In situ study of strontium segregation in La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ in ambient atmospheres using high-temperature environmental scanning electron microscopy

Abstract

Samples of the solid oxide fuel cell cathode material La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) were annealed using HT-ESEM from room temperature to 1000 °C in atmospheres of pure oxygen, pure water and ambient lab air.

Atomically resolved surface phases of La0.8Sr0.2MnO3(110) thin films

“…Moreover, the lms neither show the expected Sr segregation nor the formation of insulating, Sr-rich clusters upon prolonged (>5 h) annealing at 700 C and 0.2 mbar O 2 : the A-site-to-Mn and the La-to-Sr XPS intensity ratios stay unaltered, and the surface structures appear unchanged in STM, always retaining atomically at morphologies. It is likely that the Sr segregation observed in polycrystalline and textured lms and in ceramic samples under comparable conditions is due to grain and twin boundaries that favor Sr diffusion, 97,98 which are absent in the single-crystalline lms presented here. It is also possible that the epitaxial strain imposed by the SrTiO 3 substrate has a role in suppressing Sr segregation.…”

Surface recrystallization – an underestimated phenomenon affecting oxygen exchange activity

“…On the other hand, grain size determines the amount of grain boundaries which can act as a sink or source of impurities or nucleation sites for secondary phases. 61 Therefore, grain size can affect the rate at which the samples deactivate at 650°C following the high-temperature treatment. We estimate that the density of grain boundaries is approximately 40x greater in LSF than in LSC, which could explain why k chem values are only affected by a factor of 2-3 in case of LSC, while a 30x difference between the activated and passivated states was observed in case of LSF.…”

Effect of Microstructure on the Degradation of La0.6Sr0.4CoO3–δ Electrodes in Dry and Humid Atmospheres

“…Niania et al. analyzed the composition of crystallites formed on the surface of LSCF6428 after annealing at high temperatures in dry and humid atmospheres by Auger electron spectroscopy (AES) and low energy ion scattering (LEIS). The authors found strong segregation of strontium to the surface and concluded that the crystallites are Sr‐based compounds.…”

“…This is quite reasonable, considering that for mixed‐conducting electrodes this frequency region is commonly assigned to the actual electrode processes encompassing a series of surface‐based elementary reactions (molecular oxygen adsorption, reduction, dissociation, and incorporation into the bulk ). These reactions are strongly affected by modifications of the electrode surface induced either directly by H 2 O (g) or indirectly by poisoning with critical impurities (Cr, Si, etc. ), due to their enhanced gas phase transport in humid atmospheres .…”

“…Niania et al. heated dense samples of LSCF6428 up to 1,000 °C in dry and humid atmospheres and recorded in situ the surface growth of Sr‐based segregation phases by high‐temperature environmental SEM (HT‐ESEM). The particle growth rate was found to be strongly accelerated at higher water partial pressure.…”