Oxygen equilibration kinetics of mixed-conducting perovskites BSCF, LSCF, and PSCF at 900°C determined by electrical conductivity relaxation

For an application of mixed ionic-electronic conducting (MIEC) perovskite oxides, e.g., as solid oxide fuel cell (SOFC) cathodes, as high-temperature gas sensors or as oxygen-transport membrane (OTM) materials, the kinetics of oxygen transport is of fundamental importance. A common setup for the determination of the chemical diffusion coefficient D-delta and the surface exchange coefficient k(delta) is the electrical conductivity relaxation (ECR) method where the conductivity response of an MIEC sample is measured after the ambient oxygen partial pressure pO(2) has been abruptly changed using different gas mixtures. In the present study, however, a closed tubular zirconia "oxygen pump" setup was used which facilitates precise pO(2) control in a closed sample space with a high resolution at temperatures above 700 degrees C in atmospheres ranging from pure oxygen continuously down to pO(2) = 10(-18) bar. Reasonably fast pO(2) changes enable an application of the ECR technique on MIEC oxides down to lower partial pressures not easily accessible with gas mixtures. The oxygen transport parameters of dense ceramic bulk samples of Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF), La0.58Sr0.4Co0.2Fe0.8O3-delta (LSCF), and Pr0.58Sr0.4Co0.2Fe0.8O3-delta (PSCF) have been studied as a function of temperature (800 and 900 degrees C) in the range between 10(-6) <= pO(2)/bar <= 0.21. The D-delta and k(delta) values obtained for LSCF at 800 degrees C are in good agreement with values from literature, proving the usability of the setup for ECR measurements. For BSCF, LSCF, and PSCF, D-delta and k(delta) values could be determined for the first time at 900 degrees C as a function of pO(2). (C) 2015 Elsevier B.V. All rights reserved.