Generalized Nernst Layer Model: Application to Bromate Anion Electroreduction and Theory for the Stationary 1D Regime of Proton Transport Limitations

Theoretical analysis has been developed for the reduction process of the non-electroactive bromate anion through an autocatalytic cycle composed of the reversible Br-2-to-Br- transformation and the irreversible (owing to high acidity of the solution) comproportionation reaction. Owing to the autocatalytic character of this redox-mediating cycle (EC mechanism) even a tracer amount of Br-2 in the bulk solution may result in enormous values of the reduction current, which are determined by the diffusion-limited flux of either bromate anions or protons. The latter case has been analyzed in this study within the framework of the generalized Nernst layer model, which takes into account the non-equality of the diffusion layer thicknesses for H+, Br-, and Br-2 species because of the difference in their diffusion coefficients, as evident from the Levich formula for the rotating disk electrode (RDE). Approximate analytical expressions for all characteristics of the system have been derived for two regimes corresponding either to weak currents or to a thin kinetic layer. In particular, the maximal current for the diffusion-limited conditions turned out to depend on the ratio of the diffusion layer thicknesses for H+ and Br-2, that is, of their diffusion coefficients.