Observation of Oxygen Vacancy Filling under Water Vapor in Ceramic

Nov 20, 2013 - The signature of water in the gas phase comes up at around at 535.7 eV. ... W‴ and X‴ are signature peaks for Ce4+; these are absen...
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Observation of Oxygen Vacancy Filling under Water Vapor in Ceramic Proton Conductors in Situ with Ambient Pressure XPS Qianli 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-8600 Dübendorf, Switzerland ‡ Department of Physics, ETH Zürich, Swiss Federal Institute of Technology CH-8057 Zürich, 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 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 Xray 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. KEYWORDS: proton conductor, perovskite, proton diffusivity, oxygen vacancy, AP-XPS, ambient pressure XPS, valence band, in situ spectroscopy, impedance spectroscopy, resonant photoemission

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INTRODUCTION Protons can be structural elements in molecules and in hydrates, for example, and also ionic and electric charge carriers. In the former case they are localized; in the latter case they are delocalized. The proton is in both cases an elusive element as far as its interaction with the host lattice or molecular environment is concerned. Ceramic proton conductors are prospective solid electrolytes for intermediate temperature solid oxide fuel cells (ITSOFC1−3). For its functionality as proton conductor, the dynamics is very important with respect to lowering proton transport activation energies. Impedance spectroscopy, inelastic and quasi-elastic neutron scattering, and Raman and infrared vibrational spectroscopy are the analytical tools to address the dynamics of the proton conductivity. For improving and © 2013 American Chemical Society

optimizing proton conducting materials, detailed knowledge on the chemical interaction of the proton with the elements in the host lattice is necessary. For example, exposure of metals to hydrogen atmosphere can cause embrittlement of the metal, i.e. structural disintegration. In metal oxides, exposure to hydrogen atmosphere can form hydroxyl groups, which constitute a very rigid and polar species that may affect the integrity of the oxide and influence on charge transfer. In compounds with oxygen vacancies, the insertion of a water molecule produces two identical OHO• species in the host lattice, with both the protons of both of these species equally mobile.4 Received: June 18, 2013 Revised: November 16, 2013 Published: November 20, 2013 4690

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

In the present work we have substituted BaCeO3 to 20% with Y2O3 (BaCe0.8Y0.2O3‑δ) in order to form oxygen vacancies to be filled with oxygen from water vapor molecules. The chemical interactions of protons with the host metal oxide are investigated by ambient pressure XPS under dry and hydrated conditions, meanwhile the proton conductivity is studied using electrochemical impedance spectroscopy.

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RESULTS AND DISCUSSION Before we turn to the systematic changes in the proton conductor during hydration, we have to investigate the constitution of the proton conductor (BCY20 pellet) because it has been exposed to ambient conditions after synthesis. We have thus subjected the pellet to a controlled drying protocol and monitored this process with thermogravimetry and XPS (Supporting Information). After this drying procedure, we exposed the BCY20 pellet to 100 mTorr water vapor at around 573 K, while still recording XPS spectra. Proton conducting ceramics develop their ionic conductivity at elevated temperatures T > 500 K. A typical variation of the impedance spectra of hydrated BCY20 with temperature ranging from 320 to 820 K is shown in ref 11. Figure 2a shows a representative set of impedance spectra (out of 93) of hydrated BCY20, recorded during heating in the UHV chamber while XPS spectra were taken. The first semicircle near the origin shows up at frequencies near 1 kHz and above and originates from the bulk proton conductivity. These semicircles could be fitted with a simple model circuit (shown as inset in Figure 2b) from a serial resistance RS in series with a parallel circuit of the bulk resistance Rbulk and a constant phase element CPE. Proton bulk conductivities 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 further with the similar slope to the temperatures region of 400−450 K. Naturally, the sample undergoes thermal expansion during annealing, which is reflected by the change of the crystallographic unit cell volume (basically the thermal expansion) as determined by high temperature X-ray diffraction in air (Figure 2c). Note that the BCY20 pellet has been saturated with water vapor by the same protocol as published in ref 5. The thermal expansion profile in Figure 2d shows a similar behavior like the conductivity variation during annealing. We found recently4 that a decrease of the thermal expansion coefficient of BaZr0.9Y0.1O3‑δ (BZY10) occurs at about the same temperature of ∼650 K where the quasi elastic neutron scattering shows an onset of lateral proton mobility, revealing a correlation of proton conductivity and lattice spacing dynamics. We begin with the systematic XPS study by heating the sample under exposure to water vapor. Figure 3 shows three oxygen core level XPS spectra recorded when the sample is dried in UHV and then exposed to 100 mTorr and 200 mTorr water vapor pressure at 592 K−532 K. The spectrum recorded in dry conditions in UHV (Figure 3a) shows the O−H peak at around 533 eV and the peak from structural oxygen near 529.5 eV. The two peaks at 529 and 532.5 eV originate from structural oxygen Ox in the BCY20 perovskite lattice and from hydroxyl groups (O−H).12−15 During exposure to 100 mTorr water vapor, the spectrum is shifted by about 0.3 eV toward lower binding energy, which we have corrected for in Figure 3b. This shift is likely due to increased electronic conductivity originating from the development of the space charge region

EXPERIMENTAL SECTION

BaCe0.8Y0.2O3‑δ (BCY20) was prepared by solid state synthesis from precursors mixed in stoichiometric amounts and fired at 1473 K for 12 h in air, then ground, and calcined again at 1473 K for 12 h. The obtained powder was pressed to pellets of 1 mm thickness and 18 mm diameter at 10 kbar and sintered for 24 h at 1673 K in air.5,6 Phase purity of the BCY20 was confirmed with powder X-ray diffraction on the finally obtained pellets (Figure S1 in the Supporting Information). The sintered pellet was subject to a water vapor saturated N2 flow at 670 K for 16 h.5 Temperature dependent X-ray diffraction measurements were made on the sintered sample from room temperature up to 923 K in steps of 50 K. X-ray diffractograms were measured in a Bragg−Brentano geometry using a PANalytical X’Pert PRO θ-2θ scan system 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 holder with spring-loaded probes (Figure 1), specifically designed for the

Figure 1. BCY20 pellet with two sputtered Au current collector terminals, clamped in sample holder with 3-electrode configuration for in situ/operando high-temperature XPS and impedance spectroscopy. combination of XPS and impedance spectroscopy in situ and operando under realistic high temperature electrochemical conditions, was used to provide good electrical contact with the electrodes.8 The ionic conductivity for BCY20 was measured by electrochemical impedance spectroscopy (EIS) using a PCI4/750 Potentiostat (Gamry). The impedance 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 the Advanced Light Source in Berkeley, California.9,10 We subjected the BCY20 pellet first to a drying procedure in UHV at high temperature and exposed then the BCY20 pellet to water vapor (100 mTorr) at ambient temperature while at the same time recording XPS core level spectra and valence band (VB) spectra in situ. The temperature was increased step-by-step to 773 K under the water pressure of 100 mTorr. Again, XPS core level and VB spectra were recorded in situ together with impedance spectra. This approach warrants that we can assess the chemical state of the proton conductor surface under reaction conditions and also proton conducting operation conditions. Core level spectra for Ba 4d, Y 3d, Ce 4d, and 4691

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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. The inset shows magnified the high frequency semicircle from the proton bulk conductivity. b: Bulk proton conductivity of hydrated BCY20 as a function of temperature, derived from impedance spectra set mentioned in part a. c: Excerpt of X-ray diffractograms from hydrated BCY20 recorded from 298 to 973 K in air. d: Variation of unit cell volume of hydrated BCY20 versus temperature, as derived from X-ray diffractograms (c).

conductivity. During the injection of the water vapor into the UHV chamber, the sample temperature decreased from 592 to 545 K, an as of yet unavoidable technical side effect due to the heat 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 at around at 535.7 eV. The spectrum shows two prominent and well separated transitions at 528.5 and 532.5 eV, indicative to hydroxyl O−H and two noticeably different structural oxygen ions. We believe these different oxygen ions could be in proximity to Ce and to Y, respectively. An alternative interpretation could be that these different structural oxygens are the well-known O1 and O2 oxygen ions in orthorhombic BCY.16 The exposure to water vapor at this high temperature increases the electric conductivity of the BCY20 (Figure 2b), which manifests in an additional shift of 0.4 eV to an overall shift of the spectrum of 0.7 eV toward lower binding energies (Figure 3 shows the spectra (a) and (b) after alignment on the energy axis). More noticeable is the redistribution of spectral weight from the transition at 528.5 eV, originating from oxygen bound to Ce, toward the corresponding transition originating from oxygen bound to Y, at 532.5 eV. This reveals that the oxygen vacancies in BCY20 formed by substitution with Y are becoming filled and the corresponding states becoming more populated. A shift of 0.4 eV has been observed by Higuchi et al.17 on 10% Y-substituted barium cerate depending on whether the sample was heated in air or in hydrogen. This is considered a sign of the thermal activation of protons, preceding the onset of lateral proton diffusivity which constitutes the proton

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 are normalized and aligned by the structural Ox oxygen peak (near Ce4+).

through the grain that influences strongly the intergrain bandbending.8 Concomitantly the O−H peak height is increasing, whereas the Ox peak intensity is decreasing. This observation supports the suggestion that oxygen vacancies are filled and the concentration of protons increases and enhances the 4692

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conductivity. Since the protons draw electrons away from oxygen, when one proton is shared by two oxygen ions, each oxygen ion should be more negatively charged, and thus the O 1s core level is chemically shifted to lower binding energy. To maintain charge balance, we expect that the yttrium delivers electrons when it is getting oxidized. As we will see later, this is indeed observed. The temperature ranges that we are considering here with XPS show also characteristic changes in the thermogravimetry analyses. The mass of BCY20, as shown in Figure 4b, is

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

High resolution soft X-ray absorption and emission spectra20 show three O 2p states which have been termed “hydrogen structures” and considered direct evidence of O−H bonds in the bulk of Y-substituted SrCeO3. These so-called hydrogen structures have very small intensity.20 In order to enhance a potential spectroscopic contrast between Ce and Y, we have applied the valence band XPS experiment in the resonant mode with varied photon energy. Figure 6 shows the resonant XPS spectra in the Ce 4p→4d

Figure 4. a: TGA and mass spectrometry data of BaCe0.85Yb0.15O3 reproduced from the literature18 and b: TGA mass change and derivative from hydrated BCY20 in synthetic air.

noticeably increasing in an air filled TGA chamber at around 300 K. Comparison with literature18 (Figure 4a) shows that at such temperature surface water is being released, as shown by mass spectrometry, notwithstanding that the actual weight of the sample is increasing, possibly by hydroxylation. For BCY20, the slope of the derivative of the observed mass change (lower curve in Figure 4b) is positive between 300 and 400 K, virtually zero 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 the following: which vacancies are filled first, i.e. which vacancies are filled more easily. In the course of this in situ XPS experiment, the BCY20 always showed Ce in a mixed state of Ce3+ and Ce4+. A change in the relative spectral weight is found in the peak ratio at higher binding energies (labeled W‴ and X‴ in Figure 5a, following the notation in ref 19) and lower binding energies (labeled A-C). W‴ and X‴ are signature peaks for Ce4+; these are absent in Ce3+.15 This change of ratio in the spectra reveals that Ce3+ is partially oxidized to Ce4+ upon adding water vapor. This picture is paralleled by the evolution of Y3d core level spectra (Figure 5b) under the same conditions. The intensities of the structures at 157.5 and 159.5 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.

Figure 6. On- and off-resonance XPS spectra for dry and hydrated BaCe0.8Y0.2O3‑δ measured at ∼537 K with hν = 232 eV (Ce 4p1/2) and 223 eV, respectively. The spectra measured in water vapor are aligned to the spectrum obtained under UHV by shifting 0.6 eV to higher binding energy.

energy region. The valence band consists of a mixed state between 4d1L (A) and 4d0 (B) configurations, in analogy to ref 17. L denotes the hole in the valence band, which is mainly composed of the O 2p state. The valence band spectra show a remarkable difference between dry and hydrated state, whereas at first glance a difference between on-resonance and offresonance 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 offresonant mode two distinct peaks for Ce3+ and Ce4+ of equal 4693

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4.5 eV binding energy in Figure 6 should be assigned to a reduced Y species such as Y(3‑x)+, with x < 1. Interesting is also that this peak has slightly higher spectral weight than the neighboring conjugated Y3+ peak at around 6 eV. Upon supplying water vapor, we again notice the shift of roughly 0.6 eV toward the Fermi energy, suggesting hole doping. Hence, Ce and Y show qualitatively the identical electronic response toward hydration, this is, a slight oxidation with hole doping from the O 2p states. Moreover, it appears that at this time we are unable to discriminate between Ce and Y, despite the resonant 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, we obtain a difference spectrum which should contain the spectral signature of the oxygen defects. Figure 8 shows these difference

heights, revealing that a substantial amount of the Ce is in the Ce3+ valence at the probed region, i.e. the BCY20 surface in dry conditions. When water vapor is supplied at around 573 K, the spectral weight of the leading Ce3+ state decreases by 50%. The spectral weight of the Ce4+ state increases accordingly. A remarkable shoulder from Ce3+ spectral weight remains upon hydration at this level, but the spectral differences are striking. The Ce3+ ions are likely located on the sample surface or in an oxygen defect site. Water fills the oxygen defects, and consequently the spectral weight of Ce3+ decreases while the spectral weight of Ce4+ increases. Moreover, for the hydrated BaCe0.8Y0.2O3‑δ, the valence band shifts by 0.6 eV to lower binding energy, suggesting higher conductivity due to hole doping induced by hydration (Figure 6 shows the spectra after the aligning of the binding energy B.E. with peaks A and B). Slight shifts of the spectra on the energy axis have been observed for example in the VB and oxygen core level spectra on BaCe0.9Y0.1O3‑δ (0.4 eV) depending on whether the samples had been annealed in air or in hydrogen.21 A broad, low intensity peak with the FWHM of ∼1 eV appears at 0 eV binding energy when using hν = 232 eV (Ce resonant) as excitation energy, as shown in Figure 6. This broad peak is a spectroscopic artifact and originates from the second order effect out of the beamline grating, likewise in the Y resonance spectra (Figure 6). At the Ce resonance energy, this second 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 299 and 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

Figure 8. Difference spectra from Y and Ce resonant XPS measurements under dry and wet conditions show a chemical shift of 0.5 eV near the Fermi energy.

spectra in resonant conditions for Y and Ce. The maxima of the pronounced difference peak at a binding energy of around 5 eV are shifted by about 0.5 eV, revealing that the gap state of oxygen vacancies next to Y is by 0.5 eV closer to the Fermi level than 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 be split and their oxygen ions to fill oxygen vacancies that were formed by the substitution with yttrium, which we have sketched in Figure 9. This interpretation is indeed confirmed by the change of our spectra upon hydration. Ce3+ is on a sample surface or in an oxygen defect site. The oxygen from the water molecules fills the oxygen defects, and consequently the spectral weight of Ce3+ decreases while the Ce4+ spectral weight increases. Note while we here discuss the behavior of cerium, the oxygen vacancies should be actually concentrated at around the yttrium ions. Apparently, this makes spectroscopically little difference when water vapor is supplied. This suggestion is corroborated by the observation that the Yresonant 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 spectra are recorded contains still significant spectral weight for the O− H groups (592 and 545 K), notwithstanding that at 532 K the spectral weight from the O−H groups is clearly dominating the spectrum. The hydrogen in the O−H groups are confirmed at say 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 and 548 K (a) and Y 3p3/2 resonant XPS (photon energy = 299 eV) at 585 K and at 544 K (b). The spectra measured in water vapor are aligned to the spectrum obtained under UHV by shifting 0.6 eV to higher binding energy.

hydrated conditions at 585 and 548 K, respectively. We recall for the reader that the heat capacity of the water vapor had an effect on the temperature at the sample when the water vapor was injected in the UHV chamber. Here, too, the BCY20 has a double peak in dry UHV conditions. While we could assign the double peak to Ce3+ and Ce4+ in Figure 5, the origin of the double peak for the Y resonant spectra is not immediately clear, because we anticipate no Y4+ state. We recall that the valence band with respect to the Ce consists of a mixed state between 4d1L (A) and 4d0 (B) configurations, in analogy to ref 18. Therefore, the leading peak in the Y-resonant spectra at around 4694

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copy and electrochemical impedance spectroscopy. We observe three different temperature regimes for the proton transport along with the structural change of BCY. Applying water pressure at intermediate temperature affects the oxygen core level and Ce and Y core level as well as the valence band spectra, revealing the filling of oxygen vacancies in BCY. The corresponding increase in electric conductivity is paralleled by chemical shifts in the oxygen core level spectra. Changes in the oxygen core level spectra, particularly emerging new spectral weight, suggest that oxygen ions near Y3+ and Ce3+ can be distinguished from oxygen ions near Ce4+. The filling of oxygen vacancies with oxygen from water vapor is impressively reflected by the substantial decrease of the leading peak in the valence band spectra. Difference spectra of Ce4p1/2 and Y3p1/2 resonant VB spectra show a shift of the leading peak by 0.5 eV, which we interpret as that gap states of oxygen vacancies next to Y are 0.5 eV closer to the Fermi energy than the corresponding gap state of an oxygen vacancy next to Ce.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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

AUTHOR INFORMATION

Corresponding Author

resonant). It appears therefore that we must include in the interpretation of the VB XPS spectra and in the formulation of the electronic structure not only the B-site metal ions and the structural oxygen, the oxygen vacancies, and the oxygen from the water molecules but also the protons in the hydroxyl groups, which may be structural (localized) protons at low temperatures, and “free” or mobile (delocalized) or polaron protons at higher temperatures when the hydrogen bonds with the structural oxygen (including the oxygen ions that have become structural by filling the oxygen vacancies) “melt”. Early studies which suggest a critical role of protons on the electronic structure are based on optical spectroscopy. Sata et al.22 observed that the optical absorption edge shifted and the band gap increased depending on the Yb-doping concentration in SrZrO3, suggesting that holes are formed at the top of the VB due to Yb doping. In analogy to observations made on dry and wet CaZrSc-oxide with optical spectroscopy, doped protons from the moist environment will exchange with doped holes and oxygen vacancies that have been formed by B-site cation doping.23 The absorption in CaZr0.95Sc0.05O3‑δ is lower upon annealing in the moist atmosphere, indicating that the doped proton exchanges with a hole or an oxygen vacancy.23 An X-ray spectroscopy study conducted on In-doped CaZrO3 suggested that proton states exist in the bulk, and maybe also surface states, proton induced level at the top of the VB.24 We believe that this experimental in situ study on the chemistry and the changes of the electronic structure of proton conductors during hydration and annealing will be helpful for understanding the conditions when the proton changes from a localized state to a delocalized state where proton conductivity actually sets on.

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The research leading to these results received funding from the European Community’s Sixth Framework Marie Curie International Reintegration Programme grant no. 042095 (HiTempEchem - X-ray and Electrochemical Studies on Solid Oxide Fuel Cells and Related Materials), Swiss National Science Foundation project # 200021-124812 (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) and by the Korean-Swiss Cooperative Program in Science and Technology project “Spectroscopy on Photoelectrochemical Electrode Materials (SOPEM)” (Call 2010), NRF2013K1A3A1A14055158. We are grateful to Selma Erat (Empa, ETHZ) and William Chueh (Stanford University) for assistance at the beamline, and Songhak Yoon (Empa) for the high temperature XRD measurements. F.E.G. was supported by the Office of Basic Energy Sciences, Division of Materials and Engineering Sciences, U.S. DOE, under contract no. DE-AC0494AL85000. The ALS is supported by the Director, Office of Science/BES, of the U.S. DoE, No. DE-AC02-05CH11231.

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CONCLUSION We investigated the chemical interactions of water with the BaCe0.8Y0.2O3‑δ proton conductor under realistic working conditions at elevated temperature and high water pressure in situ combining ambient pressure X-ray photoelectron spectros4695

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