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

Abstract

Process of the bromate anion reduction in the presence of a very low bromine concentration from an acidic solution for the one-dimensional transport under steady-state conditions has been analyzed theoretically. Bromate being non-electroactive in the potential range under consideration, its transformation takes place via the redox cycle composed by the reversible couple, Br2/Br−, and the irreversible comproportionation reaction inside the solution phase between BrO3−, Br− and protons which regenerates Br2. Specific feature of this process compared to the conventional EC' mechanism is the progressive increase in the overall amount of the components of the redox couple owing to consumed bromate. As a result, this novel EC” mechanism possesses autocatalytic properties so that even a trace amount of bromine in the bulk solution may result in very high current densities, up to the those limited by the diffusion of the principal solution components, bromate and protons. Unlike our previous study of this system carried out for excess of protons compared to bromate [Electrochim. Acta, 173 (2015) 779-795] we consider the opposite situation where bromate is in great excess compared to protons so that the transport of the latter component from the bulk solution limits the maximal current. Approximate analytical formulas have been derived for all characteristics of the system based on the condition of either a relatively weak current or a thin kinetic layer compared to the diffusion one. 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, bromide anions leave the diffusion layer and react with bromate anions only in the bulk solution. Then, both the polarization curve and the maximal current correspond to the electrode reaction of Br2 molecules from the bulk solution, without a significant effect due to BrO3− presence. In the intermediate range of the diffusion layer thicknesses, zk<zd<6 zk, bromide anions generated at the electrode are consumed by the comproportionation reaction within a thin kinetic layer (located deeply inside the diffusion layer) while bromine molecules produced by this reaction diffuse partially to the electrode, generating again bromide anions. This combination of the chemical and electrochemical steps results in an autocatalytic cycle, based on the Br−/Br2 mediating redox couple, which consumes a significant amount of bromate anions. The maximal current becomes much higher, exceeding the one limited by the bromine diffusion from the bulk solution, and it depends essentially on the kinetic layer thickness. In the third range of even larger diffusion layer thicknesses, 6 zk<zd, the amounts of the accumulated redox-couple components become so high that the current is limited by the diffusion of protons from the bulk solution into the kinetic layer. This diffusion limited current is proportional to the bulk-solution proton concentration and it may reach very high values. The theory predicts a complicated behavior of the maximal current as a function of the diffusion layer thickness (or of 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”).