Ionic conductivity and fuel cell properties of apatite-type lanthanum silicates doped with Mg and containing excess oxide ions

Enhancement of the ionic conductivity of lanthanum silicate-based apatites is examined with emphasis on optimizing the La composition and the Mg doping level at the same time. La10Si5.8Mg0.2O26.8 and La9.8Si5.7Mg0.3O26.4 show the highest level of the ionic conductivities among apatite silicates, 8.8 and 7.4 × 10− 2 S cm− 1 at 800 °C, respectively, with a very low level of activation energy (0.42–0.43 eV). Their conductivities are higher than yttria stabilized zirconia (YSZ) below 900 °C and even comparable to Sr and Mg doped lanthanum gallate (LSGM) below 550 °C. A solid oxide fuel cell using La9.8Si5.7Mg0.3O26.4 as an electrolyte with Ni–ceria cermet anode and Sr doped lanthanum cobaltite cathode exhibits a remarkable improvement in power generation compared to previous data using Pt electrodes. Structural investigation by the Rietveld analysis on the powder X-ray diffraction pattern shows significant enlargement of the bottleneck triangle sizes of the conduction channel with the Mg doping.