Supervisor: prof. Enn Lust (PhD), Institute of Chemistry, University of Tartu
Opponent: Ass.prof. Irina Petrushina (PhD), Technical University of Denmark
Room Temperature Ionic Liquids (RTILs) chemistry is one of the popular trends in modern chemical research. Wide electrochemical potential and thermal windows, good ionic conductivity, acceptable viscosity, high thermal stability, extremely low vapour pressure and the ability to solubilise many chemical species make RTILs interesting electrolytes for electrochemical applications. Compared with commonly used organic compounds, they have low toxicity and are non-flammable. Furthermore, variation of the chemical composition of ions or even just the length of the alkyl groups allows fine tuning of the physicochemical properties of RTILs, such as viscosity, conductivity, catalytic activity, melting point etc. Thus, RTILs can be strategically designed for different applications. A detailed understanding of the structure, thermodynamics and kinetics of RTILs at the electrode surfaces is crucial for design of modern electrochemical devices, such as supercapacitors, actuators, batteries, solar cells, etc. Studies of RTIL based supercapacitors, actuators and sensors are performed in the University of Tartu.
In this study we applied Electrochemical Impedance spectroscopy and cyclic voltammetry methods for characterizing the processes at the interface between Cd(0001) electrode and 1-ethyl-3-methylimidazolium tetraﬂuoroborate (EMImFB4) as well as adsorption of I− anion on the Cd(0001) electrode from aqueous electrolyte solutions. We also started a series of systematic quantum chemical studies at Density Functional Theory level, which eventually lead us to a better understanding of our previous experimental studies of iodide ion adsorption at Cd(0001) and Bi(111) electrodes. Studies of the adsorption kinetics of halide ions at single-crystal electrodes was and still is a hot trend in electrochemistry. Interestingly, the adsorption process(es) in aqueous solution and room temperature ionic liquid were found to follow similar patterns, although the chemical composition and bulk properties of the liquids are completely different. We provide explanations to the differential capacitance dependences on temperature, which indeed appeared to be different for two electrolytes due to the difference in the interface structures. In order to explain the observations found for Cd(0001) | aqueous electrolyte solution interface, we were able to employ a model effectively describing the electrical double-layer as a metal-solvent dipole monolayer and metal-charged wall of the adsorbed ions. The results of extensive Density Functional Theory based calculations were used for the modelling.
In case of Cd(0001)/RTIL interface the presented theoretical model accounts multilayered structure of the electrical double-layer. Gradually "dissolving" the interfacial structure changes with the rise of temperature causing the decrease of the differential capacitance. In our experimental research we acquired new insights to the existing knowledge on the temperature dependences as well as made new observation on adsorption kinetics and structure at the electrode | RTIL interface. In this thesis we provide a comparative overview of the results for the different systems named above, presenting our most important conclusions as one logical conception of structure-property correlation as a function of temperature and electrode surface charge density.