Ionic liquids are of high interest for the development of safe electrolytes in modern electrochemical cells, such as batteries, supercapacitors and dye-sensitised solar cells. However, electrochemical applications of ionic liquids are still hindered by the limited understanding of the interface between electrode materials and ionic liquids. In this article, we first review the state of the art in both experiment and theory. Then we illustrate some general trends by taking the interface between the extremely pure ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and an Au(111) electrode as an example. For the study of this interface, electrochemical impedance spectroscopy was combined with in situ STM and in situ AFM techniques. In addition, we present new results for the temperature dependence of the interfacial capacitance and dynamics. Since the interfacial dynamics are characterised by different processes taking place on different time scales, the temperature dependence of the dynamics can only be reliably studied by recording and carefully analysing broadband capacitance spectra. Single-frequency experiments may lead to artefacts in the temperature dependence of the interfacial capacitance. We demonstrate that the fast capacitive process exhibits a Vogel-Fulcher-Tamman temperature dependence, since its time scale is governed by the ionic conductivity of the ionic liquid. In contrast, the slower capacitive process appears to be Arrhenius activated. This suggests that the time scale of this process is determined by a temperature-independent barrier, which may be related to structural reorganisations of the Au surface and/or to charge redistributions in the strongly bound innermost ion layer.

Results of potential-dependent differential capacitance measurements on the interface between six different ionic liquids and the (111) surface of single-crystalline gold are presented. The measurements were done by means of broadband impedance spectroscopy in a frequency range from 10 mHz to 1 MHz. We discuss the influence of the IL cation, the IL anion and the cations' alkyl chain length on the interfacial capacitance. Our results suggest that (i) there is no simple relationship between the cation size and the value of the differential capacitance, (ii) the general shape of the potentialdependent differential capacitance curve is more strongly influenced by the IL anion, and (iii) experimental differential capacitance curves do not exhibit a simple "camel-" or "bell-shaped" curvature as predicted by mean-field theories. Furthermore, the broadband measurements show that two capacitive processes can be distinguished, which take place on millisecond and second time scales, respectively. While a millisecond time scale is expected for double-layer charging governed by the bulk conductivity of the IL, the existence of a slow process points to additional barriers for charge transport at the interface. The capacitance contribution of the slow process is most pronounced for ILs based on the N-butyl-N-methyl-pyrrolidinium ([Pyr 1,4 ]) cation. A comparison of capacitance data with insitu STM data from previous studies suggests that the slow process is connected to herringbone-type structures at the interface. While the herringbone superstructure of the Au(111) surface is well known in aqueous electrochemistry, a herringbone-type structure of adsorbed ions was described in a recent MD simulation paper by Federov and coworkers (K. Kirchner, T. Kirchner, V. Ivaništšev, M. V. Fedorov, Electrochim. Acta 2013, in press:

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We have used broadband electrochemical impedance spectroscopy for characterizing double layer formation at the interface between the ionic liquid 1-butyl-1methylpyrrolidinium bis(trifluoromethane)sulfonimide [Py1,4]TFSI and three gold electrodes with different surface structure and roughness. Two alternative approaches for analyzing the data were compared: a) A fit in the complex impedance plane using a resistor and a constant phase element (CPE) connected in series; b) A fit in the complex capacitance plane using a Cole−Cole function. In the complex capacitance plane, a highfrequency semicircle due to double layer formation could be clearly distinguished from other capacitive or Faradaic processes detected at lower frequencies. The Cole−Cole fit of the high-frequency semicircle revealed that this semicircle is almost unsuppressed with α values close to unity, even for the rough polycrystalline Au electrode. In contrast, the CPE exponent depends much more strongly on electrode potential and electrode roughness. We show that this strong dependence is closely related to the existence of slower capacitive or Faradaic processes, and is not caused by nonideal double layer formation.

The self-reaction of state-selected HCl(+) (DCl(+)) ions with HCl has been investigated in a guided ion beam setup. The absolute cross sections for proton transfer and deuteron transfer decrease with increasing center of mass collision energy, Ec.m.. The cross section for charge transfer (DCl(+) + HCl) exhibits a maximum at Ec.m. = 0.5 eV. The cross section for PT and DT decrease significantly with increasing rotational angular momentum in the molecular ion, for the PT the cross section increases again for the highest angular momentum investigated. The rotational dependence of the cross section is rationalized by a simple model in which both the collision energy and part of the rotational energy are available for the reaction. The contribution of the rotation to the total energy available itself depends on the collision energy.

Redox-active ionic liquids are of interest for various applications in electrochemistry, for example, as electrochemically active anions and cations in supercapacitors, as redox mediators in dye-sensistized solar cells and for overcharge protection in batteries. Due to the chemical variability of redox-active ionic liquids, their electrochemical properties can be easily tuned. Here, we investigate the electrochemical kinetic properties of four ionic liquids containing sulfonium and phosphonium cations, with ferrocenyl substituents directly attached to the onium center. The redox-active ionic liquids are dissolved in the electrochemically innocent ionic liquid [EMIm]­TFSI. The results are compared to the electrochemical kinetic properties of free ferrocene standard. We obtain precise values for the heterogeneous rate constant k 0 by means of a multispectrum fit of impedance spectra measured at different overpotentials. Diffusion coefficients are derived from a convolution analysis of cylic voltammograms. The redox active cations exhibit lower k 0 values and lower diffusion coefficients than ferrocene, but the k 0 values are only weakly dependent on the chemical structure of the cations. Furthermore, we observe no correlation between k 0 and the hydrodynamic radius of the redox active cations. We offer an explanation for these observations based on kinetic barriers caused by the structure of the electrochemical double layer.

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