Oxygen nonstoichiometry of the Ruddlesden-Popper oxide Sr3Fe2O7-δ was measured at intermediate temperatures (773-1073 K) by coulometric titration and high temperature gravimetry. The oxygen nonstoichiometric behavior was analyzed using the defect equilibrium model with localized electrons. From the defect chemical analysis, estimated oxygen vacancy concentration at the O3 sites increases and at the O1 sites decreases with the increasing temperature. This characteristic behavior is considered to be caused by the redistribution of oxygen and vacancies between the O1 and O3 sites. The obtained thermodynamic quantities of the partial molar enthalpy of oxygen, h(O) - h°(O), and the partial molar entropy of oxygen, s(O) - s°(O), calculated from the Gibbs-Helmholtz equation are in good agreement with those from the statistical thermodynamic calculation based on the defect equilibrium model, indicating that the proposed defect equilibrium model is reasonable.
Factors governing the change of the surface composition and the degradation of transport properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) under low concentration sulfur-contained air (0.01 ppm) were examined as functions of water vapor pressure (p(H 2 O)) and oxygen partial pressure (p(O 2 )). Bulk LSCF samples were heat-treated at 973 K in dry or wet air containing SO 2 and more enhanced SrSO 4 formation/growth was observed under wet compared to dry air. When the p(O 2 ) was changed from 0.21 to 0.01 bar, the SrSO 4 precipitates became smaller in size and higher in number, indicating that the p(H 2 O) and the p(O 2 ) have different effects on secondary phase formation. Detailed analysis of the surface composition indicates the presence of CoFe 2 O 4 in the vicinity of the SrSO 4 particle, suggesting the Sr and Co depletion in the subsurface region. Based on the analysis, the water vapor affects the secondary phase growth due to the possibility of additional reaction pathways that may enhance the growth rate. This behavior is correlated with the logarithmic surface oxygen exchange coefficient, log k * , that was observed to decrease monotonically within 20 h of annealing time but its degradation was found to be faster under wet air, in accordance with Sr and Co depletion rate.
In order to investigate the long-term degradation of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) cathodes due to chromium (Cr) poisoning, the transport properties were investigated by two different approaches: one is electrochemical impedance spectroscopy (EIS) on porous LSCF electrodes, the other is the oxygen isotope exchange and SIMS analyses on LSCF pellets. The oxygen surface exchange coefficient (k * ) determined from EIS decreased with increasing poisoning time. For LSCF pellets, the k * values decreased linearly with poisoning time (the first stage) and then became nearly constant (the second stage). The first degradation of k * was attributed to the formation of SrCrO 4 and CoFe 2 O 4 within the initial 7∼8 h poisoning time, whereas in the second stage, LaCrO 3 formation was detected on the surface of LSCF. Those chemical behaviors were compared with thermodynamic calculations where the k * was observed to decrease in the first stage corresponding to the precipitation of SrCrO 4 and CoFe 2 O 4 , and LaCrO 3 is formed in the second stage. Those considerations lead to the conclusions that the k * decrease is dominated by the decrease in available sites such as Sr, Co or oxygen vacancies on the LSCF surface, whereas the constant k * values may be attributed to the remaining/formed LaCrO 3 component of the reacted LSCF.
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