Electrochemical Applications of Room - Temperature Ionic Liquids

R3 R2 There is good evidence that the first room-temperature ionic liquid or RTIL was identified and characterized by Walden in 1914, who examined the electrical conductivity of ethylammonium nitrate.1 Thus, it would not be an exaggeration to say that the very beginnings of electrochemistry involved room-temperature ionic liquids! Although once considered to be a curious subset of the general class of substances called “molten salts,” RTILs have taken on a life of their own and now occupy the attention of scientists worldwide. In fact, scientific interest in these interesting materials continues to grow exponentially. Today a literature search with SciFinder Scholar (Chemical Abstracts Online) employing the keywords ionic liquid or room-temperature ionic liquid produces hundreds of articles, whereas just ten years ago a search would produce only a handful of papers. From the viewpoint of electrochemistry, RTILs are under investigation as solvents for technological applications such as metal surface finishing, batteries, capacitors, fuel cells, electrosynthesis, and nuclear waste treatment. The question as to what exactly constitutes an ionic liquid or RTIL is discussed in the preceding article by Keith Johnson and will not be considered further herein. Although a large number of different RTILs have been reported in the literature, most can be classified into one of seven families on the basis of their cationic structures, as depicted in Fig. 1. Some of the common anions on which RTILs are based are also given in this figure. One advantage of having such a diverse selection of anions and cations is that it is possible to “engineer” the physicochemical properties of RTILs by the choice of the ionic constituents. For example, the combination of an imidazolium cation, such as 1-ethyl-3-methylimidazolium ion with BF4 results in a hydrophilic RTIL, whereas the combination of this cation and N(SO2CF3)2 (NTf2) produces a strongly hydrophobic ionic liquid. Although other factors may play a role, the viscosity and conductivity of such ionic liquids is largely a function of the chain length of the alkyl substituents, a factor that also carries over to the other classes of cations in Fig. 1. Electrochemical Applications of Room-Temperature Ionic Liquids