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

In this study, the effects of Fe, Mn, Ni, and Ti dopants on the densification, phase structure, microstructure, and electrical conductivity of the La9.5Ge6.0O26.25 ceramics were examined. After sintering, the La9.5Ge5.5Mn0.5O26 and La9.5Ge5.5Ti0.5O26.25 ceramics displayed hexagonal symmetry while the La9.5Ge5.5Ni0.5O25.75 and La9.5Ge5.5Fe0.5O26 ceramics produced triclinic lattice. The La9.5Ge5.5Fe0.5O26 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/La9.5Ge5.5Fe0.5O26/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 La9.5Ge5.5Fe0.5O26 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 La9.5Ge5.5Fe0.5O26 ceramic as a promising electrolyte material for SOFCs.