Modified Constructs Of Cobalt Oxide Electrode Materials And Electrochemical Properties

Among many energy storage devices,supercapacitors（SCs）stand out due to their high safety,fast charging and discharging capability,long cycle life and high power density,which can be applied to fields requiring power burst,such as backup power,high power military equipment,etc.As an electrode material for supercapacitors,cobalt trioxide（Co₃O₄）has a high theoretical capacity,low cost,environmental friendliness and excellent cycle life,but its poor conductivity limits its practical generation applications in the field of SCs.Therefore,the enhancement of supercapacitor electrochemical performance by improving the conductivity of cobalt trioxide is one of the current hot issues in the field of supercapacitor research.In this dissertation,this dissertation focuses on the conductivity of cobalt tetroxide and improves the conductivity of the electrode material by modulating the material structure,thus enhancing the electrochemical performance of SCs.The specific research contents are as follows.（1）The spinel structural Co₃O₄ with different proportions of tin doping on the graphene film was prepared by hydrothermal method to improve the conductivity of the electrode and enhance the electrochemical performance of the supercapacitors.The CSO@GF with 10%Sn doping ratio was fabricated by hydrothermal method,and the specific surface area of CSO@GF is 51.68 m² g^-1.Besides,the charge transfer resistance(R_ct)of the CSO@GF electrode was only 1.31Ω.The first-principles calculations further confirmed that an appropriate amount of tin-doped spinel structure Co₃O₄ can significantly improve the conductivity of the structure.The specific capacitance of the electrode could reach 2032.6 F g^-1 and the electrode still maintained 55.1%of the initial capacitance at 40 A g^-1.The ASC was assembled by the CSO@GF electrode and Fe₂O₃@GF electrode.The specific capacitance of the ASC device can reach 200.2 F g^-1at current density,the energy density of the ASC device can reach 62.6 Wh Kg^-1 and the power density can reach 15351.7 W kg^-1.The capacitance of the device can still maintain91.69%of the initial capacitance after 10000 galvanostatic charge-discharge（GCD）tests.（2）The pure Co₃O₄ was modified by using the hydrothermal method to prepare spinel-structured Co₃O₄ with different copper substitutions on the flexible carbon cloth to improve its conductivity.When the molar mass ratio of Co:Cu is 5:1,the spinel structural CCO@CC electrode material with a three-dimensional nanowire structure prepared by the hydrothermal method has a specific surface area as high as 53.46 m² g^-1,and its specific capacity can reach 1947.2 F g^-1.The partial substitution of copper element introduces a large number of defects in the electrode material and improves the specific capacity of the electrode material while also enhancing the R_ct of the CCO@CC electrode is significantly lower than that of the unmodified cobalt tetraoxide electrode.The CCO@CC electrode material and Fe₂O₃@CC electrode were assembled into a FSASC for electrochemical performance testing,and the specific capacity of the device could reach 182.7 F g^-1,the largest energy density of the capacitor is 57.1 Wh kg^-1 and the largest power density of the device is 15061.9 W kg^-1.After 10,000 GCD cycles at the current density of 10 A g^-1,the capacity of the device can still maintain 93.25%of the initial capacity,reflecting good electrochemical stability.Tested at different bending angles,the flexible device has almost no loss of capacity and it could be a candidate for application in flexible electronic devices.（3）Elemental selenium and oxygen defects were introduced by the gas-phase method thereby improving the conductivity of Co₃O₄ electrode materials.The three-dimensional mesh-like nanowire structure of Co₃O₄ was prepared on conductive carbon cloth by hydrothermal method,and the structure of Co₃O₄ was modified by CVD method under nitrogen gas using selenium powder,to obtain selenium partially substituted Co₃O₄（COS@CC）,and the R_ct of this electrode material is about 2.12Ω,which is significantly improved compared with the pure cobalt tetraoxide electrode（6.71Ω）.After 15,000 GCD cycles,the capacity of this electrode material is only 8%lost.In addition,the structural characterization of the electrode after the cycle test shows that the spinel structure remains stable,the microscopic morphology remains the same,and the specific surface area decreases slightly,which proves that the structural stability of the electrode material is excellent.The electrochemical performance of the electrode after the cycling test shows a slight decrease in capacity and only a slight increase in impedance.The assembled flexible device has good electrochemical properties(the largest energy density is 64.9 Wh kg^-1 and the largest power density is 15010.5 W kg^-1)and flexibility,which proves that the material has promising applications.（4）The electrode material was reduced by an electrochemical workstation to introduce abundant oxygen defects thereby further enhancing the conductivity and improving the electrochemical properties of Co₃O₄.A highly mass-loading nickel cobaltate electrode material was prepared on conductive carbon cloth by hydrothermal method,and the electrode material was electrochemically reduced by using a constant potential（-0.9 V）in a three-electrode system with 2 mol L^-1 KOH electrolyte to obtain a nickel cobaltate electrode material with a large number of oxygen defects（NCO-0.9@CC）.The R_ct of this electrode material is only 1.46Ω,which is a very significant increase in conductivity compared to the pure nickel cobaltate electrode material（9.87Ω）.The electrode material has a high capacity of 11.3 F cm^-2 at 1 m A cm^-2 and still remains 5.7 F cm^-2 at 50 m A cm^-2,with a capacity loss of only 6.7%after 10,000 cycles of testing.Assembling NCO-0.9@CC and Fe₂O₃@CC to form a FSASC,the device achieves an energy density of 3.5 m Wh cm^-3 and the largest power density is 118.9 m W cm^-3.The capacity retention of the device is about 90%at different bending angles,and after 200 bending tests,the capacity remains unchanged,demonstrating its potential for wearable flexible device applications.