Effective calcium doping at the B-site of BaFeO3−δ perovskite: towards low-cost and high-performance oxygen permeation membranes
A cost-effective doping strategy was developed to enhance the oxygen permeability and structural stability of BaFeO3−δ. We demonstrated that the alkaline earth metal element Ca, which is usually considered an A-site dopant for perovskite oxides, can be successfully introduced into the B-site of BaFeO3−δ. The cubic perovskite structure of BaFe1−xCaxO3−δ was stabilized down to room temperature for the Ca-doping concentration range from 5 to 15 at%. First principles calculations not only proved the preference of Ca at the B-site with lower defect formation energies than the A-site, but also demonstrated that the migration of the oxygens located greater distances from the Ca position is characterized by lower barrier energies than those in the Ca vicinity and even lower than that for the undoped BaFeO3−δ. We found that these favourable, low energy barrier paths away from the Ca sites exert more pronounced effects on the oxygen migration at diluted dopant concentrations, and hence, the material with x = 0.05 level of substitution shows a higher oxygen permeability with a lower activation energy compared to the undoped or highly-doped BaFeO3−δ. The BaFe0.95Ca0.05O3−δ membrane is characterized by a high oxygen permeability of 1.30 mL cm−2 min−1 at 950 °C and good long-term stability at 800/900 °C, as obtained over 200 h. Therefore, the feasibility and applicability of Ca-doping at the B-site of the perovskite can be highlighted, which allows for the enhancement of the oxygen migration ability, originating from the appropriate tuning of the lattice structure.