Transition metal-doped lanthanum germanate apatites as electrolyte materials of solid oxide fuel cells

Abstract

In this study, the effects of Fe, Mn, Ni, and Ti dopants on the densification, phase structure, microstructure, and electrical conductivity of the La 9.5 Ge 6.0 O 26.25 ceramics were examined. After sintering, the La 9.5 Ge 5.5 Mn 0.5 O 26 and La 9.5 Ge 5.5 Ti 0.5 O 26.25 ceramics displayed hexagonal symmetry while the La 9.5 Ge 5.5 Ni 0.5 O 25.75 and La 9.5 Ge 5.5 Fe 0.5 O 26 ceramics produced triclinic lattice. The La 9.5 Ge 5.5 Fe 0.5 O 26 ceramic sintered at 1400 °C showed the best electrical conductivity of 0.033 Scm − 1 and the lowest densification temperature among the apatite ceramics evaluated, and the calculated thermal expansion coefficient of this system emerged to be 10.5 × 10 − 6 K −1 , compatible with those of the common adjacent materials used in solid oxide fuel cells (SOFCs). Single cells of NiO-SDC/La 9.5 Ge 5.5 Fe 0.5 O 26 /LSCF-SDC were built and evaluated. The open circuit voltages of the single cell read 0.96 and 1.01 V respectively at 950 and 800 °C, both in accord with the redicted theoretical values and verifying the pure ionic conduction in apatites. The maximum power densities (MPDs) of the single cell were 0.384, 0.280, 0.201, and 0.125 Wcm − 2 respectively at the measurement temperatures of 950, 900, 850, and 800 °C. The MPDs of the single cell with the La 9.5 Ge 5.5 Fe 0.5 O 26 electrolyte are superior to those of the SOFCs with apatite ceramics as electrolyte previously reported in the literature, thereby testifying to the potential of the La 9.5 Ge 5.5 Fe 0.5 O 26 ceramic as a promising electrolyte material for SOFCs. • Characteristics of Fe 3 + , Mn 3 + , Ni 2 + , and Ti 4 + doped La 9.5 Ge 6 O 26 ± δ were examined. • SOFC cells of NiO-SDC/La 9.5 Ge 5.5 Fe 0.5 O 26 /LSCF-SDC were built and characterized. • MPD of the cell with La 9.5 Ge 5.5 Fe 0.5 O 26 (0.39 mm) read 0.201 Wcm − 2 Wcm − 2 at 850 °C. • The results verified the potential of La 9.5 Ge 5.5 Fe 0.5 O 26 ceramic for use in SOFCs.