Green Synthesis Of Manganese Oxide Nanomaterials And Their Application As Electrode Materials Of Supercapacitor

Ruthenium dioxide?RuO₂? and other precious metal oxides have been applied to supercapacitor electrode materials successfully due to their excellent electrochemical performance. But high cost and innate toxicity limit their wide application. Therefore, scientists are engaging in finding and developing new types of electrode materials for supercapacitor with higher cost performance ratios based on other base metal oxides such as nickel oxide, cobalt oxide and manganese and so on. Owing to low cost, environment-friendliness, high theoretical specific capacitance and wide working potential window, manganese oxide is expected to become an alternative of ruthenium dioxide.Manganese is abundant in nature, and has a variety of oxidation states such as?MnO₂, Mn₂O₃ and Mn₃O₄?. However, poor electroconductivity and cycle stability of MnO₂ seriously weaken its capacitance performance. Conductive carbon materials are usually added into MnO₂ to improve its electric conductivity. Mn₃O₄ has also attracted the wide attention as the electrode material for supercapacitor, that has an unique structure of hausmannite at room temperature. At present there have been many methods to be employed for preparing manganese oxides, but most of them need to be carried out at high temperature and high pressure. Weighty conditions limit their mass production. The preparation methods for manganese oxide varied, but most will be subject to the conditions of high temperature and high pressure. Therefore, in this thesis, we tried to study and develop some simple, easily-operated and environment-friendly methods to prepare nanoscale manganese oxides and relevant composites, characterized their structures and morphologies with IR, XRD, SEM TG etc. and investigated their electrochemical capacitance performances. The thesis consists of four parts:Chapter 1. The structures, principles for energy storage and characters of supercapacitors were introduced briefly and the advances on study of preparation methods of the electrode materials for supercapacitors were reviewed in detail.Chapter 2.With graphene oxide as raw materials, potassium permanganate as source of manganese source and glycol as a reductant agent, some nanocomposites of grapheme and MnO₂ nanoparticles were successfully prepared by using a hydrothermal method at 80?. When the molar ratio of Mn: C equaled to 1:0.8, the nanocomposite had the best capacitance performance and specific capacitance reached 224F/g at the current density of 0.5A/g. The structural analysis had proved that approximate amorphous MnO₂ nanoparticles dispersed uniquely on grapheme sheets in the composite, which was very advantageous to diffusion of the electrolyte in order to enhance specific capacitance of the electrode material.Chapter 3. Using KMnO₄ as the source of manganese, ethylene glycol as the reducing agent, SDS as a structural controller, MnO₂ nanoflakes were synthesized by a hydrothermal method. Electrochemical tests showed that when the current density was 1 A/g, the specific capacitance value is 184 F/g. At a current density, the prepared material still maintained a high specific capacitance retention rate, indicating that material has a good ratio capability. When the current density was 5 A/g, after circulation charge and discharge for 1600 cycles, the specific capacitance did not decay, which suggested the material has good cycle stability. The excellent electrochemical performance is attributed to a well-dispersed nanoflake of morphology with a low crystallinity and high specific surface area.Chapter 4. Using potassium permanganate as manganese source, glucose as the reducing agent, Mn₃O₄ nanomaterials with different morphologies were fabricated respectively by hydrothermal method and microwave assisted method. Mn₃O₄ nanocrystals obtained by the microwave assisted procedure showed a morphology of octahedral nanoblock, and the materials prepare by the hydrothermal method consisted of nanorods and irregular structural nanoblocks. Electrochemical test indicated that the specific capacitance of the hydrothermal preparation of Mn₃O₄ was higher than the microwave assisted synthesis of Mn₃O₄.