Hollow nanotube arrays of nickle–cobalt metal sulfide for high energy density supercapacitors

High energy density is still difficult to achieve using existing metal sulfides because of their low specific capacitance. To improve capacitance, a series of nickel and cobalt metal sulfides with different Ni/Co ratios were synthesized by a two-step hydrothermal method. Using the combining method of experimental research and first-principles calculation, the morphology, structural stability, electronic structure and electrochemical properties of metal sulfides were investigated systematically. The results show that the morphology of metal sulfides gradually grows from two-dimensional structure to nanotube arrays, and finally to nanorod arrays, as the Ni/Co ratios decrease. Among them, the NC24 sample with the Ni/Co ratio of 1 : 2 is a hollow nanotube array composed of NiCo(2)S(4), which shows excellent electrochemical performance. The specific capacity of the NC24 sample reaches 1527C g(−1) at 1 A g(−1), and the capacity retention is 93.81% at 10 A g(−1) after 2000 cycles. Furthermore, a symmetrical supercapacitor assembled from the NiCo(2)S(4) nanotube array shows a high energy density of 67.5 W h kg(−1). This strategy develops a nanotube array of metal sulfides and expands its application in a high energy density supercapacitor.