Synthesis And Properties Of Graphene Derivatives And Their Composites

Graphene is composed of one-atom-thick planar sheets of carbon atoms that are densely packed into a honeycomb crystal lattice. As a new member of carbon materials, graphene can be considered as the basic construction material of all other carbon dimensionalities (diamond, graphite, carbon nanotubes, etc). Since it was reported in2004, graphene has stimulated worldwide interests among scientists in various fields. As a result of its unique structural property, graphene emerges with a series of prominent intrinsic chemical and physical features, such as strong mechanical strength (～1TPa), extraordinarily high electrical and thermal conductivity, and large surface area (2675m2/g), which may rival or even surpass both single-and multi-walled carbon nantubes. These outstanding and intriguing features make this extremely versatile carbon material promising for various practical applications, including high-performance nanocomposites, transparent conducting films, sensors, nanoelectronics and energy storage devices. As we known, the unique nature of graphene makes it applicable to theoretical studies and various technologies; it has known as a sharply rising star in many frontier research fields including material science and condensed physics.Recently, utilizing graphene as supercapacitor electrode materials has become the focus of a considerable amount of research. Supercapacitor, a new-generation device, is the best known and perhaps the most promising energy storage system due to its intriguing features of high specific capability, long cyclic stability, fast charging time, convenient operation and environmentally friendly. Supercapacitors can be classified into two categories based on the storage-charge mechanisms:the electrical double layer capacitors (EDLCs) and pseudocapacitors. EDLCs store energy by forming a double-layer of electrolyte ions on the surface of conductive electrode with carbon-based active materials as the most common electrodes at present; while the capacitance of pseudocapacitors arises from Faradic reactions at the surface of active materials and the conducting polymer and transition metal oxides are the extensively applied to pseudocapacitive active materials. As a novel two-dimensional carbon member, graphene-based material is an extremely promising candidate for supercapacitors not only for its own EDLCs capacitance, but also that it can serve as an excellent conductive scaffold to be incorporated with pseudocapacitor active materials to make new composites. To further expand the application space of graphene-based materials, it is necessary to develop a series of graphene derivatives by the ways of alternation of the spatial configuration, modification of the electronic structure, functionalization, and incorporation with other materials to fabrication composites, such as design and fabrication of macroscopic two-dimensional graphene paper/film and three-dimensional graphene foam network, microscopic three-dimensional porous structures, and doping, hydrogenation or functionalized graphene, in order to improve the own EDLCs capacitance of graphene. Meanwhile, incorporation with pseudocapacitor active materials to make composites is also an attractive way to enhance the electrochemical properties for supercapacitos. Those are of momentous current significance to fully utilize the excellent properties of graphene and achieve its potential value. In addition, an important physical quantity, the "quantum capacitance" of graphene-based materials, plays an important role in the electrostatic design of devices due to its unique two-dimensional structure. Intensive study and exploration on quantum capacitance of graphene derivatives will offer an important guidance on its practical applications.Moreover, graphene is also a potential nanofiller that can dramatically improve the mechanical properties of polymer-based composites at a very low loading due to its special structure and excellent mechanical properties. A proper adjustment of graphene flakes’ orientation in polymer matrix with a better understanding of the underlying mechanisms, such as clumped, ordered or random dispersion, plays a significant role on the improvement of graphene efficiency and will boost the rational utilization of composites.Based on the above, we start our work in order to go further study on the potential application of graphene derivatives in energy storage and high-performance composites fields. In this thesis, we designed and fabricated a series of graphene derivatives and their composites via three approaches:alternation of the spatial configuration, modification of electronic structure and control of the orientation of graphene flakes in the composites. Their electrochemical properties and mechanical properties are studied. The major contents are follows:1. First, in order to pave the way for the following design of graphene derivatives and the fabrication of their composites, we synthesized graphite oxide (GO), reduced graphene oxide (RGO), microwave exfoliated graphene oxide (MEGO) and activated microwave exfoliated graphene oxide (aMEGO):through the modified Hummer’s method to prepare GO and using hydrazine as a reducing agent to make RGO; using microwave irradiation to make MEGO and KOH activation method to make aMEGO. Their morphologies and structures are also studied. It shows that exfoliated GO, RGO, MEGO flakes and aMEGO porous structures can be fabricated by different chemical methods;2. With the aim to create a new microstructure of graphene derivatives, we present a novel method to prepare highly conductive, free-standing, and flexible porous carbon thin films by chemical activation of reduced graphene oxide paper. These flexible carbon thin films possess a very high specific surface area of2400m2/g with a high in-plane electrical conductivity of5880S/m. This is the highest specific surface area for a free-standing carbon film reported to date. A two-electrode supercapacitor assembled by this carbon films as electrodes demonstrated an excellent high-frequency response, an extremely low equivalent serial resistance on the order of0.1Ω as well as a high power density of about500kW/kg (a high energy density of26Wh/kg), and an high specific capacitance of120F/g. The free standing thin films can be used as an electrode by eliminating, conducting additives and binders. The synthetic process is also compatible with existing industrial KOH activation processes and roll-to-roll thin-film fabrication technologies and is promising to realize industrialization;3. With the aim to incorporate the two charge storage mechanisms of electric double layer capacitors (EDLCs) and pseudocapacitors, we present the fabrication of aMEGO/MnO2(in short, AGMn) composites via a simple and cost-effective redox process by carbon and KMnO4. The aMEGO with its high electrical conductivity and nanoscale pore size distribution is shown to be an excellent scaffold for MnO2nanoparticles, and the loadings of MnO2can be adjusted by controlling the reaction (the highest loading can reach up to38.1wt%). The as-prepared AGMn composites maintained the three dimensional porous structure as aMEGO and MnO2nanoparticles had a homogeneous dispersion throughout the ultraporous architectures. Symmetric electrochemical capacitors were fabricated that yielded a specific capacitance of256F/g (volumetric:640F/cm3) and a good cyclic stability, and the specific capacitance normalized to MnO2was850F/g. Asymmetric electrochemical capacitors were also fabricated with AGMn composites as the positive electrode and aMEGO as the negative electrode and had a high power density of32.3kW/kg (for a energy density of20.8Wh/kg) and an energy density of24.3Wh/kg (for a power density of24.5kW/kg), showing good electrochemical properties;4. With the aim to modify the electronic structures of graphene derivatives, we introduced additional flow of ammonia gas during the activation process of graphene and fabricated lightly N-doped aMEGO (N-aMEGO). The N loadings can be controlled by changing the activation temperature and the flow rate of ammonia gas. N atom was uniformly inserted into the carbon plane and most of them existed as "N6" and "N5" configurations. The N-aMEGO can remain the porous structure of aMEGO with a high surface area. As electrodes to assemble supercapacitor, N-aMEGO showed a high gravimetric specific capacitance of420F/g and a high area-normalized specific capacitance of22μF/cm2, indicating that the insertion of N atoms into the carbon plane can alter the charge carrier density and enhance the area-normalized capacitance of lightly N-doped aMEGO;5. To get a better understanding of the physical mechanisms for the increase of the interfacial capacitance of the N-doped carbon electrodes, we fabricated single layer pristine graphene (PG) and N-doped graphene (NG) by chemical vapor deposition (CVD) method, measured the quantum capacitance of PG and NG and studied the effects of N loading on the quantum capacitance. Due to the introduction of defects and impurities, the quantum capacitance of PG was not infinite and showed a larger value than its theoretical prediction (0.8μF/cm2), that is,2.5μF/cm2. As increasing of N loadings, the quantum capacitance of NG displayed an increase trend and Dirac point shifted to right with a reducing slope, indicating that N dopants can modify the electronic structure of graphene and cause a certain degree of disorder, which lead to the increase of the quantum capacitance and the weakening of its dependence on potential. Combined with the results in last chapter, the increase of bulk capacitance with increasing N concentration (aMEGO and N-aMEGO), and the increase of quantum capacitance for PG and NG suggest that the increase in the EDLCs type of capacitance of many, if not all, N-doped carbon electrodes studied to date, is primarily due to the modification of the electronic structure of the graphene by the N dopants, which is related to the quantum capacitance of graphene;6. With the aim to control the orientation of graphene sheets in the composites, we used simple solution mixing method and layer-by-layer assembly to fabricate RGO/PVA composites and GO/PVA thin film, respectively. RGO nanosheets in polymer matrix were mostly dispersed randomly and a significant enhancement of mechanical properties of RGO/PVA composites was obtained at low RGO loading (1.8vol%), that is, a150%improvement of tensile strength and a nearly10times increase of Young’s modulus of RGO/PVA composites for1.8vol%RGO loading. A critical point was found of the mechanical improvement versus RGO loadings, which is defined as "mechanical percolation" in our lab, and the underlying mechanism of this phenomenon was also studied. While GO/PVA thin film demonstrated a well-defined layer architecture with high degree of planar orientation of GO sheets in the polymer matrix and an enhancement of elastic modulus and hardness was achieved, that is, a98.7%improvement of elastic modulus and a240.4%increase of hardness.