New theoretical insights into the competitive roles of electron transfers involving adsorbed and homogeneous phases

The traditional textbook view of adsorptive features in electrochemistry, viz., involving a bell-shaped wave prior to or after the diffusional wave is certainly right but concerns a very limited series of conditions in which adsorption kinetics are too slow vs. the scan rate. Laviron examined the converse situations in which extremely rapid adsorption kinetics make the voltammetric process follow the classical diffusional behavior though the effective electrochemical reactions proceed via adsorbed species. Thanks to a new simulation approach, implemented in KISSA (c), the present work examines intermediate situations which could not be investigated since they do not lead to analytical formulations. Besides allowing investigating the transition between the two above limiting behaviors, it is established that during such transitions voltammograms display CE-type behaviors in which the rates of the pseudo-antecedent chemical steps feature those of adsorption. An electroactive adsorbed species is indeed involved in a dynamic steady state between its adsorption and its consumption by electron transfer at the electrode surface so that its current is independent of the potential. This is a general situation presently overlooked in electrochemical theories. For example, the same CE-like behavior is also shown to occur during electropolymerization of redox polymers though it now is hidden under the monomer diffusion-controlled wave. (C) 2012 Elsevier B.V. All rights reserved.