Study On Preparation Of Carbon And MnO₂Nanofibers By Electrospinning And Their Supercapacitive Properties

Supercapacitors are highly promising in many important applications such as electric vehicles, consumer electronics, and memory backup power due to the advantages of fast charge and discharge rates, long cycle life, low cost and environmental friendliness. However, the specific energy of the present supercapacitors is usually5-10Wh/kg, which is much less than that of Li-ion batteries, and limits their widespread applications. Therefore, the urgent task in the research field of supercapacitors is to prepare novel electrode matrials to improve its properties such as the specific energy.Electrospinning is a simple method for making nanofibers. It has widely applications in the areas of tissue engineering, purification filters, catalysis, sensors and composite materials, etc.. Electrospinning technique has distinct advantages in the preparation of electrode materials of supercapacitors due to its simplicity, low cost, and no agglomeration between the produced nanofibers.In this dissertation, ultrafine carbon nanofibers (CNFs) were prepared by electrospinning. Radially oriented graphene sheets were grown on the surface of the CNFs by original process. The effects of electrochemical treatment on the electrochemical properties of the CNFs were investigated and the process for improving the performance of the CNFs by electrochemical treatment was established. Honeycomb porous MnO2nanofibers were prepared by chemical reaction in potassium permanganate (KMnO4) solutions using the CNFs as the templates and reducing agent, where their formation mechanism and structure features were investigated. The main research work and results are as follows:The effects of the carbonizing temperature on the morphology and structure in ammonia atmosphere (NH3) were investigated. The research results indicate that the application of NH3during the carbonization process is advantageous to the reduction of the CNFs. Ultrathin CNFs can be obtained by controlling the process appropriately and the thinnest CNFs with average diameter as small as11nm and surface graphene structure were obtained at1100oC. For the supercapacitor assembled using the above CNFs prepared at1100oC as the electrode materials based on aqueous electrolyte, the working voltage reaches1.8V with the specific energy being29.1Wh/kg at450W/kg. The specific capacitance can be retained for93.7%of the initial value after5000cycles. For the supercapacitor assembled using the above CNFs prepared at1100oC as the electrode materials based on aqueous electrolyte, the working voltage reaches4V, the specific capacitance and specific energy reach111F/g and61.9Wh/kg at the current density of1.67A/g, respectively.The effects of the electrochemical treatment on the performance of supercapacitors were investigated. It was found that the working voltage and specific capacitance of the supercapacitors can be increased effectively by applying alternative positive and negative voltage to treat the electrode materials repeatedly. Accordingly, a simple method to improve the performance of the supercapacitors was established. For the supercapacitor assembled using the CNFs carbonized at900oC in NH3the performance improves greatly after the electrochemical treatment of3000cycles with the specific energy increasing from0.5Wh/kg to15Wh/kg at the current density of1A/g. For the electrochemically treated supercapacitors the specific capacitance can be retained for94.5%of the initial value after15000charge/discharge tests with the specific energy being19.7Wh/kg at the current density of0.2A/g.The MnO2nanofibers were prepared by chemical reaction between the CNFs and KMnO4solutions, where the effects of the solution concentration and reaction time on the structure and property of the products were investigated. The results indicate that the solution concentration and reaction time affect the structure of the products strongly. At too high concentrations the products are core/shell structured C/MnO2composite nanofibers while at too low concentration almost no products form. With the increasing of reaction time, the diameter of MnO2nanofibers is increasing gradually and the diameter of inner carbon nanofibers is decreasing gradually, but the growth rate of MnO2nanofibers at the surface in different concentrations is different. At the KMnO4concentration of2mM the honeycomb porous MnO2nanofibers were obtained after48h reaction, which are composed of radially grown nanosheets with the thickness of3-7nm. The supercapacitors based on the honeycomb porous MnO2nanofibers and1M Na2SO4electrolyte show prominent performance with the working voltage reaching2.2V and the specific energy being41.1Wh/kg at the specific power of3.3kW/kg. The specific capacity retention after1000and3500charge/discharge tests are89%and76%, respectively.The supercapacitors with high voltage and high specific energy prepared in this dissertation are highly promising in application. The established electrochemical treatment method provides a new approach for improving the supercapacitor performance. In addition, the CNFs with radially grown graphene sheets are advantageous for applications in various areas such as wastewater treatment, catalysis, electronics, etc..