On the role of dihydrogen in the co-intercalation reactions into graphite of potassium and chalcogen

The influence of dihydrogen gas on the reactions between graphite and liquid potassium containing a very small amount of a chalcogen (O, S, Se, Te) was studied. The reactions were carried out under a pure argon atmosphere in a stainless steel reactor, between 400 and 600°C. Controlled amounts of dihydrogen gas can be added in this reactor. When dihydrogen is strictly absent, the co-intercalation of potassium and chalcogen does not take place at 400°C: only potassium intercalates, leading to the KC₈ binary compound. The same experiments carried out with controlled amounts of dihydrogen at the same temperature lead to various ternary compounds with oxygen, sulphur, selenium and tellurium. However, at 600°C, and strictly without dihydrogen, co-intercalation occurs, but only for S, Se and Te, allowing the preparation of new well-defined ternary graphite intercalation compounds. The co-intercalation of potassium and oxygen is possible only in the presence of dihydrogen, at any temperature.     

On the great difficulty of intercalating lithium with a second element into graphite

Lithium is able to intercalate into graphite leading to various binary graphite intercalation compounds, that are well defined by their stage. Concerning the ternaries, there is little literature on the subject. Thermodynamical and structural data, that differ largely from those of the other alkali metals, lead one to foresee some serious difficulties in synthesising such ternary compounds. Many experiments have attempted to synthesise ternary graphite intercalation compounds with lithium, using successively very electronegative elements, then fairly electronegative species and lastly electropositive metals. Numerous results, that are wholly negative, are described in this paper. The calcium-lithium system only allows one to prepare a novel intercalation compound, that is a first stage ternary phase exhibiting a large interplanar distance. This latter suggests that the intercalated sheets consist of several superimposed atomic layers. The synthesis of this ternary is not easy, because it needs reagents of very high purity. It possesses the brightness of metals and its strong hardness is very unusual among graphite intercalation compounds. On the other hand, the charge transfer between the graphene planes and the intercalated sheets, that just allows the intercalation, is especially high, and much higher than the LiC₆ compound.     

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.     

Synthesis and structural investigation of new graphite intercalation compounds containing the perfluoroalkylsulfonate anions C10F21SO3, C2F5OC2F4SO3, and C2F5(C6F10)SO3

The preparation of graphite intercalation compounds (GIC’s) of three perfluorinated alkylsulfonate anions, C₁₀F₂₁SO₃⁻, C₂F₅OC₂F₄SO₃⁻ and C₂F₅(C₆F₁₀)SO₃⁻ is described for the first time. Pure stage 2 GIC’s are obtained by chemical oxidation of graphite with K₂MnF₆ in a solution containing hydrofluoric and nitric acids for 72 h. One-dimensional electron density maps derived from powder diffraction data are fit to obtain models for the intercalate interlayer regions (galleries) structures: the structure models provide details on anion concentrations, orientations, and conformations. In all cases, anion bilayers are observed with anion sulfonate headgroups oriented towards graphene sheets. Compared with structures calculated for the isolated anions, the intercalated anion conformations show changes in dihedral angles, involving rotations about C-C or C-O bonds. For the GIC containing C₂F₅(C₆F₁₀)SO₃⁻, the anion conformation change is related to the more efficient packing of anions in the intercalate gallery.     

Nuclear microanalysis: An efficient tool to study intercalation compounds containing lithium

Lithium can intercalate easily into graphite leading to the LiC₆ compound but the synthesis of a ternary compound associating lithium with a second element seems to be difficult. Recently, graphite-lithium-calcium compounds were obtained by reaction of a pyrographite platelet in a molten Ca-Li alloy at 350 °C. Chemical analyses, electron microprobe, SEM and TEM give the C/Ca ratio but do not allow to determine the lithium concentration and its distribution in these compounds. Therefore, the nuclear microprobe was used to characterise more precisely these ternary intercalation compounds. Using a 3.1 MeV proton beam, the three elements can be quantified simultaneously from the ⁷Li(p,α)⁴He nuclear reaction for lithium and from elastic scattering for calcium and carbon. Among the three synthesised compounds, one of them (α phase) opposes great heterogeneities in lithium and the amount of lithium in the β phase is very high (C/Li ratio approaches 2).     

Direct conversion mechanism of fluorine-GIC into poly(carbon monofluoride), (CF)n

The reaction mechanisms of a stage-1 fluorine-graphite intercalation compound (GIC), C_2.5F, with 0.10 MPa of fluorine gas have been studied at 573-773 K. The original stage-1 structure of C_2.5F with semi-ionic C-F bonds and planar sp² carbon sheets is maintained in most part of the compound after the reaction at 573 K, although a large number of covalent C-F bonds are formed on the surface. This compound is partially or completely converted to poly(carbon monofluoride), (CF)_n, with covalent C-F bonds and puckered sp³ carbon sheets at 673 or 773 K, respectively. Single-phase (CF)_n obtained at 773 K possesses remarkably small BET specific surface area, 61 m²/g carbon, almost unchanged from the value of the precursor C_2.5F (69 m²/g carbon). In this reaction, the accommodation of fluorine atoms supplied from the atmosphere into the galleries of C_2.5F is facilitated by the rearrangement of originally intercalated fluorine atoms in the GIC, forming (CF)_n with fewer defects compared to those by the conventional direct fluorination of graphite.     

Graphite intercalation compounds as PEMFC electrocatalyst supports

Intercalation of metal chlorides into fine natural graphite flakes of 2 × 10⁻⁶ m in diameter was attempted. CuCl₂ was satisfactorily intercalated from the vapour phase. NiCl₂ and PdCl₂ were also intercalated from the liquid phase using chloroform solvent. The reduction products of the obtained graphite intercalation compounds were used as carbon supports of the platinum electrocatalyst for a proton exchange membrane fuel cell. Oxygen reduction activity was evaluated and it can be seen that natural graphite flakes treated using the intercalation technique provide different levels of activity from that of pristine natural graphite flakes. In particular, the catalytic activity was enormously improved when CuCl₂ was used as the intercalate.     

Scanning tunneling microscopy study of a BiCl3–graphite intercalation compound

In the presence of chlorine gas, BiCl₃ was intercalated from the gas phase into highly oriented pyrolytic graphite; second-stage compounds were obtained. The surface structure in air was determined with a scanning tunneling microscope. A long-periodical rectangular structure was observed with an a-axis of 1429 pm and a b-axis of 1841 pm. This commensurate structure fits very well with X-ray diffraction data for a superlattice in BiCl₃ graphite intercalation compounds published by another group.     

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.     

Characterization of n-hexadecylalkylamine-intercalated graphite oxides as sorbents

Adsorption properties of graphite oxides hydrophobized by n-hexadecylamine, (C16)_xGO, were investigated using pyrene molecules as a model of nonionic organic contaminants. A large quantity of pyrene (28.5 mg/g) was adsorbed from a water-ethanol mixture (1:2) containing 2 mM of pyrene when (C16)_0.6GO was used. The isotherm of pyrene adsorption was better described by Freundlich equation rather than Langmuir equation, which indicated a single adsorption mechanism was involved. The change in the amount of adsorbed pyrene as a function of amine content in GO was very similar to that which occurs upon introduction of pyrene into (C16)_xGO films from chloroform solution, as determined by X-ray measurements. This suggests that pyrene molecules were adsorbed not only on the outer surface but also within the interlayer space of the intercalation compound. Swelling of the intercalation compound by ethanol can render the interlayers more organophilic and make access to hexadecylamine molecules bonded to the graphite oxide layer easier for pyrene molecules, especially in (C16)_xGOs with lower amine contents.     

Aqueous electrochemical intercalation of bromine into graphite fibers

For the first time, graphite fibers have been electrochemically intercalated with Br⁻ that have the same structure and properties as those intercalated from vapor phase Br₂. This was accomplished by intercalating pitch-based Thornel^® K-1100 graphite fibers at low temperature (near 0 °C) and high currents (2 A) for long times (6 h). The mechanism appears to be that Br⁻ is oxidized to aqueous Br₂ which, when sufficient local concentration builds up, intercalates the fiber. This was confirmed by intercalating K-1100 fiber in a saturated aqueous Br₂ solution without passing an electrical current. The applied voltage does apparently lower the activation energy of the reaction as evidenced by the observation that P-120 and P-100 fibers will not intercalate in aqueous Br₂ unless a voltage is applied.     

X-ray absorption fine structure study on residue bromine in carbons with different degrees of graphitization

The structure of bromine residue compounds was investigated by X-ray absorption fine structure (XAFS) in order to interpret where and how bromine is present in carbons with different degrees of graphitization. The residue compounds can be classified into three groups, as obtained from X-ray absorption near edge structure (XANES) spectra and the values of the intramolecular distance r_Br–Br determined by extended X-ray absorption fine structure (EXAFS). In Group I, prepared from the host carbons heat treated at temperatures higher than 1900 °C, bromine exists in the interlayer space of graphite in the form of Br₂ molecules with interaction of the π electrons of graphite. In Group III, from carbon heat treated at 1000 °C, most of the bromine probably reacts with carbon atoms having a dangling bond or functional groups. For Group II, where the host carbons are heat treated at intermediate temperatures, it is likely that bromine exists in undeveloped defects with a unique electronic state.     

On the so-called “semi-ionic” C–F bond character in fluorine–GIC

The short-range structures of fluorine–graphite intercalation compounds with stage-1 structures, C_xF (x = 2.47, 2.84, 3.61), were analyzed by means of neutron diffraction. It has been shown that the so-called “semi-ionic” C–F bond character in C_xF is essentially covalent with the bond length of 0.140 nm, and the original planar graphene sheets are buckled at the sp³-hybridized carbon atoms bound to fluorine atoms. Conjugated C–C double bonds with the bond length of 0.142 nm are preserved between the carbon atoms unbound to fluorine atoms in C_xF, while other C–C bonds are single bonds with the bond lengths of 0.153 nm. The C–F bond order in C_xF is slightly lower than those in poly(carbon monofluoride) ((CF)_n) and poly(dicarbon monofluoride) ((C₂F)_n), which is explained by the hyperconjugation involving the C–C bonds on the carbon sheets and C–F bond.     

Pyrolytically prepared carbon from fluorine-GIC

Formation mechanism, crystallinity, porosity and chemical reactivity were studied on the carbon prepared by pyrolysis of single phase, stage-1 fluorine-graphite intercalation compound (fluorine-GIC; C_xF). The stage-1 C_2.5F directly decomposes to fluorocarbon gases and carbon above 650 K, without forming higher stage compounds. The pyrocarbon prepared from C_2.5F gives hkl diffraction peaks indicating graphite-like stacking order of graphene layers. This carbon possesses average crystallite sizes along the c- and a-axes (L_c and L_a) of about 5 and 50 nm, respectively. The specific surface area of the pyrocarbon (about 40 m² g⁻¹) is only twice as large as that of the original crystalline graphite. Chemical behavior of the pyrocarbon as an intercalation host for sodium and potassium is similar to that of crystalline graphite, but it is easily fluorinated by elemental fluorine even at 573 K to give poly(carbon monofluoride) [(CF)_n] probably due to the small crystallite size and the mesopores formed by formation/decomposition processes of C_2.5F.