Ionic potential window

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. 

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.

Table 3.6-2 The room-temperature electrochemical potential windows for binary and ternary chloroaluminate and related ionic liquids.

Heterogeneous ET reactions at polarizable liquid-liquid interfaces have been mainly approached from current potential relationships. In this respect, a rather important issue is to minimize the contribution of ion-transfer reactions to the current responses associated with the ET step. This requirement has been recognized by several authors [43,62,67-72]. Firstly, reactants and products should remain in their respective phases within the potential range where the ET process takes place. In addition to redox stability, the supporting electrolytes should also provide an appropriate potential window for the redox reaction. According to Eqs. (2) and (3), the redox potentials of the species involved in the ET should match in a way that the formal electron-transfer potential occurs within the potential window established by the transfer of the ionic species present at the liquid-liquid junction. The results shown in Figs. 1 and 2 provide an example of voltammetric ET responses when the above conditions are fulfilled. A difference of approximately 150 mV is observed between Ao et A" (.+. ... 

Recently, there has been considerable interest in developing molten salts that are less air and moisture sensitive. Melts such as l-methyl-3-butylimidazolium hexa-fluorophosphate [211], l-ethyl-3-methylimidazolium trifluoromethanesulfonate [212], and l-ethyl-3-methylimidazolium tetrafluoroborate [213] are reported to be hydro-phobic and stable under environmental conditions. In some cases, metal deposition from these electrolytes has been explored [214]. They possess a wide potential window and sufficient ionic conductivity to be considered for many electrochemical applications. Of course if one wishes to take advantage of their potential air stability, one loses the opportunity to work with the alkali and reactive metals. Further, since these ionic liquids are neutral and lack the adjustable Lewis acidity common to the chloroaluminates, the solubility of transition metal salts into these electrolytes may be limited. On a positive note, these electrolytes are significantly different from the chloroaluminates in that the supporting electrolyte is not intended to be electroactive. 

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. 

The preceding chapters have shown that the majority of metals can now be electrodeposited from ambient-temperature ionic liquids. However, this does not necessarily mean that the liquid with the widest potential window will negate the use of all other ionic liquids. Rather, it is most likely that ionic liquids will be task-specific with discrete anions being used for metals that cannot be electrodeposited from aqueous solutions such as Al, Li, Ti, V and W. Type I eutectics will probably be the most suitable for Al, Ga and Ge. Type II eutectics are most suitable for Cr and Type III are most suited to Zn, Cu, Ag and associated alloys. Type III will also find application in metal winning, oxide recycling and electropolishing. To date most practically important metals have been electrodeposited from ionic liquids and a comprehensive review is given in articles by Abbott [99] and Endres [100-102],... 

In general the potential windows are not as wide as those for the haloaluminates or the discrete anions and they tend to be limited by the deposition of metal at the cathodic limit and the evolution of chlorine at the anodic limit. Since ionic liquids are aprotic solvents, hydrogen evolution and hydrogen embrittlement that often occur in aqueous baths are circumvented in these liquids. Moreover, because of their thermal stability, these ionic liquids make it easier to electrodeposit crystalline metals and semiconductors through direct electrodeposition without subsequent annealing. 

Most work to date has either used soluble anodes or has not considered the anodic reaction. A limited amount of information has been collated on the electrochemical windows of ionic liquids but this tends to be on either platinum or glassy carbon, which is not necessarily suitable for practical plating systems [1], The anodic limits of most liquids are governed by the stability of the anion, although pyridinium and EMIM salts are sometimes limited by the stability of the cation. The widest electrochemical windows are obtained with aliphatic quaternary ammonium salts with fluorous anions. A selection of potential windows is given in Chapter 3. 

Although the deposition of metals from ionic liquids has been possible for over 50 years, to date no processes have been developed to a commercial scale. There are numerous technical and economic reasons for this, many of which will be apparent from the preceding chapters. Notwithstanding, the tantalizing prospect of wide potential windows, high solubility of metal salts, avoidance of water and metal/water chemistry and high conductivity compared to non-aqueous solvents means that, for some metal deposition processes, ionic liquids must be a viable proposition. 

Ionic liquids can be compared to any other liquid in that the reactivity of a species will depend upon its relative activity in solution. To this end it is important to consider the relative Lewis and Bronsted acids that can interact with the solutes to affect their activity. It is also important to remember that ionic liquids with discrete anions have wider potential windows and what we therefore hope to achieve with them is more susceptible to the presence of reactive species. The influence of impurities on the electrochemical behavior of an ionic liquid will depend upon the relative Lewis acidity/basicity of the liquid and of the redox process in question. Eutectic-based ionic liquids behave very differently from ionic liquids with discrete... 

The absorption of species from the atmosphere is common to all electrolyte solutions and clearly the absorption of water is the biggest issue. This is not solely confined to ionic liquids, however, as all electroplaters who deal with aqueous solutions of acids know, if the solution is not heated then the tank will overflow from absorption of atmospheric moisture after some time. In the aqueous acid the inclusion of water is not a major issue as it does not significantly affect the current efficiency or potential window of the solution. Water absorption is also not such a serious issue with eutectic-based ionic liquids and the strong Lewis acids and bases strongly coordinate the water molecules in solution. The presence of up to 1 wt.% water can be tolerated by most eutectic-based systems. Far from having a deleterious effect, water is often beneficial to eutectic-based liquids as it decreases the viscosity, increases the conductivity and can improve the rate of the anodic reaction allowing better surface finishes. Water can even be tolerated in the chloroaluminate liquids to a certain extent [139] and it was recently shown that the presence of trace HQ, that results from hydrolysis of the liquid, is beneficial for the removal of oxide from the aluminum anode [140]. 

---