RTILs based on the combination of a metal halide with an organic halide salt often display dramatic changes in their physicochemical properties based on the molar ratio of the components. This behavior simply reflects the fact that the anionic speciation in these ionic liquids varies with the component ratio. The most well known and thoroughly studied examples of such RTILs are chloroaluminate salts based on AlCl3 mixed with aromatic quaternary ammonium chloride salts such as 1-n-butylpyridinium chloride or 1-ethyl-3-methylimidazolium chloride. Table 9 lists a number of similar non-haloaluminate systems along with the corresponding composition-dependent anionic species. The prospects for electrodeposition/surface finishing in these RTILs are also indicated in the table. Fortunately, as a result of the development of commercially available computer modeling software, it is often possible to predict the structures of the anionic species that might exist in these RTILs. The results from these modeling programs show excellent agreement with the data obtained from spectroscopic methods. Note that all of the anions (or more accurately, complex anions) listed in Table 9 can be reduced to the corresponding metal, except those with the Td structure: AlCl4- and GaCl4-. Although AlCl4- can be reduced to Al in high temperature inorganic chloroaluminate ionic liquids or molten salts,39 it cannot be reduced to Al within the electrochemical window of chloroaluminate RTILs.
As can be seen from Tables 7 and 8, additives are often employed to improve of the surface morphology of electrodeposits. As reported for chloroaluminate RTILs,13,455-459 the electrodeposit surface quality varies greatly with the type of additive used. But in this particular case, the main role of the additive is to reduce the viscosity of the RTIL.
A number of metal and alloy coatings prepared in RTILs have been shown to have interesting properties. For example, Iwagishi, et al.440,441 electrodeposited Zn-Mg coatings on automotive steel sheet from ZnBr2–EtMeImBr–ethylene glycol (or glycerin) mixtures. For pure Zn plated steel, rust appeared after 168 hrs of immersion in a 5 mass % aqueous NaCl solution at 308 K. But if the Zn97.5Mg2.5 alloy was immersed in this same solution under the same conditions, 2184 h were required to initiate rust development.440 Not surprisingly, Zn80Mg20 alloy coatings exhibit a rust delay that exceeds 2440 hrs.441 Clearly the corrosion resistance imparted to sheet steel by Zn-Mg coatings is improved as the Mg content of the alloy is increased.
Koura et al.207,450-452 have prepared Nb3Sb superconductors in NbCl5–SnCl2–EtMeImCl (or BuPyCl). These alloys showed superconducting behavior at a temperature of 16 K (onset) and 19 K (zero).452 Electrochemical alloying/dealloying methods have been used in the RTILs to prepare porous metal surfaces.416-418,447 For example, porous Ag has been produced by the dealloying of the Zn-Ag alloy formed on a Ag substrate in the ZnCl2–EtMeImCl RTIL.417 The surface is covered with a uniform porous structure (Fig. 13), and the surface morphology depends on the quantity of the deposited Zn. The advantage of this method is that the bath can be reused completely without deterioration if the counter electrode is made from Zn.
RTILs based on fluoroanions usually exhibit outstanding electrochemical stability with electrochemical windows as large as 5.8 V. With such large electrochemical windows, it is not surprising that many attempts have been made to use these solvents to reversibly deposit active metals, such as lithium. This work is driven by the current interest in lithium batteries, and the number of reports about this work increases every year (see Section V.3). As to the surface finishing in these RTILs, several problems remain. The most significant problems are the low solubility of inorganic salts used as solutes in these ionic solvents and the difficulty of preparing thick coatings (> 1 μm) that are smooth. We think that the anodic dissolution of pure metals150,355,377,380,408 and the addition of solubility-enhancing ionophores to the RTILs are promising strategies for overcoming these limitations.289,295,368
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