Redox-additive electrolyte–driven enhancement of the electrochemical energy storage performance of asymmetric Co3O4//carbon nano-onions supercapacitors

ABSTRACT For efficient energy storage, Co3O4@nickel foam exhibiting a plate-like (p-Co3O4) and grass-like (g-Co3O4) nanostructure were prepared as binder-free supercapacitor electrode materials. The electrochemical performance of the electrodes was tested using a redox-additive electrolyte (RAE). The homogeneously grown grass-like nanostructure (g-Co3O4) exhibited superior electrochemical performance to those with the plates-like structure (p-Co3O4) in a KOH electrolyte. In addition, the electrochemical performance of g-Co3O4 was improved using an RAE in a 3 M KOH solution. Remarkably, the specific capacitance of g-Co3O4 (1560 F g–1) was increased to 6580 F g–1, approximately 4-fold just by varying the RAE concentration in KOH. Carbon nano-onion (CNO) in the form of multi-layer fullerene was introduced as a negative electrode material. Owing to the more favorable morphology and properties of CNO such as exohedral structure, small diameter, high electrical conductivity, and relatively easy aqueous media dispersion than activated carbon and graphene, the as-fabricated asymmetric Co3O4//CNO supercapacitor delivered a high energy density of 42.5 Wh kg–1 and a high power density of 12.8 kW kg–1.