Bir-MnO₂Based Composites For Electrode Materials: Design, Synthesis And Capacitive Performances

Energy is one of the most important research topics in this century. With the rapid development of global economy, the depletion of fossil fuels, as well as the increasing environmental pollution, there is an urgent need to seek renewable and clean energy sources that can substitute fossil fuels, and to develop efficient technologies associated with energy conversion and storage. In many application areas, the most effective and practical technologies for electrochemical energy conversion and storage are batteries and electrochemical supercapacitors (or ultracapacitors). In recent years, electrochemical capacitors (ECs) have attracted significant attention, mainly due to their high power density, rapid charging-discharging rates, long lifecycle, wide operating temperature range, environmental compatibility, and good operational safety.The properties of ECs intimately depend on the active materials used in electrodes. According to different energy storage mechanisms, the electrode materials for ECs can be categorized into three types:carbon materials, transition metal oxides, and conducting polymers. Amongst them, birnessite-type manganese dioxide (Bir-MnO2) is believed to be a promising electroactive material for the next generation of supercapacitors, due to its high theoretical specific capacitance, high ion permeability, environmental compatibility, low cost, and abundance in nature. However, the birnessite MnO2electrode materials are suffered from some disadvantages, such as poor electronic conductivity, low specific surface, and partial dissolution, which greatly limit its wide application in ECs. Lately, preparing nanostructures or composite materials is the major method to improve the electrochemical properties of Bir-MnO2.In order to meet the demand of high-performance electrode materials for ECs, this dissertation designed and prepared several Bir-MnO2based electrode materials with the idea of rationally designing the microstructure of materials. By systematical studie on the energy storage characteristics of these materials, the relationships between the component, structure and performance were illustrated. The concrete research contents and results are summarized as follows:1. By overcoming the disadvantages of traditional hydrothermal methods for birnessite-type MnO2nanostructures, such as high temperature, reductants, surfactants and/or templates, Bir-MnO2nanosheets were synthesized by a low-temperature hydrothermal method. Nanosized Bir-MnO2sheets with lateral size of a few hundred nm, and thickness of5-10nm were observed from SEM and TEM images. To assess the as-synthesized MnO2nanosheets for use in supercapacitors, the electrode exhibits a high specific capacitance of169F g-1at a current density of0.1A g-1, good rate capability with a capacitance of96F g-1even at a high current density of5A g-1, as well as excellent cycle stability with capacitance retention of94%at1A g-1after1,000cycles.2. There are a large number of the oxygen-containing functionalities on the surface of GO, such as epoxide, hydroxyl, car-bonyl and carboxyl groups, et al. It not only makes GO form stable suspension in water, but also can easily adsorb ions or monomers to fabricate composites. Depending on this inherent advantages of GO, we synthesized an advanced birnessite-type manganese dioxide/graphene oxide (Bir-MnO2/GO) hybrid via a simple hydrothermal methods. In this hybrid, MnO2nanosheets arrays were vertically grow on graphene oxide flakes by adjusting the concentrations of reactants, the reaction temperature and time in hydrothermal procedure. The experimental results revealed that Bir-MnO2/GO hybrids with different Bir-MnO2content could be prepared by changing the reaction time, and the hybrid with a reaction time of12h showed the best electrochemical performances. In an electrolyte of1M Na2SO4, it exhibited a high specific capacitance (213F g-1at current density of0.1A g-1), a good rate capability (even80F g-1at10Ag-1) and a long cycle life with the capacitance retention ratio of98.1%at1A g-1after1,000cycles. Furthermore, the EIS measurements demonstrated the electrochemical resistance of MnO2nanosheet which directly grows on GO is reduced, indicating easier access for intercalation/deintercalation of charges in hybrid. Thus, the Bir-MnO2/GO hybrid displayed enhanced energy storage performances compared with the GO and Bir-MnO2.3. The conductivity of GO is too bad to effectively enhance the capacitive performances of Bir-MnO2nanosheets. In order to synthesis high-performance electrode materials, it would be much better to use the reduced graphene oxide (rGO) as the substrate for Bir-MnO2nanosheets arrays. Based on this analysis, we used a facile hydrothermal method to fabricate the novel birnessite-type manganese dioxide/reduced graphene oxide (Bir-MnO2/rGO) hybrid, in which Bir-MnO2nanosheets arrays were uniform and vertically grow on rGO flakes. The formation mechanism of the hybrid is also discussed based on a series of time-dependent experiments. The results revealed that the unique structure of the hybrid provided good electronic conductivity, fast electron and ion transport, and high utilization of MnO2, which made the Bir-MnO2/rGO electrode exhibit much higher specific capacitance (315F g-1at a current density of0.2A g-1) and better rate capability (even193F g-1at6A g-1) compared with both the rGO and Bir-MnO2electrodes. Moreover, the capacitance of the electrode is still87%retained after2000cycles at a charging rate of3A g-14. One hand, the specific capacitances of the as-synthesized Bir-MnO2/rGO hybrid are far below the theoretical value of Bir-MnO2(1370F g-1). Thus, there is still room for the improvement of electrochemical capacitance performances. On the other hand, the MnO2/rGO composites have attracted considerable interest, and plenty of such composites have been reported in recent years. However, to date, researches on this field are mainly focuse on graphene-based binary composites, ternary composites based on graphene, transition metal oxides, and conducting polymers have seldom been reported for use as electrochemical capacitors. Hence, we took ternary composites as the object of the further reseach for Bir-MnO2/rGO hybrid, and designed a new ternary composite with unique structure. In this composite, Bir-MnO2nanosheets arrays were vertically grow on rGO flakes, and the Bir-MnO2/rGO nanostructure was wrapped by poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)(PEDOT:PSS). Here, the practicability of the as-designed ternary composites using as an electroactive material for electrochemical capacitors was preliminarily explored. The experimental results revealed that the Bir-MnO2/rGO/PEDOT:PSS composite containing30wt%PEDOT:PSS exhibited the best electrochemical performances. A specific capacitance of249F g-1could be obtained at a current density of0.1A g-1.