Abstract
Nickel-cobalt sulfides (NixCoyS4) are promising supercapacitor materials for their high capacitance, while the sluggish kinetics in terms of charge transfer limits their power density. To achieve both high energy and power density for the NixCoyS4 - based supercapacitor, sulfur-doped reduced graphene oxide (rGO) is incorporated into NixCoyS4 by an in situ growth – ion exchange strategy to synthesize NixCoyS4/rGO composites. Profited from the synergetic effect between rGO and NixCoyS4, the Ni1.64Co2.40S4/rGO electrode delivers high specific capacitance of 1089 F g-1 at 1 A g-1, and remains 92.6% of its capacitance at 20 A g-1 (1008 F g-1). Asymmetric supercapacitors assembled with active carbon (AC) and Ni1.64Co2.40S4/rGO (Ni1.64Co2.40S4/rGO // AC) offer both high specific capacitance (265.5 F g-1 at 1 A g-1) and superior rate capability at 50 A g-1 (recovering 63.6% of the capacitance determined at 1 A g-1). In addition, the assembled device exhibits a high capacitance retention of 92.6% after 10000 cycles at 10 A g-1, which implies an excellent cyclic stability. Ragone plot reveals that the energy density of Ni1.64Co2.40S4/rGO //AC asymmetric supercapacitor do not vanish as it delivers 30.4 Wh kg-1 at 10 kW kg-1, demonstrating its promising application.
Highlights
With gradual consumption of fuel energy and the growing environmental problems, is the need to develop renewable energies and corresponding energy storage and conversion devices is urgent (Zhong et al, 2012; Liu et al, 2018a; Zhang Q. et al, 2018)
We demonstrate that asymmetric supercapacitor (Ni1.64Co2.40S4/reduced graphene oxide (rGO)//AC) assembled with Ni1.64Co2.40S4/rGO composite and active carbon exhibits excellent electrochemical performance with outstanding capacitance retention of 92.6% after 10,000 cycles at high current density of 10 A g−1 and high energy density of 30.4 Wh kg−1 at the power density of 10 kW kg−1
Electrochemical tests were performed in three-electrode systems with 6 mol L−1 KOH as electrolyte, the working electrode was prepared by mixing NixCoyS4/rGO composites, conductive additive (Super P), and binder (60 wt% PTFE emulsion) in a ratio of 8:1:1, Hg/HgO electrode as the reference electrode and Pt electrode as the counter electrode, respectively
Summary
With gradual consumption of fuel energy and the growing environmental problems, is the need to develop renewable energies and corresponding energy storage and conversion devices is urgent (Zhong et al, 2012; Liu et al, 2018a; Zhang Q. et al, 2018). Some studies classify transition metal sulfides as battery-type electrode materials (Mohamed et al, 2018; Ma et al, 2019) Putting this controversial issue aside, it has been proven that nickelcobalt sulfides (NixCoyS4) exhibit high electronic conductivity and flexibility due to the lower optical band gap of Ni–Co and electronegativity of sulfur (Chen et al, 2013; Zhu et al, 2015). Electrochemical tests were performed in three-electrode systems with 6 mol L−1 KOH as electrolyte, the working electrode was prepared by mixing NixCoyS4/rGO composites, conductive additive (Super P), and binder (60 wt% PTFE emulsion) in a ratio of 8:1:1, Hg/HgO electrode as the reference electrode and Pt electrode as the counter electrode, respectively. Additional experimental details are described in the Supplementary Material of the work
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