NiCo2S4 nanoparticles anchored on reduced graphene oxide sheets: In-situ synthesis and enhanced capacitive performance.

A facile hydrothermal process is developed for the synthesis of NiCo2S4/reduced graphene oxide (RGO) hybrid and NiCo2S4 hollow spheres. The morphology and microstructure are characterized by powder X-ray diffraction (XRD), Raman spectra, transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED), and energy dispersive spectrometry (EDS) mapping. NiCo2S4 nanoparticles with the diameter of about 20-30nm were in-situ grown on RGO sheets. NiCo2S4 hollow spheres were obtained with the diameter of about 300-400nm and the width of shell in the range of 30-40nm in the absence of graphene oxide (GO). GO as a substrate material can offer abundant active sites for nucleation of NiCo2S4 and can be reduced to RGO, providing excellent electron transfer path and high conduction, which enable the fast surface redox reaction. Supercapacitor based on NiCo2S4/RGO hybrid shows a high specific capacitance of 1804.7F/g at a current density of 0.5A/g. Due to the high capacitive performance of NiCo2S4/RGO hybrid, the NiCo2S4/RGO//AC asymmetric supercapacitor (ASC) possesses an extended voltage window of 1.5V, high energy density of 24.4Wh/kg at a power density of 750W/kg in 2mol/LKOH electrolyte. NiCo2S4/RGO hybrid can serve as a promising electrode material for high performance supercapacitors.