Porous NiCo2O4 spheres tuned through carbon quantum dots utilised as advanced materials for an asymmetric supercapacitor

Abstract

Carbon quantum dots (CQDs) tuned porous NiCo<inf>2</inf>O<inf>4</inf> sphere composites are prepared for the first time via a reflux synthesis route followed by a post annealing treatment. Benefiting from the advantages of the unique porous structure with a large specific surface area, high mesoporosity and superior electronic conductivity, the as-obtained CQDs/NiCo<inf>2</inf>O<inf>4</inf> composite electrode exhibits high specific capacitance (856 F g^-1 at 1 A g^-1), excellent rate capability (83.9%, 72.5% and 60.8% capacity retention rate at 20, 50 and 100 A g^-1, respectively) and exceptional cycling stability (98.75% of the initial capacity retention over 10000 cycles at 5 A g^-1). Furthermore, the assembled AC//CQDs/NiCo<inf>2</inf>O<inf>4</inf> asymmetric supercapacitor manifests a high energy density (27.8 W h kg^-1) at a power density of 128 W kg^-1 or a high power density (10.24 kW kg^-1) at the reasonable energy density of 13.1 W h kg^-1 and remarkable cycling stability (101.9% of the initial capacity retention over 5000 cycles at 3 A g^-1). The results above suggest a great potential of the porous CQDs/NiCo<inf>2</inf>O<inf>4</inf> composites in the development of high-performance electrochemical energy storage devices for practical applications.