Fabrication Of Co₃S₄-Ni₃S₂ Induced By Organic Ligand And Subsequent Modification By Polypyrrole For Supercapacitor Electrode

Supercapacitors have their own advantages such as high power density,long service life and conformity with the concept of green chemistry,and have become one of the indispensable devices in energy storage materials.However,compared with secondary batteries,supercapacitors still have a bottleneck problem in energy density.In order to narrow this gap,it is mainly solved by searching for electrode materials with excellent electrochemical performance and assembling asymmetric supercapacitors.In the present work,by using sodium sulfosalicylate(SSS)as an organic ligand,Co²⁺as the central ion,and thiourea as the sulfidizing agent,Co₃S₄-Ni₃S₂@NF hybrid electrode material was fabricated by one-step hydrothermal method on nickel foam(NF)substrate.Subsequently,the PPy/Co₃S₄-Ni₃S₂@NF composite electrode material was obtained by electro-depositing conductive polypyrrole(PPy)modification.It is expected that the structure of the target object can be optimized and the electrochemical performance of the target can be optimized through the controllable hydrothermal sulfidization process and subsequent modification.The main work is as follows:(1)Preparation of Co-Ni-SSS@NF electrode material.Using NF as the current collector,a facile one-step hydrothermal method was used to prepare a stable compound of metal cobalt ions with SSS ligand.The as-synthesized electrode material shows good electrochemical stability.SEM,TEM,FT-IR,UV-Vis,XRD and XPS techniques were employed to characterize the composition and structure of as-prepared Co-Ni-SSS@NF.The electrochemical performance of as-prepared material is optimized by adjusting the hydrothermal temperature,reaction time and reactants ratio.Tests using a three-electrode system show that the specific capacity of the Co-Ni-SSS@NF electrode material is 814 mF cm^-2at a current density of1 mA cm^-2,and its rate reaches 68%within the range of 1-20 mA cm^-2.Furthermore,its specific capacity retention rate is as high as 107.8%after2000 cycles of charge and discharge at a high current density of 20 mA c m^-2,showing excellent cycle stability.It indicates that coordination of central atoms with organic ligand SSS can provide a stable and special structure as a synthesis model.(2)Preparation of Co₃S₄-Ni₃S₂@NF-SSS electrode material.Using Co-Ni-SSS@NF as a template and thiourea as sulfur source,the Co₃S₄-Ni₃S₂@NF-SSS electrode material was prepared by hydrothermal method in order to compensate the poor conductivity of Co-Ni-SSS template.Compared with the Ni-SSS@NF electrode,the electrochemical performance of the modified electrode was significantly improved.SEM,TEM,FT-IR,UV-Vis,XRD and XPS techniques were employed to characterize the composition and structure of as-prepared Co₃S₄-Ni₃S₂@NF-SSS.The specific capacity of the Co₃S₄-Ni₃S₂@NF-SSS electrode reaches 2825.5 mF cm^-2 at a current density of 2 mA cm^-2,and the rate capability amounts to 82.1%over the current density range of 1-20 mA cm^-2.Furthermore the specific capacity of the Co₃S₄-Ni₃S₂@NF-SSS electrode still retains 81%after 2000 cycles of charging and discharging at a high current density of 20 mA cm^-2.Especially when Co₃S₄-Ni₃S₂@NF-SSS was used as positive electrode and activated carbon(AC)as negative electrode,the assembled asymmetric device Co₃S₄-Ni₃S₂@NF-SSS//AC can deliver an energy density of 38.9 Wh kg^-1 at a power density of 586.1 W kg^-1.(3)Preparation of PPy/Co₃S₄-Ni₃S₂@NF-SSS electrode material.In order to further improve the cycle stability of the composite material,a thin layer of PPy was electrodeposited on Co₃S₄-Ni₃S₂@NF-SSS by electrochemical polymerization.It was found that the electrochemical performance of the composite electrode material was improved after modification.SEM,TEM,Raman,XRD and XPS techniques were employed to characterize the composition and structure of as-prepared PPy/Co₃S₄-Ni₃S₂@NF-SSS hybrid electrode.The specific capacity of the PPy/Co₃S₄-Ni₃S₂@NF-SSS electrode increased to 3953 mF cm^-2 at a current density of 2 mA cm^-2.After 2000 cycles of charging and discharging at a high current density of 20 mA cm^-2,the capacity retention rate of the PPy/Co₃S₄-Ni₃S₂@NF-SSS amounted to 92%.Compared with the previous Co₃S₄-Ni₃S₂@NF-SSS electrode,the cycle stability was greatly improved.The energy density of the assembled device PPy/Co₃S₄-Ni₃S₂@NF-SSS//AC is 39.2 Wh kg^-1 at a power density of 592.7 W kg^-1.