New insights on the structure and reducibility of 3D versus 2D La/Sr titanates for SOFC anodes

In this study, experiments carried out under air and reducing atmospheres allowed giving a new insight in the limits of the LaxSr1 − xTiO3 + δ solid solution. Diluted hydrogen at different controlled temperatures was used to simulate the SOFC anodic operating conditions (in situ reduction, T = 900 °C) and to characterize the impact of a pre-reduction at higher temperature (T = 1400 °C). LaxSr1 − xTiO3 + δ compounds (0 < x ≤ 0.40) were synthesized in air. XRD and electron diffraction are performed in order to clarify some divergences concerning the structural characterization of the series. A 3D perovskite structure (LST3D) evolving from cubic Pm 3 ¯ m for x ≤ 0.10 to tetragonal I4/mmm is observed for 0.10 < x ≤ 0.20. For 0.15 < x ≤ 0.40, the formation of lamellar phases-type impurities was observed, and interpreted as the consequence of the segregation of the randomly distributed overstoichiometric oxygen (and lanthanum) atoms, precursor to the La4SrTi5O17 and La4Sr2Ti6O20 lamellar phases, as clearly identified for x = 0.40. In the 3D solid solution, these kinds of defects deform the perovskite cell leading to a volume expansion, that can be nearly completely dissolved through a high temperature reduction process (T = 1400 °C). In situ reduction (T = 900 °C) is kinetically slower for all compositions but especially in the case of the presence of lamellar defects. Indeed, LST3D material was found thermodynamically and kinetically more reducible under H2 compared to LST2D, whose layered structure is maintained with less than 16% Ti3 + even after reduction at 1400 °C for 24 h. Electrical conductivity as a function of temperature has been measured in reducing atmosphere for both LST2D and LST3D, and discussed based on TGA measurements.