Characterization ofLa0.8Sr0.2FeO3−δand

Aniruddha Deb

¹

,

J. M. Ralph

²

,

Elton J. Cairns

³

et al.

Abstract

A temperature-dependent x-ray absorption spectroscopy ͑XAS͒ study was performed to investigate the changes in electronic and atomic structure of La-deficient La 0.7 Sr 0.2 FeO 3−␦ ͑L7S2FO3͒ and stoichiometric La 0.8 Sr 0.2 FeO 3−␦ ͑L8S2FO3͒. La 0.8 Sr 0.2 FeO 3−␦ is a promising cathode material for intermediate operating temperature ͑700-800°C͒ solid oxide fuel cells. Performance improvements have been shown by increasing the La-or A-site deficiency in this material but a clear understanding of the mechanisms responsible for this improvement are still needed. Here we report an x-ray absorption spectroscopy ͑XAS͒ study as a function of temperature to investigate electronic and atomic structure changes of La-deficient La 0.7 Sr 0.2 FeO 3−␦ ͑L7S2FO3͒ and stoichiometric La 0.8 Sr 0.2 FeO 3−␦ ͑L8S2FO3͒. In particular we have measured the temperature-dependent changes in oxidation state, bond distance, Fe coordination number, and oxygen vacancies for both compounds. L7S2FO3 contains 10% A-site vacancies compared to stoichiometric L8S2FO3, which has a fully occupied A site and thus some form of charge compensation is necessary in the former to maintain charge neutrality. X-ray absorption near edge spectroscopy shows the presence of Fe 3+ and Fe 4+ in both L7S2FO3 and L8S2FO3 ͑mixed valence͒ as established by comparison with model compounds. Studies from room temperature to 850°C show that Fe 3+ dominates over Fe 4+ in both materials with increasing dominance as the temperature is increased. Furthermore, the temperature-dependent study revealed the La-deficiency in L7S2FO3 leads to a higher concentration of both electron holes ͑i.e., more Fe 4+ created͒ and oxygen vacancies, compared to the stoichiometric L8S2FO3. Analysis of the extended x-ray absorption fine structure shows that the Fe-O bond increases with the increase in temperature for both the systems.

Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

“…TheF eK -edge energy shifts positive by 0.7 eV relative to at 0.15 Vvs. [30,31] Thei ncrease in Fe K-edge energy could also be due to oxidation of some Fe sites to Fe 6+ , Figure 1. This increase in Fe edge energy upon driving OER represents only partial oxidation of Fe 3+ sites.X AS spectra from La 1Àx Sr x FeO 3Àd show that ac hange in Fe oxidation state from Fe 3+ to Fe 4+ results in an increase in Kedge energy of about 1.3 eV.…”

Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

“…This increase in Fe edge energy upon driving OER represents only partial oxidation of Fe 3+ sites. XAS spectra from La 1− x Sr x FeO 3−δ show that a change in Fe oxidation state from Fe 3+ to Fe 4+ results in an increase in K‐edge energy of about 1.3 eV . The increase in Fe K‐edge energy could also be due to oxidation of some Fe sites to Fe 6+ , which has recently been suggested to be the active site in Ni(Fe)O x H y .…”

Correlation between structure and mixed ionic–electronic conduction mechanism for (La1−xSrx)CoO3−δ using synchrotron X-ray analysis and first principles calculations

“…35,[54][55][56][57][58][59] The normalized XANES spectra and pre-edge peaks of the Co K-edge are shown in Fig. This relationship has been extensively studied for a number of transition metal perovskite oxides.…”