Preparation And Application Of Carbon Microspheres And Manganese Dioxide Nano Material For Supercapacitors

In recent years, numerous research activities have shifted to innovative a new type of carbon-based electrode materials, carbon microspheres（CMSs）, which is a candidate for outstanding electrode material of supercapacitors and Li-ion batteries. Furthermore, carbon microspheres are considered as a promising and competitive material owing to their high packing density, good conductivity, chemical stability, mechanical strength, good liquidity and high specific surface area. In the present thesis research, we well act up to the concept of green energy. The uniform and monodisperse carbon microspheres（MCMSs） were prepared on a large scale by combining low temperature hydrothermal and high temperature carbonization process, where cheap and renewable biomass, such as potato starch and glucose, was used as starting materials. The preparing processes without any chemicals render this method green and productive. In addition, manganese dioxide nano-materials with different morphology and manganese dioxide/graphene/porous carbon（MnO₂/rGO/C） composite materials were prepared. All the as-prepared materials were measured and evaluated in the three-electrode systems in details. Moreover, the asymmetric supercapacitor with high performances was assembled by choosing the appropriate potential window and electrolyte systems as well as matching the positive and negative electrodes. The main content is summarized as follows:1.Uniform and monodiesperse carbon microspheres（CMSs） are synthesized via a catalyst-free and hydrothermal process by following a high temperature carbonization, in which glucose is used as starting materials. The morphology, microstructure, components and surface features of the as-obtained samples were characterized using SEM, TEM, XRD, Raman, FTIR and BET. Furthermore,the electrochemical behaviors of the products were evaluated by cyclic voltammetry（CV）,galvanostatic charge-discharge, and electrochemical impedance spectroscopy（EIS）. The test results indicated that the microspheres with specific area as high as 409 m2 g-1 could form porous a network structure, which provided a fast path for the insertion and extraction of electrolyte ions. A specific capacitance of 280 F g-1was obtained in the potential window from-1.3 to-0.4 V at a current density of 1 A g-1 with excellent cycle stability（The specific capacitance values keep 87.7% of the initial value after 10,000 cycles at 3 A g-1）.2. Uniform and monodisperse carbon microspheres（MCMSs） are synthesized via a catalyst-freeand hydrothermal process by following a high temperature carbonization, in which potato starch is used as raw material. The morphology and microstructure of the as-obtained samples are characterized using SEM, TEM, XRD, Raman, FTIR and BET. TEM image shows that the products consist of carbon microspheres with a mean diameter of about 250 nm. BET specific surface area of MCMSs is up to 456 m2 g-1. Furthermore, the supercapacitive performances of the MCMSs are investigated by cyclic voltammogram（CV）, galvanostatic charge-discharge, and electrochemical impedance spectroscopy（EIS）. The experimental results indicate that the MCMSs exhibits not only high specific capacitance of 245 F g-1 and good rate capability（the specific capacitance at 10 A g-1 is 61% of that at1 A g-1）, but also excellent cycle stability（no obvious capacitance decay after 4,000 cycles at 3 A g-1）.An asymmetric electrochemical capacitor is assembled by using the obtained MCMSs as negative electrode and reduced graphene oxide（RGO） as positive electrode. The as-assembled MCMSs // RGO capacitor can cycle reversibly in a voltage of 0-1.5 V and give a high energy density of 21.5 Wh kg-1 at a power density of 759 W kg-1.3. Manganese dioxide electrode materials were prepared by the method of reduction from potassium permanganate. The morphology and microstructure of the as-obtained samples are characterized using X-ray diffraction（XRD）, infrared spectrum（FTIR） and field emission scanning electron microscopy（FESEM）. Furthermore, we used cyclic voltammograms, galvanostatic charge-discharge test methods to investigate the supercapacitive performances. The tests show that all the three manganese dioxide electrode materials with different microstructures exhibit high specific capacitance within wide potential window.4. We used solvent thermal method to synthesize hexagon flake manganese dioxide/graphene/porous carbon（MnO₂/rGO/C） composites. The morphology and microstructure of the as-obtained composites are characterized using SEM, XRD, FTIR TG and EDS. The electrochemical behaviors of the composites were evaluated by cyclic voltammetry（CV）,galvanostatic charge-discharge and electrochemical impedance spectroscopy（EIS）. The test results indicated that the composites showed good electrochemical performanc.