MOF-Derived Carbon Networks with Atomically Dispersed FeâNx

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MOF-Derived Carbon Networks with Atomically Dispersed FeNx Sites for Oxygen Reduction Reaction Catalysis in Acidic Media Yuhong Qian, Qingtao Liu, Erik Sarnello, Chunhua Tang, Meilin Chng, Jianglan Shui, Tao Li, Stephen J. Pennycook, Ming Han, and Dan Zhao ACS Materials Lett., Just Accepted Manuscript • DOI: 10.1021/acsmaterialslett.9b00052 • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 4, 2019

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ACS Materials Letters

MOF-Derived Carbon Networks with Atomically Dispersed Fe-Nx Sites for Oxygen Reduction Reaction Catalysis in Acidic Media Yuhong Qian,† Qingtao Liu,‡ Erik Sarnello,§ Chunhua Tang,∥ Meilin Chng,⊥ Jianglan Shui,‡ Tao Li,§,# Stephen J. Pennycook,∥ Ming Han⊥, and Dan Zhao*,† †Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585, Singapore. ‡School of Materials Science and Engineering, Beihang University, No. 37 Xueyuan Road, Beijing, 100083, P. R. China. §Department of Chemistry and Biochemistry, Northern Illinois University, 1425 W. Lincoln Hwy., DeKalb, IL, 60115, USA. ∥Department of Materials Science and Engineering National University of Singapore, 9 Engineering Drive 1, 117575, Singapore. ⊥School of Engineering, Temasek Polytechnic, 21 Tampines Avenue 1, 529757, Singapore. #X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL, 60439, USA.

ABSTRACT: Proton exchange membrane fuel cell (PEMFC) is one of the most promising candidates for electrochemical energy conversion, but the requirement of expensive Pt-based catalysts on the cathodes prevents its large-scale application. Herein, we report the preparation of three-dimensional carbon networks with atomically dispersed Fe-N4 active sites (SA-3DNC) for oxygen reduction reaction (ORR) catalysis in the cathodes of PEMFCs. Derived from NaCl-assisted pyrolysis, the SA-3DNC has a continuous, micro-macroporous hierarchical morphology which is endowed by the modulator effect of the molten salt. In 0.1 M HClO4, the SA-3DNC has a 0.906 V onset potential for ORR, which is 30-mV more positive than that of commercial 20% Pt/C. After 10,000 cycles of accelerated degradation, the half-wave potential of SA-3DNC only drops 38 mV, compared to 125 mV of the Pt/C. Furthermore, a single cell was assembled using the SA-3DNC as the cathodic catalyst, showing only a 20% current drop after a 20-h durability test at 0.5 V. Our study has demonstrated a facile way to prepare hierarchical, stable, and noble-metal-free electrocatalysts with atomically dispersed active sites for ORR catalysis in acidic media.

an interconnected, micro-macroporous hierarchical structure can effectively reduce the internal ohmic resistance and the mass transport resistance during ORR catalysis.32-34 Also, the dispersity of active sites highly relates to the overall performance of the catalysts, and an atomic level of dispersity is preferred because it can maximize the exposure of active sites. As a result, the ideal ORR catalyst should have all the above-mentioned merits, and ideally be prepared with moderate material cost and manpower. However, the current studies have only reported catalysts with partial desired properties, which are prepared by tedious methods requiring special equipment. For example, electrospinning is a traditional method to prepare materials with a micro-macroporous hierarchical morphology, but the dispersity of the active sites in catalysts is not maximized and the method requires specific instruments.35-37 In addition, ORR catalysts with metal-containing active sites also suffer from stability issue in acidic media.38 It is therefore valuable to develop a facile method to prepare the desirable ORR catalysts with long-term stability.

The direct conversion from chemical energy into electrical energy is an appealing idea in the areas of clean energy and environmental sustainability, as this approach bypasses the Carnot limit and greatly enhances the efficiency of energy utilization.1 Among the various options of devices which are able to achieve this target, proton exchange membrane fuel cell (PEMFC) stands out due to its high energy efficiency, flexibility in size, environmental friendliness, and the feature of using hydrogen as the fuel source with only water as the oxidation product.2-3 However, costly Ptbased noble metal catalysts are needed in current PEMFCs, especially on the cathode side for oxygen reduction reactions (ORR).4 The ORR is notoriously known for its slow kinetics, which greatly reduces the overall performance of PEMFCs.5-7 Although Pt and its derivatives have shown high activity in catalyzing the ORR in PEMFCs, it has been predicted that the global reserve of Pt is unable to meet the total requirement.8 Therefore, it becomes necessary to find alternatives for Pt and other noble metal catalysts. The exploration of non-noble metal catalysts for ORR catalysis began in the 1960s, and M-N4 (M = Fe, Co) moieties have been widely regarded as one of the most active sites.9-14 Various strategies and precursors have been adopted to prepare such active sites, and the nature of the active sites and the principles for catalyst preparation have been revealed by techniques such as X-ray absorption spectroscopy and Mossbauer spectroscopy.15-16 In addition to the active sites, it has been discovered that a highly conductive carbon support is necessary for the catalyst to give a satisfying current output.17-24 Among the precursors for preparing carbon supports, polyacrylonitrile (PAN) gives a highly conductive, nitrogen-rich, and crystalline carbon support after pyrolysis.25-30 Its polymeric behavior at high temperatures is favorable to the incorporation of active sites.31 In addition to the compositions of the catalysts, the state-of-art research reveals that

Scheme 1. Schematic illustration of the preparation of SA-3DNC for ORR catalysis in acidic media.

Herein, we report the preparation of a three-dimensional, highly conductive carbon network with atomically dispersed Fe-N4 active sites (SA-3DNC) for ORR catalysis in acidic media and PEMFCs (Scheme 1). Following our previous study,39 NaCl was used as a morphological modulator, leading to the formation of SA-3DNC

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that consists of interconnected, microporous nanosheets and embeds with macropores. The highly dispersed Fe-N4 sites were introduced through a 2-step pyrolysis activation/acid wash procedure, resulting in a carbon supported single-atom catalyst with a Fe loading of 0.58 wt. %. During the half-cell electrochemical measurements in acidic media, the SA-3DNC showed an onset potential of 0.906 V and a half-wave potential of 0.729 V, which are comparable to those of 20 % Pt/C (0.876 V and 0.758 V). Synchrotron based X-ray absorption spectroscopy was used to identify the coordination environment of the active sites, suggesting that the localized structure of the Fe-N4 sites should be FeN4C12. The accelerated degradation test showed that the addition of PAN can significantly enhance the stability of SA3DNC (the decrease of half-wave potential of SA-3DNC is only 40 % of that of Pt/C). In addition, the SA-3DNC was equipped in a PEMFC single cell assembly and showed enhanced durability in a 20-h continuous discharge test. Our approach of combining saltassisted pyrolysis and secondary polymerization of PAN during high temperature paves a way for the facile preparation of efficient and durable ORR electrocatalysts in acidic media.

Figure 1a and 1b). Compared with the cube morphology of ZIF-8 and the sphere morphology of pristine PAN (Figure S1), SA3DNC has a morphology of continuous network with large pieces of carbon sheets after the salt-assisted pyrolysis. The carbon sheets have a lateral size in micrometers with a thickness of around 20 nm revealed by atomic force microscope (AFM, Figure S2). This high length-to-thickness ratio is beneficial for the exposure of active sites.40 Meanwhile, macropores with diameters ranging from 20 nm to 500 nm are scattered within the carbon sheets, providing sufficient channels for the mass transfer during the ORR catalysis.34 The selected area electron diffraction (SAED) pattern of SA3DNC in Figure 1c only has two rings from the polycrystalline carbon, confirming no metallic aggregate in the SA-3DNC.41 In order to reveal the Fe-containing sites, high-angle annular darkfield scanning transmission electron microscope (HAADF-STEM) image was taken (Figure 1d). The isolated Fe-containing sites (circled bright dots in Figure 1d) embedded in the carbon sheets can be identified by the Z-contrast difference from the difference of atomic mass. A selected area EELS-STEM elemental mapping is shown in Figure 1e and 1f. The distributions of N and Fe are even inside the SA-3DNC, and the signal of Fe is significantly lower than that of N because of its low concentration and high dispersity. In order to study the effect of each component in SA-3DNC, two samples were prepared without ZIF-8 or PAN, respectively (named as PAN-Fe-P and Z8-Fe-P). Accordingly, three catalysts without the Fe-containing active sites were prepared as well (named as PAN-P, Z8-P, and PAN-Z8-P). Their morphology was also studied under SEM and TEM (Figure S3 and S4). For the catalysts derived from ZIF-8, the salt-assisted pyrolysis transformed the separated particles into continuous networks, matching well with our previous study.39 Adding PAN into the precursors leads to the formation of interconnected sheet-like morphology after pyrolysis. This can be attributed to the fact that PAN is easy to form conjugating carbon structure under high temperature due to the cross-linking between the polymer chains.31 Although the interconnected, continuous carbon support can be prepared by either of the precursors (PAN or ZIF-8), the physical properties of the derived carbon materials vary and they can lead to different electrochemical behaviors (vide infra). The powder X-ray diffraction (PXRD) patterns of SA-3DNC and the other prepared materials are shown in Figure S5. All the samples possess a weak diffraction peak at 23 °, which is originated from the (002) plane of the carbon with low crystallinity. Interestingly, the peak of (002) plane of FeC is presented in the PXRD pattern of PAN-Fe-P, while it is missing in Z8-Fe-P or SA3DNC. The missing of FeC in the ZIF-8 derived samples suggests that the introduction of ZIF-8 suppresses the formation of iron carbide, which is less ORR-active than the Fe-N4 sites.14 Raman spectra of the samples were recorded to investigate the properties of the carbon supports (Figure S6, Table S1). Compared with carbon materials derived from ZIF-8, PAN-based materials have a higher degree of graphitization. According to previous studies, although amorphous carbon has a higher chance to expose/incorporate active sites, graphitized carbon is more resistant to corrosion thus having a better stability in long-term operations.42-44

Figure 1. Morphology studies of SA-3DNC: (a) SEM image; (b) TEM image; (c) SAED pattern; (d) High resolution HAADF-STEM image (circled areas indicate isolated Fe-containing sites); (e) Low resolution HAADF-STEM image; (f) EELS-STEM elemental mapping of the selected area in (e).

The morphology of SA-3DNC was studied by scanning electron microscope (SEM) and transmission electron microscope (TEM,


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ACS Materials Letters of our material preparation strategy. The Fe content in SA-3DNC was determined to be 0.58 wt. % by ICP-OES (Figure S11). The N2 sorption isotherms and pore size distributions of the prepared materials are shown in Figure S12. Because of the lack of pore-forming mechanism during the pyrolysis of PAN, the PANbased samples only show mediocre BET surface area after pyrolysis (44 m2 g-1 for PAN-P, Table 1). It is worth noting that the PAN-Fe has a relatively high BET surface area of 465 m2 g-1, which may have been improved by the acid wash step. Owing to the highly porous nature of ZIF-8 and the network structures formed after pyrolysis, the Z8-P and Z8-Fe-P both have a surface area of 1100 m2 g-1 and a high pore volume around 2.5 cm3 g-1. Interestingly, the introduction of PAN did not have a significant impact on the porous feature of SA-3DNC, which still shows a BET surface area of 990 m2 g-1 and an even higher pore volume of 3.45 cm3 g-1. The high surface area of SA-3DNC guarantees sufficient space for supporting the active sites, and the high pore volume is favorable to mass transport. Therefore, the SA-3DNC is expected to exhibit good ORR activity at high current density due to its improved mass transport property.

Figure 2. (a) XANES spectra and (b) k2-weighted Fourier transformed EXAFS spectra of SA-3DNC and the reference samples; (c) simulated k2-weighted Fourier transformed EXAFS spectra of the proposed localized structures of the Fe-N4 sites.

The chemical states of carbon and nitrogen in the obtained materials were studied by X-ray photoelectron spectroscopy (XPS). The C 1s XPS spectra of all samples are similar, and are all dominated by the sp2 carbon (Figure S7). Due to the higher nitrogen content in the pristine PAN than that of the ZIF-8, the PAN-derived samples show stronger signals from the C-N bonds, which was also confirmed by elemental analysis (Table S2, S3). The N 1s XPS spectra were deconvoluted into four different Ncontaining species (Figure S8, Table S2), which are classified as the pyridinic N (398.4 eV), the pyrrolic N (399.6 eV), the graphitic N (401.0 eV), and the oxidized N (> 402.0 eV). Compared with the ZIF-8 derived carbon materials, the ones prepared with PAN have a higher content of graphitic N, which can enhance the electronic conductivity by donating electrons into the conjugating sp2 carbon species.45 The introduction of Fe-TPI led to a higher pyridinic N content in the prepared materials. The pyridinic N atoms may either be on the edges of the carbon support, or be coordinated with the Fe center in the Fe-N4 sites. Based on the previous studies, both types of the pyridinic N are favorable to enhancing the ORR catalytic activity.46-47 The chemical state and coordination environment of the single-atom Fe-containing sites in SA-3DNC were investigated by synchrotron-based X-ray absorption spectroscopy (XAS). Figure 2a shows the X-ray absorption near edge structures (XANES) of SA-3DNC and the other reference samples. From the pre-edge position of Fe-TPI and SA-3DNC, it is clear that they share the same valence of Fe (II); while the Fe (III) in Fe2O3 has a pre-edge position which shifted to a higher energy. The k2-weighted Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra are shown in Figure 2b. Extracted from the reference samples, the Fe-Fe, Fe-N, and Fe-O bonds have lengths of 2.42 Å, 1.52 Å, and 1.45 Å, respectively. The SA-3DNC only shows a single peak at 1.53 Å in its EXAFS spectrum, indicating the presence of only Fe-N4 sites (Figure S9). We further performed a simulation study to reveal the localized structure of the Fe-N4 sites, and the results showed that the Fe-N4 sites in the SA3DNC have a localized structure of FeN4C12 (Figure 2c, S10).13 The XAS study showed that only Fe-N4 sites are preserved in the SA-3DNC, proving the high selectivity towards specific active sites



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Figure 3. (a) LSV plots of SA-3DNC and the other prepared materials in 0.1 M HClO4 at 1600 rpm; (b) summary of Eonset and Ehalf-wave and (c) Tafel plots of Z8-Fe-P, SA-3DNC and 20 % Pt/C; (d) peroxide yield, number of transferred electrons and (e) EIS spectra of SA-3DNC and the other prepared materials; (f) LSV plots of PAN-Fe-P, Z8-Fe-P, SA-3DNC, and 20 % Pt/C before and after the 10,000-cycle durability test.

from a mixture precursor of PAN and ZIF-8 shows the highest onset potential of 0.906 V, which is probably due to the improved electronic conductivity from PAN (Figure 3b). However, the halfwave potential of SA-3DNC is slightly lower than that of Z8-Fe-P, which may be attributed to its lower content of Fe-Nx ORR active sites due to catalyst preparation. The Tafel plots of Z8-Fe-P and SA-3DNC are shown in Figure 3c with onset potentials lower than that of 20 % Pt/C, which suggests that the intrinsic activity of the Fe-N4 sites may not be as good as that of Pt.49 We also varied the ratio between PAN and ZIF-8 in the precursor of SA-3DNC, and found that a mass ratio of 1:1 achieves the best performance (Figure S15).

The electrochemical properties of SA-3DNC and the other samples were first investigated by cyclic voltammetry (CV, Figure S13). The rectangular shape of the CV plots in Ar-saturated 0.1 M HClO4 suggests that all the samples have no red-ox behaviors or side reactions in the range of 0.2 V to 1.2 V versus the reversible hydrogen electrode (RHE). The ORR activity under steady state was studied by linear scan voltammetry (LSV) with a rotating disk electrode at 1600 rpm in O2-saturated HClO4 (Figure 3a). Due to the protonation of N-doped sites, the metal-free samples only exhibited limited current density.48 The most active Z8-P has an onset potential of 0.826 V with poor ORR kinetics indicated by its 112 mV dec-1 Tafel slope (Figure S14). After introducing the Fe-Nx sites, the PAN-Fe shows an onset potential of 0.843 V, but the lack of micropores limits the further enhancement of its ORR activity. When ZIF-8 was used as the carbon support precursor, the onset potential was further improved to 0.869 V. The SA-3DNC derived

The peroxide yield and the number of electrons transferred during ORR catalysis were measured by rotating ring-disk electrode tests. Because the formation of hydrogen peroxide may

Table 1. BET surface area and ORR activity of SA-3DNC and other prepared materials. B.E.T. surface area / m2 g-1

Pore volumea / cm3 g-1

PAN-P

44

0.18

ORR activityb

Eonset

Ehalf-waved / V

jmaxe / mA cm-2

0.748

0.412

2.42

c/V

Z8-P

1086

2.34

0.826

0.561

5.62

PAN-Z8-P

141

1.01

0.795

0.476

4.88

PAN-Fe-P

465

0.55

0.843

0.636

4.92

Z8-Fe-P

1167

2.54

0.869

0.744

5.48

SA-3DNC

990

3.45

0.906

0.729

6.13

/

0.876

0.758

5.76

20 % Pt/C a) At P / P

/ 0

-2 d) e) c) 4; Determined when j = 0.05 mA cm ; The potential when j = ½ jmax; E = 0.17 V vs RHE.

= 0.99; b) In 0.1M HClO


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ACS Materials Letters

Figure 4. (a) Polarization curves, (b) power output, and (c) 20-h cell durability test at 0.5 V of Z8-Fe-P and SA-3DNC in single cell PEMFC tests.

cause chemical degradation of the Nafion membrane, a high selectivity towards 4-e- ORR process is critical to the long-term stability of the catalysts.50 As can be seen in Figure 3d, both SA3DNC and Pt/C show numbers of electrons transferred close to 4, indicating high selectivity towards the 4-e- ORR process. The number of electrons transferred was also investigated by the Koutecky-Levich equation derived from multiple LSV plots, which also shows that SA-3DNC is selective to 4-e- ORR process (Figure S16, S17). These two results confirm that the Fe-N4 sites in SA3DNC are highly selective to the 4-e- ORR process, which is highly beneficial to the long-term durability of the cathodes in PEMFCs.

In order to measure the performance of our catalysts under actual working conditions, Z8-Fe-P and SA-3DNC were assembled into cathodes of PEMFCs. The polarization plots and the output power of the fuel cells are shown in the Figure 4a and Figure 4b, respectively. The introduction of PAN in the precursors diluted the concentration of Fe-N4 sites in the SA-3DNC, therefore the cell consisting of Z8-Fe-P shows higher voltage when the current density is lower than 1 A cm-2. However, the Z8-Fe-P has a turning point in its polarization curve at around 0.6 A cm-2, exhibiting a fast drop of voltage at higher current densities. This is due to the low electronic conductivity of the ZIF-8 derived carbon supports since the linear region of the polarization curve of Z8-Fe-P has a significantly higher slope. For SA-3DNC, its polarization curve is almost linear when the current density is higher than 0.4 A cm-2, showing that the decrease of voltage mostly comes from ohmic resistance.51 In addition, SA-3DNC is able to operate at higher voltages than Z8-Fe-P when the current density is higher than 1 A cm-2, suggesting that SA-3DNC is better than Z8-Fe-P in the transport loss region. Therefore, the physical properties of SA3DNC make it less prone to ohmic loss and transport loss. Considering the interconnected morphology and ultrahigh pore volume of SA-3DNC, the morphology produced by the saltassisted pyrolysis is highly favorable for the ORR catalysts to be operated at high current density. However, the highest power densities of Z8-Fe-P and SA-3DNC are 0.38 W cm-2 and 0.29 W cm-2, respectively, which are significantly lower than that of Pt/C (1.2 W cm-2, Figure S19). It suggests that there is still much room for the increase of active site density in our non-noble metal catalysts. The durability of the catalysts in fuel cells was tested by chronoamperometry at 0.5 V. Compared to a 50 % of current density reduction of Z8-Fe-P, the SA-3DNC only has a 25 % of current density loss (Figure 4c). In order to show the enhanced stability of SA-3DNC in a bigger picture, table S5 summarizes the performance of reported non-precious metal catalysts. Based on the current densities after the 20-hr durability tests, the performance retention of SA-3DNC is at least 10 % higher than that of the others, which suggests that introducing PAN during the preparation of catalysts may help to stabilize the active sites in advance.

In order to reveal the details during the ORR catalysis, we conducted an electrochemical impedance spectroscopy (EIS) study on the prepared samples. The EIS spectra were then fitted and deconvoluted into elements in an equivalent circuit model (Figure 3e, S18). From the equivalent circuit fitting results, the Fefree samples all show high electron transfer resistance for ORR, indicating that the N-doping sites in carbon are not efficient enough for ORR catalysis in acidic media (Table S4). After introducing the Fe-TPI into the precursors, the electron transfer resistance of ORR decreased immediately. PAN-Fe-P only shows a 1099 Ω of reaction resistance for ORR catalysis, and the resistance can be further reduced by using porous precursors during the preparation. Z8-Fe-P and SA-3DNC exhibit similar reaction resistance for ORR (668 Ω and 635 Ω, respectively), probably due to their similar surface areas and active sites. On the other hand, the EIS study also revealed the diffusional resistance of the samples. Compared with the PAN-based materials, the diffusional resistance of the ones prepared with ZIF-8 is at least 20 % lower, possibly because of the hierarchically porous structures (Table S4). Durability of the prepared catalysts was studied by accelerated degradation tests (ADT) with a continuous O2 supply (Figure 3f). The 20 % Pt/C shows a 125-mV negative shift of the Ehalf-wave, indicating a severe performance loss after 10,000 ADT cycles. Our Fe-Nx catalysts, on the contrary, show much better long-term durability with a shift of Ehalf-wave less than 60 mV. Among them, the shifts of Ehalf-wave of PAN-derived catalysts are 10 mV lower than that of the Z8-Fe-P, demonstrating that the introduction of PAN is able to substantially improve the durability of the ORR catalysts. However, by comparing the current density before and after ADT, there is still an approximate 30 % loss of Fe-Nx sites, meaning that the durability issue of Fe-based ORR catalysts has not been fully addressed by our approach.

In conclusions, PEMFC is one of the most promising candidates for electrochemical energy conversion and supply, but the high cost of Pt-based electrocatalysts prevents the large-scale application. In this study, we report a three-dimension, interconnected carbon sheet network with atomically dispersed Fe-N4 active sites named



SA-3DNC for ORR catalysis in acidic media. The PEMFC equipped with SA-3DNC shows enhanced performance at high current density and substantially improved durability in the durability test, which can be attributed to the hierarchically porous structure from the salt-assisted pyrolysis and the introduction of PAN during catalyst preparation, respectively. This study demonstrates a facile approach to prepare stable, single-atom ORR catalysts, and may serve as a cornerstone for the further development of Pt-free ORR catalysts.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxxxx.

AUTHOR INFORMATION Corresponding Author *D. Zhao. Email: [email protected]. Website: http://cheed.nus.edu.sg/stf/chezhao/home.html.

ORCID Yuhong Qian: 0000-0003-3820-8731 Erik Sarnello: 0000-0001-8357-375X Tao Li: 0000-0002-4913-4486 Dan Zhao: 0000-0002-4427-2150

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation Singapore (NRF2018-NRF-ANR007 POCEMON), the Ministry of Education - Singapore (MOE AcRF Tier 1 R-279-000-540-114), and the Agency for Science, Technology and Research (PSF 1521200078, IRG A1783c0015, and IAF-PP A1789a0024). The authors would like to acknowledge the use of Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, supported by the U.S. DOE under Contract no. DE-AC0206CH11357.

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