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Potential (Volts vs Fc/Fc*)

Fig. 10.6. (a) Cyclic voltammogram for C60 in a toluene/acetonitrile solvent using a phosphorus-hexafluoride salt electrolyte. The voltammogram shows six reversible reductions of Ca, at -0.98, -1.37, -1.87, -2.35, -2.85, and -3.26 V. Shown in (b) is the differential pulse polarogram of the same solution, which is used to identify the reduction peak voltages more accurately, (c and d) The same diagrams as in (a and b), but for C70, where the six reversible reductions occur at -0.97, -1.34, -1.78, -2.21, -2.70, and -3.07 V [10.27].

tives. Such studies can be useful in classifying categories of fullerene derivatives. For example, the addition of two phenyl groups through a carbon bridge C(C6H5)2 to C60 yields the same cyclic voltammetry wave pattern (see Fig. 10.6 for such a pattern) as the addition of isobenzofuran [(C6H4)(C4H30)] to C60 [10.7] (see §10.6). This result shows that the addition of an adduct, such as isobenzofuran or C(C6H5)2 to C60 does not significantly alter the electrochemistry of the fullerene.

Electrochemistry provides a powerful tool for the intercalation of guest species into a host material, as, for example, in layered materials such as graphite and transition metal dichalcogenides. This method has also been applied with significant success to the case of intercalation of guest species into solid C60. Because of extensive prior experience with the electrochemical intercalation of Li, Na, K, and Rb into layered host materials, and in particular the intercalation of Li into Li ion battery electrodes, the electrochemical intercalation of Li into the interstitial voids of solid C60 has received particular attention [10.33] (see also §20.4.2). The positive electrode (or working electrode) is made from a mixture (composite) containing 60% purified C60 and 40% solid electrolyte (polyethylene oxide and LiC104), presumably for better contact of the reagents with the electrolyte; metallic Li is used for the negative electrode. Furthermore, additional electrolyte material of the same composition in the form of a film was wrapped about the working electrode. Electrochemical measurements were made by sequentially applying a step potential increase of 10 mV/h to the cell, and the current was monitored as a function of cell potential. The resulting voltammogram for the first reduction cycle from 3.0-» 0.2 V is shown as the negative current trace in Fig. 10.7(a), and the subsequent oxidation cycle

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