Tailoring Electronic Structure of Atomically Dispersed Metal–N3S1

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Tailoring Electronic Structure of Atomically Dispersed Metal−N3S1 Active Sites for Highly Efficient Oxygen Reduction Catalysis Pengzuo Chen,†,§ Nan Zhang,‡,§ Tianpei Zhou,† Yun Tong,† Wensheng Yan,‡ Wangsheng Chu,*,‡ Changzheng Wu,*,† and Yi Xie† Downloaded via 95.85.68.213 on July 18, 2019 at 14:16:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), and CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei 230026, PR China ‡ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: Rational design of oxygen electrocatalysts with high catalytic activity and long-term durability is currently a severe challenge for rechargeable Zn−air batteries. Here, we highlighted an electronic structure engineering on Cu-based catalyst with atomically dispersed Cu−N3S1 active sites, representing as highly active electrode material for oxygen reduction. The structurallynew Cu−N3S1 species shows unique electron interactions with both phosphorus atoms/carbon support, which not only activate the electron transfer around the Cu−N3S1 sites but also enhance the interaction with oxygenated species, resulting in a promoting reaction kinetics process. As expected, this catalyst exhibits superior catalytic activity for oxygen reduction and evolution. A rechargeable flexible solid Zn−air battery based on this catalyst behaves superior performance of a high open-circuit voltage (1.41 V), a large power density (138.2 mW/cm2), and a small charge/discharge voltage gap (0.72 V at 2.0 mA cm−2) even under different bending angles. Specifically, this strategy is general and can be extended to other metal single atom catalyst systems (such as Fe−N3S1 species). This work will open a new avenue to design structurally-new single atom catalysts for energy-related field.

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there is an urgent demand to develop alternative, highly efficient, and durable nonprecious metal catalysts for the air cathode. Recently, a wide range of non-noble metal catalysts have been pursued and show promising ORR catalytic activity, including metal nanoparticles, metallic compounds, metal-free heteroatoms doped carbon materials and metal−nitrogen− carbon complexes (M−N−C).13−17 Density functional theory (DFT) calculations proved that M−N−C active sites possess superior ORR activity due to low free energy for O2 adsorption and O−O bond breaking.18−20 Experimental achievements demonstrated that atomically dispersed M−Nx sites in highly conductive carbon framework is effective for enhancing the catalytic activity because of increased active sites and high atom utilization efficiency.21−24 Up to now, most of research work has been devoted to improving catalytic activity based on traditional M−Nx sites. There is an open question regarding

lexible electronic devices have been developed rapidly to revolutionize many industries ranging from traditional electronics, healthcare, and aerospace in recent years.1,2 Compared with the traditional rigid format, the flexible devices exhibit unique advantages, such as being bendable, stretched, or potentially wearable and still maintaining their functions.3−5 To fulfill the demand of flexible electronic devices, tremendous efforts have been made to develop flexible energy storage systems, such as mental−air batteries, supercapacitors, and fuel cells.6−8 Among various energy storage technologies, Zn−air battery has been recognized as a potential energy source in flexible electronic devices because of its high theoretical energy density (1086 Wh kg−1), low cost, environmental friendliness, and safety.6,9 However, the output voltage and energy capacity of Zn−air battery are largely hindered by activity and stability of the catalyst during the cathodic oxygen reduction reaction (ORR) process.9,10 Platinum-based materials and its alloy shows high ORR activity and have been identified as the promising catalysts, but the high cost, scarcity, and limited stability largely obstruct their further practical implementation.11,12 Therefore, © 2019 American Chemical Society

Received: April 4, 2019 Accepted: May 28, 2019 Published: May 28, 2019 139

DOI: 10.1021/acsmaterialslett.9b00094 ACS Materials Lett. 2019, 1, 139−146

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Cite This: ACS Materials Lett. 2019, 1, 139−146

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ACS Materials Letters Scheme 1. Schematic Representation of the Synthesis for Cu-SA/NPSC

Figure 1. (a) XRD pattern and (b) TEM image of the Cu-SA/NPSC sample. (c) HAADF STEM image of Cu-SA/NPSC showing welldispersed Cu atoms in the carbon layers. (d) XANES spectra at the Cu K-edge of Cu-SA/NPSC, CuO sample, and Cu foil. (e) Fourier transforms (FTs) of Cu K-edge EXAFS oscillations of Cu-SA/NPSC, CuO sample, and Cu foil (without phase correction). (f) Comparison between the experimental data and the best fit in R space for Cu-SA/NPSC sample. Inset: Schematic model proposed for Cu-SA/NPSC (without phase correction).

the understanding of the in-depth mechanism based on the clear relationship between electronic structure and catalytic activity. Also, the discovery of structurally new M−Nx single atomic systems is essential for systematic study. Therefore, rationally design structurally new single atomic species would pave a promising way to understand reaction mechanism for electrocatalysis.

Herein, we put forward a novel electronic structure engineering on atomically dispersed Cu−N3S1 active sites (denoted as Cu-SA/NPSC) via a unique organic polymer assisted pyrolysis strategy, representing as highly active electrocatalyst for oxygen reduction for the first time. This Cu−N3S1 single atomic system behaves strong electron interactions with adjacent heteroatoms. Benefiting from the highly active Cu−N3S1 sites, strong electron transfer ability 140

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Figure 2. (a) C K-edge, (b) N K-edge, (c) S L-edge, and (d) P L-edge XANES spectra of Cu-SA/NPSC and NPSC samples.

the elements N, P, S, and Cu are homogeneously distributed on the surface of carbon nanotube (Figure S5). The chemical composition and elemental states of Cu-SA/ NPSC were investigated by X-ray photoelectron spectroscopic (XPS). As shown in Figure S7, the core-level scan spectra of N 1s were divided into three peaks, which are attributed to pyridinic-N, graphitic-N, and quaternary N+−O− species.26 Compared with nitrogen-, sulfur-, and phosphorus-doped carbon (denoted as NPSC), pyridinic-N of Cu-SA/NPSC shifted to higher binding energy, while the peaks assigned to graphitic-N (401.3 eV) and quaternary N+−O− (404.5 V) remained unchanged. This result demonstrated that Cu−N bonds were formed between pyridinic-N and Cu atoms.27 The high resolution S 2p spectrum of Cu-SA/NPSC in Figure S9 highlighted two peaks at about 164.0 and 165.2 eV, ascribed to the S 2p3/2 and S 2p1/2 peaks of thiophene-like structure (C− S−C) in the two hybrids.28,29 Furthermore, the P 2p spectra of Cu-SA/NPSC and NPSC exhibited two peaks at 132.9 and 134.0 eV, corresponding to P−C and P−O bonds, respectively (Figure S8).30 The content of Cu in Cu-SA/NPSC determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) was 1.71 at%. According to the Cu 2p and Cu LMM spectra (Figure S6), the Cu species in Cu-SA/NPSC match well with the Cu+, which further suggested the existence of Cu−N−C configuration.31 Moreover, synchrotron X-ray absorption fine structure (XAFS) measurements were employed to verify the local geometry around implanted Cu atoms. Figure 1d shows the Cu K-edge absorption energy of Cu-SA/NPSC lies between that of Cu foil and CuO, indicating that Cu atoms behave a positive valence state between 0 and +2. Moreover, the prepared CuSA/NPSC catalyst exhibited only one peak at about 1.58 Å (no phase correction) and no obvious Cu−Cu interaction peak or other higher shell peaks were detected (Figure 1e), which implied that Cu atoms existed as the isolated single-atomic form. The EXAFS result of the best fit (Figures 1f and S10 and

and preferred O2 adsorption energy, this system exhibits obviously enhanced ORR catalytic activity. When assembled into a flexible Zn−air battery, a high open-circuit voltage (1.41 V), a large power density (138.2 mW/cm2) and a small charge/discharge voltage gap (0.72 V at 2.0 mA cm−2) can be achieved. Notably, the constructed Zn−air battery displays stable discharge voltage as high as 1.22 V at 2.0 mA cm−2 under different belting angles, indicating its promising application in flexible electronics. Moreover, a structurallynew Fe−N3S1 single atom material were also developed for ORR by using this approach. This work paves a new route for developing advanced electrocatalysts for metal−air battery cathode. The synthesis of Cu-SA/NPSC consisted of two steps and was illustrated in Scheme 1. Firstly, a highly cross-linked poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) with Cu salt was coated on the surface of CNTs to form an core-shell structure (denoted as CNTs@Cu-PZS). Then the prepared CNTs@Cu-PZS was pyrolysis under Ar atmosphere and followed by acid leaching to obtain Cu-SA/NPSC catalyst. Figure 1a shows the X-ray diffraction (XRD) patterns of the Cu-SA/NPSC material. Two broad peaks at ∼26° and ∼44° were observed, which can be ascribed to (002) and (100) planes of graphite carbon.25 It is noteworthy that there was no other diffraction peak related to Cu-based compounds, suggesting that Cu species are highly dispersed in carbon framework. This result can be further confirmed by transmission electron microscopy (TEM) and high-angle annular dark field scanning transmission electron microscope (HAADF STEM) images. As shown in Figure 1b, TEM image of Cu-SA/ NPSC shows a hierarchical network structure composed of multilayered carbon nanotubes, and no nanoparticles can be found. A number of atomically dispersed bright spots on the carbon layer were observed in the HAADF STEM image, which can be assigned to the Cu atoms (Figure 1c). The energy-dispersive spectroscopy (EDS) mapping discloses that 141

DOI: 10.1021/acsmaterialslett.9b00094 ACS Materials Lett. 2019, 1, 139−146

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ACS Materials Letters

Figure 3. (a) ORR polarization curves and (b) corresponding Tafel plots of Cu-SA/NPSC, Cu-SA/NC, Cu-NP/NPSC, NPSC samples, and commercial Pt/C under a rotating rate of 1600 rpm. (c) ORR polarization curves of Cu-SA/NPSC at different rotating rates. (d) Nyquist plots of the impedance for Cu-SA/NPSC, NPSC, and Pt/C. (e) CV curves of Cu-SA/NPSC in oxygen saturated 0.1 M KOH with and without 1.0 M CH3OH. (f) ORR polarization curves of Cu-SA/NPSC before and after 10 000 CV cycles.

Table S2) indicates the copper has been bonded by 3.0 ± 0.4 nitrogen atoms at 1.96 Å and 1.0 ± 0.2 sulfur atom at 2.03 Å. And the Cu−N bonds show much lower disorder of 0.0035 Å2 than Cu−S bonds, which gives a disorder degree of 0.0080 Å2. On the basis of the HAADF-STEM and XAFS results mentioned above, it could be concluded that the Cu atoms of Cu-SA/NPSC are atomically dispersed in the carbon-based framework and present as a local configuration of Cu−N3S1. To examine the type of the chemical bonds and charge transfers related to Cu sites, C K-edge, N K-edge, P L-edge, S L-edge, and Cu L-edge XANES spectra of Cu-SA/NPSC were measured. As shown in Figure 2a, the C K-edge spectrum is dominated by the two typical features arising from the carboatomic rings.32 One is at about 284.1 eV, corresponding to the CC π* transition. The other is at about 291.5 eV, the C−C σ* transition. And a small peak between the two transitions was detected at about 287.0 eV for both of the samples, which should be attributed to the C−N−Cu/C−N chemical interaction induced in the carbon-based framework. Compared with the Cu-free sample, Cu-SA/NPSC exhibits decreased intensity in πCC * compared to NPSC, which

suggests that the unfilled state of the aromatic ring was doped by exotic electrons. The increased intensity of Cu-SA/ NPSC in π*C−N−Cu/C−N confirms that additional strong chemical bonds from in the carbon support. This change reveals a strong interaction between Cu atoms and carbon support bridged by the nitrogen, that is, Cu−N−C bonds in Cu-SA/NPSC.33 Moreover, the reduced intensity of CC π* transition revealed that the charger transferred from Cu atoms to carbon support and the electrical conductivity could be largely improved in the whole material system. Figure 2b shows the N K-edge XANES of Cu-SA/NPSC and NPSC, three obvious peaks were observed at ∼395.9, ∼399.0, and ∼405.2 eV, arising from π* transition of C−N−C, N−3C, and σ* transition of C−N, respectively.33 After Cu doping, Cu-SA/ NPSC material behaved lower absorption intensity in the three resonances. This result further suggested the formation of Cu− N−C chemical bonding, which were associated with a charge transfer from the metal atoms into N−C sites. Moreover, the S L-edge XANES presents obvious resonance for both of the samples in the region of 162−168 eV, which was a typical feature for the formation of C−S−C bonds 142

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Figure 4. (a) Comparison between the experimental data and the best fit in R space for Fe-SA/NPSC (without phase correction). (b) ORR polarization curves of Cu-SA/NPSC, Fe-SA/NPSC, and commercial Pt/C in oxygen saturated 0.1 M KOH. (c) Fit results of EXAFS for FeSA/NPSC by the IFEFFIT code.

(Figure 2c).34 However, the Cu-doped sample displays a big difference of the XANES features, in contrast to the Cu-free one, either for the shape of C−S−C resonance or for the absorption intensity. And S L-edge of the single-atom catalyst shows typical metal sulfide characteristic in the energy range of 168−185 eV. This result reveals a chemical interaction occurred between Cu and S, ascribed to Cu−S bonds. Meanwhile, S L-edge shows greater change than N and C Kedge spectra after Cu atoms introduced, as it may be due to the higher electronegativity of sulfur than nitrogen and carbon. In Figure 2d, P L-edge of Cu-SA/NPSC showed enhanced absorption intensity in both peak a (∼136.5 eV) and peak b (∼142.3 eV). This change of the P L-edge XANES features should be attributed to the electron transfer from phosphorus to Cu−N3S1 species and suggests the formation of electron interaction between Cu−N3S1 sites and P heteroatoms. These findings on the chemical configuration of atomically dispersed copper are verified by the Cu L-edge XANES spectrum (Figure S11). Cu-SA/NPSC exhibited two typical characteristic peaks at 921.7 and 941.2 eV, corresponding to the L3 and L2 edge. A sharp shoulder peak at Cu L3-edge was observed, which indicates Cu exists as a strong covalence state. This sharing 3delectrons state demonstrates that isolated single Cu site really plays the key role as the charge transfer center in the prepared catalyst. Thus, all the results above demonstrates that singleatomic Cu sites possess a Cu−N3S1 structure and would favor a superior ORR catalytic performance. Oxygen reduction activity were evaluated using a rotating disk electrode (RDE) measurement in O2-saturated 0.1 M KOH. Cu−N4 structure (denoted as Cu-SA/NC) was also synthesized as the control sample (Figure S24). As shown in Figures 3a, Cu-SA/NPSC exhibited the best electrocatalytic activity in terms of the most positive half-wave potential (∼0.84 V) under a rotating rate of 1600 rpm. Cu-SA/NC exhibit a half-wave potential of 0.80 V, which is 40 mV lower than that of Cu-SA/NPSC. The higher ORR catalytic activity of Cu-SA/NPSC demonstrate that replacement of N by S atom is an effective method to improve ORR performance for

CuN4 catalyst. Sulfur behaves higher electronegativity than that of nitrogen and carbon and Cu−S bond would endow a unique electron structure for Cu−N3S1 sites. According to the XANES analysis, Cu exists as a strong covalence state and exhibit sharing 3d-electrons state by the introduction of S atom. The strong electron interactions with adjacent S heteroatoms enable CuN3S1 to behave strong electron transfer ability and preferred O2 adsorption energy. In addition, compared to CuNP/NPSC (63 mV dec−1), Cu-SA/NC (67 mV dec−1) and NPSC (82 mV dec−1), Cu-SA/NPSC showed the smallest tafel slope of 57 mV dec−1, indicating its fast kinetics for ORR (Figure 3b). These results demonstrate structurally-new CuN3S1 sites play a critical role in improving ORR activity of NPSC. Particularly, Cu-SA/NPSC performed lower Tafel slope and superior half-wave potential to commercial 20 wt % Pt/C (80 mV dec−1 and ∼0.82 V, respectively), implying the outstanding ORR performance of Cu-SA/NPSC. Electrocatalytic kinetics of Cu-SA/NPSC was further examined by collecting the LSV curves at different rotation rates. As shown in Figure 3c, current density was increased with increasing rotation rates because of the shortened O2 diffusion distance. The electron transfer number (n) can be calculated from the corresponding Koutecky−Levich (K−L) plots (Figure S13). As a result, the electron transfer number of Cu-SA/NPSC was 3.71 at 0.7 V (vs RHE), indicative of a dominant four-electron oxygen reduction pathway, which reflected the superior selectivity of O2 reduction. Moreover, the superior ORR performance of Cu-SA/NPSC was also reflected by electrochemical impedance spectroscopy. As shown in Figure 3d, the substantially smaller semicircle in the medium frequency region reveals a decreased charge transfer resistance for Cu-SA/NPSC compared with NPSC, which indicates its outstanding electron/mass transfer ability during ORR process. The methanol tolerance of the Cu-SA/NPSC and Pt/C were also tested. A drawback in using Pt catalysts at the air cathode in metal−air battery and fuel cell is the declining activity caused by methanol poisoning. In Figure 3e, it was observed 143

DOI: 10.1021/acsmaterialslett.9b00094 ACS Materials Lett. 2019, 1, 139−146

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Figure 5. (a) Schematic illustration of flexible all-solid-state Zn−air battery. (b) Open circuit plot of Cu-SA/NPSC in a flexible Zn−air battery. Insert: Photograph of the all-solid-state Zn−air battery. (c) Charge and discharge polarization curves. (d) Discharge polarization curves and power density plots. (e) Galvanostatic discharge−charge cycling curves of flexible Zn−air battery based on Cu-SA/NPSC at 2.0 mA cm−2 under different bending angles.

behaves negligible degeneration of overpotential and current density after 1000 CV cycles, suggesting the good stability of Cu-SA/NPSC for oxygen evolution catalysis. Therefore, CuSA/NPSC would be an efficient and stable catalyst for Zn−air battery system. Fe−N3S1 structure (denoted as Fe-SA/NPSC) can also be synthesized by replacing Cu salt to Fe salt in Cu-SA/NPSC synthetic process. XAFS was also employed to verify the local geometry of Fe atoms. As shown in Figure 4a, the Fourier transform curve exhibit only one peak at 1.54 Å without phase correction and no obvious metal−metal interaction was observed. This result demonstrate that Fe atoms are atomically dispersed in the NPSC framework. And the fitting result (Figure 4a and c) suggests that single Fe atoms are coordinated with three N atoms at 2.01 Å and one S atom at 2.12 Å, which exhibit similar structure with Cu-SA/NPSC. Thus the synthetic method is universal to other metal single atom materials. The ORR catalytic activity of Fe-SA/NPSC was also evaluated by RDE measurement. As shown in Figure 4b, Fe-SA/NPSC shows a half-wave potential of 0.85 V, which is higher than that of Cu-SA/NPSC and commercial Pt/C.

that there is no obvious change after 1 M CH3OH being injected into the electrolyte. As a contrast, an inverse peak (at 1.02 V vs RHE) for CH3OH oxidation appeared instead of the cathodic peak of O2 reduction for Pt/C (Figure S18). This result suggests the superior methanol tolerance for Cu-SA/ NPSC. Moreover, the durability of Cu-SA/NPSC was also evaluated. As shown in Figure 3f, the Cu-SA/NPSC catalyst displayed negligible degeneration of current density after 10 000 CV cycles, and the slight shift of half-wave potential (∼6 mV) demonstrates the high stability of Cu-SA/NPSC for the ORR process. The OER performance of these electrocatalysts were tested in 0.1 M KOH solution. As shown in Figure S19, Cu-SA/ NPSC exhibit an overpotential of 430 mV at 10 mA cm−2, which outperforms Cu-NP/NPSC (540 mV), Cu-SA/NC (640 mV), and NPSC (640 mV). The superior OER performance of Cu-SA/NPSC demonstrates that atomically dispersed CuN3S1 sites behave high intrinsic catalytic activity and play an important role in improving OER performance of NPSC. Moreover, the stability of Cu-SA/NPSC was also evaluated. Figure S20 shows the OER stability performance of Cu-SA/NPSC. It was observed that the Cu-SA/NPSC catalyst 144

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ACS Materials Letters Author Contributions

The superior ORR performance of Fe-SA/NPSC is probably due to the higher intrinsic activity of Fe. Inspired by the outstanding electrochemical activity presented above and stimulated by the extensive attention of portable and wearable electronic devices, a rechargeable flexible solid Zn−air battery is fabricated by using the CuSA/NPSC as air-cathode (Figure 5a). As shown in Figure 5b, the fabricated battery exhibited a stable open circuit voltage as high as 1.41 V. In addition, Figure 5c shows the charge and discharge polarization curves of the all-solid-state rechargeable Zn−air batteries. Compared with Pt/C air electrode, a slightly lower charge−discharge voltage gap was observed for Cu-SA/ NPSC catalyst, suggesting its superior rechargeability. The power densities were calculated from the discharge polarization curves. As shown in Figure 5d, the maximum power density during the discharge process of Cu-SA/NPSC cathode was 138.2 mW/cm2, higher than Pt/C of 121.4 mW/cm2, which demonstrates its excellent ORR performance. Figure 5e exhibits galvanostatic discharge−charge cycling curves of flexible Zn−air battery based on Cu-SA/NPSC at 2.0 mA cm−2 under different bending angles. The initial discharging voltage of Cu-SA/NPSC-based solid Zn−air battery was 1.22 V and charging voltage is 1.94 V, respectively. The flat charge/ discharge voltage plateaus demonstrate the prominent activity for the solid Zn−air battery. Moreover, the charge/discharge voltage gap almost remained unchanged after 15 cycles under different belting angles, which further demonstrates that the outstanding durability of Cu-SA/NPSC catalyst applied in flexible solid-state battery. In conclusion, we developed a structurally new copper-based electrode material with abundant atomically dispersed Cu− N3S1 active sites for the first time. The XPS and XAS spectra demonstrated the successful replacement of traditional N sites by S heteroatoms, and XANES spectra proved the strong interaction with P and C atoms. As a result, Cu single sites can work as a charge transfer center and thereby enhancing the interaction with oxygenated species. Finally, the as-obtained catalyst exhibited Pt-like ORR performance, high product selectivity of O2 reduction and superior stability. Moreover, this Cu−N3S1 catalyst also achieved excellent performance as air cathode for flexible all solid-state Zn−air battery. This work provides a conceptual new strategy to design high-active electrocatalysts for metal−air battery.

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P.C. and N.Z. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2015CB932302), Natural Science Foundation of China (No. 21893075011, 91745113, 11621063), National Program for Support of Top-notch Young Professionals, and the Fundamental Research Funds for the Central Universities (No. WK 2060190084). We also appreciate the support from the Major/Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology. This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmaterialslett.9b00094.

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REFERENCES

(1) El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Laser scribing of high-performance and flexible graphene-based electrochemical capacitors. Science 2012, 335, 1326−1330. (2) Han, T.-H.; Kim, H.; Kwon, S.-J.; Lee, T.-W. Graphene-based flexible electronic devices. Mater. Sci. Eng., R 2017, 118, 1−43. (3) Kim, S. J.; Choi, K.; Lee, B.; Kim, Y.; Hong, B. H. Materials for flexible, stretchable electronics: graphene and 2D materials. Annu. Rev. Mater. Res. 2015, 45, 63−84. (4) Chortos, A.; Liu, J.; Bao, Z. Pursuing prosthetic electronic skin. Nat. Mater. 2016, 15, 937. (5) Yang, B.; Bi, W.; Wan, Y.; Li, X.; Huang, M.; Yuan, R.; Ju, H.; Chu, W.; Wu, X.; He, L. Surface etching induced ultrathin sandwich structure realizing enhanced photocatalytic activity. Science China Chem. 2018, 61 (12), 1572−1580. (6) Lee, K.-S.; Spendelow, J. S.; Choe, Y.-K.; Fujimoto, C.; Kim, Y. S. An operationally flexible fuel cell based on quaternary ammoniumbiphosphate ion pairs. Nat. Energy 2016, 1, 16120. (7) Shao, Y.; El-Kady, M. F.; Wang, L. J.; Zhang, Q.; Li, Y.; Wang, H.; Mousavi, M. F.; Kaner, R. B. Graphene-based materials for flexible supercapacitors. Chem. Soc. Rev. 2015, 44, 3639−3665. (8) Meng, F.; Zhong, H.; Bao, D.; Yan, J.; Zhang, X. In situ coupling of strung Co4N and intertwined N−C fibers toward free-standing bifunctional cathode for robust, efficient, and flexible Zn−air batteries. J. Am. Chem. Soc. 2016, 138, 10226−10231. (9) Nam, G.; Park, J.; Choi, M.; Oh, P.; Park, S.; Kim, M. G.; Park, N.; Cho, J.; Lee, J.-S. Carbon-coated core−shell Fe−Cu nanoparticles as highly active and durable electrocatalysts for a Zn−air battery. ACS Nano 2015, 9, 6493−6501. (10) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical energy storage for the grid: a battery of choices. Science 2011, 334, 928−935. (11) Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Recent advances in electrocatalysts for oxygen reduction reaction. Chem. Rev. 2016, 116, 3594−3657. (12) Jiang, K.; Zhao, D.; Guo, S.; Zhang, X.; Zhu, X.; Guo, J.; Lu, G.; Huang, X. Efficient oxygen reduction catalysis by subnanometer Pt alloy nanowires. Sci. Adv. 2017, 3, No. e1601705. (13) Yu, H.; Shang, L.; Bian, T.; Shi, R.; Waterhouse, G. I.; Zhao, Y.; Zhou, C.; Wu, L. Z.; Tung, C. H.; Zhang, T. Nitrogen-Doped Porous Carbon Nanosheets Templated from g-C3N4 as Metal-Free Electrocatalysts for Efficient Oxygen Reduction Reaction. Adv. Mater. 2016, 28, 5080−5086. (14) Shen, H.; Gracia-Espino, E.; Ma, J.; Zang, K.; Luo, J.; Wang, L.; Gao, S.; Mamat, X.; Hu, G.; Wagberg, T.; Guo, S. Synergistic Effects between Atomically Dispersed Fe-N-C and C-S-C for the Oxygen Reduction Reaction in Acidic Media. Angew. Chem., Int. Ed. 2017, 56, 13800−13804. (15) Yuan, K.; Zhuang, X.; Fu, H.; Brunklaus, G.; Forster, M.; Chen, Y.; Feng, X.; Scherf, U. Two-Dimensional Core-Shelled Porous

Synthetic procedures, structure characterizations, electrochemical measurements, and additional figures and tables (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wensheng Yan: 0000-0001-6297-4589 Changzheng Wu: 0000-0002-4416-6358 Yi Xie: 0000-0002-1416-5557 145

DOI: 10.1021/acsmaterialslett.9b00094 ACS Materials Lett. 2019, 1, 139−146

Letter

ACS Materials Letters Hybrids as Highly Efficient Catalysts for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2016, 55, 6858−6863. (16) Zhang, H.; Zhou, W.; Chen, T.; Guan, B. Y.; Li, Z.; Lou, X. W. A modular strategy for decorating isolated cobalt atoms into multichannel carbon matrix for electrocatalytic oxygen reduction. Energy Environ. Sci. 2018, 11, 1980−1984. (17) Yang, L.; Lv, Y.; Cao, D. Co,N-codoped nanotube/graphene 1D/2D heterostructure for efficient oxygen reduction and hydrogen evolution reactions. J. Mater. Chem. A 2018, 6, 3926−3932. (18) Guo, Y.; Yuan, P.; Zhang, J.; Hu, Y.; Amiinu, I. S.; Wang, X.; Zhou, J.; Xia, H.; Song, Z.; Xu, Q.; Mu, S. Carbon Nanosheets Containing Discrete Co-Nx-By-C Active Sites for Efficient Oxygen Electrocatalysis and Rechargeable Zn−Air Batteries. ACS Nano 2018, 12, 1894−1901. (19) Choi, C. H.; Choi, W. S.; Kasian, O.; Mechler, A. K.; Sougrati, M. T.; Brüller, S.; Strickland, K.; Jia, Q.; Mukerjee, S.; Mayrhofer, K. J.; Jaouen, F. Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts. Angew. Chem., Int. Ed. 2017, 56, 8809−8812. (20) Wang, A.; Li, J.; Zhang, T. Heterogeneous single-atom catalysis. Nat. Rev. Chem. 2018, 2, 65−81. (21) Zhang, Z.; Sun, J.; Wang, F.; Dai, L. Efficient Oxygen Reduction Reaction (ORR) Catalysts Based on Single Iron Atoms Dispersed on a Hierarchically Structured Porous Carbon Framework. Angew. Chem. 2018, 130, 9176. (22) Yang, L.; Cheng, D.; Xu, H.; Zeng, X.; Wan, X.; Shui, J.; Xiang, Z.; Cao, D. Unveiling the high-activity origin of single-atom iron catalysts for oxygen reduction reaction. P. Natl. Acad. Sci. USA 2018, 201800771. (23) Fei, H.; Dong, J.; Feng, Y.; Allen, C. S.; Wan, C.; Volosskiy, B.; Li, M.; Zhao, Z.; Wang, Y.; Sun, H.; An, P.; Chen, W.; Guo, Z.; Lee, C.; Chen, D.; Shakir, I.; Liu, M.; Hu, T.; Li, Y.; Kirkland, A. I.; Duan, X.; Huang, Y. General synthesis and definitive structural identification of MN4C4 single-atom catalysts with tunable electrocatalytic activities. Nat. Catal. 2018, 1, 63−72. (24) Zhang, H.; An, P.; Zhou, W.; Guan, B. Y.; Zhang, P.; Dong, J.; Lou, X. W. Dynamic traction of lattice-confined platinum atoms into mesoporous carbon matrix for hydrogen evolution reaction. Sci. Adv. 2018, 4, No. eaao6657. (25) Lu, Z.; Chen, G.; Siahrostami, S.; Chen, Z.; Liu, K.; Xie, J.; Liao, L.; Wu, T.; Lin, D.; Liu, Y.; et al. High-efficiency oxygen reduction to hydrogen peroxide catalysed by oxidized carbon materials. Nat. Catal. 2018, 1, 156. (26) Fei, H.; Dong, J.; Wan, C.; Zhao, Z.; Xu, X.; Lin, Z.; Wang, Y.; Liu, H.; Zang, K.; Luo, J.; et al. Microwave-Assisted Rapid Synthesis of Graphene-Supported Single Atomic Metals. Adv. Mater. 2018, 30, 1802146. (27) Li, X.; Bi, W.; Chen, M.; Sun, Y.; Ju, H.; Yan, W.; Zhu, J.; Wu, X.; Chu, W.; Wu, C.; Xie, Y. Exclusive Ni−N4 sites realize near-unity CO selectivity for electrochemical CO2 reduction. J. Am. Chem. Soc. 2017, 139, 14889−14892. (28) Yang, S.; Peng, L.; Huang, P.; Wang, X.; Sun, Y.; Cao, C.; Song, W. Nitrogen, Phosphorus, and Sulfur Co-Doped Hollow Carbon Shell as Superior Metal-Free Catalyst for Selective Oxidation of Aromatic Alkanes. Angew. Chem. 2016, 128, 4084−4088. (29) Xu, W.; Liang, Y.; Su, Y.; Zhu, S.; Cui, Z.; Yang, X.; Inoue, A.; Wei, Q.; Liang, C. Synthesis and properties of morphology controllable copper sulphide nanosheets for supercapacitor application. Electrochim. Acta 2016, 211, 891−899. (30) Li, J.-S.; Wang, Y.; Liu, C.-H.; Li, S.-L.; Wang, Y.-G.; Dong, L.Z.; Dai, Z.-H.; Li, Y.-F.; Lan, Y.-Q. Coupled molybdenum carbide and reduced graphene oxide electrocatalysts for efficient hydrogen evolution. Nat. Commun. 2016, 7, 11204. (31) Zhuang, T.-T.; Liang, Z.-Q.; Seifitokaldani, A.; Li, Y.; De Luna, P.; Burdyny, T.; Che, F.; Meng, F.; Min, Y.; Quintero-Bermudez, R.; et al. Steering post-C−C coupling selectivity enables high efficiency electroreduction of carbon dioxide to multi-carbon alcohols. Nat. Catal. 2018, 1, 421.

(32) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Hydrogen evolution by a metal-free electrocatalyst. Nat. Commun. 2014, 5, 3783. (33) Tong, Y.; Chen, P.; Zhou, T.; Xu, K.; Chu, W.; Wu, C.; Xie, Y. A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Evolution: Cobalt Oxide Nanoparticles Strongly Coupled to B, NDecorated Graphene. Angew. Chem. 2017, 129, 7227−7231. (34) Chen, P.; Zhou, T.; Xing, L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L.; Yan, W.; Chu, W.; Wu, C.; Xie, Y. Atomically dispersed iron− nitrogen species as electrocatalysts for bifunctional oxygen evolution and reduction reactions. Angew. Chem., Int. Ed. 2017, 56, 610−614.

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DOI: 10.1021/acsmaterialslett.9b00094 ACS Materials Lett. 2019, 1, 139−146