Construction And Mechanism Of Aqueous Energy Storage Devices Based On Transition Metal Compound Arrays

The warning of lacking primary energy sources（e.g.oil and coal）in the near future urgently requires to study new energy storage technologies improving the utilization efficiency.Among various types of aqueous energy storage devices,supercapacitors featuring high power density,energy density,rate performance and good cycle stability would be an ideal choice for future energy storage device.As an alternative to lithium-ion batteries,rechargeable zinc-based batteries（e.g.alkaline Ni-Zn batteries,neutral mild zinc battery）have attracted extensive interest owing to its advances including the abundant zinc source in the earth,low cost,high safety,and low redox potential.Electrode materials are an important part of energy storage devices,and the development of high-performance electrode materials can improve the energy density,power density and cycling stability of energy storage devices.Transition metal compounds are an ideal class of electrode materials because they can provide high specific capacity due to their abundant redox reaction active sites.The nanoarray structures have a high specific surface area,which can maximize the utilization of active materials and provide abundant active sites for redox reactions,thereby improving the electrochemical performance of electrode materials.Therefore,in this thesis,a transition metal compound nanoarray structure with high specific capacity,rate capability and cycle stability was prepared by hydrothermal method,chemical vapor deposition method and electrodeposition method,and its charge storage mechanism was explored.A comprehensive evaluation was carried out,and it was used as an electrode material to encapsulate a high-performance aqueous energy storage device.The main contents of this thesis are listed as below:（1）Ni Se₂nanosheet arrays were grown on carbon cloth（CF）by hydrothermal method and chemical vapor deposition method,respectively,and then nickel cobalt selenide(Ni_xCo_1-xSe₂/CF)electrodes with different Ni/Co ratios were synthesized by adjusting the doping of Co²⁺.Due to the synergistic action mechanism of Ni and Co bimetal metals,the specific capacity and rate performance of the electrode materials are effectively improved.It showed a high specific capacitance of 1019.5 C/g at 1 A/g,a good specific capacitance of703.9 C/g at 40 A/g,achieving 69%capacity retention.Finally,we assembled a hybrid supercapacitor with the Ni_0.5Co_0.5Se₂/CF array structure and the activated carbon（AC）as the positive and negative electrode,respectively,which performs excellent cycling stability,the specific capacitance was maintained at 72.2%of the initial value after 10000 charge/discharge cycles at 10 A/g.（2）A novel electrode collector was assembled by depositing highly conductive nickel-plated cotton fabric（NF）by chemically plating nickel on commercial pure cotton fabric,and the Ni-Co selenide nanowire array structure(NF@Ni_xCo_1-xSe₂)was grown in situ on this collector.The composite electrode has superior electrochemical performance than the conventional carbon cloth collector.It performed an ultra-high specific capacitance of 1333.0C/g.Finally,a flexible hybrid supercapacitor is assembled with NF@Ni_0.5Co_0.5Se₂（positive electrode）,Fe₃C/CF（negative electrode）,ultra thin and porous silk fabric（separator）and PVA/KOH（electrolyte）.The device has an energy density of 47.4 Wh/kg.After operating for4000 cycles,80%of the capacity was still maintained,exhibiting excellent cycling stability.（3）In order to further improve the energy density of the device,an alkaline Ni-Zn battery packaged with Ni_0.5Co_0.5Se₂/CF and commercial zinc as the cathode and anode,respectively.The performance difference of the battery in 6 M KOH electrolyte containing different additives,including Zn（CH₃COO）₂,Zn O and Zn Cl₂,was explored,and the mechanism was studied.In the comparison of Zn O and Zn Cl₂,we found Zn（CH₃COO）₂would bring the smallest side product,since it could effectively inhibit the corrosion of the zinc anode.In addition,with increasing the content of Zn（CH₃COO）₂in the electrolyte,the corrosion of the zinc negative electrode would diminish,and the best inhibition was achieved when it reaches the saturation state.Ni_0.5Co_0.5Se₂/CF//Zn battery in 6 M KOH electrolyte with saturated Zn（CH₃COO）₂can reach an energy density of up to 239.5 Wh/kg and a power density of 17k W/kg,and the capacity retention rate is 71.4%after 2000 cycles,exhibiting excellent electrochemical performance.（4）PET/C/Au fork-finger electrodes were prepared on flexible polyethylene terephthalate（PET）by laser etching,Mn O₂and Zn nanoarray structures were deposited as positive and negative active materials,respectively.A polyacrylamide gel electrolyte was implemented to encapsulate the flexible micro zinc ion battery.The device has a specific capacity of 0.091 m Ah/cm²at 0.1 m A/cm²and maintains 83.6%of the initial capacity after5000 cycles.In addition,the first-principles calculations proved that the carbon paste has the ability to inhibit hydrogen evolution when used as a conductive layer,which effectively improves the problem of Zn anode peeling and realizes the long-cycle stability of the device.Therefore by studying high-performance transition metal compound electrode materials,this thesis discusses the growing principle of nanostructures and electrochemical energy storage mechanisms,and then applies such nanostructure materials on aqueous energy storage devices as the electrodes,which provides an experimental and theoretical foundation for the development of high-performance electrode materials and its application on high-energy density and power density energy storage devices.