Novel strategy for improving the oxygen permeability of zirconia-based dual-phase membranes

Recently, a number of studies on oxygen transport membranes employing zirconia composites as the oxygen ion conductor have been reported, given that stabilized zirconia has superior robustness due to its high mechanical strength, a low sintering temperature, and a chemical expansion coefficient that makes it suitable for commercialization. However, the drastic decrease in the oxygen permeability due to the inter-diffusion reaction between zirconium and rare-earth metals in zirconia–perovskite composites remains a conundrum for the industrial application of oxygen permeation membranes. In this study, we propose a breakthrough strategy for dramatically enhancing the oxygen flux of a zirconia-based dual-phase membrane by applying a new coating material and minimizing the perovskite content. The membrane comprises a mixture of scandia-stabilized zirconia and strontium-doped lanthanum manganate with a volume ratio of 70 : 30. The Ruddlesden–Popper oxide structure (Nd2NiO4+δ), which exhibits low reactivity with zirconium and a high surface exchange coefficient, as well as high chemical stability in the presence of CO2, was used as a new coating material to improve the exchange reaction on the membrane surface. The highest oxygen permeation flux of up to 1.65 mL cm−2 min−1 was accomplished with the zirconia-based membrane at 900 °C under an air/He atmosphere. This value is much higher than that of La0.6Sr0.4CoO3−δ (LSC), a representative perovskite coating material with high surface exchange kinetics, used as a membrane coating. Superior stability was also observed in a long-term test where pure CO2 was used as the sweep gas.