The understanding of oxygen transport mechanisms through mixed ionic-electronic conductors is of great interest for the development of electrochemical devices at high temperature, and for energy conversion applications. Recent developments in solid oxide fuel cells (SOFCs) operating at low temperatures (500-600°C), solid oxide electrolyzer cells (SOECs), and oxygen transport membranes (OTMs) are largely based on the use of mixed ionic-electronic conductors (MIECs). Modeling the electrical charge transport process through an MIEC is an important challenge for improving oxygen transport membranes (OTM), solid oxide fuel cells (SOFC), or electrolyzers (SOEC).
Since the pioneering work of Wagner, numerous studies have been devoted to model oxygen permeation through a mixed ionic-electronic conductor (MIEC). It is now admitted that the oxygen permeation flux is simultaneously controlled by the bulk diffusion and the surface exchange for most OTM materials. However, it is difficult to obtain an explicit relationship between the oxygen flux, the rate constants of the main transport steps involved in the permeation process, and the partial pressures at the surfaces on both sides of the membrane. Then, it is generally poorly adressed to determine the kinetic parameters in MIEC, close and particularly far to equilibrium.  
This work is focused also on evaluating the oxygen exchange kinetics at the surface of MIEC close and far to equilibrium, specifically the identification the relationship between oxygen flux and the driving force, for which there is none or few data in the literature. This approach is a great importance for the determination of two coefficients — the oxygen diffusion coefficient and the surface exchange coefficient, which are obviously key parameters for the electrochemical performance of a material. The determination of these coefficients can be largely impacted by the experimental measurement conditions. This work shows that it is possible to determine the two coefficients for mixed ionic-electronic conductors, with suitable accuracy, by the oxygen semi-permeation method. However, this method requires a significant oxygen flux though the membrane sample under a low oxygen potential gradient, to obtain a suitable accuracy of the studied coefficients. This involves determining these transport coefficients at higher temperatures (> 800°C) in comparison to those required for the isotopic method (usually < 800°C).
The main advantage of the oxygen semi-permeation method is the ability to quickly determine the oxygen diffusion and oxygen surface coefficients involving a single measurement close to the working conditions, i.e. close or far from the equilibrium. This also allows the identification of the relationship between the oxygen flux and the driving force, for which there are very few data in the literature. The study of oxygen properties obtained by oxygen semi-permeation method on the mixed ionic-electronic conductors, such La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ, La_0.5Sr_0.5FeO_3-δ and Pr₆O₁₁ materials, will been discussed and compared with data obtained by isotopic exchange depth profile.
Finally, this method offers new data, and it gives new insights for a better understanding of the mechanisms that are either near and far from the equilibrium at the surface of mixed ionic or electronic conductors.