Preparation And Application Of Transition Metal Compounds As Electrode Materials For Supercapacitors

Supercapacitors,due to the high power density,good charge/discharge performance,along with long cycling stability,have a wide application prospects in the high-efficiency energy storage systems.Electrode material is one of the most vital part that can observably influence the electrochemical properties of the supercapacitors.To obtain a high energy density supercapacitor,it is of great importance to improve the specific capacitance,electrical conductivity,and cycle performance of electrode materials and provide a thorough understanding of the energy storage mechanism.Over the last couple of decades,among various energy-storage materials,transition metal nanomaterials have caused a great deal of application,because of its natural abundance,good electrical conductivity,and large theoretical specific capacitance.Unfortunately,there have been many shortcomings,such as the low specific capacitance,poor energy density,and the short lifetimes,which limit the amount of commercial development.Based on the above defects,on the basis of environmental protection,low-cost,high power density,and prominent electrical conductivity,this thesis focuses on the improvement of specific capacitance,cycling stability of electrode materials.In addition,to enhance the the working voltage,the asymmetrical supercapacitors were set up which was a critical factor to improve the energy density.And the main research contents are as follows:（1）The NiCo₂S₄/NiS hollow nanospheres and NiCo₂S₄/Co₉S₈ nanorods based on the nickel foam as the substrate have been produced by hydrothermal synthesis and changing the concentration of cationic reactants.Besides,a series of electrochemical tests were also conducted.Thanks to the reasonable crystal structure and the outstanding electronic conductivity,the NiCo₂S₄/NiS//AC ASC displayed the energy density of 35.6 Wh·kg^-1 and 94.8%capacitance retention even after 3000 cycles;the NiCo₂S₄/Co₉S₈//AC ASC exhibited an energy density of 49.6 Wh·kg^-1 and loosed only 11%of its initial specific capacitance after 5000 cycles.（2）On the basis of the study of the first system,NiMoO₄/NiO nanoflowers were successfully synthesized via hydrothermal reaction and annealing treatment.NiMoO₄/NiO showed the relatively large specific surface area and the high surface roughness which has provided a considerable amount of active sites.In a three-electrode system,NiMoO₄/NiO displayed the specific capacitance of 10.3F·cm^-2/1982.3 F·g^-1,and the as-assembled NiMoO₄/NiO//AC ASC also obtained the energy density of 38.0 Wh·kg^-1.（3）CoMoO₄/Co₉S₈ nanorod arrays grown on a nickel foam substrate with the relatively high electronic conductivity were successfully assembled by one-step hydrothermal route and one-step vulcanization process,which achieved the outstanding specific capacitance(9.2 F·cm^-2/2059.3 F·g^-1)and a good capacity retention of 91.4%after 3000 cycles by the electrochemical measurement.Furthermore,the as-prepared CoMoO₄/Co₉S₈//AC ASC also displayed the energy density of 42 Wh·kg^–1.（4）Ni/Zn-MOFprecursorwassuccessfullysynthesisedfromthe2-methylimidazole as an organic ligand in the solution.Then,Zn-doped NiSe₂/ZnSe with the relatively large specific surface area and high conductivity was obtained via heat treatment and selenization.Electrochemical properties of the NiSe₂/ZnSe electrodes which were manufactured using pressure-rolling process were carried out by electrochemical test,and the specific capacitance of 884.50 F·g^-1 has acquired at the current density of 3 A·g^-1.Besides,NiSe₂/ZnSe//AC ASC based on the as-synthesized NiSe₂/ZnSe as the active electrode also provided a outstanding energy density of 45 Wh·kg^-1 at a power density of 400 W·kg^-1.