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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6245−6252

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Metal−Organic Framework-Derived Co3ZnC/Co Embedded in Nitrogen-Doped Carbon Nanotube-Grafted Carbon Polyhedra as a High-Performance Electrocatalyst for Water Splitting Zhou Yu,†,‡ Yu Bai,*,‡,§ Shimin Zhang,† Yuxuan Liu,† Naiqing Zhang,‡ Guohua Wang,∥ Junhua Wei,∥ Qibing Wu,∥ and Kening Sun*,‡ †

School of Chemistry and Chemical Engineering and ‡Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, P. R. China § Advanced Research Institute for Multidisciplinary Science, Beijing Institute of Technology, Beijing 100081, P. R. China ∥ State Key Laboratory of Advanced Chemical Power Sources, Zunyi 563000, P. R. China S Supporting Information *

ABSTRACT: The development of efficient, low-cost, and stable electrocatalysts for overall water splitting is of great significance for energy conversion. Transition-metal carbides (TMCs) with high catalytic activity and low cost have attracted great interests. Nevertheless, utilizing an efficient catalyst for overall water splitting is still a challenging issue for TMCs. Herein, we report the synthesis of a highperformance electrocatalyst comprising Co3ZnC and Co nanoparticles embedded in a nitrogen-doped carbon nanotube-grafted carbon polyhedral (Co3ZnC/Co-NCCP) by the pyrolysis of bimetallic zeolitic imidazolate frameworks in a reductive atmosphere of Ar/H2. The Co3ZnC/Co-NCCP exhibits remarkable electrochemical activity in catalyzing both the oxygen evolution reaction and hydrogen evolution reaction, in terms of low overpotential and excellent stability. Furthermore, the Co3ZnC/Co-NCCP catalyst leads to a highly performed overall water splitting in the 1 M KOH electrolyte, delivering a current density of 10 mA cm−2 at a low applied external potential of 1.65 V and shows good stability without obvious deactivation after 10 h operation. The present strategy opens a new avenue to the design of efficient electrocatalysts in electrochemical applications. KEYWORDS: oxygen evolution reaction, transition-metal carbides, carbon nanotube, bimetallic zeolitic imidazolate frameworks, alkaline media

1. INTRODUCTION

been investigated extensively as OER and HER electrocatalysts. Among these materials, transition-metal carbides (TMCs) have attracted wide interest in the electrochemical applications because of their high electron conductivity, low cost, and chemical stability.24−26 Nevertheless, there are only a few reports focused on overall water-splitting properties for TMCbased catalysts. More importantly, TMCs generally suffer from high overpotential and low durability in the harsh electrolyte.27,28 Notably, experimental results have shown that the combination of TMCs and the heteroatom-doped (e.g., N, S, and P) carbon materials represents a unique strategy to resist corrosion in alkaline solution, providing more exposed active sites and high surface area to significantly enhance the electrocatalytic performance.29−31 Lan et al.32 reported Co6Mo6C2 coated by N-doped porous carbon and anchored

Hydrogen has been considered as a green renewable energy carrier, which can be a promising alternative for future energy strategies because of the burning of fossil fuel and environmental contamination.1,2 Electrochemical water splitting provides an attractive and sustainable approach to obtain clean hydrogen fuel.3,4 The water-splitting reaction involves oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), both of which require high-performance electrocatalysts to facilitate the charge-transfer kinetics and improve the energy-transfer efficiency.5−8 Although platinumbased material and precious metal oxides (e.g., RuO2 and IrO2) have been approved to be highly efficient HER and OER catalysts, their scarcity and high cost have hindered their largescale applications.9−13 Thus, the development of efficient, earth-abundant, and cost-effective catalytic materials is in urgent demand.14 Recently, transition-metal material-based catalysts, such as metal oxides,15,16 metal phosphides,17−19 metal chalcogenides,20,21 and metal carbides/nitrides,22,23 have © 2018 American Chemical Society

Received: October 24, 2017 Accepted: January 29, 2018 Published: January 29, 2018 6245

DOI: 10.1021/acsami.7b16130 ACS Appl. Mater. Interfaces 2018, 10, 6245−6252

Research Article

ACS Applied Materials & Interfaces Scheme 1. Illustration of the Synthesis Procedure for Co3ZnC/Co-NCCP

Figure 1. (a) XRD patterns and (b) EDS spectra of Co3ZnC/Co-NCCP.

on a N-doped reduced graphene oxide film (Co6Mo6C2/ NCRGO) as excellent OER electrocatalysts. Zou et al.33 synthesized ultrasmall molybdenum carbide nanoparticles embedded within nitrogen-rich carbon, which served as an excellent HER electrocatalyst. Besides, creating heterostructures is another way to improve the electrocatalytic performance owing to the synergistic effect of different materials which could promote charge separation and enhances the exposure of active sites.34−37 For example, Wang et al.38 synthesized CoOx@CN with outstanding HER and OER performances because of the synergistic interaction between metallic Co and cobalt oxide. The catalytic performance greatly depends on the synthetic method and morphological characteristics. Therefore, it is urgent to develop an efficient method for the preparation of heterostructure TMCs and heteroatom-doped carbon composites with simple step processes and well-defined architectures. Recently, metal−organic frameworks (MOFs) have received great interest because of their large surface area, uniform ordered pores, and easy synthesis without surfactant and tunable structure, which are promising for high-performance electrocatalytic progress.39−43 In particular, zeolitic imidazolate frameworks (ZIFs) as a class of MOF have been confirmed to be ideal precursors for the preparation of heteroatom-doped carbon species-based nanocomposites by in situ carbonization to enhance the relevant properties. Besides, compared with bulk materials, ZIF-derived nanocomposites have a strong ability to resist drastic structural collapse caused by hightemperature calcination and the specific surface area and pore volume can also benefit the catalytic reactions.44−46 For example, Muhler et al.47 developed a facile approach for the synthesis of Co@Co3O4 nanoparticles encapsulated in a Ndoped carbon nanotube (CNT)-grafted carbon polyhedron and exhibited their remarkable capability as a bifunctional oxygen electrocatalyst. Nonetheless, to the best of our knowledge, heterostructure TMCs and heteroatom-doped carbon composites derived from ZIFs as highly efficient electrocatalysts for overall water splitting have been rarely explored so far.

Herein, we report the synthesis of Co3ZnC and Co nanoparticles embedded in a nitrogen-doped CNT-grafted carbon polyhedral (Co3ZnC/Co-NCCP) by reductive carbonization of bimetallic Zn and Co bimetallic ZIFs (ZnCo-ZIFs). The resulting Co3ZnC/Co-NCCP exhibits excellent activity and stability for both the OER and HER processes. As expected, the Co3ZnC/Co-NCCP presents an outstanding performance toward overall water splitting under alkaline condition. The prominent activity would be ascribed to the unique structure and the chemical composition that provide large surface area, highly active species, and robust pore structure.

2. RESULTS AND DISCUSSION The synthetic process for Co3ZnC/Co nanoparticles embedded in the nitrogen-doped CNT-grafted carbon polyhedra (Co3ZnC/Co-NCCP) is described in Scheme 1. The ZnCoZIFs were obtained via a facile room-temperature coprecipitation reaction. Then, the as-prepared ZnCo-ZIFs were pyrolyzed in a reductive atmosphere of Ar/H2 to obtain Co3ZnC/CoNCCP. In the pyrolysis process, the cobalt and zinc ions from the ZnCo-ZIFs are converted into Co3ZnC and Co NPs and the organic ligands from the MOFs are converted into a nitrogen-doped CNT (NCNT)-grafted carbon polyhedral under the catalysis of Co nanoparticles in a reductive atmosphere.48 The interconnected CNTs within the polyhedron can significantly improve the electrical conductivity and structure stability of the resultant product. As shown in scanning electron microscopy (SEM) (Figure S1a,b), the as-synthesized ZnCo-ZIFs exhibit a well-defined rhombic polyhedral with sizes of about 500 nm and the exterior surface is smooth. The transmission electron microscopy (TEM) image (Figure S1c) further confirms that the ZnCoZIFs are solid. The measured powder X-ray diffraction (XRD) patterns of ZnCo-ZIFs show strong diffraction peaks at identical positions similar to the previously reported zeolitetype structure, confirming the good crystallinity and pure-phase zeolite-type structure (Figure S1d). After carbonization under 6246


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ACS Applied Materials & Interfaces

Figure 2. High-resolution XPS spectra of (a) Co 2p, (b) Zn 2p, (c) N 1s, and (d) C 1s of Co3ZnC/Co-NCCP.

Figure 3. (a,b) SEM images, (c−e) TEM images, (f) HRTEM image, and (g) EDX element mapping of Co3ZnC/Co-NCCP.

the Ar/H2 flow at 600 °C, the ZnCo-ZIFs were transformed into Co3ZnC/Co-NCCP. The corresponding XRD patterns reveal that the characteristic peaks of Co3ZnC/Co-NCCP can be well-indexed to Co3ZnC (PDF 29-0524), Co (PDF 150806), and C (PDF 41-1487) standards (Figure 1a). The energy-dispersive X-ray (EDX) spectroscopy (Figure 1b) further confirms the coexistence of Co, Zn, C, and N elements in the Co3ZnC/Co-NCCP (Figure 1b). X-ray photoelectron spectroscopy (XPS) was further employed to clarify the chemical composition of the Co3ZnC/Co-NCCP. Survey spectrum in Figure S2 shows the existence of Zn, Co, N, and C elements, which coincides with the XRD result. As shown in Figure 2a, the high-resolution Co 2p spectrum can be fitted into two major peaks (Co2p3/2 and

Co2p1/2) and two shakeup satellites (indicated as “Sat.”). The peaks at around 778.0 and 793.6 eV are corresponding to Co0, the two peaks at 779.5 and 794.8 eV can be ascribed to Co3+, whereas the peaks around 780.8 and 796.3 eV are assigned to Co2+.49 In Figure 2b, the Zn 2p spectrum reveals two evident peaks at 1021.1 and 1044.2 eV, which correspond to Zn 2p3/2 and Zn 2p1/2 of Zn species, respectively. The N 1s spectrum can be deconvolved into three peaks that are assigned to pyridinic nitrogen (398.0 eV), pyrrolic nitrogen (398.6 eV), and graphitic nitrogen (400.2 eV) in Figure 2c. It is worth noting that the N species doping not only enhances the electron transfer but also serves as an active site for electrochemical performance.50 As shown in Figure 2d, the fitted C 1s peak clearly indicates the peak located in 284.1 eV 6247


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Figure 4. (a) LSV curves and (b) Tafel plots of Co3ZnC/Co-NCCP, Co-NCCP, Co3ZnC/Co-NCP, and RuO2 in 1 M KOH. (c) LSV curves of the Co3ZnC/Co-NCCP obtained before and after 1000 potential cycles. (d) Time-dependent current density (i−t) curves of the Co3ZnC/Co-NCCP at an overpotential of 300 mV.

for typical C−C bond of sp3 carbon and the peaks at 284.9 and 288.1 eV with respect to C−N and CN species, respectively.51 All of the above results manifest that the Co3ZnC/Co-NCCP has been fabricated successfully by a simple annealing method. The structural and morphological characteristics of Co3ZnC/ Co-NCCP were further investigated by SEM and TEM. As shown in Figure 3a,b, the as-prepared Co3ZnC/Co-NCCP shows a uniform polyhedral morphology with a diameter of about 400 nm, which is well-inherited from the ZnCo-ZIFs. TEM further reveals that Co3ZnC/Co-NCCP possesses a porous structure, in which the Co3ZnC and Co nanoparticles with the size of a few nanometers are homogeneously rooted in the carbon framework. Note that a large number of NCNTs with the lengths of 30−60 nm are deeply embedded in the polyhedral surface (Figure 3c−e). It is commonly accepted that the cobalt nanoparticles are quickly formed at a high temperature and a reductive atmosphere, followed by the catalytic growth of NCNTs.48 The polyhedral shell with interconnected NCNTs is very critical for the enlarged electrochemical surface area and the enhanced charge transport. In high-resolution TEM (HRTEM) images (Figure 3f), the lattice fringes of 0.187 and 0.264 nm are consistent with the (200) plane of Co3ZnC and (220) plane of Co, respectively. Moreover, the Co3ZnC/Co nanoparticles are coated with ultrathin (∼2 nm) graphitic carbon layers. The elemental mapping results (Figure 3g) further confirm that the C, Co, Zn, and N elements are homogeneously distributed within the Co3ZnC/Co-NCCP. As shown in Figure S3a−c, the Co3ZnC/ Co nanoparticles were distributed in nitrogen-doped carbon polyhedra (denoted as Co3ZnC/Co-NCP) with a diameter of ∼400 nm; unlike the Co3ZnC/Co-NCCP, the Co3ZnC/CoNCP was synthesized by the conventional carbonization method using argon gas as inert atmosphere led to the CNT-

free polyhedra structure decorated with uniformly distributed Co3ZnC/Co nanoparticles. The measured XRD pattern of Co3ZnC/Co-NCP matches well with the Co3ZnC/Co-NCCP pattern (Figure S3d). In addition, the single-metal Co-ZIF precursor (ZIF-67) was synthesized (Figure S4a,b) and then directly carbonized at 600 °C for 2 h under a flow of Ar/H2 to obtain Co nanoparticles embedded in the nitrogen-doped CNT-grafted carbon polyhedral structure (denoted as CoNCCP). The Co-NCCP is similar to the morphology and the size observed on Co3ZnC/Co-NCCP (Figure S4c,d). The crystalline nature of the Co-NCCP is further confirmed by XRD in Figure S5. The peaks can index with standard metallic Co (PDF 15-0806) and C (PDF 41-1487). The structural features of Co3ZnC/Co-NCCP, Co-NCCP, and Co3ZnC/Co-NCP were also investigated by Raman spectroscopy (Figure S6). The intensity ratios of ID/IG value is determined to be 1.02 for the Co3ZnC/Co-NCCP, larger than those of the Co-NCCP (0.98) and Co3ZnC/Co-NCP (0.94). This indicates a higher graphitic degree and surface defect of the Co3ZnC/Co-NCCP, suggesting a higher amount of N doping into the carbon layer.52 The specific surface area and porosity of all of the samples were investigated by N2 adsorption−desorption measurement. As shown in Figure S7, all of the samples exhibit a typical type-IV isotherm, indicating the existence of the mesoporous structure. The specific surface areas of Co3ZnC/Co-NCCP, Co-NCCP, and Co3ZnC/CoNCP are determined to be 249.6, 185.1, and 151.3 m2·g−1, respectively, and pore sizes are about 4.5, 4.6, and 8.5 nm, respectively. The relative large surface area and unique porous structure of Co3ZnC/Co-NCCP tend to offer more accessible active sites, which is likely to enhance the electrocatalytic performance.53 The electrochemical activity of Co3ZnC/Co-NCCP toward OER was tested in a typical three-electrode system in 1 M 6248


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Figure 5. (a) LSV curves and (b) Tafel plots of Co3ZnC/Co-NCCP, Co-NCCP, Co3ZnC/Co-NCP, and Pt/C in 1 M KOH. (c) LSV curves of the Co3ZnC/Co-NCCP before and after 1000 potential cycles. (d) Time-dependent current density (i−t) curves over the Co3ZnC/Co-NCCP at the overpotential of 200 mV.

activity, stability is another significant standard to assess the electrocatalyst. As displayed in Figure 4c, after 1000 cycles, the linear sweep voltammetry (LSV) curves of the Co3ZnC/CoNCCP show a very small negative shift compared to that of the first cycle. Meanwhile, chronoamperometric measurement was evaluated at an overpotential of 300 mV. It can be seen in Figure 4d, the Co3ZnC/Co-NCCP can retain 94% of its initial current after 10 h testing, implying the high stability of the Co3ZnC/Co-NCCP in OER. Besides the excellent OER performance, Co3ZnC/Co-NCCP also shows desirable HER activity in 1.0 M KOH. It can be seen in Figure 5a, the bare GC is ineffective toward hydrogen evolution. The as-prepared Co3ZnC/Co-NCCP exhibits superior HER performance, with a low overpotential of 188 mV to reach a current density of 10 mA cm−2, which is much lower than that of Co-NCCP (265 mV) or Co3ZnC/Co-NCP (337 mV), and close to the commercial Pt/C catalyst (93 mV) at the same conditions. The corresponding Tafel slopes (Figure 5b) are 159, 171, and 59 mV dec−1 for Co-NCCP, Co3ZnC/ Co-NCP, and Pt/C catalyst, respectively. The Tafel slope of Co3ZnC/Co-NCCP is about 108 mV dec−1, implying a fast HER kinetics. The low overpotential and Tafel slope confirm its higher HER activity than those of many other previously reported HER catalysts (Table S2). After 1000 cycles in alkaline solution, the LSV polarization of Co3ZnC/Co-NCCP exhibits an indistinguishable shift in current flow compared with the initial one (Figure 5c), suggesting its outstanding stability. Meanwhile, chronoamperometric measurement was measured at the potential of 200 mV. After 10 h, the Co3ZnC/Co-NCCP product exhibits good long-term stability with 92.5% current retention (Figure 5d). We further investigated the effect of various carburization temperatures and Zn/Co molar ratios for ZnCo-ZIFs. The electrochemical results (Figure S10) con-

KOH. As shown in Figure 4a, glassy carbon (GC), Co-NCCP, Co3ZnC/Co-NCP, and commercial RuO2 are also performed under the same conditions. As expected, the bare GC electrode exhibits little activity toward OER. The Co3ZnC/Co-NCCP showed the highest activity for oxygen evolution, getting a current density of 10 mA cm−2 at the overpotential of 295 mV, which is lower than those of Co-NCCP (387 mV), Co3ZnC/ Co-NCP (400 mV), and even RuO2 (340 mV). Such an outstanding OER electrocatalytic activity is also manifested by the Tafel analysis (Figure 4b), where the Tafel slope value of Co3ZnC/Co-NCCP (70 mV dec−1) is much lower than those of Co-NCCP (90 mV dec−1), Co3ZnC/Co-NCP (98 mV dec−1), and RuO2 (82 mV dec−1), indicating its efficient reaction kinetics for OER. Meanwhile, the performance of Co3ZnC/Co-NCCP is superior to some of the previously reported noble metal-free electrocatalysts for OER in alkaline solution, which is summarized in Table S1. Furthermore, electrochemical impedance spectroscopy was used to identify the reaction kinetics during the electrochemical process. As shown in Figure S8, Co3ZnC/Co-NCCP possessed a chargetransfer resistance (Rct of 17.5 Ω), which is smaller than those for Co-NCCP (22.4 Ω) and Co3ZnC/Co-NCP (28.4 Ω), respectively. This suggested that the Co3ZnC/Co-NCCP has a much faster electron-transfer process. We also investigated the electrochemically active surface area of various catalysts by evaluating the double-layer capacitance (Cdl) using the cyclic voltammetry technique (Figure S9a−c). As shown in Figure S9d, the Cdl value of Co3ZnC/Co-NCCP is 10.4 mF cm−2, which is higher than those of Co-NCCP (6.2 mF cm−2) and Co3ZnC/Co-NCP (3.9 mF cm−2). This implies that the Co3ZnC/Co-NCCP features higher effective surface area and more active sites, which would lead to the enhanced catalytic performance. In addition to the remarkable electrocatalytic 6249


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Figure 6. (a) Polarization curves for water electrolysis of Co3ZnC/Co-NCCP||Co3ZnC/Co-NCCP, Pt/C||RuO2, and carbon cloth at a scan rate of 5 mV s−1 in 1 M KOH solution. (b) Time-dependent current density curve of Co3ZnC/Co-NCCP as a high-performance catalyst for the overall water splitting.

firmed that the best performance is achieved by the product obtained at 600 °C with the Zn/Co molar ratio of 1:2. On the basis of the aforementioned result, the superior catalytic activity of Co3ZnC/Co-NCCP is mainly ascribed to the following reasons: (1) The Co3ZnC/Co interfaces could be synergistically active sites for OER and HER catalysis. In addition, the N-doped carbon frameworks have been proved to be also HER and OER active.50 (2) The unique structure composed of N-doped CNT-grafted carbon polyhedral derived from ZnCo-ZIFs leads to a large electrochemical surface area and structural defects with more active sites for water splitting. (3) The uniformly distributed Co3ZnC/Co nanoparticles coated by graphene layers enable it to avoid the corrosion in the alkaline media. Because of the above reasons, Co3ZnC/CoNCCP is expected to provide large surface area, efficient electron transfer, and abundant catalytic sites for highperformance water splitting. On the basis of the above results, the Co3ZnC/Co-NCCP is demonstrated to be highly active and stable toward OER and HER in alkaline media. Therefore, we constructed a twoelectrode configuration for overall water splitting with 1 M KOH electrolyte, where Co3ZnC/Co-NCCP coated onto a carbon cloth was used as both the anode and cathode. The reference system using Pt/C-RuO2 and carbon cloth as a cathode and an anode was also tested. As shown in Figure 6a, Co3ZnC/Co-NCCP requires an applied voltage of 1.65 V to achieve 10 mA cm−2. This result is very close to 1.62 V of the Pt/C-RuO2 system and much lower than 1.95 V for the bare carbon cloth. The obvious H2 and O2 bubbles from both electrodes can be clearly observed in the inset of Figure 6b. The long-term durability of Co3ZnC/Co-NCCP for overall water splitting was performed in 1 M KOH solution. The chronoamperometric response (Figure 6b) shows that the current density can be well-maintained without obvious deactivation after 10 h. As verified by TEM image (Figure S11), the rhombic polyhedral shape with small embedded CNTs can be still obtained, revealing excellent structure robust after 10 h of electrolysis. All of these results demonstrate the high electrochemical stability of the Co3ZnC/Co-NCCP catalyst.

highly efficient and robust catalyst for both the HER and OER in alkaline media, affording a current density of 10 mA cm−2 at overpotentials of 295 mV for the OER and 188 mV for the HER. Furthermore, Co3ZnC/Co-NCCP exhibits overall water splitting at 10 mA cm−2 with only 1.65 V, which is close to 1.62 V of Pt/C-RuO2, along with excellent durability. The outstanding electrocatalytic performance of Co3ZnC/CoNCCP is considered to be a promising candidate to replace the conventional noble metal in water splitting devices, and this strategy can be extended to fabricate other nanomaterials for energy fields.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16130. Further details about experimental section, XRD, SEM, TEM, Brunauer−Emmett−Teller, Raman, and electrochemical data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.B.). *E-mail: [email protected] (K.S.). ORCID

Yu Bai: 0000-0003-2617-7536 Naiqing Zhang: 0000-0002-9528-9673 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the National Natural Science Foundation of China (grant no. 51203036) and the Scientific Research Foundation of Beijing Institute of Technology.

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3. CONCLUSIONS In summary, we have successfully prepared an efficient electrocatalyst (Co3ZnC/Co-NCCP) by a simple and easily scalable one-pot thermal treatment of ZnCo-ZIFs. The prepared porous Co3ZnC/Co-NCCP catalyst performed as a

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Research Article


Information. The corrected file was reposted on February 9, 2018.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on February 8, 2018, with Figures S8, S10, and S11 missing from the Supporting 6252


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