Ni–Co sulfide nanowires on nickel foam with ultrahigh capacitance for asymmetric supercapacitors

Abstract

In this paper we synthesize well aligned Ni–Co sulfide nanowire arrays (NWAs) with a Ni–Co molar ratio of 1 : 1 on 3D nickel foam by a facile two-step hydrothermal method. Owing to the low electronegativity of sulfur, Ni–Co sulfide NWAs exhibit a more flexible structure and much higher conductivity compared with Ni–Co oxide NWAs when used as active materials in supercapacitors. The electrochemistry tests show that these self-supported electrodes are able to deliver ultrahigh specific capacitance (2415 F g⁻¹ and 1176 F g⁻¹ at a current density of 2.5 mA cm⁻² and 30 mA cm⁻², respectively), together with a considerable areal capacitance (6.0 F cm⁻² and 2.94 F cm⁻² at a current density of 2.5 mA cm⁻² and 30 mA cm⁻², respectively), and good rate capability. More importantly, the asymmetric supercapacitor, composed of Ni–Co sulfide NWAs as the positive electrode and activated carbon as the negative electrode, reaches up to an energy density of 25 W h kg⁻¹ and a power density of 3.57 kW kg⁻¹ under a cell voltage of 1.8 V. Furthermore, the two assembled supercapacitors in series can power a 3 mm diameter red (2.0 V, 20 mA) round light-emitting diode (LED) indicator for more than 30 minutes after charging separately for a total time of 6 min. The superior electrochemistry capacity demonstrates that the self-standing Ni–Co sulfide nanowire arrays are promising for high-performance supercapacitor applications.