Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy

Transcription

1 Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy Chunjuan Zhang, 1 Michael E. Grass, 3 Anthony H. McDaniel, 5 Steven C. DeCaluwe, 2 Farid El Gabaly, 5 Zhi Liu, 3 Kevin F. McCarty, 5 Roger L. Farrow, 5 Mark A. Linne, 5 Zahid Hussain, 3 Gregory S. Jackson, 2, * Hendrik Bluhm 4, * and Bryan W. Eichhorn 1, * 1 Department of Chemistry and Biochemistry, 2 Department of Mechanical Engineering, University of Maryland, College Park, MD Advanced Light Source, 4 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA Sandia National Laboratories, Livermore, CA Photoelectron spectroscopic measurements have the potential to provide detailed mechanistic insight by resolving chemical states, electrochemically-active regions and local potentials or potential losses in operating solid oxide electrochemical cells (SOCs), such as fuel cells. However, high vacuum requirements have limited XPS analysis of electrochemical cells to ex situ investigations. Using a combination of ambient pressure XPS and specially fabricated CeO 2-x /YSZ/Pt single-chamber cells, we perform in situ spectroscopy to probe oxidation states of all exposed surfaces in operational SOCs at 750 C in 1 mbar reactant gases H 2 and H 2 O. Kinetic energy shifts of core level photoelectron spectra from the electrolyte and electrodes provide a direct measure of the local surface potentials and a basis for calculating local overpotentials across exposed interfaces. The mixed ionic / electronic conducting CeO 2-x electrodes undergo Ce 3+ /Ce 4+ oxidation-reduction changes with applied bias, which signifies the region of electrochemical activity. The simultaneous measurements of local surface Ce oxidation states and electric potentials reveal the active ceria regions during H 2 electro-oxidation and H 2 O electrolysis. The measurements indicate that the electrochemically active regions extend ~150 µm from the current collectors and are not limited by the three-phase-boundary interfaces associated with other SOC materials. The persistence of the Ce 3+ /Ce 4+ shifts in the ~150 µm active region during electrochemical operation suggests that -1-

2 the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes. Solid oxide electrochemical cells (SOCs) are among the most promising technologies for clean, efficient fuel generation and electric power production from both traditional and renewable energy sources. 1,2 These devices employ solid oxide electrolyte membranes (such as yttria-stabilized zirconia, YSZ), that facilitate oxide ion transport between anode and cathode while electronically isolating the electrodes Oxide transport through the solid-state electrolyte requires high temperature (>650 C), which limits the materials that can be used as SOC components. 7-9 Electrodes must combine oxide-ion conduction with catalytically-active electronically-conducting materials, and thus metal-oxide composites (such as Ni/YSZ) are often employed as electrodes. However, mixed ionic / electronic conducting (MIEC) materials such as ceria have also attracted attention as SOC electrodes. Theory predicts that the mixed conduction expands the electrochemically active region away from the electronic current collector interface by facilitating both O 2- hopping) ion conduction and electron conduction (polaron With its mixed oxidation states (e.g. CeO 2-x, 0<x<0.5), ceria can provide MIEC functionality as well as surface catalytic activity However, the current understanding of elementary processes and length scales of the electrochemically active regions of MIEC electrodes in SOCs is limited because of the paucity of suitable in situ techniques to probe these issues. Measuring local overpotentials associated with individual cell components (anode, cathode, electrolyte and their interfaces) is challenging due to the high operating temperatures and the difficulty of attaching probes across each component. Traditional electrochemical evaluation of electrode overpotentials includes voltammetric techniques and electrochemical impedance spectroscopy (EIS) employing reference electrodes These techniques provide valuable information regarding global electrode overpotentials and resistances. However, local voltage distributions throughout the cell, including local voltage losses across interfaces, and the associated changes in chemical states are not well understood Recently, in-situ diagnostic techniques such as in-situ Raman spectroscopy have been employed to provide valuable chemical information regarding local oxidation-reduction mechanistic processes and material chemical states, but

3 correlations between these chemical processes and the local potentials associated with such changes are difficult to study by these techniques. To address the aforementioned questions, we fabricated SOCs suitable for simultaneous electrochemical evaluation and X-ray photoelectron spectroscopy (XPS). In situ ambient pressure XPS (APXPS) 31,32 of these cells enables us to spatially resolve the chemical states of SOC component surfaces during operation. The combination of the high brightness 3 rd generation synchrotron source at the Advanced Light Source (ALS) and differentially pumped electron lens system circumvents the high vacuum requirement of standard XPS experiments and allows for the collection of XPS data in the presence of reactive gasses. The SOC cells consist of Au/CeO 2-x working electrodes (WE), YSZ electrolytes and Pt counter electrodes (CE) on polycrystalline YSZ electrolyte disks (Figs. 1 and S1). All components are exposed and located on the same side of the electrolyte disk to enable APXPS access from anode to cathode. Photoelectron spectra of the SOC components show a shift in electron kinetic energies when measured as a function of applied bias. The shifts originate from changes in the local Fermi level, which changes with the local electric potential. The magnitude of APXPS KE shifts (ΔKE in electron volts) are related to the local potential (V L ) relative to ground as ΔKE = e V L This correlation between KE shifts and local surface potential allows for a direct, contactless measure of local surface potentials in SOCs. Ladas et al. 36 proposed such experiments while performing in situ diagnostics of non-faradaic electrochemical modification of catalytic activity (NEMCA) under ultrahigh vacuum conditions. We describe here the direct measurement of the local surface potentials and chemical states of all SOC components during electrochemical operation by utilizing in situ synchrotronbased APXPS. Figure 1a shows the schematic ceria/ysz/pt cell geometry and simplified experimental setup. We fabricated three separate SOCs in which films of 50 nm, 100 nm or 250 nm of dense ceria WEs were sputtered on top of gold current collectors. The ceria extends onto the YSZ electrolytes only on the side closest to the counter electrode to create well-defined current pathways. An insulating alumina layer deposited underneath each Au pad blocks direct ionic transport between the YSZ electrolyte and the Au current -3-

4 Figure 1. Electrochemical activity spreads over 100 µm of ceria anode. (a) Solid oxide cells have a 200 nm Pt counter electrode, 300 nm Au current collector on top of a 30 nm alumina film (black), and 50, 100, or 250 nm ceria working electrode patterned onto a YSZ single crystal substrate. This geometry exposes all cell components to the X-ray beam. The drawing is not to scale. (b) During operation, we heat the cell close to the detector aperture in 1 mbar H 2 and H 2 O. (c) A 250-nm-thick ceria anode converts H 2 O to H 2 and O 2- in a 100-µm region at +1.2V cell potential. In situ APXPS reveals local surface potentials (red squares) and the relative shift of Ce oxidation state out of equilibrium (green circles) in this region. collector. Sputter-deposited dense platinum, located 0.5 mm away from the ceria edge, acts as a counter electrode. The cells were mounted for APXPS and heated to 750 C in 1:1 H 2 and H 2 O at a total pressure of 1 mbar (Fig. 1b). In each experiment, we grounded the Au current collector of the WE and controlled the bias on the Pt CE with a potentiostat. We conducted two-probe linear sweep voltammetry (LSV) experiments between -1.2V to +1.2V where V cell = V L(Pt) V L(Au). In these experiments, V L(Au) is always zero (i.e. grounded to the XPS chamber) and V L(Pt) is the applied potential. At open circuit voltage (V cell = 0.0 V) with no net current in the cell, the ceria electrode surface is in equilibrium with H 2 and H 2 O in the gas phase (eq. 1). 2Ce 3+ + V O + H 2 O(g) 2Ce 4+ + O 2 + H 2 (g) V O = oxide vacancy (1) Under applied bias, charged species (oxide ions and polarons) move in the WE, driving the surface away from equilibrium and promoting the surface chemistry in eq. 1. For example, under positive bias, oxide ions move from the ceria surface to the YSZ and are conducted to the Pt CE where H 2 oxidation occurs (see Fig. 1c) At negative bias, the -4-

5 flux of oxide ions and polarons are reversed, and H 2 oxidation (eq. 1, reverse reaction) takes place on the ceria surface while electrolysis occurs on the Pt electrode. Figure 2 illustrates how the cell voltage is distributed for a range of positive and negative biases by traversing a single pathway from anode to cathode across the surface of the cell. We recorded Au 4f, Ce 3d and 4d, Pt 4f and Zr 3d spectra at various positions on the cell surface (Fig. 2 inset) with an X- ray beam size of ~75 µm in the lateral dimension (ALS, beamline ). Across the Au current collector, Ce 4d and Au 4f (from exposed edges) remain un-shifted, confirming that the Au and the ceria directly above it are grounded (Fig. S3). In contrast, the Pt 4f ΔKE captures Preprint of article published in Nature Materials Potential (V) Figure 2. In situ XPS directly measures local potentials on the surface of the 50-nm-thick ceria cell. Local APXPS evaluates the individual potential distributions in a single pathway across the cell surface (inset, not to scale). Along this pathway, we derived from the XPS KE shifts the potential losses associated with individual processes including charge transfer at the Pt-YSZ interface (spot 1) and lateral charge transport within the ceria film (between spots 2 and 6) and the YSZ electrolyte (between spots 1 and 2), measured as a function of cell current. Full-cell V-I curves (black line) were simultaneously measured by linear sweep voltammetry (LSV). APXPS measurement positions for determining local potentials are shown as red circles. The contributions from ΔV L(YSZ-Ceria) (spot 2) (< 0.1V) and ΔV L(Ceria-Au) (spot 6) (< 0.05V) are negligible. the full potential difference across the entire SOC, which validates the relationship between ΔKE and the local potentials. The ΔKE values of the CeO 2-x (Fig. S2) and YSZ between the Au and Pt electrodes represent the local potentials (V L ) relative to the grounded Au. For the cell design (Fig. 1), electrochemical processes proceed in parallel in the WE, such that WE overpotentials for a given process vary with the path taken between the WE and CE. Regardless, for a given path line, spatially resolved ΔKE values allow us to extract the local potential losses (ΔV L ) for individual SOC processes necessary to drive eq. 1. These processes include interfacial charge transfer (for example at the Pt-YSZ interface) and the ohmic losses associated with the transport of charged species along the cell surface. For the specific pathway shown in Fig. 2, there is no local surface activity (local current) or charge transfer at the exposed YSZ-ceria interface (spot Pt ECHEM!iR bulk!lsv XPS(Probe(Spots(on(SOC CeO x Au YSZ XPS! V L(YSZ)(((((((((( =V L(Zr) (1)!!5!V L(Zr) (2) V L(Ceria)((((((((( =V L(Ce) (2)!5!V L(Ce) (6) V L(Pt0YSZ)(((((( =V L(Pt) (1)!5!V L(Zr) (1) V L(YSZ0Ceria)( =V L(Zr) (2)!5!V L(Ce) (2) Sum!( V L(YSZ)( +! V L(Ceria)( +! V L(Pt0YSZ)( +! V L(YSZ0Ceria) ) Current (ma)

6 2), therefore negligible potential loss (ΔV L(YSZ-Ceria), spot 2) was observed in the APXPS measurements. The sums of the individual losses (Fig. 2, cyan circles) are in excellent agreement with the electrochemically measured potentials from the LSV measurements (Fig. 2, black line). The global electrochemical overpotential (η) needed to drive a reaction at a certain rate is the difference between the potential (ΔV) and the thermodynamic equilibrium potential (ΔV eq ), i.e. η = ΔV ΔV eq. 37,38-6- For the singlechamber cells used in the experiments, both electrodes are exposed to both fuel and oxidizer and the open circuit voltage is 0.0 V at all locations (i.e. there is no thermodynamic driving force, ΔV eq = 0.0 V). Therefore, the local voltage drops (ΔV L ) across interfaces provide a measure of local overpotentials (η L = ΔV L ΔV L,eq = ΔV L ) during the cell operation. For example, the difference in V L for Pt and Zr at the Pt-YSZ interface (ΔV L(Pt-YSZ) at spot 1, Fig. 2) represents a local Pt-YSZ overpotential, η L,Pt-YSZ. This correlation provides a non-contact method to measure local electrochemical overpotentials for single-chamber cells when a charge-transfer interface is optically accessible. The cell design mandates that oxide ions move in a net vertical direction between the ceria surface and the YSZ electrolyte whereas electrons (polarons) primarily move laterally between the ceria surface and the Au current collector (Fig. 1c). Even though YSZ and ceria have similar ionic resistivities, 39,40 the oxide ions are primarily transported laterally through the YSZ because it has a thickness several orders of magnitude larger than the ceria WE. Oxide ions (and oxide vacancies) move vertically through the ceria film only when there is a large potential gradient between the YSZ and ceria surface, which occurs near the Au current collector (see Fig. S5, supporting information). The lack of an oxide source or drain at the Au current collector prevents lateral oxide transport to the Au electrode. Likewise, the lack of an electron drain at the ceria / YSZ interface inhibits vertical transport of polarons. The lateral evolution of the measured electric potential on the ceria surface, V L(ceria), are shown in Figs. 1 and S5. These values include the interfacial potential loss between ceria and gold (ΔV L(Ceria-Au) ). Because the measured ΔV L(Ceria-Au) values are very small (< 0.05V; spot 6 in Fig. S3), they are neglected in subsequent discussions. Note that

7 Figure 3. The cerium oxidation state changes in a µm region near the Au current collector. (a) The Ce 3d spectrum changes based on % Ce 3+ in the probed area are shown for the 50-nm-thick ceria cell under different applied biases. The spectra are linear combinations of Ce 3+ (green line) and Ce 4+ (shaded cyan) contributions. For instance, at -1.2V cell bias, the surface of a 50 nm ceria film contains 38% Ce 3+ at spot 3 (inset, Fig. 2). (b) For all the three cells with different thicknesses of ceria films (blue diamonds 50 nm, red triangles 100 nm, and black squares 250 nm), the Ce oxidation state at +1.2V relative to OCV only changes in a µm region near the edge of the Au current collector (data recorded at spots 2-7, Fig. 2 inset). Thinner ceria anodes shift further out of equilibrium. The inset shows V-I curves for the three cells with the same color and symbol identities. V L(ceria) varies from zero on top of the Au pad (Fig. S3, spot 7) to +0.8 V at the outer edge of the ceria film (Fig. 2, spot 2). Resolution of this local potential provides a basis for defining electrochemically active regions. The Ce 3+ /Ce 4+ ratio across the CeO 2-x surface dramatically changes from the edge of the Au current collector extending out ca. 150 µm at both positive and negative biases. CeO 2-x surface oxidation states are measured using Ce 3d core-level APXPS at various applied cell potentials, as shown in Figs. 3 and S4. Despite the continuous increase in V L(ceria) from the Au current collector to the edge of the ceria film (Fig. 1c), the Ce 3+ /Ce 4+ ratios are significantly shifted from their equilibrium positions only in this ~150 µm region. Outside this region, the films maintain their equilibrium distributions in all three SOCs tested. In a comparison of the three cells with different ceria electrode thicknesses, -7-

8 the 250 nm CeO 2-x films produce the largest total currents, whereas the 50 nm CeO 2-x films generate the smallest (Fig. 3b, inset). In contrast, the 50 nm CeO 2-x films give the largest Ce 3+ /Ce 4+ variations whereas the 250 nm films have the smallest (Fig. 3b). Understanding how the oxide ion current couples with the pronounced oxidation state changes at the surface provides insight into the behavior of MIEC electrodes. Oxide-ion fluxes across the MIEC ceria films occur in regions of large potential gradients between the ceria surface and the YSZ electrolyte. The total cell current must flow through the ceria film in the form of oxide flux between the ceria surface and the YSZ electrolyte. The local vertical oxide currents across the films (i local ) increase with the local potential differences between the ceria surface and the YSZ electrolyte; ΔV L(vertical) = V L(YSZ) V L(ceria). The local potentials of ceria are measured directly from the APXPS measurements whereas the local YSZ potentials are calculated from electrochemical measurements (see supporting information). Because i local increases or decreases monotonically with ΔV L(vertical), ΔV L(vertical) is a direct indicator of local current generation at the surface of the active electrochemical region. The plot of calculated ΔV L(vertical) indicates electrochemical activity (large ΔV L(vertical) ) in a region that is remarkably similar to the Ce 3+ /Ce 4+ redox active ~150 µm region measured from APXPS (Fig. S5). These data demonstrate the ability of APXPS to spatially resolve electrochemically-active regions under operating conditions by monitoring both oxidation shifts from equilibrium and spatially resolved potential gradients. To improve the spatial resolution of measurements in the electrochemically-active regions, we conducted additional APXPS experiments on ALS beamline equipped with an area detector 41. Due to the limitation of the energy range at beamline 9.3.2, we monitored the Ce 4d core level across the active region instead of the Ce 3d levels accessible on beamline However, the use of the 2-D detector allows us to image the region of interest and gives a greater data density at a spatial resolution of 20 µm. Figure 4 shows the rigid shifts of the Ce 4d spectra for a similar 250 nm cell (in this cell, the ceria extended 300 µm from the Au current collector rather than 500 µm), which shows a similar ~150 µm region of electrochemical activity (large ΔV L(vertical) ). The plots of ceria surface potential (Fig. 4e) clearly illustrate the dramatic potential drop across the -8-

9 Au (a) Ce 4d +1.2V (c) Ce 4d OCV (b) Ce 4d -1.2V Intensity (A.U.) BE (ev) BE (ev) BE (ev) (d) Ce4+ Potential (V) YSZ Ce2Ox Distance (mm) Preprint of article published in Nature Materials 0.8 (e) V Hemispherical Analyzer 0.0 Entrance Slit V BE (ev) (f) Distance (mm) x Electrostatic Lens BE Detector x y Sample Figure 4: Ce 4d spectra obtained with a 2-D area detector (ALS Beamline 9.3.2) reveal the 100 µm active region of the ceria electrode near the edge of the Au current collector. Plots (a) (c) show distance-resolved XPS spectra of the Ce 4d region at -1.2, 0, and +1.2 V applied potential recorded with 490 ev photon energy. The ceria is 50 nm thick on the working electrode. Plot (d) shows integrated spectra taken from 50 micron slices of (a) starting at x = -0.1 mm; spectra from the active region are displayed in red. The peaks marked labeled Ce4+ are associated only with Ce4+ species, indicating that the active region is reduced under positive applied potential. The potential across the ceria surface calculated from plots (a) (c) is shown in plot (e). (f) A schematic representation of the spatially resolved photoelectron detection. Spectra (a) (c) were normalized so that each row of pixels has the same integrated intensity to remove artifacts from the analyzer and/or surface roughness. 250 nm ceria film in the active region of the electrode. The oxidation/reduction of ceria in the active region is also apparent (Fig. 4d), but because of the difficulty of comparing Ce 4d and Ce 3d spectra, we cannot quantitatively compare the oxidation state data from the different beamlines. Changes in the Ce3+/Ce4+ ratios in the active regions are a consequence of oxide and polaron surface concentration shifts caused by interaction between the surface chemical kinetics and the lateral and vertical potential losses. The applied cell bias drives the system away from equilibrium and generates large vertical and lateral potential losses -9-

10 in the active region (see supporting information). The resulting fluxes of charged species establish concentration shifts at the surface, relative to the equilibrium state. Chemical reactions at the ceria surface reduce the oxide ion and polaron shifts at the surface and drive the system back towards equilibrium. For example, at positive applied cell bias, Ce 3+ and oxide vacancies (V O ) are driven to the ceria surface (Fig. 1), which promotes water electrolysis. The electrolysis reaction (eq. 1, forward reaction) generates Ce 4+ and oxide ions to reduce the surface concentration shifts and drive the system back towards equilibrium. The persistence of the Ce 3+ /Ce 4+ shifts in the ~150 µm electrochemically active regions suggests that the electrochemical rates are co-limited by the surface reaction kinetics and the lateral electron transport. If electron (polaron) transport were significantly slower than the surface chemical kinetics, the electrochemically-active region would be restricted to regions very close to the current collectors to minimize the distance of electron transport. On the other hand, if the surface reaction kinetics were the principal rate-limiting step (i.e., faster lateral electron transport), the entire ceria surface (500 µm wide) would show evidence of non-equilibrium oxidation states and surface activity. Furthermore, the stable Ce 3+ /Ce 4+ shifts with bias in the electrochemically active region indicate that the surface kinetics on the ceria film are slow in comparison with oxide ion or vacancy transport between the ceria and the YSZ. Fast surface kinetics relative to oxide and vacancy transport would eliminate these shifts and re-establish equilibrium conditions. Finally, the thinnest ceria film (50 nm) has the smallest total cell current and the largest Ce 3+ /Ce 4+ shifts. The smaller currents in the thin films correspond to slower surface reactions, which maintain the Ce 3+ /Ce 4+ equilibrium values. ratios farther from their The Ce 3+ /Ce 4+ shifts on the ceria surface have similar magnitudes under positive (electrolysis) and negative (H 2 electro-oxidation) bias, while the corresponding surface reaction rates (currents) are significantly different (Figs. 3b and S4). This observation indicates that larger ceria surface concentration shifts are needed to sustain H 2 oxidation currents, relative to H 2 O electrolysis currents. Therefore, ceria is a better catalyst for water electrolysis than for H 2 electro-oxidation under the same operating conditions. The observed trends in total SOC currents at the same applied bias could be due to several -10-

11 factors. These include changes in surface activity, subsurface processes, 42 and changes in the charge-transfer resistance at the YSZ-ceria interface. Detailed structural and electronic data below the ceria surface are needed to answer these important questions and new experiments are being designed to address these issues. In situ ambient pressure XPS measurements of specially engineered singlechamber solid oxide cells directly identify electrochemically-active surface regions and allow for surface mapping of local electric potentials and potential losses during electrochemical operation. We have shown that the active electrochemical region extends ~150 µm away from the current collector and that significant shifts from the equilibrium surface Ce 3+ /Ce 4+ concentrations are needed to drive the electro-oxidation of H 2 and the electrolysis of H 2 O. The correlations between local potential losses and chemical state changes provide mechanistic insight into working electrochemical devices. The application of this technique on high-temperature oxide materials for electrochemical devices, such as solid oxide fuel cells or electrochemical sensors, can provide fundamental mechanistic information that is inaccessible by other means. Such studies will facilitate rational advances in high temperature oxide devices and are currently being pursued. Author contributions: All coauthors contributed to the conception and design of experiments. The ALS team collected and analyzed the XPS data. The Sandia team collected the electrochemical data. The Maryland team fabricated and characterized cells and collated data analysis. Z.H. and M.L. initiated and partially funded the collaboration. Acknowledgments: This work was funded by the ONR through Contract number N The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH Work at Sandia National Laboratories supported by the Laboratory Directed Research and Development program under contract DE-AC04-94AL85000 of the United States Department of Energy. References: -11-

12 1 Atkinson, A. et al., Advanced anodes for high-temperature fuel cells. Nat. Mater. 3, (2004). 2 Minh, N.Q., Ceramic fuel-cells. J. Am. Ceram. Soc. 76, (1993). 3 Huang, Y.H., Dass, R.I., Xing, Z.L., & Goodenough, J.B., Double perovskites as anode materials for solid-oxide fuel cells. Science 312, (2006). 4 Zhan, Z.L. & Barnett, S.A., An octane-fueled olid oxide fuel cell. Science 308, (2005). 5 Adler, S.B., Factors governing oxygen reduction in solid oxide fuel cell cathodes. Chem. Rev. 104, (2004). 6 Haile, S.M., Fuel cell materials and components. Acta Mater. 51, (2003). 7 Steele, B.C.H. & Heinzel, A., Materials for fuel-cell technologies. Nature 414, (2001). 8 Brandon, N.P., Skinner, S., & Steele, B.C.H., Recent advances in materials for fuel cells. Annu. Rev. Mater. Res. 33, (2003). 9 Shao, Z.P. & Haile, S.M., A high-performance cathode for the next generation of solid-oxide fuel cells. Nature 431, (2004). 10 Esch, F. et al., Electron localization determines defect formation on ceria substrates. Science 309, (2005). 11 Fleig, J., On the width of the electrochemically active region in mixed conducting solid oxide fuel cell cathodes. J. Power Sources 105, (2002). 12 Mogensen, M., Sammes, N.M., & Tompsett, G.A., Physical, chemical and electrochemical properties of pure and doped ceria. Solid State Ionics 129, (2000). 13 Campbell, C.T. & Peden, C.H.F., Chemistry - oxygen vacancies and catalysis on ceria surfaces. Science 309, (2005). 14 Fu, Q., Saltsburg, H., & Flytzani-Stephanopoulos, M., Active nonmetallic Au and Pt species on ceria-based water-gas shift catalysts. Science 301, (2003). 15 Yang, L. et al., Enhanced Sulfur and Coking Tolerance of a Mixed Ion Conductor for SOFCs: BaZr 0.1 Ce 0.7 Y 0.2 x Yb x O 3 d. Science 326, 126 (2009). 16 Murray, E.P., Tsai, T., & Barnett, S.A., A direct-methane fuel cell with a ceriabased anode. Nature 400, (1999). 17 Ganduglia-Pirovano, M.V., Hofmann, A., & Sauer, J., Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges. Surf. Sci. Rep. 62, (2007). 18 Trovarelli, A., Catalysis by Ceria and Related Materials, edited by Trovarelli, A. (Imperial College Press, London, 2002), Ch Hsieh, G., Mason, T.O., Garboczi, E.J., & Pederson, L.R., Experimental limitations in impedance spectroscopy.3. Effect of reference electrode geometry/position. Solid State Ionics 96, (1997). 20 Krishnan, V.V., McIntosh, S., Gorte, R.J., & Vohs, J.M., Measurement of electrode overpotentials for direct hydrocarbon conversion fuel cells. Solid State Ionics 166, (2004). 21 Schichlein, H., Muller, A.C., Voigts, M., Krugel, A., & Ivers-Tiffee, E., Deconvolution of electrochemical impedance spectra for the identification of -12-

13 electrode reaction mechanisms in solid oxide fuel cells. J. Appl. Electrochem. 32, (2002). 22 Metzger, P., Friedrich, K.A., Muller-Steinhagen, H., & Schiller, G., SOFC characteristics along the flow path. Solid State Ionics 177, (2006). 23 Chan, S.H., Chen, X.J., & Khor, K.A., Reliability and accuracy of measured overpotential in a three-electrode fuel cell system. J. Appl. Electrochem. 31, (2001). 24 Winkler, J., Hendriksen, P.V., Bonanos, N., & Mogensen, M., Geometric requirements of solid electrolyte cells with a reference electrode. J. Electrochem. Soc. 145, (1998). 25 Pomfret, M.B. et al., Hydrocarbon fuels in solid oxide fuel cells: In situ Raman studies of graphite formation and oxidation. J. Phys. Chem. C 112, (2008). 26 Liu, D.J. & Almer, J., Phase and strain distributions associated with reactive contaminants inside of a solid oxide fuel cell. Appl. Phys. Lett. 94, 3 (2009). 27 Sumi, H. et al., Changes of internal stress in solid-oxide fuel cell during red-ox cycle evaluated by in situ measurement with synchrotron radiation. J. Fuel Cell Sci. Technol. 3, (2006). 28 Cheng, Z. & Liu, M.L., Characterization of sulfur poisoning of Ni-YSZ anodes for solid oxide fuel cells using in situ Raman micro spectroscopy. Solid State Ionics 178, (2007). 29 Wilson, J.R. et al., Three-dimensional reconstruction of a solid-oxide fuel-cell anode. Nat. Mater. 5, (2006). 30 Fister, T.T. et al., In situ characterization of strontium surface segregation in epitaxial La0.7Sr0.3MnO3 thin films as a function of oxygen partial pressure. Appl. Phys. Lett. 93, 3 (2008). 31 Ogletree, D.F., Bluhm, H., Hebenstreit, E.D., & Salmeron, M., Photoelectron spectroscopy under ambient pressure and temperature conditions. Nucl. Instrum. Methods Phys. Res. Sect. A-Accel. Spectrom. Dect. Assoc. Equip. 601, (2009). 32 Salmeron, M. & Schlogl, R., Ambient pressure photoelectron spectroscopy: A new tool for surface science and nanotechnology. Surf. Sci. Rep. 63, (2008). 33 Siegbahn, H. & Lundholm, M., A method of depressing gaseous-phase electron lines in liquid-phase esca spectra. J. Electron Spectrosc. Relat. Phenom. 28, (1982). 34 Doron-Mor, H. et al., Controlled surface charging as a depth-profiling probe for mesoscopic layers. Nature 406, (2000). 35 Ertas, G. & Suzer, S., Surface Chemistry in Biomedical and Environmental Science, edited by Blitz, Jonathan P. & Vladimir M. Gun'ko (Springer, Dordrecht, The Netherlands, 2006), Ch Ladas, S., Kennou, S., Bebelis, S., & Vayenas, C.G., Origin of non-faradaic electrochemical modification of catalytic activity. J. Phys. Chem. 97, (1993). 37 Bard, A.J. & Faulkner, L.R., Electrochemical methods, fundamentals and applications (John Wiley & Sons, Inc., New York, 2001). -13-

14 38 Vetter, K.J., Electrochemical kinetics: theoretical and experimental aspects. (Academic Press, New York, 1967). 39 Mogensen, M., The CRC handbook of solid state electrochemistry, edited by Trovarelli, A. (CRC Press, Boca Raton, 1997), Ch Kwon, O.H. & Choi, G.M., Electrical conductivity of thick film YSZ. Solid State Ionics 177, (2006). 41 Grass, M.E. et al., New ambient pressure photoemission endstation at Advanced Light Source beamline Rev. Sci. Instrum. 81, (2010). 42 Fleig, J., On the current-voltage characteristics of charge transfer reactions at mixed conducting electrodes on solid electrolytes. Phys. Chem. Chem. Phys. 7, (2005). -14-