Electrochemical intercalation of aluminium chloride in graphite in the molten sodium chloroaluminate medium

Aluminium chloride intercalation in graphite was studied by anodic oxidation of compacted graphite (rod) and graphite powder electrodes in sodium chloroaluminate melt saturated with sodium chloride at 175 °C. The studies carried out by employing both galvanostatic and cyclic voltammetric techniques had shown that the intercalation reactions take place only beyond the chlorine evolution potential of +2.2 V vs. Al on both the electrodes. The extent of intercalation reaction was directly related to the anodic potential and probably to the amount of chlorine available on the graphite anodes. In the case of graphite powder electrode, a distinctly different redox process was observed at sub-chlorine evolution potentials and this was attributed to the adsorption of chlorine on its high surface area. This finding contradicts a report in the literature that the intercalation reactions occur at potentials below chlorine evolution in the chloroaluminate melt.     

Exfoliation of nitric acid intercalated carbon fibers: effects of heat-treatment temperature of pristine carbon fibers and electrolyte concentration on the exfoliation behavior

Effect of heat treatment temperature of mesophase pitch-based carbon fibers on the exfoliation behavior of derived intercalation compounds with nitric acid was studied. Carbon fibers heat-treated above 2500 °C gave intercalation compounds with mass increase of more than 80 mass% and resulted in a marked exfoliation by a rapid heating to 1000 °C, where no memory of original single fiber was observed. On those below 2000 °C, on the other hand, their residue compounds showed mass increase less than 80 mass% and the appearance after exfoliation at 1000 °C was similar to the original single fiber. On 1150 °C-treated carbon fibers, mass increase was only 13 mass% and no evidence of intercalation was detected even after electrolysis and, as a consequence, the formation of only small fissures along their fiber axis was observed, with no apparent exfoliation. The dependence on electrolyte concentration was also examined on 3000 °C-treated carbon fibers.     

Characterizations of expanded graphite/polymer composites prepared by in situ polymerization

Poly(styrene-co-acrylonitrile)/expanded graphite composite sheets with very low in-plane (8.5 × 10⁻³ Ω cm) and through-thickness (1.2 × 10⁻² Ω cm) electrical resistivities have been prepared. The expanded graphite was made by oxidation of natural graphite flakes, followed by thermal expansion at 600 °C. Microscopic results disclosed that the expanded graphite has a legume-like structure, and each “legume” has a honeycomb sub-structure with many diamond-shaped pores. After soaking the expanded graphite with styrene and acrylonitrile monomers, the polymer/expanded graphite composite granules were obtained by in situ polymerization of the monomers inside the pores at 80 °C. The functional groups and microstructures of the oxidized graphite, expanded graphite and composites in the forms of particles or sheets were carefully characterized using various techniques, including X-ray powder diffraction, thermogravimetry, optical and electron microscopy. It was found that the honeycomb sub-structure survived after hot-pressing, resulting in a graphite network penetrating through the entire composite body, which produces a composite with excellent electrical conductivity.     

Electrochemical insertion of lithium ions into disordered carbons derived from reduced graphite fluoride

Both covalent (obtained by direct fluorination at high temperature) and semi-ionic carbon fluorides (synthesized at room temperature) were reduced in order to obtain disordered carbons containing very small content of fluorine and different physical properties according to the reduction treatment (chemical, thermal or electrochemical). After a physical characterization (X-ray diffraction, electron spin resonance and FT-IR spectroscopies), the electrochemical behaviours of the pristine carbon fluorides and of the treated samples were investigated during the insertion of lithium using liquid carbonate-based electrolytes (LiClO₄-EC/PC, 50:50%, v/v). Both galvanostatic and voltammetric modes were performed and revealed that the voltage profiles and the capacities differed according to the starting material and the reduction treatment. Semi-ionic carbon fluoride treated in F₂ atmosphere for 2 h at 150 °C and then chemically reduced in KOH exhibits high reversible capacities (the reversible capacity is 530 mAh g⁻¹ in the second cycle); in this case, the voltage profiles show a large flat portion at potentials lower than 0.3 V which is attributed to the insertion/deinsertion of lithium ions between the small graphene sheets and/or the absorption of pseudo metallic lithium into the microporosity of the sample. Nevertheless, a part of the lithium ions are removed at potentials higher than 0.5 V versus Li⁺/Li limiting the useful capacity.     

Surface characteristics of fluorine-modified PAN-based carbon fibers

Different fluorination methods were applied to modify the surface properties of carbon fibers. The relationship between the degree of fluorination and the physicochemical properties of carbon fibers was studied using a combination of mechanical tests, elemental analysis (EA), X-ray photoelectron spectrometry (XPS), and X-ray diffraction (XRD). EA and XPS analyses of fluorinated carbon fibers showed that treatment with mixtures of F₂/O₂ introduced a much higher fluorine concentration than that with F₂ only. However, XRD analysis showed that there was no increase in the interlayer distance, due to the mild fluorination condition applied. Consequently, the oxyfluorination was one of the more effective methods to increase surface polarity of carbon fibers, which probably played an important role in improving the tensile properties of the fibers in the epoxy resin system.     

The electrical and thermal conductivity of woven pristine and intercalated graphite fiber-polymer composites

A series of woven fabric laminar composite plates and narrow strips were fabricated from a variety of pitch-based pristine and bromine intercalated graphite fibers in an attempt to determine the influence of the weave on the electrical and thermal conduction. It was found generally that these materials can be treated as if they are homogeneous plates. The rule of mixtures describes the resistivity of the composite fairly well if it is realized that only the component of the fibers normal to the equipotential surface will conduct current. When the composite is narrow with respect to the fiber weave, however, there is a marked angular dependence of the resistance which was well modeled by assuming that the current follows only along the fibers (and not across them in a transverse direction), and that the contact resistance among the fibers in the composite is negligible. The thermal conductivity of composites made from less conductive fibers more closely followed the rule of mixtures than that of the high conductivity fibers, though this is thought to be an artifact of the measurement technique. Electrical and thermal anisotropy could be induced in a particular region of the structure by weaving together high and low conductivity fibers in different directions, though this must be done throughout all of the layers of the structure as interlaminar conduction precludes having only the top layer carry the anisotropy. The anisotropy in the thermal conductivity is considerably less than either that predicted by the rule of mixtures or the electrical resistivity.     

Chemical and electrochemical intercalation of lithium into boronated carbons

Boron-substituted carbons have been produced by chemical vapour deposition from acetylene and boron trichloride precursors at 1140°C. Li intercalation was investigated, chemically from the Li vapour and electrochemically in Li–carbon cells. In both cases, the amount of intercalated Li increases with the boron content of the carbon, up to a value of 13 at.% boron. Above this value, the intercalation of lithium is not so efficient. This behaviour is understood by assuming that boron, which acts as an electron acceptor and hence favours the intercalation of electron donors like lithium, can enter substitutionally into the carbon lattice up to a certain limit, which is close to 13 at.% for the materials deposited at 1140°C.     

Short residence time graphitization of mesophase pitch-based carbon fibers

The effects of graphitization time and temperature on the properties of three mesophase pitch-based carbon fibers have been characterized. Graphitization temperatures studied were 2400, 2700, and 3000 °C and residence times ranged from 0.7 to 3600 s. Helium pycnometry, measurements of fiber tow resistance, and X-ray diffraction were employed to study fiber properties. As anticipated, substantial variations in fiber properties were noted for the range of graphitization conditions studied and among the three fiber types. Significant structural evolution and property development occurred even at the shortest furnace residence times. For example, for one of the fibers, a furnace residence time of 0.7 s at 3000 °C resulted in a degree of graphitization value of ∼50%, a density of 1.98 g/cm³, and an electrical resistivity of 6.3 μΩ m (corresponding thermal conductivity ∼200 W m⁻¹ K⁻¹). A simple energy consumption analysis suggests that short residence time graphitization at high temperature may result in both lower costs and substantially higher production rates for fibers prepared from mesophase pitch.     

Exfoliated carbon fibers as an electrode for electric double layer capacitors in a 1 mol/dm H2SO4 electrolyte

Exfoliated carbon fibers (ExCFs) synthesized through the rapid heating of intercalation compounds of carbon fibers were examined as electrodes of electric double layer capacitors (EDLC). The measurement of EDLC was performed using a standard three-electrode cell with 1 mol/dm³ sulfuric acid electrolyte. The capacitance of as-prepared ExCFs reached 117 F/g, even though they had a relativity small surface area of about 330 m²/g. After air activation of ExCFs, the BET surface area increased slightly, but the capacitance of the EDLC increased up to 160 F/g. Capacitance of ExCFs strongly depended on their BET surface area, having a different dependence from that reported on activated carbon fibers.     

Surface properties of oxyfluorinated PAN-based carbon fibers

In this study, the oxyfluorination of PAN-based carbon fibers was undertaken at room temperature using fluorine-oxygen mixtures, and the influence of oxyfluorination on properties such as wettability, surface polarity, surface free energy, conductivity and tensile strength was investigated. As the oxyfluorination time increased at a total pressure of 5 kPa, both the fluorine/carbon and oxygen/carbon ratios increased. The contribution of semicovalent C-F bond to F1s spectra is considerably decreased with increasing total pressure from 5 to 80 kPa, and at the same time, the contribution of covalent C-F bond is increased. As the total pressure of fluorine-oxygen mixtures increased, the contact angle of water significantly decreased and again increased to a similar value to that of as-received carbon fiber. A short oxyfluorination of carbon fibers considerably increased the wettability, that is, hydrophilicity.The electrical conductivity of oxyfluorinated carbon fiber is larger than that of the as-received fiber. This is because the surface region of carbon fiber is fluorinated. An increase in the tensile strength of about 18% after oxyfluorination is observed. The increase in tensile strength of oxyfluorinated carbon fibers can be understood as being due to a decrease in the diameter of the fiber.     

Textural characterisation of graphite matrices using electrochemical methods

The present work describes a novel approach for the textural characterisation of a carbonaceous porous medium. Due to the electronic conduction properties of expanded natural graphite (ENG), electrochemical determination was proposed and the use of cyclic voltammetry enables determination of the surface area and porous volume of ENG. First, experiments were performed without the electrochemical electron transfer reaction. Then, the volume of the entire transformation of potassium hexacyanoferrate into the pores of the material was deduced by applying the rules of thin-layer electrochemistry. For the specific surface area and the porosity, the results are satisfying, but in comparison with classical methods, the electrochemical procedure does not allow the determination of the pore size distribution of the material. It is possible to evaluate the mean pore diameter and to assume that the double-layer capacitance may depend on the kind of graphite surface (basal or edge plane).     

Biopitch-based general purpose carbon fibers: Processing and properties

Eucalyptus tar pitches are generated on a large scale in Brazil as by-products of the charcoal manufacturing industry. They present a macromolecular structure constituted mainly of phenolic, guaiacyl, and siringyl units common to lignin. The low aromaticity (60-70%), high O/C atomic ratios (0.20-0.27%), and large molar mass distribution are peculiar features which make biopitches behave far differently from fossil pitches. In the present work, eucalyptus tar pitches are evaluated as precursors of general purpose carbon fibers (GPCF) through a four-step process: pitch pre-treatment and melt spinning, and fiber stabilization and carbonization. Homogeneous isotropic fibers with a diameter of 27 μm were obtained. The fibers had an apparent density of 1.84 g/cm³, an electrical resistivity of 2 × 10⁻⁴ Ω m, a tensile strength of 130 MPa, and a tensile modulus of 14 GPa. Although the tensile properties advise against using the produced fibers as structural reinforcement, other properties give rise to different potential applications, as for example in the manufacture of activated carbon fibers or felts for electrical insulation.     

Surface compatibility in a carbon-alloy composite and its influence on the electrochemical performance of Li/ion batteries

Carbon-alloy composites were prepared by coating carbon materials with different surfaces with Sn, Sb or SnSb. The SnSb-hard carbon spherule (HCS) composite electrode shows the best cycling performance. The matching of crystalline parameters between SnSb and the carbon ensures a good dispersion of SnSb alloy on the surface. The high density of the nucleation centers on the HCS surface leads to a small SnSb crystallite size. Open pores on the surface of HCS, into which the alloy crystals are allowed to grow, act as pinning centers, which further stabilize the composite. These three factors are supposed to be responsible for the best cyclic performance of SnSb-HCS composite. The latter two factors also result in a large BET surface area, which leads to a large initial irreversible capacity loss, because more solid electrolyte interface film is formed. It seems that the best cyclic performance and the highest initial efficiency are paradoxical in the SnSb-carbon composite. Further surface modification should be conducted to obtain better electrode materials.     

Reversible intercalation of HF in fluorine-GICs

Reversible intercalation/deintercalation processes of HF were characterized for first stage fluorine-graphite intercalation compounds with semi-ionic C-F bonds (fluorine-GICs; C_xF) by X-ray diffraction and infrared absorption spectroscopy. C_xF(HF)_δ prepared by the intercalation of HF to C_xF possesses the interlayer distance of about 0.60 nm in the composition range of 2.1<x<2.6. Absorption peaks ascribed to the vibrational modes of carbon sheets in HF-intercalated C_xF are observed at higher wavenumbers with stronger intensities than those for HF-free C_xF. Attractive interaction is suggested to exist between the intercalated HF molecules and C_xF caused by the polar characters of both the H-F bonds in the former and the semi-ionic C-F bonds in the latter.     

Electrochemical and morphological study of the effect of polymerization conditions on poly(tetrathiophene) with emphasis on carbon fiber microelectrodes: A cyclic voltammetry and atomic force microscopy study

Electropolymerization of tetrathiophene was carried out in various solvents on carbon fiber microelectrodes and platinum electrodes. Acetonitrile, dichloroethane and propylene carbonate were investigated as solvent systems for film formation, and solvent effect as well as the effect of other experimental conditions (scan rate, supporting electrolyte) on the electropolymerization has been discussed on the basis of the electropolymerization mechanism. Surface morphology was investigated by atomic force microscopy (AFM). Carbon fiber electrodes were found to be an effective microelectrode system for the electropolymerization of tetrathiophene, which was compared with polyterthiophene growth on carbon fibers.     

Flexible graphite as a heating element

Flexible graphite is an effective heating element. It provides temperatures up to 980 °C (though burn-off occurs in air at 980 °C), response half-time down to 4 s, and heat output at 60 s up to 5600 J. The electrical energy for heating by 1 °C is 1-2 J in the initial portion of rapid temperature rise. The temperature and heat output increase with decreasing thickness and with increasing power.