Observation of Oxygen Vacancy Filling under Water Vapor inCeramic Proton Conductors in Situ with Ambient Pressure XPSQianli Chen,†,‡ Farid El Gabaly,§ Funda Aksoy Akgul,∥,⊥ Zhi Liu,∥ Bongjin Simon Mun,#

Shu Yamaguchi,▲ and Artur Braun*,†

†Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Science and Technology, CH-8600Dubendorf, Switzerland‡Department of Physics, ETH Zurich, Swiss Federal Institute of Technology CH-8057 Zurich, Switzerland§Sandia National Laboratories, Livermore, California 94551, United States∥Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States⊥Physics Department, Nigde University, 51240 Nigde, Nigde, Turkey#Department of Physics and Photon Science, School of Physics and Chemistry, Ertl Center for Electrochemistry and Catalysis,Gwangju Institute of Science and Technology, Gwangju, Chonnam 500-712, Republic of Korea▲Department of Materials Engineering, University of Tokyo, 113-8656 Tokyo, Japan

*S Supporting Information

ABSTRACT: The interaction of metal oxides with their ambientenvironment at elevated temperatures is of significant relevance forthe functionality and operation of ceramic fuel cells, electrolyzers, andgas sensors. Proton conductivity in metal oxides is a subtle transportprocess which is based on formation of oxygen vacancies by cationdoping and substitution and oxygen vacancy filling upon hydration inwater vapor atmosphere. We have investigated the conductivity andelectronic structure of the BaCeY-oxide proton conductor underrealistic operation conditions from 373 to 593 K and water vaporpressures up to 200 mTorr in situ by combining ambient pressure X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. We provide element specific spectroscopicevidence 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 theY dopant are by around 0.5 eV closer to the Fermi level than the corresponding hole states from Ce. Both hole states becomesubstantially depleted upon hydration, while the proton conductivity sets on and increases systematically. Charge redistributionbetween lattice oxygen, Ce, and Y when BCY is exposed to water vapor at ambient and high temperature provides insight in thecomplex mechanism for proton incorporation in BCY.

KEYWORDS: proton conductor, perovskite, proton diffusivity, oxygen vacancy, AP-XPS, ambient pressure XPS, valence band,in situ spectroscopy, impedance spectroscopy, resonant photoemission

■ INTRODUCTION

Protons can be structural elements in molecules and inhydrates, for example, and also ionic and electric chargecarriers. In the former case they are localized; in the latter casethey are delocalized. The proton is in both cases an elusiveelement as far as its interaction with the host lattice ormolecular environment is concerned.Ceramic proton conductors are prospective solid electrolytes

for intermediate temperature solid oxide fuel cells (IT-SOFC1−3). For its functionality as proton conductor, thedynamics is very important with respect to lowering protontransport activation energies. Impedance spectroscopy, inelasticand quasi-elastic neutron scattering, and Raman and infraredvibrational spectroscopy are the analytical tools to address thedynamics of the proton conductivity. For improving and

optimizing proton conducting materials, detailed knowledge onthe chemical interaction of the proton with the elements in thehost lattice is necessary. For example, exposure of metals tohydrogen atmosphere can cause embrittlement of the metal, i.e.structural disintegration. In metal oxides, exposure to hydrogenatmosphere can form hydroxyl groups, which constitute a veryrigid and polar species that may affect the integrity of the oxideand influence on charge transfer. In compounds with oxygenvacancies, the insertion of a water molecule produces twoidentical OHO• species in the host lattice, with both theprotons of both of these species equally mobile.4

Received: June 18, 2013Revised: November 16, 2013

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In the present work we have substituted BaCeO3 to 20% withY2O3 (BaCe0.8Y0.2O3‑δ) in order to form oxygen vacancies to befilled with oxygen from water vapor molecules. The chemicalinteractions of protons with the host metal oxide areinvestigated by ambient pressure XPS under dry and hydratedconditions, meanwhile the proton conductivity is studied usingelectrochemical impedance spectroscopy.

■ EXPERIMENTAL SECTIONBaCe0.8Y0.2O3‑δ (BCY20) was prepared by solid state synthesis fromprecursors mixed in stoichiometric amounts and fired at 1473 K for 12h in air, then ground, and calcined again at 1473 K for 12 h. Theobtained powder was pressed to pellets of 1 mm thickness and 18 mmdiameter at 10 kbar and sintered for 24 h at 1673 K in air.5,6 Phasepurity of the BCY20 was confirmed with powder X-ray diffraction onthe finally obtained pellets (Figure S1 in the Supporting Information).The sintered pellet was subject to a water vapor saturated N2 flow at670 K for 16 h.5 Temperature dependent X-ray diffraction measure-ments were made on the sintered sample from room temperature upto 923 K in steps of 50 K. X-ray diffractograms were measured in aBragg−Brentano geometry using a PANalytical X’Pert PRO θ-2θ scansystem with the X-ray wavelength of 1.5406 Å (Cu−Kα1).Au electrodes were deposited on the proton conducting electrolyte

by evaporation in vacuum from crucibles heated by an electron beam,with a mask to control the shape of the electrode.7 A sample holderwith spring-loaded probes (Figure 1), specifically designed for the

combination of XPS and impedance spectroscopy in situ and operandounder realistic high temperature electrochemical conditions, was usedto provide good electrical contact with the electrodes.8 The ionicconductivity for BCY20 was measured by electrochemical impedancespectroscopy (EIS) using a PCI4/750 Potentiostat (Gamry). Theimpedance spectra were analyzed with ZView (Scribner Associates).X-ray photoelectron spectra were recorded at the ambient pressure

photoemission spectrometer chamber at Beamline 9.3.2 at theAdvanced Light Source in Berkeley, California.9,10

We subjected the BCY20 pellet first to a drying procedure in UHVat high temperature and exposed then the BCY20 pellet to water vapor(100 mTorr) at ambient temperature while at the same time recordingXPS core level spectra and valence band (VB) spectra in situ. Thetemperature was increased step-by-step to 773 K under the waterpressure of 100 mTorr. Again, XPS core level and VB spectra wererecorded in situ together with impedance spectra. This approachwarrants that we can assess the chemical state of the proton conductorsurface under reaction conditions and also proton conductingoperation conditions. Core level spectra for Ba 4d, Y 3d, Ce 4d, and

O 1s were recorded for ambient temperature, 373 and 573 K invacuum, and 573 K with 100 mTorr and 200 mTorr water vaporpressure, respectively. The temperature was measured with a 2-colorpyrometer (Mikron M90-H1). Ce 4p1/2 resonant XPS spectra wererecorded in vacuum and with the water vapor pressures mentionedabove; Y resonant XPS spectra were recorded with respect to the Y3p1/2 and 3p3/2 from 299 to 310 eV, respectively.

■ RESULTS AND DISCUSSION

Before we turn to the systematic changes in the protonconductor during hydration, we have to investigate theconstitution of the proton conductor (BCY20 pellet) becauseit has been exposed to ambient conditions after synthesis. Wehave thus subjected the pellet to a controlled drying protocoland monitored this process with thermogravimetry and XPS(Supporting Information). After this drying procedure, weexposed the BCY20 pellet to 100 mTorr water vapor at around573 K, while still recording XPS spectra.Proton conducting ceramics develop their ionic conductivity

at elevated temperatures T > 500 K. A typical variation of theimpedance spectra of hydrated BCY20 with temperatureranging from 320 to 820 K is shown in ref 11. Figure 2ashows a representative set of impedance spectra (out of 93) ofhydrated BCY20, recorded during heating in the UHV chamberwhile XPS spectra were taken.The first semicircle near the origin shows up at frequencies

near 1 kHz and above and originates from the bulk protonconductivity. These semicircles could be fitted with a simplemodel circuit (shown as inset in Figure 2b) from a serialresistance RS in series with a parallel circuit of the bulkresistance Rbulk and a constant phase element CPE. Proton bulkconductivities σ = 1/RS are plotted in Figure 2b.Rather, the conductivity increases linear from 400 to 450 K,

increases then steep from 450 to 550 K, and increases furtherwith the similar slope to the temperatures region of 400−450K.Naturally, the sample undergoes thermal expansion during

annealing, which is reflected by the change of the crystallo-graphic unit cell volume (basically the thermal expansion) asdetermined by high temperature X-ray diffraction in air (Figure2c). Note that the BCY20 pellet has been saturated with watervapor by the same protocol as published in ref 5. The thermalexpansion profile in Figure 2d shows a similar behavior like theconductivity variation during annealing. We found recently4

that a decrease of the thermal expansion coefficient ofBaZr0.9Y0.1O3‑δ (BZY10) occurs at about the same temperatureof ∼650 K where the quasi elastic neutron scattering shows anonset of lateral proton mobility, revealing a correlation ofproton conductivity and lattice spacing dynamics.We begin with the systematic XPS study by heating the

sample under exposure to water vapor. Figure 3 shows threeoxygen core level XPS spectra recorded when the sample isdried in UHV and then exposed to 100 mTorr and 200 mTorrwater vapor pressure at 592 K−532 K. The spectrum recordedin dry conditions in UHV (Figure 3a) shows the O−H peak ataround 533 eV and the peak from structural oxygen near 529.5eV. The two peaks at 529 and 532.5 eV originate fromstructural oxygen Ox in the BCY20 perovskite lattice and fromhydroxyl groups (O−H).12−15 During exposure to 100 mTorrwater vapor, the spectrum is shifted by about 0.3 eV towardlower binding energy, which we have corrected for in Figure 3b.This shift is likely due to increased electronic conductivityoriginating from the development of the space charge region

Figure 1. BCY20 pellet with two sputtered Au current collectorterminals, clamped in sample holder with 3-electrode configuration forin situ/operando high-temperature XPS and impedance spectroscopy.

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through the grain that influences strongly the intergrain band-bending.8 Concomitantly the O−H peak height is increasing,whereas the Ox peak intensity is decreasing. This observationsupports the suggestion that oxygen vacancies are filled and theconcentration of protons increases and enhances the

conductivity. During the injection of the water vapor into theUHV chamber, the sample temperature decreased from 592 to545 K, an as of yet unavoidable technical side effect due to theheat capacity of water vapor.In a further step, we increased the water pressure to 200

mTorr. The signature of water in the gas phase comes up ataround at 535.7 eV. The spectrum shows two prominent andwell separated transitions at 528.5 and 532.5 eV, indicative tohydroxyl O−H and two noticeably different structural oxygenions. We believe these different oxygen ions could be inproximity to Ce and to Y, respectively. An alternativeinterpretation could be that these different structural oxygensare the well-known O1 and O2 oxygen ions in orthorhombicBCY.16 The exposure to water vapor at this high temperatureincreases the electric conductivity of the BCY20 (Figure 2b),which manifests in an additional shift of 0.4 eV to an overallshift of the spectrum of 0.7 eV toward lower binding energies(Figure 3 shows the spectra (a) and (b) after alignment on theenergy axis). More noticeable is the redistribution of spectralweight from the transition at 528.5 eV, originating from oxygenbound to Ce, toward the corresponding transition originatingfrom oxygen bound to Y, at 532.5 eV. This reveals that theoxygen vacancies in BCY20 formed by substitution with Y arebecoming filled and the corresponding states becoming morepopulated.A shift of 0.4 eV has been observed by Higuchi et al.17 on

10% Y-substituted barium cerate depending on whether thesample was heated in air or in hydrogen. This is considered asign of the thermal activation of protons, preceding the onset oflateral proton diffusivity which constitutes the proton

Figure 2. a: Representative set of impedance spectra of hydrated BCY20 pellet recorded during heating in UHV at 520 K, 570 K, and 620 K. Theinset shows magnified the high frequency semicircle from the proton bulk conductivity. b: Bulk proton conductivity of hydrated BCY20 as a functionof temperature, derived from impedance spectra set mentioned in part a. c: Excerpt of X-ray diffractograms from hydrated BCY20 recorded from 298to 973 K in air. d: Variation of unit cell volume of hydrated BCY20 versus temperature, as derived from X-ray diffractograms (c).

Figure 3. O 1s core level XPS for BCY20 at (592 to 532 K,temperature changes due to heat capacity of injected water vapor) in(a) UHV and in water vapor with (b) p(H2O) = 100 mTorr and (c)p(H2O) = 200 mTorr. Photon energy = 700 eV. The spectra arenormalized and aligned by the structural Ox oxygen peak (near Ce4+).

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conductivity. Since the protons draw electrons away fromoxygen, when one proton is shared by two oxygen ions, eachoxygen ion should be more negatively charged, and thus the O1s core level is chemically shifted to lower binding energy. Tomaintain charge balance, we expect that the yttrium deliverselectrons when it is getting oxidized. As we will see later, this isindeed observed.The temperature ranges that we are considering here with

XPS show also characteristic changes in the thermogravimetryanalyses. The mass of BCY20, as shown in Figure 4b, is

noticeably increasing in an air filled TGA chamber at around300 K. Comparison with literature18 (Figure 4a) shows that atsuch temperature surface water is being released, as shown bymass spectrometry, notwithstanding that the actual weight ofthe sample is increasing, possibly by hydroxylation. For BCY20,the slope of the derivative of the observed mass change (lowercurve in Figure 4b) is positive between 300 and 400 K, virtuallyzero from 450 to 650 K, and negative for T > 800 K.It is expected that exposure to water fills oxygen vacancies

and thus oxidizes the BCY20. One interesting question is thefollowing: which vacancies are filled first, i.e. which vacanciesare filled more easily. In the course of this in situ XPSexperiment, the BCY20 always showed Ce in a mixed state ofCe3+ and Ce4+. A change in the relative spectral weight is foundin the peak ratio at higher binding energies (labeled W‴ andX‴ in Figure 5a, following the notation in ref 19) and lowerbinding energies (labeled A-C). W‴ and X‴ are signature peaksfor Ce4+; these are absent in Ce3+.15 This change of ratio in thespectra reveals that Ce3+ is partially oxidized to Ce4+ uponadding water vapor. This picture is paralleled by the evolutionof Y3d core level spectra (Figure 5b) under the sameconditions. The intensities of the structures at 157.5 and159.5 eV are increasing during supply of the water vapor,whereas the structures at 156.5 and 158.5 eV are decreasing.As stated in the Introduction, proton conductivity is a subtle

process. Weak spectral signatures are a manifestation of that.

High resolution soft X-ray absorption and emission spectra20

show three O 2p states which have been termed “hydrogenstructures” and considered direct evidence of O−H bonds inthe bulk of Y-substituted SrCeO3. These so-called hydrogenstructures have very small intensity.20

In order to enhance a potential spectroscopic contrastbetween Ce and Y, we have applied the valence band XPSexperiment in the resonant mode with varied photon energy.Figure 6 shows the resonant XPS spectra in the Ce 4p→4d

energy region. The valence band consists of a mixed statebetween 4d1L (A) and 4d0 (B) configurations, in analogy to ref17. L denotes the hole in the valence band, which is mainlycomposed of the O 2p state. The valence band spectra show aremarkable difference between dry and hydrated state, whereasat first glance a difference between on-resonance and off-resonance cannot be made out.The leading peak A at around 5 eV in Figure 5 therefore

represents the Ce3+ state and peak B represents the Ce4+ state.The BCY20 dried in UHV shows in the resonant and off-resonant mode two distinct peaks for Ce3+ and Ce4+ of equal

Figure 4. a: TGA and mass spectrometry data of BaCe0.85Yb0.15O3reproduced from the literature18 and b: TGA mass change andderivative from hydrated BCY20 in synthetic air.

Figure 5. Ce 4d (a) and Y 3d (b) core level XPS for BCY20 at ∼530K, dry and hydrated conditions, Ce and Y become oxidized uponaddition of water. hν = 700 eV.

Figure 6. On- and off-resonance XPS spectra for dry and hydratedBaCe0.8Y0.2O3‑δ measured at ∼537 K with hν = 232 eV (Ce 4p1/2) and223 eV, respectively. The spectra measured in water vapor are alignedto the spectrum obtained under UHV by shifting 0.6 eV to higherbinding energy.

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heights, revealing that a substantial amount of the Ce is in theCe3+ valence at the probed region, i.e. the BCY20 surface in dryconditions. When water vapor is supplied at around 573 K, thespectral weight of the leading Ce3+ state decreases by 50%. Thespectral weight of the Ce4+ state increases accordingly. Aremarkable shoulder from Ce3+ spectral weight remains uponhydration at this level, but the spectral differences are striking.The Ce3+ ions are likely located on the sample surface or in anoxygen defect site. Water fills the oxygen defects, andconsequently the spectral weight of Ce3+ decreases while thespectral weight of Ce4+ increases. Moreover, for the hydratedBaCe0.8Y0.2O3‑δ, the valence band shifts by 0.6 eV to lowerbinding energy, suggesting higher conductivity due to holedoping induced by hydration (Figure 6 shows the spectra afterthe aligning of the binding energy B.E. with peaks A and B).Slight shifts of the spectra on the energy axis have beenobserved for example in the VB and oxygen core level spectraon BaCe0.9Y0.1O3‑δ (0.4 eV) depending on whether the sampleshad been annealed in air or in hydrogen.21

A broad, low intensity peak with the FWHM of ∼1 eVappears at 0 eV binding energy when using hν = 232 eV (Ceresonant) as excitation energy, as shown in Figure 6. This broadpeak is a spectroscopic artifact and originates from the secondorder effect out of the beamline grating, likewise in the Yresonance spectra (Figure 6). At the Ce resonance energy, thissecond order effect can be significant.The observation that we make on the cerium resonant VB

spectra is paralleled by the yttrium resonant VB spectra at 299and 311 eV.Figure 7 shows the Y-resonant VB spectra of the BCY20

recorded at the 3p3/2 and 3p1/2 resonant energies under dry and

hydrated conditions at 585 and 548 K, respectively. We recallfor the reader that the heat capacity of the water vapor had aneffect on the temperature at the sample when the water vaporwas injected in the UHV chamber. Here, too, the BCY20 has adouble peak in dry UHV conditions. While we could assign thedouble peak to Ce3+ and Ce4+ in Figure 5, the origin of thedouble peak for the Y resonant spectra is not immediately clear,because we anticipate no Y4+ state. We recall that the valenceband with respect to the Ce consists of a mixed state between4d1L (A) and 4d0 (B) configurations, in analogy to ref 18.Therefore, the leading peak in the Y-resonant spectra at around

4.5 eV binding energy in Figure 6 should be assigned to areduced Y species such as Y(3‑x)+, with x < 1. Interesting is alsothat this peak has slightly higher spectral weight than theneighboring conjugated Y3+ peak at around 6 eV. Uponsupplying water vapor, we again notice the shift of roughly 0.6eV toward the Fermi energy, suggesting hole doping. Hence,Ce and Y show qualitatively the identical electronic responsetoward hydration, this is, a slight oxidation with hole dopingfrom the O 2p states. Moreover, it appears that at this time weare unable to discriminate between Ce and Y, despite theresonant excitation that we want to take advantage of.When we subtract the VB XPS spectra recorded under wet

conditions from those recorded under dry conditions, weobtain a difference spectrum which should contain the spectralsignature of the oxygen defects. Figure 8 shows these difference

spectra in resonant conditions for Y and Ce. The maxima of thepronounced difference peak at a binding energy of around 5 eVare shifted by about 0.5 eV, revealing that the gap state ofoxygen vacancies next to Y is by 0.5 eV closer to the Fermi levelthan a gap state from an oxygen vacancy next to Ce.The temperature at which we study the BCY20 with VB XPS

is the temperature where we expect the water molecules to besplit and their oxygen ions to fill oxygen vacancies that wereformed by the substitution with yttrium, which we havesketched in Figure 9. This interpretation is indeed confirmed bythe change of our spectra upon hydration. Ce3+ is on a samplesurface or in an oxygen defect site. The oxygen from the watermolecules fills the oxygen defects, and consequently thespectral weight of Ce3+ decreases while the Ce4+ spectralweight increases. Note while we here discuss the behavior ofcerium, the oxygen vacancies should be actually concentrated ataround the yttrium ions. Apparently, this makes spectroscopi-cally little difference when water vapor is supplied. Thissuggestion is corroborated by the observation that the Y-resonant and Ce-resonant VB XPS show identical behavior.The oxygen core level spectra in Figure 3 show that the

temperature range (600 to 500 K) where the VB XPS spectraare recorded contains still significant spectral weight for the O−H groups (592 and 545 K), notwithstanding that at 532 K thespectral weight from the O−H groups is clearly dominating thespectrum. The hydrogen in the O−H groups are confirmed atsay 545 K (Figure 3) in the VB XPS spectra recorded at 537 K(on- and off-Ce resonant) and at 548 and 544 K (on- and off-Y

Figure 7. Y 3p1/2 resonant XPS (photon energy = 311 eV) at 585 and548 K (a) and Y 3p3/2 resonant XPS (photon energy = 299 eV) at 585K and at 544 K (b). The spectra measured in water vapor are alignedto the spectrum obtained under UHV by shifting 0.6 eV to higherbinding energy.

Figure 8. Difference spectra from Y and Ce resonant XPSmeasurements under dry and wet conditions show a chemical shiftof 0.5 eV near the Fermi energy.

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resonant). It appears therefore that we must include in theinterpretation of the VB XPS spectra and in the formulation ofthe electronic structure not only the B-site metal ions and thestructural oxygen, the oxygen vacancies, and the oxygen fromthe water molecules but also the protons in the hydroxylgroups, which may be structural (localized) protons at lowtemperatures, and “free” or mobile (delocalized) or polaronprotons at higher temperatures when the hydrogen bonds withthe structural oxygen (including the oxygen ions that havebecome structural by filling the oxygen vacancies) “melt”. Earlystudies which suggest a critical role of protons on the electronicstructure are based on optical spectroscopy. Sata et al.22

observed that the optical absorption edge shifted and the bandgap increased depending on the Yb-doping concentration inSrZrO3, suggesting that holes are formed at the top of the VBdue to Yb doping. In analogy to observations made on dry andwet CaZrSc-oxide with optical spectroscopy, doped protonsfrom the moist environment will exchange with doped holesand oxygen vacancies that have been formed by B-site cationdoping.23 The absorption in CaZr0.95Sc0.05O3‑δ is lower uponannealing in the moist atmosphere, indicating that the dopedproton exchanges with a hole or an oxygen vacancy.23 An X-rayspectroscopy study conducted on In-doped CaZrO3 suggestedthat proton states exist in the bulk, and maybe also surfacestates, proton induced level at the top of the VB.24 We believethat this experimental in situ study on the chemistry and thechanges of the electronic structure of proton conductors duringhydration and annealing will be helpful for understanding theconditions when the proton changes from a localized state to adelocalized state where proton conductivity actually sets on.

■ CONCLUSIONWe investigated the chemical interactions of water with theBaCe0.8Y0.2O3‑δ proton conductor under realistic workingconditions at elevated temperature and high water pressure insitu combining ambient pressure X-ray photoelectron spectros-

copy and electrochemical impedance spectroscopy. We observethree different temperature regimes for the proton transportalong with the structural change of BCY. Applying waterpressure at intermediate temperature affects the oxygen corelevel and Ce and Y core level as well as the valence bandspectra, revealing the filling of oxygen vacancies in BCY. Thecorresponding increase in electric conductivity is paralleled bychemical shifts in the oxygen core level spectra. Changes in theoxygen core level spectra, particularly emerging new spectralweight, suggest that oxygen ions near Y3+ and Ce3+ can bedistinguished from oxygen ions near Ce4+. The filling of oxygenvacancies with oxygen from water vapor is impressivelyreflected by the substantial decrease of the leading peak inthe valence band spectra. Difference spectra of Ce4p1/2 andY3p1/2 resonant VB spectra show a shift of the leading peak by0.5 eV, which we interpret as that gap states of oxygenvacancies next to Y are 0.5 eV closer to the Fermi energy thanthe corresponding gap state of an oxygen vacancy next to Ce.

■ ASSOCIATED CONTENT

*S Supporting InformationFigures S1−S3. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: artur.braun@alumni.ethz.ch.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The research leading to these results received funding from theEuropean Community’s Sixth Framework Marie Curie Interna-tional Reintegration Programme grant no. 042095 (HiTempE-chem - X-ray and Electrochemical Studies on Solid Oxide FuelCells and Related Materials), Swiss National ScienceFoundation project # 200021-124812 (Effect of lattice volumeand imperfections on the proton-phonon coupling in protonconducting lanthanide transition metal oxides: High pressureand high temperature neutron and impedance studies) and bythe Korean-Swiss Cooperative Program in Science andTechnology project “Spectroscopy on PhotoelectrochemicalElectrode Materials (SOPEM)” (Call 2010), NRF-2013K1A3A1A14055158. We are grateful to Selma Erat(Empa, ETHZ) and William Chueh (Stanford University) forassistance at the beamline, and Songhak Yoon (Empa) for thehigh temperature XRD measurements. F.E.G. was supported bythe Office of Basic Energy Sciences, Division of Materials andEngineering Sciences, U.S. DOE, under contract no. DE-AC04-94AL85000. The ALS is supported by the Director, Office ofScience/BES, of the U.S. DoE, No. DE-AC02-05CH11231.

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Figure 9. Sketch of the BCY20 structure showing Y, Ce, O, andprotons from the water or from the hydroxyl groups.

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