Electroreduction of halogen oxoanions via autocatalytic redox mediation by halide anions: novel EC” mechanism. Theory for stationary 1D regime

• Published in: Electrochimica acta Vol. 173; pp. 779 - 795
• Main Authors: Vorotyntsev, Mikhail A., Konev, Dmitry V., Tolmachev, Yuriy V.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 10.08.2015
• Subjects: autocatalytic cycle; comproportionation; diffusion layer; halogen oxoanions; kinetic layer; redox mediation; autocatalytic cycle; redox mediation; kinetic layer; comproportionation; halogen oxoanions; diffusion layer
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/j.electacta.2015.05.099

Theoretical analysis of the system with coupled electrochemical and chemical steps has been carried out where bulk solution contains non-electroactive halogen oxoanions, XOn−, with a very small addition of halogen molecules, X2. The latter are electroreduced rapidly at the electrode surface, generating halide anions, X−, which diffuse towards solution, wherein they comproportionate with the principal oxidant, XOn−, yielding electroactive halogen. Unlike the well-known catalytic EC’ mechanism where both the electrochemical reaction and the chemical step retain the total amount of the mediating redox couple, the passage of the electroreduction cycle in the system under our study results in an increase of the total content of its components, halogen and halide anion, via the consumption of halate anions. We propose to denote this new autocatalytic EC mechanism as EC”. Approximate analytical formulas have been derived for all characteristics of this system under steady state conditions at the uniformly accessible electrode surface. We found that the behavior of the system depends crucially on the relation between the diffusion layer thickness, zd, and the kinetic layer thickness, zk (determined by the rate of the homogeneous reaction). For a very thin diffusion layers: zd<zk, halide anions leave the diffusion layer and react with XOn− anions only in the bulk solution. Both the polarization curve and the maximal current correspond to the electrode reaction of halogen molecules from the bulk solution, without a significant contribution due to the comproportionation reaction. In the intermediate range of the diffusion layer thickness: zk<zd<2n zk, the halide anions generated at the electrode are consumed mostly by this homogeneous reaction within a thin kinetic layer (located deeply inside the diffusion layer) while the halogen molecules produced by this reaction diffuse partially to the electrode, generating again halide anions. This combination of the chemical and electrochemical steps results in an autocatalytic cycle, based on the X−/X2 mediating redox couple, which consumes a significant amount of XOn− anions. The maximal current becomes much higher than the mass-transport of halogen from the bulk can sustain, depending essentially on the kinetic layer thickness. In the third range of the diffusion layer thickness: 2n zk<zd, the amounts of the accumulated redox-couple components are so high that the principal (but non-electroactive) oxidant, XOn− is consumed within the external part of the kinetic layer with the maximal rate determined by the XOn− anion diffusion across the diffusion layer, which results in a very high maximal current proportional to the bulk concentration of XOn− anions. The theory predicts a complicated behavior of the maximal current as a function of the diffusion layer thickness (or the disk rotation rate for the RDE technique), with a maximum and a minimum separated by the range with an anomalous variation: increase of the maximal current with increase of the diffusion layer thickness (“autocatalytic interval”).