Separation of Double-Layer Charging and Faradaic Processes at Electrodes

Abstract

In studying transient electrode processes in aqueous solutions, the current density is often expressed in terms of separate capacitive charging and faradaic contributions, in = C(∂η/∂t) + if without a critical evaluation of the underlying assumptions. This paper investigates rigorously the basic concepts and assumptions involved in justifying the use of this equation, starting from the basic principles of electrochemical thermodynamics, kinetics, and mass transfer. The equation derives from a species mass balance for the electrochemical interface, which justifies already an a priori separation of the double layer and faradaic charges. An important assumption required in further analysis to obtain this equation is to neglect time dependent variations in the concentrations of species, which take part in the electrode reaction, adjacent to the electrode surface in the solution. It is further necessary to neglect variations in the conductivity with position near the electrode and assume small concentrations of the species taking part in the electrode reaction.