Phase equilibria, structure and properties of complex oxides in the NdFeO3 − δ – SrFeO3 − δ – SrCoO3 − δ – NdCoO3 − δ system as potential cathodes for SOFCs

Abstract

The phase equilibria in the NdFeO3−δ – SrFeO3−δ – SrCoO3−δ – NdCoO3−δ system were studied at 1373K in air. The homogeneity ranges and crystal structure of Nd1−xSrxFe1−yCoyO3−δ solid solutions were determined by the X-ray powder diffraction, the unit cell parameters were refined by the Rietveld analysis. It was found that Nd1−xSrxFe1−yCoyO3−δ with 0.0≤x≤0.6 and 0.1≤y≤0.9 possesses orthorhombic structure (sp. gr. Pbnm) that reveals O-type→O′-type→O-type structural transition. The orthorhombic distortion decreases with the increasing of strontium content, as a result Nd1−xSrxFe1−yCoyO3−δ solid solutions with 0.7≤x≤0.9 and 0.1≤y≤0.9 possess the cubic structure (sp. gr. Pm3m). Isothermal-isobaric phase diagram for the NdFeO3−δ – SrFeO3−δ – SrCoO3−δ – NdCoO3−δ system at 1373K in air has been presented. A series of samples with the overall composition Nd0.2Sr0.8Fe1−yCoyO3−δ (0.0≤y≤1.0) were characterized by the thermogravimetric analysis, iodometric titration, dilatometry and electrical conductivity measurements. The oxygen nonstoichiometry in Nd0.2Sr0.8Fe1−yCoyO3−δ (0.0≤y≤1.0) increases with increasing temperature and cobalt content. Gradual substitution of iron by cobalt in the Nd0.2Sr0.8Fe1−yCoyO3−δ oxides leads to the increase of the average thermal expansion coefficients. The electrical conductivity exhibits maximum within the range of 600–750K and significantly increased with cobalt content at T<800K. Chemical compatibility of Nd0.2Sr0.8Fe1−yCoyO3−δ with Ce0.8Sm0.2O2−δ and La0.88Sr0.12Ga0.82Mg0.12O3−δ solid electrolytes has been studied. The single cells based on the LSGM electrolyte, Sr2Ni0.75Mg0.25MoO6 anode and Nd0.2Sr0.8Fe1−yCoyO3−δ (y=0.3 and 1.0) cathode were fabricated and examined.