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 () and reaction layer thickness (). Also, we have demonstrated that the characteristic frequency of the reaction impedance loop is a function of 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.