Oxygen Semi-permeation Research Articles

Symmetric and asymmetric membranes based on La0.5Sr0.5Fe0.7Ga0.3O3-δ perovskite with high oxygen semipermeation flux performances and identification of the rate-determining step of oxygen transport

This work focuses on identifying the rate-determining step of oxygen transport through La0.5Sr0.5Fe0.7Ga0.3O3-δ membranes with symmetric and asymmetric architectures. The best oxygen semipermeation fluxes are 3.4 10−3 mol. m-2.s-1 and 6.3 10−3 mol. m-2.s-1 at 900 °C for the symmetric membrane and asymmetric membrane with a modified surface. The asymmetric membrane with a modified surface leads to an increase of approximately 7 times the oxygen flux compared to that obtained with the La0.5Sr0.5Fe0.7Ga0.3O3-δ dense membrane without surface modification. This work also shows that the oxygen flux is mainly governed by gaseous oxygen diffusion through the porous support of asymmetric La0.5Sr0.5Fe0.7Ga0.3O3-δ membranes.

Oxygen semi-permeation properties of Ba1-xSrxFeO3-δ perovskite membranes

This work is focused on the evaluation of oxygen semi-permeation performances of free cobalt perovskite membrane materials, Ba1-xSrxFeO3-δ perovskites. A wide range of Ba and Sr contents was studied. The best performances were obtained for Ba0.9Sr0.1FeO3-δ perovskite membranes with oxygen fluxes of 4.1 10−3 mol.m−2.s−1 at 850 °C. For a better understanding of the rate determining step of oxygen transport through Ba1-xSrxFeO3-δ perovskite membranes (with x = 0.1, 0.5, and 0.7), the oxygen diffusion and oxygen desorption coefficients were determined using a specific method based on oxygen semi-permeation.

Oxygen semi-permeation properties of La1−xSrxFeO3−δ perovskite membranes under high oxygen gradient

Abstract

Determination of Oxygen Diffusion and Surface Exchange Coefficients of Mixed Ionic-Electronic Conductors by Oxygen Semi-Permeation Methods

The understanding of oxygen transport mechanisms through mixed ionic-electronic conductors is of great interest for the development of electrochemical devices at high temperature for energy conversion applications. This study is focused on the determination of two coefficients—the oxygen diffusion coefficient and the surface exchange coefficient which are key parameters for the electrochemical performance of a material. These coefficients can be largely impacted by the measurement conditions. The oxygen semi-permeation method is an adequate method to determine these two fundamental coefficients close to the working conditions of the material, i.e., under small or large oxygen partial pressure gradients.

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.

Determining the rate-limiting step during oxygen semi-permeation of CaTi0.9Fe0.1O3-δ oxygen transport membranes

Oxygen transport membranes are dense ceramic oxides that have the ability to exchange oxygen with the surrounding atmosphere and to transport it along a chemical potential gradient. To improve performance, it is important to determine whether the oxygen transport is limited by the surface exchange or by bulk diffusion. Because this analysis involves very specific testing conditions, it is difficult to generalize and compare literature results. Recent studies have shown that CaTi0.9Fe0.1O3−δ (CTF) offers a reasonable compromise between performance and long-term stability, making it a good candidate for the rigorous conditions of industrial applications. Although the baseline flux performance of CTF has been established, the rate-limiting step for oxygen transport remains a matter of debate. In the present investigation, the oxygen activity on both surfaces of the membrane is measured simultaneously with the permeation flux to determine the limiting step in the oxygen semipermeability of CTF. The limiting step varies with temperature: below 750°C oxygen transport is limited both by bulk diffusion and by surface exchanges, with bulk diffusion limitations dominating at higher temperatures, although surface polarization remains influential. Moreover, the flux is limited by ionic conductivity at low temperatures and by electron hole conductivity at high temperatures.

Effect of Porous Layer on Oxygen Semi Permeation and Surface Exchange of CaTi0.9Fe0.1O3-δ Membranes

Oxygen transport membranes are dense ceramic oxides that allow oxygen diffusion along a chemical potential gradient. CaTi0.9Fe0.1O3−δ(CTF) exhibits an interesting tradeoff between performance and stability; despite a lower oxygen flux, it exhibits a superior thermal, mechanical, and chemical stability. To optimize the performance of CTF, it is important to establish the rate-limiting step of oxygen transport mechanism, either surface oxygen exchange reactions, or bulk diffusion. We prepared CTF membranes with and without a porous layer. The limiting step was determined using the ratio between chemical potential drop of oxygen near the surface and across the bulk. We find that bare CTF membranes exhibit a mixed-control below 750 °C, changing to a bulk-limited process with increasing temperature. The porous layer mitigates surface limitation and increases the flux by 35% at lower temperature.

Effect of Porous Layer on Oxygen Semi Permeation and Surface Exchange of CaTi0.9Fe0.1O3-δ Membranes

Oxygen transport membranes (OTM) are dense ceramics oxides that can move oxygen along a chemical potential gradient. The membrane must be oxygen-ion conductive to transport oxygen, but should also be electronically conductive in order to maintain internal charge neutrality. In the simplest design, the ceramic membrane separates two atmospheres with different oxygen partial pressures, creating the gradient which drives oxygen flux. At the oxygen-rich side, oxygen is adsorbed on membrane surface, then diffuses through the bulk, and is finally released at the oxygen-lean side. OTMs have been used to precisely control the amount of oxygen in the mixture supplied for the partial oxidation of methane. In oxy-fuel combustion, using oxygen supplied by an OTM instead of air increases combustion temperature and decreases NOx emissions. These, and most other OTM applications, operate under conditions of temperature, gas pressure and oxygen partial pressures, which can lead to thermo-chemical degradation and mechanical failure of the OTM. Therefore, to be an economically viable option, OTMs must excel in two requirements which fundamentally drive cost: performance and stability. In perovskite materials, the oxygen diffusion is proportional to the concentration of mobile vacancies in the structure. However, vacancies are also responsible for creep, which often limits the lifetime of the ceramic. Consequently, the improvement of performance comes at the expense of stability, and vice-versa. CaTi0.9Fe0.1O3−δ (CTF) exhibits an interesting tradeoff between performance and stability, where its lower oxygen flux can be compensated by its superior thermal, mechanical and chemical stability [3]. To optimize flux performance of CTF it is important to establish the rate limiting step during the transport of the oxygen, which may be limited by the rate of the oxygen exchange at either side, or by bulk diffusion. In this study, we will characterize the oxygen semi-permeability, and establish the rate limiting step of CTF membranes as a function of temperature. CaTi0.9Fe0.1O3−δ powder was prepared by electrofusion and checked for chemical purity and composition by X ray fluorescence. The resulting powders have an orthorhombic crystal structure, increasing symmetry with temperature increase, becoming cubic at 1300 °C. Sintered pellets with > 96 %TD retain the same crystalline structure, and were used for conductivity and oxygen flux measurements. Conductivity was measured between 150 and 700 °C and between 1 and 10-27atm. CTF is a p-type conductor, limited mostly by ionic conductivity at lower temperature, transitioning to electronic conductivity above 700 °C, also verified during flux measurements. Regardless, the conduction behavior is never really dominated by either type of charge carrier, and can be better characterized as mixed-limited under the experimental conditions. Semi-permeability measurements were performed under synthetic air (0.21 bar) and argon (10-5 bar), between 550 and 950 °C. Oxygen semi permeation at 900 °C is 3.2 × 10-3 ml.min-1.cm-1, consistent with the literature [4]. Additionally, electrochemical surface probes were used to measure the oxygen activity of both surfaces of the membrane. The surface polarization of the oxygen-rich side is negligible compared to the oxygen-lean side, indicating that the former is not limiting. The limiting step on the lean side can be inferred from the ratio between chemical potential drop of oxygen near the surface and across the bulk. Figure 1 show that below 750 °C CTF is mixed limited by the surface exchange as well as bulk diffusion, and that at higher temperature it is increasingly limited by bulk diffusion. Assuming that conductivity is limited by ionic charge carriers, we calculated oxygen diffusion, D*, and surface exchange, k, coefficients and compared them with isotopic oxygen exchange between 700 and 900 °C. The characteristic thickness, Lc = D*/k, is superior to 0.6 mm above 700 °C, which confirms that the oxygen semi-permeability is limited by bulk diffusion at OTM operation temperatures.

Effect of Porous Layer on Oxygen Semi Permeation and Surface Exchange of CaTi0.9Fe0.1O3-δ Membranes

Effect of Porous Layer on Oxygen Semi Permeation and Surface Exchange of CaTi0.9Fe0.1O3-δ Membranes

Identification of the rate-determining step in oxygen transport through La(1−x)SrxFe(1−y)GayO3−δ perovskite membranes

La(1−x)SrxFe(1−y)GayO3−δ perovskites are some of the most promising materials for oxygen transport membranes due to their high oxygen semi-permeation and good chemical stability at high temperature under a wide range of oxygen partial pressures (from 0.21 to 10−19atm.). This paper is focused on a better understanding of the influence of the Ga and Sr substitution ratios in the La(1−x)SrxFe(1−y)GayO3−δ perovskite series on oxygen semi-permeation performances, oxygen diffusion and surface exchange coefficients. This study identifies the optimal compositions of the La(1−x)SrxFe(1−y)GayO3−δ perovskite series. In addition, this study shows that the rate-determining step of oxygen transport through the membrane depends strongly on the Ga and Sr substitution ratios.

Surface Exchange Model for MIEC Membrane in Transient Stage

Mixed Ionic and Electronic Conductors are promising materials for oxygen transport membranes at high temperature. The mixed conduction leads to oxygen semi-permeation properties. This oxygen transport properties is linked to the ability of the membrane material to reversibly adsorb and desorb oxygen and to diffuse oxygen through the lattice [1]. However, the defect equilibrium with the surrounding atmosphere induces a “chemical” expansion in the same order than the thermal expansion. Mechanically, the transient stage is critical due to the stress induced by the chemical and thermal strain [2]. To predict this strain, the oxygen activity field through the membrane needs to be known.Usually, to model, the membrane is divided into three zones: the bulk, where oxygen bulk diffusion takes place and the two surfaces where oxygen exchanges between atmosphere and membrane take place. Oxygen bulk diffusion is well described by the Wagner theory [3]. The surface exchanges include the adsorption/desorption, dissociation/association and incorporation/exclusion of oxygen. A consensus has not yet emerged regarding the oxygen surface exchange models proposed in the literature. Two categories of models are reported: one corresponds to an extension of the Wagner theory to the surface [4] and the second corresponds to a chemical approach by kinetic laws [1, 5].Different experiments have been performed to characterize the surface exchange and bulk diffusion parameters. The measures reported by Geffroy et al. on membrane surfaces during steady oxygen flow exhibits a large chemical potential gap between the membrane surfaces and the surrounding gases [6]. From our knowledge, this experimental data cannot be reproduced by a single model from literature. Moreover, although the combination of Wagner extension approach on the high oxygen side and kinetic approach on the low oxygen side makes it possible to reproduce results in steady state, it does not allowed to investigate transient stage.The proposed talk deals with a new macroscopic surface exchange model describing both oxidizing and reducing surface for transient stage. This model assumes that the oxygen flow is governed by the association/dissociation of adsorbed oxygen and by the high energetic cost of oxygen reduction/oxidation. Then, the balance of a transient species at the membrane surface is introduced to account for these two phenomena. The first results obtained are in accordance with oxygen semi-permeation measurements reported by Geffroy et al. [6]. The computation of isothermal expansion tests realized by Adler [7] in transient stage, using the proposed exchange model and the chemical expansion model proposed by Valentin et al. [8], present also a good correlation with experimental results.[1] S.B. Adler et al, Mechanisms and rate laws for oxygen exchange on mixed-conducting oxide surfaces, J Catal, (245), 91-109, 2007.[2] O. Valentin et al, Chemical expansion of La0.8Sr0.2Fe0.7Ga0.3O3-δ, Solid State Ionics, (193), 23-31, 2011.[3] C. Wagner, Equations for transport in solid oxides and sulfides of transition metals, Prog Solid State Ch, (10), 3-16, 1975.[4] H.J.M. Bouwmeester et al, Importance of the surface exchange kinetics as rate limiting step in oxygen permeation through mixed-conducting oxides, Solid States Ionics, (72), 185-194, 1994.[5] S. J. Xu et al, Oxygen permeation rates through ion-conducting perovskite membranes, Chem Eng Sci, (54), 3839-3850, 999.[6] P.M. Geffroy et al, Influence of oxygen surface exchanges on oxygen semi-permeation through La(1-x)SrxFe(1-y)GayO3-δ dense membrane, J Electrochem Soc, (158), 1-9, 2011.[7] S.B. Adler, Chemical expansivity of electrochemical Ceramics, J. Am. Ceram. Soc., (84), 2117-2119, 2004.[8] O. Valentin et al., Thermo-Chemo-Mechanical Modelling of Mixed Conductors in Transient Stages, Adv. Sci. Rech., (65), 232-237, 2010.

Effect of cation substitution in the A site on the oxygen semi-permeation flux in La0.5A0.5Fe0.7Ga0.3O3−δ and La0.5A0.5Fe0.7Co0.3O3−δ dense perovskite membranes with A = Ca, Sr and Ba (part I)

Numerous mixed ionic electronic conducting materials with a perovskite structure have been investigated in the literature as potential membrane materials for oxygen separation applications. However, the effect of cation substitution in the A site of a perovskite structure on the oxygen semi-permeation flux is not clearly understood. The goal of this paper is to provide insight into the effect of cation substitution in the A-site on oxygen semi-permeation flux in La0.5A0.5Fe0.7Ga0.3O3−δ and La0.5A0.5Fe0.7Co0.3O3−δ perovskite membranes with A = Ca, Sr, and Ba. In addition, the rate-determining step in the trans-membrane oxygen transport was clearly identified based on oxygen diffusion and oxygen surface exchange coefficients for these two perovskite membrane series.

Determination of Oxygen Diffusion Coefficients in La1-xSrxFe1-yGayO3-δ Perovskites Using Oxygen Semi-Permeation and Conductivity Relaxation Methods

This paper reports new evidence that oxygen surface exchange and bulk diffusion in a mixed conductor can be simultaneously determined via the oxygen semi-permeation method. Herein, we report the use of an original apparatus for oxygen activity measurements at both membrane surfaces to evaluate the oxygen surface exchange and bulk diffusion coefficients. Oxygen surface exchange and bulk diffusion in the La1-xSrxFe1-yGayO3-δ perovskite series are also determined and compared with the results from three different methods: isotopic exchange, conductivity relaxation, and oxygen semi-permeation. Although the thermodynamic conditions for these methods are not exactly the same, the values obtained for the oxygen surface exchange and bulk diffusion coefficients are in good agreement.

Oxygen semi-permeation performances of La1−xAxCo1−y−zFeyTazO3−δ perovskite (with A=Sr and Ba)

Cobalt-based perovskite oxides like La1−xSrxCo1−yFeyO3−δ or Ba1−xSrxCo1−yFeyO3−δ are considered as promising materials, because of very high oxygen semi-permeation. However, this perovskite material series presents a low chemical stability for industrial application due to the flexible redox behavior of cobalt. Then, recent studies show that the Ta co-doping on the B-site of perovskite structure improves the chemical stability under reducing conditions at high temperature. This paper is devoted to the study of La1−xAxCo1−y−zFeyTazO3−δ perovskite systems (with A=Sr and Ba), and specifically the impact of Ta and Fe doping on the chemical stability and oxygen semi-permeation performances. Besides, this work aims at better understanding of oxygen transport mechanism through the La1−xAxCo1−y−zFeyTazO3−δ perovskite membranes (with A=Sr or Ba) that is not yet clearly identified. This better understanding involves dissociating the oxygen transport steps linked to oxygen diffusion in the membrane bulk and to the surface exchange on both membrane surfaces.

Evaluating oxygen diffusion, surface exchange and oxygen semi-permeation in Ln2NiO4+δ membranes (Ln=La, Pr and Nd)

To clearly identify the rate determining step (r.d.s.) in the oxygen semi-permeation of dense Ln2NiO4+δ membranes (with Ln=La, Pr and Nd), a specific apparatus was designed to measure the oxygen semi-permeation flux and activity gradient at both membrane surfaces. From these measurements, the coefficients for the oxygen diffusion and surface exchange at both membrane surfaces were determined. Their values were compared to the values reported in the literature for similar membrane materials. We concluded that the oxygen flux through these membranes was limited by the oxygen surface exchange at the oxygen-lean face and not oxygen volume diffusion, as previously claimed.

The Impact of Experimental Factors on Oxygen Semi-Permeation Measurements

Many recent studies have focused on the development of perovskite membranes with high oxygen semi-permeation performance. However, the usual experimental setups used to measure oxygen semi-permeation values may lead to significant errors in the actual performance of the membrane materials. In this paper, special attention is paid to the experimental conditions and the associated impacts on oxygen semi-permeation measurements collected from the membrane. Furthermore, the experimental setups described in the literature do not allow the direct identification of the limiting step for oxygen transport through the membrane, i.e., either through bulk oxygen diffusion or oxygen surface exchange on both membrane surfaces. In this work, a specific experimental setup is proposed for the discrimination of oxygen bulk diffusion and surface exchange phenomena.

Influence of Oxygen Surface Exchanges on OxygenSemi-Permeation through La(1−x)SrxFe(1−y)GayO3−δDense Membrane

Because of high oxygen semi-permeation and very good chemical stability, La0.8Sr0.2Fe0.7Ga0.3O3−δ perovskite is one of the most promising materials as a catalytic membrane reactor for hydrogen production. In this paper, oxygen semi-permeation of La0.8Sr0.2Fe0.7Ga0.3O3−δ membranes with different architectures is studied over a large range of temperatures in order to identify the rate determining step of the oxygen flux through the membrane. We show that the oxygen flux through the membrane is governed by the surface exchanges on the membrane reducing face. Besides, these results are confirmed by potentiometric measurements of oxygen activity on both membrane faces under oxygen partial pressure gradient. Finally, new architectures with La0.8Sr0.2Fe0.7Ni0.3O3−δ porous catalytic layer on membrane reducing surface are tested to improve the performances of oxygen semi-permeation.

La (1− x) Sr xFe (1− y) Ga yO 3− δ perovskite membrane: Oxygen semi-permeation, thermal expansion coefficient and chemical stability under reducing conditions

The La (1− x) Sr x Fe (1− y) Ga y O 3− δ perovskites are one of the most promising materials for catalytic membrane reactor because of their excellent oxygen semi-permeation, low thermal expansion coefficient and good chemical stability at high temperature (700–1000 °C) under a wide range of oxygen partial pressure (from 0.21 to 10 −19 atm). In this paper, the thermal expansion coefficient (TEC) and the oxygen semi-permeation of La (1− x) Sr x Fe (1− y) Ga y O 3− δ perovskite in relation with Sr and Ga substitutions have been measured and discussed. The chemical stability of La (1− x) Sr x Fe (1− y) Ga y O 3− δ perovskite formulations under diluted methane atmosphere is tested and discussed. This study leads to identify the Sr and Ga substitution ratios presenting the best compromise between high oxygen semi-permeation, low thermal expansion coefficient and good chemical stability in working conditions. In this way, one of the best compromises between good oxygen permeation fluxes, dimensional and chemical stabilities are La 0.6Sr 0.4Fe 0.6Ga 0.4O 3− δ and La 0.6Sr 0.4Fe 0.7Ga 0.3O 3− δ .

Influence of glass and gold sealants materials on oxygen permeation performances in La 0.8Sr 0.2Fe 0.7Ga 0.3O 3− δ perovskite membranes

Gas dissociation with a ceramic membrane is an economical and environmental attractive process for methane gas conversion into syngas (mixture of H 2 and CO). For this application, the membrane materials require mixed ionic and electronic conductivities in a large range of pO 2 resulting in an excellent oxygen semi-permeation of the membrane to improve methane conversion. In this paper, particular attention is given to the influence of sealant materials on oxygen semi-permeation performances of membrane materials. For instance, oxygen semi-permeation measurements have been performed with La 0.8Sr 0.2Fe 0.7Ga 0.3O 3− δ membranes under an oxygen partial pressure gradient at temperatures from 700 °C to 1000 °C with different sealing materials (glass or gold ring sealant) between the membrane and the support. The result leads to a better understanding of the effects of sealing on oxygen transport through the La 0.8Sr 0.2Fe 0.7Ga 0.3O 3− δ membrane. Indeed, a gold sealing material is required for accurate measurements of oxygen semi-permeation flux.

Oxygen semi-permeation, oxygen diffusion and surface exchange coefficient of La (1− x) Sr xFe (1− y) Ga yO 3− δ perovskite membranes

Mixed ionic and electronic conductors (MIEC) have presented great economical and environmental interests for these last years because of their potential applications for electrode materials in solid oxide fuel cell and for oxygen separation form air such as in catalytic membrane reactors for methane conversion into syngas (H 2–CO mixture) (A.F. Sammells, M. Schwartz, R.A. Mackay, T.F. Barton, D.R. Peterson [1], U. Balachandran, J.T. Dusek, R.L. Mieville, R.B. Poeppel, M.S. Kleefisch [2], H.J.M. Bouwmeester, B.A. Boukamp [3]). A good compromise between oxygen permeability, chemical stability and physical properties is required to optimize the process. La (1− x) Sr x Fe (1− y) Ga y O 3− δ materials fulfill this requirement and were retained as membrane for catalytic membrane reactor (CMR) (Y. Teraoka [4], G. Etchegoyen, T. Chartier [5]). Oxygen semi-permeations through La (1− x) Sr x Fe (1− y) Ga y O 3− δ membranes have been measured under various oxygen partial pressure gradients from 973 K to 1273 K, and compared with the values obtained by isotopic oxygen exchange depth experiments (S. Kim, S. Wang, X. Chen, Y.L. Yang, N. Wu, A. Ignatiev, A.J. Jacobson, and B. Abeles [6]). Those results lead to a better understanding of the oxygen transport through the membrane and the influence of Sr and Ga amounts on oxygen semi-permeation through the membrane. The influence of Ga amount is not limited to the improvement of dimensional stabilities but it also increases oxygen diffusion and surface exchange kinetic. This paper suggests that La 0.6Sr 0.4Fe 0.6Ga 0.4O 3− δ perovskite is a very good compromise for membrane reactor materials and opens new perspectives on membrane architecture development.