Outstanding performances of Ni2CoS4/expanded graphite with ultrafine Ni2CoS4 particles for supercapacitor applications

Abstract

Nickel sulfides are desirable electrode materials for supercapacitors, while low electronic conductivity and poor cyclic stability restrict their wide applications. Herein, Ni2CoS4/expanded graphite (Ni2CoS4/EG) composite was prepared in mixed solvents of ethylene glycol and H2O via a rapid and energy-saving microwave heating method. Scanning transmission electron microscopy image shows that Ni2CoS4 particles are ultrafine with an average diameter of 2 nm and uniformly distributed on expanded graphite. The specific capacitance of the Ni2CoS4/EG composite can reach up to 2056.8 F g−1 at 5 A g−1 as compared to 1574.4 F g−1 of Ni3S4, 229.1 F g−1 of CoS and 1516.6 F g−1 of Ni2CoS4; and even at higher current density of 30 A g−1, the specific capacitance can still demonstrates 1923.3 F g−1, thus 92.5% of rate capability can be achieved as the current density increases from 5 to 30 A g−1. Moreover, it exhibits an excellent stability of 94.4% after cycling at current density of 30 A g−1 for 2000 cycles. The composite delivers high initial capacitance, excellent rate capability, and fantastic stability. Furthermore, the fabricated AC//Ni2CoS4/EG asymmetric supercapacitor also exhibits a high specific capacitance of 120.3 F g−1 at 0.5 A g−1, an superior cycle life (91% at 5 A g−1 for 5000 cycles), and an extremely high energy density of 52 Wh kg−1 at 477 W kg−1. This work offers a new insight to synthesize ultrafine bimetallic sulfides, and the superior high performances of the Ni2CoS4/EG composite can provide practical applications in supercapacitors.