Studying Electrochemical Interfaces

At any electrochemical interface, charge transfer occurs only at the end of a succession of the following coupled elementary phenomena:

1. Transport of reactive species in the bulk of the solution (mass transport).

2. Adsorption of the reactive species on to the electrode.

3. Electrochemical and chemical interfacial reactions.

Both adsorption and reactions occur on the electrode surface; but, mass transport is a homogeneous phase phenomenon. The electrochemist has to isolate each of the elementary phenomenons from others to study them. Hence, he has to use a technique which is capable of separating the elementary phenomenons and extract the data for each of them separately.

Some techniques like AUGER that are able to characterize the adsorbed species on the interface require the use of vacuum techniques; therefore they can’t be for “in-situ” analysis of the electrochemical interface. Other techniques using electromagnetic waves like SEM are being used to study the electrochemical interface. But these cannot be used when dissolution of the surface or deposition on the surface occurs. Hence electrical methods are often the only possible methods for “in-situ” investigations of electrochemical interface.

Use of electrical methods allows studying the reaction kinetics, which permits the separation of the coupling between the elementary phenomena by control of the reaction rates. This enables the individual electronic steps in the reaction mechanism to be distinguished and the often unstable reaction intermediates involved in these reactions to be counted. Even if these techniques do not allow a real identification of the reaction intermediates and bondings from a chemical point of view, they at least give information on the kinetics of the reaction mechanism governing the behavior of the electrochemical interface and some characterization of these intermediates.

Steady state techniques are used to study simple processes; where as non-steady state techniques are necessary to study more complex electrochemical systems. Disturbing the reaction from the steady state by applying a small perturbation to the electrochemical system allows the system to attain a new steady state. As the various elementary processes change at different rates, their response can be analyzed to separate the overall electrochemical process. The choice of the technique to be used depends on the application. Transient (time analysis) techniques are generally used for testing models or determining kinetic parameters of a known mechanism, as they are well suited for extracting kinetic parameters when the mass transport is negligible. However, when complex heterogeneous reactions interact with mass transport, transient analysis will lead to very poor results and a frequency analysis is more efficient. Hence, the use of impedance measurements over a wide frequency range is a better technique for studying the electrochemical interface.

Devising the measurement procedure and elaborating the models which have to be compared with the experimental data require an accurate description of the kinetic and electric laws governing the interface.