The Preparation Of Nanostructured Manganese Dioxide As The Electrode Materials Of Electrochemical Capacitors

MnO2 has become one of the promising alternative candidates as the electrode materials for supercapacitors, because of its abundance, low cost and environmental friendliness. However, there are still several problems to be overcome for the application of MnO2 in supercapacitor:(1) the real capacitance is much lower than the theoretic capacitance, (2) lower rate capacibility because of low electronic conductivity of MnO2 materials, (3) the cycling instability caused by the dissolution of Mn. At present, the researches on MnO2 supercapacitor are focused on preparation of nanostructured MnO2 or MnO2 films to improve the electrochemical performances. Based on literatures, the objective of this study is to find the factors influencing the electrochemical performances of MnO2 insupercapacitor, such as BET area, electronic conductivity and valence of manganese. The main contents of this thesis include:(1)α-andγ-MnO2 were synthesized by co-precipitation and ultrasonic method using Pluronic P123 and sodium dodecyl sulfate (SDS) as surfactants. Nanowires were obtained by co-precipitation method using P123 as surfactant, nanosheet by ultrasonic method using SDS as surfactant and nanorod by co-precipitation method using SDS as surfactant. The BET results showed that higher BET area can be obtained via the surfactant-assisted process, and the highest BET area is 279 m2 g-1, when using 0.02% P123 as surfactant. From the plot of surfactant's influence on the BET area and discharge capacitance, it can be observed that BET areas of samples increase with increasing of concentration of P123 firstly and further increasing of P123 resulted in decrease of BET surface area. The electrochemical tests showed that the discharge capacitance has the same tendency as BET area, and the polymorphic forms of MnO2 only play a minor role.(2) A series of mesoporous manganese oxides were synthesized and their electrochemical properties were tested. The precursor of manganese oxalate was prepared via solid-state reaction method. From XRD, SEM and TG-DTA analysis, the as-synthesized manganese oxalate precursor is in pure phase and has morphology of nanoplates. The decomposition of precursor occurs at 245℃. The influence of calcined temperature and time on the formation and diameter of mesoporous was discussed. After calcination at 200℃for 5 h, the mesoporous MnO2 was obtained which was confirmed by TEM. The BET areas of samples increase with increasing of calcination temperature and time and during heat treatment, the formation of mesoporous structure is closely related with the release of CO2 and H2O.(3) The relationship between the conductivity of electrode and electrochemical performances of MnO2 electrode was investigated. Poor crystallinedα-MnO2 grown on multi-walled carbon nanotubes (MWCNTs) by reducing KMnO4 in ethanol was characterized by XRD, SEM and BET surface area measurement. The results showed that MWCNTs is wrapped up by poor crystalline MnO2 and BET areas of the composites maintain the same level of 200 m2g-1 as the content of MWCNTs in the range of 0-30%. The discharge capacitance of carbon nanotubes is much lower than manganese oxide. At lower content of MWCNTs, the electrode exhibits a high polarization, but at higher content, the capacitance decreases. The effect of additional conductive agent KS6 on the electrochemical behavior of the composites was also studied. With a fixed carbon content of 25% (MWCNTs included), MnO2 with 20% MWCNTs and 5% KS6 showed the highest specific capacitance, excellent cyclability and best rate capability, which gives the specific capacitance of 171 F g-1 at a scan rate of 5 mV s-1, and remains 114.6 F g-1 at 100 mV s-1.(4) Two heating processes, heating up solid-state reactions and drying products at various temperatures were applied to prepare MnO2. From XRD, SEM and XPS, it can be concluded that the crystalline is strengthened by both heat treatments and the manganese valence is more closed to 4 for the sampled with the solid-state reaction at 80℃. The sample synthesized at 80℃has the specific capacitance of 193 F g-1 at a current density of 0.5 A cm-2. The factors influencing its electrochemical behavior such as microstructure, BET surface area and the valence of manganese were studied.