Cleaning up their act: A novel CO2‐stable and cobalt‐free dual‐phase membrane for oxygen separation from air has been developed that consists of 40 wt % NiFe2O4 and 60 wt % Ce0.9Gd0.1O2−δ. This membrane shows a steady oxygen permeation flux over 100 h using CO2 as sweep gas at 1000 °C.

The cubic perovskite Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (denoted BSCF) is the state-of-the-art ceramic membrane material used for oxygen separation technologies above 1150 K. BSCF is a mixed oxygen-ion and electron conductor (MIEC) and exhibits one of the highest oxygen permeabilities reported so far for dense oxides. Additionally, it has excellent phase stability above 1150 K. In the intermediate temperature range (750-1100 K), however, BSCF suffers from a slow decomposition of the cubic perovskite into variants with hexagonal stacking that are barriers to oxygen transport. To elucidate details of the decomposition process, both sintered BSCF ceramic and powder were annealed for 180-240 h in ambient air at temperatures below 1123 K and analyzed by different transmission electron microscopy techniques. Aside from hexagonal perovskite Ba 0.6 Sr 0.4 CoO 3-δ , the formation of lamellar noncubic phases was observed in the quenched samples. The structure of the lamellae with the previously unknown composition Ba 1-x Sr x Co 2-y Fe y O 5-δ was found to be related to the 15R hexagonal perovskite polytype. The valence and spin-state transition of cobalt leading to a considerable diminution of its ionic radius can be considered a reason for BSCF's inherent phase instability at intermediate temperatures.

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Highly metastable Ba 1Àx Sr x LiF 3 (0 < x # x max z 0.4) with an inverse perovskite structure analogous to that of BaLiF 3 was synthesized by soft mechanical treatment of BaF 2 and LiF together with SrF 2 at ambient temperature. Ex as well as in situ X-ray powder diffraction (XRPD) measurements show that heat treatment at 393 K initiates the decomposition of the mixed phase into BaLiF 3 , LiF and (Sr,Ba)F 2 . Structural details of the metastable compound (Ba,Sr)LiF 3 were investigated by ultrafast 19 F magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Interestingly, five magnetically inequivalent F sites were identified which correspond to fluorine anions coordinated by a variable number of Ba and Sr cations, respectively. Details from XRPD and NMR spectroscopy are discussed with respect to the formation mechanisms and thermal stability of the as prepared fluorides. Impedance spectroscopy is used to characterize (long-range) ionic transport properties. Results are compared with those obtained recently on mechanosynthesized BaLiF 3 .

CO 2 -stable oxygen-permeable Fe 2 O 3 (FO) -Ce 0.9 Gd 0.1 O 2-δ (CGO) dual phase composite membranes of the composition χ wt % FO -(100 À χ) wt % CGO with χ = 25, 40, 50 were successfully prepared via a one-pot single-step method. X-ray diffraction (XRD) demonstrated that all FO -CGO composite membranes after sintering at 1300 °C for 5 h represent a microscale mixture of only the two pure phases FO and CGO. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), and oxygen permeation revealed that the microstructure of the composition of the χ FO -(100 À χ) CGO dual phase membranes has a great influence on the oxygen permeability including its time-dependence. It was found that the composition of 40FO -60CGO displays the highest oxygen permeability. An oxygen permeation flux of 0.18 mL/min 3 cm 2 was obtained through the uncoated 40FO -60CGO membrane with a thickness of 0.5 mm under an air/He oxygen gradient at 1000 °C. In situ XRD demonstrates that the 40FO -60CGO material possesses a good phase stability not only in an atmosphere of 50 vol % CO 2 /50 vol % Ar but also in other atmospheres with a low oxygen partial pressure like reduced pressure (vacuum) and 5 vol % H 2 / 95 vol % He. After coating the 40FO -60CGO dual phase membrane with a porous La 0.6 Sr 0.4 CoO 3-δ (LSC) layer of a few μm thicknesses on the air side, the oxygen permeation flux reaches the steady state immediately. This steady oxygen permeation flux of the LSC coated membrane was found to be 0.20 mL/min 3 cm 2 unchanged for more than 150 h even when pure CO 2 was used as the sweep gas, which indicates that the coated 40FO -60CGO dual phase membrane is CO 2 stable.