Oxide ionic conductivities of apatite-type lanthanum silicates and germanates and their possibilities as an electrolyte of lower temperature operating SOFC

Abstract Electrical properties of La x M 6 O 12+1.5 x (M = Si, Ge) as an electrolyte for solid oxide fuel cell (SOFC) have been investigated. In La x Si 6 O 12+1.5 x and La x Ge 6 O 12+1.5 x of x = 8–11, the highest conductivities were achieved at x = 9.7 (La 9.7 Si 6 O 26.55 ) and x = 9.0 (La 9.0 (GeO 4 ) 6 O 1.5 ), respectively. The conductivity of La 9.0 (GeO 4 ) 6 O 1.5 was higher than that of La 9.7 Si 6 O 26.55 in a temperature region higher than 700 °C, and the conductivity (2.4 × 10 −3 S cm −1 ) of La 9.7 Si 6 O 26.55 at 400 °C was higher than that (8.3 × 10 −5 S cm −1 ) of La 9.0 (GeO 4 ) 6 O 1.5 . The power densities of SOFC (H 2 | Pt | electrolyte (thickness: 1 mm) | Pt | O 2 ) using La 9.0 (GeO 4 ) 6 O 1.5 as an electrolyte were 14.3 mW cm −2 (700 °C) and 24.0 mW cm −2 (800 °C). The corresponding SOFC using La 9.7 Si 6 O 26.55 was found to work even at lower temperatures of 400 and 500 °C with power densities of 0.011 and 0.12 mW cm −2 . The SOFC (H 2 | Ni–Sm 0.2 Ce 0.8 O 1.9 | electrolyte | Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 2.5 | air) using 0.3 mm thickness La 9.7 Si 6 O 26.55 electrolyte gave the 3.4 mW cm −2 power density at 500 °C.