Synthesis And Characterization Of Manganese Oxide And Its Hierarchical Composite For High Performance Electrochemical Electrodes

Manganese oxide is one of the most promising pseudocapacitance electrode materials due to its high specific capacitance, environmental compatibility and cost effectiveness. Up to now, many manganese oxides with various structures have been extensively developed, using various structures including nanowires, nanotubes, and nanoflakes prepared via electrochemical and chemical routes to enhance electrochemical properties. The investigated materials mainly focus on the amorphous or poorly crystallized manganese oxides, single-crystalline manganese oxides and manganese oxide thin films because these material cathodes show ideally capacitive behavior such as large capacity, good electrochemical reversibility and high pulse-power property in a potential window of1.0V in Na2SO4solution. The practical use of MnO2as electrode materials is largely weight-down due to its poor electrical conductivity (10-5-10-6S/cm), which limits the charge-discharge rate of the electrochemical capacitance; as well as a relatively small surface area of bulk MnO2, which constraints the energy that can be stored in a particular electrode. The major strategy to improve the electrical conductivity of the MnO2electrodes is to combine conductive materials (e.g., carbon-based materials, metals, and oxides) to MnO2, forming MnO2composite electrodes with improved conductivitie. Taken together, new designs of high-performance MnO2-based electrodes must feature with a desired combination of small-sized MnO2nanoparticles and a good electrical conductivity with the help from the conductive additives.In this study, MnO2, CuBi2O4-doped MnO2, hierarchical TiO2/Cu2O-Mn3O4nanoflakes composite have been synthesized by chemical bath deposition (CBD) technique and successive ionic layer adsorption and reaction (SILAR) method. The effects of doping, substrates influence of the reaction bath time and bath temperature and synergistic of hierarchical ternary composite on electrochemical properties of MnO2, CuBi2O4-doped MnO2thin films and TiO2/Cu2O-Mn3O4nanoflakes were investigated. The significant results achieved in this dissertation are given as follows:1) Manganese dioxide (MnO2) films with different nanostructures were deposited on indium tin oxide (ITO) glasses by using chemical bath deposition (CBD). Deposition temperature and time were varied from60℃to90℃and from2h to72h, respectively. The films deposited at60℃for8h showed that obtained nanoflowers had an amorphous nature, while those deposited at higher temperatures of70,80and90℃showed a well-developed nanowire and nanorod morphology. However, those which were deposited at60℃showed the best electrochemical properties, including a higher specific capacitance, good rate of performance and a cycling stability (93%loss of the initial value after10,000cycles).2) Manganese dioxide (MnO2) and CuBi2O4-doped MnO2thin films with different nanostructures were deposited on indium tin oxide (ITO) glass and Ti foil substrates by using chemical bath deposition (CBD) technique. The effects of doping and substrates on electrochemical properties of MnO2and CuBi2O4-doped MnO2thin films on ITO glass and Ti foil were investigated. Capacitive properties of MnO2and CuBi2O4-doped MnO2thin films electrodes were studied using cyclic voltammetry and electrochemical impedance spectroscopy in a three-electrode experimental setup using0.1M Na2SO4aqueous solution as electrolyte. Specific capacitance, obtained from electrochemical measurement for the CuBi2O4-doped MnO2exhibited a higher value of338F g-1 compared to the MnO2exhibits value of135F g-1. Additionally, CuBi2O4-doped MnO2thin films on ITO electrode had a better and satisfactory specific capacitance value, and exhibited more excellent electrochemical stability and reversibility than Ti foil substrates.3) The novel successive ionic layer adsorption and reaction (SILAR) method has been developed for the synthesis of hierarchical nanostructured material by growing Cu2O-Mn3O4nanoflakes into TiO2crumpled-like paper ball micro-nanospheres. This was prepared using hydrothermal method on Ti foil substrate. The nanocomposites of TiO2/Cu2O-Mn3O4were strategically combined into a single entity to synergize and construct a high electrochemical performance of pseudocapacitors. The specific capacitance of the ternary composite electrode exhibited a capacitance of259F g-1at3A g-1. With an excellent cycling performance of no capacity loss over5000cycles, it was better than those of TiO2/Mn3O4, TiO2/Cu2O and other individual components of Mn3O4.