Intensified Synthesis And Electrochemical Properties Of MnO₂Supercapacitor Electrode Materials

Manganese dioxide is one of the most promising electrode materials for supercapacitors. So far, the energy density of supercapacitors is too small to compete with that of rechargeable batteries. According to the energy storage mechanism, improving the energy storage capability of manganese dioxide can be achieved by designing manganese dioxide powers with large specific surface area and proper pore structure. In this research, assisted by ultrasound-microwave and micro-impinging, manganese dioxide of mesoporous sphere-network structure and uniform ultra-fine spheres were synthesized, respectively. The as-prepared samples are of large specific surface area and high specific capacitance.Firstly, by applying ultrasound and microwave in the polymer P123templating redox reaction between manganese sulfate and potassium permanganate, sphere-network manganese dioxide with mesopores were synthesized. Ultrasound was used to mix P123-ethanol solution and manganese sulfate solution, and it turned out that ultrasound had direct effects on the template shape of P123. Only mixed by certain power of ultrasound can the template introducing sphere-network structure be formed. During the synthesis process, the gradientless temperature field created by microwave and the ultrasound of proper intensity ensure the manganese dioxide be precipitated and grow on P123templates, creating mesoporous sphere-network. The porous structure results in a specific surface area as large as245m2g-1and allows ions to move from electrolyte to electrode surface much more easily. Therefore, the specific capacitance of this porous manganese dioxide material reached222F g-1.Second, by adjusting pH, reaction time of the redox between methenamine and potassium permanganate to prepare manganese dioxide under ultrasound and microwave interactions, the synthesis time was reduced from more than3hours to10minutes and the reaction temperature also dropped down from95℃to50℃. The effects of reaction conditions (reaction temperature, the amount of sulfuric acid, reaction time and dispersing intensity) on morphologies and the specific capacitances were investigated. According to the analysis of ultrasound and microwave intensification and the precipitation process, the dispersing behavior of the added sulfuric acid is one of the critical parameters which decide the synthesis process,Finally, micro-impinging was applied in the reaction process of methenamine and potassium permanganate. In this way, the added sulfuric acid was dispersed much more homogenously and immediately by micro-impinging and therefore uniform manganese dioxide ultrafine powders of200nm were formed. The reactant ratio, the amount of sulfuric acid, the dispersing method and the aging time were investigated, respectively, and the optimal reaction conditions were obtained. The specific capacitance of the micro-impinging sample measured by constant current charging discharging test was230F g-1.