Activation of Catalytically Active Edge-Sharing Domains in

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Activation of catalytically active edge sharing domains in Ca2FeCoO5 for oxygen evolution reaction in highly alkaline media Damian Kowalski, Hisao Kiuchi, Teruki Motohashi, Yoshitaka Aoki, and Hiroki Habazaki ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06854 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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ACS Applied Materials & Interfaces

Activation of Catalytically Active Edge Sharing Domains in Ca2FeCoO5 for Oxygen Evolution Reaction in Highly Alkaline Media Damian Kowalski,*,† Hisao Kiuchi,‡ Teruki Motohashi,⸶ Yoshitaka Aoki,† and Hiroki Habazaki† † Faculty

of Engineering, Hokkaido University, N13W8, Sapporo 060-8628, Japan Office of Society-Academia Collaboration for Innovation, Kyoto University, Kyoto 611-0011, Japan ⸶ Department of Materials and Life Chemistry, Kanagawa University, Yokohama, Kanagawa 221-8686, Japan ‡

KEYWORDS: OER, Brownmillerite, perovskite, Ca2FeCoO5, electrocatalysis

ABSTRACT: Recharchable zinc-air-battery is considered as one of the possible candidates to replace conventional lithium ion battery. One of the requirements for effective battery operation is an oxygen evolution reaction (OER) which needs to be provided in highly alkaline electrolyte. The A2BBÒ5 Brownmillerite-type Ca2FeCoO5 electrocatalyst having 57 Pbcm symmetry demonstrates very high electrocatalytic activity towards OER in 4 mol dm-3 KOH. Our studies show that the electrocatalyst undergoes bulk amorphisation upon OER and adequately activate catalytically active domains. The synchrotron radiation studies using X-ray absorption fine structure (EXAFS) technique show that the central structural unit found in the polarized Ca2FeCoO5 is a cluster of edge-sharing CoO6 octahedra. The electrochemical data imply that OER preferentially takes place on the edge-sharing CoO6 octahedra catalytic centers reconstructed in Brownmillerite type electrocatalyst. The EXAFS second shell peaks at interatomic distance of 2.8 Å are the fingerprints of those catalytically active domains.

INTRODUCTION. Recharchable zinc-air-battery is considered as one of the possible candidates to replace conventional lithium ion battery system due to its relatively high theoretical energy density of 1300 W.h.kg-1.1 One of the main shortcomings hampering development and commercialization of the zinc-air-battery is an electrocatalytic reaction which occurs at the positive electrode leading oxygen evolution/reduction upon charging/discharging process. Various types of non-precious metal electrocatalysts, including ABO3 perovskites, have been proposed for oxygen evolution reaction (OER), oxygen reduction reaction (ORR) as well as those with bifunctional properties.2-18 Providing an alternating oxygen evolution and oxygen reduction reactions on the same surface of electrocatalyst is typically problematic as different catalytic active sites need to be provided for OER and ORR. The access of their catalytic activity to bifunctional operation is challenging as the ones typically suffer from pure cyclability. One of the possible alternatives to bifunctionality is providing pair of air electrodes in which OER and ORR are taking place separately in three electrode configuration. Herein our focus is on OER19 dedicated to reduce overpotential aspects in zinc air battery while separate research on ORR electrocatalysts such as LSMN20 is ongoing in our group. The perovskites are the compounds of ABO3 structure having BO6 units constructing 3D network of cornersharing octahedra with A cations located in the interstitial spaces. The perovskites in the form of superstructures varying in crystallographic order such as A2BBÒ6 have been also widely investigated for OER reactions. One of the oxygen deficient structures of this type having A2BBÒ5 configuration is Brownmillerite-type structure not really explored for OER so far. The correlation between perovskites and Brownmillerite can be described in terms of cell parameters in the following scheme: ab  2½ap, bb  4ap, cb  2½ap.21 Very recently our group reported very high

catalytic activity towards OER for Brownmillerite-type Ca2FeCoO5 electrocatalyst.19 The crystallographic structure of this compound is shown in Fig. 1.

Figure 1. Crystallographic structure of the A2BBÒ5 Brownmillerite-type Ca2FeCoO5 compound having 57 Pbcm symmetry. The unit cell dimensions are a=5.367 Å , b=11.107 Å, c=14.7787 Å and 881.08 Å3. Light blue spheres: Ca, brown spheres: Fe, dark blue spheres: Co, red spheres: O.

The layers of corner-sharing CoO6 octahedra are separated by chains of corner-sharing tetrahedra of FeO4 with Ca located in the interstitial spaces establishing orthorhombic structure having 57 Pbcm symmetry. The unit cell has lattice parameters of a=5.367 Å, b=11.107 Å, c=14.7787 Å and cell volume 881.08 Å3.21 The catalytic activity of this compound at specific OER conditions much exceeds the catalytic activity of state-of-the-art electrocatalysts composed of precious-metals such as RuO2.22

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In the present study we look in depth into the structural transformations of Ca2FeCoO5 compound (Fig. 1) upon polarization for better understanding the nature of catalytically active sites which are responsible for very high activity towards OER. In view of the charge transfer characteristics in many functional materials, high degree of crystallinity is considered to be required to provide sort of electrochemical reactions in the fields of electrocatalysis, photoelectrochemistry, batteries, intercalation, etc. The recent studies on series of perovskites, however, suggest that some of the electrocatalysts may undergo surface amorphisation upon OER reaction.4,7 In contrast to electrochemical catalysis such as OER, the amorphous structures are well known in heterogeneous and homogeneous catalysis. Herein we provide direct experimental evidence on bulk reconstruction of the Brownmillerite-type Ca2FeCoO5 compound leading to formation of amorphous phase having edge sharing CoO6 catalytic clusters and their involvement in the OER reaction. RESULTS Co dependent OER kinetics in strongly alkaline electrolyte. The catalytic activity of Ca2Fe2-xCoxO5 system for 0≤x≤1 was systematically studied by linear sweep voltammetry on glassy carbon using rotating disk electrode at 1600 RPM in highly alkaline electrolyte of 4 mol dm-3 KOH with the potential sweep rate of 1 mV s-1 (Fig. 2c,d). The use of concentrated alkaline solution is essential for constructing practical metal-air secondary battery in order to enhance ionic conductivity of the electrolyte and to suppress corrosion of the negative electrode. The critical point for electrochemical studies in 4 mol dm-3 KOH is the reference on the RHE scale in highly alkaline electrolytes; herein commercial Hydroflex ET070 RHE electrode was used as a reference point to remain equilibrium of O2/H2O redox potential identical across all the RDE measurements (Fig. S1). The cobalt free Ca2Fe2O5 compound (Fig. 2 c,d) shows no activity towards oxygen evolution reaction in the potential range of 1.1-1.7 V vs. RHE which indicates fundamental role of atomic cobalt in the studied electrocatalytic reaction. The magnitude of the catalytic activity for cobalt free electrocatalyst is of the order of activity for carbon sheet electrode shown as a reference in the same figure. The catalytic activity towards OER for Co:Fe ratio 0.25:1.75 is strongly enhanced in terms of current density and onset potential (Eonset) equal to 1.54 V vs. RHE. The reduction of iron content (Co:Fe 0.5:1.5) results in the shift of thermodynamic value of Eonset of 38 mV towards lower potential. The fastest kinetics of oxygen evolution reaction was observed for the electrocatalyst with highest cobalt content, i.e. Co:Fe ratio 1:1 for which the Eonset was reduced to 1.50 V vs. RHE. The OER catalytic activity for the most active electrocatalyst Ca2FeCoO5 is further compared with the state-of-the-art RuO2 electrocatalyst as a reference at pH from 12.8 to ≥14 in Fig. 2 a,b. The effect of pH is clearly visible as the OER kinetics is more rapid in stronger alkaline media as a result of increased ionic conductivity indicating the presence of proton-electron transfer steps during OER wherever the rate-limiting step could be proton transfer step or acid-base equilibrium.23-25 Taking into account BET surface area of 2.9 and 8.4 m2 g-1 for Ca2FeCoO5 and RuO2,

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respectively, the OER kinetics is significantly faster for studied electrocatalyst (Table S1). It is worth nothing that at pH≥14 the catalytic activity for Ca2FeCoO5 is raised with the Eonset shifted towards lower values. Whereas it is difficult to assess accurate reaction order from pH related measurements as many parameters can influence pHdependence, the shift of the thermodynamic value (Eonset) and kinetic parameter (j) suggest change of the reaction order for Ca2FeCoO5 compound at pH≥14.23 Herein, our focus is on OER reaction at pH≥14. To understand the very high catalytic activity of the Ca2FeCoO5 compound let us first consider the structure changes upon the OER conditions in alkaline electrolyte at pH≥14. A series of Ca2Fe2-xCoxO5 electrocatalysts with cobalt content 0≤x≤1 were deposited on carbon sheet electrodes and polarized at 1.7 V vs. RHE for 1h in 4 mol dm-3 KOH. The primary reason for study of the electrocatalyst on carbon sheet instead of glassy carbon RDE electrode is in-depth investigation under long term polarization and further study of physicochemical changes by means of synchrotron radiation discussed in the next sections of this paper. The corresponding current density response is shown in Fig. 2 e,f. The current was kept at 95, 90, 55 and ∽0.3 mA for x equal to 1.0, 0.5, 0.25, and 0 showing stable response upon oxygen evolution reaction for cobalt containing catalysts, reflecting the trend obtained by RDE measurements. After polarization the electrocatalysts were systematically investigated by means of TEM/EDS shown in Fig. 3. Surface/bulk reconstruction upon OER reaction. The TEM images for as received and after polarization of catalyst at 1.7 V for 60 min in oxygen saturated 4 mol dm-3 KOH are compared in Fig. 3 a-c. The as prepared Ca2Fe2xCoxO5 is composed of monocrystalline regions with constant lattice spacing. The polarization of electrocatalyst results in crystalline-to-amorphous phase transition at the surface of the electrocatalyst as well as the bulk of oxide as indicated in Fig 3 a-c. The diffraction patterns show diffraction spots and diffraction concentric rings indicating crystalline and amorphous nature of the Ca2Fe2-xCoxO5 compound before and after polarization at OER conditions, respectively. The surface/bulk reconstruction is observed for full range of cobalt content Brownmillerite-type compound as demonstrated in Fig. 3 a-c and Fig. S2. Nearly complete amorphisation is observed upon polarization of electrocatalyst having cobalt content 0.25≤x≤1, whereas cobalt free electrocatalyst, shows the fraction of crystalline regions having the longitudinal shape located in the core of amorphous domain (Fig. S2). High angle annular dark field (HAADF) transmission electron spectroscopy (TEM) images and corresponding elemental energy dispersive X-ray spectrometry (EDS) mapping of Ca, Fe, Co (Fig. 3 d-g), and O (Fig. S3) indicate that all elements were well distributed through the oxide and no regions with segregation of Ca, Co and Fe were found for 0.25≤x≤1. The enhancement of the Fe intensity for electrocatalysts with reduced cobalt content is clearly visible. The longitudinal regions enriched in Fe for cobalt free electrocatalyst (x=0) are evident on HAADF images and corresponding EDS mapping indicating that the corresponding crystalline region is enriched in iron. The bulk amorphisation of the electrocatalyst upon polarization at 1.7 V is confirmed by XRD studies in Fig. 3h. The XRD


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content. The as formed Ca2Fe2-xCoxO5 0≤x≤1 powder electrocatalyst has the most intense characteristic peaks at ca. 33 and 46-47 2 shown zoomed in XRD profile. Apart from the main peaks at

spectrum for the powder type as formed electrocatalyst is in line with the theoretical predictions21 and shows analogous crystallographic structure indicating formation of Brownmillerite-type structure for full spectrum of cobalt

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Figure 2 a-b) the effect of pH for the magnitude of oxygen evolution reaction (OER) for Ca2FeCoO5 and RuO2 shown as a state-of-theart reference catalyst, c,d) linear sweep voltammograms for Ca2Fe2-xCoxO5 0≤x≤1 compound in 4 mol dm-3 KOH electrolyte (pH≥14.0), e-f) polarization of for Ca2Fe2-xCoxO5 0≤x≤1 compound at 1.7 V vs. RHE in 4 mol dm-3 KOH electrolyte for 1h.

ca. 33 and 46-47 2 the electrocatalyst shows XRD pattern coming from the carbon sheet electrode which is shown as a reference in the upper part of spectrum. The polarization of the electrocatalyst for 60 min results in complete reduction of the peaks at ca. 33 and 46-47 2 which confirms bulk crystalline-to-amorphous transition. From the practical point of view, the fundamental question here is if the crystalline-to-amorphous transition has chemical or electrochemical nature. By studding the set of experimental conditions, we found out that the transition into amorphous phase occurs by means of chemical treatment in 4 mol dm-3 KOH. Figure 3i shows the XRD spectrum of Ca2FeCoO5 powder type electrocatalyst kept in 4 mol dm-3 KOH at 50 C for 3 days indicating its amorphisation without providing electrochemical reaction. The amorphized compound shows similar electrocatalytic activity (Fig. 3j) as the as-prepared crystalline electrocatalyst. In both cases the bulk reconstruction in highly alkaline electrolyte is associated with very high OER kinetics (Fig. 3j) and is of our interest in further X-ray absorption near edge structure (XANES) and X-ray absorption fine structure (EXAFS) investigations. Evidence on the clusters formation during amorphisation. The surface/bulk reconstruction of Brownmillerite-type electrocatalyst initially observed by TEM was studied in detail at Spring8 synchrotron by means of X-ray absorption near edge structure (XANES) and X-ray absorption fine structure (EXAFS) to obtain more detailed information about the local structure changes of electrocatalyst upon OER. The previous X-ray absorption near edge structure (XANES) studies indicated that the

oxidation state for iron and cobalt is Fe3+ and Co3+. Figure 4a shows the normalized Co K-edge XANES spectra for as prepared and after 3h of polarization at 1.7 V vs. RHE Ca2FeCoO5 compounds. All spectra show a small pre-edge shoulder caused by dipole-allowed electronic transitions. The position of Co K-edge for Ca2FeCoO5 compound is 7726 eV and shifts of approximately 3 eV in the edge position towards higher energy indicating a partial oxidation of cobalt upon oxygen evolution reaction, with the energy position close to compounds such as CoOOH (7730.4 eV) and Co3O4 (7730.1 eV). The edge position would be therefore consistent with Co valency ≥3, although difficult in precise determination due to sensitivity to sample geometry, etc. The Fe K-edge spectrum (Fig 4c) for as formed Ca2FeCoO5 compound shows pre-edge shoulder at 7112 eV and K-edge position at 7127 eV. Polarization of sample at 1.7 V vs. RHE for 3h results in shift of ca. 3 eV towards higher energy. The Fourier-transform EXAFS spectra (Fig. 4b) show the peak at the shorter distance side would be ascribed to the first shell of Co-O bonds at interatomic distance of 1.9 Å originating from octahedral CoO6 units in Brownmillerite structure. After polarization of the sample in 4 mol dm-3 KOH a second shell peak at interatomic distance of 2.8 Å appears in the Fourier transforms which is highly surprising for Brownmilleritetype compound. The second shell peak is characteristic for layered oxides such as CoOOAx where A stands for H, Li, Na compounds.26-28 The second shell peak can be assigned to Co-Co interatomic distances of 2.8 Å in edge-sharing octahedra. No significant changes in the Fourier



media. The compositional studies on Ca2Fe2-xCoxO5 show optimum catalytic activity towards OER at x=1, its slight drop as x is reduced to 0.5, significant reduction of catalytic activity for x=0.25, whereas Ca2Fe2O5 is practically inactive. The reduction of catalytic activity towards OER with decrease of

transformed EXAFS spectra for Fe were observed upon amorphisation of the electrocatalyst (Fig. 4d). DISCUSSION. The main goal of this study was to get insight into the atomic structure changes upon oxygen evolution reaction for Brownmillerite-type electrocatalyst which shows very high OER activity in strongly alkaline

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200 150 100

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Figure 3 a-c) transmission electron microscopy (TEM) images of crystalline-to-amorphous transition upon polarization of Ca2Fe2-3 xCoxO5 compound at 1.7 V vs. RHE in 4 mol dm KOH for 1h, d-g) high angle annular dark field (HAADF) images and corresponding elemental energy dispersive X-ray spectrometry (EDS) mapping for Ca2Fe2-xCoxO5 electrocatalyst polarized at 1.7 V vs. RHE in 4 mol dm-3 KOH for 1h, h) X-ray diffraction (XRD) spectra for as-prepared powder type Ca2Fe2-xCoxO5, as-prepared Ca2Fe2-xCoxO5 deposited on carbon sheet electrode, and Ca2Fe2-xCoxO5 deposited on carbon sheet electrode and polarized at 1.7 V vs. RHE in 4 mol dm-3 KOH for 1h; the XRD spectrum for carbon sheet electrode is shown as a reference, i) XRD spectrum and j) linear sweep voltammogram for Ca2FeCoO5 compound chemically treated (without providing electrochemical reaction) with 4 mol dm-3 KOH at 50 C for 72 h.

cobalt content reveals the beneficial role of atomic cobalt for the presence of catalytic centers at which oxygen

evolution reaction preferentially takes place. Very high OER kinetics for Brownmillerite-type electrocatalyst is


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associated with surface/bulk reconstruction upon cycling in highly alkaline electrolyte as evidenced by TEM and XRD studies. The electrocatalyst undergoes quick amorphisation during cycling or potentiostatic polarization in KOH electrolyte, with an excessive degree of amorphisation for x=0.25÷1, whereas incomplete amorphisation for cobalt free electrocatalyst is observed. The surface reconstruction was very recently reported for series of perovskite-type electrocatalysts by the other groups leading to irreversible amorphisation of a few nanometers at the surface of catalyst.4,7,29 More interestingly in-situ x-ray studies at the Co K-edge for Co3O4 show structurally reversible amorphisation-crystallization 1.6

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Ca2FeCoO5  + + + xO2 + (2) Both electrochemical polarization (Fig. 3h) as well as chemical treatment (Fig. 3i) lead to amorphisation of the 2Ca2+

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crystallographic structure in Fig. 5. There is a very good correlation between experimental and fitted spectrum for two paths up to 3 Å giving the number of neighbors in each shell as evidenced in Fig. 5. Two fundamental pathways are possible for oxide reconstruction and formation of the clusters detected by EXAFS technique: i) chemical mechanism and/or ii) electrochemical reconstruction, via following possible reactions:

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Figure 4. a,c) normalized Co K-edge and Fe K-edge XANES spectra and b,d) Fourier transforms obtained for Ca2FeCoO5 electrocatalyst polarized at 1.7 V for 3h in 4 mol dm-3 KOH electrolyte.

of sub-nanometer shell of the cobalt oxide catalyst upon resting and oxygen evolution electrochemical states.30 Our TEM and XRD investigations for Brownmillerite-type oxide apparently show that reconstruction at electrochemical potential for OER is more pronounced comparing with the state-of-the-art perovskites and extends to the bulk of oxide. The observed amorphisation is an irreversible process. Except reconstruction, the electrocatalyst does not show significant compositional changes observed by high resolution EDS mapping (Fig. S3). Those results put forth the question regarding changes of local atomic structure under electrochemical reaction conditions. The EXAFS spectra for the as prepared electrocatalyst show that the electrocatalyst structure is initially composed of domains having corner-sharing CoO6 octahedra within the molecular dimensions as demonstrated in Fig. 1. The polarization of the electrocatalyst at 1.7 V vs. RHE in 4 mol dm-3 KOH results in near completed amorphisation of the electrocatalyst with rearranged CoO6 octahedra positions. The second shell peak which appears at 2.8 Å indicates formation of domains containing edge-sharing CoO6 octahedra. One of the well-known model compounds constructed of edge-sharing CoO6 octahedra is CoOOH.26 We therefore fitted the observed Fourier transformed spectra for Ca2FeCoO5 compound polarized at 1.7 V vs. RHE in 4 mol dm-3 KOH for 3h with model -CoOOH compound of known

Figure 5. EXAFS spectra and Fourier transforms for Ca2FeCoO5 electrocatalyst polarized at 1.7 V for 3h in in 4 mol dm-3 KOH . Black solid line and red dashed line correspond to experimental and fitted data, respectively. Fitting parameters are included in Table S2.

electrocatalyst. The chemical mechanism (1) appears to be favored as the chemical treatment in KOH leads to formation similar Co-Co clusters as detected by EXAFS technique (Fig. S5). If true electrochemical process for



clusters formation exist, the oxygen evolution reaction would be affected by reaction (2), which is not the case as evidenced in Fig. 3j. Therefore, we rule out the concept of clusters formation by means of electorochemical mechanism herein. The chemical nature of the reconstruction indeed has some implications; i) the surface/bulk reconstruction is an irreversible process, ii) faradaic efficiency for OER is not affected by reaction (2). The fundamental question which arises here is the size of domain containing edge-sharing CoO6 octahedra. This information cannot be directly obtained from EXAFS studies, however, by looking at atom-atom distances in Fig. 5 one may notice the absence of peaks for distance over 3Å. This suggests that the edge-sharing CoO6 octahedra are rather in the form of clusters having fever cobalt atoms in lattice domain (Fig. 6). The ideal cluster would be composed of x-number of edge-sharing CoO6 octahedra, however the presence of defects such as corner-sharing CoO6 octahedra

Figure 6. Visualization of the cluster structure composed of four units of edge sharing CoO6 octahedra. Dark blue spheres: Co, red spheres: O.

in the cluster cannot be excluded. Another important aspect is the population of the domains and type of their distribution in the amorphous structure of polarized Ca2FeCoO5 as this is in direct correlation with efficiency of the OER reaction. Further modelling studies such as pair distribution function analysis31 may shed a light on that issue. Our studies show that OER preferentially proceed on the edge-sharing CoO6 octahedra catalytic centers reconstructed in Brownmillerite type electrocatalyst. One of the requirements for the electrocatalyst in applications such as zinc-air battery is that it should show good performance towards OER and ORR reactions. In practice, most of the presently investigated OER perovskites show good catalytic activity in OER followed by rather poor activity in ORR and vice versa. This common problem could be solved by designing electrocatalyst with reversible reconstruction of the catalytic centers such as those presented here. This is the key point to obtain efficient electrocatalyst with bifunctional property as the ones generally suffer from poor cycling stability. CONCLUSIONS. The central structural unit found in the bulk of Ca2Fe2-xCoxO5 electrocatalyst undergoing quick amorphisation upon OER conditions is a cluster of edgesharing CoO6 octahedra. Those edge-sharing domains

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appear to be a catalytic centers for oxygen evolution reaction. The EXAFS peaks at interatomic distance of 2.8 Å are the fingerprints for formation of those catalytic clusters. Further computational studies may address size and population of CoO6 domains in this highly active electrocatalyst. EXPERIMENTAL SECTION. The details of Ca2Fe2-xCoxO5 synthesis have been described elsewhere, except as specified below.19 Characterization. The X-ray diffraction analysis (XRD) was performed with Rigaku, Ultima IV using CuKα radiation (wavelength 1.5405). The morphology of the catalyst was characterized by scanning electron microscope Zeiss Sigma 500 operated at acceleration voltage of 1 kV. Transmission electron micrographs and corresponding high-resolution EDS mapping were performed with a Cscorrected S/TEM Titan3TM G2 60-300, FEI Co. operated at acceleration voltage of 300 kV. The X-ray absorption near edge structure (XANES) measurements were performed at SPring-8 Hyogo, Japan. A Si(111) double-crystal monochromator was used. The XAFS data were processed using Athena software. The BET surface areas of the electrocatalysts were evaluated by means of nitrogen gas adsorption/desorption isotherms at 77 K using Belsorpmax instrument, Bel Japan. Figures 1 and 6 were drawn using Vesta software. Electrochemical measurements for OER activity. Glassy carbon (GC) electrodes, 5 mm in diameter were prepared for OER measurements using 50 mg of the catalyst, 10 mg of acetylene black (AB) treated by nitric acid, and 60 mg of 5 wt.% Na+-exchanged Nafion dispersed in 5 mL of ethanol solution according to procedure described elsewhere.19 The electrochemical measurements were carried out utilizing a potentiostat (Princeton Applied Research PMC CHS08H12) combined with a rotating disk electrode (RDE) Pine Instrument Co. Reversible hydrogen electrode RHE ET070 HydroFlex (Gaskatel) electrode or Hg/HgO/4M KOH were used as a reference electrodes; The platinum served as a counter electrode and GC-RDE loaded with 1.0 mg cm-2 of the electrocatalyst at a rotation rate of 1600 rpm was the working electrode. The electrochemical measurements were performed in 0.1 - 4 mol dm-3 KOH aqueous solution saturated with oxygen at room temperature. Use of concentrated alkaline solution is essential for constructing practical metal-air secondary battery in order to enhance ionic conductivity of the electrolyte and to suppress corrosion of the negative electrode. The potential calibration of Hg/HgO/4M KOH was performed with ET070 HydroFlex RHE reference electrode to get the correct potential values on RHE scale (Fig. S1). The potential of the working electrode was swept from 1.10 V to 1.70 V vs RHE with a potential sweep rate of 1 mV s-1. Prior to the correct OER measurement, the surface of the catalyst was conditioned by applying 30 potential cycles between 1.3 and 0.6 V vs. RHE with a sweep rate of 50 mV s-1. For pH effect measurements, the potential was iR-corrected with a solution resistance measured by AC impedance spectroscopy before each experiment. Polarization test. The electrode used for durability test consisted carbon sheet and electrocatalyst deposited on the electrode. The OER polarization test was conducted in a 4 mol dm-3 KOH


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aqueous solution saturated with oxygen at room temperature. The electrochemical cell was made of acrylic resin was equipped with a 1.0 cm2 carbon sheet working electrode, a platinum counter electrode, and an Hg/HgO/4M KOH reference electrode. Princeton Applied Research PMC CHS08H12 potentiostat/galvanostat was used to apply a constant potential of 1.70 V vs RHE. The chemical treatment experiment (Fig. 3 i,j) was papered in 4 mol dm-3 KOH at 50 C for 72 h with Ca2FeCoO5 calcinated at 800 C, whereas the other electrochemical measurements were obtained for Ca2Fe2-xCoxO5 calcinated at 600 C.

ASSOCIATED CONTENT Supporting Information. RHE calibration, TEM, EDS, BET, EXAFS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Damian Kowalski Faculty of Engineering, Hokkaido University, N13W8 Kita-ku, Sapporo, 060-8628 Japan. tel: +81-11-706-6752. E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT This work was supported by the “Research and Development Initiative for Science Innovation of New Generation Battery (RISING Project)” and “Research and Development Initiative for Science Innovation of New Generation Battery (RISING2 Project)” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The synchrotron radiation experiments were performed at the BL28XU in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2017B7612 and 2018A7612).

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Activation of Catalytically Active Edge-Sharing Domains in

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