Synthesis MOFs-derived Metal Sulfide Electrode Materials And Application In Supercapacitors

With the excessive consumption of non-renewable resources and the aggravation of global warming,energy storage equipment technology urgently needs to be broken through to meet the needs of sustainable energy and low-carbon economy in the future.Integrating high energy density,power density and inherent safety into a single energy storage element is an urgent need,but also a great challenge.To solve this problem,it largely depends on designing novel energy storage systems using novel electrode materials with excellent electrochemical properties.Supercapacitors（SCs）have the advantages of excellent inherent safety,remarkable cycle stability,high charge-discharge efficiency,so they have great development potential in the field of high-efficiency energy storage.According to the energy density formula:E=1/2 CV²（E,energy density）,it can be seen that specific capacitance（C）and potential window（V）are proportional to E.SCs with high energy density can be obtained by preparing nano-electrode materials with high specific capacitance and then assembling asymmetric supercapacitors（ASCs）to expand its potential window.Metal-organic frameworks（MOFs）and MOFs derivatives are considered as ideal electrode materials due to their large specific surface area and adjustable porosity.Transition metal sulfides are used for SCs due to their excellent electrical conductivity and high theoretical specific capacitance.Therefore,MOFs can be used as sacrificial templates to obtain various porous transition metal sulfide nanocomposites through different curing treatments.This dissertation focuses on improving the specific capacitance and cycle life of the electrode material.The relationship between the structure,composition,morphology and electrochemical performance of the electrode material is discussed by means of XRD,SEM,TEM,XPS,etc.,and its working principle is explained.This paper mainly carries out the following three aspects of research:（1）Preparation of Fe.₉₂Co.₀₈S@NiS/NiO composite electrode by two-step solvothermal methodFirstly,the Fe.₉₂Co.₀₈S@NiS/NiO nanocomposites prepared by hydrothermal vulcanization using the spindle-shaped FeCo-MIL-88 as the precursor have the unique morphology of stacked nanosheets.The electrochemical test results show that as a cathode material,when the current density is 1 A?g^-1,the specific capacitance reaches1868 F?g^-1.At a constant current density of 10 A?g^-1,the initial capacitance retention was 62.5%after 5 000 charge-discharge cycles.The asymmetric capacitor assembled with Fe.₉₂Co.₀₈S@NiS/NiO as the cathode achieves a power density of 251.1 W?kg^-1 at an energy density of 38.2 Wh?kg^-1.Two ASCs devices in series can keep a red LED light on for about 30 minutes,which has practical application value.（2）Synthesis of CoNi₂S₄@NiCo₂O₄ composite nanomaterials by hydrothermal methodUniform spherical nanoflower ZIF-67@α-Co/Ni（OH）₂ precursor was prepared by adjusting the ratio of cobalt and nickel ions through simple hydrothermal reaction.The reaction of ZIF-67@α-Co/Ni（OH）₂ with Na₂S?9H₂O at high temperature yields lamellar CoNi₂S₄@NiCo₂O₄.When used as a cathode material,the specific capacitance of CoNi₂S₄@NiCo₂O₄ flakes reaches 1904 F?g^-1 at a current density of 1 A?g^-1,and at a current density of 14 A?g^-1,the CoNi₂S₄@NiCo₂O₄ electrode retains 57.2%of the initial specific capacitance after 5 000 charge-discharge cycles.The CoNi₂S₄@NiCo₂O₄//AC ASCs can maintain an energy density of 74.8 Wh?kg^-1 at the high power density of 800W?kg^-1.Two tandem CoNi₂S₄@NiCo₂O₄//AC ASCs devices can power a small LED bulb for about 60 minutes,demonstrating the possibility of practical applications.（3）Amorphous Co₉S₈@Fe S/Fe₃O₄ composite electrode was prepared by hydrothermal methodUsing a new MOF-to-MOF strategy,ZIF-67@Fe-PBA precursor was synthesized by ligand exchange reaction at room temperature using single metal cobalt based ZIF-67 as cobalt source,and then irregular nanosheet deposition Co₉S₈@Fe S/Fe₃O₄ was synthesized by sulfurization treatment.The prepared Co₉S₈@Fe S/Fe₃O₄ exhibits a variety of excellent electrochemical properties,including synergistic bimetal composition,abundant active sites,short electron and ion transport paths,and high structural strength.Electrochemical test results showed that as a positive electrode material,the specific capacitance reached 1434.3 F?g^-1 and the cyclic stability increased to 104.2%.An asymmetric capacitor assembled with Co₉S₈@Fe S/Fe₃O₄ as a positive electrode achieves power density of 800 W?kg^-1 at energy density of 25.9 Wh?kg^-1.The assembled asymmetric capacitor can power a small green LED bulb for 20 minutes,demonstrating its potential application in future energy storage devices.