Correspondence between Experiment and Theory of Bulk Electrocrystallisation at Solid Electrodes in Aqueous Electrolyte

A theory of bulk-metal electrocrystallisation at solid-metal surfaces in aqueous electrolytes is presented. The electrochemical processes in the vicinity of the electrode surface are dynamic interactions between charged and uncharged species. Redox processes in the classical sense constitute only part of the explanation for the build-up of a diffusion layer leaving the Helmholtz layer unchanged, however. It was disclosed by experiments that, even in the case of fully reversible processes, a significant degree of hysteresis prevailed, particularly at fast-potential sweep rates towards high overpotentials in cyclic voltammetry (dc). The lack of complete reversibility in nernstian systems is a key topic of the present model, and the description also involves a prediction of the properties observed in non-reversible systems. These considerations lead to a novel concept for reversibility that is based on the relation between thermodynamic behavior and kinetics. Some of the parameters characterizing the process of electrocrystallisation are thus reconsidered. The extension of the electrical double layer and the site of reduction and oxidation of electroactive species are modeled. As a consequence of the species position relative to the electrode, the rate of reduction and oxidation may increase thus leading to current densities that exceed the magnitude of conventional diffusion current densities observed in cyclic voltammetry. This result was accomplished by including in the description a depletion layer devoid of charged species distant from the electrode surface. The treatment shows that long-range reduction and oxidation is likely to proceed solely as a result of thermodynamic arguments. The dependence on distance was compared to earlier predictions of long-distance tunneling phenomena. Since the kinetics at the electrode is also included in the treatment of electroactivity, the position of the peak-current density versus potential-sweep rate was also estimated. Finally, the trace of the cyclic voltammogram was calculated by a simplified version of convolution methodology. The impact of the model on the composition and structure of electrode materials with respect to current yield is discussed.