EFFECT OF THE ELECTRICAL DOUBLE-LAYER ON VOLTAMMETRY AT MICROELECTRODES

An analysis of transport of charged and uncharged species associated with a steady-state faradaic process at a spherical microelectrode is reported. We examine systems comprising various relative concentrations of a redox species and, if charged, its counterion and an inert electrolyte. Of particular interest is the behavior of these systems when the thickness of the diffuse double layer (characterized by the Debye length, κ-1) and the radius of the electrode (r0) are comparable. Transport of each species is assumed to be governed by the Nernst-Planck equation. A generalized solution obtained by using finite-difference simulations demonstrates that significant enhancement or inhibition of the steady-state flux can occur and will depend upon the dimensionless parameter r0κ, upon the relative values of the applied potential (Eapp), the formal redox potential (E°′), and the potential of zero charge (Epzc), upon the charges and relative concentrations of the species in solution, and upon the distance of closest approach of the reactant to the electrode surface. Analytic solutions for several limiting cases are discussed and serve as simple expositions of the phenomena as well as a verification of the simulations. In infinitely dilute ionic solutions, i.e., r0κ → 0, the limiting flux of ionic species may be computed directly from the Smoluchowski-Debye theory for ionic bimolecular reaction rates. Computation of theoretical voltammograms in the limit of infinite dilution (i.e., r0κ → 0) reveals the surprising result that under certain conditions the steady-state current-voltage curve will be peaked rather than sigmoidal, giving the appearance that electrochemical activity occurs only within a small (several hundred millivolt) potential window. The effect is easily explained when the electric field and the charge of the reacting species are considered. © 1990 American Chemical Society.