Modelling of dc and ac responses of a planar mixed conducting oxygen electrode

A one-dimensional model is developed to describe the oxygen reduction reaction on a dense Mixed Ionic Electronic Conductor (MIEC) electrode. It takes into account surface oxygen adsorption, oxygen incorporation inside the electrode, bulk diffusion of oxygen ions through the MIEC and subsequent exchange of oxygen ions with the purely ionic conducting electrolyte. The electrochemical step (ion transfer) is assumed to be fast compared to surface exchange and transport processes. The simulated polarisation curves exhibit a Tafel-like behaviour which corresponds to a limitation by bulk diffusion of oxygen ions. Under high cathodic overpotentials, a limiting current is predicted and can be related to a surface step. The faradaic impedance, corresponding to the proposed reaction mechanism, is calculated and simulated as a function of the electrode polarisation. The influence of kinetic and transport parameters as well as that of the oxygen partial pressure on the shape of both polarisation curves and impedance diagrams is discussed.