Room-temperature electrochemistry

However, ionic liquids containing other classes of organic cations are known. Room-temperature ionic liquids containing organic cations including quaternary ammonium, phosphonium, pyridinium, and - in particular - imidazolium salts are currently available in combination with a variety of anions (Figure 3.1-1 provides some common examples) and have been studied for applications in electrochemistry [7, 8] and in synthesis [9-11]. 

Jeng EGS, Sun IW (1997) Electrochemistry of tellurium (IV) in the basic aluminum chloiide-l-methyl-3-ethylimidazolium chloride room temperature molten salt. J Electrochem Soc 144 2369-2374... 

This equation was hrst obtained by Gabriel Lippmann in 1875. The Lippmann equation is of basic importance for electrochemistry. It shows that surface charge can be calculated thermodynamically from data obtained when measuring ESE. The values of ESE can be measured with high accuracy on liquid metals [e.g., on mercury (tf= -39°C)] and on certain alloys of mercury, gallium, and other metals that are liquid at room temperature. 

Skotheim et al. [286, 357, 362] have performed in situ electrochemistry and XPS measurements using a solid polymer electrolyte (based on poly (ethylene oxide) (PEO) [363]), which provides a large window of electrochemical stability and overcomes many of the problems associated with UHV electrochemistrty. The use of PEO as an electrolyte has also been investigated by Prosperi et al. [364] who found slow diffusion of the dopant at room temperature as would be expected, and Watanabe et al. have also produced polypyrrole/solid polymer electrolyte composites [365], The electrochemistry of chemically prepared polypyrrole powders has also been investigated using carbon paste electrodes [356, 366] with similar results to those found for electrochemically-prepared material. 

Hussey, C. L., The Electrochemistry of Room-Temperature Haloaluminate Molten Salts, in Chemistry of Nonaqueous Solutions, G. Mamantov and A. I. Popov, Editors. 1994, YCH Publishers New York. p. 227. 

Wilkes, J. S. Levisky, J. A. Wilson, R. A. Hussey, C. L. DialkylimidazoUum chlor-oaluminate melts—A new class of room-temperature ionic liquids for electrochemistry, spectroscopy, and synthesis, Inorg. Chem., 1982, 21(3), 1263-1264. 

In another study involving C78, a pure sample of the C2v isomer was prepared using the cyclopropanation-retro-cyclopropanation reaction sequence [44, 64]. This reaction scheme consists of a controlled potential electrolytic (CPE) reduction of a previously synthesized cyclopropane derivative of the isomer, leading to removal of the cyclopropane moiety (s), (see Sect. 6.1.5). A pure sample of the D3 isomer was obtained by high performance Kquid chromatography (HPLC) as previously described [49, 65]. The redox behavior of both isomers, in DCM at room temperature, reveals that their cathodic electrochemistry is indeed very similar (although not identical) in this solvent [44]. The first two reductions are easier for the D3 isomer by 60 and 100 mV, respectively, while the third and fourth reductions are nearly identical for the two... 

The electrochemistry of Cd(II) was investigated at different electrodes (GC, polycrystalline tungsten, Pt, Ni) in a basic l-ethyl-3-methylimidazolium chloride/tet-rafluoroborate, at room temperature molten salt [312], and in acidic zinc chloride-l-ethyl-3-methylimidazolium [284]. 

Wang, S.F., Chen, T., Zhang, Z.L., Pang, D.W., and Wong, K.Y., Effects of hydrophobic room-temperature ionic liquid l-butyl-3-methylimidazolium tetrafluoroborate on direct electrochemistry and biocatalysis of heme proteins entrapped in agarose hydrogel films, Electrochem. Commun., 9, 1709-1714, 2007. 

The main consensus seems to be that the first major studies of room temperature molten salts were made in the 1940s by a group led by Frank Hurley and Tom Weir at Rice University. When they mixed and gently warmed powdered pyridinium halides with aluminum chloride, the powders reacted, giving a clear, colorless liquid [4-7]. These mixtures were meant to be used in electrochemistry, particularly in electroplahng with aluminum. 

Warburg impedance is a well-known term in the field of impedance spectroscopy because of the early date at which it was published, the formulation came before the rest of the properties of the interface were known. In fact, for nearly all real situations examined in electrochemistry, the Warburg impedance is relatively small. Thus, for a concentration of 1 mol liter and a frequency of 1 kilocycle s l, and using the normal parameters for room temperature, the resistance is in the milliohm cm-2 range. 

We summarize what is special with these prototype fast ion conductors with respect to transport and application. With their quasi-molten, partially filled cation sublattice, they can function similar to ion membranes in that they filter the mobile component ions in an applied electric field. In combination with an electron source (electrode), they can serve as component reservoirs. Considering the accuracy with which one can determine the electrical charge (10 s-10 6 A = 10 7 C 10-12mol (Zj = 1)), fast ionic conductors (solid electrolytes) can serve as very precise analytical tools. Solid state electrochemistry can be performed near room temperature, which is a great experimental advantage (e.g., for the study of the Hall-effect [J. Sohege, K. Funke (1984)] or the electrochemical Knudsen cell [N. Birks, H. Rickert (1963)]). The early volumes of the journal Solid State Ionics offer many pertinent applications. 

In this chapter, we first discuss the principal motivations for doing electrochemistry at other than room temperature and attempt to delineate the type of chemical information that can be obtained from such measurements. The emphasis is on measurements at reduced temperatures, though the principles apply to high-temperature electrochemistry as well. 

Molten salts or ionic liquids (also referred to as fused salts by some authors) were among the very first media to be employed for electrochemistry. In fact, Sir Humphrey Davy describes electrochemical experiments with molten caustic potash (KOH) and caustic soda (NaOH) [1] as early as 1802 A wide variety of single molten salts and molten salt mixtures have been used as solvents for electroanalytical chemistry. These melts run the gamut from those that are liquid well below room temperature to those melting at more than 2000°C. The former present relatively few experimental challenges, whereas the latter can present enormous difficulties. For example, commercially available Teflon- and Kel-F-shrouded disk electrodes and Pyrex glass cells may be perfectly adequate for electrochemical measurements in ambient temperature melts such as the room-temperature chloroaluminates, but completely inadequate for use with molten sodium fluoroaluminate or cryolite (mp = 1010°C), which is the primary solvent used in the Hall-Heroult process for aluminum electrowinning. 

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