Investigation of the permeation behavior and stability of a Ba0.5Sr0.5Co0.8Fe0.2O3-δ oxygen membrane

SrCo0.8Fe0.2O3-delta (SCFO) and Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCFO) oxides were successfully prepared, using a combined citrate-EDTA complexing method. The results of O-2-TPD and XRD showed that the introduction of barium into SCFO could effectively suppress the oxidation of Co3+ and Fe3+ to higher valence states of Co4+ and Fe4+ in the lattice, and stabilize the perovskite structure under lower oxygen partial pressures. Oxygen permeation experiment showed that BSCFO membrane also had higher oxygen permeation flux than that of SCFO under air/He oxygen partial pressure gradient. At 950 degrees C, the permeation flux through 1.80 mm BSCFO membrane exposed to flowing predried air (P-O2' = 0.21 atm) and helium (P-O2 = 0.037 atm) is ca. 1.4 ml/cm(2) min and the activation energy for oxygen transportation is 40.9 kJ/mol within the temperature range of 775-950 degrees C. The permeation flux of BSCFO was less sensitive to minor amounts of CO2 and water vapor presented in the air than that of SCFO. Long-term oxygen permeation study of more than 1000 h at 850 degrees C indicated that the BSCFO membrane could operate stably as an oxygen generator at that temperature. A very slow exponential decay in the measured oxygen permeation flux occurred at temperatures lower than 825 degrees C, which was caused by a phase transition. The phase transition was found to be reversible at higher temperatures, but a long time was needed for the equilibration. (C) 2000 Elsevier Science B.V. All rights reserved.