Preparation And Electrochemical Properties Of Nanostructured Manganese Dioxide Used For Supercapacitor

Supercapacitors have been extensively studied as the energy storage devices. And the electrode material has been the focus of research and exploitation in the field of supercapacitor. It is the transition metal oxide manganese dioxide that has been applied widely as the electrode material. So we chose inexpensive manganese dioxide as our target product used for the supercapacitor electrode material. And a series of modern physical methods are taken to characterize the properties of the prepared materials. Power X-ray diffraction (XRD), scanning electron microscope (SEM) and N2 adsorption and desorption measurements are employed to investigate crystalline structure, surface morphology, the specific surface area and the pore size distribution. And it is discussed that how the synthesized conditions and the heat-treatment effect the capacitance of manganese dioxide materials.Different nanostructured manganese dioxide was prepared by hydrothermal and oxidation reduction method. The major research contents are as follows:First.α-MnO₂ powders were prepared by hydrothermal, in which KMnO₄ oxidized the reductant (CH₃COO)₂Mn that leads the formation of MnO₂. The SEM pictures depict the typical nano-rod of morphologies, and the nano-rods show a uniform distribution. The effect on characteristics of materials by reaction time and temperature were studied. The results show that the better capacitance and the large aspect ratio are obtained when the reaction temperature is 140℃and the reaction time is 2 h. The best specific capacitance is 145 F/g. The nanorods have a diameter about 20-40 nm and the lengths up to a region of hundreds of nanometers to several micrometers. The specific capacitance decreased as the reaction time prolonged, and the charge transfer resistance increased.Second. Oxalic acid was used as reductant to prepare MnO₂ powders. And the electrochemical properties were studied. The XRD results indicate that the product is amorphousα-MnO₂. The SEM pictures show that the surface morphologies are consist of nano-spheres. The optimal prepared conditions are as follows: reaction temperature is 60℃, reaction time is 3h and the molar ratio is KMnO₄:H₂C₂O₄=2:3. The specific capacitance of the product is 158 F/g. The specific capacitance increased by 68%, from 158 F/g to 265 F/g, when the product was heat-treated at 400℃for 2h. And the influence of different electrolyte on the capacitance property was studied. The cyclic voltammetry results indicate that neutral electrolyte is more suitable for the MnO₂ electrode working than basic electrolyte.Last. Amorphous manganese dioxide is prepared by reaction of potassium permanganate with an organic reductant triethanolamine for the first time. XRD and SEM results show that the prepared material is nanoscaled MnO₂ powders, but there is some particle agglomeration. The effect of heat-treatment temperature is studied on the characteristics of the materials. It is found that when the heat-treatment temperature rising to 400℃, the crystalline convert toα-MnO₂ from amorphous MnO₂. The electrochemical characteristics of the prepared MnO₂ powder are characterized by means of cyclic voltammetry experiments in 1 mol/L Na₂SO₄ electrolyte. The specific capacitance value is 251 F/g that is obtained from the product annealing at 350℃at a scan rate of 2 mV/s. And charging-discharging measurement reveals the good stability of the prepared material.