Hydrogen in alkali-metal-graphite intercalation compounds

The sorption and desorption of hydrogen by alkali-metal-graphite intercalation compounds (AGICs), MC₈ and MC₂₄ (M = K, Rb and Cs), were studied by the constant-volume closed method and mass-analyzed thermal desorption spectroscopy. Three forms of hydrogen were detected in KC₈H_x and KC₂₄H_x, while two forms in RbC₈H_x and one form in RbC₂₄H_x were detected. The energy diagrams for the systems composed of AGICs and hydrogen demonstrate that the type of alkali metal intercalated in graphite and the stage structure of AGICs have greater influence on the absorption process than on the adsorption process of hydrogen.     

Electrochemical applications of graphite intercalation compounds

Real and potential applications of graphite intercalation compounds in electrochemical processes are critically surveyed. Special attention is given to the fields of “batteries” and “chemically modified carbon electrodes”.Some recent results concerning both the discharge mechanism of graphite oxide positives in organic electrolyte-lithium batteries, and the preparation of metal-doped carbon electrodes via graphite compounds with ion exchange behaviour, are presented in detail.     

Synthesis of ternary intercalation compounds of carbon fiber

Ternary intercalation compounds of carbon fiber (ICCF) were successfully synthesized by soaking pitch-based carbon fibers in solvents such as 1,2-dimethoxyethane (DME) or tetrahydrofuran (THF) containing dissolved alkali metals (lithium, sodium, or potassium). ICCF with stage-1 and random stage was synthesized in alkali metal–DME and potassium–THF systems using different types of carbon fibers. However, ICCF with stage-1 could not be synthesized in lithium– or sodium–THF systems using carbon fibers with a low graphitization degree. Furthermore, the influence of the graphitization degree in the synthesis of ICCF was discussed. The graphitization degree of the host carbon fiber, in addition to the dimensions and steric structure of the intercalated complex affected the formation of TICCF.     

EXAFS study of Br2-graphite intercalation compounds

Extended X-ray absorption fine structure (EXAFS) measurements are presented for dilute graphite-Br₂ intercalation compounds containing 0.27 mole % and 0.75 mole % Br₂. Intercalated Br₂ molecules are found to have a BrBr distance of $\text{2.34 ± 0.02}\text{A}\text{?}$ and predominate in the 0.75% sample. A unique feature of the present work is the discovery of another type of bromine molecule with a BrBr distance of $\text{2.53 ± 0.03}\text{A}\text{?}$. This molecule seems to be associated with defect or edge sites and predominates in the 0.27% sample. It is reasonable to speculate that this molecule acts like a “can opener”, preparing the graphite planes for intercalation. The implication of these results with respect to charge transfer is discussed, and it is estimated that roughly 0.16 electrons are transferred to each intercalated molecule, while ～0.6 electrons are transferred to each “can opener” molecule.     

Small-angle x-ray scattering of graphite-ferric chloride intercalation compounds

Small-angle X-ray scattering data have been acquired for a sample of highly-oriented pyrolytic graphite and for two graphite-FeCl₃ compounds. The scattering patterns show considerable structure; model calculations indicate that a range of in-plane heterogeneities (with radii from ～ 20 to 200 Å) are required to reproduce the observed data.     

Graphite intercalation compounds as electrode materials in batteries

The increasing need for energy storage systems has stimulated research for new batteries and improvements in old ones. A brief evaluation of the available batteries and systems under development will first be made. Owing to their particular properties, lamellar compounds can have interesting applications in this field. After a comparison between intercalation compounds of graphite and other layer compounds, trying to distinguish their respective advantages, the various proposals for graphite intercalation compounds as electrode materials in batteries will be discussed. It will be shown that their potentialities require further work.     

Synthesis of cupric chloride-graphite intercalation compounds by the molten salt method

By using the CuCl₂KCl molten salt system, the stage two and three CuCl₂-graphite intercalation compounds were synthesized at 380 °C after only two hours. The reaction temperature and the initial graphite/chloride ratio have a strong influence on the stage of the compounds obtained. The compounds washed out from excess molten salt by water are very stable in air and even in water, which seems to be due to the coexistence of a small amount of graphite.     

Formation of metal chloride-graphite intercalation compounds in molten salts

Three molten salt systems, FeCl₃KCl, CuCl₂KCl and CuCl₂NaCl, were used to synthesize metal chloride-graphite intercalation compounds. Binary compounds, FeCl₃GICs and CuCl₂CICs, were obtained in the corresponding molten salts. The stage structure of GICs obtained was found to be governed by the composition of the molten salt and the reaction temperature. The behaviour of the formation of GICs in a molten salt has a close relation to its phase diagram: the formation of a complex chloride, e.g., KFeCl₄, reduces the amount of reagent chloride and its reactivity, and consequently has a strong influence on the final stage of the compound and the intercalating rate.     

Polymerization of benzene occluded in graphite-alkali metal intercalation compounds

Benzene accommodated within the interlayer space of graphite-alkali metal (potassium and rubidium) intercalation compounds was found to be polymerized not only to biphenyl, but also to terphenyl and quaterphenyl, while only biphenyl was formed by the action of potassium or rubidium metal alone under the same conditions. The formation of higher oligomers of benzene in the interlayer space of graphite intercalation compounds is to be ascribed to the amphoteric nature of the compounds, which are capable of both donating and accepting electrons.     

Staging influence on the magnetic properties of NiCl2 intercalation compounds

A first-stage NiCl₂ graphite intercalation compound (GIC) was prepared using very fine natural graphite powder under a high chlorine pressure and at high temperature. A second-stage NiCl₂-GIC was also prepared on highly oriented pyrolytic graphite (HOPG). The structure of the compounds consists of NiCl₂ layers inserted between the graphene ones. It is shown that the magnetic behaviour is three dimensional for both compounds. The ordering temperature decreases with the stage due to the increase of the interlayer distance.     

LixZrS2 intercalation compounds

The disulfide ZrS₂ has been intercalated with lithium by means of the butyllithium method. Two phases have been characterized. The first (0? x < 0.20), of the NiAs type, presents no parameter variation. The second (0.30 < x ? 1) is rhombohedral at room temperature but undergoes a phase transition to a spinel structure in the 0.30 < x < 0.50 range at 250°C. Electrical and magnetic measurements have shown that the first phase is semiconducting, the second being of a metallic type. Comparisons are made with the Li_xZrSe₂ system.     

Electrochemical synthesis of graphite intercalation compounds with nickel and hydroxides

Graphite intercalation compounds (GIGs) with nickel and iron hydroxides were synthesized from the corresponding GICs with metal chlorides by galvanostatic oxidation and also by repeated charge-discharge cycles in an alkaline secondary battery with KOH aqueous solution. The stage-one structure in the original chloride-GICs was found to disappear after the first charge-discharge process and the compounds kept the stage-two structure. A gradual increase in the capacity of the battery with charge-discharge repetition suggests a slow replacement of chloride ions by hydroxide ones in the graphite interlayer space.     

Molecular motions in the ternary graphite intercalation compounds with potassium and methylbenzenes

Molecular dynamics in the ternary graphite intercalation compounds (GICs) with potassium and methylbenzenes (toluene and o-xylene) have been investigated. It was found that the proton spin-lattice relaxation is determined by two different molecular motions. At low temperatures the relaxation process is monitored by the three-fold methyl group reorientations. At high temperatures the motion is most likely to be the rotation of the phenyl ring around the two-fold axes. The relaxation measurements are discussed in terms of the previously proposed structure model of the ternary GICs with potassium and aromatic hydrocarbons.     

Optical study of electron back-donation in tetrahydrofuran potassium graphite intercalation compounds

We report the results of optical reflectance studies of charge transfer in the stage-one tetrahydrofuran (THF) graphite intercalation compounds (GICs) K(THF)_xC₂₄ (x = 1 and 2). For both x = 1 and 2, intercalation of THF into KC₂₄ was observed to lower the free-carrier plasma frequency, consistent with a lowering of graphitic carbon π electron concentration from one electron per K atom (e/K) in KC₂₄ to a value of ∼0.51 ± 0.07 (e/K) for the x = 1, 2 K(THF)_xC₂₄ compounds. This amount of electron back-donation to the intercalate layers (∼49%) is significantly larger than reported recently in the K(NH₃)_x GICs, where values of 20% (stage one) and 38% (stage two) are obtained upon NH₃ uptake. In contrast to previous studies in the stage-one K(NH₃)_4.1C₂₄ compound, no optical evidence was found for localized (or solvated) electrons in the intercalate layers of K(THF)_xC₂₄.     

Electrochemical and chemical reactivity of graphite intercalation compounds with CrO3

The electrochemical oxidation of graphite-CrO₃ intercalation compound (GIC-CrO₃) prepared by the impregnation-dry method allowed lower chromium oxides to be removed, while they are unaffected by chemical treatment in hot 6 N HCl. The Cr(VI) formed from these oxides due to anodization in 0.5 N H₂SO₄ is equal to only 0.11% of the total chromium. The removal of the lower oxides restores the electrochemical properties of the host graphite, which suggests these oxides are bonded to the graphite structure. SEM and electron microprobe analysis showed that the concentration of the intercalated chromium oxides is higher near the flake edges. The behaviour of GIC-CrO₃s prepared by the dry, the impregnation-dry and the solvent methods was then examined in cold and hot solutions of KOH of different concentrations. On boiling with 5 N KOH, the sample prepared by the solvent method appeared to be more readily deintercalated than that of the impregnation-dry method. The reasons for this behaviour are considered in terms of the effect of the intercalation conditions on the structural properties.