In Situ Characterization of Ceria Oxidation States in High-Temperature

Nov 2, 2010 - Phone: 1-301-405-2368. Fax: 1-301-405-2025., †. University .... was not measured directly. In the subsequent two-electrode experiments...
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J. Phys. Chem. C 2010, 114, 19853–19861

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In Situ Characterization of Ceria Oxidation States in High-Temperature Electrochemical Cells with Ambient Pressure XPS Steven C. DeCaluwe,†,| Michael E. Grass,‡ Chunjuan Zhang,† Farid El Gabaly,§ Hendrik Bluhm,‡ Zhi Liu,‡ Gregory S. Jackson,*,† Anthony H. McDaniel,§ Kevin F. McCarty,§ Roger L. Farrow,§ Mark A. Linne,§,⊥ Zahid Hussain,§ and Bryan W. Eichhorn† UniVersity of Maryland, College Park, Maryland 20742, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Sandia National Laboratories, LiVermore, California 94551 ReceiVed: August 14, 2010; ReVised Manuscript ReceiVed: October 12, 2010

Ambient pressure X-ray photoelectron spectroscopy (XPS) is used to measure near-surface oxidation states and local electric potentials of thin-film ceria electrodes operating in solid oxide electrochemical cells for H2O electrolysis and H2 oxidation. Ceria electrodes which are 300 nm thick are deposited on YSZ electrolyte supports with porous Pt counter electrodes for single-chamber tests in H2/H2O mixtures. Between 635 and 740 °C, equilibrium (zero-bias) near-surface oxidation states between 70 and 85% Ce3+ confirm increased surface reducibility relative to bulk ceria. Positive cell biases drive H2O electrolysis on ceria and further increase the percentage of Ce3+ on the surface over 100 µm from an Au current collector, signifying broad regions of electrochemical activity due to mixed ionic-electronic conductivity of ceria. Negative biases to drive H2 oxidation decrease the percentage of Ce3+ from equilibrium values but with higher electrode impedances relative to H2O electrolysis. Additional tests indicate that increasing H2-to-H2O ratios enhances ceria activity for electrolysis. 1. Introduction Simple and complex oxide materials containing metal cations with multiple valence states can exhibit mixed ionic-electronic conductivity (MIEC). MIEC materials transport both oxygen and electrons (i.e., polarons) through the bulk lattice via the incorporation of oxygen vacancies, which are accommodated by multiple oxidation states in the metal cations. Reversible oxidation and MIEC behavior are useful in electrodes for hightemperature electrochemical devices (notably solid oxide fuel cells and solid oxide electrolysis cells), where both oxide ion (O2-) and electron conductivity facilitate reactions on either side of an O2--conducting electrolyte membrane.1 MIEC oxides have been adapted for high-temperature solid oxide cells (SOCs), both for O2 reduction1-4 and for fuel oxidation,5-7 but there is still uncertainty in the literature over how to model MIEC behavior. This uncertainty stems in part from limited in situ data to correlate metal oxidation states for active surfaces during electrochemical excitation in reactive high-temperature environments. Although recent studies with in situ Raman spectroscopy have provided insight into the behavior of high-temperature electrodes,8,9 the lack of distinct Raman signatures for relevant transitions in many MIEC oxides limits the insight gained from these studies. In contrast, ambient pressure X-ray photoelectron spectroscopy (XPS) probes the surface and near-surface oxidation states of materials under high-temperature reacting conditions.10-12 This study furthers the development of ambient * Corresponding author. E-mail: [email protected]. Phone: 1-301-4052368. Fax: 1-301-405-2025. † University of Maryland. | Current address: NIST Center for Neutron Research, Gaithersburg, MD 20899. ‡ Lawrence Berkeley National Laboratory. § Sandia National Laboratories. ⊥ Current address: Chalmers University of Technology, 41296 Gothenburg, Sweden.

pressure XPS to study high-temperature electrochemistry by using synchrotron radiation from the Advanced Light Source (ALS) in Lawrence Berkeley National Laboratory.13,14 In this current study, ambient pressure XPS has been applied to active MIEC ceria (CeO2-x) electrodes to explore correlations between electrochemical behavior and surface oxidation states. As a simple MIEC material, ceria has received attention as an SOC electrocatalyst, both as a carbon-tolerant catalyst in fuelcell anodes15-18 and as an active component in cathodes for electrolysis cells.19-22 Because the cerium cation can undergo facile redox between Ce4+ and Ce3+ valence states at elevated temperatures and very low oxygen partial pressure (PO2),23,24 it has unique catalytic properties that have led to its broad application in electrochemical devices25 and catalytic reactors.26,27 Many ceria phases of the form CeO2-x have been identified28 and can be modeled as mixtures of the two valence states Ce3+ (Ce2O3) and Ce4+ (CeO2) if local charge neutrality is assumed. In high-temperature SOCs, the mixed valence states of ceria facilitate MIEC behavior and surface activity for oxidation/ reduction reactions. However, the link between the MIEC behavior of ceria and electrochemical activity is not fully understood, in part because of the unknown distribution of Ce3+ and Ce4+ oxidation states at the nonequilibrium conditions of high-temperature electrochemistry. Developing a more quantitative understanding of ceria behavior in SOCs will require understanding of how CeO2-x oxidation states correlate with catalytic activity and conductivities and how these conditions change with temperature, gas composition, and electrochemical potential. By tracking changes in the Ce3d core level, XPS has been used extensively as an ex situ tool to characterize Ce oxidation states.29-35 In contrast to previous ex situ XPS studies of ceria oxidation states under thermodynamic equilibrium at low temperatures, this study characterizes an operating CeO2-x electrode during electrochemical oxidation of H2 as well as

10.1021/jp107694z  2010 American Chemical Society Published on Web 11/02/2010

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Figure 1. (a) Schematic of a single-chamber electrochemical cell with thin-film CeO2-x WE with a patterned Au current collector, a supporting single-crystal YSZ electrolyte, and a porous Pt CE with a Pt mesh current collector. An Al2O3 heater on the backside of the YSZ provided heating for the cell in the XPS chamber with reactive gas mixtures of H2 and H2O between 0.5 and 1.0 Torr. XPS measurements are primarily taken in the ∼200 µm wide border of ceria outside the Au pattern closest to the Pt CE. A scale bar of 2 mm is shown on the electrode. (b) Photo of the working cell at temperature inside the XPS chamber during electrochemical testing.

reduction of H2O at elevated temperatures up to 740 °C. Such in situ characterization is essential for understanding the nonequilibrium behavior of ceria electrodes during hightemperature electrochemical-cell operation. Analyses of the simultaneous XPS and electrochemical measurements, as collected in this study, provide a foundation for understanding H2 oxidation and H2O electrolysis on CeO2-x anodes. 2. Experimental Methods The single-chamber cells in this study were created by patterning all electrodes on the same side of a supporting singlecrystal yttria-stabilized zirconia (YSZ) electrolyte as pictured in Figure 1a. Dense ceria thin films were sputter-deposited in the center of the YSZ support to form the working electrode (WE). The final ceria film thickness is approximately 300 nm as measured by SEM imaging. The cross-section in Figure S1 in the Supporting Information shows the good contact between the ceria and YSZ thereby allowing for facile O2- transport across the phase boundary. A patterned dense Au current collector (250-300 nm thick) as illustrated in Figure 1a was sputter-deposited over the ceria films and patterned via optical lithography. Pt-slurry was used to form a porous counter electrode (CE), with Pt gauze pressed into the slurry paste for current collection. Electric leads to the Au pattern on the WE and the porous Pt CE shown in Figure 1b serve as clamps to hold the YSZ electrolyte firmly to the button heater. Additional details regarding the cell fabrication and geometry are provided in the Supporting Information. For the experiments at ALS, the electrochemical cells were mounted onto an Al2O3 heater with internal Pt coils, which heated the cell to temperatures as high as 740 °C, as measured by a two-color pyrometer and validated by total bulk resistance (Rbulk) measurements, which were correlated with temperatures in separate high-temperature furnace experiments. H2 and H2O mixtures were backfilled into the single XPS chamber through separate leak valves. For simultaneous XPS/electrochemical testing, the Au current collector was grounded so that any shifts of the Ce core-level peaks at bias corresponded to potential

differences between the ceria and Au.36-38 The source for water vapor in the XPS chamber was HPLC grade water that was degassed in three freeze-pump-thaw cycles prior to the experiments. The total pressure in the experimental cell was measured by using an absolute membrane pressure gauge. The cells were tested at the ALS beamline 11.0.2 in the ambient pressure XPS endstation, which employs a differentially pumped electrostatic lens system that allows for photoelectronbeam measurements at Torr pressures in the experimental cell.39 Figure 1b shows a photograph of the entrance aperture to the electron analyzer positioned just above an operating cell at the beamline. Ce3d core-level spectra were taken with a photon energy of 1180 eV, which corresponds to a mean free path of order 1 nm for the photoelectrons emitted from Ce3d states (∼280 eV KE). The size of the incident X-ray measurement spot was ∼200 µm, and the relative sample position was controlled with a precision of ∼50 µm to probe the spatial distribution of cerium oxidation states. 2-D modeling of the electric field in the YSZ and bulk ceria (see Supporting Information) predicts that most of the current collection occurs at the Au bar closest to the Pt CE, implying that changes in near-surface ceria oxidation states due to electrochemical activity are most readily observed by in situ XPS in the 200 µm width of ceria extending beyond the Au bar closest to the CE. Hence, XPS measurements focused primarily on two principal locations in that 200 µm width: position 0, on the edge of the ceria film closest to the Pt CE with a beam center close to 200 µm from the Au current collector, and position 1, at the ceria/Au interface nearest the Pt CE with a beam center less than 100 µm from the Au current collector. Electrochemical measurements at the ALS, including linearsweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were conducted by using a Gamry MultiEChem System. Initial three-electrode experiments at the ALS with two Pt electrodes were wired such that the applied bias was between the two Pt electrodes, with one as a reference electrode and the other through which current flowed to/from the ceria electrode of interest. In these experiments, the actual cell voltage Vcell ≡ φPt -

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TABLE 1: Key Surface and Electrochemical Reactions for H2 Oxidation and H2O Electrolysis on the Active Ceria Electrodes reaction R1: Hydrogen oxidation (forward) and H2O electrolysis (reverse) on ceria surface R2: Oxide-ion charge-transfer reactions at ceria/YSZ interface

+

Oxo(CeOx)

+ H2(g) T 2CeCe ′ (CeOx) + V••o (CeOx) + H2O(g)

•• x Oxo(YSZ) + V•• o (CeOx) T Vo (YSZ) + Oo(CeOx) x CeCe (CeOx) + e-(Au) ′ (CeOx) T CeCe

R3: Electron charge-transfer reaction at ceria/Au interface

φAu (where φPt and φAu are electric potentials of the Pt CE and the Au current collector on the WE, respectively) was not measured directly. In the subsequent two-electrode experiments at ALS, the ceria served exclusively as the WE, and the single Pt electrode was wired as both the reference and CE such that Vcell was measured by the Gamry system. To complement the simultaneous electrochemistry and XPS experiments at the ALS, electrochemical characterization (LSV and EIS) of the single-chamber cell over a broader range of partial pressures, PH2 and PH2O (with Ar dilution), was undertaken. PH2 and PH2O ranging from 2 to 10 Torr were supplied to the cell placed on a horizontal surface at the base of a tubular reactor with the reacting gases and diluent supplied through a central feed tube. H2 and Ar flows were provided by mass flow controllers (Brooks 5850E) with appropriate fractions of Ar passed through a temperature-controlled Nafion-tube humidifier to provide the desired PH2O. Electrochemical data for both EIS and LSV measurements were collected by using an EcoChemie Autolab 30 system. For a given set of measurements, the overpotential associated with the ceria electrode ηWE (defined by φAu - φYSZ@WE) varies with total cell bias Vcell and total current Itot as follows:

Vcell ) -(ηWE + ItotRbulk,YSZ + ηCE)

x 2CeCe (CeOx)

of Ce3+ cations but the transfer of electrons from one cerium cation to a neighboring cation. These ionic fluxes are balanced locally at the ceria surface by reaction R1 being driven out of equilibrium, which can change the ceria near-surface oxidation state (represented here by XCe3+(s) or % Ce3+). A major point of this study is to measure the ceria oxidation state change, defined by ∆% Ce3+ ) 100% · (XCe3+(s) - XCe3+(s),eq), and understand its relation to electrochemical processes. The total WE overpotential ηWE in eq 1 can be represented as the summation of overpotentials related to the various processes occurring in the WE:

ηWE ) ηYSZ-Ceria + ηi,Ceria + ηe,Ceria + ηCeria-Au

(2)

In eq 2, ηYSZ-Ceria represents the overpotential associated with the buildup of charged species at the ceria/YSZ interface to

(1)

where the bulk YSZ resistance Rbulk,YSZ is taken from the highfrequency intercept in full-cell EIS measurements and ηCE is the total CE overpotential. Given the cell geometry in Figure 1 and the MIEC nature of the WE, the specific values of the individual overpotentials in eq 1 will vary with the path chosen between the WE and the RE. For a given Itot, however, the overpotentials for various path lines will always sum to the same Vcell. Although the ItotRbulk,YSZ is determined from experimental measurements, insufficient measurements were made of ηCE to determine with confidence the total ηWE from eq 1. Thus, the electrochemical measurements in the current study are plotted as IRbulk-corrected biases (i.e., Vcell + ItotRbulk,YSZ). 3. Results and Discussion The ceria electrode surfaces in this study promote heterogeneous H2 oxidation/H2O reduction, as expressed in Table 1 by reaction R1 in Kro¨ger-Vink notation. In Table 1, OOx and VO•• represent oxide ions and vacancies in the ceria lattice, respecx and CeCe ′ represent Ce4+ and Ce3+. Electrons tively, and CeCe x 2released by OO (i.e., O ) in reaction R1 are strongly localized on nearby cerium lattice sites to form Ce3+ which effectively act as localized negative charges or polarons.40 R1 likely proceeds by several elementary reaction steps with intermediate species (such as surface OH-),41 but the global equilibrium between the principal gas-phase species and the ceria surface as described by R1 is sufficient for the current discussion. When an electrical bias is applied to the cell, electrochemical potential gradients through the ceria drive fluxes of oxide ions (JO2-) and polarons (JCe3+) as depicted qualitatively in Figure 2. Polaron hopping (characterized by JCe3+) is not the movement

Figure 2. (a) 2-D schematic showing transport and reaction processes in the ceria electrode, counter Pt electrode, and YSZ electrolyte support. Dashed area shows region expanded in panels b and c. (b) Processes in ceria electrode under negative bias with H2 oxidation at the ceria surface. Plot on right shows qualitative profiles of voltage φ - φAu and Ce3+ fraction XCe3+ (at equilibrium and at bias) along line 1 (dashed red in diagram) with surface values indicated by tick marks at the surface. (c) Processes in ceria electrode under positive bias with H2O electrolysis at the ceria surface. Plot on right shows qualitative profiles of φ - φAu and XCe3+ along line 1 with surface values indicated by tick marks.

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Figure 3. (a) Ce3d XPS spectra from CeO2-x closest to Pt cathode with varying current density during cell operation at 635 °C, PH2 ) 0.25 Torr, and PH2O ) 0.25 Torr. The fitted contributions from Ce3+ (green hashes) and Ce4+ (solid cyan) are shown for the spectrum at Itot ) 0.123 mA. The incident-photon energy is 1180 eV. (b) The measured percentage of Ce3+ and φCe-Au (i.e., φCeria) from the XPS spectra fitting as a function of total current through the ceria electrode for cell operation at 635 °C, PH2 ) 0.25 Torr, and PH2O ) 0.25 Torr.

facilitate transfer of O2- ions across the interfacial double layer, as indicated by reaction R2 in Table 1. ηi,Ceria refers to the change in potential across the thickness of the ceria film associated with ion flux JO2- between the YSZ-ceria interface and the ceria surface. JO2- is related to the potential drop through the Nernst-Planck equation expressed in eq 3.

JO2- ) -νO2-[CO2-]∇(µO2- - 2Fφ)

(3)

where VO2-, [CO2-], and µO2-, are respectively the mobility, concentration, and chemical potential of the oxide ions. F is Faraday’s constant. An applied electrical bias also creates a transverse overpotential, represented by ηe,Ceria, to drive electron flux (via polaron hopping) between the reactive ceria surface and the Au current collector. Because polaron mobility VCe3+ is significantly larger than VO2-,28 O2- transport in this same lateral direction is likely relatively minor. At the ceria-Au interface, electrons are transferred to/from the ceria surface across a double layer via the electron charge-transfer reaction R3 in Table 1, which results in an overpotential ηCeria-Au. The magnitude of ηWE and the constituent overpotetials in eq 2 can vary with the path line chosen between the YSZ and Au current collector. Furthermore, because local potentials are measured only on exposed surfaces, ηYSZ-Ceria and ηi,Ceria are not obtained directly. However, measurements of the local ceria surface voltage relative to the grounded Au current collector provide a measure for the local value of ηe,Ceria + ηCeria-Au. As illustrated in Figure 2b, a negative bias drives O2- ions from the YSZ to the ceria surface, a net forward reaction rate (H2 oxidation) for R1, and a flow of electrons/polarons from the ceria surface to the Au current collector. With net H2 oxidation on the ceria electrode, XCe3+(s) tends to decrease from its equilibrium value. Conversely, for a positive bias with H2O electrolysis on the ceria surface (net negative rate for R1), the flow of ions are reversed (as illustrated in Figure 2c), and XCe3+(s) tends to increase from its equilibrium value. The changes in surface Ce3+, i.e. ∆% Ce3+, with current reflect the importance of the surface oxidation state in determining the net rate of R1 as well as the magnitude of an effective reaction rate constant for R1 (kR1). For highly reactive surfaces and/or conditions, relatively small changes in surface concentrations (oxidation state) will be needed to drive R1 (as measured by current). In a local sense, nonuniformities in ∆% Ce3+ for a given cell as measured by ambient pressure XPS reflect the distribution of

electrochemical activity.13 The XCe3+ profiles in Figure 2b,c for H2 oxidation and H2O electrolysis, respectively, serve only as qualitative representations of how the vertical XCe3+ and electric potential profiles in the ceria electrodes respond to cell bias. Two cells were characterized at ALS. A first cell was characterized by using a three-electrode geometry with a porous Pt reference electrode pasted on one side of the ceria film and a second L-shaped porous Pt electrode placed on two other sides of the ceria electrode (inset, Figure 3b). Because of initial concerns about simultaneously grounding the potentiostat, the WE, and the electron energy analyzer, a fixed bias was maintained across the two Pt electrodes, and the Au current collector on the ceria electrode was wired as the CE while grounded to the XPS chamber. In these measurements, the recorded potential differences between the working Pt electrode and the Au current collector on the ceria were larger in magnitude than the voltage difference across the two Pt electrodes because of overpotentials associated with the ceria/ Au electrode through which the measured current Itot flowed. For this reason, Itot through the ceria electrode has been used to specify conditions and facilitate comparison with the twoelectrode geometry used in all other tests. In all of the measurements, the rigid shifts in the Ce3d peaks in the XPS spectra provided a measure of the surface electrical potential of the ceria electrode to evaluate ηe,Ceria + ηCeria-Au. Figure 3a shows a series of Ce3d spectra recorded at the edge of the ceria electrode closest to the Pt electrode, more than 100 µm from the Au current collector (as indicated by the square symbol, i.e., position 0, on the layout in Figure 3b). The Ce3d spectra were recorded for a range of voltages from -1.2 to +1.0 V maintained across the two Pt electrodes for constant PH2 ) PH2O ) 0.25 Torr and T ) 635 °C. The individual spectrum at each Itot consists of contributions from 10 peaks, each of which is attributed to either Ce3+ or Ce4+, as indicated in the reference spectra at the bottom of Figure 3a (solid blue spectrum, Ce3+ reference; green line peaks, Ce4+ reference). As discussed in the Supporting Information, fitting the measured spectra to reference spectra of Ce3+ and Ce4+ allowed the fraction of each oxidation state to be quantified in the near-surface region. The spectra in Figure 3a demonstrate that the ceria surface goes from a more reduced state at large positive bias (Itot ) -0.348 mA) to a more oxidized state at large negative bias (Itot ) 0.123 mA). Figure 3b shows the results of fitting the Ce3d spectra in Figure 3a in terms of XCe3+(s), and these results are compared

In Situ Characterization of Ceria Oxidation States with results obtained by fitting similar spectra measured at a point adjacent to the Au current collector (shown as position 1 in the insert diagram). At open circuit, the ceria surface at both positions between the first Au bar and the Pt CE was substantially reduced, 82 ( 3% Ce3+. By applying cathodic current (H2O electrolysis on the ceria surface), the degree of reduction increased to roughly 90% Ce3+. With anodic currents (H2 oxidation on the ceria), the surface became significantly less reduced relative to OCV (61% Ce3+ at position 0 and 49% Ce3+ at position 1). The relatively large changes in XCe3+(s) with Itot in comparison to subsequent experiments discussed below indicate that, at the relatively low temperature but high bias (due to the three electrode arrangement), the surface was driven further out of equilibrium in order to support the similar magnitudes of net rates for R1 (i.e., Itot). The changes in XCe3+(s) with bias were accompanied by nonzero electric potential on the ceria surface φCeria, which are also plotted versus Itot for both positions in Figure 3b. As stated earlier, φCeria estimates the local overpotentials associated with electron/polaron transport between the ceria surface and the Au current collector; that is, φCeria ≈ -(ηe,Ceria + ηCeria-Au). φCeria varied with location and increased in magnitude further from the Au current collector (from position 1 to position 0). The increased voltage with distance from the current collector is likely associated with electrons/polarons produced/destroyed in that region and the resistance to their transport parallel to the ceria surface. The departure of XCe3+(s) from its equilibrium values at the WE edge (position 1) closest to the Pt CE suggests that the electrochemically active region extended out beyond 100 µm adjacent to the Au current collector. Such a broad region of electrochemical activity is facilitated by the MIEC behavior of the ceria and in particular its ability to conduct electrons/ polarons parallel to the surface. Also of note in Figure 3b, the changes in both XCe3+(s) and φCeria were much larger for the negative bias even though Itot were significantly lower. These results suggest that, at this low temperature, the ceria served as a superior electrode for H2O electrolysis than for H2 oxidation. Furthermore, the differences in φCeria must be attributed in part to ηe,Ceria or ηCeria-Au. A possible explanation is that, at these conditions, there is strong coupling between the electron transport and the surface reactions in such a way that impedes electro-oxidation of H2 on the surface more so than H2O electrolysis, but further experiments are needed to verify this hypothesis. Although these initial experiments provided some insight into the role of ceria in operating SOFC anodes, the electrode configuration and unmeasured cell biases limit the utility of these measurements. Subsequent measurements were conducted at higher temperatures with a two-probe configuration (inset, Figure 4) that allowed measurement of both Itot and Vcell. Figures 4-6 present the XPS and corresponding electrochemical data for a second set of measurements, taken adjacent to the Au current collector (position 1) at T ) 650-740 °C with Vcell ranging between +1.0 and -1.2 V. Figure 4 shows the fitted percentage Ce3+ values as a function of temperature, with PH2 ) PH2O unless otherwise noted. In one set of measurements, the gas-phase composition was also varied by increasing PH2/ PH2O from 1.0 to 4.0 while maintaining a total pressure of 0.8 Torr. As indicated in Figure 4, it was difficult to control the temperature with changes in PH2 and PH2O in the ambient pressure XPS chamber, and as such, the change in PH2/PH2O resulted in a drop in temperature to 650 °C. The IRbulk-corrected V-I curves in Figure 5 show that these tests explored a range of Itot values similar to that of the low-temperature measurements

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Figure 4. Percentage of Ce3+ (as determined by XPS spectral fits) at the WE surface between the Au current collector and the Pt CE for a range of operating temperatures and three total cell biases. All measurements with PH2 ) PH2O ) 0.40 Torr except at lowest temperature where PH2 ) 0.64 Torr and PH2O ) 0.16 Torr.

Figure 5. IRbulk-corrected V-I curves for a range of temperatures at fixed Ptot ) 0.80 Torr with different PH2/PH2O ratios.

shown in Figure 3, by maintaining smaller overall cell biases during the higher temperature tests. Although the different conditions and Pt electrode geometries in Figures 3 and 4 make definitive comparison between the two sets problematic, the results in Figure 4 show that the variation of ceria near-surface oxidation states, particularly in the direction of H2 oxidation, was significantly decreased at temperatures above 635 °C. For T < 720 °C, the results in Figure 4 show a fairly consistent trend with cell bias, with the fraction of Ce3+ ranging between roughly 69 and 76%, and an open-circuit oxidation state of percentage of Ce3+ ) 70-72%. Only for the measurements at T ) 740 °C did the near-surface percentage of Ce3+ values vary by more than 5% with bias in either direction, compared to changes of 10-30% for similar Itot values in Figure 3b. Furthermore, increasing the reducing potential of the gas (either with increased PH2/PH2O or with increased T) did not lead to appreciable increases in the equilibrium percentage of Ce3+ values (collected at Vcell ) 0.0 V) over the time scales investigated (the cell spent several hours at each set of conditions). One consistent feature in Figures 3b and 4 is the overall high degree of reduction observed at the ceria near-surface, relative to the predicted bulk composition for the specified temperature

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Figure 6. Percentage of Ce3+ and potential at ceria electrode surface between Au current collector and ceria-film border (position 1) as a function of total current Itot: (a) for temperatures between 670 and 710 °C and PH2 ) PH2O ) 0.40 Torr and (b) for a broader range of temperatures and PH2 + PH2O ) 0.80 Torr.

and effective partial pressure of oxygen PO2,eq. Numerous prior experiments have demonstrated the enhanced reducibility of ceria surfaces, particularly at the intermediate temperatures explored in this study, on both single-crystal34,42 and polycrystalline31,43-49 samples. Previous measurements have utilized a wide range of techniques, including XPS,31,34,42-46 temperatureprogrammed desorption/reduction,31,42,43 magnetic susceptibility,43 Fourier-transform infrared,43,47 UV-vis reflectance,43 thermogravimetry,48,49 scanning electron microscopy,48 and X-ray absorption spectroscopy.49 Although the use of ambient pressure XPS in this study allows for measurement at conditions not accessible in previous XPS measurements, prior quantitative

analysis of ex situ ceria surfaces via XPS after reduction at a similar range of temperatures (500-800 °C) reveals ceria nearsurface Ce3+ fractions ranging from 40 up to 96%,42,44,46 a range entirely consistent with the values in this study. Table 2 compares the observed near-surface equilibrium Ce3+ fractions (XCe3+(s)) to the predicted bulk fractions (XCe3+(b)). Bulk values are based on fits (described in detail in the Supporting Information) to previously published data, as presented in the previous literature.25,28,50 As seen in Table 2, the observed XCe3+(s) values are more than two orders of magnitude larger than bulk equilibrium values. The fits were also used to estimate the free energy of reduction at the ceria surface ∆Gred(s), relative to the bulk value ∆Gred(b). Results in Table 2 suggest that the energy of reduction at the ceria near-surface region is 2.09-2.20 eV lower than in the bulk. Previous Born potential simulations of ceria surfaces predict that the energy of formation for Ce2O3 is 3-6 eV lower at the surface, relative to the bulk, but gradually approaches the bulk value over a depth on the order of 10 Å.51 The smaller change in ∆Gred derived from the XPS measurements in Table 2, relative to the atomistic simulation values, is likely explained by the mean free path of detected photoelectrons, which probes ceria oxidation states through a significant depth of the reduced surface layer. Figure 5 shows the corresponding IRbulk-corrected V-I curves collected for three representative sets of conditions: (i) a baseline case with T ) 700 °C and PH2 ) PH2O ) 0.4 Torr; (ii) a hightemperature case at T ) 740 °C also with PH2 ) PH2O ) 0.4 Torr; and (iii) a more reducing case, with PH2 ) 0.64 Torr, PH2O ) 0.16 Torr, and a lower T ) 650 °C. For each of the three conditions, the polarization resistance Rpol (≈ -δVcorr/ δItot) shows three general regions over the range of currents in Figure 5: a relatively low-resistance region at negative currents (electrolysis on the ceria WE), a region of high Rpol at smallto-moderate positive currents, and a region of reduced Rpol at the very highest positive currents (H2 oxidation on the ceria WE). Comparison of the baseline and T ) 740 °C results reveals that, though improved kinetics with higher T decreases Rpol in all regions, the higher T has a larger effect on H2 oxidation currents, relative to electrolysis currents. For the highly reducing case, the polarization results are very similar to the baseline case at low currents but show only minimal reductions in Rpol with increasing currents in either direction. Figure 6 shows the collected results from the in situ XPS data near position 1, ∆% Ce3+ (upper panel) and φCeria (lower panel), as a function of Itot for several two-electrode measurements. Figure 6a shows data collected at or near the baseline conditions, for T ) 700 °C and PH2 ) PH2O ) 0.40 Torr. Figure 6b shows the data for the varying gas-phase conditions discussed with regard to Figure 5. Results for T ) 700 °C from Figure 6a are reproduced for comparison in Figure 6b as a baseline condition. In marked contrast to the results in Figure 3b, the data in Figure 6a show very similar rates of change for ∆% Ce3+ with both positive and negative Itot at these temperatures. The smaller magnitude currents for Itot > 0 correlate with smaller changes in percentage of Ce3+, relative to Itot < 0. Figure 6b shows that the rate of change for percentage of Ce3+ with Itot remains similar in the positive and negative current

TABLE 2: Comparison of Calculated Equilibrium Ce3+ Bulk Mole Fractions XCe3+(b) with XPS-Measured Ce3+ Surface Fractions XCe3+(s) for Different Gas-Phase Conditions T (deg C) 650 700 740

PH2 (Torr) 0.64 0.40 0.40

PH2O (Torr) 0.16 0.40 0.40

PO2,eq (bar) -24

3.4 × 10 1.5 × 10-21 1.7 × 10-20

XCe3+(b) (%)

XCe3+(s) (%)

∆Gred(s) - ∆Gred(b) (eV)

0.0015 0.0016 0.0060

0.72 0.72 0.70

-2.09 -2.20 -2.13

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Figure 7. (a) IRbulk-corrected V-I curves at 700 ( 5 °C for a range of PH2 and PH2O at H2/H2O ratio )1.0. The lowest-pressure data were taken at the ALS, and all other data were taken in separate experiments at atmospheric pressure with Ar dilution. (b) IRbulk-corrected V-I curves at 700 ( 5 °C for a fixed PH2O ) 2 Torr and a range of PH2.

directions at higher T ) 740 °C. However, for the increased PH2/PH2O at T ) 650 °C, larger rates of changes in percentage of Ce3+ with negative Itot (H2O electrolysis) were observed, but these increased rates of change in percentage of Ce3+ with Itot were not observed for positive currents. These observations are consistent with the expected impact of increasing PH2/PH2O on the net rates of R1. Mass action kinetics for R1 suggest that, at the smaller PH2O, larger departures from equilibrium surface configuration are required to drive H2O electrolysis. On the other hand, higher PH2 tends to reduce the need for the surface to depart from equilibrium for net H2 oxidation. At present, it is unclear whether changes in the relationship between ∆% Ce3+ and Itot with varying conditions are influenced by changes in surface kinetics and/or local current distribution or by other underlying phenomenon. Further studies with more detailed mapping of the ceria surface may resolve the distribution of local currents in thin-film ceria electrodes. The lower panels of Figure 6a,b show φCeria, the sum of ηe,Ceria and ηCeria-Au, as a function of Itot. Figure 6a shows a larger slope for φCeria versus Itot for H2 oxidation on the ceria than for H2O electrolysis, which is consistent with the trend seen at lower temperature in Figure 3b. Similarly in Figure 6b, φCeria trends at T ) 740 °C are consistent with the baseline results, with larger variations in φCeria and smaller Itot magnitudes with negative bias relative to positive bias. Like the low-temperature data in Figure 3b, these results suggest that the difference in electrochemical performance between positive and negative bias can be attributed at least partially to ηe,Ceria or ηCeria-Au. In contrast, results for the highly reducing case in Figure 6b showed significantly larger φCeria values for positive bias, as a function of Itot. The results for this condition had very similar trends for ∆% Ce3+ and φCeria, with larger departure from equilibrium for H2O electrolysis per unit of Itot than for H2 oxidation. Given this result, one might predict larger Itot magnitudes with negative bias relative to positive bias for this condition. Figure 5, however, suggests that this is not the case. It is important to emphasize that the electrochemical measurements in Figure 5 reflect performance of both the ceria WE and Pt CE, whereas the XPS measurements are inherently local in nature, and thus changes in the XPS results at one position may result from changes in the distribution of local currents as much as from global changes in the ceria surface activity. To further assess electrochemical performance of the ceria cells, additional voltammetry measurements were performed on the same single-chamber cells (outside of ALS) for a range of

PH2 and PH2O up to 10 Torr at a cell temperature of 700 °C. These experiments were done at atmospheric pressure with Ar dilution. Initial experiments explored the effect of PH2 and PH2O while keeping the ratio fixed at 1.0. The resulting IRbulk-corrected V-I curves are compared in Figure 7a with similar curves taken from the cells in the ambient pressure XPS chamber without Ar dilution. Figure 7a shows a general increase in electrochemical performance with increasing reactant pressures because of a reduction in activation overpotentials. In addition, the electrochemical performance at increased partial pressures further confirms that, at these conditions, the ceria electrodes are more active for H2O electrolysis than for H2 oxidation.13 Additional electrochemical characterization of the ceria WE explored the effects of the gas reducing potential by varying PH2/PH2O at a constant cell temperature of 700 °C. Figure 7b shows the effect of varying PH2 while maintaining PH2O fixed at 2 Torr. The results indicate that increasing PH2/PH2O from 1.0 to 5.0 not only improves H2 oxidation rates on the ceria WE but also increases H2O electrolysis rates to an even greater degree. Although it was possible that changes in the Pt CE performance influenced this result, similar experiments in twochamber cells (not shown here), where changes in the anode feed gas do not impact cathode performance, also showed improvements in electrolysis activity with increasing PH2 for a fixed PH2O. Thus, the observed increase in H2O electrolysis with increased PH2 can be partly attributed to improvements in the ceria WE electrochemical activity, likely because of increased oxide vacancies at the surface. Conversely, increasing PH2O from 2 to 5 to 10 Torr while holding PH2 fixed at 10 Torr did not show significant changes in corrected V-I curves (not shown in Figure 7) for either positive or negative currents. This suggests that electrolysis and H2 oxidation on ceria do not have strong order dependencies on PH2O. On the other hand, the strong dependency of the electrochemical behavior with PH2 suggests the importance of near-surface states on the effectiveness of MIEC materials such as ceria for promoting electrochemical reactions in solid oxide cells. 4. Conclusions Ce3d core-level XPS of active ceria electrodes in a H2/H2O atmosphere has revealed how near-surface ceria oxidation states change (between Ce4+ and Ce3+) with electrochemical activity in solid oxide cells operating between 630 and 740 °C. The results have confirmed the significant increase in reducibility

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of the ceria near-surface region relative to bulk phase ceria, as proposed by atomistic models.51 Under equilibrium, zero-current conditions at PH2 ) PH2O ) 0.25 to 0.40 Torr, the near surface contained between 70 and 85% Ce3+, over two orders of magnitude more than bulk-phase thermodynamics predict. Measurements of percentage of Ce3+ under different cell biases revealed how far out of equilibrium selected locations of the ceria surface were driven in order to sustain a net reaction, either positive current (net H2 oxidation on the ceria) or negative current (net H2O electrolysis on the ceria). Positive currents resulted in decreases in percentage of Ce3+ from its equilibrium value in the near-surface region, and negative currents increase percentage of Ce3+. The rate at which the local percentage of Ce3+ changes with cell current provides a unique measure for assessing the activity of the surface and/or the importance of the ceria surface reactions. The ambient pressure XPS measurements provided additional insight into the ceria electrodes with their MIEC behavior. Local surface electrical potentials given by kinetic energy shifts of Ce core-level peaks are correlated to the overpotential associated with electron (or polaron) transport along the ceria surface and with the charge transfer between the ceria electrode and Au current collector. These measurements along with the simultaneous measurements of percentage of Ce3+ in the near-surface region revealed that electrochemical activity in the MIEC ceria electrode extends out over 100 µm from the Au current collector. Furthermore, electrochemical characterization under more reducing environments (with increased PH2/PH2O) increased surface activity for H2O electrolysis in spite of the increased product (H2) concentrations. In general, electrochemical results showed that, for similar magnitudes of cell bias, positive currents for H2 oxidation were significantly smaller than negative currents for H2O electrolysis. The simultaneous electrochemical and XPS measurements indicated that the highly reduced ceria surface is more active for electrolysis and, furthermore, that ceria surface reactions play some role in limiting the electrochemical currents. This study demonstrates the power of ambient pressure XPS to assess local oxidation states and surface electric potentials as a function of electrochemical activity in operating hightemperature solid oxide electrochemical cells. Qualitative and quantitative information about the near-surface oxidation state of ceria has revealed new insight into the mechanisms and energetics of H2 electrochemical oxidation and H2O electrolysis on ceria surfaces. These results have provided the basis for future extensive 2-D mapping of active ceria electrode surfaces as well as of other MIEC electrode surfaces where electrochemical activity and conductivity properties may vary strongly with surface oxidation state. The simultaneous surface and electrochemical data, coupled with additional electrochemical investigations and computational modeling, will provide mechanistic insight into operational solid oxide cells that were previously unattainable with standard electrochemical measurements and ex situ material characterization. Acknowledgment. UMD participants acknowledge the support of the Office of Naval Research through Contract No. N000140510711 (Dr. Michele Anderson, program manager). Work at LBNL and the ALS was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, and Chemical Sciences Division of the U.S. Department of Energy under contract No. DEAC02-05CH11231. Work by Sandia National Laboratories was supported by the Laboratory Directed Research and Development program through Contract No. DE-AC04-94AL85000 of the United States Department of

DeCaluwe et al. Energy. UMD authors acknowledge the assistance of Mr. Tom Loughran of the Nanocenter who facilitated in cell fabrication. Supporting Information Available: Cell fabrication details, including SEM cross sections of the ceria electrodes, are presented. In addition, 2-D simulations of the O2--ion conduction through the YSZ and ceria electrode are shown to assess the regions of high current density in the ceria electrode. The Supporting Information also presents the method for establishing XPS reference spectra to interpret the experimental spectra. Calculations for determining equilibrium bulk ceria oxidation show the method for extrapolating experimental measurements from earlier literature. Finally, further electrochemical data associated with electrochemical impedance spectra are provided. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Adler, S. B.; Chen, X. Y.; Wilson, J. R. J. Catal. 2007, 245, 91. (2) Adler, S. B. Chem. ReV. 2004, 104, 4791. (3) Baumann, F. S.; Fleig, J.; Cristiani, G.; Stuhlhofer, B.; Habermeier, H. U.; Maier, J. J. Electrochem. Soc. 2007, 154, B931. (4) Fleig, J.; Maier, J. J. Euro. Ceram. Soc. 2004, 24, 1343. (5) Goodenough, J. B.; Huang, Y. H. J. Power Sources 2007, 173, 1. (6) Huang, Y. H.; Dass, R. I.; Xing, Z. L.; Goodenough, J. B. Science 2006, 312, 254. (7) Primdahl, S.; Mogensen, M. Solid State Ionics 2002, 152, 597. (8) Pomfret, M. B.; Owrutsky, J. C.; Walker, R. A. J. Phys. Chem. B 2006, 110, 17305. (9) Cheng, Z.; Liu, M. L. Solid State Ionics 2007, 178, 925. (10) Knop-Gericke, A.; Kleimenov, E.; Havecker, M.; Blume, R.; Teschner, D.; Zafeiratos, S.; Schlogl, R.; Bukhtiyarov, V. I.; Kaichev, V. V.; Prosvirin, I. P.; Nizovskii, A. I.; Bluhm, H.; Barinov, A.; Dudin, P.; Kiskinova, M. AdV. Catal. 2009, 52, 213. (11) Grass, M. E.; Zhang, Y. W.; Butcher, D. R.; Park, J. Y.; Li, Y. M.; Bluhm, H.; Bratlie, K. M.; Zhang, T. F.; Somorjai, G. A. Angew. Chem., Intl. Ed. 2008, 47, 8893. (12) Ketteler, G.; Ogletree, D. F.; Bluhm, H.; Liu, H. J.; Hebenstreit, E. L. D.; Salmeron, M. J. Am. Chem. Soc. 2005, 127, 18269. (13) Zhang, C.; Grass, M. E.; McDaniel, A. H.; DeCaluwe, S. C.; El Gabaly, F.; Liu, Z.; McCarty, K. F.; Farrow, R. L.; Linne, M. A.; Hussain, Z.; Jackson, G. S.; Bluhm, H.; Eichhorn, B. W. Nat. Mater. 2010, 9, 949. (14) El Gabaly, F.; Grass, M. E.; McDaniel, A. H.; Farrow, R. L.; Linne, M. A.; Hussain, Z.; Bluhm, H.; Liu, Z.; McCarty, K. F. Phys. Chem. Chem. Phys. 2010, 12, 12138. (15) Ahn, K. Y.; He, H. P.; Vohs, J. M.; Gorte, R. J. Electrochem. Solid State Lett. 2005, 8, A414. (16) Gorte, R. J.; Vohs, J. M. J. Catal. 2003, 216, 477. (17) He, H. P.; Gorte, R. J.; Vohs, J. M. Electrochem. Solid State Lett. 2005, 8, A279. (18) Kim, T.; Ahn, K.; Vohs, J.; Gorte, R. J. J. Power Sources 2007, 164, 42. (19) Marina, O. A.; Pederson, L. R.; Williams, M. C.; Coffey, G. W.; Meinhardt, K. D.; Nguyen, C. D.; Thomsen, E. C. J. Electrochem. Soc. 2007, 154, B452. (20) Osada, N.; Uchida, H.; Watanabe, M. J. Electrochem. Soc. 2006, 153, A816. (21) Uchida, H.; Osada, N.; Watanabe, M. Electrochem. Solid State Lett. 2004, 7, A500. (22) Zhu, B.; Albinsson, I.; Andersson, C.; Borsand, K.; Nilsson, M.; Mellander, B. E. Electrochem. Commun. 2006, 8, 495. (23) Tuller, H. Solid State Ionics 2000, 131, 143. (24) Stubenrauch, J.; Vohs, J. M. J. Catal. 1996, 159, 50. (25) Mogensen, M. In Catalysis by Ceria and Related Materials; Trovarelli, A., Ed.; Imperial College Press: London, 2002, Ch.15. (26) Nunan, J. G.; Robota, H. J.; Cohn, M. J.; Bradley, S. A. J. Catal. 1992, 133, 309. (27) Trovarelli, A.; de Leitenburg, C.; Boaro, M.; Dolcetti, G. Catal. Today 1999, 50, 353. (28) Mogensen, M.; Sammes, N. M.; Tompsett, G. A. Solid State Ionics 2000, 129, 63. (29) Tsunekawa, S.; Fukuda, T.; Kasuya, A. Surf. Sci. 2000, 457, L437. (30) Tsunekawa, S.; Ishikawa, K.; Li, Z. Q.; Kawazoe, Y.; Kasuya, A. Phys. ReV. Lett. 2000, 85, 3440. (31) Holgado, J. P.; Alvarez, R.; Munuera, G. Appl. Surf. Sci. 2000, 161, 301. (32) Liu, G.; Rodriguez, J. A.; Hrbek, J.; Dvorak, J.; Peden, C. H. F. J. Phys. Chem. B 2001, 105, 7762.

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