate novel hybrids using reversible vanadium (V) redox couples along with functionalized graphene electrodes. Redox couples of V(III)/V(IV) and V(IV)/V(V) are formed upon initial charging of the V(IV) electrolyte present in both half-cells, leading to a Galvani potential difference during operation. Moreover, functionalized electrodes with high surface area facilitate both facile redox reactions and electric double layer formation. The unique chemistry offers a straightforward device regeneration step which significantly enhances the cycling stability. Optimized hybrids exhibit a battery-like specific capacity of 700 mAh g−1 at 0.5 A g−1, a supercapacitor-like capacitance of 850 F g−1 over 10,000 cycles at 10 A g−1, and a very low self-discharge rate. This corresponds to power densities of 0.3 kW kg−1 and 9.2 kW kg−1 and energy densities of 363.8 Wh kg−1 and 24.9 Wh kg−1, respectively, outperforming similar aqueous redox hybrid systems.