High-temperature characterization of oxygen-deficient K2NiF4-type Nd2−xSrxNiO4−δ (x = 1.0–1.6) for potential SOFC/SOEC applications

Abstract

Previously unexplored oxygen-deficient Ruddlesden–Popper Nd_2−xSr_xNiO_4−δ (x = 1.0–1.6) nickelates were evaluated for potential use as oxygen electrode materials for solid oxide fuel and electrolysis cells, with emphasis on structural stability, oxygen nonstoichiometry, dimensional changes, and electrical properties. Nd_2−xSr_xNiO_4−δ ceramics possess the K₂NiF₄-type tetragonal structure under oxidizing conditions at 25–1000 °C. Acceptor-type substitution by strontium is compensated by the generation of electron–holes and oxygen vacancies. Oxygen deficiency increases with temperature and strontium doping reaching ∼1/8 of oxygen sites for x = 1.6 at 1000 °C in air. Strongly anisotropic expansion of the tetragonal lattice on heating correlated with oxygen nonstoichiometry changes results in an anomalous dilatometric behavior of Nd_2−xSr_xNiO_4−δ ceramics under oxidizing conditions. Moderate thermal expansion coefficients, (11–14) × 10⁻⁶ K⁻¹, ensure however thermomechanical compatibility with common solid electrolytes. Reduction in inert atmosphere induces oxygen vacancy ordering accompanied by a contraction of the lattice and a decrease of its symmetry to orthorhombic. Nd_2−xSr_xNiO_4−δ ceramics exhibit a p-type metallic-like electrical conductivity at 500–1000 °C under oxidizing conditions, with the highest conductivity (290 S cm⁻¹ at 900 °C in air) observed for x = 1.2. The high level of oxygen deficiency in Sr-rich Nd_2−xSr_xNiO_4−δ implies enhanced mixed ionic–electronic transport favorable for electrode applications.