Preparation And Electrochemical Properties Of Trimanganese Tetraoxide Nanocrystal/graphene Composite Electrode Material

As a new energy storage device, electrochemical capacitor has attracted a great deal attention because of its rapid charge-discharge ability, long cycle life and high power density. Research results indicate that the capacitive property of electrochemical capacitor is influenced by the electrode materials, electrolyte, and the assembled devices, with the most important factor being the electrode materials. Therefore, what is a way to improve the capacitance of electrochemical capacitor is developing a kind of electrode materials with good capacitance. Mn₃O₄ has been widely used as electrode materials for electrochemical capacitor, which has a lot of advantages, such as high theoretical capacitance, environment-friendly and nature abundance. However, its electrochemical performance is limited due to its poor electrical conductivity, small surface area and low active utilization. In order to overcome these problems, the Mn₃O₄ can be made into nano-size and hybridized with carbon-based materials which have excellent electrical conductivity, like graphene. Therefore, the Mn₃O₄/graphene hybrid electrode materials with high capacitance are expected to be prepared. In this thesis, by using ion-exchange reaction, graphene nano-layer holey processing and high-temperature calcination process, manganese tetroxide nanocrystalline/reduced graphene ?holey graphene? hybrid electrode materials are prepared, and their morphology, structure and electrochemical peorperty are systematically investigated.The whole thesis consists of four chapters. The structure, energy storage principle, the electrode materials, and the application of the electrochemical capacitor are systematically reviewed, and also the structure, preparation methods and applications of graphene, Mn₃O₄, and Mn₃O₄/graphene hybrid materials are discussed in Chapter 1. The second part is the experimental parts and result discussion. By using ion exchange method and heat treatment, the nanocrystalline manganese tetroxide/reduced graphene electrode material ?Mn₃O₄/RGO? are prepared, and their structure, morphology and electrochemical properties are systematically investigated in Chapert 2. By using H₂O₂ as etchant and oxidant, nanoporous is introduced into graphene oxide nanosheets and holey graphene oxide is obtained, then the nanocrystalline manganese tetroxide/holey reduced graphene electrode material ?Mn₃O₄/HRGO? are prepared, and their structure, morphology and electrochemical properties are systematically investigated in Chapert 3. The conclusion of this thesis is given in Chapter 4.?1? The MnOx/graphene hybrid material is prepared by exchanging GO with Mn2+ ions and followed by calcining the Mn2+-exchanged material at different temperatures for 4 h in N2 atmosphere. The effect of the calcination temperatures on the phase and morphology of the obtained materials is investigated. The phase and size of MnOx nanocrystalline and reduction degree of GO in the hybrid material are affected by calcining temperature. The Mn₃O₄/RGO hybrid sample can be obtained at 450?, in which Mn₃O₄ nanocrystalline with average size of 13.5 nm are homogeneously distributed on the surface of RGO. The electrochemical properties of Mn₃O₄ nanocrystalline/RGO are investigated by cyclic voltammetry and galvanostatic charge-discharge in 1 M KOH electrolyte. Their electrochemical performance is highly structure dependent and highly utilization of Mn₃O₄ nanocrystalline. The Mn₃O₄ nanocrystalline/RGO hybrid electrode exhibits a high specific capacitance of 519 F g-at a current density of 1 A g^-1. This method can be expanded to control the size of other transition-metal oxide nanocrystalline on the graphene nanosheets and improve the capacitance and urilization of the metal oxide-graphene nanohybrids.?2? By using H₂O₂ as etchant and oxidant, nanoporous is introduced into graphene oxide nanosheets and holey graphene oxide is obtained ?HGO? at 100? for 4 h. Then the nanocrystalline manganese tetroxide/holey reduced graphene electrode material ?Mn₃O₄/HRGO? is prepared by exchanging HGO with Mn2+ ions and followed by calcining the Mn2+-exchanged material at different temperatures for 4 h in N2 atmosphere. The effect of the calcining temperatures on the phase and morphology of the obtained materials is investigated. The phase and size of MnOx nanocrystalline and reduction degree of HGO in the hybrid material are affected by calcining temperature, by which the Mn₃O₄/HRGO hybrid sample can be obtained at 450?. By comparing with Mn₃O₄/RGO-450 electrode material, the effect of the holey treatment for graphene oxide on the electrode capacity, cycling stability and rate performance of the obtained MnOx/HRGO hybrid electrodes are investigated, and their electrochemical properties are also discussed.