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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*,† Downloaded via 146.185.205.153 on July 20, 2019 at 04:28:03 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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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 Highway, DeKalb, Illinois 60115, United States ∥ 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, Illinois 60439, United States S Supporting Information *

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 SA3DNC 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 noblemetal-free electrocatalysts with atomically dispersed active sites for ORR catalysis in acidic media.

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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 have been

he 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 that are able to achieve this target, proton exchange membrane fuel cell (PEMFC) stands out because of 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 Pt-based 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 © 2019 American Chemical Society

Received: March 3, 2019 Accepted: May 1, 2019 Published: May 1, 2019 37

DOI: 10.1021/acsmaterialslett.9b00052 ACS Materials Lett. 2019, 1, 37−43

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combining salt-assisted 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. The morphology of SA-3DNC was studied by scanning electron microscope (SEM) and transmission electron microscope (TEM, Figure 1a and 1b). Compared with the cube

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 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 metalcontaining 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. 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 Scheme 1. Schematic Illustration of the Preparation of SA3DNC for ORR Catalysis in Acidic Media

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) lowresolution HAADF-STEM image, and (f) EELS-STEM elemental mapping of the selected area in panel e.

study,39 NaCl was used as a morphological modulator, leading to the formation of SA-3DNC 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 and 0.758 V). Synchrotron based Xray 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 SA-3DNC (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

morphology of ZIF-8 and the sphere morphology of pristine PAN (Figure S1), SA-3DNC has a morphology of continuous network with large pieces of carbon sheets after the saltassisted 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 lengthto-thickness ratio is beneficial for the exposure of active sites.40 Meanwhile, macropores with diameters ranging from 20 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 SA-3DNC in Figure 1c only has two rings from the polycrystalline carbon, confirming no metallic aggregate in the SA-3DNC.41 To reveal the Fe-containing sites, high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image was taken (Figure 1d). The isolated Fe-containing sites (circled 38

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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

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. To study the effect of each component in SA3DNC, 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 (Figures 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 crosslinking 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 SA3DNC 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 SA-3DNC. 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 ORRactive than the Fe-N4 sites.14 Raman spectra of the samples were recorded to investigate the properties of the carbon supports (Figure S6 and 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 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). Because of 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 (Tables S2 and S3). The N 1s XPS spectra were deconvoluted into four different N-containing species (Figure S8 and 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. On the basis of the previous studies, both types of the pyridinic N are favorable to enhancing the ORR catalytic

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.

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, 1.52, 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 SA-3DNC have a localized structure of FeN4C12 (Figures 2c and 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 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 PAN-based 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-FeP 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 39

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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 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 half-wave 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 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 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 SA-3DNC and Pt/C show numbers of electrons transferred close to 4, indicating high selectivity towards the 4e− 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 (Figures S16 and S17). These two results confirm that the Fe−N4 sites in SA-3DNC are highly selective to the 4-e− ORR process, which is highly

Table 1. BET Surface Area and ORR Activity of SA-3DNC and Other Prepared Materials ORR activityb

PAN-P Z8-P PAN-Z8-P PAN-Fe-P Z8-Fe-P SA-3DNC 20% Pt/C

BET surface area (m2 g−1)

pore volumea (cm3 g−1)

44 1086 141 465 1167 990

0.18 2.34 1.01 0.55 2.54 3.45

Eonsetc (V)

Ehalf‑waved (V)

jmaxe (mA cm−2)

0.748 0.826 0.795 0.843 0.869 0.906 0.876

0.412 0.561 0.476 0.636 0.744 0.729 0.758

2.42 5.62 4.88 4.92 5.48 6.13 5.76

a At P/P0 = 0.99. bIn 0.1 M HClO4. cDetermined when j = 0.05 mA cm−2. dThe potential when j = 1/2 jmax. eE = 0.17 V vs RHE.

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. 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 Arsaturated 0.1 M HClO4 suggests that all the samples have no red-ox behaviors or side reactions in the range of 0.2−1.2 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). Because of 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

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. 40

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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.

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 salt-assisted 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 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 nonnoble 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). To show the enhanced stability of SA-3DNC in a bigger picture, Table S5 summarizes the performance of reported non-precious metal catalysts. On the basis of the current densities after the 20-h 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 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 threedimension, interconnected carbon sheet network with atomically dispersed Fe−N4 active sites named SA-3DNC for ORR catalysis in acidic media. The PEMFC equipped with SA3DNC 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.

beneficial to the long-term durability of the cathodes in PEMFCs. 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 (Figures 3e and S18). From the equivalent circuit fitting results, the Fe-free 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 and 635 Ω, respectively), probably because of 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 PANderived 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. 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 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 41

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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/acsmaterialslett.9b00052. Preparation of SA-3DNC, details of material characterization, details of electrochemical experiments, SEM images of ZIF-8 and pristine PAN, AFM image and height profile of SA-3DNC, SEM and TEM images of AN-P, Z8-P, PAN-Z8-P, PAN-Fe-P, Z8-Fe-P, and SA3DNC, PXRD patterns, Raman spectra, and C and N 1s XPS spectra of SA-3DNC and other prepared materials, Fe K-edge EXAFS oscillations, Fe content in the SA3DNC determined by ICP-OES, N2 sorption isotherms and pore size distributions, CV plots of SA-3DNC and the other prepared materials, Tafel plots of PAN-P, Z8P, and PAN-Z8-P, LSV plots of SA-3DNCs and other prepared materials, Koutecky−Levich equations of SA3DNC and the other prepared materials, equivalent circuit model for fitting the EIS spectra of CN-LDH composites, power output of a single PEMFC, deconvolution results of the Raman and C and N 1s spectra, elemental analysis results and equivalent circuit fitting results of SA-3DNC and the other prepared materials, and performance summary of nonprecious metal ORR catalysts in PEMFCs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Website: http://cheed.nus.edu. sg/stf/chezhao/home.html. ORCID

Jianglan Shui: 0000-0002-5935-5733 Dan Zhao: 0000-0002-4427-2150 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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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DOI: 10.1021/acsmaterialslett.9b00052 ACS Materials Lett. 2019, 1, 37−43