Bromate electroreduction from sulfuric acid solution at rotating disk electrode: Experimental study
Résumé
Our recent theoretical analysis of the bromate reduction from acidic media at rotating disk electrode (RDE) under steady-state conditions gave astonishing predictions for the current at the plateau of the voltammogram (called “the maximal current”, jmax) which deviated cardinally from those for the previously known mechanisms of electrochemical processes. Because of the non-electroactivity of the bromate anion itself, its transformation (without an added catalyst) can only take place owing to the redox cycle composed of the rapid reduction of bromine species (which are always present in low amounts inside strongly acidic solutions of bromates) into bromide ions at the electrode and of the comproportionation reaction between bromate and bromide ions inside the solution phase which regenerates bromine molecules. According to this theory, owing to the autocatalytic features of this mechanism the dependence of the maximal current density, jmax, on the RDE revolution frequency, f, is complicated. In particular, it includes a range of relatively low frequencies where the maximal current can exceed the bromate diffusion-limited one, even for tracer amounts of bromine in the bulk solution. Another surprising conclusion of the theory is the existence of an intermediate range of frequencies (“anomalous region”) where the maximal current increases if the rotation frequency diminishes, i.e. for the weaker agitation intensity. This study presents the first experimental verification of these predictions for a series of bromate solutions of various concentrations in 2 M sulfuric acid. Qualitative analysis of these experimental data has confirmed the principal theoretical expectations, first of all the existence of the anomalous region of frequencies. At the same time it has been found that the previously published theoretical model based on the literature data for the parameters of the system (diffusion coefficients of solution components, rate constant of the comproportionation reaction, etc.) corresponding to dilute solutions is not able to provide a quantitative interpretation of experimental data. An advanced variant of the theory has been proposed that takes into account the dependence of these parameters on the solution composition, first of all on the effect of the acid concentration. Comparison of predictions of this model with experimental data has shown their quantitative agreement, i.e. the simulated plots for the dependence, jmax(f), turned out to be close to experimental data within the whole range of frequencies available experimentally. This result represents an unambiguous proof of validity both of the principles of the underlying theory of this process and of the predictions derived within its framework.