Inappropriate Use of the Quasi-Reversible Electrode Kinetic Model in Simulation-Experiment Comparisons of Voltammetric Processes That Approach the Reversible Limit

Many electrode processes that approach the "reversible" (infinitely fast) limit under voltammetric conditions have been inappropriately analyzed by comparison of experimental data and theory derived from the "quasi-reversible" model. Simulations based on "reversible" and "quasi-reversible" models have been fitted to an extensive series of a.c. voltammetric experiments undertaken at macrodisk glassy carbon (GC) electrodes for oxidation of ferrocene (Fc(0/+)) in CH3CN (0.10 M (n-Bu)(4)NPF6) and reduction of [Ru(NH3)(6)](3+) and [Fe(CN)(6)](3-) in 1 M KCl aqueous electrolyte. The confidence with which parameters such as standard formal potential (E-0), heterogeneous electron transfer rate constant at E-0 (k(0)), charge transfer coefficient (alpha), uncompensated resistance (R-u), and double layer capacitance (C-DL) can be reported using the "quasi-reversible" model has been assessed using bootstrapping and parameter sweep (contour plot) techniques. Underparameterization, such as that which occurs when modeling C-DL with a potential independent value, results in a less than optimal level of experiment-theory agreement. Overparameterization may improve the agreement but easily results in generation of physically meaningful but incorrect values of the recovered parameters, as is the case with the very fast Fc(0/+) and [Ru(NH3)(6)](3+/2+) processes. In summary, for fast electrode kinetics approaching the "reversible" limit, it is recommended that the "reversible" model be used for theory-experiment comparisons with only E-0, R-u, and C-DL being quantified and a lower limit of k(0) being reported; e.g., k(0) >= 9 cm s(-1) for the Fc(0/+) process.