Study On The Preparation And Electrochemical Performance Of Manganese Dioxide And Its Composites

In this paper,the current developments in characteristics of supercapacitor,electrode materials and corresponding applications have been reviewed.Meanwhile,MnO₂ in a variety of morphologies has been successfully synthesized by solvothermal and self-sacrificing template methods and used as electrode materials for supercapacitor.Besides,spheres C@MnO₂ composites were prepared by liquid redox reaction in different concentration of KMnO₄.The hollow spheres carbon@MnO₂?HC@MnO₂?were obtained by template method and redox reaction with KMnO₄ solution.The microstructure and morphology,composition were investigated by X-ray diffraction?XRD?,scanning electron microscope?SEM?,transmission electron microscope?TEM?.The specific surface area was confirmed by N₂ adsorption/desorption isotherm.Furthermore,cyclic voltammetry?CV?,galvanostatic charge/discharge?GCD?and electrochemical impedance spectroscopy?EIS?were applied with three-electrode configuration by electrochemical workstation.The main contents of this thesis are as follows:1.MnO₂ has been found to possess high specific capacitance.However,low conductivity limits its capacitive performance.Morphology control and nanocrystallization are effective approaches to improve the electrochemical properties.The precursor of manganese carbonate was prepared by solvothermal method.MnO₂ in different morphologies was obtained at the sacrifice of as-prepared MnCO₃ through thermal transformation manipulation.The result of XRD measurement indicated the high phase purity of the as-obtained MnCO₃ and MnO₂.The SEM photographs of MnO₂indicated that a framework microstructure consisting of many interlaced MnO₂ crystal grains was obtained.The sample of MnO₂ used as the electrode materials possessed high specific capacitance and excellent rate capability,which was ascribed to its porous hierarchical structural design.Electrochemical measurements show that the highest specific capacitance of as-prepared MnO₂ can reach up to 152 F g^-1 at the scan rate of 2mV s^-1.When the current density increased from 0.2 A g^-1 to 2 A g^-1,the specific capacitance decreased slightly,which proved that the electrode material had a good rate performance.After 1000 cycles,the capacitance retention can still maintain more than85%at the current density of 2 A g^-1,which indicates an excellent cycle performance.2.The proportion of C/Mn composition has a great influence on the electrochemical performance of C@MnO₂ composites.Well-dispersing carbon microspheres with uniform size have been prepared by hydrothermal-calcination method.C@MnO₂ in various mass ratio of C/Mn were prepared by liquid KMnO₄solution and as-prepared carbon spheres.XRD test results show that the as-prepared MnO₂ corresponds to the?-MnO₂.It can be observed from SEM images that the surface of carbon microspheres were covered by MnO₂ layer.Moreover,the coverage and thickness are different in diverse system.Electrochemical tests show that the conductivity of C/MnO₂ has been significantly improved compared with MnO₂ alone and the capacitive characteristics exhibited obviously.The highest specific capacitance of CSM-3 can reach up to 259 F g^-1 at the scan rate of 5 mV s^-1.In a limited concentration range,the specific capacitance of C/Mn composites increases with the ratio of C/Mn increasing.Nevertheless,the excessive manganese dioxide have adverse effects on the electrochemical performance of composites.After 400 cycles,the capacitance retention can still maintain more than 96%at the current density of 1 A g^-1,which indicates an excellent cycle performance.3.High specific area is beneficial to electric double layer capacitance?EDLC?,which is part of the total capacitance in C@MnO₂ composites.The colloidal silica sol adopted to be the hard template was coated by phenolic resin.Then the product was treated with carbonization at 600 ^oC and etched by NaOH solution.The hollow carbon spheres@MnO₂ composites with high specific surface were prepared by the redox reaction of hollow carbon spheres and KMnO₄ solution.XRD characterization results show that the prepared MnO₂ corresponds to the?-MnO₂.SEM and TEM measurements together identified the hollow structure of carbon microspheres.BET results show that the hollow carbon spheres alone possess a high specific surface area with 382 m² g^-1,while the composite materials?specific surface area is 152 m² g^-1.Electrochemical tests show that the conductivity of C/MnO₂ has been significantly improved compared with MnO₂ alone and the capacitive characteristics exhibited obviously.The composites possessed desired specific surface area and abundant porous structure,which resulted in a higher electronic double layer capacitance.The highest specific capacitance of composites can reach up to 258 F g^-1 at the scan rate of 2 mV s^-1.Meanwhile,the capacitance retention can still maintain more than 80%at the current density of 1 A g^-1 compared with the initial value,which indicated an excellent rate performance and remarkable capacitive behavior.After 1000 cycles,the capacitance decay rate was less than 3%at the current density of 1 A g^-1,which indicated an excellent cycle performance.