Square Wave Voltcoulommetry of two-electron molecular electrocatalytic processes with adsorbed species. Application to the surface O2 reduction in acetonitrile at anthraquinone-modified glassy carbon electrodes

Abstract

An adequate analysis of the performance and efficiency of a given multi-redox molecular electrocatalyst versus a particular substrate of interest requires the knowledge of the overall reaction pathway and the determination of the rate constants of the catalytic steps taking place. For this purpose, a theoretical model is presented for the Square Wave Voltcoulommetry response of surface-confined two-electron catalysts, that is, molecular species with two different redox states that can act in different electrocatalytic forms as a function of the applied potential. The rigorous theoretical model incorporates convective mass transport effects and non-idealities in the electrochemical behaviour of the immobilized redox probes. The results obtained have been applied to the study of the surface oxygen reduction in an aprotic medium (acetonitrile) at an anthraquinone-modified glassy carbon electrode. The charge-potential responses indicate that the reaction pathway involves two different catalytic routes due to the two reduction products of the anthraquinone moiety, that is, the semiquinone anion radical and the dianion. A Koutecky-Levich approach, combined with the fittings of the experimental responses to the theoretical expressions deduced here, has allowed us to obtain the values of the different thermodynamic (formal potential, interaction parameters, surface coverage) and kinetic (rate constant) parameters of the process.