Solvothermal synthesis of NiCo2O4 nanocomposites on liquid-phase exfoliated graphene as an electrode material for electrochemical capacitors

Abstract

Graphene was synthesised by an efficient, facile and green method of liquid phase exfoliation of highly pyrolytic graphite in an apt ratio of water and ethanol and subsequently used to produce graphene-NiCo2O4 nanocomposites via solvothermal method using urea as a hydrolysis agent. X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and Raman spectroscopy revealed the formation of pure NiCo2O4 composites. Transmission electron microscope (TEM), field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscope (HR-TEM) demonstrated that NiCo2O4 was successfully anchored on the graphene sheets. The electrochemical analysis performed on the prepared graphene-NiCo2O4 electrodes showed that the addition of graphene to the NiCo2O4 nanomaterials is paramount to the charge storage performance. Different ratios of graphene to NiCo2O4 varied to form nanocomposites under the same reaction conditions were analysed to reveal the best synergistic effect of the pseudocapacitance of NiCo2O4 nanomaterial and the excellent conductivity of graphene. The capacitive performance of the graphene-NiCo2O4 composites electrode prepared at a mass ratio (1:8) exhibited superior specific capacitance of 698 F/g at a current density of 0.5 A/g, with an energy density of 96.9 Wh/kg at power density of 13.9 kW/kg. The electrode also showed good cycling performance, with about 74% retention of the capacitance after 5000 cycles.