Oxy-apatites based on rare earth silicates (A10x(SiO4)6O2±x, A=rare earth cation) are of increasing interest for solid oxide fuel cell (SOFC) application due to the high conductivity at moderate temperatures (<1000 K). Here, we report on the intermediate-temperature synthesis (973 K) of thin film oxy-apatites and high total conductivity of the cerium silicate-based apatites at temperatures <750 K: the Ce_4.67(SiO₄)₃O-based apatites (~80 nm-thick) were synthesized at 973 K at very low oxygen partial pressure (p(O2)<10^-17atm) and the incorporation of ZnO into the cerium silicate system leads to the high total conductivity of ~0.05-0.2 S/cm. The formation of oxy-apatites was identified by in-situ conductivity measurements as a function of p(O2), x-ray diffraction analysis and x-ray photoelectron spectroscopy. In particular, the in-situ conductivity measurements confirm that the dominant conduction mechanisms of this class of materials are mainly dependent on oxygen interstitials.

Since these materials are prepared at low p(O2) and are stable in reducing atmospheres, Ce_4.67(SiO₄)₃O-based thin film apatites exhibiting high conductivity are of relevance as anodes for intermediate temperature thin film SOFC application. In order to evaluate the performance of the apatite anode in SOFC devices, thin film apatites were grown on Sc-stabilized ZrO₂/LSCF electrolyte/cathode substrates. Bilayer anode of apatite/Pt and Ni-apatite composite anode were also utilized in the identical electrolyte/cathode system and the performance were compared. The noteworthy SOFC performance (e.g., peak power density of ~100 mW/cm² at 748 K) and superior stability were observed in Ni-apatite SOFCs due to higher catalytic activity and low contact resistance at the electrolyte/anode interface compared to those with a single layer apatite anode and bilayers of apatite/Pt anode. The present study demonstrating intermediate temperature synthesis of Ce_4.67(SiO₄)₃O-based oxy-apatites and their high conductivity may significantly contribute to the field of intermediate temperature thin film SOFC application.