Ln2NiO4+δ (Ln = La, Pr) As Suitable Cathodes for Metal Supported Cells

In the last decade, thanks to their mixed ionic and electronic conductivity (i.e. MIEC properties), the lanthanide nickelates, Ln₂NiO_4+δ (Ln = La, Pr, Nd) have attracted much attention as cathode for solid oxide fuel cells. Simultaneously, Metal Supported Cells (MSCs) for IT-SOFC application (typically 600 – 700 °C) have been developed in respect of several advantages including reduced cost and better thermal cycling resistivity. Nevertheless, the use of porous metal implies to sinter the constitutive elements of the cells under low pO₂ (e.g. under N₂, pO₂ » 10^-4 atm). In that respect, the chemical stability, thermal and chemical expansion when exposed to temperatures as high as 1400 °C, under pO₂ as low as 10^-4 atm of Ln₂NiO_4+δ phases were studied through TGA (Thermal Gravimetry Analysis), XRD analyses, high temperature X-ray diffraction and Thermal Expansion Coefficient measurements. 

Electrochemical Impedance Spectroscopy measurements on Ln₂NiO_4+δ//GDC//YSZ symmetrical half-cells sintered either in air or under argon at various temperatures have been performed in the range 500 °C to 800 °C, in air, at i_dc = 0 A. Pr₂NiO_4+δ, either sintered in air or under nitrogen at 1150 °C and then re-oxidized, shows the lowest polarization resistance values (R_p = 0.12 Ω.cm² at 600 °C) while La₂NiO_4+δexhibits smaller polarization resistances when sintered under nitrogen, compared to air. Finally, the suitability of lanthanide nickelates in MSCs will be demonstrated and the relationship between cathode performances and structural and microstructural features will be discussed in details. Besides,modelling of the impedance data will be emphasized.