Research On Preparation And Aqueous Energy Storge Performance Of Modified Manganese Dioxide Composites

As an emerging energy storage technology,aqueous energy storage devices have outstanding advantages such as high safety,high energy density and environmental friendliness,which have attracted the close attention of many researchers.Among the aqueous energy storage devices,aqueous zinc ion batteries（AZIBs）and aqueous supercapacitors（ASCs）stand out from them for their high energy density and high power density,respectively,and are expected to be practically applied to large-scale electric grid energy storage.Among them,manganese dioxide（MnO₂）is a popular electrode material for aqueous energy storage devices due to its non-toxic,high operating voltage,high theoretical capacity and low production cost.However,pure MnO₂ is subject to repeated insertion/extraction of ions and Jahn-teller effect during charge/discharge,which gradually dissolves in the electrolyte and eventually leads to poor rate performance and short cycle life.Therefore,in this paper,we propose to modify the structure and surface of manganese dioxide using nitrogen-doped mesoporous carbon and dopamine,respectively,to address the shortcomings of manganese dioxide and prepare high-performance composites for aqueous energy storage devices.The study details and conclusions are summarized as follows:（1）Study on the aqueous energy storage performance of carbon grown manganese dioxide compositesIn this work,MnO₂ was grown on nitrogen-doped mesoporous carbon（NMC）by hydrothermal method to obtain MnO₂@NMC composites.NMC and MnO₂@NMC exhibit good aqueous energy storage properties.On the one hand,NMC exhibits high specific capacitance,high rate performance and excellent cycling performance in ASCs,and shows typical dual-layer capacitance characteristics.On the other hand,MnO₂@NMC-x exhibits high electrochemical performance and kinetics in AZIBs.Specifically,the specific capacity of MnO₂@NMC-20 increased from 247.8 m Ah g^-1（MnO₂）to 325.6 m Ah g^-1 at a current density of 100 m A g^-1.The specific capacity retention in the rate test(100 m A g^-1 to 2000 m A g^-1)increased from 42.19%（MnO₂）to 51.02%.The capacity retention after 3500 cycles increased from 47.15%（MnO₂）to 56.63%.Notably,R_ct of MnO₂@NMC from 113.6Ωdecreased to 75.16Ωand exhibited enhanced surface capacitance characteristics and faster ion diffusion rate.It indicates the NMC substrate can provide abundan t surface active sites for MnO₂nanowires and enhance the conductivity of the electrode for the purpose of promoting fast ion and electron transport.（2）Study of aqueous energy storage properties of polydopamine surface-modified manganese dioxide compositesBased on the previous chapter,the surface modification of MnO₂@NMC with polydopamine（PDA）was used to reasonably construct PDA@MnO ₂@NMC composites with thorn-like micro-nanostructures.Due to the incorporation of PDA,PDA@MnO₂@NMC exhibited enhanced electrochemical properties and fast diffusion kinetics in both AZIBs and ASCs.The generation of MnOOH and Zn Mn₂O₄ discharge products was observed by ex-situ X-ray diffraction and Raman characterization,indicating that MnOOH and Zn Mn₂O₄ correspond to H⁺and Zn²⁺insertion in the two discharge platforms of AZIBs,respectively.Density flooding theory（DFT）calculations also indicate that PDA can be firmly immobilized on the MnO ₂ surface,thus preventing the dissolution of MnOOH.In addition,PDA with surface hydrophilic groups can capture more H⁺,thereby increasing the surface capacitance and extending the first discharge platform.Further EIS and GITT test results showed that the ohmic resistance of PDA@MnO₂@NMC decreased almost by half,especially the ion diffusion coefficients increased more than 30 times compared to pure MnO ₂.The results indicate that the surface PDA layer can ultimately improve the electrochemical performance of the electrode by enhancing its electrochemical kinetics.In summary,compared with pure MnO₂,the MnO₂-based composites modified by methods,such as composite with carbon materials and surface modification by polymers,which exhibit excellent aqueous energy storage performance.And the assembled AZIBs and ASCs show enhanced electrochemical kinetics and structural stability,showing great competitiveness in the field of aqueous energy storage.