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.
Pd-impregnated Sr1.9VMoO6– double perovskite as an efficient and stable anode for solid-oxide fuel cells operating on sulfur-containing syngas
Abstract The development of carbon- and sulfur-tolerant anode materials is highly desirable for the commercial application of solid-oxide fuel cells (SOFCs). We report herein the performance of Sr 1.9 VMoO 6− δ (SVMO) double perovskite as a potential anode material for SOFCs and the improvement of its electrochemical performance on hydrogen and H 2 S-containing syngas operation. SVMO has a cubic structure and thermal expansion coefficient of 13.3 × 10 −6  K −1 between 30 and 1000 °C in 5% H 2 /Ar. The electrical conductivities of SVMO in H 2 are considerably higher than those of existing double-perovskite anodes and traditional Ni–YSZ (40 vol% Ni) anode. The impregnation of Pd nanoparticles to form a composite anode (Pd–SVMO) or the addition of a Ce 0.8 Sm 0.2 O 1.9 buffer layer between anode and electrolyte significantly improves the electrochemical performance of SVMO for hydrogen oxidation, and the former is considerably more effective than the latter. The Pd–SVMO composite anode exhibits good stability and resistance to carbon deposition and sulfur poisoning for an SOFC operated on H 2 S-containing syngas based on an 80 h test, suggesting the potential of this anode material for SOFCs running on hydrocarbon fuels. The mechanisms for improving the electrochemical performance of the anodes and the resistance to carbon deposition and sulfur poisoning are also discussed.
Defect formation and mechanical stability of perovskites based on LaCrO3 for solid oxide fuel cells (SOFC)
Abstract Perovskites on the basis of LaCrO3 are of interest as ceramic interconnect materials for the development of solid oxide fuel cells (SOFCs). The interconnects are exposed to oxidising and reducing atmospheres under operating conditions. Oxygen vacancy formation was determined as a function of oxygen partial pressure between 1 and 10−22 bar at temperatures between 900 and 1100 °C. Different perovskite compositions made of (La,Ca/Sr)CrO3−δ, La(Cr,Mg)O3−δ, La(Cr,Mg/Cu/Co,Al)O3−δ, and (La,Ca)(Cr,Al)O3−δ were investigated. Defect models were evaluated to describe the oxygen vacancy formation and the respective thermodynamic data were determined. The results are used to explain existing literature data on the isothermal expansion of LaCrO3 based perovskites under reducing conditions. Complementary mechanical measurements with selected perovskite compositions revealed that lower oxygen partial pressure causes higher stiffness, strength and fracture toughness. The change in properties is discussed in terms of the observed ferroelastic domains and the interaction of the domain wall motion with the oxygen vacancies.