Liquids for Electrochemical Applications
Chemfiles Volume 5 Article 6
Over the past decade, Ionic Liquids have attracted much interest for their use as non-aqueous electrolytes in electrochemical applications. In this context, their conductivity as well as their electrochemical stability are the most important physical properties. Together with other interesting properties such as their negligible vapor pressure and their non-flammability, they appear to be ideal electrolytes for many interesting applications as already described and discussed in a growing number of publications.1
As mentioned above, one very interesting property is their conductivity. Typical values are in the range from 1.0 mS/cm to 10.0 mS/cm. Recently, interesting materials with conductivities above 20 mS/cm based on the imidazolium-cation were described: 1-ethyl-3-methylimidazolium thio-cyanate (Prod. # 07424) and 1-ethyl-3-methylimidazolium dicyanamide (Prod. # 00796).
Of course, a solution of a typical inorganic salt such as sodium chloride in water shows a higher conductivity. But if we compare other properties of this solution with an Ionic Liquid, significant disadvantages become obvious: aqueous electrolytes are liquid over a smaller temperature range and the solvent water is volatile!
Another very important property of Ionic Liquids is their wide electro-chemical window, which is a measure for their electrochemical stability against oxidation and reduction processes:
Obviously, the electrochemical window is sensitive to impurities: halides are oxidized much easier than molecular anions (e.g., stable fluorine-containing anions such as bis(trifluoromethylsulfonyl)imide), where the negative charge is delocalized over larger volume. As a consequence, contamination with halides leads to significantly lower electrochemical stabilities.
- High Conductivity The materials showing the highest conductivities, 1-ethyl-3-methylimi-dazolium thiocyanate and dicyanamide exhibited the lowest electro-chemical stabilities. Nevertheless, these materials are good candidates for use in any application where a high conductivity combined with thermal stability and non-volatility is necessary, e.g., 1-dodecyl-3-methylimidazolium iodide (Prod. # 18289) in dye-sensitized solar cells.2
- High Stability The electrochemically most stable materials having comparable small conductivities (N-butyl-N-methylpyrrolidinium bis(trifluoromethyl-sulfonyl)imide (Prod. # 40963), triethylsulphonium bis(trifluoromethyl-sulfonyl)imide (Prod. # 08748), and N-methyl-N-trioctylammonium bis(trifluoromethylsulfonyl)imide (Prod. # 00797). These materials are good electrolytes for use in batteries,3 fuel cells,4 metal deposition,5 and electrochemical synthesis of nano-particles.6
- Combined Properties For applications where conductivity and electrochemical stability are needed (e.g., supercapacitors7 or sensors8), imidazolium-based Ionic Liquids with stable anions (e.g., tetrafluoroborate or trifluoromethylsulfonate) are the materials of choice.
Sigma-Aldrich now offers a set of Ionic Liquids especially useful for your electrochemical applications.
- For a general overview: Trulove, C.; Mantz, R. A. “Ionic Liquids in Synthesis”, Chapter 3.6: Electrochemical Properties of Ionic Liquids, (P. Wasserscheid, T. Welton eds.), Wiley-VCH, Weinheim, 2003.
- Yamanaka, N.; Kawano, R.; Kubo, W.; Kitamura, T.; Wada, Y.; Watanabe, M.; Yanagida, S. Chem. Commun. 2005, 740–742.
- Wilkes, J. S.; Levisky, J. A.; Wilson, R. A. Inorg. Chem. 1982, 21, 1263–1264. Garcia, B.; Lavallée, S.; Perron, G.; Michot, C.; Armand, M. Electrochim. Acta 2004, 49, 4583–4588.
- Yanes, E. G.; Gratz, S. R.; Baldwin, M. J. Anal. Chem. 2001, 73, 3838–3844.
- (a) Zell, C. A.; Freyland, W. Langmuir 2003, 19, 7445–7450. (b) Scheeren, C. W.; Machado, G.; Dupont, J.; Fichtner, P. F. P.; Texeira, S. R. Inorg. Chem. 2003, 42, 4738–4742. (c) Huang, J.-F.; Sun, I.-W. J. Chromat. A 2003, 1007, 39–45.
- (a) Zhou, Y.; Antonietti, M. J. Am. Chem. Soc. 2003, 125, 14960–14961. (b) Zhou, Y.; Antonietti, M. Chem. Mater. 2004, 16, 544–550.
- Sato, T.; Masuda, G.; Takagi, K. Electrochim. Acta 2004, 49, 3603–3611.
- See: http://www.iolitec.de.