Novel cobalt-free BaFe1-: XGdxO3- δ perovskite membranes for oxygen separation

Abstract

A cobalt-free perovskite-type mixed ionic and electronic conductor (MIEC) is of technological and economic importance in many energy-related applications. In this work, a new group of Fe-based perovskite MIECs with BaFe1-xGdxO3-δ (0.025 ≤ x ≤ 0.20) compositions was developed for application in oxygen permeation membranes. Slight Gd doping (x = 0.025) can stabilize the cubic structure of the BaFe1-xGdxO3-δ perovskite. The Gd substitution of BaFe1-xGdxO3-δ materials increases the structural and chemical stability in the atmosphere containing CO2 and H2O, and decreases the thermal expansion coefficient. The BaFe0.975Gd0.025O3-δ membrane exhibits fast oxygen surface exchange kinetics and a high bulk diffusion coefficient, and achieves a high oxygen permeation flux of 1.37 mL cm-2 min-1 for a 1 mm thick membrane at 950 °C under an air/He oxygen gradient, and can maintain stability at 900 °C for 100 h. Compared to the pristine BaFeO3-δ and the well-studied Ba0.95La0.05FeO3-δ membranes, a lower oxygen permeation activation energy and higher oxygen permeability are obtained for the 2.5 at% Gd-doped material, which might be attributed to the expanded lattice by doping large Gd3+ cations and a limited negative effect from the strong Gd-O bond. A combination study of first principles calculation and experimental measurements was further conducted to advance the understanding of Gd effects on the oxygen migration behavior in BaFe1-xGdxO3-δ. These findings are expected to provide guidelines for material design of high performance MIECs.