Preparation Of Three-dimensional Electrodes Of Graphene Hydrogels / Graphene Quantum Dots And The Application In Supercapacitors

Supercapacitor is a new type of energy storage device with long cycle life, high energy density, power density, short charging time and low production cost, and its capacitive performance is between traditional capacitor and secondary battery. Supercapacitors have attracted much attention for a wide range of applications from electrical or hybrid vehicles, renewable energy generation devices, portable devices and so on. Compared to the secondary batteries such as lead-acid, although supercapacitors have high power density and good cycle stability, their low energy storage ability is difficult to meet the growing demand of people’s daily work and life practically.Graphene materials, due to their excellent chemical stability, good electrical conductivity, high specific surface area and strong mechanical properties, have become one of the most popular electrode materials for supercapacitors. In this dissertation, graphene hydrogel with three dimensional network structures was prepared using hydrothermal method. The network structure can restrain effectively the restack of graphene, therefore, the electrodes of graphene hydrogel demonstrate high electric double layer capacitance and long cycle life. However, the specific capacity and energy density of electric double layer capacitor materials are far lower than the pseudocapacitance materials, which cannot meet the practical requirements. How to improve substantially the specific capacity and energy density of graphene hydrogel electrodes, and meanwhile maintaining their high cycle stability and fast charge- discharge ability, is the key scientific problem to be solved in this dissertation. We prepared amine-functionalized graphene quantum dots with high specific surface area, high stability, good electrical conductivity and a large number of redox-active positions by using a novel bottom-up approach—the alkali-catalyzed molecular fusion. Using in-situ hydrothermal method and electrophoretic deposition respectively, three-dimension supercapacitors of amine-functionalized GQDs/graphene hydrogels have been assembled with high capacity, high energy density, high power density and good cycle stability. The main contents are described as follows:1. The preparation and electrochemical properties of three-dimensional graphene hydrogels(3DGH) were studied. Using single layer graphene oxide prepared by modified Hummer method as the reactant, 3DGH with porous structure were prepared by the hydrothermal method through adjusting the reaction temperature and time. The structure and morphology of the graphene hydrogels were characterized by SEM, FTIR, XRD, TEM, Raman and XPS analysis. Two electrodes of graphene hydrogels were assembled into symmetrical supercapacitor, and then its capacitive performances are researched by using 1 M H2SO4 solution as the electrolyte. The electrode area specific capacitance of symmetric supercapacitor reached 536.5 m F/cm2(233.2 F/g), and its energy density was 9.31 u Wh/cm2 at the current density of 1 m A/cm2. Moreover, the 3DGH supercapacitor showed a long-term cycling stability, and it retained 100% of the initial capacitance after 10000 charge-discharge tests at a current density of 20 m A/cm2.2. Using in-situ hydrothermal method, three-dimension electrodes of amine-functionalized GQDs@graphene hydrogel have been prepared. We prepared amine-functionalized single crystal GQDs by the alkali-catalyzed molecular fusion method, and GQDs were characterized by AFM, TEM, XRD, Raman, IR, XPS and so on. With the adjustment of the amount of GQDs, the temperature and time of hydrothermal reaction, amine-functionalized GQDs were assembled into 3DGH in situ. Amine-GQDs@3DGH composite materials were characterized by SEM, EDS, XRD and IR. Under the three-electrode system, we tested their electrochemical properties. The specific capacitance of amine-GQDs@3DGH electrode is 409 F/g, which is much higher than 3D GH at the scan rate of 10 m V/s. The CV curve of amine-functionalized GQDs@3DGH exhibits Faradic redox peaks, suggesting a predominant pseudocapacitative feature, which is the main contribution by amine-functionalized GQDs. Symmetrical supercapacitors of amine-GQDs@3DGH were assembled and the capacitive performances were investigated using using 1 M H2SO4 solution as the electrolyte. At a current density of 1 A/g, the specific capacitance of the symmetrical supercapacitor was 312.8 F/g, the energy density of total supercapacitor was 10.9 Wh/kg with the power density 250 W/kg. The capacitance retention rate of supercapacitor reached 94% after 5000 charge-discharge tests at the current density of 2 A/g.3. Using electrophoretic deposition hydrothermal method, three-dimension electrodes of amine-functionalized GQDs@3DGH with high capacitive performance have been prepared. Choosing graphene hydrogel as the anode, and through adjusting the time and voltage of depositon, amine-functionalized GQDs with negative charge were loaded onto graphene hydrogels by electrophoretic deposition technology. The structure and morphology of amine-functionalized GQDs@3DGH were characterized by XRD, SEM, TEM, XPS, BET, Raman and FTIR analysis. Electrochemical measurements of the amine-functionalized GQDs@3DGH as the work electrode were performed in a three-electrode system using Ag/Ag Cl electrode as the reference electrode, a platinum wire as the counter electrode, and 1M H2SO4 as the electrolyte. The specific capacitance of electrode reached up to 2537 m F/cm2. Symmetric supercapacitors were assembled by two amine-functionalized GQDs@3DGH electrodes. At a current density of 1 m A/cm2, the specific capacitance of electrode was 2322.1 m F/cm2, which was three times higher than that of the 3DGH supercapacitors. The energy density of total supercapacitor was 83.3 u Wh/cm2 with the power density 250 u W/cm2. Meanwhile, at the current density of 30 m A/cm2, the capacitance of supercapacitor remained 900 m F/cm2 and the capacitance retention rate of supercapacitor reached 100% after 10000 charge-discharge tests. The supercapacitors of amine-functionalized GQDs@3DGH showed an excellent long-term cycling stability.