The Effect of Schmidt Number on the Faradaic Impedance of the Dissolution of a Copper Rotating Disk

Abstract

Frequency‐response results, as calculated by the Stefan‐Maxwell macroscopic impedance model, are presented for a copper rotating disk in chloride solutions. The working algorithm uses concentrated‐solution theory and accounts for multicomponent diffusion, migration, and homogeneous and hetereogeneous reactions, as well as a finite Schmidt number and interfacial velocity. The validity of the general program was checked by comparing the concentrated‐solution model in the limit of dilute solutions, excess of supporting electrolyte, and infinite Schmidt number to known analytic solutions. Excellent agreement was obtained. Results for copper dissolution are plotted in various forms, enabling the effect of the Schmidt number (Sc) on the frequency‐response of the faradaic impedance to be studied. Specifically, plotting the low‐frequency data according to vs. pIm and the high‐frequency data as vs. allows the entire frequency spectrum to be utilized in the determination of unknown diffusion coefficients for a given electrochemical system. Additionally, the effect of electrode kinetics on the faradaic impedance as applied to the Sc‐determination procedure has been studied. Finally, new ways of plotting the dimensionless convective‐diffusion impedance reduce the Schmidt number dependence of the frequency response nearly to one curve by stretching the abscissa using and . These plots confirm that the slope of the low‐frequency linear region is proportional to and that the slope of the high‐frequency linear region is proportional to .