6. Electrochemical reaction kinetics

Like all heterogeneous processes, an electrochemical reaction consists of several reaction steps. First, a reactant must be transported to the electrode surface; that was considered in Chapter 3. Prior to a reaction, a reactant may be adsorbed on the surface and the adsorbed species undergoes electron transfer. A reaction product must desorb from the surface and be transported away, in order to yield to other species to react. The effect of adsorption on the reaction rate has been considered in Chapter 5 and 9, while this Chapter concentrates on the rate and mechanisms of electron transfer.

An electrochemical reaction has the special feature that its rate can be externally controlled in situ by varying the electrode potential or electric current. This is possible because of Faraday’s law (see Chapter 1), which states that the reaction rate is proportional to the electric current. The core of an electrochemical reaction is electron transfer that is basically a quantum mechanical phenomenon but is traditionally modeled analogously to the activated complex theory14F*


*Rudolph Marcus received the Nobel prize in chemistry in 1992 for his theory of electron transfer, known as the Marcus theory; it will be addressed briefly later.