Preparation and electrochemical capacitive performance of graphene/carbon nanotube composite

Abstract

The development of more advanced and environmentally friendly energy storage devices is an urgent request to meet future societal and environmental needs. Supercapacitor is one of the most promising electrochemical energy storage devices owning to its long cycle life, high dynamic propagation, quickly response and low maintenance cost compared to the traditional batteries and capacitors. However, their energy density is far less batteries, which limits their application in energy storage. In order to break through the limitation, exploring novel electrode materials with both high energy densities and power densities is extremely urgent. At present, it is a hot topic to produce composite electrode materials with synergistic effect. Graphene, a one-atom-thick two dimensional single layer of sp2-bonded carbon, owning to its unique structural properties, has exhibited many advantages, such as extraordinarily high electrical and thermal conductivity, great mechanical strength and high surface area, making it a potential candidate for applications in energy storage field. Therefore, graphene based composite materials have been utilized in various practical applications, including energy storage and conversion, transparent conducting films, chemical sensors, and actuators, etc. Given the extraordinary properties of graphene, such as the low mass density, good compatibility, highly conductive, large specific surface area and excellent flexibility, it is considered as one of the most suitable substrate materials for preparing supercapacitor electrodes. In recent years, graphene composites as supercapacitor electrode material have been widely investigated. Here, three- dimensional (3D) graphene/carbon nanotube (CNT) nanocomposites were prepared by in-situ compound method using as-synthesized graphene as matrix, which prepared by chemical vapor deposition (CVD). Firstly, graphene was prepared on Ni foam by CVD method using CH4 as carbon sources and H2/Ar as buffer gas at 850°C. Then a solution carbon source was obtained by ultrasonic vibration using a mixture solution of ferrocene and dimethylbenzenes. Finally, 3D graphene/CNT nanocomposite was synthesized by CVD method using Ni foam deposited graphene as supporter substrate at 750°C for 30 min. The morphology and microstructure of the resulting samples were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the electrochemical capacitive performance of the samples were investigated using CHI 660D chemical workstation to analyze the cyclic voltammetry and AC impedance. The results indicated that large amounts of graphenes had been formed on the Ni foam and 3D graphene/CNT nanocomposites also had been synthesized. The 3D lamellar structures between graphenes formed in the synthesis process could densely upload active substance, which was very different in structure and performance with individual graphene. The cyclic voltammetry (CV) curves of 3D graphene/CNT composite exhibited that the electrochemical capacitive performance has good reversibility. The Nyquist plots also displayed that the 3D graphene/CNT composite has excellent performance of electric charge transportation and double layer capacitance. The electrochemical measurements indicated that the maximum specific capacitance of 3D graphene/CNT composite, as the supercapacitor electrode material, exhibited a maximum specific capacitance of 289.8 F/g using 1.5 mol/L Li2SO4 as the electrolyte in the system and also showed excellent high-capacity and cycling stability. The specific capacitance of the 3D graphene/CNT composite remains 92% after 2000 cycles.