In situ Characterization of Ceria Oxidation States in. High-Temperature Electrochemical Cells with Ambient. Pressure XPS

Transcription

1 In situ Characterization of Ceria Oidation States in High-Temperature Electrochemical Cells with Ambient Pressure XPS Steven C. DeCaluwe a, Michael E. Grass b, Chunjuan Zhang a, Farid El Gabaly c, Hendrik Bluhm b, Zhi Liu b, Gregory S. Jackson a *, Anthony H. McDaniel c, Kevin F. McCarty c, Roger L. Farrow c, Mark A. Linne c, Zahid Hussain c, and Bryan W. Eichhorn a a University of Maryland, College Park, MD 20742, USA b Lawrence Berkeley National Laboratory, Berkeley, CA USA c Sandia National Laboratories, Livermore, CA USA Current address: Colorado School of Mines, Golden, CO, 80401, USA Current address: Chalmers University of Technology, Gothenburg, Sweden AUTHOR ADDRESS: Gregory S. Jackson: gsjackso@umd.edu CORRESPONDING AUTHOR: Prof. Gregory S. Jackson, Dept. of Mechanical Engineering Colorado School of Mines, Golden, CO, USA gsjackso@mines.edu phone:

2 Abstract Ambient pressure X-ray photoelectron spectroscopy (XPS) is used to measure near-surface oidation states and local electric potentials of thin-film ceria electrodes operating in solid oide electrochemical cells for H 2O electrolysis and H 2 oidation. 300 nm thick ceria electrodes are deposited on YSZ electrolyte supports with porous Pt counter electrodes for single-chamber tests in H 2/H 2O mitures. Between 635 and to 740 ºC, equilibrium (zero-bias) near-surface oidation states between 70 and 85% Ce 3+ confirm increased surface reducibility relative to bulk ceria. Positive cell biases drive H 2O electrolysis on ceria and further increase % Ce on the surface over 100 µm from an Au current 3+ collector, signifying broad regions of electrochemical activity due to mied ionic-electronic conductivity of ceria. Negative biases to drive H2 oidation decrease % Ce from equilibrium values but 3+ with higher electrode impedances relative to H2O electrolysis. Additional tests indicate that increasing H2 to H2O ratios enhance ceria activity for electrolysis. Keywords: ceria, electrodes, XPS, MIEC, fuel cells. 2

3 Introduction Simple and comple oide materials containing metal cations with multiple valence states can ehibit mied ionic-electronic conductivity (MIEC). MIEC materials transport both oygen and electrons (i.e., polarons) through the bulk lattice via the incorporation of oygen vacancies, which are accommodated by multiple oidations states in the metal cations. Reversible oidation and MIEC behavior are useful in electrodes for high-temperature electrochemical devices (notably solid oide fuel cells and solid oide electrolysis cells) where both oide ion (O 2- ) and electron conductivity facilitate reactions on either side of an O 2- -conducting electrolyte membrane. 1 MIEC oides have been adapted for high-temperature solid oide cells (SOCs), both for O 2 reduction 1-4 and for fuel oidation, 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 oidation states for active surfaces during electrochemical ecitation 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 oides limits the insight gained from these studies. In contrast, ambient pressure X-ray photoelectron spectroscopy (XPS) probes the surface and near-surface oidation states of materials under high-temperature reacting conditions This study furthers the development of ambient pressure XPS to study high-temperature electrochemistry 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 (CeO 2- ) electrodes to eplore correlations between electrochemical behavior and surface oidation states. As a simple MIEC material, ceria has received attention as an SOC electrocatalyst, both as a carbon tolerant catalyst in fuel cell anodes and as an active component in cathodes for electrolysis cells Because the cerium cation can undergo facile redo between Ce and Ce valence states at elevated temperatures and very low oygen partial pressure (PO2), it has unique catalytic properties that have 23,24 3

4 Figure 1. (a) Schematic of a single-chamber electrochemical cell with thin-film CeO2- working electrode with a patterned Au current collector, a supporting single-crystal YSZ electrolyte, and a porous Pt counter electrode 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 mitures of H2 and H2O between Torr. XPS measurements are primarily taken in the ~200 µm wide border of ceria outside the Au pattern closest to the Pt counter electrode. 2 mm scale bar shown on the electrode. (b) Photo of the working cell at temperature inside the XPS chamber during electrochemical testing. led to its broad application in electrochemical devices and catalytic reactors. Many ceria phases of 25 26,27 the form CeO2- have been identified and can be modeled as mitures of the two valence states Ce ceria phases of the form CeO2- have been identified and can be modeled as mitures of the two valence 28 states Ce (Ce2O3) and Ce (CeO2) if local charge neutrality is assumed. In high-temperature SOCs, the mied valence states of ceria facilitate MIEC behavior and surface activity for oidation/reduction reactions. However, the link between the MIEC behavior of ceria and electrochemical activity is not fully understood, due in part to the unknown distribution of Ce and Ce oidation states at the non equilibrium conditions of high-temperature electrochemistry. Developing a more quantitative understanding of ceria behavior in SOCs will require understanding of how CeO 2- oidation 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 etensively as an e situ tool to characterize Ce oidation states In contrast to previous e situ XPS studies of ceria oidation states 4

5 under thermodynamic equilibrium at low temperatures, this study characterizes an operating CeO 2- electrode during electrochemical oidation of H 2 as well as reduction of H 2O at elevated temperatures up to 740 C. Such in situ characterization is essential for understanding the non-equilibrium behavior of ceria electrodes during high-temperature electrochemical-cell operation. Analyses of the simultaneous XPS and electrochemical measurements, as collected in this study, provide a foundation for understanding H 2 oidation and H 2O electrolysis on CeO 2- anodes. 2. Eperimental Methods The single chamber cells in this study were created by patterning all electrodes on the same side of a supporting single-crystal 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 approimately 300 nm as measured by SEM imaging. The crosssection in Figure S1 shows the good contact between the ceria and YSZ thereby allowing for facile O 2- transport across the phase boundary. A patterned dense Au current collector ( 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 eperiments at ALS, the electrochemical cells were mounted onto an Al 2O 3 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 (R bulk) measurements, which were correlated with temperatures in separate high-temperature furnace eperiments. H 2 and H 2O mitures were back-filled 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. The source for water vapor in the

6 XPS chamber was HPLC grade water that was degassed in three freeze-pump-thaw cycles prior to the eperiments. The total pressure in the eperimental cell was measured using an absolute membrane pressure gauge. The cells were tested at the ALS beamline in the ambient pressure XPS endstation, which employs a differentially pumped electrostatic lens system that allows for photoelectron-beam measurements at Torr pressures in the eperimental cell. Figure 1b shows a photograph of the entrance 39 aperture to the electron analyzer positioned just above an operating cell at the beamline. Ce3d corelevel spectra were taken with photon energy 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 oidation 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 oidation states due to electrochemical activity are most readily observed by in situ XPS in the 200 µm width of ceria etending 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 linear-sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were conducted using a Gamry MultiEChem System. Initial three-electrode eperiments at the ALS with the 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 eperiments, the actual cell voltage V cell φ Pt φ Au (where φ Pt and φ Au 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 eperiments at ALS, 6

7 the ceria served eclusively as the WE and the single Pt electrode was wired as both the reference and counter electrode such that Vcell was measured by the Gamry system. To complement the simultaneous electrochemistry and XPS eperiments at the ALS, electrochemical characterization (LSV and EIS) of the single-chamber cell over a broader range of partial pressures, P H2 and P H2O, (with Ar dilution) was undertaken. P H2 and P H2O 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 being supplied through a central feed tube. H 2 and Ar flows were provided by mass flow controllers (Brooks 5850E) with appropriate fractions of Ar passed through a temperaturecontrolled Nafion-tube humidifier to provide the desired P H2O. Electrochemical data both EIS and LSV measurements were collected 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) (1) where the bulk YSZ resistance Rbulk,YSZ is taken from the high-frequency 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 I tot, however, the overpotentials for various path lines will always sum to the same Vcell. While the ItotRbulk,YSZ is determined from eperimental measurements, insufficient measurements were made of η CE to determine with confidence the total η WE from equation 1. Thus, the electrochemical measurements in the current study are plotted as IR bulk corrected biases (i.e., V cell + I totr bulk,ysz). 3. Results and Discussion The ceria electrode surfaces in this study promote heterogeneous H2 oidation/h2o reduction, as epressed in Table 1 by reaction R1 in Kröger-Vink notation. In Table 1, O O and V O represent oide 7

8 O O Preprint of article published in Journal of Physical Chemistry C Table 1 -- Key surface and electrochemical reactions for H 2 oidation and H 2O electrolysis on the active ceria electrodes R1 R2 R3 Hydrogen oidation (forward) and H2O electrolysis (reverse) on ceria surface: 2Ce Ce (CeO ) + OO (CeO ) + H 2 (g) 2CeʹCe (CeO ) + VO (CeO ) + H 2O(g) Oide-ion charge transfer reactions at ceria/ysz interface: OO (YSZ) + VO (CeO ) VO (YSZ) + O (CeO Electron charge transfer reaction at ceria/au interface: CeʹCe (CeO ) Ce Ce (CeO ) + e - O (Au) ) ions and vacancies in the ceria lattice respectively, and Ce Ce and ' Ce Ce represent Ce and Ce Electrons released by O (i.e., O 2- ) in reaction R1 are strongly localized on nearby cerium lattice sites to O form Ce. which effectively act as localized negative charges or polarons. R1 likely proceeds by several elementary reaction steps with intermediate species (such as surface OH), but the global - 41 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 flues of oide ions (JO2-) and polarons (JCe3+) as depicted qualitatively in Figure 2. Polaron hopping (characterized by JCe3+) is not the movement of Ce cations but the transfer of electrons from one cerium 3+ cation to a neighboring cation. These ionic flues are balanced locally at the ceria surface by reaction R1 being driven out of equilibrium, which can change the ceria near-surface oidation state (represented here by XCe3+,(s) or % Ce ). A major point of this study is to measure the ceria oidation state change, 3+ defined by Δ% Ce = 100% (X 3+ Ce3+,(s) X Ce3+,(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 ocurring in the WE: η WE = η YSZ-Ceria + ηi,ceria + ηe,ceria + η Ceria-Au (2) 8

9 In equation 2, η YSZ-Ceria represents the overpotential associated with buildup of charged species at the ceria/ysz interface to facilitate transfer of O ions across the 2- interfacial double layer, as indicated by reaction R2 in Table 1. η i,ceria refers to the change in potential across the thickness of the ceria film associated with ion flu J O2- between the YSZ-ceria interface and the ceria surface. JO2- is related to the potential drop through the Nernst-Planck equation epressed in equation 3. J O2 = ν O2 [ C O2 ] ( µ O2 2Fφ ) (3) where vo2-, [CO2-], and µο2, are respectively the 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 epanded in b) and c). b) Processes in ceria electrode under negative bias with H2 oidation at the ceria surface. Plot on right shows qualitative profiles of voltage φ φ Au and Ce 3+ fraction X Ce3+ (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 H 2O electrolysis at the ceria surface. Plot on right shows qualitative profiles of φ φ Au and X Ce3+ along line 1 with surface values indicated by tick marks. mobility, concentration, and chemical potential of the oide ions. F is Faraday s constant. An applied electrical bias also creates a transverse overpotential, represented by ηe,ceria, to drive electron flu (via polaron hopping) between the reactive ceria surface and the Au current collector. Since polaron mobility v Ce3+ is significantly larger than v 28 O2-, O transport in 2- 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 9

10 reaction R3 in Table 1, which results in an overpotential η Ceria-Au. The magnitude of η WE 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 eposed 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 + η Ceria-Au. As illustrated in Figure 2b, a negative bias drives O ions from the YSZ to the ceria surface, a net 2- forward reaction rate (H 2 oidation) for R1, and a flow of electrons/polarons from the ceria surface to the Au current collector. With net H 2 oidation on the ceria electrode, X Ce3+,(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 Ce fractions (defined by Δ% Ce = % (X Ce3+,(s) X Ce3+,(s),eq)) with current reflect the importance of the surface oidation state in determining the net rate of R1 as well as the magnitude of an effective reaction rate constant for R1 (k R1). For highly reactive surfaces and/or conditions, relatively small changes in surface concentrations (oidation state) will be needed to drive R1 (as measured by current). In a local sense, non-uniformities in Δ% Ce 3+ for a given cell as measured by ambient pressure XPS reflect the distribution of electrochemical activity. 13 The X Ce3+ profiles in Figures 2b and 2c for H 2 oidation and H 2O electrolysis, respectively, serve only as qualitative representations of how the vertical X Ce3+ and electric potential profiles in the ceria electrodes respond to cell bias. Two cells were characterized at ALS. A first cell was characterized 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). Due to initial concerns about simultaneously grounding the potentiostat, the working electrode and the electron energy analyzer, a fied bias was maintained across the two Pt electrodes, and the Au current collector on the ceria electrode was wired as the counter electrode while grounded to the XPS chamber. In these measurements, the recorded potential differences between the working Pt electrode and the Au current 10

11 Figure 3. (a) Ce3d XPS spectra from CeO 2- closest to Pt cathode with varying current density during cell operation at 635 C, P H2 = 0.25 Torr, and P H2O = 0.25 Torr. The fitted contributions from Ce (green 3+ hashes) and Ce (solid cyan) are shown for the spectrum at I 4+ tot = ma. The incident photon energy is 1180 ev. (b) The measured % of Ce and 3+ φce-au 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. 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 I tot flowed. For this reason, I tot through the ceria electrode has been used to specify conditions and facilitate comparison with the two-electrode geometry used in all other tests with the second cell as described hereafter. In all of the measurements, the rigid shifts in the Ce3d peaks in the XPS spectra provided a measure of the surface potential of the ceria electrode to evaluate ηe,ceria + η Ceria-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 to +1.0 V maintained across the two Pt electrodes for constant PH2 = PH2O = 0.25 Torr and T = 635 ºC. The individual spectrum at each Itot consists of contributions from 10 peaks, each of which is attributed to either Ce or Ce, as indicated in the reference spectra at the bottom of Figure 3a (solid blue spectrum Ce reference and green line peaks Ce reference). As discussed in the supporting information,

12 fitting the measured spectra to reference spectra of Ce and Ce allowed the fraction of each oidation 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 (I tot = ma) to a more oidized state at large negative bias (Itot = ma). Figure 3b shows the results of fitting the Ce3d spectra in Figure 3a in terms of X Ce3+,(s), and these results are compared with 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% Ce 3+. By applying cathodic current (H 2 O electrolysis on the ceria surface), the degree of reduction increased to roughly 90% Ce 3+. With anodic currents (H 2 oidation on the ceria), the surface became significantly less reduced relative to OCV (61 % Ce 3+ at position 0 and 49 % Ce 3+ at position 1). The relatively large changes in the X Ce3+,(s) with I tot in comparison to subsequent eperiments 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., I tot ). The changes in X Ce3+,(s) with bias were accompanied by non-zero electric potential on the ceria surface φ Ceria, which are also plotted vs. I tot for both positions in Figure 3b. As stated earlier, φ Ceria estimates the local overpotentials associated with electron/polaron transport between the ceria surface and the Au current collector; i.e., φ Ceria (η e,ceria + η Ceria-Au ), φ Ceria 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 X Ce3+,(s) from its equilibrium values at the WE edge (position 1) closest to the Pt CE suggest that the electrochemically active region etended 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. 12

13 Also of note in Figure 3b, the changes in both X Ce3+,(s) and φ Ceria were much larger for the negative bias even though I tot were significantly lower. These results suggest that at this low temperature, the ceria served as a superior electrode for H 2 O electrolysis than for H 2 oidation. Furthermore, the differences in φ Ceria must be attributed in part to η e,ceria or η Ceria-Au. A possible eplanation is that at these conditions, there is strong coupling between the electron transport and the surface reactions in such a way that impedes electro-oidation of H 2 on the surface more so than H 2 O electrolysis, but further eperiments are needed to verify this hypothesis. Although these initial eperiments 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 2-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 Figure 4. Percentage of Ce (as determined by XPS spectral fits) at the WE surface between the Au 3+ current collector and the Pt CE for a range of operating temperatures and 3 total cell biases. All measurements with P H2 = P H2O = 0.40 Torr ecept at lowest temperature where P H2 = 0.64 Torr and P H2O = 0.16 Torr. 13

14 collector ( position 1 ) at T = ⁰C with V cell ranging between +1.0 and -1.2 V. Figure 4 shows the fitted % Ce values as a function of temperature, with P 3+ H2 = P H2O unless otherwise noted. In one set of measurements, the gas-phase composition was also varied by increasing P H2 /P H2O 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 P H2 and P H2O in the ambient pressure XPS chamber, and as such, the change in P H2 /P H2O resulted in a drop in temperature to 650 ºC. The IRbulk-corrected V-I curves in Figure 5 show that these tests eplored a similar range of I tot values as the low-temperature measurements shown in Figure 3, by maintaining smaller overall cell biases during the higher temperature tests. While 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 oidation states particularly in the direction of H 2 oidation was significantly decreased at temperatures above 635 ⁰C. For T < 720 ⁰C, the results in Figure 4 show a fairly consistent trend with cell bias, with the fraction of Ce ranging between roughly 69 76%, and an open circuit oidation state 3+ of % Ce = 70 72%. Only for the measurements at T = 740 ºC did the near-surface % Ce values vary by more than 5% with bias in either direction, compared to changes of 10 30% for similar I tot values in Figure 3b. Furthermore, increasing the reducing potential of the gas (either with increased P H2/P H2O or with increasing T) did not lead to appreciable increases in the equilibrium % Ce 3+ values (collected at V cell = 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 and effective partial pressure of oygen P O2,eq. Table 2 compares the observed near-surface equilibrium Ce fractions 3+ (X Ce3+(s)) to the predicted bulk fractions (X Ce3+(b)). Bulk values are based on fits (described in detail in the supporting information) to previously published data, as presented in the previous literature 25,28,42. As seen in Table 2, the observed X Ce3+(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 ΔG red(s), relative to the bulk value ΔG red,(b). Results in Table 2 suggest that the energy of reduction at the 14

15 Table 2 Comparison of calculated equilibrium Ce 3+ mole fractions XCe3+,(b) with XPS-measured Ce 3+ surface fractions X Ce3+,(s) for different gas-phase conditions Temp P H2 P O2,eq X Ce3+,(b) (C) (Torr) P H2O (Torr) (bar) (%) XCe3+,(s) (%) ΔGred,(s) ΔGred,(b) (ev) * ceria near-surface region is ev lower than in the bulk. Previous Born potential simulation 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. The smaller 43 change in ΔG red derived from the XPS measurements in Table 2, relative to the atomistic simulation values, is likely eplained by the mean free path of detected photoelectrons, which probes ceria oidation 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 high temperature case at Figure 5. IR bulk-corrected V-I curves for a range of temperatures at fied P tot = 0.80 Torr with different P H2/P H2O ratios. 15

16 T = 740 ⁰C also with P H2 = P H2O = 0.4 Torr; and (iii) a more reducing case, with P H2 = 0.64 Torr, P H2O = 0.16 Torr, and a lower T = 650 ⁰C. For each of the three conditions, the polarization resistance R pol ( - dvcorr/ditot) 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 small-to-moderate positive currents, and a region of reduced Rpol at the very highest positive currents (H2 oidation on the ceria WE). Comparison of the baseline and T = 740 ⁰C results reveals that, while improved kinetics with higher T decreases R pol in all regions, the higher T has a larger effect on H 2 oidation 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 R pol at large currents in either direction. Figures 6a and 6b show the collected results from the in situ XPS data near position 1 Δ% Ce (upper pane) and 3+ φceria (lower pane) as a function of I tot 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 Figure 6. Percentage of Ce and potential at 3+ ceria electrode surface between Au current collector and ceria-film border (Position 1) as a function of total current I tot: (a) for temperatures between 670 and 710 C and P H2 = P H2O = 0.40 Torr, and (b) for broader range of temperatures and P H2 + P H2O = 0.80 Torr. 16

17 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 Δ% Ce with both positive and negative I 3+ tot at these temperatures. The smaller magnitude currents for Itot > 0 correlate with smaller changes in % Ce, 3+ relative to I tot < 0. Figure 6b shows that the rate of change for % Ce with I 3+ tot remains similar in the positive and negative current directions at higher T = 740 ºC. However, for the increased P H2/P H2O at T = 650 ºC, larger rates of changes in % Ce with negative I 3+ tot (H 2O electrolysis) were observed, but these increased rates of change in % Ce with I 3+ tot were not observed for positive currents. These observations are consistent with the epected 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 oidation. At present, it is unclear whether changes in the relationship between Δ% Ce and 3+ 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 panes of Figures 6a and 6b show φceria, the sum of ηe,ceria and ηceria-au, as a function of Itot. Figure 6a shows a larger slope for φceria vs. Itot for H2 oidation on the ceria than for H2O electrolysis, which is consistent with the trend seen at lower temperature in Figure 3b. Similarly in Figure 6b, φ Ceria trends at T = 740 ºC are consistent with the baseline results, with larger variations in φ Ceria and smaller I tot magnitudes with negative bias, relative to positive bias. As with 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 η Ceria-Au. In contrast, results for the highly reducing case in Figure 6b showed significantly larger φ Ceria values for positive bias, as a function of I tot. The results for this condition had very similar trends for Δ% Ce 3+ and φceria, with larger departure from equilibrium for H2O electrolysis per unit of Itot than for H2 oidation. 17

18 Given this result, one might predict larger I tot 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, while 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 P H2 and P H2O up to 10 Torr at a cell temperature of 700 ºC. These eperiments were done at atmospheric pressure with Ar dilution. Initial eperiments eplored the effect of P H2 and P H2O pressure while keeping the ratio fied at 1.0. The resulting IR bulk -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 due to a reduction in activation overpotentials. In addition, the electrochemical performance at increased partial Figure 7. (a) IR bulk-corrected V-I curves at 700 ± 5 C for a range of P H2 and P H2O at H 2/H 2O ratios = 1.0. The lowest pressure data was taken at the ALS and all other data was taken in separate eperiments at atmospheric pressure with Ar dilution. (b) IR bulk-corrected V-I curves at 700 ± 5 C for a fied P H2O = 2 Torr and a range of P H2. 18

19 pressures further confirms that at these conditions, the ceria electrodes are more active for H 2 O electrolysis than for H 2 oidation. 13 Additional electrochemical characterization of the ceria WE eplored the effects of the gas reducing potential by varying P H2 /P H2O at a constant cell temperature of 700 ºC. Figure 7b shows the effect of varying P H2 while maintaining P H2O fied at 2 Torr. The results indicate that increasing P H2 /P H2O from 1.0 to 5.0 improves not only H 2 oidation rates on the ceria WE, but increases H 2 O electrolysis rates to an even greater degree. While it was possible that changes in the Pt CE performance influenced this result, similar eperiments in 2-chamber cells (not shown here), where changes in the anode feed gas do not impact cathode performance, also showed improvements in electrolysis activity with increasing P H2 for a fied P H2O. Thus, the observed increase in H 2 O electrolysis with increased P H2 can be partly attributed to improvements in the ceria WE electrochemical activity, likely due to increased oide vacancies at the surface. Conversely, increasing P H2O from 2 to 5 to 10 Torr while holding P H2 fied 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 H 2 oidation on ceria do not have strong order dependencies on P H2O. On the other hand, the strong dependency of the electrochemical behavior with P H2 suggests the importance of near-surface states on the effectiveness of MIEC materials like ceria for promoting electrochemical reactions in solid oide cells. 4. Conclusions Ce3d core-level XPS of active ceria electrodes in a H 2 /H 2 O atmosphere has revealed how nearsurface ceria oidation states change (between Ce 4+ and Ce 3+ ) with electrochemical activity in solid oide cells operating between C. The results have confirmed the significant increase in reducibility of the ceria near-surface region relative to bulk phase ceria, as proposed by atomistic 19

20 models. 43 Under equilibrium, zero-current conditions at P H2 = P H2O = 0.25 to 0.40 Torr, the near surface contained between 70 85% Ce 3+, over two orders of magnitude larger than bulk phase thermodynamics predict. Measurements of % Ce 3+ 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 H 2 oidation on the ceria) or negative current (net H 2 O electrolysis on the ceria). Positive currents resulted in decreases in % Ce 3+ from its equilibrium value in the near surface region, and negative currents increase % Ce 3+. The rate at which local % Ce 3+ 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 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 % Ce 3+ in the near surface region revealed that electrochemical activity in the MIEC ceria electrode etends out over 100 µm from the Au current collector. Furthermore, electrochemical characterization under more reducing environments (with increased P H2 /P H2O ) increased surface activity for H 2 O electrolysis in spite of the increased product (H 2 ) concentrations. In general, electrochemical results showed that for similar magnitudes of cell bias, positive currents for H 2 oidation were significantly smaller than negative currents for H 2 O 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 oidation states and surface electric potentials as a function of electrochemical activity in operating high-temperature solid oide electrochemical cells. Qualitative and quantitative information about the near-surface oidation 20

21 state of ceria has revealed new insight into the mechanisms and energetics of H 2 electrochemical oidation and H 2 O electrolysis on ceria surfaces. These results have provided the basis for future etensive 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 oidation state. The simultaneous surface and electrochemical data, coupled with additional electrochemical investigations and computational modeling, will provide mechanistic insight into operational solid oide cells that were previously unattainable with standard electrochemical measurements and e situ material characterization. Acknowledgement: UMD participants acknowledge the support of the Office of Naval Research through Contract # N (Dr. Michele Anderson program manager). LBNL work was supported by Contract # DE-AC02-05CH Work by Sandia National Laboratories was supported by the Laboratory Directed Research and Development program through Contract # DE-AC04-94AL85000 of the United States Department of Energy. UMD authors acknowledge the assistance of Mr. Tom Loughran of the Nanocenter who facilitated in cell fabrication.. Supporting Information Available: In supporting information, cell fabrication details, including SEM cross sections of the ceria electrodes, are presented. In addition, 2-D simulations of the O -ion 2- 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 eperimental spectra. Calculations for determining equilibrium bulk ceria oidation show the method for etrapolating eperimental measurements from earlier literature. Finally, further electrochemical data associated with electrochemical impedance spectra are provided. This supporting information material is available free of charge via the Internet at 21

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