Ionic Liquids: Promising Solvents for Electrochemistry

Abstract

Ionic liquids are solvents that are solely composed of ions. By definition their melting points are below 100 °C. Typical cations are substituted imidazolium ions, like 1-butyl-3-methylimidazolium, or tetraalkylammonium ions, like e.g. trioctyl-methyl-ammonium. Some important anions are hexafluorophosphate, trifluoromethylsulfonate, bis(trifluoromethylsulfonyl)imide. Many ionic liquids have negligible vapour pressures even at temperatures of 300 °C and more, they can have viscosities similar to water, ionic conductivities of up to 0.1 (Ω cm)⁻¹, and, which makes them interesting for electrochemistry, wide electrochemical windows of more than 6 Volt. In this review article recent results of the author are summarized. It is shown that with the scanning tunneling microscope the processes during phase formation can be probed in situ with high quality. An important result is that semiconductors, shown at the example of germanium, can be made electrochemically on the nanoscale and that the electronic properties (band gap) can be measured in situ with current/voltage tunneling spectroscopy. Ionic liquids will gain a rising interest in electrochemistry as elements and compounds can be made electrochemically which are not accessible by conventional aqueous or organic electrochemistry.