SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy Chunjuan Zhang, 1 Michael E. Grass, 3 Anthony H. McDaniel, 5 Steven C. DeCaluwe, 2 Farid El Gabaly, 5 Zhi Liu, 3 Kevin F. McCarty, 5 Roger L. Farrow, 5 Mark A. Linne, 5 Zahid Hussain, 3 Gregory S. Jackson, 2, * Hendrik Bluhm 4, * and Bryan W. Eichhorn 1, * 1 Department of Chemistry and Biochemistry, 2 Department of Mechanical Engineering, University of Maryland, College Park, MD Advanced Light Source, 4 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA Sandia National Laboratories, Livermore, CA SEM images SEM images are shown in Fig. S1 for the three cells with 50 nm, 100 nm and 250 nm thick ceria films. All images show ~20 nm grain sizes for sputtered ceria films, and there is no observable difference for the ceria mophology. Figure S1. SEM images for (a) 50-nm, (b) 100-nm and (b) 250-nm-thick ceria cells. The YSZ substrate grain boundaries are easily seen in these images. The sputtered ceria films show ~20 nm grain sizes. 2. Calculating surface potential and oxidation state changes from XPS measurements When a potential is applied to a surface, the kinetic energy of the photoelectrons increases or decreases with the local surface potential in the area from which the photoelectrons are emitted. For example, when a +1.2V potential is applied to the Pt electrode of our experimental cell, the kinetic energy of the photoelectrons emitted from the Pt electrode is reduced by 1.2 ev, corresponding to an equivalent increase in the apparent binding energy. The Au electrode, however, is grounded and as such the associated photoelectron peaks do not shift. Therefore, when the Pt electrode is biased, the surface potential changes across the cell, revealing the local potential (relative to open circuit conditions) of the probed surface. This point is illustrated in Fig. S2, which shows part of the Ce 4d spectrum recorded at 490 ev at beamline at five different applied potentials, all recorded at spot 3 for the 50 nm ceria cell (see Fig. 2 inset). The surface potential changes at spot 3 can be measured using the changes in the apparent binding energy as a function of applied bias. The difference between the peak energy at OCV (zero volts) and at an applied bias therefore gives the cumulative potential drop between the grounded Au pad and the location where the spectrum was taken. nature materials 1

2 supplementary information We considered the error associated with the surface potential measurements taken at beamline (Fig. 4) because there is a greater data density than for that taken at beamline The error associated with the local potential measurement can be estimated by taking the standard deviation of the potentials measured over the region on top of the Au pad (not shown, similar to Figure 4a-c). The standard deviation is 16 mv when each fit spectrum covers 30 pixels from the detector CCD (ca. 30 microns at the sample). This error is mainly due to uncertainty in spectrum fitting. Other factors, such as inhomogeneity in the samples and chemical state changes during the experiments, can also contribute to the overall experimental error. The overall accuracy associated with data taken over a long duration appears to be ca. 50 mv. For the data presented in Figure 4, the resolving power of beamline was E/ΔE = 3000, which means the beamline energy resolution was ca. 160 mev at 490 ev photon energy; the resolution of the spectrometer does not affect the overall resolution in this case. For all other data presented, the resolving power of beamline was also E/ΔE = 3000, leading to an energy resolution of about 160 mev at 490 ev photon energy. The potential accuracy for the GAMRY PCI4/750 potentiostat is ±2 mv. The percentage of the ceria electrode in the Ce 3+ state is determined by analysis of the Ce 3d spectra taken at an incident photon energy of 1180 ev. Due to final state effects, the Ce 4+ spectrum consists of three spin orbit doublets, which have a spin-orbit splitting of ~18.4eV. The consensus in previous studies 1-4 is that the highest binding energy peaks result from a Ce3d 9 O2p 6 Ce4f 0 final state. The lower binding energy peaks result from a mixture of Ce3d 9 O2p 5 Ce4f 1 and Ce3d 9 O2p 4 Ce4f 2 final states. Similarly, the Ce 3d spectrum of Ce 3+ consists of two doublets and one satellite doublet. These doublets correspond to a mixture of Ce3d 9 O2p 5 Ce4f 1 and Ce3d 9 O2p 4 Ce4f 2 final states. Ce 4+ and Ce 3+ reference spectra were created using maximally oxidized and reduced spectra from our measurements. The generated reference spectra are in excellent agreement with literature results. 2,4 All subsequent spectra were fit with a linear combination of the self-generated reference Ce 3+ and Ce 4+ spectra. To account for apparent binding energy shifts with applied bias, the binding energy of the reference spectra is allowed to shift, although the relative position of the two spectra is held constant. The Ce 3+ percentage reported in the text is determined by the ratio of the Ce 3+ spectrum area to the total fitted area. Fitted Ce 3d spectra recorded on the 50 nm ceria cell at spot 3 are shown in Fig. 3a. The ceria at that spot at -1.2 V is significantly oxidized (38% Ce 3+ ) relative to open circuit (51% Ce 3+ ), while it is only slightly reduced at +1.2 V (56% Ce 3+ ). 2 nature MATERIALS

3 supplementary information &9$;.+6!%&!"&!&& / !9".6 8&9$.6 &.6 :&9$.6 :!9".6 &9<#.+6 )'"$*!ν"#"$%&"'(!"#!"$!"%!""!"&!!# '(()*+,-./0,10,2.3,+* Figure S2. Spectra recorded at spot 3 on the 50 nm cell show rigid shifts in the apparent binding energy of Ce 4d (490 ev incident photon energy) at different biases (indicated with text labels in the same color of the corresponding spectrum). Shifts in kinetic energy relative to open circuit indicate cumulative potential drops from the Au pad to spot 3. The inset shows a wider energy window with the full Ce 4d region and the Si 2p peak from Si 4+ at ~101 ev. a 1.0 b KE (ev) 0.0 KE (ev) Bias Pt4f_spot1 Zr3d_spot2 Ce4d_spot3 Au4f_spot6 or Bias Ce4d_spot2 Ce4d_spot3 Ce4d_spot6 Ce4d_spot Bias (V) Bias (V) Figure S3. (a) Kinetic energy shifts (ΔKE) of Au4f, Ce4d, Zr3d and Pt4f spectra at different biases for selected spots recorded on the 50 nm ceria cell. (b) ΔKE of Ce4d spectra at selected spots vs. applied bias. The ceria above the gold pad does not shift (grounded) while ceria above the YSZ shifts with applied bias. The XPS sampling positions across the cell are shown in the inset of Fig. 2. nature materials 3

4 supplementary information XPS spectra were collected approximately in a line along the cell current direction between the Pt CE and the Au current collector. Fast surveys were conducted while moving the XPS sample manipulator (x,y,z coordinates were recorded). Phase boundaries exposed on the cell surface, such as Pt- YSZ and ceria-ysz, were easily located upon the appearance / disappearance of one material or the other from the observed survey spectra. Although the gold pad was buried underneath ceria films, some gold signals can still be collected due to 1) exposed side edges and 2) spilled Au species observed as isolated domains on the ceria films. The transition from isolated Au patches to the buried Au current collector was observed as the location where the apparent Au binding energy no longer varied with applied bias, and served as a reference location for all other measurements across the cell. Finally, the testing spot positions were located on the cell schematic drawings by examing the manipulator coordinates and the optical cell images. The x axis in the figures of this manuscript represents the distance of any probing spot to the gold edge (facing the Pt CE), where x is set to zero. Negative x values are toward the Pt CE.!"# $% &'()*+#&,-.&/"0 $ &*6 311&*6 &%3.2&0 &/"0 &43.2& &* & Figure S4. Spatially-resolved percentage of Ce 3+ changes under applied bias relative to equilibrium states at OCV for the three cells with difference ceria film thicknesses (left: 250 nm; center: 100 nm; right: 50 nm). The sampling spots are marked for the 50-nm-thick ceria film (the cell geometry and spot positions are shown in Fig. 2 inset). Both the 100-nm-thick and 50-nm-thick ceria films show similar magnitude of % Ce 3+ changes for electrolysis (at +1.2V) and H 2 oxidation (at -1.2V). Larger cerium oxidation state changes were observed on thinner cells. All the three cells show a consistent trend in the surface redox changes for both electrolysis and H 2 oxidation, though no obvious changes were observed for H 2 oxidation on the 250-nm-thick ceria film. 3. Calculations of ΔV L(vertical), V L(YSZ) under a cell bias of +1.2V Since the ionic species essentially move in a net vertical direction through the ceria films, the local potential losses between the top and bottom of the ceria films (ΔV L(vertical) ) in the vertical direction represent a local measure of driving force for net surface reactions (the local currents). The values of ΔV L(vertical) can be derived from the difference between the V L(Ceria) and V L(YSZ) at any point of lateral displacement from the Au current collector (x) as shown in Eq. S1. ΔV L(vertical) = V L(YSZ) V L(Ceria) (S1) 4 nature MATERIALS

5 supplementary information V L(Ceria) is directly measured from the XPS experiments and V L(YSZ) is derived from approximation of linear potential distribution on YSZ that has boundary conditions from electrochemistry (at the gold edge, x = 0 mm, Eq. S2) and XPS (V L(Zr) at spot 2, x = -0.5 mm vs. the gold edge, Eq. S3), which can be calculated from Eq. S4. V L(YSZ) (0) = V cell ΔV L(Pt-YSZ) - ir Bulk V L(YSZ) (-0.5) = V L(Zr) (spot 2) (S3) V L(YSZ) (x) = V L(YSZ) (0) + 2[V L(YSZ) (0) - V L(YSZ) (-0.5)]x = ΔV L(vertical) + V L(Ceria) where V L(YSZ) (0) and V L(YSZ) (-0.5) are plotted as gray dots in Fig. S5, and V L(YSZ) (x) are the gray solid lines between them. The green open squares are the XPS-detected V L(Ceria) values, and the green solid lines are the smooth fittings for V L(ceria). With this information, the local vertical potential losses (ΔV L(vertical), red solid curves in Fig. S5) to drive oxide ions from the ceria surface into the YSZ electrolyte can be calculated as a function of x according to Eq. S1. The cell currents were obtained from the LSV measurements (holding the applied potentials at V cell = +1.2V), while the V L(Ceria) values were recorded from the APXPS Ce4d KE shifts. The ΔV L(Pt-YSZ) (spot 1) and V L(Zr) (spot 2) were obtained from the KE shifts in the Pt4f and Zr3d spectra (shown in Fig. 2). The ohmic resistances (R bulk ) of YSZ were detected in separate EIS measurements under the same conditions. (S2) (S4)!!""!#!"# #"$ #"% #"& #"' 250 nm thick ceria V L(YSZ) V L(Ceria) V L(vertical) $ %&' () *,-./ $ %&' () *+,-./ 50 nm thick ceria &# -# '#!# %&' () #"# (#"& (#"' #"# #"' (#"& (#"' #"# #"' x)*++, x)*++, Fig. S5. Spatially-resolved lateral surface potentials and calculated overall vertical potential losses through the ceria films (left: 250-nm-thick; right: 50-nm-thick) based on simultaneous XPS and LSV measurements under a cell bias +1.2V. The spatially-resolved ceria surface % Ce 3+ shifts away from the equilibrium values, recorded in APXPS Ce3d spectra under the same conditions, are shown for the 250- nm-thick (black) and 50-nm-thick (blue) ceria films respectively to the right axis. Here, all the point data were from direct XPS or electrochemical measurements, while all the solid curves were smooth fittings based on experimental data. Fitting the V L(Ceria) curve for the 50-nm-thick ceria cell required the use of an arbitrary point at x ~ -0.2 mm. # nature materials 5

6 supplementary information Reference: 1 Burroughs, P., Hamnett, A., Orchard, A.F., & Thornton, G., Satellite structure in x-ray photoelectron-spectra of some binary and mixed oxides of lanthanum and cerium. J. Chem. Soc., Dalton Trans., (1976). 2 Mullins, D.R., Overbury, S.H., & Huntley, D.R., Electron spectroscopy of single crystal and polycrystalline cerium oxide surfaces. Surf. Sci. 409, (1998). 3 de Groot, F. & Kotani, A., A Core Level Spectroscopy of Solids (CRC Press, Taylor & Francis Group, Boca Raton, 2008). 4 Trudeau, M.L., Tschöpe, A., & Ying, J.Y., XPS investigation of surface oxidation and reduction in nanocrystalline Ce x La 1-x O 2-y. Surf. Interf. Anal. 23, (1995). 6 nature MATERIALS