Impact of Sr segregation on the electronic structure and oxygen reduction activity of SrTi[subscript 1−x]Fe[subscript x]O[subscript 3] surfaces
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STF Manuscript revised final 03262012 yildz.pdf
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Author(s)
Chen, Yan
Cai, Zhuhua
Kim, Jae Jin
Tuller, Harry L.
Yildiz, Bilge
Jung, Woo Chul
Date Issued
April 2012
Journal
Energy & Environmental Science
Publisher
Royal Society of Chemistry
Citation
Chen, Yan, WooChul Jung, Zhuhua Cai, Jae Jin Kim, Harry L. Tuller, and Bilge Yildiz. “Impact of Sr Segregation on the Electronic Structure and Oxygen Reduction Activity of SrTi1−xFexO3 Surfaces.” Energy & Environmental Science 5, no. 7 (2012): 7979.
Version
Author's final manuscript
Abstract
The correlation between the surface chemistry and electronic structure is studied for SrTi[subscript 1−x]Fe[subscript x]O[subscript 3] (STF), as a model perovskite system, to explain the impact of Sr segregation on the oxygen reduction activity of cathodes in solid oxide fuel cells. Dense thin films of SrTi[subscript 0.95]Fe[subscript 0.05]O[subscript 3] (STF5), SrTi[subscript 0.65]Fe[subscript 0.35]O[subscript 3] (STF35) and SrFeO[subscript 3] (STF100) were investigated using a coordinated combination of surface probes. Composition, chemical binding, and valence band structure analysis using angle-resolved X-ray photoelectron spectroscopy showed that Sr enrichment increases on the STF film surfaces with increasing Fe content. In situ scanning tunnelling microscopy/spectroscopy results proved the important and detrimental impact of this cation segregation on the surface electronic structure at high temperature and in an oxygen environment. While no apparent band gap was found on the STF5 surface due to defect states at 345 °C and 10[superscript −3] mbar of oxygen, the surface band gap increased with Fe content, 2.5 ± 0.5 eV for STF35 and 3.6 ± 0.6 eV for STF100, driven by a down-shift in energy of the valence band. This trend is opposite to the dependence of the bulk STF band gap on the Fe fraction, and is attributed to the formation of a Sr-rich surface phase in the form of SrO[subscript x] on the basis of the measured surface band structure. The results demonstrate that Sr segregation on STF can deteriorate oxygen reduction kinetics through two mechanisms – inhibition of electron transfer from bulk STF to oxygen species adsorbing onto the surface and the smaller concentration of oxygen vacancies available on the surface for incorporating oxygen into the lattice.
MIT Department
Massachusetts Institute of Technology. Department of Materials Science and Engineering
Massachusetts Institute of Technology. Department of Nuclear Science and Engineering
Massachusetts Institute of Technology. Laboratory for Electrochemical Interfaces
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DOI of Published Version
https://doi.org/10.1039/c2ee21463f
