Development of novel low-temperature SOFCs with co-ionic conducting SDC-carbonate composite electrolytes

A series of ceria-based composite materials consisting of samaria doped ceria (SDC) and binary carbonates(Li₂CO₃-Na₂CO₃) were examined as functional electrolytes for low-temperature solid oxide fuel cells (SOFCs). DTA and SEM techniques were applied to characterize the phase- and micro-structural properties of the composite materials. Conductivity measurements were carried on the composite electrolytes with a.c. impedance in air. A transition of ionic conductivity with temperature was occurred among all samples with different carbonate content, which related to the interface phase. Single cells based on the composite electrolytes, NiO as anode and lithiated NiO as cathode, were fabricated by a simple dry-pressing process and tested at 400-600 °C. The maximum output power at 600 °C increased with the carbonate content in the composite electrolytes, and reached the maximum at 25 wt.%, then decreased. Similar trend has also shown at 500 °C, but the maximum was obtained at 20wt.%. The best performances of 1085 mW cm^-2 at 600 °C and 690 mW cm^-2 at 500 °C were achieved for the composite electrolytes containing 25 and 20 wt.% carbonates, respectively. During fuel cell operation, it found that the SDC-carbonate composites are co-ionic (O^2-/H⁺) conductors. At lower carbonate contents, both oxide-ion and proton conductions were significant, when the content increased to 20-35 wt.%, proton conduction dominated. The detailed conduction mechanism in these composites needs further investigation.