Graphite intercalation compounds electron transfer

Graphite intercalate compounds are among the best known carbonaceous catalysts, and there is good evidence that electron transfer is important in these systems. The systems were reviewed by Boersma in 1974 and attention will be focused on a few important papers before and after then. 

Another important feature for lithium graphite intercalation compounds in Li -containing electrolytes is the formation of solid electrolyte interface (SEI) film. During the first-cycle discharge of a lithium/carbon cell, a part of lithium atoms transferred to the carbon electrode electrochemically will react with the nonaque-ous solvent, which contributes to the initial irreversible capacity. The reaction products form a Lb-conducting and electronically insulating layer on the carbon surface. Peled named this film as SEI. Once SEI formed, reversible Lb intercalation into carbon, through SEI film, may take place even if the carbon electrode potential is always lower than the electrolyte decomposition potential, whereas further electrolyte decomposition on the carbon electrode will be prevented. 

The properties of the intercalation compound, potassium graphite, KCg, have been detailed in several review articles.34/35 The bonding in potassium graphite is described in terms of the limiting structure, K+Cg", and it is believed that the anion forms as a result of the transfer of an electron from the alkali metal to the conduction band of graphite. Novikov and Volpin35 have noted a similarity between aromatic radical-anions and alkali metal-graphite intercalation compounds. Their observation was based on inspection of reduction potentials of aromatic hydrocarbons relative to biphenyl, Table 9.2 ... 

The structure of the intercalates are of considerable interest, in that the intercalate material enters the graphite layers to form in the final analysis, a one-to-one graphite-intercalate layer structure. Intermediate compounds involve inclusion of the material in, for example, a second, fourth, or fifth layer rather than dilution of the amount in the same layer. The chemical bonding in the compounds appears to be ionic and certainly involves electron transfer probably, for example, from potassium to the upper pi-band of graphite. ... 

Like the covalent graphite compounds, the intercalation compounds are formed by the insertion of a foreign material into the host lattice. The structure however is different as the bond, instead of being covalent, is a charge-transfer interaction. This electronic interaction results in a considerable increase in electrical conductivity in the ab directions. 

There is relevance of these considerations to the processes of activation of carbons (discussed in Chapters 5 and 6). A series of compounds, known as intercalation compounds, exists in which ions and molecules are located between the graphene layers of carbon materials, principally graphitic structures, but not necessarily so. These reactions, leading to the formation of intercalation compounds, involve electron transfer processes. For example, electron transfer from the graphene layer forms a bromine anion (Br ), or electron transfer to the graphene layer forms a potassium cation (K ). Ferric chloride also forms an intercalation compound. 

Although there are numerous families of lamellar solids, only a handful of them exhibit the kind of versatile intercalation chemistry that forms the basis of this book. In arriving at the content of this volume, the editors have accurately identified six classes of versatile layered compounds that are at the forefront of materials intercalation chemistry, namely, smectite clays, zirconium phosphates and phos-phonates, layered double hydroxides (known informally as hydrotalcites or anionic clays ), layered manganese oxides, layered metal chalcogenides, and lamellar alkali silicates and silicic acids. Graphite and carbon nanotubes have not been included, in part because this specialty area of intercalation chemistry is limited to one or two molecular layers of comparatively small guest species that are capable of undergoing electron transfer reactions with the host structure. 

As already mentioned the fundamental condition which must be fulfilled for intercalation to occur is electron transfer from the graphite macromolecule to intercalate or vice versa. This quantity determines directly many physiocochemical properties of GICs. For example, it is obvious that for an acceptor compound the quantity of electrons lost by the graphene layers (some other time understood as the hole concentration), must exactly be compensated by the amount of electrons accumulated in the intercalate layers to assure the electrical balance of the intercalation system. The formation of acceptor and donor-type compounds in the reactions of anodic oxidation and cathodic reduction may be represented by the following equations, respectively,... 

Ohana, I., I. Palchan, Y. Yacoby, and D. Davidov. 1988. Electronic charge transfer in stage-2 fluorine-intercalated graphite compounds. Phys. Rev. B 38 12627-12632. 

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