Co-doped ceria-based solid solution in the CeO 2–M 2O 3–CaO, M = Sm, Gd system

Abstract

The Pechinni method (A) as well as hydrothermal treatment (B) of co-precipitated CeO 2-based gels with NaOH solution were used to synthesise pure CeO 2, and CeO 2-based solid solutions with formula Ce 1− x M x O 2, Ce 1− x (M 0.5Ca 0.5) x O 2 M = Gd, Sm for 0.15 < x < 0.3 nanopowders. The thermal evolution of CeO 2-based precursors during heating them up to 1000 °C was monitored by thermal (TG, DTA) analysis and X-ray diffraction method. All nanopowders and samples sintered were found to be pure CeO 2 or ceria-based solutions with fluorite-type structure. The microstructure of CeO 2-based sintered samples at 1500 °C (A) or 1250 °C (B) was observed for 2 h under the scanning electron microscope. The electrical properties of singly Ce 1− x M x O 2 or doubly doped CeO 2-based samples with formula Ce 1− x (M 0.5Ca 0.5) x O 2, M = Gd, Sm, 0.15 < x < 0.30 were investigated by means of the ac impedance spectroscopy method throughout the temperature range of 600–800 °C. It has been stated that partial substitution of calcium by samarium or calcium by gadolinium in the Ce 1− x (M 0.5Ca 0.5) x O 2, M = Gd, Sm solid solutions leads to ionic conductivity enhancement comparable with only samaria- or gadolina-doped ceria. The CeO 2-based samples with small-grained microstructures obtained from powders synthesised by hydrothermal method exhibited better ionic conductivity than samples with the same composition obtained from powders synthesised by the Pechinii method. The stability of the electrolytic properties of selected co-doped ceria sinters in fuel gases (H 2, CH 4) as well as exhaust gases from diesel engine was also investigated. The co-doped Ce 0.8(Sm 0.5Ca 0.5) 0.2O 2 or Ce 0.85(Gd 0.5Ca 0.5) 0.15O 2 dense samples would appear be to more adequate oxide electrolytes than Ce 1− x M x O 2, M = Sm, Gd and x = 0.15 or 0.2 for electrochemical devices operating at temperatures ranging from 600 to 700 °C.