Synthesis And Capacitive Performance Of Three-dimensional Electrodes Of Graphene Quantum Dots/TiO₂Nanotube Arrays

Electrochemical supercapacitors are emerging energy storage devices capable ofoffering high power density, long cycle life, low self-discharge rate, and good safetyfor hybrid electric vehicles, memory back-up systems, energy managements, andmobile electronic devices. However, Low energy density of supercapacitor hasrestricted its application. Therefore, Preparation of supercapacitors with high powerdensity, high energy density and long cycle life is the main research direction of thisfield. In this dissertation, we develop the growth, on a gram scale, of nearlymonolayer and single-crystalline graphene quantum dots (GQDs) with high specificsurface area, high stability, good electrical conductivity and high water-solubility by anovel bottom-up approach—the alkali-catalyzed molecular fusion route. A largenumber of redox active positions in the edge are introduced through the in-situ aminofunctional. We design that high conductivity super-long titanium dioxide nanotubearrays act as the three-dimensional ordered current collector, and GQDs of rich innitrogen active positions act as highly redox-active material. Three-dimensionalelectrodes of amino-functionalized GQDs/TiO2nanotube arrays with the ultra-highcapacity and long cycle life are manufactured by the controllable methods of in situ orelectrophoresis assembly. The main contents are as follows:1. TiO2nanotube arrays were grown on Ti foils by two-step anodization，andthe methods of electrochemical reduction and vacuum annealing were used in themodification of TiO2nanotube arrays for improving the conductibility of TiO2nanotube arrays. Characterization tests such as SEM, XRD and XPS were conducted.And the capacitance properties of TiO2nanotube arrays were studied inthree-electrode system. The results showed that the oxygen vacancies of TiO2nanotube arrays annealing in air increased after electrochemical reduction. The arealcapacitance was achieved as11.37mF/cm2at a scan rate of100mV s1, much higherthan that of the pristine TiO2nanotube arrays (0.004mF/cm2). The temperature ofannealing and pH value of electrolytes have an impact on the performance ofcapacitor. The insertion of hydrogen ion plays an important role on the improvement of electrical capacity. We prepared titanium dio xide nanotube arrays with highlyc-axis preferentia lly oriented crystallization and a high concentration of oxygenvacancies by second anodization in ethyle ne glycol and annealing underpoor-oxygen conditions. After vacuum anneal treatment, the TiO2nanotube arraysshowed the color of black and achieved an areal capacitance of7.41mF/cm2. Bycomparison of two kinds of modification methods, we chose the vacuum annealingTiO2nanotube arrays as the more suitable load carrier for GQDs.2. Hydro-thermal in-situ method was used to synthesize GQDs/TiO2nanotubearrays three-dimensional electrodes. The TiO2nanotube arrays conducted as a carrierand conductive network, with amino-functionalized GQDs of about3nm loading onits tube and pipe wall. This structure can prevent the aggregation of nanomaterals,enlarge specific surface area and improve wetting ability of the composite electrode.The redox reactions peak appeared on the cyclic voltammogram curve, which wasinvolved with the pseudocapacitance reaction between amino functional groups andions of electrolyte. When the concentration of precursor solution was the half of theoriginal concentration, we obtained the best capacitance performance of compositeelectrode. The areal capacitance of GQDs/TiO2electrode was achieved as227mFcm2at a current density of1mA cm2in acid electrolyte, and showed a long-termcycling stability with only10.1%reduction of capacitance after1000cycles.3. Electrophoresis assembly was used to synthesize GQDs/TiO2nanotubearrays three-dimensional electrodes. Compared to the hydro-thermal in-situ method,the electrophoretic deposition method shows milder and higher controllability. Byadjusting the voltage and time of the electrophoretic deposition process, we acquiredan areal capacitance of280mF cm2at a current density of1mA cm2in acidelectrolyte, and also showed a long-term cycling stability with only4.35%reductionof capacitance after5000cycles in three-electrode system. Finally we assembledGQDs/TiO2nanotube arrays three-dimensional electrodes as symmetricalsupercapacitors with the specific capacitance of142.3F/g.