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Jul 31, 2016 - ... of the Advanced Light Source (Lawrence Berkeley National Laboratory), which is equipped with a Scienta R4000 HiPP electron analyzer...
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Origin of Overpotential-Dependent Surface Dipole at CeO2−x/Gas Interface During Electrochemical Oxygen Insertion Reactions Zhuoluo A. Feng,†,‡ Chirranjeevi Balaji Gopal,§ Xiaofei Ye,§ Zixuan Guan,† Beomgyun Jeong,∥ Ethan Crumlin,∥ and William C. Chueh*,‡,§ †

Department of Applied Physics, Stanford University, Stanford, California 94305, United States Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States § Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ∥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡

S Supporting Information *

ABSTRACT: Ion insertion at the interfaces of batteries, fuel cells, and catalysts constitutes an important class of technologically relevant, charge-transfer reactions. However, the molecular nature of charge separation at the adsorbate/solid interface remains elusive. It has been hypothesized that electrostatic dipoles at the adsorbate/solid interface could result from adsorption-induced charge redistribution, preferential segregation of charged point defects in the solid, and/or intrinsic dipoles of adsorbates. Using operando ambient-pressure X-ray photoelectron spectroscopy, we elucidate the coupling between electrostatics and adsorbate chemistry on the surface of CeO2−x, an excellent electrocatalyst and a model system for studying oxygen-ion insertion reactions. Three adsorbate chemistries were studiedOH−/CeO2−x (polar adsorbate), CO2− 3 /CeO2−x (nonpolar adsorbate), and Ar/CeO2−x (no adsorbate)under several hundred mTorr of gas pressure relevant to electrochemical H2/CO oxidation and H2O/CO2 reduction. By integrating core-level spectroscopy and contact-potential difference measurements, we simultaneously determine the chemistry and coverage of adsorbates, Ce oxidation state, and the surface potential at the gas/solid interface over a wide range of overpotentials. We directly observe an overpotential-dependent surface potential, which is moreover sensitive to the polarity of the adsorbates. In the case of CeO2−x covered with polar OH−, we observe a surface potential that increases linearly with OH− coverage and with overpotential. On the other hand, for CeO2−x covered with nonpolar CO2− 3 and free of adsorbates, the surface potential is independent of overpotential. The adsorbate binding energy does not change systematically with overpotential. From these observations, we conclude that the electrostatic dipole at the adsorbate/ CeO2−x interface is dominated by the intrinsic dipoles of the adsorbates, with the solid contributing minimally. These results provide an atomistic picture of the gas/solid double layer and the experimental methodology to directly study and quantify the surface dipole.

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INTRODUCTION Electrical double layers and interfacial charge-transfer reactions are central to electrochemistry and are found widely in physical, chemical, and biological systems.1 The interfacial electrostatic dipole constituted by two layers of opposite charges brings adjacent phases in contact and modulates the rate of chargetransfer reactions. At the solid/liquid junction in the fuel cell, electrolyzers, and Li-ion batteries, one of the two charges is located in the solid (e.g., metal or semiconductor), while the other is located in the liquid as compact (Stern) or diffuse (Gouy−Chapman) layers.2 Another example is found in organic electronics, where self-assembled monolayers tune the charge injection barrier between electrodes and organic molecules.3−7 Charge-transfer reactions at an electrochemical interface can be broadly classified into three types, according to the type of charge transferred across the interface (Figure 1). The first and most studied charge transfer reaction is electron-transfer © 2016 American Chemical Society

between a solid and a liquid electrolyte (e.g., Pt/aqueous electrolyte and Si/aqueous electrolyte interfaces found in electrolyzers and photoelectrochemical cells).8,9 This unipolar charge transfer reaction only involves the transfer of electrons across the interface, because the electrode cannot accommodate ionic species, such as H+ and OH− (Figure 1A). The second type is an ion-transfer reaction between a liquid electrolyte and an insertion solid, typically a mixed ionic and electronic conductor (MIEC, e.g. battery electrodes such as LixC6 and LixCoO210,11). Unlike the electron-transfer reaction, ions are transferred across the interface and subsequently incorporated into the bulk of the electrode (Figure 1B). To maintain electroneutrality, electrons are transferred elsewhere. The third and final type is an ambipolar charge-transfer reaction, Received: June 16, 2016 Revised: July 21, 2016 Published: July 31, 2016 6233

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Figure 1. Schematics of different types of charge-transfer reactions at electrochemical interfaces. “CC” denotes current collector, and the red lines denote interfaces. (A) Electron-transfer reaction between a solid and a liquid electrolyte. (B) Ion-transfer reaction between a mixed ionic electronic conductor (MIEC) and a liquid electrolyte. (C) Ambipolar charge-transfer reaction where both electrons and ions are exchanged across the interface.

Figure 2. Three plausible origins of interfacial dipoles. (A) Charge redistribution between the adsorbates and the electrode surface. (B) Preferential accumulation/depletion of oxygen vacancies, electrons, and/or dopants. (C) Intrinsic dipole moment of the adsorbates. × × 2Ce′Ce + V •• O + 1/2O2(g) ⇌ 2CeCe + OO

involving the simultaneous transfer of both electrons and ions across the interface (Figure 1C). Strictly speaking, such an interface is not electrochemical but rather chemical in nature since there is no net charge transferred. This type of interface is commonly found in high-temperature fuel cells where the MIEC electrodes are in contact with the gas phase, in addition to the electrolyte and the current collector. As exemplified in Figure 1C, oxygen molecules dissociate into oxygen ions and electron−holes at the MIEC/gas interface, and migrate separately to the MIEC/electrolyte and MIEC/current collector interface, respectively. This type of interface is being studied extensively in the fields of solid-oxide fuel cells, permeation membranes, and catalysis (nonfaradaic electrochemical modification of catalytic activity).12,13 The double-layer structure and electrostatic dipole across the metal/electrolyte and semiconductor/electrolyte interfaces is relatively well-studied. Less is known about the double-layer across the MIEC/electrolyte ion-transfer interface, though there has been some recent progress in the context of battery electrodes.14 The double-layer across a MIEC/gas interface is by far the least understood and is the subject of this work. In a pioneering theoretical work, Fleig modeled the effect of oxygen adsorbate on the surface dipole of a metallic oxide electrode, related the surface potential to the overpotential, and demonstrated how it modifies the Butler−Volmer reaction kinetics.15 In practice, however, due to the challenges associated with characterization and isolating various contributions to the surface potential, only a few experiments have studied the surface electrostatic dipoles,16,17 and their origin remains unclear. One of the most studied electrochemical MIEC/gas model systems is the CeO2−x (ceria)/gas interface, typically on yttriastabilized zirconia solid electrolyte.18−23 The main charge carriers in ceria are localized electrons and oxygen vacancies, and the defect equilibrium reaction is given by

(1)

× where, following the Kroger−Vink notation, CeCe ′ , V•• O , CeCe, and OO× denote Ce3+, oxygen vacancy, Ce4+ and O2−, respectively. The ceria/gas interface is encountered in a variety of electrochemical and chemical energy conversion applications, such as solid-oxide fuel cells anodes (H2, CO, and fuel oxidation),24−26 oxygen storage capacitors in automotive catalytic converters,27 and reactive media for solar thermochemical cycles (H2O dissociation).28 At a mechanistic level, the exchange of oxygen ions and electrons between the electrode surface and the gas phase is strongly influenced by the charge distribution at the interface and surface potentials. Previous in situ investigations of the electrochemical CeO2−x/ gas interface have focused on the overpotential-dependent oxidation state of Ce, including those carried out by the present authors.21,22 Using ambient pressure X-ray photoelectron spectroscopy (APXPS), Zhang et al. quantified the overpotential-dependent binding energy of OH− adsorbate relative to that of the surface lattice oxygen.20 However, the binding energy shifts could be attributed to both chemical as well as electrostatic changes. Additionally, the work did not quantify the surface potential or the work function, both of which are important properties of the double layer. To date, the doublelayer structure of this important class of interfaces remains elusive, largely due to experimental challenges of simultaneously determining the adsorbate coverage, surface potential, and overpotential at the MIEC/gas interface. In this work, we elucidate the double-layer structure and determine the origin of the surface dipole at the ceria/gas interface operando under conditions relevant to CO/H2 oxidation and CO2/H2O dissociation. We consider three plausible double-layer structures for interfacial dipoles, though it is unclear which one dominates under reaction conditions. First, electrostatic dipoles can result from charge-redistribution between the adsorbates and the electrode surface (Figure 2A).

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Chemistry of Materials Second, preferential accumulation/depletion of oxygen vacancies, localized electrons, and/or dopants at the ceria/gas can give rise to surface charge, even in the absence of charged adsorbates (Figure 2B). Third, dipoles can stem solely from the intrinsic dipole moment of adsorbates (Figure 2C), such as that of hydroxyl ions and molecular water. Combining a solid-state electrochemical cell and ambient-pressure X-ray photoelectron spectroscopy (APXPS), we simultaneously quantified the overpotential-dependent adsorbate coverage, Ce oxidation state, and surface dipole potential in well-defined, dense thin film electrodes at ∼500 °C and a few hundred mTorr gas pressure. To unambiguously determine the origin of the surface dipole, we compared the behavior of three types of adsorbate chemistries: polar OH− (in a H2O/H2/Ar gas mixture), nonpolar tridentate CO2− 3 (in a CO2/CO gas mixture), and no adsorbate (in Ar gas). With OH− adsorbates on the ceria electrode surface, a strong linear correlation is observed between the surface potential and the adsorbate coverage. When the polar OH− is replaced by the nonpolar CO2− 3 , the surface potential becomes independent of adsorbate coverage. Likewise, in an inert atmosphere, the surface potential does not change with overpotential. These self-consistent observations point to the intrinsic dipole moment of polar adsorbates as the dominant contributor to the surface dipole (as proposed in Figure 2C), with other sources such as adsorbate/MIEC bonding (Figure 2A) and preferential accumulation of oxygen vacancies, electrons, and/or dopant (Figure 2B) contributing minimally.

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Figure 3. Experimental setup: The solid-state electrochemical cell is based on a yttria-stabilized-zirconia single crystal solid electrolyte. The bottom electrode is porous Pt sputtered onto a thin Sm-doped ceria interlayer. The top electrode is a thick PLD-grown Sm-doped ceria film with embedded Pt current collectors. While the overpotential is applied between the top and bottom electrodes, the top one is electrically connected with the spectrometer and grounded. Tunable soft X-rays shine onto the sample at grazing incidence, so that only a thin layer of gas molecules close to solid/gas interface are excited. See Experimental Section and Figure S1 for more details. voltage bias between the gold foil and the electron analyzer. A 1 eV shift of the gas phase core level corresponds to 0.96 eV change in the work function. The methods for quantifying the VB maxima and CL binding energy positions are described in a previous publication.21

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RESULTS AND DISCUSSION Oxygen CL spectroscopy was used to identify the adsorbate chemistry and to quantify the coverage on the surface of the ceria electrode (Figure 4). Taken at an information depth of 0.6 nm, the O 1s spectra are sensitive to the adsorbates. In H2O/ H2/Ar atmosphere (0.3 Torr), the O 1s spectra (Figure 4A) show principally two peaks: one at 528.8−529.6 eV binding energy corresponding to surface lattice oxygen, and the other at 531.1−531.9 eV corresponding to polar hydroxyl adsorbates incorporated on surface oxygen vacancies. The details of the assignment and quantification were reported in previous publications.21,22 The adsorbate coverage increases with cathodic overpotential, as the number of oxygen vacancies (serving as adsorption sites) increases. In CO2/CO atmosphere (0.3 Torr), the O 1s peak at a binding energy of 531.3− 532.1 eV corresponds to tridentate carbonate adsorbate (Figure 4B), which is essentially nonpolar. This assignment, validated in a recent work,22 is based on both C 1s and O 1s CL spectra. Like the hydroxyl adsorbate, the carbonate adsorbate coverage also increases with cathodic overpotential, though it saturates at 17% coverage due to adsorbate−adsorbate interaction.21,22 Finally, in Ar (0.1 Torr), as expected, no significant oxygencontaining adsorbate is detected spectroscopically (Figure 4C). To probe the electrostatics of the double layer at the electrochemical ceria/gas interface and study how it changes with the adsorbate chemistry, we measure the overpotential dependence of the work function (Φs), which is given by30 −ΔΦs = Δμe + eΔψSC + eΔχ (2)

EXPERIMENTAL SECTION

Sample Preparation. The fabrication procedure of the electrochemical cell has been reported previously.21 To briefly summarize, the samples are based on 10 × 10 × 0.5 mm Y0.16Zr0.84O1.92 (YSZ) (100) single-crystal substrates (MTI Corp.). The working electrode is a 450 nm thick Sm0.2Ce0.8O1.9 (SDC) thin film grown by pulsed-laser deposition (PLD/MBE 2300, PVD Products), with micropatterned Pt stripes embedded as the current collector. These interconnected stripes ensure a uniform current density along the sample surface. On the bottom side, the counter electrode is made of porous Pt sputtered onto a 50 nm thick SDC interlayer. A schematic of the sample cross section is shown in Figure 3. Electrochemical APXPS. Operando APXPS experiments were performed at the beamline 9.3.2 of the Advanced Light Source (Lawrence Berkeley National Laboratory), which is equipped with a Scienta R4000 HiPP electron analyzer.29 First, the SDC-based electrochemical cell was annealed at 500 °C in 5 × 10−5 Torr O2 to remove adventitious carbon and sulfur before dosing the following three gas mixtures in different experiments: a) 260 mTorr H2O/H2/Ar (1:8:4), b) 270 mTorr CO2/CO (25:2), and c) 110 mTorr Ar. A BioLogic SP-300 potentiostat was employed to bias the cell (Figure 3). The sample temperature was maintained at 500 °C and monitored using the potentiostatically determined ohmic resistance of the YSZ electrolyte. At each voltage bias, we waited for the system to reach equilibrium and then collected the valence-band (VB) and core-level (CL) spectra to monitor the changes of the electrochemical interface. The working electrode was electrically connected to the electron analyzer with a piece of gold foil, which was also grounded. The geometry of the electrochemical cell and testing conditions are such that the applied overpotential is dropped entirely at the ceria/gas interface. The overpotential at the working electrode was quantified via the rigid shift of the Ce 4d5/2 core level photoelectron peak. The binding energies of the photoemission spectra were calibrated to the Au 4f7/2 peak (83.8 eV). To calibrate binding energy shifts of the gas-phase core level with work function changes in the sample, we altered the electrical connections as shown in Supplementary Figure S1 and applied a

where Δ indicates variation relative to open-circuit condition. On the right-hand side, μe, ψSC, and χ correspond to the electron chemical potential, space-charge potential, and surface potential, respectively. Using CL and VB photoelectron spectroscopy, we deconvolve these contributions, while directly measuring the adsorbate coverage at the same time. We will 6235

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Figure 4. Rigid shifts of O 1s and Ce 4d core-level binding energy and valence-band maximum (VBM) with overpotential in A. H2O/H2/Ar, B. CO2/CO, and C. Ar. Upper panels: XPS spectra of O 1s, Ce 4d CLs and VB; vertical bars pinpoint the peak positions and VBM. Lower panels: CL and VBM binding energy as a function of overpotential. A 1 eV to 1 V dashed line is plotted for reference. The O 1s CL spectra have been normalized by the pre-edge background, while the VB spectra has been normalized by the integrated area of Ce 4d spectra. O 1s and VB spectra were measured at a photon energy of 790 and 250 eV, respectively, to attain the same information depth.

first discuss the quantification of Δμe from APXPS (typically the largest contribution to the overpotential-dependent work function), and subsequently the measurement of ΔΦS and eΔψSC, and finally that of eΔχ. When an overpotential is applied to the ceria thin film electrode, it exchanges oxygen with the gas phase in the process of equilibration (i.e., gaining or losing oxygen vacancies and electrons). Figure 4 shows the CL spectra of O 1s and VB spectra for the ceria electrode in H2O/H2/Ar, CO2/CO, and Ar. At cathodic polarizations, both the CL and VB spectra shift to higher binding energies (equivalently, lower electron chemical potential). It has been shown that the rigid shift of CL photoelectron peaks is a direct measure of Δμe.31−33 Quantification of binding energies at various overpotentials confirms that all peaks shift rigidly with one another, and that electrons are added/removed (effecting a change in the Ce oxidation state between 4+ and 3+). We use this rigid shifting of CL photoelectron peaks to directly obtain Δμe. Also evident in Figure 4, the binding energy shifts are linear with the overpotential, with a one-to-one correlation. In other words, we show experimentally that the electron chemical potential μe changes by approximately 1 eV per 1 V of overpotential applied to the ceria electrode. This is true regardless of adsorbate chemistry, because the electron chemical potential is only a function of the oxygen activity in the dense electrode. Our finding also agrees with the defect chemical prediction that, for a highly doped MIEC in which oxygen vacancy concentration is approximately fixed by doping (such as Sm-doped ceria), Δμe is precisely equal to the overpotential.33

At sufficiently cathodic overpotentials, the Ce 4f photoelectron peak, corresponding to the Ce3+ localized electrons,34,35 can be measured directly.36 Indeed, under both H2O/H2/Ar and CO2/CO atmospheres, the Ce 4f peak intensity increases with cathodic overpotential (Figure 4A,B), consistent with an increase of the electron chemical potential discussed above. Under pure Ar, the open-circuit condition is far more oxidizing than under H2O/H2/Ar and CO2/CO. Hence, the Ce 4f peak could not be detected directly. We estimate that the fractional Ce3+ concentration is no greater than 2.7% (calculated using a dilute solution approximation) under the most cathodic condition. Next, to measure the overpotential-dependent work function, we use the gas molecules (Ar or CO2) that lie between the electrode surface and the APXPS electron analyzer as probes.37−40 Specifically, the binding energy shifts of the gas molecule reflect the overpotential-dependent contact potential difference (CPD), which is the work function difference between the electron analyzer and the sample (CPD = ΦS − ΦA).30 With the analyzer work function approximately constant, ΔCPD yields ΔΦS. To avoid averaging the electrostatic potential gradient between the sample and the electron detector, we use a grazing-incidence X-ray to illuminate the gas molecules in the vicinity of the sample surface (Figure 3). We calibrate the gas phase peak shift to the voltage between a metallic sample (Au) and the electron analyzer: for every 1 V applied, the gas phase peak (and thus the CPD) shifted by 0.96 eV (Figure S1). After accounting for 6236

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Figure 5. Shift of gas and solid phase XPS CL peaks with overpotential in (A) H2O/H2/Ar, (B) CO2/CO, and (C) Ar. Upper panels: XPS spectra of gas phase CLs; vertical bars pinpoint the peak positions. Ar 2p, Ce 4d, and C 1s were measured using a photon energy of 490, 370, and 490 eV, respectively. Lower panels: CL shifts of the gas phase (Ar 2p or C 1s), the solid phase (Ce 4d), and the difference between the two. The errors of the reported slopes are estimated from the fitting error and the spectrometer resolution limit, which we take to be 0.1 eV.

free of adsorbates, the work function change is induced primarily by the change of the electron chemical potential and has negligible contribution from that of the outer work function. We first examine the possibility that the adsorbate-ceria polarization (Figure 2A) contributes to the outer work function changes by inspecting the chemical shift of the adsorbate relative to the lattice oxygen. This could be attributed to a change in the adsorbate geometry or in the bonding with the electrode surface. As shown in Figure 7A, neither OH− nor CO2− 3 binding energies exhibit a strong chemical shift with overpotential, despite the significantly different behavior of the outer work function. Of the minor variations observed, they are nonmonotonic with respect to the overpotential and are similar between the two types of adsorbates, unlike the behavior of the outer work function. A similarly weak and nonmonotonic correlation is observed when the chemical shifts are plotted against the adsorbate coverage (Figure 7B). Combined, these observations suggest that polarization of the adsorbate-ceria bond is independent of overpotential, and not likely linked to the outer work function. We briefly compare our results to literature reports of OH− on ceria. The observed OH− binding

this correction, the gas peak binding energy shifts directly yield the change in the work function of the electrode. For all three adsorbate environments (OH−, CO2− 3 , and no adsorbate), the binding energy of the gas phase peaks, and hence the work function, shifts linearly with overpotential (Figure 5). Specifically, for the OH− adsorbate, the electrode work function shifts by 1.6 eV per 1 V overpotential, while for the CO2− 3 adsorbate and no adsorbate, the work function shifts 1 eV per 1 V overpotential. Using eq 2, we subtract the contribution of electron chemical potential to the work function, leaving only the contributions from the gas/solid interface, namely, the electrostatic potential changes due to the space-charge layer and to adsorbate-induced dipoles (Figure 6). We denote this quantity as the outer work function. This contrasts with the electron chemical potential, which is sometimes called the inner work function.41 The outer work function changes by 0.6 ± 0.1 V per volt of overpotential for ceria covered with OH− adsorbates (Figure 6A), by 0.0 ± 0.1 V per volt of overpotential for ceria covered with CO32− adsorbates (Figure 6B), and by 0.1 ± 0.2 V per volt of overpotential in the absence of any adsorbate (Figure 6C). In other words, for ceria electrode surface covered with CO2− 3 and 6237

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Figure 6. Overpotential-dependent shifts of total work function (ΔΦS), and the corresponding inner work function (Δμe, electron chemical potential) and outer work function (eΔψSC + Δχ, space-charge and surface potential) contributions in (A) H2O/H2/Ar, (B) CO2/CO, and (C) Ar atmosphere. The values are plotted relative to open-circuit conditions. The top x-axis indicates the equivalent oxygen partial pressure, calculated by the Nernst equation.

between probing depths of 0.6 and 1.2 nm (obtained by varying the X-ray photon energy), Figure S2. The binding energies exhibit identical values, within error. This observation holds regardless of the nature of adsorbates (polar OH− vs nonpolar CO2− 3 ), and in the absence of any adsorbates (Ar atmosphere). Also, the change in fwhm of lattice O 1s peaks is less than 50 meV in magnitude for a 0.6 V applied bias for the three gas chemistries (Figure S3). The magnitude of the BE shift and full width at half maximum changes suggest that the surface space charge thickness is either much smaller or much larger than the XPS probing depth. The latter is inconsistent with our recent report of simultaneous enrichment of positively charged oxygen vacancy and negatively charged electrons near the surface of doped ceria, indicating that the system is close to being electroneutral within the probing depth.33 Thus, we conclude that the space-charge potential does not change significantly with overpotential (i.e., eΔψSC ∼ 0). We note that the probing depth here, though shallow, corresponds well to the expected space-charge layer thickness for doped ceria.45 The high concentration of oxygen vacancies (∼1027 m−3) effectively screens the surface charge without building up a significant electrostatic potential gradient. Additional support for this conclusion comes from the dependence of the outer work function on the adsorbate chemistry. With no adsorbates on the surface, the surface potential does not change. Thus, the only contribution to the outer work function change is that due to space charge potential. The fact that there is no change in the outer work function in Figure 6C suggests that a space charge potential, if present, is invariant with the applied overpotential. Furthermore, an overpotential-dependent outer function is observed only in the presence of OH− and not for CO2− 3 , in spite of both adsorbates being charged. This strongly suggests that an adsorbate-induced space charge potential in the nearsurface region of doped ceria is insignificant. Having ruled out adsorbate-ceria bonding and space-charge as significant contributor to the outer work function, we turn our attention to the intrinsic dipoles of the adsorbates as the source of the surface potential. Figure 8 plots the relative surface dipole potential against the adsorbate coverage. The

Figure 7. Binding energy (BE) difference between adsorbates and lattice O as a function of (A) overpotential and (B) adsorbate coverage.

energy here (2.1 to 2.3 eV vs lattice oxygen) is consistent with nonelectrochemical studies.42−44 In a previous electrochemical XPS study on adsorbate/ceria, a binding energy of 1.2 to 1.5 eV versus lattice oxygen was observed when the overpotential between +1.2 V and −1.2 V was applied.20 The difference is likely caused by a different type of adsorbate and/or the surface chemistry of ceria. Nonetheless, the weak overpotential dependence of the adsorbate binding energy reported in ref 20 and that of the outer work function observed in this work are consistent with electrostatic effects dominating chemical shifts. Next, we examine the possibility that the polarization in the space-charge region (ΔψSC) is the main contributor to the outer work function. The space-charge potential arises due to diffuse charge within the ceria electrode which screens the excess charge near the surface, e.g., due to preferential segregation of oxygen vacancies, localized electrons, and/or dopant to the surface (Figure 2B) or those of the adsorbates (Figure 2C). The effect of space charge potential on photoelectron spectra is most apparent when the XPS information depth and the surface space charge thickness are comparable. We study this by plotting the CL and VB shift 6238

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density of the total available adsorption sites, θ is the adsorbate coverage, and ε0 is the vacuum permittivity. This model is valid for dilute adsorbate coverage where the adsorbate−adsorbate interaction is weak. Using eq 3, the slope in Figure 8 is simply eΔχ/Δθ = e μ⊥ ρ0/ε0. For the OH− adsorbate, the slope is 5.2 ± 0.5 eV/ML giving a μ⊥ of 0.92 ± 0.09 D, which is about 55% of the dipole moment of a free OH− ion (1.66 D).46,47 The strong linearity in Figure 8 suggests that the model is valid, and that the surface potential arises mostly from the intrinsic dipole. A number of possibilities may account for the smaller OH− dipole moment along the surface normal direction. The OH− group could be similar to a free OH radical in terms of charge distribution but is tilted by about 30° relative to the surface normal. This has been reported in prior density functional calculations studying H2 and H2O adsorption on ceria.44,48−50 Another possibility is that the O−H bond is weakened as a result of charge transfer from the ceria electrode surface. For the CO2− 3 adsorbate, on the other hand, the slope is 0.0 ± 0.3 eV/ML, giving a μ⊥ of 0.00 ± 0.05 D. A recent study combining angle-dependent near-edge X-ray absorption fine structure and density-functional calculation revealed that the most stable form of adsorbate when CO2 molecules interact with CeO2 (100) surface is a flat-lying tridentate CO2− 3 , which has a negligible dipole moment in the surface normal direction.51 The consistency between the surface potential and the polarity of the adsorbate provides additional support to our proposal that the intrinsic dipoles dominate the electrostatic double layer. Our work clarifies the role of the electrostatic double layer during ion incorporation reactions in ceria-based electrodes. In

Figure 8. Dependence of surface dipole energy on the coverage of OH− and CO2− 3 adsorbates. The slopes of fitted lines are reported in the unit of Debye, and the slope errors are estimated from the fitting errors.

OH− coverage increases systematically with cathodic overpotential, from 0.04 to 0.13 monolayer. At the same time, the surface potential increases by 0.5 eV. The CO2− 3 coverage, on the other hand, increased from 0.07 to 0.17 monolayer but the surface potential changed negligibly. From the linear curves in Figure 8, we extract the out-of-plane dipole moment of the adsorbates. We employ a simple Helmholtz model3 eΔχ = eμ⊥ (ρ0 Δθ )/ε0

(3)

where χ is the surface dipole potential, μ⊥ is the component of the adsorbate’s dipole moment normal to the surface, ρ0 is the

Figure 9. Elucidating the origin of surface dipole potential by comparing the three adsorbate chemistries on the surface of ceria. (A) H2O/H2/Ar, (B) CO2/CO, and (C) Ar atmosphere. The first column shows the physical structure of the interfacial region that connects the gas and solid phases. The second column shows how the adsorbate coverage and surface electron concentration change with overpotential. The third column shows the corresponding changes of electrostatic potential. 6239

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Chemistry of Materials the case of OH− adsorbates, the average surface dipole increases in magnitude with increasingly cathodic polarizations during water dissociation (and the reverse for anodic polarizations). Chemically, this reflects the increase in OH− coverage with increasing cathodic overpotential, which is coupled to the increase in oxygen vacancy adsorption sites. Electrostatically, the change of the surface dipole can modulate the rate of charge-transfer reactions in order to equate the ion and electron transfer rates, analogous to the role of electrostatic-field-induced drag in bulk ambipolar diffusion. The increase in dipole strength with overpotential during water dissociation facilitates the transfer of electrons from the oxide surface to the oxygen−hydrogen bond in OH−. This is consistent with our previous speculation that the electron transfer between the ceria and OH− is the rate-determining step.21 In the case of CO2− 3 , the lack of a significant dipole could imply that ion and electron transfer rates are similar (analogous to the specific case of ambipolar diffusion having comparable diffusivity for the positive and negative species).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

Z.A.F., X.Y., and W.C.C. conceived the experiment. Z.A.F., Z.G., B.J., and E.C. performed the experiments. Z.A.F. prepared the samples and analyzed the data. All authors contributed to writing the manuscript. W.C.C. supervised the project. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Award No. 1336835, and by the Global Climate Energy and Project at Stanford. Z.A.F. was supported additionally by the Gordon and Betty More Foundation through Grant GBMF2573, and X.Y. was supported additionally by the Stanford Graduate Fellowship. The Advanced Light Source is supported by the Director, Office of Science, and Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Z. Liu, R. Chang, B. Mao, and S. Axnanda for their support at the Advanced Light Source. Lastly, we are grateful for the insightful discussion with Prof. Z.-X. Shen.

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CONCLUSIONS Using operando ambient pressure XPS, we quantified the surface dipole potential on SDC and its dependence on overpotential under three gas environments, as summarized in Figure 9. The main contributions to the overpotentialdependent work function are the shift in the Fermi level (inner work function) and in the surface potential (outer work function). In all three adsorbate chemistries (OH−, CO2− 3 , no adsorbate), the near-surface Fermi level determined within the top 0.6 nm behaves independently of the adsorbate. On cathodic polarization, the electrode forms Ce3+ and oxygen vacancies, and the Fermi level moves toward higher energy, filling the Ce 4f electronic state. On anodic polarization, the reverse takes place. For both cathodic and anodic overpotentials, the Fermi level shifts 1:1 with the applied overpotential, as a result of the high Sm doping level. In H2/ H2O (Figure 9A), the strong intrinsic dipole of the OH− adsorbates contributes an additional 0.6 eV per V of overpotential to the work function. The surface potential contribution vanishes (i.e., the work function changes by 1 eV per V of overpotential) upon switching the gas to CO2/CO mixtures (Figure 9B), where the adsorbate is nonpolar tridentate CO2− 3 . Despite having comparable O 1s binding energy shifts with respect to lattice oxygen, the OH− and CO2− 3 exhibit very different surface potentials. This suggests that the dipole does not stem from the adsorbate-MIEC bonding. On the adsorbate-free surface, the surface dipole is independent of the overpotential (Figure 9C), indicating that a double layer potential from localized electrons (Ce3+)/dopant (Sm3+) and oxygen vacancies accumulation in the near surface region is negligible. Further corroborating this interpretation is the similar absence of overpotential-dependent dipole for a CO2− 3 covered surface. Our work advances the understanding of the double layer in electrochemical MIEC/gas interfaces and paves the way toward understanding how electrostatics affect ioninsertion kinetics.

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Experimental setup; Ar 2p spectra; gas-phase peak shift; average change in binding energy for H2O/H2/Ar, CO2/ CO, and Ar atmosphere; fwhm of lattice O 1s peak relative to OCV in H2O/H2/Ar, CO2/CO, and Ar atmosphere (PDF)

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DOI: 10.1021/acs.chemmater.6b02427 Chem. Mater. 2016, 28, 6233−6242

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