Numerical Analysis of CE Processes in RDE Systems: Diffusion and Electro-hydrodynamic Impedances

Abstract

This paper presents a numerical analysis of the effect of different parameters (rotation speed, equilibrium constant and Schmidt numbers) on the diffusion (ZD) and electro-hydrodynamic (ZEHD) impedances of chemical-electrochemical (CE) systems in a rotating disk electrode (RDE) configuration. For this purpose, we used a finite difference algorithm to discretize and solve the governing equations. Our results show that the separation between convection-diffusion and reaction impedance loops depends on the ratio between diffusion layer thickness ( δ N ) and reaction layer thickness ( δ R ). Also, we have demonstrated that the characteristic frequency of the reaction impedance loop is a function of δ R − 2 . As for ZEHD data, we found that, for slow kinetics, the plots do not overlap for different rotation speeds. Further, the upper limit of the negative phase is different for both, slow and fast kinetics, from the usual 180° value found for single charge transfer systems. The increment of the equilibrium constant, obtained via increasing the reaction rate constant of the electroactive species, caused the magnitude ZD to decrease and that of ZEHD to increase. Lastly, we found that changing ScA mainly affects the concentration gradient at the surface while the effect of ScB will depend on the kinetic regime.

Impedance Analysis of Electrochemical Systems

“…They first calculated the axial velocity as a function of the Schmidt number, thus allowing the convective-diffusion impedance to be calculated as a function of tabulated parameters. Even though this development was shown to be more accurate than the Nernst approximation layer in both the medium- and low-frequency ranges of the diffusion impedance, , its use remains infrequent.…”

Calculation of Cathodic Limiting Current Density in Weak Acids: Part I. Aqueous CO2 Solutions

“…To the best of our knowledge, there are no similar studies that were done using transient techniques such as EIS or EHD that would provide additional experimental data useful in this verification. The existing studies using EIS and EHD are either pure theoretical models of the CE mechanism, 22 or they are limited to determining transport properties of electrolytes in pure mass transfer controlled systems, such as diffusion coefficient or Schmidt number. 53,54 The few graphs presented below indicate the performance of the model.…”