ABSTRACT
Figure 3.1 Structure of ionic liquids.The electrochemical window of electrolytes is very crucial in electrochemical applications. ILs are electrochemically rather stable having an electrochemical window typically from 2 to even 6 V. The electrochemical stability of the ILs depends on redox reactions of both the anion and the cation. Anions such as Tf2N-oxidize at relatively high anodic potentials compared with, for example, BF4-resulting in extended stability of ILs containing Tf2N-[6, 29]. Also, tetraalkylammonium cation-based ILs show extended electrochemical window on the negative side of the potential scale [30]. Another important issue connected with the electrochemical properties of ILs is the influence of moisture. In chloride-based ILs, the addition of water produces HCl, which is then reduced further, greatly changing the electrochemical response of IL [31]. Also, the residual water can drastically decrease the electrochemical window. Shröder et al. studied the impact
of water on cyclic voltammetry of the electrolytes [BMIM][BF4] and [BMIM][PF6] [9]. For both of those ILs, a decrease in the electrochemical window was observed with increasing concentration of water. Also, under exposure of [BMIM][PF6] to water-saturated argon, the electrochemical window decreased from ca. 4 to 2 V within less than 1 h. Even ppm levels of water can affect the electrochemical response [32]. Hence, great care must be taken in synthesis of ILs as well as in their electrochemical studies. Viscosity of ILs ranges from ca. 30 cP to 600 cP [5]. These relatively high values of viscosities have strong influence on the conductivity of ILs and mass transport in them. For conventional electrolytes conductivity is proportional to the concentration of charge carriers. However, in ILs the apparent cation-anion interaction makes it difficult to exactly define the number of charge carriers. Hence, ILs have much lower ionic conductivities than could be expected when compared with some high-temperature molten salts [33]. High viscosities of ILs also result in relatively low self-diffusion and difficulties in determination of the exact transport numbers.
