An evaluation of Ce-Pr oxides and Ce-Pr-Nb oxides mixed conductors for cathodes of solid oxide fuel cells: Structure, thermal expansion and electrical conductivity

Ceria doped with praseodymia and niobia has been studied in order to evaluate the possibility of applying these materials as cathodes for solid oxide fuel cells. The content of crystallographic phases and their lattice parameters have been determined by X-ray diffraction, the thermal expansion coefficient has been characterised by the dilatometry technique and the electrical conductivity has been measured by complex impedance spectroscopy and by the four probe DC technique. The results have shown the presence of one fluorite phase in binary compositions (CeO₂-PrO_2−x) for PrO_2−x concentrations up to about 20 mol% and two fluorite phases with different lattice parameters for higher PrO_2−x concentrations. The addition of 3 mol% NbO_2·5 has allowed the stabilisation of a single fluorite phase up to 50 mol% PrO_2−x. The thermal expansion coefficient varies between 0 and 30 × 10⁻⁶/K depending on composition and temperature. The electrical conductivity is mainly electronic and thermally activated. The conductivity exceeds 0·1 S/cm at 800°C for compositions with 40 to 50 mol% PrO_2−x.