Perovskite-like and perovskite-related oxides based on variable-valence transition metals may exhibit substantial changes in oxygen content in the course of temperature cycling or under variation of oxygen chemical potential at elevated temperatures. This gives rise to a chemical contribution to the thermal expansion (chemical expansion/contraction or chemical strain) originating mainly from the accompanying changes in the average oxidation state of transition metal cations and, consequently, their ionic radii. Excessive dimensional changes caused by the chemical expansion impose constraints on the applicability of functional oxides at elevated temperatures and under variable redox conditions (due to thermomechanical instability or incompatibility with other materials) and complicate the processing of oxide ceramics at high temperatures. Another unfavorable phenomenon is microcracking caused by a strongly anisotropic expansion of crystal lattice, with effects on the microstructural and mechanical integrity and electrical and electrochemical properties. The present work is a representative study case that focuses on layered Ln₂NiO₄-based ceramic materials for oxygen electrodes of solid oxide fuel/electrolysis cells (SOFC/SOEC).

Ln₂NiO_4+δ–based oxides (Ln = La, Pr, Nd) 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 SOFC/SOEC [1]. Acceptor-type substitution of Ln³⁺ by Sr²⁺ in Ln₂NiO_4+δ is compensated by comparatively high oxygen deficiency in Sr-rich Ln_2-xSr_xNiO_4-δ phases at elevated temperatures [2,3]. The transition from oxygen excess to oxygen deficiency is expected to be accompanied by a change in the ionic transport mechanism from prevailing interstitial oxygen diffusion in rock-salt layers to oxygen vacancy diffusion in perovskite-type layers.

Ceramic powders of Ln_2-xSr_xNiO_4-δ (Ln = La, Nd, Pr, x = 1.0-1.6) were prepared by the Pechini method with final calcination steps at 1150-1200°C in oxygen. Thermogravimetric studies confirmed that acceptor-type substitution by strontium is compensated by the formation of oxygen vacancies and electron-holes and progressively increases high-temperature oxygen nonstoichiometry (Fig.1A), which reaches as high as δ = 0.36-0.40 for x = 1.6 at 900°C in air. High-temperature XRD studies demonstrated that Ln_2-xSr_xNiO_4-δ nickelates exhibit strongly anisotropic lattice expansion interrelated with oxygen deficiency changes on heating (Fig.1B). Highly anisotropic expansion induces microcracking phenomenon, which, in turn, results in a strong hysteresis in dimensional changes (Fig.1C) and also in variations of electrical conductivity (Fig.1D) on cycling in air for the ceramic samples sintered at 1200-1300°C.

An appropriate approach to minimize or neglect the undesirable effects of anisotropic expansion and microcracking is to preserve the grain size in Ln_2-xSr_xNiO_4-δ ceramics or porous electrodes at ~1 micron or submicron level. In particular, spark plasma sintering with subsequent careful oxidation treatment was adopted to avoid grain growth and to produce mechanically stable ceramics with smooth reversible thermochemical expansion and enhanced electrical properties.

Acknowledgments: This work was supported by the FCT/MCTES/FEDER (projects CARBOSTEAM (POCI-01-0145-FEDER-032295) and CICECO-Aveiro Institute of Materials (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020), and PhD grant SFRH/BD/138773/2018).

References:

[1] A.P. Tarutin, J.G. Lyagaeva, D.A. Medvedev, L. Bi, A.A. Yaremchenko, Recent advances in layered Ln₂NiO_4+δ nickelates: fundamentals and prospects of their applications in protonic ceramic fuel and electrolysis cells, Journal of Materials Chemistry A 9 (2021) 154-195. https://doi.org/10.1039/D0TA08132A

[2] E. Kravchenko, D. Khalyavin, K. Zakharchuk, J. Grins, G. Svensson, V. Pankov, A. Yaremchenko, High-temperature characterization of oxygen-deficient K₂NiF₄-type Nd_2-xSr_xNiO_4-d (x = 1.0-1.6) for potential SOFC/SOEC applications, Journal of Materials Chemistry A 3 (2015) 23852-23863. https://doi.org/10.1039/C5TA06779K

[3] E. Kravchenko, K. Zakharchuk, A. Viskup, J. Grins, G. Svensson, V. Pankov, A. Yaremchenko, Impact of oxygen deficiency on the electrochemical performance of K₂NiF₄-type (La_1-xSr_x)₂NiO_4-δ oxygen electrodes, ChemSusChem 10 (2017) 600-611. https://doi.org/10.1002/cssc.201601340

Figure 1. (A) Oxygen nonstoichiometry of Nd_2-xSr_xNiO_4-δ nickelates in air; (B) relative elongation of Nd_2-xSr_xNiO_4-δ tetragonal lattice along the crystallographic a and c axes on heating in air; (C) dimensional changes and (D) variations in electrical conductivity of Nd_0.6Sr_1.4NiO_4-δ ceramics (sintered either at 1200 or 1300°C) on thermal cycling in air.