Potential-Dependent Restructuring and Hysteresis in the Structural

Article pubs.acs.org/JPCC

Potential-Dependent Restructuring and Hysteresis in the Structural and Electronic Transformations of Pt/C, Au(Core)-Pt(Shell)/C, and Pd(Core)-Pt(Shell)/C Cathode Catalysts in Polymer Electrolyte Fuel Cells Characterized by in Situ X‑ray Absorption Fine Structure Shin-ichi Nagamatsu,† Takashi Arai,‡ Masakuni Yamamoto,‡ Takuya Ohkura,‡ Hiroyuki Oyanagi,‡ Takayuki Ishizaka,§ Hajime Kawanami,§ Tomoya Uruga,∥ Mizuki Tada,⊥ and Yasuhiro Iwasawa*,† †

Innovation Research Center for Fuel Cells, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan ‡ Honda R&D Co., Ltd., Hagamachi, Hagagun, Tochigi 321-3393, Japan § AIST, Miyagino, Sendai 983-8551, Japan ∥ Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Koto, Sayo, Hyogo 679-5198, Japan ⊥ Institute for Molecular Science, 38 Nishigo-naka, Myodaiji, Okazaki, Aichi 444-8585, Japan S Supporting Information *

ABSTRACT: Potential-dependent transformations of surface structures, Pt oxidation states, and Pt−O bondings in Pt/C, Au(core)-Pt(shell)/C (denoted as Au@Pt/C), and Pd(core)Pt(shell)/C (denoted as Pd@Pt/C) cathode catalysts in polymer electrolyte fuel cells (PEFCs) during the voltagestepping processes were characterized by in situ (operando) Xray absorption fine structure (XAFS). The active surface phase of the Au@Pt/C for oxygen reduction reaction (ORR) was suggested to be the Pt3Au alloy layer on Au core nanoparticles, while that of the Pd@Pt/C was the Pt atomic layer on Pd core nanoparticles. The surfaces of the Pt, Au@Pt and Pd@Pt nanoparticles were restructured and disordered at high potentials, which were induced by strong Pt−O bonds, resulting in hysteresis in the structural and electronic transformations in increasing and decreasing voltage operations. The potential-dependent restructuring, disordering, and hysteresis may be relevant to hindered Pt performance, Pt dissolution to the electrolyte, and degradation of the ORR activity.

1. INTRODUCTION Polymer electrolyte fuel cells (PEFCs) are one of the most efficient clean energy technologies1−9 and are considered to be suitable for automotive applications due to high power density. PEFCs can form electric power even at low temperatures (333−373 K) using hydrogen at the anode and oxygen (air) at th ecathode with 40−60% practical efficiency though the theoretical value is 83% (ΔG/ΔH (H2)). A membrane electrode assembly (MEA) as an active unit of PEFCs has a stacking structure with an anode catalyst layer (e.g., Pt/C), a proton conducting membrane electrolyte, and a cathode catalyst layer (e.g., Pt/C). At the cathode catalyst surface, oxygen reacts with hydrogen (O2 + 4H+ + 4e− → 2H2O) to form water (oxygen reduction reaction: ORR). Although a variety of catalysts have been proposed as promising PEFC cathode catalysts, carbon-supported Pt catalysts are still considered to be the most active and durable cathode catalysts for PEFCs. Main drawbacks to develop fuel cells for automobiles are insufficient ORR activity (surface-specific activity per 1 cm2 of © 2013 American Chemical Society

Pt and practically, mass-specific activity per 1 g Pt, related to cost performance) and poor durability (mainly resistant to unfavorable dissolution and sintering of expensive Pt catalysts) of Pt/C cathode catalysts under PEFC operating conditions. There is a challenge to improve those parameters to use for different PEFCs.10−14 Recently density functional theory (DFT) studies showed that direct correlation between ORR activity and OH coverage from water activation is not a straightforward process.15 Therefore, the decreasing activity is attributed partly to the hindered O2 reduction caused by adsorbed oxygenates like OH species on Pt in the potential region of about 0.75−1.0 V.16−19 The other notable problem associated with harsh operating conditions in PEFCs is the corrosion of carbon support, which thermodynamically may start at 0.207 V,20 resulting in eventually sintering of Pt nanoparticles over long cycling hours.21 Received: March 10, 2013 Revised: May 26, 2013 Published: June 5, 2013 13094

dx.doi.org/10.1021/jp402438e | J. Phys. Chem. C 2013, 117, 13094−13107

The Journal of Physical Chemistry C

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insight may be obtained to the mechanisms and guidelines for promoting activity and improving durability of cathode catalysts to develop next generation PEFCs. There are few suitable in situ techniques for measuring the dynamic chemical events at cathode surfaces in MEA under the PEFC working conditions. It is in general hard to observe the MEA cathode catalysts with small nanometer dimensions in situ in the complex, wet, heterogeneous, and multiphasic reaction field involving catalyst nanoparticles, carbon support, ionomer, polymer electrolyte, and an abundance of water and fuel gases, which prevents in situ measurements of the catalyst nanoparticles by spectroscopic methods such as FT-IR, ultrafast laser spectroscopies, electron spectroscopies, and so forth and by analysis techniques such as STEM/EDS, SPM, and so forth. X-ray absorption fine structure (XAFS) analysis has become a powerful tool for investigating the local coordination structures and oxidation states of supported metal nanoparticle catalysts under in situ reaction conditions.50,74,83−107 The mechanisms of the electrochemical processes involved in rapid voltage-controlled processes on Pt/C and Pt3Co/C catalysts have been investigated by a real-time XAFS technique, showing the kinetics and rate constants of elementary reaction steps for structural and electronic transformations of the cataysts.89,106 In the present study we report new aspects of the restructuring and hysteresis in the transformations of surface structures, Pt oxidation states, and Pt−O bondings of the Pt/C, Au(core)Pt(shell)/C (denoted as Au@Pt/C), and Pd(core)-Pt(shell)/C (denoted as Pd@Pt/C) cathode catalysts in PEFC MEAs during the voltage-stepping processes based on a systematic molecular-level study by in situ (operando) XAFS.

The DFT simulations of the mechanism for the ORR on Pt(111) surface have been performed to understand the reaction pathways involving O, OOH, H2O2, and so forth as intermediates and the transition state for the ORR.22,23 The mechanisms and parameters for the MEA performance and durability must be understood more thoroughly in order to development of next generation PEFCs for automobiles. In comparison to the constant potential hold process, Pt dissolution is 3−4 times higher in voltage cycling, which is a major problem because automobiles require continual repetition of the power-on/off processes with rapid changes in cell voltages for driving cars.24 Although the Pt dissolution potential (>1.1 V) is higher than the operation potential of PEFCs (0.6− 1.0 V), ramping of the potential to open circuit voltage (OCV) around 1.0 V during the power-on/off drive and possible undesirable ramping to the higher potentials (>1.1 V) at unidentified places in MEA cathode catalyst layers with illdefined heterogeneous environments may promote the Pt leaching, resulting in the degradation of catalyst performances. The mechanism of the Pt dissolution to the electrolyte at an atomic scale still remains unclear. A promising approach to reduce Pt loadings and to improve fuel-cell performance and durability of cathode catalysts is alloying of Pt with transition metals such as Co, Ni, Cu, Fe, Cr, Ti, and so forth. These Pt-M alloys have been demonstrated to be 2−10 times more active than polycrystalline Pt for the ORR.25−43 Another strategy to obtain high-performance is depositing thin Pt overlayers onto less or non-noble metal substrates to make Pt surface-enriched nanoparticles called as skin, skeleton, and core−shell structures, where Pt can be sufficiently used and hence catalyst cost can be reduced.16,44−52 Among examined metal core substrates (Ru, Ir, Rh, Au, and Pd) for Pt monolayer shell, Pd showed the highest ORR activity. The Pt mass-specific activity of a Pd(core)-Pt monolayer(shell)/C cathode catalyst has been reported to be 5−8 times higher than that of Pt/C, and the noble metal (Pt +Pd) mass-specific activity has also been reported to be two times higher than that of Pt/C.49,50 But most of those bimetal systems may not be stable due to segregation and leaching over time.53 The Pt surface-specific ORR activity in the bimetallic systems has been demonstrated to depend on a variety of key issues, such as nanoparticle size and shape (plane),18,27,28,39,43,54−60 surface roughness,30,35,58,61−64 compressive strain effect (bond distance),44,58,65−70 downward Pt d-band center, 27,43,52,62,63,68,69,71,72 ligand effect (electron transfer ), 1 9 , 2 5 , 2 7 , 3 1 , 5 8 , 6 2 , 6 4 − 6 9 , 7 3 , 7 4 met al−oxy gen bond strength, 8,36,53,54,75,76 anion adsorption, 18 oxide formation,22−24,29,77 and so forth.43,48,57,78−82 Although many investigations on electrochemical surface events have been performed for both model systems such as single crystals, polycrystals, or carbon-supported nanoparticles and practical PEFCs, fundamental issues such as the voltage-dependent structures and electronic states of cathode catalyst surfaces, the structural kinetics of the transformations of catalyst themselves, the molecular-level mechanism for the Pt dissolution, and the origin of the enhancement mechanism for the ORR are not clearly understood yet. Little is known about molecular-level information on the transformation of structures and chemical compositions of the surfaces of promising core−shell cathode catalysts and the behavior of Pt−O bonds with oxygenates on the catalyst surfaces under voltage operating conditions, by which the

2. EXPERIMENTAL SECTION 2.1. Preparation of the Catalysts Loaded with Core− Shell Metal Nanoparticles. The core−shell bimetallic nanoparticles were prepared as follows; Au or Pd nanoparticles as core were prepared by the chemical reduction of Au and Pd precursors (HAuCl4 or Na2PdCl4) in water. After forming the suspension of reduced Au or Pd nanoparticles, a given amount of well-dispersed Ketjen Black support was added with vigorous stirring. After the complete adsorption of metal nanoparticles on the carbon support, a given amount of Pt precursor was added at room temperature and reduced to form the monolayer shell on the surface of core metal nanoparticles. After a few hours, the carbon-supported core−shell nanoparticles were centrifuged and filtrated by a membrane filter. Finally, the obtained catalysts were washed with deionized water at least three times and dried under reduced pressure at room temperature and at 323 K. The loaded Pt shells on the core−shell catalysts have 0.23 mg Pt/cm2 (13 wt % Pt) for Au@Pt/C and 0.27 mg Pt/cm2 (15 wt % Pt) for Pd@Pt/C, respectively. The Au/Pt and Pd/Pt atomic ratios of these two samples with 1 ML Pt were 1:1 and 2:1, respectively. As a reference sample, 30 wt % Pt/C (0.60 mg Pt/ cm2) was also prepared following the same procedure. 2.2. Assembling Procedures To Prepare the Membrane-Electrode Assembly (MEA) for in Situ XAFS Measurements. The prepared catalysts were coated on to MEAs, where Nafion112 membranes with 50 μm thickness were used as a polymer electrolyte, and the prepared MEA was sandwiched in a handmade single cell (Figure S1 of Supporting Information). The MEA anode catalysts of Au@Pt/C and Pd@ Pt/C used in XAFS measurements were 50 wt % Pd/C (0.5 mg Pd/cm2) and 50 wt % Ru/C (0.4 mg Ru/cm2), respectively. 13095



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The gas flows of H2 (99.99999%) for anode and dry air (99.99995%) for cathode were regulated by mass-flow controllers and were bubbled through humidifiers at 351 K. The humidified gases were supplied to the in situ XAFS cell heated at 353 K. Gas pressures and flow rates were 130 kPa and 100 mL min−1 for the anode (H2) and 100 kPa and 300 mL min−1 for the cathode (air), respectively. A cell voltage between the anode and the cathode was controlled by a PotentioGalvano-stat (HOKUTO DENKO HZ-5000) with a current amplifier (Hokuto Denko HAG1550A power unit). Aging of the MEA samples was always conducted by flowing dry air at cathode and was repeated 150 times under the following current density steps (0.2 → 0 → 0.05 → 0.1 → 0.2 → 0.3 → 0.4 → 0.5 → 0.6 → 0.7 → 0.8 → 0.9 → 1.0 A/cm2). 2.3. Cyclic Voltammetry Measurements. The cyclic voltammograms (CVs) were recorded with H2 flow (anode) and N2 flow (cathode) at 50 mV/s in the range 0.05−1.4 V at 353 K. The electrochemical surface areas (ECSAs) of the core− shell nanoparticles at cathodes were calculated by charge density of hydrogen adsorption on a Pt surface (210 μC/ cm−Pt2) in the hydrogen underpotential deposition region (0.05 V, ca. 0.3 V of the onset of the double-layer region). Typical CVs for Pt/C, Au@Pt/C, and Pd@Pt/C are shown in Figure S2, and the specific power densities of the samples are plotted against the mass activity in Figure S3, where the Pd@Pt/C and Au@Pt/C showed a higher power density at the higher mass activity than the Pt/C. 2.4. In Situ XAFS Measurements. In situ XAFS spectra at Pt LIII-edge, Au LIII-edge, and Pd K-edge for Pt/C, Au@Pt/C, and Pd@Pt/C cathode catalysts in PEFC MEA were measured at BL01B1 station in Spring-8 using a Si(111) double-crystal monochromator in a fluorescence mode as shown in Figure S1. The detectors for Pt LIII-edge and Au LIII-edge XAFS measurements were an ion chamber (I0: Ar 15%/N2 85%) for incident X-rays and a Lytle detector (If: Kr 100%) for fluorescent X-rays, respectively. The detectors for Pd K-edge XAFS measurements were an ion chamber (I0: Ar 100%) for incident X-rays and a Lytle detector (If: Kr 100%) for fluorescent X-rays, respectively. The collected XAFS spectra were analyzed in Ifeffit (Athena and Artemis).108 Background subtraction was performed using Autobk.109 The white-line peak of a normalized Pt LIII-edge XANES (X-ray absorption near-edge structure) spectrum was analyzed by curve fitting with an arctangent function and a Lorentzian function. The background in XAFS spectra were removed by using REX2000 software (ver.2.5, RIGAKU). The Victoreen function was employed for the background subtraction, and the spline smoothing method with Cook and Sayers criteria was used as the μ0 method. The extracted EXAFS (extended X-ray absorption fine structure) oscillations were k3-weighted and Fourier transformed to r-space over k = 30−160 nm−1 or 30−150 nm−1, respectively for Pd K-edge data and Pt LIII-edge data. The fittings of k3-weighted EXAFS data in r-space were performed with Artemis.108 Fitting parameters for each shell were the coordination number (CN), interatomic distance (R), Debye−Waller factor (σ2), and correction-of-edge energy (ΔE0). The phase shift and amplitude functions for Pt− O, Pt−Pt, Pt−Au, and Pt−Pd were obtained from FEFF (Ver 8.4).108 Error ranges of the curve-fitting analysis of EXAFS Fourier transforms were based on the definition of the Feffit code in the Artemis program. XAFS spectra were measured under the stepwise voltage operation from 0.4 to 1.4 V and from 1.4 to 0.4 V every 0.2 V

(voltages referred to RHE). In the stepwise voltage operating process, the fuel cells (anode: H2; cathode: air) were kept typically for 60 min at each voltage for XAFS measurements. The Pt LIII-edge XAFS spectra at each voltage were measured at 50 min acquisition (5 times of 10 min acquisition) starting the measurement at 5 min after a voltage step-up to the next higher voltage or a voltage step-down to the next lower voltage. The XAFS spectra every 10 min at a constant voltage were almost the same, which suggests that the catalyst surface event at each voltage observed by XAFS is in an equilibrium state. 2.5. TEM Observations. STEM photographs were taken using Hitachi HD-2700 STEM with a CCD camera at an accelerating voltage of 300 kV (Gatan, Inc.).

3. RESULTS AND DISCUSSION 3.1. STEM Images and EDS Profiles. Core−shell Au@Pt and Pd@Pt bimetallic nanoparticles on carbon were characterized by STEM (scanning transmission electron microscope) images and EDS (energy dispersive X-ray spectroscopy) line profiles (Figure 1). The EDS line profile of a Au@Pt particle on

Figure 1. STEM images and EDS line profiles of Au@Pt/C (a) and Pd@Pt/C (b). STEM photographs were taken using a Hitachi HD2700 STEM.

carbon showed the existence of Au as a major element, which was distributed as a core (Figure 1a). The concentration of Au steeply decreased to a negligible level at the boundary (both sides of the line profile) of the nanoparticle, where the principal metal was Pt. Pt was observed over the whole nanoparticle, covering the nanoparticle surface. Also for the line profiles of Pd and Pt of a Pd@Pt particle on carbon (Figure 1b), the intensity of Pd was large inside the nanoparticle and negligible at the boundary. Pt was also observed over the whole nanoparticle, covering the nanoparticle surface. At the boundary of the nanoparticle (both sides of the line profile) Pt was observed as a major metal. Thus it is suggested by the STEM-EDS line profiles that the Au@Pt and Pd@Pt nanoparticles prepared by the sequential synthesis method possess Pt-enriched surfaces similar to previous reports.40,45,46,54 The core−shell type structures were also supported by the results of in situ XAFS measurements under voltage operating processes as discussed hereinafter. Averaged sizes of Pt nanoparticles of the Pt/C and bimetallic nanoparticles of the Au@Pt/C and Pd@Pt/C were estimated as 2.8, 6.2, and 3.6 nm, respectively, by TEM images (Figure S4). After the stepwise voltage operation between 0.4 and 1.4 V, the Pt particle size in the Pt/C increased from 2.8 to 3.6 nm, but the change was not so significant. The particle sizes of 6.5 and 3.0 nm in the Au@Pt/C and Pd@Pt/C also remained almost unchanged (6.2 and 3.6 nm) after the stepwise voltage operations. These stable bimetallic core−shell and mono13096



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respectively, in Figure S6a and b revealed no electronic change in the Au and Pd atoms in the power-off and -on conditions. The Au and Pd atoms in Au@Pt and Pd@Pt made no direct bonding with oxygen in the potential stepping ORR processes as suggested by the EXAFS analysis at Au LIII-edge and Pd Kedge, respectively. Figure S6c is a typical series of EXAFS Fourier transforms for the Pd@Pt/C in the stepwise voltage operation, where the Fourier transform for Pd foil is also shown for comparison. The EXAFS data revealed almost no change in the local structure around Pd atoms. Thus, it is concluded that the Au and Pd atoms in Au@Pt/C and Pd@Pt/C do not act as reaction sites for the ORR in the potential range 0.4−1.4 V and the Au and Pd cores are stable and not degraded under the potential operations. The white-line responses in Pt/C, Au@Pt/C and Pd@Pt/C in the every 0.2 V stepwise voltage operation are summarized in Figure 3a−c. The white line intensities at 0.4 V for the Au@Pt/

metallic nanoparticles on carbon supports were subjected to XAFS measurements. 3.2. In Situ XANES under Stepwise Voltage Operation. Figure 2 shows the series of in situ XANES spectra for Au@Pt/

Figure 2. A series of Pt LIII-edge XANES spectra for Au@Pt/C (a) and Pd@Pt/C (b) in the stepwise voltage operation; 0.4 V → 1.4 V → 0.4 V vs RHE, every 0.2 V.

C and Pd@Pt/C in PEFC MEA under the stepwise voltage operation 0.4 V → 0.6 V → 0.8 V → 1.0 V → 1.2 V → 1.4 V → 1.2 V → 1.0 V → 0.8 V → 0.6 V → 0.4 V vs RHE. The intensity of white lines (2p → 5d transition) in the XANES spectra at Pt LIII-edge reflects the degree of the vacancy of Pt 5d orbitals near the Fermi level for the Au@Pt/C and Pd@Pt/C catalysts, and hence one can estimate the oxidation state of Pt atoms of the catalysts. The white lines of the XANES spectra for the Au@Pt/C catalyst became larger with increasing voltages from 0.4 to 1.4 V and smaller with decreasing voltages from 1.4 to 0.4 V reversely as shown in Figure 2a. The variation of the XANES spectra for Pd@Pt/C was similar to that for Au@Pt/C (Figure 2b). The XANES spectra for Pt/C in the voltage operating process were also measured for comparison. The variation of the XANES spectra with the potentials in the voltage operating process 0.4 V → 0.6 V → 0.8 V → 1.0 V → 1.2 V → 1.4 V → 1.2 V → 1.0 V → 0.8 V → 0.6 V → 0.4 V vs RHE is shown in Figure S5, which is similar to those of the Au@Pt/C and Pd@ Pt/C. However, the change in the XANES spectra of the Pt/C with the bulk Pt and the smaller Pt fraction at the surface was less than those of the Au@Pt/C and Pd@Pt/C with Ptenriched surfaces. In situ Au LIII-edge and Pd K-edge XANES spectra for Au@ Pt/C and Pd@Pt/C MEA catalysts, respectively, in the stepwise voltage operation 0.4 V → 1.4 V → 0.4 V vs RHE every 0.2 V were also measured to examine the electronic changes in Au and Pd sites in the core−shell type nanoparticles. The series of Au LIII-edge and Pd K-edge XANES spectra,

Figure 3. Comparison of the white-line responses among Pt/C (a), Au@Pt/C (b), and Pd@Pt/C (c) in the stepwise voltage operation 0.4 V → 1.4 V → 0.4 V vs RHE, every 0.2 V.

C and Pd@Pt/C cathode catalysts in PEFC were similar to that for the Pt/C cathode with metallic Pt state at 0.4 V, which indicates that electron transfer from Au or Pd core to Pt shell at 0.4 V is negligible. All of the Pt/C, Au@Pt/C, and Pd@Pt/C showed an increase of the white line intensity above 1.0 V in the potential gain process, compared to the intensity for the metallic Pt state at 0.4 V. As for the Au@Pt/C and Pd@Pt/C 13097



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catalysts with the Pt surface-enriched structures the white line intensity at 1.4 V was larger than the corresponding intensity of the Pt/C with the Pt bulk. It is to be noted that the white line responses in Figure 3a−c exhibited a distinct hysteresis behavior in voltage-up and down processes. A similar hysteresis in XANES was observed for 1 ML Pt/Rh(111) in 0.01 M HClO4 solution.91 The white line intensity increased with an increase of the voltage above 1.0 V up to 1.4 V. By decreasing voltage from 1.4 to 1.0 V the intensity remained almost unchanged, keeping the high intensity, and it did not reduce back to the initial intensities at 1.2 and 1.0 V. The white line intensity drastically reduced at 0.8 V, and it became the initial intensity for the metallic Pt state at 0.4 V eventually. The hysteresis in the change of Pt oxidation states was observed commonly with Pt/C, Au@Pt/C, and Pd@Pt/C as shown in Figure 3. The enhanced ORR activity of Pt bimetallic nanoparticles has been attributed to electronic effect (ligand effect) such as a change in the Pt 5d valence band vacancy induced by underlying metals.36,65 In the present Au@Pt/C and Pd@Pt/ C catalysts the electron transfer from Au and Pd to the Ptenriched surface to influence the unoccupied d orbitals was negligible as mentioned above, but this does not naturally mean that the electronic states of the Pt-enriched surfaces are not influenced from the underlying Au and Pd. The weighted center of the d-band of the Pt-enriched surfaces may shift upward/downward by the underlying Au or Pd due to strain effects, ligand effects, and so forth.68,69 The 5d vacancy increased by increasing potentials, and the white line intensity at 1.4 V did not decrease until 0.8−1.0 V in the voltage decreasing operation, showing irreversible change in the 5d vacancy against the PEFC potentials. It is to be noted that there existed an isosbestic point at 11.571 keV in a series of XANES spectra for the Au@Pt/C in Figure 2a. Also in Figure 2b for the Pd@Pt/C an isosbestic point was observed at 11.571 keV. The presence of the isosbestic point indicates the direct transformation between an initial electronic state of Pt atoms at 0.4 V and a final electronic state at 1.4 V without any stable intermediate states. The isosbestic point in the XANES spectra for the Pt/C was observed at the energy of 11.569 keV for in both increasing and the decreasing voltage operations, indicating direct and reversible transformation between the two electronic states of Pt at 0.4 and 1.4 V. The valences of Pt atoms in Pt, Au@Pt, and Pd@Pt at 0.4 and 1.4 V were estimated by the XANES white line peak intensity. The white line intensities of the XANES spectra for Pt foil, Pt(acac)2, PtO, and PtO2 in Figure S7 are regarded to be proportional to their Pt valences as shown in Figure S8. The XANES spectra of the Pt/C, Au@Pt/C, and Pd@Pt/C at 0.4 V vs RHE were the same as that of metallic Pt foil as shown in Figure 4, where the XANES spectrum of the Pt/C at 0.4 V is typically shown. By using the linear relationship in Figure S8, the averaged valence of the Pt atoms in the PEFC Pt/C catalyst at 1.4 V is estimated to be 1+. It is to be noted that the XANES spectrum of Pt/C at 1.4 V was well fitted by a linear combination of the XANES spectra of Pt0 foil (67%) and Pt2+O (33%) as shown in Figure 4, which indicates the Pt valence of 0.7+. We assume an averaged valence of 0.85+ here. If the positive charge of 0.85+ is assigned to the two surface layers (adsorbed oxygen atoms and subsurface oxygen atoms) of Pt nanoparticles with 3.6 nm dimension as suggested by the EXAFS analysis in section 3.3, the oxidation state of the

Figure 4. Pt LIII-edge XANES spectra for Pt foil (1; green solid line), Pt/C at 0.4 V (2: red dotted line) and 1.4 V (3: red dotted line), and Pd@Pt/C at 1.4 V (4: red dotted line), fitting (5: blue solid line) of the XANES (3: red dotted line) by a linear combination of Pt foil XANES and PtO XANES, and fitting (6: blue solid line) of the XANES (4: red dotted line) by a linear combination of Pt foil XANES and PtO XANES.

outermost surface Pt atoms at 1.4 V is estimated to be approximately 2.2+ because the Pt atoms of the outermost layer make direct bonds with O atoms twice the second layer Pt atoms. The valences of Pt atoms in the Au@Pt/C and Pd@Pt/ C cathode catalysts at 1.4 V are also estimated to be 1.4+ and 1.7+, respectively (Figure S8). The Pt shell of the Pd@Pt/C at 1.4 V was more positive than that of the Au@Pt/C at 1.4 V, and this difference is referred to the difference in the number of O atoms bonded to a Pt atom as shown in Tables S1, S2, and S3. It is thus demonstrated that the direct transformation from metallic Pt to mostly Pt2+ at the nanoparticle surfaces occurs at the higher potentials above 1.2 V, exhibiting the isosbestic point in the XANES series, and no further oxidation to Pt4+ was observed under the fuel cell operating conditions. To understand the behavior of catalysts in more detail, we characterized structural transformations of the cathode catalysts under the stepwise voltage operation by in situ EXAFS. 3.3. In Situ EXAFS under Stepwise Voltage Operation. Pt LIII-edge EXAFS spectra for Pt/C, Au@Pt/C, and Pd@Pt/C were measured in situ under the stepwise voltage operation 0.4 V → 0.6 V → 0.8 V → 1.0 V → 1.2 V → 1.4 V → 1.2 V → 1.0 V → 0.8 V → 0.6 V → 0.4 V vs RHE to determine the potential-dependent structural parameters around Pt atoms of the PEFC cathode catalysts. The EXAFS Fourier transforms (FT) for Pt/C, Au@Pt/C, and Pd@Pt/C in the voltage increasing and decreasing processes are shown in Figure 5a−a′, b−b′, and c−c′, respectively. The curve-fittings for the EXAFS data were performed with both oscillations and Fourier transforms (Figure 5, Figures S9, S10, and S11). In the case of the Au@Pt/C, the k-range for FTs was 30−90 nm−1, which was smaller than the k-range (30−120 nm−1) for the Pt/C and Pd@Pt/C because Pt LIII-edge energy (11.564 keV) is close to Au LIII-edge energy (11.919 keV). As a common feature in the FTs for the three catalysts, the FT peak intensity around R = 0.2−0.3 nm due to Pt-M contributions reduced by increasing voltages from 0.4 to 1.4 V, while the FT peak around R = 0.15 nm due to Pt−O contribution evolved at the higher voltages (Figure 5). 13098



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Figure 5. k3-weighted EXAFS oscillations χ(k) at Pt LIII-edge (a, b, c) and their Fourier transforms (a′, b′, c′) for Pt/C (a, a′), Au@Pt/C (b, b′), and Pd@Pt/C (c, c′) under the stepwise voltage operation 0.4 V → 1.4 V → 0.4 V vs RHE; solid lines: observed, dotted lines: curve fitted; k-range: 30− 120 nm−1 (a, c) or 30−90 nm−1 (b), R-range: 0.12−0.32 nm (a) or 0.12−0.35 nm (b, c).

Pt at 0.4 V was 11.2 (±1.6), which is among the values expected from the particle size around 3.0 nm, and it did not change so much up to 1.0 V. Above 1.2 V the Pt nanoparticles were oxidized to form Pt−O bonds at the distance around 0.202 nm, and the CN of Pt−O increased from 0 at 1.0 V to 0.8 (±0.4) at 1.2 V and to 1.2 (±0.4) at 1.4 V. Simultaneously, the CN of Pt−Pt decreased to 8.7 (±1.3) at 1.2 V and remarkably to 7.0 (±1.1) at 1.4 V. In the voltage decreasing steps from 1.4 to 0.4 V every 0.2 V, the change in the CNs of Pt−Pt and Pt−O bonds did not retrace their voltage gaining steps but showed a definite hysteresis (Figure 6). They did not change significantly in the range from 1.4 to 1.0 V but changed drastically at 0.8 V and came back to the initial values at 0.4 V eventually. On the basis of a fcc structure of Pt nanoparticles with 3 nm dimensions, we can estimate the number of Pt atoms in the outermost surface layer, the second layer, the third layer, the fourth layer, the fifth layer, and the central atom to be 181, 116, 65, 29, 7, and 1, respectively. Hence, the CN for Pt−Pt bonds is estimated by the eq (1 + 7 + 29 + 65 + 116)/399 × 12 + 181/ 399 × 9 = 10.6, where 399 is the total number of Pt atoms and 12 is the CN of Pt in fcc Pt crystal and 9 is the CN of Pt atoms

The structural parameters determined by the EXAFS curve fitting analysis for the Pt/C, Au@Pt/C, and Pd@Pt/C are shown in Tables S1−S3, where the estimated standard deviations (error ranges) are also shown. In the EXAFS analysis for the Au@Pt/C, it is hard to discriminate the contribution of Pt and Au from each other due to their similar phase shift and backscattering amplitude functions. Hence, the structural parameters for Pt−Pt determined for the Au@Pt/C in Table S2 involve the contribution of Pt−Au. However, fortunately the discussion on the surface structures was not hindered. As for the Pd@Pt/C in Table S3, the EXAFS technique can determine the structural information of Pt−Pt and Pt−Pd bonds separately. The variation of the R and CN of Pt−Pt and Pt−O bonds in the Pt/C catalyst with the potential (V) vs RHE is shown in Figure 6 with the error bars and discussed here in detail because the discussion on the Pt/C catalyst provides the basis for the structural analysis of Au@Pt and Pd@Pt later. In the voltage gaining operation only Pt−Pt bonds at 0.276 (±0.001) nm were observed in the range 0.4−1.0 V, indicating the metallic state of Pt nanoparticles at the low potentials. The CN of Pt− 13099



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should be disordered due to the small domain and curvature for the nanoparticles, and indeed the observed CN of Pt−O bonds at 1.4 V was 1.2 (±0.4) smaller than the expected CN of 1.6 (Figure 6 and Table S1). The Pt−O bond distance (0.202 nm) determined by the EXAFS analysis (Figure 6 and Table S1) was almost the same as 0.200 nm for the PtO tetragonal monoxide structure (Figure S12). The PtO phase at the surface produced in situ under the voltage operation cannot be detected by XRD due to the small quantity and domain and the disordered structure. At 1.2 V the PtO was estimated to cover half the surface, where the estimated CNs of Pt−Pt and Pt−O are 8.1 and 1.2, respectively, which reproduce the observed values of 8.7 (±1.3) and 0.8 (±0.4), respectively (Table S1). The PtO phase was hard to be reduced to metallic state at 1.4−1.0 V in the voltage decreasing operation, which would suppress the overall ORR activity. Thus, in the stepwise voltage operation of the PEFC MEA we found the hysteresis in the transformation of structures and electronic states of the Pt/C cathode catalyst involving the disordered Pt2+-oxide formation by the in situ XAFS characterization (Figure 7). A lattice distortion of the surface Pt layer due to oxygen absorption into the subsurface at 1.3 V was observed with Pt(111) surface by STM, and in the decreasing potential operation a change back to the flat surface morphology was observed at 0.5 V.110 This observation with Pt(111) surface indicates an aspect of hysteresis though the potentials where the structures transform are different from the potentials for the hysteresis in the real Pt/C cathode catalyst in PEFC MEA (Figures 6 and 7). Oxide growth first begins with the formation of a thin layer having a PtO composition, on which a PtO2composed oxide continues growing above around 1.2 V. The PtO bulk-oxide requires higher electrode potential >1.3 V. However, considering thin oxide layers (1−2 layers), DFT calculations indicate the possibility of a stable phase having a PtO composition with the (100) plane.111 With increasing thickness of the oxide layer, α-PtO2 and β-PtO2 having the (001) and (110) planes, respectively, are formed, which is quite unlike the PEFC MEA Pt/C operation conditions. The formation of a PtO2 layer was also ruled out on Pt/Rh(111) based on the white line intensity and width.90 The present XAFS analysis of oxygen incorporation into the subsurface of Pt nanoparticles to form disordered PtO layer at the surface is also supported by a DFT study.112

Figure 6. Variation of the interatomic distance and coordination number of Pt−Pt and Pt−O bonds for Pt/C determined by the Pt LIIIedge EXAFS analysis with the potential (V) vs RHE.

at the outermost surface. The estimated value of 10.6 reproduces the observed CN of 11.2 (±1.6). The surface of the Pt nanoparticles on carbon was oxidized to form Pt−O bonds at 1.0−1.4 V, and the CNs of Pt−Pt (7.0 ± 1.1) and Pt− O (1.2 ± 0.4) at 1.4 V can be reproduced by the following equations. Considering the formation of Pt2+O phase with the CN of 4 for Pt−O (Figure S12) in the surface layer of Pt nanoparticles as suggested by the XANES analysis in section 3.2, we estimated the CN of Pt−Pt to be 7.3 by the equation [(1 + 7 + 29 + 65)/399 × 12 + 116/399 × 9] × 2/3 + 10.6 × 1/3 = 7.3, where two-third of the surface layers were conceived to be oxidized to PtO phase and the remaining one-third of the surface was regarded to be still metallic as shown in Figure 7. Then the CN of Pt−O was estimated as 1.6 by the equation 181/399 × 4 × 2/3 + 116/399 × 2 × 2/3 = 1.6. The PtO phase at the surface of the Pt nanoparticles with 3 nm dimensions

Figure 7. Structure transformations of Pt/C under stepwise voltage operation between 0.4 and 1.4 V (power-off and power-on) proposed by in situ XAFS analysis. 13100



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conditioning cycles as a result of balancing of Pt−Au bonding more preferable than Au−Au bonding and less surface energy of Au than Pt. In the voltage gaining process from 0.4 to 1.0 V only Pt− Pt(Au) bonds at 0.280 (±0.001) nm were observed without any Pt−O bonds, indicating the metallic state of the Ptenriched surfaces at the low potentials. It is notable that the determined Pt−Pt(Au) bond distance (Robs: 0.280 nm) is longer than 0.277 for Pt metal and shorter than 0.288 nm for Au metal. The intermediate bond distance of 0.280 nm (±0.001 nm) also indicates the alloy formation of Pt and Au. Assuming a linear equation for the observed Pt−Pt(Au) bond distance (Robs), Robs = 0.288XAu + 0.277(1 − XAu), where XAu is the fraction of Au, and also assuming a random mixing of Pt and Au due to the preferable Pt−Au bonding, we estimated the fraction of Pt in the alloy to be Pt:Au = 3:1 though there is a certain error and a part of the alloy layer might be Pt:Au = 2:1. Thus, the majority of the alloy composition was estimated to be Pt3Au. As the total amount of Pt atoms is 1 ML at the surface of Au@Pt, the Pt3Au phase may be located in the top two layers. The ORR activity at 0.4−1.0 V is referred to the Pt3Au alloy surface. The CN (10.2−10.9) of Pt−Pt(Au) bonds at 0.4−1.0 V did not significantly change, and the R (0.280 nm) of Pt−Pt(Au) also remained almost unchanged at these potentials as shown in Figure 8. However, Pt atoms of the Pt3Au phase at the surface began to be oxidized at 1.2 V, making Pt−O bonds at 0.202 (±0.007) nm as determined by the EXAFS analysis. It is to be noted that the CN of Pt−Pt(Au) decreased largely to 7.5 (±2.1) at 1.4 V and the R of Pt−Pt(Au) also reduced to 0.276 (±0.001) nm which is almost the same as 0.277 nm for pure Pt metal as shown in Figure 8 and Table S2. When the Pt3Au alloy structure transforms to a Au(core)-Pt(monatomic-layer shell) structure, one expects that the drastic decrease in the CN of Pt−Pt(Au) from ca. 10.6 to 9.0 in the case of fcc(111) arrangement (6 for Pt−Pt(topmost surface) and 3 for Pt− Au(underlying layer)). This is the case observed with the Au@ Pt/C at 1.4 V though the CNPt−Pt(Au) is a little smaller than the expected value due to the Pt(shell)-Au(core) lattice mismatch and disordering of the Pt−Au interface bonding. It is to be noted that the Au(core)-Pt3Au(alloy shell) structure was restructured to the Au(core)-Pt(1 ML shell) structure at 1.4 V vs RHE. The restructuring of dealloy may be induced by strong oxygen adsorption on the dealloyed Pt surface (RPt−O = 0.201 ± 0.002; CNPt−O = 1.3 ± 0.3). The cartoon of the structural transformation of the Au@Pt/C cathode catalysts in PEFC under the stepwise voltage gaining operation is illustrated in Figure 10. It is not possible to measure STEM/ EDS for the Au@Pt/C catalyst in MEA at 1.4 V because, when the PEFC operation of the Au@Pt/C catalyst in MEA at 1.4 V was stopped and the catalyst was removed out of the MEA for STEM measurement, the catalyst had changed to an unidentified sample at the lower potential. On the other hand, XAFS can provide catalyst information in situ at 1.4 V and any potentials as shown in this study. In the voltage decreasing steps from 1.4 to 0.4 V, the change in the R and CN of Pt−Pt(Au) and Pt−O bonds did not retrace their voltage gaining steps but showed a hysteresis (Figures 5b, 8, and 10). The RPt−Pt, CNPt−Pt, and CNPt−O did not change significantly in the range from 1.4 to 1.0 V and changed back to the initial values of the voltage gain steps at 0.8 V by alloying to the Pt3Au phase (Figure 10). The change of the structural parameters determined by EXAFS and the

The variation of the R and CN of Pt−Pt(Au) and Pt−O bonds in the Au@Pt/C catalyst with the potential (V) vs RHE is shown in Figure 8. In the core−shell nanoparticles with an

Figure 8. Variation of the interatomic distance and coordination number of Pt−Pt(Au) and Pt−O bonds for Au@Pt/C determined by the Pt LIII-edge EXAFS analysis with the potential (V) vs RHE.

averaged particle size of 6.2−6.5 nm (Figure S4), the CN of Pt−Pt(Au) bonds should be 9 for an ideal fcc(111)-1 × 1 surface or less for a defect surface with steps, kinks, and so forth, whereas the observed CNs at 0.4−1.0 V were as large as around 10.6 (±1.9), which is closer to CN around 11 expected for Pt−Au alloy phase at the surface layers. We have measured an STEM/EDS profile for a Au@Pt/C sample at low potential as shown in Figure 9. Comparing with Figure 1a for Au@Pt/C

Figure 9. STEM image and EDS line profile of Au@Pt/C at a low potential after the XAFS measurement. STEM photographs were taken using a Hitachi HD-2700 STEM.

with a core−shell structure, Figure 9 does not show a Au(core)Pt(shell) feature but indicates an aspect of Au−Pt alloy. The assynthesized Au@Pt/C sample with the monatomic Pt shell layer was prepared by postsynthesis of Pt shell on premade Au core, but the Au(core)-Pt (monolayer shell) structure transformed to the surface alloy layers at low potentials after the 13101



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Figure 10. Structure transformations of Au@Pt/C under stepwise voltage operation between 0.4 and 1.4 V (power-off and power-on) proposed by in situ XAFS analysis.

hysteresis behavior are similar to the change in the XANES white line intensity (Figure 3). Hence, these changes in both the XANES and EXAFS may originate from the structural and electronic rearrangements of the Au@Pt surface at 1.4 and 1.0 V in the potential increasing and decreasing processes, respectively. The structure and electronic state of the Au@ Pt/C cathode catalyst in PEFC transform dynamically between the Pt3Au alloy surface and Pt monolayer in the power-off and -on operations (Figure 10). Similar to our findings in a PEFC MEA Au@Pt/C system, surface exchange reactions between Au and Pt in a Pt-on-Au RDE system in 0.5 M H2SO4 were reported in CV analysis for hydrogen evolution and oxidation reactions.113 Pt segregation was caused by Pt−O bonding at high potentials in the both systems, while in the RDE system at low potentials Au migrated to cover the surface due to the lower surface energy and deactivated the electrode,113 but in the present MEA Au@Pt/C system the Au−Pt alloy surface layer was produced at the low potentials, showing a good performance as shown in Figures S2 and S3. The variation of the R and CN of Pt−Pt, Pt−Pd, and Pt−O bonds in the Pd@Pt/C catalyst with the potential (V) vs RHE is shown in Figure 11 and Table S3. In the voltage gaining process from 0.4 to 0.8 V, Pt−Pt bonds at 0.274 (±0.002) nm and Pt−Pd bonds at 0.272 (±0.001) nm were observed without any Pt−O bonding. The Pt−Pt bond distance was a little shorter than 0.277 nm for Pt metal, and the Pt−Pd bond distance was also a little shorter than 0.276 nm for PtPd alloy due to a characteristic aspect of bimetal nanoparticles. The CNs of Pt−Pt and Pt−Pd bonds at 0.4−0.8 V were 4.9 (±1.0) and 2.6 (±0.8), respectively. When Pt constitutes a monolayer shell on Pd core with about 3 nm dimensions, the CNs of Pt−Pt and Pt−Pd are expected to be 6 and 3, respectively, assuming a fcc(111) arrangement as shown in Figure 12. The CNs values determined by in situ EXAFS almost coincide with the expected values, but the observed CN (4.9) of Pt−Pt bonds in the 2D shell is a little smaller than 6 for a full-coverage monolayer shell, which suggests the formation of submonolayer Pt shell with vacancies on the Pd core. At 1.0 V the oxidation of the Pt shell began to form Pt−O bonds, and the CN of Pt−O bonds increased to 1.0 (±0.2) at 1.2 V and 2.4 (±0.8) at 1.4 V. When oxygen atoms adsorb on 3-fold-hollow sites at the Pt monatomic layer surface, the CN of Pt−O would be 3, which

Figure 11. Variation of the interatomic distance and coordination number of Pt−Pt, Pt−Pd, and Pt−O bonds for Pd@Pt/C determined by the Pt LIII-edge EXAFS analysis with the potential (V) vs RHE.

is similar to the experimentally determined value 2.4 (±0.8). At the onset of the Pt−O CN change, the CN of Pt−Pt bonds changed from 4.9 (±1.0) at 0.4 V to 2.4 (±1.2) at 1.4 V. It is suggested that the Pt submonolayer shell became a disordered monolayer by making Pt−O bonds, resulting in the reduction of the observed CNPt−Pt. The Pt(surface)-Pd(core) bond distance at the interface decreased significantly from 0.272 (±0.001) nm at 0.4 V to 0.270 (±0.001) nm at 1.4 V (Figure 11). Positively charged Pt shell with adsorbed oxygen may make stronger interaction with the underlying Pd core, which would increase the durability of the Pd@Pt/C catalyst at the high potentials. In the voltage decreasing steps from 1.4 to 0.4 V every 0.2 V, the changes in the CNs of Pt−Pt and Pt−O bonds and the R of Pt−Pd bond did not retrace their voltage gaining steps but showed a hysteresis (Figure 11) similar to the hysteresis in the 13102



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Figure 12. Structure transformations of Pd@Pt/C under stepwise voltage operation between 0.4 and 1.4 V vs RHE (power-off and power-on) proposed by in situ XAFS analysis.

demanded for further discussion on the black box of PEFC cathode catalysis in conjunction with the restructuring and hysteresis. Nevertheless, the present in situ XAFS study on the Pt/C, Au@Pt/C, and Pd@Pt/C in voltage stepping operation processes provided new and unique insights into the promotion and suppression of the ORR activity and Pt dissolution to the electrolyte of the core−shell catalysts in PEFCs.

change of the XANES white line intensities for the Pd@Pt/C (Figure 3c). Thus, the both structural and electronic hysteresis observed with the EXAFS and XANES, respectively, originates from the restructuring of the Pd@Pt due to strong Pt−O bonding. The structural parameters did not change significantly in the range from 1.4 to 1.0 V but came back to the initial values at 0.4 V eventually (Figure 11 and Table S3). The overall cartoon of the structural transformation of the Pd@Pt/C cathode catalyst in PEFC under the stepwise voltage operation is illustrated in Figure 12. The systematic in situ XAFS analysis revealed the restructuring and hysteresis in the transformations of the structures and electronic states of Pt/C, Au@Pt/C, and Pd@ Pt/C cathode catalysts in PEFCs in the stepwise voltage operations for the first time. The surface restructuring and hysteresis are induced by the strong Pt−O bond formation at 1.2−1.4 V vs RHE. The stable oxidized structures form accompanied with the rearrangements of the Pt, Au@Pt, and Pd@Pt nanoparticle surfaces. Once such new restructured/ disordered structures are produced at 1.4 V, the oxidized surfaces cannot readily be reduced under the fuel cell ORR conditions at 1.0−1.2 V, and the nanoparticle surfaces are hard to reconstitute back to the original structures at the potential gain steps at 1.0−1.2 V, where a large kinetic barrier for the reduction of the strongly bound oxygen atoms on the rearranged cathode surfaces may exist. The Pt, Au@Pt, and Pd@Pt show the feature of biphasic stability at 0.4−1.4 V, which may cause the hysteresis in the potential increasing and decreasing operations. The hard reduction of the Pt2+-O species may in turn increase the possibility of Pt dissolution to the electrolyte and degrade the ORR activity eventually. Timeresolved XAFS of rapid voltage-controlled ORR processes on Pt/C and Pt3Co/C between 0.4 and 1.0 V vs RHE decided 10 elementary steps and their rate constants, involving charge transfers, charging/discharging, Pt−O bonding/dissociation, and Pt−Pt dissociation/rebonding.89,106 The better activity of the Pt3Co/C was referred to the larger rate constants for each step, and the higher durability of the Pt3Co/C was referred particularly to the promotion of the Pt−O dissociation and Pt− Pt rebonding steps compared to the case of the Pt/C. Further study on the Au@Pt/C and Pd@Pt/C catalysts from timeresolved viewpoint will help kinetic understanding of the catalyst surface events in PEFC MEAs. Progress of the other sophisticated techniques which can be applied to wet, heterogeneous, and multiphasic PEFC catalyst layers is also

4. CONCLUSION The transformations of surface structures, Pt oxidation states, and Pt−O bonding of the cathode catalysts in PEFC MEAs during voltage operating processes may be relevant to the ORR activity and degradation of the cathode catalysts in PEFC systems. Particularly, the voltage-dependent structural and electronic transformations at higher potentials provide new insights into the molecular-level origin of promotion and suppression of the ORR activity as well as Pt sintering and dissolution by possible undesirable ramping to the higher potentials. We suggested by using in situ XAFS analysis that the structures, Pt oxidation states, and Pt−O bondings of Pt/C, Au@Pt/C, and Pd@Pt/C dynamically transform during the fuel cell power-off/on processes. We found the potentialdependent restructuring and hysteresis in the transformations of the structures and electronic states of the catalysts in the voltage increasing and decreasing steps, which may be induced by Pt−O bonding and due to the biphasic stability of the cathode surfaces. The active phase of the Au@Pt/C for the ORR was suggested to be the Pt3Au alloy layer on Au core, while the active phase of the Pd@Pt/C was the Pt atomic layer on Pd core. The hard reduction of the Pt2+-O species may suppress the ORR activity and promote the Pt dissolution to the electrolyte, resulting in degradation of the PEFC cathode catalysis eventually. In situ real-time XAFS of the core−shell catalysts in rapid voltage-controlled processes will provide further insight into the mechanism and deeper understanding of the ORR activity and durability of the cathode catalysts to develop next generation PEFCs.

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ASSOCIATED CONTENT

S Supporting Information *

Structural parameters by EXAFS curve fitting, XANES spectra at Pt LIII-edge, Au LIII-edge and Pd K-edge, cyclic voltammograms, and size distribution by TEM. This material is available free of charge via the Internet at http://pubs.acs.org. 13103



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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-42-443-5921. E-mail: [email protected]. Present Address

M.T.: Research Center for Materials Science and Department of Chemistry, Graduate School of Science, Nagoya University, Chigusa, Nagoya 464-8602, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS XAFS measurements were performed with the approval of SPring-8 (Nos. 2009B1368, 2010A1555, 2010B1532, and 2011A1544).

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