Electrochemical methods - Impedance measurements

Whether in the case of processes of academic interest, i.e. often simplified in relation to the real world so as to describe the latter in fundamental terms, or of more applied interest, the charge transfer that takes place at an interface between an electrode and an electrolyte occurs as a result of a succession of elementary phenomena that are more or less strongly coupled:

• transport of reactive species within the electrolyte, often associated with chemical reactions in the electrolyte;

• adsorption of reactive species on the electrode ;

• interfacial chemical and electrochemical reactions often taking place in several monoelectronic steps.

The electrochemist's aim is either to analyze an interfacial mechanism by chemical identification and kinetic characterization of reaction intermediates, or to estimate a quantity characteristic of a process (e.g. corrosion or deposition rate) from the measurement of a well-defined quantity. So, to unravel couplings between material transport and interfacial reactions, or to carry out a test, the electrochemist must often use a technique capable of extracting information during the course of the electrochemical process.

Some of the techniques used to characterize the state of the surface or the species adsorbed at the interface require the electrode to be placed in a vacuum (slow electron diffraction, Auger electron spectroscopy, etc.), and are therefore not suitable for in situ studies. Electromagnetic radiation techniques (optical: ellipsometry, or X-ray: EXAFS) are beginning to be used to study the electrochemical interface, but run into major difficulties as soon as the surface is altered (dissolved, deposited, etc.). As a result, electrical techniques are often the only ones that can be used to study the electrochemical interface in situ.

By influencing the speed of electrochemical reactions, the use of electrical quantities enables kinetic studies to be carried out, dissociating the couplings between the various elementary phenomena. This makes it possible to distinguish between the monoelectronic stages of reaction mechanisms and to count the often unstable intermediates involved in these reactions. While these techniques do not enable the identification of bonds and reaction intermediates in the chemical sense of the term, they do provide information on the kinetics of the reaction mechanism governing the behavior of the electrochemical interface, and provide some characterization of these intermediates.

In addition to stationary techniques, which can be used to study the simplest processes, non-stationary techniques are needed to analyze more complex...