A reasoned general explanation about the concepts of diffusion and reaction layers

Abstract

The understanding of the implication of the transport of matter is fundamental in the teaching of heterogeneous charge transfers, which are the central element of the electrochemical science and technologies. Thus, even in the absence of intrinsic kinetic complications, the overall rate of such events is frequently conditioned by the mass transport and also by the occurrence of homogeneous coupled chemical reactions. Upon comprehending and accounting for these two effects, the emergent and widely used concepts of the diffusion and reaction layers will be discussed and disentangled under potential-controlled conditions. First, some basic concepts about the transport of charged species in solution are recalled, in order to establish the experimental conditions under which diffusion-only transport can be considered. Under these premises, a fast electron transfer will be considered to introduce the concept of “diffusion layer”, discussing its relationship with the time of the experiment and with the characteristics of the diffusive field that is strongly influenced by the geometry and size of the electrode considered. Next, fast electron transfer reactions “conditioned” by homogeneous chemical reactions undergone by the reactant and/or the product of the redox couple will be analysed. A historical introduction to the concept of “reaction layer” is carried out, pointing out its value in processes with chemical regeneration of the electroactive species and its non-negligible interplay with the mode of diffusion.