Electrochemistry of Room-Temperature/ Articles about substances designated as ionic liquids have begun to appear with increasing regularity in chemistry journals around the world, страница 10


In 1914, Walden published an article describing the electrical conductivity of some molten salts or ionic liquids.162 Thus, it would not be an exaggeration to say that electrochemistry in ionic liquids has a very long history. Since the publication of that article, the field of electrochemistry in ionic liquids has become a mature science, and almost every electrochemical technique that has been applied to aqueous electrolyte solutions or solutions prepared from aprotic organic solvents and organic salts has been applied to solutions derived from ionic liquids as well. In the case of reactive or high-temperature ionic liquids, it has been necessary to overcome numerous experimental obstacles. The purpose of this Section is to review a few of the more common electrochemical methods that have been commonly applied to RTILs. For more complete information, the reader is advised to consult one of the many excellent comprehensive texts on electrochemistry such as the classic text coauthored by Allen Bard and Larry Faulkner.321

             1.    Electrodes and Experimental Considerations

Electrochemical experiments with RTILs are normally carried out in three-electrode cells. Working electrodes (WE) that have been used for such experiments in RTILs include polycrystalline metals such as gold, platinum, and tungsten, and non-metals, principally glassy (vitreous) carbon and pyrolytic graphite. Various single crystal metals have also been used for specialized surface studies

and will not be discussed here.322

It is important that the surface of the WE be carefully prepared before use by polishing with aqueous slurries containing successively finer grades of alumina, electrochemical polishing under appropriate conditions that depend on electrode material, and finally, degreasing with ethanol or acetone. The procedures for carrying out such polishing are well established. However, because such polishing involves exposure of the electrode surface to water, the electrode should be carefully dried under vacuum before use in dry RTILs. In some cases, it may be advantageous to electrochemically precondition the WE before use. For example, the electrode might be poised at a potential close to the positive limit of the solvent to remove surface impurities such as adsorbed water. However, all preconditioning methods must be carefully evaluated to ensure that they produce reproducible results and do not diminish the active surface area of the electrode. Cyclic voltammetry is a good technique for assessing such preconditioning methods. In some cases, it may be necessary to employ more drastic cleaning procedures that would seldom be used during electrochemical experiments in aqueous solutions such as cleaning the WE surface with an abrasive tissue after each electrochemical experiment. However, if electrochemical experiments performed in RTILs result in intractable surface films, there is often no other recourse except to re-polish the electrode surface. 

At the present time, there is no way to determine the active surface areas of working electrodes used in RTILs. Most investigators simple refer all quantitative data, e.g., current densities, to the geometrical area of the electrode. This matter is further complicated by the tendency of organic salt-based ionic liquids to form films at their anodic and/or cathodic potential limits. It is doubtful that surface area measurements derived from experiments carried out in aqueous solutions are applicable to electrodes used in RTILs. Furthermore, it is often assumed that the electrode surface area does not vary with the type of ionic liquid under investigation; this may be a false assumption. At the present time, there are few answers for these questions. The best strategy is to polish the electrode until the maximum voltammetric current is obtained for a test redox couple such as ferrocenium/ferrocene (Fc+/Fc). By referring to the original scan at the fresh electrode, it is possible to track any electrode surface area changes that occur during the course of a series of experiments.