Synthesis And Electrochemical Properties Of Sandwich-type Core-shell Transition Metal Oxides

With the excessive consumption of non-renewable resources worldwide,mainly represented by fossil fuels,it is of great importance to explore energy storage and conversion devices with high energy density and high-power density.Supercapacitors have become one of the most promising energy storage devices in recent years because they have the advantages of high-power density,a fast charge-discharge rate and long-term cycle.In order to promote the application of supercapacitors in production and life,the design and synthesis of excellent performance electrode materials with large capacitance and high energy density is a major problem that needs to be solved urgently.Currently,the three common electrode materials are transition metal oxides/sulfides,conductive polymers and carbon-based materials.Transition metal oxides are the most widely used electrode materials except carbon-based materials because of their high specific capacitance.However,transition metal oxides as electrode materials had some disadvantages such as unsatisfactory conductivity and insufficient electrochemical reaction sites.In order to overcome the above disadvantages,a novel sandwich-type core-shell nanomaterials emerged.This thesis aimed to improve the overall performance of electrode materials.Starting from improving the morphology and structure of electrode materials,a sandwich-type transition metal oxides electrode material with unique structure,which was superior to most previous studies,was constructed.The sandwich-type transition metal oxides electrode material was combined with capacitive electrode activated carbon as the positive and negative electrode of battery-supercapacitor hybrid device,respectively.The electrochemical performance of the assembled battery-supercapacitor hybrid device was also investigated.The detailed research results of this thesis were shown as follows:1.The sandwich-type MnMoO₄@Ni Mo O₄@Mn₂O₃ core-shell nanosheet arrays,which had more advantages than the traditional core-shell structures,was synthesized directly on the nickel foams through a two-step controllable hydrothermal method,and the three core-shell nanosheet arrays had synergistic effect.First,Mn-Mo precursor was obtained by growing MnMoO₄ nanosheets directly on the surface of NFs with a solvent of the mixture of deionized water（DIW）and ethylene glycol.Second,Ni Mo O₄ was coated on the surface of MnMoO₄ nanosheets with DIW as solvent.Due to the transformation of solvent,the state of MnMoO₄ changed,and during the process,some MnMoO₄ was transformed into Mn₂O₃,thus forming a unique sandwich-type core-shell nanostructure.The hydrothermal reaction time of the second step was controlled to be 1 h,2 h and 3 h respectively,and the obtained MnMoO₄@Ni Mo O₄@Mn₂O₃ series materials were denoted MMNM-1,MMNM-2 and MMNM-3 respectively.Through the test,it was found that MMNM-2 had the most superior electrochemical performance.When the current density was 3 m A cm^-2,the specific capacitance of MnMoO₄@Ni Mo O₄@Mn₂O₃-2 was 3053.82F g^-1.After 10,000 cycles of charge and discharge,the MnMoO₄@Ni Mo O₄@Mn₂O₃-2electrode could maintain 100.00%of the initial capacitance(I=10 m A cm^-2),indicating excellent cycling stability.2.MnMoO₄ precursor loaded on NFs was soaked in a beaker of HCl-tris buffer（p H=8.4）containing a certain amount of dopamine hydrochloride and ammonia for 24 h.During the process,MnMoO₄ was converted to Ni Mn₂O₄.The advantages of the synthesis method made the combination between Ni Mn₂O₄ and nickel foams substrate extremely firm,which provided an extremely reliable base for the subsequent electrochemical reaction.A novel sandwich-type core-shell nanomaterial with better electrochemical performance was obtained by introducing a microvillous nitrogen-doped carbon（N-C）organic sandwich layer into the traditional transition metal oxides core-shell nanomaterials,which was denoted Ni Mn₂O₄@N-C.A layer of Mn O₂ was coated on Ni Mn₂O₄@N-C surface through the controllable hydrothermal reaction.The density of Mn O₂ shell could be controlled by changing the hydrothermal reaction time.The results showed that when the reaction time was 2 h,the Mn O₂ shell density was appropriate,the specific surface area was the largest,and the electrochemical performance was the best（Ni Mn₂O₄@N-C@Mn O₂-2）.This was the result of the synergistic effect of Ni Mn₂O₄ backbone with a tight connection to the NFs substrate,N-C fine villous interlayer with excellent electrical conductivity,and Mn O₂ shell with suitable density,which ensured the maximum specific capacitance while avoiding structural collapse during repeated charge and discharge.Its specific capacitance was1180.82 F g^-1 at a current density of 3 m A cm^-2.The battery-supercapacitor hybrid device（Ni Mn₂O₄@N-C@Mn O₂-2//AC BSH device）was assembled with Ni Mn₂O₄@N-C@Mn O₂-2 as the positive electrode and activated carbon as the negative electrode.Its energy density could achieve 34.29 W h kg^-1(I=3 m A cm^-2).When the current density was 10 m A cm^-2,it could still maintain 96.68%of the initial capacitance after 30,000charge-discharge cycles,which was better than the results of most related materials previously reported.