Graphene oxide-drove transformation of NiS/Ni3S4 microbars towards Ni3S4 polyhedrons for supercapacitor

Ni3S4 is regarded as one of the promising electrode materials for energy storage, but the difficulty in obtaining its pure phase hinders its wide applications. In this work, we introduced a novel method to in-situ synthesize Ni3S4@reduced graphene oxide (rGO) composite, where graphene oxide (GO) was found to induce the oxidation of Ni2+ to Ni3+ and the morphology transformation from microbar to polyhedron during the hydrothermal process. The influence of the content and oxidation degree of GO on the phase composition and morphology of nickel sulfide is investigated. It is found that the oxygen-containing functional group of GO is responsible for the change of valence state, which thus drives the transformation of NiS/Ni3S4 towards Ni3S4. The obtained Ni3S4@rGO composite shows a high energy storage capacity (1830 F g-1 at 2 A g-1), remarkably higher than the unpurified phase NiS/Ni3S4 (830 F g-1). Correspondingly, the assembled asymmetry supercapacitor indicates a high energy density of 37.3 Wh kg-1 at a power density of 398 W kg-1. More importantly, the capacitance retention reaches 91.4 % after 10,000 cycles at a current density of 2 A g-1. Thus, this research overcomes the difficulty of synthesizing the pure Ni3S4 phase, which provides a new available pathway for constructing high-performance electrode materials.