Morphological Modulation And Electrochemical Properties Of Nickel-based Bimetallic Oxides/Selenides

The development of new energy sources and the design of new energy storage and conversion devices have always been a hot topic of interest and research.Among the various types of electrochemical energy storage devices,supercapacitors have been widely studied for their high power density,long cycle life and fast charging and discharging advantages.However,the lower energy density has hindered its practical application.The energy density of supercapacitors depends on the specific capacitance and operating voltage window of the material,so electrode materials are responsible for widening the voltage window and increasing the specific capacitance of energy storage devices.The aim of this thesis is to develop nickel-based bimetallic oxide/selenide electrode materials and to improve the electrochemical performance of the electrode materials through morphology modulation,component optimisation,structural design and ion exchange.In addition,the prepared electrodes were assembled into asymmetric hybrid capacitors to achieve increased energy density of energy storage devices.The main research contents is as follows:The NiMo-based oxide nanomaterials with different morphologies were prepared by adjusting the pH of the precursor solution.The prepared NiO/Mo O₃composites with interconnected nanosheet structures have a large specific surface area and the formed open spaces between the nanosheets provide convenient channels for the penetration and rapid transport of electrolyte ions.The specific capacity of NiO/Mo O₃electrode was as high as210.6 m Ah g^-1at 1 A g^-1,and the capacity retention rate was 83.9%after 5000 cycles of charging and discharging,indicating that the NiO/Mo O₃electrode possess good cycling stability.The energy density of the asymmetric hybrid capacitor assembled with NiO/Mo O₃and activated carbon was 50.1 Wh kg^-1.In response to the poor conductivity and low rate performance of NiMo-based oxide electrodes,Self-supported Ni Se₂/Mo Se₂/NF micron hollow sphere composites with heterogeneous structures were prepared on nickel foam substrates using hydrothermal and selenization strategy.The hollow structure of Ni Se₂/Mo Se₂/NF materials facilitates electrolyte penetration and shortens the ion transport path.In addition,the introduction of Se can improve the electrical conductivity of the material and accelerate the charge transfer,thus improving the electrochemical performance.The Ni Se₂/Mo Se₂/NF micron hollow spheres have a specific capacity of 275.0 m Ah g^-1at 1 A g^-1.The asymmetric hybrid capacitor assembled with Ni Se₂/Mo Se₂/NF as the positive electrode and AC as the negative electrode has an energy density of 52.6 Wh kg^-1at power density of 749.6 W kg^-1.After 5000 cycles,the specific capacity of Ni Se₂/Mo Se₂/NF//AC maintained at 75.9%,showing a good cycling stability.The electrochemical properties of the material can be improved by introducing another metal compound with excellent pseudocapacitive properties to compound with the Ni-based material.By using the strategy of elemental component optimization,the Co substituted for Mo in the composite with Ni to prepare self-supporting NiCo₂O₄nanomaterials with diverse morphology and controllable structure.A series of NiCo₂O₄nanomaterials with diverse morphologies were prepared by modulating the dielectric constant and viscosity of the reaction solvent.Using a mixture of propylene glycol and water as the reaction solvent,the prepared NiCo₂O₄nanoear electrode have a relatively rough surface and possess a large specific surface area,which can provide more active sites for the redox reaction and thus increase the specific capacity of the materials.The NiCo₂O₄nanoear electrode is 250.5 m Ah g^-1at 1 A g^-1.When the current density is 20 A g^-1,the capacity retention rate is as high as73.0%,indicating a good rate performance.After 5000 cycles,the retention of specific capacity was 91.5%,which has good cycling stability.The assembled NiCo₂O₄//AC supercapacitor exhibited good electrochemical performance with an energy density of 801 W kg^-1at 46.4 Wh kg^-1.Enhanced performance of electrode materials through the construction of hierarchical layered structures.The hydrothermal and roasting methods were used to prepare hierarchical layered NiCo₂O₄nanowire/NC(NCO_NWs/NC)and NiCo₂O₄nanosheet/NC(NCO_NSs/NC)arrays on a porous NC conductive substrate derived from Ni-based metal-organic skeletons（Ni-MOF）.The unique hierarchical structure not only improves the electrical conductivity and reduces the internal resistance of the composite,but also allows more electrochemically active sites to participate in the electrochemical reaction,which is conducive to improving the performance of the composite.The specific capacity of the NCO_NWs/NC electrode was 270.3m Ah g^-1at 1 A g^-1,with a specific capacity retention of 80.0%at 10 A g^-1.The power density of the NCO_NWs/NC//CNT was 749.7 W kg^-1and the energy density was as high as 58.3 Wh kg^-1.In order to further improve the electrical conductivity and electrochemical properties of NiCo₂O₄/NC composites,the NiCo-OH/NC precursor nanowire materials were tuned by a selenization reaction strategy to prepare NiCo₂Se₄/NC nanotube arrays with hierarchical hollow structures.The introduction of Se improves the electrical conductivity of the composite and accelerates the transfer of charge;the hollow structure facilitates the penetration of electrolyte ions and accelerates the diffusion of ions,improving the electrochemical properties.The specific capacity of hollow NiCo₂Se₄/NC nanotube was 306.4m Ah g^-1at 1 A g^-1.The specific capacity retention is 81.4%with good multiplicative performance at 20 A g^-1.The asymmetric hybrid capacitor assembled with NiCo₂Se₄/NC as the positive electrode can achieve a energy density of 69.2 Wh kg^-1at 750.0 W kg^-1.After5000 cycles,the specific capacity maintained at 86.6%,showing good cycle durability.