Electrochemical Potential Windows

Ionic liquids possess a variety of properties that make them desirable as solvents for investigation of electrochemical processes. They often have wide electrochemical potential windows, they have reasonably good electrical conductivity and solvent transport properties, they have wide liquid ranges, and they are able to solvate a wide variety of inorganic, organic, and organometallic species. The liquid ranges of ionic liquids have been discussed in Section 3.1 and their solubility and solvation in... 

A key criterion for selection of a solvent for electrochemical studies is the electrochemical stability of the solvent [12]. This is most clearly manifested by the range of voltages over which the solvent is electrochemically inert. This useful electrochemical potential window depends on the oxidative and reductive stability of the solvent. In the case of ionic liquids, the potential window depends primarily on the resistance of the cation to reduction and the resistance of the anion to oxidation. (A notable exception to this is in the acidic chloroaluminate ionic liquids, where the reduction of the heptachloroaluminate species [Al2Cl7] is the limiting cathodic process). In addition, the presence of impurities can play an important role in limiting the potential windows of ionic liquids. 

Table 3.6-1 The room-temperature electrochemical potential windows for non-haloaluminate...

As shown in Figure 3.6-1, GC and Pt exhibit anodic and cathodic potential limits that differ by several tenths of volts. However, somewhat fortuitously, the electrochemical potential windows for both electrodes in this ionic liquid come out to be 4.7 V. What is also apparent from Figure 3.6-1 is that the GC electrode exhibits no significant background currents until the anodic and cathodic potential limits are reached, while the Pt working electrode shows several significant electrochemical processes prior to the potential limits. This observed difference is most probably due to trace amounts of water in the ionic liquid, which is electrochemically active on Pt but not on GC (vide supra). 

Tables 3.6-1 and 3.6-2 contain electrochemical potential windows for a wide variety of ionic liquids. Only limited information concerning the purity of the ionic liquids listed in Tables 3.6-1 and 3.6-2 was available, so these electrochemical potential windows must be treated with caution, as it is likely that many of the ionic liquids would have had residual halides and water present.

The solvents used for electroanalytical determinations vary widely in their physical properties liquid ranges (e.g., acetamide, N-methyl-acetamide and sulfolane are liquid only above ambient temperatures), vapour pressures (Table 3.1), relative permittivities (Table 3.5), viscosities (Table 3.9), and chemical properties, such as electron pair and hydrogen bond donicities (Table 4.3), dissolving ability of the required supporting electrolyte to provide adequate conductivity, and electrochemical potential windows (Table 4.8). A suitable solvent can therefore generally be found among them that fits the electroanalytical problem to be solved. 

Physicochemical properties of ILs can be changed by variation of the component ions. There are important studies to achieve ILs having excellent properties such as low Tm, low viscosity, high ionic conductivity and wide electrochemical potential windows. It is generally understood that ILs are difficult to apply as electrolyte solution substituents because they contain a large number of ions which cannot work as carrier ions for electrochemical devices such as secondary batteries. Therefore, structural design of ions for particular applications is important for ILs. Selective ion conduction is one of the attractive and challenging tasks for IL science. 

Polymeric chlorocomplex anions are also known for some metals. Since the electrodeposition of A1 does not occur within the electrochemical potential window of known alkylated imidazolium and pyridinium cations under the basic condition of room temperature, an electrodeposit of the pure metal can be obtained. 

There are some reports on the electrodeposition of cobalt [105], nickel, zinc, and magnesium [106] in N(CF3S02)2 ionic liquids. The electrochemical behavior of lanthanide elements has been reported in EMIN(CF3S02)2 and BMPN(CF3S02)2 ionic liquids, though their metals were not obtained within the electrochemical potential windows of these ionic liquids [107]. 

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