Role of specifically adsorbed electrochemically inert species in the electrode kinetics

Abstract

The problem of decoding the physico-chemical mechanism of interaction between the elementary act of an electrochemical reaction and the adsorption phenomena at a metal-solution interface is of major importance because it directly concerns the interpretation of electrochemical kinetics data and the proper mathematical description of the effect of the electric double layer on electrode processes. In connection with the latter aspect the fundamental problem arises, namely, the problem of adequate statistico-mechanical approaching the transition from the description of the elementary act of an electrochemical process occurring at the interface in the presence of electroinactive specifically adsorbed species, ions or dipole molecules, to the description of the resulting heterogeneous process the rate of which is experimentally measurable. The advances made in recent years in interpretation of the data on the kinetics of electrode processes involving the specific adsorption of ionic and dipole solution components relate mainly to substantiation of the role of the discreteness effects. The local electric field acting near the interface on discrete particles possessing electric charge or dipole moment differs, due to polarization of the interface by these particles, from the average field described by the macroscopic equations of electrodynamics with a continuous charge and dipole moment distribution. The characteristic relaxation times of the ‘the electrostatic image» forces, i.e. the redistribution times of free and bound charges at the interface due to its polarization by discrete particles present in solution, are of the order to 10 −15 −10 −14 sec. The characteristic times of many types of electrochemical reactions, however, are comparable with the fluctuation reorientation time of dipole molecules of the solvent in the solvation shells of reactants (∼10 −11 sec), i.e. they are by three–four orders of magnitude higher than the relaxation times of ‘the electrostatic self-image’ forces. For this reason, the discrete nature of charges and dipole moments should affect the probability of the elementary act of an electrochemical process. Moreover, the local electric field associated with the discreteness effect can influence the rate of electrochemical processes by changing the concentration of reactants and reaction products inside the layer adjacent to the electrode. In the present paper the problem of statistico-mechanical averaging the probability of the elementary act of an irreversible electron transfer reaction occurring on a metal-solution interface in the presence of specifically adsorbed electrochemically inert ions or dipole molecules is discussed in detail. The analytical expressions are derived for the polarization curve of the interface in cases of localization of the reactants both inside and outside the compact part of the electric double layer. Within the framework of the theory presented the available experimental data on electrochemical kinetics complicated by the specific adsorption of charged and neutral components of the solution are analysed.