Evaluation of apatite silicates as solid oxide fuel cell electrolytes

Apatite-type silicates have been considered as promising electrolytes for Solid Oxide Fuel Cells (SOFC); however studies on the potential use of these materials in SOFC devices have received relatively little attention. The lanthanum silicate with composition La₁₀Si_5.5Al_0.5O_26.75 has been evaluated as electrolyte with the electrode materials commonly used in SOFC, i.e. manganite, ferrite and cobaltite as cathode materials and NiO-CGO composite, chromium-manganite and Sr₂MgMoO₆ as anode materials. Chemical compatibility, area-specific resistance and fuel cell studies have been performed. X-ray powder diffraction (XRPD) analysis did not reveal any trace of reaction products between the apatite electrolyte and most of the aforementioned electrode materials. However, the area-specific polarisation resistance (ASR) of these electrodes in contact with apatite electrolyte increased significantly with the sintering temperature, indicating reactivity at the electrolyte/electrode interface. On the other hand, the ASR values are significantly improved using a ceria buffer layer between the electrolyte and electrode materials to prevent reactivity. Maximum power densities of 195 and 65 mWcm⁻² were obtained at 850 and 700 °C, respectively in H₂ fuel, using an 1 mm-thick electrolyte, a NiO-Ce_0.8Gd_0.2O_1.9 composite as anode and La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ as cathode materials. This fuel cell was tested for 100 h in 5%H₂-Ar atmosphere showing stable performance.