Oxygen permeation properties and the stability of La0.6Sr0.4Fe0.8Co0.2O3 studied by Raman spectroscopy

A further investigation of the kinetic demixing/decomposition of La0.6Sr0.4Co0.2Fe0.8O3−δ oxygen separation membranes

“…It is very likely that the B-site cations Co and Fe diffuse faster than the Sr as can be inferred from the results from the 3672 and 5512-h membranes where the Fe/Sr and Co/Sr ratios are higher at the air side than that in the bulk. This observation is consistent with previous studies by other groups [30,33,34].…”

“…On the air side surface of the 5512-h, however, cobalt ferrite was found rather than the cobalt oxide. Since both cobalt oxide and cobalt ferrite were also observed on the air side surface of similar La 0.5 Sr 0.5 Co 0.5 Fe 0.5 O 3−ı and LSCF membranes by other researchers [30,34], we can postulate that a transition from cobalt oxide to cobalt ferrite occurred on the outer surface between 1128 and 5512 h of operation. This transition indicates that the cobalt enrichment on the outer surface thermodynamically favoured the formation of cobalt oxide and afterwards iron enrichment resulted in favourable thermodynamics for the formation of cobalt ferrite.…”

“…Diethelm et al studied the change of La 0.6 Ca 0.4 Fe 0.75 Co 0.25 O 3−ı membranes after use as membrane reactors for the partial oxidation of methane (POM) and found cobalt enrichment on the air side and lanthanum enrichment on the methane side; a secondary phase of cobalt oxide appeared in the bulk near the air side surface [33]. Iguchi et al also studied LSCF under POM conditions and a cobalt ferrite secondary phase was found on the air side by Raman spectroscopy; they also noted a SrO secondary phase and La depletion at the air side [34]. Lein et al studied kinetic demixing and decomposition of a cobalt slightly excessive La 0.5 Sr 0.5 Co 0.5 Fe 0.5 O 3−ı membrane under an air/He difference, and observed cobalt oxide on the air side surface.…”

“…Besides the above concerns, another inevitable phenomenon which might cause severe performance degradation of MIEC membranes must be addressed, that is, the kinetic demixing and decomposition of membrane materials, such processes having been extensively observed in perovskite-type MIEC membranes in air separation operations [28][29][30][31][32][33][34][35]. The kinetic demixing happens due to the different oxygen chemical potentials of the feed side and permeate side when MIEC membranes are operated under airseparation conditions.…”

Thermodynamic and kinetic considerations on degradations in solid oxide fuel cell cathodes

“…Even so, some perovskite oxides exhibit the same order of diffusivity like in (La,Sr)(Ga,Mg)O 3 [41] or even faster B-site cation diffusivity like in (La,Sr)FeO 3 [42]. In (La,Sr)(Co,Fe)O 3 and related perovskites, actual kinetic demixings have been reported recently [43,44] resulting in precipitation of Co/Fe-rich phases on the oxidative side. For (La,Sr)FeO 3 or (La,Sr)(Co,Fe)O 3 , the cathode reactions are not restricted to the three-phase boundaries but take place mainly at the cathode surface.…”