Chemical and structural changes in Ln2NiO4+δ (Ln=La, Pr or Nd) lanthanide nickelates as a function of oxygen partial pressure at high temperature

•The structure of Ln2NiO4+δ compounds are studied vs. temperature and pO2•Structural transitions are evidenced in air as well as in low pO2 atmosphere•The structural transitions do not significantly affect their TECs values•Up to 1200 °C, they show good chemical stability and no chemical expansion vs. pO2•TECs of nickelates, 8YSZ and GDC are compared in air and in low pO2 atmosphere

The chemical stability of lanthanide nickelates Ln2NiO4+δ (Ln=La, Pr or Nd) has been studied in the temperature range 25–1300 °C, either in air or at low pO2 (down to 10−4 atm). Thermal gravimetry analysis (TGA) measurements coupled with X-ray diffraction (XRD) characterization have shown that all compounds retain their K2NiF4-type structure in these conditions, while remaining over-stoichiometric in oxygen up to 1000 °C. Only Nd2NiO4+δ starts to decompose into Nd2O3 and NiO above 1000 °C, at pO2=10−4 atm. In addition, a careful analysis of the lanthanide nickelates structural features has been performed by in situ XRD, as a function of temperature and pO2. For all compounds, a structural transition has been always observed in the temperature range 200–400 °C, in air or at pO2=10−4 atm. In addition, their cell volume did not vary upon the variation of the oxygen partial pressure. Therefore, these materials do not exhibit a chemical expansion in these conditions, which is beneficial for a fuel cell application as cathode layers. Additional dilatometry measurements have revealed that a temperature as high as 950 °C for Pr2NiO4+δ or 1100 °C for La2NiO4+δ and Nd2NiO4+δ has to be reached in order to begin the sintering of the material particles, which is of primary importance to obtain an efficient electronic/ionic conduction in the corresponding designed cathode layers. Besides, excellent matching was found between the thermal expansion coefficients of lanthanide nickelates and SOFC electrolytes such as 8wt% yttria stabilized zirconia (8YSZ) or Ce0.8Gd0.2O2−δ (GDC), at least from 400 °C up to 1400 °C in air or up to 1200 °C at pO2=10−4 atm.

Graphical abstractThis study reports the good chemical stability of oxygen overstoichiometric Ln2NiO4+δ(Ln = La, Pr or Nd) at high temperatures (up to 1300 °C), eitherin air or at pO2down to 10-4 atm. In addition, these MSC cathode materials show a small chemical expansion as well as a good TEC compatibility with electrolyte materials (GDC or YSZ).Figure optionsDownload full-size imageDownload as PowerPoint slide