Redox couples with unequal diffusion coefficients: effect on redox cycling.

Redox cycling between two electrodes separated by a narrow gap allows dramatic amplification of the faradaic current. Unlike conventional electrochemistry at a single electrode, however, the mass-transport-limited current is controlled by the diffusion coefficient of both the reduced and oxidized forms of the redox-active species being detected and, counterintuitively, by the redox state of molecules in the bulk solution outside the gap itself. Using a combination of finite-element simulations, analytical theory, and experimental validation, we elucidate the interplay between these interrelated factors. In so doing, we generalize previous results obtained in the context of scanning electrochemical microscopy and obtain simple analytical results that are generally applicable to experimental situations where efficient redox cycling takes place.