Stable Oxygen Permeation Research Articles

Effects of copper doping on microstructural evolution and oxygen permeability of Ce0.8Sm0.2O2-δ-Sm0.6Sr0.4Fe1-xCuxO3-δ dual-phase oxygen permeable membranes

Ce0.8Sm0.2O2-δ(wt.70 %)-Sm0.6Sr0.4Fe1-xCuxO3-δ(wt.30 %) (SDC-SSFC) dual-phase oxygen permeable membranes were prepared by one-pot EDTA-citric acid combined complexing method. Effects of copper doping content on crystallographic phase, microstructure and oxygen permeability were deeply investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDXS). Systematic studies reveal that SDC as oxygen ion conductors and SSFC as electron conductor display an excellent synergistic effect only if a suitable doping content of copper was adopted. It can be found that Ce0.8Sm0.2O2-δ-Sm0.6Sr0.4Fe0.9Cu0.1O3-δ exhibits the highest oxygen permeation flux of up to about 0.61 ml cm−2 at 950 °C and lower oxygen permeation activation energy, which displays nearly a two-fold increase in oxygen permeability compared with the sample without any copper doping. If the copper doping content in B-site of SSFC perovskite exceeds about 30 %, a third phase, i.e., CuO, would appear in the bulk of SDC-SSFC dual-phase membrane and the grain size of CuO tend to gradually increase which would degrade the oxygen permeability of SDC-SSFC dual-phase membrane. Kinetical analysis showed that the critical membrane thickness (Lc) of Ce0.8Sm0.2O2-δ-Sm0.6Sr0.4Fe0.9Cu0.1O3-δ membrane is close to approximately 0.6 mm, i.e., if the membrane thickness is larger than 0.6 mm, its rate-determining step would transform into the bulk-diffusion exchange from surface exchange. Meanwhile, Ce0.8Sm0.2O2-δ-Sm0.6Sr0.4Fe0.9Cu0.1O3-δ membrane exhibits a stable oxygen permeation flux of up to 0.32 ml cm−2·min−1 in pure CO2 atmosphere for more than 120 h without any degradation.

Role of Fe/Co Ratio in Dual Phase Ce0.8Gd0.2O2-δ-Fe3-xCoxO4 Composites for Oxygen Separation.

Dual-phase membranes are increasingly attracting attention as a solution for developing stable oxygen permeation membranes. Ce0.8Gd0.2O2-δ-Fe3-xCoxO4 (CGO-F(3-x)CxO) composites are one group of promising candidates. This study aims to understand the effect of the Fe/Co-ratio, i.e., x = 0, 1, 2, and 3 in Fe3-xCoxO4, on microstructure evolution and performance of the composite. The samples were prepared using the solid-state reactive sintering method (SSRS) to induce phase interactions, which determines the final composite microstructure. The Fe/Co ratio in the spinel structure was found to be a crucial factor in determining phase evolution, microstructure, and permeation of the material. Microstructure analysis showed that all iron-free composites had a dual-phase structure after sintering. In contrast, iron-containing composites formed additional phases with a spinel or garnet structure which likely contributed to electronic conductivity. The presence of both cations resulted in better performance than that of pure iron or cobalt oxides. This demonstrated that both types of cations were necessary to form a composite structure, which then allowed sufficient percolation of robust electronic and ionic conducting pathways. The maximum oxygen flux is jO2 = 0.16 and 0.11 mL/cm2·s at 1000 °C and 850 °C, respectively, of the 85CGO-FC2O composite, which is comparable oxygen permeation flux reported previously.

Oxygen transport kinetics of BSCF-based high entropy perovskite membranes

Three high entropy BSCF perovskite membranes were designed by doping various cations into A- and/or B-sites to improve the oxygen permeation stability at 700–800 °C. The membrane with the highest mixed entropy (ΔSmix), i.e. HEBSCF-AB, can be operated stably at 750 and 800 °C, but unstable at 700 °C. To understand the degradation mechanism, the oxygen transport kinetics of the three high entropy perovskite materials were analyzed using the permeation model established by our group. An easy and feasible strategy was proposed to improve the stability of oxygen permeation. We find that the doping site has a significant influence on the bulk diffusion resistance of the membranes but has a weak effect on the interfacial exchange resistances. The continuous attenuation of HEBSCF-AB membrane at 700 °C is mainly caused by the increase of interfacial exchange resistance of the sweeping side, revealing the HEBSCF-AB oxide is not a stable catalyst for the oxygen evolution reaction when it is coated on the HEBSCF-AB membrane surfaces. By coating Sm0.5Sr0.5CoO3-δ (SSC) as the porous layer on the HEBSCF-AB surfaces for oxygen activation, the oxygen permeation flux at 700 °C becomes stable. ​It confirms that the simple approach is effective to improve the permeation stability and the high entropy strategy works well to restrain the phase transition of materials.

Zirconium and Yttrium Co-Doped BaCo0.8Zr0.1Y0.1O3-δ: A New Mixed-Conducting Perovskite Oxide-Based Membrane for Efficient and Stable Oxygen Permeation.

Oxygen permeation membranes (OPMs) are regarded as promising technology for pure oxygen production. Among various materials for OPMs, perovskite oxides with mixed electron and oxygen-ion (e−/O2−) conducting capability have attracted particular interest because of the high O2− conductivity and structural/compositional flexibility. However, BaCoO3−δ-based perovskites as one of the most investigated OPMs suffer from low oxygen permeation rate and inferior structural stability in CO2-containing atmospheres. Herein, zirconium and yttrium co-doped BaCoO3−δ (BaCo1−2xZrxYxO3−δ, x = 0, 0.05, 0.1 and 0.15) are designed and developed for efficient and stable OPMs by stabilizing the crystal structure of BaCoO3−δ. With the increased Zr/Y co-doping content, the crystal structural stability of doped BaCoO3−δ is much improved although the oxygen permeation flux is slightly reduced. After optimizing the co-doping amount, BaCo0.8Zr0.1Y0.1O3−δ displays both a high rate and superior durability for oxygen permeation due to the well-balanced grain size, oxygen-ion mobility, crystal structural stability, oxygen vacancy concentration and surface exchange/bulk diffusion capability. Consequently, the BaCo0.8Zr0.1Y0.1O3−δ membrane delivers a high oxygen permeation rate of 1.3 mL min−1 cm−2 and relatively stable operation at 800 °C for 100 h. This work presents a promising co-doping strategy to boost the performance of perovskite-based OPMs, which can promote the industrial application of OPM technology.

CO2-stable and cobalt-free Ce0.8Sm0.2O2-δ-La0.8Ca0.2Al0.3Fe0.7O3-δ dual-phase hollow fiber membranes for oxygen separation

In this work, 60 wt% Ce0.8Sm0.2O2-δ-40 wt% La0.8Ca0.2Al0.3Fe0.7O3-δ (SDC-LCAF) dual-phase oxygen permeable hollow fiber membrane precursors were prepared via a combined sol–gel and phase inversion technique. Dual-phase SDC-LCAF hollow fiber membrane with compatible fluorite and perovskite phases was obtained after sintering at 1450 °C. The chemical compatibility, thermal expansion, conductivity, oxygen permeation, CO2 tolerance, and short-term regenerative durability regarding the phase structure and composition were systematically studied. The oxygen flux of the SDC-LCAF dual-phase membrane was significantly increased compared to that of LCAF single phase membrane. At 950 °C, the oxygen flux of the SDC-LCAF hollow fiber membrane reached 1.84 mL min−1 cm−2 when both the He sweep and the air feed flow rates were 100 mL min−1, and the dual-phase membrane exhibited stable oxygen permeation fluxes in pure CO2 atmosphere. When pure CO2 was used as the sweep gas in a 225-h short-term stability test, the oxygen permeation fluxes decreased slightly because of the inhibiting effect of CO2 on oxygen surface-exchange reaction. Nevertheless, the ability of the SDC-LCAF membrane to recover the original oxygen permeation performance indicated that the membrane surface did not react irreversibly with CO2. The CO2 stability was not reflected in the LCAF single phase membrane, thus indicating the superior membrane stability following the incorporation of the SDC phase.

Improving intermediate-temperature stability of BSCF by constructing high entropy perovskites

High entropy perovskites bring more space for materials design in many fields. The stability of materials in thermodynamics can be improved by increasing the mixed entropy. In this work, a series of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) based high entropy perovskite (HEBSCF) were designed to improve the stability of BSCF at intermediate temperatures. The influence of high entropy composition on the lattice parameter, microstructure and stability of HEBSCF were investigated. The results show that HEBSCF can accommodate cations with large size differences. Compared with BSCF, doping elements at A site (HEBSCF-A), B site (HEBSCF-B) or both sites (HEBSCF-AB) can improve the mixed entropy. Among the three doped BSCF, HEBSCF-AB has the highest mixed entropy and shows stable oxygen permeation flux at 750 and 800 °C up to 300 h. No phase transition was observed on HEBSCF-AB after the long-term tests at intermediate temperatures. This research indicates that the high entropy stabilization strategy is feasible to improve the permeation stability of perovskite membranes by inhibiting phase transition.

Zirconium and Yttrium Co-Doped Baco0.8zr0.1y0.1o3-Δ: A New Mixed-Conducting Perovskite Oxide-Based Membrane for Efficient and Stable Oxygen Permeation

Zirconium and Yttrium Co-Doped Baco0.8zr0.1y0.1o3-Δ: A New Mixed-Conducting Perovskite Oxide-Based Membrane for Efficient and Stable Oxygen Permeation

Oxygen-permeable ceramic membrane with improved mediate-temperature stability based on partially A-site doped KxSr1-xCo0.8Fe0.2O3-δ

ABSTRACT Practical application of SrCo0.8Fe0.2O3-–δ(SCF) perovskite oxide for oxygen separation is hindered by poor stability. Here, we developed new K x Sr1–x Co0.8Fe0.2O3–δ membrane material by optimizing K+ doping in A-site of prototypical SCF. The partial substitution of Sr2+ with K+ can lead to the enhanced mechanical strength and intermediate-temperature oxygen permeation stability. It is found that the doping content of K+ should be lower than 20% because the excessive doping of K+ in A-site of SrCo0.8Fe0.2O3–δcan destroy their cubic perovskite structure. While x ≥ 0.2, some diffraction peaks different from cubic perovskite can be evidenced by X-ray diffraction and their oxygen permeation flux also decreases rapidly. Among the fabricated membranes, K0.1Sr0.9Co0.8Fe0.2O3–δexhibits the highest oxygen permeation flux (1.5 cm3 min−1 cm−2 at 950°C) under an air/He gradient. More important is that K0.1Sr0.9Co0.8Fe0.2O3–δ displays a stable oxygen permeation performance during over 120 h permeation operation at 750°C.

CO2‐resistant SDC‐SSAF oxygen selective dual‐phase hollow fiber membranes

AbstractCe0.85Sm0.15O1.925–Sm0.6Sr0.4Al0.3Fe0.7O3‐δ (SDC‐SSAF) hollow fiber membranes aimed for oxyfuel application are fabricated via a phase inversion‐sintering technique using powders synthesized from EDTA–citrate complex method. Oxygen ions transport is revealed to be the limiting step that determines the overall oxygen transport rate in SDC‐SSAF. SDC‐SSAF hollow fiber membrane attained a maximum flux of 2.6 ml min−1 cm−2 at 950°C under an oxygen partial pressure difference of 1/0.02 atm (feed/permeate). Variation in oxygen partial pressure on the permeate side exerts larger influence on the oxygen permeation flux relative to the change on the feed side. Stability tests over 100 h demonstrate stable oxygen permeation flux under both He and CO2 sweep conditions.

Oxygen permeability and structural stability of CO2-stable SDC–SSCFG dual-phase membrane

Ce0·8Sm0·2O2−δ –Sm0·3Sr0·7Co0·6Fe0·3Ga0·1O3−δ dual-phase membranes with different weight ratios were successfully developed, and the microstructure, structural stability, oxygen (O2) permeability and oxygen permeation stability using helium (He) or carbon dioxide (CO2) as sweeping gas were systematically investigated. It is found that the oxygen fluxes through dual-phase membranes decrease with the increase in Ce0·8Sm0·2O2−δ content. The oxygen permeation flux of 80Ce0·8Sm0·2O2−δ –20Sm0·3Sr0·7Co0·6Fe0·3Ga0·1O3−δ dual-phase membrane reaches 0·48 ml/(min cm2) with pure carbon dioxide as the sweeping gas. By reducing the thickness of the membrane and porous Sm0·7Sr0·3CoO3−δ activation layer coating on the permeate side, a stable flux of 0·62 ml/(min cm2) can be obtained during a long-term oxygen permeation operation for 100 h. The Ce0·8Sm0·2O2−δ –Sm0·3Sr0·7Co0·6Fe0·3Ga0·1O3−δ dual-phase membranes show excellent oxygen permeability and structural stability in carbon dioxide atmosphere and could have great potential application in oxy-fuel techniques for carbon dioxide capture and storage.

A novel lanthanum strontium cobalt iron composite membrane synthesised through beneficial phase reaction for oxygen separation

Abstract Composite ceramic membrane is one of the most attractive concepts which combines the advantages of different phases into a single membrane matrix. Recently, the reported significant increased oxygen surface kinetics on the Perovskite/Ruddlesden-Popper composite system because of the formation of novel and fast oxygen transport paths along the hetero-interface has been implanted into the oxygen permeation membrane system. In this work, a novel La0.6Sr0.4Co0.2Fe0.8O3-δ-(La0.5Sr0.5)2CoO4+δ (LSCF-LSC) composite hollow fiber membrane is synthesized with oxygen permeation flux of 4.52 mL min−1 cm−2 at 950 °C. It presents round 4 times and 2.3 times of that of the single LSCF membrane and LSC-coated LSCF membrane at 900 °C. For better comparison, (La0.576Sr0.424)1.136Co0.3Fe0.7O3-δ (LSCF-new) is prepared based on the composition of LSCF-LSC composite. The enhanced oxygen permeability was further investigated through electrochemical impedance spectroscopy (EIS) measurements. We also confirm that LSCF-LSC shows significantly lower area specific resistance (ASRs) for LSCF-LSC|Ce0.8Sm0.2O1.9 (SDC)|LSCF-LSC symmetrical cell relative to other symmetrical cells. This novel LSCF-LSC composite membrane also presents high CO2 tolerance, with stable oxygen permeation fluxes round 2.6 mL min−1 cm−2 at 900 °C for 100 h.

High-performance oxygen permeation membranes: Cobalt-free Ba0.975La0.025Fe1-xCuxO3-δ ceramics

A new group of cobalt-free perovskite oxides, Ba0.975La0.025Fe1-xCuxO3-δ (BLFC, x = 0.05–0.15), was designed, characterized and applied as oxygen permeation membranes. It was found that BLFC oxides with Cu doping range of 0.075–0.15 maintain cubic perovskite phase in a wide range of temperatures. More Cu introduced at the B-site results in a gradual increase of the electrical conductivity, which is attributed to the denser overlapping of electron clouds of Cu–O bonds. With increasing Cu content, the oxygen vacancy concentration increases and the oxygen ion migration energy decreases, leading to the highest oxygen permeation flux of 1.59 mL cm−2 min−1 recorded for Ba0.975La0.025Fe0.9Cu0.1O3-δ 1 mm thick membrane at 950 °C. However, the oxygen permeability decreases with further Cu doping, which may be correspond to a presence of defect association. Ba0.975La0.025Fe0.9Cu0.10O3-δ membrane with 0.7 mm thickness delivers stable oxygen permeation flux of 1.57 mL cm−2 min−1 for 200 h at 900 °C. All of the obtained results indicate that the developed BLFC with optimized Cu content (i.e. x = 0.1) is a very promising material for usage in oxygen separation applications.

Various influence of surface modification on permeability and phase stability through an oxygen permeable membrane

Good oxygen permeability and stability of oxygen transport membrane are highly necessary for practical applications. Herein, through different theories, the oxygen transport limitation step through the Ruddlesden-Popper (Pr0.9La0.1)1.9Ni0.74Cu0.21Ga0.05O4+δ ((PL)1.9NCG) membrane was demonstrated firstly, which suggest surface modification can be an effective approach to improve the oxygen separation performance. After coating, various influences of different side surface modification on permeability and phase stability were observed. The sweep-side coated membrane exhibits largely enhanced permeation fluxes than feed-side coated membrane, and the feed-side coated membrane shows better phase stability under the same conditions (these results reveal that the sweep side is the permeability limitation side and the feed is the phase stability limitation side). The both side coated membrane combines abovementioned advantages which shows 57% enhanced and stable oxygen permeation flux at 800 °C. The observed various modification effects on the permeation performance are discussed based on the surface exchange properties and the mechanism of the oxygen transporting membrane in detail.

Materials design for ceramic oxygen permeation membranes: Single perovskite vs. single/double perovskite composite, a case study of tungsten-doped barium strontium cobalt ferrite

Pure oxygen is an important raw material with many important applications. The production of oxygen via a conducting ceramic membrane is a new, cost-effective and advanced technology with the advantage of continuous oxygen production. The perovskite-type mixed-conducting oxide Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) exhibits superb oxygen permeability, yet it suffers from poor phase stability. In this study, we aim to improve the operational stability of the BSCF membrane by introducing a high-valence W6+ ion as a B-site dopant. Its effect on the phase composition, structure, structural stability, electrical conductivity, oxygen transfer rate and oxygen permeability as a membrane is systematically investigated. Upon the partial substitution of cobalt and iron in the W6+-doped BSCF, single/double perovskite composites are formed instead of single perovskite composites. Remarkably, the formation of the single/double perovskite composites enhances the oxygen permeation stability without obviously compromising the oxygen permeability. Among the various materials, the composite with the nominal composition of Ba0.5Sr0.5Co0.8Fe0.1W0.1O3-δ shows the best performance in terms of stability and oxygen permeability. These findings thus introduce a new way to design conducting ceramic membranes for oxygen separation at high temperatures.

Effects of B-site Nb doping on the CO2 resistance and rate-controlling step of Ce0.8Gd0.2O2−δ–Pr0.6Sr0.4Co0.5Fe0.5O3−δ dual-phase membranes

Dense fluorite–perovskite-type dual-phase membranes of 60% Ce0.8Gd0.2O2−δ–40% Pr0.6Sr0.4Co0.5Fe0.4Nb0.1O3−δ (CG–PSCF0.4N0.1) were prepared successfully via ethylenediaminetetraacetic acid (EDTA)–citric acid method. Subsequently, the effects of B-site Nb doping on the CO2 resistance, oxygen permeability, and rate-controlling step of Ce0.8Gd0.2O2−δ–Pr0.6Sr0.4Co0.5Fe0.5O3−δ (CG–PSCF) were systematically investigated. The results of high-temperature in situ X-ray diffraction under pure CO2 atmosphere, X-ray photoelectron spectra, and oxygen permeation tests indicated that the thermal stability and CO2 resistance of CG–PSCF could be improved significantly via Nb doping. Furthermore, by measuring the distributions of permeation resistances at certain temperatures using the adopted permeation model, the rate-controlling step was determined, and the oxygen permeability degradation mechanism of CG–PSCF was attributed to the increased bulk diffusion resistance caused by phase transition of PSCF and formation of carbonate. All the results demonstrated that Nb doping has a positive effect on the oxygen permeation stability of CG–PSCF, and CG–PSCF0.4N0.1 dual-phase membranes have promising potential for oxy-fuel combustion.

High oxygen permeation through A-site deficient K2NiF4+δ-type oxide hollow-fiber membrane

A U-shaped hollow-fiber membrane of A-site deficient K2NiF4+δ-type oxide (Pr0.9La0.1)1.9(Ni0.74Cu0.21Ga0.05)O4+δ ((PL)1.9NCG) was developed through the phase inversion spinning method. At 975 °C, 2.51 mL/min cm2 oxygen permeation flux can be achieved under air/He. And stable oxygen permeation performances were obtained during the 285 h long-term tests. The electrical conductivity, surface morphology and oxygen permeation properties of (PL)1.9NCG hollow-fiber membrane were studied in detail.

Induction brazing BaCo0.7Fe0.2Nb0.1O3-δ membrane tubes to steel supports with Ag-based filler in air

An induction brazing process has been developed to seal BaCo0.7Fe0.2Nb0.1O3-δ (BCFN) membrane tubes to metal supports using Ag-Cu alloy in air. The closed-one-end BCFN membrane tubes with an outer diameter of 16mm and a length of 560mm were prepared by slip casting. The thermal expansion behaviors of the BCFNO membrane, Ag braze and 310S stainless steel were characterized. A sleeve joint was optimized to reduce the stresses caused by the mismatching in thermal expansions. Ag-1mol%Cu braze was chosen as brazing filler duo to its good wettability for both the membranes and the metal substrates and thin and continuous reaction layers. Based on the characteristics of the BCFN membrane, a temperature schedule was exploited to successfully achieve the induction brazing of the BCFN membrane tubes to the 310S supports using Ag-1mol%Cu in air. The entire time needed is about 80min. The brazed assemble subjected to 15 thermal cycles still exhibits no degradation in joint hermeticity and stable oxygen permeation flux. The result indicates that induction brazing can efficiently join large membrane tubes with high thermal expansion to their metal supports.

Effects of B‐site doped elements in the electronic‐conducting perovskite phase on the property of dual‐phase membranes

AbstractA series of dense oxygen permeable dual‐phase membranes with a composition of 60 wt% Ce0.8Gd0.2O2−δ‐40 wt% Ba0.95La0.05Fe0.9M0.10O3−δ (CGO‐ BLFM0.10, M=Fe, Nb, Zr, Zn, Sc, Y) were successfully synthesized and evaluated as potential ceramic membranes for oxy‐fuel combustion. The effects of B‐site doped elements in electronic‐conducting phase (BLFM0.10) on the crystal structure, microstructure, chemical compatibility, oxygen permeability, as well as chemical stability of CGO‐BLFM0.10 were systematically investigated. All electronic‐conducting phase BLFM0.10 oxides exhibited a pure cubic perovskite structure and showed good chemical compatibility with ionic‐conducting phase CGO. CGO‐BLFSc0.10 showed the best oxygen permeation stability under a pure CO2 atmosphere. CO2‐corrosion on the perovskite phase is the main reason for the property deterioration of fluorite‐perovskite‐typed dual‐phase membrane materials. The stability of dual‐phase membrane materials can be effectively enhanced by reducing the basicity of electronic‐conducting phase of perovskite materials.

Iron-doping effects on the CO2 tolerance of a perovskite oxygen permeable membrane

Bao.8La0.2Co0.88−xFexNb0.12O3−δ membranes (BLCFN) with different Fe-doping were successfully prepared by solid-state reaction method. The microstructure, oxygen permeability, thermal analysis, and oxygen permeation stability using CO2 as sweep gas were systematically investigated. After being calcined under pure CO2, BLCFN membranes remain their major perovskite phase but diffraction peaks of BaCO3, CoO appeared on the membranes which Fe content is less than 0.2. Apparent activation energy of oxygen permeation are around 60–70 kJ/mol for all membranes. With the increase of Fe-doping content, the flux through BLCFN membranes decreases but the degradation of oxygen flux becomes less pronounced when CO2 is used as sweep gas at 850 °C. Enhanced CO2 resistance of the perovskite would be resulted from an increasing average binding energy due to the Fe-doping. For the membrane where the Fe-doped content equals to 0.2, the oxygen permeation flux is 0.96 mL cm−2 min−1 at 900 °C with He sweeping. When using pure CO2 as sweep gas, the oxygen permeation flux decreases slightly in the first 50 h and then reaches a steady state of ~0.31 mL cm−2 min−1 for more than 60 h in a prolonged continuous oxygen operation. The observations indicated that a stable oxygen permeation could be realized by suitable elemental doping in a single perovskite membrane.

Novel cobalt-free tantalum-doped perovskite BaFe1−yTayO3−δ with high oxygen permeation

Cobalt-free perovskite-type oxides BaFe1−yTayO3−δ (0≤y≤0.2) were synthesized via a simple solid state reaction. The cubic perovskite structure can be obtained when y is over 0.1. BaFe0.9Ta0.1O3−δ (BFT0.1) membrane shows the highest oxygen permeation flux, which can reach 1.6ml·min−1·cm−2 at 950°C under the gradient of air/He. The O2-TPD results reveal that BaFe0.9Ta0.1O3−δ material shows an excellent reversibility and phase structure stability in air. The oxygen permeation flux is limited by the bulk diffusion when the membrane thickness is over 0.8mm, and it is limited by both the bulk diffusion and the surface exchange when the membrane thickness is below 0.5mm. Stable oxygen permeation fluxes are obtained during 180h operation.