(31f) Oxygen Separation Using Mixed Ionic-Electronic Conducting Perovskite Membranes: Present and Prospects

Among novel technologies under development as cost-effective
alternatives to conventional oxygen production methods, mixed ionic?electronic
conducting ceramic membranes offer great promise. These oxygen-transport membranes
selectively separate oxygen from an air supply, or other source, at elevated
temperature (700-1000°C) under an oxygen chemical potential gradient. Besides direct
use in schemes for oxygen production, the membranes have attracted much
interest for the conversion of natural gas into syngas (CO + H₂).

Membranes fabricated from oxygen-deficient perovskites La_1-xSr_xCo_1-yFe_yO_3-d can be
considered as prototypes. Very high oxygen transport rates have been achieved,
which relates to the high concentrations of mobile oxygen vacancies in the
perovskite oxides at elevated temperatures. The presence of multivalent cations
ensures a high, often predominating, electronic conductivity. Crucial to the
performance as oxygen transport membrane is that the materials are capable of
rapid oxygen exchange and diffusion of oxygen, and maintain structural and
chemical integrity under the conditions of application.

SrCo_0.8Fe_0.2O_3-d (SCF) is
reported to provide one of the largest membrane oxygen fluxes in the series La_1-xSr_xCo_1-yFe_yO_3-d. However,
undesirable ordering of SCF into a brownmillerite type structure, Sr₂Co_1.6Fe_0.4O₅,
has been reported to occur below 1073 K at a pO₂ lower
than 0.1 atm. This ordered state reduces the oxygen flux, whereas the
associated lattice expansion leads to large mechanical stresses across the
membranes. Literature reports have demonstrated an increase in oxygen flux and
an apparent increase in the stability of the cubic perovskite phase upon 50%
substitution of Ba for Sr, Ba₀₅Sr_0.5Co_0.8Fe_0.2O_3-d (BSCF).

The present
paper discusses the oxygen transport properties, oxygen stoichiometry, and phase
stability of SCF and BSCF, and related compositions. Particular attention is
drawn to the influence of CO₂ adsorption on the kinetics of
surface oxygen exchange, chemical expansion and the phenomenon of kinetic
demixing. Prospects are discussed to use these materials as oxygen transport
membranes.