Reference Electrodes for Use in Room-temperature Ionic Liquids

Reference Electrodes for Use in Room-temperature Ionic Liquids 

Voltammetric, electrodeposition, electrosynthetic and electroanalytical studies are carried out in room-temperature ionic liquids (RTILs) by a significant and increasing number of both industrial and academic laboratories [23-25], Such studies, when carried out at anything other than a very empirical level, require the use of a reference electrode . The purpose of this chapter is to address the special problems this poses and their solutions. First, however, we start by considering the essential features of a reference electrode in general. 

Consider a generalized electrode process A we — B, in which A is electrolytically converted into B at a suitable electrode. The rate at which this happens is measured by the current, I, flowing through the electrode, via. Faraday s Law  

The potential difference ( J m — t s)ref is established by means of a suitable electrochemical equilibrium being established at the surface of the reference electrode  

Ox +e -B- Red, so that the reference potential difference of interest is quantified by means of the Nernst equation [27]  

---

Silvester DS, Ward KR, Aldous L, Hardacre C, Compton RG (2008a) The electrochemical oxidation of hydrogen at activated platinum electrodes in room temperature ionic liquids as solvents. J Electroanal Chem 618 53-60 Silvester DS, Rogers El, Compton RG, McKenzie KJ, Ryder KS, Endres F, MacFarlane D, Abbott AP (2008b) Reference electrodes for use in room-temperature ionic liquids. In Endres F, MacFarlane DR, Abbot A (eds) Electrodeposition from ionic liquids. Wiley, Weinheim, pp 296-309... 

Occasionally also the use of so-caUed pseudo reference electrodes has been reported (see Chap. 14). Such pseudo reference electrodes became popular in polarography but especially in cyclic voltammetry, employing three electrode arrangements. They consist of a silver or platinum wire or activated carlxMi dipping into the electrolyte. They substitute for a reference electrode. Such electrodes were reported to exhibit very stable potentials. The ease of such an arrangement was also used in electrochemical studies in (room temperature) ionic liquids. It must be pointed out that electrode potentials versus such electrodes are meaningless as such arrangements do not constitute thermodynamic values. 

The redox reaction between iodides, I and I3, is also used as the reference electrodes in nonchloroaluminate ionic liquids especially for the N(CF3S02)2 ionic liquids. Platinum wire is used for the electrode and immersed in an ionic liquid that contains iodine and an iodide salt having the same cation as the ionic liquid. The concentration ratio of the iodine salt to iodide is often specified as 1 4, namely [I3 ] [I ] = 1 3. The potential of this l /l3 electrode is reported as —0.16 V in TMHAN(CF3S02)2 at 50°C [15] (TMHA = trimethyl-n-hexylam-monium) and —0.21 V in EM1N(CF3S02)2 at room-temperature [16] against Fc/Fc. ... 

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. 

---