Structural and defect chemistry guidelines for Sr(V,Nb)O3-based SOFC anode materials.

Structural and defect chemistry guidelines were used for Nb-substituted SrVO3-δ materials, designed to meet SOFC anode requirements, with emphasis on redox tolerance, thermochemical compatibility with other SOFC materials, electrical conductivity and adjustable changes in oxygen stoichiometry for their prospective impact on electrocatalytic performance. SrV1-xNbxO3-δ (x = 0-0.30) ceramics were prepared by solid-state synthesis and sintered at 1773 K in a reducing atmosphere. XRD and SEM/EDS showed that under these conditions a single-phase cubic perovskite structure appears up to x ≈ 0.25. Electrical conductivity is metallic-like and nearly p(O2)-independent. Although substitution by niobium decreases the conductivity, which still exceeds 100 S cm(-1) for x ≤ 0.20 at temperatures below 1273 K, it also expands the stability domain of the cubic perovskite phase and suppresses partly high thermochemical expansion characteristic of parent SrVO3-δ. The upper p(O2) limit of phase stability was found to shift from ∼2 × 10(-15) atm for the undoped material to ∼2 × 10(-12) atm for x = 0.30, whereas the average thermal expansion coefficient at 773-1223 K decreased from 22.7 × 10(-6) to 13.3 × 10(-6) K(-1). SrV1-xNbxO3-δ perovskites undergo oxidative decomposition in air, which causes dimensional and microstructural changes. However, sluggish kinetics of oxidation under inert gas conditions results in nearly reversible behavior in relatively short-term redox cycles between reducing and inert atmospheres. Subtle structural changes and a close correlation with point defect chemistry clarify these sluggish changes and provide guidelines to retain the metastability.

Towards a high thermoelectric performance in rare-earth substituted SrTiO3: effects provided by strongly-reducing sintering conditions.

Donor-substituted strontium titanate ceramics demonstrate one of the most promising performances among n-type oxide thermoelectrics. Here we report a marked improvement of the thermoelectric properties in rare-earth substituted titanates Sr0.9R0.1TiO3±Î´ (R = La, Ce, Pr, Nd, Sm, Gd, Dy, Y) to achieve maximal ZT values of as high as 0.42 at 1190 K < T < 1225 K, prepared via a conventional solid state route followed by sintering under strongly reducing conditions (10%H2-90%N2, 1773 K). As a result of complex defect chemistry, both electrical and thermal properties were found to be dependent on the nature of the rare-earth cation and exhibit an apparent correlation with the unit cell size. High power factors of 1350-1550 µW m(-1) K(-2) at 400-550 K were observed for R = Nd, Sm, Pr and Y, being among the largest reported so far for n-type conducting bulk-ceramic SrTiO3-based materials. Attractive ZT values at high temperatures arise primarily from low thermal conductivity, which, in turn, stem from effective phonon scattering in oxygen-deficient perovskite layers formed upon reduction. The results suggest that highly-reducing conditions are essential and should be employed, whenever possible, in other related micro/nanostructural engineering approaches to suppress the thermal conductivity in target titanate-based ceramics.

Methane oxidation over mixed-conducting SrFe(Al)O3-delta-SrAl2O4 composite.

The steady-state CH4 conversion by oxygen permeating through mixed-conducting (SrFe)0.7(SrAl2)0.3Oz composite membranes, comprising strontium-deficient SrFe(Al)O3-delta perovskite and monoclinic SrAl2O4-based phases, occurs via different mechanisms in comparison to the dry methane interaction with the lattice oxygen. The catalytic behavior of powdered (SrFe)0.7(SrAl2)0.3Oz, studied by temperature-programmed reduction in dry CH4 at 523-1073 K, is governed by the level of oxygen nonstoichiometry in the crystal lattice of the perovskite component and is qualitatively similar to that of other perovskite-related ferrites, such as Sr0.7La0.3Fe0.8Al0.2O3-delta. While extensive oxygen release from the ferrite lattice at 700-900 K leads to predominant total oxidation of methane, significant selectivity to synthesis gas formation, with H2/CO ratios close to 2, is observed above 1000 K, when a critical value of oxygen deficiency is achieved. The steady-state oxidation over dense membranes at 1123-1223 K results, however, in prevailing total combustion, particularly due to excessive oxygen chemical potential at the membrane surface. In combination with surface-limited oxygen permeability, mass transport limitations in a porous layer at the membrane permeate side prevent reduction and enable stable operation of (SrFe)0.7(SrAl2)0.3Oz membranes under air/methane gradient. Taking into account the catalytic activity of SrFeO3-delta-based phases for the partial oxidation of methane to synthesis gas and the important role of mass transport-related effects, one promising approach for membrane development is the fabrication of thick layer of porous ferrite-based catalyst at the surface of dense (SrFe)0.7(SrAl2)0.3Oz composite.