ac Impedance of Faradaic reactions involving electrosorbed intermediates—I. Kinetic theory

The evaluation of the electrochemical adsorption behaviour of chemisorbed intermediates generated in a multi-step Faradaic reaction at appreciable net currents is important for understanding the reaction mechanism of the overall process involved. Measurement of ac impedance of the reaction at controlled potentials provides an important experimental route to the required information about the “overpotential-deposited” ad-atom species. Interpretation of the measurements however, requires further examination. Based on an extension of Armstrong's treatment, it is shown that interpretation of ac impedance measurements directly in terms of the components of an intuitively assumed equivalent circuit is rarely correct; only in the case of underpotential-deposition of an ad-species, where no continuous Faradaic currents pass, in such an approach satisfactory. Kinetic analysis is given for the behaviour of a multistep process with examples from the cathodic H2 evolution reaction where electrochemical and H-recombination desorption pathways are involved. The kinetic analysis enables the steady-state adsorption pseudocapacitance, Cφ, for H to be evaluated as a function of overpotential. Its behavior is clearly distinguishable from the quantity Cp commonly written as the pseudocapacitance element in the equivalent circuit for this type of reaction.