Oxygen permeation behavior of La0.6Sr0.4Co0.8Fe0.2O3 hollow fibre membranes with highly concentrated CO2 exposure

Rational design of mixed ionic-electronic conducting membranes for oxygen transport

The mixed ionic-electronic conducting (MIEC) oxides have generated significant research efforts in the scientific community during the last 40 years. Since then, many MIEC compounds, most of which are based on perovskite oxides, have been synthesized and characterized. These compounds, when heated to high temperatures, form solid ceramic membranes with high oxygen ionic and electrical conductivity. The driving force for oxygen ion transport is the ionic transfer of oxygen from the air as a result of the differential partial pressure of oxygen across the membrane. Electronic and ionic transport in a range of MIEC materials has been studied using the defect theory, particularly when dopants are introduced to the compound of interest. As a result, many types of ionic oxygen transport limits exist, each with a distinct phase shift depending on the temperature and partial pressure of oxygen in use. In combination with theoretical principles, this work attempts to evaluate the research community's major and meaningful achievements in this subject throughout the preceding four decades.

Deciphering the Interaction of Single-Phase La0.3Sr0.7Fe0.7Cr0.3O3-δ with CO2/CO Environments for Application in Reversible Solid Oxide Cells

A detailed study aimed at understanding and confirming the reported highly promising performance of a La_0.3Sr_0.7Fe_0.7Cr_0.3O_3-δ (LSFCr) perovskite catalyst in CO₂/CO mixtures, for use in reversible solid oxide fuel cells (RSOFCs), is reported in this work, with an emphasis on chemical and performance stability. This work includes an X-ray diffraction (XRD), thermogravimetric analysis (TGA), and electrochemical study in a range of pO₂ atmospheres (pure CO₂, CO alone (balance N₂), and a 90-70% CO₂/10-30% CO containing mixture), related to the different conditions that could be encountered during CO₂ reduction at the cathode. Powdered LSFCr remains structurally stable in 20-100% CO₂ (balance N₂, pO₂ = 10^-11-10^-12 atm) without any decomposition. However, in 30% CO (balance N₂, pO₂ ∼ 10^-26 atm), a Ruddlesden-Popper phase, Fe nanoparticles, and potentially some coke are observed to form at 800 °C. However, this can be reversed and the original perovskite can be recovered by heat treatment in air at 800 °C. While no evidence for coke formation is obtained in 90-70% CO₂/10-30% CO (pO₂ = 10^-17-10^-18 atm) mixtures at 800 °C, in 70 CO₂/30 CO, minor impurities of SrCO₃ and Fe nanoparticles were observed, with the latter potentially beneficial to the electrochemical activity of the perovskite. Consistent with prior work, symmetrical two-electrode full cells (LSFCr used at both electrodes), fed with the various CO₂/CO gas mixtures at one electrode and air at the other, showed excellent electrochemical performance at 800 °C, both in the SOFC and in SOEC modes. Also, LSFCr exhibits excellent stability during CO₂ electrolysis in medium-term potentiostatic tests in all gas mixtures, indicative of its excellent promise as an electrode material for use in symmetrical solid oxide cells.

Development of a Membrane Module Prototype for Oxygen Separation in Industrial Applications

The integration of oxygen transport membranes in industrial processes can lead to energy and economic advantages, but proof of concept membrane modules are highly necessary to demonstrate the feasibility of this technology. In this work, we describe the development of a lab-scale module through a comprehensive study that takes into consideration all the relevant technological aspects to achieve a prototype ready to be operated in industrial environment. We employed scalable techniques to manufacture planar La0.6Sr0.4Co0.2Fe0.8O3-δ membrane components suitable for the application in both 3- and 4-end mode, designed with a geometry that guarantees a failure probability under real operating conditions as low as 2.2 × 10−6. The asymmetric membranes that act as separation layers showed a permeation of approx. 3 NmL/min/cm2 at 900 °C in air/He gradient, with a remarkable stability up to 720 h, and we used permeation results to develop a CFD model that describes the influence of the working conditions on the module performance. The housing of the membrane component is an Inconel 625 case joined to the membrane component by means of a custom-developed glass–ceramic sealant that exhibited a remarkable thermo-chemical compatibility both with metal and ceramic, despite the appearance of chemical strain in LSCF at high temperature. The multi-disciplinary approach followed in this work is suitable to be adapted to other module concepts based on membrane components with different dimensions, layouts or materials.

Recent Advances in Molten-Carbonate Membranes for Carbon Dioxide Separation: Focus on Material Selection, Geometry, and Surface Modification

Membranes for carbon dioxide permeation have been recognized as potential candidates for CO₂ separation technology, particularly in the energy sector. Supported molten-salt membranes provide ionic routes to facilitate carbon dioxide transport across the membrane, permit the use of membrane at higher temperature, and offer selectivity based on ionic affinity of targeted compound. In this review, molten-carbonate ceramic membranes have been evaluated for CO₂ separation. Various research studies regarding mechanisms of permeation, properties of molten salt, significance of material selection, geometry of support materials, and surface modifications have been assessed with reference to membrane stabilities and operational flux rates. In addition, the outcomes of permeation experiments, stability tests, selection of the compatible materials, and the role of interfacial reactions for membrane degradation have also been discussed. At the end, major challenges and possible solutions are highlighted along with future recommendations for fabricating efficient carbon dioxide separation membranes.

A CO2 -Tolerant Perovskite Oxide with High Oxide Ion and Electronic Conductivity

Mixed ionic-electronic conductors (MIECs) that display high oxide ion conductivity (σ_o ) and electronic conductivity (σ_e ) constitute an important family of electrocatalysts for a variety of applications including fuel cells and oxygen separation membranes. Often MIECs exhibit sufficient σ_e but inadequate σ_o . It has been a long-standing challenge to develop MIECs with both high σ_o and stability under device operation conditions. For example, the well-known perovskite oxide Ba_0.5 Sr_0.5 Co_0.8 Fe_0.2 O_{3- δ} (BSCF) exhibits exceptional σ_o and electrocatalytic activity. The reactivity of BSCF with CO₂ , however, limits its use in practical applications. Here, the perovskite oxide Bi_0.15 Sr_0.85 Co_0.8 Fe_0.2 O_{3- δ} (BiSCF) is shown to exhibit not only exceptional bulk transport properties, with a σ_o among the highest for known MIECs, but also high CO₂ tolerance. When used as an oxygen separation membrane, BiSCF displays high oxygen permeability comparable to that of BSCF and much higher stability under CO₂ . The combination of high oxide transport properties and CO₂ tolerance in a single-phase MIEC gives BiSCF a significant advantage over existing MIECs for practical applications.

Enhancing Oxygen Permeation via the Incorporation of Silver Inside Perovskite Oxide Membranes

As a possible novel cost-effective method for oxygen production from air separation, ion-conducting ceramic membranes are becoming a hot research topic due to their potentials in clean energy and environmental processes. Oxygen separation via these ion-conducting membranes is completed via the bulk diffusion and surface reactions with a typical example of perovskite oxide membranes. To improve the membrane performance, silver (Ag) deposition on the membrane surface as the catalyst is a good strategy. However, the conventional silver coating method has the problem of particle aggregation, which severely lowers the catalytic efficiency. In this work, the perovskite oxide La0.8Ca0.2Fe0.94O3−a (LCF) and silver (5% by mole) composite (LCFA) as the membrane starting material was synthesized using one-pot method via the wet complexation where the metal and silver elements were sourced from their respective nitrate salts. LCFA hollow fiber membrane was prepared and comparatively investigated for air separation together with pure LCF hollow fiber membrane. Operated from 800 to 950 °C under sweep gas mode, the pure LCF membrane displayed the fluxes from 0.04 to 0.54 mL min−1 cm−2. Compared to pure LCF, under similar operating conditions, the flux of LCFA membrane was improved by 160%.

Designing CO2-resistant oxygen-selective mixed ionic-electronic conducting membranes: guidelines, recent advances, and forward directions

CO₂ resistance is an enabling property for the wide-scale implementation of oxygen-selective mixed ionic-electronic conducting (MIEC) membranes in clean energy technologies, i.e., oxyfuel combustion, clean coal energy delivery, and catalytic membrane reactors for greener chemical synthesis. The significant rise in the number of studies over the past decade and the major progress in CO₂-resistant MIEC materials warrant systematic guidelines on this topic. To this end, this review features the pertaining aspects in addition to the recent status and advances of the two most promising membrane materials, perovskite and fluorite-based dual-phase materials. We explain how to quantify and design CO₂ resistant membranes using the Lewis acid-base reaction concept and thermodynamics perspective and highlight the relevant characterization techniques. For perovskite materials, a trade-off generally exists between CO₂ resistance and O₂ permeability. Fluorite materials, despite their inherent CO₂ resistance, typically have low O₂ permeability but this can be improved via different approaches including thin film technology and the recently developed minimum internal electronic short-circuit second phase and external electronic short-circuit decoration. We then elaborate the two main future directions that are centralized around the development of new oxide compositions capable of featuring simultaneously high CO₂ resistance and O₂ permeability and the exploitation of phase reactions to create a new conductive phase along the grain boundaries of dual-phase materials. The final part of the review discusses various complimentary characterization techniques and the relevant studies that can provide insights into the degradation mechanism of oxide-based materials upon exposure to CO₂.

High-Performance Microchanneled Asymmetric Gd(0.1)Ce(0.9)O(1.95-δ)-La(0.6)Sr(0.4)FeO(3-δ)-Based Membranes for Oxygen Separation

A microchanneled asymmetric dual phase composite membrane of 70 vol % Gd(0.1)Ce(0.9)O(1.95-δ)-30 vol % La(0.6)Sr(0.4)FeO(3-δ) (CGO-LSF) was fabricated by a "one step" phase-inversion tape casting. The sample consists of a thin dense membrane (100 μm) and a porous substrate including "finger-like" microchannels. The oxygen permeation flux through the membrane with and without catalytic surface layers was investigated under a variety of oxygen partial pressure gradients. At 900 °C, the oxygen permeation flux of the bare membrane was 1.6 (STP) ml cm(-2) min(-1) for the air/He-case and 10.10 (STP) ml cm(-2) min(-1) for the air/CO-case. Oxygen flux measurements as well as electrical conductivity relaxation show that the oxygen flux through the bare membrane without catalyst is limited by the oxygen surface exchange. The surface exchange can be enhanced by introduction of catalyst on the membrane surface. An increase of the oxygen flux of ∼1.49 (STP) mL cm(-2) min(-1) at 900 °C was observed when catalyst is added for the air/He-case. Mass transfer polarization through the finger-like support was confirmed to be negligible, which benefits the overall performance. A stable flux of 7.00 (STP) ml cm(-2) min(-1) was observed between air/CO/CO2 over 200 h at 850 °C. Partial surface decomposition was observed on the permeate side exposed to CO, in line with predictions from thermodynamic calculations. In a mixture of CO, CO2, H2, and H2O at similar oxygen activity the material will according to the calculation not decompose. The microchanneled asymmetric CGO-LSF membranes show high oxygen permeability and chemical stability under a range of technologically relevant oxygen potential gradients.

A novel CO2- and SO2-tolerant dual phase composite membrane for oxygen separation

Here we report a novel, cheap, non-toxic, CO₂- and SO₂-stable dual phase composite oxygen transport membrane (Al_0.02Ga_0.02Zn_0.96O_1.02–Gd_0.1Ce_0.9O_1.95−δ) for oxygen separation.

Enhanced oxygen separation through robust freeze-cast bilayered dual-phase membranes

Dual-phase oxygen-permeable asymmetric membranes with enhanced oxygen permeation were prepared by combining freeze-casting, screen-printing, and constraint-sintering techniques. The membranes were evaluated under oxyfuel operating conditions. The prepared membranes are composed of an original ice-templated La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3-δ) support with hierarchically oriented porosity and a top fully densified bilayered coating comprising a 10 μm-thick La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3-δ) layer and a top protective 8 μm-thick layer made of an optimized NiFe2O4/Ce(0.8)Tb(0.2)O(2-δ) composite synthesized by the one-pot Pechini method. Preliminary analysis confirmed the thermochemical compatibility of the three involved phases at high temperature without any additional phase detected. This membrane exhibited a promising oxygen permeation value of 4.8 mL min(-1)  cm(-2) at 1000 °C upon using Ar and air as the sweep and feed gases, respectively. Mimicking oxyfuel operating conditions by switching argon to pure CO2 as a sweep gas at 1000 °C and air as feed enabled an oxygen flux value of 5.6 mL min(-1)  cm(-2) to be reached. Finally, under the same conditions and increasing the oxygen partial pressure to 0.1 MPa in the feed, the oxygen permeation reached 12 mL min(-1)  cm(-2). The influence of CO2 content in the sweep gas was studied and its reversible and positive effect over oxygen permeation at temperatures equal to or above 950 °C was revealed. Finally, the membrane stability over a period of 150 h under CO2-rich sweep gas showed a low degradation rate of 2.4×10(-2)  mL min(-1)  cm(-2) per day.

High-flux oxygen-transporting membrane Pr(0.6)Sr(0.4)Co(0.5)Fe(0.5)O(3-δ): CO2 stability and microstructure

High oxygen permeability and good thermochemical stability of oxygen-transporting membranes (OTMs) are two main requirements concerning the applicability of these devices in chemical processes, such as CO2 capture using the oxyfuel concept or catalytic membrane reactors. In this work, a single-phase perovskite-type membrane Pr0.6Sr0.4Co0.5Fe0.5O3-δ (PSCF) with 0.6-mm thickness was subjected to periodic thermal cycling in the temperature range between 850 and 1000 °C in a 1000-h long-term permeation test with pure CO2 as the sweep gas. The results of this long-term permeation operation revealed a stepwise increase in oxygen permeation values at 1000 °C after each thermal cycle, reaching from 1.38 cm(3) (STP) min(-1) cm(-2) in the first cycle to 1.75 cm(3) (STP) min(-1) cm(-2) in the fourth cycle. Furthermore, the membrane showed very good CO2 stability at 900 °C and above. Despite a partial decrease in oxygen permeation fluxes at 850 °C, a steady state of 0.25 cm(3) (STP) min(-1) cm(-2) was reached and maintained for more than 100 h. The newly developed PSCF membrane also exhibited a higher oxygen permeation flux with He and CO2 sweeping at all measured temperatures compared to a similar La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) membrane.

La0.6Sr0.4Co0.2Fe0.8O3−δ hollow fibre membrane performance improvement by coating of Ba0.5Sr0.5Co0.9Nb0.1O3−δ porous layer

The oxygen permeation performance of perovskite La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) hollow fibre membranes was enhanced by surface modification via coating of a Ba_0.5Sr_0.5Co_0.9Nb_0.1O_3−δ (BSCN) porous layer.

Oxidative CO2 reforming of methane in La0.6Sr0.4Co0.8Ga0.2O3-δ (LSCG) hollow fiber membrane reactor

CO2 utilization in catalytic membrane reactors for syngas production is an environmentally benign solution to counter the escalating global CO2 concerns. In this study, integration of a La0.6Sr0.4Co0.8Ga0.2O3-δ (LSCG) hollow fiber membrane reactor with Ni/LaAlO3-Al2O3 catalyst for the oxidative CO2 reforming of methane (OCRM) reaction was successfully tested for 160 h of reaction. High CH4 and CO2 conversions of ca. 94% and 73% were obtained with O2 flux ca. 1 mL·min(-1)·cm(-2) at 725 °C for the 160-h stability test. Surface temperature programmed desorption studies of the membrane were conducted with H2, CO, and CO2 as probe gases to facilitate understanding on the effect of H2 and CO product gases as well as CO2 reactant gases on the membrane surface. Scanning electron microscopy-energy dispersive X-ray (SEM-EDX), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) analysis of the postreacted membrane after 160-h stability tests suggests Sr-enriched phases with the presence of adsorbed carbonate and hydrogenated carbon. This shows the subsequent reactant spillover on the membrane surface from the catalyst bed took place due to the reaction occurring on the catalyst. However, XRD analysis of the bulk structure does not show any phase impurities, thus confirming the structural integrity of the LSCG hollow fiber membrane.

A novel cobalt-free, CO2-stable, and reduction-tolerant dual-phase oxygen-permeable membrane

A novel CO2-stable and reduction-tolerant Ce0.8Sm0.2O(2-δ)-La0.9Sr0.1FeO(3-δ) (SDC-LSF) dense dual-phase oxygen-permeable membrane was designed and evaluated in this work. Homogeneous SDC-LSF composite powders for membrane fabrication were synthesized via a one-pot combustion method. The chemical compatibility and ion interdiffusion behavior between the fluorite phase SDC and perovskite phase LSF during the synthesis process was studied. The oxygen permeation flux through the dense dual-phase composite membranes was evaluated and found to be highly dependent on the volume ratio of SDC and LSF. The SDC-LSF membrane with a volume ratio of 7:3 (SDC70-LSF30) possessed the highest permeation flux, achieving 6.42 × 10(-7) mol·cm(-2)·s(-1) under an air/CO gradient at 900 °C for a 1.1-mm-thick membrane. Especially, the membrane performance showed excellent durability and operated stably without any degradation at 900 °C for 450 h with helium, CO2, or CO as the sweep gas. The present results demonstrate that a SDC70-LSF30 dual-phase membrane is a promising chemically stable device for oxygen production and CO2 capture with sufficiently high oxygen permeation flux.