Construction And Capacitive Properties Of High Performance Manganese Oxide Nanosheet Electrode System

As a type of electrode materials for the new energy storage device of supercapacitor,manganese oxide has attracted extensive attention from researchers at home and abroad due to its rich content,environmental friendliness and high theoretical specific capacitance.This paper scientifically summarizes the research progress of supercapacitors,and focuses on the research status of manganese oxide materials in the application of supercapacitors.High capacitance and long-term cycling stability are two major issues in the application of manganese oxide electrodes in supercapacitors.In this paper,in-depth study on the preparation of manganese oxide nanosheet electrodes together with the design of electrolytes have been carried out.A high performance manganese oxide electrode system has been constructed.The specific works are listed as follows:?1?Mn₃O₄ nanospheres are grown on nickel foam by hydrothermal reaction in an alcoholic water system.The spinel Mn₃O₄ nanospheres agglomerated by nanoparticles with smaller size have a rough surface,which is beneficial to electrolyte immersion and ion diffusion.The Mn₃O₄ nanospheres are directly coated on the surface of nickel foam having a close contact with the current collector,which can effectively promote charge transport.Mn₃O₄ nanospheres electrode exhibits a good specific capacitance(117.8 F g^-1 at 1 A g^-1)and a good rate capability(46.2%retention at 30 A g^-1).During the charge and discharge cycle,Mn₃O₄ nanospheres electrode shows twice increase on the specific capacitance.And nanospheres are in-situ transformed into nanosheets.?2?Manganese oxide nanosheets assembled porous honeycomb architecture is in situ formed on nickel foam by electrochemical induction of Mn₃O₄ nanospheres electrode in Na₂SO₄ electrolyte.The nanosheets morphology and the capacitive performance of the electrode are optimized by adjusting the parameters in the electrochemical induction process,including potential range,current and number of cycles.The nanosheets formation is studied by scanning electron microscopy.And a phase transition from spinel to layered structure is proposed through electrochemical study and other physicochemical characterizations.The optimal honeycomb manganese oxide nanosheets electrode exhibits a high specific capacitance of 376.6 F g^-1 at 1 A g^-1,which is more than three times higher than the initial value.And it exhibits a superior rate capability of 60%retention at 30 A g^-1.This can be attributed to the wide layered interspace of manganese oxide,the pre-intercalation of H₂O and Na⁺in the layered structure after electrochemical induction,and its unique nanosheets assembled porous honeycomb architecture.?3?In-situ electrochemical induction of Mn₃O₄ electrode is carried out in three different cationic electrolytes,including Li₂SO₄,MgSO₄ and K₂SO₄.Based on the electrochemical induction in Na₂SO₄,the effect of electrolyte cation on electrochemical induction and electrochemical performance of the electrode are investigated.In different cationic electrolytes,phase transition efficiency of Mn₃O₄ and peeling-off process of manganese oxide nanosheets are affected by electrolyte cations,thereby preparing nanosheets with different lateral dimensions.The lateral dimensions of the nanosheets increase sequentially in the order of Li⁺,K⁺,Na⁺to Mg²⁺.At the same time,the electrodes exhibit different capacitive properties and cycling performance.The specific capacitances of the electrodes increase sequentially in the order of Li⁺,K⁺,Na⁺to Mg²⁺,and the cycling stabilities decrease in this order.The electrode exhibits considerable capacitance and cycling performance in K₂SO₄electrolyte,which is a preferred electrolyte for electrochemical induction.?4?The cycling stability of manganese oxide nanosheets electrode is greatly enhanced by pre-adding a small amount of MnSO₄ in Na₂SO₄ electrolyte.The cycling performance of the electrode and the morphology of birnessite change with Mn²⁺concentration,which are ascribed to the electrochemical oxidation of Mn²⁺or the reduction of manganese oxide during the charge/discharge cycling process.By controlling a low concentration of Mn²⁺,the electrode exhibits enhanced capacitance,while a hierarchical nanosheets assembled nanowall array architecture is achieved due to the secondary growth of manganese oxide nanosheets.Compared with the case of electrolyte without addition of Mn²⁺?64%retention after 4,000cycles?,enhanced cycling stability with capacitance retention of 93%after 10000 cycles is achieved resulting from the increase of Mn valence state and the unique hierarchical structure.