Mass Transport in (La0.8Sr0.2)0.95CrxFe1–xO3−δ–Scandia-stabilised zirconia dualphase composite as a dense layer in oxygen transport membranes

Abstract

Electrical and oxygen-ion transport in the dual-phase composite systems (La0.8Sr0.2)0.95CrxFe1–xO3−δ (LSCrF) (x = 0.3, 0.5, 0.7)–10 mol % Sc2O3–1 mol % CeO2–89 mol % ZrO2 (10Sc1CeSZ) have been investigated. In these three (x = 0.3, 0.5, 0.7) dual-phase systems, the pure ionic conductor 10Sc1CeSZ dominates the oxygen bulk diffusion whereas the mixed electronic and ionic conductor LSCrF is the predominant phase for oxygen surface exchange and provides pathways for a counter flow of electrons to maintain electrical neutrality. Hence, the electrical conductivity of the dual-phase composite materials increases whereas the diffusion coefficient decreases with increase of the LSCrF content, as expected. However, the surface exchange coefficients as a function of the LSCrF composition show significant scatter. For both phases, once the volume fraction is lower than 30%, the continuous network starts to disconnect and percolation thresholds were observed for both electrical conductivity and oxygen diffusion coefficients in the composites. For the composites with three-dimensional networks of both phases, no obvious difference was observed for the electrical conductivity and oxygen tracer diffusion behavior and it was also confirmed that the microstructures may have a minor effect on the oxygen diffusion behavior of the dual-phase materials. Furthermore, the microscale studies of oxygen diffusion in each phase of the dual-phase composite reveal a synergistic effect between these two phases: the surface exchange coefficient, k, of LSCrF decreases while that for the 10Sc1CeSZ phase k increases when compared with their corresponding isolated single-phase materials.