Leveraging the A-site Ba2+–Sr2+ ratio in the designated perovskite to enhance oxygen transport and structural/interfacial stability

Abstract

The stoichiometric ratio of Ba²⁺ to Sr²⁺ in the A-site of the designated perovskite oxide, La_0.2Ba_0.8−xSr_xFe_0.8Zn_0.2O_3−δ (LBSFZ), clearly influences the oxygen transport and structural integrity of the membrane at high temperatures. Ba²⁺ ion favors lattice oxygen transfer whilst the smaller Sr²⁺ ion alleviates distortions of the LBSFZ crystalline structure. In particular, the LBSFZ membranes (x = 0.2 to 0.4) exhibit higher lattice oxygen permeability than the pristine La_0.2Ba_0.8Fe_0.8Zn_0.2O_3−δ (x = 0) membrane at temperatures above 880 °C due to lattice expansion incurred at high temperatures. Amid the LBSFZ membranes, the membrane with x = 0.2 manifests the highest oxygen flux (J_O2 = 1.1 cm³ cm⁻² min⁻¹) at 950 °C driven by He purging. However, the composition stress built-up in the membrane after 50 h examination caused micro-cracks. Such structural vulnerability was effectively overcome by compositing LBSFZ with a fluorite oxide, Ce_0.2Gd_0.8O_2−δ (CGO). Compared to the single-phase perovskite membrane, the composite membrane is critically affected by the growth of an interface, which is detrimental to oxygen transport. Integrating the highest Sr-doped LBSFZ (x = 0.6) with CGO gave rise to the maximum oxygen permeation flux because of the lowest extent of interfacial diffusion. Moreover, this minimum interfacial diffusion ensured an intimate boundary between the perovskite and fluorite phases, which is crucial in realizing a mechanically sound and gastight matrix. As a consequence, a J_O2 of 6.14 cm³ cm⁻² min⁻¹ was attained through carrying out the partial oxidation of methane in the permeate side, through which the membrane displayed adequate crystalline phase stability.