Preparation And Application Of MoO₃Nanocomposite Electrode Materials For Electrochemical Capacitors

With the rapid development of industrial society, the human demand for energyhas risen sharply. Moreover, energy resources and environment now becomehigh-profile topics over the world, due to the decreasing amount of nonrenewableenergy, such as coal, petrol, and natural gas, as well as environmental problems.Consequently, development of low-carbon economy and clean renewable energy hasbecome the significant topics globally. This paper aims to explore innovative aqueouselectrochemical capacitors.According to charge storage mechanisms, electrochemical capacitors can bedivided into three types, i.e., electrochemical double layer capacitors (EDLCs),faradic pseudo-capacitors, and hybrid supercapacitors. EDLCs store energy throughthe charge separation at the electrode/electrolyte interface, and they show very highpower density and excellent cycling performance. Pseudocapacitors characterized bythe presence of faradic current, store energy by using the ion fastintercalation/deintercalation or redox reactions on the electrode surface, and theirtheoretical specific capacitance and energy is10-100times higher than that of EDLC.The two electrodes used in hybrid supercapacitors store energy in different manners.One of them uses EDLCs type electrode materials, such as activated carbon (AC), andthe other electrode uses pseudocapacitors type or secondary battery type electrodematerials.Compared with organic electrolyte, aqueous electrochemical capacitors have animportant position in the energy storage system. The power capability of aqueouselectrolyte is much higher than that of organic electrolyte due to their higher ionicconductivity. Moreover, aqueous electrolytes are cheaper, safer, and moreenvironmentally friendly than organic electrolytes. Therefore, aqueous electrolyteseems to be more suitable as electrolyte for electrochemical capacitors. Currently,extensive studies have been focused on the cathode materials for aqueouselectrochemical capacitors while relative less reports on the anode materials, whichmake it difficult to match the cathode materials because of low capacity and poorcycle performance.As a result, this dissertation mainly studies on the virginal MoO3nanobelts and PPy@MoO3nanocomposites as anode materials for electrochemical capacitors. Thepreparation, characterization and electrochemical performance of the MoO3nanobeltsand PPy@MoO3nanocomposites for electrochemical capacitors are illustrated asfollows:Chapter3concerns preparation of MoO3nanobelt anode material byhydrothermal method, and synthesis of a nanocomposite of PPy-coated MoO3nanobelts via a low-temperature in situ oxidative polymerization route. Thesupercapacitor based on the PPy@MoO3nanocomposite as the anode, activatedcarbon as the cathode and0.5M K2SO4aqueous solution as the electrolyte exhibitsbetter rate capability as well as excellent cycling performance compared to thevirginal MoO3. The energy density of the supercapacitor consisting of thePPy@MoO3nanocomposite is20W h kg-1at the power density of75W kg-1;Moreover, it keeps excellent rate behavior with an energy density of12W h kg-1evenat3kW kg-1. After600cycles, there is only17%capacitance loss and the main losshappens at the initial several cycles.The work mentioned in Chapter4is that a faradic pseudocapacitor based on thePPy@MoO3nanocomposite anode and the nanowire Na0.35MnO2cathode using0.5MNa2SO4electrolyte was built up, which exhibits excellent electrochemicalperformance. The charge and discharge voltage range for this pseudocapacitor is from0to1.7V. However, even at2.6kW kg-1, its energy density can be18Wh kg-1,90%of that at power density of80W·kg-1. After1000cycles, there is only21%capacityloss.