A.L. Shaula

Oxygen transport in Ce0.8Gd0.2O2−δ-based composite membranes

Abstract Gadolinia-doped ceria electrolyte Ce 0.8 Gd 0.2 O 2− δ (CGO) and perovskite-type mixed conductor La 0.8 Sr 0.2 Fe 0.8 Co 0.2 O 3− δ (LSFC), having compatible thermal expansion coefficients (TECs), were combined in dual-phase ceramic membranes for oxygen separation. Oxygen permeability of both LSFC and composite LSFC/CGO membranes at 970–1220 K was found to be limited by the bulk ambipolar conductivity. LSFC exhibits a relatively low ionic conductivity and high activation energy for ionic transport (∼200 kJ/mol) in comparison with doped ceria. As a result, oxygen permeation through LSFC/CGO composite membranes, containing similar volume fractions of the phases, is determined by the ionic transport in CGO. The permeation fluxes through LSFC/CGO and La 0.7 Sr 0.3 MnO 3− δ /Ce 0.8 Gd 0.2 O 2− δ (LSM/CGO) composites have comparable values. An increase in the p-type electronic conductivity of ceria in oxidizing conditions, which can be achieved by co-doping with variable-valence metal cations, such as Pr, leads to a greater permeability. The oxygen ionic conductivity of the composites consisting of CGO and perovskite oxides depends strongly of processing conditions, decreasing with interdiffusion of the phase components, particularly lanthanum and strontium cations from the perovskite into the CGO phase.

Oxygen Ionic and Electronic Transport in Apatite-Type Solid Electrolytes

The oxygen ionic conductivity of apatite-type La 9.83 Si 4.5 Al 1.5-y Fe y O 26±δ (y = 0-1.5), La 10-x Si 6-y Fe y O 26±δ (x = 0-0.77; y = 1-2), and La 7-x Sr 3 Si 6 O 26-δ (x = 0-1) increases with increasing oxygen content. The ion transference numbers, determined by faradaic efficiency measurements at 973-1223 K in air, are close to unity for La 9.83 Si 4.5 Al 1.5-y Fe v O 26+δ and La 10 Si 5 FeO 26.5 , and vary in the range 0.96-0.99 for other compositions. Doping of La 9.83 (Si, Al) 6 O 26 with iron results in an increasing Fe 4+ fraction, which was evaluated by Mossbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric La 10 (Si, Fe)O 26+δ apatites stabilize the Fe 3+ state. Among the studied materials, the highest ionic and electronic transport is observed for La 10 Si 5 FeO 26.5 , where oxygen interstitials are close neighbors of Si-site cations. Data on transference numbers, total conductivity, and Seebeck coefficient as a function of the oxygen partial pressure confirm that the ionic conduction in Fe-substituted apatites remains dominant under solid oxide fuel cell operation conditions. However, reducing p (O 2 ) leads to a drastic decrease in the ionic transport, presumably due to a transition from the prevailing interstitial to a vacancy diffusion mechanism, which is similar to the effect of acceptor doping. Iron additions improve the sinterability of silicate ceramics, increase the n-type electronic conductivity at low p(O 2 ), and probably partly suppress the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air are (8.8-9.9) X 10 -6 K -1 at 300-1250 K.

Electron-hole transport in (La0.9Sr0.1)0.98Ga0.8Mg0.2O3-δ electrolyte: effects of ceramic microstructure

The oxygen ion transference numbers of a series of (La0.9Sr0.1)0.98Ga0.8Mg0.2O3−δ (LSGM) ceramics with different microstructures, prepared by sintering at 1673 K for 0.5–120 h, were determined at 973–1223 K by a modified Faradaic efficiency technique, taking electrode polarization into account. In air, the transference numbers vary in the range 0.984–0.998, decreasing when temperature or oxygen partial pressure increases. Longer sintering times lead to grain growth and to the dissolution of Sr-rich secondary phases and magnesium oxide, present in trace amounts at the grain boundaries, into the major perovskite phase. This is accompanied with a slight decrease of the total grain-interior resistivity and thermal expansion, while the boundary resistance evaluated from impedance spectroscopy data decreases 3–7 times. The electron-hole transport in LSGM ceramics was found to decrease when the sintering time increases from 0.5 to 40 h, probably indicating a considerable contribution of acceptor-enriched boundaries in the hole conduction. Due to reducing boundary area in single-phase materials, further sintering leads to higher p-type conductivity. The results show that, as for ionic conductivity, electronic transport in solid electrolytes significantly depends on ceramic microstructure.

Fe4+ formation in brownmillerite CaAl0.5Fe0.5O2.5+δ

A Mossbauer spectroscopy study of brownmillerite Ca2FeAlO5+δ, in combination with the structure refinement from X-ray powder diffraction data, showed that hyperstoichiometric oxygen incorporation into the lattice is accompanied with Fe4+ formation in the perovskite-like layers, which are built of oxygen–metal octahedra and preferably occupied by iron cations. This behavior differs from that of another layered ferrite, Sr4Fe6O13+δ, where the point defects formed due to oxidation are primarily accumulated in non-perovskite layers. The incorporation of even minor amounts of extra oxygen in Ca2FeAlO5+δ leads to decreasing water absorption by the lattice, probably due to blocking of oxygen vacancies located in the tetrahedral layers of brownmillerite phase.

Oxygen permeability of LaGa0.65Ni0.20Mg0.15O3−δ ceramics: effect of synthesis method

Oxygen ionic transport in dense LaGa{sub 0.65}Ni{sub 0.20}Mg{sub 0.15}O{sub 3-{delta}} membranes, prepared by the standard ceramic synthesis technique and via glycine-nitrate process (GNP), was studied using measurements of the total conductivity, oxygen permeation and faradaic efficiency (FE). At 1223 K oxygen transfer through LaGa{sub 0.65}Ni{sub 0.20}Mg{sub 0.15}O{sub 3-{delta}} ceramics is mainly determined by the bulk ambipolar conductivity, while decreasing temperature leads to a greater role of the surface exchange rate. In spite of moderate difference in the ceramic microstructures, the surface exchange limitations are considerably higher for the membranes prepared by the standard ceramic route compared to GNP-synthesized material. Thermal expansion and partial ionic and electronic conductivities were found essentially independent of the synthesis method. The level of oxygen ionic conduction in LaGa{sub 0.65}Ni{sub 0.20}Mg{sub 0.15}O{sub 3-{delta}}, characterized by the activation energy of about 150 kJ/mol and ion transference numbers in the range 1x10{sup -3}-5x10{sup -2} at 973-1223 K, is higher than that in La(Ga,Ni)O{sub 3-{delta}} perovskites and comparable to La{sub 2}NiO{sub 4}-based phases.

Oxygen Permeability and Thermal Expansion of Ferrite-Based Mixed Conducting Ceramics

In order to evaluate promising directions in the development of mixed-conducting membrane materials for oxygen separation and partial oxidation of natural gas, a series of ferritebased ceramics were studied, including La1-xSrxFe1-yGayO3-δ (x = 0.5 0.8; y = 0 0.4), La1-xSrxFe1-yAlyO3-δ (x = 0.7 1.0; y = 0 0.5), La0.3Sr0.7Fe0.7-xAl0.3CrxO3-δ (x = 0.1 0.2), (Sr2Fe3)1-x(SrCo)xOz (x = 0 0.8), CaFe0.5Al0.5O2.5+δ and Ln3-xCaxFe5O12-δ (Ln = Gd, Y; x = 0 0.5). The maximum oxygen permeation is observed for perovskite-type solid solutions with high oxygen deficiency, which exhibit, however, excessive thermal and chemically induced expansion. As for cobaltiteand gallate-based mixed conductors, the increase in ionic transport is accompanied with increasing limiting role of the surface exchange processes. The stability of perovskite-related ferrites in reducing atmospheres, which is comparable to that of LaFeO3-δ and iron oxide, may be moderately increased or decreased by donoror acceptor-type doping, respectively. In addition, the substitution of iron with cations having a more stable oxidation state, such as Ga 3+ , Al 3+ or Cr 3+/4+ , partly prevents the lattice expansion induced by oxygen nonstoichiometry variations, although the solubility of these dopants in the ferrite lattice is limited. Introduction Dense ceramic membranes with mixed oxygen ionic and electronic conductivity are of significant interest for numerous electrochemical applications, including oxygen generators and electrocatalytic reactors for the conversion of natural gas to synthesis gas [1-5]. Synthesis gas (syngas), a mixture of hydrogen and carbon monoxide, is used as a feedstock for commercial Fischer-Tropsh synthesis of hydrocarbons, methanol synthesis, ammonia synthesis, etc. The conventional industrial technologies for syngas generation are based on catalytic steam reforming and/or partial oxidation of methane. Whilst steam reforming is energy intensive due to a highly endothermic reaction, the main cost of partial oxidation is associated with the oxygen plant. Nowadays the cost for synthesis gas production comprises up to 50 % of the total capital investment in gas-to-liquid (GTL) plants [3,4]. Dense ceramic membranes may combine air separation and partial oxidation in one single reactor, and, thus, substantially decrease the investment for the GTL technology [2-4]. The requirements to membrane materials include high oxygen permeability, stability in a wide range of oxygen chemical potential and in the presence of CO2, SOx and steam, compatibility with catalysts and construction materials, low thermal and chemically induced expansion. A special attention is focused on perovskite-related oxides since their transport properties and stability in various atmospheres vary in a wide range. Perovskite-type mixed conductors with highest oxygen Defect and Diffusion Forum Online: 2004-05-20 ISSN: 1662-9507, Vols. 226-228, pp 141-160 doi:10.4028/www.scientific.net/DDF.226-228.141

Oxygen-ionic conductivity of perovskite-type La1−xSrxGa1−yMgyM0.20O3−δ (M=Fe, Co, Ni)

The oxygen-ionic conductivity of perovskite-type La 1-x Sr x Ga 0.80-y Mg y M 0.20 O 3-δ (x = 0-0.20, y = 0.15-0.20, M = Fe, Co, Ni) and La 0.50 Pr 0.50 Ga 0.65 Mg 0.15 Ni 0.20 O 3-δ in air, determined by the measurements of total conductivity. faradaic efficiency and oxygen permeability at 973-1223 K, increases with increasing concentration of the acceptor-type dopants, within the solid solution formation limits. The level of ionic conduction in these phases is, however, lower than that in parent compounds, La 1-x Sr x Ga 1-y Mg y O 3-δ , probably due to partial oxygen-vacancy ordering and a higher average cation-anion bond energy in transition metal-containing gallates. The activation energy for ionic transport in La 1-x Sr x Ga 0.80-y Mg y M 0.20 O 3-δ varies in the range of 126-171 kJ mol -1 . For compositions containing different transition metal cations, the ionic transport increases in the sequence M = Co < Fe < Ni, with the maximum ionic conductivity observed for La 0.90 Sr 0.10 Ga 0.05 Mg 0.15 Ni 0.20 O 3-δ perovskite.

Structural characterization of mixed conducting perovskites La(Ga,M)O3−δ (M=Mn, Fe, Co, Ni)

Abstract Comparative analysis of the structure refinement results of perovskite-like LaGa0.5M0.5O3−δ (M=Mn, Fe, Co, Ni) and data on other LaGaO3-based phases, heavily doped with transition metal cations, shows that on doping the structural changes in these oxides follow common trends for the perovskite-type systems. The maximum ionic conductivity, observed in various perovskites when the tolerance factor values are approximately 0.96–0.97, was found to correlate with the transition from orthorhombic to rhombohedral structure and maximum lattice distortion. The perovskite unit cell distortion near the orthorhombic–rhombohedral phase boundary may hence play a positive role in the ionic transport processes.

Defect formation and transport in SrFe1-xAlxO3-δ

Perovskite-related phases derived from SrFeO3-δ are among known mixed conductors with highest oxygen permeability and are thus of interest as the ceramic membrane materials for oxygen separation and partial oxidation of light hydrocarbons. Dense ceramics of SrFe1-xAlxO3-δ (x=0.1–0.5) were prepared via the glycine-nitrate process. The cubic solid solution formation was found to occur in the concentration range x=0–0.35. Increasing aluminum content leads to decreasing thermal expansion coefficients (TECs), relative fraction of Fe4+ under oxidizing conditions, and also the total conductivity, predominantly p-type electronic at oxygen pressures close to atmospheric. The TECs vary in the range (13.5–16.4)×10−6 K−1 at 373–923 K and increase up to (18.6–31.9)×10−6 K−1 at 923–1273 K. The oxygen permeation fluxes decrease moderately with aluminum additions. The Mössbauer spectroscopy data and p(O2) dependencies of electrical properties indicate a small-polaron mechanism of electronic transport in SrFe1-xAlxO3-δ. Reducing oxygen partial pressure results in transition from dominant p- to n-type electronic conduction. The low-p(O2) stability limit of SrFe1-xAlxO3-δ perovskites lies between that of LaFeO3-δ and Fe/Fe1-γO boundary.

Ion Transport and Thermomechanical Properties of SrFe ( Al ) O3 − δ – SrAl2O4 Composite Membranes

Moderate additions of monoclinic SrAl 2 O 4 to perovskite-type SrFe(Al)O 3-δ mixed conductors improve the sinterability and thermomechanical properties, including the thermal shock resistance, Vickers hardness, and fracture toughness, and decrease thermal expansion. The iron solubility in SrAl 2 O 4 , a mixed ionic and n-type electronic conductor with insulating properties, is lower than 5%. The total conductivity of SrAl 2 O 4 ceramics in air varies in the range 10 -7 -10 -5 S/cm at 973-1223 K. The transport properties and phase stability of dual-phase (SrFe) 1-x (SrAl 2 ) x O 2 (x = 0.3-0.7) composite membranes, where the partial dissolution of strontium aluminate in the ferrite phase leads to formation of A-site-deficient Sr 1-y Fe 1-2y Al 2y O 3-δ (y ≈ 0.08-0.12), are determined by the perovskite component. The total conductivity and Seebeck coefficient oxygen partial pressure dependencies exhibit general trends typical for SrFeO 3 -based solid solutions. Although the conductivity and oxygen permeability of (SrFe) 1-x (SrAl 2 ) x O z composites decrease with increasing x, the permeation fluxes through (SrFe)o.7(SrAl 2 ) 0.3 O z ceramics are comparable to those through single-phase SrFe 0.7 Al 0.3 O 3-δ . Under high p O2 gradients such as air/(H 2 -H 2 O), the oxygen transport is limited by surface-related processes, enabling stable operation of (SrFe) 0.7 (SrAl 2 ) 0.3 O 2 membranes. This composition was selected for fabrication of tubular membranes by the cold isostatic pressing. Surface modification of (SrFe) 0.7 (SrAl 2 ) 0.3 O z in order to enhance the exchange kinetics was found inappropriate from a stability point of view.