How to obtain kinetic information in thin-film voltammetry from the comparison of SCV and SWV responses

Abstract

Thin-film voltammetry (TFV) has one of the simplest experimental setups for studying the electrochemical processes that take place at liquid/liquid interfaces. This is because in TFV it is possible to neglect the iR drop of the non-aqueous solvent, when it corresponds to an organic thin film attached to the working electrode. In this manuscript the theory of TFV is used for describing how the electrochemical responses of staircase cyclic voltammetry (SCV) and square-wave voltammetry (SWV) are affected by the confinement of electroactive species in a thin film that is in contact with the working electrode. The reason for comparing the responses of these two techniques is established on the different concentration profiles that the electroactive species present within the thin-film and the implications that it has on the voltammetric responses of each technique. According to our analysis, the presence of the so-called “quasi-reversible maximum” cannot be used in TFV for estimating kinetic parameters since it would lead to erroneous results. This is because both the apparent kinetic and the apparent thickness change simultaneously with the variation of the scan-rate of each technique. Instead of this, it is proposed a strategy based on the variation of the scan-rate of SCV and SWV. Thus, it is possible to obtain 2 sets of voltammograms whose peak currents and peak potentials characterize the kinetic and thickness of the system. The estimation can be made when all the peak parameters fit properly to the theoretical data provided in this manuscript. Besides, it is shown that the results of SCV and SWV complement each other providing the way for estimating the formal charge transfer rate constant and the thickness of this kind of systems.