Abstract
A novel, powerful method based on a microkinetic approach is described for the estimation of the oxygen transport parameters of mixed electronic conducting materials (MIECs). This method is validated on the perovskite La0.6Sr0.4Co0.2Fe0.8O3-δ and has been applied on Ba0.5Sr0.5Co0.8Fe0.2O3-δ. This approach is original and relevant in that the surface kinetic rate constants are measured using a sample in powder form. In contrast to methods previously used, such as isotope exchange depth profiling (IEDP) and electrical conductivity relaxation (ECR), which determine the global exchange kinetic parameter, our microkinetic modelling approach allows the estimation of the forward and reverse kinetic rates accounting for the oxygen vacancy concentration. Also, the self-diffusion rate coefficient has been estimated at different oxygen partial pressures. This microkinetic approach, which combines SSITKA (steady-state isotopic transient kinetic analysis) and thermogravimetric measurements at controlled oxygen partial pressure, has the potential to significantly accelerate the characterization of oxygen transport in perovskites and related materials in the future. In this study, the kinetic parameters were measured in a temperature window between 873 K and 1173 K, and at two oxygen pressure conditions (21 kPa and 1 kPa) that are appropriate for simulating the semi-permeability of oxygen in a membrane in a process of oxygen separation from air.
Highlights
Mixed ionic and electronic conductors (MIECs) are an important class of materials that include oxides of different structures such as perovskites, brownmillerite and fluorites
The evolution of the oxygen non-stoichiometry (d) and the oxygen vacancies of La0.6Sr0.4Co0.2Fe0.8O3Àd and Ba0.5Sr0.5Co0.8Fe0.2O3Àd with the temperature at fixed oxygen partial pressure can be followed by thermogravimetric analysis (TGA)
We have described an original method based on a microkinetic approach for the estimation of the oxygen transport parameters from powder
Summary
Mixed ionic and electronic conductors (MIECs) are an important class of materials that include oxides of different structures such as perovskites, brownmillerite and fluorites. They can potentially be employed in high-temperature applications for which simultaneous reaction and separation is advantageous.[1] Research using MIECs has been conducted on partial oxidation of methane to syngas,[2,3] oxidative coupling of methane,[4] and oxidative dehydrogenation of ethane to ethylene.[5] These materials are proposed for oxygen separation applications and cathode materials for the latest SOFC technologies.[6,7,8,9,10]. The first step is the dissociative adsorption of O2 on the surface of the perovskite, leading to the formation of anionic oxygen; the second step is oxygen diffusion by an activated hopping mechanism with oxygen vacancies VO;s ; and the final step is the desorption/
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