Coupling of lateral electric field and transversal faradaic processes at the conductor/electrolyte solution interface

A quantitative theory is presented for the bipolar behavior of conducting planar surfaces in a thin-layer cell of a type commonly used in electrokinetic studies. The lateral current density distribution in the cell, as dictated by the externally applied field in the solution, is formulated for the situation in which depolarization of the interface arises from transversal electron-transfer processes that occur at the two sides of the conducting surface. The treatment explicitly analyses the two limiting cases of bipolar electrodic behavior, i.e., totally irreversible electron transfer and Nernstian (mass-transfer-limited) electrodics. The spatial distribution of the electric field is calculated by means of Poisson's equation under conditions of a finite current. The results allow for a rigorous estimation of the overall bipolar faradaic current. Analytical expressions are given for the electric parameters (potential, field, local current, and bipolar faradaic current) in the case of irreversible electron transfer, and numerical analysis is performed for the reversible, Nernstian case. Deviations of the conductivity curves from the trend expected on the basis of a linear potential profile are discussed in terms of the local ohmic and faradaic contributions to the total current. The theory is supported and illustrated by experimental data for gold and aluminum surfaces in KNO3 solution, in the absence and presence of the electroactive species Fe(CN)(6)(3-)/Fe(CN)(6)(4-).