Dual-phase ceramic membranes with very high oxygen flux have been designed by taking into account the volume fraction of the fluorite phase, membrane thickness, and surface modification. The oxygen flux of Ce 0.9 Gd 0.1 O 2−δ −La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ (GDC−LSCF) dual-phase membranes has been systematically investigated as a function of membrane thickness and volume fraction of the fluorite phase with or without surface modification. The percolation threshold of the composites for electronic conduction has been determined to be about 20 vol % of LSCF by general effective-medium theory. The oxygen flux of uncoated fluorite phase-rich membrane (80 vol % GDC−20 vol % LSCF) with 30-μm thickness exhibits a low oxygen flux (8.0 × 10 −3 mL• cm −2 •min −1 at 850 °C) under an air/He gradient, indicating that the permeation is controlled by only the surface-exchange kinetics of GDC. With both sides coated with La 0.6 Sr 0.4 CoO 3−δ (LSC), the flux of the membrane (3.6 mL•cm −2 •min −1 at 850 °C) has been dramatically enhanced by about 3 orders of magnitude in comparison with the oxygen flux of the membrane with a nonmodified surface. This observation implies that surface modification has a decisive role in dramatically enhancing the contribution of the GDC to the fluorite rich dual-phase membrane.

The oxygen permeation flux of dual-phase membranes, Ce0.9Gd0.1O2-δ-La0.7Sr0.3MnO3±δ (GDC/LSM), has been systematically studied as a function of their LSM content, thickness, and coating material. The electronic percolation threshold of this GDC/LSM membrane occurs at about 20 vol % LSM. The coated LSM20 (80 vol % GDC, 20 vol % LSM) dual-phase membrane exhibits a maximum oxygen flux of 2.2 mL·cm(-2)·min(-1) at 850 °C, indicating that to enhance the oxygen permeation flux, the LSM content should be adjusted to the minimum value at which electronic percolation is maintained. The oxygen ion conductivity of the dual-phase membrane is reliably calculated from oxygen flux data by considering the effects of surface oxygen exchange. Thermal cycling tests confirm the mechanical stability of the membrane. Furthermore, a dual-phase membrane prepared here with a cobalt-free coating remains chemically stable in a CO2 atmosphere at a lower temperature (800 °C) than has previously been achieved.

SrCo0.9Nb0.1O3−δ (SCN) has been investigated for oxygen transport membrane and solid oxide fuel cell applications as it is more stable than the state-of-the-art Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) material. Here, the crystal structure and transport properties of SCN were systematically investigated. Combined neutron and high-temperature X-ray powder diffraction analyses over the temperature range of 25–800 °C showed that the crystal structure of SCN belongs to the tetragonal space group P4/mmm with the unit cell a c × a c × 2a c, where a c is the lattice parameter of the cubic perovskite at temperatures <160 °C. The tetragonal (P4/mmm) to cubic (Pm3̅m) phase transition occurred in SCN with increasing temperature. This phase transition affected the electrical conductivity, whereas the impact on the magnetic susceptibility was not significant. The surface exchange and diffusion coefficients of SCN in the temperature range of 600–850 °C were characterized by electrical conductivity relaxation and oxygen permeation measurements. The self-diffusion and surface exchange coefficient of SCN are comparable with those of BSCF. Hence, SCN shows promise as a next-generation mixed ionic–electronic conducting ceramic.

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