CHEM-E4185 - Electrochemical Kinetics, 25.02.2019-29.05.2019

Electrochemistry studies the physical properties of electrically charged particles and their systems as well as their reactions and transport in the system. An electrochemical system thus consists of charged particles and a medium, i.e. a solvent, most commonly water. Particles and the medium form an electrolyte solution. Particles are usually ions, ion complexes or macromolecules, but also a free solvated electron can exist in a solution. Solid electrolytes and molten salts are special cases of electrolyte solutions; they require very high operation temperatures, typically approximately 800 ºC. The former are used in solid oxide fuel cells and the latter in the production of, for example, aluminum. Recently, ionic liquids aka room temperature molten salts have received a lot of attention. They are salts assembled from large organic molecules that can be liquid at room temperature or at slightly elevated temperatures (less than 100 °C).

An electrochemical system also contains electrodes. They are needed because an electrochemical reaction is always heterogeneous, taking place on a solid surface containing free electrons or at some other interface between the electrolyte solution and a phase containing charged species. Electrodes are typically metals or other solids with good electrical conductivity, such as graphite. In order to understand the nature of an electrode reaction, the electron structure of solid materials is briefly addressed in the end of this chapter. Electrodes are connected to each other via an external electrical circuit that can be as simple as a copper wire, but most commonly a load (a resistor) and/or a measurement instrument. An ensemble formed of an electrolyte solution, electrodes and an external circuit is called as an electrochemical cell.

Thermodynamics is a discipline with which the properties of electrolyte solutions can be studied. Due to long-range electrical (coulombic) interactions, the properties of electrolyte solutions are very different from solutions of electrically neutral solutes; this is addressed in detail in Chapter 2. Electrode reactions create local concentration differences in an electrolyte solution that transport processes try to level out. The analysis of transport processes therefore constitutes an essential part of the study of an electrochemical system. Transport is addressed in Chapter 3.

Yet another special feature of electrochemical reactions is that in addition to temperature, pressure and concentration, their rate depends, like most reactions, on the potential of the electrode. Potential has several bearings in electrochemistry as addressed in the end of this chapter. Since reaction rates depend on potential in an exponential manner as explained in Chapter 6, reaction rates can be controlled over several orders of magnitude.