| Graphene refers totwo-dimensional carbon materials consisting of single- or few-layer of graphitic sheets where the carbon atoms residing in hexagonal lattice structures. Graphene has many excellent properties:high carrier mobility (?200,000 cm2/V · s); superior conductivity (?106 S/cm); high thermal conductivity (?5300 W/m · K); high light transmittance (only?2.3%); high mechanical strength (to withstand the extreme pressure of up to 1.06 T Pa) and a large specific surface area (-2600 m2/g). Because of these superior characteristics, graphene has great prospect in many areas, and received wide attention and research interest from scientific community. Graphene and graphene-based composite materials have been widely applied to super capacitors, lithium ion batteries, solar cells, biosensors, nano-electronic component, field effect transistor, fuel cells and transparent conductive films, etc.The wide applications of graphene and its composites should be based on the large scale preparation of graphene. In recent years, researchers have proposed various methods for the preparation of graphene, including micromechanical exfoliation, chemical vapor deposition, silicon carbide epitaxial growth, arc discharge, electrochemical exfoliation of graphite,high temperature thermal exfoliation of graphite oxide (GO) and chemical reduction graphene oxide, etc. Among these methods, the graphite oxide routes are considered to be most promising for mass production of graphene. However, they suffer from many difficulties. First, preparation of GO is difficult. GO is prepared by oxidation of graphite powder in concentrated sulfuric acid. After the oxidation, the separation of GO from sulfuric acid is proceeded by repeatedly washing with vast amounts of water and hydrochloric acid. Second, a chemical reduction and exfoliation of GO needs both extra mechanical energy and expensive and/or toxic reductant, while a high temperature decomposition is energy intensive; thirdly, both chemical and thermal reductions usually introduce extra defects in graphene. Therefore, it is necessary to find an economic, environmental friendly, energy saving approach for the mass-production of graphene. Herein, we proposed anelegant method for graphene and graphene-based composite preparation throughthermal exfoliation of sulfuric acid intercalated graphite oxide (SIGO) at temperatures low to 110 ?. We call this novel method as SIGO process. We also studied the electrochemistry application of the graphene and its compositespreparation by SIGO method. The results are as follows:1. Preparation of graphene by SIGO process and the mechanism.If look into the structure of the graphene oxide sheet, it can be found that although a significant fraction of the original sp2-C in graphitic sheet have been oxidized to sp3-C bonding to the 0 atoms (predominantly hydroxyl and epoxy groups) and H atoms (CH and CH2), they are still staying in the arrayed hexagonal structures. A direct dehydration between the O and the H atoms should be desirable for a perfect restoration of the graphitic sheet. We hope the intercalated sulfuric acid in the SIGO can catalyze a rapid and complete dehydration. We found SIGO is easily available as the immediate oxidation descendent of graphite in sulfuric acid, and can undergo rapid reduction and expansion exfoliation to graphene at temperature just above 110 ? in ambient atmosphere. The sulfuric content in SIGO has great influence on the expansion temperature of SIGO. A content range between 3-6 wt.% could resulting the lowest expansion temperature, namely, about 110 ?. The oxygenic and hydric groups in SIGO are mainly removed through dehydration.The yield of graphene with reference to GO of the SIGO process was about 52%. In comparison, the the traditional thermal exfoliation of GO at temperatures higher than 900 ?led to a signicantly lower graphene yield (38%), because of the decomposition of GO to CO2. This SIGO process is reductant free, easy operation, low-energy, environmental friendly and generates graphene with low oxygen content, less defect and high conductivity. The provided synthesis route from graphite to graphene via SIGO is more compact and readily scalable.2. Supercapacity behavior of the graphene prepared by SIGO method. SIGO samples containing different amounts of sulfuric acid, were used as the precursors to prepare graphene. Several factors, such as sulfuric acid content, the expansion temperature and the electrolytes that may have influence on the capacitance of graphene electrodewere investigated. By comparing the electrochemical capacity of different graphene electrode, we found that graphene prepared from SIGO with 4.0wt% sulfur content at 120? (ESIGO-120) exhibited the best performance. ESIGO-120 electrode deliverd the highest specific electrochemical capacitance in 6 M KOH solution, which reached 267 F/g at the current density of 1 A.g-1.The electrochemical capacity was about 252F/g at 0.5 A/gin the 0.6M tetraethyl ammonium tetrafluoroborate (TEATFB) electrolyte.3. Synthesis of Mn3O4-graphene composites by the SIGO process for electrochemical capacitors. Mn3O4-SIGO composites were prepared by coating of Mn3O4 nano-particles on SIGO, where the Mn3O4 was synthesized by chemical redox reaction. The resulting Mn3O4-SIGO composites with different Mn3O4 content were thermally expanded to Mn3O4-graphene. Electrochemical supercapacitorbehavior ofall of the composite materials were tested in 1M Na2SO4 and 1M KOH electrolytes. It was found that the Mn3O4-Graphene compositeelectrode containing 49wt.% Mn3O4 exhibited the best electrochemical performance and a capacity up to 198F/g at acurrent densityof 0.2A/g.4. Synthesis of Fe2O3-graphene composites by the SIGO process for lithium batteries. Fe2O3-SIGO composites were prepared prepared by hydrolysis of FeCl3 in a SIGO solution. Fe2O3-SIGO composites were then thermally exfoliated to Fe2O3-Graphenecomposites with different iron oxide contents. The ferric oxide particles in composite materials were about 5-10 nm in sizes and evenly distributed. Fe2O3-GNS composite with iron oxide content of 40wt.% exhibits the best electrochemical performance. At current density of 0.1 A/g, the charge capacity of first cycle is about 839mAh/g and maintains 798mAh/g after 100 cycles. 5. Graphene film was prepared by sulfuric acid catalyzed dehydration of graphene oxide film, and the obtained graphene film was tested for supercapacitor electrodes. The synthesized graphene film was porous and exhibited high capacitance; the capacitance is up to 287F/g at a current density of 1A/g. When used 0.6M tetraethyl ammonium tetrafluoroborate (TEATFB) as the electrolyte, the capacitance was up to 267F/g at a current density of 1 A/g.6. Tin dioxide-graphene film (SnO2-GNS-film) was prepared through sulfuric acid catalytic deoxygenation of graphene oxide. This film material without adding conductive agent and binder can be directly used for lithium-ion battery anode material, and showed high capacity and good cycle stability. At the current density of 0.2A/g, the charge capacity of the composite film is of up to 808mAh/g and maintains 693mAh/g after 200 cycles. The tin dioxide particles in the film material are about 3nm, which can effectively alleviate the electrode volume expansion and structure collapse in lithium alloying process. |
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