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

The phase equilibria in the NdFeO3 (-) (delta) -SrFeO3 (-) (delta) -SrCoO3 (-) (delta) -NdCoO3 (-) (delta) system were studied at 1373 K in air. The homogeneity ranges and crystal structure of Nd-1 (-) xSrxFe1 (-) yCoyO3 (-) (delta) solid solutions were determined by the X-ray powder diffraction, the unit cell parameters were refined by the Rietveld analysis. It was found that Nd-1 (-) xSrxFe1 (-) yCoyO3 (-) (delta) 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 Nd-1 (-) xSrxFe1 (-) yCoyO3 (-) (delta) 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 (-) (delta) - SrFeO3 (-) (delta) - SrCoO3 (-) (delta) - NdCoO3 (-) (delta) system at 1373 K in air has been presented. A series of samples with the overall composition Nd0.2Sr0.8Fe1 (-) yCoyO3 (-) (delta) (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 (-) (delta) (0.0 <= y <= 1.0) increases with increasing temperature and cobalt content. Gradual substitution of iron by cobalt in the Nd0.2Sr0.8Fe1 (-) yCoyO3 (-) (delta) oxides leads to the increase of the average thermal expansion coefficients. The electrical conductivity exhibits maximum within the range of 600-750 K and significantly increased with cobalt content at T < 800 K. Chemical compatibility of Nd0.2Sr0.8Fe1 (-) yCoyO3 (-) (delta) with Ce0.8Sm0.2O2 (-) (delta) and La0.88Sr0.12Ga0.82Mg0.12O3 (-) (delta) solid electrolytes has been studied. The single cells based on the LSGM electrolyte, Sr2Ni0.75Mg0.25MoO6 anode and Nd0.2Sr0.8Fe1 (-) yCoyO3 (-) (delta) (y = 0.3 and 1.0) cathode were fabricated and examined.