Preparation And Performance Of Cobalt-based Sulfide As Electrode Materials For Supercapacitors

In recent years,environmental pollution and shortage of fossil fuels have received worldwide attention,thus,research and design of pollution free and highly efficient energy storage devices are of particular significance.Because of their high power density,rapid charging/discharging ability and long cycle life,supercapacitors have been extensively studied,and electrode materials directly affect the electrochemical properties of supercapacitors.In this paper,cobalt-based sulfides（CNTs and different metal elements were modified）were prepared and used as electrode materials for supercapacitors,including CoS_1.29/CNTs,CuCo₂S₄/CNTs and Zn_0.76Co_0.24S/CoS.The main research includes:（1）Precursor of Co₂（CO₃）（OH）₂/CNTs were prepared using hydrothermal method,and then were transformed into CoS_1.29/CNTs via an ion-exchange process.The CNTs can provide an effective electron transfer and ion diffusion paths in the matrix of CoS_1.29,which significantly reduces the series resistance during the energy storage process,and improves the electrochemical performance.When the amount of CNTs was6.1 wt%,the electrode exhibited a higher specific capacitance(717.9 F g^-1 g^-1 at 1 A g^-1)measured in 2 M KOH electrolyte.The asymmetric supercapacitor assembled with the CoS_1.29/CNTs-6.1%electrode and an activated carbon（AC）electrode exhibited an energy density of 39.1 Wh kg^-1 at a power density of 399.9 W kg^-1,and the specific capacitance of the CoS_1.29/CNTs-6.1%//AC device maintained 91.3%of its original value after 2000 cycles at 3 A g^-1.（2）Nanocomposite of CuO/Co₃O₄/CNTs were prepared using hydrothermal method,and then were transformed into CuCo₂S₄/CNTs via an ion-exchange process.The embedded CNTs in the CuCo₂S₄ matrix enhanced rate of electron transfer and ion diffusion,and thus promoted the faradaic reactions between materials surface and electrolyte in the energy storage processes.When the amount of CNTs was 3.2 wt%,the electrode exhibited a higher specific capacitance of 557.5 F g^-1 at 1 A g^-1.The asymmetric supercapacitor of CuCo₂S₄/CNTs-3.2%//AC exhibited an energy density of23.2 Wh kg^-1 at a power density of 402.7 W kg^-1,and the residual specific capacitance of this device retained 85.7%of its original value after tested for 10,000 GCD cycles at3 A g^-1.（3）ZnCo₂O₄ nanocomposites were firstly synthesized via a hydrothermal method assisted by hexadecyltrimethyl ammonium bromide,and then transformed into Zn_0.76Co_0.24S/CoS nanoparticles（ZCS NPs）using a hydrothermal sulfuration process.The average diameter of ZCS NPs is estimated to be about 15.0 nm,which is beneficial for Faradaic redox reactions in energy storage process due to their numerous active surfaces.Because the ZCS NPs has lower series and charge transfer resistance,and also higher ion diffusion rate than that of the ZnCo₂O₄,it achieved a large specific capacitance of 1269.1 F g^-1 at 0.5 A g^-1.In addition,ZCS NPs//AC device exhibited a high energy density of 45.4 Wh kg^-1 at a power density of 805.0 W kg^-1,with an excellent cycling stability(91.6%retention of the initial capacitance over 5000 GCD cycles at 10 A g^-1).