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.     

Kinetic demixing/decomposition of Ba0.5Sr0.5CoxFe1−xO3−δ (x = 0.2 and 0.8)

Microstructural changes due to kinetic demixing within sintered BSCF ceramics (Ba_0.5Sr_0.5Co_xFe_1?xO_3?δ, x = 0.2 and 0.8: BSCF5528 and BSCF5582, respectively) have been investigated. When the specimens were subjected to 2 A/cm² at 1000 °C and pO₂ = 10^?5 atm, there was a significant enhancement of grain growth as well as 2nd phase formation observed in BSCF5528. At the anode, cobalt deficient aggregates within the grains; and, at the cathode, cobalt rich 2nd phase particles were observed on the grain surfaces of the microstructure. Such phenomena were not observed in BSCF5582, even under higher current density (7 A/cm²) and longer delay time. These results were explained by the kinetic demixing/decomposition.     

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 (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.     

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.     

Production and properties of substituted LaFeO3-perovskite tubular membranes for partial oxidation of methane to syngas

Tubular membranes of La_0.6Ca_0.4Fe_0.75Co_0.25O_3−δ and La_0.5Sr_0.5Fe_1−yTi_yO_3−δ (y = 0, 0.2) for the application of partial oxidation of methane to syngas were produced by thermoplastic extrusion and investigated by oxygen permeation measurements. The optimum ceramic content in the feedstock for extrusion was found to be 51 vol% as a result of rheology measurements. Tubes with an outer diameter of 4.8–5.5 mm and thickness of 0.25–0.47 mm were produced with densities higher than 95% of the theoretical density. The oxygen permeation flux of the tubular membranes was measured with air on one side and Ar or Ar + CH₄ mixture on the other side. The oxygen permeation rate decreased with Ti-substitution while it was considerably increased by introduction of 5% methane into the system. The normalized oxygen fluxes in air/Ar gradient at 900 °C were measured to be 0.06, 0.051, and 0.012 μmol cm⁻² s⁻¹ for LCFC, LSF, and LSFT2, respectively, and 0.18 μmol cm⁻² s⁻¹ for LSFT2 with 5% methane.     

Electrochemical and microstructural characteristics of nanoperovskite oxides Ba0.2Sr0.8Co0.8Fe0.2O3−δ (BSCF) for solid oxide fuel cells

Nanoperovskite oxides, Ba_0.2Sr_0.8Co_0.8Fe_0.2O_3?δ (BSCF), were synthesized via the co-precipitation method using Ba, Sr, Co, and Fe nitrates as precursors. Next, half cells were fabricated by painting BSCF thin film on Sm_0.2Ce_0.8O_x (samarium doped ceria, SDC) electrolyte pellets. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS) measurements were carried out on the BSCF powders and pellets obtained after sintering at 900 °C. Investigations revealed that single-phase perovskites with cubic structure was obtained in this study. The impedance spectra for BSCF/SDC/BSCF cells were measured to obtain the interfacial area specific resistances (ASR) at several operating temperatures. The lowest values of ASR were found to be 0.19 Ω cm², 0.14 Ω cm² 0.10 cm², 0.09 Ω cm² and 0.07 Ω cm² at operating temperatures of 600 °C, 650 °C, 700 °C, 750 °C and 800 °C, respectively. The highest conductivity was found for cells sintered at 900 °C with an electrical conductivity of 153 S cm^?1 in air at operating temperature of 700 °C.     

Oxygen permeation through dense La0.1Sr0.9Co0.8Fe0.2O3−δ perovskite membranes: Catalytic effect of porous La0.1Sr0.9Co0.8Fe0.2O3−δ layers

The oxygen permeation performance of a number of La_0.1Sr_0.9Co_0.8Fe_0.2O_3−δ (LSCF1982)-based membranes, consisting of dense LSCF1982 layer with/without porous LSCF1982 layer, was analyzed on the basis of the thickness of the dense layer and catalytic effect of the porous layer. A 0.27 mm thick dense membrane gives oxygen permeation flux ($J_{O_2}$) of 2.33 sccm min⁻¹ cm⁻² at 900 °C, which is increased to 3.55 sccm min⁻¹ cm⁻² on applying a porous layer of LSCF1982 onto the dense membrane. The membrane gives a stable flux for 300 h. The flux was further improved by reducing the thickness of the dense LSCF1982 layer and at 950 °C a flux of 4.47 sccm min⁻¹ cm⁻² is obtained with 0.012 mm thick membrane.     

SrCo0.9Sc0.1O3−δ perovskite hollow fibre membranes for air separation at intermediate temperatures

SrCo_0.9Sc_0.1O₃ (SCSc) perovskite powders with sub-micron particle size were synthesized by a modified Pechini method combined with a post-treatment of sintering and ball-milling. From the prepared powders, the SCSc hollow fibre membranes with asymmetric structure and gas-tight property were fabricated by spinning a polymer solution containing 58.4 wt% SCSc followed by sintering at 1200 °C for 5 h. The oxygen permeation properties of the obtained SCSc fibres were measured under air/He gradients at 500–800 °C. This showed the oxygen flux of 1 mL cm^?2 min^?1 at 750 °C and 4.41 mL cm^?2 min^?1 at 900 °C. Modeling analysis reveals that the oxygen permeation process is predominated by oxygen surface exchange kinetics with an activation energy of 95.0 kJ mol^?1. The SCSc membranes showed excellent oxygen permeation performance while exhibiting high structural and permeating stability at intermediate temperatures (500–800 °C).     

Improving the POM performance of YBa2Cu3O7−δ membrane reactor by Co doping

The performance of Co doped YBa₂Cu₃O_7−δ (YBCO) membrane reactor have been investigated in a process of the partial oxidation of methane (POM) to syngas. The results shows that doping YBCO membrane with a little Co can enhance its oxygen permeation flux and improve its stability in reducing atmosphere noticeably. At 900 °C, with feed flow at 50 ml/min, CH₄ 6.0 v%, SV = 12,000 h⁻¹, and Ni/ZrO₂ catalyst, CH₄ conversion rate, CO selectivity, and oxygen permeation flux can reach to 98%, 92% and 1.41 ml min⁻¹ cm⁻² respectively.     

Preparation of dual-phase composite BaCe0.8Y0.2O3/Ce0.8Y0.2O2 and its application for hydrogen permeation

Dual-phase ceramic membranes composed of BaCe_0.8Y_0.2O₃ (BCY) and Ce_0.8Y_0.2O₂ (CYO) were successfully synthesized by solid state reaction method for hydrogen permeation. The influences of the BCY/CYO volume ratios on phase composition, microstructure, chemical stability and electrical property were investigated. The hydrogen permeation of the dual-phase composite was characterized as a function of temperature and feed side hydrogen partial pressure. The results showed that there was no reaction between the two constituent oxides observed under the preparation conditions. The dual-phase composite with different BCY/CYO volume ratios after sintering at 1550 °C exhibited dense structure, as well as good stability in 4% H₂/Ar, wet Ar and pure CO₂ atmosphere. The conductivity of the dual-phase composite increased with the content of CYO increasing and 30BCY–70CYO exhibited the highest total conductivity of 2.6×10⁻² S cm⁻¹ at 800 °C in 4% H₂/Ar. The hydrogen permeability of 30BCY–70CYO sample was improved as the temperature and the hydrogen partial pressure in feed gas increased. The hydrogen permeation flux of 1.7 μmol cm⁻² s⁻¹ was achieved at 850 °C.     

CO2-tolerant Ni-La5.5WO11.25-δ dual-phase membranes with enhanced H2 permeability

Enhancing the ambi-polar conductivity of the ceramic hydrogen permeable membrane by introducing an electron conductive metallic phase is quite effective, which is helpful for the hydrogen permeation flux improvement. To develop CO₂-tolerant hydrogen permeable membranes with better hydrogen permeability, Ni-La_5.5WO_11.25-δ (Ni-LWO) cermet membranes are fabricated. The alkaline earth metal-free ceramic LWO is used as the main proton-conductive phase and Ni is used as the main electron-conductive phase. The Ni-LWO membrane exhibits good chemical stability in CO₂-containing atmosphere since its hydrogen permeability maintains well in the measurement for about 180 h. Compared with the LWO ceramic membrane, the hydrogen permeability of the Ni-LWO membrane has been improved significantly. The Ni/LWO ratio has great impact on the performance of the cermet membrane. Meanwhile, among all the dual-phase Ni-LWO membranes with different Ni/LWO volume ratios, the membrane with 60 vol% Ni shows the highest hydrogen permeation flux of 0.18 ml min⁻¹ cm⁻² at 1000 °C when the feed gas contains 50% H₂.     

A novel series of Ba0.5Sr0.5Al0.2−xMgxFe0.8O3−ξ (x ≤ 0.2) membranes for oxygen permeation application

The pervoskite-type oxides have received attention due to their potential applications in catalysis, fuel cells, sensors, gas separable membranes, and electrolytes. In view of the importance of oxygen separation from air, stable Ba_0.5Sr_0.5Al_0.2−xMg_xFe_0.8O_3−ξ (x = 0–0.2) powders have been synthesized by decomposition of sol–gel derived oxalate at 950 °C for 5 h and characterized with regard to formation, nature of iron species, oxygen permeation, and electrical conductivity. It is shown that magnesium substitution leads to (i) a stable perovskite-type cubic phase with ‘a’ = 3.953–3.978 Å, (ii) weakening of metal–oxygen bond, (iii) reduction of Fe⁴⁺ ions, and (iv) enhancement of oxygen deficiency and electrical conductivity. Their compact discs act as stable oxygen permeable filters with flux density of ∼3.013–3.355 μmol cm⁻² s⁻¹ at 1000 °C. The maximum value corresponds to composition x = 0.2 and hence can be a potential membrane for oxygen separation technology.     

Electrical characterization of multidoped ceria ceramics

Ceria ceramics was obtained from multi-doped nanosized ceria powders prepared by both modified glycine nitrate procedure (MGNP) and self-propagating reaction at room temperature (SPRT). Rare earth elements such as Nd, Sm, Gd, Dy, Y, Yb were used as dopants. The overall mole fraction of dopants was 0.2. One-hour long sintering of powder compacts was performed at 1500 °C in oxygen atmosphere. Phase composition, microstructure and ionic conductivity of sintered samples were analysed. Single-phase ceria was detected in all samples. In general, the increase in the number of dopants improved the ionic conductivity. The samples doped simultaneously with five dopants had the highest ionic conductivity, as evidenced by the impedance measurements. At 450 °C, the conductivity of sample obtained by MGNP was 3.94×10^?3 Ω^?1 cm^?1 whereas the conductivity of sample obtained by SPRT was 2.61×10^?3 Ω^?1 cm^?1. The conductivity activation energy for MGNP and SPRT samples was measured to be 0.348 and 0.385 eV, respectively. Finally, the conductivity decreased as the number of dopants increased to six.     

Ionic/electronic mixed conduction relations in perovskite-type oxides by defect structure

Perovskite-type oxides La_1-aA_aM_1-bB_bO_3-x with A=Sr²⁺, Ln³⁺, Ce⁴⁺ and M=Fe, Co, Ga; B=Co, Fe, Mg were prepared in the concentration range a=0·1 to 1 mol and b=0·1 to 0·5 mol. Additionally, A-substoichiometric compositions were prepared. Preparation conditions for monophase materials and structure types of the perovskite were determined by X-ray investigation. The electrical conductivity as a function of pO₂ in the range 10⁵>pO₂>10⁻¹⁴ Pa and temperature (500 to 1000°C) was measured on ceramic shapes by a dc four-point technique in combination with solid electrolyte coulometry. The ionic part of conductivity in mixed conductors was determined by oxygen permeation measurements. The II–III-perovskites Sr(Co,Fe)O_3-x in their stabilized form are excellent mixed conductors (maximum 500 S cm⁻¹ at 400°C) and have up to 2 orders of magnitude higher oxygen ionic conductivity than the preferred III–III-perovskite La(Sr)Mn(Co)O_3-x. The oxygen ionic conductivity of the electrolyte La(Sr)Ga(Mg)O_3-x, was increased by doping with 0·1 mol Co. By applying higher Co or Fe doping concentrations the lanthanum gallate, becomes a mixed conductor.     

Partial oxidation of methane in BaCe0.1Co0.4Fe0.5O3−δ membrane reactor

A new perovskite material, BaCe_0.1Co_0.4Fe_0.5O_3?δ used as dense oxygen permeable membrane for partial oxidation of methane (POM) reaction was investigated. In order to improve the synergetic effects between membrane and catalyst, LiLaNiO/γ-Al₂O₃ catalyst was directly packed onto the surface of the membrane to carry out POM. In BaCe_0.1Co_0.4Fe_0.5O_3?δ membrane reactor, high oxygen permeation flux, high CH₄ conversion and CO selectivity were obtained. At 950 °C, oxygen flux of 9.5 ml cm^?2 min^?1, CH₄ conversion of 99% and CO selectivity of 93% were achieved with a membrane thickness of 1.0 mm. There was an induction process at the initial stage of POM, which was related to the reduction of NiO to Ni⁰ in LiLaNiO/γ-Al₂O₃ catalyst. Experiments illustrated that higher reaction temperature would shorten the induction time. During continuously operating for 1000 h at 875 °C, no degradation of performance of the membrane reaction was observed. SEM characterization also demonstrated that the membrane disc maintained an integral structure without any cracks after long-term operation.     

Influence of sintering aids and excess Lithium on the conductivity of perovskite-type Li3/8Sr7/16Zr1/4Nb3/4O3 solid electrolyte

Perovskite-structured Li_3/8Sr_7/16Zr_1/4Nb_3/4O₃ solid-state Lithium-conductors were prepared by conventional solid-state reaction method. Influence of sintering aids (Al₂O₃, B₂O₃) and excess Lithium on structure and electrical properties of Li_3/8Sr_7/16Zr_1/4Nb_3/4O₃ (LSNZ) has been investigated. Their crystal structure and microstructure were characterized by X-ray diffraction analysis and scanning electron microscope, respectively. The conductivity and electronic conductivity were evaluated by AC-impedance spectra and potentiostatic polarization experiment. All sintered compounds are cubic perovskite structure. Optimal amount of excess Li₂CO₃ was chosen as 20 wt% because of the total conductivity of LSNZ-20% was as high as 1.6×10⁻⁵ S cm⁻¹ at 30 °C and 1.1×10⁻⁴ S cm⁻¹ at 100 °C, respectively. Electronic conductivity of LSNZ-20% is 2.93×10⁻⁸ S cm⁻¹, nearly 3 orders of magnitude lower than ionic conductivity. The density of solid electrolytes appears to be increased by the addition of sintering aids. The addition of B₂O₃ leads to a considerable increase of the total conductivity and the enhancement of conductivity is attributed to the decrease of grain-boundary resistance. Among these compounds, LSNZ-1 wt%B₂O₃ has lower activation energy of 0.34 eV and the highest conductivity of 1.98×10⁻⁵ S cm⁻¹ at 30 °C.     

High performance BaCe0.8Y0.2O3−a (BCY) hollow fibre membranes for hydrogen permeation

In this work, BaCe_0.8Y_0.2O_3−α (BCY) perovskite hollow fibre membranes were fabricated by a phase inversion and sintering method. BCY powder was prepared by the sol–gel technique using ethylenediaminetetraacetic acid (EDTA) and citric acid as the complexing agents. Gel calcination was carried out at high temperature to form the desired crystal structure. The qualified BCY hollow fibre membranes could not be achieved even the sintering was carried out at temperatures up to 1550^oC due to the poor densification behavior of the BCY material. The addition of sintering aid (1 wt% Co₂O₃) inside BCY powder as the membrane starting material significantly improved the densification process, leading to the formation of gas-tight BCY hollow fibres. The optimum sintering temperature of BCY hollow fibre membrane was 1400 °C to achieve the best mechanical strength. H₂ permeation through the BCY hollow fibre membranes was carried out between 700 and 1050 °C using 25% H₂–He mixture as feed gas and N₂ as sweep gas, respectively. For comparison purpose, the disk-shaped BCY membrane with a thickness of 1 mm was also prepared. The measured H₂ permeation flux through the BCY hollow fibres reached up to 0.38 mL cm⁻² min⁻¹ at 1050 °C strikingly contrasting to the low values of less than 0.01 mL cm⁻² min⁻¹ from the disk-shaped membrane. After the permeation test, the microstructure of BCY hollow fibre membrane was still maintained well without signals of membrane disintegration or peeling off.     

Oxygen ionic transport in SrFe1−yAlyO3−δ and Sr1−xCaxFe0.5Al0.5O3−δ ceramics

The oxygen permeability of mixed-conducting Sr_1−xCa_xFe_1−yAl_yO_3−δ (x=0–1.0; y=0.3–0.5) ceramics at 850–1000 °C, with an apparent activation energy of 120–206 kJ/mol, is mainly limited by the bulk ionic conduction. When the membrane thickness is 1.0 mm, the oxygen permeation fluxes under pO₂ gradient of 0.21/0.021 atm vary from 3.7×10⁻¹⁰ mol s⁻¹ cm⁻² to 1.5×10⁻⁷ mol s⁻¹ cm⁻² at 950 °C. The maximum solubility of Al³⁺ cations in the perovskite lattice of SrFe_1−yAl_yO_3−δ is approximately 40%, whilst the brownmillerite-type solid solution formation range in Sr_1−xCa_xFe_0.5Al_0.5O_3−δ system corresponds to x>0.75. The oxygen ionic conductivity of SrFeO₃-based perovskites decreases moderately on Al doping, but is 100–300 times higher than that of brownmillerites derived from CaFe_0.5Al_0.5O_2.5+δ. Temperature-activated character and relatively low values of hole mobility in SrFe_0.7Al_0.3O_3−δ, estimated from the total conductivity and Seebeck coefficient data, suggest a small-polaron mechanism of p-type electronic conduction under oxidising conditions. Reducing oxygen partial pressure results in increasing ionic conductivity and in the transition from dominant p- to n-type electronic transport, followed by decomposition. The low-pO₂ stability limits of Sr_1−xCa_xFe_1−yAl_yO_3−δ seem essentially independent of composition, varying between that of LaFeO_3−δ and the Fe/Fe_1−γO boundary. Thermal expansion coefficients of Sr_1−xCa_xFe_1−yAl_yO_3−δ ceramics in air are 9×10⁻⁶ K⁻¹ to 16×10⁻⁶ K⁻¹ at 100–650 °C and 12×10⁻⁶ K⁻¹ to 24×10⁻⁶ K⁻¹ at 650–950 °C. Doping of SrFe_1−yAl_yO_3−δ with aluminum decreases thermal expansion due to decreasing oxygen nonstoichiometry variations.     

Ionic conductivity of tetragonal ZrO2 polycrystal doped with TiO2 and GeO2

The effects of the co-doping and the resultant co-segregation of 2 mol% TiO₂ and 2 mol% GeO₂ on the ionic conductivity and on the chemical bonding state in a tetragonal ZrO₂ polycrystal were investigated. The conductivity data and grain boundary microstructure showed that the doped Ti⁴⁺ and Ge⁴⁺ cations segregate along the grain boundary, and this segregation causes a reduction in the conductivity of both the grain interior and grain boundary and an increase in the activation energy of the grain boundary conductivity. Overall, the data indicate that the segregation retards the diffusion of oxygen anions. A first-principle molecular orbital calculation explains the retarded diffusion of the oxygen anion from a change in the covalent bonds around the dopant cations; an increase in the strength of the covalent bond between the oxygen and doped cation should work to suppress the diffusion of the oxygen anion.