Determination of Factors Governing Surface Composition and Degradation of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ Electrode under Sulfur-Contained Air

Abstract

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.

Achievements of NEDO Durability Projects on SOFC Stacks in the Light of Physicochemical Mechanisms

“…When Comparison is made between LSC(SSC) and LSCF, differences can be seen in kinetics despite the thermodynamic features are similar: The total reaction speed is high in LSS(SSC). This is understandable from the thermodynamic reactivity of the Sr component. The SrSO 4 formation in LSCF exhibits more high kinetic barriers; number and size of formed SrSO 4 are smaller and larger, respectively, in LSCF, providing some heterogenous features on the reacted LSCF surface . Thus, the SrSO 4 formation in LSCF can be influenced by many factors such as availability of protons/water vapors etc.…”

“…Thus, the SrSO 4 formation in LSCF can be influenced by many factors such as availability of protons/water vapors etc. .…”

“…The SrSO 4 formation in LSCF exhibits more high kinetic barriers; number and size of formed SrSO 4 are smaller and larger, respectively, in LSCF, providing some heterogenous features on the reacted LSCF surface . Thus, the SrSO 4 formation in LSCF can be influenced by many factors such as availability of protons/water vapors etc.…”

“…This can be represented in terms of Δ μ (O 2 ) derived from cathode overpotential. Current density and air utilization determine a flow rate of air. Local temperature can be influenced by Joule heats and reaction heats. If water is involved, there must appear a gap in water chemical potential, Δ μ (H 2 O) between in gases and in cathodes. Two effects of compound formations are considered, trapping and poisoning. When gaseous impurities are reacted with cathodes to form SrSO 4 /SrCrO 4 outside the electrochemically active area, this is regarded as trapping ; however, when the Sr component in SrSO 4 /SrCrO 4 comes from the active area, this may cause degradations because the electrochemical performance strongly depends on the Sr content in the perovskite oxide surface . This strongly suggests that composition of the cathode particle surface in the active area is crucially important in determining performance and its degradations. Such surface concentrations are changed by various processes: Chemical reactions to extract the Sr component out of the perovskite cathodes. The Sr volatilization from the cathodes. Sr (Co) diffusion from deep inside to cathode surface to compensate the Sr(or Co) depletion on surface. …”

“…Two effects of compound formations are considered, trapping and poisoning. When gaseous impurities are reacted with cathodes to form SrSO 4 /SrCrO 4 outside the electrochemically active area, this is regarded as trapping ; however, when the Sr component in SrSO 4 /SrCrO 4 comes from the active area, this may cause degradations because the electrochemical performance strongly depends on the Sr content in the perovskite oxide surface . This strongly suggests that composition of the cathode particle surface in the active area is crucially important in determining performance and its degradations.…”