SOME STRUCTURAL ASPECTS OF IONIC CONDUCTIVITY IN CO-DOPED CERIA-BASED ELECTROLYTES

The sinters of co-doped ceria solid solutions with the formula of Ce0.85Sm0.15-xRxO1.9, where R = Y, Gd, Pr, Tb, 0 < x < 0.15, were obtained from powders synthesised by Pechini method. The linear variation of cell parameter a vs. chemical composition was observed for Ce0.85Sm0.15-xRxO1.9, where R = Y, Gd, Tb, 0 < x < 0.15 samples. However, the introduction of Pr3+ into Ce0.85Sm0.15-xPrxO1.9 caused a small deviation from linearity due to possible changes in the valence from Pr3+ to Pr4+. The determined values of oxide transference number t(ion) for Ce0.85Sm0.15-xRxO1.9, R = Y, Gd in the temperature range 400-750 degrees C and partial oxygen pressure from 10(-6) to 1 atm were close to 1, which indicated that materials investigated exhibited practically pure ionic oxide conductivity. On the other hand, the introduction of Tb3+ or Pr3+ higher than x > 0.05 into solid solution Ce0.85Sm0.15-xRxO1.9, R = Tb, Pr caused a decrease in the ionic transference number t(ion) below 1 due to an increase in partial electronic conduction. This fact limiting investigated co-doped terbia and samaria or samaria and praseodymia ceria-based solid solutions for the further application as oxide electrolytes in solid oxide fuel cells. The analysis of bulk and grain boundary values indicated that partial substitution of Sm3+ by Y3+ or Gd3+ caused slight improvements in the ionic conductivity of Ce0.85Sm0.15-xRxO1.9. The highest ionic conductivity was found for solid solution with chemical composition Ce0.85Sm0.1Y0.05O1.9. The selected co-doped ceria samples were tested as solid electrolytes in solid oxide fuel cells operating in the intermediate temperature range 500-750 degrees C.