Preparation Of Manganese Oxide Micro-nano Electrode Materials And Study On Performance Of Asymmetric Supercapacitors With Water System

Considering the rapid depletion of fossil fuels and gradual global warming,the conversion/storage of green energy has attracted considerable attention worldwide.In a variety of energy storage equipment,supercapacitors have been widely used in the fields of portable electronics,heavy industry,national defense,electric vehicles and electronic applications based on their advantages of high power density,rapid charge-discharge capability and excellent long-term cycle stability.In recent years,manganese oxide has become a star material in the field of supercapacitors due to its advantages such as low cost,high theoretical capacitance and large voltage window in water system electrolyte.However,its further development is hindered by the poor conductivity of manganese oxide electrode material and the instability in the cycle process.First,the poor electrical conductivity of manganese oxide limits its rapid ion and electron transport,thus limiting its electrochemical properties.In this paper,porous biomass carbon（BC）from grapefruit peel was prepared by pyrolytic activation method,and then sea urchin-like MnO₂/BC nanocomposites were prepared by combining it with MnO₂.The structure and morphology of the product were characterized by XRD,SEM,TEM and nitrogen adsorption/desorption.The electrochemical performance shows that the specific capacitance of MnO₂/BC composites can reach 205.5 F g^-1 when the charge-discharge current density is 0.5 A g^-1.At the same time,compared with MnO₂,the lower resistance indicates that the introduction of biomass carbon overcomes the disadvantage of poor electrical conductivity of MnO₂,indicating that the high performance MnO₂/BC electrode material has been successfully obtained.Secondly,the electronic structure of manganese oxides can be adjusted by properly adjusting the content of Mn^x+in the mixed valence manganese oxides to further improve their electrical conductivity and properties.In this chapter,the internal electronic structure of the mixed valence manganese oxides is adjusted through the over-reduction（OR）strategy,and manganese oxides with different Mn²⁺/Mn³⁺ratios are prepared,which makes these oxides become a good platform for studying the structure-property relationship.The Mn²⁺/Mn³⁺ratio of manganese oxides can be accurately controlled between 0.6 and 1.7 by controlling the amount of reducing agent to control the redox process.The most suitable Mn²⁺/Mn³⁺ratio Mn oxide electrode OR4 shows a high specific capacitance of 274 F g^-1.The asymmetric supercapacitor constructed by combining OR4（positive electrode）and commercial activated carbon（negative electrode）achieves a high energy density of 27.7 Wh·kg^-1 due to its large voltage window of 2.0 V.Because of the Jahn-Teller distortion of Mn（III）O₆ octahedron,the cycle life of OR4//AC has been maintained at about 92.9%after 10,000 cycle tests,making it more competitive than other work.In addition,two all-solid ultracapacitor devices connected in series can easily light a red LED for 15 minutes.Finally,we directly grow binderless electrode materials MnO₂/CC and MnO₂/g-C₃N₄/CC on the surface of flexible carbon cloth by simple hydrothermal method.The effect of g-C₃N₄ on the electrochemical performance of supercapacitors was investigated by adding ultra-thin nanosheets of g-C₃N₄.Through electrochemical test,we obtained that the specific capacitance of MnO₂/g-C₃N₄/CC of composite g-C₃N₄ultra-thin nanosheets was higher than that of MnO₂/CC.Asymmetric supercapacitors composed of MnO₂/g-C₃N₄/CC and Fe₂O₃/CC also show higher energy density.The significant improvement of electrochemical performance may be attributed to its self-supporting binder-free characteristic.Meanwhile,after the introduction of g-C₃N₄ nanosheets,theπ-πconjugate effect between g-C₃N₄ and carbon cloth was formed,which enhanced the stability of electrode materials.