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The Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) mixed ionic electronic conductor (MIEC) is considered to be one of the most promising perovskite-based materials for oxygen transport exhibiting high oxygen permeability. Such oxygen-permeable perovskite membrane maybe applied to increase efficiency of combustion or reforming processes of new fuels such as green ammonia. An approach to elevate the oxygen permeability of BSCF membranes is reduction of membrane thickness, as the permeation process is mostly controlled by bulk diffusion. Since self-standing thin membranes are not sufficiently mechanically stable, asymmetric membranes consisting of a thin layer membrane deposited onto a porous bulk support are usually employed. . In this work, planar BSCF thin film membranes of 1 to 2 and 15 to 20 µm thickness were successfully synthesized by a combination of magnetron sputtering (MS) and thermal annealing (TA) and/or selective laser annealing (SLA) processes. Thin films appeared to be pinhole-free with a high crystallinity of a single perovskite phase and a microstructure that showed suitability for oxygen permeation applications. Correlations between thin film properties and MS process parameters (e.g. power, Ar/O2 ratio, pressure, etc.) and TA&/SLA parameters were established and optimised as key factors for producing thin film membrane exhibiting high oxygen flux and good stability. A variation of the oxygen flux through the BSCF asymmetric membrane as well as the porous substrate is presented as a function of temperature and also oxygen partial pressures applied to both sides of the membrane allowing for calculation of permeability of the deposited thin film. Figure 1

The influence of elevated oxygen pressure (from 1 to 50 bar) on phase separation in Ba0.5Sr0.5Co0.8Fe0.2O3-delta(BSCF) cubic perovskite was investigated in the range of 300to 1300 K. X-ray diffraction and thermogravimetric analysis revealed two separation processes occurring in the high temperature (HT), 1043 K, and low temperature (LT), 713 K, ranges. Increasing oxygen pressure shifts slightly the LT phase separation to lower temperature, but has a rather minor effect on the volume fraction growth of the secondary phase. Phase transformation in the HT range is a slow process and the degree of decomposition is strongly influenced by the cooling rate from higher temperatures. On the contrary, the LT separation is relatively fast and is essentially independent of the cooling rate. The BSCF ceramics thermally treated at p(O-2)=50 bar was analyzed by different electron microscopy techniques. The structures of separation products were identified as trigonal Ba3Co10O17-like lamellae and 2H-hex-agonal Ba-0.5 (+/-) Sr-x(0.5) (+/-) xCoO3-delta perovskite, respectively. (C) 2016 Elsevier B.V. All rights reserved.

The membrane microstructure and phase composition of Ruddlesden–Popper-type La 2 NiO 4+ δ ceramics, which were prepared by field-assisted sintering technique/spark plasma sintering (FAST/SPS) process or by conventional pressing and pressureless sintering were investigated. As starting material, a La 2 NiO 4+ δ powder, with an average particle size of 0.2 μm was used. The grain-size distribution of the resulting membranes varied from 0.015 μm 2 for FAST/SPS sintered ceramic to 6.11 μm 2 for pressureless sintered membrane. The microstructure analysis of membranes was performed by transmission and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. Phase transition from orthorhombic to tetragonal crystallographic structure in FAST/SPS material was investigated by temperature-dependent X-ray powder diffraction. The effect of sintering technique and grain size on the oxygen permeation performance of the membranes is discussed with respect to impurities in the material.

The performance of perovskite-type oxygen-transporting membranes during CO 2 sweeping has been investigated. The series of La 1− x Sr x Co0 .8 Fe 0.2 O 3− δ (LSCF) materials with x =1, 0.8, 0.6 and 0.4 (SCF, LSCF2882, LSCF4682, LSCF6482) were synthesized by the sol–gel method and characterized by X-ray diffraction (XRD). To investigate the poisoning effect of CO 2 on the phase composition and microstructure of the membranes after long-term oxygen permeation experiments (more than 200 h) at 1173 K under a CO 2 atmosphere, transmission electron microscopy (TEM) investigations were carried out. Electron-energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDXS) revealed the formation of carbonate at the outer surface of the LSCF materials with low lanthanum content, which correlates with a decrease in oxygen permeation flux. Nevertheless, the LSCF 6482 material is tolerant to CO 2 gas. No carbonate layer was found after 200 h of operation, and the oxygen flux remained stable.

The effect of grain size on oxygen permeation properties of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and SrCo0.8Fe0.2O3−δ (SCF) membranes was investigated by variation of the dwell time. The membrane microstructure was examined by field-emission scanning microscopy (FE-SEM) and then evaluated using a statistical approach. With longer dwell times the grain growth was stimulated and leaded to grains with a narrower size distribution. The grains of SCF (average size from 11.3 to 19.9 μm) were found to be smaller than those of BSCF (average size from 13.9 to 41.3 μm). The oxygen permeation flux of BSCF membranes was found to be independent of grain size in the range from 24 to 42 μm. However, membranes with smaller grains (13.9 μm) show a decreased oxygen permeation flux. For the SCF membranes a decrease in permeation flux with larger grains was observed for average grain sizes between 11.3 and 19.9 μm. By transmission electron microscopy (TEM) formation of an oxygen ordered SrCo0.8Fe0.2O2.5 brownmillerite by-phase could be observed at the oxygen-depleted sweep side of the membrane.

The SrCo0.8Fe0.2O3−δ (SCF) perovskite was systematically doped with increasing lanthanum content up to 60 wt% on the A-site to investigate the effect on CO2 tolerance. Different powders were prepared by a sol-gel method and the materials were characterized by in-situ X-ray diffraction (XRD) and long-term oxygen permeation measurements in CO2-containing atmospheres. The microstructure was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All powders exhibit cubic perovskite structure except the rhombohedral La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF 6482), which however, shows a phase transition into cubic perovskite structure at higher temperature. By doping 20 wt% lanthanum, the tolerance against CO2 is considerably increased and doping with 60 wt% lanthanum resulted in a stable oxygen permeation performance in CO2 atmosphere for at least 200 h. Oxygen permeation experiments in an air/helium gradient showed that with increasing lanthanum content the oxygen permeation flux decreases. Microstructure analysis of the membranes after CO2 operation showed that the carbonate preferentially forms a dense layer at the carbon dioxide exposed sweep side of the membranes.

The series of (Ba₀.₅Sr₀.₅)(Co₀.₈Fe₀.₂){sub 1–z}Zr{sub z}O{sub 3–δ} (z=0, 0.01, 0.03, 0.05, 0.07, and 0.09) was synthesized by a sol–gel method. The materials with a zirconium content up to 3 mol% were found to be single phase. Further increase results in formation of a mixed (Ba,Sr)ZrO₃ by-phase, which was found along the grain boundaries and in the grains. With increasing zirconium content the oxygen permeation flux decreases considerably. The effect of the zirconium substitution on the long-term phase stability was investigated by long-term oxygen permeation experiments and X-ray diffraction. A slight stabilization of the oxygen flux of (Ba₀.₅Sr{sub 0.5})(Co₀.₈Fe₀.₂)₀.₉₇Zr₀.₀₃O{sub 3–δ} was found after 180 h at 1023 K. However, all compositions show a decrease in permeation flux with time, but the pure BSCF membrane exhibited the strongest drop after 180 h of operation. The decomposition products of the cubic perovskite phase were found to be a hexagonal Ba{sub 0.5±x}Sr{sub 0.5±x}CoO₃ and a rhombohedral Ba{sub 1–x}Sr{sub x}Co{sub 2–y}Fe{sub y}O{sub 5–δ}. - Graphical abstract: Backscattered-electron channeling contrast image of BSCF membrane cross-section after long-term oxygen permeation at 1023 K showing different phases in different colors. Highlights: • Ba₀.₅Sr₀.₅Co₀.₈Fe₀.₂O{sub 3–δ} systematically doped with increasing amount of zirconium. • Cubic single-phase materials up to 3more » wt% zirconium. • Mixed (Ba,Sr)ZrO₃ by-phase formed mainly in the grain boundaries. • Janecke prism was proposed by XRD and EDXS data. • (Ba₀.₅Sr₀.₅)(Co₀.₈Fe₀.₂)₀.₉₇Zr₀.₀₃O{sub 3–δ} showed a slight stabilization of oxygen flux as compared to pure Ba₀.₅Sr₀.₅Co₀.₈Fe₀.₂O{sub 3–δ}.« less

U-shaped K 2 NiF 4 -type oxide hollow-fiber membranes based on (Pr 0.9 La 0.1 ) 2 (Ni 0.74 Cu 0.21 Ga 0.05 )O 4+ δ (PLNCG) were successfully prepared through a phase-inversion spinning process. The effect of CO 2 and SO 2 on the oxygen permeation, phase structure and the microstructure of the material were investigated. In situ XRD patterns of PLNCG under air, Ar and CO 2 atmosphere in the range from room temperature to 1000 °C were performed and indicate a good phase stability under oxidizing and reducing conditions as well as chemical stability against CO 2 . Effects of SO 2 concentration in the sweep gas and the feed gas on the oxygen permeation flux through the PLNCG hollow-fiber membrane, as well as on membrane's microstructure were also investigated. Pr 2 O 2 SO 4 , La 2 O 2 SO 4 and NiO formed after SO 2 treatment and the material developed some porosity on the side exposed to SO 2 . The observations indicate that the PLNCG membrane is highly stable against CO 2 but sensitive to SO 2 .

High oxygen permeability and good thermochemical stability of oxygen-transporting membranes (OTMs) are two main requirements concerning the applicability of these devices in chemical processes, such as CO2 capture using the oxyfuel concept or catalytic membrane reactors. In this work, a single-phase perovskite-type membrane Pr0.6Sr0.4Co0.5Fe0.5O3-δ (PSCF) with 0.6-mm thickness was subjected to periodic thermal cycling in the temperature range between 850 and 1000 °C in a 1000-h long-term permeation test with pure CO2 as the sweep gas. The results of this long-term permeation operation revealed a stepwise increase in oxygen permeation values at 1000 °C after each thermal cycle, reaching from 1.38 cm(3) (STP) min(-1) cm(-2) in the first cycle to 1.75 cm(3) (STP) min(-1) cm(-2) in the fourth cycle. Furthermore, the membrane showed very good CO2 stability at 900 °C and above. Despite a partial decrease in oxygen permeation fluxes at 850 °C, a steady state of 0.25 cm(3) (STP) min(-1) cm(-2) was reached and maintained for more than 100 h. The newly developed PSCF membrane also exhibited a higher oxygen permeation flux with He and CO2 sweeping at all measured temperatures compared to a similar La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) membrane.

By cobalt-doping of the mixed conducting phase PSFC, a good combination of high CO2 stability and high oxygen permeability is obtained for the 60 wt% Ce(0.9)Pr(0.1)O(2-δ)-40 wt% Pr(0.6)Sr(0.4)Fe(0.5)Co(0.5)O(3-δ) (CP-PSFC) dual phase membrane, which suggests that CP-PSFC is a promising membrane for industrial applications in the oxyfuel process for CO2 capture.