Review
Graphite intercalation compounds with large fluoroanions

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Abstract

Over the past 20 years, a number of relatively large, perfluorinated anions have been found to intercalate into graphite. This review will describe the background, some syntheses and structures of these graphite intercalations compounds (GIC's). The fluoroanion intercalates include perfluoroalkylimides, perfluoroalkylsulfonates, and perfluoroalkylborate esters. Synthetic methods can include either chemical oxidation, for example using K2MnF6 in anhydrous or hydrous hydrofluoric acids or electrochemical oxidation. These gallery structures present in these GIC's are larger and more complex than for most previously-known GIC's, and therefore may be important in advancing the known chemistry of graphite. In this review, the relationship of intercalate packing, orientation and conformation is discussed.

Graphical abstract

This review describes the syntheses and structures of graphite intercalation compounds with some large fluoroanions, including perfluoroalkylsulfonates and perfluoroalkylborates.

Introduction

Graphite is a unique layered host in that it is electro-active and can be either oxidized or reduced to form intercalation compounds [1]. During oxidation, anions intercalate between the sheets, and the graphene sheets are separated and the structure expands along the stacking direction. The strong, aromatic carbon–carbon bonding remains intact in most cases, although the formation of graphite fluorides or graphite oxides can result in direct covalent bonding between intercalate and graphene carbon. Intercalation usually occurs to generate long-range order in the sequence of expanded galleries—this phenomenon is known as staging. Stage 1 has the greatest intercalate content, in this case the intercalate forms galleries between all the graphene sheets. In a stage 2 graphite intercalation compound (GIC), alternate galleries are occupied, in stage 3 every third gallery is occupied, and so on. The long-range ordering appears to be related to mechanical energy minimization, and requires a mechanical flexibility that is unique to graphene sheets [2].

Another difference between graphite intercalation and that of many other layered hosts is the very high potential required to oxidize graphite. The onset potential for graphite intercalation is above 4 V versus Li/Li+, and this potential increases as a function of the extent of intercalation, so that potentials above 5 V can be required to achieve a stage 1 GIC. These very highly oxidative potentials greatly limit the synthetic approaches available, and candidate intercalate anions, in the formation of GIC's. It is for this reason that larger anions, containing alkyl groups, must be perfluorinated. To date there is not much evidence that the C–H bond can remain intact in an intercalate anion under the highly oxidative conditions found in acceptor-type GIC's.

Even so, there has been a wide range of GIC's prepared containing oxidatively stable intercalate anions, these include the tetrahedral or octahedral fluoro-, chloro-, bromo- or oxo-metallates [3], [4], trifluoroacetate [5], perfluoroalkyl-substituted sulfonyl imides or methides [6], [7], perfluoroalkylsulfonates [8], [9], [10], [11], and perfluoroalkylborate esters [12], [13]. The gallery heights, or distance along the stacking direction between encasing graphene sheet centers, in these GIC's range from ≈0.6 nm for graphite bifluoride, CxHF2 and graphite nitrate, CxNO3, to ≈3 nm for CxC10F21SO3 [14]. The intercalation of the larger anions, forming highly expanded galleries, may well be the first step in developing a much broader range of graphite compounds where selective sorption, catalysis, and nanocomposite assembly are possible as for other layered hosts.

The first (and smallest) perfluoroalkylsulfonate intercalate, CF3SO3, has been produced with a gallery height of 0.8 nm, consistent with a monolayer arrangement of these intercalate anions. In contrast, electrochemical oxidation of graphite with the chemical series of perfluoroalkylsulfonates CnF2n+1SO3 (n = 4, 6, 8, 10) produces GIC's with a bilayer arrangement of anion intercalates [8], [9], [10], [11]. A chemical method was also employed to obtain new GIC's containing perfluoroalkylimides and methide C(SO2CF3)3 [11]. A chemical method for some of these syntheses can readily generate multiple gram quantities without the need for binders or additives; and some of these GIC's show remarkable ambient stability. Due to these advantages, detailed structural analyses from relatively high-quality powder X-ray diffraction (XRD) data have been obtained in several cases. An example discussed below involves the study of helical twisting of fluorocarbon chains in the intercalate layer in CxC8F17SO3 [11]. This level of detail often cannot be obtained for GIC's that are difficult to obtain in gram quantities, high purity, or are very reactive/unstable in air.

This paper reviews recent results in the chemical intercalation of the perfluoroalkylsulfonate anions and the borate ester of B[OC(O)C(O)O]2 [15] and the electrochemical intercalation of the borate esters B[OC(CF3)2C(CF3)2O]2 [12], and B[OC(CF3)2C(O)O]2 [13]. The GIC's obtained were characterized by a number of methods, with powder XRD being most important in order to develop detailed structural models for the compounds obtained.

Section snippets

Graphite borates

During electrochemical preparation, the potential–charge curve obtained using the B[OC(CF3)2C(CF3)2O]2 intercalate presents a series of plateaus and ascending regions that is highly characteristic of graphite intercalation (Fig. 1, line i). The required electrochemical potential increases during oxidation of a single phase, while the voltage plateaus occur where one stage in converted to another (during the preparation reaction this means that a higher stage is converted to one lower, for

Conclusion

Graphite chemistry has expanded recently (literally and figuratively) to include larger and more complex anion intercalates between graphene layers. We can expect that along with these larger galleries, our ability to modify and re-organize graphene layers will also improve, as has been seen with other layered hosts.

Acknowledgement

The authors gratefully acknowledge support from NSF grant DMR-9900390.

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