Reference electrodes ionic liquid electrochemistry

As has been shown in the preceding sections, there is no universal reference electrode system which works for all kinds of molten salts or ionic liquid electrochemistry. Indeed the reference electrodes that can be used are dependent upon the solvent system. Consequently, careful consideration should be made for choice of reference electrode and thought given to its use within the solvent system under investigation. 

Molten salts or ionic liquids (also referred to as fused salts by some authors) were among the very first media to be employed for electrochemistry. In fact, Sir Humphrey Davy describes electrochemical experiments with molten caustic potash (KOH) and caustic soda (NaOH) [1] as early as 1802 A wide variety of single molten salts and molten salt mixtures have been used as solvents for electroanalytical chemistry. These melts run the gamut from those that are liquid well below room temperature to those melting at more than 2000°C. The former present relatively few experimental challenges, whereas the latter can present enormous difficulties. For example, commercially available Teflon- and Kel-F-shrouded disk electrodes and Pyrex glass cells may be perfectly adequate for electrochemical measurements in ambient temperature melts such as the room-temperature chloroaluminates, but completely inadequate for use with molten sodium fluoroaluminate or cryolite (mp = 1010°C), which is the primary solvent used in the Hall-Heroult process for aluminum electrowinning. 

The electrochemistry of ionic liquids is different in some essential features from the electrochemistry of aqueous electrolytes. Particularly for electrodeposition, which involves charge transfer from the electrolyte to the electrode, the double layer on the electrode is of great importance. In general the cathode is negatively charged for the electrodeposition of metals and therefore coated with a (Helmholtz-) layer of cations at least 0.5 nm thick but the metal species in most ionic liquids is anionic (for instance AlCh ). This makes the metal deposition process complicated, for more details we refer to Chapter 2. 

One of the open problems of electrochemistry in ionic liquids nowadays is the choice of a reference electrode. Chapter 3 discusses the latest understanding and practical approaches to the apphcation of reference electrodes and internal reference redox scales in ionic hquids. Problems and limitations of reference systems are presented with the help of experimental examples. Comparison of the behavior of internal reference redox systems in organic solvents with added supporting electrolyte and ionic hquids is provided and new observations critically discussed. This chapter is very important for any experimentalist starting work in the electrochemical and IL helds. Further information about internal reference redox scales can also be found in Volume 2 (Chap. 14). 

One of the open problems of electrochemistry in ionic liquids (ILs) nowadays is the choice of a reference electrode. In order to carry out reliable electrochemical measurements in ILs, a robust and stable reference electrode is required. Unfortunately, many literature reports describe the use of poorly chosen, unreliable reference electrodes or the use of quasi-reference electrodes of unknown potential versus any standard potential scale, so they could not be reproduced, being therefore, of little or no value [1]. In addition, the potential differences of reliable reference electrodes in ILs have rarely been measured, preventing the comparison of potential data measured by different research groups in the same IL. A similar situation was reported by lUPAC almost 30 years ago for nonaqueous systems [2]. As a consequence, it is necessary to agree for ILs systems on some procedures and limitations to measure and report electrode potentials versus generally accepted standards. There are in general two possible ways (a) to use a reference electrode or (b) to refer aU potentials versus internal reference redox systems. 

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