Yttrium doping of Ba0.5Sr0.5Co0.8Fe0.2O3-δ part I: Influence on oxygen permeation, electrical properties, reductive stability, and lattice parameters

Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) exhibits a very high oxygen permeability in its cubic perovskite phase, making it a promising candidate for high-temperature energy-related applications such as oxygen-transport membranes. It suffers, however, from a pronounced phase instability at application-relevant temperatures below 840 °C which is presumed to result from a valence change of B-site cobalt. In an attempt to stabilize the cubic BSCF phase, monovalent Y3+ was doped in small concentrations (1-10 mol-% yttrium) onto its B-site. The influence of this doping on the physico-chemical properties (electrical conductivity, reductive stability, lattice constant), on the sintering behavior, and on the oxygen permeation of BSCF has been systematically investigated. Despite a slightly adverse effect to permeability (decrease in oxygen permeation by about 20-30%), a doping concentration of 10 mol-% Y is found to completely suppress secondary-phase formation and, hence, stabilize the cubic BSCF system at 800 °C. These findings are extremely promising with regard to a long-term operation of BSCF in atmospheres free of acidic impurity gases.