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 BaFe_1−xGd_xO_3−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 BaFe_1−xGd_xO_3−δ perovskite. The Gd substitution of BaFe_1−xGd_xO_3−δ materials increases the structural and chemical stability in the atmosphere containing CO₂ and H₂O, and decreases the thermal expansion coefficient. The BaFe_0.975Gd_0.025O_3−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 BaFeO_3−δ and the well-studied Ba_0.95La_0.05FeO_3−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 BaFe_1−xGd_xO_3−δ. These findings are expected to provide guidelines for material design of high performance MIECs.