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

Abstract

Structural and defect chemistry guidelines were used for Nb-substituted SrVO_3−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. SrV_1−xNb_xO_3−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 SrVO_3−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. SrV_1−xNb_xO_3−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.