Dense Ceramic Membranes Research Articles

Structural evolution of BaCe0.65Zr0.20Y0.15O3-δ-Ce0.85Gd0.15O2-δ composite MPEC membrane by in-situ synchrotron XRD analyses

Nowadays, dense ceramic membranes based on mixed ionic and electronic conductors are considered very promising materials for H2 separation at T > 600 °C. Among these, BaCe0.65Zr0.2Y0.15O3-δ-Ce0.85Gd0.15O2-δ (BCZ20Y15-GDC15) composite combine an acceptable H2 flux and good chemical stability under CO2- and H2S-containing atmospheres. However, a clear understanding of its crystal structure, phase stability and mechanical stability under real working conditions could not yet be obtained. In this work, its structural evolution was investigated from room temperature to 800 °C by in-situ synchrotron XRD analyses under dry and wet H2. No chemical interaction between the BCZ20Y15 and GDC15 phases occurred in the composite, thus demontrating its excellent chemical stability under operating conditions. However, some phase transitions were observed for the BCZ20Y15 phase, under both dry and wet H2: i.e., it showed an orthorhombic Imma structure from room temperature to 100 °C, trigonal R-3c up to 700 °C and cubic Pm-3m up to 800 °C. On the other hand, the GDC15 phase did not display any phase transition, remaining in a cubic Fm-3m structure under all tested conditions. Moreover, a synergistic effect of the BCZ20Y15 and GDC15 phases in the volume expansion of the composite was revealed: indeed, BCZ20Y15 and GDC15 lattice expansion rates tend to approach each other in the composite under reducing conditions. This synergistic effect is very important for the mechanical performances of BCZ20Y15-GDC15 composite. The similar expansion rate observed for BCZ20Y15 and GDC15 may reduce the strain and prevent failure of this ceramic membrane under operating conditions.

Influence of the La6W2O15 Phase on the Properties and Integrity of La6-xWO12-?–Based Membranes

The aim of the present work is to evaluate the influence of La 6 W 2 O 15 secondary phase on the properties and integrity of La 6-x WO 12-? –based membranes. Structural, microstructural and thermo–chemical study was carried out evidencing significant crystallographic and thermal expansion anisotropy: the reason for poor thermo–mechanical stability of La 6 W 2 O 15 . Conductivity of La 6 W 2 O 15 was one to two orders of magnitude lower compared to the phase pure La 6-x WO 12-? in the range of 300 to 900 °C. The relaxation study showed that the hydration process was faster for the La 6 W 2 O 15 compared to the LWO phase, due to the higher electronic contribution to the total conductivity. Short–term stability tests in H 2 at 900 °C and in a mixture of CO 2 and CH 4 at 750 °C were conducted and material remained stable. Remarkable reactivity with NiO and YSZ at elevated temperatures was further evidenced compared to the relative inert behavior towards MgO and CGO. Keywords: La 6 W 2 O 15 , La 6-x WO 12-? , mixed protonic–electronic conductor, dense ceramic membrane, hydrogen separation.

Effects of membrane thickness and structural type on the hydrogen separation performance of oxygen-permeable membrane reactors

Membrane thickness and structural type, which are closely related to the species diffusion in the membrane bulk and reactions at the gas-solid interfaces, have important influences on the performance of a dense ceramic membrane. Recently, oxygen-permeable dense ceramic membrane reactors have been suggested for hydrogen purification at elevated temperatures. Mixed conducting composite with composition of 75 wt% Ce0.85Sm0.15O2-δ-25 wt% Sm0.6Sr0.4Al0.3Fe0.7O3-δ (SDC-SSAF) was fabricated into oxygen-permeable membranes with different thicknesses and structural types. The effects of membrane thickness and structural type on the hydrogen separation performance were tested under different hydrogen concentrations and flow rates on side I, different flow rates of steam/He mixed gas on side II as well as different temperatures. The effect of concentration polarization on hydrogen separation rate and the rate determining steps were discussed by comparing the performances of the symmetric membranes and the asymmetric membranes. The apparent activation energies of the membrane reactors for hydrogen separation were analyzed. This study will give some constructive guides to the subsequent work on how to effectively improve the hydrogen separation performance.

Perovskite foams used in combination with dense ceramic membranes for oxygen transport membrane applications

The future industrial implementation of membranes for oxygen transport requires new designs to increase oxygen semi-permeation, together with robust performance under operating conditions. This work describes the development of innovative membrane designs based on perovskite foams with a very large porosity (above 90%) supporting thin dense perovskite membranes (thickness of 50–100 µm). The preparation of such foam-supported membranes is described in this paper. The performances in terms of oxygen semi-permeation are also measured and compared with the best results reported in the literature up to date.

Ohmically heated ceramic asymmetric tubular membranes for gas separation

Mixed conducting dense ceramic gas separation membranes can be used in air separation for oxy-fuel and pre-combustion processes in CO2 capture schemes for emission-free power plants and chemical industries. Such membranes operate at high temperatures, and the energy penalty associated with heating the membranes with external electrical heaters or burners via the feed or sweep gas can be significantly reduced by adopting direct ohmic heating of the membrane. We demonstrate that tubular asymmetric gas separation membranes of La2NiO4+δ and La0.87Sr0.13CrO3-δ can be heated to temperatures in excess of 800 °C by passing current through them, and that such ohmically heated tubular membranes can be operated in an oxygen potential gradient to selectively separate oxygen from a feed gas mixture. We highlight the associated challenges with heat transport and thermal gradients and undertake numerical simulations to investigate the effect of materials properties on heating power and heat distribution in the tubes.

Microstructure and Electrochemical Properties of La0.8Sr0.2Ga0.8Mg0.2O3 Thin Film Deposited by Vacuum Cold Spray for Solid Oxide Fuel Cells

Doped lanthanum gallate (La0.8Sr0.2Ga0.8Mg0.2O3, LSGM) is considered as a promising electrolyte material for the intermediate temperature-solid oxide fuel cell (IT-SOFC), due to its high ionic conductivity at 600~800 °C. However, the common methods to fabricate the LSGM electrolyte (e.g., solid phase sintering) often lose their advantages owing to the depletion of Ga and the interfacial reaction between the LSGM and electrodes. Vacuum cold spray (VCS) is an emerging method to fabricate thin (﹤20 μm) and dense ceramic electrolyte membrane. In the present study, the LSGM electrolyte layer with a thickness of ~10 μm is prepared by VCS system assisted with in-situ particle heating approach (25~400 °C). The coating density and the ionic conductivities were increased with heating temperature. The results showed that VCS is a promising method to prepare the electrolyte film applied in the IT-SOFC.

On the compatibility of dual phase BaCe0.65Zr0.2Y0.15O3-based membrane for hydrogen separation application

Abstract Among dense ceramic membranes for H 2 separation, the ceramic-ceramic composite ones have recently gained the interest of the scientific community thanks to their enhanced performances. However, the ceramic phases constituting the composite must show high chemical and thermomechanical compatibility to form a crack-free and robust membrane. This work reports the compatibility between the BaCe 0.65 Zr 0.2 Y 0.15 O 3−δ (BCZY) proton conductive system and various electron conducting systems belonging to four different families: ceria-based (CeO 2 and Ce 0.8 Gd 0.2 O 1.9 ), lanthanum nickelate cobaltite (La 0.95 Ni 0.6 Co 0.4 O 3 − δ ), strontium titanate (Sr 0.9 La 0.1 TiO 3 − δ ) and the non-oxidic α-SiC. This work highlights how the coupling of BCZY with chemically different electron-conductive ceramics represents a challenging task, since La 0.95 Ni 0.6 Co 0.4 O 3 , Sr 0.9 La 0.1 TiO 3 and α-SiC react with the proton conductive perovskite producing many secondary phases in air and in reducing atmosphere after 4 h of permanence at 1400 °C. As may be expected, BCZY is chemically compatible with CeO 2 and Ce 0.8 Gd 0.2 O 1.9 in air and in reducing atmosphere after 4 h of permanence at 1400 °C. Dilatometric results prove the thermomechanical compatibility between BCZY and the ceria-based systems in the operation temperature range.

Investigation of oxygen transport membrane reactors for oxy-fuel combustion and carbon capture purposes

There is an increasing pressure for industry to reduce carbon dioxide (CO2) emissions from combustion processes, resulting in an increased interest in the development of methods to sequester and recycle CO2 from flue gases. Current methods available to separate nitrogen and CO2 for environmental benefits, such as chemical looping combustion (CLC), are expensive at the high flow rates encountered in industry. One potential alternative is a ceramic membrane catalytic reactor, which produces pure oxygen and simultaneously conducts oxy-fuel combustion, thus, CO2in the product stream can successfully be separated from the nitrogen in air. This work investigates the performance of a perovskite-type SrSc0.1Co0.9O3-δ(SSC) and La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) membrane reactors for the combustion of methane in various configurations. The ceramic membranes exploited here are oxygen semi-permeable, dense ceramic membranes based on the composite oxides with a high mixed oxygen ionic and electronic conductivity at high temperatures. The prepared SSC and LSCF hollow fibre membranes with catalysts were used to perform reactions with a methane fuel. The oxygen permeability feasibility of the membrane reactors were studied and confirmed. The CO2selectivity at various test conditions were also reported with the maximum selectivity achieved for SSC membrane to be 85.4% while LSCF membrane achieved 87.1% CO2selectivity.

Influence of a La0.6Sr0.4CoO3-δ Functional Layer on (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ Oxygen Transport Membranes (OTMs)

Nanoporous La0.6Sr0.4CoO3-δ (LSC) thin films with a thickness between 100 and 500 nm were derived from metal-organic precursors. These layers were applied onto a thin Ba0. 5Sr0. 5Co0.8Fe0.2O3-δ substrate (thickness ≈ 400 µm) in order to enhance oxygen surface exchange. The surface exchange coefficient k δ of uncoated and LSC-coated dense BSCF ceramic bulk samples were determined by electrical conductivity relaxation (ECR) measurements. By combining the results from these ECR measurements under varying oxygen partial pressures with transmission electron microscopy (TEM) analyses the influence of the nanostructured functional LSC layer on the dense BSCF ceramic membrane was investigated and discussed. Elemental distributions before and after aging obtained by analytical scanning TEM were carried out to address cation interdiffusion between LSC and BSCF.

Influence of a La0.6Sr0.4CoO3-δ Functional Layer on (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ Oxygen Transport Membranes (OTMs)

The performance of electroceramic high temperature oxygen-transport membranes (OTM) is deter­mined by ionic and electronic transport processes. Oxygen transport kinetics is determined by the transport parameters, namely the surface exchange coefficient k δ and the bulk diffusion coefficient D δ. Mixed-conducting perovskite oxides, e.g., A1-xSrxCo1-yFeyO3- δ (A = Ba, La, Pr…) [1], are materials with excellent oxygen transport properties. Especially Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) shows an excellent OTM performance [2] in its cubic phase [3] at high temperatures (T > 850 °C). For an energy-efficient application it makes sense to use these oxygen separation membranes at lower temperatures (T< 700 °C). However, the occurrence of secondary phases in this temperature range [4] leads to a decreased oxygen transport through the BSCF membrane. Secondary phase formation can, though, be suppressed by suitable doping strategies [5,6]. OTM permeation occurs due to an oxygen partial pressure (pO2) gradient applied across the dense membrane. Reducing the thickness of the membrane increases the oxygen flux. However, below a certain thickness, the surface transfer of oxygen becomes rate-limiting; an improvement in oxygen flux can only be realized by surface activation, e.g., by a nanostructured layer. A promising candidate for such a layer is La0.6Sr0.4CoO3-δ (LSC) [7]. On the one hand, nanoporous LSC can enlarge the geo­metrical surface area and consequently increase the effective k δ value which is important especially at the low-pO2 side of the membrane because oxygen transport parameters strongly decrease with lower pO2 [8]. On the other hand, the occurrence of chemical “hetero-interfaces” as previously reported in LSC [9,10] may also improve oxygen surface exchange. The oxygen transport parameters (k δ, D δ) of uncoated and LSC-coated dense BSCF ceramic bulk samples are determined by electrical conductivity relaxation (ECR) measurements. Combining the results from these ECR measurements under varying partial pressures (pO2 = 10-2…1 bar) and transmission electron microscopy (TEM) analysis the influence of the nanostructured functional LSC layer on the dense BSCF ceramic membrane is investigated and discussed. Elemental distributions before and after aging obtained by analytical scanning TEM are presented to address cation interdiffusion between LSC and BSCF.

Gas to Liquids: Natural Gas Conversion to Aromatic Fuels and Chemicals in a Hydrogen-Permeable Ceramic Hollow Fiber Membrane Reactor

The performance of a dense ceramic hydrogen-permeable membrane reactor for the nonoxidative methane dehydroaromatization (MDA), according to the equilibrium reaction 6CH4 ⇆ C6H6 + 9H2 with a 6 wt % Mo/HZSM-5 bifunctional catalyst was investigated. A U-shaped ceramic hollow fiber membrane of the composition La5.5W0.6Mo0.4O11.25−δ (LWM0.4) has been used for the in situ removal of H2 to overcome thermodynamic constraints. The yield of aromatics (benzene, toluene, naphthalene) in the MDA could be increased in the beginning of the aromatization reaction by ∼50%–70%, in comparison with the fixed-bed reactor, because 40%–60% of the H2 abstracted have been extracted at 700 °C with a weight hourly space velocity (WHSV) of 840 cm3 gcat–1 h–1. These advantages of the membrane reactor operation decrease with time on stream, since the removal of H2 boosts not only CH4 conversion and yield of aromatics, but also catalyst deactivation by deposition of carbonaceous deposits. However, the catalyst system could be regenera...

Joining of ceramic Ba0.5Sr0.5Co0.8Fe0.2O3 membranes for oxygen production to high temperature alloys

The possibility of joining dense ceramic BCSF tubular membranes to metal alloys using a silver braze was investigated. Four different alloys (Crofer 22 APU®, Kanthal APM®, Haynes 214® and EN 1.4841) were considered and the influence of their oxide scale stability/reactivity and their thermal expansion coefficient on the stability of the final joint was evaluated. Leak tight assemblies were obtained only for steels with a thermal expansion coefficient of >16 [10−6K−1] and protective coating. Proof-of-concept oxygen flux measurements up to 830°C were performed on these assemblies, demonstrating the functionality of the developed hot sealing. In addition a simulation of the stresses occurring in the joint assembly during use was performed for different materials and geometries. The obtained results fit well with the experimental findings.

Autothermal reforming of ethanol in dense oxygen permeation membrane reactor

Perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) material was synthesized by the combined EDTA–citric acid complexing method and the disk-type BSCF dense membrane reactor (DMR) was constructed for H2 production by ethanol oxidative steam reforming reaction. The autothermal reaction conditions in the DMR were obtained through optimizing the ethanol oxidative steam reforming reaction (EOSR). The results showed that the hydrogen yield decreases (3.5–3.2mol/molEtOH) with the net heat of reaction Q decreases from 0 to −257J/molEtOH because the oxygen permeation flux for the membrane increases with temperature. The hydrogen production is 3.5mol/molEtOH at autothermal temperature of 750°C and the oxygen permeation flux of the dense ceramic membrane reactor is 6.5ml(STP)/cm2min. The activation energy for oxygen permeation was 46.4kJ/mol under the air/Ar gradient, as compared to about 20.1kJ/mol under EOSR reaction, respectively. The BSCF DMR performed stably under the autothermal reaction condition was stable for at least 180h. These results indicate the BSCF membrane holding the high oxygen permeation flux, the excellent phase reversibility and the good stability under the highly reducing atmosphere, shows great application potential for H2 production via the autothermal reaction.

Shape Forming of Ceramic Tubes for Electrochemical Reactors by Gel-Casting Method

Three-dimensional structures are required as electrodes to increase the space time yield of electrochemical reactors when the reactions take place at low current densities. However, a tubular arrangement is necessary in some cases even in the industrial practice. Tubular Electrochemical Reactors (TERs) consist of a dense co-ionic ceramic membrane and two porous electrodes [1,2]. Gel-casting is a promising method of producing tailor-made dimensions of electrolyte and/or electrode tubes for the needs of the above reactors. The basic principle of gel-casting concerns a combination of a solvent, a dispersant, organic monomers and low particle sized ceramic powders to form a high-solids-content, fluid slurry. The slurry is poured under conditions into a casting mold where by an initiator and a catalyst the organic monomers polymerize to form a 3-D polymer network of a solid gel in the shape of the mold. In particular, an aqueous solution of water soluble monomer acrylamide (AM) and cross-linker methylene–bis-acrylamide (MBAM) in appropriate ratio was prepared. Darvan C was used as dispersant and added to the AM and MBAM solution. Subsequently, the ceramic powder was added (i.e 50-80 wt%) and the system was mixed thoroughly to a stable slurry. Finally initiator and catalyst were added to the slurry, respectively ammonium persulfate (APS) and tetra methyl ethylene diamine (TEMED)/g of slurry. The casting slurry is poured into molds of various shapes to form within minutes a gelled green tubular body. Prior any further heating treatment the green bodies may subject to machining i.e cutting and drilling. All volatiles are removed by a heating treatment up to 6000C. Calcination at higher temperatures was applied in order to introduce mechanical strength to the shaped structures through controlled sintering [3]. The sintering temperature is totally depended on the composition of the ceramic powder. Major crucial factors in the overall success of the process are particle size distribution of the ceramic powder, drying conditions, slurry rheology, mold features and sintering parameters. In this study, ceramic powders of an oxygen ion conductor such as apatite-type lanthanum silicates (ATLS) and a proton conductor such as yttrium-doped barium zirconate (BZY) where prepared, optimized and used in gel-gasting methods. They are considered promising electrolytes for intermediate temperature (IT-SOFCs) solid oxide fuel cells. ATLS have attracted interest as promising electrolytes due to their thermodynamic stability, robustness (as ceramics) and their ability to maintain distorted crystal structure by doping but more important due to their high oxide ion conductivity at intermediate temperatures. Additionally they exhibit satisfactory refractoriness with high ionic but low electronic conductivity, low cost and good availability of raw materials [4,5]. On the other hand BZY proton conducting ceramics are regarded as promising electrolyte materials for low temperature proton conducting solid oxide fuel cells due to their higher ionic conductivities and lower activation energy as compared to conventional oxygen-ion conducting electrolytes [6-8]. The above materials were synthesized from various routes, providing thus singled-phased structures and a suitable particle sized distribution for their use in the shape-forming with gel-casting. Powders are verified by XRD, SEM and particle size laser distribution analysis. In addition, rheological measurements were performed at the slurries in order to evaluate the suitable viscosity for a successful shape-forming casting. Different sintering protocols were applied. The porosity of the produced tubes was evaluated by Archimedes method while the microstructure was studied by SEM.

Hydrogen separation membranes based on dense ceramic composites in the La27W5O55.5–LaCrO3 system

Some compositions of ceramic hydrogen permeable membranes are promising for integration in high temperature processes such as steam methane reforming due to their high chemical stability in large chemical gradients and CO2 containing atmospheres. In the present work, we investigate the hydrogen permeability of densely sintered ceramic composites (cercer) of two mixed ionic-electronic conductors: La27W3.5Mo1.5O55.5−δ (LWM) containing 30, 40 and 50wt% La0.87Sr0.13CrO3−δ (LSC). Hydrogen permeation was characterized as a function of temperature, feed side hydrogen partial pressure (0.1–0.9bar) with wet and dry sweep gas. In order to assess potentially limiting surface kinetics, measurements were also carried out after applying a catalytic Pt-coating to the feed and sweep side surfaces. The apparent hydrogen permeability, with contribution from both H2 permeation and water splitting on the sweep side, was highest for LWM70-LSC30 with both wet and dry sweep gas. The Pt-coating further enhances the apparent H2 permeability, particularly at lower temperatures. The apparent H2 permeability at 700°C in wet 50% H2 was 1.1×10−3mLmin−1cm−1 with wet sweep gas, which is higher than for the pure LWM material. The present work demonstrates that designing dual-phase ceramic composites of mixed ionic-electronic conductors is a promising strategy for enhancing the ambipolar conductivity and gas permeability of dense ceramic membranes.

Characterization of oxygen-dependent stability of selected mixed-conducting perovskite oxides

For high-temperature energy applications, e.g., as solid oxide fuel cell cathodes or as dense ceramic oxygen-transport membranes, mixed ionic-electronic conducting (MIEC) perovskites, namely of the composition (AxSr1−x)(CoyFe1−y)O3−δ (A=La, Pr, Ba), are promising candidates given their excellent oxygen-ionic and electronic transport properties. A high level of oxygen non-stoichiometry is an important prerequisite for a high performance; however, the structural stability of the oxide is also affected by oxygen deficiency.By coulometric titration experiments performed on selected state-of-the-art MIEC powder samples in a tubular zirconia “oxygen pump” setup, facilitating precise control of the oxygen partial pressure in the entire range between pO2=10−18…1bar at temperatures between 700…1000°C, the low-pO2 stability limits can be determined. By monitoring the electrical current necessary for pumping oxygen through the solid electrolyte into or out of the measurement chamber, the amount of oxygen transported and, thus, changes in oxygen stoichiometry can be determined. The low-pO2 stability limits for all investigated high-performance MIEC oxides were similar (below 10−10bar at 900°C).

Oxygen permeability of Ce0.8Sm0.2O2 − δ–LnBaCo2O5 + δ (Ln=La, Nd, Sm, and Y) dual-phase ceramic membranes

Ce0.8Sm0.2O2 − δ–LnBaCo2O5 + δ (SDC-LnBCO, Ln=La, Nd, Sm, and Y) composites were prepared and characterized as dense ceramic membranes for oxygen separation. Ce0.8Sm0.2O2 − δ (SDC) shows a good chemical compatibility with LaBaCo2O5 + δ (LBCO), NdBaCo2O5 + δ (NBCO), and SmBaCo2O5 + δ (SBCO), while a little of impurity is detected in the sample with YBaCo2O5 + δ (YBCO). The SDC-SBCO and SDC-YBCO samples exhibit larger grain size than SDC-LBCO and SDC-NBCO. The incorporation of SDC increases the structural stability and decreases the thermal expansion coefficient of LnBCO at high temperature. The oxygen permeation flux of SDC-LnBCO dual-phase membrane decreases with decreasing Ln3+ radius, which is 2.57 × 10−7, 1.58 × 10−7, 8.05 × 10−8, and 7.57 × 10−8 mol cm−2 s−1 at 925 °C for SDC-LBCO, SDC-NBCO, SDC-SBCO, and SDC-YBCO membranes, respectively. Increasing the flow rate of feeding and sweeping gas can create oxygen partial pressure gradient, and thus, increase the oxygen permeability. These results indicate that SDC-LBCO and SDC-NBCO composites are promising candidates for oxygen permeation membrane.

(Invited) MIEC Materials for Membrane Applications: Enhancing the Oxygen Transport

Mixed ionic-electronic conducting (MIEC) materials are of great interest for a variety of high-temperature applications, such as solid oxide fuel cell (SOFC) cathodes or dense ceramic membranes for gas separation. Several MIEC perovskite oxides, namely of the composition AxSr1-xCoyFe1-yO3- d (A = La, Ba), exhibit excellent oxygen-ionic and electronic transport properties. Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF), or La0.6Sr0.4CoO3-δ (LSC) are amongst these state-of-the-art high-flux materials and, hence, very promising candidates for a high-permeation oxygen transport membrane (OTM).In order to assess their applicability for OTMs under operating conditions, it is essential, though, to determine their electrochemical transport properties not only under oxidizing conditions (pure oxygen, air), but also as a function of oxygen partial pressure pO2 down to low values of, e.g., 10-6 bar. This can be achieved in a closed tubular zirconia “oxygen pump” setup [1,2] facilitating precise and continuous pO2 control in the entire range between pO2 = 10-20...1 bar above 700 °C. By performing fast pO2 step changes and applying the electrical conductivity relaxation (ECR) method to selected MIEC compositions, the oxygen equilibration kinetics of dense ceramic bulk samples of BSCF, LSCF, or LSC have been studied, yielding chemical diffusion coefficients, D δ, and surface exchange coefficients, k δ , as f(T,pO2) for 700 ≤ T / °C ≤ 900 and 10-5 ≤ pO2 / bar ≤ 0.21. Furthermore, chemical stability issues of MIEC oxides are also addressed over a wide pO2 range using this “oxygen pump”.By reducing the thickness of an OTM, the oxygen flux will be increased. However, upon reducing the thickness below a “characteristic” membrane thickness [3], given by the ratio of D δ and k δ , surface exchange becomes rate-determining for thin membranes. No enhancement of the oxygen flux is achieved by thinning the membrane any further if one cannot enhance the surface oxygen exchange. This, however, is possible by modifying the membrane surfaces with a porous functional/catalytic layer that provides more surface area for the exchange of oxygen between the gas phase and the membrane material. This becomes especially important for the permeate-side OTM surface where k δ values are reduced as a result of the low pO2. This concept has already been successfully applied to OTMs [4], facilitating unprecedented oxygen fluxes in the case of a dense BSCF membrane covered with a microporous BSCF functional layer [5], but also to SOFCs where nanoscaled LSC thin-film cathodes with a nanoporous microstructure [6] led to a large enhancement of oxygen surface reduction and, in combination with the occurrence of chemical hetero-interfaces [7], thus resulted in the best performance of SOFC cathodes reported so far in literature.With the help of a 3D FEM OTM transport model [8] the interplay of transport parameters (D δ and k δ ) and functional-layer microstructure (thickness, porosity, particle sizes) can be readily assessed. In our present study it is shown that the choice of materials in an OTM depends not only on their pO2- and temperature-dependent transport parameters, but also on phase stability, chemical compatibility and the optimum functional-layer microstructure. To this end high-resolution structural analyses (TEM) are also required.

Synthesis of Yttria-Doped Bi2o3 Nanopowders Via Sol Gel Used in Electrolyte of Solid Oxide Fuel Cell

Oxide ion conductors have been studied for many years because of their application in devices with high economical interest such as solid oxide fuel cells, oxygen sensors , dense ceramic membranes for oxygen separation. In this paper nanopowders were synthesized via sol gel method. The sintering temperature to complete phase transition are above 500˚c. As-prepared yttria doped Bi2O3 nanocrystals were characterized by X-ray diffraction (XRD) a nd transmission electron microscope (TEM). Transmission electron microscope (TEM) investigations revealed that the average particle size is less than 30 nm for these powders. It's worth noting that results show a good agreement of both methods. The morphology of the powders were observed on Field Emission scanning electron microscope (FESEM). Ionic conductivity measured by AC impedance spectroscopy.

Development of Mixed-Conducting Membranes for Hydrogen Separation

As the clean energy demands of the world continue to increase, so too will the use of hydrogen containing fuels. For harvesting “green” energy, hydrogen of sufficient purity will need to be extracted either from hydrogen-rich fuel sources or post-processing flue gases, requiring efficient separation of hydrogen from the many byproducts that arise from hydrogen production. This pure hydrogen can then be utilized for various power generation processes. While there are many different materials to be chosen as membrane constituents for this separation, proton-conducting ceramics represent a particularly appealing option for purification of hydrogen as a fuel source of solid oxide fuel cells. In these applications, the relatively high operating temperature of the fuel cell allows the integration of ceramic membranes with promising proton conductivity that is higher than or comparable to other hydrogen separation material systems. Additionally, these proton conductors need to attain high permeation rates with low fabrication costs[1]. Recently, a ceramic electrolyte composed of BaZr0.1Ce0.7Y0.1Yb0.1O3-d (BZCYYb) has demonstrated high proton conductivity that has spurred research interest on its application in ceramic electrolyte and hydrogen membrane fields. [2]. In this work, we examine two main features of membrane performance, optimal hydrogen flux of the membrane and improved microstructure through scalable manufacturing techniques.The membrane consists of a Ni-BZCYYb dense layer supported by a porous Ni-BZCYYb. The first part of the work identifies the optimal hydrogen flux through variation of the nickel content in an asymmetric dense Ni-BZCYYb pellet. The determination of the transference number for the ceramic phase allows us to optimize the electrical conductivity brought on by the nickel. The hydrogen flux through the Ni-BZCYYb was determined as a function of membrane thickness and hydrogen partial pressure. The hydrogen flux was determined by using a mass spectrometer, which correlates the measured hydrogen partial pressure to amount of hydrogen present in a known sweeping gas atmosphere. Results have shown a hydrogen flux of 0.714 ml/cm2min under 100% hydrogen at 750˚C.The microstructure also plays an important role in the performance of an asymmetric permeation membrane. For dense ceramic membranes, the ohmic permeation resistance from the proton conduction is inversely proportional to the thickness of the membrane. By using a tape-cast method, the thickness of the dense layer can be tailored down to about 10 microns. The results demonstrate an optimized membrane in terms of nickel content and thickness of asymmetric dense/porous layers. This demonstrates the ability to fabricate a cermet hydrogen permeation membrane using a large-scale, industry method.Further, various catalysts that may enhance hydrogen adsorption, dissociation, and oxidation on the surface of the hydrogen separation membranes are also explored and will discussed in the presentation.