Fluorine-graphite Intercalation Compounds Research Articles

Contribution of Embedded Bromine Molecules to the Dielectric Properties of Fluorinated Graphite Intercalation Compounds

Here, we study the dielectric properties of fluorinated graphites C2Fx with different fluorine contents (x ≈ 1.05, 0.85, and 0.60) and embedded bromine molecules by impedance spectroscopy. An analysis of the results using the Maxwell-Garnett approximation made it possible to estimate the contributions of the fluorinated graphite matrix and bromine guests to the dielectric permittivity of the compound. The permittivity of the C2Fx matrix increases with decreasing fluorine content and does not depend on temperature. The change in the permittivity upon cooling/heating of the samples is provided by the polarization of bromine molecules Br2. The significant temperature-dependent dielectric response of C2F0.60 is associated with the formation of bromine and polybromide ions, such as Br2– and Br3–, in the interlayer space of the matrix. The decrease in the permittivity at low temperature is explained by the freezing of the ion mobility.

Redox reactions between acetonitrile and nitrogen dioxide in the interlayer space of fluorinated graphite matrices.

The interlayer space of 2D materials can be a slit reactor where transformations not typical for the gas phase occur. We report redox reactions involving acetonitrile and nitrogen oxide guests in galleries of fluorinated graphite. Fluorinated graphite intercalation compounds with acetonitrile are treated with dinitrogen tetraoxide and the synthesis products are studied by a set of experimental methods. Data analysis reveals that N2O4 dissociates in fluorinated graphite matrices to form nitrogen-containing species NO3, NO2, NO, and N2. The interaction of NO3 with acetonitrile yields HNO3, which predominates as a guest in the synthesis products independently of the fluorination degree of the matrix. This reaction is accompanied by the removal of fluorine atoms weakly bonded to the graphite layers, leading to partial defluorination of the matrices. Our work demonstrates the possibility of using fluorinated graphite as a test nanoreactor whose dimension can be controlled by fluorination of the layers.

Room temperature synthesis of fluorinated graphite intercalation compounds with low fluorine loading of host matrix

The interaction of graphite with a gaseous mixture of BrF3 and Br2 at room temperature produced fluorinated graphite intercalation compounds with bromine. By changing the concentration of BrF3 in Br2 from 6.2 to 2.9 % we were able to obtain matrices of fluorinated graphene layers (C2Fx)n with x = 0.76, 0.66, 0.35, 0.25. It was shown that bromine was completely replaced by acetonitrile in the (C2Fx)n with x > 0.6, while two types of guests (CH3CN and Br2) were present between the layers with the lower fluorine loading. The Van der Waals forces hold CH3CN molecules in the (C2Fx)n host. The interaction of Br2 molecules with sp2-carbon regions remained in the C2Fx layers has donor-acceptor type. The models of guest molecules packing in host matrices (C2Fx)n are considered using the data on the stoichiometry and X-ray diffraction analysis.

Microscopic characterization of the C–F bonds in fluorine–graphite intercalation compounds

The structures of fluorine–graphite intercalation compounds (F-GICs, C2.8F and C3.5F) have been analyzed by high-resolution transmission electron microscopy (TEM). Cross-sectional TEM images of the F-GICs indicate that the interlayer distance increases by insertion of fluorine with randomly buckled carbon layers. Such a structure can form by alternation in the bond angle at a carbon atom covalently bonded with fluorine. Electron energy loss spectroscopy combined with TEM indicates that the π-orbital network over the graphitic carbon layer reduces with fluorination. The C–F bond is essentially covalent.

Tuning fluorine and oxygen distribution in graphite oxifluorides for enhanced performances in primary lithium battery

Fluorine Graphite Intercalation Compounds are one of the most common cathode used in primary lithium batteries, mainly for their high delivering potential. Nevertheless, to increase their capacities, fluorine can be partially substituted by oxygen. Oxifluorides offer then versatility to design cathode materials delivering the upmost performances. Combining the choice of the precursor (either graphite oxide or sub-fluorinated graphite fluoride) and two-step synthesis processes, which include Hummers' oxidation and direct fluorination, allow the tuning of the fluorine and oxygen atoms distribution. According to the sequence, i.e. fluorination/oxidation or oxidation/fluorination, several oxifluorides were then prepared. On one hand, direct fluorination using F2 gas resulted in homogeneous fluorine dispersion when precursor was graphite oxide. On the other hand, Hummers’ oxidation was modulated according to the temperature parameter. COH, COC, COOH, CF, CF2 and CF3 were identified through multinuclear solid-state MAS-NMR. Covalence and environment, especially in-plane organization of those chemical groups, were assessed. When oxifluorides are then used as cathode in primary lithium battery, the better performances are obtained for 3 phases compounds where fluorinated part insured high potential whereas oxygenated part allowed durability to be reached; carbonaceous region providing conductivity. A maximum of about 2400 Wh/kg of energy density is attempted.

Structure and supercapacitor properties of few-layer low-fluorinated graphene materials

Graphene materials including porous few-layered graphenes are under focus of scientific community today. The great attention towards them is caused by their remarkable properties. Here, we use two different fluorinated graphite intercalation compounds with different fluorine content in their matrices to prepare thermally exfoliated fluorinated graphites and to investigate them in terms of their structural, spectroscopic and electrochemical properties as materials for use in supercapacitors. A higher temperature of low-fluorinated graphenes preparation resulted in higher exfoliation degree and higher gravimetric capacitance in supercapacitor appliances. More dramatic leap of gravimetric capacitance was observed after a treatment in a concentrated acid mixture of graphene material samples prepared at the highest temperature of thermolysis with highest exposition time.

Fluorine-graphite intercalation compound (C4F)n at high pressure: Experimental and theoretical study

In this paper the structural and vibrational properties of intercalated fluorine graphite (C4F)n under compression up to 40 GPa are investigated by means of Raman spectroscopy, X-ray diffraction and ab initio calculations. The theoretical study was performed within the framework of the density functional theory, taking into account van der Waals corrections to the total energy. We studied the effect of anisotropic and isotropic deformation on the crystal properties, in relation to the layered structure of the sample and in particular to the CF bonding. The evolution with pressure of the phonon modes at the center of the Brillouin zone was calculated and compared with experimental data. Both our experimental data and theoretical models indicate that a structural transition occurs around 10 GPa. Above this pressure the (C4F)n structure becomes dynamically unstable, characterized by a “soft mode” and a change in symmetry due to anisotropic strain components.

Nitrogen species in few-layer graphene produced by thermal exfoliation of fluorinated graphite intercalation compounds

Few-layer graphene materials have been produced by decomposition of fluorinated graphite intercalated with acetonitrile (CH3CN) or isopropylamine ((CH3)2CHNH2) in the conditions of thermal shock. Electronic structure of the samples before and after exfoliation was studied using near-edge X-ray absorption fine structure and X-ray photoelectron spectroscopy. Analysis of the spectroscopic data revealed the same set of nitrogen species in both exfoliation products that indicates decomposition of the intercalated molecules to the atomic state. Most of the nitrogen atoms were incorporated at the edges of graphene layers in pyridinic and protonated nitrogen forms. A difference in the electrochemical performance of the graphene materials was related with the exfoliation degree and the defect density in the graphene layers. Schematic representation of a process of thermal exfoliation of acetonitrile-intercalated fluorinated graphite.

Long-term stability of fluorine–graphite intercalation compound prepared under high pressure of fluorine

Fluorine–graphite intercalation compounds were prepared from highly oriented pyrolytic graphite (HOPG) and natural graphite under high pressure of fluorine (16MPa) at room temperature and were analyzed by means of X-ray diffraction. The rate of intercalation under high pressure of fluorine was higher than that under ordinary pressure. The compositions of the compounds obtained from HOPG and natural graphite were estimated by the weight change before and after fluorination to be C3.41F and C5.62F, respectively. They were blue–black compounds composed of two different phases. One phase is a conventional stage II CxF with the repeat distance of ca. 0.93–0.96nm which was the same compound as that obtained under ordinary pressure of fluorine, and the other was also a stage II with a different repeat distance of ca. 1.03–1.18nm. The fluorine species intercalated between the graphene layers can easily move between both phases and the phase transition occurs periodically. The decomposition rate of fluorine-intercalated HOPG was extremely low and it shows a long-term stability in air for 26years.

Nitrogen inserting in fluorinated graphene via annealing of acetonitrile intercalated graphite fluoride

Fluorinated graphite intercalation compounds (FGICs) with acetonitrile C2Fx · yCH3CN (x = 0.92, 0.69, and 0.49) have been synthesized using a gaseous mixture of BrF3 and Br2 at room temperature and subsequent replacement of the reaction medium components on acetonitrile by isopiestic method. The change in functional composition and electronic structure of FGICs under annealing at 250–260 °C was studied by infrared, X‐ray photoelectron, and near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS) methods. Analysis of the data revealed interaction of acetonitrile with a C2Fx matrix. The X‐ray spectra detected formation of new nitrogen species attributed to two‐coordinated and mono hydrogenated three‐coordinated atoms. Mechanism of nitrogen incorporation in the fluorinated graphene layers was proposed. Schematic structure of graphite fluoride C2Fx intercalated with acetonitrile and NEXAFS N K‐edge spectrum of C2F0.69 · 0.137CH3CN before and after annealing at 250 °C.

Polyethyleneimine functionalised nanocarbons for the efficient adsorption ofcarbon dioxide with a low temperature of regeneration

Polyethyleneimine (PEI) conjugates with a range of nanocarbons (NCs) have been prepared, and their performances with regard to carbon dioxide absorption and liberation are compared. PEI-functionalised multi-walled carbon nanotubes (PEI-MWNTs) prepared by the reaction of branched PEI (25,000 Da) with F-MWNTs in the presence of pyridine, showed a lower CO2 capacity at 25 °C (5 wt%, 1.1 mmol CO2/g adsorbent) as compared to PEI-SWNTs (9.2 wt%, 2.1 mmol CO2/g adsorbent), consistent with the interior layers of the MWNTs adding weight to the base NC without adding functionality. PEI-functionalised graphite/graphene was prepared by three routes: fluorinated graphite intercalation compounds, prepared from natural graphite powder, were reacted with PEI in EtOH with pyridine; exfoliated natural graphite powder was reacted with Boc–Phe(4-N3)–OH, and subsequently PEI to give PEI-Phe(4-N-G); graphite oxide (GO) was reacted with PEI in the presence of NEt3 to give PEI-GO. The CO2 capacity of PEI-GO at 25 °C (8 wt%, 1.8 mmol CO2/g adsorbent) was comparable to that of PEI-SWNTs making GO a valid and cheaper alternative to the SWNT scaffold. The temperature of CO2 desorption of the PEI-NCs was 75 °C, providing a lower energy load for regeneration compared to current amine-based scrubbing units. The rate of CO2 uptake is seen to depend on the curvature of the NC substrate.

ChemInform Abstract: Fluorine Compounds as Energy Conversion Materials

AbstractReview: 35 refs.

Synthesis and IR spectroscopic characterization of fluorinated graphite intercalation compounds with chlorinated derivatives of methane and ethane

We have synthesized (C2FxBr0.01 · yG)n intercalation compounds with (C2FxBr0.01)n as a host and G (dichloromethane, chloroform, carbon tetrachloride, and dichloroethane) as a guest (x and y are stoichiometric coefficients). The behavior of the functional groups in the guest and host subsystems has been studied by IR spectroscopy. We have examined the influence of the degree of fluorination of the polymer host (0.5 < x < 1.0) and the nature of the guest on the vibrational frequencies of the C-F and C-Cl bonds.

The synthesis and properties of highly exfoliated graphites from fluorinated graphite intercalation compounds

A series of new highly exfoliated graphites were synthesized from fluorinated graphite intercalation compounds of compositions C 2F· xR (R = ClF 3, (CH 3) 2CO, CH 3CN, C 6H 6, CCl 4). The exfoliation degree of these materials reaches more than 2000 times, indicating a significantly higher exfoliated state than observed for the conventional expanded graphites derived from graphite bisulphate or nitrate. These exfoliated graphites have increased interlayer distances, specific surface areas as high as 170–370 m 2/g, low bulk densities of 0.4–0.7 g/L, and good sorption capacities towards a range of organic and inorganic liquids. The influence of the nature of the intercalated molecules on the properties of the exfoliated graphite is discussed.

Chemically modified graphene sheets by functionalization of highly exfoliated graphite

Highly exfoliated graphite (HEG) containing multi-, bi- and monolayer graphenes has been prepared from an alternative class of precursors – fluorinated graphite intercalation compounds, namely, poly(dicarbon fluoride) intercalation compounds C2F·xR. Treatment of HEG by mixture of concentrated nitric and sulfuric acids results in chemically modified graphenes (CMG) dispersible in water or alcohol medium without any surfactants or stabilizers. The samples were examined by elemental analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), IR, Raman and UV-vis spectroscopies. Elemental analysis and IR data indicate that covalent functionalization with oxygen-containing groups, such as carboxyl group, occurs upon acid treatment. AFM images show thin sheets of about 1 nm, confirming the well-dispersed state of the material. Dispersions of CMG sheets from highly exfoliated graphite are very stable colloids (over a year) and can serve for further functionalization.

Performances of metal fluoride added carbon anodes with pre-electrolysis for electrolytic synthesis of NF 3

Metal fluoride added carbon anodes treated by pre-electrolysis were investigated for electrolytic production of nitrogen trifluoride (NF 3) in molten NH 4F·KF·4HF at 100 °C. The conditions for pre-electrolysis were first optimized using a graphite sheet anode as a model anode. The formation of fluorine-graphite intercalation compounds (fluorine-GICs) with semi-covalent C–F bonds, (C x F) n , on the MgF 2 and CaF 2 added carbon anode surface was accelerated by pre-electrolysis at potentials less than 4.0 V. Critical current densities (CCD) on the MgF 2 added carbon anodes pre-electrolyzed under various conditions were determined, and the highest CCD was 290 mA cm −2 obtained for that pre-electrolyzed at 3.5 V for 500 C cm −2. This anode was successfully used in the electrolysis at 100 mA cm −2 for 290 h and the maximum NF 3 current efficiency was 55%. From these results, it was concluded that the metal fluoride added carbon anode treated by pre-electrolysis has a high potential for electrolytic production of NF 3 at higher current density.

X-ray study of fluorine-graphite intercalation compounds CF0.25and CF0.33

X-ray study of fluorine-graphite intercalation compounds CF_0.25and CF_0.33

Characteristics of the Fluorocarbon Surface Film Generated on Carbon Anode during F2 Evolution in Molten KF-2HF

During the fluorine evolution reaction occurring in the electrolysis of molten KF-2HF, a thin, solid fluorocarbon layer is formed on the carbon anode. The high anodic overvoltage which characterized this process is mainly due to the presence of such a film which drastically changes the wettability of the electrode by the melt. Activation of carbon at in KF-2HF modifies strongly the surface morphology (scanning electron microscopy) and composition (energy-dispersive X-ray and X-ray photoelectron spectroscopy) of the surface fluorocarbon film. In addition, it enhances the electrochemical performances of the carbon electrodes vs fluorine evolution. Local microconductivity measurements carried out by atomic force microscopy have revealed heterogeneities of composition of the carbon-fluorine film due to the presence of conducting fluorine-graphite intercalation compounds and insulating graphite fluorides within the film. Finally, the peculiar shape of the fluorine bubbles observed during electrolysis of molten KF-2HF was studied, taking into account the presence of these two fluorocarbon compounds on the surface.

Preparation of low-temperature graphite fluorides through decomposition of fluorinated-graphite intercalation compounds

The decomposition of C x FCl y · zR (R = N2O4, N2O, SO2) fluorinated graphite intercalation compounds at 370–420 K leads to the formation of C x FCl y graphite fluorides with x close to 2 and y close to 0.1. The C x FCl y fluorides are first-stage compounds with an interlayer spacing of 0.60–0.61 nm. According to the synthesis conditions and their structure, these compounds can be classed with low-temperature C2F graphite fluorides. They are stable in air and inert atmospheres up to 720–750 and 750–780 K, respectively, which is comparable to the thermal stability of high-temperature CF and C2F graphite fluorides.

X-ray photoelectron spectroscopy study of intercalated compounds of fluorinated graphite C2FxBr0.01·yCH3CN

The energy level structure of fluorinated graphite intercalation compounds C2FxBr0.01·yCH3CN (x = 0.49–0.87, y = 0.084–0.136) has been studied by X-ray photoelectron spectroscopy providing the information on the electronic structure of compounds in question. The analysis of variations of the binding energy of core levels C1s, F1s, and O1s opens the possibility to explore the nature of the chemical bond C-F in the fluorographite matrix with a varying degree of fluorination, as well as to model the structure of these compounds. The examination of decomposition results of spectra into components has revealed the occurrence of C-F fragments, carbon atoms not bonded directly to fluorine atoms, and “graphite-like” areas, whose contribution to the overall structure increases with the degree of matrix fluorination decreasing. The presence of oxygen was considered from the viewpoint of surface phenomena characteristic of low-temperature carbon materials.