Electrochemical Double Layers in Ionic Liquids Investigated by Broadband Impedance Spectroscopy and Other Complementary Experimental Techniques

Abstract

Ionic liquids consist of large asymmetric organic cations and of weakly coordinating anions, i.e., anions with a highly delocalized negative charge. Ionic liquids (IL) exhibit remarkable physicochemical and electrochemical properties, in particular, high thermal stability, low vapor pressure, high ionic conductivity, and broad electrochemical window (Buzzeo and Evans in ChemPhysChem 5:1106, 2004 [1], Endres and Zein El Abedin in Phys Chem Chem Phys 8:2101, 2006 [2], Galinski et al. in Electrochim Acta 8:2101 [3]). By changing the functional groups of cations and by varying the cation/anion combination, the properties of ionic liquids can be adjusted to particular requirements. Thus, ionic liquids are being called designer solvents. They are considered as promising electrolytes for different kinds of electrochemical cells, e.g., for electrosynthesis (Buzzeo and Evans in ChemPhysChem 5:1106, 2004 [1], Hapiot and Lagrost in Chem Rev 108:2238, 2008 [4]), for electroanalysis (Buzzeo and Evans in ChemPhysChem 5:1106, 2004 [1], Baker et al. in Analyst 130:800, 2005 [5]), for electrodeposition of metals (Endres and Zein El Abedin in Phys Chem Chem Phys 8:2101, 2006 [2], Endres et al. in Phys Chem Chem Phys 12:1724, 2010 [6], Endres et al. in Angewandte Chemie (International ed. in English) 42:3428, 2003 [7]), for energy storage in batteries and supercapacitors (Sillars et al. in Phys Chem Chem Phys 14:6094, 2012 [8], Simon and Gogotsi in Nat Mater 7:845, 2008 [9], Lewandowski and Świderska-Mocek in J Power Sources 194:601, 2009 [10], Srour et al. in 200th ECS meeting. ECS, pp 23–28, 2012 [11], for energy conversion in dye-sensitized solar cells (Gratzel in Acc Chem Res 42:1788, 2009 [12]) and for double layer field-effect transistors (Yuan et al. in Adv Funct Mater 19:1046, 2009 [13]). For all these electrochemical applications, the structure and dynamics of the interfacial double layer between ionic liquids and electrode materials plays a crucial role, see for instance Ref. (Endres et al. in Phys Chem Chem Phys 12:1724, 2010 [6]). Since ILs are highly concentrated ionic fluids, the classical Stern model for double layers in diluted electrolytes, in which the ions are treated as point charges, is not applicable. In the case of ionic liquids, the finite volume of the ions, the chemical structure of the ions, and specific interactions of the ions with the electrode surface have to be taken into account. In order to obtain information about the structure and dynamics of the double layer, various experimental techniques have been applied. Broadband impedance spectroscopy in a three-electrode setup yields electrode potential-dependent double layer capacitance values of the electrode | IL interface. Complementary information has been obtained from other techniques, such as scanning tunneling microscopy, atomic force microscopy , surface force apparatus measurements, X-ray reflectivity measurements, surface-enhanced Raman spectroscopy , and sum-frequency generation vibrational spectroscopy . In this chapter, we describe the current level of understanding of the electrode | IL interface based on the combination of the complementary techniques.