Electrochemical kinetics in inorganic systems

Kinetics plays an essential role in the preparative electrochemical industry, in the field of electrochemical generators, in electroanalysis and in corrosion, but it is not an easy subject to tackle, as the equations involved sometimes put off neophytes. Reaction mechanisms are often complex, and may involve a succession of electrochemical or chemical steps. The slowest of these (known as the kinetically decisive step) dictates the overall kinetics. It should also be remembered that, unlike chemical reactions which take place throughout the volume of a reactor, electrochemical reactions take place at the surface of an electrode. Their kinetics will therefore also depend on the speed of transport of the electroactive material between the electrolyte core and the electrode.

The oxidation and reduction reactions of organic compounds studied in molecular electrochemistry often involve short-lived reaction intermediates. Their detection requires sophisticated techniques. For the purposes of this article, we have focused on inorganic systems for two reasons: firstly, they often give rise to simpler mechanisms; secondly, the examples cited will be of greater use to the engineer. Indeed, today's major industrial processes are essentially in the field of inorganic electrochemistry, even if certain high-value-added organic compounds can be synthesized by electrochemistry.

It's easy to measure the speed of an electrochemical reaction: a simple ammeter is all you need, since the speed is proportional to the electric current flowing. Studying the kinetics of an electrochemical reaction as a function of the potential applied to the electrode thus amounts to drawing a "current-potential" curve.

This article begins with a brief review of thermodynamics. Then, the precise definitions of the quantities commonly used in electrochemical kinetics, their physical meaning and the main applications that derive from them in electroanalysis, corrosion and industrial processes, are set out. The equations of stationary, i.e. time-independent, I-E curves are described in detail, as they form the necessary basis of electrochemical kinetics, which the engineer must know before attempting to unravel the elementary mechanisms through the study of short-time transient phenomena. These are briefly discussed at the end of the article. The discussion is illustrated by a series of examples and a compilation of experimental data useful to the engineer. Finally, the main electrochemical techniques available for experimentation are also described.

Thermodynamic data for a given redox system are generally readily available in the literature. The same cannot be said for kinetic data, which depend on the reaction medium and electrode material.