In Situ X-ray Absorption Fine Structure Analysis of PtCo, PtCu, and

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In Situ X-Ray Absorption Fine Structure Analysis of PtCo, PtCu, and PtNi Alloy Electrocatalysts: The Correlation of Enhanced Oxygen Reduction Reaction Activity and Structure Takahiro Kaito, Hiroyuki Tanaka, Hisashi Mitsumoto, Seiho Sugawara, Kazuhiko Shinohara, Hiroko Ariga, Hiromitsu Uehara, Satoru Takakusagi, and Kiyotaka Asakura J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01736 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 16, 2016

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The Journal of Physical Chemistry C 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.

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The Journal of Physical Chemistry

In Situ X-Ray Absorption Fine Structure Analysis of PtCo, PtCu, and PtNi Alloy Electrocatalysts: The Correlation of Enhanced Oxygen Reduction Reaction Activity and Structure Takahiro Kaito1,2, Hiroyuki Tanaka1, Hisashi Mitsumoto1, Seiho Sugawara1, Kazuhiko Shinohara1, Hiroko Ariga3, Hiromitsu Uehara3, Satoru Takakusagi3, and Kiyotaka Asakura3 1) Nissan Motor Co., LTD. Natsushima-cho Yokosuka-shi Kanagawa 237-8523, Japan. 2) Department of Quantum Chemistry and Technology, Graduate School of Engineering, Hokkaido University, Kita21 Nishi10, Kita-ku, Sapporo, Hokkaido, 001-0021, Japan. 3) Institute for Catalysis, Hokkaido University, Kita21 Nishi10, Kita-ku, Sapporo, Hokkaido, 001-0021, Japan. Abstract In order to examine the relationship between the oxygen reduction reaction (ORR) activity of a fuel cell catalyst and its structure and/or electronic state, carbon-supported Pt and Pt alloys having various structures, compositions and morphologies were studied. Regardless of the atomic ordering or morphology (core-shell or random alloy) of the catalyst, the ORR activity was primarily dependent on the Pt-Pt bond distance. Among these materials, Pt2Co, having the shortest Pt-Pt distance, exhibited the highest ORR activity. The activities of this catalyst per unit surface area and per unit mass were approximately ten times and six times higher than those of a commercially-available carbon supported Pt electrocatalyst (Pt/C). This work also found a monotonic increase in catalytic activity with decreasing Pt-Pt distance. 1. Introduction Polymer electrolyte fuel cells (PEFCs) show significant promise as future power sources for vehicles because they do not emit greenhouse gases during their operation. However, there are several issues to be solved to allow the widespread use of fuel cell electric vehicles (FCEVs). Among these challenges, the most important is the high cost of FC systems resulting from the use of a significant quantity of platinum (Pt) in each cell. In order to reduce the amount of Pt required, it is necessary to study the factors that govern the activity of the oxygen reduction reaction (ORR). Pt alloys with 3d transition metal atoms additives such as PtNi, PtCo, PtMn, PtFe, PtCr, PtCoCr and PtVCr have been reported to have higher ORR activities than standard Pt/C electrocatalysts.1-9 In addition, some of these PtM electrocatalysts show much higher area- and mass-specific ORR activities.9 Studies of such materials have shown that the factors governing their activities may include atomic ordering and morphology in addition to the Pt-Pt distance, Pt 5d vacancy, and d-band center.2,3,9,11,13 The d-band center theory appears to be the most widely accepted ideas of 1 ACS Paragon Plus Environment


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explaining the ORR activity of these catalysts,2,12 which indicates the presence of valcano-type dependence on the d-band center. The ORR activity depends on the balance of the activity to generate -OOH groups from O2 and the stability of the oxide or the strong adsorption of OH to poison the Pt surface which are related to the d-band center.12,14-21 However, to the best of our knowledge, there are no experimental methods that can directly determine the d-band center of a catalyst under in situ conditions. The microstructure of Pt is often examined by X-ray absorption fine structure (XAFS) because of the significant penetration of this technique.22 XAFS studies have elucidated catalytic properties and reaction mechanisms based on dynamic structural changes23-25 in actual fuel cells26-29 and in three-electrode electrochemical cells.11,30-38 One achievement of XAFS to date has been the discovery of a volcano-like activity dependence based on Pt-Pt bond distance and the vacancy in the d-band.2 However, Pt/Au core shell electrocatalysts have exhibited the highest ORR activities, which is an exception to the volcano-like dependence on the Pt-Pt bond distance. That is, the Pt-Pt distance in such materials should be longer than that in pure Pt because of the effect of Au core, since the Au-Au and Pt-Pt distances in the bulk are 0.288 and 0.277 nm, respectively. Our group has previously carried out in situ K-edge EXAFS measurements of Pt/Au core shell electrocatalysts. The results confirmed the core shell structure and also identified contractions of both Au-Au and Pt-Pt bond distances in the Au core and Pt shell.39 We therefore proposed that the activity enhancement associated with the Pt/Au core shell structure should be related to the Pt-Pt distance. In the present study, we studied PtM alloys synthesized using different treatments, with various added metals and ordering. The resulting Pt-Pt distances and Pt 5d vacancies were subsequently compared to the catalytic activities obtained from these materials. We found a strong correlation between the Pt-Pt bond distance and activity, in contrast to previous literature reports.2,32 The results of this work suggest that the Pt-Pt bond distance is an important parameter to monitor when assessing the ORR activity, and also demonstrate that the Pt-Pt bond distances can be elucidated from EXAFS data. 2. Material and methods 2.1 Preparation of PtCo/C, PtCo/C-HT, PtCu/C, PtCu/C-HT, and PtNi/C electrocatalysts We prepared samples with two different Pt/Co ratios using different treatment temperatures, as well as PtCu/C with and without heat treatment and PtNi/C without heat treatment. Herein, the suffix -HT indicates that the material underwent heat treatment. The details of the synthetic procedures are described in the Supporting Information (SI). A commercially-available Pt/C catalyst was used as a reference sample (nominal Pt loading: 50 wt%, Ketjenblack carbon support, TEC10E50E, Tanaka Kikinzoku Kogyo K. K.) as reported previously.39 2.2 Characterization of Pt/C, PtCo/C, PtCo/C-HT, PtCu/C, PtCu/C-HT, and PtNi/C electrocatalysts 2 ACS Paragon Plus Environment

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The prepared electrocatalysts were characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (STEM-EDX), high-angle annular dark-field (HAADF)-STEM, inductively-coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD) and electrochemical methods. The particle diameter distributions and the average diameters of the electrocatalysts were obtained using TEM (HF-2000 field emission TEM Hitachi High-Technologies Corp.). The sectional elemental distributions of the PtCo/C-HT and PtNi/C electrocatalyst particles were assessed by STEM-EDX and STEM-HAADF (HD-2700 Hitachi High-Technologies Corporation). The mass proportions of Pt, Co, Cu, and Ni were determined from ICP-MS analysis (SPS-3520, SII NanoTechnology Inc.). Electrochemical analysis was conducted with a rotating disk electrode (RDE) measurement apparatus (HZ-5000, Hokuto Denko Corp.). During the electrochemical experiments, a reversible hydrogen electrode (RHE) and a Pt mesh were employed as the reference electrode and the counter electrode, respectively. Cyclic voltammograms (CVs) were obtained in deaerated 0.1 M HClO4 at a sweep rate of 50 mVs-1. Polarization curves were recorded in O2-saturated 0.1 M HClO4 at a sweep rate of 10 mVs-1. 2.3 XAFS measurements XAFS data were acquired at the X-ray bending magnet beamline BL16B2 of SPring-8 in Japan. The ring was operated at 8 GeV with a ring current of 100 mA in the top-up mode. The X-ray beam was monochromatized using a Si (111) double crystal monochromator. The incident and transmitted X-ray intensities were monitored using two ionization chambers filled with nitrogen and a mixture of 85% nitrogen and 15% argon for incident and transmitted measurements, respectively. Samples were positioned in in situ measurement cells as described in a previous paper.40 Prior to the in situ measurements, air was removed by passing pure nitrogen (99.99995%) through the electrolyte solution in the XAFS cell for 30 minutes. During the experiments, nitrogen gas was continuously passed over the electrolyte solution to prevent air contamination. The electrode potential was swept cathodically from the rest potential (ca. 1.0 V vs. RHE) to 0.05 V vs. RHE at a slow scan rate (1 mVs-1) to avoid any significant current and potential inhomogeneity in the catalyst layer. Subsequently, three oxidation-reduction cycle (ORC) treatments (0.05–1.2 V vs. RHE) were conducted to remove contamination from the surface of the electrocatalyst. Following the ORC treatments, XAFS measurements were performed at 0.4 V vs. RHE after holding the electrode potential at 0.4 V vs. RHE for 20 minutes. Data were also acquired for Pt, Co, and Cu foils and for PtCo, PtCu, and PtNi random alloy foils containing 90 atomic percent (at%) Pt as reference samples for curve-fitting analyses, to determine the EXAFS fitting parameters (the amplitude reduction factor, Sj, and the energy difference, ∆E0j) as well as to confirm the validity of the theoretical phase shift and amplitude parameters derived from FEFF8. 2.4 XAFS analyses The XAFS analyses were conducted with the REX2000 software package (Rigaku Co.).41

The 3

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detailed analysis method is described in the SI. During curve-fitting analyses of bimetallic alloys in particular, the following equations must be satisfied within an error bar.42-44

(1)

(2)

Here M is the metal added to Pt, NXY is the coordination number of atom Y around the central atom X, CX is the mole fraction of X, and RXY is the interatomic distance between X and Y on X’s absorption edge. Errors in these structural parameters were determined at the 90% confidence level by the Hamilton ratio method.45 3. Results 3.1 ICP-MS analyses following electrochemical treatments Table 1 summarizes the sample preparation conditions, along with the average diameters and atomic percentages of Pt, Co, Cu, and Ni after in situ XAFS analyses. The atomic percentages in Table 1 were calculated from the mass concentrations determined by ICP-MS. Although the same PtCo/C was used for the preparation of the PtCo/C-HT-600, -700 and -800 specimens, the atomic ratio of the PtCo/C was different from that of the PtCo/C-HT series. This resulted from the dissolution of Co atoms in the PtCo/C during the preparation of the electrocatalyst ink samples, due to the use of an acidic solution. The ink preparation method is described in the SI. In addition, the Co atoms were evidently not well alloyed with the Pt in the absence of heat treatment. Finally, a significant amount of Ni was removed from the PtNi/C during the ink preparation process, since Ni was easily dissolved by the acid solution. In contrast, Cu remained in the PtCu /C after the preparation of the ink, such that the Pt to Cu ratios of the PtCu/C and PtCu/C-HT were the same. The atomic percentages of those electrocatalysts before electrochemical testing are shown in Table S8 of supporting information.

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Table 1. Heat treatment conditions, average diameters, and atomic percentages of Pt/C, PtCo/C, PtCo/C-HT, PtCu/C, PtCu/C-HT, and PtNi/C. PtxM atomic ratio

Temperature / ºC

Heating time

Average diameter

/ hour

/ nm

Pt

at% Co Cu

Ni

Pt/C PtCo/C PtCo/C-HT-600

Pt Pt5Co Pt3Co

600

2.0

2.2 2.4 4.5

100 82 75

-

-

-

18 25

-

-

PtCo/C-HT-700 PtCo/C-HT-800

Pt3Co Pt3Co

700 800

2.0 1.0

4.4 4.2

74 74

26 26

-

-

PtCo/C-HT-600h PtCu/C

Pt2Co Pt3Cu

600 -

2.0 -

3.9 2.2

70 73

30 -

27

-

PtCu/C-HT-600

Pt3Cu

PtNi/C

Pt9Ni

600 -

2.0 -

4.0 6.8

73 88

-

27 -

12

3.2 Characterization of PtCo/C, PtCo/C-HT, PtCu/C, PtCu/C-HT, and PtNi/C electrocatalysts Figures 1(a)–1(c) present typical TEM images of the PtCo/C, PtNi/C, and PtCo/C-HT-600h, respectively, while Figure 2 summarizes their particle diameter distributions. Figures S1 and S2 show the TEM images and diameter distribution of the other samples, demonstrating that the particle size of the PtCo/C-HT was larger and more widely distributed after the heat treatment. In addition, the PtNi/C had a larger average particle size than that of the PtCo/C. Figures 3 and 4 provide STEM-HAADF images and STEM-EDX line analyses for the PtCo/C-HT-600 and PtNi/C samples after the RDE trials. STEM-EDX examinations of all the samples did not identify any monometallic particles or any particles in which the atomic ratio greatly deviated from the average value. Figure 3 shows a HAADF image and a STEM-EDX line profile for the PtCo/C-HT-600 after RDE testing, demonstrating that the Pt and Co atoms were well mixed in the particle, although the surface Pt concentration appears to be greater than that of Co. Figure 4 presents a HAADF image and a STEM-EDX line profile of a single PtNi/C particle after RDE testing. The core of the particle is dark, indicating the absence of Pt. The EDX data also show that few Ni atoms were present in this particle. This PtNi particle therefore had a hollow structure with a shell thickness of approximately 2.5 nm, similar to the Pt hollow structure previously reported by Wang et al.46 Figure S3 shows X-ray diffraction patterns obtained from the PtCo/C series of samples. The PtCo/C diffraction peaks are initially broad but are sharpened by heat treatment, indicating that the heat treatment increased the crystallinity of the samples. The corresponding lattice constants are summarized in Table S1. The lattice constants were decreased following heat treatment, indicating PtCo alloy formation. In addition, broad peaks appeared at 33° and 23°, both characteristic of a 5 ACS Paragon Plus Environment


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Pt3Co intermetallic compound. 3.3 ORR activities of Pt/C, PtCo/C, PtCo/C-HT, PtCu/C, PtCu/C-HT, and PtNi/C electrocatalysts Figure S4 and Table S2 respectively provide CV data and electrochemical effective areas (ECAs) for Pt/C, PtCo/C, PtCo/C-HT, PtCu/C, PtCu/C-HT, and PtNi/C specimens, together with the ORR activities of these materials. Figure S5 presents the linear sweep voltammograms (LSVs). In each case, the specific activity at a corrected electrode potential of 0.9 V vs. RHE was acquired from the intercept of the Koutecky-Levich plot, as provided in Table S2 (see the SI for details). The PtCo/C-HT-600h had the highest area-specific activity, exhibiting a value an order of magnitude greater than that of Pt/C. 3.4 XAFS analysis 3.4.1 XANES analysis of PtCo/C, PtCo/C-HT, PtCu/C, PtCu/C-HT, and PtNi/C electrocatalysts The Co-K, Cu-K, and Ni-K edge spectra of the electrocatalysts are respectively compared in Figures S6, S7, and S8 of the SI. The Co K-edge data for the PtCo/C shows an edge position similar to that of the CoO, while the PtCo/C-HT samples generated K-edge spectra identical to the spectrum of PtCo, as shown in Figure S6. The Cu K-edge data obtained from the PtCu/C and PtCu/C-HT indicate PtCu alloy formation while the Ni K-edge data for the PtNi/C suggest a greater extent of oxidation. Figures 5 and 6 respectively present the Pt LIII and LII XANES spectra of the Pt foil, PtCo, PtCu, PtNi alloy foils, Pt/C, PtCo/C, PtCo/C-HT series, PtCu/C, PtCu/C-HT, and PtNi/C. Figure 5 shows the edge peak commonly known as the white line, which can be assigned to the transition from the 2p to the 5d empty state. In order to determine the Pt 5d vacancies in the Pt/C, PtCo/C, PtCo/C-HT, PtCu/C, PtCu/C-HT, and PtNi/C from the data in Figures 5 and 6, we analyzed the Pt LII and LIII-edge XANES spectra according to the method proposed by Mansour et al.47 The results showed that the differences in the XANES edge heights fell within the limit of the error bars and exhibited no significant dependence of the d vacancy activity. The correlation between the degree of Pt 5d vacancies (Table S3 in the SI) and the area-specific activities is summarized in Figure S9. 3.4.2 EXAFS analysis We initially analyzed the reference foils to confirm the validity of the analysis method and of the derived S and ΔE values. The fitting results using FEFF-derived phase shifts and amplitude functions are shown in Table S4. Figures 7 and 8 respectively present the k3-weighted Pt-LIII in situ EXAFS oscillations and their Fourier transforms for the Pt/C, PtCo/C, PtCo/C-HT, PtCu/C, PtCu/C-HT, and PtNi/C at 0.4 V vs. RHE, together with those of Pt and Pt alloy foils as references. Table 2 summarizes the curve fitting results for the prepared electrocatalysts. Equations (1) and 6 ACS Paragon Plus Environment

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(2) were well satisfied within the error bars. Co and Ni K-edge curve fittings for the PtCo/C and PtNi/C samples were conducted based on the Co-O and Co-Pt bonds or Ni-O and Ni-Pt bonds, respectively, assuming that the Co and Ni atoms were oxidized, as indicated by the XANES spectra in Figures S6 or S8. Table 2 contains the coordination numbers determined by assuming random alloy particles and the diameters derived from the TEM data in Table 1. The shape of each PtM/C particle was assumed to be a cuboctahedron. A comparison of the total coordination numbers and particle diameters expressed by the edge atoms is shown in Table S7. The PtCo/C, PtCu/C, and Pt/C are classified as having 5 edge atoms. Other heat-treated samples contained particles with 8 atoms on the edge. The EXAFS-determined partial coordination numbers of all electrocatalyst samples coincided with the expected coordination numbers derived from the cuboctahedral random alloyed nanoparticles model shown in Figure S11(b), indicating that the PtCu and PtCo alloys were composed of randomly-mixed structures. The hollow structure proposed for the PtNi/C is illustrated in Figure S11(a), as determined from the HAADF image and the STEM-EDX intensity line profile. The EXAFS results agree with a hollow cuboctahedral particle 7 nm in diameter with 13 edge atoms. The thickness of the shell was assumed to be 2.5 nm and the hollow portion was assumed to be a smaller cuboctahedron with 5 atoms on the edge.



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Table 2. Curve fitting results for heat-treated PtCo/C, PtCo/C-HT, PtCu/C, PtCu/C-HT, and PtNi/C at 0.4 V vs. RHE. Coordination number Sample

Edge

Pt LIII

Bond

Atomic

Debye-Waller

ΔE0 / eV

Expected

Obtained

distance / nm

factor / nm

Pt-Pt

-

5.5 ± 0.8

0.273 ± 0.001

0.0086 ± 0.0002

10.8

Pt-Co

-

1.2 ± 1.0

0.268 ± 0.004

0.0161 ± 0.0056

-8.3

Co-O

-

1.4 ± 0.6

0.199 ± 0.002

0.0100 ± 0.0026

-0.6

Co-Pt

-

5.5 ± 0.7

0.268 ± 0.001

0.0960 ± 0.0070

-1.8

Pt-Pt

8.2

8.2 ± 1.0

0.270 ± 0.001

0.0078 ± 0.0002

10.8

Pt-Co

2.7

2.7 ± 1.0

0.264 ± 0.002

0.0107 ± 0.0013

-8.3

Co-Co

2.7

2.8 ± 0.9

0.261 ± 0.001

0.0088 ± 0.0012

8.5

Co-Pt

8.2

8.2 ± 0.9

0.264 ± 0.001

0.0079 ± 0.0003

1.2

Pt-Pt

8.1

8.0 ± 1.0

0.270 ± 0.001

0.0075 ± 0.0002

10.8

Pt-Co

2.8

2.8 ± 1.0

0.263 ± 0.002

0.0099 ± 0.0008

-8.3

Co-Co

2.8

2.7 ± 1.0

0.260 ± 0.002

0.0100 ± 0.0016

8.5

Co-Pt

8.1

8.2 ± 0.9

0.263 ± 0.001

0.0055 ± 0.0004

1.2

Pt-Pt

8.1

8.2 ± 1.7

0.270 ± 0.001

0.0086 ± 0.0005

10.8

Pt-Co

2.8

2.8 ± 1.3

0.263 ± 0.003

0.0103 ± 0.0018

-8.3

Co-Co

2.8

3.0 ± 0.8

0.259 ± 0.003

0.0098 ± 0.0013

8.5

Co-Pt

8.1

8.0 ± 1.9

0.265 ± 0.001

0.0078 ± 0.0007

1.2

Pt-Pt

7.6

7.6 ± 1.4

0.269 ± 0.001

0.0079 ± 0.0003

10.8

Pt-Co

3.2

3.2 ± 1.6

0.263 ± 0.003

0.0106 ± 0.0018

-8.3

Co-Co

3.2

3.2 ± 1.2

0.261 ± 0.003

0.0122 ± 0.0017

8.5

Co-Pt

7.6

7.7 ± 0.9

0.263 ± 0.001

0.0064 ± 0.0003

1.2

Pt-Pt

7.0

7.0 ± 1.0

0.272 ± 0.001

0.0077 ± 0.0003

12.0

Pt-Cu

2.6

2.5 ± 1.5

0.266 ± 0.003

0.0107 ± 0.0016

-3.5

Cu-Cu

2.6

2.6 ± 0.7

0.263 ± 0.003

0.0123 ± 0.0014

-2.0

Cu-Pt

7.0

7.0 ± 0.9

0.267 ± 0.003

0.0108 ± 0.0008

1.4

Pt-Pt

7.7

7.7 ± 1.0

0.272 ± 0.001

0.0073 ± 0.0003

12.0

Pt-Cu

2.8

2.7 ± 1.1

0.267 ± 0.002

0.0092 ± 0.0019

-3.5

Cu-Cu

2.8

2.7 ± 0.8

0.264 ± 0.002

0.0093 ± 0.0010

4.5

Cu-Pt

7.7

7.7 ± 1.3

0.267 ± 0.001

0.0082 ± 0.0005

5.9

Pt-Pt

-

10.2 ± 1.1

0.273 ± 0.001

0.0085 ± 0.0002

12.0

Pt-Ni

-

1.4 ± 0.6

0.266 ± 0.004

0.0107 ± 0.0017

-14.5

Ni-O

-

1.5 ± 0.3

0.202 ± 0.001

0.0055 ± 0.0014

18.1

R -factor 7.3

PtCo/C Co K

Pt LIII PtCo/C -HT-600

8.2

4.4

Co K


3.2

6.1

Co K


2.5

4.3

Co K

Pt LIII PtCo/C -HT-600h

7.9

8.1

Co K

Pt LIII

2.5

5.6

PtCu/C Cu K

3.8

5.4

Pt LIII PtCu/C -HT-600 Cu K

PtNi/C

Pt LIII Ni K

3.2

4.0 2.6


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Ni-Pt

-

10.2 ± 1.2

0.266 ± 0.001

0.0110 ± 0.0006

3.7

4. Discussion 4.1 ORR activity enhancement factors In the literature, there are several proposed enhancement factors for ORR. i) electronic effects based on d band theory, ii) core/shell structures29,10 and

These are

11,12,2,3

iii) intermetallic compound formation.12,9 When considering practical applications, it is important to correlate the ORR activity with the measured physical properties of the catalyst, and STEM can provide the morphology and inner structure of the nanoparticles. In this work, the prepared PtM were of several types, including a random alloy (PtCo, PtCu), a core hollow structure (PtNi), and a core shell structure (PtAu). Comparing the PtCo alloy particles, the ORR activities were found to change with the treatment temperature even though the alloy structure and surface composition were unchanged. These results demonstrate that the morphology and inner structure may not only be the only factors affecting the catalytic activity. The d band density of state values can be estimated from the white line peak intensities of the Pt LII, III XANES data in Figures 5 and 6. We therefore estimated the empty d orbitals and found little correlation between the intensities and ORR activities, as shown in Figure S9 and Table S3, in contrast to previously reported results.32 It appears that the L2, 3 edge intensity may not be a good indicator for high activity. Wang et al. reported that the formation of a PtCo intermetallic structure is crucial for high ORR activity.9 In the present study, we identified a peak at 33°, characteristic of the Pt3Co intermetallic structure (as shown in the XRD in Figure S3), even though the coordination numbers derived from EXAFS analyses of the heat-treated PtCo/C-HT materials indicated the formation of random alloys. As discussed in the SI, the formation of intermetallic structures as the major component of the catalytic material can be rejected based on the EXAFS data because in this case only Pt should be found in the vicinity of the Co K-edge EXAFS, which was not observed. We conclude that the majority of the material was present as a PtCo random alloy, even though a Pt3Co intermetallic peak is observed in XRD patterns. PtNi/C (Pt9Ni/C) showed a characteristic hollow structure and higher activity than the Pt/C. Such high activity in the case of hollow structures has also been reported by Wang et al.46 Finally, we attempted to establish a correlation between ORR activity and Pt-Pt bond length. Figure 9 plots the area-specific activity against the bond length. Pt/C has a Pt-Pt bond length of 0.275 nm, a little shorter than that of Pt foil (0.276 nm). In the case of the PtNi/C and PtCo/C we found a still shorter Pt-Pt distance of 0.273 ± 0.001 nm. Although the Pt/C and PtM/C materials examined in this paper and the previous one,39 have different morphologies from each other, the 9 ACS Paragon Plus Environment


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area-specific activities of the PtM/C were approximately five times higher than that of Pt/C. The contraction of the Pt-Pt distance did not result from the formation of an alloy because the Pt-Pt distance of the PtNi (9/1) alloy foil containing the same Ni atomic percent as the PtNi/C was determined to be 0.275 ± 0.001 nm. The additional reductions in the Pt-Pt distance may be due to the hollow structure, as suggested by Wang et al.46 PtCu/C and PtCu/C-HT-600 had the same Pt-Pt distance of 0.272 nm and showed similar ORR activities, although the PtCu/C-HT-600 evidently had larger particles than the PtCu/C. Conversely, the PtCo/C-HT series had shorter bond distances and higher ORR activity than the PtCo/C. PtCo/C-HT-600h had the shortest bonds and the highest activity.

The

Therefore, it seems that there is a correlation between the bond distance and the ORR activity. In fact, the correlation coefficient obtained from linear regression of the data in Figure 9 was R2=0.82. We also plotted previous results for PtAu/C39 on Figure 9, and these data appear in the same linear region as the PtM/C data. Thus, the ORR activity can be roughly estimated based solely on the Pt-Pt bond distances. Mukerjee and Adzic have reported the dependence of ORR activity on bond length.32 They found both a volcano-type dependence and an optimal Pt-Pt distance, whereas our work determined a monotonic increase. Figure S12 presents a plot of the ORR activity as a function of the bond length based on reported data.9,48-51 The R2 value determined from all the electrocatalysts data plotted in Figure S12 was 0.43, hence there is a distinct correlation though the data were measured by the different authors. Wang et al. claimed that the formation of a well-ordered Pt3Co intermetallic compound core is an important requirement for high activity,9 but their data also fits this same general linear trend. Thus, the bond length dependence appears to be a universal relationship and an important factor related to the ORR activity. As such, this work has demonstrated a correlation between the bond distance and the ORR activity, although further theoretical work is necessary to rationalize this correlation and to find the relation between the well-known d-band center theory.52 Nevertheless, we can safely say that the Pt-Pt bond distance may be a good indicator of ORR activity. 5. Conclusions Carbon-supported PtM (PtCo/C, PtCu/C, and PtNi/C) alloy electrocatalysts were prepared that showed much higher area-specific activities than Pt/C. XAFS analysis demonstrated that there is a strong correlation between the Pt-Pt bond distance and the area-specific activity, regardless of the particle morphologies and the presence of ordered or disordered structures or of hollow or core-shell structures. Shortened Pt-Pt distances were found to enhance the ORR activity in all types of PtM alloy electrocatalysts. The next step of this work will be to obtain an understanding of the correlation between Pt-Pt distance and each of the elementary ORR steps through theoretical calculations, as a means of elucidating the reason why activity is enhanced by Pt-Pt contraction. 10 ACS Paragon Plus Environment

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Supporting information Following information is available free of charge via the Internet at http://pubs.acs.org” 1.

Preparation of PtCo/C, PtCo/C-HT, PtCu/C, PtCu/C-HT, and PtNi/C electrocatalysts

2.

Electrochemical measurements

3.

XAFS analyses

4.

Transmission electron microscopy

5.

X-ray diffraction

6.

ORR activities of Pt/C, PtCo/C, PtCo/C-HTs, PtCu/C, PtCu/C-HT, and PtNi/C electrocatalysts

7.

XANES analyses of PtCo/C, PtCo/C-HTs, PtCu/C, PtCu/C-HT, and PtNi/C electrocatalysts

8.

XAFS analyses of reference foils and a Pt/C base electrocatalyst

9.

Detailed discussions of the PtCo/C electrocatalyst based on XAFS and XRD data

10. Pt3Co intermetallic compound structure 11. Model structures for PtNi, PtCo, and PtCu electrocatalysts 12. Numbers of edge atoms, particle sizes, and coordination numbers of electrocatalyst particles 13. Correlation between ORR activity and bond length based on literature data

14. Atomic ratio of prepared electrocatalyst before the electrochemical testing

6. Acknowledgments The authors wish to express their gratitude to Nissan ARC, Ltd. for assistance during XAFS data acquisition. Synchrotron radiation experiments were performed at the BL16B2 beam line of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposals No. 2012A5390, 2012A5391). This work was supported by the Polymer Membrane Fuel Cell Project of the New Energy and Industrial Technology Development Organization (NEDO) of Japan.



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7. References Mukerjee, S.; Srinivasan, S., Enhanced Electrocatalysis of Oxygen Reduction on Platinum Alloys in Proton Exchange Membrane Fuel Cells. J. Electroanal. Chem. 1993, 357, 201-224. 2 Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J., Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction. J.Electrochem. Soc. 1995, 142, 1409-1422. 3 Xiong, L.; Manthiram, A., Influence of Atomic Ordering on the Electrocatalytic Activity of Pt-Co Alloys in Alkaline Electrolyte and Proton Exchange Membrane Fuel Cells. J. Mater. Chem. 2004, 14, 1454-1460. 4 Chen, S.; Sheng, W.; Yabuuchi, N.; Ferreira, P. J.; Allard, L. F.; Shao-Horn, Y., Origin of Oxygen Reduction Reaction Activity on “Pt3Co” Nanoparticles: Atomically Resolved Chemical Compositions and Structures. J.Phys. Chem. C 2009, 113, 1109-1125. 5 Rao, C. S.; Singh, D. M.; Sekhar, R.; Rangarajan, J., Pt–Co Electrocatalyst with Varying Atomic Percentage of Transition Metal. Int. J.Hydrogen Energ. 2011, 36, 14805-14814. 6 W Wanjala, B. N.; Fang, B.; Shan, S.; Petkov, V.; Zhu, P.; Loukrakpam, R.; Chen, Y.; Luo, J.; Yin, J.; Yang, L.; Shao, M.; Zhong, C.-J., Design of Ternary Nanoalloy Catalysts: Effect of Nanoscale Alloying and Structural Perfection on Electrocatalytic Enhancement. Chem. Mater. 2012, 24, 4283-4293. 7 Jeon, M. K.; McGinn, P. J., Co-Alloying Effect of Co and Cr with Pt for Oxygen Electro-Reduction Reaction. Electrochim. Acta 2012, 64, 147-153. 8 Oezaslan, M.; Hasché, F.; Strasser, P., Oxygen Electroreduction on PtCo3, PtCo and Pt3Co Alloy Nanoparticles for Alkaline and Acidic PEM Fuel Cells. J. Electrochem. Soc. 2012, 159, B394-B405. 9 Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D., Structurally Ordered Intermetallic Platinum-Cobalt Core–Shell Nanoparticles with Enhanced Activity and Stability as Oxygen Reduction Electrocatalysts. Nat Mater 2013, 12 , 81-87. 10 Shao, M.; Sasaki, K.; Marinkovic, N. S.; Zhang, L.; Adzic, R. R., Synthesis and Characterization of Platinum Monolayer Oxygen-Reduction Electrocatalysts with Co–Pd Core–Shell Nanoparticle Supports. Electrochem. Commun. 2007, 9 , 2848-2853. 11 Mukerjee, S.; McBreen, J., Effect of Particle Size on the Electrocatalysis by Carbon-Supported Pt Electrocatalysts: an in situ XAS Investigation. J. Electroanal.Chem. 1998, 448 , 163-171. 12 Koh, S.; Toney, M. F.; Strasser, P., Activity–Stability Relationships of Ordered and Disordered Alloy Phases of Pt3Co Electrocatalysts for the Oxygen Reduction Reaction (ORR). Electrochim. Acta 2007, 52, 2765-2774. 13 Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M., Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6, 241-247. 14 Zhang, J.; Vukmirovic, M.B. ; Sasaki, K;Uribe, F; Adzic, R.R., Platinum Monolayer Electrocatalysts for Oxygen Reduction: Effect of Substrates, and Long-Term Stability. J. Serb. Chem. Soc. 2005, 70, 525. 15 Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; MarkoviāE N. M., Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability. Science 2007, 315, 493-497. 16 Zhang, J.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R., Controlling the Catalytic Activity of Platinum-Monolayer Electrocatalysts for Oxygen Reduction with Different Substrates. Angew. Chem. Int.Ed. 2005, 44, 2132-2135. 17 Jia, Q.; Caldwell, K.; Ramaker, D. E.; Ziegelbauer, J. M.; Liu, Z.; Yu, Z.; Trahan, M.; Mukerjee, S., In situ Spectroscopic Evidence for Ordered Core–Ultrathin Shell Pt1Co1 Nanoparticles with Enhanced Activity and Stability as Oxygen Reduction Electrocatalysts. J.Phys. Chem. C 2014, 118, 20496-20503. 18 Hammer, B.; Nørskov, J. K., Electronic Factors Determining the Reactivity of Metal Surfaces. Surf. Sci. 1995, 343, 211-220. 19 TripkoviāE V.; Skúlason, E.; Siahrostami, S.; Nørskov, J. K.; Rossmeisl, J., The Oxygen 1


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Reduction Reaction Mechanism on Pt(111) from Density Functional Theory Calculations. Electrochim. Acta 2010, 55, 7975-7981. 20 Kitchin, J.R.; Norskov, J.K.; Barteau, M. A.; Chen, J.G., Role of Strain and Ligand Effects in the Modification of the Electronic and Chemical Properties of Bimetallic Surfaces. Phys. Rev.Lett. 2004, 93 , 156801. 21 Z Zhang, J.; Lima, F. H. B.; Shao, M. H.; Sasaki, K.; Wang, J. X.; Hanson, J.; Adzic, R. R., Platinum Monolayer on Nonnoble Metal−Noble Metal Core−Shell Nanoparticle Electrocatalysts for O2 Reduction. J.Phys. Chem. B 2005, 109, 22701-22704. 22Lytle, F. W.; Via, G. H.; Sinfelt, J. H., New Application of Extended Xray Absorption Fine Structure (EXAFS) as a Surface Probe-Nature of Oxygen Interaction with a Ruthenium Catalyst. J.Chem. Phys. 1977, 67, 3831-3832. 23 van't Blik, H. F. J.; Prins, R., Characterization of Supported Cobalt and Cobalt-Rhodium Catalysts: I. Temperature-Programmed Reduction (TPR) and Oxidation (TPO) of CoRh/Al2O3 J. Catal. 1986, 97, 188-199. 24 Iwasawa, Y.; Asakura, K.; Ishii, H.; Kuroda, H., Dynamic Behaviour of Active Sites of a SiO2-Attached Mo(VI)-Dimer Catalyst during Ethanol Oxidation Observed by means of EXAFS. Z. Phys.Chem. Neue Fol. 1985, 144, 105-115. 25 Asakura, K.; Kitamura-Bando, K.; Isobe, K.; Arakawa, H.; Iwasawa, Y., Metal-Assisted CO Insertion Reaction on a New Surface Rhodium Dimer Catalyst Observed by an in situ EXAFS Technique. J. Am. Chem. Soc. 1990, 112, 3242-3244. 26 Allen, P. G.; Conradson, S. D.; Wilson, M. S.; Gottesfeld, S.; Raistrick, I. D.; Valerio, J.; Lovato, M., In situ Structural Characterization of a Platinum Electrocatalyst by Dispersive X-Ray Absorption Spectroscopy. Electrochim. Acta 1994, 39, 2415-2418. 27 Allen, P. G.; Conradson, S. D.; Wilson, M. S.; Gottesfeld, S.; Raistrick, I. D.; Valerio, J.; Lovato, M., Direct Observation of Surface Oxide Formation and Reduction on Platinum Clusters by Time-Resolved X-Ray Absorption Spectroscopy. J.Electroanal. Chem. 1995, 384, 99-103. 28 Wiltshire, R. J. K.; King, C. R.; Rose, A.; Wells, P. P.; Hogarth, M. P.; Thompsett, D.; Russell, A. E., A PEM Fuel Cell for in situ XAS Studies. Electrochim. Acta 2005, 50, 5208-5217. 29 Ishiguro, N.; Saida, T.; Uruga, T.; Nagamatsu, S.-i.; Sekizawa, O.; Nitta, K.; Yamamoto, T.; Ohkoshi, S.-i.; Iwasawa, Y.; Yokoyama, T.; Tada, M., Operando Time-Resolved X-Ray Absorption Fine Structure Study for Surface Events on a Pt3Co/C Cathode Catalyst in a Polymer Electrolyte Fuel Cell during Voltage-Operating Processes. ACS Catal. 2012, 2, 1319-1330. 30 Yoshitake, H.; Mochizuki, T.; Yamazaki, O.; Ota, K.-I., Study of the Density of the d-State and Structure Transformation of Pt FinePparticles Dispersed on Carbon Electrodes by in situ X-Ray Absorption Spectroscopy. J. Electroanal. Chem. 1993, 361, 229-237. 31 Yoshitake, H.; Yamazaki, O.; Ota, K.-I., Novel Spectroelectrochemical Cell for in situ XAFS Spectroscopy on Gas Generating Electrodes. J. Electroanal. Chem. 1994, 371, 287-290. 32 Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J., Effect of Preparation Conditions of Pt Alloys on their Electronic, Structural, and Electrocatalytic Activities for Oxygen Reduction - XRD, XAS, and Electrochemical Studies. J.Phys. Chem. 1995, 99, 4577-4589. 33 McBreen, J.; Mukerjee, S., In situ X-ray Absorption Studies of a Pt-Ru Electrocatalyst. J. Electrochem. Soc. 1995, 142, 3399-3404. 34 Maniguet, S.; Mathew, R. J.; Russell, A. E., EXAFS of Carbon Monoxide Oxidation on Supported Pt Fuel Cell Electrocatalysts. J. Phys. Chem. B 2000, 104, 1998-2004. 35 Russell, A. E.; Rose, A., X-Ray Absorption Spectroscopy of Low Temperature Fuel Cell Catalysts. Chem. Rev. 2004, 104, 4613-4636. 36 Imai, H.; Izumi, K.; Matsumoto, M.; Kubo, Y.; Kato, K.; Imai, Y., In situ and Real-Time Monitoring of Oxide Growth in a Few Monolayers at Surfaces of Platinum Nanoparticles in Aqueous Media. J.Am. Chem. Soc. 2009, 131, 6293-6300. 37 Sasaki, K.; Wang, J. X.; Naohara, H.; Marinkovic, N.; More, K.; Inada, H.; Adzic, R. R., Recent Advances in Platinum Monolayer Electrocatalysts for Oxygen Reduction Reaction: Scale-Up Synthesis, Structure and Activity of Pt Shells on Pd Cores. Electrochim. Acta 2010, 55, 2645-2652. 38 Uehara, H.; Uemura, Y.; Ogawa, T.; Kono, K.; Ueno, R.; Niwa, Y.; Nitani, H.; Abe, H.; Takakusagi, S.; Nomura, M.; Iwasawa, Y.; Asakura, K., In situ Back-Side Illumination Fluorescence XAFS 13 ACS Paragon Plus Environment


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(BI-FXAFS) Studies on Platinum Nanoparticles Deposited on a HOPG Surface as a Model Fuel Cell: a New Approach to the Pt-HOPG Electrode/Electrolyte Interface. Phys. Chem. Chem. Phys. 2014, 16, 13748-13754. 39 Kaito, T.; Mitsumoto, H.; Sugawara, S.; Shinohara, K.; Uehara, H.; Ariga, H.; Takakusagi, S.; Hatakeyama, Y.; Nishikawa, K.; Asakura, K., K-Edge X-Ray Absorption Fine Structure Analysis of Pt/Au Core–Shell Electrocatalyst: Evidence for Short Pt–Pt Distance. J. Phys. Chem. C 2014, 118 , 8481-8490. 40 Kaito, T.; Mitsumoto, H.; Sugawara, S.; Shinohara, K.; Uehara, H.; Ariga, H.; Takakusagi, S.; Asakura, K., A New Spectroelectrochemical Cell for in situ Measurement of Pt and Au K-Edge X-Ray Absorption Fine Structure. Rev.Sci. Instrum. 2014, 85, 084104. 41 K. Asakura, Analysis of XAFS, in X-ray Absorption Fine Structure for Catalysts and Surfaces, edited by Y. Iwasawa, World Scientific , Singapore, (1996) 35-58. 42 Bian, C.-R.; Suzuki, S.; Asakura, K.; Ping, L.; Toshima, N., Extended X-ray Absorption Fine Structure Studies on the Structure of the Poly(vinylpyrrolidone)-Stabilized Cu/Pd Nanoclusters Colloidally Dispersed in Solution. J. Phys. Chem. B 2002, 106, 8587-8598. 43 Asakura, K.; Yamazaki, Y.; Kuroda, H.; Harada, M.; Toshima, N., A “Cluster-in-cluster” Structure of the SiO2-Supported PtPd Clusters. Jpn. J. of Appl. Phys. 1993, 32-S2, 448. 44 Via, G. H.; Drake, K. F.; Meitzner, G.; Lytle, F. W.; Sinfelt, J. H., Analysis of EXAFS Data on Bimetallic Clusters. Catal. Lett. 1990, 5, 25-33. 45 Hamilton, W., Significance Tests on the Crystallographic R Factor. Acta Crystallogr 1965, 18, 502-510. 46 Wang, J. X.; Ma, C.; Choi, Y.; Su, D.; Zhu, Y.; Liu, P.; Si, R.; Vukmirovic, M. B.; Zhang, Y.; Adzic, R. R., Kirkendall Effect and Lattice Contraction in Nanocatalysts: A New Strategy to Enhance Sustainable Activity. J. Am. Chem. Soc. 2011, 133 , 13551-13557. 47 Mansour, A. N.; Cook Jr, J. W.; Sayers, D. E.; Emrich, R. J.; Katzer, J. R., Determination of Support and Reduction Effects for PtAl2O3 and PtSiO2 by X-Ray Absorption Spectroscopy. J. Catal. 1984, 89, 462-469. 48 Dutta, I.; Carpenter, M. K.; Balogh, M. P.; Ziegelbauer, J. M.; Moylan, T. E.; Atwan, M. H.; Irish, N. P., Electrochemical and Structural Study of a Chemically Dealloyed PtCu Oxygen Reduction Catalyst. J. Phys.Chem. C 2010, 114, 16309-16320 49 Wanjala, B. N.; Fang, B.; Shan, S.; Petkov, V.; Zhu, P.; Loukrakpam, R.; Chen, Y.; Luo, J.; Yin, J.; Yang, L.; Shao, M.; Zhong, C.-J., Design of Ternary Nanoalloy Catalysts: Effect of Nanoscale Alloying and Structural Perfection on Electrocatalytic Enhancement. Chem. Mater. 2012, 24, 4283-4293. 50 Yu, Z.; Zhang, J.; Liu, Z.; Ziegelbauer, J. M.; Xin, H.; Dutta, I.; Muller, D. A.; Wagner, F. T., Comparison between Dealloyed PtCo3 and PtCu3 Cathode Catalysts for Proton Exchange Membrane Fuel Cells. J.Phys. Chem. C 2012, 116, 19877-19885. 51 Loukrakpam, R.; Shan, S.; Petkov, V.; Yang, L.; Luo, J.; Zhong, C.-J., Atomic Ordering Enhanced Electrocatalytic Activity of Nanoalloys for Oxygen Reduction Reaction. J. Phys. Chem. C.2013, 117, 20715-20721. 52 Nilekar, A. U.; Mavrikakis, M., Improved Oxygen Reduction Reactivity of Platinum Monolayers on Transition Metal Surfaces. Surf. Sci. 2008, 602, L89-L94.


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5 nm

5 nm

5 nm

(a) PtCo/C

(b) PtNi/C

(c) PtCo/C-HT-600h

Figure 1 TEM images of (a) PtCo/C, (b) PtNi/C without heat treatment and (c) PtCo/C-HT-600h with heat treatment process.



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(a) PtCo/C

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(b) PtNi/C

(c) PtCo/C-HT-600h Figure 2 Diameter distributions of (a) PtCo/C, (b) PtNi/C without heat treatment and (c) PtCo/CHT-600h with heat treatment process.


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(a) HAADF-STEM image

(b) STEM-EDX profile of yellow line in (a)

Figure 3 (a) HAADF-STEM image of heat treated PtCo/C-HT-600 after RDE experiments and (b) Pt and Co STEM-EDX intensity profile of yellow line shown in the HAADF image.



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(a) HAADF-STEM image

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(b) STEM-EDX profile of yellow line in (a)

Figure 4 (a) HAADF-STEM image of PtNi/C after RDE experiments and (b) Pt and Ni STEM-EDX intensity profile of yellow line shown in HAADF image.


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Figure 5 Pt LIII-edge XANES spectra of Pt foil, PtCo, PtCu, PtNi alloy foils, Pt/C, PtCo/C, PtCo/CHTs, PtCu/C, PtCu/C-HT and PtNi/C at 0.4V vs. RHE.



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Figure 6 Pt LII-edge XANES spectra of Pt foil, PtCo, PtCu, PtNi alloy foils, Pt/C, PtCo/C, PtCo/CHTs, PtCu/C, PtCu/C-HT and PtNi/C at 0.4V vs. RHE.


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Figure 7 k3-weighted Pt-LIII edge XAFS oscillation of Pt foil, PtCo, PtCu, PtNi alloy foils, Pt/C, PtCo/C, PtCo/C-HTs, PtCu/C, PtCu/C-HT and PtNi/C at 0.4V vs. RHE.



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Figure 8 Fourier transform of k3-weighted Pt-LIII edge XAFS oscillation of Pt foil, PtCo, PtCu, PtNi alloy foils, Pt/C, PtCo/C, PtCo/C-HTs, PtCu/C, PtCu/C-HT and PtNi/C at 0.4V vs. RHE.


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Figure 9 Correlation between Pt-Pt distances at 0.4 V vs. RHE and area specific activities of Pt/C, PtCo/C, heat treated PtCo/C-HTs, PtCu/C, heat treated PtCu/C-HT, PtNi/C hollow structure and PtAu/C39.



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In Situ X-ray Absorption Fine Structure Analysis of PtCo, PtCu, and

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