Ceramic Membranes For Oxygen Separation Research Articles

Thermochemically stable ceramic composite membranes based on Bi2O3 for oxygen separation with high permeability.

Ceramic oxygen separation membranes can be utilized to reduce CO2 emissions in fossil fuel power generation cycles based on oxy-fuel combustion. State-of-the-art oxygen permeable membranes based on Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) offer high oxygen permeability but suffer from long-term instability, especially in the presence of CO2. In this work, we present a novel ceramic composite membrane consisting of 60 vol% (Bi0.8Tm0.2)2O3-δ (BTM) and 40 vol% (La0.8Sr0.2)0.99MnO3-δ (LSM), which shows not only comparable oxygen permeability to that of BSCF but also outstanding long-term stability. At 900 °C, oxygen fluxes of 1.01 mL min-1 cm-2 and 1.33 mL min-1 cm-2 were obtained for membranes with thicknesses of 1.35 mm and 0.75 mm, respectively. Moreover, significant oxygen fluxes were obtained at temperatures down to 600 °C. A stable operation of the membrane was demonstrated with insignificant changes in the oxygen flux at 750 °C for approx. one month and at 700 °C with 50% CO2 as the sweep gas for more than two weeks.

Materials design for ceramic oxygen permeation membranes: Single perovskite vs. single/double perovskite composite, a case study of tungsten-doped barium strontium cobalt ferrite

Pure oxygen is an important raw material with many important applications. The production of oxygen via a conducting ceramic membrane is a new, cost-effective and advanced technology with the advantage of continuous oxygen production. The perovskite-type mixed-conducting oxide Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) exhibits superb oxygen permeability, yet it suffers from poor phase stability. In this study, we aim to improve the operational stability of the BSCF membrane by introducing a high-valence W6+ ion as a B-site dopant. Its effect on the phase composition, structure, structural stability, electrical conductivity, oxygen transfer rate and oxygen permeability as a membrane is systematically investigated. Upon the partial substitution of cobalt and iron in the W6+-doped BSCF, single/double perovskite composites are formed instead of single perovskite composites. Remarkably, the formation of the single/double perovskite composites enhances the oxygen permeation stability without obviously compromising the oxygen permeability. Among the various materials, the composite with the nominal composition of Ba0.5Sr0.5Co0.8Fe0.1W0.1O3-δ shows the best performance in terms of stability and oxygen permeability. These findings thus introduce a new way to design conducting ceramic membranes for oxygen separation at high temperatures.

Oxygen transport properties of Ca-doped Pr2NiO4

Praseodymium nickelate Pr2NiO4+δ with layered Ruddlesden–Popper (R–P) structure is a promising material for the ceramic membranes for oxygen separation and for SOFC cathodes due to its mixed ionic-electronic conducting nature and high oxygen mobility and surface reactivity. However, a low thermal stability and the inadequate electronic conductivity of Pr2NiO4+δ require its doping for a more efficient performance. This work presents results of study of the effect of Ca doping on the structural and transport properties of Pr2NiO4. Pr2−xCaxNiO4 oxides (x = 0–0.6) were synthesized by a co-precipitation method and characterized by XRD, XPS and TEM methods. The oxygen content in these materials and its variation with increasing temperature was evaluated using TGA. The oxygen transport properties of powdered samples were studied by the temperature-programmed oxygen isotope heteroexchange with C18O2 (TPIE). Electrical conductivity of compact samples was measured by a dc four-probe technique. Electrochemical measurements were performed using an impedance spectroscopy method with symmetrically arranged electrodes on the Ce0.8Sm0.2O1.9 electrolyte. It was shown that doping resulted in enhanced electrical conductivity and at a low Ca content the electrochemical activity of electrodes increased while the interstitial oxygen content and oxygen diffusivity gradually decreased. For x > 0.3 increasing the lattice anisotropy resulted in the co-existence of 2–3 channels of oxygen diffusion revealed as separate peaks in TPIE curves. The fast diffusion channel (D⁎ ~ 10−8 cm2/s at 700 °C), with a share in the total diffusion drastically decreasing at big doping levels, corresponds to the fast interstitial oxygen diffusion via the cooperative mechanism while the slow channels (D⁎ < 10−10 cm2/s) are probably related to the oxygen transport in perovskite layers and the complicated transport involving interlayer positions near the dopant cation sites.

Microchannel structure of ceramic membranes for oxygen separation

Microchanneled ceramic membranes have demonstrated superior performance in oxygen separation from air over conventional membranes. In this study, the contributions of the microchannel structure to the superior performance were investigated. Compared with supported membranes, the microchanneled membranes provide fast pathways within the channels for gas diffusion as compared to the tortuous interconnection of pore channels in the supported membranes. The walls of the numerous channels provide a large surface for facilitating oxygen dissociation, which was confirmed by varying the channel wall surface using mesh templates with different aperture sizes. In summary, the microchannel structure facilitates gas diffusion and provides a large membrane active surface, resulting in high performance in oxygen separation.

Novel Approach for Developing Dual-Phase Ceramic Membranes for Oxygen Separation through Beneficial Phase Reaction.

A novel method based on beneficial phase reaction for developing composite membranes with high oxygen permeation flux and favorable stability was proposed in this work. Various Ce0.8Sm0.2O2-δ (SDC) + SrCO3+Co3O4 powders with different SDC contents were successfully fabricated into membranes through high temperature phase reaction. The X-ray diffraction (XRD) measurements suggest that the solid-state reaction between the SDC, SrCO3 and Co3O4 oxides occurred at the temperature for membrane sintering, leading to the formation of a highly conductive tetragonal perovskite phase SmxSr1-xCoO3-δ. The morphology and elemental distribution of the dual-phase membranes were characterized using back scattered scanning electron microscopy and energy dispersive X-ray spectroscopy (BSEM-EDX). The oxygen bulk diffusivity and surface exchange properties of the materials were investigated via the electrical conductivity relaxation technique, which supported the formation of conductive phases. The SDC+20 wt % SrCO3+10.89 wt % Co3O4 membrane exhibited the highest permeation flux among the others, reaching 0.93 mL cm(-2) min(-1) [STP = standard temperature and pressure] under an air/helium gradient at 900 °C for a membrane with a thickness of 0.5 mm. In addition, the oxygen permeation flux remained stable during the long-time test. The results demonstrate the beneficial phase reaction as a practical method for the development of high-performance dual-phase ceramic membranes.

Oxygen permeability of Ce0.8Sm0.2O2 − δ–LnBaCo2O5 + δ (Ln=La, Nd, Sm, and Y) dual-phase ceramic membranes

Ce0.8Sm0.2O2 − δ–LnBaCo2O5 + δ (SDC-LnBCO, Ln=La, Nd, Sm, and Y) composites were prepared and characterized as dense ceramic membranes for oxygen separation. Ce0.8Sm0.2O2 − δ (SDC) shows a good chemical compatibility with LaBaCo2O5 + δ (LBCO), NdBaCo2O5 + δ (NBCO), and SmBaCo2O5 + δ (SBCO), while a little of impurity is detected in the sample with YBaCo2O5 + δ (YBCO). The SDC-SBCO and SDC-YBCO samples exhibit larger grain size than SDC-LBCO and SDC-NBCO. The incorporation of SDC increases the structural stability and decreases the thermal expansion coefficient of LnBCO at high temperature. The oxygen permeation flux of SDC-LnBCO dual-phase membrane decreases with decreasing Ln3+ radius, which is 2.57 × 10−7, 1.58 × 10−7, 8.05 × 10−8, and 7.57 × 10−8 mol cm−2 s−1 at 925 °C for SDC-LBCO, SDC-NBCO, SDC-SBCO, and SDC-YBCO membranes, respectively. Increasing the flow rate of feeding and sweeping gas can create oxygen partial pressure gradient, and thus, increase the oxygen permeability. These results indicate that SDC-LBCO and SDC-NBCO composites are promising candidates for oxygen permeation membrane.

Facile fabrication and improved carbon dioxide tolerance of a novel bilayer-structured ceramic oxygen permeating membrane

An asymmetrical Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF)|Pr0.5Ce0.5O2−δ (PrCe) bilayer-structured ceramic membrane is fabricated by a facile technique involving dry pressing of BSCF substrate, wet spraying of PrCe thin film layer (~10μm) and co-sintering of the bilayer structures. The phase reaction, sintering behavior and oxygen permeability of the membrane are systematically investigated using XRD, ESEM, EDX and oxygen permeation studies. The CO2 poisoning resistance of the layer materials is characterized by FTIR and CO2-TPD experiments. The large shrinkage of BSCF substrate and the diffusion of compositional elements from BSCF substrate into the PrCe layer promote the densification of PrCe layer at a temperature as low as 1100°C. The as-fabricated bilayer membrane exhibits not only high oxygen permeation flux, reaching 1.6mLcm−2min−1 at 850°C, but also favorable permeation stability even after the introduction of 10% CO2 into helium sweep gas. After the continuous operation at 850°C under the CO2-containing atmosphere for 800min, the permeation flux remains stable at around 1.35mLcm−2min−1. The facile fabrication and superiority of the bilayer-structured membrane in resistance towards CO2 poisoning thus provides a new way for the development of ceramic membranes for oxygen separation.

Synthesis of Yttria-Doped Bi2o3 Nanopowders Via Sol Gel Used in Electrolyte of Solid Oxide Fuel Cell

Oxide ion conductors have been studied for many years because of their application in devices with high economical interest such as solid oxide fuel cells, oxygen sensors , dense ceramic membranes for oxygen separation. In this paper nanopowders were synthesized via sol gel method. The sintering temperature to complete phase transition are above 500˚c. As-prepared yttria doped Bi2O3 nanocrystals were characterized by X-ray diffraction (XRD) a nd transmission electron microscope (TEM). Transmission electron microscope (TEM) investigations revealed that the average particle size is less than 30 nm for these powders. It's worth noting that results show a good agreement of both methods. The morphology of the powders were observed on Field Emission scanning electron microscope (FESEM). Ionic conductivity measured by AC impedance spectroscopy.

Cobalt-free niobium-doped barium ferrite as potential materials of dense ceramic membranes for oxygen separation

Cobalt-free perovskite-type oxides with the nominal composition of BaNbyFe1−yO3−δ (y=0.025–0.20) are synthesized and evaluated as materials used in ceramic membranes for oxygen separation. The effects of Nb-doping on the crystal structure, surface morphology, electrical conductivity, chemical bulk diffusion and surface exchange, and oxygen permeability of the oxides are systematically investigated using XRD, SEM, four-probe DC conductivity, electrical conductivity relaxation technique, and oxygen permeation studies. A small amount of Nb-doping induces a sharp increase in electrical conductivity. A further increase in the Nb-doping amount, however, lowers the electrical conductivity as a result of the blocking effect of Nb5+ on electronic conduction. A small amount of Nb-doping has less impact on the sintering capability. From the oxygen permeation test, it was found that Nb-doping could significantly enhance the oxygen permeability, especially below 750°C. Among all of the compositions, BaNb0.05Fe0.95O3−δ shows the highest oxygen permeation fluxes, reaching 1.35 and 0.61mLcm−2min−1 for a membrane with a thickness of 1.0mm at 900 and 700°C, respectively. Furthermore, the membrane is rate-controlled mainly by bulk diffusion, indicating the potential to further improve the oxygen permeation flux via a thinner membrane.

Phase-inversion tape casting and oxygen permeation properties of supported ceramic membranes

A variant of tape casting, involving phase inversion, was explored for the preparation of supported ceramic oxygen separation membranes in one step. A slurry of Zr0.84Y0.16O1.92 (YSZ) and La0.8Sr0.2MnO3−δ (LSM) powders in a N-methyl-2-pyrrolidone solution of polyethersulfone was tape cast, and immersed in water bath for solidification into a green tape. After sintering at 1320°C a ceramic membrane was obtained with a total porosity of ~35%, consisting of a porous layer with thickness ~0.85mm on a gas-tight top layer with thickness ~0.15mm. An oxygen permeation flux of 1.90×10−3molm−2s−1 was observed at 900°C by exposing the porous side of the membrane to air and the other side to helium. A supported membrane with the identical YSZ/LSM ratio, made by sequential two-step tape casting, with graphite as pore former, showed a flux of 3.31×10−4molm−2s−1. The difference in oxygen flux between the two membranes is attributed to the difference in resistance towards gas transport in the support layer. It is concluded that the phase inversion tape casting is a simple and effective method for preparation of supported oxygen separation membranes.

A Bifunctional Air Electrode Catalyzed by Ba0.5Sr0.5Co0.8Fe0.2O3–δ for Zinc-Air Batteries

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Processing of thin film ceramic membranes for oxygen separation

Abstract Processing of dense and thin ceramic membrane layers for high temperature selective oxygen separation is addressed in this study. Mixed oxygen-ionic and electronic conducting perovskite oxide system based on La 0.2 Sr 0.8 Fe 0.8 Ta 0.2 O 3− δ composition is employed for processing of structural and functional layers. Special focus is aimed at obtaining thin layer and final microstructure with particle size in the sub-micron range. Thin layer deposition is performed by dip coating technique using stable colloidal suspension of perovskite particles dispersed within ethanol media. Two polymer based surfactants were screened for their effect on particle agglomeration and rheological response. By using optimum quantity of 2.5 wt.% addition of selected surfactant it is possible to obtain dense 15–60 μm thick functional layers. The thermal cycle applied resulted in final particle sizes within sub-micron range. By employing suspension with pore former it was possible to significantly increase the surface area of the functional layer.

Effect of foreign oxides on the phase structure, sintering and transport properties of Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3−δ as ceramic membranes for oxygen separation

A series of foreign oxides including CuO, ZnO, NiO, Co 2O 3 and Al 2O 3 were investigated as sintering aids to facilitate the densification of Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ (BSCF) membranes for oxygen separation. The effects of these oxides on the phase structure, sintering behavior, electrical conductivity and oxygen permeability of BSCF oxide were systematically studied by X-ray diffraction (XRD), oxygen temperature-programmed desorption (O 2-TPD), environmental scanning electron microscopy (ESEM), four-probe direct current (DC) conductivity and gas permeation measurements. The results showed that reactions between the membrane material and the foreign oxides took place at elevated temperatures. These oxides displayed different sintering behaviors during the sintering process of BSCF perovskite, with obvious improvement demonstrated in the case of CuO, minimal effects observed from ZnO, Co 2O 3 and NiO samples, and suppressing effects from Al 2O 3. The electrical conductivity was clearly increased by introducing Co 2O 3 as a sintering aid, yielding a maximum value of 70 S cm −1 at 900 °C. The oxygen permeation fluxes of the membrane samples from BSCF + NiO (5 wt.%) and BSCF + ZnO (5 wt.%) are comparable to those of pristine BSCF membranes, whereas the fluxes decreased noticeably when the other foreign oxides were introduced.

Atomic-scale insight into the oxygen ionic transport mechanisms in La 2NiO 4-based materials

The computer simulation studies employing both static lattice and molecular dynamics (MD) methods, were used to identify anion migration pathways, relevant energetic parameters and effects of the transition metal cation dopants on oxygen ion transport in La 2Ni(M)O 4+ δ (M = Fe, Co, Cu) solid solutions, a family of promising oxide materials for fuel cell electrodes and dense ceramic membranes for oxygen separation. The factors related to different oxygen sublattices in the K 2NiF 4-type structure of La 2Ni(M)O 4+ δ were appraised analyzing the MD data. The results show, in particular, that the incorporation of dopants having 3+ oxidation state leads to higher ionic charge-carrier concentration affecting the overall anion diffusivity, which is essentially determined by cooperative mechanisms involving oxygen interstitials and anions occupying regular apical sites in the layered lattices. However, these dopants tend to decrease anion mobility, both in the rock-salt and perovskite-like layers of the K 2NiF 4-type structure. The likely microscopic mechanisms of anion diffusion in oxygen-hyperstoichiometric La 2Ni(M)O 4+ δ are determined.

Layered perovskite Y1−Ca BaCo4O7+ as ceramic membranes for oxygen separation

Abstract Layer-structured mixed conducting oxides with the composition Y 1− x Ca x BaCo 4 O 7+ δ ( x = 0.0–0.4) were synthesized and their performance as oxygen separating membranes was explored. The crystal structure, phase stability, oxygen absorption/desorption properties, and electrical conductivity of the oxides, as well as oxygen permeation fluxes of the corresponding membranes, were systematically investigated. The properties of YBaCo 4 O 7+ δ -based oxides were greatly improved by Ca incorporation into the lattice structure at an optimal concentration. In the series Y 1− x Ca x BaCo 4 O 7+ δ , the Y 0.8 Ca 0.2 BaCo 4 O 7+ δ membrane exhibited the highest oxygen permeation flux, reaching a value of 0.75 × 10 −6 mol cm −2 s −1 at 900 °C under the conditions of air/helium oxygen gradient and a membrane thickness of 1.0 mm. Studies of the effect of membrane thickness on the oxygen fluxes implied that at membrane thicknesses larger than 0.7 mm, the permeation process was mainly controlled by bulk diffusion, indicating the potential to further improve the oxygen fluxes via thinner membranes of asymmetric structure.

Composite Ceramic Membranes for Oxygen Separation in Carbon Capture and Storage Systems

CGO-LSCF composite membranes have been characterised with a view to applications in oxygen separators for oxyfueled boilers in Carbon Capture and Storage (CCS) systems. Although commonly believed to be chemically stable together, evidence is presented that an interdiffusion reaction occurs on sintering at 1250ºC for 4 hours. Isotopic oxygen exchange measurements show that this does not appear to have any detrimental effect on the oxygen exchange properties. Similarly, electrical conductivity was also measured, and no detrimental effect was observed for these properties.

Spin-state transition of iron in ( [formula omitted] perovskite

The redox behavior of iron during heating of a high-performance perovskite for ceramic oxygen separation membranes was studied by combined electron energy-loss (EELS, esp. ELNES) and Mössbauer spectroscopical in situ methods. At room temperature, the iron in ( Ba 0.5 Sr 0.5 ) ( Fe 0.8 Zn 0.2 ) O 3 - δ (BSFZ) is in a mixed valence state of 75% Fe 4 + in the high-spin state and 25% Fe 3 + predominantly in the low-spin state. When heated to 900 ∘ C , a slight reduction of iron is observed that increases the quantity of Fe 3 + species. However, the dominant occurrence is a gradual transition in the spin-state of trivalent iron from a mixed low-spin/high-spin to a pure high-spin configuration. In addition, a remarkable amount of hybridization is found in the Fe–O bonds that are highly polar rather than purely ionic. The coupled valence/spin-state transition correlates with anomalies in thermogravimetry and thermal expansion behavior observed by X-ray diffraction and dilatometry, respectively. Since the effective cationic radii depend not only on the valence but also on the spin-state, both have to be considered when estimating under which conditions a cubic perovskite will tolerate specific cations. It is concluded that an excellent phase stability of perovskite-based membrane materials demands a tailoring, which enables pure high-spin states under operational conditions, even if mixed valence states are present. The low spin-state transition temperature of BSFZ provides that all iron species are in a pure high-spin configuration already above ca. 500 ∘ C making this ceramic highly attractive for intermediate temperature applications ( 500 – 800 ∘ C ).

CO 2-tolerant oxygen separation membranes targeting CO 2 capture application

Burning fossil fuels with O 2/CO 2 mixture produces a concentrated CO 2 stream and thus enables efficient CO 2 capture. In an attempt to improve the economic competiveness of this oxy-fuel combustion technology, intensive efforts have been made to develop ceramic oxygen separation membranes that have a potential to reduce the oxygen production costs over the present cryogenic air separation process. In this paper it is proposed that the O 2/CO 2 mixture could be supplied by using CO 2 as sweeping gas to remove the oxygen separated from air by an oxygen selective membrane. State-of-the-art membranes with the perovskite structure, such as SrCo 0.8Fe 0.2O 3− δ (SCF), suffer degradation upon exposure to the acidic gas CO 2. This paper reports that the stability of SCF in CO 2 can be improved remarkably by reducing its surface basicity through the introduction of Ti ions, while the oxygen permeability is largely retained. The membrane-based O 2/CO 2 production concept is also verified with a CO 2-tolerant membrane tube.

Composite Ceramic Membranes for Oxygen Separation in Carbon Capture and Storage Systems

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Structural and electrochemical features of Bi 2O 3-based fast oxide ion conductors

Oxide ion conductors have been increasingly studied for many years because of their application in devices with high economical impact such as solid oxide fuel cells (SOFC), oxygen sensors, and dense ceramic membranes for oxygen separation, and membrane reactors for oxidative catalysis. New fast oxide ion conductors for low temperature applications (400–600 °C) have been proposed during recent years. The influence of Sb 2O 3 and Ta 2O 5 as on the Bi 2O 3 structures and the corresponding electrical behavior were investigated. The powder products, obtained by solid state reactions, were characterized by XRD and SEM. They were sintered at different temperatures and the bulk ceramics were characterized and their electrical behavior versus temperature were recorded. This investigation underlines the influence of the nature of dopants on the structures and electrical performances bismuth based oxides. For microstructure analysis the theory of fractals was used.