Table of Contents:
This cumulative dissertation can be divided into two main topics: The investigation of the structure and the dynamics of the interface between ionic liquids and metal electrodes, and the study of the kinetics of electrochemical reactions in ILs.
Both topics are of great interest for technical applications based on ILs. Some examples of such applications are metal deposition, batteries and super capacitors or dye-sensitized solar cells.
The main experimental techniques used were cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Initially, the basic principles of both methods are introduced. CV was mainly used to determine the electrochemical stability windows of ILs, to calibrate pseudo- and quasireference electrodes and to determine diffusion coefficients of electrochemically active species. The latter was typically done by calculating the convolution of the CVs with t^(-0.5) to eliminate the need for data correction due to kinetic inhibition of the investigated slow reactions. This allowed the determination of accurate diffusion coefficients. EIS was used to measure the differential double-layer capacitance of the IL | Metal interface. To this end, potential dependent impedance spectra were measured and fitted with the Cole-Cole-equation in the complex capacitance plane. This method allows not only the determination of the capacitive contribution of different processes but also of their time constants. Furthermore, EIS was used to measure the heterogeneous rate constant of electrochemical reactions. Here, potential dependent impedance spectra in the region around the equilibrium potential of the reaction were used. These spectra were fitted in one step with a shared set of parameters. All impedance analysis was done in a newly developed software called RelaxIS. The development of a custom software allowed the implementation of new algorithms to perform the complicated Multi-Spectrum-Fits. The development history and some features of RelaxIS are described in this dissertation. Alongside of this, the general principles behind the evaluation of impedance spectra are discussed.
After the presentation of the experimental techniques, the current state of research of the two main topics are given. Regarding the investigation of the interfacial structure and dynamics first classical double-layer models and their enhancements for ionic liquids are introduced. Due to the high ion density in ILs and the complex interactions between the ions the classical models fail for these systems and a general model for ILs is still an active research topic. A seemingly universal property of these interfaces seems to be the tendency to form potential dependent multi-layer structures. This was shown both in experimental as well as simulation studies. For instance, force-distance curves measured by in-situ-AFM or molecular dynamics simulations gave strong evidence for the existence of these multi-layer structures. The differential double-layer capacitance only shows a mild potential dependence. In the region around the potential of zero charge, no clear indication about either a parabolic-, camel- or bell-shape was found. Interestingly, for some ionic liquids a second, strongly potential dependent capacitive process besides the double-layer charging was found. The time constant of this process was in the range of seconds and it may be explained by the surface reconstruction of the gold electrode or by the reorientation of the ion-layers. Studies of the temperature-dependence of the differential capacitance show large discrepancies, which is most likely due to disagreement about the best measurement and evaluation methods. Some studies show an increase of the differential capacitance with increasing temperature while other studies show only a mild temperature dependence or slight decrease with increasing temperature.
Two publications which are part of this cumulative dissertation are about the study of interfacial processes in ionic liquids. In the first publication, the potential-dependent differential capacitance of several ionic liquids in contact with a Au(111)-electrode was measured. By varying both the cations as well as the anions general trends of the differential capacitance in relation to the ion sizes could be deduced. Surprisingly, in the cathodic regime the double-layer capacitance was proportional to the cation size, while in the anodic regime the expected, inverse proportional dependence on anion size was found. Furthermore, the slow capacitive process was studied in detail. It was found, that it was mainly visible for ionic liquids with pyrrolidinium-based cations, while it was not observed to any significant extent in ILs with imidazolium cations.
The second publication about this topic investigated the influence of surface roughness of the metal electrode on the ideality of the double-layer charging process. Often, non-ideal behavior of this process is explained by surface roughness. This study could show that the non-ideality is virtually independent of the surface roughness. If evaluated correctly, the double-layer charging behaved as an almost ideal capacitor on all tested electrodes. Instead the apparent non-ideality observed in other studies is due to slow capacitive and faradaic processes which are often treated incorrectly in the evaluation of the spectra.
The investigation of the kinetics of electrochemical reactions in ionic liquids mainly treated simple heterogeneous 1-electron reactions of an electrochemically active species at a metal electrode. The theoretical part of this dissertation first describes fundamental kinetic formulations, like the Butler-Volmer- or Marcus theories. Following this, the theories are then applied to impedance spectroscopy where the ratio of the Warburg coefficient and the charge transfer resistance is introduced as the central quantity Φ_f. This quantity eliminates the influence of the surface concentration and is applied in a corresponding study, which is part of the cumulative part. Previous studies found that both the heterogeneous rate constant as well as the diffusion coefficients are significantly lower in ILs as compared to classical electrolytes based on organic solvents with a conducting electrolyte. Furthermore, relations e.g. between the rate constant and the hydrodynamic Stokes-Einstein-radius did not seem to be applicable in ionic liquids.
In the corresponding study, which is part of this cumulative dissertation, both the diffusion coefficients as well as the heterogeneous rate constants of four redox-active ionic liquids on ferrocene basis were measured in solution of the inert IL [EMIm]TFSI. To measure the rate constants, the development of a new analysis method based on Φ_f was required, since the accurate determination of the charge transfer resistance was not possible due to the strong overlap between the charge transfer semicircle and the diffusion branch. The measurements could verify earlier measurements, in that the redox-ILs also showed low k_0-values. Remarkably, only minor differences between the different redox-active ILs could be found, even though their equilibrium potentials are different. This result was discussed with regard to additional barriers for the approach of a charged particle to the electrode caused by the double-layer structure.
For the study of faradaic reactions in ILs it was necessary to work with finite-element simulations. This knowledge allowed a contribution to another research paper about anode materials for lithium-ion-batteries. This study, as well as two book chapters are presented in the cumulative part. The book chapters give reviews about impedance spectroscopy and complementary techniques for the investigation of the structure and processes at the interface between ILs and metal electrodes.