SrIrO3 Catalysts

Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 1516−1522

pubs.acs.org/JPCL

Oxygen Evolution Reaction Activity in IrOx/SrIrO3 Catalysts: Correlations between Structural Parameters and the Catalytic Activity Kyuho Lee,*,§,† Motoki Osada,†,‡ Harold Y. Hwang,†,¶ and Yasuyuki Hikita¶ §

Department of Physics, Stanford University, Stanford, California 94305, United States Geballe Laboratory for Advanced Materials, Department of Applied Physics, Stanford University, Stanford, California 94305, United States ‡ Materials Science & Engineering, Stanford University, Stanford, California 94305, United States ¶ Stanford Institute for Materials & Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States

J. Phys. Chem. Lett. Downloaded from pubs.acs.org by TULANE UNIV on 03/20/19. For personal use only.

†

S Supporting Information *

ABSTRACT: Understanding how structural properties affect the oxygen evolution reaction (OER) of a catalyst can reveal important information not only on the catalytic mechanism but also on the general design strategy of OER catalysts. We report a variation of ∼0.15 V in the overpotential of the recently discovered IrOx/SrIrO3 OER catalysts, which directly correlates with the structural parameters of the as-synthesized SrIrO3 epitaxial films. This variation is caused by both extrinsic area enhancement and intrinsic electronic structure modification driven by defect formation. These correlations not only indicate that microscopic film defects play an important role in the activity of the IrOx/ SrIrO3 catalyst but also provide readily accessible parameters predictive of the activity posttransformation to IrOx/SrIrO3. Establishing strong associations between the catalytic activity and key structural and electronic parameters, rather than synthetic variables, provides important guidance to control and study these complex catalysts independent of the synthetic technique.

T

operation.9 However, the OER mechanism behind the observed activity enhancement or the structure of the active IrOx layer are yet to be well understood. This calls for a systematic study on the microscopic formation process and the atomic and electronic structure of the IrOx/SIO catalyst, as it will help unveil the mechanisms behind the high catalytic activity in IrOx/SIO and give new insights for further OER catalyst development. However, such an investigation requires that a stable synthetic platform for the catalyst be established and that the as-synthesized structural variables relevant to the catalytic activity be identified. This is because the catalytic activity in general is nontrivially influencedon the order of 450 mV in the overpotential for some cases10by the structural properties of the catalyst, such as the average stoichiometry and crystallinity.10−14 Considering the wide range of structural and physical properties that can be sensitively varied in SIO epitaxial thin films,15,16 understanding how these properties affect the catalytic activity is especially crucial for probing the IrOx/SIO catalyst. In addition, identification of the relationships between the as-synthesized structural properties and the catalytic activity can provide

he increasing demand toward clean, sustainable energy has motivated vast research in energy storage mechanisms to overcome the drawbacks of the intermittent nature of leading sources of renewable energy, such as solar and wind.1−4 Among various approaches, the conversion of electrical energy into hydrogen gas via water electrolysis stands out as a promising way to store energy.5,6 The key to improving the overall efficiency of water electrolysis lies in the development of high-performance catalysts for the oxygen evolution reaction (OER), the rate-limiting half-reaction of water electrolysis involving transfer of four electrons.4,7 In particular, the development of acid-stable OER catalysts is of great interest due to their compatibility with polymer electrolyte membranebased water electrolyzers, which show higher current density output and lower gas crossover rate than conventional alkaline electrolyzers.8 A recent finding is the observation of remarkably high OER activity in the perovskite-SrIrO3 (SIO) epitaxial thin film catalyst synthesized by pulsed laser deposition (PLD).9 Contrary to theoretical predictions, this catalyst exhibits the highest OER activity in acid, with current output surpassing that of IrOx or RuOx by more than an order of magnitude.9 This surprising result is attributed to the operando formation of an intrinsically more active iridium−oxygen composite surface layer on the perovskite−SIO film (IrOx/SIO), driven by Sr leaching during the initial period of electrochemical © XXXX American Chemical Society

Received: January 20, 2019 Accepted: March 13, 2019

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DOI: 10.1021/acs.jpclett.9b00173 J. Phys. Chem. Lett. 2019, 10, 1516−1522

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The Journal of Physical Chemistry Letters

Figure 1. Characterization of a representative sample grown with substrate temperature of 680 °C and oxygen partial pressure of 300 mTorr at laser fluence of 1.76 J cm−2. (A) XRD 2θ-ω scan, showing clear SIO 00l peaks along with STO 00l substrate peaks (indicated by *). (B) Rocking curve measurement around the SIO film 002 peak (circle markers) with two fit curves (dashed and dot-dashed curves) and their sum (solid curve). (C) AFM surface topography of the as-synthesized SIO thin film. (D) Plot of the six CV curves of the electrochemical sequence, showing the second sweep for each CV sequence. (E) EIR;10 versus operating time. The values extracted from the 6 CV measurements are shown as markers, and the CP measurement is shown as a solid curve. Error bars of the markers are the standard deviations from the 3 CV sweeps. (F) XPS Sr3d and Ir4f spectra before (dotted curve, top) and after (solid curve, bottom) electrochemical operation.

in the rocking curve around a symmetrical diffraction peak corresponds to the distribution of crystalline planes slightly tilted from the nominal c-axis, this indicates that the narrower peak corresponds to the strained part of the film closer to the substrate, while the broader peak corresponds to the mosaic part of the film closer to the surface. The FWHM of the broader peak gives, therefore, a measure of the film mosaicity. The large variation in FWHM from the varied growth conditions shows that SIO can be stabilized over a wide range of crystalline quality. While the variation in FWHM is typically associated with the film lattice parameters and the film thickness by the introduction of dislocations due to in-plane lattice relaxation,19 we observed weak correlations with respect to these two parameters (Figure S2). This suggests that the film mosaicity in the present case is likely driven by the kinetics during growth, modifying the Sr/Ir ratio in the deposited films. Flat film surfaces were confirmed from atomic force microscopy (AFM) measurements, with an average roughness Ra of 0.97 ± 0.84 nm over all samples (Figure 1C). Furthermore, the previously observed enhancement in the activity over operating time9 was reproduced, as seen from the cyclic voltammetry (CV) curves (Figure 1D) and the decreasing trend in the plot of the IR-corrected (i.e., subtracting the potential from the uncompensated resistance) potential required to reach a current density of 10 mA cm−2geo versus time (EIR;10) (Figure 1E); here, cm−2geo corresponds to the geometrically projected electrode surface area. The associated loss of Sr from the film during electrochemical operation9 was also confirmed from X-ray photoelectron spectroscopy (XPS) (Figure 1F). While it could be possible

mechanistic insights on the transformation of SIO into IrOx/ SIO, which can inform the development of other complex oxide electrocatalysts as well. Correlating the catalytic activity with the structural properties rather than growth parameters is also important, as it can resolve discrepancies in the catalytic activity arising from different growth techniques.17 With these motivations, we have systematically synthesized SIO epitaxial films, with controlled variations in the structural properties, on a series of single-crystal oxide substrates using PLD.15 We characterized the film thickness, in-plane film electrical resistivity at room temperature (ρ), tilt mosaicity, in-plane (af) and out-of-plane (cf) lattice constants, and surface strontium-to-iridium (Sr/Ir) stoichiometry of the films. By comparing these results to the electrochemical properties of the same films, we have determined the key structural and electronic parameters relevant to the high catalytic activity and identified strong correlations among these parameters. The growth of single-phase c-axis oriented perovskite−SIO films was confirmed by the series of SIO 00l peaks in the X-ray diffraction (XRD) 2θ-ω scans of the films (Figures 1A and S1A). As previously reported,18 rocking curve measurements around the SIO 002 peaks showed a superposition of two peaks, one narrow and one broad (Figure 1B). The full width at half-maximum (FWHM) values of the two peaks were obtained from the curve by fitting the data with two mixed Gaussian-Lorentzian curves. An average FWHM of 0.06 ± 0.02° was obtained for the narrower peak for all samples, slightly larger than the average FWHM of 0.03 ± 0.01° for the substrates, while the broader peaks showed varying FWHMs ranging from 0.04° to 1.21° (Figure S2). Because the FWHM 1517

DOI: 10.1021/acs.jpclett.9b00173 J. Phys. Chem. Lett. 2019, 10, 1516−1522

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The Journal of Physical Chemistry Letters

Figure 2. Characterization results of four representative samples, indicated by circle, triangle, square, and diamond markers. (A) Tafel plot of the fourth CV measurement for each sample. (B) XRD 2θ-ω scans, showing variation in the SIO 002 film peak positions. (C) Rocking curves around the SIO 002 film peaks. (D) XPS Sr3d and Ir4f spectra, showing a larger Sr/Ir ratio for more active samples. (E) XPS Sr3d peak deconvolution of the four samples, showing larger SrO signal (dashed doublet) for more active samples.

that the remaining Sr3d signal after electrochemical operation could be due to some of the film surface retaining the original SIO form, the increase in film surface roughness after electrochemical measurement indicates that this is rather unlikely (Figure S3). Instead, the remaining Sr3d signal after electrochemical operation is attributed to the contribution of the bulk of the film in the XPS spectrum due to the estimated X-ray penetration depth of 7 nm at the incident photon energy of 1486.60 eV. The film thickness was obtained from X-ray reflectivity measurements (Figure S4). Structural and electrochemical characterization of the samples revealed nontrivial variations in the structural properties and the catalytic activity, as illustrated by four representative samples with different activity (Figure 2). The growth conditions of the four samples are shown in Table S1. A spread of ∼0.15 V in EIR;10 was observed over all samples (Figures 2A and S5). Variations in cf and film mosaicity were observed from the shifts in XRD 002 film peaks (Figure 2B) and different rocking curve FWHMs (Figure 2C) respectively. Interestingly, XPS measurements revealed that the samples with higher activity have a larger Sr/Ir ratio (Figure 2D). Furthermore, the Sr3d peak deconvolution showed existence

of two doublets (Figure 2E), the lower binding energy doublet corresponding to the perovskite Sr and the higher binding energy doublet corresponding to the rock-salt SrO,20−22 consistent with previous reports.9 As observed in other Srbased perovskite thin films,22−24 these results indicate the presence of a SrO segregation layer on top of the as-grown SIO film, the thickness of which is shown to scale with the catalytic activity (Figure 2E). Out of the series of six CV measurements, we selected EIR;10 for the fourth CV as the benchmark for the catalytic activity of each sample. (All CVs showed identical trends; the plot of EIR;10 for all six CVs is shown in Figure S5.) Plots of the catalytic activity against representative structural properties of the as-synthesized films revealed that the activity is linearly proportional to the FWHM (Figure 3A) and the logarithm of ρ (Figure 3B). Meanwhile, the activity has minimal correlation with the film thickness (Figure 3C) or cf (Figure 3D). The samples grown on different substrates do not show notable deviations from the trends established by the samples grown on STO substrates (Figures 3A−D). The plot of the catalytic activity against af on these samples (Figure 3E) does not show a notable trend either, indicating that af also plays a minimal 1518

DOI: 10.1021/acs.jpclett.9b00173 J. Phys. Chem. Lett. 2019, 10, 1516−1522

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Figure 3. EIR;10 obtained from the fourth CV measurement for all samples plotted against (A) FWHM of the broader peak in the rocking curve around the SIO 002 peak, (B) in-plane electrical resistivity at room temperature (ρ), (C) film thickness, and (D) out-of-plane lattice constant (cf). Samples in Figure 2 are indicated by their corresponding markers. (E) EIR;10 obtained from the fourth CV measurement plotted against in-plane lattice constant (af) for the samples grown on different substrates labeled on the graph.

strongly correlated parameters, which has been explicitly verified through regression analysis (see the Supporting Information), both effectively predicting the changes in the catalytic activity induced by mosaicity-driven alterations in the film texture. This can be verified by plotting EIR;10 against the geometricarea-normalized double-layer capacitance (Cdl/Ageo), which is directly proportional to the number of active sites per unit geometrical area (Figures 4B and S6). Due to the direct proportionality of the exchange current with the electrochemically active surface area (ECSA), one expects from the Tafel equation that for a fixed current density the overpotential scales as the negative logarithm of ECSA, which is precisely what is observed in Figure 4B. This is also confirmed from the vertical trend in the plot of Cdl/Ageo versus the Cdl/Ageonormalized current at a fixed potential (Figure S7). Hence, this is a strong indication that the ECSA enhancement due to defect formation and continuous Sr leaching plays a significant role in the observed variations in the OER activity of this catalyst. However, this does not necessarily mean that the activity enhancement over the samples can be wholly attributed to area enhancement. The vertical spread of ∼50 mV in Figure 4B suggests that some films are intrinsically more active than others. Namely, the IR-corrected potential required to reach the ECSA-normalized current density of 5 mA cm −2 (EIR;5,ECSA), which reflects the intrinsic catalytic activity, plotted against the measured as-grown film parameters reveals that the intrinsic catalytic activity reasonably correlates with ρ (Figure 4C), film mosaicity (Figure 4D), and film thickness (Figure 4E) but weakly with the lattice constants (Figure S8). (We note that we took the ECSA-normalized current density

role in the enhancement of the catalytic activity. This insensitivity of the catalytic activity on the lattice constants is somewhat surprising, considering how many of the 3d transition metal oxide catalysts show strain-dependent activity.14,25 This difference implies that the Sr-deficient surface IrOx, which would be less affected by substrate strain than SIO itself, is the catalytically active species for OER.9 The observed relationships of the catalytic activity with the SrO segregation layer thickness, FWHM, and ρ of the assynthesized SIO films can be understood coherently by linking them with defect formation in the SIO films. The increase in the FWHM of the as-synthesized SIO thin films indicates a larger population of crystallites with different tilt angles, inducing a higher density of domain boundaries. This results in larger ρ due to enhanced scattering from these domain boundaries26,27 and the formation of a thicker SrO surface segregation layer due to accelerated diffusion of Sr via domain boundaries toward the surface in the as-synthesized state (Figure 4A, bottom).28 Statistical analysis between ρ and FWHM has been included in the Supporting Information. The drastic decrease of the XPS Sr3d signal during the initial stages of electrochemical operation indicates that the SrO segregation layer is removed at the early stages of operation, as expected from its highly hygroscopic property, revealing the film beneath which will have a larger effective surface area due to increased mosaicity (Figure 4A, middle). This results in a larger number of IrOx active species exposed to the electrolyte and, thus, higher catalytic activity. The enhancement of the catalytic activity during operation can be understood as a result of continuous formation of IrOx active species via Sr leaching from the perovskite structure (Figure 4A, top). This interpretation is in agreement with the FWHM and ρ being 1519

DOI: 10.1021/acs.jpclett.9b00173 J. Phys. Chem. Lett. 2019, 10, 1516−1522

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Figure 4. (A) Schematic of a high-activity film as-grown (bottom), at the initial stages of electrochemical operation (middle), and during steadystate electrochemical operation (top). The gray arrow indicates the temporal direction. (B) EIR;10 obtained from the six CV measurements versus the double-layer capacitance (Cdl) normalized by the geometric area of films (Ageo), followed by plots of EIR;5,ECSA obtained from six CV measurements versus (C) ρ, (D) FWHM of the broader peak in the rocking curve around the SIO 002 peak, and (E) film thickness. Solid markers are samples grown on non-STO substrates.

of 5 mA cm−2 instead of 10 mA cm−2 as the reference to avoid loss of data points due to the decreased current density when normalized by a larger area; see Figure S9.) The three correlating parameters are impacted by the defect-driven changes in the electronic structure of the IrOx active species. The increase in ρ, while reflecting an increase of domain boundaries, could also be an indication of a change in the intrinsic electronic structure of the SIO film, especially given the complexity of the band structure around the Fermi level in SIO, which is sensitively dependent on the local configuration of the IrO6 network.16,29 Formation of defects, which are probed by ρ and the FWHM and encouraged by thicker film growth, can also affect the local electronic structure of the SIO film.30,31 Such modifications to SIO could propagate to atomic and electronic structures of the IrOx active species, driving intrinsic catalytic activity enhancement. Small fluctuations over time in the intrinsic activity of individual samples (Figures 4C−E and S10) also suggest that the intrinsic activity is sensitive to the structural changes of the IrOx active species. Further studies are called for to understand the atomic and electronic structure of this catalyst and its relation to the other recently reported acid-stable ternary oxide catalysts.32−36 In summary, we examined the relationships between the assynthesized structural variables of SIO epitaxial thin films and their OER catalytic activity post electrochemical transformation into IrOx/SIO. The overall catalytic activity revealed minimal correlation with the film thickness or the lattice constants but exhibited systematic variation with film mosaicity, ρ, and surface SrO segregation layer thickness,

suggesting that the mosaicity-induced increase in the number of exposed IrOx active species plays a critical role in the activity enhancement. When subtracting the area enhancement contributions to the catalytic activity, however, the catalytic activity showed correlations with film mosaicity, ρ, and film thickness, indicating that the defect-driven changes in the electronic structure of the IrOx active species intrinsically affect the catalytic activity. As demonstrated through this study, the identification of correlations between the catalytic activity and key as-synthesized structural variables, rather than purely synthetic parameters, provides important guidance to systematically develop catalysts independent of the synthetic technique. In this regard, epitaxial thin films with controllably varied parameters such as orientation, thickness, and crystallinity of a fixed phase and surface structure serve as an ideal platform in electrocatalysis research.

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EXPERIMENTAL METHODS

Sample Preparation. A pulsed excimer laser (KrF, λ = 248 nm) and a SIO polycrystalline target were used for the PLD growth of SIO films. The films were grown on 5 × 5 mm2 SrTiO3 (STO) (001) substrates with substrate temperature (Ts) ranging from 620 to 680 °C, oxygen partial pressure (PO2) ranging from 50 to 300 mTorr, and laser fluence (F) ranging from 0.8 to 2.5 J cm−2, with a laser repetition rate of 3 Hz. These ranges of growth conditions are within the growth phase space of SIO previously reported.15 The film thickness was controlled by changing the laser pulse counts. 1520

DOI: 10.1021/acs.jpclett.9b00173 J. Phys. Chem. Lett. 2019, 10, 1516−1522

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The Journal of Physical Chemistry Letters SIO films were also grown with fixed growth conditions (Ts = 650 °C, PO2 = 90 mTorr, F = 1.11 J cm−2) on each of 5 × 5 mm2 KTaO3 (KTO) (001), DyScO3 (DSO) (110), STO (001), (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) (001), and LaSrAlO4 (LSAO) (001) substrates. The respective in-plane pseudocubic lattice constants of 3.99 Å (KTO), 3.95 Å (DSO), 3.91 Å (STO), 3.87 Å (LSAT), and 3.76 Å (LSAO) induce lattice mismatch ranging from −4.8 to +1.0%.37 Sample Characterization. XRD 2θ-ω scans and reciprocal space map measurements were carried out using a monochromated Cu Kα1 (λ = 1.5406 Å) source to confirm epitaxial growth of SIO (001) on the substrates. Rocking curve measurements were taken around the SIO 002 film peaks to evaluate the film mosaicity from the FWHM. X-ray reflectivity (XRR) was used to extract the fabricated SIO film thickness (Figure S4). AFM measurements in non-contact mode were performed to confirm flat, uniform film surfaces for all samples studied. In-plane electrical resistivity was measured at room temperature by establishing four-point probes on the films. Given the insulating character of the substrates used, the measured ρ is solely due to the SIO film resistivity. For selected samples, XPS measurements were taken using an Al Kα source (photon energy = 1486.60 eV) and a circular spot size of 200 μm in diameter. The binding energies were calibrated by setting the measured adventitious C1s peak to 284.8 eV. MultiPak (Physical Electronics, Inc.) software was used for the background subtraction and the deconvolution of the Sr3d core-level spectra. Electrochemical Measurements. For each sample, silver paste (Epoxy Technology Epo-Tek H20E) was applied on the four edges of the film and the back of the sample to establish good Ohmic contact. A copper wire was connected to the back of the sample, and the entire sample and the wire were covered with chemically resistant and highly insulating epoxy (Loctite EA 9462 A resin), except for a portion of the SIO film with a typical area of 4 mm2 for electrochemical measurements.25,38,39 We note that Sr leaching leads to the formation of an additional IrOx/SIO interface. However, given the similar work functions for SrIrO3 and IrOx,40−42 we do not expect a significant charge transfer impedance. A three-electrode setup was employed with the sample as the working electrode, Hg/Hg2SO4 in saturated K2SO4 as the reference electrode, and a platinum coil as the counter electrode. For each sample, six CVs were taken, with each CV consisting of three potential sweeps at a rate of 10 mV s−1. For each CV, electrochemical impedance spectroscopy measurements were taken at open circuit from 100 kHz to 1 Hz with an AC amplitude of 10 mV to obtain the uncompensated resistance of the cell. The double-layer capacitance (Cdl) was monitored after each CV as well by running potential sweeps at a 150 mV window on a nonfaradaic region with sweep rates of 5, 10, 20, 40, 60, 80, 100, 250, and 500 mV s−1. Between the fifth and sixth CVs, a 2-hour chronopotentiometry (CP) measurement with a set current density of 10 mA cm−2geo was taken to measure the stability of the samples. All measurements were performed in 0.5 M H2SO4 under oxygen gas purge. The Hg/Hg2SO4 reference electrode was precalibrated against standard hydrogen electrode, and the potential values in the acquired data were converted to the reversible hydrogen electrode (RHE) scale. The potential values were also postcorrected for 100% of the uncompensated resistance.

Conversion from Cdl to ECSA was done by comparing Cdl with the electrochemically exposed surface area of a highly crystalline 5 nm film (FWHM = 0.045°, ρ = 1.024 mΩ cm) grown on STO, which was obtained by applying the roughness factor from AFM measurement to the geometrically projected area.9 For a highly crystalline sample epitaxially strained to STO, the number of Ir atoms, assumed to be the active site, initially exposed to the electrolyte per unit area is estimated to be 6.56 × 1012 mm−2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00173. Table summarizing growth conditions of the four representative samples in Figure 2, regression analysis of correlations among the catalytic activity, ρ, and FWHM, XRD characterization of SIO films grown on varying substrates, correlations among FWHMs of the broader peak in the rocking curve around the SIO 002 peak, film thickness, cf, af, and ρ, AFM measurement before and after electrochemical operation, XRR measurement of a representative SIO film, EIR;10 for all six CV measurements plotted against characterized structural parameters, CV curves and EIR;10 versus Cdl/ Ageo plot of two representative samples, plot of Cdl/Ageo versus I (Cdl/Ageo)−1 for all samples, EIR;5,ECSA for all six CV measurements plotted against cf and af, ECSA measurements of a representative sample, and plot of the ECSA-normalized current density versus the IRcompensated potential for two representative samples (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kyuho Lee: 0000-0002-0817-0499 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Contract DE-AC02-76SF00515 and the Laboratory Directed Research and Development program at SLAC National Accelerator Laboratory (Y.H.).

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REFERENCES

(1) Ibrahim, H.; Ilinca, A.; Perron, J. Energy Storage Systems Characteristics and Comparisons. Renewable Sustainable Energy Rev. 2008, 12, 1221−1250. (2) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474−6502. (3) Montoya, J. H.; Seitz, L. C.; Chakthranont, P.; Vojvodic, A.; Jaramillo, T. F.; Nørskov, J. K. Materials for Solar Fuels and Chemicals. Nat. Mater. 2017, 16, 70−81. (4) Fabbri, E.; Habereder, A.; Waltar, K.; Kötz, R.; Schmidt, T. J. Developments and Perspectives of Oxide-Based Catalysts for the Oxygen Evolution Reaction. Catal. Sci. Technol. 2014, 4, 3800−3821. 1521

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(24) Fister, T. T.; Fong, D. D.; Eastman, J. A.; Baldo, P. M.; Highland, M. J.; Fuoss, P. H.; Balasubramaniam, K. R.; Meador, J. C.; Salvador, P. A. In Situ Characterization of Strontium Surface Segregation in Epitaxial La0.7Sr0.3MnO3 Thin Films as a Function of Oxygen Partial Pressure. Appl. Phys. Lett. 2008, 93, 151904. (25) Petrie, J. R.; Jeen, H.; Barron, S. C.; Meyer, T. L.; Lee, H. N. Enhancing Perovskite Electrocatalysis through Strain Tuning of the Oxygen Deficiency. J. Am. Chem. Soc. 2016, 138, 7252−7255. (26) Kennedy, R. J.; Madden, R.; Stampe, P. A. Effects of Substrate Temperature on the Growth and Properties of SrRuO3 and CaRuO3 Thin Films. J. Phys. D: Appl. Phys. 2001, 34, 1853−1861. (27) Yamada, T.; Makino, H.; Yamamoto, N.; Yamamoto, T. Ingrain and Grain Boundary Scattering Effects on Electron Mobility of Transparent Conducting Polycrystalline Ga-Doped ZnO Films. J. Appl. Phys. 2010, 107, 123534. (28) Lee, D.; Jacobs, R.; Jee, Y.; Seo, A.; Sohn, C.; Ievlev, A. V.; Ovchinnikova, O. S.; Huang, K.; Morgan, D.; Lee, H. N. Stretching Epitaxial La0.6Sr0.4CoO3−δ for Fast Oxygen Reduction. J. Phys. Chem. C 2017, 121, 25651−25658. (29) Nie, Y. F.; King, P. D. C.; Kim, C. H.; Uchida, M.; Wei, H. I.; Faeth, B. D.; Ruf, J. P.; Ruff, J. P. C.; Xie, L.; Pan, X.; et al. Interplay of Spin-Orbit Interactions, Dimensionality, and Octahedral Rotations in Semimetallic SrIrO3. Phys. Rev. Lett. 2015, 114, 016401. (30) Di Valentin, C.; Pacchioni, G.; Selloni, A. Electronic Structure of Defect States in Hydroxylated and Reduced Rutile TiO2 (110) Surfaces. Phys. Rev. Lett. 2006, 97, 166803. (31) Xu, P. S.; Sun, Y. M.; Shi, C. S.; Xu, F. Q.; Pan, H. B. The Electronic Structure and Spectral Properties of ZnO and Its Defects. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 199, 286−290. (32) Sun, W.; Liu, J.-Y.; Gong, X.-Q.; Zaman, W.-Q.; Cao, L.-M.; Yang, J. OER Activity Manipulated by IrO6 Coordination Geometry: An Insight from Pyrochlore Iridates. Sci. Rep. 2016, 6, 38429. (33) Diaz-Morales, O.; Raaijman, S.; Kortlever, R.; Kooyman, P. J.; Wezendonk, T.; Gascon, J.; Fu, W. T.; Koper, M. T. M. Iridium-Based Double Perovskites for Efficient Water Oxidation in Acid Media. Nat. Commun. 2016, 7, 12363. (34) Lebedev, D.; Povia, M.; Waltar, K.; Abdala, P. M.; Castelli, I. E.; Fabbri, E.; Blanco, M. V.; Fedorov, A.; Copéret, C.; Marzari, N.; Schmidt, T. J. Highly Active and Stable Iridium Pyrochlores for Oxygen Evolution Reaction. Chem. Mater. 2017, 29, 5182−5191. (35) Kim, J.; Shih, P.-C.; Tsao, K.-C.; Pan, Y.-T.; Yin, X.; Sun, C.-J.; Yang, H. High-Performance Pyrochlore-Type Yttrium Ruthenate Electrocatalyst for Oxygen Evolution Reaction in Acidic Media. J. Am. Chem. Soc. 2017, 139, 12076−12083. (36) Wu, Y.; Sun, W.; Zhou, Z.; Zaman, W. Q.; Tariq, M.; Cao, L.; Yang, J. Highly Efficient Oxygen Evolution Activity of Ca2IrO4 in an Acidic Environment due to Its Crystal Configuration. ACS Omega 2018, 3, 2902−2908. (37) Longo, J. M.; Kafalas, J. A.; Arnott, R. J. Structure and Properties of the High and Low Pressure Forms of SrIrO3. J. Solid State Chem. 1971, 3, 174−179. (38) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977−16987. (39) Stoerzinger, K. A.; Qiao, L.; Biegalski, M. D.; Shao-Horn, Y. Orientation-Dependent Oxygen Evolution Activities of Rutile IrO2 and RuO2. J. Phys. Chem. Lett. 2014, 5, 1636−1641. (40) Shimizu, T.; Yamaguchi, T.; Nishikawa, Y. Semiconductor Device and Method for Manufacturing Same. U.S. Patent US 20050139926, 2005. (41) Banerjee, W.; Maikap, S.; Rahaman, S. Z.; Prakash, A.; Tien, T.C.; Li, W.-C.; Yang, J.-R. Improved Resistive Switching Memory Characteristics Using Core-Shell IrOx Nano-Dots in Al2O3/WOx Bilayer Structure. J. Electrochem. Soc. 2011, 159, H177−H182. (42) Prakash, A.; Maikap, S.; Lai, C. S.; Lee, H. Y.; Chen, W. S.; Chen, F. T.; Kao, M. J.; Tsai, M. J. Improvement of Uniformity of Resistive Switching Parameters by Selecting the Electroformation Polarity in IrOx/TaOx/WOx/W Structure. Jpn. J. Appl. Phys. 2012, 51, 04DD06.

(5) Suen, N.-T.; Hung, S.-F.; Quan, Q.; Zhang, N.; Xu, Y.-J.; Chen, H. M. Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46, 337−365. (6) Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972−974. (7) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, No. eaad4998. (8) Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A Comprehensive Review on PEM Water Electrolysis. Int. J. Hydrogen Energy 2013, 38, 4901−4934. (9) Seitz, L. C.; Dickens, C. F.; Nishio, K.; Hikita, Y.; Montoya, J.; Doyle, A.; Kirk, C.; Vojvodic, A.; Hwang, H. Y.; Norskov, J. K.; Jaramillo, T. F. A Highly Active and Stable IrOx/SrIrO3 Catalyst for the Oxygen Evolution Reaction. Science 2016, 353, 1011−1014. (10) Matsumoto, Y.; Yamada, S.; Nishida, T.; Sato, E. Oxygen Evolution on La1‑xSrxFe1‑yCoyO3 Series Oxides. J. Electrochem. Soc. 1980, 127, 2360−2364. (11) Forgie, R.; Bugosh, G.; Neyerlin, K. C.; Liu, Z.; Strasser, P. Bimetallic Ru Electrocatalysts for the OER and Electrolytic Water Splitting in Acidic Media. Electrochem. Solid-State Lett. 2010, 13, B36−B39. (12) Kubicek, M.; Limbeck, A.; Frömling, T.; Hutter, H.; Fleig, J. Relationship Between Cation Segregation and the Electrochemical Oxygen Reduction Kinetics of La0.6Sr0.4CoO3−δ Thin Film Electrodes. J. Electrochem. Soc. 2011, 158, B727−B734. (13) Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y.-L.; Risch, M.; Hong, W. T.; Zhou, J.; Shao-Horn, Y. Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution. Nat. Commun. 2013, 4, 2439. (14) Stoerzinger, K. A.; Choi, W. S.; Jeen, H.; Lee, H. N.; ShaoHorn, Y. Role of Strain and Conductivity in Oxygen Electrocatalysis on LaCoO3 Thin Films. J. Phys. Chem. Lett. 2015, 6, 487−492. (15) Nishio, K.; Hwang, H. Y.; Hikita, Y. Thermodynamic Guiding Principles in Selective Synthesis of Strontium Iridate RuddlesdenPopper Epitaxial Films. APL Mater. 2016, 4, 036102. (16) Gruenewald, J. H.; Nichols, J.; Terzic, J.; Cao, G.; Brill, J. W.; Seo, S. S. A. Compressive Strain-Induced Metal-Insulator Transition in Orthorhombic SrIrO3 Thin Films. J. Mater. Res. 2014, 29, 2491− 2496. (17) Yagi, M.; Tomita, E.; Kuwabara, T. Remarkably High Activity of Electrodeposited IrO2 Film for Electrocatalytic Water Oxidation. J. Electroanal. Chem. 2005, 579, 83−88. (18) Chung, J.-S.; Jang, S. Y.; Moon, S. J.; Jeon, B. C. PLD Growth of Epitaxially-Stabilized 5d Perovskite SrIrO3 Thin Films. J. Korean Phys. Soc. 2010, 56, 1814−1817. (19) Rao, R. A.; Lavric, D.; Nath, T. K.; Eom, C. B.; Wu, L.; Tsui, F. Three-Dimensional Strain States and Crystallographic Domain Structures of Epitaxial Colossal Magnetoresistive La0.8Ca0.2MnO3 Thin Films. Appl. Phys. Lett. 1998, 73, 3294−3296. (20) Bertacco, R.; Contour, J. P.; Barthélemy, A.; Olivier, J. Evidence for Strontium Segregation in La0.7Sr0.3MnO3 Thin Films Grown by Pulsed Laser Deposition: Consequences for Tunnelling Junctions. Surf. Sci. 2002, 511, 366−372. (21) Ito, A.; Masumoto, H.; Goto, T. Microstructure and Electrical Conductivity of SrRuO3 Thin Films Prepared by Laser Ablation. Mater. Trans. 2006, 47, 2808−2814. (22) Crumlin, E. J.; Mutoro, E.; Liu, Z.; Grass, M. E.; Biegalski, M. D.; Lee, Y.-L.; Morgan, D.; Christen, H. M.; Bluhm, H.; Shao-Horn, Y. Surface Strontium Enrichment on Highly Active Perovskites for Oxygen Electrocatalysis in Solid Oxide Fuel Cells. Energy Environ. Sci. 2012, 5, 6081−6088. (23) Dulli, H.; Dowben, P. A.; Liou, S.-H.; Plummer, E. W. Surface Segregation and Restructuring of Colossal-Magnetoresistant Manganese Perovskites La0.65Sr0.35MnO3. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, R14629−R14632. 1522

DOI: 10.1021/acs.jpclett.9b00173 J. Phys. Chem. Lett. 2019, 10, 1516−1522