Oxygen Release from Grossly Nonstoichiometric SrCo 0.8 Fe 0.2 O 3-δ Perovskite in Isostoichiometric Mode Ilya A. Starkov, Sergey F. Bychkov, Stanislav A. Chizhik, and Alexandr P. Nemudry* Institute of Solid State Chemistry and Mechanochemistry, SB RAS, 630128 Kutateladze 18, Novosibirsk, Russia * S Supporting Information ABSTRACT: The kinetics of oxygen release from grossly nonstoichiometric perovskite SrCo 0.8 Fe 0.2 O 3-δ (SCF) with mixed conductivity chosen as a model object was studied for the first time using the oxygen partial pressure relaxation technique. The use of isostoichiometric conditions during the investigation of the oxygen exchange in SCF characterized by a wide homogeneity region made it possible to get information that cannot be obtained using traditional measurements under isobaric conditions. This approach allowed us to discover a kinetic compensation ef fect: successive oxygen release from SCF was found to result simultaneously in the increase of the apparent activation energy and pre-exponential factor of the reaction rate. ■ INTRODUCTION During the past decade, much attention has been paid to the study of the oxygen exchange mechanism in materials with mixed ionic-electronic conductivity (MIEC). 1-7 MIEC materi- als are of great practical interest for development of electrochemical devices and membrane reactors. Their func- tional properties, such as response time of sensors, efficiency of oxygen sorbents and SOFC electrodes, oxygen permeability of ceramic membranes, etc. are determined by the oxygen exchange of MIEC oxides with the gas phase. In the papers by Bouwmeester et al., 1 Steele, 2 and Qiu et al. 3 considerable progress was achieved in understanding the significance of surface reactions for the oxygen transport in MIEC oxides. It was shown that if the thickness of an oxygen permeable membrane (L) is below critical L c = D 0 */k s the surface exchange kinetics limits the oxygen flux J O 2 . 1 Different reaction mechanisms that can account for the observed pO 2 and δ-dependence of the surface exchange coefficient were analyzed. 7 The observed change of the power index n in J O 2 = k(pO 2 ′ n - pO 2 ″ n ) with temperature growth was related by Huang and Goodenough 5 to the involvement both the surface oxygen exchange reactions and bulk oxygen diffusion in the overall permeation process. A new concept on the influence of the charge of adsorbed oxygen species on the surface coverage and power index n, and hence, on the oxygen exchange rate was developed by Fleig et al. 8 The effect of the oxide band structure on rate laws for oxygen exchange was analyzed, 6 and DFT calculations of di fferent reaction pathways for oxygen incorporation in MIEC oxides were carried out. 9 In this paper, we would like to attract attention to another aspect associated with MIEC oxides. Typically, MIEC oxides are grossly nonstoichiometric compounds (i.e., solid solutions with a wide range of homogeneity). So, their properties (structural, thermodynamic, and transport) significantly depend on the oxygen content. 10 For oxygen exchange, this effect is demonstrated by the dependence of the surface exchange and diffusion coefficients of nonstoichiometric perovskites on oxygen partial pressure (in other words on the oxygen nonstoichiometry δ). 7,11 Thus, variation of temperature and/ or oxygen partial pressure pO 2 during the experiment can result in a significant change of the stoichiometry. Hence, it can change the kinetic parameters of MIEC oxides during the data acquisition. Usually, this aspect is not taken into account during investigation of the oxygen exchange in grossly nonstoichio- metric oxides. In our opinion, this leads to incorrect conclusions and notions about the oxygen exchange mechanism in MIEC oxides. For example, the activation energy of the oxygen exchange of MIEC oxide with the gas phase (or oxygen permeability of a ceramic membrane) can be underestimated if it is determined from the kinetic data obtained in the isobaric regime or at constant ΔpO 2 range. This error is caused by a significant change of the stoichiometry and hence the transport properties of the material due to the temperature variation. To avoid misconceptions, we propose to conduct kinetic studies of MIEC oxides at an almost constant stoichiometry. Since the oxygen uptake or release is accompanied by stoichiometry changes in kinetic experiments, it is necessary to narrow down the Δδ = δ i - δ f range as much as possible and fix the initial δ i and final δ f stoichiometry by selecting appropriate conditions (pO 2 , T). For such measurements, we will use the term “isostoichiometric”. To test this approach, well-known nonstoichiometric perov- skite SrCo 0.8 Fe 0.2 O 3-δ (SCF) was chosen as a model object. A simple and effective method for determination of the oxygen stoichiometry of MIEC oxides as a continuous function of pO 2 Received: December 20, 2013 Revised: February 24, 2014 Published: February 28, 2014 Article pubs.acs.org/cm © 2014 American Chemical Society 2113 dx.doi.org/10.1021/cm4040775 | Chem. Mater. 2014, 26, 2113-2120