Synthesis And Electrochemical Properties Of Cobalt And Molybdenumbased Metal Oxides With Shell Nanostructure

With the massive consumption of fossil energy and non-renewable energy,as well as the increasingly serious environmental pollution problem,the demand for renewable clean novel energy gradually increases.Therefore,environmentally friendly energy storage and conversion equipment has been widely concerned.Among them,supercapacitors（SCs）have been widely studied for their high-power density,fast charging and discharging speed,and good cycling stability.However,its relative low energy density limits its wide application.In this paper,the capacitive electrode activated carbon（AC）is combined with the synthesized transition metal oxide electrode material with Faraday charge storage mechanism to assemble the battery-hybrid supercapacitors（BSHs）.The results of this paper are shown as follows:1.Zn Co₂O₄/Ni Mo O₄ core-shell nanostructure materials with different morphologies were synthesized on the surface of nickel foams（NFs）by a simple hydrothermal synthesis method,and their electrochemical properties were directly tested as non-adhesive electrode materials.In this paper,not only the influence of different morphologies of Zn-Co precursor on the material,but also the influence of reaction time of Ni Mo O₄ shell material on the electrode material was explored.The results show that the structure of Zn-Co nanosheet precursor is more stable,and the optimal reaction time of Ni Mo O₄ shell material is 2 h.Core-shell nanostructures can have more active reaction sites and store more electric charges,thus increasing the specific capacity of battery-type electrode materials.The electrode synthesized by this method has a specific capacity of 1238.1 C g^-1(3 m A cm^-2),and has good cycling stability.The stable structure of Zn-Co nanosheet precursors provides a possibility for further study.2.Based on the Zn-Co nanosheet precursor supported on NFs,Zn Co₂O₄/Co Mo O₄core-shell nanosheet structure was directly grown on the surface of NFs with three-dimensional skeleton by a two-step controlled hydrothermal synthesis reaction.The effect of the reaction time of Co Mo O₄ on the material thickness,morphology and properties was investigated.The results show that Zn Co₂O₄/Co Mo O₄ composite electrode（ZCCM-3）has the highest specific capacity and the best cycling stability when the reaction time of Co Mo O₄ is 3 h,and can still retain 104.1%of the initial capacity（5000 cycles）.The ZCCM-3//AC BSH device was assembled using the ZCCM-3 electrode as the positive electrode,achieving a maximum energy density of29.24 W h kg^-1 and maintaining 90.95%of the initial capacitance after 5000 cycles.3.The structure is very important to the electrochemical performance of the material,and the composition of the material also has a great influence on the performance.Although the traditional molybdate contains two metals,Mo does not participate in the redox reaction,while the battery-type electrode stores charge through the Faraday process.Therefore,in the fifth chapter,the hydrothermal synthesis method is used to synthesize multiple redox species of bimetallic cationic molybdate Zn-Ni-Mo-O and Co-Ni-Mo-O on NFs.The effect of the relative content of different metal nitrates on the morphology and electrochemical properties of the materials was investigated.By comparing the comprehensive properties of the synthesized series of electrode materials,it was found that when the reactants n（Zn（NO₃）₂·6H₂O）:n（Ni（NO₃）₂·6H₂O）or n（Co（NO₃）₂·6H₂O）:n（Ni（NO₃）₂·6H₂O）=1:2,the formed materials have the best electrochemical properties（ZNM2 and CNM2）.The maximum energy densities of the two electrodes were 13.4 W h kg^-1 and 31.1 W h kg^-1,respectively.4.In addition to being widely used in SCs,transition metal oxides can also be used as catalysts to improve OER catalytic activity.In chapter six of this paper,Ni Co₂O₄/Ni Mo O₄ core-shell nanoarray materials with different morphologies were formed on the surface of NFs by a two-step hydrothermal synthesis method.After comparing the specific capacity,cycle stability and rate performance,the two groups of materials（NCNMS-2 and NCNMW-2）with the best performance were selected for assembling BSH device and OER performance test.Because the special core-shell nanosheet arrays provides more active reaction sites and a stable structure,the stable operating voltage of the BSH device assembled by NCNMS-2 is 1.6 V,and its highest energy density can reach 30.57 W h kg^-1,and the over-potential measured by LSV of NCNMS-2 is 175 m V.The slope of Tafel is 59 m V Dec^-1,which indicates that it has good OER catalytic activity.Meanwhile,the NCNMW-2//AC BSH device has a maximum energy density of 26.51 W h kg^-1 and a Tafel slope of 79 m V Dec^-1.5.Hollow nanostructured materials are considered as good electrode materials because of their large specific surface area,exposure of more active reaction sites,and effective reduction of ion transport distance.In chapter seven,ZIF-67 was firstly formed at room temperature,and then the double-shelled nanocage hollow layer Co₃O₄/Ni Co₂O₄ powder electrode material was formed by chemical bath deposition method.It was coated on the surface of NFs by coating method,and the assembled BSH device and OER performance was tested.The maximum energy density of the BSH device assembled with the positive electrode is 12.03 W h kg^-1.When used as OER catalyst,the potential is 158 m V.In this paper,metal oxide materials which can be used in SC are modified by different strategies,and the extended application of some transition metal oxides in OER catalysis is also measured,which provides new possibilities for the design and development of the next generation of energy storage and conversion devices.