Preparation And Assembly Of Chemically Reduced Graphene And Its Electrochemical Performance

Graphene, a kind of two-dimensional carbon material constructed by layers of sp2-bonded carbon atom, exhibits applications in fields of electronics, composite materials, catalysis, energy generation and storage etc owing to its extreme mechanical strength, exceptionally high electron mobility and thermal conductivities and has stimulated tremendous attention for both the experimental and theoretical scientific communities in recent years since it was discovered at2004by Geim. Graphene produced through chemically reducing graphene oxide obtained from graphite is considered to be the more efficient, inexpensive and simpler approach to large-scale use, it is still the hot issues to seek the safe, efficient, pollution-free reducing agent and study its mechanism. However, graphene dissolved in water prone to an irreversible coagulation and affect the performace of the graphene. Therefore, the morphology control, chemical doping and etching are effective approach to tune the property of graphene and greatly expand their applications. In this dissertation, based on the materials chemistry of graphene, the preparation, doping, assembly and their electrochemical properties of graphene-based materials were investigated. The main contents and results are summarized as follows:(1) GO being exposed to a vapor and liquid phase of formaldehyde and formic acid at a reaction temperature below200℃were studied using XRD, XPS and Raman spectroscopy. The reducing agent concentration, reducing temperatures and time intensively affect the electrical conductivity of chemically reduced graphene (rGOs). Significant reduction of GO was observed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), with the largest extent of reduction obtained at150℃in vapor phase and at175℃in liquid phase. In addition, with the increase of reaction time such as24h, O content of rGO increase obviously when it obtained in vapor phase while the O content increases slightly or even decrease for rGO obtained in solution of the reductant. It is reveals that the atomic ratio of C to O has some correlation to the electrical conductivity of rGO.(2) N-doped graphene hydrogels (NGHs) could be obtained by using hydroxylamine hydrochloride and Hydroxylamine (HA) as the chemical dopant and reductant through a simple solvothermal reaction. The products were characterized by scanning electron microscope, XRD, XPS, Raman spectroscopy and electrochemistry. The results showed that the electrical conductivity, microstructure and doping level of NGHs were influenced by the type and quantity of reductants, temperature and time of the reaction. The NGHs prepared at150℃for12h using HA as reductant (NGH-HA12) had a N-doping level of4.32%in atom and exhibited a specific capacitance of205F g-1and good cycling stability. The energy density and power density could reach3.65Wh kg-1and20.5kW kg-1at a discharge of100A g-1for the symmetric capacitor assembled by NGH-HA12in25%KOH electrolyte using the two-electrode symmetric capacitor test.(3) Various N-doped graphene-based materials with large area, including N-doped graphene paper (NG-P), N-doped graphene and carbon nanotubes composite paper (NG-CNT-P), N-doped graphene transparent films (NG-TF) and the composite transparent films deposited on substrates, are prepared using a facile approach named as "hydroxylamine diffusing inducing assembly (HDIA)". The molding of GO is carried out at room temperature, the reduction of graphene oxide and the N-doping are achieved simultaneously under atmospheric pressure and at low temperature (100℃). The NG-P treated at100℃for10h possesses a tensile stress of about70.0MPa and Young’s modulus of about17.7GPa. Using the two-electrode symmetric capacitor test, ultrahigh-rate capacitors assembled by NG-P treated at300℃for2h (NG-P300) still show rectangle-liked CV curves at scan rate of800V s-1and exhibit a phase angle of-77.1°at frequency of120Hz in1M H2SO4electrolyte, thermal conductivity of NG-P300is calculated to be about3403.39W m-1K-1. The surface resistivity of NG-TP can reach up to4000Ω/a at78%transmittance (550nm).(4) Using the similar process, larger-scale N-doped graphene fiber mats (NG-FM) are prepared using the "hydroxylamine diffusing inducing assembly (HDIA)" The obtained NG-FM exhibits good flexibility. Using the two-electrode symmetric capacitor test, the obtained NG-P treated at300℃for2h (NG-FM300) possesses a specific capacitance of188F g-1at scan rate of5mV s"1and can retain Ccs51%at10V s-1, the energy density of capacity assembled by NG-FM300can reach to3.1Wh kg-1at a power density of114,000W kg-1in25%KOH.(5) Carbon fibre mats modified by graphene (GCFMs) have been fabricated by combining the electrospinning, thermal treatment and surface decoration. Pt particles were deposited on the GCFM using formaldehyde vapor react with H2PtCl6·6H2O on the GCFM. The electrochemical catalytic electrodes are characterized and evaluated. The results show that the Pt catalyst loaded on GCFM possesses high electrocatalytic activity, good tolerance towards reaction intermediates and unusually high stability towards methanol electrocatalytic oxidation compared with Pt-CFM (the Pt catalyst loaded on CFM) and Pt/C-GCFM(commercial Pt/C doped to GCFM)catalytic electrodes. It is found that the special structure of the GCFM is favorable for the activity and long-term stability of the Pt catalyst towards methanol electro-oxidation, indicating that the GCFM is a hopeful candidate to be developed as an excellent supporting material for electrochemical catalysts.