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The integration of mixed ionic–electronic conducting separation membranes in catalytic membrane reactors can yield more environmentally safe and economically efficient processes. Concentration polarization effects are observed in these types of membranes when O2 permeating fluxes are significantly high. These undesired effects can be overcome by the development of new membrane reactors where mass transport and heat transfer are enhanced by adopting state-of-the-art microfabrication. In addition, careful control over the fluid dynamics regime by employing compact metallic reactors equipped with microchannels could allow the rapid extraction of the products, minimizing undesired secondary reactions. Moreover, a high membrane surface area to catalyst volume ratio can be achieved. In this work, a compact metallic reactor was developed for the integration of mixed ionic–electronic conducting ceramic membranes. An asymmetric all-La0.6Sr0.4Co0.2Fe0.8O3–δ membrane was sealed to the metallic reactor by the reactive air brazing technique. O2 permeation was evaluated as a proof of concept, and the influence of different parameters, such as temperature, sweep gas flow rates and oxygen partial pressure in the feed gas, were evaluated.

Powders of constant morphology and quality are indispensable for reproducible ceramic manufacturing. In this study, commercially available powders were characterized regarding their microstructural properties and screened for a reproducible membrane manufacturing process, which was done by sequential tape casting. Basing on this, the slurry composition and ratio of ingredients were systematically varied in order to obtain flat, crack-free green tapes suitable for upscaling of the manufacturing process. Debinding and sintering parameters were adjusted to obtain defect-free membranes with diminished bending. The crucial parameters are the heating ramp, sintering temperature, and dwell time. The microstructure of the asymmetric membranes was investigated, leading to a support porosity of approximately 35% and a membrane layer thickness of around 20 &micro, m. Microstructure and oxygen flux are comparable to asymmetric La0.6Sr0.4Co0.2Fe0.8O3-&delta, (LSCF) membranes manufactured from custom-made powder, showing an oxygen flux of &gt, 1 mLcm&minus, 2min at 900 &deg, C in air/Ar gradient.

Considering the challenging conditions imposed by application of membranes in an asymmetric design, in particular creep resistance of the substrate material is an important parameter for the stability in long-term operation. As promising material, in terms of chemical stability, the perovskite SrTi0.75Fe0.25O3-δ has been identified in previous works. Porous supports with different microstructures have been produced using different manufacturing methods and compared to the material in its fully dense state regarding creep behaviour. The creep deformation of pressed, porous tape-cast and freeze-dried SrTi0.75Fe0.25O3-δ specimens has been investigated in the application relevant temperature range of 850–1000 °C under compressive stresses of 15, 30 and 45 MPa. A global fitting method considering all experimental data was used to derive stress exponent and activation energy of SrTi0.75Fe0.25O3-δ, which are 2.9 ± 0.4 and 402 ± 25 kJ/mol, respectively. Thus, it is suggested that the mechanism controlling creep is mainly related to dislocation climb/glide.

Perovskite-structured powders of SrTi1-xCoxO3-δ (STC-x) with nominal stoichiometry of x = 0–0.75 as well as SrTi0.75Co0.25-yFeyO3-δ (STCF-y) where y = 0–0.25 were synthesized using the Pechini method. Thermal/chemical expansion behaviour, total electrical conductivities, and oxygen permeation rates were investigated. The substitution of Ti with Co leads to an increase in both electronic and ionic conductivities and, therefore, oxygen permeability. Thermal and chemical expansions also increase slightly. The optimum Co content was found to be 25–35% due to the trade-off between phase stability and permeability. The oxygen permeation rate of STC35 is comparable to that of state-of-the-art (La,Sr)(Co,Fe)O3-δ, whereas the expansion coefficients are lower. Co-doping in STCF-y did not produce any significant differences in oxygen permeability at both high temperature and sample thickness (1.0 mm), i.e. in a solid-state diffusion-limited regime. At lower temperatures (

Hierarchically porous SrTi 0.75 Fe 0.25 O 3-δ specimens were produced through a freeze drying procedure, which yielded a channel-like porosity, reaching a value of 32%. Compressive testing was used to determine apparent elastic modulus and fracture stresses in the transverse (out-of-plane) and longitudinal (in-plane) direction, revealing a strong dependence onto pore orientation. The lower mechanical stability in the in-plane direction appears to be associated with bending mode of the pore walls, being a result of a lower resistance to crack initiation. Acoustic emissions recorded during compressive tests indicated continuous damage of the pore walls before complete failure of the specimen, which could be also confirmed by complementary in-situ compressive tests in a scanning electronic microscope.

Advanced oxygen transport membrane designs consist of a thin functional layer supported by a porous substrate material that carries mechanical loads. Creep deformation behavior is to be assessed to warrant a long-term reliable operation at elevated temperatures. Aiming towards an asymmetric composite, the current study reports and compares the creep behavior of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) perovskite porous substrate material with different porosity and pore structures in air for a temperature range of 800–1000 °C. A porosity and pore structure independent average stress exponent and activation energy are derived from the deformation data, both being representative for the LSCF material. To investigate the structural stability of the dense layer in an asymmetric membrane, sandwich samples of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) with porous substrate and dense layers on both side were tested by three-point bending with respect to creep rupture behavior of the dense layer. Creep rupture cracks were observed in the tensile surface of BSCF, but not in the case of LSCF.

The performance of ceramic gas separation membranes can be increased by reducing their thickness. To maintain the mechanical stability of the membrane, a supporting structure is needed. In this work magnesium oxide (MgO) is identified as support material for H 2 -separation membranes made of La 6-x WO 12-δ (LaWO). Therefore MgO is tested under application-related conditions in a water-gas-shift (WGS) catalytic membrane reactor for hydrogen production and shows a good stability. The sequential tape casting serves as the manufacturing method for the preparation of the asymmetric membrane structures. To show the compatibility of the two materials, detailed investigations of the sintering behaviour and stability tests are performed and show the well-matching properties of the two materials. Microstructural analyses reveal the inter-diffusion between LaWO and MgO.

Ceramic membranes can be divided into dense and porous membranes. The materials, microstructure, and manufacturing methods are described and insights into current research topics are given. By adapting the material properties and tailoring microstructures, membranes and components suitable for a variety of processes can be developed. In applications, a distinction can be made between pure gas separation and membrane reactors. In the latter ones, in addition to gas separation, a chemical reaction takes place on one or both sides of the membrane. Membrane reactors can be used to produce basic chemicals or synthetic fuels. The supply of gases can be of interest for power plants, cement, steel or glassworks as well as for the medical sector or for mobile applications.

Membrane-based oxy-combustion is a promising technology for energy efficient combustion of carbon-containing fuels with the simultaneous opportunity to capture CO2 from the resulting exhaust gas. However, oxy-combustion conditions result in special demands on the design of the ceramic membrane components due to the high pressure and temperature applied. Therefore, we have developed a planar membrane design for 4-end operation using asymmetric membranes of La0.6Sr0.4Co0.2Fe0.8O3−δ. FEM and CFD simulations have been performed in order to develop an internal channel structure that allows withstanding pressures of 5 bar on the feed side while achieving the desired O2 concentrations of 27% in the sweep gas, i.e. CO2, and an oxygen recovery rate from the feed gas of 86% at the same time. Due to the symmetric design of the membrane components, they are scalable and adaptable in size. This design has been realized in a process chain from powder to the final component consisting of thin 20 µm Membrane layer, support with 38% porosity, an inner channelled architecture and a thin (3–5 µm) porous activation layer. Particular emphasis was laid on scalable manufacturing processes in order to ensure transferability to industrial scale. The process chain is also applicable to other membrane materials suitable for any application of interest. Finally, the reproducible processing was successfully demonstrated by the fabrication of membrane components in lengths of 100 mm and widths of 70 mm.

The possibility of joining dense ceramic BCSF tubular membranes to metal alloys using a silver braze was investigated. Four different alloys (Crofer 22 APU®, Kanthal APM®, Haynes 214® and EN 1.4841) were considered and the influence of their oxide scale stability/reactivity and their thermal expansion coefficient on the stability of the final joint was evaluated. Leak tight assemblies were obtained only for steels with a thermal expansion coefficient of >16 [10−6 K−1] and protective coating. Proof-of-concept oxygen flux measurements up to 830 °C were performed on these assemblies, demonstrating the functionality of the developed hot sealing. In addition a simulation of the stresses occurring in the joint assembly during use was performed for different materials and geometries. The obtained results fit well with the experimental findings.

Aiming toward an optimization of dual phase oxygen transport membrane materials for oxygen separation applications, ceramic composites consisting of a Ce1−xGdxO2−δ (0 Ce1−xGdxO2−δ ≈ FexCo3−xO4 > CoO, and furthermore, hardness values are also in the same order. The hardness values of the obtained composites at higher impression loads reveal a stronger dependency on porosity than on composition due to similar hardness values of the main phases. Any compositional effect appears to diminish above a porosity of ≈1%.

Asymmetric membranes of mixed ionic-electronic conducting perovskite-type oxides SrTi1-xFexO3-δ (STF, x = 0.3, 0.5 and 0.7) were prepared by inverse sequential tape-casting. Both porous support (~600 μm) and functional membrane layer (~20 μm) for a given membrane assembly were made from the same composition to ensure thermochemical compatibility between the layers. Oxygen fluxes were assessed in the range 650 -1020 °C, using either (non-pressurized) ambient air or pure oxygen as feed gas at the support side of the asymmetric membrane and argon as sweep gas. Notably, similar oxygen fluxes (~1.2 × 10−6 mol cm−2 s−1) are measured through the membranes of different compositions above 950 °C when using ambient air as feed gas. This observation is interpreted to reflect the major role of the support layer resistance in rate-limiting the oxygen fluxes through the STF asymmetric membranes, which conclusion is supported by comparison of the oxygen fluxes with those measured previously through asymmetric membranes of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF). A simple diffusion-convection model is used to account for the observed gas phase polarization in the porous support layers limiting the oxygen fluxes.

The overall permeation rate through asymmetric oxygen transport membranes is significantly governed by the porous support. Therefore, the microstructuring of the support's pore structure is essential to achieving the highest performances. Freeze casting is already proven to obtain hierarchical porous structures with low tortuosity, which potentially enhances the oxygen flux of oxygen transport membranes. Although a performance improvement has been reported, such improvement is not self-evident. There has yet to be a detailed comparison of the achieved microstructures in order to identify the relevant microstructural parameters. Asymmetric membranes from Ba0.5Sr0.5(Co0.8Fe0.2)0.97Zr0.03O3-δ consisting of a surface-activated 20 µm membrane layer with tape- or freeze-cast supports that have identical pore volume and layer thickness were manufactured, characterized, and compared by means of oxygen flux measurements. They were also microstructurally investigated via computed X-Ray tomography and flow simulation experiments. In the air/Ar gradient, the freeze-cast support membrane performs below the tape-cast-supported membrane. In particular, the transition zone close to the membrane, which is caused by the freezing process, significantly constrains the diffusivity and permeability of the support, and therefore leads to concentration polarizations. At temperatures below 800 °C, surface exchange kinetics at the membrane-support interface become rate-limiting.

In dual-phase oxygen transport membranes we noticed that there is fast-growing interest in research for oxyfuel combustion process application. One such potential candidate is CGO-FCO (60wt% Ce0.8Gd0.2O2-δ-40wt% FeCo2O4) identified to provide good oxygen permeation flux with substantial stability in harsh atmosphere. Dense CGO-FCO membranes of 1mm thickness were fabricated by sintering dry pellets pressed from powders synthesized by one-pot method (modified Pechini process) at 1200 degrees C for 10h. Microstructure analysis indicates presence of a third orthorhombic perovskite phase in the sintered composite. We also identified that the spinel phase tends to form an oxygen deficient phase at the grain boundary of spinel and CGO phases. Surface exchange limitation of the membranes was overcome by La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) porous layer coating over the composite. Moreover, the oxygen permeation flux of the CGO-FCO screen printed with a porous layer of 10mthick LSCF is 0.11mL/cm2 per minute at 850 degrees C with argon as sweep and air as feed gas at the rates of 50 and 250mL/min.

Oxygen transport membranes (OTMs) are attracting great interest for the separation of oxygen from air in an energy efficient way. In the last decade one major driver was the development of CO2 mitigation scenarios utilizing carbon capture and storage (CCS) technology for large point sources. One very promising concept is the oxyfuel technology, which realizes the combustion of fossil fuels with oxygen-enriched recirculated flue gas thus requiring enormous quantities of oxygen. OTMs can deliver this oxygen efficiently if the required heat is delivered by the process itself as it is in the case of power, cement, glass, or steel plants. Another possible application is the operation of a membrane reactor, in which oxygen is directly consumed by a chemical reaction. In this context, the research focuses on the partial oxidation of methane or even the oxidative coupling of methane to higher hydrocarbons such as ethylene, propylene, aromatics etc. Such OTMs consist of mixed ionic electronic conductors (MIEC), which can consist of a single phase MIEC material or a composite of two separate phases, each phase providing electronic or ionic conductivity, respectively. In general, the bulk transport is based on defects in the crystal lattice, i.e. oxygen vacancies and electron holes. This leads to a trade-off between permeability requiring a high defect concentration and stability, requiring a low defect concentration. Therefore, materials with limited permeability have to be used in many applications to provide a long term stable operation. In order to still reach sufficiently high permeation rates, thin supported membrane layers are developed in different shapes, mainly planar or as capillaries. In case that the membrane thickness is below a characteristic thickness, surface exchange kinetics become rate limiting because of the relatively fast diffusion through the thin bulk membrane. Consequently, the solid-gas electrochemical interfaces become more and more important, making catalytically active layers, such as porous electrodes, a key element in the future development of long term stable, high performance membranes. In this presentation the development of thin (8µm – 400µm) supported membrane layers is described using high performance perovskites, i.e. Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and La0.6-xSr0.4Co0.2Fe0.8O3-δ (LSCF). Oxygen permeation measurements with varying conditions, i.e. temperature as well as feed gas composition and sweep gas flow rates, are used to identify limiting transport processes. The overall oxygen transport process of BSCF membranes is analyzed by combining different modelling approaches. While the gas diffusion in the gas phase and the porous support is modelled by cfd and binary friction model respectively, the transport through the bulk and the surface exchange is addressed in combination using a modified Wagner approach, accounting for the electrochemically active surface areas. According to this model, either gas diffusion in the porous support or the oxygen surface exchange is limiting the transport depending on experimental conditions. Surface exchange limitations can be partly overcome using porous electrode layers made of the membrane material. Due to the high electrochemical activity of the perovskites used, an additional permeation enhancement using noble metal catalysts cannot easily be realized at high temperatures. In addition, dual phase membranes were investigated, which are much more prone to surface exchange limitations because of the limited length of the active triple phase boundaries. Electrodes made of a single phase MIEC material, i.e. LSCF, show evidence of these limitations even using 1 mm thick samples.

Doped ceria pellets with the composition Ce0.8Gd0.2-xPrxO2-δ with x = 0.15, 0.1, 0.05, and 0.03 were investigated with a special focus on the partial conductivities in the temperature range of 200–600°C. Temperature dependent conductivity provided by impedance spectroscopy in air was compared to measurements of the oxygen partial pressure dependent electronic conductivity. The electronic conductivity was analyzed down to 200°C by using a modified Hebb-Wagner setup with encapsulated Pt microcontacts. A small polaron hopping process introduced by reduction of praseodymium was found to have a pronounced influence on the electronic conductivity at low temperatures. A splitting of the maximum introduced by praseodymium small polaron hopping was observed. Especially for the compositions x ≤ 0.1, a strong deviation of the electronic conductivity curves from the standard acceptor doping case was measured due to superimposed electronic conductivity from the Pr3+/4+ redox reaction.

New oxygen transport membrane materials based on SrTi1-xFexO3-δ, synthesized through solid state reaction and processed via tape casting were characterized with respect to their mechanical behaviour via depth-sensitive indentation and ring-on-ring flexural testing. The elastic moduli obtained by indentation with 1 N load for SrTi1-xFexO3-δ (x = 0.25, 0.35, 0.5) specimens were 147 ± 10 GPa, 123 ± 6 GPa and 158 ± 10 GPa, respectively. Fracture stress was accessed by ring-on-ring testing performed at 100 N/min and the obtained results were 92 ± 9 MPa, 117 ± 15 MPa, and 100 ± 15 MPa for SrTi0.75Fe0.25O3, SrTi0.65Fe0.35O3, and SrTi0.5Fe0.5O3 respectively. Ring-on-ring tests conducted at different loading rates gave access to subcritical crack growth sensitivity and aided the prediction of the materials’ lifetime through stress-time-probability diagrams, where SrTi1-xFexO3-δ (x = 0.25, 0.35, 0.5) may resist for 1 year with a failure probability of 0.1% at least 15 MPa, 22 MPa, and 12 MPa respectively.

Ceramic oxide membranes are widely being researched for Carbon Capture and Storage/Utilization sector applications. Foreseen applications of these membranes are oxygen generation for oxyfuel combustion in e.g. power plants, glass-, cement- or steel production. Major drawback with Mixed Ionic and Electronic Conducting (MIEC) perovskite structure membranes is their limited long term stability at high temperatures in aggressive atmospheres. Dual phase composite membranes have been reported to excel overcoming this drawback. In addition to performance evaluation, Ce0.8Gd0.2O2-δ – FeCo2O4 (CGO-FCO) membranes were subjected to stability test in flue gas conditions closely mimicking industrial flue gas atmosphere. The dual phase composites are investigated for their phase stability at the operating temperature of 850 °C in a gradient of oxygen chemical potential. The composites were also exposed to a series of gas mixtures over a period of time at their operating temperature to test for the chemical stability. CGO-FCO membranes are identified to possess chemical stability in gas mixtures of CO2, SO2 along with oxygen over a period of 200 h at 850 °C under oxygen partial pressure gradient.

Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3− δ (BSCF) is widely known as a promising candidate material for oxygen transport membranes (OTMs). In order to maximize the oxygen permeation through such a membrane, the membrane layer should be as thin as possible, which requires a porous support. Because of the expansion behavior of BSCF, porous supports of the same material were developed to avoid failure due to mismatches in the thermal expansion coefficients. For the purpose of minimizing concentration polarization in the support pores, the microstructure of these support-layers has been optimized. For that reason supports with a porosity of up to 41% were developed. Membrane curvature caused by different shrinkage rates during co-firing could be minimized by the use of corn starch as pore former. By increasing the support porosity from 26% to 41%, the oxygen permeation of a supported 20 μm membrane in an air–argon gradient at 800 °C was increased by 50%. Compared to a disc membrane of 0.9 mm thickness the permeation enhancement is 90%.

[EN] One of the most promising materials for oxygen separation amongst ceramic oxygen transport membranes (OTMs) is La0.6Sr0.4Co0.2Fe0.8O3 (-delta) (LSCF) due to its relatively high oxygen permeability combined with high stability. In this work, asymmetric thin-film LSCF membranes supported over a porous LSCF support were manufactured by inverse sequential tape casting. Moreover, surface activation was accomplished by depositing a porous LSCF activation layer in order to promote oxygen evolution reaction, i.e. O2- to O-2 oxidation, in the permeate membrane side. In this case, the porous layer allows the surface area available for oxygen activation to be enlarged. Both the manufacturing of the asymmetric thin-film membranes and the surface activation are described in detail. A thorough study of the oxygen permeation is presented for disk-shaped 30 mu m thick LSCF-supported membranes considering the following operating parameters: temperature (1000-600 degrees C), sweep flow rate (300-750 ml min(-1) argon) and oxygen partial pressure in the feed (0.21-1 atm). High permeation fluxes were achieved, e.g., 11.87 ml min(-1) cm(-2) at 1000 degrees C and 300 ml min(-1) argon sweep when using pure oxygen as feed. A change in the apparent activation energy at about 850 degrees C was related to a reversible structural change in the perovskite symmetry (cubic rhombohedral), as revealed by XRD measurements. Furthermore, the application of an activation layer allowed the permeation process to be improved, especially at low temperatures, i.e. below 800 degrees C. Specifically, an improvement of up to about 300% at 600 degrees C is observed upon application of the activation layer. The activated membrane reached a flux of 13.3 ml min(-1) cm(-2) at 1000 degrees C under an O-2/Ar gradient. Additional permeation tests using CO2-rich sweep gas demonstrated the good stability and performance of these LSCF asymmetric membranes at 900 and 1000 degrees C. (c) 2013 Elsevier B.V. All rights reserved., Financial support by the Spanish Ministry for Science and Innovation (ENE2011-24761 and SEV-2012-0267 Grants), by the EU through FP7 NASA-OTM Project (NMP3-SL-2009-228701), and by the Helmholtz Association of German Research Centres through the Helmholtz Portfolio MEM-BRAIN is gratefully acknowledged. The authors would like to express their gratitude to Dr. W. Fischer for crystal structure analysis by X-ray diffraction. Mrs. J. Carter-Sigglow has contributed to this work with the careful revision of the English usage.

[EN] Oxygen transport membranes made of Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) were manufactured by tape casting and co-firing. The disk-shaped membranes consisted of a top gastight layer (70 mu m thick) and a porous substrate (830 mu m thick) with 34% open porosity. The variation of the permeation operation conditions allowed (i) the identification of the different limitations steps in the permeation process, i.e., bulk oxygen ion diffusion, catalytic surface exchange and gas phase diffusion in the membrane compartments and porous substrate, and (ii) the ultimate optimization of the oxygen flux. The variables considered in the systematic permeation study included the inlet gas flow rate of the sweep and air feed, the temperature and the nature of the oxygen feed gas (air or pure oxygen). Moreover, the influence of the deposition of a catalytic activation layer (17 mu m thick) made of BSCF on top of the thin gastight layer was investigated. As a result of this parametric study, unpreceded oxygen flux values were achieved, i.e., a maximum flux of 67.7 ml(STP) min(-1) cm(-2) was obtained at 1000 degrees C using pure oxygen as the feed and argon as the sweep, while a flux of 12.2 ml(STP) min(-1) cm(-2) at 1000 degrees C was obtained when air was used as the feed. (C) 2011 Elsevier BM. All rights reserved., Financial support from the Spanish Ministry for Science and Innovation (Project ENE2008-06302 and FPI Grant JAE-Pre 08-0058), EU through FP7 NASA-OTM Project (NMP3-SL-2009-228701), and the Helmholtz Association of German Research Centres through the Helmholtz Alliance MEM-BRAIN (Initiative and Networking Fund) is kindly acknowledged. Mrs H. Burlet has contributed to this work with the careful revision of the English language.