Oxygen Transport Membranes Research Articles

Stable Ce0.8Gd0.2O2-δ oxygen transport membrane reactor for hydrogen production

Hydrogen production using oxygen transport membrane reactors has attracted widespread attention. However, the structural stability of membrane materials under harsh reducing atmospheres is still a significant challenge. Gadolinium doped cerium oxide (CGO) presents high ionic conductivity and good reducing resistance but is limited by its poor electronic conductivity. Herein, a 2 mol.% cobalt-doped Ce0.8Gd0.2O2-δ (CoCGO) ultrathin membrane was manufactured by thin-film technology and applied to hydrogen production from water splitting (WS) with simultaneous syngas production through partial oxidation of methane (POM). Two catalysts, La0.4Sr0.6CoO3-δ (LSC) and Ni/Al2O3, were utilized for promoting WS and POM, respectively. Hydrogen production rate above 1.8 mL min-1 cm-2 and methane conversion of around 80 % were achieved, and no noticeable degradation was detected during 100 h operation, suggesting its prospective stability advantages as a membrane reactor for hydrogen production from water.

Failure Mechanisms of Ba0.5Sr0.5Co0.8Fe0.2O3-δ Membranes after Pilot Module Operation.

The step from the testing of oxygen transport membranes on a lab scale to long-term operation on a large scale is a challenge. In a previous study, membrane failure was observed at defined positions of one end of the cooled tubular Ba0.5Sr0.5Co0.8Fe0.2O3-δ membranes after an emergency shutdown. To understand the failure mechanisms, strength degradation and transient stress distribution were investigated by brittle-ring tests and finite element simulations, respectively. A 15% decrease in the characteristic strength of 162 MPa was proven after aging at 850 °C and was attributed to grain coarsening. The reduction in characteristic strength after thermal shock ranged from 5 to 90% depending on the cooling rates, and from 5 to 40% after the first and 20th soft thermal cycling. Simulations indicated the chemical strains induced by a 10-bar feed air and 50 mbar permeate pressure, which caused tensile stresses of up to 70 MPa at the outer surface. These stresses relaxed to 43 MPa by creep within a 1000 h operation. A remaining local stress maximum seemed to be responsible for the fracture. It evolved near the experimentally observed fracture position during a 1000 h permeation and exceeded the temperature and time-dependent strength. The maximum stress was formed by a chemical strain at temperatures above 500 °C but effective creep relaxation needed temperatures above 750 °C.

Thermal Properties and Joinability Investigation of BaO–SrO–SiO2–B2O3 Glasses for Oxygen Transport Membrane Application

Three new BaO–SrO–SiO2–B2O3 (BS) glasses with different SrO contents (6–25 mol%) are developed for oxygen transport membrane (OTM) joining application. The content of strontium is investigated first in terms of its effect on the glass‐forming tendency, thermal expansion coefficient, crystallization, shrinkage behavior, and viscous flow properties. Differential scanning calorimetry (DSC) is carried out. Dilatometric tests are performed to obtain coefficients of thermal expansion (CTEs) of BS glasses. The crystallization behavior of the BS glasses is investigated by X‐ray powder diffraction (XRD). Sinking dilatometric measurements simulate the joining procedure and observe the shrinkage behavior of the BS glasses. The viscous flow behavior of the BS glasses is examined via hot stage microscopy. The glass with 15 mol% SrO (BS15) glass shows the best glass‐forming tendency, most matching CTE (11.9 × 10−6 K−1), densest microstructure, highest shrinkage rate (24%), and good viscous behavior at high joining temperatures compared with other BS glasses. BS15 glass is chosen for helium leak test and assembly test joining with Aluchrom and SrTi0.75Fe0.25O3−δ membrane (STF25). The sandwiched sample with two Aluchrom plates sealed by BS15 glass at 1075 °C for 5 min achieves good gas‐tightness with low helium leakage rate <10−9 mbar·l s−1.

Reduced strength degradation of reactive air brazed Ba0.5Sr0.5Co0.8Fe0.2O3-δ membranes by pre-oxidation of metallic components

Oxygen transport membranes can be joined leak-tight and high-temperature resistant to metal components by reactive air brazing. To overcome the typical strength degradation of these braze joints during aging at the application temperature of 850 °C, the diffusion-controlled growth of mixed oxide layers at the interface between metal and braze alloy has to be reduced. In this study, we investigated how a pre-oxidation of the chromia forming austenitic steel X15CrNiSi25-21 and alumina forming ferritic steel X8CrAl25-5 influences the bending strength of BSCF-Ag3CuO-steel joints. Brazed double joint specimens were aged for 1000 h at 850 °C in air prior to 4-point bending and subsequent macroscopic as well microscopic analyses.Pre-oxidation of steel X15CrNiSi25-21 reduced the strength degradation during aging from 75 to 59% and effectively suppressed chromium poisoning of BSCF. The minimum fracture stress was raised from 0 to 24 MPa which significantly increased the reliability. For the first time, it was possible to obtain mixed and ceramic fracture types in addition to poor-strength delamination fractures. The strength of pre-oxidized X8CrAl25-5 joints was affected by axial cracks and pores at the mixed oxide layer and lower compared to joints with pre-oxidized X15CrNiSi25-21. In contrast to pre-oxidized X15CrNiSi25-21, no acicular phases were identified in the BSCF. Nevertheless, the apparently dense Al2O3 layer could not avoid chromium diffusion, as Ba–Cr oxides were detected in the braze.

Recent Progress and Challenges in Mixed Ionic–Electronic Conducting Membranes for Oxygen Separation

Carbon neutrality refers to the state of making net‐zero emissions of carbon dioxide (CO2), which is a key concept in tackling global warming. It can be achieved by offsetting carbon emissions as well as balancing reduced and emitted emissions. Globally, CO2 is emitted mainly from fossil fuel power plants. The use of carbon capture and storage technology, including pre‐, post‐, and oxy‐fuel combustion capture, in power plants can provide a carbon‐neutral strategy that allows for the sustainable use of fossil fuels while potentially reducing CO2 emissions. Oxy‐fuel combustion capture facilitates CO2 capture by simplifying combustion products through the reaction of recirculated flue gas with a high‐purity oxygen. Oxygen transport membranes, which produce pure oxygen by oxygen transport via oxides at high temperatures, have attracted increased interest because they can improve overall efficiency when integrated with oxy‐fuel combustion capture. Dual‐phase membranes with fluorite structure have greater potential for commercialization than perovskite‐based single‐phase membranes, which have poor chemical and mechanical properties. However, despite these advantages, their low oxygen permeability remains a critical issue. This review focuses on progress in the development of dual‐phase membranes and summarizes various approaches that can facilitate bulk diffusion and surface exchange, which affect the oxygen permeability.

Oxygen permeability and surface kinetics of composite oxygen transport membranes based on stabilized δ-Bi2O3

Composite ceramic membranes based on the ionic conducting Tm-stabilized δ-Bi2O3 (BTM) and the electronic conducting (La0.8Sr0.2)0.99MnO3-δ (LSM) exhibit among the highest oxygen flux values reported for Bi2O3-based membranes. Here, we use pulse-response isotope exchange (PIE) and oxygen flux measurements to elaborate on limiting factors for the oxygen permeation in BTM - 40-70 vol% LSM composites. Once both phases percolate, between 30 and 50 vol% BTM, the flux is essentially independent of the BTM/LSM volume ratio. The oxygen permeability is under mixed diffusion- and surface control, gradually becoming more bulk-limited with increasing temperature. The oxygen exchange coefficients of BTM-LSM are significantly higher than its constituent phases, revealing that a cooperative surface exchange mechanism enhances the kinetics. Some of the Tm was substituted with Pr to introduce electronic conductivity in BTM. (Bi0.8Tm0.15Pr0.05)2O3-δ (BTP) exhibits higher surface exchange coefficients compared to BTM, but the oxygen flux remains one order of magnitude lower than that of percolating BTM-LSM composites.

Stable, asymmetric, tubular oxygen transport membranes of (Sc2O3)0.10(Y2O3)0.01(ZrO2)0.89 – LaCr0.85Cu0.10Ni0.05O3-δ

Oxygen transport membranes have the potential to deliver pure and cheap oxygen to chemical reactors, combustors, gasifiers etc., given that geometry, microstructure and material properties are optimized. This work demonstrates the first successful preparation of dual-phase tubular, asymmetric oxygen transport membranes consisting of a new electronic conductor LaCr0.85Cu0.10Ni0.05O3-δ mixed with (Sc2O3)0.10(Y2O3)0.01(ZrO2)0.89 as ionic conductor. Challenges related to Cr-volatility were overcome by using Fe2O3 as a sintering aid. The sintering aid decreased mismatches in shrinkage and thermal expansion between the four layers in the asymmetric membrane and decreased the sintering temperature such that the Cr-volatility was suppressed. The membranes reached an oxygen flux of 0.28 ml∙min−1∙cm−2 in an air/N2 atmosphere at 950 °C. Furthermore, the membranes showed a stable oxygen flux after exposure to different atmospheres, including air/CO2 and air/H2 gradients. The successful fabrication of stable, asymmetric, tubular membranes opens the possibility for future integration in syngas or oxy-combustion applications.

Microstructure coarsening in Ba0.5Sr0.5Co0.8Fe0.2O3-δ during reactive air brazing

Reactive air brazing of the ceramic oxygen transport membrane material BSCF causes microstructural modifications. The trend towards minimizing of membrane thicknesses requires a physical understanding of these modifications since they may influence both oxygen permeation and mechanical properties. To this purpose, wetting samples with variations in the Ag-xCuO braze alloy (1 < x < 25 at.-%) and the brazing time (0 < t < 120 min) were characterized by quantitative image analysis. We found that the pore size in reactive air brazed BSCF increases with increased brazing time as well as CuO-content in the braze. A local porosity minimum is always observed at the end of the reaction zone close to the unaffected bulk BSCF. Additionally, a zone with unidirectionally elongated grains at the end of which silver residues were found is observed. The observed coarsening effects are explained by liquid melt film penetration along the grain boundaries which increases the grain boundary mobility. Phase field simulations qualitatively confirmed the experimentally observed elongated grain structure.

Sensitivity of Material, Microstructure and Operational Parameters on the Performance of Asymmetric Oxygen Transport Membranes: Guidance from Modeling.

Oxygen transport membranes can enable a wide range of efficient energy and industrial applications. One goal of development is to maximize the performance by the improvement of the material, microstructural properties and operational conditions. However, the complexity of the transportation processes taking place in such commonly asymmetric membranes impedes the identification of the parameters to improve them. In this work, we present a sensitivity study that allows identification of these parameters. It is based on a 1D transport model that includes surface exchange, ionic and electronic transport inside the dense membrane, as well as binary diffusion, Knudsen diffusion and viscous flux inside the porous support. A support limitation factor is defined and its dependency on the membrane conductivity is shown. For materials with very high ambipolar conductivity the transport is limited by the porous support (in particular the pore tortuosity), whereas for materials with low ambipolar conductivity the transport is limited by the dense membrane. Moreover, the influence of total pressure and related oxygen partial pressures in the gas phase at the membrane’s surfaces was revealed to be significant, which has been neglected so far in permeation test setups reported in the literature. In addition, the accuracy of each parameter’s experimental determination is discussed. The model is well-suited to guiding experimentalists in developing high-performance gas separation membranes.

Oxygen production routes assessment for oxy-fuel combustion

Oxyfuel combustion is a promising alternative to decarbonize the power sector. However, the main barrier to commercial deployment of the technology is the high energy consumption associated with oxygen production (∼200–300 kWh per ton of O2), which penalizes the thermal-to-electric efficiency of 8.5–12% compared to traditional air combustion plants. Typically, oxygen is obtained from a cryogenic air separation process. However, other technologies have been gaining momentum in recent years, such as membrane technologies, chemical looping air separation, and renewable-driven electrolysis. The present work evaluates all these options for O2 production to retrofit a 550 MWe coal-fired power plant with oxyfuel combustion. A techno-economic assessment is carried out to estimate the energy penalty, the O2 production cost (€/ton) and the Levelized Cost of Electricity. The best results are obtained by combining oxygen transport membranes and electrolysis since the energy consumption has been reduced to 98.56 kWh/ton of O2, decreasing by 59.31% the cryogenic distillation energy consumption (242.24 kWh/ton O2), reducing the overall energy penalty compared to cryogenic air separation from 8.88% points to 7.56%points. The oxygen transport membrane presents the lowest cost of electricity in retrofitting cases, 51.48 $/MWh, while cryogenic distillation estimated cost is 52.7 $/MWh. Their costs of avoided CO2 are 31.79 $/ton CO2 and 34.15 $/ton CO2 respectively.

The in situ generated emerging phase inside dual phase oxygen transport membranes

The in situ generated emerging phase inside the dual-phase oxygen transport membranes (DP-OTMs) plays a crucial role in boosting the overall performance of DP-OTMs. However, its detailed structure and properties are still not fully understood. Utilizing advanced transmission electron microscopy (TEM) techniques, the emerging phase GdxCe1-xFeyCo1-yO3-δ (GCFCO) inside the CexGd1-xO2-δ-FeCo2O4 (CGO-FC2O) OTMs was successfully characterized at the atomic scale. The newly formed GCFCO is primarily surrounded by the CGO, and contributes to a significant reduction of non-solute segregation at the CGO grain boundaries. Electronic characteristics of the GCFCO shows a sensitive dependence on its chemical composition, including the valence state of Ce and Fe as well as the oxygen vacancies. Additional CGO-GCFCO interfaces were introduced, where almost intact crystal structures were observed with slight Gd and Co segregation ∼1 nm at the edges. Approaching the interface, on the CGO side, only a minimum drop of the Ce valence was determined. On the GCFCO side, mixed Ce3+ and Ce4+ are partially occupying the Gd sites, while Fe and Co valence stay constant until the edge. Our study provides novel insight into the phase information within CGO-FC2O composites, which paves the path towards superior performance of various DP-OTMs.

Performance study of asymmetric oxygen transport membranes with vertically channelled pores by phase inversion tape casting

Asymmetric oxygen transport membranes have been extensively studied revealing the importance of the support porosity. In this study, asymmetric membranes with vertically channelled pores were prepared from Ba0.5Sr0.5(Co0.8Fe0.2)0.97Zr0.03O3-δ (BSCF-Z) by phase inversion tape casting. The rate-limiting effects of the membrane layer thickness, the support structure, and the activation layers were thoroughly analysed. This entails comparing the permeation rate of the samples with the membrane layer on different sides of the channelled supports. Pore tortuosity and computed permeability were evaluated from 3D-X-ray computed tomography and compared to previously reported asymmetric membranes prepared by tape-casting and freeze-drying. Due to a lower pore tortuosity, the channelled supports have an advantage in gas transport over the tape cast and freeze cast supports. However, this is compensated by a thicker membrane layer (∼54 μm) resulting in similar performance compared to the thinner membrane layers (∼20 μm) with freeze cast and tape cast supports.

Permeability and stability enhancement of dual-phase membrane by nickel-based porous layer for water splitting

The high oxygen permeability and stability of oxygen transport membrane (OTM) materials remain critical issues in the implementation of hydrogen production via thermochemical water splitting. In this study, a novel design of a fluorite phase Ce0.85Pr0.15O2-δ (CP) porous layer coupled with a nickel-based catalyst anchored to a dual-phase Ce0.85Pr0.15O2-δ-Pr0.6Sr0.4Fe0.9Al0.1O3-δ (CP-PSFA) ceramic membrane is developed owing to its excellent tolerance in the reductive atmosphere during static experiments. The oxygen permeability and hydrogen production performance of the modified CP-PSFA OTM were improved significantly with the coating of the CP porous layer; that is, a high O2 permeation flux of 3.7 mL cm-2 min-1 at 925 °C under a reductive atmosphere was achieved, which was approximately 43% higher than that of its counterpart. Meanwhile, after the nickel-based catalyst was loaded, the hydrogen production rate reached 1.79 mL cm-2 min-1 at 925 °C with the condition of optimal catalyst loading (30 wt% NiO), which shows a 64% increase compared to its counterpart. In addition, the long-term stability of the modified membranes remained intact. The results prove that the CP porous layer loaded with a nickel-based catalyst improves the permeability of the CP-PSFA ceramic membrane as well as the stability, providing an effective strategy for the design of high-performance OTMs for thermochemical water splitting for hydrogen production.

Testing of high performance asymmetric tubular BSCF membranes under pressurized operation – A proof-of-concept study on a 7 tube module

Oxygen transport membranes are a possible low-cost alternative to cryogenic air separation units or pressure swing adsorption for oxygen production, and could facilitate carbon capture by oxy-fuel combustion processes. In this work, a proof-of-concept module with 7 ceramic tubular membranes was designed and tested in a high pressure test stand. To ensure high oxygen production rates, asymmetric membranes with a length of 70 cm consisting of a porous Ba0.5Sr0.5(Co0.8Fe0.2)0.97Zr0.03O3-δ support and a dense Ba0.5Sr0.5(Co0.8Fe0.2)0.97Zr0.03O3-δ thin separation layer were used. The module was successfully operated at up to 5.4 bar for 280 h in a temperature range from 650 °C to 850 °C. Oxygen purities up to 96.5% and flow rates up to 6.2 ml/cm2/min were measured, and a total maximum oxygen production of 227 NLPH was demonstrated.

Evaluation of Er Doped CeO2-δ as Oxygen Transport Membrane.

Ceria based materials are robust candidates for a range of applications involving redox reactions and high oxygen activity. The substitution of erbium in the ceria lattice introduces extrinsic oxygen vacancies. Further addition of Co introduces electronic carriers. We have studied the structural and redox behavior of Ce1−xErxO2-δ (x = 0.1 and 0.2) and the influence of adding 2 mol% of Co in the electrochemical properties. A limitation in the solubility of Er cation is found. Diffusion and surface exchange coefficients have been obtained by electrical conductivity relaxation and the DC-conductivity and O2 permeation measurements show the importance of the electronic component in the transport properties, obtaining an oxygen permeation flux of 0.07 mL·min−1·cm−2 at 1000 °C, for a 769 μm thick membrane.

Sealing behaviour of glass-based composites for oxygen transport membranes

In this study, seven different filler materials in different proportions were added to a Ba-Ca-Si glass matrix “H” to investigate new sealant with higher thermal expansion coefficient (CTE) value and good sealing performance for application in oxygen transport membrane (OTM). SrTi0.75Fe0.25O3-δ (STF25) was used as an OTM, and the sealing partners were ferritic steel Aluchrom and pre-oxidized Aluchrom. Compatibility tests were carried out to investigate the feasibility of the composites. Higher CTE values were found in dilatometer tests on composite samples by adding 40 wt% Ag (HAg40) and 30 wt% Ni-Cr (HNC30). Gas-tightness measurements of sandwiched samples produced appropriate helium leakage rates in the range of 10−6 mbar·l·s−1. Sealing behaviour of sealants HAg40 and HNC30 were investigated by joining STF25 and as-delivered/pre-oxidized Aluchrom together. Scanning electron microscopy (SEM) on cross-sections of the joints revealed a homogeneous microstructure and good adherence of the glass sealants to support metals and STF25.

Development of a Membrane Module Prototype for Oxygen Separation in Industrial Applications.

The integration of oxygen transport membranes in industrial processes can lead to energy and economic advantages, but proof of concept membrane modules are highly necessary to demonstrate the feasibility of this technology. In this work, we describe the development of a lab-scale module through a comprehensive study that takes into consideration all the relevant technological aspects to achieve a prototype ready to be operated in industrial environment. We employed scalable techniques to manufacture planar La0.6Sr0.4Co0.2Fe0.8O3-δ membrane components suitable for the application in both 3- and 4-end mode, designed with a geometry that guarantees a failure probability under real operating conditions as low as 2.2 × 10−6. The asymmetric membranes that act as separation layers showed a permeation of approx. 3 NmL/min/cm2 at 900 °C in air/He gradient, with a remarkable stability up to 720 h, and we used permeation results to develop a CFD model that describes the influence of the working conditions on the module performance. The housing of the membrane component is an Inconel 625 case joined to the membrane component by means of a custom-developed glass–ceramic sealant that exhibited a remarkable thermo-chemical compatibility both with metal and ceramic, despite the appearance of chemical strain in LSCF at high temperature. The multi-disciplinary approach followed in this work is suitable to be adapted to other module concepts based on membrane components with different dimensions, layouts or materials.

High CO2 resistance of indium-doped cobalt-free 60 wt.%Ce0.9Pr0.1O2-δ-40 wt.%Pr0.6Sr0.4Fe1-In O3-δ oxygen transport membranes

The oxygen transport membrane (OTM) has huge application prospects in gas separation and carbon neutralization based on oxygen enriched combustion. In this paper, the family 60 wt.%Ce0.9Pr0.1O2-δ-40 wt.%Pr0.6Sr0.4Fe1-xInxO3-δ (CPO-PSF1-xIxO, x = 0.01, 0.025, 0.05, 0.075, 0.1) cobalt-free dual-phase MIEC OTMs doped with indium have been successfully prepared by Pechini method. The phase structure, surface morphology, element distribution, oxygen permeability, and long-term operation stability of these OTMs are systematically explored. Among these OTMs, the champion oxygen permeable flux of CPO-PSF0.99I0.01O reaches 1.07 mL min−1·cm−2 and 0.80 mL min−1·cm−2 at 1000 °C under air/He gradient and air/CO2 gradient. Meanwhile, CPO-PSF0.99I0.01O maintains the value of 0.80 mL min−1·cm−2 steadily at 1000 °C for 100 h when pure CO2 as the sweep gas. The surface element distribution and phase structure of the OTMs after long-term oxygen permeability reaction are investigated by XRD, SEM combining with EDS, where the spent membranes retain the same structure and component as the fresh membranes, demonstrating that the In-doped OTMs have an excellent CO2 tolerance. Suitable indium substitution for iron of these OTMs not only improves the oxygen permeability, but also maintains the long-term reaction stability of the material.

Simultaneously Enhanced Permeability and Stability in Al Doped Pr0.6sr0.4feo3-Δ Oxygen Transport Membrane for Co2 Capture

Simultaneously Enhanced Permeability and Stability in Al Doped Pr0.6sr0.4feo3-Δ Oxygen Transport Membrane for Co2 Capture

Oxygen Permeability and Surface Kinetics of Composite Oxygen Transport Membranes Based on Stabilized Δ-Bi2o3

Oxygen Permeability and Surface Kinetics of Composite Oxygen Transport Membranes Based on Stabilized Δ-Bi2o3