Synergistically Enhanced Oxygen Evolution Reaction Catalysis for

Subscriber access provided by TUFTS UNIV

Article

Synergistically Enhanced Oxygen Evolution Reaction Catalysis for Multi-Element Transition-Metal Oxides Ikuya Yamada, Akihiko Takamatsu, Kaisei Asai, Hideo Ohzuku, Takuto Shirakawa, Tasuku Uchimura, Shogo Kawaguchi, Hirofumi Tsukasaki, Shigeo Mori, Kouhei Wada, Hidekazu Ikeno, and Shunsuke Yagi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00511 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Abstract Transition metal oxides have been extensively investigated as novel catalysts for oxygen evolution reaction (OER). Partial elemental substitutions are effective ways to increase catalytic performance and such electronic interactions between multiple elements are known as synergistic effects. However, serious issues such as random atomic arrangement and ambiguous roles of constituent elements humper theoretical investigations for rational materials design. Herein, we describe systematic study on OER activity of AA'3B4O12-type quadruple perovskite oxides, in which multiple transition metal ions are located at distinct crystallographic sites. Electrochemical measurements demonstrate that OER catalytic activities of quadruple perovskite oxide series, CaCu3B4O12 (B = Ti, V, Cr, Mn, Fe, and Co), are all superior to those of simple perovskite counterparts CaBO3. The order of activity of B-site transition metal ions for CaBO3 (Fe4+ > Co4+ >> Ti4+, V4+, Cr4+, Mn4+) is retained in CaCu3B4O12, indicating that B-site ions play a primary role whereas A'-site Cu ions secondarily contribute to OER activity for CaCu3B4O12. Charge-transfer energies, energy differences between oxygen 2p band center and unoccupied 3d band center of B-site transition metal obtained from first-principles electronic-state calculations, illustrate that OER overpotentials of quadruple perovskite oxides are lower than simple perovskite oxides by ~150 mV. These findings propose a simple avenue to realize enhanced OER activity for multiple transition-metal ions. 1. Introduction Electrochemical oxygen evolution/reduction reactions (OER/ORRs) are essential processes for energy conversion technologies utilizing renewable sources;1,2 OER is applied for water

ACS Paragon Plus Environment

2

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

electrolysis and charge of metal-air batteries while ORR is adopted for fuel cells and discharge of metal-air batteries. Intrinsic large overpotentials in these reactions cause huge amount of energy loss in worldwide scale, thus development of cost-effective catalysts is desired. Precious metals (Ru, Ir, and Pt) and their compounds are known as superior catalysts for OER and ORR,36

but the high cost and less earth-abundance remain unsolved problems. Transition metal oxides consisting of non-precious metal elements in various structure types

such as perovskite,7-10 spinel,11,12 and pyrochlore13-15 are considered as promising candidates for OER and ORR catalysts owing to their flexibility in chemical compositions and electronic states. In particular, a wide range of perovskite and related oxides have been extensively investigated to increase catalytic performance.9,10,16-18 Recently, multi-transition-element perovskite oxides like Ba0.5Sr0.5Co0.8Fe0.2O3–δ (BSCF), La1–xSrxCoO3–δ, and Sr(Co0.7Fe0.2Nb0.1)O3–δ were proposed as highly active OER catalysts.9,19-22 Positive interactions between constituent elements to increase catalytic activity are interpreted as synergistic effects.23-26 State-of-the-art techniques revealed new insights into OER mechanisms concerning lattice-oxygen activation and electron transfer in these oxides.27,28 However, most transition metal oxide catalysts ever studied are involved in severe structure randomness derived from off-stoichiometry (see a structure model of BSCF in Figure 1a drawn by using the VESTA-3 software29), in addition to unobvious electronic state of each constituent ion in mixed state. First-principles electronic-state calculations using oversimplified structure models obscure electronic interactions between multiple transition metal ions.23,24,30-33 Hence, simple materials design is desired for further improvement of catalytic performance. Recently, highly active and stable OER catalysis were reported for multiple-element transition metal oxides crystallizing in AA'3B4O12-type quadruple perovskite structure (Figure 1b).34-36 In


3

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


this structure, multiple transition metal ions occupy distinct crystallographic sites; Jahn-Teller active Cu2+ and Mn3+ ions occupy A'-sites in pseudosquare coordination whereas typical 3d metal ions occupy B-sites in octahedral coordination.37,38 Such an intelligible structure excludes substantial structural randomness and reasonably defines electronic states of constituent elements, enabling systematic investigations based on experimental and computational methods. Herein, we describe a comparative study on OER catalysis for simple and quadruple perovskite oxides, CaBO3 and CaCu3B4O12 (B = Ti, V, Cr, Mn, Fe, and Co), respectively. Electrochemical measurements demonstrate that OER catalytic activities monotonically increased from simple to quadruple perovskite oxide series. Considering intrinsic lower OER activities of single transition metal ions, enhanced OER activities of quadruple perovskite oxides are attributed to synergistic effects between A'- and B-site ions. The systematic increase in OER overpotentials from simple to quadruple perovskite oxide series is well interpreted by using charge-transfer energies obtained from density-functional-theory (DFT) calculation, proposing a rational design for highly active OER catalysts. 2. Experimental and Computational Procedures Sample Preparation The perovskite oxide catalysts CaBO3 (B = Ti, V, Cr, Mn, Fe, and Co), LaFeO3, TeCuO3, LaCoO3, LaCuO3, CaCu3B4O12 (B = Ti, V, Cr, Mn, Fe, and Co), LaCu3Fe4O12, LaTi0.5Cu0.5O3, and LaMn0.5Cu0.5O3 were synthesized by solid-state reaction according to literatures.39-50 CaVO3, CaCrO3, CaFeO3, CaCoO3, TeCuO3, LaCuO3, CaCu3B4O12 (B = V, Cr, Mn, Fe, and Co), and LaCu3Fe4O12 were synthesized under high pressures up to 12 GPa, whereas others under ambient pressure. Detailed synthesis conditions are listed in Table S1. Basic Characterization


4


Page 4 of 31

X-ray powder diffraction (XRD) patterns were collected by using X-ray diffractometer with Cu Kα radiation (Ultima IV, Rigaku, Japan). Synchrotron X-ray powder diffraction (SXRD) patterns at room temperature were collected using a Debye-Scherrer camera installed at the BL02B2 beamline of SPring-8, Japan.51 The wavelength was determined to approximately 0.5 Å using CeO2 as reference. The SXRD data were analyzed using Rietveld refinement program RIETAN-FP.52 High-resolution transmission electron microscope (HRTEM) images were collected using a transmission electron microscope JEM-2100F (JEOL Ltd.). Specific surface area was estimated by the Brunauer-Emmett-Teller analysis of Kr gas adsorption data (BELSORP-max, MicrotracBEL, Japan). X-ray absorption near edge structure (XANES) data at room temperature were collected by transmission method at the BL14B2 beamline of SPring-8, Japan. Electrochemical Characterization OER catalytic activities were evaluated by using rotating ring/disk electrode system. A 5 wt% proton-type Nafion suspension (Sigma-Aldrich), 0.1 M KOH aqueous solution (Nacalai Tesque, Inc., Japan), and tetrahydrofuran (THF, Sigma-Aldrich) were mixed in a 2:1:97 volume ratio. The catalyst ink was prepared by mixing 5 mg of catalyst, 1 mg of acetylene black (Denka Co., Ltd., Japan), and 1mL of the above THF solution. A 6.4 µL of catalyst ink was taken with stirring and drop cast onto the glassy-carbon disk (4 mm in diameter). Electrochemical measurements were performed using a rotating-disk electrode rotator (RRDE-3 A, BAS Inc., Japan) in combination with a bipotentiostat (Model-2325, BAS Inc., Japan). A Pt wire electrode and an Hg/HgO electrode (International Chemistry Co., Ltd., Japan) filled with a 0.1 M KOH aqueous solution (Nacalai Tesque, Inc., Japan) were used as the counter and reference electrodes, respectively. All electrochemical measurements were conducted under O2 saturation at room


5

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature. This fixed the equilibrium potential of the O2/H2O redox couple to 0.304 V versus Hg/HgO. The disk potential was controlled between 0.3 and 0.9 V versus Hg/HgO at a scan rate of 10 mV s–1. The disk potentials are represented in those versus reversible hydrogen electrode (RHE), together with iR-compensation (R ~43 Ω). The capacitive effect was compensated by averaging the cathodic and anodic scans. DFT calculation DFT calculations were performed for simple and quadruple perovskite oxides, CaBO3, CaCu3B4O12 (B = Ti, V, Cr, Mn, Fe, and Co), LaBO3, (B = Mn, Fe, Co), LaMn7O12, CaMn7O12, and LaCu3Fe4O12. The space groups and experimental lattice constants of those compounds are summarized in Table S2, except for LaMn7O12 and CaMn7O12. Structure data for LaMn7O12 and CaMn7O12 were adopted from a previous report.36 The magnetic structures considered in DFT calculations are summarized in Table S7 (see also Supporting Information about the treatment of magnetic structures in calculations). The calculations were performed using the plane-wave based projector augmented wave (PAW) method with the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) functional as implemented in the Vienna Ab-initio Simulation Package (VASP).53-55 Radial cutoffs and valence electrons for PAW potentials adopted in this work are summarized in Table S6. The on-site Coulombic interactions on the localized 3d electrons were treated with the GGA+U approach56 with Ueff = 2, 3.3, 3.5, 4, 3.9, 3.3, and 7 eV for Ti, V, Cr, Mn, Fe, Co, and Cu, respectively, which were chosen to reproduce the experimental oxidation enthalpy, as reported previously.57,58 The plane-wave cutoff energy was set to 500 eV. Brillouin zones were sampled following the Monkhorst-Pack scheme, except for CaMn7O12 where a gamma-centered scheme was adopted.59 For k-point sampling, the mesh k1×k2×k3 was prepared as the near natural number of 40 per lattice parameter (1 Å–1) to each


6


Page 6 of 31

direction. The lattice constants and internal coordinates were optimized until the total energy difference and residual forces converged to less than 10–5 eV and 10–2 eV Å–1. The oxygen 2p band centers and unoccupied 3d band centers of transition metal atoms were computed from the projected DOS. The definition of band centers is described in Supporting Information. The number of conduction bands were increased until the shapes of projected DOS were converged. Band centers and density of states are summarized in Table S5, and Figure S5 and S6, respectively. 3. Results and discussion Synthesis and basic characterization. Figure 1c and 1d illustrate XRD patterns of CaBO3 and CaCu3B4O12 (B = Ti, V, Cr, Mn, Fe, and Co), respectively. All samples crystallized in simple/quadruple perovskite-type structures and considerable impurity phases were not observed. Rietveld analysis based on SXRD data exhibited that occupancy factors at oxygen sites were refined to near the unity for oxides containing unusual high-valence metal ions (i.e. Fe4+ and Co4+). This means that no significant amount of oxygen vacancy existed in the samples synthesized under strong oxidizing conditions using high-pressure apparatus in this study, although much amount of oxygen vacancy is usually included when synthesized at ambient conditions.22,60,61 Lattice constants refined by Rietveld analysis of SXRD data are listed in Table S2. HRTEM images of as-synthesized quadruple perovskite oxide samples displayed that well crystalline structures retained near the surface (Figure S1). Valence states of transition metal ions were investigated by XANES spectroscopy. Figure 2a– 2f show K-edge XANES spectra of B-site transition metal elements (Ti, V, Cr, Mn, Fe, and Co) for CaBO3 and CaCu3B4O12. For B = Ti, V, Cr, and Mn, no significant differences in absorption edge position between CaBO3 and CaCu3B4O12, indicating isovalent states of Ca2+B4+O3 and


7

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ca2+Cu2+3B4+4O12. Although valences lower than +4 were reported for CaCu3Fe4O12 and CaCu3Co4O12 (Ca2+Cu~2.4+3Fe~3.65+4O12 and Ca2+Cu~3+3Co~3.25+4O12),46,62,63 clear negative shifts were not observed for XANES spectra of Fe and Co K-edges (Figure 2e and 2f). This is probably because changes in coordination environment and spin-state could transform spectral shapes.46,47 Hence, to evaluate valence states of these oxides precisely, additional XANES spectra were investigated. Figure 2g displays Fe K-edge XANES spectra for ACu3Fe4O12 (A = Ca, Y, and La). The edge position for CaCu3Fe4O12 was close to that for YCu2+3Fe3.75+4O12 rather than LaCu3+3Fe3+4O12. Figure 2h shows Cu K-edge XANES spectra of CaCu3Fe4O12, CaCu3Co4O12, and Cu2+/3+-references. A slight positive shift of the Cu K-edge from CaCu3Ti4O12 to CaCu3Fe4O12 indicates valence transition from Cu2+ to Cu~2.4+, as reported previously.62 CaCu3Co4O12 shows a further positive shift of Cu K-edge, almost identical to that of LaCu3+3Fe3+4O12.

Therefore,

we

concluded

that

appropriate

valence

states

are

CaCu~2.4+3Fe~3.65+4O12 and CaCu~3+3Co~3.25+4O12 for the samples tested in this study. OER Activity. Figure 3a–3f show linear sweep voltammograms of CaBO3 and CaCu3B4O12 in alkaline conditions (0.1 M KOH). Current densities per surface areas of oxides (in a unit of mA cm–2oxide) were adopted to exclude geometric factors of catalysts. Specific surface areas were determined by Brunauer-Emmett-Teller analysis of Kr-gas absorption data, ranging from ~0.15 to ~1.6 m2 g–1 (see Table S2). We determined potentials reaching a current density of 0.05 mA cm–2oxide as OER onset potentials (EOER), and OER overpotentials (ηOER) were calculated: ηOER = EOER – 1.23 V. Note that CaCu3B4O12 were superior to the corresponding CaBO3 in all couples; lower overpotentials and higher current densities (see Table S3). Since CaCu3Cr4O12 exhibited considerable redox signals in the vicinity of the OER onset possibly attributed to oxidation and reduction between Cr4+ and Cr6+ ions of the catalyst surface (see a single measurement of cyclic


8


Page 8 of 31

voltammogram in Figure S2), the EOER could not be defined for this oxide. The current densities did not reach 0.05 mA cm–2oxide for CaTiO3 and CaCrO3 in the potential range measured, thus EOER could not be determined for them also. Figure 4a–4e show systematic comparisons of OER specific activities (defined as current density at 1.6 V vs. RHE) and overpotentials between CaBO3 and CaCu3B4O12 (B = Ti, V, Mn, Fe, Co). Data of a perovskite oxide TeCuO3 are also included as a reference of OER activity of Cu2+-oxide. For any comparisons, CaCu3B4O12 displayed much higher OER activities than CaBO3. Despite of smaller specific surface areas of quadruple perovskite oxides compared to those of simple perovskite counterparts, the superiority was roughly retained in the comparison of mass activity except for B = V (Figure S3). Although TeCuO3 exhibited an intrinsically poor activity, Cu-added oxides CaCu3B4O12 showed higher activities compared with single-transitionmetal oxides CaBO3. This is a simple indication of synergistic effects of Cu and B ions on OER catalysis. OER activity of CaCu3+3Co3.25+4O12 was compared with those of reference catalysts with closer valences, LaCo3+O3 and LaCu3+O3 (Figure 5a). This also displays significant enhancement of OER activity as a copper-cobalt complex oxide. To further investigate synergistic interactions between Cu3+ and B3+ ions in this framework, OER activity of LaCu3+3Fe3+4O12 was compared with those of LaFe3+O3 and LaCu3+O3 references (Figure 5b). This comparison evidences effective enhancement of OER activity for copper-iron complex oxide. Structural effects on OER activity were studied by comparing OER activities of quadruple perovskite and mixed-metal perovskite oxides with identical transition metal ions. Figure 5c compares OER activities between CaCu3Ti4O12 and LaTi0.5Cu0.5O3, latter of which randomly contains Cu2+ and Ti4+ ions at B-sites.64 LaTi0.5Cu0.5O3 displayed higher activity than Cu2+- and


9

Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ti4+-references, TeCuO3 and CaTiO3, respectively. CaCu3Ti4O12 displayed much higher activity, thus the framework of quadruple perovskite structure is favorable for further enhancement of OER catalysis. Figure 5d compares OER activities between CaCu3Mn4O12 and LaMn0.5Cu0.5O3, latter of which consists of randomly mixed Mn4+ and Cu2+ ions at B-sites.65 The increased OER activity of LaMn0.5Cu0.5O3 and much more enhanced activity of CaCu3Mn4O12 are explained in line with synergistic effects and advantage of quadruple perovskite structure. Figure 6a displays dependence of ηOER upon B-site metal ion for CaBO3 and CaCu3B4O12 series. The order of ηOER is Fe4+ < Co4+ 150 mV decade–1) for less active CaBO3 (B = Ti, V, Cr, and Mn), whereas much small for CaFeO3 and CaCoO3 (46 and 88 mV decade–1, respectively). This is consistent with higher OER activities of the CaFeO3 and CaCoO3, as observed in overpotentials and specific activities. Figure 6e shows B-site metal ion dependence of Tafel slopes for CaCu3B4O12 and CaBO3. CaCu3B4O12 (B = Ti, Cr, and Mn) displays almost monotonic decrease in Tafel slopes compared to corresponding CaBO3. CaCu3Fe4O12 and CaCu3Co4O12 exhibit slight decrease in Tafel slopes than CaFeO3 and CaCoO3, respectively. These are another indication of enhancement of OER activities for CaCu3B4O12. Stability of quadruple perovskite oxides was tested in 100 continuous OER measurements (Figure S4). CaCu3Ti4O12, CaCu3V4O12 CaCu3Co4O12, and LaCu3Fe4O12 almost retained their initial activities during 100 cycles as well as CaCu3Fe4O12, in which a robust framework through covalent bonding among Cu, Fe, and O ions retains the activity.34 In contrast, CaCu3Mn4O12 exhibited a gradual degradation in activity, which is similar to CaMn7O12 and LaMn7O12 quadruple perovskite oxides.36 Furthermore, CaCu3Cr4O12 demonstrated prominent redox waves to increase activity in early cycles, followed by a monotonic degradation. The differences in stability for quadruple perovskite series is probably attributed to the intrinsic nature of constituent elements, that is, significant oxygen deficiency and reconstruction of atoms may proceed on the catalyst surface for Cr, Mn-based quadruple perovskite oxides. DFT Calculations. The above experimental study demonstrates that combinations of multiple transition metal ions for quadruple perovskite structures effectively increase OER activities,


11

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


which could be called quadruplication effect on OER catalysis. Since electric conductivity of quadruple and simple perovskite oxides with identical B-site metals is similar (Table S4), the differences in OER activities between these perovskite oxides are not predominated by the electrical conductivity of catalysts. To interpret the enhanced OER activities for quadruple perovskite oxides, we investigated bulk electronic structures by using DFT calculation. Figure 7a and 7b show band centers (centroids of density of states) for O 2p band (ε2p) and unoccupied 3d band of B-site ions (ε3d-un), respectively (also see Table S5). ε2p changed in dependent on B-site metal ions and almost no significant difference between CaBO3 and CaCu3B4O12 was confirmed, except for B = Cr and Co. ε3d-un roughly decreased from Ti to Co and almost the same for CaBO3 and CaCu3B4O12. Figure 7c displays ηOER as a function of ε2p, where LaFeO3–LaCu3Fe4O12, (Ca/La)MnO3–(Ca/La)Mn7O12, and LaCoO3–CaCu3Co4O12 pairs are included.36 In accordance with a previous report,10 we explored a possible relationship between ηOER and ε2p. However, no clear correlation over all data points was found. Apparently, a near linear curve could be observed for simple perovskite series (dashed line). This indicates that the ε2p-descriptor is applicable for selected compounds but does not reasonably explain the difference in ηOER between simple and quadruple perovskite series. Recently, Hong et al.28 proposed that chargetransfer energies from oxygen 2p band to unoccupied transition metal 3d band, which were obtained from X-ray absorption/emission spectroscopy experiments, are closely related to the OER mechanism and specific activities. Hence, we estimated DFT-based charge-transfer energy (∆) from the difference between ε2p and ε3d-un: ∆ = ε3d-un – ε2p. Figure 7d illustrates ∆ for CaBO3 and CaCu3B4O12 series. In each series, ∆ exhibited a monotonic decrease from Ti to Co. An exception, a small rise at CaCu3Co4O12, is attributed to significant valence lowering to Co3.25+. This is in stark contrast to unsystematic behavior of ε2p and ε3d-un (Figure 7a and 7b). Figure 7e


12


Page 12 of 31

shows ηOER as a function of ∆. Most data points of quadruple perovskite series followed a linear curve. Except for CaCu3Mn4O12, all the data points are included in a line within a deviation of ±0.03 V. For simple perovskite series, most data points also followed a linear relationship, where CaMnO3 and CaFeO3 are protruded from a ±0.03 V deviation. The line for quadruple perovskite series is offset to lower side by ~0.15 V from that of simple perovskite series, indicating a systematic decrease in OER overpotential from simple to quadruple perovskite oxides, regardless of combinations of A'- and B-site metals. This trend demonstrates that quadruple perovskite oxides provide a suitable framework for multiple transition metal ions to enhance OER activity. We propose that the DFT-based charge-transfer energy serves to design highly active catalysts including hypothetical compounds, unlike a previous study based on spectroscopy experiment.28 4. Conclusion We performed comparative study on OER catalytic activities of simple and quadruple perovskite oxides. The OER activity is monotonically enhanced for CaCu3B4O12 (B = Ti, V, Cr, Mn, Fe, and Co), in which the order of OER activity (Fe > Co >> others) is mutual for simple and quadruple perovskite oxides. The DFT calculation demonstrates that the OER overpotentials of simple and quadruple perovskite oxides consisting of various A'- and B-site transition metal ions follow a linear relationship with the charge-transfer energy defined by the electronic energy levels of O 2p and unoccupied B-site 3d band centers. The quadruple perovskite framework provides suitable synergistic interactions to increase OER catalytic activity. These findings propose a simple way to increase OER activity over transition metal oxides consisting of multiple transition metal elements. On the other hand, significant development is desired to apply quadruple perovskite oxide catalysts to practical use because of undesirable properties of


13

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


high-pressure synthesis such as high cost, different morphology control, and small amount of sample. For further study, morphology control under high pressure is in progress.

ASSOCIATED CONTENT Supporting Information. Computational details, all relevant experimental data, DFT calculation data (PDF) Corresponding Authors *[email protected] (I.Y.) *[email protected] (H.I.) *[email protected] (S.Y.) ACKNOWLEDGMENTS The authors thank Dr. Naoto Umezawa and Dr. Takashi Kamegawa for fruitful discussion. The synchrotron radiation experiments were performed at SPring-8 with the approval of JASRI (Proposal numbers 2017B1076, 2017B1077, and 2017B1900). This work was supported by JSPS KAKENHI (Grant Numbers 15H04169, 16H00893, 16H02393, 16H04220, 17K18973, 17K19182, 18H03835, and 26106518) and Toray Science Foundation.


14


Page 14 of 31

Figure Caption Figure 1. (a) Crystal structure of Ba0.5Sr0.5Co0.8Fe0.2O3–δ as a typical example of structurally disordered perovskite oxides with high OER activities. Both A-site (Ba: orange spheres; Sr: green spheres) and B-site (Co: blue spheres; Fe: red spheres) ions are randomly distributed, in addition to disordered arrangement of oxygen vacancies. (b) Crystal structures of simple orthorhombic CaBO3 (left) and quadruple cubic CaCu3B4O12 (right) perovskite oxides. Green, blue, red, and aqua spheres represent Ca, Cu, B, and O atoms, respectively. XRD patterns of (c) CaBO3 and (d) CaCu3B4O12. The Bragg reflection indices are based on space groups of Pnma and Im3ത for CaBO3 and CaCu3B4O12, respectively. Figure 2. Transition metal K-edge XANES spectra of (a) Ti, (b) V, (c) Cr, (d) Mn, (e) Fe, and (f) Co for CaBO3 (black) and CaCu3B4O12 (red). (g) Fe K-edge XANES spectra for CaCu3B4O12, YCu3Fe4O12, and LaCu3Fe4O12. (h) Cu K-edge XANES spectra for CaCu3B4O12 (B = Ti, Fe, and Co) and LaCu3Fe4O12. Figure 3. Linear sweep voltammograms in OER conditions for CaBO3 and CaCu3B4O12 perovskites with B = (a) Ti, (b) V, (c) Cr, (d) Mn, (e) Fe, and (f) Co. Figure 4. OER specific activities at 1.6 V vs. RHE and overpotentials (ηOER) for CaBO3 and CaCu3B4O12 perovskite oxides with B = (a) Ti, (b) V, (c) Mn, (d) Fe, and (e) Co. ηOER for CaTiO3, is not displayed because it could not be determined in this study. Figure 5. (left) Linear sweep voltammograms, (right) OER specific activities and overpotentials (ηOER) for (a) CaCu3Co4O12, (b) LaCu3Fe4O12, (c) CaCu3Ti4O12, and (d) CaCu3Mn4O12. Data of their reference catalysts are also displayed.


15

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. (a) OER overpotentials (ηOER) and (b) specific activities at 1.6 V vs. RHE for CaCu3B4O12 and CaBO3 series. Tafel plots of (c) CaCu3B4O12 and (d) CaBO3. (e) Tafel slopes for CaCu3B4O12 and CaBO3 series. Figure 7. Band centers of (a) O 2p band (ε2p) and (b) B-site metal unoccupied 3d band (ε3d-un) for CaCu3B4O12 and CaBO3 series obtained from DFT calculations. (c) OER overpotentials (ηOER) as a function of O 2p band center (ε2p). The Fermi energies were set to 0 eV. The dashed line is a guide to the eye. (d) Charge-transfer energies (∆) for CaCu3B4O12 and CaBO3 series obtained from DFT calculations. (e) OER overpotentials (ηOER) as a function of charge-transfer energy (∆). The red and black lines are the guides to the eye for quadruple and simple perovskite series, respectively. The shaded areas represent deviation of ±0.03 V from the lines.


16


Page 16 of 31

Figure 1


17

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2


18


Page 18 of 31

Figure 3


19

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4


20


Page 20 of 31

Figure 5


21

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6


22


Page 22 of 31

Figure 7


23

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Hong, W. T.; Risch, M.; Stoerzinger, K. A.; Grimaud, A.; Suntivich, J.; Shao-Horn, Y. Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis. Energy Environ. Sci. 2015, 8, 1404-1427. (2) Fabbri, E.; Habereder, A.; Waltar, K.; Kötz, R.; Schmidt, T. J. Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction. Catal. Sci. Technol. 2014, 4, 3800-3821. (3) Stoerzinger, K. A.; Diaz-Morales, O.; Kolb, M.; Rao, R. R.; Frydendal, R.; Qiao, L.; Wang, X. R.; Halck, N. B.; Rossmeisl, J.; Hansen, H. A.; Vegge, T.; Stephens, I. E. L.; Koper, M. T. M.; Shao-Horn, Y. Orientation-Dependent Oxygen Evolution on RuO2 without Lattice Exchange. ACS Energy Lett. 2017, 2, 876-881. (4) Rao, R. R.; Kolb, M. J.; Halck, N. B.; Pedersen, A. F.; Mehta, A.; You, H.; Stoerzinger, K. A.; Feng, Z.; Hansen, H. A.; Zhou, H.; Giordano, L.; Rossmeisl, J.; Vegge, T.; Chorkendorff, I.; Stephens, I. E. L.; Shao-Horn, Y. Towards identifying the active sites on RuO2(110) in catalyzing oxygen evolution. Energy Environ. Sci. 2017, 10, 2626-2637. (5) Gasteiger, H. A.; Marković, N. M. Just a Dream—or Future Reality? Science 2009, 324, 48. (6) Kuo, D. Y.; Kawasaki, J. K.; Nelson, J. N.; Kloppenburg, J.; Hautier, G.; Shen, K. M.; Schlom, D. G.; Suntivich, J. Influence of Surface Adsorption on the Oxygen Evolution Reaction on IrO2(110). J. Am. Chem. Soc. 2017, 139, 3473-3479. (7) Bockris, J. O.; Otagawa, T. Mechanism of oxygen evolution on perovskites. J. Phy. Chem. 1983, 87, 2960-2971. (8) Bockris, J. O. M.; Otagawa, T. The Electrocatalysis of Oxygen Evolution on Perovskites. J. Electrochem. Soc. 1984, 131, 290-302. (9) 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, 13831385.


24


Page 24 of 31

(10) Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y. L.; Risch, M.; Hong, W. T.; Zhou, J.; Shao-Horn, Y. Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution. Nat. Commun. 2013, 4, 2439. (11) Maiyalagan, T.; Jarvis, K. A.; Therese, S.; Ferreira, P. J.; Manthiram, A. Spinel-type lithium cobalt oxide as a bifunctional electrocatalyst for the oxygen evolution and oxygen reduction reactions. Nat. Commun. 2014, 5, 3949. (12) Kim, T. W.; Woo, M. A.; Regis, M.; Choi, K. S. Electrochemical Synthesis of Spinel Type ZnCo2O4 Electrodes for Use as Oxygen Evolution Reaction Catalysts. J. Phys. Chem. Lett. 2014, 5, 2370-2374. (13) Lebedev, D.; Povia, M.; Waltar, K.; Abdala, P. M.; Castelli, I. E.; Fabbri, E.; Blanco, M. V.; Fedorov, A.; Copéret, C.; Marzari, N.; Schmidt, T. J. Highly Active and Stable Iridium Pyrochlores for Oxygen Evolution Reaction. Chem. Mater. 2017, 29, 5182-5191. (14) Kim, J.; Shih, P. C.; Tsao, K. C.; Pan, Y. T.; Yin, X.; Sun, C. J.; Yang, H. High-Performance Pyrochlore-Type Yttrium Ruthenate Electrocatalyst for Oxygen Evolution Reaction in Acidic Media. J. Am. Chem. Soc. 2017, 139, 12076-12083. (15) Hirai, S.; Yagi, S.; Chen, W. T.; Chou, F. C.; Okazaki, N.; Ohno, T.; Suzuki, H.; Matsuda, T. NonFermi Liquids as Highly Active Oxygen Evolution Reaction Catalysts. Adv. Sci. 2017, 4, 1700176. (16) Kim, J.; Yin, X.; Tsao, K. C.; Fang, S.; Yang, H. Ca2Mn2O5 as oxygen-deficient perovskite electrocatalyst for oxygen evolution reaction. J. Am. Chem. Soc. 2014, 136, 14646-14649. (17) Cheng, X.; Fabbri, E.; Nachtegaal, M.; Castelli, I. E.; El Kazzi, M.; Haumont, R.; Marzari, N.; Schmidt, T. J. Oxygen Evolution Reaction on La1–xSrxCoO3 Perovskites: A Combined Experimental and Theoretical Study of Their Structural, Electronic, and Electrochemical Properties. Chem. Mater. 2015, 27, 7662-7672. (18) Hwang, J.; Rao, R. R.; Giordano, L.; Katayama, Y.; Yu, Y.; Shao-Horn, Y. Perovskites in catalysis and electrocatalysis. Science 2017, 358, 751.


25

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Jung, J. I.; Jeong, H. Y.; Kim, M. G.; Nam, G.; Park, J.; Cho, J. Fabrication of Ba0.5Sr0.5Co0.8Fe0.2O3–δ catalysts with enhanced electrochemical performance by removing an inherent heterogeneous surface film layer. Adv. Mater. 2015, 27, 266-271. (20) Li, M.; Zhou, W.; Peterson, V. K.; Zhao, M.; Zhu, Z. A comparative study of SrCo0.8Nb0.2O3–δ and SrCo0.8Ta0.2O3–δ as low-temperature solid oxide fuel cell cathodes: effect of non-geometry factors on the oxygen reduction reaction. J. Mater. Chem. A 2015, 3, 24064-24070. (21) Wang, J.; Gao, Y.; Chen, D.; Liu, J.; Zhang, Z.; Shao, Z.; Ciucci, F. Water Splitting with an Enhanced Bifunctional Double Perovskite. ACS Catal. 2017, 8, 364-371. (22) Mefford, J. T.; Rong, X.; Abakumov, A. M.; Hardin, W. G.; Dai, S.; Kolpak, A. M.; Johnston, K. P.; Stevenson, K. J. Water electrolysis on La1–xSrxCoO3–δ perovskite electrocatalysts. Nat. Commun. 2016, 7, 11053. (23) Yang, F.; Sliozberg, K.; Sinev, I.; Antoni, H.; Bähr, A.; Ollegott, K.; Xia, W.; Masa, J.; Grünert, W.; Cuenya, B. R.; Schuhmann, W.; Muhler, M. Synergistic Effect of Cobalt and Iron in Layered Double Hydroxide Catalysts for the Oxygen Evolution Reaction. ChemSusChem 2017, 10, 156-165. (24) Dutta, A.; Mutyala, S.; Samantara, A. K.; Bera, S.; Jena, B. K.; Pradhan, N. Synergistic Effect of Inactive Iron Oxide Core on Active Nickel Phosphide Shell for Significant Enhancement in Oxygen Evolution Reaction Activity. ACS Energy Lett. 2017, 3, 141-148. (25) Zhu, Y.; Zhou, W.; Chen, Y.; Yu, J.; Liu, M.; Shao, Z. A High-Performance Electrocatalyst for Oxygen Evolution Reaction: LiCo0.8Fe0.2O2. Adv. Mater. 2015, 27, 7150-7155. (26) Diaz-Morales, O.; Ledezma-Yanez, I.; Koper, M. T. M.; Calle-Vallejo, F. Guidelines for the Rational Design of Ni-Based Double Hydroxide Electrocatalysts for the Oxygen Evolution Reaction. ACS Catal. 2015, 5, 5380-5387. (27) Grimaud, A.; Diaz-Morales, O.; Han, B.; Hong, W. T.; Lee, Y. L.; Giordano, L.; Stoerzinger, K. A.; Koper, M. T. M.; Shao-Horn, Y. Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution. Nat. Chem. 2017, 9, 457-465.


26


Page 26 of 31

(28) Hong, W. T.; Stoerzinger, K. A.; Lee, Y.-L.; Giordano, L.; Grimaud, A.; Johnson, A. M.; Hwang, J.; Crumlin, E. J.; Yang, W.; Shao-Horn, Y. Charge-transfer-energy-dependent oxygen evolution reaction mechanisms for perovskite oxides. Energy Environ. Sci. 2017, 10, 2190-2200. (29) Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44, 1272-1276. (30) Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; Garcia-Melchor, M.; Han, L.; Xu, J.; Liu, M.; Zheng, L.; Garcia de Arquer, F. P.; Dinh, C. T.; Fan, F.; Yuan, M.; Yassitepe, E.; Chen, N.; Regier, T.; Liu, P.; Li, Y.; De Luna, P.; Janmohamed, A.; Xin, H. L.; Yang, H.; Vojvodic, A.; Sargent, E. H. Homogeneously dispersed multimetal oxygen-evolving catalysts. Science 2016, 352, 333-337. (31) Zhang, C.; Bristowe, P. D. First principles calculations of oxygen vacancy formation in bariumstrontium-cobalt-ferrite. RSC Adv. 2013, 3, 12267-12274. (32) Gangopadhyay, S.; Masunov, A. E.; Inerbaev, T.; Mesit, J.; Guha, R. K.; Sleiti, A. K.; Kapat, J. S. Understanding oxygen vacancy migration and clustering in barium strontiumcobalt iron oxide. Solid State Ionics 2010, 181, 1067-1073. (33) Lumeij, M.; Koettgen, J.; Gilleßen, M.; Itoh, T.; Dronskowski, R. Detailed insights into the structural properties and oxygen-pathways in orthorhombic Ba0.5Sr0.5Co0.8Fe0.2O3–δ by electronic-structure theory. Solid State Ionics 2012, 222-223, 53-58. (34) Yagi, S.; Yamada, I.; Tsukasaki, H.; Seno, A.; Murakami, M.; Fujii, H.; Chen, H.; Umezawa, N.; Abe, H.; Nishiyama, N.; Mori, S. Covalency-reinforced oxygen evolution reaction catalyst. Nat. Commun. 2015, 6, 8249. (35) Takamatsu, A.; Yamada, I.; Yagi, S.; Ikeno, H. Oxygen Evolution via the Bridging Inequivalent Dual-Site Reaction: First-Principles Study of a Quadruple-Perovskite Oxide Catalyst. J. Phys. Chem. C 2017, 121, 28403-28411. (36) Yamada, I.; Fujii, H.; Takamatsu, A.; Ikeno, H.; Wada, K.; Tsukasaki, H.; Kawaguchi, S.; Mori, S.; Yagi, S. Bifunctional Oxygen Reaction Catalysis of Quadruple Manganese Perovskites. Adv. Mater. 2017, 29, 1603004.


27

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37) Vasil’ev, A. N.; Volkova, O. S. New functional materials AC3B4O12. Low Temperature Physics 2007, 33, 895. (38) Yamada, I. Novel catalytic properties of quadruple perovskites. Sci. Technol. Adv. Mater. 2017, 18, 541-548. (39) Goodenough, J. B.; Longo, J. M.; Kafalas, J. A. Band antiferromagnetism and the new perovskite CaCrO3. Mater. Res. Bull. 1968, 3, 471-481. (40) Takano, M.; Nakanishi, N.; Takeda, Y.; Naka, S.; Takada, T. Charge disproportionation in CaFeO3 studied with Mössbauer effect. Mater. Res. Bull. 1977, 12, 923-928. (41) Demazeau, G.; Parent, C.; Pouchard, M.; Hagenmuller, P. Sur deux nouvelles phases oxygenees du cuivre trivalent: LaCuO3 et La2Li0.50Cu0.50O4. Mater. Res. Bull. 1972, 7, 913-920. (42) Subramanian, M. A.; Marshall, W. J.; Calvarese, T. G.; Sleight, A. W. Valence degeneracy in CaCu3Cr4O12. J. Phys. Chem. Solids 2003, 64, 1569-1571. (43) Kadyrova, N. I.; Tyutyunnik, A. P.; Zubkov, V. G.; Zakharova, G. S.; Volkov, V. L.; Dyachkova, T. V.; Zainulin, Y. G. Thermobaric synthesis and crystal structure of new compounds, NaCu3V4O12 and CaCu3V4O12. Russ. J. Inorg. Chem. 2003, 48, 1785-1790. (44) Chenavas, J.; Joubert, J. C.; Marezio, M.; Bochu, B. The synthesis and crystal structure of CaCu3Mn4O12: A new ferromagnetic-perovskite-like compound. J. Solid State Chem. 1975, 14, 25-32. (45) Yamada, I.; Takata, K.; Hayashi, N.; Shinohara, S.; Azuma, M.; Mori, S.; Muranaka, S.; Shimakawa, Y.; Takano, M. A perovskite containing quadrivalent iron as a charge-disproportionated ferrimagnet. Angew. Chem. Int. Ed. 2008, 47, 7032-7035. (46) Yamada, I.; Ishiwata, S.; Terasaki, I.; Azuma, M.; Shimakawa, Y.; Takano, M. Synthesis, Structure, and Physical Properties of A-site Ordered Perovskites ACu3Co4O12 (A = Ca and Y). Chem. Mater. 2010, 22, 5328-5332. (47) Osaka, T.; Takahashi, H.; Sagayama, H.; Yamasaki, Y.; Ishiwata, S. High-pressure synthesis of an unusual antiferromagnetic metal CaCoO3 with GdFeO3-type perovskite structure. Phys. Rev. B 2017, 95, 224440.


28


Page 28 of 31

(48) Falcón, H.; Alonso, J. A.; Casais, M. T.; Martı́nez-Lope, M. J.; Sánchez-Benı́tez, J. Neutron diffraction study, magnetism and magnetotransport of stoichiometric CaVO3 perovskite with positive magnetoresistance. J. Solid State Chem. 2004, 177, 3099-3104. (49) Long, Y. W.; Hayashi, N.; Saito, T.; Azuma, M.; Muranaka, S.; Shimakawa, Y. Temperatureinduced A-B intersite charge transfer in an A-site-ordered LaCu3Fe4O12 perovskite. Nature 2009, 458, 6063. (50) Kohn, K.; Inoue, K.; Horie, O.; Akimoto, S.-I. Crystal chemistry of MSeO3 and MTeO3 (M = Mg, Mn, Co, Ni, Cu, and Zn). J. Solid State Chem. 1976, 18, 27-37. (51) Kawaguchi, S.; Takemoto, M.; Osaka, K.; Nishibori, E.; Moriyoshi, C.; Kubota, Y.; Kuroiwa, Y.; Sugimoto, K. High-throughput powder diffraction measurement system consisting of multiple MYTHEN detectors at beamline BL02B2 of SPring-8. Rev. Sci. Instrum. 2017, 88, 085111. (52) Izumi, F.; Momma, K. Three-Dimensional Visualization in Powder Diffraction. Solid State Phenom. 2007, 130, 15-20. (53) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953-17979. (54) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169-11186. (55) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15-50. (56) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study. Phys. Rev. B 1998, 57, 1505-1509. (57) Wang, Y.; Guo, G. Robust half-metallic antiferromagnets LaAVOsO6 and LaAMoYO6 (A = Ca, Sr, Ba; Y=Re, Tc) from first-principles calculations. Phys. Rev. B 2006, 73, 064424. (58) Hu, Z.; Metiu, H. Choice of U for DFT+U Calculations for Titanium Oxides. J. Phys. Chem. C 2011, 115, 5841-5845. (59) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188-5192.


29

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(60) Björketun, M. E.; Castelli, I. E.; Rossmeisl, J.; Olsen, T.; Ukai, K.; Kato, M.; Dennler, G.; Jacobsen, K. W. Defect Chemistry and Electrical Conductivity of Sm-Doped La1–xSrxCoO3–δ for Solid Oxide Fuel Cells. J. Phys. Chem. C 2017, 121, 15017-15027. (61) Kuklja, M. M.; Mastrikov, Y. A.; Jansang, B.; Kotomin, E. A. The Intrinsic Defects, Disordering, and Structural Stability of BaxSr1–xCoyFe1–yO3–δ Perovskite Solid Solutions. J. Phys. Chem. C 2012, 116, 18605-18611. (62) Yamada, I.; Murakami, M.; Hayashi, N.; Mori, S. Inverse Charge Transfer in the Quadruple Perovskite CaCu3Fe4O12. Inorg. Chem. 2016, 55, 1715-1719. (63) Mizokawa, T.; Morita, Y.; Sudayama, T.; Takubo, K.; Yamada, I.; Azuma, M.; Takano, M.; Shimakawa, Y. Metallic versus insulating behavior in theA-site ordered perovskite oxides ACu3Co4O12 (A = Ca and Y) controlled by Mott and Zhang-Rice physics. Phys. Rev. B 2009, 80, 125105. (64) Palacin, M. R.; Bassas, J.; Rodriguez-Carvajal, J.; Gomez-Romero, P. Syntheses of the perovskite La2CuTiO6 by the ceramic, oxide precursors and sol-gel methods, and study of the structure and Cu-Ti distribution by X-ray and neutron diffraction. J. Mater. Chem. 1993, 3, 1171-1177. (65) Cortés-Gil, R.; Ruiz-González, M. L.; Alonso, J. M.; García-Hernández, M.; Hernando, A.; González-Calbet, J. M. Magnetoresistance and Ferromagnetism in Disordered LaCu0.5Mn0.5O3 Perovskite. Chem. Mater. 2013, 25, 2100-2108. (66) Tahini, H. A.; Tan, X.; Schwingenschlögl, U.; Smith, S. C. Formation and Migration of Oxygen Vacancies in SrCoO3 and Their Effect on Oxygen Evolution Reactions. ACS Catal. 2016, 6, 5565-5570.


30


Page 30 of 31

Table of Contents Graphic


31

Synergistically Enhanced Oxygen Evolution Reaction Catalysis for

Recommend Documents