Study On Preperation And Electrochemical Characteristics Of MnO₂Electrode Material Foraqueous Supercapacitors

The manganese dioxide（MnO₂） has been considered as one of the supercapacitorelectrode materials duo to its low cost, environment friendly nature, wide dischargepotential window, and superior capacitance performance. On account of employingneutral inorganic electrolyte, the MnO₂supercapacitors are more secure andconvenient than other electrode materials, such as acitived carbon, in process ofassembling and application. In the dissertation, the massive MnO₂was synthesized byanode electro-deposition, and was nano-fefined through ball grinding method. Thestructures, particle sizes and morphologies of the as-synthesized MnO₂werecharacterized by XRD, TG/DSC, powder coulter, FE-SEM （Field Emission ScanningElectron Microscopy）, et al. The electrochemical performances of MnO₂eletrodeswere evaluated by cyclic voltammetry（CV） and galvanostatic charge-discharge（TV）in0.5mol/L Na2SO₄solution.The massive MnO₂was milled from5hours to80hours with the rotate of250r/min. With the increase of milling time, the MnO₂average grain diameter firstdcreased rapidly and then increased. The MnO₂milled for10hours exhibited aminimum size of about200nm. The electrochemical measurement showed that, withthe increasing of milling time, the specific capacitance of MnO₂electrode firstincreased and then decreased from5hours to20hours. When MnO₂milled more than20hours, the milled time had hardly influence on the specific capacitance. When themilling time was equal to10hours, MnO₂eletrode processed a maximum specificcapacitance of55F/g at a constant current of25mA/g. After10000cycles forcharge-discharge at the current density of25mA/g, the specific capacitanceattenuation was less than2%, and the charge-discharge efficiency keep at98.5%.The MnO₂power of milling10hours was prepared by heat treatment from200℃to560℃. With the increasing of temperature from room temperature to450℃,the MnO₂gradually turned from γ o β type.When temperature increased to560℃,MnO₂transformed to Mn2O3.With the rising of the heat treatment temperature, thespecific capacitance of MnO₂electrode first increased and then decreased. Through300℃heat treatmented, MnO₂eletrode processed a maximum specific capacitanceof85F/g at a constant current of25mA/g, compared with the intally MnO₂electrodecapacitance increased by54.5%. After10000cycles for charge-discharge at thecurrent density of25mA/g, the capacitance attenuation was less than3%, and thecharge-discharge efficiency keep98.5%. Composite electrode materials with different ratios were formed by mixingMnO₂（10h,300℃） and activated carbon （AC）. The results showed that with thecontent of AC increased, the specific capacitance complex electrode first increasedand then decreased. When the mass ratio of MnO₂to AC was6:4, MnO₂eletrodeprocessed a maximum specific capacitance of103.7F/g at a constant current of100mA/g, compared with the pure MnO₂electrode capacitance increased by23%.Hybrid supercapacitor MnO₂-Na2SO₄-AC was assembled through using MnO₂aspositive electrode, AC electrode as negative electrode, and0.5mol/L Na2SO₄aselectrolyte solution. There was2.0V operating voltage of the hybrid supercapacitor.The specific capacitance and specific energy of hybrid supercapacitor reached10.2F/g and5.5Wh/Kg at a constant current of100mA/g, respectively. After20000cycles for charge-discharge at the current density of100mA/g, the capacitanceattenuation was only10%, and the charge-discharge efficiency keep95%.