Preparation Of Nanostructured Manganese Dioxide-based Composites And Their Electrochemical Properties

Manganese dioxides （MnO₂） hold a significant promise as electrode materials forsupercapacitors, but the electrochemical performance is ultimately limited by its poorelectrical conductivity. It is therefore the aim of this work to fully exploit the potentialof MnO₂-based electrode materials. The dissertation proposes strategic designs andfabrication of high-performance nanocomposites by incorporating MnO₂nanostructuresinto conducting materials, and then explores the close relationship between synthesistechnique, microstucture and the electrochemical properties. From a point view ofelectrode configuration based on these new electrode materials, asymmetricsupercapacitors are finally constructed to harvest both high power density and highenergy density.In view of the good pseudocapacive behavior and good electrical conductivity ofconducting polymers, two kinds of MnO₂/conducting polymers nanocomposites wereprepared. Firstly, mesoporous MnO₂/polyaniline composites with unique morphology ofhierarchical hollow submicron spheres were prepared by modified interfacial synthesis.The three-dimentional architecture provides not only the “ion-buffering reservoirs” forrapid Faradic reactions but also the rich and uniform meso-channels for fast electrolyticdiffusion. The hierarchical nanocomposites electrode exhibits high specific capacitanceof262F/g with good rate capability. Secondly, a novel approach based on a reactivetemplate of MnO₂was proposed for the first time to in-situ polymerize polypyrrolenanostructures. By controlling the molar ratio of MnO₂and pyrrole monomers,one-dimentional nanotubular MnO₂/polypyrrole nanocomposites were prepared. Andthe synergistic effect between each component ensures a maximum specific capacitanceof318F/g with excellent rate capability and cycling stability.Electrospun carbon nanofibers （CNF） fabric was used as conductive substrates foruniform grow MnO₂nanostructures on the surface of CNF by in-situ redox deposition.The as-prepared MnO₂@CNF nanocomposites showed unique coaxial configurationand exhibited good electrochemical performance when used as freestanding electrodes.Afterwards, three effective sythesis techniques were developed to further improve theutilization of MnO₂nanostructures, including the introduction of porous, ultrathinMnO₂nanflakes, the increase in the electrical conductivity of CNF cores by embedding carbon nanotubes and the nano-combination with polypyrrole. The three resultedfreestanding nanocomposites electrodes showed better electrochemical prepertiescompared to the counterparts. Moreover, the effect of temperature on thepseudo-capacitive behavior of the freestanding MnO₂@CNF was extensively studied.Finally, a novel aqueous asymmetric sucapacitors was proposed and designedrationally by using two completely freestanding electrodes, i.e., MnO₂@CNF as thepositive electrode and activated CNF as the negative electrode. The influence ofpositive electrode and negative electrode on the electrochemical performance ofthe asymmetric supercapacitors was investigated. The as-assembled asymmetricsupercapacitor with optimal mass ratio can be operated reversibly over a widevoltage range of0-2.0V, and presented both high energy density of30.6Wh/kgand high power density of20.8kW/kg. The high-performance asymmetricsupercapacitors based on freestanding electrodes may find great applications inthe increasing demands on energy storage systems with high energy/powerdelivery.