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     

Oxygen transport kinetics of MIEC membranes coated with different catalysts

Oxygen permeation through mixed ionic‐electronic conducting membrane may be controlled by oxygen bulk diffusion and/or oxygen interfacial exchange kinetics. In this article, we chose BaCe_0.05Fe_0.95O_3‐δ (BCF) as a representative to study the oxygen transport resistances of the membrane coated with different porous catalysts, including BCF itself, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF) and Sm_0.5Sr_0.5CoO_3‐δ (SSC). The oxygen transport resistances of bulk, gas‐solid interfaces of feed‐side and sweep‐side of the catalyst‐coated membranes can be separately obtained through a linear regression of experimental data according to an oxygen permeation model. The three resistances of the membrane coated with BCF catalyst are smaller than those of the membrane coated with BSCF and SSC catalysts, although BSCF catalyst itself has the fastest bulk diffusion and interfacial exchange kinetics. The catalytic activities of BSCF and SSC catalysts on BCF membranes are impacted by the transport kinetics of catalysts, microstructure of catalyst layers, and cationic inter‐diffusion between the membrane and catalysts. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2803–2812, 2016     

Tuning Ba0.5Sr0.5Co0.8Fe0.2O3-δ cathode to high stability and activity via Ce-doping for ceramic fuel cells

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) fuel-cell cathode stands out because of its ultrahigh ionic conductivity and excellent electrocatalytic activity, but it is still very subject to instability. Here, a new strategy of Ce doping is proposed to boost the stability and activity of the BSCF cathode. A one-pot combustion method is employed to synthesize (Ba_0.5Sr_0.5)_1–xCe_xCo_0.8Fe_0.2O_3-δ (x=0–0.2) cathodes. Both BSCF and (Ba_0.5Sr_0.5)_0.9Ce_0.1Co_0.8Fe_0.2O_3-δ have a cubic perovskite structure. (Ba_0.5Sr_0.5)_0.8Ce_0.2Co_0.8Fe_0.2O_3-δ shows two phases of cubic perovskite and fluorite ceria. Proper Ce doping can boost the electrical conductivity of BSCF, and can dramatically reduce the polarization resistance of BSCF cathode. Ce doping significantly improved BSCF cathode long-term stability by 160 h. Moreover, ten-percent Ce doping in BSCF highly improves single-cell output performance from 516.33 mW cm^?2 to 629.75 mW cm^?2 at 750 °C. The results reveal that Ce doping as a potential strategy for enhancing the stability and activity of BSCF cathode is promising.     

Effect of microstructure on oxygen permeation of Ba0.5Sr0.5Co0.8Fe0.2O3−δ and SrCo0.8Fe0.2O3−δ membranes

The effect of grain size on oxygen permeation properties of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) and SrCo_0.8Fe_0.2O_3?δ (SCF) membranes was investigated by variation of the dwell time. The membrane microstructure was examined by field-emission scanning microscopy (FE-SEM) and then evaluated using a statistical approach. With longer dwell times the grain growth was stimulated and leaded to grains with a narrower size distribution. The grains of SCF (average size from 11.3 to 19.9 μm) were found to be smaller than those of BSCF (average size from 13.9 to 41.3 μm). The oxygen permeation flux of BSCF membranes was found to be independent of grain size in the range from 24 to 42 μm. However, membranes with smaller grains (13.9 μm) show a decreased oxygen permeation flux. For the SCF membranes a decrease in permeation flux with larger grains was observed for average grain sizes between 11.3 and 19.9 μm. By transmission electron microscopy (TEM) formation of an oxygen ordered SrCo_0.8Fe_0.2O_2.5 brownmillerite by-phase could be observed at the oxygen-depleted sweep side of the membrane.     

Fabrication of BSCF-based mixed oxide ionic-electronic conducting multi-layered membrane by sequential electrophoretic deposition process

The Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF)-based multi-layered oxygen separation membrane was fabricated by the sequential electrophoretic deposition (EPD) process. A thin porous/dense bi-layer of BSCF was formed on a thick porous support of BSCF. The porous support prepared by a sacrificial template method using BSCF powder mixed with wheat starch (30 wt%) as a pore-forming agent, followed by uniaxial pressing and low-temperature sintering, was directly used as an EPD electrode. A thin BSCF layer was first formed on the porous support, and then a thin BSCF + PMMA (polymethyl methacrylate) layer was sequentially formed on the thin BSCF layer using a bimodal suspension of BSCF and PMMA. A 30-μm thin porous/dense bi-layer of BSCF of which the total thickness was obtained by optimizing the processes of EPD and subsequent co-sintering. The oxygen separation performance of 3.7 ml (STP) min^?1 cm^?2 at 860 °C was achieved for the BSCF-based multi-layered oxygen separation membrane.     

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.     

New morphological Ba0.5Sr0.5Co0.8Fe0.2O3−α hollow fibre membranes with high oxygen permeation fluxes

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?α (BSCF) hollow fibre membranes were fabricated by a combined phase inversion and sintering technique. The membranes were characterised by XRD, SEM and tested for air separation. The membrane possesses a novel morphology consisting of one dense layer and one porous layer. Oxygen permeation fluxes through the obtained hollow fibre membranes were measured in the temperature range 650–950 °C using helium sweep gas rates from 50 to 200 mL min^?1. Experimental results indicated the oxygen permeation flux through the BSCF hollow fibre membrane sintered at 1050 °C was approximately 11.46 mL min^?1 cm^?2 at 950 °C when the helium sweep rate was kept at 200 mL min^?1. The BSCF hollow fibre membrane showed a stable oxygen permeation flux of 8.60 mL min^?1 cm^?2 over the investigated period of 120 h at 900 °C.     

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     

Assessment of Ba0.5Sr0.5Co1−yFeyO3−δ (y = 0.0-1.0) for prospective application as cathode for IT-SOFCs or oxygen permeating membrane

The influence of iron doping level in Ba_0.5Sr_0.5Co_1−yFe_yO_3−δ (y = 0.0-1.0) (BSCF) oxides on their phase structure, oxygen nonstoichiometry, electrical conductivity, performance as symmetrical cell electrode and oxygen permeating membranes was systematically investigated. A cubic perovskite structure was observed for all the compositions with the presence of iron. The increase of iron doping level resulted in the decrease of the lattice constant, room-temperature oxygen nonstoichiometry, total electrical conductivity, and the increase of area specific resistance (ASR) as cathode with samaria doped ceria electrolyte. However, promising cathode performance with an ASR as low as 0.613 Ω cm² was still obtained at 600 °C for Ba_0.5Sr_0.5FeO_3−δ (BSF). The ceramic membranes composing of BSCF with various iron doping level are all oxygen semi-permeable at elevated temperatures. The increase of iron doping level resulted in the decrease of oxygen permeation flux from JO₂ = 2.28 μmol cm⁻² s⁻¹ (STP) for Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF5582) to ∼0.45 μmol cm⁻² s⁻¹ (STP) at 900 °C for BSF (y = 1.0) with the same membrane thickness of 1.1 mm, alongside with the change of the rate-determination step from the oxygen surface exchange to the slow oxygen bulk diffusion. The formation of composite oxide with a proper electronic conducting phase and the thin film technology are important for their prospective application as cathode in IT-SOFCs and oxygen permeating membrane, respectively.     

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.     

Evaluation of hydrogen separation performance of Ni-BaCe0.85Fe0.15O3-δ cermet membranes

Adding metal phase as the electronic transport channels in mixed protonic and electronic conductors is an effective method to enhance the conductivity and hydrogen permeability. Ceramic-metal (cermet) Ni-BaCe_0.85Fe_0.15O_3-δ (Ni-BCF8515) dual-phase membranes were successfully fabricated in this article, where BaCe_0.85Fe_0.15O_3-δ (BCF8515) served as the protonic conductor and Ni acted as the electronic conductor. The hydrogen permeability of Ni-BCF8515 dual-phase membrane was improved apparently, reaching 0.325?mL?min^?1?cm^?2 at 1000?°C, which is four times higher than that of the single phase BCF8515 membrane. Hydrogen permeability of Ni-BCF8515 dual-phase membrane under different hydration conditions was studied here. Moreover, Ni-BCF8515 dual-phase membrane showed stable hydrogen permeability under the reducing atmosphere for 50?h.     

Evaluation of (Ba0.5Sr0.5)0.85Gd0.15Co0.8Fe0.2O3−δ cathode for intermediate temperature solid oxide fuel cell

A perovskite-type (Ba_0.5Sr_0.5)_0.85Gd_0.15Co_0.8Fe_0.2O_3?δ (BSGCF) oxide has been investigated as the cathode of intermediate temperature solid oxide fuel cells (IT-SOFCs). Coulometric titration, thermogravimetry analysis, thermal expansion and four-probe DC resistance measurements indicate that the introduction of Gd³⁺ ions into the A-site of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) leads to the increase in both oxygen nonstoichiometry at room temperature and electrical conductivity. For example, the conductivity of BSGCF is 148 S cm^?1 at 507 °C, over 4 times as large as that of BSCF. Furthermore, the electrochemical activity toward the oxygen reduction reaction is also enhanced by the Gd doping. Impedance spectra conducted on symmetrical half cells show that the interfacial polarization resistance of the BSGCF cathode is 0.171 Ω cm² at 600 °C, smaller than 0.297 Ω cm² of the BSCF cathode. A Ni/Sm_0.2Ce_0.8O_1.9 anode-supported single cell based on the BSGCF cathode exhibits a peak power density of 551 mW cm^?2 at 600 °C.     

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.     

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     

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.     

Thermochemical expansion of mixed-conducting (Ba,Sr)Co0.8Fe0.2O3−δ ceramics

The thermal and chemical expansion of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and SrCo_0.8Fe_0.2O_3−δ (SCF) mixed ionic-electronic conductors were studied in combination with oxygen nonstoichiometry (δ) at 298–1223 K and p(O₂) = 10⁻⁴ to 1.00 atm. In order to minimize the effects of phase separation or oxygen-vacancy ordering processes, the data were collected in dynamic cooling mode using dense ceramic samples. The procedure was justified by a very fast equilibration at given p(O₂) in high-temperature range demonstrated for ceramics samples with different specific surface area. The difference in nonstoichiometry of BSCF and SCF at temperatures ≥973 K was found to be ≤0.03 oxygen atoms per formula unit. BSCF demonstrates favorably smaller chemical expansion compared to SCF and many other mixed conductors, originating from smaller δ variations and larger unit cell less sensitive to temperature and nonstoichiometry changes. Excessive thermochemical expansion impedes however the use of BSCF in single-phase fuel cell cathodes and planar mixed-conducting membranes.     

Modeling of the performance of BSCF capillary membranes in four-end and three-end integration mode

Owing to their high surface-to-volume ratio, there has been an increasing research interest in mixed ionic–electronic conducting (MIEC) capillary membranes for large-scale high temperature oxygen separation applications. They offer an energy-efficient solution for high temperature combustion processes in oxy-fuel and pre-combustion CO₂ capture technologies used in fossil fuel power plants. In order to assess the effectiveness of these membranes in power plant applications, the impact of the geometry of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) capillaries on their performance in the three-end and four-end integration modes has been investigated and thoroughly discussed. The model's parameters were derived from four-end mode lab-scale experiments using gas-tight, macrovoid free and sulfur-free BSCF capillary membranes that were prepared by a phase-inversion spinning technique. The results of this modeling study revealed that in the four-end mode higher average oxygen fluxes and smaller total membrane areas can be obtained than in the three-end mode. This is due to the higher pO₂ gradient across the membrane wall.     

Fabrication of BSCF-based mixed ionic-electronic conducting membrane by electrophoretic deposition for oxygen separation application

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF), which exhibits a high mixed oxide ionic-electronic conduction, was used for the fabrication of an oxygen separation membrane. An asymmetric structure, which was a thin and dense BSCF membrane layer supported on a porous BSCF substrate, was fabricated by the electrophoretic deposition method (EPD). Porous BSCF supports were prepared by the uniaxial pressing method using a powder mixture with BSCF and starch as the pore-forming agent (0–50 wt.%). The sintering behaviors of the porous support and the thin layer were separately characterized by dilatometry to determine the co-fired temperature at which cracking did not occur. A crack-free and thin dense membrane layer, which had about a 15 μm thickness and >95% relative density, was obtained after optimizing the processes of EPD and sintering. The dense/porous interface was well-bonded and the oxygen permeation flux was 2.5 ml (STP) min⁻¹ cm^-2 at 850 °C.     

The effect of an oxygen partial pressure gradient on the mechanical behavior of perovskite membrane materials

In application of perovskite as oxygen conducting materials the membrane is operated at elevated temperatures under an oxygen gradient. The effect of the partial pressure difference on the mechanical properties is reported in the current work. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) samples were annealed under an oxygen gradient. The mechanical properties of cross-sections were characterized using indentation testing. Chemical strains for BSCF and LSCF were too small to detect them after cooling to RT by XRD; however, the results suggest that the indentation crack length is affected by chemical strains for LSCF, but not for BSCF. An anisotropy of the indentation crack length and corresponding apparent fracture toughness is related with the interaction of domain switching and residual strain that is probably also associated with the observation that vacuum (10⁻⁵ mbar) annealed LSCF showed surface cracking on heating in air, whereas for BSCF such fracture features were not observed.