Oxygen-deficient K2NiF4-type Ln1-xSr1+xNiO4-δ nickelates: Ceramic processing and oxygen-ionic transport

Ln2NiO4+δ–based oxides with perovskite-related K2NiF4-type structure demonstrate high oxygen diffusivity and surface exchange kinetics in combination with comparatively high electronic conductivity, and are considered,... [ view full abstract ]

Ln₂NiO_4+δ–based oxides with perovskite-related K₂NiF₄-type structure demonstrate high oxygen diffusivity and surface exchange kinetics in combination with comparatively high electronic conductivity, and are considered, therefore, as attractive mixed ionic-electronic conductors for oxygen electrodes of high-temperature solid oxide fuel/electrolysis cells. Acceptor-type substitution of Ln³⁺ by Sr²⁺ in Ln₂NiO_4+δ is compensated by generation of electron-holes and oxygen vacancies; this results in comparatively high oxygen deficiency in Sr-rich Ln_1-xSr_1+xNiO_4-δ phases at elevated temperatures. The transition from oxygen excess to oxygen deficiency is expected to be accompanied by a change of ionic transport mechanism from prevailing interstitial oxygen diffusion in rock-salt layers to oxygen vacancy diffusion in perovskite-type layers. The ionic transport in oxygen-deficient Sr-rich nickelates however was never studied, mostly due to difficulties in preparation of ceramic samples.

This work aimed at the fabrication of dense Ln_1-xSr_1+xNiO_4-δ (Ln = Nd or Pr, x = 0.2-0.4) ceramics and studies of oxygen-ionic permeability. Powders of Ln_1-xSr_1+xNiO_4-δ were prepared by Pechini method with final calcination steps at 1100-1200°C in oxygen. High-temperature XRD studies demonstrated that Ln_1-xSr_1+xNiO_4-δ nickelates exhibit strongly anisotropic lattice expansion interrelated with oxygen deficiency changes on heating. Highly anisotropic expansion made impossible the fabrication of dense ceramics employing traditional compaction and sintering due to microcracking effects. Attempts to employ hot pressing (5 GPa at 1100°C) also were unsuccessful: prepared dense samples underwent mechanical degradation on thermal cycling. Finally, spark plasma sintering with subsequent careful oxidation treatment was adopted to avoid grain growth and to produce gas-tight mechanically stable disk-shaped samples with relative density ≥ 91%. Oxygen permeation studies at 850-975°C confirmed that oxygen-ionic transport in Ln_1-xSr_1+xNiO_4-δ indeed increases with increasing oxygen deficiency. At the same time, the estimated ionic conductivity of these ceramics is lower compared to oxygen-overstoichiometric Ln₂NiO_4+δ counterparts despite a higher concentration of mobile ionic charge carriers.