Transient Response of Chemical-Electrochemical (CE) Processes in Rotating Disk Electrode (RDE) Systems: Diffusion and Electrohydrodynamic Impedances

Abstract

We present a transient treatment of chemical-electrochemical (CE) processes in rotating disk electrode systems as an extension to a steady-state analysis already initiated by our group (results have been submitted to the Journal of the Electrochemical Society). Our numerical procedure can investigate systems with different diffusion coefficients over a wide range of homogenous reaction rate constants and was used to investigate both diffusion and electrohydrodynamic impedance responses. Two sets of rate constants (termed “slow kinetics” and “fast kinetics”) were used to assess the impact of the chemical step on both impedances behavior. For diffusion impedances, slow kinetic systems displayed two impedance loops, one associated with the homogenous reaction and the other to the convective diffusion. As the value of the electroactive species rate constant grew relative to its chemical counterpart, the homogenous reaction loop started to dominate the overall impedance. For systems with fast kinetics, only the loop corresponding to the convective diffusion was visible (for frequencies up to 100 Hz) and the overall impedance decreased rapidly with an increase on the reaction rate of the electroactive species. Results for electrohydrodynamic impedance corroborated the previous findings: two time constants could be observed for slow kinetic systems, but only one in the case of fast kinetics. These findings show that impedance techniques can be of great help on establishing reaction mechanisms for complex systems, since they are sensitive to the presence of non-electrochemical activities as long as they interfere with the concentration profile of the electroactive species.