| Supercapacitors are emerging energy storage devices between conventional capacitors and batteries because of their high power density, fast charge and discharge and long cycle life. They have attracted much attention for a wide range of applications from electrical or hybrid vehicles, renewable energy generation devices, and portable devices. Compared with batteries, supercapacitors have high power density, but low energy density restricts their applications. Therefore, developing supercapacitors with high energy density, high power density and long cycle life is becoming a research hotspot by far. Due to their high conductivity, high specific surface area, high chemical stability and abundant functional groups, graphene quantum dots(GQDs) can be used as highly active substances in supercapacitor field. In this dissertation, we prepared amine-functionalized graphene quantum dots with high specific surface area, high stability, good electrical conductivity and water soluble by using a novel bottom-up approach—the alkali-catalyzed molecular fusion. The surface of GQDs had a large number of redox-active positions. After vacuum anneal treatment, Ti O2 nanotube arrays with high conductivity were used as a three dimensional conductive substrate, and amine-functionalized graphene quantum dots were loaded into nanotubes by electrophoretic deposition technology. Amine-functionalized GQDs/Ti O2 nanotube arrays supercapacitors 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 functionalized-GQDs/Ti O2 nanotube arrays composite materials were studied. We prepared hydroxyl-functionalized GQDs and amine-functionalized GQDs by the alkali-catalyzed molecular fusion method, and GQDs were charactered by TEM, Raman and IR and so on. Two kinds of graphene quantum dots were loaded into Ti O2 nanotubes by electrophoretic deposition technology respectively. Under the three-electrode system, we tested their electrochemical properties respectively. The results showed that amine-GQDs/Ti O2 nanotube arrays composite materials were ideal electrode materials with highly electrochemical activity. In order to optimize the preparation conditions of composite materials, operation voltage and the time of electrophoretic deposition and annealing temperatures were changed. Finally, 3V and 150 oC 3 h were chosen as the optimal deposition voltage and annealing temperature.2. The capacitive performance of symmetrical supercapacitors assembled by amine-GQDs/Ti O2 nanotube arrays was studied. Amine-GQDs/Ti O2 nanotube arrays were charactered by SEM, EDS, Raman, TGA and XPS. Two electrodes of amine-GQDs/Ti O2 nanotube arrays were assembled into a symmetrical supercapacitor, and then its capacitive performances were researched by using 1 M H2SO4 solution and H3PO4-PVA gel electrolyte respectively. Using 1 M H2SO4 solution as the electrolyte, the specific capacitance of amine-GQDs/Ti O2 nanotube arrays electrodes loaded quantum dots for 1 h reached up to 595 F/g at current density of 0.5 A/g, which exceeded the theory capacity of graphene(550 F/g). The supercapacitor with high energy density of 21.8 W h/kg at a current density of 0.5 A/g could light red and green LED lights. Moreover, the supercapacitor of Amine-GQDs/Ti O2 nanotube arrays showed excellent cylic stability, and it retained 90% of the initial capacitance after 10000 cyclic voltammetry tests at a current density of 10 A/g. In addition, the electrode materials also had high areal capacitance. The areal capacitance of Amine-GQDs/Ti O2 nanotube arrays electrodes loaded GQDs for 3 h was 194.4 m F/cm2 at a current density of 0.2 m A/cm2. In H3PO4-PVA gel electrolyte system, the electrode loaded GQDs for 1 h acquired an specific capacitance of 332.5 F/g and the energy density of 11.5 W h/Kg at a current density of 0.2 A/g.3. In order to improve the energy density of supercapacitors by increasing the voltage window, asymmetrical supercapacitors of amine-GQDs/Ti O2 nanotube arrays//graphene hydrogel were assembled. Amine-GQDs/Ti O2 nanotube arrays electrode was prepared as working electrode, and the 3D porous graphene hydrogel electrode was prepared as the counter electrode. 1 M H2SO4 solution and H3PO4-PVA gel were used as electrolyte respectively and the electrochemical properties of asymmetrical supercapacitors were discussed. In 1 M H2SO4 solution, the working voltage of asymmetrical supercapacitor was expanded to 1.2 V. At a current density of 0.5 A/g, the specific capacitance of the asymmetrical supercapacitor was 298.7 F/g and the energy density was up to 59.7 W h/kg, which was much higher than the symmetrical supercapacitor(21.8 W h/kg). In H3PO4-PVA gel electrolyte system, the working voltage window of the asymmetrical supercapacitor reached 1.6 V and at a current density of 0.5 A/g the specific capacitance was 102.4 F/g with the energy density of 36.5 W h/Kg. |
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