ИСТИНА

Recently, mixed electronic and ionic conductors with layered structures have been regarded as promising cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) operated at 500-750oC. It was previously showed [1] that the presence of rock-salt type slabs in the structure of layered facilitates oxygen-on diffusion. Therefore, the oxides relating to the K2NiF4-type structure are the most attractive to be considered as fast oxide-ion conductors. Lanthanum cuprate (La2CuO4) with K2NiF4 structure displays both high oxygen diffusion and surface exchange coefficients and low thermal expansion coefficient (TEC), which is comparable with ceria solid electrolyte Ce0.9Gd0.1O1.95. Substitution of Sr for La in La2CuO4 allows enhancing electrical conductivity. At the same time introduction of praseodymium into the La2CuO4 structure may improve the catalytic activity and electrical conductivity. In this work we present the study of high-temperature thermal and electrical conductive properties of Pr- and Sr- doped La2CuO4. Samples of La1.8-xPrxSr0.2CuO4-δ (x = 0.2; 0.4) were prepared by annealing of stoichiometric amounts of La2O3, Pr6O11, SrCO3 и CuO at 1000°C for 50h in air. Phase purity of the sample was checked by X-ray powder diffraction (XRD) using Huber G670 Guinier diffractometer (CuKα1 radiation, image foil detector). Oxygen content of the samples was determined by iodometric titration. Thermogravimetric (TG) studies were performed at 25-900oC in air with a heating rate of 10K/min using Netzsch STA 449C thermoanalyser. High-temperature electrical conductivity of the ceramic samples was measured by four-probe DC technique at 100-900oC and pO2=10-3-1 atm. XRD patterns of the samples were fully indexed in the tetragonal I-centered (I4/mmm) unit cell with parameters: а = 3.7636(5) Å, с = 13.183(2) Å for x = 0.2 and а = 3.7645(3) Å, с = 13.163(2) Å for x = 0.4. Increase of Pr content leads to unit cell compression along the c-axis concerned with smaller Pr radius (r(Pr3+) = 1.179 Å) in comparison with La one (r(La3+) = 1.216 Å). The oxygen content in the La1.8-xPrxSr0.2CuO4-δ coincides with stoichiometric within a measurement error. Heating of the both samples up to 600-700°C in air leads to weight loss the and change in oxygen content due to the formation of oxygen vacancies that can promote oxygen mobility in the crystal structure. However, increase of praseodymium content is conducive to keeping oxygen in the crystal structure. According to the XRD data recorded after thermogravimetric analysis in air the formation of new phases was not detected. The electrical conductivity of both compounds demonstrates metallic-like behavior within studied temperature and oxygen partial pressure ranges. Conductivity of the samples weakly depends on pO2. The composition with x=0.2 shows the highest conductivity (up to 70 S/cm) in the temperature range of 500-900oC in air.