Construction Of Arrayed Nanomaterials And Their Application In Flexible Aqueous Energy Storage Devices

Aqueous electrochemical energy storage system has widely attracted researchers’interests due to its superior safety,easier assembly procedure,less restrictions to the assembly environment,when compared with the Lithium-ion battery and alkaline ion batteris.Nanomaterials are usually anchored/grown on a substrate to achieve arrayed structures by orderly self-assemble.The as-obtained arrayed nanomaterials with a large specific area guarantee the full exposure of active sites with electrolyte,thus facilitating electron transfer and ions diffusion.Furthermore,they grown on the substrate will not only simplify the assembly procedure of energy storage devices,but also avoid using of conductive additives and non-active binders.In this dissertation,a series of orderly nanoarrayed electrode materials are prepared on the flexible carbon cloth,and their morphologies and structures are characterizchaed and analysized,electrochemical energy storage performance are also systematically studied.The research contents are summarized as follows:（1）NiCo₂O₄ nanowires grown on the carbon cloth is prepared and modified with oxygen vacancies engineering to increase its specific surface area and electrical conductivity,thus enhancing the specific capacitance of NiCo₂O₄ and improving its cycle stability.Firstly,NiCo-LDH on the carbon cloth is synthesized through hydrothermal reaction,NiCo₂O₄@CC is obtained by calcination of NiCo-LDH@CC in the air,NiCo₂O₄@CC with high contents of oxygen vacancies（Vo-NiCo₂O₄@CC）,large surface area and good electrochemical energy storage performance is successfully prepared through the solvothermal treatment of NiCo₂O₄@CC.After the hydrothermal treatment with ethylene glycol/Na OH solvent,Vo-NiCo₂O₄@CC basically retains the original nanowires morphology of NiCo₂O₄@CC,but the specific surface area is increased by about 50%to that of pristine NiCo₂O₄.Electrochemical test reveals that the intrinsic conductivity of Vo-NiCo₂O₄@CC is enhanced after hydrothermal treatment and the Vo-NiCo₂O₄@CC-16h with reaction time for 16 h reached a specific areal capacitance of 1389 m F cm^-2.This value is 2.5 times higher than that of pristine NiCo₂O₄@CC(564 m F cm^-2)under the same conditions.When the Vo-NiCo₂O₄@CC-16h is used as cathode and active carbon on carbon cloth as anode,1.0 M KOH as electrolyte to assemble the aqueous asymmetric supercapacitor（ASC）,the device outperform a specific areal capacitance of 699m F cm^-2 at 2 m A cm^-2,and achieved impressive energy density of 43.6 Wh kg^-1,which exceeds those of commercial lead-acid batteries(35–40 Wh kg^-1).（2）Co₃O₄@CC nanosheets arrays are obtained by the pyrolysis of cobalt based metal organic framework in-situ grown on carbon cloth substrate（Co-MOF@CC）.Quenching Co₃O₄@CC in Ni²⁺-containing cool solution for seconds can result in the formation of Ni-Co₃O₄@CC.During the quenching process,not only additional oxygen vacancies are introduced,but also Ni²⁺doping is achieved.The specific surface area of the quenched sample increased by 51%when compared with that of the unquenched sample,which may result in the Ni-Co₃O₄@CC with nanoarray flakes structures possessing excellent electrochemical energy storage performance.The electrochemical results of single electrode test in 1.0 M KOH electrolyte show that the specific capacity of Ni-Co₃O₄@CC electrode is up to 0.55 m Ah cm^-2at 0.1 m A cm^-2,which is much higher than that of Co₃O₄@CC(0.02 m Ah cm^-2)under the same condition.In addition,after 5000 cycles of consecutive charge and discharge,the coulombic efficiency of Ni-Co₃O₄@CC electrode remains at 100%with the capacity retention of 84.2%.When an aqueous Co-Zn battery assembled based on the Ni-Co₃O₄@CC cathode and Zn@CC anode,the specific areal capacity of the device was as high as 0.7 m Ah cm^-2,which is much higher than the results reported in many literatures.（3）Through the calcination of cobalt-based metal-organic framework on carbon cloth（Co-MOF@CC）to obtain N-C nanoarray structure,followed by loading iron oxide nanoneedles on the N-C nanosheets to achieve a hierarchical three-dimensional Fe₂O₃/N-C@CC electrode for aqueous supercapacitor.This three-dimensional composite electrode can make full use of the synergistic effect of N-C nanosheets and Fe₂O₃ nanoneedles,thus giving Fe₂O₃/N-C@CC electrode a desirable capacitance.Introducing N-C nanoarray structures not only improve the conductivity of Fe₂O₃/N-C@CC electrode,but also increase the mass loading of Fe₂O₃.In addition,the porosity of N-C structure increases the ion diffusion channel,thus facilitating the rapid transport of electrolyte ions.Fe₂O₃ is loaded on the N-C nanosheets to form hierarchical nanoarrays,which ensures the reaction sites of Fe₂O₃ nanoneedles can be fully utilized.Therefore,the area capacitance of as-prepared Fe₂O₃/N-C@CC electrode reaches 183.3 m F cm^-2.Similarly,NiO/N-C@CC cathode is prepared by loading NiO on the N-C skelecton,and then matched with Fe₂O₃/N-C@CC anode to assemble a quasi-solid state ASC.The resulting ASC device delivers an energy density of 14.1 Wh kg^-1 at 1500 W kg^-1.（4）Built on the high electrical conductivity and excellent structural stability of orderedly nanoarrayed nitrogen-doped carbon nanotubes（CNTs）,the composite electrodes of CNT/electrochemically active transition metal oxides are constructed.The synergistic effect between the CNTs and transition metal oxides can be fully integrated to ensure both physical stability and chemical activity,contributing good flexibility and high energy density of energy storage device.Vertically-aligned CNTs nanoarrays are firstly anchored on carbon cloth,then PPy/Fe₂O₃/CNT@CC and Mn O₂/N-CNT@CC nanocomposite electrodes are successfully fabricated through the electrodeposition method,respectively.Additionally,an ASC is constructed by using Mn O₂/CNT@CC as cathode,PPy/Fe₂O₃/CNT@CC as anode,and 1 M Na₂SO₄ as electrolyte.Electrochemical test results proves that the as-assembled device can work steadily under the work window range of 0–2.6 V,and achieve a high energy density of72.3 Wh kg^-1,which outperforms most of literature results related to iron-based or manganese-based ASCs.