Electrodeposition Controling Synthesis And Electrochemical Properties Of Nanostructured MnO₂

Manganese dioxide was widely used in the field of chemical power, separation and catalysis as a common manganese oxide for its special structure and performance. In order to synthesize the manganese with particular structure and morphology, its preparation methods has been researched widely and deeply. The method of electrochemical deposition attracts common attention for the reason of its gentle preparation conditions, controllable components and morphology, and high crystallization capacity. Changing the electrodepositing conditions of temperature, pH value, electrolyte type and current density will deeply influence the components, Mn average degree of oxidation, crystal structure, and morphology of the product.Electrocapacitors is a new type of electrochemical energy storage devices. They have high power density, high charge/discharge efficiency and long cycle life in comparison with rechargeable batteries and traditional electrostatic capacitors. The study of super capacitors mainly concentrated on the electrode materials. Amongst so many kind of electrode materials, manganese dioxide aroused most attention because of its low price, environmental friendly and excellent performance, which is considered as the most potential electrode material of electrochemical capacitors.Through changing the reaction conditions such as temperature, voltage, concentration of H⁺ and mixing Mg²⁺, we prepared a series of different structures and morphologies of MnO₂ by electrolysis in 0.05 mol/L MnCl₂ electrolyte, and selected the representative MnO₂ as the electrode materials of electrocapacitors and took measure of discharging specific capacitance of these MnO₂ by constant-current charge/discharge experiment. According to the experimental results, we discussed the effects of temperature, voltage, H⁺ ion concentration and mixing Mg²⁺ for the stucture, morphology, particle size, specific surface area and specific capacitance of deposited MnO₂.The experimental results are as follows:1. The crystal structures of electrolytic products were mainly affected by temperature and concentration of H⁺.α-MnO₂ can be prepared only in the conditions of 100℃and C[HC1]=2 mol/L, and in the other electrolyse conditions of temperature and concentration of H⁺, all the MnO₂ prepared are similar to Nsutite (γ-MnO₂).2. The morphology, particle size and specific surface area of EMD were mainly affected by temperature, voltage and concentration of H⁺, has the following rules:The morphology of EMD showed a rule of phasic change as the voltage rising when the temperature and concentration of H⁺ in a certain condition.The particle size of EMD is mainly controlled by temperature. Generally, the particle size was smaller (between 30 nm～60 nm) at high temperature than that of at low temperature.The specific surface area of EMD regularly affected by the changes of temperature and voltage: at the same temperature, the specific surface area of EMD was increased with the rise of voltage; in the same voltage, the specific surface area of EMD was decreased with the rise of temperature3. The representative EMD were used as the active material of the positive electrode for electrocapacitors and showed stable cycle properties and capacitance characteristics through constant current charging/discharging experiments at the current density of 200 mA/g and under the voltage window of 0 V～0.8 V. According to the specific capacitance of EMD prepared in different conditions, it was found that the deposited conditions of low temperature and high voltage are good for increasing the specific capacitance of EMD. The largest specific capacitance of EMD which was prepared in 1.1 V at 35℃was near 133.5 F/g.