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Abstract: Porous substrates are a prerequisite for advanced oxygen transport membranes. In particular, phase stability and mechanical robustness are of concern for long term performance and reliability. These aspects were investigated for porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ material using annealing experiments along with microstructural investigations, depth-sensitive micro-indentation and ring-on-ring bi-axial bending tests. Annealing studies revealed phase instabilities at elevated temperatures that were also characterized by X-ray diffraction analysis. Elastic modulus and fracture stress were strongly affected by the porosity, whereas their temperature dependency agreed with the behavior of dense material. [Copyright &y& Elsevier]

Abstract: Ba0.5Sr0.5Co0.8Fe0.2O3−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 20μm membrane in an air–argon gradient at 800°C was increased by 50%. Compared to a disc membrane of 0.9mm thickness the permeation enhancement is 90%. [Copyright &y& Elsevier]

Abstract: Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) exhibits high oxygen permeability, which is why it is being discussed for gas separation (oxygen transport membrane, OTM) in zero-emission power plants using oxyfuel technology when the membrane is operated in a clean environment, i.e. no flue gas contact. We investigate the influence of membrane processing on microstructure and oxygen permeation. Pure-phase BSCF powder is synthesized using a modified Pechini method. For comparison, commercially available powder is also used, synthesized by a solid-state reaction. Disk-shaped membranes of various microstructures, i.e. closed porosities and grain sizes, are prepared by uniaxial pressing and sintering of the powders processed in different ways. The powders and membranes are characterized by methods including BET, SEM, XRD, and DSC. The microstructures obtained by different sintering conditions are investigated by SEM and TEM. Sintering at 1150°C leads to incongruent melting of BSCF indicated by DSC. The liquid phase appears at three-phase boundaries grain–grain–air and consists of nearly pure cobalt oxide with small impurities of barium and strontium detected by TEM/EDX analysis. Oxygen permeation of the membranes is measured in an air/Ar gradient depending on temperature and membrane microstructure. The closed porosity of different processed membranes is varied between 2 and 15% with uniform grain sizes in the range of approx. 10μm. The average grain size is increased from 10 to 45μm by increasing the sintering temperature. Neither porosity nor the grain size significantly influences the oxygen permeation rate of 1-mm-thick disks in the investigated parameter range. [Copyright &y& Elsevier]

Asymmetric membranes of mixed ionic-electronic conducting perovskite-type oxides SrTi 1-x Fe x O 3- 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 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). A simple diffusion-convection model is used to account for the observed gas phase polarization in the porous support layers limiting the oxygen fluxes. Image 1 • Asymmetric all-perovskite membranes were prepared by inverse sequential tape-casting. • Support layer resistances limit the oxygen fluxes when using air as feed gas. • Diffusion-convection simulations account for the observed gas phase polarization. [ABSTRACT FROM AUTHOR]

Highlights: [•] Membrane microstructure was analyzed using SEM and X-ray computer tomography. [•] Different limitations for the oxygen transport were identified. [•] The porous substrate has a significant influence on the permeation. [•] A model approach was developed to account for surface exchange. [•] The characteristic thickness is the governing factor for modeling oxygen transport. [ABSTRACT FROM AUTHOR]

One of the most promising materials for oxygen separation amongst ceramic oxygen transport membranes (OTMs) is La0.6Sr0.4Co0.2Fe0.8O3−δ (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 O2 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 30µm thick LSCF-supported membranes considering the following operating parameters: temperature (1000–600°C), sweep flow rate (300–750mlmin−1 argon) and oxygen partial pressure in the feed (0.21–1atm). High permeation fluxes were achieved, e.g., 11.87mlmin−1 cm−2 at 1000°C and 300mlmin−1 argon sweep when using pure oxygen as feed. A change in the apparent activation energy at about 850oC 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°C. Specifically, an improvement of up to about 300% at 600°C is observed upon application of the activation layer. The activated membrane reached a flux of 13.3mlmin−1 cm−2 at 1000°C under an O2/Ar gradient. Additional permeation tests using CO2-rich sweep gas demonstrated the good stability and performance of these LSCF asymmetric membranes at 900 and 1000oC. [ABSTRACT FROM AUTHOR]

Abstract: Oxygen transport membranes made of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) were manufactured by tape casting and co-firing. The disk-shaped membranes consisted of a top gastight layer (70μm thick) and a porous substrate (830μm 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 (17μm 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 67.7ml(STP)min−1 cm−2 was obtained at 1000°C using pure oxygen as the feed and argon as the sweep, while a flux of 12.2ml(STP)min−1 cm−2 at 1000°C was obtained when air was used as the feed. [Copyright &y& Elsevier]

The utilization of oxygen transport membranes enables the production of high-purity hydrogen by the thermal decomposition of water below 1000 °C. This process is based on a chemical potential gradient across the membrane, which is usually achieved by introducing a reducing gas. Computational fluid dynamics (CFD) can be used to model reactors based on this concept. In this study, a modelling approach for water splitting is presented in which oxygen transport through the membrane acts as the rate-determining process for the overall reaction. This transport step is implemented in the CFD simulation. Both gas compartments are modelled in the simulations. Hydrogen and methane are used as reducing gases. The model is validated using experimental data from the literature and compared with a simplified perfect mixing modelling approach. Although the main focus of this work is to propose an approach to implement the water splitting in CFD simulations, a simulation study was conducted to exemplify how CFD modelling can be utilized in design optimization. Simplified 2-dimensional and rotational symmetric reactor geometries were compared. This study shows that a parallel overflow of the membrane in an elongated reactor is advantageous, as this reduces the back diffusion of the reaction products, which increases the mean driving force for oxygen transport through the membrane.

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 O 2 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-La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ membrane was sealed to the metallic reactor by the reactive air brazing technique. O 2 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 µm. Microstructure and oxygen flux are comparable to asymmetric La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) membranes manufactured from custom-made powder, showing an oxygen flux of > 1 mLcm -2 min at 900 °C in air/Ar gradient., Competing Interests: The authors declare no conflict of interest.