Effects of Nb⁵⁺, Mo⁶⁺, and W⁶⁺ dopants on the germanate-based apatites as electrolyte for use in solid oxide fuel cells

Rare information is available in the literature on the cell performance of the solid oxide fuel cells (SOFCs) using apatites known for their good electrical conductivity as electrolyte materials. In this study, La _9.5Ge_5.5Nb_0.5O_26.5, La _9.5Ge_5.5Mo_0.5O_26.75, and La _9.5Ge_5.5W_0.5O_26.75 ceramics were prepared and characterized. The results indicated that the La _9.5Ge_5.5Nb_0.5O_26.5 and La _9.5Ge_5.5W_0.5O_26.75 ceramics reported hexagonal phase, while the La_9.5Ge_5.5Mo _0.5O_26.75 ceramic demonstrated triclinic symmetry. Among the apatities evaluated, La_9.5Ge_5.5Nb_0.5O _26.5 sintered at 1450 C showed the best conduction with an electrical conductivity value of 0.045 S/cm at 800 C. Button cells of NiO-SDC/La _9.5Ge_5.5Nb_0.5O_26.5/LSCF-SDC were built and revealed good structural integrity. The total ohmic resistance (R ₀) and interfacial polarization resistance (R_P) of the cell read 0.428 and 0.174 Ω cm² and 0.871 and 1.164 Ω cm², respectively at 950 and 800 C. The maximum power densities (MPD) of the single cell at 950 and 800 C were respectively 0.363 and 0.095 W cm ^-2. Without optimizing the anode and cathode as well as hermetic sealing of the cell against the gas, the study found the performance of the single cell with the pure La_9.5Ge_5.5Nb_0.5O _26.5 as its electrolyte material superior to those of the SOFC cells with a YSZ electrolyte of comparable thickness shown in the literature.