Regulating the oxidation degree of nickel foam: a smart strategy to controllably synthesize active Ni3S2 nanorod/nanowire arrays for high-performance supercapacitors

A novel strategy is described in this study to construct three-dimensional (3D) nickel foam (NF) supported active Ni3S2 arrays with varieties of morphologies using a controllable pre-oxidation method followed by a post-hydrothermal treatment. A systematic study of the pre-oxidation effect is presented, and the possible reaction mechanism is discussed. Specifically, through accurately controlling the oxidation degree of the NF precursor, grass-like Ni3S2 nanorod/nanowire arrays grown in situ on the NF network are synthesized. Benefiting from the unique architecture to provide a large interfacial area, effective pathway for charge transport and high electrochemical activity from Ni3S2, the binder-free electrode exhibits remarkable electrochemical performances with a specific areal capacitance of 4.52 F cm(-2) at 1.25 mA cm(-2), as well as superior cycling stability (108.3% capacitance retention after 2000 cycles). An aqueous asymmetric supercapacitor device composed of such an electrode as the positive electrode and nitrogen-doped porous graphitic carbon (NPGC) as the negative electrode achieves a high energy density of 48.5 W h kg(-1) and power density of 4.8 kW kg(-1). Further, two such 4 cm(2) devices connected in series can successfully illuminate three color-changing LEDs for a long time, demonstrating the great potential for high-performance supercapacitors. The smart strategy described herein could be widely adopted to create other composites with desirable nanostructures.