A.V. Kovalevsky

Ionic transport in oxygen-hyperstoichiometric phases with K2NiF4-type structure

Abstract Results on oxygen permeation through dense ceramics of La2−xSrxNi1−y−zFeyCuzO4+δ (x=0–0.10; y=0.02–0.10; z=0–0.10), LaPrNi0.9Fe0.1O4+δ, La2Cu1−xCoxO4+δ (x=0.02–0.30) and Ln2CuO4+δ (Ln=Pr, Nd) at 973–1223 K suggest two significant contributions to the ionic conductivity of the oxygen-hyperstoichiometric phases with K2NiF4-type structure. The relative role of the first of them, oxygen interstitial migration in the rock-salt-type layers of the K2NiF4-like lattice, increases with increasing temperature; the role of oxygen vacancy diffusion in the perovskite layers increases when temperature decreases. This behavior was attributed to the lower activation energy for ionic conduction via the vacancy diffusion mechanism. The oxygen permeability of the title materials was found to be limited by both bulk ionic conductivity and surface exchange rates and may thus be enhanced by catalytically active layers, including Pt, Ag and praseodymium oxide, deposited on the membrane surface. Oxygen permeability of K2NiF4-type phases exhibiting maximum ionic transport, such as La2Ni0.98Fe0.02O4+δ, La2Ni0.88Fe0.02Cu0.10O4+δ and La2Cu0.90Co0.10O4+δ, is about one order of magnitude lower than that of most permeable perovskite-type materials. Decreasing radii of the rare-earth cations in the A-sublattice of cuprates and nickelates leads to a dramatic decrease in ionic transport, similar to perovskite oxides. Thermal expansion coefficients of the title materials vary in the range (10.1–13.4)×10−6 K−1.

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.

The stability and mixed conductivity in La and Fe doped SrTiO3 in the search for potential SOFC anode materials

Both physical properties and the level of mixed conduction obtained in La and Fe doped SrTiO3 are widely influenced by composition. In contrast to La free compositions, La containing compositions show high stability against reaction with yttria stabilised zirconia (YSZ) and a closely matching thermal expansion coefficient (∼1×10−5 K−1). Faradaic efficiency measurements for Sr0.97Ti0.6Fe0.4O3–δ and La0.4Sr0.5Ti0.6Fe0.4O3–δ show ionic transference numbers in air between 5 × 10−3 to 4 × 10−2, and 2 × 10−4 to 6 × 10−4 respectively, decreasing with decreasing temperature. The substitution of La for Sr is observed to deplete the level of both ionic and total conductivity obtained in air.

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.

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.

Processing, microstructure and properties of LaCoO3-δ ceramics

Abstract Dense lanthanum cobaltite ceramics with different microstructures were prepared using several processing procedures, including chemical and ceramic synthesis routes. XRD, SEM, dilatometry, total electrical conductivity and oxygen permeability measurements were used for the characterization of these materials. Submicrometer size LaCoO 3− δ powders obtained via a cellulose-precursor technique or a combustion synthesis process showed much higher sinterability and poor compactability with respect to the powder prepared by the standard ceramic procedure. The influence of the processing route on crystal lattice, electronic conductivity and thermal expansion of LaCoO 3− δ ceramics was negligible. At the same time, the preparation technique significantly affects the ceramic microstructure and the oxygen ionic conductivity. LaCoO 3− δ membranes prepared via the standard ceramic technique, involving higher sintering temperatures, exhibit significantly higher oxygen permeation fluxes than ceramics prepared from organic precursors. This behavior was attributed to the effect of grain-boundary resistivity to ionic transport, which decreases with increasing sintering temperature and grain size, as commonly found for oxide solid electrolytes.

Faradaic efficiency and oxygen permeability of Sr0.97Ti0.60Fe0.40O3-δ perovskite

Abstract Oxygen ionic conduction in the perovskite-type Sr 0.97 Ti 0.60 Fe 0.40 O 3− δ was studied using oxygen permeability, Faradaic efficiency and total electrical conductivity measurements at 973–1223 K. The ion transference numbers of the strontium titanate–ferrite in air vary from 0.005 to 0.08, decreasing with decreasing temperature. The electron–hole conductivity of the oxide is relatively low but exceeds the ionic conductivity. The activation energy for the electronic conductivity is 35±3 kJ/mol at 470–890 K and drops at higher temperatures. Studying the oxygen permeation through dense Sr 0.97 Ti 0.60 Fe 0.40 O 3− δ ceramic membranes as a function of membrane thickness showed that at temperatures above 1170 K the permeation fluxes are limited by both bulk ionic conductivity and surface exchange rates. Depositing of porous layers of the same material or a mixture of platinum and praseodymium oxide onto the membrane feed-side surface leads to a significant increase in the oxygen permeability. Decreasing temperature results in increasing role of the bulk ionic transport in Sr 0.97 Ti 0.60 Fe 0.40 O 3− δ as the permeation-determining factor. The oxygen permeation fluxes at 1073 K are limited predominantly by the oxygen ionic conductivity of the ceramics. The thermal expansion coefficients of the ceramic material in air were calculated from dilatometric data to be 11.7×10 −6 K −1 in the temperature range 300–720 K and 16.6×10 −6 K −1 at 720–1070 K.

Oxygen permeability of La2Cu(Co)O4+δ solid solutions

Abstract Formation of the La 2 Cu 1− x Co x O 4+ δ solid solutions with orthorhombic K 2 NiF 4 -type structure was found to be in the range of 0≤ x ≤0.30 at temperatures above 1270 K. Incorporating cobalt into the copper sublattice of lanthanum cuprate leads to increasing oxygen hyperstoichiometry and decreasing electrical conductivity. Thermal expansion coefficients of the La 2 Cu 1− x Co x O 4+ δ ( x =0.02–0.30) ceramics at 470–1100 K were calculated from the dilatometric data to vary in the range (12.2–13.2)×10 − 6 K − 1 . Studying the dependence of oxygen permeation fluxes through La 2 Cu(Co)O 4+ δ on the membrane thickness demonstrated that the oxygen transport at the thickness values below 1 mm is limited by both surface exchange rate and bulk ionic conductivity. Oxygen permeability of the La 2 Cu 1− x Co x O 4+ δ solid solutions was ascertained to increase with cobalt concentration at x =0.02–0.10 and to decrease with further dopant additions, indicating a participation of interstitial oxygen in the ionic transport.