Pyridinic-N-dominated Doped Defective Graphene as Superior

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Pyridinic-N-dominated Doped Defective Graphene as Superior Oxygen Electrocatalyst for Ultrahigh-Energy-Density Zn-Air Batteries Qichen Wang, Yujin Ji, Yongpeng Lei, Yaobing Wang, Yingde Wang, Youyong Li, and Shuangyin Wang ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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Pyridinic-N-dominated Doped Defective Graphene as Superior Oxygen Electrocatalyst for Ultrahigh-Energy-Density Zn-Air Batteries Qichen Wang,†‡&+ Yujin Ji,%+ Yongpeng Lei,* ,†‡ Yaobing Wang,£ Yingde Wang,§ Youyong Li,* , % and Shuangyin Wang*,＃ †

School of Aeronautics and Astronautics & Science and Technology on High Strength Structural

Materials Laboratory, Central South University, Changsha 410083 China. ‡

College of Basic Education, National University of Defense Technology, Changsha 410073

China. &

College of Materials Science and Engineering, Central South University of Forestry and

Technology, Changsha 410004 China. %

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-

Based Functional Materials & Devices, Soochow University, Jiangsu 215123 China. £

Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key

Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 China.. §

Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National

University of Defense Technology, Changsha 410073 China. ＃

State Key Laboratory of Chem/Bio-Sensing and Chemometrics, Provincial Hunan Key

Laboratory for Graphene Materials and Devices, College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082 China.

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ABSTRACT: Identification of catalytic sites for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in carbon materials remain a great challenge. Here, we construct a pyridinic-N-dominated doped graphene with abundant vacancy defects. The optimized sample with an ultrahigh pore volume (3.43 cm3 g−1) exhibits unprecedented ORR activity with a halfwave potential of 0.85 V in alkaline. For the first time, density functional theory results indicate that the quadri-pyridinic N-doped carbon site synergized with vacancy defect is the active site, which presents the lowest overpotential of 0.28 V for ORR and 0.28 V for OER. The primary Zn-air batteries display a maximum power density of 115.2 mW cm-2 and an energy density as high as 872.3 Wh kg-1. The rechargeable Zn-air batteries illustrate a low discharge-charge overpotential and high stability (> 78 h). This work provides the new insight into the correlation between N configuration synergized with vacancy defect in electrocatalysis.


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Electrochemical oxygen electrode catalysis, such as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), is especially crucial for next-generation renewable energy conversion applications.1-5 But the sluggish kinetics of ORR/OER restrict the overall energy conversion efficiency. At present, the commonly used platinum (Pt) and ruthenium (Ru)/iridium (Ir) oxides cannot simultaneously catalyze ORR/OER or present unsatisfactory performance.6-12 To date, enormous efforts have been therefore spent on the development of bifunctional ORR/OER catalysts based on earth-abundant elements.13-20 Metal-free catalysts (e.g., heteroatoms-doped carbon materials) have been experimentally and theoretically proved to efficiently catalyze reversible ORR/OER in alkaline electrolyte.21-25 Currently, it is regretful that there is no consensus on the actual catalytic mechanism of metal-free N-doped nanocarbon system.26-28 It is reported that N doping induce uneven charge distribution of adjacent C atoms, facilitating ORR catalysis.29 Whilst conversely, other reports believe that doped pyridinic-N create the active sites (e.g. pyridinic-N based mechanism for ORR).30 Recently, the defectactivity relationship for ORR has been carefully discussed and summarized.31-34 For example, Yao et al. proposed that catalytic activity was dependent on the carbon defects (e.g. edge pentagon and 5-8-5 defect, etc) within the structure instead of heteroatom doping.35-37 And Zhang et al. concluded that the a curved configuration that five-carbon ring adjacent to seven-carbon ring (C5+7) exhibited small overpotential. In generally, there is a fact that defects often induced upon heteroatom doping cannot be ignored, which poses a huge challenge to the confirmation of active sites. Therefore, a clear understanding of the active sites (heteroatom doping, defective effect or the synergistic effect of doping and defects) is essential for the synthesis of efficient ORR/OER catalysts.


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Besides, insufficient mass transport undoubtedly degrades the catalytic performance at high overpotential, during which the transport and consumption of O2 are vast. It is known that rotating disk electrode (RDE) measurements significantly improve O2 diffusion and minimize the mass transfer resistance owing to the forced convection at high rotating speeds (such as 1600 rpm).38, 39 However, full-cell measurement is generally performed under static condition, where reactants diffusion resistance is an extremely critical factor to deliver energy output. To solve this problem, constructing hierarchically macro/mesoporous nanostructure with an open framework is necessary to promote fast mass exchanges and improve transport kinetics.40,

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Recently, Duan et al. reported a three-dimensional (3D) Nb2O5/HGF composite for ultrahigh-rate energy storage at practical levels of mass loading, giving emphasis on the critical role of porosity in mass transport.42 Thus, a rational design of highly efficient metal-free carbon-based catalysts featured with hierarchical porous structure and prominent activity is preferentially desirable but still a fundamental challenge. As known, the reduced graphene oxides have plenty of structural defects (such as vacancies, etc).43-45 It is anticipated to introduce more targeted pyridinic-N to defective graphene. Herein, we report a novel 3D defective graphene enriched with pyridinic-N, exhibiting superior bifunctional ORR/OER performance. For the first time, density functional theory (DFT) calculations reveal that the quadri-pyridinic N-doped carbon site synergized with vacancy defect is the reactive site with the lowest overpotential for ORR (0.28 V) and OER (0.28 V). As a proof of concept, the constructed primary/rechargeable Zn-air batteries display better performance than those of noble-metal-based Zn-air batteries.


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Figure 1. (a) Schematic illustration of the fabrication process series of samples. (b) SEM, (c) TEM, (d) Highresolution TEM, (e) AFM image, (f) Elemental mapping images and (g) N2 adsorption/desorption isotherms of NDGs-800.

The fabrication process is illustrated in Figure 1a and Figure S1. Field-emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM) images in Figure 1b and 1c reveal the open porous structure and typical graphene character of the as-prepared NDGs-800. Further high-resolution TEM (HRTEM) characterization in Figure 1d indicates the few-layer feature. The atomic force microscopy (AFM) analysis in Figure 1e shows nanosheets structure with a thickness of 3.0 nm, corresponding to about 9 layers of graphene sheets. And the


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elemental mapping images of NDGs-800 (Figure 1f) confirm the homogeneous distribution of C, N and O species on graphene scaffold. Local graphene-like structure with N doping is believed to alter its properties. For instance, in comparison to the electrical conductivity (1.81 S cm-1) of DGs-800 prepared via heating GO at 800 °C in N2, the electrical conductivity of NDGs800 was much higher (3.07 S cm-1). Furthermore, the porosity of NDGs-x was studied in Figure S2. All samples show a typical type IV isotherm curve with an obvious hysteresis, implying the presence of mesoporous structure.46,47 Figure 1g displays a BET surface area of 443.2 m2 g−1 and an ultrahigh pore volume of 3.43 cm3 g−1 for NDGs-800. And the rapid N2 rise at P/P0 >0.9 region indicates the existence of much larger pores, arising from the 3D self-assembled DGs sheets. In contrast, two comparison samples (NDGs-800-1# and NDGs-800-2#) show decreased BET surface area and pore volume (Figure S3). We believe the highly interconnected 3D graphene framework with excellent charge (electron and ion) transport capability facilitate effective exposure of more accessible active sites. A broad peak at 25.5° in X-ray diffraction (XRD) pattern was assigned to the (002) of graphitic carbon for NDGs-800 (Figure S4).48,49 Figure 2a presents Raman spectra of NDGs-x under the 532 nm laser, with typical D-band (~1350 cm-1) and G-band (~1580 cm-1) corresponding to the disorder and the vibration of sp2-bonded carbon atoms, respectively.50,51 The ratio value of ID/IG were calculated to be 1.23 ~ 1.09, indicating the highly defective structure including vacancies, etc. In the X-ray photoelectron spectroscopy (XPS) spectra (Figure S5), no signal of metal species (Mn) were detected because that the GO was prepared by improved Hummers’ method. In Figure 2b, the C 1s peak of NDGs-800 can be fitted to C-C (284.7 eV), C-N (285.9 eV) and C=O (288.6 eV).52 Meanwhile, high-resolution N 1s spectrum (Figure 2c) shows four fitted peaks around 398.4, 399.7, 400.9 and 403.0 eV, associated to


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pyridinic-N, pyrrolic-N, graphitic-N and pyridinic N+-O-, respectively.53 The overall N content decreases from 11.04 to 1.48 at%. Specifically, NDGs-800 possesses the highest proportion of pyridinic-N (47.9 %) among all the NDGs-x samples (Figure 2d). We speculate that g-C3N4 prefer to react with defects of GO to promote the formation of pyridinic-N during thermal treatment.54,55 Such unique pyridinic-N-dominated doping and rich defects are expected to make a great contribution for ORR/OER.

Figure 2. (a) Raman spectra of different samples. (b) XPS survey spectra, (c) The high-resolution N 1s spectrum of ＋

－

the NDGs-800. (d) The distribution of pyridinic-N, pyrrolic-N, graphitic-N and pyridinic N -O obtained from the N 1s spectra of different samples.

The catalytic ORR activity of NDGs-x and Pt/C catalysts was measured. The linear sweep voltammetry (LSV) result demonstrates that NDGs-800 has the unprecedented ORR activity


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with an onset potential (Eonset) of 0.98 V (vs. reversible hydrogen electrode) and half-wave potential (E1/2) of 0.85 V, respectively (Figure 3a). Additionally, the NDGs-800 also reaches a higher current density of 5.6 mA cm-2 at 0 V. The Jk of NDGs-800 (13.91 mA cm−2) at 0.8 V is higher than that of Pt/C (13.32 mA cm−2), NDGs-900 (6.03 mA cm−2), NDGs-600 (5.55 mA cm−2) and NDGs-700 (2.80 mA cm−2) (Figure 3b). Such superior performance makes NDGs800 one of the best ORR metal-free catalysts, even surpassing most of non-precious metal-based electrocatalysts reported.17, 37, 56-60 The increased current plateau ranging from 0.6 to 0 V with the increment of rotating speed represents a surface-controlled kinetics process (Figure 3c). Accordingly, the value of electron transfer number (n) based on Koutecky-Levich (K-L) equation for NDGs-800 was estimated to close to 4.0 in Figure 3d, implying an ideal fourelectron ORR pathway with a high catalytic efficiency. In addition, the superior catalytic performance is also confirmed by lower Tafel slope of 81 mV dec-1 (Figure S6). The chronoamperometric response (i-t) and accelerated degradation test demonstrates the excellent stability of NDGs-800 (Figure S7). And the acid ORR activity of NDGs-800 was also measured in 0.5 M H2SO4 (Figure S8).


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Figure 3. (a) LSV curves of NDGs-x and Pt/C catalysts for ORR in 0.1 M KOH. Scan rate: 5 mV s−1. (b) The comparison of kinetic current density (Jk) and E1/2 of NDGs-x and Pt/C catalysts. (c) LSV curves at different rotation speeds from 400 to 1600 rpm for NDGs-800. (d) The corresponding K-L plots of NDGs-800. (e) LSV curves of DGs-800, NDGs-800, RuO2/C and Pt/C catalysts for OER in 1 M KOH. Scan rate: 2 mV s−1. (f) The corresponding Tafel plots for OER catalysis.

We then investigated the electrocatalytic OER performance. The required overpotential to reach a current density of 10 mA cm−2 is 450 mV (Figure 3e), which is slightly larger than RuO2/C (375 mV). The Tafel slope of 132 mV dec−1 (Figure 3f) for NDGs-800 demonstrates good kinetic process. The NDGs-800 also maintain a good catalytic stability at least 40000 s (Figure S9). The OER activity of NDGs-800 still can’t catch up with RuO2/C, which is a crucial problem needed to be tackled in future study. The excellent ORR/OER could was comparable to those of known bifunctional electrocatalysts reported previously. (Table S1).17, 34, 59-67


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Figure 4 (a) The seven types of pyridinic-N-contained sites (1N, 2N, 3N-1, 3N-2, 4N, 5N and 6N) in graphene model and (b) the corresponding overpotential versus adsorption energy of *OH along ORR and OER pathway without considering the effect of pH. (c) Calculated Gibbs free energy diagrams of ORR and OER in 4N (quadripyridinic N) site and (d) the optimized adsorption configurations of ORR/OER intermediates (*OOH, *O, and *OH). (e) The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution in 4N site. Gray, blue, red and white balls represented the C, N, O and H atoms respectively.

To identify the N doping states (p-type or n-type), the slopes of Mott-Schottky plots (Figure S10) in p-type region are much lower than that of n-type region, indicating more positive charge carrier density due to the electron-withdrawing capability of pyridinic-N.60 In order to unfold the distinguished performance of NDGs-800, we performed a series of experiments described in Supporting Information (from Figure S11 to Figure S15). Furthermore, the mechanism of ORR and OER is investigated based on first-principles calculations in different pyridinic-N-contained configurations synergized with the common vacancy defects. As shown in Figure 4a, seven


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types of pyridinic-N configurations (i.e. 1N, 2N, 3N-1, 3N-2, 4N, 5N and 6N) in graphene model were constructed at the edge of vacancy defect. According to the analysis of molecular orbital, the C and N at the edge mainly contributes to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) shown in Figure 4e, while the carbon atom bonded with pyridinic N will become the potential active site due to the electron transfer from N to C atom. Then the optimal free energy reaction pathway was calculated and the overpotential of each site is given to reflect the practical performance of ORR and OER. Figure 4b demonstrates that 4N (quadri-pyridinic N) configuration exhibited the best OER/ORR performance due to the lowest overpotential 0.28/0.28 V while the 2N site is followed because the overpotential (0.33 V) of OER is relatively higher than that (0.28 V) of ORR, indicating that the 2N site is propitious for ORR. However, according to the formation energy shown in Table S2, 3N-2 configurations are the most common with the overpotential of OER/ORR corresponding to 0.52/0.52 V, which might be the reason why our NDG-catalysts are better than other traditional N-doped nanocarbon system and the more exposure of 2N/4N site will be responsible for the distinguished performance. Further, the ORR/OER reaction pathway was given in Figure 4c and Figure S16. The formation of intermediate *OOH adsorption and *OH desorption tend to be the rate-limiting step during ORR and OER (Figure 4d). Previous research shows that edge effect of pyridinic N (1N-R) would enhance the catalytic activities of graphene.34,37 In order to reveal the distinct chemical activity of quadri-pyridinic N, the difference of HOMO and LUMO was calculated because the HOMO-LUMO gap plays a vital role in the catalytic performance of ORR/OER.68,69 A smaller HOMO-LUMO gap will lead to stronger adsorption of *OOH and *OH with lower overpotential due to the more filling of bonding orbital. In Figure S17, It is found that the HOMO-LUMO gap has a linear relationship


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with the performance of ORR/OER in N-doped configurations and the distinct activity of quadripyridinic N is attributed to its lower HOMO-LUMO gap compared other pyridinic-N and graphitic-N configurations. The seven pyridinic-N configurations in our work could further improve OER/ORR performance and thus demonstrating excellent performance. Besides, it is widely accepted that the electrochemical active surface area (ECSA), estimated from the doublelayer capacitance (Cdl), make a great contribution to enhanced electrochemical activity for nanostructured catalysts.70 As a result, NDGs-800 displays the highest Cdl of 18.2 mF cm-2 in Figure S18, larger than that of NDGs-600 (13.0 mF cm-2), NDGs-700 (8.3 mF cm-2) and NDGs900 (17.0 mF cm-2), confirming the better exposure and enhanced utilization of active sites of NDGs-800.

Figure 5. (a) Galvanostatic discharge voltage and power density curves of the single Zn-air battery with NDGs-800 and Pt/C as air cathodes. (b) Maximum power density and corresponded discharge current density as well as the comparison with previous works. (c) Charge and discharge polarization curves of rechargeable Zn-air batteries. (d) Charge-discharge cycling performance of rechargeable Zn-air batteries at a constant charge-discharge current density of 10 mA cm-2. (e) Photograph of two-series liquid Zn-air light a red LED (≈ 3.0 V).


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As a proof of concept, we assembled primary Zn-air batteries (Figure S19) with NDGs-800loaded carbon cloth/gas diffusion layer as the air cathode. The open-circuit voltage (OCV) and maximum power density are 1.45 V and 115.2 mW cm−2, respectively, superior to those of Pt/C (1.43 V; 110.3 mW cm−2) (Figure 5a). In Figure 5b, we also compared the maximum power density vs. discharge current density with other advanced ORR catalysts.34,56,60,67,71-77 Moreover, the discharge voltage platforms at different current densities are more stable than those of Pt/Cbased Zn-air batteries (Figure S20). The specific capacities normalized to the weight of consumed Zn electrode was calculated to be 750.8 mAh g-1 at constant current densities of 10 mA cm-2, corresponding to a much higher energy density of 872.3 Wh kg-1, which is one of the highest value among carbon-based materials. Meanwhile, we also constructed rechargeable Znair batteries owing to the excellent ORR/OER activity of NDGs-800. And a rechargeable Zn-air batteries based on the mixture of Pt/C + Ir/C (1 : 1 by weight) catalysts was also tested as a reference. As shown in Figure 5c, the discharge-charge overpotential of NDGs-800 is 0.76 V at a current density of 10 mA cm-2, slowly larger than that of Pt/C + Ir/C counterpart (0.68 V), suggesting efficient reversibility of the rechargeable Zn-air batteries. In Figure 5d, when cycled at the constant current density of 10 mA cm−2 at 20 min per cycle, the voltage difference retains stable after more than 78 h (up to 234 cycles). Oppositely, the Pt/C + Ir/C show fast activity decay at the same condition. The batteries with Pt/C + Ir/C electrode exhibited a voltage gap increase of 0.24 V, almost three times of NDGs-800 electrode (0.08 V). These results sufficiently prove the robust stability of NDGs-800. In general, such a distinguished result highlights that 3D hierarchical porous architecture in NDGs-800 electrode leads to a much lower internal resistance, thus showing much less discharge-charge voltage degradation.[32] As an illustration, a


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red light-emitting diodes (LED, 3.0 V) can be powered by two series-connected Zn-air batteries using the NDGs-800 as air-cathode (Figure 5e). In summary, we have prepared a novel pyridinic-N-dominated doped defective graphene towards efficient oxygen electrocatalysis. The quadri-pyridinic N-doped carbon site synergized with vacancy defect acts as the active site, displaying the lowest overpotential for ORR (0.28 V) and OER (0.28 V). Furthermore, the assembled Zn-air batteries could deliver a maximum power density of 115.2 mW cm−2 and an energy density as high as 872.3 Wh kg-1. The corresponding rechargeable Zn-air batteries display low discharge-charge overpotential value and excellent stability (more than 78 h). In addition, the significance of this work is the new insight into the correlation between quadri-pyridinic N-doped carbon site synergized with vacancy defect and ORR/OER catalysis, which also provides a platform to support various electrochemically active species (e.g., oxides, carbides, sulfides, etc) to fabricate more efficient and robust electrocatalysts applied in energy conversion devices.

ASSOCIATED CONTENT Supporting Information. Additional: Experimental section, computation details, photograph, SEM, BET, XRD, XPS, Tafel plots, stability data for ORR and OER, Mott-Schottky plots, Raman spectra, LSV curves, Schematic and photograph of the rechargeable Zn-air battery, galvanostatic discharge curves and Table included (S1) The electrocatalytic activities of the recently reported bifunctional catalysts for ORR/OER; (S2) The formation energy of seven types of pyridinic-N-doped active sites are provided in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.P. Lei). *E-mail: [email protected] (Y.Y. Li) *E-mail: [email protected] (S.Y. Wang) ORCID Y.P. Lei: 0000-0002-8061-4808 Y.Y. Li: 0000-0002-5248-2756 S.Y. Wang: 0000-0001-7185-9857 Q.C. Wang: 0000-0001-8991-915X Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Q.C. Wang and Y.J. Ji contributed equally to this work. Yongpeng Lei thanks the Research Project of NUDT (ZK16-03-32). Yingde Wang thanks the support from the National Natural Science Foundation of China (51773226). REFERENCES (1) Li, Y. G.; Dai, H. J. Recent advances in zinc-air batteries. Chem. Soc. Rev. 2014, 43, 52575275.


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(2) Cui, H.; Zhou, Z.; Jia, D. Heteroatom-doped graphene as electrocatalysts for air cathodes. Mater. Horiz. 2017, 4, 7-19. (3) Qu, K.; Zheng, Y.; Jiao, Y.; Zhang, X.; Dai, S.; Qiao, S. Z. Polydopamine-inspired, dual heteroatom-doped carbon nanotubes for highly efficient overall water splitting. Adv. Energy Mater. 2017, 7, 1602068-1602075. (4) Fu, J.; Cano, Z. P.; Park, M. G.; Yu, A.; Fowler, M.; Chen, Z. W. Electrically rechargeable zinc-air batteries: progress, challenges, and perspectives. Adv. Mater. 2017, 29, 16046851604718. (5) Dou, S.; Tao, L.; Huo, J.; Wang, S. Y.; Dai, L. Etched and doped Co9S8/graphene hybrid for oxygen electrocatalysis. Energy Environ. Sci. 2016, 9, 1320-1326. (6) Liang, Y. Y.; Li, Y. G.; Wang, H. L.; Zhou, J. G.; Wang, J.; Regier, T.; Dai, H. J. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater. 2011, 10, 780-786. (7) Wu, G.; Santandreu, A.; Wang, H. L.; Dai, L. M. Carbon nanocomposite catalysts for oxygen reduction and evolution reactions: from nitrogen doping to transition-metal addition. Nano Energy 2016, 29, 83-110. (8) Chen, Y. J.; Ji, S. F.; Wang, Y. G.; Dong, J. C.; Chen, W. X.; Li, Z.; Shen, R.; Zheng, L. R.; Zhuang, Z. B.; Wang, D. S., et al. Isolated single iron atoms anchored on N-doped porous carbon as an efficient electrocatalyst for the oxygen reduction reaction. Angew. Chem. Int. Ed. 2017, 129, 7041-7045. (9) Ling, T.; Yan, D. Y.; Jiao, Y.; Wang, H.; Zheng, Y.; Zheng, X.; Mao, J.; Du, X. W.; Hu, Z.; Jaroniec, M., et al. Engineering surface atomic structure of single-crystal cobalt (II) oxide nanorods for superior electrocatalysis. Nat. Commun. 2016, 7, 12876-12883.


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(10) Guo, C.; Zheng, Y.; Ran, J.; Xie, F.; Jaroniec, M.; Qiao, S. Z. Engineering high-energy interfacial structures for high-performance oxygen-involving electrocatalysis. Angew. Chem. Int. Ed. 2017, 56, 8539-8543. (11) Zhu, Y. P.; Jing, Y.; Vasileff, A.; Heine, T.; Qiao, S. Z. 3D synergistically active carbon nanofibers for improved oxygen evolution. Adv. Energy Mater. 2017, 1602928-1602935. (12) Zhou, R.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Determination of the electron transfer number for the oxygen reduction reaction: from theory to experiment. ACS Catal. 2016, 6, 4720-4728. (13) Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X. A metal-organic frameworkderived bifunctional oxygen electrocatalyst. Nat. Energy 2016, 1, 15001-15008. (14) Gupta, S.; Zhao, S.; Wang, X. X.; Hwang, S.; Karakalos, S.; Devaguptapu, S. V.; Mukherjee, S.; Su, D.; Xu, H.; Wu, G. Quaternary FeCoNiMn-based nanocarbon electrocatalysts for bifunctional oxygen reduction and evolution: promotional role of Mn doping in stabilizing carbon. ACS Catal. 2017, 7, 8386-8393. (15) Cheng, H.; Chen, J.; Li, Q.; Su, C.; Chen, A.; Zhang, J.; Tong, Y.; Liu, Z. A modified molecular framework derived highly efficient Mn-Co-carbon cathode for a flexible Zn-air battery. Chem. Commun. 2017, 53, 11596-11599. (16) Deng, Y. P.; Jiang, Y.; Luo, D.; Fu, J.; Liang, R.; Cheng, S.; Bai, Z.; Liu, Y.; Lei, W.; Yang, L., et al. Hierarchical porous double-shelled electrocatalyst with tailored lattice alkalinity toward bifunctional oxygen reactions for metal-air batteries. ACS Energy Lett. 2017, 2, 2706-2712. (17) Wang, Q.; Chen, Z.; Lei, Y.; Wu, N.; Wang, Y.; Wang, B.; Wang, Y. Fe/Fe3C@C nanoparticles encapsulated in N-doped graphene-CNTs framework as an efficient


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Pyridinic-N-dominated Doped Defective Graphene as Superior

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