Perparation Of Graphene And Graphene/Carbon Nanotubes Composites With Their Applications In Energy Storage

Electric double layer supercapacitor (ESC) is one novel type of energy storage devices. It has been attracting ever-increasing concern because of its high energy density, fast charging rate, long service time and stability, etc. Since electrodes are crucial components of these supercapacitors, the choice of the electrode material has a significant impact on the overall performance of these devices. Graphene is a unique 2-dimensional material with only one carbon atom thickness, possessing a wide range of advantages such as supreme conductivity, high specific surface area and excellent chemical stability, which makes it an outstanding candidate material for the electrodes. The most common synthesis method of graphene begins with oxidation treatment of graphite, followed by adopting suitable reductants, such as hydrazine hydrate, dimethylhydrazine, to obtain chemically reduced graphene oxide. Due to the toxicity of these reductants, it is, therefore, necessary to find out more environmental friendly ways to reduce graphene oxide. Despite the prominent electric capacity of graphene, its easy tendency of aggregation largely diminishes its specific surface area, which becomes a great concern of the electrode synthesis. Carbon nanotubes have emerged as one-dimensional carbon nanomaterials, which has high aspect ratio, and superior conductivity, making it another alternative for electrodes. However, its weak dispersibility and high in-system resistance limits its application in electrode materials to some extent. To make the most of the individual characteristics, a composite that is composed of graphene and carbon nanotube, is obtained to form a 3-dimensional interconnected network, improving the conductivity and specific surface area at the same time, which provides a promising approach to high-performance electrode synthesis.In this work, the application of graphene-based electrode in supercapacitors has been investigated. The morphological, structural and compositional characteristics of the graphene-based electrode has studied with infrared absorption spectroscopy, X-ray diffraction, ultraviolet-visible absorption spectroscope, scanning electron microscope, high resolution transmission electron microscope and nitrogen adsorption-desorption analyser. In addition, the electrical and electrochemical properties of the material have been measured with four-probe tester, battery tester, and electrochemistry workstation respectively. Details are as follows:1. Improved Hummers method has been adopted to synthesize graphene oxide solution, followed by reduction by three different methods, i.e. ascorbic acid reduction, hydrothermal technique and sodium borohydride reduction. The structural, morphological and electrochemical properties of the three corresponding materials have been measured. And the results showed that the RGO obtained by L-ascorbic acid reduction and hydrothermal technique had comparable specific capacitance,118.71 F/g and 114.56, which were much higher than that of materials reduced by NaBH4,101.87 F/g, to be exact.2. By controlling the GO/water ratio, an un-bonded high-quality graphene airgel electrode material had been obtained through a combination of freeze-drying and high temperature hydrogen reduction. The experimental results showed that the obtained GA is almost oxygen-function groups and micro-poles free, with few layers and relative large surface area. The corresponding specific capacitance can be as high as 131.32 F/g.3. A macroscopic composite of RGO-CNT had been obtained by hydrothermal reduction of a mixture of GO solution with carboxylic multi-walled carbon nanotubes. The experimental results showed that, RGO-CNT has the smallest resistance, better recycle lifetime and highest specific capacitance of 157.05 F/g, which is much higher than their individual components.