Oxygen Transport Membranes Research Articles

Hebb-Wagner polarization assessment of enhanced oxygen permeability for surface modified oxygen transport membranes

Tri-layered “porous | dense | porous” La0.8Sr0.2Cr0.5Fe0.5O3−δ-Zr0.84Y0.16O2−δ (LSCrF-YSZ) oxygen transport membranes (OTMs) were fabricated and permeation resistances from oxygen reduction and evolution reactions were determined by using Hebb-Wagner polarization method after introducing additional electron-blocking YSZ thin layers within the dense LSCrF-YSZ layers. Adding nano-scale catalysts, i.e. La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCoF) on air side and Ce0.8Sm0.2O1.9−δ/Ni (SDC/Ni) on CH4 side, into the porous LSCrF-YSZ layers yielded substantially reduced interfacial polarization resistances, and thereby allowed for high oxygen permeability at reduced temperatures under the air/CH4 gradient, e.g., 1.1 and 4.3 ml cm−2 min−1 at 650 and 800 °C, respectively. Analysis of the impedance spectra suggest that the oxygen reduction kinetics on air side was probably limited by charge transfer reaction at T ≥ 750 °C and surface oxygen exchange at T ≤ 700 °C. Meanwhile, oxygen evolution reactions on CH4 side dominated the total resistances to oxygen permeation through the tri-layered OTMs.

Microstructure, mechanical behavior and flow resistance of freeze-cast porous 3YSZ substrates for membrane applications

Freeze-cast porous 3YSZ with different porosities were characterized as mechanical load carrying supports for oxygen transport membrane applications. Porosity influence on mechanical properties, i.e. elastic modulus and fracture stresses was assessed with biaxial ring-on-ring bending tests. The flow resistance was characterized in terms of the pressure drop using different gases to reveal the effect of the porous support on the accessing of the inlet gas flow to the functional dense membrane layer. Both properties were discussed in terms of the influence of porosity and pore structure, and compared with the properties of porous 3YSZ produced via pressing and sintering.

Mechanical properties and lifetime predictions of dense SrTi1-xFexO3-δ (x = 0.25, 0.35, 0.5)

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 1N load for SrTi1-xFexO3-δ (x=0.25, 0.35, 0.5) specimens were 147±10GPa, 123±6GPa and 158±10GPa, respectively. Fracture stress was accessed by ring-on-ring testing performed at 100N/min and the obtained results were 92±9MPa, 117±15MPa, and 100±15MPa 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 1year with a failure probability of 0.1% at least 15MPa, 22MPa, and 12MPa respectively.

Morphology, performance and stability of multi-bore capillary La0.6Sr0.4Co0.2Fe0.8O3-δ oxygen transport membranes

Mixed ionic-electronic conducting 3, 4, 7-bore capillary membranes made of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) were successfully prepared by the combined phase inversion/sintering technique. The membranes fabricated have asymmetric wall structures with micro-channels formed in between surfaces, and dense layers sandwiched in between the micro-channels. By changing the solvent from DMSO to NMP, changes in the morphology of the 7-bore membrane were observed, where the separation layer has reduced its effective thickness. The multi-bore membranes exhibited 3-point bending fracture loads of 10.4, 13.5, 15.4 and 11.7 N with a 3cm testing span for the 3-bore, 4-bore, 7-bore-DMSO and 7-bore-NMP samples, respectively, which are much stronger than single-bore hollow fibre membranes. Oxygen permeation of the multi-bore membranes was measured with a sweep gas flow through lumen and the effect of operating temperature has on the performance was studied between 750°C to 1000°C. Oxygen fluxes measured are comparable to typical sandwich-like structured single-bore hollow fibres at temperatures below 900°C, but are notably higher at higher temperatures owe to their thinner membrane walls. The 200-h long-term permeation test conducted on the 7-bore membrane showed a slight increase in permeation flux, but the sign of kinetic demixing/decomposition appeared on the outer surface, where the surface of the thinnest membrane walls underwent faster demixing/decomposition than the thickest walls. In summary, the results demonstrated that multi-bore configurations can achieve optimised material distribution during the fabrication, and can obtain strong mechanical property, high permeation flux for the final products whilst maintaining high membrane area to volume ratios.

Oxy-fuel combustion technology: current status, applications, and trends

Summary The increased level of emissions of carbon dioxide into the atmosphere due to burning of fossil fuels represents one of the main barriers toward the reduction of greenhouse gases and the control of global warming. In the last decades, the use of renewable and clean sources of energies such as solar and wind energies has been increased extensively. However, due to the tremendously increasing world energy demand, fossil fuels would continue in use for decades which necessitates the integration of carbon capture technologies (CCTs) in power plants. These technologies include oxycombustion, pre-combustion, and post-combustion carbon capture. Oxycombustion technology is one of the most promising carbon capture technologies as it can be applied with slight modifications to existing power plants or to new power plants. In this technology, fuel is burned using an oxidizer mixture of pure oxygen plus recycled exhaust gases (consists mainly of CO2). The oxycombustion process results in highly CO2-concentrated exhaust gases, which facilitates the capture process of CO2 after H2O condensation. The captured CO2 can be used for industrial applications or can be sequestrated. The current work reviews the current status of oxycombustion technology and its applications in existing conventional combustion systems (including gas turbines and boilers) and novel oxygen transport reactors (OTRs). The review starts with an introduction to the available CCTs with emphasis on their different applications and limitations of use, followed by a review on oxycombustion applications in different combustion systems utilizing gaseous, liquid, and coal fuels. The current status and technology readiness level of oxycombustion technology is discussed. The novel application of oxycombustion technology in OTRs is analyzed in some details. The analyses of OTRs include oxygen permeation technique, fabrication of oxygen transport membranes (OTMs), calculation of oxygen permeation flux, and coupling between oxygen separation and oxycombustion of fuel within the same unit called OTR. The oxycombustion process inside OTR is analyzed considering coal and gaseous fuels. The future trends of oxycombustion technology are itemized and discussed in details in the present study including: (i) ITMs for syngas production; (ii) combustion utilizing liquid fuels in OTRs; (iii) oxy-combustion integrated power plants and (iv) third generation technologies for CO2 capture. Techno-economic analysis of oxycombustion integrated systems is also discussed trying to assess the future prospects of this technology. Copyright © 2017 John Wiley & Sons, Ltd.

Fabrication of ice-templated tubes by rotational freezing: Microstructure, strength, and permeability

We demonstrate a facile and scalable technique, rotational freezing, to produce porous tubular ceramic supports with radially aligned porosity. The method is based on a conventional ice-templating process in a rotatory mold and demonstrated here with yttria-stabilized zirconia (YSZ). We investigated the effects of solid loading, freezing temperature, and volume of the slurry on the microstructure, strength (o-ring test and four-point bending), and air permeability. The results show that pore volume and pore size can be controlled by the solid loading and freezing temperature respectively, and overall tube thickness can be adjusted by the volume of slurry initially poured into the mold. Decreasing pore size and pore volume increases the mechanical properties but decreases the air permeability. These tubes could be particularly interesting as tubular membrane supports such as oxygen transport membranes.

Timing of sexual abuse, sexual risk behaviors and substance use among US women

In this study, for application in oxygen transport membranes, an LSCF (La0.6Sr0.4Co0.2Fe0.8O3−δ)–GDC (Ce0.9Gd0.1O2−δ) composite was manufactured, and its mechanical properties were studied. Generally, it is known that LSCF has a nonlinear modulus due to changes in its microstructure when a critical stress is loaded. To improve the mechanical properties of this material, which has a perovskite structure, a study was conducted to evaluate whether its properties change according to the rule of mixtures when it is formed into a composite with GDC. The results showed that the nonlinearity of the modulus of the composite for each specimen composition was largely reduced and stable under fatigue loading. The fracture toughness was superior to that of the LSCF (1.05 MPa m1/2) or GDC (1.28 MPa m1/2) monolithic materials when the composites (1.63 MPa m1/2) was manufactured. The mechanisms for these were explained by finite element analysis and the observation of crack propagation. It was also confirmed that when these composite materials are used for oxygen transport membranes, they show stable properties.

Ionic/Electronic Conductivity, Thermal/Chemical Expansion and Oxygen Permeation in Pr and Gd Co-Doped Ceria PrxGd0.1Ce0.9-xO1.95-δ

The oxygen permeation flux of Ce0.9Gd0.1O1.95-δ (CGO)-based oxygen transport membranes under oxidizing conditions is limited by the electronic conductivity of the material. This work aims to enhance the bulk ambipolar conductivity of CGO by partial substitution of Ce with the redox active element Pr. A series of compositions of PrxGd0.1Ce0.9-xO1.95-δ (x = 0, 0.02, 0.05, 0.08, 0.15, 0.25, 0.3 and 0.4) was prepared by solid state reaction. X-ray powder diffraction (XPD) indicates that Pr is completely dissolved in the fluorite structure up to 40 at.%. Pronounced nonlinear thermal expansion behavior was observed as a function of temperature, due to the simultaneous contributions of both thermal and chemical expansion. The electronic and ionic conductivities were measured as a function of temperature and oxygen partial pressure. Within the range from 10 to 15 at.% Pr, a drastic drop of the activation energy of the hole mobility and an abrupt increase of the hole conductivity at low temperature was observed. The behavior could be rationalized by a simple percolation model. Oxygen permeation fluxes through disk shaped samples fed with air on one side and N2 on the other side were also measured. The oxygen flux through Pr0.05Gd0.1Ce0.85O1.95-δ was higher than that for CGO by one order of magnitude owing to the enhanced electronic conductivity albeit the flux is still limited by the electronic conductivity. In terms of the electronic and ionic conductivity, the estimated maximum oxygen permeation flux of a 10 μm Pr0.4Gd0.1Ce0.9O1.95-δ -based membrane exceeds 10 Nml cm−2 min−1 at 900°C under a small oxygen potential gradient (0.21/10−3 bar) which is promising for use in oxygen production and in oxy-fuel combustion. Also the material may be well applicable to SOFC/SOEC composite electrodes where mixed conductivity is also desirable.

Exploring the performance of dual-phase oxygen transport membranes for carbon capture purposes

Exploring the performance of dual-phase oxygen transport membranes for carbon capture purposes

Electrochemical-Mechanical Coupling in Composite Planar Structures that Integrate Flow Channels and Ion-Conducting Membranes

Ceramic oxygen-transport membranes, such as the doped perovskite La0.6Sr0.4Co0.8Fe0.2O3 − δ (LSCF6482) considered in the present paper, are effective in applications such as air separation. The present paper considers a planar configuration that is composed of a thin (order tens of microns) ion-transport membrane, a relatively thick (order millimeter) porous-ceramic support structure, and millimeter-scale oxygen-collection flow channels. The lattice-scale strain associated with charged defects (oxygen vacancies and small polarons) within ion-transport membranes causes macroscopic stress that could distort or damage the assembly. The modeling approach is based on an extended two-dimensional Nernst–Planck–Poisson (NPP) formulation that is developed and applied to evaluate the effects of chemically induced stress within a planar oxygen-separation assembly. The computational model predicts two-dimensional distributions of steady-state defect concentrations, electrostatic potentials, and stress. Parameter studies consider the effects of support-membrane dimensions, materials mechanical properties, and operating conditions. Although the stress is found to have a negligible influence on the defect transport, the defect transport is found to significantly affect the stress distributions. Such results can play important roles in the design and development of planar ion-transport membranes and their support structures.

Determining the rate-limiting step during oxygen semi-permeation of CaTi0.9Fe0.1O3-δ oxygen transport membranes

Oxygen transport membranes are dense ceramic oxides that have the ability to exchange oxygen with the surrounding atmosphere and to transport it along a chemical potential gradient. To improve performance, it is important to determine whether the oxygen transport is limited by the surface exchange or by bulk diffusion. Because this analysis involves very specific testing conditions, it is difficult to generalize and compare literature results. Recent studies have shown that CaTi0.9Fe0.1O3−δ (CTF) offers a reasonable compromise between performance and long-term stability, making it a good candidate for the rigorous conditions of industrial applications. Although the baseline flux performance of CTF has been established, the rate-limiting step for oxygen transport remains a matter of debate. In the present investigation, the oxygen activity on both surfaces of the membrane is measured simultaneously with the permeation flux to determine the limiting step in the oxygen semipermeability of CTF. The limiting step varies with temperature: below 750°C oxygen transport is limited both by bulk diffusion and by surface exchanges, with bulk diffusion limitations dominating at higher temperatures, although surface polarization remains influential. Moreover, the flux is limited by ionic conductivity at low temperatures and by electron hole conductivity at high temperatures.

Oxygen permeation and stability of CaTi0.73Fe0.18Mg0.09O3−δ oxygen-transport membrane

Oxygen permeation membranes consisting of gastight mixed ionic-electronic conductors (MIEC) allow the diffusion of oxygen through the crystal lattice via oxygen vacancies. An extended screening of doped CaTiO3 materials is reported here with the aim of improving the ionic and the electronic conductivity of the material. The dopants were selected taking into account the ionic radii affinity and their aliovalence or multivalence. Moreover, this work presents the oxygen permeability of CaTi1−xFexO3−δ co-doped with Mg. The structural and electrochemical properties of the nominal perovskite CaTi0.73Fe0.18Mg0.09O3−δ (CTFM) are studied. X-ray diffraction demonstrates that the material crystallizes as a single perovskite structure with orthorhombic distorted symmetry, although the MgO segregates as a minor separated phase. The partial addition of Mg2+ to the perovskite results in an increase of oxygen vacancies, and therefore the total conductivity at high temperatures with respect to the non-doped CaTi0.8Fe0.2O3 (CTF). On the contrary, below 800°C the conductivity of CTFM is lower than for CTF, attributed to the lower amount of Fe in the CTFM. Permeation measurements were performed on CTFM gastight membranes of 900 μm thickness, reaching oxygen fluxes of up to 0.95mLmin−1cm−2 at 1000°C. Stability of the membrane to CO2 was evaluated by exposing the membrane to 15% CO2 in Ar atmosphere for 24h at 750°C and O2 fluxes were measured by using this atmosphere as sweep gas. Initial O2 fluxes were maintained after the CO2 treatment. In addition, thermogravimetric (TG) measurements performed on powder CTFM under dry CO2 (5% in Ar) did not show any carbonate formation, confirming the stability of the material in this atmosphere.

Simulation of the effect of the porous support on flux through an asymmetric oxygen transport membrane

Asymmetric membranes provide a low ionic resistance of the functional separation layer together with a high mechanical stability. However, the microstructure of the porous support in the membrane assembly affects the overall flux significantly. This effect was studied by applying the binary friction model (BFM) for the support together with a modified Wagner equation for the dense membrane using transport relevant parameters obtained from micro computed tomography data of a tape cast Ba0.5Sr0.5Co0.8Fe0.2O3–δ support. The influence of different pore diameters and thicknesses of the support were compared for different feed gases (oxygen and air) and flow configurations (3-end, 4-end, assembly orientation).The effect of the support at large pore diameters (>35µm) for the 3-end mode transport process using oxygen as feed gas, was negligible. This was not the case for the 4-end mode irrespective of the feed gas, and for the 3-end mode using air as feed gas. This was attributed to the binary diffusion term in the BFM. Thin small-pored supports yield the same flux as thick large-pored supports considering a non-linear relationship between thickness and pore size. This can be used for the optimization of the support's microstructure with regards to mechanical strength and permeability.

Mechanical properties of LSCF (La0.6Sr0.4Co0.2Fe0.8O3−δ)–GDC (Ce0.9Gd0.1O2−δ) for oxygen transport membranes

In this study, for application in oxygen transport membranes, an LSCF (La0.6Sr0.4Co0.2Fe0.8O3−δ)–GDC (Ce0.9Gd0.1O2−δ) composite was manufactured, and its mechanical properties were studied. Generally, it is known that LSCF has a nonlinear modulus due to changes in its microstructure when a critical stress is loaded. To improve the mechanical properties of this material, which has a perovskite structure, a study was conducted to evaluate whether its properties change according to the rule of mixtures when it is formed into a composite with GDC. The results showed that the nonlinearity of the modulus of the composite for each specimen composition was largely reduced and stable under fatigue loading. The fracture toughness was superior to that of the LSCF (1.05MPam1/2) or GDC (1.28MPam1/2) monolithic materials when the composites (1.63MPam1/2) was manufactured. The mechanisms for these were explained by finite element analysis and the observation of crack propagation. It was also confirmed that when these composite materials are used for oxygen transport membranes, they show stable properties.

Investigation of oxygen transport membrane reactors for oxy-fuel combustion and carbon capture purposes

There is an increasing pressure for industry to reduce carbon dioxide (CO2) emissions from combustion processes, resulting in an increased interest in the development of methods to sequester and recycle CO2 from flue gases. Current methods available to separate nitrogen and CO2 for environmental benefits, such as chemical looping combustion (CLC), are expensive at the high flow rates encountered in industry. One potential alternative is a ceramic membrane catalytic reactor, which produces pure oxygen and simultaneously conducts oxy-fuel combustion, thus, CO2in the product stream can successfully be separated from the nitrogen in air. This work investigates the performance of a perovskite-type SrSc0.1Co0.9O3-δ(SSC) and La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) membrane reactors for the combustion of methane in various configurations. The ceramic membranes exploited here are oxygen semi-permeable, dense ceramic membranes based on the composite oxides with a high mixed oxygen ionic and electronic conductivity at high temperatures. The prepared SSC and LSCF hollow fibre membranes with catalysts were used to perform reactions with a methane fuel. The oxygen permeability feasibility of the membrane reactors were studied and confirmed. The CO2selectivity at various test conditions were also reported with the maximum selectivity achieved for SSC membrane to be 85.4% while LSCF membrane achieved 87.1% CO2selectivity.

Graphite and PMMA as pore formers for thermoplastic extrusion of porous 3Y-TZP oxygen transport membrane supports

A gas permeable porous support is a crucial part of an asymmetric oxygen transport membrane (OTM). Here, we develop feedstocks for thermoplastic extrusion of tubular, porous 3Y-TZP (partially stabilized zirconia polycrystals, (Y2O3)0.03(ZrO2)0.97)) ceramics, using graphite and/or polymethyl methacrylate (PMMA) as pore formers. The influence of pore former content and type, 3Y-TZP particle size and support sintering temperature on the microstructure, porosity and gas permeability were studied. Using at least 40 vol% pore former, consisting of graphite and PMMA in the volume ratio 2:1, tubes with gas permeability exceeding the target of 10−14m2 are obtained. In the temperature range 1250–1400°C the support gas permeability is insensitive to the sintering temperature, and the feedstocks shrink more than 15% during sintering, making them ideal for co-sintering with functional OTM layers. This demonstrates the suitability of thermoplastic extrusion for fabrication of porous 3Y-TZP OTM supports, or for other technologies requiring porous ceramics.

Surface Diffusion of Oxygen Transport Membrane Materials Studied by Grain‐Boundary Grooving

Mass transport mechanism responsible for grain‐boundary grooving during thermal annealing of polished ceramics of Ba0.5Sr0.5Co0.8Fe0.2O3‐δ (BSCF) and La2NiO4+δ (LN) was revealed by atomic force microscopy. Surface diffusion mechanism was confirmed for both materials by the evolution of the grain‐boundary width (w) with annealing time (t), and the surface diffusion coefficient was determined from the slope of w versus t1/4 following the theory by Mullins. An Arrhenius temperature dependence of the surface diffusion was observed, and the activation energy was determined to be 220 ± 30 and 450 ± 30 kJ/mol, respectively, for BSCF and LN. The surface diffusion data are discussed with respect to similar data for other oxide materials and cation and oxygen anion diffusion in BSCF and LN. Finally, the dihedral angle for both LN and BSCF was determined, and these are typical in the range reported for other oxide materials.

A comparative study of La0.6Sr0.4Co0.2Fe0.8O3−δ oxygen transport membranes deposited on porous metal supports prepared by supersonic air-gas plasma spraying (SAPS) and low pressure plasma spraying-physical vapor deposition (PS-PVD)

Abstract Perovskite-type La0.6Sr0.4Co0.8Fe0.2O3 − δ (LSCF) oxygen transport membranes were successfully deposited on porous austenitic stainless steel by SAPS. A performance comparison between SAPS membrane and the one prepared by the reported PS-PVD was carried out. X-ray diffraction (XRD) result demonstrates that the degree of perovskite crystallinity of the PS-PVD membrane is better than that of the SAPS membrane. However, scanning electron microscope (SEM) and gas-tightness measurement results indicate that the SAPS membrane shows a much denser microstructure than that of the PS-PVD membrane. The thermal stability result reveals that some vertical through cracks appears on the PS-PVD membrane, while no obvious vertical cracks can be observed on the SAPS membrane. Then the oxygen permeation flux through the membranes was measured under high temperature (700 °C and 900 °C). It is found that the oxygen permeation flux through the PS-PVD membrane is higher than that of the SAPS membrane. Further studies demonstrate that the oxidation of porous metallic substrates greatly decrease the oxygen permeation flux.

Designing Fe-Based Oxygen Catalysts by Density Functional Theory Calculations

A cheap and effective oxygen catalyst can find a variety of applications including cathodes for solid oxide fuel cells, oxygen transport membranes, cathodes for metal air batteries, and catalysts for thermal water splitting. This work studies BaFeO3 (BFO)-based perovskites as oxygen catalysts from density functional theory calculations by substituting the A and B sites of the BFO perovskite structure. Na, K, Rb, Ca, Sr, Y, La, and Pb are substituted into the A site, and Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb, Ag, In, and Ce are substituted into the B site. We point out that there is a trade-off among stability, electronic conductivity, and ionic conductivity, where the material selection needs to be based on the maximization of the desired property for a given application. Specifically, the calculation results show that the A site substitution with Na and K lowers the vacancy formation energy while preserving the electronic conductivity. However, the introduction of such elements may destabilize the...

Unraveling Crystal Structure and Transport Properties of Fast Ion Conducting SrCo0.9Nb0.1O3−δ

SrCo0.9Nb0.1O3−δ (SCN) has been investigated for oxygen transport membrane and solid oxide fuel cell applications as it is more stable than the state-of-the-art Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) material. Here, the crystal structure and transport properties of SCN were systematically investigated. Combined neutron and high-temperature X-ray powder diffraction analyses over the temperature range of 25–800 °C showed that the crystal structure of SCN belongs to the tetragonal space group P4/mmm with the unit cell ac × ac × 2ac, where ac is the lattice parameter of the cubic perovskite at temperatures <160 °C. The tetragonal (P4/mmm) to cubic (Pm3m) phase transition occurred in SCN with increasing temperature. This phase transition affected the electrical conductivity, whereas the impact on the magnetic susceptibility was not significant. The surface exchange and diffusion coefficients of SCN in the temperature range of 600–850 °C were characterized by electrical conductivity relaxation and oxygen permeation m...