Oxygen permeation and stability of CaTi0.73Fe0.18Mg0.09O3−δ oxygen-transport membrane

Abstract

Oxygen permeation membranes consisting of gastight mixed ionic-electronic conductors (MIEC) allow the diffusion of oxygen through the crystal lattice via oxygen vacancies. An extended screening of doped CaTiO3 materials is reported here with the aim of improving the ionic and the electronic conductivity of the material. The dopants were selected taking into account the ionic radii affinity and their aliovalence or multivalence. Moreover, this work presents the oxygen permeability of CaTi1−xFexO3−δ co-doped with Mg. The structural and electrochemical properties of the nominal perovskite CaTi0.73Fe0.18Mg0.09O3−δ (CTFM) are studied. X-ray diffraction demonstrates that the material crystallizes as a single perovskite structure with orthorhombic distorted symmetry, although the MgO segregates as a minor separated phase. The partial addition of Mg2+ to the perovskite results in an increase of oxygen vacancies, and therefore the total conductivity at high temperatures with respect to the non-doped CaTi0.8Fe0.2O3 (CTF). On the contrary, below 800°C the conductivity of CTFM is lower than for CTF, attributed to the lower amount of Fe in the CTFM. Permeation measurements were performed on CTFM gastight membranes of 900 μm thickness, reaching oxygen fluxes of up to 0.95mLmin−1cm−2 at 1000°C. Stability of the membrane to CO2 was evaluated by exposing the membrane to 15% CO2 in Ar atmosphere for 24h at 750°C and O2 fluxes were measured by using this atmosphere as sweep gas. Initial O2 fluxes were maintained after the CO2 treatment. In addition, thermogravimetric (TG) measurements performed on powder CTFM under dry CO2 (5% in Ar) did not show any carbonate formation, confirming the stability of the material in this atmosphere.