Mediator reduction of bromate anion at rotating disk electrode under steady-state conditions for high current densities

Theoretical study of the bromate anion reduction under steady-state conditions is performed for rotating disk electrode. Transport of the components in solution is described within the framework of the Nernst stagnant layer model. Numerical calculations carried out recently for the same system confirmed the validity of our previous approximate analytical approaches for the weak current and thin kinetic layer regimes for small and moderate values of the principal parameters of the system: ratio of the diffusion and kinetic layer thicknesses, x (dk) = z (d)/z (k), for the whole range of possible currents. At the same time, these numerical results showed a pronounced change of the calculated concentration distributions, compared to the predictions of the thin kinetic layer model, for very large values of the x (dk) parameter. A new theoretical analysis performed in this study provides approximate analytical expressions for the concentration distributions under conditions of very strong current exceeding the bromate diffusion-limited one. These expressions demonstrate that the passing of such currents results in a cardinal change of the kinetic layer structure, compared to that for weaker currents. The comproportionation reaction takes place mainly inside a layer near the electrode surface for moderate current densities while for strong currents a BrO3 (-)-free layer is formed near the surface, so that the reaction is localized within a narrow ``reaction zone'' displaced from the electrode surface.