Synthesis Of MnO₂@Carbon Nanocomposites And Their Electrochemical Performance For Supercapacitors

The global energy crisis caused by the rapid economic growth has attracted intense attention to the rational use of energy. Energy storage, as an effective approach of energy utilization, has been one of the most important issues in the world. As a new type of energy storage component, supercapacitors have been deemed to the greatest potential in information domain, transportation and eletronic field for their excellent rate ability, high power density and long cyclic life. The electrochemical properties are often influenced by the liveness and kinetic characteristics of the electrode materials. Transition metal oxides are considered to be the most promising materials for supercapacitors for their high theoretical specific capacitance and favourable reversibility. Among them, manganese oxides were, in particular, attractive due to their high theoretical specific capacitance, low cost and environmental friendliness. However, the conglomeration of Mn O2 nanoparticles and invalid contact with electrolyte led to the poor capacitance performance due to the noneffective utilization of manganese atom.In this work, various C-Mn O2 nanostructures prepared by using different carbon substrate has been synthesized and tested as electrode materials. The purposes of these designs are to enhance the electric conductivity and the utilization of manganese atoms. The crystal structure, morphologies and specific surfaces area were investigated by X-ray diffraction, focused ion beam scanning electron microscopy, transmission electron microscopy and N2 adsorption-desorption measurement. Electrochemical properties were characterized by cyclic voltammetry(CV), galvanostatic chargedischarge and electrochemical impendance spectroscopy(EIS). The research content and the main conclusion are summarized as below:Three kinds of nanostructures with ultrathin Mn O2 nanosheets wrapping carbon spheres forming core-shell yolk-shell and hollow structures have been prepared by tailoring the concentration of KMn O4 solutions. When tested as electrode materials, the yolk-shell structure obtained the highest capacitance of 273 F g-1 with favourable rate ability and cycling abilities(85.9% remain after 1000 cycles).A series of mesoporous carbon coupled with Mn O2 nanocomposites has been synthesized by tailoring the p H of the mixture resulting in the regulation of the reaction between carbon and KMn O4. The nanocomposites kept the mesoporous features resulting in a high specific surfaces area. When tested as electrode materials, the nanocomposites exhibited excellent rate ability. Furthermore, the capacitance is enlarged to 122.8 % after 1000 cycles.The AMC-Mn O2 nanocomposites were prepared by hydrothermal reaction with KMn O4, in which AMC were utilized as the template. The AMC were wrapped with ultrathin Mn O2 nanosheets uniformly resulting in a high specific surfaces area and excellent electrical conductivity. The electrochemical measurements showed that the capacitance of AMC-Mn O2 nanocomposites wais 297.8 F g-1 and retained about 50.3% of its in initial capacity when the current density increasing from 0.2 A g-1 to 20 A g-1. When assembled as an asymmetric supercapacitor, an energy density of 18.4 Wh kg-1 were obtained, rendering the AMC-Mn O2 nanocomposites promising as the electrode materials for high performance supercapacitors.Coaxial mesoporous Mn O2/amorphous-carbon nanotubes have been synthesized via a facile and cost-effective strategy at room temperature. The coaxial double nanotubes of inner(outer) Mn O2 and outer(inner) amorphous carbon can be obtained via fine tuning the preparative factors(e.g., deposition order and processing temperature). The electrochemical test showed the capacitance of the nanotubes was 362 F g-1 and retained about 41.7% of its in initial capacity when the current density increasing from 0.5 A g-1 to 20 A g-1. When assembled as an asymmetric supercapacitor, an energy density of 22.56 Wh kg-1 were obtained at a power density of 224.9 W kg-1, rendering the coaxial mesoporous Mn O2/amorphous-carbon nanotubes promising as the electrode materials for high performance supercapacitors.In summary, these facile synthetic strategies could be useful to design other novel carbon-Mn O2 nanostructures. Simultaneously, the obtained nanocomposites in this work exhibited excellent electrochemical performance and they could be the optimized nanomaterials for supercapacitors.