cm401977p_si_001.pdf (332.67 kB)
Observation of Oxygen Vacancy Filling under Water Vapor in Ceramic Proton Conductors in Situ with Ambient Pressure XPS
journal contribution
posted on 2013-12-10, 00:00 authored by Qianli Chen, Farid El Gabaly, Funda Aksoy Akgul, Zhi Liu, Bongjin
Simon Mun, Shu Yamaguchi, Artur BraunThe 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.