Effect of Sr and Al or Fe co-doping on the sinterability and conductivity of lanthanum silicate oxyapatite electrolytes for solid oxide fuel cells

The effect of co-doping of Sr and Al or Fe on the microstructure, sinterability and oxide-ion conductivity of lanthanum silicate oxyapatites is investigated in detail at 300-800 °C by the electrochemical impedance spectroscopy. The oxide-ion conductivity is 1.46 × 10−2 S cm−1 for La9.5Sr0.5Si5.5Fe0.5O26.5 (LSSFO) and 1.34 × 10−2 S cm−1 at 800 °C for La9.5Sr0.5Si5.5Al0.5O26.5 (LSSAO), respectively, which is one order of magnitude higher than 6.16 × 10−3 S cm−1 measured on La9.67Si6O26.5 (LSO) oxyapatite under the identical test conditions. The grain bulk and grain boundary resistances of co-doped oxyapatite are significantly smaller than that of LSO oxyapatite, and decrease significantly with the increase of the sintering temperature. LSSFO and LSSAO also show significantly higher density as compared to that of LSO. The results indicate that co-doping of Sr and Al or Fe significantly improves the densification, sinterability and oxide-ion conductivity of lanthanum silicate oxyapatites.