Outstanding performances of Ni2CoS4/expanded graphite with ultrafine Ni2CoS4 particles for supercapacitor applications
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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.
This work was supported by the Open Project of Fuel Cells & Hybrid Electric Power Key Lab, Chinese Academy of Sciences (KLFC201702), the Open Project from State Key Lab of Catalysis (N-14-1), and the Innovative Research Team of Southwest Petroleum University (2015CXTD04).
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