(The Max Bredig Award In Molten Salts and Ionic Liquid Chemistry) Design and Electrochemical Application of Ionic Liquids Based on an Understanding of Their Nature
Authors: Watanabe Masayoshi 
Published: February 2020

Ionic liquids (ILs) are now being recognized as a third group of solvents, following water and organic solvents. They are readily available and possess unique properties like nonvolatility, high thermal stability, and designability that make them possible to use on demand and under unusual conditions. We have focused on understanding the unique properties of ILs and their utilization as neoteric solvents and electrolytes. More specifically, ILs can serve as neoteric electrolytes in electrochemical energy conversion systems. H+- and Li+-conducting and electron-transporting ILs are being designed to develop innovative fuel cells, batteries, and dye-sensitized solar cells. ILs also exhibit unique polymer solubilities. These challenges encourage the development of new materials and devices for a sustainable society based on a thorough understanding of ILs. In this presentation, I will focus on unique transport properties of certain ILs and their interesting applications by utilizing such properties.

The Walden rule correlates the molar conductivity Λ of an ionically conducting liquid to its viscosity by Λ
η
= const. In most cases, the molar conductivity of a given ionic liquids (ILs) lies below the logΛ
- log(1/η) line of an ideal KCl aqueous solution [1], which indicates the ionic mobility is governed by the viscosity and also some ionic association may occur in the liquids. We have quantified the deviation from the ideal line by using “ionicity” parameter, which is derived from the molar conductivity ratio based on conductometry and PGSE-NMR (Λ
imp/Λ
NMR) [2]. In the case of aprotic ILs, the ionicity is affected by subtle balance between Coulombic and van der Waals interactions [2, 3]. In certain cases, the Walden plots of ILs lie above the ideal line; in such cases there should be super-diffusing mechanisms of ions where the activation barrier for conductivity is lower than that for viscosity. We found such phenomena in certain protic ILs having proton donating/accepting anions (e.g. HSO4
-, H2PO4
-) [4] and ILs containing an I-/I3
- redox couple [5]. Even in the cases where Λ
imp/Λ
NMR < 1 or the Walden plots lie below the ideal KCl line, unique transport phenomena are seen when ionic liquids are used as electrolyte of electrochemical systems and electrochemical reactions occur at the electrode/electrolyte interfaces, as discussed below.

Proton conduction occurs mainly via the vehicle mechanism in a prtotic IL, diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), and the proton transference number (t
+) is 0.5~0.6. However, fast proton-exchange reactions appear to occur between ammonium cations and amines[6] when [dema][TfO] is applied to fuel cell electrolyte, where de-protonated amines are continuously generated by the cathodic reaction. Another example is equimolar glyme/Li salt complexes that can be categolized into solvate ILs. The experimental results strongly suggest that Li+ cation conduction in the equimolar complex takes place by the migration of [Li(glyme)1]+ cations, whereas the ligand exchange mechanism is overlapped when interfacial electrochemical reactions of [Li(glyme)1]+ cations occur [7].

We have also proposed soft materials containing ionic liquid (ion gels) [8]. Ion gels are composed of ionic liquids (ILs) immobilized within a three-dimensional molecular network. Such gels preserve the attractive physicochemical properties of ILs, while maintaining a soft solid consistency. Ion gels are a novel platform for many applications such as electrolyte membranes, actuators, gas-separation membranes, and organic thin-film transistors.

References

1. W. Xu, E. I. Cooper, C. A. Angell, J. Phys. Chem. B, 2003, 107, 6170.

2. H. Tokuda, S. Tsuzuki, M. A. B. H. Susan, K. Hayamizu, M. Watanabe, J. Phys. Chem. B, 2006, 110, 19593.

3. K. Ueno, H. Tokuda, M. Watanabe, Phys. Chem. Chem. Phys., 2010, 12, 1649.

4. H. Nakamoto, M. Watanabe, Chem. Commun.
2007, 2539.

5. R. Kawano, M. Watanabe, Chem. Commun.
2003, 330.

6. S.-Y. Lee, A. Ogawa, M. Kanno, H. Nakamoto, T. Yasuda, M. Watanabe, J. Am. Chem. Soc.
2010, 132,9764.

7. K. Yoshida, M. Nakamura, Y. Kazue, N. Tachikawa, S. Tsuzuki, S. Seki, K. Dokko, M. Watanabe, J. Am. Chem. Soc.
2011, 133, 13121.

8. T. Ueki, M. Watanabe, Bull. Chem. Soc. Jpn. (Accounts), 2012, 85, 33.