Mixed Ionic-electronic Conducting Membrane Research Articles

BaxSr1-xCoyFe1-yO3-δ (BSCF) mixed ionic-electronic conducting (MIEC) materials for oxygen separation membrane and SOFC applications: Insights into processing, stability, and functional properties

Cubic perovskite BaxSr1-xCoyFe1-yO3-δ (BSCF), with x = 0.5 and y = 0.8, is one of the oxygen permeable mixed ionic-electronic conducting (MIEC) membrane materials having the highest oxygen permeation flux reported. The material has potential for high-temperature electrochemical applications such as oxygen separation membrane and cathode for Solid Oxide Fuel Cells (SOFCs). The material has been highly studied, focusing on the oxygen transport properties, thermochemical expansion, mechanical properties, high temperature creep behavior, chemical sealing, etc. Therefore, a review article that consolidates the existing knowledge is necessary. The available review articles have focused on mechanical properties and applications as cathode for intermediate temperature (IT) SOFCs. Hence, this review article has covered additional information about crystal structure and oxygen non-stoichiometry of BSCF, material synthesis, membrane fabrication, thermomechanical stability, phase stability with respect to temperature and oxygen partial pressure, mechanical properties as well as the performance analysis in terms of oxygen permeation flux and failure analysis. It also emphasizes the potential of BSCF as MIEC membrane material. The content is equally helpful for researchers who are newcomers to the field and have the intention to provide a basic understanding of MIEC membrane materials.

Thermochemical solar-driven reduction of CO2 into separate streams of CO and O2 via an isothermal oxygen-conducting ceria membrane reactor

• Demonstration of single-step CO 2 thermolysis in a novel solar-driven membrane reactor. • In-situ separation of CO and O 2 via lattice oxygen transfer through ceria dense membrane. • Continuous and isothermal operation of the MIEC tubular membrane reactor at 1450–1550 °C. • Increasing the temperature, CO 2 concentration or flow rate enhanced CO and O 2 production rates. • The fuel production rate was up to 4.26 μmol/cm 2 /min at 1550 °C under pure CO 2 feeding. CO 2 single-step thermolysis was achieved using oxygen permeable MIEC (mixed ionic-electronic conducting) membranes made of ceria for separate production of CO on the feed side and O 2 on the sweep side. The CO 2 -dissociation reaction was driven by concentrated solar energy as a renewable thermal energy source and by applying a chemical potential gradient between both membrane sides. A continuous oxygen transfer across the membrane was achieved thanks to a flow of inert gas on the permeate side. This created the required oxygen partial pressure gradient and favored oxygen permeation via oxygen ion diffusion through the ceria membrane thickness. A novel solar chemical reactor integrating the reactive ceria membrane was designed and tested under real concentrated solar radiation, with operating temperatures up to 1550 °C. The reactive part of the tubular redox membrane was located inside a well-insulated cavity receiver for homogeneous heating, which was fed with a carrier argon flow on the sweep side to facilitate the transport and removal of the permeated oxygen. The dynamic response of the solar fuel production upon changing the operating conditions (temperature, CO 2 mole fraction, and feed gas flow rate) in the membrane reactor was investigated by quantifying the evolved gas production rates. Continuous CO 2 dissociation was achieved on the feed side inside the tubular membrane with in-situ spatial separation of O 2 and CO streams across the membrane. Reliable solar membrane reactor operation under real concentrated sunlight was successfully demonstrated for the first time, with stable and unprecedented CO production rates up to 0.071 μmol/cm 2 /s at 1550 °C and CO/O 2 ratio of 2.

Inhibiting in situ phase transition in Ruddlesden-Popper perovskite via tailoring bond hybridization and its application in oxygen permeation

Inhibiting in situ phase transition in Ruddlesden-Popper perovskite via tailoring bond hybridization and its application in oxygen permeation

Universally applicable kinetic model for mixed ionic-electronic conducting membranes

Kinetic model is significant for disclosing the oxygen permeation process through mixed ionic-electronic conducting (MIEC) membranes. However, current kinetic models are only applicable for certain materials. As a result, it is difficult to select a proper model to compare kinetic parameters of different membranes. In this paper, based on the dependence of oxygen vacancy concentration on oxygen partial pressure in the bulk of MIEC membrane, an extending of Zhu′s model was established for extensive MIEC membranes by revising one assumption in the original model. La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) membranes with different thickness and coated/uncoated LSCF catalyst layers and La0.7Sr0.3CoO3-δ (LSC) membrane were chosen to verify the correctness of the extended model. The fitting results of LSCF and LSC are reasonable and exclusive compared to the original model and Xu-Thomson′s model. In a word, the extended model can be applied to various MIEC materials.

Status of CO<sub>2</sub>-stable dual-phase mixed conductor oxygen permeable membrane materials

Dense ceramic mixed ionic-electronic conducting membranes (MIECMs) are renowned for their rich and varied properties as admirable candidates in energy industries, such as oxygen separation from air, cathode materials for solid fuel oxide cells (SOFCs) or membrane reactors for partial oxidation of light hydrocarbons. Especially, the MIECMs integrated oxy-fuel combustion is considered to be a promising cost-effective and efficient method for CO2 capture and sequestration. During the oxy-fuel process, high purity oxygen production has been put forward to use OTMs technology instead of the conventional cryogenic technique, which can benefit from low costs and energy consumption and high efficiency. According to the phase numbers, the OTMs can be classified into single phase and composite OTMs. Extensive research activities have been directed towards the single phase perovskite-type oxides due to their high oxygen permeability. However, their industrial practical application was limited since they suffer from deactivation in the presence of CO2 due to alkaline earth carbonates forming on their surfaces. In order to overcome these issues, focus has shifted to the development of dual-phase membranes consisting of an oxygen ion conducting phase and an electron ion conducting phase, which benefit from the contribution of each phase and overcome the limitations of perovskite materials. Dual-phase OTM typically composed of an oxygen conducting phase (OCP) and an electronic conducting phase (ECP), which displays a better structural and mechanical stability comparing with the single phase OTM under reducing atmosphere due to the incorporation of fluorite phase. The first generation dual-phase OTMs are usually made from the fluorite oxides and noble metals, where fluorite oxides are used as the OCP and noble metals are adopted as the ECP. However, the noble metal-containing dual-phase OTM is too expensive. In order to lower the costs, researchers have developd the second generation dual-phase OTMs, which are consisted of fluorite oxides and perovskite oxides. Although the costs have been reduced, reactions between two phases were usually observed in the second generation dual-phase OTMs. Based on the previous findings and driven by the demands for oxy-fuel combustion for CO2 capture, research put forwards to the new CO2-stable dual-phase OTMs without noble metals and alkaline earth elements. Until now, already a large number of CO2-stable dual-phase membrane materials have been explored. However, their CO2 stabilities have been enhanced, but most of their oxygen permeabilities are too low for practical applications. Therefore, continues research efforts have to be made since most of available OTMs materials can′t satisfy the practical applications. In order to enhance the oxygen permeability and/or improve material thermochemical stability of dual-phase OTMs, the large majority of studies focused on tuning the composition through doping, synthesized temperatures, modified the membrane surfaces with coating or development of new synthesis methods. Meanwhile, considerable theoretical efforts have been put into the dual-phase OTMs, a number of models for permeation process such as Bouwmeester model, Xu-Thomson model, and Zhu model et al. are proposed. Up to now, countless publications has been produced in this field. In order to give the guidelines for the development of the dual-phase OTM materials and promote the process of industrialization of the OTMs technology in the oxy-fuel process, we summarize the status of CO2-stable dual-phase mixed conductor oxygen permeable membrane materials. In this review, the oxygen permeability mechanism and research progress of the dual-phase OTM materials are briefly introduced firstly. Subsequently, the influence of the preparation methods, sintering temperatures, ratios of two phases and the composition of the two phases on the oxygen permeability and stability are analyzed systematically. Next, the applications of dual phase MIECM reactor in partial oxidation of methane to syngas (POM), coupling reactions, water decomposition and oxy-fuel combustion are introduced and analyzed. Finally, the existing scientific problems and the possible research directions have been pointed out.

Accelerated Oxygen Mass Transfer in Copper and Vanadium Oxide-Based MIEC-Redox Membrane

Mixed ionic-electronic conducting (MIEC) membranes have a great potential for use in gas separation technology in the production of pure oxygen for metallurgy, micro- and nanoelectronics, biotechnology, pharmaceutics, etc. Before this happens, an important issue has to be addressed. This issue involves the design of novel MIEC membranes with enhanced oxygen transport. Recently, a concept of thick two-layer solid/liquid composite MIEC-Redox membranes with accelerated diffusion-bubbling oxygen transfer was proposed (see Belousov and Fedorov in ACS Appl Mater Interfaces 10:21794–21798, 2018). These diffusion-bubbling membranes could serve for efficient oxygen separation from air. However, there is no enough fundamental understanding of the mass transport processes involved. This understanding is important for evaluating the potential of MIEC-Redox membranes for practical application. The presented article gives a theory of oxygen mass transfer in two-layer copper and vanadium oxide-based MIEC-Redox membranes. The theory describes both the transport of oxygen ions in the outer layer of the membrane and the nucleation, growth, and transport of oxygen gas bubbles in the inner layer of the membrane. Kinetic regularities of oxygen mass transfer are established. The ways to enhance the productivity of MIEC-Redox membranes are outlined.

Asymmetric dual-phase MIEC membrane reactor for energy-efficient coproduction of two kinds of synthesis gases

An asymmetric 75 wt% Sm0.15Ce0.85O1.925-25 wt% Sm0.6Sr0.4Al0.3Fe0.7O3-δ (SDC-SSAF) dual-phase mixed ionic-electronic conducting (MIEC) oxygen-permeable membrane reactor was applied to coproduce ammonia synthesis gas (ASG, H2/N2 = 3) and liquid fuels synthesis gas (LFSG, H2/CO = 2). The effects of CH4 concentration, CH4 flow rate, steam flow rate and temperature on the performance of the membrane reactor were studied. The SDC-SSAF membrane reactor showed an excellent performance for the coproduction of ASG and LFSG. An ASG production rate of 20.7 mL cm−2 min−1, a LFSG production rate of 51.0 mL cm−2 min−1 and an oxygen permeation rate of 9.1 mL cm−2 min−1 were achieved at 925 °C. Compared with traditional industrial processes, the energy saving of this membrane reactor process is expected as high as 66.5%. The post-mortem of the membrane reactor using scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) characterization revealed that the membrane has an excellent structural stability under operation condition.

Towards improved partial oxidation product yield in mixed ionic-electronic membrane reactors using CSTR and CFD modelling

Mixed ionic-electronic conducting (MIEC) membrane reactors are attractive for partial oxidation and oxidative coupling of methane (OCM) because of their ability to separate oxygen from air with a low energetic penalty, and introduce the oxygen into a reaction zone with spatial and temporal control. To facilitate design and optimization of such reactors, two numerical models for MIEC membrane reactors for methane reforming are introduced: a computationally inexpensive, coupled, two-chamber CSTR model, and a CFD model for reacting flow. The CFD model considers diffusive transport with a mixture averaged model, and was shown to agree well with previous experimental and modelling works. Both models utilize a detailed gas-phase chemical kinetic mechanism and a membrane oxygen permeation model that considers the local oxygen concentration. It is demonstrated that the CSTR model can be used to evaluate a large number of reactor parameters efficiently to identify optimal conditions for OCM. The most promising reactor operating points identified using the CSTR for a particular reactor size are further considered with spatial detail using the CFD model. It is shown that the coupling between oxygen permeation, gas phase chemical kinetics, and flow field can have a significant impact on the reactor performance. Here the flow direction in a typical button reactor was reversed, which resulted in a 40% increase in predicted C2 yield. The increase is attributed to the shorter post-reaction residence time of the C2 products in the reactor, thereby inhibiting deeper oxidation. This significant impact on the reactor performance can be utilized to improve the yield of partially oxidized products from membrane reactors.

Hydrogen and Ethylene Production through Water-Splitting and Ethane Dehydrogenation Using BaFe0.9Zr0.1O3- δ Mixed-Conductors

Hydrogen production from water-splitting has attracted significant interest because of its use in refining, chemicals’ production and as an alternative fuel. A promising technology for hydrogen production through water-splitting at moderate temperatures is the use of mixed ionic-electronic conducting (MIEC) membranes [1-2]. Using an inert gas on the oxygen-lean side, oxygen permeation rates are slow unless vacuum is used (or higher pressure on the feed side). Fuel addition in the oxygen-lean stream raises the oxygen chemical potential difference and hence the oxygen permeation and hydrogen production rate increase significantly [1-5]. One such fuel is ethane whose partial dehydrogenation leads to a valuable chemical, namely ethylene. Coupling water-splitting and ethane dehydrogenation using a MIEC membrane can reduce the complexity and capital cost of producing both (process intensification). This study investigated the co-production of hydrogen and ethylene using BaFe0.9Zr0.1O3- δ membranes. Experimental measurements performed in a button-cell reactor showed significant oxygen permeation, ethane conversion and selectivity to ethylene. The performance of a 1.1 mm thick membrane operating at inlet XH2O=50% at the steam side (balance is nitrogen) was investigated as a function of temperature and inlet ethane mole fraction at the oxygen-lean side (balance is helium). At T=900 °C and XC2H6=10%, the oxygen permeation flux (JO2) was ≈ 2.0 μmole/cm2/sec, while the ethane conversion and selectivity to ethylene were 95% and 83%, respectively. At these conditions, the combination of gas-phase and surface reactions lead to the production of other products, such as hydrogen, methane, acetylene, carbon monoxide and carbon dioxide. When using ethane, the oxygen permeation through BaFe0.9Zr0.1O3- δ increases due to electrochemical reactions of these products with oxygen ions on the membrane surface, while the electron transfer process takes place through a redox mechanism that involves iron and its different oxidation states [5]. Lowering the temperature to T=850 °C decreased the oxygen permeation flux to ≈ 1.0 μmole/cm2/sec and conversion of ethane to 79% while ethylene selectivity increased to 93%. Under long-term operation, BaFe0.9Zr0.1O3- δ shows good stability. To further increase the performance of the material, we investigated the limitations imposed by surface reactions and charged species diffusion in an effort to identify the rate-limiting step in the overall oxygen permeation process.

Unprecedented Perovskite Oxyfluoride Membranes with High-Efficiency Oxygen Ion Transport Paths for Low-Temperature Oxygen Permeation.

Unprecedented perovskite oxyfluoride membranes, a new generation of mixed ionic-electronic conducting (MIEC) membranes, feature extraordinary performance for low-temperature oxygen permeation, which transcend the performance of state-of-the-art MIEC membranes and fulfil commercial requirements. These results provide important progress for MIEC membranes and will potentially open the door to exploring high-performance MIEC compounds.

Stability of sulfate doped SrCoO3−δ MIEC membrane

A sulfate doped SrCoO3−δ (SC) mixed ionic-electronic conducting (MIEC) oxide, SrCo0.97S0.03O3−δ (SCS), was prepared to study the dependence of its stability on the incorporation of sulfate group into perovskite lattice. The high-temperature cubic perovskite phase can be stabilized down to room temperature by replacing 3% of cobalt-sites with sulfur. However, permeation experiments show that oxygen permeation fluxes through SC and SCS membranes experienced similar degradation processes at 850°C. Hysteresis loops of oxygen permeation fluxes appear on the both membranes indicate a phase transition from cubic to hexagonal during cooling/heating circle of oxygen permeation tests. The results of permeation experiments, annealing treatments in an air or helium atmosphere and the EDS analysis show that more SrSO4 particles are enriched on the sweep side surface. A mechanism analysis on the sulfate group migration from the membrane bulk to surfaces was proposed to help understanding the experimental results. It is concluded that the sulfate group can migrate outside of perovskite lattice and reach to the gas–solid interfaces, so the stabilization function of sulfate to the cubic perovskite phase loses easily as the sulfate doped perovskite membrane encountering annealing treatments or oxygen partial pressure gradients.

(Invited) The Role of Solid-Gas Electrochemical Interfaces for Mixed Ionic Electronic Conducting Oxygen Transport Membranes

Oxygen transport membranes are of great interest for different applications requiring oxygen due to their high efficiency and infinite selectivity. Potential membrane materials are mixed ionic-electronic conducting (MIEC) ceramics enabling oxygen transport by ion diffusion without creating a voltage due to the electrical conductivity. High performance membranes are developed consisting of thin layers on a porous support. Oxygen transport through such asymmetric membranes can be considered as a series of different processes, each of which can be rate limiting depending on operation conditions. One major resistance is concentration polarization in the support pores, which can be eliminated experimentally by using pure oxygen on the support side of the membrane. By doing so the role of oxygen surface exchange in thin perovskite layers is investigated. Different modelling approaches are described. Finally dual phase MIEC membranes, in which oxygen ions and electrons are conducted through separate phases, are briefly discussed.

Mixed ionic-electronic conducting (MIEC) membranes for hydrogen production from water splitting

Hydrogen as a clean energy carrier for power generation through fuel cells has attracted much attention. Water is a suitable source for hydrogen production by the splitting reaction. Significant amount of hydrogen can be produced at moderate temperature if a mixed ionic-electronic conducting (MIEC) membrane is used to remove the produced oxygen from water splitting reaction, although the equilibrium constant for water splitting is small. In this review, the principle of hydrogen production via water splitting in MIEC membrane reactor is illustrated, and the factors affecting the hydrogen production rate (such as membrane materials, thickness, modification of membrane surface) and the stabilities of membranes under high oxygen chemical potential gradient are discussed. Furthermore, following the concept of process intensification, the researches on water splitting coupling with other catalytic reactions in membrane reactors are summarized.

Current developments of mixed conducting membranes on porous substrates

The fabrication of mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen separation has extensively increased in the last three decades as a promising alternative of clean energy delivery. In recent years, the interest on supported MIEC membranes has increased due to their attractive properties such as higher mechanical strength and oxygen flux compared to self-supported membranes. This work presents a literature review on the development of supported MIEC membranes of perovskite structure. The concepts and transport mechanism of those membranes are explained and recent works on supported MIEC membranes are presented. Finally, manufacturing methods of self-supported membranes and their influence on oxygen permeation are discussed.

Rare Earth‐doped Ceria Catalysts for ODHE Reaction in a Catalytic Modified MIEC Membrane Reactor

AbstractAn intensification process for the selective oxidation of hydrocarbons integrates a catalytic reactor and an oxygen separation membrane. This work presents the study of oxidative dehydrogenation of ethane at 1123 K in a catalytic membrane reactor based on mixed ionic‐electronic conducting (MIEC) membranes. The surface of a membrane made of Ba0.5Sr0.5Co0.8Fe0.2O3−δ has been activated using different porous catalytic layers based on rare earth‐doped cerias (fluorite structure) and the porous catalytic coating was deposited by screen printing (coating around 15 μm). The different catalyst formulations were developed by partial substitution of Ce and were synthesized by co‐precipitation followed by cobalt impregnation when required. Specifically, seven different catalysts based on the system Ce1−xLnxO2−δ (x=0.1 or 0.2; Ln=Tb, Pr, Er, Gd, and Tb+Er), including the effect of cobalt addition (2 % molar) in Ce0.8Tb0.2O2−δ, were studied. The ceria catalysts were studied by XRD, SEM, DC‐conductivity as a function of oxygen partial pressure, and the high‐temperature stability in a CO2 environment was assessed using thermogravimetry. Then, the influence of the ceria catalytic coating on the oxygen permeation flux through the MIEC membrane was studied using argon and methane as the sweep gas in the permeate side. Finally, oxidative dehydrogenation of ethane reaction tests were performed at 1123 K, as a function of the ethane concentration in the feed. The use of a disk‐shaped membrane in the reactor made it possible to prevent the direct contact of gaseous oxygen and hydrocarbons and thus to increase the ethylene yield. High ethylene yields (up to ≈84 %) were obtained using a catalytic coating based on 20 % Tb‐doped ceria including macropores produced by the addition of graphite platelets in the screen printing ink. The high yields obtained in this kind of catalytic membrane are attributed to the combination of: the high catalytic activity; the control of the oxygen concentration in the gas phase (reaction chamber); and the appropriate fluid dynamics, enabling the fast ethylene evacuation.

Development of CO2 Protective Layers by Spray Pyrolysis for Ceramic Oxygen Transport Membranes

AbstractCeramic mixed ionic–electronic conducting (MIEC) membranes enable very selective oxygen separation from air at high temperatures. Two major potential applications of oxygen‐transport membranes are: i) oxygen production for oxyfuel power plants, and, ii) integration within high‐temperature catalytic membrane reactors for methane or alkane upgrading by selective oxidative conversions. However, these applications involve contact with carbon‐bearing atmospheres and most state‐of‐the‐art highly permeable MIEC membranes do not tolerate operation under CO2‐rich environments due to carbonation processes. The present contribution shows our first attempts in the development of ceria‐based protective thin layers on monolithic LSCF membranes.Gd‐doped ceria (CGO) deposition is carried out by air blast spray pyrolysis on mirror‐polished LSCF disc membranes. The layer thickness is maintained below 0.4 μm in order to prevent the formation of cracks during thermal cycling and minimize limitations caused by the reduced oxygen permeability through the ceria layer. After optimization of the spraying process, smooth crack‐free dense coatings are obtained with high crystallinity in the as‐deposited state. The layers are characterized by XRD, SEM, AFM, DC‐conductivity measurements, interferometry and optical microscopy. Oxygen separation is studied on coated LSCF using air as the feed and argon/CO2 mixtures as the sweep gas in the temperature range 650–1000°C. The protected membrane exhibits a higher stability than the uncoated LSCF membrane, although the nominal oxygen flux is slightly reduced at temperatures below 850°C due to the limited ambipolar conductivity of doped ceria in the range of oxygen partial pressures investigated. Moreover, the protective layer (250 nm thickness) remains stable after the permeation testing.

Reduction-Tolerant Oxygen-Permeable Perovskite-Type Oxide Sr0.7Ba0.3Fe0.9Mo0.1O3−δ

The mixed ionic-electronic conducting (MIEC) materials that have been developed for catalytic membrane reactors are vulnerable to a reducing atmosphere, especially H2. We reported on a MIEC oxide, Sr0.7Ba0.3Fe0.9Mo0.1O3−δ (SBFM), with a cubic perovskite structure (a = 3.91 Å at room temperature). SBFM can maintain full perovskite structure in a 5 vol % H2/He atmosphere at 900 °C even for 50 h. The high reduce-tolerant property was mainly attributed to the synergistic effect between Mo(VI)/Mo(V) and Fe3+/Fe2+ couples. SBFM also exhibited good tolerance to a CO2 atmosphere at high temperature (≥900 °C) but reacted with CO2 at relatively low temperature (<800 °C). The introduction of Mo ion into the Fe site greatly improved the thermal stabilization of the SBFM oxide. The average thermal expansion coefficients in nitrogen atmosphere were 12.8 × 10−6 and 20.8 × 10−6 °C −1 in the temperature ranges of 200−600 and 600−900 °C, respectively. At 900 °C, the oxygen permeation flux of SBFM membrane (1 mm in thickness) was 4.39 × 10−7 mol cm−2 s−1. The oxygen permeation activation energy (Ea) was 42 kJ mol−1, which was significantly lower than that of commonly used MIEC membranes.

Oxidative coupling of methane (OCM) in a catalytic membrane reactor and comparison of its performance with other catalytic reactors

The oxidative coupling of methane (OCM) was studied in a catalytic membrane reactor (CMR), catalyst packed bed reactor (PBR) and catalyst packed bed membrane reactor (PBMR) respectively. The CMR consists of a mixed ionic-electronic conducting membrane (MIECM) Ba0.5Ce0.4Gd0.1Co0.8Fe0.2O3−δ (BCGCF) coating on the outer surface of ceramic tubular support using sol–gel method. The inner surface of the membrane tube was coated with 3 components (Na-W-Mn) catalyst using mixture slurry dip coating method. Prior to the OCM reaction, separation of oxygen from the mixture of nitrogen and oxygen similar to air composition was performed in the membrane reactor. In the absence of chemical reaction, the oxygen permeation flux was 0.6 cm3/min cm2 at 900 °C with sweep gas flow rate 100 cm3/min. The oxygen permeation flux increased significantly during the oxidative coupling of methane reaction. 67.4% of C2+ selectivity and methane conversion of 51.6% with oxygen permeation flux 1.4 cm3/min cm2 was obtained at 850 °C with sweep gas flow rate of 100 cm3/min. The performances of PBR and PBMR using Na-W-Mn/SiO2 were compared with the performance of CMR. The catalytic membrane reactor performed best among three reactors with C2+ yield of 34.7%. The deterioration of the catalytic membrane performance after reaction was investigated by SEM and XRD analyses.

Directional dependence of oxygen ionic transport in a tubular MIEC membrane

Surface exchange and bulk diffusion resist ionic oxygen transport through a dense mixed ionic-electronic conducting (MIEC) membrane. In the regime of mixed resistances, the outwards oxygen ionic flow rate per unit length from the tube to the shell (F out) is predicted to exceed that from the shell to the tube (F in) for equivalent rich and lean oxygen partial pressures. Numerical simulations for typical parameter values predict enhancement factors (F out / F in) of about 1.05. This directional dependence is not predicted to occur when either the surface of exchange or bulk diffusion are the controlling resistance. Experimental data are in qualitative agreement with the theoretical predictions. The observed ratio between the outwards and inwards oxygen ionic flow at 1000°C through a La 0.5Sr 0.5Fe 0.8Ga 0.2O 3-δ (LSFG) membrane tube was in the range of 1.14 to 1.38.