Vladimir Pankov

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

Previously unexplored oxygen-deficient Ruddlesden–Popper Nd2−xSrxNiO4−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. Nd2−xSrxNiO4−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 Nd2−xSrxNiO4−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. Nd2−xSrxNiO4−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 Nd2−xSrxNiO4−δ implies enhanced mixed ionic–electronic transport favorable for electrode applications.

Impact of Oxygen Deficiency on the Electrochemical Performance of K2NiF4-Type (La1−xSrx)2NiO4−δ Oxygen Electrodes

Perovskite-related (La1-x Srx )2 NiO4-δ (x=0.5-0.8) phases were explored for possible use as oxygen electrodes in solid electrolyte cells with a main focus on the effect of oxygen deficiency on the electrocatalytic activity. (La1-x Srx )2 NiO4-δ solid solutions were demonstrated to preserve the K2 NiF4 -type tetragonal structure under oxidizing conditions. Acceptor-type substitution by Sr is compensated by the formation of oxygen vacancies and electron holes and progressively increases high-temperature oxygen nonstoichiometry, which reaches as high as δ=0.40 for x=0.8 at 950 °C in air. The electrical conductivity of (La1-x Srx )2 NiO4-δ ceramics at 500-1000 °C and p(O2 )≥10-3  atm is p-type metallic-like. The highest conductivity, 300 S cm-1 at 800 °C in air, is observed for x=0.6. The average thermal expansion coefficients, (14.0-15.4)×10-6  K-1 at 25-900 °C in air, are sufficiently low to ensure the thermomechanical compatibility with common solid electrolytes. The polarization resistance of porous (La1-x Srx )2 NiO4-δ electrodes applied on a Ce0.9 Gd0.1 O2-δ solid electrolyte decreases with increasing Sr concentration in correlation with the concentration of oxygen vacancies in the nickelate lattice and the anticipated level of mixed ionic-electronic conduction. However, this is accompanied by increasing reactivity between the cell components and necessitates the microstructural optimization of the electrode materials to reduce the electrode fabrication temperature.