Electroreduction of bromate anion on inactive RDE under steady-state conditions: Numerical study of ion transport processes and comproportionation reaction

Abstract

International audienceThe study is devoted to analyzing the electroreduction of bromate anion BrO (3) (-) on catalytically inactive (e.g., carbon) electrodes by numerical methods. This process is realized due to the combination of the reversible mediator pair Br-2/Br- and the process of comproportionation (reaction of bromate and bromide anions) in solution phase. These reactions increase the concentration of bromine and bromide near the electrode surface; hence, this process is autocatalytic (ECaEuro(3) mechanism). Within the framework of this study, a numerical algorithm which allows the system of diffusion-kinetic equations to be solved for this system is proposed for one-dimensional transport and the process under steady-state conditions. The results are compared with the conclusions of the approximate analytical theory published in Electrochim. Acta, 2015, vol. 173, p. 779, which allows inferring that both approaches are correct. The deviation between the latter is observed only in the cases of violation of approximations lying in the basis of the corresponding analytical relationships. Thus, the predictions of the analytical theory of practical interest can be considered as reliably confirmed by numerical calculations, particularly, the prediction on the nonmonotonous dependence of the maximum current on the diffusion layer thickness (or the RDE rotation rate) including the anomalous region of this dependence in which the current increases with the increase in the diffusion layer thickness

Bromate electroreduction via autocatalytic redox mediation: EC” mechanism. Theory for stationary 1D regime. Current limitation by proton transport

“…In practical terms it is seen from the analysis in section A1.3 of Appendix 1 for weak currents that this approximation reproduces reasonably well all characteristics already for x dk =2, the precision increasing rapidly for larger x dk values. Besides, a similar study for the case of the proton excess compared to bromate [4] demonstrated that the approximation of a thin kinetic layer resulted in predictions for both weak and strong currents which were in conformity with those on the basis of numerical calculations.…”

“…outside the kinetic layer. To facilitate the comparison of the results in this study with those in Refs [3,4] we introduce in a similar formal manner the diffusion-limited current of bromate for its transformation to bromine:…”

“…Our theoretical studies [3,4] have revealed its highly unusual features, in particular an explosive increase of the steady-state cathodic current if the diffusion layer thickness increases with a narrow interval near its critical value, this novel and astonishing feature originating from the autocatalytic nature of the process, which was termed as EC" mechanism [3], to distinguish it from the well-known catalytic EC' mechanism [5]. Besides, it has been found that despite the electrochemical inactivity of the bromate ion at the electrode the current approaches under certain conditions to the its diffusion-limited current, i.e.…”

“…Theoretical analysis 2.1. Description of the system under study: bromate anion electroreduction As in our previous study [3,4] we deal with the steady-state situation where all characteristics do not vary in time. The electrode is considered as equally accessible which means the identical convective conditions for every point of its surface.…”

“…It is this autocatalytic property of the reaction scheme composed of Eqs (1) and (2) that results in the above quoted surprising results. The theoretical analysis in Refs [3,4] was performed the case where the process of bromate reduction was carried at a constant pH value, including the kinetic layer near the surface, despite the consumption of protons by reaction (2). This condition may be ensured by addition of an excessive amount of the strong acid so that the proton concentration in the bulk solution exceeds significantly the bromate concentration.…”