E.N. Naumovich

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.

Materials of high-temperature electrochemical oxygen membranes

Oxygen permeability of Ln1-xMxCoO3-δ (Ln = La, Pr, Nd; M = Sr, Ca, Bi, Pb; x = 0–0.9) and SrCo1-xMexO3-δ (Me = Cr, Mn, Fe, Ni, Cu; x = 0–0.5) perovskite-like oxide ceramics, which are promising materials for high-temperature electrochemical oxygen membranes where matter is transferred owing to conjugate transport of oxide ions O2− and electrons through a gas-tight ceramic material, has been investigated. Dependencies of the density of the molecular oxygen flow passing throuth the membrane on the chemical potential gradient of O2 in the gas phase and temperature have been analyzed. Physicochemical models of such dependencies are proposed. It is shown that complex oxides SrCo1-xFexO3-δ (x = 0.2–0.35) and La1-xSrxCoO3-δ (x = 0.65–0.75) having the highest oxide ionic conductivity can be used as materials for electrochemical oxygen membranes.

Perovskite-like system (Sr,La)(Fe,Ga)O3−δ: structure and ionic transport under oxidizing conditions

Abstract The maximum solid solubility of gallium in the perovskite-type La 1− x Sr x Fe 1− y Ga y O 3− δ ( x =0.40–0.80; y =0–0.60) was found to vary in the approximate range y =0.25–0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O 3− δ in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7–16.0)×10 −6 K −1 at 300–800 K and (19.3–26.7)×10 −6 K −1 at 800–1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La 1− x Sr x Fe 1− y Ga y O 3− δ ( x =0.7–0.8; y =0.2–0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La 1− x Sr x Fe 1− y Ga y O 3− δ increases with strontium content up to x =0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport.

Ionic conductivity of La(Sr)Ga(Mg,M)O3−δ (M=Ti, Cr, Fe, Co, Ni): effects of transition metal dopants

Abstract Oxygen-ion conductivity of the perovskite-type solid solutions (La,Sr)Ga1−zM2O3−δ (M=Ti, Cr, Fe, Co; z=0–0.20), LaGa1−y−zMgyMzO3−δ (M=Cr, Fe, Co; y=0.10–0.20, z=0.35–0.60) and LaGa1−zNizO3−δ (z=0.20–0.50) was studied using the techniques of oxygen permeation, Faradaic efficiency, ion-blocking electrode and the e.m.f. of oxygen concentration cells. Oxygen-ion transference numbers vary from 2×10−6 to 0.98 throughout the series and p-type electronic conductivity increases with increasing transition metal content. Substitution of Ga with higher valence cations (Ti, Cr) decreases ionic conductivity whereas small amounts of Fe or Co (∼5%) increase ionic conductivity. For higher transition metal contents, lower levels of oxygen-ion conductivity and an increase in the activation energy, EA, for ionic transport, from 60 (5%-doped) to 230 kJ/mol (>40%-doped) are observed. In heavily doped phases, EA tends to decrease with temperature and, above 1170 K, values are similar to the undoped phase suggesting that an order–disorder transition takes place. Factors affecting the observed ionic conductivity trends are discussed.

Mixed electronic and ionic conductivity of LaCo(M)O3 (M=Ga, Cr, Fe or Ni): I. Oxygen transport in perovskites LaCoO3–LaGaO3

Abstract The formation of a continuous series of solid solutions with a rhombohedrally-distorted perovskite type structure has been found in the pseudobinary oxide system LaCoO 3 –LaGaO 3 . Substitution of gallium with cobalt in the lanthanum gallate results in blocking oxygen ionic transport and increasing electronic conductivity. The activation energy of the electrical conductivity of the LaGa 1− x Co x O 3 ceramics ( x =0.2–0.6) is in the range 57–65 kJ mol −1 . The thermal expansion coefficients increase regularly with the cobalt content and lie in the range from 11.2×10 −6 to 22.3×10 −6 K −1 . Doping LaCoO 3 with gallium has been ascertained to lead to lower electronic conductivity and oxygen permeability. The results of electrical conductivity and electron paramagnetic resonance (EPR) studies suggest that introduction of gallium into the cobalt sublattice leads to insulating cobalt ions.

Surface modification of La0.3Sr0.7CoO3−δ ceramic membranes

The dependence of oxygen permeability of dense La 0 . 3 Sr 0 . 7 CoO 3 - Φ ceramics on membrane thickness indicates significant surface exchange limitations to the permeation fluxes, which suggests a possibility to increasemembrane performance by surface activation. The cobaltite membranes with various porous layers applied onto the permeate-side surface were tested at 850-1120 K. Silver-modified La 0 . 3 Sr 0 . 7 CoO 3 membranes showed enhanced permeation at temperatures above 950 K; deposition of porous layers of PrO x and Pr 0 . 7 Sr 0 . 3 CoO 3 - Φ had no positive effect. The maximum oxygen permeability at 850-1120 K was observed in the case of porous La 0 . 3 Sr 0 . 7 CoO 3 - Φ layers with surface density about 10 mg cm - 2 . These results suggest that the surface exchange of lanthanum-strontium cobaltite membranes under an oxygen chemical potential gradient is limited by both oxygen sorption at the surface and ion diffusion through the surface oxide layers. Oxygen permeability of La 0 . 3 Sr 0 . 7 CoO 3 - Φ ceramics was found to increase with increasing grain size due to decreasing grain-boundary resistance to ionic transport.

Oxygen permeability of LaFe1−xNixO3−δ solid solutions

Incorporation of nickel into the iron sublattice of LaFeO3−δ was found to result in increasing oxygen nonstoichiometry, electrical conductivity, and thermal expansion of the perovskites. Activation energy for electrical conductivity of the LaFe1−xNixO3−δ (x = 0.2–0.5) decreases with x in the range from 23.9 to 6.6 kJ/mol. Thermal expansion coefficients of the ceramic materials calculated from the dilatometric data were (8.9–11.9) × 10−6 K−1. Oxygen permeation through LaFe1−xNixO3−δ membranes was found to increase with nickel content due to increasing oxygen vacancy concentration and bulk ionic conductivity, which are the permeation flux-limiting factors.

Mixed electronic and ionic conductivity of LaCo(M)O3 (M=Ga, Cr, Fe or Ni): IV. Effect of preparation method on oxygen transport in LaCoO3−δ

Abstract Measurements of oxygen permeation through dense LaCoO 3− δ membranes prepared by different routes (standard ceramic or from organic precursors) showed a considerable role of processing conditions and microstructure on permeation fluxes. The higher permeability observed in ceramics produced by the ceramic route was attributed to a lower grain-boundary resistance to oxygen transport. Experimental data were also compared with results of numerical modeling of oxygen transport in LaCoO 3− δ , based on isotopic diffusion data in single crystals. Ionic conduction in ceramics with smaller grain size is lower than in single crystals, suggesting a significant grain boundary resistance. In contrast, oxygen permeability of LaCoO 3− δ prepared by the standard ceramic synthesis route, involving higher sintering temperatures is higher than expected. This suggests easy diffusion along grain boundaries. The influence of the ceramic microstructure on total electrical conductivity and thermal expansion of lanthanum cobaltite ceramics was found negligible.

Oxygen permeability and Faradaic efficiency of Ce0.8Gd0.2O2–δ–La0.7Sr0.3MnO3–δ composites

Composite Ce0.8Gd0.2O2−δ (CGO, solid electrolyte) and La0.7Sr0.3MnO3−δ (LSM, electronic conductor) ceramics were tested as dual-phase membranes for oxygen separation. Oxygen permeation through CGO–LSM composite ceramics containing similar percentages of both phases is limited by bulk ionic transport. In contrast to electronic transport, oxygen ion transport in these composites depends strongly on processing conditions, decreasing with interdiffusion of components. Oxygen ions are blocked by low ionic conductivity layers formed by diffusion of cations of LSM to the contacts between CGO grains. Testing of CGO–LSM membranes at high oxygen pressures (1–50 atm) showed that the composite ceramics are stable in these conditions and exhibit Wagner-type permeation fluxes.

Mixed electronic and ionic conductivity of LaCo(M)O3 (M=Ga, Cr, Fe or Ni): II. Oxygen permeation through Cr- and Ni-substituted LaCoO3

The oxygen permeation fluxes, electrical conductivity and thermal expansion of LaCo1−xCrxO3 (x=0.1–0.4) solid solutions have been ascertained to decrease with increasing chromium concentration. The dependence of the oxygen permeability on thickness of the LaCo1−xCrxO3 (x<0.3) dense ceramic membranes suggests a presence of surface-exchange limitations of the oxygen transport. Introduction of nickel into the cobalt sublattice of LaCo1−xNixO3 has been found to result in a sharp decrease of the oxygen ionic conductivity which is the flux-limiting factor. Electronic conductivity of the solid solutions increases with nickel content. A prolonged stabilization process to attain stationary oxygen flux through LaCo(Cr)O3 and LaCo(Ni)O3 membranes has been pointed out. Thermal expansion coefficients of the ceramics have been calculated from the dilatometric data to be in the range (17.3–23.7)×10−6 K−1.