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Observation of oxygen vacancy filling under water vapor in ceramic proton conductors in-situ with ambient pressure XPS

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Publication date 2013
Author Chen Qianli, El Gabaly Farid, Aksoy Akgul Funda, Liu Zhi, Mun Bongjin Simon, Yamaguchi Shu, Braun Artur,
Project Effect of lattice volume and imperfections on the proton-phonon coupling in proton conducting lanthanide transition metal oxides: High pressure and high temperature neutron and impedance studies
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Original article (peer-reviewed)

Journal Chemistry of Materials
Volume (Issue) 25(23)
Page(s) 4690 - 4696
Title of proceedings Chemistry of Materials
DOI 10.1021/cm401977p

Abstract

The interaction of metal oxides with their ambient environment at elevated temperatures is of significant relevance for the functionality and operation of ceramic fuel cells, electrolyzers, and gas sensors. Proton conductivity in metal oxides is a subtle transport process which is based on formation of oxygen vacancies by cation doping and substitution and oxygen vacancy filling upon hydration in water vapor atmosphere. We have investigated the conductivity and electronic structure of the BaCeY-oxide proton conductor under realistic operation conditions from 373 to 593 K and water vapor pressures up to 200 mTorr in situ by combining ambient pressure X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. We provide element specific spectroscopic evidence that oxygen vacancies are filled by oxygen upon water exposure and partly oxidize Ce3+ and Y2+ toward Ce4+ and Y3+. Moreover, the resonant valence band spectra of dry and hydrated samples show that oxygen ligand holes in the proximity of the Y dopant are by around 0.5 eV closer to the Fermi level than the corresponding hole states from Ce. Both hole states become substantially depleted upon hydration, while the proton conductivity sets on and increases systematically. Charge redistribution between lattice oxygen, Ce, and Y when BCY is exposed to water vapor at ambient and high temperature provides insight in the complex mechanism for proton incorporation in BCY.
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