Selection of oxygen permeation models for different mixed ionic‐electronic conducting membranes

Permeation data of several mixed ionic‐electronic conducting (MIEC) membranes were analyzed by two oxygen permeation models (i.e., Zhu's model and Xu–Thomson's model), respectively, to find a concise method to guide the choice of permeation models. We found that Zhu's model can well fit the permeation data of perovskite‐type membranes, like Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF) and BaCe_0.05Fe_0.95O_3‐δ (BCF), and dual‐phase membranes, like 75 wt % Ce_0.85Sm_0.15O_1.925–25 wt % Sm_0.6Sr_0.4Al_0.3Fe_0.7O_3‐δ (SDC‐SSAF), whose oxygen vacancy concentrations are almost independent of the oxygen partial pressure at elevated temperatures. However, Zhu's model was not appropriate for membranes whose oxygen vacancy concentration changed obviously with oxygen partial pressure at elevated temperatures, such as La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ (LSCF) and La_0.7Sr_0.3CoO_3‐δ (LSC). On the contrary, Xu–Thomson's model can fit the data of LSCF and LSC well, but it is inapplicable for BSCF, BCF, and SDC‐SSAF. Therefore, the dependence of oxygen vacancy concentration on oxygen partial pressure was suggested as an index for the selection of the permeation models. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4043–4053, 2017     

Degradation mechanism analysis of Ba0.5Sr0.5Co0.8Fe0.2O3‐δ membranes at intermediate‐low temperatures

The degradation of the permeation flux of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ membranes has typically been attributed to the phase transformation of the material at intermediate temperatures. In this study, the effect of the interfacial oxygen exchange steps was considered to give an overall view of the degradation mechanism. The changes in the interfacial exchange resistances, bulk resistance, and morphologies of the membranes were investigated via physical characterizations and a permeation model. The interfacial oxygen exchange resistances increased more quickly with time than bulk resistance. Meanwhile, BaSO₄ particles were detected on both surfaces of the membranes, and their contents reached maximum at 650°C. However, after the membrane surfaces were coated by Sm_0.5Sr_0.5CoO_3‐δ porous layers, the interfacial oxygen exchange resistances kept constant and the degradation rates were slowed down. The degradation was predominated by the increase of interfacial oxygen exchange resistances induced by the enrichment of BaSO₄ particles on membrane surfaces. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3879–3888, 2015     

Co‐firing technology for the preparation of asymmetric oxygen transporting membranes based on BSCF and Zr‐doped BSCF

Asymmetric tubular membranes with a length of 450 mm were prepared in one step by co‐firing of a green support coated by slurry. BSCF (Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ) and Zr‐doped BSCF3Zr (Ba_0.5Sr_0.5(Co_0.8Fe_0.2)_0.97Zr_0.03O_3‐δ) were used for the separation layer as well as for the porous support, latter one together with PMMA microspheres as a pore forming agent. The gas leakage at room temperature was below 0.003 mL STP/(cm²·min). Oxygen fluxes up to 12 mL STP/(cm²·min) were observed at 900°C in vacuo operation mode. The oxygen flux increased with growing driving force but the slope of the curve flattened at higher driving forces probably caused by limiting surface exchange and pressure losses inside the porous support. The oxygen permeation of asymmetric BSCF tubes was slightly higher compared to BSCF3Zr and exceeded the oxygen flux of a monolithic BSCF membrane by a factor of 4 at comparable operation conditions. © 2013 American Institute of Chemical Engineers AIChE J, 60: 15–21, 2014     

Permeation model and experimental investigation of mixed conducting membranes

A simple oxygen permeation model was developed based on the theoretical analysis of the role of interfaces of mixed conducting membranes. The developed model equations contain three resistance constants, which can be determined by correlating oxygen permeation flux to oxygen partial pressure on each side. A series of experimental measurements of oxygen fluxes of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ membranes over a wide range of temperature and oxygen partial pressures were tested for the regression of three resistance constants with good correlation (R > 0.997). With this model, the interfacial exchange resistances of each side can be well distinguished from the bulk‐diffusion resistance under a wide‐temperature range. The kinetics parameters, including interfacial exchange coefficients on each side and ionic diffusion coefficient, can be obtained through the three resistance constants. Parametric studies can predict the influences of membrane thickness, oxygen partial pressures on oxygen flux, distribution of permeation resistances, and characteristic thickness. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1744–1754, 2012     

A permeation model study of oxygen transport kinetics of BaxSr1-xCo0.8Fe0.2O3-δ

Ba_xSr_1-xCo_0.8Fe_0.2O_3-δ (BSCF) materials with different Ba doping amounts are widely applied as membranes for oxygen separation and electrodes in solid oxide cells. Different opinions were put forward about the effect of Ba contents on the oxygen transport kinetics. In this work, an oxygen permeation model was first used to investigate the effect of Ba content in BSCF (x = 0, 0.1, 0.3, 0.5, 0.7) on the oxygen transport kinetics. The permeation resistance constants were obtained separately through fitting the condition experimental data to the model. With the increase of Ba content, the interfacial oxygen exchange resistance constants of both sides decrease quickly and then increase slightly, while bulk diffusion resistance remains monotone decreasing. The improvement of oxygen permeation flux is mainly contributed by the enhancement of interfacial oxygen exchange kinetics. It is inferred that Ba element may play an important role in the interfacial oxygen reactions.     

Enhancing oxygen permeation via multiple types of oxygen transport paths in hepta‐bore perovskite hollow fibers

The multiple types of efficient oxygen transport paths were demonstrated in high‐mechanical‐strength hepta‐bore Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ hollow fiber membranes. These types of paths play a prominent role in enhancing oxygen permeation fluxes (17.6 mL min^?1 cm^?2 at 1223 K) which greatly transcend the performance of state‐of‐the‐art Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ hollow fiber membranes, showing a good commercialization prospect. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4273–4277, 2017     

Evaluation of mixed‐conducting lanthanum‐strontium‐cobaltite ceramic membrane for oxygen separation

In this study, La_0.4Sr_0.6CoO_3‐δ (LSC) oxide was synthesized via an EDTA‐citrate complexing process and its application as a mixed‐conducting ceramic membrane for oxygen separation was systematically investigated. The phase structure of the powder and microstructure of the membrane were characterized by XRD and SEM, respectively. The optimum condition for membrane sintering was developed based on SEM and four‐probe DC electrical conductivity characterizations. The oxygen permeation fluxes at various temperatures and oxygen partial pressure gradients were measured by gas chromatography method. Fundamental equations of oxygen permeation and transport resistance through mixed conducting membrane were developed. The oxygen bulk diffusion coefficient (D_v) and surface exchange coefficient (K_ex) for LSC membrane were derived by model regression. The importance of surface exchange kinetics at each side of the membrane on oxygen permeation flux under different oxygen partial pressure gradients and temperatures were quantitatively distinguished from the oxygen bulk diffusion. The maximum oxygen flux achieved based on 1.6‐mm‐thick La_0.4Sr_0.6CoO_3‐δ membrane was ～4.0 × 10^?7 mol cm^?2 s^?1at 950°C. However, calculation results show theoretical oxygen fluxes as high as 2.98 × 10^?5 mol cm^?2 s^?1 through a 5‐μm‐thick LSC membrane with ideal surface modification when operating at 950°C for air separation. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

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 Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF) and La₂NiO_4+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 t^1/4 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.     

Experimental investigation and mathematical modeling of oxygen permeation through dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite-type ceramic membranes

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) perovskite powder was synthesized via EDTA/citrate complexation method. BSCF membranes were formed by pressing powder at 400 MPa and sintering at 1100 °C for 10 h. XRD patterns showed that a high pure powder with cubic structure was obtained. SEM micrographs revealed that the membranes are dense with large grains. Effects of temperature, feed and permeate side oxygen partial pressures, flow rates and membrane thickness on oxygen permeation flux were studied experimentally. A Nernst–Planck based mathematical model, including surface exchange kinetics and bulk diffusion, was developed to predict oxygen permeation flux. Considering non-elementary surface reactions and introducing system hydrodynamics into the model resulted in an excellent agreement (RMSD = 0.0617, AAD = 0.0487 and R² = 0.985) between predicted and measured fluxes. The results showed that oxygen permeation flux increases with temperature, feed side oxygen partial pressure and flow rates, however decreases with permeate side oxygen partial pressure and membrane thickness. Contribution of feed side surface exchange reactions, bulk diffusion and permeate side surface exchange reactions resistances in the total resistance are in the range of 8–32%, 10–81% and 11–59%, respectively. Permeation rate-limiting step was determined using the membrane dimensionless characteristic thickness.     

Single- and dual-phase capillary membranes prepared through plastic extrusion method for oxygen permeation

Two capillary membranes, single-phase Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) and dual-phase 75 wt% Ce_0.85Sm_0.15O_1.925 - 25 wt% Sm_0.6Sr_0.4Cr_0.3Fe_0.7O_3-δ (SDC-SSCF), with dense cross section, were successfully prepared through the plastic extrusion method. The dual-phase capillary membrane shows higher strength compared to the BSCF counterpart, while the two capillary membranes exhibit much higher fracture strength than those of hollow fiber membranes. The oxygen permeation fluxes of both membranes increase with the increase of temperature and flow rate of sweep gas at the ambient pressure, and can be greatly improved by applying high pressures to the feed side. The oxygen permeation flux of BSCF capillary membrane is up to 19.5 mL cm^?2 min^?1 when 0.5 MPa air was applied to the feed side at 900 °C, which is one order of magnitude higher than that of SDC-SSCF capillary membrane. Thus, both capillary membranes have their own advantages and meet applications under different operation conditions.     

Further performance improvement of Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite membranes for air separation

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) is a promising mixed conducting ceramic membrane material for air separation. In this work, BSCF powder was synthesized by a modified Pechini sol–gel technique at relatively lower temperature. The O₂ permeation through a series of BSCF membranes has been tested at different temperatures and various O₂ partial pressure gradients. Theoretical investigation indicated that bulk diffusion and the O₂ exchange reactions on membrane surfaces jointly controlled the O₂ permeation through BSCF membranes with thickness of between 1.1 and 0.75 mm. To further improve the O₂ fluxes, effective efforts are made on membrane thickness reduction and surface modification by spraying porous BSCF layers on both surfaces. When the membrane thickness was reduced from 0.75 to 0.40 mm, the O₂ fluxes were increased by 20–60% depending on the operating conditions. The surface modification further improved the O₂ flux by another 20–40%. The high O₂ fluxes achieved in this work are quite encouraging with a maximum value reaching 6.0 mL min⁻¹ cm⁻² at 900 °C.     

Preparation and oxygen permeation of U‐shaped perovskite hollow‐fiber membranes

The oxygen permeation of dense U‐shaped perovskite hollow‐fiber membranes based on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ prepared by a phase inversion spinning process is reported. The perovskite hollow fibers with totally dense wall were obtained with the outer diameter of 1.147 mm and the inner diameter of 0.691 mm. The dependences of the oxygen permeation on the air flow rate on the shell side, the helium flow rate on the core side, the oxygen partial pressures, and the operating temperatures were experimentally investigated. According to the Wagner theory, it follows that the oxygen transport through the U‐shaped hollow‐fiber membrane is controlled by both surface reaction and bulk diffusion at the temperature ranges of 750–950°C. High oxygen permeation flux of 3.0 ml/(min cm²) was kept for about 250 h at 950°C under the conditions of the air feed flow rate of 150 ml/min and the helium flow rate of 50 ml/min. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Preparation and Oxygen Permeation Properties of BaCrOx Coated Ba0.5Sr0.5Co0.8Fe0.2O3- Tubular Membrane

Barium-chromium oxide (BaCrO_x) coated Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) tubular membranes were successfully prepared and evaluated for oxygen separation applications under high pressure–temperature conditions. The oxygen permeation flux was measured in accordance with the temperature, air pressure, and retentate flow rate, ranging from 750–950°C, 3–9 atm, and 200–1000 mL/min, respectively. The permeation testing on the BaCrO_x coated BSCF tubular membranes showed that the oxygen flux increased as the temperature, pressure, and retentate flow rate increased. The oxygen permeation flux was 5.7 mL/(min cm²) with temperature, pressure, and retentate flow rate of 900°C, 9 atm, and 1000 mL/min, respectively. The temperature dependence of the oxygen permeation process is further investigated, and the Arrhenius pre-exponential factor, as well as the apparent activation energy, is determined.     

Yttrium doping of Ba0.5Sr0.5Co0.8Fe0.2O3-δ part I: Influence on oxygen permeation,electrical properties,reductive stability,and lattice parameters

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) exhibits a very high oxygen permeability in its cubic perovskite phase, making it a promising candidate for high-temperature energy-related applications such as oxygen-transport membranes. It suffers, however, from a pronounced phase instability at application-relevant temperatures below 840?°C which is presumed to result from a valence change of B-site cobalt. In an attempt to stabilize the cubic BSCF phase, monovalent Y³⁺ was doped in small concentrations (1–10?mol-% yttrium) onto its B-site. The influence of this doping on the physico-chemical properties (electrical conductivity, reductive stability, lattice constant), on the sintering behavior, and on the oxygen permeation of BSCF has been systematically investigated. Despite a slightly adverse effect to permeability (decrease in oxygen permeation by about 20–30%), a doping concentration of 10?mol-% Y is found to completely suppress secondary-phase formation and, hence, stabilize the cubic BSCF system at 800?°C. These findings are extremely promising with regard to a long-term operation of BSCF in atmospheres free of acidic impurity gases.     

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 Ba_xSr_1-xCo_yFe_1-yO_3-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.     

A novel porous‐dense dual‐layer composite membrane reactor with long‐term stability

A porous‐dense dual‐layer composite membrane reactor was proposed. The dual‐layer composite membrane composed of dense 0.5 wt % Nb₂O₅‐doped SrCo_0.8Fe_0.2O_3‐δ (SCFNb) layer and porous Ba_0.3Sr_0.7Fe_0.9Mo_0.1O_3‐δ (BSFM) layer was prepared. The stability of SCFNb membrane reactor was improved significantly by the porous‐dense dual‐layer design philosophy. The porous BSFM surface‐coating layer can effectively reduce the corrosion of the reducing atmosphere to the membrane, whereas the dense SCFNb layer permeated oxygen effectively. Compared with single‐layer dense SCFNb membrane reactor, no degradation of performance was observed in the dual‐layer membrane reactor under partial oxidation of methane during continuously operating for 1500 h at 850°C. At 900°C, oxygen flux of 18.6 mL (STP: Standard Temperature and Pressure) cm^?2 min^?1, hydrogen production of 53.67 mL (STP) cm^?2 min^?1, CH₄ conversion of 99.34% and CO selectivity of about 94% were achieved. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4355–4363, 2013     

Partial oxidation of methane to syngas in BaCe0.15Fe0.85O3−δ membrane reactors

Dense planar Ba_0.15Ce_0.85FeO_3−δ (BCF1585) membrane reactors were investigated to produce syngas from methane. Firstly, the membrane itself catalytic activity to methane was investigated using a blank BCF1585 without any catalysts. Then a LiLaNi/γ-Al₂O₃ catalyst was packed on the BCF1585 membrane surface to test the synergetic effects of the membrane and catalyst. It was found that the membrane itself has a poor catalytic activity to methane. The main products are CO₂ and C₂, and methane conversion is low due to the low oxygen permeation flux. However, after the catalyst was packed on the membrane surface, both methane conversion and oxygen permeation flux were greatly improved by the synergetic effect between the membrane and catalyst. Carbon monoxide selectivity reached at 96% with methane conversion of up to 96%. The oxygen permeation flux reached at 3.0 mL/cm² min at 850 °C for a 1.5 mm disk membrane and can effectively be increased by reducing the thickness of the membranes. After operation for 140 h at 850 °C, the used membrane was examined with SEM and EDXS. The results revealed that the decomposition of the membrane materials could not be avoided under such conditions. Oxygen partial pressure gradient across the membranes is suggested as a critical factor to accelerate the kinetic decomposition of the materials.     

Performance control of dead-end tubular membranes fabricated with a modified phase inversion casting method

A modified phase inversion casting method is employed for the formation of dead-end tubular membrane shape in a single step. Ce_0.9Gd_0.1O_1.95-δ (CGO)-Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) composites are applied as the membrane material. Performance of the membrane is optimised by adjusting the processing conditions in the fabrication process. Long finger-like channels, which were found to promote oxygen permeation flux without decrease of the membrane mechanical strength, were obtained in the dead-end tubes by adjusting the ceramic loadings of the casting slurries. Slurries with lower viscosity provided reduced resistance for the channel growth during the phase inversion in water. The performance of the dual-phase membranes with varied CGO:BSCF ratios were compared. The membranes containing more BSCF show higher oxygen permeation fluxes with helium as sweep gas. It was also verified that CGO content played an important role in enhancing the mechanical strength of the CGO-BSCF membranes.     

Oxygen permeability and structural stability of a novel tantalum‐doped perovskite BaCo0.7Fe0.2Ta0.1O3−δ

Dense BaCo_0.7Fe_0.2Ta_0.1O_3?δ (BCFT) perovskite membranes were successfully synthesized by a simple solid state reaction. In situ high‐temperature X‐ray diffraction indicated the good structure stability and phase reversibility of BCFT at high temperatures. The thermal expansion coefficient (TEC) of BCFT was determined to amount 1.02 × 10^?5 K^?1, which is smaller than those of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) (1.15 × 10^?5 K^?1), SrCo_0.8Fe_0.2O_3?δ (SCF) (1.79 × 10^?5 K^?1), and BaCo_0.4Fe_0.4Zr_0.2O_3?δ (BCFZ) (1.03 × 10^?5 K^?1). It can be seen that the introduction of Ta ions into the perovskite framework could effectively lower the TEC. Thickness dependence studies of oxygen permeation through the BCFT membrane indicated that the oxygen permeation process was controlled by bulk diffusion. A membrane reactor made from BCFT was successfully operated for the partial oxidation of methane to syngas at 900°C for 400 h without failure and with the relatively high, stable oxygen permeation flux of about 16.8 ml/min cm². © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Fabrication of lanthanum-based perovskites membranes on porous alumina hollow fibre (AHF) substrates for oxygen enrichment

In this work, two types of lanthanum-based MIEC perovskite oxides, namely La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and La_0.6Sr_0.4Co_0.2Ni_0.8O_3-δ (LSCNi), were deposited onto porous alumina hollow fibre (AHF) substrates and used for oxygen enrichment. Such structure was developed to shorten oxygen ion diffusion distances in dense membranes and simultaneously leading to higher oxygen flux. The perovskite oxides were prepared using Pechini sol-gel method and deposited via a vacuum-assisted technique. The deposition of lanthanum-based membranes onto the outer and inner sides of the porous AHF has been facilitated through numerous microchannels in the AHF substrates. The effects of operating temperature and argon sweep gas flowrate on oxygen permeation flux of lanthanum-based AHF membrane were investigated. The results revealed that the oxygen permeation flux of LSCF-AHF and LSCNi-AHF increased with operating temperatures due to the improvement of bulk diffusion and surface exchange properties after the lanthanum-based perovskite deposition. Higher oxygen flux was observed for LSCNi-AHF as LSCNi possessed balanced oxygen ionic and electronic conductivities as compared to LSCF membranes. Benefitting from improved oxygen activation and vacancy generation properties after Ni substitution into the B-site ion of LSC perovskite, a dramatic increased oxygen fluxes up to 4.5 mL/min·cm² was observed at 950 °C. The present work demonstrated a feasible method for fabricating oxygen transport membrane (OTM) using porous AHF substrates