Oxygen permeation through tape-cast asymmetric all-La0.6Sr0.4Co0.2Fe0.8O3−δ membranes

Multiplying Oxygen Permeability of a Ruddlesden-Popper Oxide by Orientation Control via Magnets

Ruddlesden-Popper-type oxides exhibit remarkable chemical stability in comparison to perovskite oxides. However, they display lower oxygen permeability. We present an approach to overcome this trade-off by leveraging the anisotropic properties of Nd2 NiO4+δ . Its (a,b)-plane, having oxygen diffusion coefficient and surface exchange coefficient several orders of magnitude higher than its c-axis, can be aligned perpendicular to the gradient of oxygen partial pressure by a magnetic field (0.81 T). A stable and high oxygen flux of 1.40 mL min-1  cm-2 was achieved for at least 120 h at 1223 K by a textured asymmetric disk membrane with 1.0 mm thickness under the pure CO2 sweeping. Its excellent operational stability was also verified even at 1023 K in pure CO2 . These findings highlight the significant enhancement in oxygen permeation membrane performance achievable by adjusting the grain orientation. Consequently, Nd2 NiO4+δ emerges as a promising candidate for industrial applications in air separation, syngas production, and CO2 capture under harsh conditions.

Thermal Swing Reduction-Oxidation of Me(Ba, Ca, or Mg)SrCoCu Perovskites for Oxygen Separation from Air

The climate change impact associated with greenhouse gas emissions is a major global concern. This work investigates perovskite compounds for oxygen separation from air to supply oxygen to oxyfuel energy systems to abate these significant environmental impacts. The perovskites studied were Me0.5Sr0.5Co0.8Cu0.2O3−δ (MeSCC) where the A-site substitution was carried out by four different cations (Me = Ca, Mg, Sr, or Ba). SEM analysis showed the formation of small particle (<1 µm) aggregates with varying morphological features. XRD analysis confirmed that all compounds were perovskites with a hexagonal phase. Under reduction and oxidation reactions (redox), Ba and Ca substitutions resulted in the highest and lowest oxygen release, respectively. In terms of real application for oxygen separation from air, Ba substitution as BaSCC proved to be preferable due to short temperature cycles for the uptake and release of oxygen of 134 °C, contrary to Ca substitution with long and undesirable temperature cycles of 237 °C. As a result, a small air separation unit of 0.66 m3, containing 1000 kg of BaSCC, can produce 18.5 ton y−1 of pure oxygen by using a conservative heating rate of 1 °C min−1. By increasing the heating rate by a further 1 °C min−1, the oxygen production almost doubled by 16.7 ton y−1. These results strongly suggest the major advantages of short thermal cycles as novel designs for air separation. BaSCC was stable under 22 thermal cycles, and coupled with oxygen production, demonstrates the potential of this technology for oxyfuel energy systems to reduce the emission of greenhouse gases.

Perovskite oxides for oxygen transport: Chemistry and material horizons

Oxygen permeable membrane, which has the advantages of high separation selectivity, low energy consumption and simple process in oxygen separation, can be used in the fields of environment and energy, such as pure oxygen preparation, fuel cell, oxygen-enriched combustion and chemical reactor for methane catalytic conversion (e.g. partial oxidation of methane, carbon dioxide reforming with methane). New materials and technological development are needed to achieve this target for GHG reformation. In this direction, mixed ionic-electronic conducting (MIEC) oxides based on perovskite oxides are one of the prominent materials for oxygen transport at high temperatures. These compounds were created into solid ceramic membranes with considerable electronic and oxygen ionic conductivity. As a result of the differential partial pressure of oxygen across the membrane, this process enables the ionic transfer of oxygen from the air, providing the driving force for oxygen ion transport. Notably, over the last 40 years, the defect theory has been applied to a wide range of MIEC materials, providing insight into electronic and ionic transport, widely applied to designing catalysts for wastewater treatment and gas purification. However, a critical review by in-depth analysis from the material side on perovskite oxides for oxygen transport is needed. This work evaluates the research community's significant and relevant contributions in the perovskite oxides for gas separation domain over the previous four decades in conjunction with theoretical concepts, which would give rise to the fundamental understanding of the impact of various transitional metal elements on oxygen transport performance and stability in a different atmosphere.

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.

Defect engineering of oxide perovskites for catalysis and energy storage: synthesis of chemistry and materials science

Oxide perovskites have emerged as an important class of materials with important applications in many technological areas, particularly thermocatalysis, electrocatalysis, photocatalysis, and energy storage. However, their implementation faces numerous challenges that are familiar to the chemist and materials scientist. The present work surveys the state-of-the-art by integrating these two viewpoints, focusing on the critical role that defect engineering plays in the design, fabrication, modification, and application of these materials. An extensive review of experimental and simulation studies of the synthesis and performance of oxide perovskites and devices containing these materials is coupled with exposition of the fundamental and applied aspects of defect equilibria. The aim of this approach is to elucidate how these issues can be integrated in order to shed light on the interpretation of the data and what trajectories are suggested by them. This critical examination has revealed a number of areas in which the review can provide a greater understanding. These include considerations of (1) the nature and formation of solid solutions, (2) site filling and stoichiometry, (3) the rationale for the design of defective oxide perovskites, and (4) the complex mechanisms of charge compensation and charge transfer. The review concludes with some proposed strategies to address the challenges in the future development of oxide perovskites and their applications.

Development and Proof of Concept of a Compact Metallic Reactor for MIEC Ceramic Membranes

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-La_0.6Sr_0.4Co_0.2Fe_0.8O_3–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.

Oxygen Transport Membranes for Efficient Glass Melting

Glass manufacturing is an energy-intensive process in which oxy-fuel combustion can offer advantages over the traditional air-blown approach. Examples include the reduction of NO_x and particulate emissions, improved furnace operations and enhanced heat transfer. This paper presents a one-dimensional mathematical model solving mass, momentum and energy balances for a planar oxygen transport membrane module. The main modelling parameters describing the surface oxygen kinetics and the microstructure morphology of the support are calibrated on experimental data obtained for a 30 μm thick dense La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) membrane layer, supported on a 0.7 mm porous LSCF structure. The model is then used to design and evaluate the performance of an oxygen transport membrane module integrated in a glass melting furnace. Three different oxy-fuel glass furnaces based on oxygen transport membrane and vacuum swing adsorption systems are compared to a reference air-blown unit. The analysis shows that the most efficient membrane-based oxyfuel furnace cuts the energy demand by ~22% as compared to the benchmark air-blown case. A preliminary economic assessment shows that membranes can reduce the overall glass production costs compared to oxyfuel plants based on vacuum swing adsorption technology.

Asymmetric LSCF Membranes Utilizing Commercial Powders

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_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) membranes manufactured from custom-made powder, showing an oxygen flux of > 1 mLcm^-2min at 900 °C in air/Ar gradient.

Ice-Templating for the Elaboration of Oxygen Permeation Asymmetric Tubular Membrane with Radial Oriented Porosity

An original asymmetric tubular membrane for oxygen production applications was manufactured in a two-step process. A 3 mol% Y2O3 stabilized ZrO2 (3YSZ) porous tubular support was manufactured by the freeze-casting technique, offering a hierarchical and radial-oriented porosity of about 15 µm in width, separated by fully densified walls of about 2 µm thick, suggesting low pressure drop and boosted gas transport. The external surface of the support was successively dip-coated to get a Ce0.8Gd0.2O2−δ – 5mol%Co (CGO-Co) interlayer of 80 µm in thickness and an outer dense layer of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) with a thickness of 30 µm. The whole tubular membrane presents both uniform geometric characteristics and microstructure all along its length. Chemical reactivity between each layer was studied by coupling X-Ray Diffraction (XRD) analysis and Energy Dispersive X-Ray spectroscopy (EDX) mapping at each step of the manufacturing process. Cation interdiffusion between different phases was discarded, confirming the compatibility of this tri-layer asymmetric ceramic membrane for oxygen production purposes. For the first time, a freeze-cast tubular membrane has been evaluated for oxygen permeation, exhibiting a value of 0.31 ml·min−1·cm−2 at 1000ºC under air and argon as feed and sweep gases, respectively. Finally, under the same conditions and increasing the oxygen partial pressure to get pure oxygen as feed, the oxygen permeation reached 1.07 ml·min−1·cm−2.

Mixed Ionic-Electronic Conduction in NiFe2 O4 -Ce0.8 Gd0.2 O2-δ Nanocomposite Thin Films for Oxygen Separation

NiFe₂ O₄ -Ce_0.8 Gd_0.2 O_2-δ (NFO/CGO) nanocomposite thin films were prepared by simultaneously radio-frequency (RF) magnetron sputtering of both NFO and CGO targets. The aim is the growth of a CO₂ -stable composite layer that combines the electronic and ionic conduction of the separate NFO and the CGO phases for oxygen separation. The effect of the deposition temperature on the microstructure of the film was studied to obtain high-quality composite thin films. The ratio of both phases was changed by applying different power to each ceramic target. The amount of each deposited phase as well as the different oxidation states of the nanocomposite constituents were analyzed by means of X-ray photoelectron spectroscopy (XPS). The transport properties were studied by conductivity measurements as a function of temperature and pO₂ . These analyses enabled (1) selection of the best deposition temperature (400 °C), (2) correlation of the p-type electronic behavior of the NFO phase with the hole hopping between Ni³⁺ -Ni²⁺ , and (3) following the conductivity behavior of the grown composite layer (prevailing ionic or electronic character) attained by varying the amount of each phase. The sputtered layer exhibited high ambipolar conduction and surfaceexchange activity. A 150 nm-thick nanograined thin film was deposited on a 20 μm-thick Ba_0.5 Sr_0.5 Co_0.8 Fe_0.2 O_3-δ asymmetric membrane, resulting in up to 3.8 mL min^-1  cm^-2 O₂ permeation at 1000 °C under CO₂ atmosphere.

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.

Elucidation of the Oxygen Surface Kinetics in a Coated Dual-Phase Membrane for Enhancing Oxygen Permeation Flux

The dual-phase membrane has received much attention as the solution to the instability of the oxygen permeation membrane. It has been reported that the oxygen flux of the dual-phase membrane is greatly enhanced by the active coating layer. However, there has been little discussion about the enhancement mechanism by surface coating in the dual-phase membrane. This study investigates the oxygen flux of the Ce_0.9Gd_0.1O_2-δ-La_0.7Sr_0.3MnO_3±δ (GDC 80 vol %/LSM 20 vol %) composite membrane depending on the oxygen partial pressure (P_O2) to elucidate the mechanism of enhanced oxygen flux by the surface modification in the fluorite-rich phase dual-phase membrane. The oxygen permeation resistances were obtained from the oxygen flux as a function of P_O2 using the oxygen permeation model. The surface exchange coefficient (k) and the bulk diffusion coefficient (D) were calculated from these resistances. According to the calculated k and D values, we concluded that the active coating layer (La_0.6Sr_0.4CoO_3-δ) significantly increased the k value of the membrane. Furthermore, the surface exchange reaction on the permeate side was more sluggish than that at the feed side under operating conditions (feed: 0.21 atm/permeate side: 4.7 × 10^-4 atm). Therefore, the enhancement of the oxygen surface exchange kinetics at the permeate side is more important in improving the oxygen permeation flux of the thin film-based fluorite-rich dual-phase membrane. These results provide new insight about the function of the surface coating to enhance the oxygen permeation flux of the dual-phase membrane.

Hierarchy in inorganic membranes

Inorganic membranes usually possess an unsymmetrical cross section showing a hierarchical structure: the ZIF-8 separation layer is deposited on a graded titania support.

Dual-Phase Oxygen Transport Membranes for Stable Operation in Environments Containing Carbon Dioxide and Sulfur Dioxide

Dual-phase membranes are appealing candidates for oxygen transport membranes owing to their unique combination of ambipolar electron-ion transport and endurance. However, O2 separation in industrial environments demands very high stability and effectiveness in the presence of CO2- and SO2-bearing process gases. Here, the composition of dual-phase membranes based on NiFe2O4-Ce(0.8) Tb(0.2)O(2-δ) (NFO-CTO) was optimized and the effective performance of catalytically-activated membranes was assessed in presence of CO2 and SO2. Further insight into the limiting mechanisms in the permeation was gained through electrical conductivity studies, permeation testing in several conditions and impedance spectroscopy analysis. The dual-phase membranes were prepared by one-pot sol-gel method and their permeability increases with increasing fluorite content. An O2 flux of 0.25 (ml min(-1)  cm(-2)) mm at 1000 °C was obtained for a thick self-standing membrane with 40:60 NFO/CTO composition. An in-depth study mimicking typical harsh conditions encountered in oxyfuel flue gases was performed on a 50:50 NFO/CTO membrane. CO2 content as well as SO2 presence in the sweep gas stream were evaluated in terms of O2 permeation. O2 fluxes of 0.13 and 0.09 mL min(-1)  cm(-2) at 850 °C were obtained for a 0.59 mm thick membrane under CO2 and 250 ppm SO2 in CO2 sweep conditions, respectively. Extended periods at work under CO2- and SO2-containing atmospheres revealed good permeation stability over time. Additionally, XRD, backscattered electrons detector (BSD)-SEM, and energy-dispersive X-ray spectroscopy (EDS) analysis of the spent membrane confirmed material stability upon prolonged exposure to SO2.

Nanoparticles at Grain Boundaries Inhibit the Phase Transformation of Perovskite Membrane

The high-energy nature of grain boundaries makes them a common source of undesirable phase transformations in polycrystalline materials. In both metals and ceramics, such grain-boundary-induced phase transformation can be a frequent cause of performance degradation. Here, we identify a new stabilization mechanism that involves inhibiting phase transformations of perovskite materials by deliberately introducing nanoparticles at the grain boundaries. The nanoparticles act as "roadblocks" that limit the diffusion of metal ions along the grain boundaries and inhibit heterogeneous nucleation and new phase formation. Ba0.5Sr0.5Co0.8Fe0.2O3-δ, a high-performance oxygen permeation and fuel cell cathode material whose commercial application has so far been impeded by phase instability, is used as an example to illustrate the inhibition action of nanoparticles toward the phase transformation. We obtain stable oxygen permeation flux at 600 °C with an unprecedented 10-1000 times increase in performance compared to previous investigations. This grain boundary stabilization method could potentially be extended to other systems that suffer from performance degradation due to a grain-boundary-initiated heterogeneous nucleation phase transformations.

The effect of microstructure and surface decoration with K2NiF4-type oxide upon the oxygen permeability of perovskite-type La0.7Sr0.3FeO3−δ hollow fiber membranes

Dense La_0.7Sr_0.3FeO_3−δ (LSF) hollow fiber membranes with two kinds of microstructures (LSF-a and LSF-b) were prepared by the phase inversion/sintering method, and the outside surfaces were modified with K₂NiF₄-type oxide in discontinuous method.