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WW8.03/AAA10.03 - Operando X-Ray Absorption Spectroscopy of LSCF Thin Film Cathode During Its Degradation in Carbon Dioxide and Water 
Date/Time:
April 24, 2014   3:45pm - 4:00pm
 
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Information from traditional ex situ characterization techniques falls short in accurately describing complex materials systems, such as solid oxide fuel cells, which require specific conditions for optimal operation. Operando x-ray absorption spectroscopy (XAS) fills the gap by providing element-specific information on the chemical phenomena responsible for performance deficiencies. A La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) thin film quasi-symmetrical cathode cell was tested in air contaminated with H2O and CO2 at 400 and 700°C and simultaneously examined with glancing angle operando XAS using a custom built testing platform, which is also described. Whereas impedance spectroscopy indicates increased polarization resistance in the cathode in both atmospheres at 750°C, XAS near-edge and extended fine structure data indicated that the oxidative behavior of H2O and CO2 on Fe and Co cations have diverging behaviors with increasing temperatures. In particular, the degree of oxidation according to absorption edge shift varies with temperature and cation and comparison with the impedance data showed some correlation between the degree of oxidation and amount of increase in the polarization resistance as a function of temperature. The local atomistic structure of the Co was also observed to be more severely affected by H2O and CO2 than that of the Fe, which showed stability in its local structure under all of the testing conditions.Synchrotron-based in situ x-ray photoelectron spectroscopy was performed to complement the study with surface-specific information. Spectroscopy at high temperature and post-gas exposure discovered the removal and formation of a carbonate species, the irreversible formation of a new Co phase, and the related evolution of different oxygen moieties, such as surface, lattice, and hydroxide oxygen, all as a function of temperature and atmosphere. The combined information suggests that the segregation of the Co cation from the perovskite phase into an oxide phase via the formation and subsequent decomposition of a carbonate phase at operating temperature is strongly suspected as a mechanism by which H2O and CO2 cause degradation in the LSCF cathode.Additionally, a comparison of the results from operando and in situ testing revealed that cathodic bias has some interaction with the CO2 gas. While cathodic bias caused a slight reduction of Fe and Co and CO2 gas caused an oxidation of Fe and Co, the combination of those parameters in the operando experiment caused more severe oxidation of both Fe and Co. In conclusion, the operando results cannot be predicted by their in situ components, strengthening the rationale for conducting operando experiments, despite their challenges.
 


 
 
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