Electrochemical Properties of Ionic Liquids

The early history of ionic liquids research was dominated by their application as electrochemical solvents. One of the first recognized uses of ionic liquids was as a solvent system for the room-temperature electrodeposition of aluminum [1]. In addition, much of the initial development of ionic liquids was focused on their use as electrolytes for battery and capacitor applications. Until recently, electrochemical studies in the ionic liquids were primarily carried out in the haloaluminate-based systems, and this work has been extensively reviewed [2-9]. Development of non-haloaluminate ionic liquids over the past fifteen years, however, has led to an explosion of research in these systems [10,11]. Much of the initial interest in these new ionic liquids has been in areas other than electrochemistry. However, this initial slight has been largely corrected, as evidenced by the dramatic growth over the past five years in electrochemically related publications involving non-haloaluminate ionic liquids and the appearance of several good reviews on the subject [12-17]. 

Ionic liquids possess a variety of properties that make them desirable as solvents for investigating 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 ofionic liquids have been discussed in Section 3.1 and the solubility and solvation in Section 3.3. In this section we will deal specifically with the electrochemical properties of ionic liquids (electrochemical window, conductivity, and transport properties). We 

A key criterion for selection of a solvent for electrochemical studies is the electrochemical stability of the solvent [28,29]. This is most dearly 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 hquids, 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 addic chloroaluminate ionic hquids where the reduction of the heptadiloroaluminate spedes, [AI2CI7], is the limiting cathodic prcxiess). In addition, the presence of impurities can play an important role in limiting the potential window of ionic hquids. 

It must be noted that impurities in the ionic hquids can have a profound impact on the potential limits and the corresponding electrochemical window. During the synthesis of many of the non-haloaluminate ionic hquids residual halide and water may remain in the final product [30]. Hahde ions (Cl, Br, I ) are more easily oxidized than the fluorine-containing anions used in most non-haloaliuninate ionic liquids. Consequently, the observed anodic potential limit could be appreciably reduced if significant concentrations of hahde ions are present. 

As shown in Fig. 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 Fig. 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 likely due to trace amounts of water in the ionic liquid, which is electrochemically active on Pt but not on GC vide supra). 

Huddleston, G. A. Broker, H. D. Willauer, R. D. Rogers, in Green (or greener) industrial applications of ionic liquids. , ACS Symposium Series, in press. 

Bonhote, A.-P. Dias, N. Papageor-giou, K. Kalyanasundaram, M. Gratzel, Inorg. Chem., 1996, 35, 1168. 

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