Oxygen self-diffusion and conductivity measurements in apatite type electrolyte materials for SOFCs

Dense apatite type lanthanum silicate LSO (La9.33Si6O26) electrolyte and the Al-, Fe- and Mg-doped compositions LASO (La9.83Al1.5Si4.5O26), LFSO (La9.83Fe1.5Si4.5O26), and LMSO (La9.93Mg0.9Si5.1O26) have been prepared and investigated in this work in terms of their oxygen self-diffusion and total conductivity. Oxygen self-diffusivities and surface exchange coefficients and the respective activation enthalpies have been estimated and discussed. All samples exhibit a typical Arrhenius behaviour of the surface exchange coefficients meaning that the surface incorporation reaction is thermally activated. The doped samples exhibit higher surface concentrations as well as higher surface exchange coefficients as compared to the undoped sample. This indicates that doping has a direct influence to the oxygen incorporation reaction. It has been found that doping enhances the self-diffusion in all cases as compared to the undoped material. The trend found in this study is LSO «LFSO < LASO ≤ LMSO. The diffusivities of the apatite type lanthanum silicate (ATLS) electrolyte materials are higher compared to yttria stabilized zirconia which is the state-of-the-art electrolyte material. The activation enthalpy for oxygen diffusion in YSZ is higher as compared to the ATLS used in this study. Conductivity studies were also made on samples of the same composition and density. We observed that aluminium- and magnesium-doped samples have better conductivity than the undoped counterpart. The role of iron on the conduction mechanism is unclear, but it shows a positive influence on the conductivity. This behaviour is in agreement with conductivity measurement results in the literature, where it has been found that doping with aluminium and magnesium leads to an increased ionic conductivity.