Unraveling Thermodynamics, Stability, and Oxygen Evolution Activity

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Unraveling Thermodynamics, Stability, and Oxygen Evolution Activity of Strontium Ruthenium Perovskite Oxide Bae Jung Kim, Daniel F. Abbott, Xi Cheng, Emiliana Fabbri, Maarten Nachtegaal, Francesco Bozza, Ivano E. Castelli, Dmitry Lebedev, Robin Schaeublin, Christophe Copéret, Thomas Graule, Nicola Marzari, and Thomas J. Schmidt ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03171 • Publication Date (Web): 21 Mar 2017 Downloaded from http://pubs.acs.org on March 25, 2017

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Unraveling Thermodynamics, Stability, and Oxygen Evolution Activity of Strontium Ruthenium Perovskite Oxide Bae-Jung Kim,§ Daniel F. Abbott,§ Xi Cheng,§ Emiliana Fabbri,§,* Maarten Nachtegaal,⊥ Francesco Bozza,‡ Ivano E. Castelli,¶ Dmitry Lebedev, † Robin Schäublin, ∥ Christophe Copéret, ∥ Thomas Graule,‡ Nicola Marzari,¶ Thomas J. Schmidt §,† §

Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen, Switzerland

Paul Scherrer Institut, 5232 Villigen, Switzerland

⊥

‡

Laboratory for High Performance Ceramics, EMPA, Swiss Federal Laboratories for Materials

Testing and Research, 8600 Dübendorf, Switzerland ¶

Theory and Simulation of Materials (THEOS), and National Centre for Computational Design

and Discovery of Novel Materials (MARVEL), EPFL, 1015 Lausanne, Switzerland ∥Scientific

Center for Optical and Electron Microscopy, ETH Zurich, 8093 Zurich, Switzerland

†

Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland

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ABSTRACT Extensive investigations in understanding the functional mechanisms of metal oxides behind oxygen evolution have been carried out since electrolyzer has demonstrated promising possibilities as a device to produce hydrogen for electrochemical energy conversion systems. In particular, perovskite oxides are reputable for high activity towards the oxygen evolution reaction (OER). Here, we revisited the list of active perovskite oxides constructed based on theoretical oxygen binding energies of reaction intermediates to the catalyst surface. From this list, Ru-based perovskites, i.e. SrRuO3 and LaRuO3 have been predicted as active perovskites to exhibit a particularly high OER activity. We report on the stability of nano-scaled SrRuO3 perovskite prepared by a simple and scalable flame synthesis method. Attempts to attain LaRuO3 were made; however, its DFT calculated phase diagram suggests that its perovskite phase is not thermodynamically stable, which supports our experimental results such that only a mixture of different La-Ru-O phases has been obtained. Nano-scaled SrRuO3 is evaluated for its electrochemical activity with a particular emphasis pointed towards stability in both alkaline and acidic media. Through conjoining electrochemical methods, operando X-ray absorption spectroscopy (XAS), and theoretical calculations, we show that SrRuO3 exhibit trivial activity towards OER that decreases promptly. The loss in activity is rationalized through DFT based computations, which corroboratively suggest the poor chemical stability of both selected perovskites. Regardless of the predicted theoretical OER activity, the intrinsic instability strongly suggests that Sr- and La-based ruthenium oxides are not viable catalysts for OER in aqueous media. This further suggests that their activities are independent of their binding energies between intermediates and catalyst surface but rather closely associated with material dissolution. We highlight that understanding the origin of stability under real operating


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environment is absolutely essential for the design of a sustainable electrocatalyst with optimal balance between activity and stability. Keywords: perovskite, oxygen evolution reaction, strontium, lanthanum, ruthenium oxide, thermodynamic stability, Pourbaix diagram, X-ray absorption spectroscopy


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1. INTRODUCTION As technologies are evolving to become more energy-intensive, the demands for clean and sustainable energy have continued to increase due to negative impacts on the environment from extensive use of non-renewable fuels. As a result, developments in fuel cell technologies are gaining much attention as alternative route to produce energy. With this reasons, the development of water electrolysis process has become a major focus as means to provide pure hydrogen.1-6 The efficiency of water electrolysis is strongly limited by the oxygen evolution reaction (OER) as it suffers from sluggish kinetics causing high overpotentials on the electrolyzer anode. Various electrocatalysts have been studied in the past towards improving the OER activity, where the oxides of precious metals – iridium– have been accepted as the most active catalysts for OER.1, 5, 7-11 However, in the prospect of industrialization, they are unsuitable for commercialization considering their high costs and scarcity. Thus, the design of an efficient, stable and viable electrocatalyst is in demand to remedy some of these drawbacks for commercial uses. Among possible candidates for metal oxide catalysts, perovskites with the general formula of ABO3 have been known for their promising potential in a wide range of applications. Generally, the A-site is occupied by an alkaline-earth metal (e.g. Sr or Ba), a rare-earth metal (e.g. La or Pr), or a combination of the two. The B-site is generally a transition metal (e.g. Ti or Co) in a 6-fold octahedral coordination with oxygen ions. Conventionally, the BO6 octahedral mainly influences electronic and magnetic properties of perovskites. The A-site metal is known to affect the conduction band energy of the compound and the degree of deviation from the ideal cubic structure.1, 12 Extensive studies have been carried out to understand the origin of catalytic activities of perovskites. Matsumoto et al.13 first proposed that the OER takes place at the surface


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of the perovskite oxide when its B-site is occupied by a transition metal. The authors claimed that the formation of the σ* band within the lattice and acquiring a high oxidation state of the Bsite metal would lead to improved catalytic activity towards OER. Otagawa and Bockris14-15 also proposed a relationship between the electronic structure and the OER activity of perovskite oxides using the bond strength of surface oxygenated intermediates as the descriptor. They have displayed this correlation in a volcano plot of activity in terms of the bond strength (exhibits a linear relationship with the number of d-electrons). More recently, Suntivich et al.16 have reported a different volcano-type relationship based on molecular orbital principles drawing a relationship between activities and eg band fillings of perovskites. Additionally, computational studies using density functional theory (DFT) have been explored to attain more in-depth understanding of the OER activity of perovskite oxides in relation to their electronic properties.4, 17-18

Man et al.4 proposed a correlation between catalytic activity and binding energies of surface

intermediates (O* and OH*) suggesting a list of active OER perovskite catalysts. Past studies point out that the best catalyst should exhibit an optimal balance between surface-oxygen interaction (adsorption vs. desorption) energies. Although the mentioned studies have described in detail the descriptors for oxygen evolution activity and provided fundamental concepts behind designing an efficient catalyst, the stability of metal oxides is rarely discussed, even if stability is probably one of the most critical parameters to select an OER catalyst. Only few recent studies have established insights into relationships between activity of OER and stability of catalysts.19-27 These experimental evidences propose possible relationships between electrochemical activity and stability of perovskite oxides. Particularly, Chang et al.21-22 have investigated model SRO electrodes such as epitaxial films with different orientations with focus in the relation between surface defects/orientation, stability,


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and OER activity. However, their investigations exclude standardized end-of-life stability tests and thermodynamic aspects with regards to stability. In our recent work, thermodynamics behind the stability of metal oxides was further assessed describing that metal oxides are subjected to inseparable structural disintegration during the oxygen evolution process, and established fundamental qualities for an ideal metal oxide catalyst.3 Overall, these studies suggest that future research should be directed towards augmenting both activity and stability together through optimizing composition and structure in order to attain superior oxygen evolution activity within the thermodynamic stability boundaries. Approaching this resolution, we have chosen two perovskites – SrRuO3 and LaRuO3; which are anticipated to be highly active toward the OER from the volcano plot by Man et al.4 – to clarify their overall activity and stability during oxygen evolution. Herein, we take an approach of conjoining experimental evidences and theoretical calculations in order to elucidate the stability of Sr- and La- ruthenium oxides, specifically focusing on their perovskite structure (SRO-p and LRO-p). To serve this purpose, we prepared nanoparticles of these materials via simple and scalable flame spray synthesis (FSS) method. Through this method of synthesis, we were able to attain SRO-p with a high surface area, which enabled us to directly relate its behavior during oxygen evolution to its changes at the surface.11 We could not, however, obtain a single phase LRO-p but rather a mixture of different La-Ru-O compounds, consistent with the predicted instability of LRO-p according to the DFT computed phase diagram. The changes in the functional behavior of SRO-p is experimentally observed on the atomic scale through operando X-ray absorption spectroscopy in both alkaline and acidic media.28 Corroborative DFT based computational evidences are also presented to support our findings relating the overall thermodynamic stability of these compounds. On the basis of our findings


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from the conjoined approaches via computational analysis and experimental results, we prove the inadequacy of SrRuO3 as OER catalysts, despite of the projection of their high oxygen evolution activities, owing to their unappealing stability in aqueous electrolyte solutions. 2. METHODS 2.1 Material Synthesis Liquid-feed flame spray synthesis (FSS) technique was used to synthesize Sr and La-ruthenium oxides. The same FSS setup as described in the reference29 was used for the syntheses. For the synthesis of SRO, the precursors solution was prepared by dissolving stoichiometric amounts of strontium nitrate (≥ 99.0%, Sigma Aldrich) and ruthenium nitrosyl-nitrate (aqueous solution, Ru 1.5% w/v, Alfa Aesar) in a solution composed of deionized water, 20 vol. % acetic acid (≥ 99.0%, Fluka) and 50 vol. % N,N-Dimethylformamide (DMF, ≥ 99.8%, Roth). The total metal concentration in solution was fixed to 0.045 M. The flow rate of the dispersing oxygen was fixed at 35 ml min-1, while the flow rates of the combustion gases were fixed at 17 ml min-1 and 13 ml min-1 for oxygen and acetylene, respectively. The similar procedure was used for LRO, except dissolving lanthanum oxide (> 99.99 %, Auer Remy) in diluted nitric acid instead of dissolving strontium nitrate in acetic acid. The flow rate of the dispersing oxygen was fixed at 50 ml min-1, while the flow rates of the combustion gases were fixed at 30 ml min-1 and 13 ml min-1 for oxygen and acetylene, respectively. RuO2 was prepared using a simple one-pot Adams method described previously.30 Hydrated ruthenium chloride (RuCl3·xH2O, 99.9%-Ru, Strem Chemicals) was used as the Ru precursor.


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2.2 Material Characterization Phase analysis of prepared materials was performed using powder X-ray diffraction (XRD, Bruker D8 system in Bragg–Brentano geometry, CuKα radiation (λ = 0.15418 nm). Specific surface area was calculated by Brunauer-Emmett-Teller analysis of N2 adsorption / desorption isotherms (AUROSORB-1, Quantachrome). Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) (TECNAI F30 operated at 300 kV) were used to study the surface morphology and composition of the prepared materials. For X-ray absorption near-edge structure (XANES) spectroscopy measurements, catalyst powders were dispersed in the mixture of isopropanol and Milli-Q water in the equal ratio sonicated for 60 min. The ink was then spray coated on Kapton film. XANES spectra at the Ru K-edge were recorded at the SuperXAS beamline of the Swiss Light Source (PSI, Villigen, Switzerland). The incident photon beam provided by a 2.9 T superbend magnet source was collimated by a Pt-coated mirror at 2.5 mrad and monochromatized by a Si (311) channel-cut monochromator. Focusing was performed by a Pt-coated toroidal mirror at 2.5 mrad to a spot size of 100 x 100 micro-meter. Spectra were collected in QEXAF mode using Ar filled 15 cm long gridded ionization chambers.31 Reference spectra of a Ru foil were simultaneously collected for absolute energy calibration. 2.3 Electrochemical Characterization The electrochemical activities of the prepared catalysts were evaluated in a standard threeelectrode electrochemical cell using the thin-film rotating disk electrode (RDE) methodology.32 The setup for OER and cyclic voltammetry (CV) consists of a potentiostat (Biologic VMP-300) and a rotation speed controlled motor (Pine Instrument Co., AFMSRCE). All the electrochemical


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measurements were performed at the standard room temperature using Reversible Hydrogen Electrode (RHE) and Silver Chloride Electrode (Ag+/AgCl) as the reference electrode in alkaline and acidic media, respectively. A piece of platinum mesh was used as the counter electrode. A homemade Teflon cell was used to contain aqueous electrolytes (0.1 M KOH and 0.1 M HClO4) with the working electrode immersed under the potential control. A porous thin film of materials was prepared by drop-casting a catalyst ink on a polished glassy carbon electrode (5 mm OD/0.196 cm2). The catalyst ink was prepared from a catalyst suspension made from sonicating (Bandelin, RM 16 UH, 300 Weff, 40 kHz) 10 mg of catalyst in a solution mixture of 4 mL isopropyl alcohol and 1 mL of Milli-Q water (ELGA, PURELAB Chorus), and 20 µL of Nafion (Sigma-Aldrich, 5 wt. %). The same method of preparation of the working electrode was used for all the samples. The 0.1 M KOH electrolyte was prepared from mixing KOH pellets (SigmaAldrich, 99.99%), and the acid electrolyte, 0.1 M HClO4, was prepared by diluting concentrated HClO4 (Sigma-Aldrich, 70 %) in Milli-Q water. Initially, 5 reverse potentiostatic sweeps of CV was performed in the synthetic air-saturated electrolytes (in KOH or in HClO4) between 1.0 and 1.5 VRHE (in alkaline media), and 1.4 VRHE (in acid) at a scan rate of 50 mV s-1. Subsequently, chronoamperometric measurements were carried out to measure the OER activity holding each potential step for 60 seconds in the same range of potentials as in CV while rotating the working electrode at 900 rpm. The resulting OER curves were compared by observing their current densities at a potential (ca. 1.425 VRHE) near the OER onset. The chemical stability of the catalysts was assessed using the same setup, except agitation was not applied to eliminate possible physical disturbance. A cycle of CV was conducted at every 2 hours up to 8 hours, additionally at 12 hours and 24 hours at a sweep rate of 10 mV sec-1. In addition, the open circuit potential (OCP) was recorded in between every CV during the time of the contact. All measured


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currents were normalized by the mass of catalyst loading, and potentials were corrected for the ohmic-drop measured by the electrochemical impedance spectroscopy. 2.4 Operando Flow Cell Test The homemade operando XAS flow cell was described in detail previously.28 The electrode materials were air-spray coated at the center of a Kapton film. Vulcan carbon was used as the counter electrode material. A silver chloride electrode (Ag+/AgCl) was used as the reference electrode. During electrochemical testing, the electrolyte was drawn into the cell and collected in a syringe at the flow rate of 0.1 mL/min for the quantification of the dissolved species. The chronoamperometry measurement was carried out holding for 60 seconds at each increasing potential step from 1.2 VRHE to 1.6 VRHE, and again during the reverse sequence back to 1.2 VRHE. The absorption spectra were collected throughout each potential step. The results of the cathodic polarization are not included in our discussion. The electrolyte was collected at the end of each in situ flow cell test for the inductively-coupled plasma – optical emission spectrometry measurement (ICP-OES; Varian – Agilent Technologies Inc., VISTA Pro AX). 2.5 Density Functional Theory Calculations We investigated the phase stability and stability in an aqueous environment of SrRuO3 and LaSrO3 by means of ab-initio quantum mechanics simulations in the framework of densityfunctional theory (DFT) using the Quantum ESPRESSO package33 to fully relax the two perovskite structures as well as all the competing phases, as described in the Materials Project database34, in in which the two perovskite oxides could separate. We used the PBEsol exchangecorrelation functional in the Generalized Gradient Approximation35 and the pseudopotentials


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from the recently proposed Standard Solid State Pseudopotential library (SSSP accuracy).36-37 The phase and Pourbaix diagrams are then generated using the ASE module.38 In the phase stability diagram, the total energy of the candidate perovskite oxide is compared with the total energies of all possible competing phases in which the material under investigation can separate. A candidate material is stable when the compound is at a vertex of the calculated convex hull, which defines the stability frontier at 0K (stable materials are indicated with black dots). Unstable materials are, instead, above the 2-dimensional convex hull (or inside the Gibbs hyperplanes in a multidimensional hull) and are indicated by red dots. We address the stability in an aqueous environment against dissolved and solid phases and ions by means of Pourbaix diagrams. In a Pourbaix diagram, stable phases are mapped as a function of pH and electrochemical potential. In this scheme,39-40 we combine calculated densityfunctional theory energies (for the solid species) with experimentally measured dissolution energies (for the dissolved phases).41-42 These methods can provide a descriptor for the stability in aqueous which has been recently used to identify novel light harvesting materials for water splitting.43


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3. RESULTS AND DISCUSSION 3.1 Physico-chemical and Thermodynamic Characterization Sr- and La- ruthenium oxide catalysts (SRO and LRO, generally referring to oxide phases in SrRu-O and La-Ru-O systems, respectively) were prepared in nano-scale by the liquid-feed flame spray synthesis (FSS) technique. X-ray diffraction (XRD) was performed in order to investigate the crystalline structures of the synthesized materials (Figure 1a and b, respectively). First, Figure 1a presents the XRD pattern of SRO, in which the peaks are well indexed to those characteristic of the SrRuO3 perovskite structures (SRO-p) [JCPDS PDF Card: 80-1529 and 01075-2831] with untraceable amount of side products. The broadness of the peaks does not allow us to distinctively distinguish between the cubic and orthorhombic phases of SRO-p; the presence of the different SRO-p symmetry could be caused by the temperature gradient in the luminescent tip of the flame. This thermal window leads the constituents of SRO-p to coordinate with numerous degrees of symmetry (Tetragonal above 820K and Cubic above 950K).12, 44-45 In contrast the XRD pattern of the prepared LRO shows a number of indiscernible diffraction peaks (see Figure 1b), indicating the presence of numerous different crystalline structures. However, the XRD profile reveals no peaks that are distinctive of the targeted LaRuO3 perovskite structure (LRO-p). The pronounced diffraction peaks suggest the presence of a significant portion of La2O3 [JCPDS PDF Card: 01-083-13] along with La2RuO5 and La3RuO7, indicating a mixture of different lanthanum ruthenium oxides of different stoichiometric ratios (in the following indicated as LRO-m). In order to elucidate the appearance of various side-products obtained from the synthesis, the thermodynamic stability of each possible crystalline phase was calculated and presented in an


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equilibrium triangle with the respective elemental constituents (i.e. Sr/La, Ru, and O) on the vertices; Figure 1c and d show phase diagrams for SRO and LRO, respectively. The phase diagrams are constructed to display thermodynamically stable phases at intersecting vertices of convex-hulls, which are marked by black dots. The red dots that lie outside of the vertices and/or lines represent thermodynamically instable phases in a fully relaxed crystalline structure. Abiding to this convention, Figure 2c reveals that thermodynamically stable phases of SRO are: Sr2Ru3O10, Sr4Ru2O9, Sr2RuO4, SrRuO3 and all of the single-metal oxides – SrO, SrO2, RuO2, and RuO4. Exceptionally, Sr3Ru2O7 is revealed as the only unstable phase (shown in red), which indicates that SRO is thermodynamically unstable in this compositional ratio (Sr : Ru : O = 3 : 2 : 7) and decomposes to a more stable phase.46 Revisiting the XRD pattern of SRO (Figure 1a) with the information regarding the phase stability, the detection of its perovskite phase is thermodynamically validated based on its formation energy calculations. In a similar manner, the thermodynamic stability of each LRO phase is computed and plotted in the phase diagram following the same convention (Figure 1d). It is noteworthy that there are three unstable phases (La2RuO5, La3Ru3O11, and LaRuO3) including La2RuO5 and the targeted perovskite structure (LRO-p). Although the FSS technique offers the capability to attain a targeted phase, the calculated phase diagram is consistent with the absence of LRO-p in the XRD profile, presumably due to its unfavorable formation energy. Furthermore, the phase diagram shows that La2RuO5 is thermodynamically unstable, yet its presence could be plausible (as observed from the XRD pattern) based on the fact that the calculated energy for La2RuO5 deviates from the convex-hull (presenting stable phases) only by 0.05 eV, which defines the stability threshold.


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Figure 1. X-Ray Diffraction analyses of (a) SRO (Miller indexes are given for the cubic structure [JCPDS PDF Card: 80-1529]), and (b) LRO. Phase stability diagram of all possible (c) Sr, Ru, and O and (d) La, Ru, and O phases computed using DFT calculations. Black and red dots represent thermodynamically stable phases and unstable phases, respectively. The physical and structural traits of the prepared metal oxide nanoparticles are observed using transmission electron microscopy (TEM) (Figure 2). The TEM image (Figure 2a) clearly shows that the prepared SRO-p nanoparticles are consistent in size (below 10 nm) and shape and uniformly distributed, which are ideal features of a catalyst for assessing its surface chemistry during a reaction. The high-resolution TEM (HRTEM) image of SRO-p nanoparticles reveals


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clear fringes indicating formation of distinctive crystalline structures (Figure 2b). It is known that the FSS method promotes the formation of nano-scale particles driven by Brownian motion, coagulation, and coalescence.47-48 Consequently, the formed particles tend to agglomerate via Van der Waals forces.47 Figure 2c clearly shows that LRO-m is composed of particles that range from nano-scale to micro-scale. The formation of different sized particles could be attributed to the presence of different crystalline structures of LRO-m. In order to identify the large particles, compositional analysis was carried out using energy dispersive X-ray (EDX) spectroscopy (Table 1). The percentage of the large dark particle (visible in Figure 1c) shows a large content of La (87 wt. %) compared to Ru (5 wt. %), which confirms that it is a La-O species (i.e. La2O3). In contrast, EDX analysis of the LRO-m nanoparticles (see Figure 2d) reveals that they are not comprised solely of La-O species, but the particles rather contain an even amount of La and Ru. As shown in Figure 2c, the undesired La2O3 side product occupies a large portion of the total particle population due to its relatively large size. Consequently, these large La2O3 particles induce a comparably smaller BET surface area of 16 m2 g-1 for LRO-m, whereas SRO-p shows a significantly higher BET surface area of 76 m2 g-1 owing to its uniform size of the nanoparticles. At this point it is worth mentioning that past studies using different methods of LRO syntheses report the presence of La2O3, ruthenium oxides, and various lanthanum ruthenium oxide phases as inseparable impurities.49-56 Supported by the above findings, LRO-p can be categorized as a thermodynamically infeasible material. Thereby, it is most likely to expect that the prepared LRO would consist of various phases other than LRO-p that are pointed out in the phase diagram. Thermodynamically unstable phases shown in the phase diagram (i.e. marked in red dots) may be found present as meta-


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stables, however they would evolve to a more thermodynamically stable phase (i.e. phases marked in black dots).

Figure 2. (a) TEM image of SRO-p particles exhibiting similar particles sizes (< 10 nm). (b) High-resolution TEM (HRTEM) image of SRO-p nanoparticles. (c) TEM image of LRO-m composed of multiple species. The apparent large particles presented in the darkest contrast are attributed to La2O3. (d) HRTEM image of nanoparticles of LRO-m.


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Table 1. Energy Dispersive X-Ray Spectroscopy (EDX) analysis of LRO-m nanoparticles and the large La2O3 particle.

3.3 Electrochemical Study Electrochemical characterizations were conducted in both 0.1 M KOH and 0.1 M HClO4 to assess the oxygen evolution activity and stability of the synthesized SRO-p in two different pH values. Note that further assessment regarding electrocatalytic activity towards OER for LRO was excluded in the discussion due to the presence of various crystalline structures and phases, none of which corresponds to the perovskite phase of our interest. Figure 3 shows constructed Tafel plots from measured steady-state currents from chronoamperometry studies (refer to Figure S2) of all materials in order to evaluate their catalytic activities. Figure 3a and b compare the Tafel plots of SRO-p and RuO2 in alkaline and in acidic electrolytes, respectively. As summarized in Table 2, the apparently higher oxidation Tafel slope value for SRO-p (74 mV dec1

) in alkaline electrolyte suggest much slower reaction kinetics in comparison to that of RuO2 (54


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mV dec-1). Additionally, notably lower current densities are observed at 1.425 VRHE for SRO-p (7.8×10-2 A g-1) than that of RuO2 (14.2 A g-1). Comparatively, in acidic electrolyte, the Tafel slope values for all materials under investigation are revealed to be much lower than in alkaline (52 and 42 mV dec-1 for SRO-p and RuO2, respectively). The comparably smaller Tafel slope values for SRO-p in acidic electrolyte indicate a decrease in overpotential, which leads to a higher current density at the comparing potential of 1.425 VRHE (1.8 and 9.9 A g-1 for SRO and RuO2, respectively), which could possibly resemble a higher OER activity than in alkaline electrolyte. Yet, as polarization continues, the polarization behavior of SRO-p suggests that the observed current density may not solely arise from the OER.

Figure 3. Tafel plots for OER of SRO-p and RuO2 calculated from the CA at increasing potentials (a) in KOH and (b) in HClO4.


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Table 2. Summary of Tafel plots of SRO and LRO presented in Figure 3a and b, respectively. Tafel Slope [mV dec-1]

Activity at 1.425 VRHE [A g-1]

Catalysts KOH

HClO4

KOH

HClO4

RuO2

57.2

41.7

14.2

9.9

SrRuO3

74.2

51.5

7.8×10-2

1.8

As described in the experimental section, the cyclic voltammetry (CV) for SRO-p was conducted by sweeping at 50 mV sec-1 before and after the chronoamperometry (CA) and compared to those of RuO2 (see Figure S1). A wider potential window was assigned when testing in KOH for possible positive potential shifts of the OER onset.21 Referring to Figure S1, all materials feature different redox characteristics in both KOH and HClO4. Yet, the potentialinduced current densities describing these features diminish after the series of CA. This openly manifests the materials’ instability, which indicates the deactivation (i.e. loss) of active sites during polarization. In our recent study3, an in-depth discussion about thermodynamics behind the set of mechanisms of metal oxides during oxygen evolution was presented. Therefore, at this stage, the origin of the observed potential-induced current of SRO-p is questionable considering the concomitance of perovskite oxide OER and dissolution: ABO3 + H2O / 2H+ (in acid) ⇔ A(aq)2+ + BO2 + 2OH– (in alkaline) / H2O (in acid) In order to elucidate this inquiry, we first relate to the widely conceded oxygen evolution mechanisms of RuO2 in both alkaline and acid electrolyte.57-58 Past studies have studied different mechanism pathways of metal oxides for the oxygen evolution reaction. Table S1 briefly


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summarizes these electrochemical oxide pathways for the oxygen evolution of RuO2 in both alkaline and acidic media. As shown, RuO2 readily forms a hydrous oxide surface layer resulting with RuO2(OH)2, an oxidation state of VI, and is further oxidized throughout the process of the anodic sweep to form RuO4.1, 59-60 The RuO4 corrosion potential coincides with the onset of the oxygen evolution at 1.4 VRHE.1, 60 Therefore, it is difficult to decouple multiple processes occurring concomitantly above this potential, which complicates the process of exclusively evaluating the intrinsic catalytic activity towards OER. Thus, the relatively higher current responses from all materials in acid may not solely correspond to OER, but also to Ru corrosion or the combination of OER and Ru corrosion. In fact, the measured current is attributed to the OER (90 %) and the Ru corrosion (10 %) according to a study on SRO thin-film using the rotating ring disk electrode (RRDE) with a ruthenized platinum ring.22 Returning to the Tafel plots and accepting the mentioned contention, the changes in Tafel slope at a higher overpotential in the acid electrolyte might be associated with a more aggressive RuO4 corrosion. Additionally, Burke et al.61 explained the the relatively lower Tafel slope of RuO2 in acid by the additional contribution from readily increasing coverage of the surface radicals (O*; * indicates species adsorbed at the surface) with increasing potential. 3.4 Operando X-ray Absorption Spectroscopy – XANES The results of the electrochemical characterization depict the substantial activity loss for SRO-p, revealing its short-lasting activity for an adequate OER catalyst. Such observations could only point to the materials’ insufficient stability under the necessary conditions for OER in practical applications. XANES was carried out in order to further understand the relationship between physical transformation and stability. In the following section, we will present electronic properties of the prepared SRO-p during the oxidation process, and relate to their catalytic


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activities. XANES spectra of the dry catalyst samples were examined in order to comprehend the initial valence states of the materials before conducting in situ electrochemical characterization (Figure S3 and Table S2). Extended X-ray absorption fine structure (EXAFS) spectroscopy results have not been included in this work because of imprecision in assessing their atomic structures due to the instability of the resulted absorption spectra from the material during the series of potential steps. The potential-induced changes in electronic properties of SRO-p from its initial state were observed in a flow cell for better resemblance of a practical application via series of operando XANES measurements. Figure 4 displays the obtained normalized Ru K-edge XANES spectra of SRO-p electrode in the flow cell with KOH and HClO4 (Figure 4a and b, respectively). Insets in Figure 4 show the non-normalized XANES spectra as a function of the applied potential. Figure 5 facilitates the reading of the changes in the white-line intensities of non-normalized XANES spectra as a function of the applied potential. Additionally, Figure 6 is constructed for the better illustration of energy shifts of SRO-p during anodic polarization, which will be referred to throughout the discussion.


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Figure 4. Operando XAS results of the catalysts in flow cell. Normalized XANES spectra of SRO-p (a) in KOH, and (b) in HClO4. The insets illustrate the loss in absorption edge-step intensity in the non-normalized XANES data.


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Figure 5. Changes in the intensity (%) of RuO2 in HClO4 (black), SRO-p in KOH (red) and in HClO4 (blue) with reference to the initial white-line intensity height (at 22117 eV) at OCV.

Figure 6. Changes in the X-ray energy position from the position at OCV for SRO-p (a) in KOH and (b) in HClO4; (c) operando XANES of RuO2 was observed only in HClO4. For SRO-p in 0.1 M KOH electrolyte, the continuous shift of the absorption edge to higher energies upon each increasing potential step is observed, which can be ascribed to the


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simultaneous increase of the Ru oxidation state (see Figure 4a). The early increase of oxidation state directly manifests the initial oxidative transformation of the Ru species of SRO toward the hydrous RuO2 (RuO2(OH)2) in oxidation state of VI as previously mentioned (Table S1).42 Likewise, the subsequent oxidation state increases can be ascribed to the rest of previously described OER pathway of RuO2, which eventually further oxidizes to a higher valence state as polarization continues into the oxygen evolution regime. As shown in Figure 6a, for SRO in alkaline electrolyte, the energy shift reached its maximum (ca. 2.14 eV) by the end of the anodic sweep. Particularly, when the potential is held above 1.3 VRHE, the edge position is shifting more rapidly indicative of the increase in oxidation state reaching its maximum at 1.425 VRHE. This behavior of changes in the oxidation state is in good agreement with previously reported XANES spectra of thin film SrRuO3 model electrodes.21-22 Note that only a small increase of Ru oxidation state (from ~IV to ~V of oxidation state) is deduced from this insignificant edge-shift on the basis of reference curves established in past studies62-63. This indicates that the Ru species in SRO system deviates from the aforementioned OER pathway (Table S1) after its oxidation state increases from its initial IV. Intriguingly, at that very potential (1.425 VRHE), we observe a rapid increase in current density from its CV (Figure S1a, red solid line), which indicates the increase of the oxidation state of Ru species of SRO-p. Given the estimated small change of Ru oxidation state deduced from the edge-shifts, it is certain that this increase of current during the initial cycle is not related to the OER process as described in Table S1. Furthermore, the white-line intensity of the non-normalized spectra significantly decreases above 1.425 VRHE, which signifies the loss of the absorbing species (i.e. Ru) from the electrode (~30 %; see Figure 5). The correlation between the potential-induced current and the absorption spectra of SRO-p (i.e. the progressive loss of amplitude and the shift towards higher energy above 1.425


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VRHE) suggests that the dissolution of Ru species is principally responsible for the current responses upon polarization. Thus, the observed current density loss in the CV after CA measurements (Figure S1 dashed line) can be rationally related to the loss of Ru active sites. Upon the subsequent cathodic sweep, the spectra remain rather consistent without any additional shifts. This irreversible increase of oxidation state of Ru suggests that irreversible changes (i.e. dissolution) have taken place during the first anodic polarization in alkaline electrolyte. Additionally, the appearance of a pre-edge peak is detected upon polarizing above 1.4 VRHE. The presence of pre-edge peak in the XANES spectra of other SRO systems was already reported in past studies22, 63-65. The emerging pre-edge peak accompanied by the construction of an isosbestic point suggests that SRO perovskite structure may have gone through distortion.63, 66 The operando XANES spectra of SRO-p (Figure 4b) in the acid flow cell exhibits similar magnitude of peak shifts as in KOH (compared in Figure 6b). Referring to Figure 5 (blue line), the fact that the absorption intensity severely diminishes above 1.425 VRHE to an extent that the electronic state could not be inferred accurately indicates that the Ru is spontaneously removed from the path of X-ray beam. Thus, only the operando XANES spectra up to those which exhibit sufficient intensity were taken into the consideration for the reasonable measure of oxidation state. As speculated from its increase of current density, it is certain that the SRO-p is more readily dissolved in acid during the polarization than in alkaline electrolyte. Yet, the comparable energy shift of spectra below 1.425 VRHE in acid implies that Ru is going through similar extent of transformations as in alkaline in terms of its oxidation state. In contrast, the XANES spectra of RuO2 do not reveal a prompt dissolution as SRO-p owing to having invariable RuO2 layers throughout the coated thickness on Kapton film, which induces


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rather consistent absorption spectra continuously reflecting on the changes in Ru. Nevertheless, after a repetitive polarization, a similar decaying behavior would be expected from the absorption spectra of RuO2. Overall, our analyses of the operando XANES spectra of SRO-p in both alkaline and acid electrolytes support its inability to withstand the polarization into the oxygen evolution regime (above 1.4 VRHE) regardless of being in either basic or acidic media. Especially, SRO-p showed a loss of Ru species via dissolution throughout the oxidation process in acid. This provides reasonable explanations for the observations made from the CVs. Unlike a single metal oxide, the dissolution of a perovskite is intricate by having two metal cations, in which the A-site metal (i.e. Sr) follows the dissolution route of the B-site metal.3 The dissolution of the metal cations were monitored through inductively-coupled plasma optical emission spectrometry (ICP-OES) of the electrolyte collected from the operando XANES flow cells tests (see Table 3). Comparing ratios between the dissolved Ru and A-site metal (i.e. Ru/Sr) from both electrolytes reveal that much more Sr (by about an order of magnitude) was dissolved in acid compared in alkaline. Expectedly, this additionally supports that the dissolution of Sr into aqueous phase occurs more rapidly in acid due to the higher proton concentration than in alkaline. It should be noted that ICP-OES results represents only the concentration of dissolved metal contents in aqueous phase, not the bulk solid phase that also may have been dispatched from Kapton film.


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Table 3. The summary of ICP-OES measurements. Indicated concentrations are normalized by the weight of the catalyst loading

Sr

Ru

Electrolyte

[ppm -1 mgloading ]

[ppm -1 mgloading ]

KOH

13.93

19.90

1.43

HClO4

70.50

11.18

0.16

‫ܝ܀‬ൗ ‫ܚ܁‬

In order to assess the thermodynamic stability of the perovskite, Pourbaix diagrams are constructed to learn about their phase stability in aqueous solutions. Thereby, we can better estimate the intrinsic stability of a material at the necessary potentials for OER in a specified pH level (pH 1 for 0.1 M HClO4 and pH 13 for 0.1 M KOH). Figure 7a depicts thermodynamically favored phases for SRO-p for the range of potentials in which the flow cells were polarized during the operando XANES measurements (indicated by thick vertical solid lines; 1.2 – 1.6 VRHE) in both alkaline and acidic electrolyte (vertical dashed lines; red for acidic and blue for alkaline electrolyte). The standard hydrogen scale was used in the Pourbaix diagram. As a result, Figure 7a evidently highlights the dissolution of Sr cation (Sr(aq)2+) in most of the pH domains in the working potential ranges in both acidic and alkaline electrolyte (red and blue thick vertical solid lines, respectively), which underlines the spontaneous dissociation of SRO from its perovskite structure (or in any other phases) via immediate release of Sr species into the aqueous phase even at its open circuit potential (OCP). Meanwhile, within the range of potentials in both acid and alkaline media, only the dissolved Ru reveals to be subjected to the phase transformation from RuO2 (IV) to RuO4 (VIII) at ca. 1.3 VRHE (corresponds to 1.24 VSHE and 0.54 VSHE at pH 1 and 13, respectively). This Ru transition is also consistent with the Pourbaix


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diagram of RuO2 (see Figure S4), which shows the abovementioned transitions at the same potentials as in the SRO-p’s Pourbaix diagram. In solution, these species would be present in their aqueous phase (RuO42- and its derivatives).42 Thereby, the Ru from SRO would be solely responsible for its OER while also being subjected to dissolution throughout the oxidation process. This serves as the supporting evidence to the operando XANES results, with the rapid peak shift observed above 1.3 VRHE along with the decrease of absorption spectra intensity. In summary, the computed Pourbaix diagram supports that SRO-p is readily decomposed through chemical dissolution in any aqueous solution, thus, incapable of retaining its solid perovskite phase characteristics for any applications involving a contact with water-based solutions, which primarily includes any oxygen evolution reactions. Additional to the dissolution of SRO-p, we also describe the thermodynamic stability of LRO-p in aqueous environment – under hypothetical conditions where solid LRO-p phase is stable – to further draw remarks on LRO-p’s properties to support its unsuitability as an OER catalyst. Similar to the case of SRO-p, Figure 7b clearly shows that La is spontaneously dissolved from the solid to the aqueous phase via chemical dissolution within in a wide potential range. This indicates that the phase integrity of LRO-p would not be conserved in contact with aqueous solutions, even if the LRO-p phase was hypothetically stable. Consequently, as explained in the case of SRO-p, only the exposed Ru atoms at the surface would serve as active sites for the oxygen evolution reaction. Thus, when potential is applied, the consequential currents would be ascribable to the similar dissolution process of SRO-p as described, in which the dissolved Ru would principally be responsible.


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Figure 7. Pourbaix diagrams of (a) SRO-p and (b) LRO-p based on the DFT calculations. The vertical lines indicate the pH levels of the electrolytes that the materials were tested; and the marked show the potential range of the in-situ flow cell testing. Although the computed Pourbaix diagram shows that the dissolved RuO2 from SRO-p undergoes the same extent of transformation in its oxidation state in both pH 1 and 13, we observed from the operando XANES spectra that SRO-p is subjected to a more aggressive dissolution in acidic than in alkaline electrolyte (Figure 5, blue line). The hampering of Ru dissolution in alkaline electrolyte has been explained in earlier studies57, 61 in which the authors explain that the presence of oxygen-bridged perruthenate (RuO4-) or ruthenate oxy-hydroxide (RuO4(OH)2-) formed at the surface of catalyst in alkaline media is supposedly more stable than RuO4. The above findings suggest that the activity of SRO-p is dependent on its thermodynamic nature under OER conditions, such that the dissolution of metals would play a key role. Previously, we inferred from the Pourbaix diagram that the Sr cation would be dissociated from the structure in aqueous electrolytes independent of the pH within our working potential. In order to elaborate on the activity with regards to dissolution, we have observed the changes in CV at different


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electrolyte contact periods. Figure 8 shows decrease of current density as a function of contact period in different electrolytes. This test explicitly manifests the relationship between the activity loss and the dissolution of materials. The observed current density over the contact time for SRO-p in both alkaline and acidic electrolytes (Figure 8a and b, respectively) shows similar behavior of losses as its respective absorption intensities during the operando XANES measurements. Particularly, for SRO-p in acid (Figure 8b inset), the open circuit potential (OCP) value is decreased in a similar behavior as the current density (measured at 1.45 VRHE) showing half of its initial OCP after 24 hours (OCPinitial = ~1.25 VRHE ; OCP24 hours = ~0.65 VRHE). The loss of OCP in acid additionally highlights the severe loss of material from the current collector as we have deduced from Figure 5 (blue line). In contrast, Figure 8a (inset) reveals the stable OCP for SRO in alkaline media while the current density has decreased by about 10 folds after 24 hours. This may be due to the nature of alkaline media in which the metal oxide is less prone to dissolution as compared to acidic. Yet, the behavior of its current density still signifies the loss of active species over time.


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Figure 8. The chemical stability of SRO in contact with electrolytes. CV at 10 mV sec-1 was executed at 0, 2, 4, 6, 8, 12, and 24 hours after the initial contact with synthetic air saturated electrolyte without agitation (a) in KOH and (b) in HClO4. Insets show the decrease of the current density at 1.45 VRHE for every time interval (indicated by colored dots in the black line), and the OCP collected during those contact period (hollow blue dots in the blue line).


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All of the presented evidences together attest to the vulnerable characteristics of SRO-p as the candidate for OER. Essentially, the thermodynamic stability of SRO is determined by the computed phase diagram; its presence in perovskite structure (SrRuO3) is confirmed by XRD analyses. However, the severe loss of both absorption edge-step intensity (in situ XANES) and current density upon polarization point toward the fact that SRO-p swiftly follows the chemical dissolution route once in contact with aqueous electrolyte, which leads to the dissociation of the perovskite into RuO2 and Sr(aq)2+. Subsequently, the dissolved RuO2 undergoes several potentialinduced transformations following its typical electrochemical oxide pathway to a higher valence state (RuO4), which is subjected to dissolution (in form of ruthenates, RuO42-) above 1.4 VRHE as it simultaneously participates in the OER. Based on the DFT calculated Pourbaix diagram, the dissolution to Sr(aq)2+ takes place in a wide range of potentials in the pH domains below 13. Finally, these revealed properties, together, suggest that the measure of the OER activity of SRO is impracticable attempt based on its thermodynamic instability under the aqueous environment leading to complete degradation of its solid phase integrity through the development of dissolution. In addition to the case of SRO-p, the DFT computed phase diagram and the XRD results point out the physical instability of the targeted LRO-p, thereby rendering other LRO crystalline structures (LRO-m). Thus, the infeasibility of attaining LRO-p, as expected from its phase diagram, only allowed us to project its intrinsic behavior during the electrochemical reactions. Similar as to SRO-p, the Pourbaix diagram of LRO-p predicts that it would be chemically dissolved in aqueous solutions, in which the compositional metals would dissociate into La3+ cations and RuO2. Thereby, complete disintegration of LRO-p by the electrode would be expected over time.


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4. CONCLUSION In summary, we have investigated the integrity of SrRuO3 and LaRuO3, which were expected to deliver low overpotential for the OER based on theoretical calculations. Throughout the work, we were able to establish thermodynamic insights into the stability of these Ru-based perovskites built from the consolidated approach of theoretical computations and experimental analyses. We utilized theory-based phase diagrams constructed by DFT simulations to corroboratively confirm the identified phases of synthesized materials by XRD analysis. The results have revealed that SRO is thermodynamically stable in its perovskite phase based on the reaction energy. In contrast, the perovskite phase of LRO is found to be thermodynamically unstable as confirmed by XRD analysis of the synthesized powders. The decay of activity of the prepared SrRuO3was monitored throughout the oxidation process in both alkaline and acidic electrolyte via RDE tests and operando XANES spectroscopy. Through operando XANES spectroscopy, we were able to confirm the further dissolution of Ru species upon anodic polarization. From our analysis, we have gained insights into intrinsic features of SrRuO3 which served as aids to unravel the true origin of its ambiguous activity towards OER as reported so far. We have learned that studied strontium ruthenium oxide in perovskite structure is subjected to spontaneous chemical dissolution in an aqueous solution, which dissociates Sr cation into the aqueous phase while the Ru cation follows its electrochemical oxide pathway of its rutile form. Additionally, the calculated Pourbaix diagram was utilized as the verifying evidence to support its phase instability in aqueous environment. We conclude that SrRuO3 is an inadequate candidate for OER considering their thermodynamic instability in aqueous solutions. Furthermore, our analyses revealed that the measure of true


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intrinsic activities of such materials is inaccessible due to the concurrent dissolution under the essential conditions for the OER. Therefore, it is critical to understand and address the origin of stability under the real operating conditions in order to design an ideally sustainable oxygen catalyst.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Cyclic voltammetry and chronoamperometry of RuO2 and SRO-p; Oxygen evolution pathways for RuO2 in alkaline and acidic electrolytes; ex-situ X-ray absorption near edge spectra of dry electrodes; DFT computed Pourbaix diagram of RuO2.

AUTHOR INFORMATION Corresponding Author *E. F.: E-mail: [email protected]. Telephone: +41 (0)56 310 45 80 Author Contributions B.K., E.F., and T.J.S. developed the concept. F.B. and T.G. synthesized the nano-scaled perovskites. D.L. and C.C. synthesized RuO2. B.K. carried out physical characterizations, electrochemical measurements, and analyses. I.E.C. and N.M. carried out DFT computations. R.S. provided access and supervised transmission electron microscope sessions. B.K., M.N., E.F., D.F.A., and X.C. carried out the experiments at the SuperXAS beamline. M.N. guided the operando XAS measurements. B.K., D.F.A., E.F., X.C. and T.J.S. discussed the results. F.B., I.E.C., M.N., R.S., T.G., C.C., and N.M. revised the manuscript.


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ACKNOWLEDGMENT The authors gratefully acknowledge the Swiss National Science Foundation through its Ambizione Program, CCEM (RENERG2), the Swiss Competence Center for Energy Research (SCCER) Heat & Electricity Storage, and the Swiss National Science Foundation within NCCR Marvel and Paul Scherrer Institute for financial contributions to this work. The authors thank the Swiss Light Source for providing beamtime at the SuperXAS beamline.


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Unraveling Thermodynamics, Stability, and Oxygen Evolution Activity

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