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Letter

Same-View Nano-XAFS/ STEM-EDS Imagings of Pt Chemical Species in Pt/C Cathode Catalyst Layers of a Polymer Electrolyte Fuel Cell Shinobu Takao, Oki Sekizawa, Gabor Samjeské, Shin-ichi Nagamatsu, Takuma Kaneko, Takashi Yamamoto, Kotaro Higashi, Kensaku Nagasawa, Tomoya Uruga, and Yasuhiro Iwasawa J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b00750 • Publication Date (Web): 21 May 2015 Downloaded from http://pubs.acs.org on May 25, 2015

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The Journal of Physical Chemistry Letters

Same-View Nano-XAFS/ STEM-EDS Imagings of Pt Chemical Species in Pt/C Cathode Catalyst Layers of a Polymer Electrolyte Fuel Cell Shinobu Takao,a Oki Sekizawa,a Gabor Samjeské,a Shin-ichi Namagatsu,a Takuma Kaneko,a Takashi Yamamoto,b Kotaro Higashi,a Kensaku Nagasawa,a Tomoya Uruga,a, c Yasuhiro Iwasawa a, d* a

Innovation Research Center for Fuel Cells, The University of Electro-Communications,

Chofugaoka, Chofu, Tokyo 182-8585, Japan b

Department of Mathematical and Material Sciences, Faculty of Integrated Arts and Sciences,

The University of Tokushima, Minamijosanjima, Tokushima 770-8502, Japan c

Japan Synchrotron Radiation Research Institute, Spring-8, Sayo, Hyogo 679-5198, Japan

d

Department of Engineering Science, Graduate School of Information Engineering Science, The

University of Electro-Communications, Chofugaoka, Chofu, Tokyo 182-8585, Japan E-mail address of Corresponding Author: [email protected]

ACS Paragon Plus Environment

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ABSTRACT.

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We have made the first success in the same-view imagings of two-dimensional

nano-XAFS and TEM/STEM-EDS under humid N2 atmosphere for Pt/C cathode catalyst layers in membrane electrode assemblies (MEAs) of polymer electrolyte fuel cells (PEFCs) with Nafion membrane to examine the degradation of Pt/C cathodes by anode gas exchange cycles (startup/shut-down simulations of PEFC vehicles). The same-view imaging under the humid N2 atmosphere provided unprecedented spatial information on the distribution of Pt nanoparticles and oxidation states in the Pt/C cathode catalyst layer as well as Nafion ionomer-filled nano-holes of carbon support in the wet MEA, which evidences the origin of the formation of Pt oxidation species and isolated Pt nanoparticles in the nano-hole areas of the cathode layer with different Pt/ionomer ratios, relevant to the degradation of PEFC catalysts.

TOC GRAPHICS

KEYWORDS. Nano-XAFS mapping, TEM/STEM-EDS, Same-view nano-imaging, Mapping of Pt species degradation, Pt/C fuel cell catalyst


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Polymer electrolyte fuel cell (PEFC) (polymer proton exchange membrane fuel cell, PEMFC) is a clean energy-converting device with high power density and efficiency at low temperatures, which brings low or even zero emission vehicles into reality.1, 2 Despite the announcement of launching PEFC vehicles onto the market, for widely spread commercialization of PEFC vehicles, further improvement of the performance and durability of PEFCs for low cost by new guideline and concept is demanded. To achieve the tasks, test and diagnosis methods, which can directly characterize catalysts and prove fundamental issues for development of next-generation PEFCs, are essential and mandatory. Particularly, high durability of carbon-supported platinum (Pt/C) cathode catalysts in membrane electrode assemblies (MEAs) of PEFCs is a key issue to reduce Pt utilization with sufficient performances for the low-cost next-generation PEFCs, which may be adequately unraveled by the spatially resolved imaging of Pt chemical species and carbon support in the Pt/C catalyst layers of MEAs. The oxygen reduction reaction (ORR) events in MEAs have extensively been studied so far, and recently the elementary reaction steps and rate constants for the cathode catalysis in stepwise potential operations and potential-jump transient response processes have been determined by insitu time-resolved x-ray absorption fine structure (XAFS) techniques.3-8 The chemical events at the cathode surfaces are suggested to occur heterogeneously in the space of the Pt/C cathode catalyst layers due to the spatially heterogeneous property and distribution of Pt nanoparticles and carbon supports, which should cause microscopically nonuniform potential-loads and hence heterogeneous degradations at different places of the catalyst layers in the PEFC potential operations.9-17 Therefore, the development of a new method for spatially resolved visualization of the atomic geometry and oxidation state of Pt/C cathode catalysts in PEFC MEAs may be a


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significant challenge to elucidate the origin and mechanism of the Pt dissolution and deterioration of the cathode catalysts. Recently, we succeeded in the first spatial approaches to the non-destructive two-dimensional observation of Pt chemical species in the Pt/C cathode catalysts of PEFC MEAs by the scanning nano-XAFS mapping method and the nano-quick XAFS (QXAFS) method at the spatial resolution of 570 nm x 540 nm or 228 nm x 225 nm.18 By using the new nano-XAFS techniques we gained new insights into the distribution of metallic and oxidized Pt species as well as the preferential places and mechanism for Pt oxidation and dissolution as the key issues for the PEFC degradation in accelerated durability test (ADT) by 0.6 V-1.0 V (vs RHE) rectangle load cycles.18 However, the degraded Pt/C cathode catalysts were suggested to involve both sub-nano sized Pt oxidized species and Pt metallic nanoparticles heterogeneously in nano〜micro sized cracks and voids in carbon supports, which attempted us to develop a new method that can directly observe both Pt species/nanoparticles and carbon support with Nafion ionomers in the degraded MEA Pt/C cathode catalyst layers under humid atmosphere. Electron microscopic techniques (e.g. transmission electron microscope (TEM), scanning transmission electron microscope (STEM), and energy dispersive x-ray spectroscopy (EDS)) have many benefits for sub-nanometer characterization of MEAs if the electron microscopic images are obtained with the same field of vision (same view) as that for the nano-XAFS chemical mapping under humid atmospheric conditions. The STEM/EDS can give morphological information on atomic arrangement and element distribution, while the nanoXAFS can give molecular-level chemical information on electronic (oxidation) states and coordination structures with chemical bonding.18-26 Therefore, we have developed a new same-


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view method to obtain complementary information on nano-XAFS and TEM/STEM-EDS imagings for MEA Pt/C cathode layers in sub-nano〜nano scales, designing a stacking membrane cell. The same-view stacking membrane cell has allowed us to achieve nondestructive TEM/STEM-EDS measurements under the humid N2 atmosphere to avoid MEA transformations and destructions which are easily caused by shrinking of ionomers and electrolytes by drying and water loss under high vacuum conditions taken for usual TEM/STEMEDS observations.27 By using the same-view nano-XAFS/STEM-EDS imaging technique we have also found the origin and nanoscopic aspect of the degradation of PEFCs by anode gas exchange (AGEX) cycles (start-up/shut-down simulations of PEFC vehicles) due to the formations of monomeric Pt2+ ions and isolated metallic Pt nanoparticles in nano-hole areas of the cathode carbon layer depending on the Pt/ionomer ratios in the nano-hole areas. We have designed a new same-view stacking membrane cell for the nano-XAFS and TEM/STEM-EDS imagings under the humid N2 atmosphere as shown in Figure 1 A. The cell was composed of a stacked 100 nm-thick SiN membrane with 100 m-thick Si flame and a 0.5 mm x 0.5 mm window for the same-view measurements of nano-XAFS and TEM/STEM-EDS. The detailed information about the stacked SiN membrane cell is described in the experimental part hereinafter and Figure SI 1 (Supporting information). By using the designed cell we have imaged two sorts of MEAs; one is an initial MEA after aging (initial conditioning) and the other is a degraded MEA after 300 cycles of the AGEX treatment. Their cyclic voltammograms and averaged power densities are shown in Figure SI 2. The MEA treatment with the 300 AGEX cycles decreased the electrochemical surface area (ECSA) to 62 % of that of the aging MEA, and the maximum power density after the 300 AGEX


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cycles decreased to 66 % of the initial value. Many round-shaped cracks appeared in the cathode catalyst layer after the AGEX cycles. After the electrochemical procedures and AGEX cycles, both anode and cathode gases were replaced by N2 and the PEFC was left until it reached an open circuit voltage (OCV) to prevent the sample from being exposed to high potentials. The MEA was sliced to a small piece with 200 nm thickness (4-6 layers of carbon particles) by an ultra-microtome for nano-XAFS and TEM/STEM-EDS measurements under the humid N2 atmosphere, and the sliced MEA piece was putted on a 100 nm thick SiN membrane under the humid N2 atmosphere. The resultant nano-XAFS spectra and TEM/STEM-EDS images for the sliced MEA in the SiN membrane stacking cell are regarded to be equivalent to those measured in situ after the aging and the AGEX cycles because all the procedures are conducted under the humid N2 atmosphere without exposing air and the degradation of MEAs is irreversible in the present time scale. For the nano-XAFS measurements we used a nano-beam of 225 nm x 228 nm because the MEA after the 300 AGEX cycles possessed small cracks of 200 nm–500 nm sizes due to carbon corrosion. The nano-XAFS spectra were obtained by scanning a given area of the sample with the nanobeam at different 220 energies by the similar method to that described in our previous report (Experimental of SI).18 The positions in each of the 220 energy maps were calibrated by map fitting before data treatments for the nano-XAFS mapping (Figures SI 3 and SI 4). For the position calibration of nano-XAFS map and STEM image we used the orthogonal distance regression for the estimation of fitting parameters p0, p1 and p2 to give a minimum residual; A(x,y) - p0 * B(xp1,y-p2) (A(x,y): absorbance of nano-XAFS map and B(x,y): contrast of the STEM image for coordinate point (x,y)). Thus, the positions were calibrated by using the calculated p1 and p2 (Figures SI 3 and 4).


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It was found that Pt nanoparticles moved on the support carbon in the TEM/STEM-EDS observations under the humid N2 atmosphere without any temperature control, whereas such Pt migrations were never observed with dried MEA samples under high vacuum. The Pt nanoparticles in the MEA with the humid N2 atmosphere should be more easily excited by the irradiation of accelerated electrons at 200 kV,28, 29 and hence the location and size distribution of Pt nanoparticles may change during the TEM/STEM-EDS observations. Moreover, some bubbles by water boiling in the specimen appeared during the observations under the humid N2 atmosphere. However, we have succeeded in overcoming these problems by regulating the sample temperature at 300.5 K by a Cryo-holder, in which Pt nanoparticles neither moved nor aggregated. As the result, the same-view imagings of TEM/STEM-EDS and nano-XAFS were achieved under the humid N2 atmosphere without any destruction and with good spatial resolution enough to identify not only Pt nanoparticles but also cathode layer structures (Figure SI 5). Figure 1 B and C exhibit the nano-XAFS (11.600 keV) mapping, which corresponds to a map of the Pt quantity, and the white line peak top (WLPT) intensity mapping of the normalized nano-XAFS, which corresponds to a map of the Pt oxidation states, respectively for the Pt/C cathode catalyst layer in the degraded PEFC MEA after the AGEX treatments. The nano-XAFS mappings revealed the heterogeneous distribution of both Pt amounts and oxidation states in the Pt/C cathode catalyst layer. This is contrasted to the homogeneous property of the Pt/C cathode layer in an initial MEA after the aging as shown in Figure SI 6. After the nano-XAFS measurements of the degraded MEA we measured the TEM/STEM-EDS in the same region by using the same-view cell (Figure 1 D). The geometrical structure of the cathode catalyst layer involving nano-holes and the Pt distribution in the STEM imaging well corresponded to those in


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the XAFS mapping (Figure 1 E). On the other hand, when the STEM image of the MEA was measured in high vacuum, some broken areas with nano〜micro-cracks and voids, which were caused by water loss and shrinking of ionomers and electrolytes, were observed (Figure 2). These results indicate the importance of both temperature control and voidance of water loss to achieve the equivalence to in-situ observations without any structural destructions and transformations of MEA. Moreover, we found two sorts of specific Nafion ionomer-filled nanoholes in the XAFS mapping, which showed high and low WLPT intensities of the normalized XAFS (red squares 1 and 2 in Figure 1 B-D), where the normal area 3 without holes in the cathode layer is also shown. The nano-XAFS spectra and normalized nano- x-ray absorption near edge structure (XANES) spectra for the areas 1 - 3 are shown in Figure 3 A and B, respectively. The Pt quantity (Pt LIII-edge jump) involved in the nano-hole areas 1 and 2 was much lower than that in the normal area 3 without carbon corrosion; the Pt amounts in the areas 1 and 2 were estimated to be, respectively 22 % and 18 % of that in the area 3. The Pt oxidation states in the cathode layer were estimated from a linear relationship between the Pt valence and the white line peak area of the normalized XANES spectra.18 The Pt oxidation state in the nano-hole area 1 was estimated as +1.9 (±0.15) from Figure 3 B, which indicates that the majority of Pt species in the nano-hole area 1 is situated in a Pt2+ state, leaching from the carbon support. In the nano-hole area 1, neither nano size nor sub-nano size particles were observed even by the highest magnification 1,500,000 of TEM (Figure 4 a and b), but the certain amount of Pt species and Nafion ionomers existed as proved by the STEM-EDS for the Pt, C, F, and S line profiles in Figure 4 c. For the nano-hole area 1 the nano-QEXAFS oscillation and the associated Fourier transform and curve-fitting are shown in Figure 3C 1-a and 1-b, respectively. The Pt species in the area 1 possessed only Pt-O bonds with the coordination number of 2.3 (±0.8) at 0.211 nm


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(±0.007 nm) and no Pt-Pt bonds were observed as summarized in Table S2, which also revealed the formation of monomeric Pt2+ ions by oxidative Pt dissolution from the Pt/C cathode layer by the AGEX treatments. The bond distance of and coordination number of Pt-O indicate monomeric Pt(OH)2 species, but the Pt2+ coordination sphere should be in equilibrium with the Nafion ionomer(Nf-SO3 groups) and water in the nano-hole area; e.g. [Pt(OH)2(NfSO3)x(H2O)y]. Previously, we reported the formation of monomeric Pt2+-O4 species with a planar coordination structure in micro-crack areas of the Pt/C cathode layer degraded by accelerated durable test rectangle cycles.18 The Pt2+ species in the nano-hole area of the Pt/C cathode layer degraded by the AGEX cycles may be loosely coordinated with the ionomer and/or water, which coordination number is hard to decide exactly and becomes very small in the EXAFS analysis. In contrast, it was observed that Pt nanoparticles, probably detached from the carbon support by the AGEX load cycles, were distributed in the nano-hole area 2 (Figure 4 d and e) and their Pt valence was estimated to be zero (Figure 3 B). The metallic Pt nanoparticles in the area 2 were also confirmed by the nano-QEXAFS analysis (Figure 3 C, 2-a, and 2-b), which exhibited only Pt-Pt bonds with the coordination number of 11.7 (±1.7) at 0.277 nm (±0.001 nm) (Table S2), and no Pt-O bonds were observed with the detached Pt nanoparticles. The EDS line profiles in the nano-hole area 2 (Figure 4 f) showed much lower F content (F element originates from the Nafion ionomers) than that in the nano-hole area 1, while the Pt contents in both nano-hole areas were similar to each other as above mentioned. The Pt/ionomer ratios (wt/wt) were estimated as 0.02 and 0.12 for the areas 1 and 2, respectively, whereas that in the normal catalyst layer like the area 3 was calculated as 1.07 (similar to the original value) by the EDS. Thus, it is to be noted that the Pt/ionomer ratio caused the different types of degradation of the Pt/C catalysts in the MEA cathode layer; leaching as Pt2+ ions and detaching as metallic Pt nanoparticles from the


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carbon support. The normalized nano-XANES spectra in Figure 3 B showed an isosbestic point at 11.570 keV among them. This indicates the direct transformation of Pt0 to Pt2+ species during the AGEX treatments. On the other hand, Figure 1 B-F, Figure 3 B and 3 C 3-a and 3-b, and Figure 4 g and h consistently evidence that the metallic Pt nanoparticles in the cathode area 3 without carbon corrosion remained unchanged in the oxidation state and dimension after the 300 AGEX cycles. In conclusion, we made the first success in the non-destructive same-view nano-XAFS and TEM/STEM-EDS imagings for the PEFC MEA Pt/C cathode catalyst layers under humid N2 atmosphere by using the new same-view stacking membrane cell. The complementary nanoXAFS and STEM-EDS imagings allowed to find the unprecedented aspect of the formations of the leached Pt2+ oxidation species and detached metallic Pt nanoparticles in the Nafion ionomerfilled nano-hole areas of the degraded Pt/C cathode layers due to the carbon corrosion by the AGEX treatments. The two ways of either leaching or detaching of the Pt nanoparticles from the carbon support relevant to the irreversible degradation of PEFCs depended on the Pt/ionomer ratios in the nano-hole areas. The study will be further extended to MEA samples with Pt alloy cathode catalysts to clarify spatially heterogeneous dealloyed phenomenon relevant to the alloy/C catalyst deactivation under the PEFC operation conditions.


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100 mthick Si

high

(11.600 keV) mapping 1 2 m

nano-XAFS and EDS 100 nm-thick SiN

spacer Humid N2 ambient spacer

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Figure 1. Same-view measurements of nano-XAFS and TEM/STEM-EDS for the degraded MEA sample after the 300 AGEX cycles under humid N2 atmosphere. A: Schematic arrangement of the same-view stacking membrane cell for nano-XAFS and TEM/STEM-EDS measurements under humid N2 atmosphere (the picture does not show proportional scales) (see Figure SI 1) B: (11.600 keV) mapping of the cathode catalyst; C: WLPT intensity mapping of the normalized XANES; D: STEM image after the nano-XAFS measurement for the same area as B and C at 300.5 K; E: the superposition of B and D; F: the superposition of C and D.


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Inserted figure: a designed cell for nano-focused XAFS measurements (the stacking membrane cell (A) was put on a 1 m-thick SiN membrane (10 mm x 10 mm) and the 1 m-thick SiN membrane was arranged on the engraved part of the center of the cell).

Figure 2. a: Nano-XAFS ((11.600 keV)) mapping of the MEA Pt/Ccathode catalyst layer under humid N2 atmosphere; b: STEM image of the MEA under high vacuum after the nano-XAFS measurements for the same region as a. Yellow arrows indicate broken areas with nano〜 microcracks and voids; c: the superposition of a and b.


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A

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PtO2

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Figure 3. A: Nano-XAFS spectra for the 335 nm x 338 nm area 1 (red), the 225 nm x 228 nm area 2 (green), and the 225 nm x 228 nm area 3 (blue) in Figure 1. B: Their normalized nanoXANES spectra for the areas 1 (red), 2 (green), and 3 (blue), and reference spectra of Pt-foil (black), PtO (purple) and PtO2 (pink). C: k2-weighted nano QEXAFS oscillations (a) and associated Fourier transforms (b); 1, 2, and 3: areas 1, 2, and 3, respectively; curve-fitting results in R-space: Δk=3-9 Å-1, ΔR=1.4-3.0 Å for area 1, and Δk=3-10 Å-1, R=1.6-3.2 Å for areas 2 and 3.


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Figure 4. STEM and TEM images and EDS profiles for the areas 1, 2, and 3. a and b: STEM and TEM images of the area 1 in Figure 1 B, respectively; c: the EDS line profiles for the blue line in a; d and e: STEM images of the area 2 in Figure 1 B; f: the EDS line profiles for the blue line in d; g and h: STEM images of the area 3 in Figure 1 B.


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Experimental Preparation of sliced MEA samples for nano-XAFS and TEM/STEM-EDS observations. For preparation of sliced MEAs for the same-view observations after the aging and 300 AGEX cycles, we used a glove bag which was filled by humid N2 during putting the MEAs out of the PEFC and slicing the MEAs to small pieces with a given dimension. Then a sliced MEA piece was put on a SiN membrane substrate (100 nm thickness) with 100 m thick Si flame (Alliance Bio, Inc.) in the humid N2-filled glove bag. The sliced sample on the SiN membrane was surrounded by a TEFLON tube (300 nm thickness), and covered with another SiN membrane (Figure SI 1). The SiN stacking membrane cell with a Cryo-holder (kept at 300.5 K) or normal holder was employed for TEM/STEM-EDS observations. In nano-XAFS measurements, the SiN stacking membrane cell with a sealed MEA sample was put on a large SiN membrane with 1 m thickness (NTT Advance Technology, Corp.) and arranged in our specially designed nano-XAFS cell as shown in Figure SI 1. Nano-focused beam XAFS mapping. The Pt LIII-edge nano-XAFS spectra were measured at BL36XU by using a Si(111) double crystal monochromator. X-ray beam (11.390–12.200 keV) was focused to 228 nm x 225 nm size via a pair of elliptically bent Kirkpatrick-Baez (KB) mirrors. The nano-XAFS spectra were measured in a fluorescence mode using Vortex-ME IV detector, where the sample was inclined to the X-ray nanobeam by 30º. In the scanning nanoXAFS method, a XAFS spectrum was obtained from 220 maps corresponding to 220 energy points (see Experimental of SI).18 To avoid a sample damage by the nano-focused X-ray beam, a beam stay time at each pixel point was shortened to 1.5 s/pixel in the mapping.


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TEM/STEM-EDS observations. TEM and STEM-EDS images were measured on a JEM-2100F equipped with an energy dispersive spectrometer (EDS) at 200 kV. In the STEM-EDS observation, we used 0.7 nm of electron-beam size, which was sufficiently smaller than nanocracks/ nanoholes (200-500 nm) in the degraded MEAs. For the TEM/STEM-EDS observation of the same sample and region as those for the XAFS mapping, the TEM/STEMEDS measurement conditions were adjusted using both SiN membrane (100 nm thickness) and Cu grid (150 mesh). The sample temperature was controlled to be 300.5 K by a Cryo holder. In order to examine if any nanocracks/nanoholes were not artificially made in the additional procedures such as slicing and cutting of the MEAs, we estimated F and S contents and confirmed if the F/S ratios had the same value as that for Nafion-212 or Nafion (EW-1100).

Supporting Information Available: (Experimental descriptions; same-view measurement cell design; electrochemical property; position calibration; nano-EXAFS analysis) This material is available free of charge via the Internet http://pubs.acs.org.

Corresponding Author *Yasuhiro Iwasawa; Fax number: +81-42-443-5483, E-mail: [email protected]

ACKNOWLEDGEMENT This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. We thank Mr. Yoshiki Miura for his help in designing a XAFS


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cell with a SiN membrane plate holder. XAFS measurements were performed with the approval of SPring-8 subject number 2013A7803, 2013B7803, 2014A7802, 2014A7806, 2014A7807, 2014B7802, and 2014B7804.

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STEM-EDS Imagings of Pt ... - ACS Publications

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