Oxygen Evolution Reaction on La1âxSrxCoO3 Perovskites: A

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The oxygen evolution reaction on La1-xSrxCoO3 perovskites: A combined experimental and theoretical study of their structural, electronic, and electrochemical properties. xi cheng, Emiliana Fabbri, Maarten Nachtegaal, Ivano E. Castelli, Mario El Kazzi, Raphael Haumont, Nicola Marzari, and Thomas J. Schmidt Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03138 • Publication Date (Web): 04 Nov 2015 Downloaded from http://pubs.acs.org on November 5, 2015

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Chemistry of Materials

The oxygen evolution reaction on La1-xSrxCoO3 perovskites: A combined experimental and theoretical study of their structural, electronic, and electrochemical properties. Xi Cheng,*,† Emiliana Fabbri,*,† Maarten Nachtegaal,‡ Ivano E. Castelli,# Mario El Kazzi,† Raphael Haumont,& Nicola Marzari,# and Thomas J. Schmidt†,§ †

Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen, Switzerland

‡

Paul Scherrer Institut, 5232 Villigen, Switzerland

#

Theory and Simulation of Materials (THEOS), and National Centre for Computational Design and Discovery of Novel Materials (MARVEL), École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland &

SP2M, ICMMO, Université Paris-Sud XI, 91405 Orsay, France

§

Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland

ABSTRACT: The bulk electronic structure, surface composition, conductivity and electrochemical activity towards the oxygen evolution reaction for the La1-xSrxCoO3 perovskite series (with x=0, 0.2, 0.4, 0.6, 0.8, 1) are investigated experimentally and theoretically. It is found that Sr substitutions have the effect of straightening the octahedral cage, aligning atoms along the Co-O-Co axis and increasing the average oxidation state of the Co cations. As a consequence, both the ex-situ electronic conductivity as well as the activity towards the oxygen evolution reaction are considerably improved. According to density-functional theory calculations, the alignment of the Co-O-Co bonds and the oxidation of the Co cations enhance the overlap between the occupied O-2p valence bands and the unoccupied Co-3d conduction bands, rationalizing the improvement of the conductivity as a function of the Sr fraction. Additionally, a study of the surface properties as a function of the Sr fraction, carried out by X-ray photoelectron spectroscopy (XPS), provides insight both on surface composition and its effects on the OER activity.

Introduction

B-site have shown the potential of being efficient oxygen electrodes in alkaline solutions.4, 6 Furthermore, it was demonstrated that their physico-chemical properties as well as their catalytic activity can be significantly influenced by substitution or partial substitution of the A and/or B-site by other elements resulting in (AxA’1-x)(ByB’1-y)O3 compositions. By virtue of this flexibility, extensive studies have been carried out on perovskites in order to understand the parameters determining their electrocatalytic activity,6-9 with the further purpose of improving known materials or predicting new active materials. For example, an early study of Matsumoto et al.10 proposed that the OER activity of perovskite oxides would be enhanced by widening the σ∗ band. The authors suggested that an extended σ∗ band should enhance the electron transfer between the OH- and the oxide catalyst surface. A different view was proposed by Bockris and Otagawa,11 which claimed that the OER activity would increase as the B-OH bond strength decreases. These

Direct electrochemical water splitting is considered a key process in the development of novel energy storage systems, crucial for a sustainable and environmentally friendly energy economy. Water electrolyzers can convert water into hydrogen and oxygen through an electrochemical process, allowing H2 to be stored as an energy vector.1 In addition, the O2 that is concomitantly produced can be used for several processes.2, 3 However, the overpotentials at the anode side where the oxygen evolution reaction (OER) takes place are substantial, even when highly active, precious-metal catalysts are used.4 Therefore, the development of anode materials based on inexpensive and abundant elements, displaying both high OER activity and stability appears to be a crucial point towards the development of new generation hydrogen-based storage systems.4, 5 Perovskite oxides (ABO3) with alkaline or rare-earth cations in the A-site and first-row transition-metal cations in the 1

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authors surmised that the increase in the number of d-electrons of the perovskite transition metal cations (B-site) would weaken the B-OH bond strength. A couple of decades later, Rossmeisl and co-workers8 and Shao-Horn and co-workers6 suggested taking the difference between the surface binding energies of O* and HO* (∆GO*-∆GHO*) and the eg filling of surface transition-metal cations, respectively, as descriptor of the OER activity. More recently, Koper and co-workers7 showed a correlation between the bulk oxide formation energy and the OER activity. However, the descriptors above were proposed considering a single physico-chemical property of perovskites from different families (i.e. varying the A-site and B-site at the same time).6, 8, 11 This might reduce the predictive power of the chosen descriptor, since varying both the A-site and B-site divers several changes in the physico-chemical properties of the considered oxides, making it more challenging to find out how the different properties relate to each other and to catalytic activity.

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spectroscopy (XPS). These results show a considerable degree of La and/or Sr excess at the surface, which suggests that the activity of La1-xSrxCoO3 might be further improved by reducing the segregation of La and Sr towards the surface.

Experimental Material synthesis La1-xSrxCoO3 powders were synthesized with a modified sol-gel process. In brief, stoichiometric quantities of commercial La2O3 (Aldrich, 99.9%), Sr(NO3)2 (Aldrich, 99%), Co(NO3)2·6H2O (Aldrich, 99%) precursors were prepared in an aqueous solution of 0.2 M nitric acid, to which citric acid (Aldrich, 99.5%) was added as a chelating agent in a 2:1 ratio with respect to the total metal cations. After a transparent solution was achieved, the pH was adjusted between 9 and 10 by NH4OH additions. The solution was then heated under stirring to evaporate water until it changed into a viscous gel and finally ignited to flame, resulting in a black ash. To obtain a single-phase perovskite material, the LaCoO3 powder was calcined at 1000°C for 4h in O2. The SrCoO2.5 powder was calcined at 900°C for 8h in O2, while other La1-xSrxCoO3 (with x= 0.2, 0.4, 0.6, 0.8) powder were calcined at 1050°C for 4h in O2.

Thus, in the present work, we select a relatively simple system to carry out our study: the La1-xSrxCoO3 series (with x=0, 0.2, 0.4, 0.6, 0.8, 1). This choice is based on several reasons: first, by only varying the A-site composition, one can closely monitor the relative tuning of bulk structure,12-14 electronic properties12, 13, 15 and conductivity.16-18 Second, according to earlier density-functional theory calculations,7, 8 SrCoO3 and LaCoO3 are located on and below the top of a volcano plot for the OER activity. Both of these aspects make the La1-xSrxCoO3 system a key example to study the influence of bulk properties on the electrochemical activity towards the OER.

Physico-chemical characterization Phase identification for the La1-xSrxCoO3 powders was carried out by X-ray diffraction (XRD, Bruker D8 system) with Cu Kα polychromatic radiation (λ = 0.15418 nm) in a Bragg-Brentano geometry. To obtain the X-ray Rietveld refinements, diffraction patterns were collected with a X’celerator Phillips expert diffractometer between 2θ = 10° and 100°, with a step of 0,004°, using Cu Kα1 monochromatic radiation (λ = 0.15406 nm). Rietveld refinements were done with the XND software.19 The specific surface area of the powder was determined by Brunauer-Emmett-Teller (BET) analysis. To measure the ex-situ conductivity for each material, impedance spectroscopy measurements were performed. The oxide powders were kept under a constant pressure of 0.6 MPa for 5 minutes and the electrical resistivity was evaluated by 4-wire impedance spectroscopy measurements at room temperature applying a bias of 100 mV in the frequency range between 1 MHz and 1 Hz. X-ray photoelectron spectroscopy (XPS) measurements were performed using a VG ESCALAB 220iXL spectrometer (Thermo Fischer Scientific) equipped with an Al Kα monochromatic source and a magnetic lens system. The binding energies of the acquired spectra were referenced to the C 1s line at 284.9 eV. Background subtraction has been performed according to Shirley,20 and the atomic sensitivity factors (ASF) of Scofield were applied to estimate the atomic composition.21 For X-ray absorption near-edge

In the following we first provide X-ray Rietveld refinements and X-ray absorption near-edge structure (XANES) spectra to show how the progressive substitution of La by Sr aligns the atoms along the Co-O-Co bonds and oxidizes the Co cations, respectively. Simultaneously, we note an improvement of the ex-situ electronic conductivity as a function of the Sr fraction. Density-functional theory calculations allow to link these physico-chemical properties to each other, since the alignment of the Co-O-Co atoms and the oxidation of the Co cations enhance the overlap between the occupied O-2p valence bands and the unoccupied Co-3d conduction bands, thus improving the ex-situ electronic conductivity. Further investigations show that a correlation does exist between the OER activity and the conductivity of La1-xSrxCoO3 oxides, which allows us eventually to relate the OER activity and the electronic structure of this perovskite series. Additionally, since the structure and the composition of the oxide surface might differ from that of the bulk, and influence the OER activity, we also present a surface characterization by X-ray photoelectron 2


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Chemistry of Materials between 1-1.7 V (RHE) at 10 mVs-1 and 1600 rpm in synthetic air-saturated electrolyte, chronoamperometry measurements holding each potential for 30 sec (synthetic air-saturated and 1600rpm) were performed. The chronoamperometry measurements allow obtaining an almost steady-state current with no capacitive contribution (See supporting information Figure S1 and Figure S2). The steady-state current (noted from the chronoamperometry measurement at an overpotential of 0.3 V vs. RHE) normalized by the mass as well as the BET surface area of the perovskite oxide was taken as a parameter of OER activity. All the potentials were corrected for the ohmic-drop measured by impedance spectroscopy.

structure (XANES) spectroscopy measurements, La1xSrxCoO3 powders were dispersed in isopropanol and sonicated for 45 minutes (Bandelin, RM 16 UH, 300 Weff, 40 kHz). The ink was dried at ~40°C and the powder was diluted in cellulose and prepared as pellets for XANES measurements in transmission mode. XANES spectra at the Co 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 Si-coated mirror (at 2.8 mrad which also rejects higher harmonics), monochromatized by a Si (111) channel-cut monochromator. The rejection of higher harmonics and focusing were achieved by a Si-coated collimating mirror at 2.8 mrad and focused by a Rh-coated toroidal mirror at 2.8 mrad to a spot size of Ø1mm, respectively. Spectra were background corrected and normalized. Reference spectra of a Co foil were simultaneously collected for absolute energy calibration.

Results and discussion Structure characterization Figure 1 shows the X-ray diffraction (XRD) patterns of La1-xSrxCoO3 (with x=0, 0.2, 0.4, 0.6, 0.8, 1). An almost pure perovskite phase is achieved for 0≤x≤0.8; the tiny peak denoted by (°) corresponds to a (LaySrx)2CoO4 secondary phase.27 We also observe a phase transition from a rhombohedral (LaCoO3) to a cubic structure (La0.2Sr0.8CoO3). For x=1, the XRD pattern shows a hexagonal structure with a composition close to SrCoO2.5,28 in coexistence with a small amount (less than 5%) of spinel Co3O4 phase (peaks denoted by *). Note that the asymmetric shape of Bragg peaks suggests a complex mixture of SrCoO2.5+δ with different oxygen content (δ) (see supporting information Figure S3).

Density-functional theory calculations We complemented the experimental analysis with first-principles calculations, using density-functional theory as implemented in the Quantum-ESPRESSO distribution22 to investigate the electronic structure of the La1-xSrxCoO3 series. We used the generalizedgradient approximation as implemented in the PBEsol exchange-correlation functional22, 23 and pseudopotentials from the recently proposed Standard Solid State Pseudopotential library (SSSP accuracy).24 To reproduce correctly the semiconducting behavior of LaCoO3, we have included a Hubbard correction U = 3.5 eV to the Co sites.

For a more precise crystal structure analysis of the solid solutions, X-ray Rietveld refinements are

Electrochemical characterization For the electrochemical characterization, thin-film rotating disk electrode (RDE) measurements were performed.25, 26 Thin-film electrodes were prepared from an ink suspension consisting of 7.5 mg of the oxide powder, 2.5 mL isopropanol and 10µL Na+-exchanged Nafion as a binder. The ink was sonicated for 30 mins and 20 µL of the ink were dropped and then dried on a rotating mirror polished glassy carbon electrode (0.196 cm2), resulting in an oxide catalyst loading of 300 µg.cm-2. The OER was investigated using a home-made Teflon cell with a Biologic VMP-300 potentiostat system. The working electrodes were immersed under potential control (1.0 V vs. RHE) in a 0.1 M KOH electrolyte at room temperature and the measurements were performed using a hydrogen reference electrode (RHE) separated by a salt bridge with diffusion barrier and a gold counter electrode in a three electrode configuration. The 0.1 M KOH electrolyte was prepared from Milli-Q water and KOH pellets (Sigma Aldrich, 99.99%). After 30 reverse scan sweeps

Figure 1. X-ray diffraction (XRD) patterns for La1-xSrxCoO3 (x=0, 0.2, 0.4, 0.6, 0.8, 1); the tiny peak denoted by ° indicates a (LaySrx)2CoO4 secondary phase; the peaks denoted by * indicate the Co3O4 spinel phase. 3



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undertaken, except for the case x=1 due to its mixture of compositions (SrCoO2.5+δ). For x≤0.4, the structure is well described by the R-3c space group, while for x≥0.6, the symmetry becomes cubic (Pm-3m space group). Figure 2 shows an example of the observed and calculated diffraction patterns for these two domains (x=0.2 and x=0.8). Table 1 presents the refinement agreement factor, cell parameters, atomic coordinates, and selected interatomic distances. The

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Co-O-Co angle along chains of CoO6 octahedra is also shown, whereas the evolution of the pseudocubic (x≤0.4) and cubic (x≥0.6) cell parameters and Co-O-Co angles as a function of x is plotted in Figure 3 (for the pseudocubic cell, see supporting information Figure S4). We observe that the more La is replaced by Sr, the less is the difference between the lattice constants apc and cpc, suggesting a decrease of the rhombohedral distortion as a function of x. The

Figure 2. X-ray Rietveld refinements of La0.8Sr0.2CoO3 (left) and La0.2Sr0.8CoO3 (right).

Table 1. Results of Rietveld refinement

La1-xSrxCoO3 x=0 R-3c

x=0.2 R-3c

x=0.4 R-3c

x=0.6 Pm-3m

x=0.8 Pm-3m

a* c*

5.4467 Å 13.3011 Å

5.4489 Å 13.1876 Å

5.4364 Å 13.2340 Å

apc# cpc#

3.8513 Å 3.7819 Å

3.8529 Å 3.8069 Å

3.8441 Å 3.8203 Å

3.8327 Å

3.8352 Å

La/Sr Co O

6a (0 0 ¼) 6b (0 0 0) 18e (0.5467 0 ¼)

6a (0 0 ¼) 6b (0 0 0) 18e (0.5452 0 ¼)

6a (0 0 1/4) 6b (0 0 0) 18e (0.5367 0 ¼)

1a (0 0 0) 1b (½½ ½) 3c (½ ½ 0)

1a (0 0 0) 1b (½½ ½) 3c (½ ½ 0)

d(La/Sr-O)

2.4688 Å 2.7028 Å 2.9780 Å

2.4783 Å 2.7140 Å 2.9708 Å

2.5187 Å 2.7144 Å 2.9179 Å

2.7102 Å

2.7120 Å

d(Co-O)

1.9311 Å

1.9346 Å

1.9285 Å

1.9164 Å

1.9177 Å

Co-O-Co angle

164.83°

165.43°

168.09°

180°

180°

Rwp RBragg DoF

4.15% 3.05% 1.26%

4.52% 3.50% 1.36%

4.29% 4.37% 1.43%

4.09% 3.25% 1.47%

3.30% 3.15% 1.34%

(* a and c are cell parameters expressed with hexagonal setting R-3c. #Pseudocubic parameters apc and cpc are calculated with following equations: apc = a/√2, cpc = c/2√3, see supporting information Figure S4) 4


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transition between the two phases is expected at 0.4≤x≤0.6, in agreement with the value x=0.55 reported in the study of Mineshige et al.,14 about La1-xSrxCoO3 (0≤x≤0.7) series. Moreover, the cubic cell parameter extrapolated in Figure 3a (a ≈ 3.84 Å) for SrCoO3 agrees with the value reported by Nemudry et al.,29 who successfully synthesized cubic SrCoO3+δ with different oxygen content (δ) by electrochemical oxidation of brownmillerite SrCoO2.5. An abrupt reorganization of ionic bonds occurs at the phase transition: in the R-3c phase, the (La/Sr)O12 cubooctahedron exhibits 3‘short’, 3 ‘long’, and 6 ‘medium’ bonds, while in the cubic state, these 12 distances are equal (Table 1). This phenomenon seems to be a common feature for the rhombohedral perovskite family with R1-xR’xCoO3 composition,30 where the R-3c symmetry allows a certain flexibility of the oxygen position in the lattice, inducing different(La/Sr)-O distances, but also tilting the BO6 octahedra.

decreases rapidly with the increase of the Sr content (see Table 1, the difference between different (La/Sr)-O distances decreases as a function of the Sr fraction). As a consequence, the Co-O-Co angle increases up to 180° (Figure 3b). The oxygen shift leads to the phase transition, and the abrupt feature of this transition around x ≈ 0.5 indicates a drastic change in the local chemistry of the material. The chemical substitution of La by Sr is also well known to create oxygen vacancies in the compound.14 Such vacancies cannot be taken into account by XRD analysis with accuracy and neutron diffraction refinement would be necessary to quantify the concentration of oxygen vacancies, and to investigate their role in the R-3c to Pm-3m phase transition. Ex-situ electronic structure

The Rietveld refinement shows that the oxygen shift

conductivity

and

electronic

Figure 4 shows the ex-situ electronic conductivity of La1-xSrxCoO3 compressed powders measured by impedance spectroscopy at room temperature in a tailored 4-probe electrode set up. The conductivity significantly increases from LaCoO3 to La0.8Sr0.2CoO3 by a factor of 300. However, as the value of x rises, the incremental increase of the conductivity becomes less pronounced, e.g. from x=0.4 to x=0.8 the increase of the conductivity is only by a factor of 3. Furthermore, SrCoO2.5 presents a relatively high resistivity which is mostly assigned to its non-perovskite-type. The previous study on the La1-xSrxCoO3 series10, 14 reported a similar evolution for the electronic conductivity as a function of x. However, the electronic conductivity of the present La1-xSrxCoO3 samples is much lower than that reported by Matsumoto et al.14 (0.2 vs. 2000 S cm-1). This is due to the fact that in the present work the conductivity is measured on slightly compressed powder, which

Figure 3. Evolution of the pseudocubic (x≤0.4) and cubic (x≥0.6) cell parameter (a) and Co-O-Co angle (b) as a function of the Sr fraction. (The open circle in figure 3a indicates the value of SrCoO3+δ cubic reported by 29 Nemudry et al. )

Figure 4. Ex-situ electronic conductivity of La1-xSrxCoO3 powders as a function of the Sr fraction.

5



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presents a higher contact resistance than the sintered samples used by Matsumoto et al.. Matsumoto et al.10 attributed the evolution of the conductivity as a function of x to the broadening of the σ∗ band. However, according to other reports,14, 16, 31 the effect of Sr substitution on the conductivity of La1-xSrxCoO3 might be more complex than this. In fact, LaCoO3 oxide is a charge transfer (CT) insulator, which has a relatively wide band gap (∆) between the occupied O-2p valence bands and the unoccupied Co-3d conduction bands. According to our XRD results, the substitution of La3+ by the larger Sr2+ cations aligned the Co-O-Co atoms (from 164.83° of LaCoO3 to ideal 180° of La0.4Sr0.6CoO3), resulting in a structural change from a rhombohedral distorted perovskite to a cubic structure. Mineshige et al.16 demonstrated that the increase of the Co-O-Co angle would broaden both the occupied O-2p valence bands and the unoccupied Co-3d conduction bands, thus enhancing their overlap and increasing the electronic conductivity (See supporting information Figure S5). These authors showed further that the critical value of the Co-O-Co angle for the insulator to metallic transition was 165°, which corresponded to the value of x≈0.25 in La1-xSrxCoO3.

Figure 5. Co K-edge XANES spectra of the La1-xSrxCoO3 (x=0, 0.2, 0.8) at room temperature; the inset shows the main peak region.

La1-xSrxCoO3 with x≥0.2, and that it increases with the increase of the Sr fraction. Prompted by these results, we perform density-functional theory (DFT) calculations in order to investigate further and rationalize the evolution of the ex-situ electronic conductivity of La1-xSrxCoO3 as a function of the Sr fraction. In the first set of calculations, we fix the stoichiometry to that of LaCoO3 and only change the Co-O-Co angle (θ).

Moreover, when La3+ is replaced with Sr2+, the excess negative charge induced by the Sr substitution could be compensated for by the creation of holes, i.e. an oxidation of Co3+ to Co4+ (equation 1 and 2), or by the creation of oxygen vacancies (equation 3). LaLax + SrO h. + CoCox LaLax + SrO

SrLa’ + h. +OOx

(1)

CoCo.

(2)

Figure 6a shows that when the Co-O-Co angle increasesfrom 164.8° to 168.1°, the O-2p valence band shifts up while the Co-3d conduction band stays unchanged, reducing the band gap (∆) and improving the conductivity. At 180°, the Co-3d conduction band broadens and the gap is completely closed. In the second set of calculations (Figure 6b), the rhombohedral structure is instead kept fixed, while the effect of different oxidation states for Co is studied (the oxidation state of Co changes with the increase of the Sr fraction). Since in the calculations a hexa-rhombohedral unit (6xABO3) is used, therefore the changes in composition for the A-cations can take place only in steps of 1/6=0.167. Figure 6b shows that as the Sr fraction increases, the Co-3d conduction band shifts down while the position of the O2p valence band remains unchanged. The DFT gap (underestimating the experimental one) is almost closed for x=0.833. Therefore, from Figure 6a and 6b, we deduce that both the alignment of the Co-O-Co bonds and the increase of Co oxidation state can reduce the band gap and improve the conductivity. However, neither of these two effects can explain the large change in conductivity in going from LaCoO3 to La0.6Sr0.4CoO3 observed in our experimental data, since for the latter composition the band gap depicted in Figure 6a and 6b remains still too large.

SrLa’ + ½ VO.. +OOx (3)

While some authors supported the existence of Co4+ ions throughout the La1-xSrxCoO3 when x > 0,12-15 others 31, 32 reported little or no changes in the Co oxidation state. With Hueso et al.32 reporting that the differences in the cationic charges of La3+ and Sr2+ might be mainly compensated for by the formation of oxygen vacancies. To clarify the effect of Sr substitution on the Co oxidation state in La1-xSrxCoO3, we performed X-ray absorption near-edge structure (XANES) spectroscopy measurements on selected La1-xSrxCoO3 (x=0, 0.2, 0.8). Figure 5 shows the Co K edge XANES spectra of La1-xSrxCoO3 (x=0, 0.2, 0.8) at room temperature. A close examination of the spectra reveals a shift of the main peak towards a higher energy range concomitantly with the increase of the Sr fraction. According to the literature,12, 13, 33 a shift of the main peak of the Co K edge towards higher energy range indicates an oxidation of Co. Thus, the XANES results confirm that the average of the Co oxidation state is higher than 3+ for

Therefore, we perform a third set of calculations which take into account the combined effects of the Co-O-Co angle, structure and Co oxidation state (Figure 6c). A hexa-rhombohedral unit (6xABO3) is used for x≤0.4, while a supercell of 2x2x2 cubic primitive cell is used for x≥0.6, 6


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in this latter case which the composition steps for theA-cation can only be multiples of 1/8=0.125. We observe in Figure 6c that the gap is significantly reduced

at x=0.333, suggesting that the insulator-to-metallic transition might occur around x=0.333. This finding agrees with the experimental results. Therefore, we propose that the significant increase of ex-situ electronic conductivity from LaCoO3 to La0.6Sr0.4CoO3 is due to the combination of the alignment of the CoO-Co bonds and the further oxidation of Co (beyond Co3+). Furthermore, Figure 6c shows that in changing the composition from x=0.333 to x=0.625, there is a remarkable up-shift of the O-2p valence band and a down-shift of the Co-3d conduction band resulting in a closing of the gap and an overlap between these two bands. However, the increase of the ex-situ electronic conductivity from x=0.4 to x=0.8 in the experimental data is small. This discrepancy might be explained by the formation of oxygen vacancies, which slow down the increase in electronic conductivity. As shown before, the XAS results confirm that the average of the Co oxidation state is higher than 3+ in La1-xSrxCoO3 with x≥0.2. However, part of the negative excess charges might be compensated for by the formation of oxygen vacancies (Equation 3), especially for La1-xSrxCoO3 with a high Sr fraction.13 A high amount of oxygen vacancies in the lattice would decrease the electronic conductivity at the expense of an increased ionic conductivity (oxygen ion conductivity), because the oxygen vacancies would reduce the interaction between the Co cations and O2- ions in the lattice, thus inhibiting the increase of the electronic conductivity. Based on the above considerations it is then reasonable to predict that a highly electronic conductive La1-xSrxCoO3 would be favoured by: (i) Co cations with a high oxidation state, (ii) a structure where the Co-O-Co angle equals 180° and (iii) a lower amount of oxygen vacancies in the lattice. In this case, the SrCoO3 oxide with a stoichiometric composition and a cubic structure would present the highest electronic conductivity in the La1-xSrxCoO3 series. Oxygen evolution reaction activity The electrochemical activity towards the OER for the La1-xSrxCoO3 electrodes was evaluated by cyclic voltammetry and chronoamperometric measurements using a thin-film rotating disk electrode configuration. Concerning the OER measurements, carbon was often added to the electrode composition in order to enhance the conduction pathway within the electrode and, thus, to maximize the perovskite utilization.34-36 However, recent studies have shown that carbon plays a more complex role than just a simple conductive support.37, 38 Our previous study on a Ba0.5Sr0.5Co0.8Fe0.2O3/carbon composite electrode by X-ray absorption near-edge spectroscopy (XANES) demonstrated that a reduction of the Co oxidation state occurs during the composite preparation

Figure 6. Projected density of states for the O-2p and Co-3d states, as obtained in the DFT (PBEsol) calculations: in (a) the stoichiometry is fixed at LaCoO3 and we only change the Co-O-Co angle; in (b) the rhombohedral structure is fixed and we only change the Co oxidation state by varying the Sr fraction. In the calculations a hexa-rhombohedral unit (6xABO3) is used, therefore the changes in composition for the A-cations can only take place in steps of 1/6=0.167; in (c) the calculations are left fully free to relax, and a hexa-rhombohedral unit (6xABO3) is used for x≤0.4, followed by a supercell of the 2x2x2 cubic primitive cell for x≥0.6, in this latter case the composition steps for the A-cations can only be multiples of 1/8=0.125). 7



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process.33 Thus, in this paper, we prepared the thin-film electrodes without carbon to obtain an accurate relationship between the oxide properties and the OER activity.

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OER current of SrCoO2.5 is quite stable (Figure 7f). After 30 reverse scans, chronoamperometry measurements are performed to obtain an almost steady-state current with no capacitive contribution (See supporting information Figure S1 and Figure S2). In the following, the steady-state current (noted from the chronoamperometry measurement at an overpotential of 0.3 V vs. RHE) normalized by the mass as well as the BET surface area (See supporting information Table S2) of the perovskite oxide is taken as a parameter of the OER activity (See supporting information Table S1).

At first, 30 reverse scan sweeps between 1-1.7 V (RHE) at 10 mVs-1 and 1600 rpm in synthetic air-saturated electrolyte are performed to obtain a stable voltammogram. As shown in figure 7, the OER currents of La1-xSrxCoO3 with 0≤x≤0.8 are found to gradually decrease with increasing CV scans; however, the rate of decrease becomes less and less evident with the number of cycles. In contrast, the

-1

Figure 7. Cyclic voltammograms for La1-xSrxCoO3 oxide electrode (30 cycles at 10 mVs in synthetic air-saturated 0.1

M KOH at 1600 rpm).

8


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Figure 8a shows the evolution of the current density normalized by the oxide mass (mA mgoxide-1) as a function of the Sr fraction. The mass activity is enhanced by about 6 times in going from LaCoO3 to La0.2Sr0.8CoO3. However, in going from La0.8Sr0.2CoO3 to La0.2Sr0.8CoO3 no significant variation in mass activity is observed, with slightly lower mass activity for the SrCoO2.5 sample. Note that the mass activity does not take into account the effect of the specific surface. Therefore, we also calculate the current density normalized by the BET surface area of La1-xSrxCoO3 while being aware that the BET surface area does not always correspond to the electrochemical active surface area of the catalyst. We believe that the specific current density expressed in µA cmoxide-2 can give a rough estimation of the effect of the specific surface area on OER activity. Moreover, herein we try to build a correlation between different perovskites intrinsic properties and OER activity. Thus, taking the specific current density in µAcmoxide-2 as a parameter of OER activity may increase the reliability of the correlation. From Figure 8b we can deduce a different trend compared to that of the mass activity vs. Sr content. The specific current density (µA cmoxide-2) increases with the increase of the Sr content from x=0 to x=0.8; for SrCoO2.5, a decrease in current density is observed. The trend of the activity (µA cmoxide-2) vs. conductivity depicted in Figure 9 suggests that a correlation does exist between the ex-situ electronic conductivity and the OER activity of La1-xSrxCoO3 (0≤x≤0.8). Being the ex-situ electronic conductivity of La1-xSrxCoO3 related to its electronic structure, it is reasonable to suggest that there is a correlation between the electronic structure and the OER activity of La1-xSrxCoO3. That is the enhancement of the overlap between the occupied O-2p valence bands and the unoccupied Co-3d conduction bands improves the ex-situ electronic conductivity of La1-xSrxCoO3, thus increasing their OER activity. One possible explanation for this finding is that the improved conductivity of La1-xSrxCoO3 might enhance the electron transfer between the adsorbed OH* and the oxide catalyst surface. Studies to further understand the effect of the electronic structure and the conductivity on the OER activity of perovskite catalysts are currently in process.

-1

-2

Figure 8. Current density mA mgoxide (a) and µA cmoxide (b) as a function of Sr fraction x of La1-xSrxCoO3 series, the red circle represents the SrCoO2.5.

For SrCoO2.5, even though the correlation between its conductivity and its electronic structure is not clear due to its non-perovskite structure, still we observe that the correlation between its OER activity and its conductivity somewhat fits the trend of the activity vs. conductivity. We suggest that the overlap between Co cations and O2- ions in the lattice of SrCoO2.5 might be deteriorated by the non-aligning Co-O-Co and by the high amount of oxygen vacancies, degrading its conductivity and OER activity. In the correlation between activity vs. conductivity another important parameter should be taken into account, i.e., the formation of oxygen vacancies. As we discussed before, the small conductivity increase

-2

Figure 9. Current density (µA cmoxide ) as a function of the ex-situ electronic conductivity of La1-xSrxCoO3 series; the red circle represents the SrCoO2.5.

between x=0.4 and x=0.8 might be assigned to the formation of oxygen vacancies which would reduce the interaction between Co cations and O2- ions. Therefore, the present results suggest that if La1-xSrxCoO3 (0.4≤x≤0.8) could be synthesized with 9



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oxygen stoichiometry, the ex-situ electronic conductivity and, thus, the OER activity would befurther improved. Particularly, SrCoO3 with a cubic structure and a stoichiometric composition would display both the highest conductivity as well as the highest OER activity within the La1-xSrxCoO3 series. This assumption, based on the experimental results, is in agreement with the DFT calculations reported by Man et al.,8 who also predict SrCoO3 as one of the most active catalyst within the perovskite oxide family.

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atomic composition is always around 70%, independently of the Sr fraction, the formation of surface oxygenated species seems to be favored by a higher Sr fraction. This can be explained by the high reactivity of Sr cations with the environment, leading to the formation of carbonates and/or hydroxide surface species. This assumption can be confirmed by considering the Sr 3d spectra of La1-xSrxCoO3 (x=0.2, 0.8, 1) (Figure 11). The Sr 3d region has well resolved spin-orbit components, which can be fitted by two sets of Sr 3d5/2 and 3d3/2 doublets with an energy separation of 1.76 eV and a branching ratio of 3d3/2/3d5/2 = 0.69.42 Each spectrum can be well fitted considering the presence of two different chemical environments for the Sr cations. The low-energy component is referred to as lattice Sr (SrL), and the high-energy component is referred to as surface Sr (SrS).39 Similar to O 1s spectra, the ratio of surface Sr/lattice Sr (SrS/SrL) seems to be favored by a higher Sr fraction, especially for SrCoO2.5. The large difference between the cases of La0.2Sr0.8CoO3 and SrCoO2.5 may be due to the lower reactivity of La cations with the environment, when compared to Sr ones.7, 43, 44 The present results indicates that a composition of 20 at% of La in the A-site can already significantly reduce the amount of surface species in La1-xSrxCoO3.

Surface characterization So far we have reported that a correlation exists between the OER activity of La1-xSrxCoO3 and its electronic structure. However, the structure and the composition of the surface might differ from that of the bulk of the oxides, thus, influencing the OER activity. Here, we present an X-ray photoelectron spectroscopy (XPS) study of the La1-xSrxCoO3 series to provide an overview of the surface composition and to discuss its effect on the OER activity. Table 2 shows the XPS-derived atomic compositional data for La1-xSrxCoO3 powders, taking Co3p%+La4d%+Sr3d%=100% (the value in brackets represent the stoichiometric value). We observe that the measured Co atomic composition is around 30% instead of 50% for all the samples. In contrast, the measured La and Sr atomic compositions are both higher than their stoichiometric values. The segregation of La and Sr towards the surface is considered to be responsible for the excess of A-site metals. Figure 10 shows the O 1s spectra of La1-xSrxCoO3 oxides in which several different oxygen species are evident. Generally, in the literature the low-energy component (~528.5 eV) is assigned to as the lattice oxygen (OL) while the high-energy components (529-532 eV) are assigned to as surface phases such as hydroxides and carbonates (OS).39-41 From Figure 9, we can observe that for compositions with higher La content, the OL component is predominant, while it decreases for compositions with higher Sr fraction (x≥0.4). For SrCoO2.5 the OS peak is significantly more intense than the OL one, indicating a strong presence of oxygenated surface species. These data indicate that although the A-site

Figure 10. XPS Oxygen 1s spectra of La1-xSrxCoO3.

Table 2. XPS-derived atomic compositional data for La1-xSrxCoO3 powders, taking Co3p%+La4d%+Sr3d%=100% (the value in brackets represent the stoichiometric value). LaCoO3

La0.8Sr0.2CoO3

La0.6Sr0.4CoO3

La0.4Sr0.6CoO3

La0.2Sr0.8CoO3

SrCoO2.5

La4d

68±6%(50%)

51±5%(40%)

42±4%(30%)

30±3%(20%)

13±1%(10%)

--

Sr3d

--

21±2%(10%)

28±3%(20%)

41±4%(30%)

59±6%(40%)

72±7%(50%)

Co3p

32±3%(50%)

28±3%(50%)

30±3%(50%)

29±3%(50%)

28±3%(50%)

28±3%(50%)

10


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Chemistry of Materials only make plausible assumptions, since the effect of the presence of surface oxygenated species on the OER activity is still poorly understood. First, due to the observed surface segregation of Sr and La, the Co surface concentration falls well below the bulk concentration (with the B-site Co being considered as the active site). Thus, the La1-xSrxCoO3 series might still show potential of enhancing the OER activity if the Co atomic composition at the surface could be improved by physical or chemical treatment of the oxides. Second, in the previous section we predicted that SrCoO3 with a cubic structure and a stoichiometric composition would present the highest OER activity of the La1-xSrxCoO3 series. According to the XPS results, even though the SrCoO3 could successfully be synthesized, a high amount of surface oxygenated species would be expected. Under the assumption that the surface oxygenated species might have a negative effect on the OER activity, La-containing La1-xSrxCoO3 might have a higher OER activity than SrCoO3, since 20 at% of La can already significantly reduce the amount of the surface oxygenated species. Studies to further understand the effect of the surface oxygenated species on the OER activity of La1-xSrxCoO3 are currently in progress.

Conclusions The electronic structure, conductivity and electrochemical properties of the La1-xSrxCoO3 perovskite series (with x=0, 0.2, 0.4, 0.6, 0.8, 1) have been studied experimentally and theoretically. We find that as La is gradually replaced by Sr a phase transition from a rhombohedral (LaCoO3) to a cubic structure (La0.2Sr0.8CoO3) occurs, accompanied by a progressive alignment of the Co-O-Co bonds and an oxidation of Co beyond Co3+. Calculations predict that both the alignment of the Co-O-Co bonds and the oxidation of Co contribute to increasing the overlap between the occupied O-2p valence bands and the unoccupied Co-3d conduction bands, thus also increasing the ex-situ electronic conductivity as a function of the Sr faction. Since as the electronic conductivity increases the OER activity of La1-xSrxCoO3 is enhanced, we suggest that the OER activity is driven by its electronic structure and benefits by an increased overlap between the occupied O-2p valence bands and the unoccupied Co-3d conduction bands.

Figure 11. XPS Sr 3d spectra of La0.8Sr0.2CoO3, La0.2Sr0.8CoO3 and SrCoO2.5. Each spectrum could be fitted with two different components: The low-energy component is referred to as lattice Sr, and the high-energy component is referred to as surface Sr.

Furthermore, and based on the experimental results, we predict that cubic SrCoO3 would display the highest OER activity within the La1-xSrxCoO3 series, in agreement with the results obtained by DFT.7, 8 However, XPS results suggest that even though SrCoO3 might be successfully synthesized, a high amount of oxygenated surface species is to be expected. Under the assumption that these species have a negative effect on the OER activity, SrCoO3

In principle, we would like to relate the surface properties of La1-xSrxCoO3 to their OER activity. However, based on the current XPS results, we can 11



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might be less active than La containing La1-xSrxCoO3, since the presence of La in the A-site of the perovskite was shown to significantly reduce the amount of oxygenated surface species. XPS results also suggest that the activity of La1-xSrxCoO3 could be further improved by reducing the segregation of La and Sr towards the surface.

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based catalysts for the oxygen evolution reaction. Catal Sci Technol 2014, 4, 3800-3821. 5. Gupta, S.; Kellogg, W.; Xu, H.; Liu, X.; Cho, J.; Wu, G., Bifunctional Perovskite Oxide Catalysts for Oxygen Reduction and Evolution in Alkaline Media. Chem Asian J 2015. DOI: 10.1002/asia.201500640 6. Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y., A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383-1385. 7. Calle-Vallejo, F.; Diaz-Morales, O. A.; Kolb, M. J.; Koper, M. T. M., Why Is Bulk Thermochemistry a Good Descriptor for the Electrocatalytic Activity of Transition Metal Oxides? Acs Catal 2015, 5, 869-873. 8. Man, I. C.; Su, H. Y.; Calle-Vallejo, F.; Hansen, H. A.; Martinez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Norskov, J. K.; Rossmeisl, J., Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. Chemcatchem 2011, 3, 1159-1165. 9. Rossmeisl, J.; Qu, Z. W.; Zhu, H.; Kroes, G. J.; Norskov, J. K., Electrolysis of water on oxide surfaces. J Electroanal Chem 2007, 607, 83-89. 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. Bockris, J. O.; Otagawa, T., The Electrocatalysis of Oxygen Evolution on Perovskites. J Electrochem Soc 1984, 131, 290-302. 12. Efimova, E.; Efimov, V.; Karpinsky, D.; Kuzmin, A.; Purans, J.; Sikolenko, V.; Tiutiunnikov, S.; Troyanchuk, I.; Welter, E.; Zajac, D.; Simkin, V.; Sazonov, A., Shortand long-range order in La1−xSrxCoO3 and La1−xBaxCoO3. J Phys Chem Solids 2008, 69, 2187-2190. 13. Hanashima, T.; Azuhata, S.; Yamawaki, K.; Shimizu, N.; Mori, T.; Tanaka, M.; Sasaki, S., Compositional Dependence of X-Ray Absorption Spectra on Magnetic Circular Dichroism and Near-Edge Structure at CoKEdge in La1-xSrxCoO3(0≤x≤0.6). Jpn J App Phys 2004, 43, 4171-4178. 14. Mineshige, A.; Inaba, M.; Yao, T. S.; Ogumi, Z.; Kikuchi, K.; Kawase, M., Crystal structure and metalinsulator transition of La1-xSrxCoO3. J Solid State Chem 1996, 121, 423-429. 15. Sikolenko, V. V.; Sazonov, A. P.; Efimov, V. V.; Efimova, E. A.; Kriventsov, V. V.; Kochubei, D. I.; Zimmermann, U., Phase separation in La1-xSrxCoO3 solid solutions with a perovskite structure. Crystallogr Rep+ 2006, 51, S67-S75. 16. Mineshige, A.; Kobune, M.; Fujii, S.; Ogumi, Z.; Inaba, M.; Yao, T.; Kikuchi, K., Metal-insulator transition and crystal structure of La1-xSrxCoO3 as functions of Srcontent, temperature, and oxygen partial pressure. J Solid State Chem 1999, 142, 374-381. 17. Sarma, D. D.; Chainani, A.; Cimino, R.; Sen, P.; Carbone, C.; Mathew, M.; Gudat, W., Electronic-Structure of and the Metal-Insulator-Transition in La1-Xsrxcoo3Delta - a Soft-X-Ray Absorption Study. Europhys Lett 1992, 19, 513-518.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. Comparison between cyclic voltammetry vs. chronoamperometry measurements for SrCoO2.5 electrode; Crystal structure of pseudocubic; current density and the associated Tafel slopes, BET surface aera and correlation of electronic structure of La1-xSrxCoO3 and the Co-O-Co angle

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the Swiss National Science Foundation through its Ambizione Program, the Swiss Competence Center for Energy Research (SCCER) Heat & Electricity Storage and the Commission for Technology and Innovation (CTI) Switzerland as well as 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.

REFERENCES 1. Edwards, P. P.; Kuznetsov, V. L.; David, W. I. F.; Brandon, N. P., Hydrogen and fuel cells: Towards a sustainable energy future. Energy Policy 2008, 36, 43564362. 2. Buchi, F. N.; Hofer, M.; Peter, C.; Cabalzar, U. D.; Bernard, J.; Hannesen, U.; Schmidt, T. J.; Closset, A.; Dietrich, P., Towards re-electrification of hydrogen obtained from the power-to-gas process by highly efficient H-2/O-2 polymer electrolyte fuel cells. Rsc Adv 2014, 4, 56139-56146. 3. Demirbas, A., Biomass resource facilities and biomass conversion processing for fuels and chemicals. Energ Convers Manage 2001, 42, 1357-1378. 4. Fabbri, E.; Habereder, A.; Waltar, K.; Kotz, R.; Schmidt, T. J., Developments and perspectives of oxide12


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Chemistry of Materials 31. Jiang, Y.; Bridges, F.; Sundaram, N.; Belanger, D. P.; Anderson, I. E.; Mitchell, J. F.; Zheng, H., Study of the local distortions of the perovskite system La1-xSrxCoO3 (0

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