Doped CeO2 is widely considered as the only feasible alternative to YSZ as electrolyte material in SOFC. In fact, ionic conductivity of RE-doped CeO2 can exceed 0.01 S/cm already at 600 °C , which would allow a decrease in the standard operating temperature of cells. Thanks to the fluorite structure, ionic conduction mechanism relies on oxygen vacancies which are formed to compensate charge unbalance due to the incorporation of RE3+ ions at Ce4+ sites and their equilibrium concentration can be extrinsically modified just by changing the amount of doping agent. However, increasing dopant concentration beyond a certain threshold reduces ionic conductivity, due to defect clustering and structural modifications. In order to investigate the correlation between structure and electrical conduction at different doping levels, a detailed structural-electrical study was carried out in the system CeO2-Sm2O3. Ternary oxides of nominal composition Ce1-xSmxO2-x/2 with 0.1<=x<=0.4 were prepared by co- precipitation of mixed oxalates and subsequent thermal treatment in air at 1200, 900, or 800 °C; the structure was then analyzed at room temperature both by synchrotron X-ray diffraction and ?-Raman spectroscopy, while conductivity was studied in the temperature range 300 - 800 °C by electrochemical impedance spectroscopy (Fig. 1) on samples sintered in air at 1400°C. Results show that for x<=0.3 association enthalpy of defects follows regular solutions model, while for x=0.4 a deviation from the model is observed with anomalous increase of the association enthalpy. This is explained with the formation of a hybrid structure, intermediate between the CeO2 and the Sm2O3 cubic structure.
Electrical properties of ternary oxides in the CeO2-Sm2O3 system
S Presto;M Viviani;C Artini
2017
Abstract
Doped CeO2 is widely considered as the only feasible alternative to YSZ as electrolyte material in SOFC. In fact, ionic conductivity of RE-doped CeO2 can exceed 0.01 S/cm already at 600 °C , which would allow a decrease in the standard operating temperature of cells. Thanks to the fluorite structure, ionic conduction mechanism relies on oxygen vacancies which are formed to compensate charge unbalance due to the incorporation of RE3+ ions at Ce4+ sites and their equilibrium concentration can be extrinsically modified just by changing the amount of doping agent. However, increasing dopant concentration beyond a certain threshold reduces ionic conductivity, due to defect clustering and structural modifications. In order to investigate the correlation between structure and electrical conduction at different doping levels, a detailed structural-electrical study was carried out in the system CeO2-Sm2O3. Ternary oxides of nominal composition Ce1-xSmxO2-x/2 with 0.1<=x<=0.4 were prepared by co- precipitation of mixed oxalates and subsequent thermal treatment in air at 1200, 900, or 800 °C; the structure was then analyzed at room temperature both by synchrotron X-ray diffraction and ?-Raman spectroscopy, while conductivity was studied in the temperature range 300 - 800 °C by electrochemical impedance spectroscopy (Fig. 1) on samples sintered in air at 1400°C. Results show that for x<=0.3 association enthalpy of defects follows regular solutions model, while for x=0.4 a deviation from the model is observed with anomalous increase of the association enthalpy. This is explained with the formation of a hybrid structure, intermediate between the CeO2 and the Sm2O3 cubic structure.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
