Nano CoO/3D Graphene Composites For Supercapacitors Application

When the need to release large amounts of energy in the short time, the battery will not provide power output to meet the requirements; the supercapacitor as a new energy storage technology can completely discharged the stored energy in the range of microseconds to milliseconds. Supercapacitors fill the performance space between ordinary capacitors and batteries. It can be used in electric or gas-electric hybrid vehicles in the recovery of waste energy during braking, or provide energy for portable communication tools, and even in some areas to replace the battery. In order to meet future applications, preparations of high energy density, power density, long cycle life and low cost electrode materials will be the critical study of supercapacitors.To improve the specific capacitance, rate capability and cycling stability of supercapacitors, we engineered and prepared novel supercapacitor electrodes. First, the freestanding3D graphene foams (GF) were prepared by chemical vapor deposition (CVD). The3D GF work as current collectors for supercapacitors electrodes replacing the usual metal materials. Then the different CoO nanostructures were grown on the3D GF forming composite materials to produce high performance electrodes materials. The main contents of this thesis are as follow:(1) Muti-layer graphenes were deposited on porous Ni foam by CVD using ethanol as carbon source at atmospheric pressure. Subsequently the Ni foams coated with graphene were soaked in HC1to dissolve the Ni foams completely. Eventually, the freestanding3D graphene foams were gained. The GFs which inherit the macrostructures of Ni foams with high specific surface areas, good electrical conductivity, electrochemical stability and ultra-light mass are excellent electrode materials for supercapacitors replacing the metal materials. The cyclic voltammetry (CV) curves of3D graphene show a pair of obvious redox peaks, which are similar with CV curves of Ni foam in NaOH electrolyte. This is likely origin from the residual metal ions in3D graphene foams.(2) To enhance the specific capacitance of3D GF, porous CoO nanowalls were grown in situ on graphene by a simple hydrothermal process and succedent annealing treatment. The CoO nanowalls/3D GF hybrid electrodes with ultra-light current collectors that eliminate the use of less active materials such as binder materials, conductive additives obtained a high specific capacitance. When the lA/g current density was applied, capacitances of231.87F/g (139.47F/g even scaled for total mass of electrode) were fulfilled. Owing to the good electrical conductivity of3D GF and the robust mechanical adhesion of CoO nanowalls with graphene foam, the composite electrodes suggested good rate performance. As well as the high surface area and three-dimensional porous architectures can short ion diffusion distance and electron-transfer resistances. Even the charge-discharge current increases from1to10A/g, there is still～79%remain of the capacitance. The cycling stability of the CoO@GF hybrid electrode was examined over1000charge-discharge cycles at the current density of7A/g. The capacitance maintained is over98%and without noticeable change of curves which reveals good cycling performance.(3) On the basis of the works of CoO nanowalls/3D graphene, novel CoO nanobundles materials were synthesized on the surface of3D graphene through altering reactants and conditions of hydrothermal reactions. Every CoO nanobundle is composed of narrow (-600nm) and porous CoO nanoflakes. Then detailed characterizations of structures and supercapacitor performances of CoO NB@GFs were conducted. Capacitances of231.87F/g (scaled for total mass of CoO) were fulfilled at lA/g current density. When the charge-discharge current increases from1to10A/g, the remain of the capacitance exceed88%. These indicate better performance of CoO nanobundles materials than CoO nanowall which stem from smaller size and higher porosity of CoO nanobundles materials than CoO nanowalls. The CoO NB@GFs hybrid electrodes were charge/discharge at10A/g over1000cycles with nearly no attenuation of specific capacitances which improves excellent cycling performance.