Combined Electron and Structure Manipulation on Fe-Containing N

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Energy, Environmental, and Catalysis Applications

Combined electron and structure manipulation on Fe containing N-doped CNTs to boost bifunctional oxygen electrocatalysis Lei Zhao, Qichen Wang, Xinqi Zhang, Cheng Deng, Zhihong Li, Yongpeng Lei, and Mengfu Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09197 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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ACS Applied Materials & Interfaces

Combined electron and structure manipulation on Fe containing N-doped CNTs to boost bifunctional oxygen electrocatalysis Lei Zhao,a+ Qichen Wang,b+ Xinqi Zhang,a Cheng Deng,*a Zhihong Li,a Yongpeng Lei*bc, and Mengfu Zhu*a a

Institute of Medical Support Technology, Academy of Military Science of Chinese PLA, Tianjin

300161, China. E-mail: [email protected]; [email protected] b

State Key Laboratory of Powder Metallurgy & Hunan Provincial Key Laboratory of Chemical Power

Sources, College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China. E-mail: [email protected] c

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai 200050,

China.

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Abstract: It is a challenge to synthesize highly efficient non-precious metal electrocatalysts with well-defined nanostructure and rich active species. Herein, through electron engineering and structure manipulation simultaneously, we constructed Fe embedded pyridinic-N-dominated CNTs on ordered mesoporous carbon, showing excellent oxygen reduction reaction activity (half-wave potential, 0.85 V) and an overpotential of 420 mV to achieve 10 mA cm-2 for oxygen evolution reaction in alkaline (potential difference, 0.80 V). Density functional theory calculation indicates those Fe@N4 clusters improve charge transfer, and further promote the electrocatalytic reactivity of the functionalized region in CNTs. The rechargeable Zn-air batteries were assembled, displaying robust charging-discharging cycling performance (over 90 h) with voltage gap of only 0.08 V, much lower than that of Pt/C + Ir/C electrode (0.29 V). This work presents a highly active non-precious metal-based bifunctional catalyst towards air electrode for energy conversion.

Keywords: bifunctional oxygen electrocatalysts, non noble-metal, charge transfer, mass transport, Zn-air batteries,

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1. Introduction Rechargeable Zn-air batteries, with a theoretical energy density of as high as 1084 Wh kg-1, show promising potential owing to low cost, resource abundance, eco-friendliness and safety.1,2 It is noteworthy that the slow oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) kinetics in air cathode are of critical importance to determine the final energy output efficiency.3-5 Besides, the inadequate reserve and unaffordable price of Ir/Ru and Pt-based catalysts limit the application of OER & ORR catalysts.6 Hence, rational design and development of bifunctional OER and ORR electrocatalysts based on earth-abundant elements is urgently required. As far as we know, ORR and OER processes involving oxygen consuming or formation,

occur

at

the

triple-phase

electrochemical

interface

between

reactant-electrolyte-catalysts.7, 8 Thus, engineering abundant active phase and construction of high-speed diffusion channels, allowing sufficient charge (electron and ion) transport to accessible active sites timely, are essential to weaken the polarization of ORR/OER under lower overpotential. Previous studies demonstrate that the Fe, Co and their alloys encapsulated in carbon nanotubes (CNTs) show remarkable performance for electrocatalysis.9-12 Nevertheless, there is still a certain gap between this kind of catalysts and noble-metal-based materials with respect to catalytic reactivity and durability due to individual active centres and insufficient transfer efficiency.13-15 The ordered mesoporous carbon (OMC) shows great potential in electrochemical reaction owing to its well-defined inner channels, large surface area, excellent electronic conductivity and stability, etc.16-18 Spatially confined hybridization strategy is an efficient way to realize the in situ synthesis 3



of catalytic active phases in intimate touch with the original nanocarbon framework, which can fully exploit the advantages of both.19-21 Herein, through electron engineering and structure manipulation simultaneously, we constructed Fe embedded pyridinic-N-dominated CNTs on OMC (denoted as Fe-N-CNT/OMC), exhibiting excellent ORR with the onset potential (Eonset) and half-wave potential (E1/2) of 1.01 and 0.85 V (vs. RHE), respectively. The overpotential of 420 mV to achieve 10 mA cm-2 for OER in alkaline was achieved. And the potential difference of bifunctional ORR/OER activity is as low as 0.80 V, which is one of the best oxygen catalysts reported currently. Density functional theory calculation explains the important role of Fe@N4 in charge transfer. The rechargeable Zn-air batteries display robust charging-discharging cycling performance (over 90 h) with almost ignorable voltage gap of 0.08 V, much lower than that of Pt/C + Ir/C electrode (0.29 V). 2. Experimental section Ordered mesoporous silica (SBA-15, XFF01) was purchased from Nanjing XFNANO Co., Ltd. The carbon molecular sieve (HTCMS-260, 0.4 nm) and silica spheres (MS-65-11, 65 nm) were bought from Weihai Huatai Molecular Sieve Co., Ltd. and Nanjing nanorainbow Biotechnology Co., Ltd., respectively. Hydrofluoric acid (HF, 40%), ferric acetylacetonate (Fe(Acac)3), sulfuric acid (H2SO4, 98%), sucrose (C12H22O11), melamine (99 %), ethanol (EtOH), KOH (99.5%) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Nafion solution (5 wt%, Dupont D520) and Pt/C (20 wt%, JM) were supplied by Shanghai Hesen Electric Co., Ltd. N2 and O2 with a purity of 99.99% were supplied by Tianjin Fengchuan Chemical Reagent Factory. g-C3N4 was synthesized 4


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by heating melamine in air at 550 °C for 4 h.22 The OMC sample was prepared according to literature 23. The obtained OMC (0.1 g) was finely ground with melamine (0.5 g), g-C3N4 (0.5 g) and Fe(Acac)3 (0.15 g for Fe-N-CNT/OMC, 0.05g for Fe-N-CNT/OMC-1# and 0.25g for Fe-N-CNT/OMC-2#) in agate mortar. Finally, the powder was heated in N2 to prepare Fe-N-CNT/OMC. The temperature was raised to 600 °C (2 °C min-1, 2 h), and then continually heated to 800~1000 °C (5 °C min-1, 3 h). The comparison samples were also obtained using a similar procedure. The Fe-OMC was prepared in the absence of melamine and g-C3N4. The N-OMC was prepared without adding Fe(Acac)3. The Fe-N-CNT was prepared in the absence of OMC. The OMacC (Mac, macro-) was prepared using 65 nm silica spheres as hard template and the OMicC (Mic, micro-) was obtained from 0.4 nm carbon molecular sieves, followed by the same grinding and pyrolysis procedure. The relevant characterization, electrochemical measurements and Zn-air batteries measurements were given in Supplemental Information. 3. Results and Discussion A series of samples were prepared by one-step solid-state reaction using OMC, Fe(Acac)3 and melamine as raw materials. Among them, the sample obtained at 900 °C was named as Fe-N-CNT/OMC. The OMC (average pore size ~4.7 nm) shows a surface area as high as 742.3 m2 g-1, and the mesoporous channels are clearly demonstrated by TEM and BET analysis (ESI, Figure S1 and S2†). The XRD pattern of Fe-N-CNT/OMC is shown in Figure S3 (ESI†). Apart from the peaks of graphite carbon at 26.1o and α-Fe at 45.0o, the other diffraction peaks can be assigned to Fe3C phase (PDF #65-2412). The 5



ordering of Fe-N-CNT/OMC is also reflected in the small-angle XRD pattern (ESI, Figure S4†). A remarkable diffraction peak at 1.1° assigned to the (100) reflection of hexagonal mesostructure, suggests the highly-ordered periodic structure.24 The SEM image in Figure 1a shows that a large amount of CNTs are homogeneously anchored on OMC. The TEM images in Figure 1b and Figure S5 (ESI†) exhibit that the encapsulated nanoparticles (black dots) with sizes of 20~40 nm appeared on tip or inside of the bamboo-like CNTs. As known, the bamboo-like CNTs can expose more electroactive graphitic edges, which could be recognized as high-energy sites and thus be beneficial for electrocatalysis.25-27

Figure 1 (a) SEM image of Fe-N-CNT/OMC. Inset shows the schematic model. (b) TEM image, (c) HRTEM image of Fe-N-CNT/OMC. (d) EDS elemental mapping images.

The HRTEM image in Figure 1c confirms the Fe/Fe3C core and graphitic carbon shell with the lattice fringes of 0.21 and 0.36 nm, respectively.28, 29 This unique geometric confinement avoids the aggration and dissolution of the core to a great extent, boosting the 6


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electrocatalytic ORR and OER.30, 31 Moreover, the EDS elemental mapping (Figure 1d) shows that the Fe, N and C in Fe-N-CNT/OMC distribute uniformly after carbothermal reduction process.32 During thermal treating, the carbonaceous gases (e.g., C2N2+, C3N2+, C3N3+) decomposed from raw materials were catalysed by iron-containing NPs to form bamboo-like CNTs on the surface of OMC. Meanwhile, the CNTs/OMC framework was simultaneously doped by nitrogenous gases. It is noted that the iron-containing NPs could also enhance the graphitization of the neighbouring C species, making the simultaneous formation of Fe NPs encapsulated in graphitic carbon shell.33 The pore structure was characterized by N2 adsorption-desorption method. In Figure S6 (ESI†), Fe-N-CNT/OMC exhibits a type-III isotherm curve with a clear hysteresis loop, indicating the mesopores. The SBET was determined as 504.4 m2 g-1 and the pore size mainly centred at ~3.2 nm. Such large SBET and unique mesoporous structure are believed to facilitate reactant diffusion, the intimate contact between reactant and accessible active sites, and so on. The Raman spectrum displays D-band (1340 cm-1, disordered carbon) and G-band (1580 cm-1, ordered carbon) (Figure 2a).34, 35 The ratio of ID/IG is 1.16, suggesting the abundant defects in Fe-N-CNT/OMC.

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Figure 2 (a) Raman spectrum. High-resolution (b) N 1s XPS spectrum and (c) Fe 2p XPS spectrum of Fe-N-CNT/OMC. (d) Fe K-edge of XANES spectra (the blue area emphasizes the near-edge absorption energy). (e) EXAFS spectra for the Fe K-edges. (f) The wavelet transform of Fe K-edge EXAFS for Fe-N-CNT/OMC.

The X-ray photoelectron spectra (XPS) of Fe-N-CNT/OMC reveal four distinct peaks corresponding to C 1s, N 1s, O 1s and Fe 2p (ESI, Figure S7†) with the relative N content of 3.65 at. %. Figure 2b gives the deconvoluted four N species, namely pyridinic-N (398.5 eV), pyrrolic-N (399.6 eV), graphitic-N (401.2 eV) and oxidized-N (403.6 eV), with content of 48.9, 12.5, 27.1 and 11.5%, respectively.36 In the high-resolution Fe 2p spectrum (Figure 2c), a noticeable Fe0 2p3/2 peak at 707.1 eV is obviously observed, attributing to the presence of metallic Fe or Fe3C in Fe-N-CNT/OMC. The signals at 710.7 (Fe 2p3/2) and 721.5 eV (Fe 2p1/2) and a weak peak at 716.0 eV is assigned to a satellite peak, confirm the presence of Fe(II) and Fe(III).37, 38 It is rational to conclude that these Fe 8


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species could coordinate with pyridinic-N to generate Fe-Nx configuration owing to their lone-pair electrons, which generally act as catalytic active sites towards ORR and OER. The local atomic structure around Fe species were further exmained by the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. The intensity of Fe-N-CNT/OMC is higher than that of standard Fe foil (Figure 2d), indicating that Fe species may stabilize N or C atoms and possess positive charges.39 The main peak at 1.47 Å is assigned to Fe-N coordination and the Fe-Fe peak is near 2.04 Å (Figure 2e and S8, ESI, Table S1†). Besides, the wavelet transform (WT) analysis was provided in Figure 2f. The intensity maxima at 5 and 11 Å-1 were caused by the Fe-N and Fe-Fe, separately, agreeing well with the EXAFS fitting results in R space. Therefore, we infer that the Fe-Nx and Fe@C would endow Fe-N-CNT/OMC excellent ORR/OER activity.40, 41 The ORR activity was firstly evaluated. The LSV curves (ESI, Figure S9†) of the samples synthesized at various temperatures show that Fe-N-CNT/OMC, which was obtained at 900 °C, displays the highest ORR activity. Hence, we chose 900 °C as the optimal synthesis temperature below. Then the optimal mass ratio of reactants (OMC: melamine: g-C3N4 : Fe(Acac)3) was also identified to be 2:10:10:3 by comparing the LSV curves (ESI, Figure S10†). Fe-OMC, N-OMC and Fe-N-CNT were prepared by the same procedure as comparison samples. The cyclic voltammetry (CV) result shows that Fe-N-CNT/OMC has a highest positive peak potential of 0.82 V in Figure 3a. The Eonset and E1/2 were 1.01 and 0.85 V (Figure 3b), respectively, exceeding those of the Pt/C (Eonset: 0. 97 V, E1/2: 0.82 V). In Figure 3c, the fitted Kouteckye-Levich plots show good linearity 9



between J-1 vs ω-1/2. A typical 4e− reduction pathway is also confirmed. In Figure 3d, the negligible Tafel slope difference between Fe-N-CNT/OMC (75 mV dec-1) and Pt/C (79 mV dec-1) suggests the excellent ORR activity. In addition, only 13.3% current degradation (Pt/C, 22.4%) after 2000 s test was observed (ESI, Figure S11†). And the ORR activity of Fe-N-CNT/OMC under acidic environment was also measured (ESI, Figure S12†).

Figure 3 (a) CV curves (O2-saturated 0.1 M KOH), (b) LSV curves. Scan rate: 5 mV s-1. (c) LSV curves at different rotation speed for Fe-N-CNT/OMC. Inset: K-L plots. (d) Tafel plots.

Moreover, to deeply understand the relationship between pore structure and catalytic activity, we also synthesized Fe-N-CNT/OMacC and Fe-N-CNT/OMicC using 65 nm 10


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silica spheres and 0.4 nm carbon molecular sieves (ESI, Figure S13†), respectively. The influence of pore size on the ORR activity was also studied (ESI, Figure S14†). As seen, the E1/2 of Fe-N-CNT/OMicC (0.69 V) and Fe-N-CNT/OMacC (0.74 V) are both lower than that of Fe-N-CNT/OMC. Accordingly, the Tafel slopes (ESI, Figure S15†) of Fe-N-CNT/OMicC and Fe-N-CNT/OMacC were determined to be 124 and 115 mV dec-1, separately. The EIS result (ESI, Figure S16†) shows that the charge transfer resistance of Fe-N-CNT/OMC (33.9 Ω) is smaller than that of Fe-N-CNT/OMicC (55.4 Ω) and Fe-N-CNT/OMacC (45.3 Ω).42, 43 All these results confirm that the mesoporous structure seems to be more favourable for reaction kinetics compared to micropore and macropore.44

Figure 4 (a) LSV curves, (b) The corresponding Tafel plots, (c) OER polarization curves of 11



Fe-N-CNT/OMC. (d) The overall polarization curves.

Moreover, in Figure 4a, Fe-N-CNT/OMC displays an overpotential of 420 mV to achieve the current density of 10 mA cm-2 (Ej=10), slightly higher than that of RuO2/C, but superior to Pt/C and other samples. Besides, the small Tafel slope of 75 mV dec-1 (Figure 4b) illustrates a faster OER kinetic process. After 2000 cycles, the LSV curve just displays a slight shift to a higher potential (Figure 4c). As known, the smaller ∆E (∆E=Ej=10 - E1/2) reflects the better bifunctional performance.45-47 The Fe-N-CNT/OMC shows the significantly lowest value of only 0.80 V (Figure 4d), which is comparable to the recently reported advanced bifunctional electrocatalysts (ESI, Table S2†). Furthermore, the mechanism of ORR & OER is investigated based on first-principles calculation. As we know, 4N (quadri-pyridinic N) configuration in graphene demonstrates the best OER/ORR performance among various pyridinic-N configurations, due to an overpotential of as low as 0.28/0.28 V.6 Therefore, the 4N sites are also created in SWNT(6,6) at first, labeled as N4-SWNT(6,6). Then both the experiments and DFT calculations show that single Fe atom will be captured in the centre of the 4N sites with strong Fe-N covalent bonds, and this configuration is called Fe@N4-SWNT(6,6), as illustrated in inset I in Figure 5. Although the concentration of Fe@N4 embedded in SWNT considered in the first-principles calculations may be different with that of observed experiments, the essential effect and underlying mechanism can be extracted from this simple model. Figure 5 exhibits the comparison of density of state (DOS) for the Fe@N4-SWNT(6,6), N4-SWNT(6,6) and pure SWNT(6,6), and from which four reasons can be concluded to explain the enhanced activeness in Fe@N4-SWNT. Firstly, the energy 12


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gap of pure SWNT(6,6), N4-SWNT(6,6) and Fe@N4-SWNT(6,6) are about 0.34, 0.24 and 0.18 eV, respectively. The above results demonstrate that the band gap width of SWNT can be significantly reduced via introducing N or Fe atoms, which can improve the electrical conductivity and accelerate the speed of electron flow. Secondly, an obvious charge transfer appears around the interface between Fe@N4(N4) and SWNT, which suggests that the strong interactions between them and this interface force can be utilized to modify the physical properties of SWNT. Our Bader charge analysis further gives the quantitative results. For N4-SWNT configuration, 4.48 electrons are transferred to 4 N atoms from the surrounding C atoms. For Fe@N4-SWNT configuration, C and Fe atoms lose 3.77 and 0.92 electrons, respectively, and N atoms obtain 4.69 electrons. From which we can see that the induced Fe atoms can substantially enhance the charge transfer and interface interactions in Fe@N4-SWNT. Besides, the strong charge transfer will also generate a local dipole near the interface. More importantly, compared with that of pure SWNT(6,6), the N4-SWNT(6,6) has a larger work function. However, inducing a Fe@N4 cluster can reduce the work function and promote the chemical reactivity of the functionalized region in SWNT. First-principles calculations (Figure 5b) demonstrate that upon creating N4 cluster in SWNT(6,6), its work function increases up to 4.53 eV from the initial 4.31 eV. On the contrary, once inducing Fe@N4 cluster in SWNT(6,6), its work function decreases to 4.18 eV. The reduced work function in Fe@N4-SWNT(6,6) is likely to improve its chemical activity.

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Figure 5 (a) Results of DFT calculations: DOS. The vacuum level is aligned at 0. Inset I denotes the optimized geometric structure of Fe@N4-SWNT(6,6), and inserted plot II and III show the 3D charge density difference of N4-SWNT(6,6) and Fe@N4-SWNT(6,6), respectively. The cyan and red regions represent charge accumulation and depletion, respectively. (b) The electrostatic potential profiles averaged on the plane as a function of position in the a-axis. The geometric structure of Fe@N4-SWNT(6,6) is shown in the back ground.

To unfold the excellent electrocatalytic performance of Fe-N-CNT/OMC, we propose the following factors should be considered: (1) The N-doped CNTs in situ grown on OMC not only improve the conductivity, but also enhance the exposure of active sites and promote the contact between reactant and active centre. (2) The simultaneous presence of Fe-Nx and Fe@C ensure the highly efficient ORR/OER activity. (3) The Fe@N4 cluster enhances charge transfer, and further promote the electrocatalytic reactivity of the functionalized region in CNTs. Besides, the synthesis of low-cost bifunctional Fe-N-CNT/OMC is simple, highly efficient and no special equipment needed, which is beneficial to large-scale production.

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Figure 6 (a) Galvanostatic discharge curves and corresponding power density curves of different primary Zn-air batteries. (b) Galvanostatic discharge voltage platform curves of the Fe-N-CNT/OMC electrode at different current densities. (c) Specific capacities of different primary Zn-air batteries. (d) Schematic illustration of the rechargeable Zn-air batteries. (e) Charging-discharging polarization curves and (f) cycling performance (20 min / cycle).

As

an

illustration,

the

primary

and

rechargeable

Zn-air

batteries

with

Fe-N-CNT/OMC as the air-cathode were assembled. In Figure 6a, an open circuit voltage (OCV) of 1.38 V and maximum power density (Pmax) of 96.1 mW cm-2 were noticed, respectively, close to that of Pt/C + Ir/C (OCV: 1.40 V, Pmax: 101.9 mW cm-2). In Figure 6b, when Zn-air batteries were discharged at 2, 5 and 10 mA cm-2, the discharge voltage plateaus are 1.27, 1.24 and 1.19 V, respectively. The stable voltage curves verify the robust stability. The specific capacity and energy density normalized to the consumed Zn electrode were 680 mAh g-1 and 816 Wh kg-1 (Figure 6c), which are comparable to those of Pt/C + Ir/C (730 mAh g-1, 828 Wh kg-1). Furthermore, the rechargeable Zn-air batteries 15



were also assembled (Figure 6d). In Figure 6e, the typical discharging-charging polarization curves were given. Remarkably, it is found in Figure 6f that the voltage during charging-discharging remained stable up to 90 h, with only a small voltage gap of 0.08 V (obviously lower than that of Pt/C + Ir/C electrode, 0.29 V). Considering the good performance and simple synthesis, Fe-N-CNT/OMC shows competitive advantages among various non-precious metal electrocatalysts for rechargeable Zn-air batteries reported recently (ESI, Table S3†). 4. Conclusion In summary, we constructed Fe embedded pyridinic-N-dominated CNTs on OMC. DFT calculation indicates that the Fe@N4 clusters accelerate charge transfer. The large SBET as well as the unique OM structure provides efficient reactant diffusion channel and effective utilization of accessible active sites. The simultaneous electron engineering and structure manipulation deliver Fe-N-CNT/OMC excellent oxygen catalytic activity. The rechargeable Zn-air batteries display much higher charging-discharging cycling stability (90 h) than that of Pt/C + Ir/C electrode. Based on this simple and effective design, we go one step further in highly efficient non-precious metal bifunctional electrocatalysts for renewable energy conversion applications.

ASSOCIATED CONTENT Supporting Information Details of Characterization, Electrochemical measurements and Zn-air batteries measurements; supplementary structure characterization of Fe-N-CNT/OMC and 16


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comparison samples; LSV curves of comparison samples; Chronoamperometric response, Tafel plots and EIS spectra of Fe-N-CNT/OMC and comparison samples; Structural parameters extracted from the Fe K-edge EXAFS fitting; comparison of ORR/OER activity and performance of Zn-air batteries between Fe-N-CNT/OMC and other electrocatalysts reported recently. Acknowledgements L. Zhao and Q. Wang contributed equally to this work. Y.P. Lei thanks the financial support from the Opening Project of State Key Laboratory of High Performance Ceramics (SKL201701SIC) and Changsha Science and Technology Plan. M.F. Zhu thanks National Natural Science Foundation of China (51878659). We thank the 1W1B station in BSRF. We also thank Dr. L. Xu’s valuable discussion on DFT and Dr. P. X. Cui’s suggestion on the analysis of XAFS’s result.

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[4] Wang, Q.; Shang, L.; Shi, R.; Zhang, X.; Waterhouse, G. I. N.; Wu, L. Z.; Tung, C. H.; Zhang, T. 3D Carbon Nanoframe Scaffold-Immobilized Ni3FeN Nanoparticle Electrocatalysts for Rechargeable Zinc-Air Batteries’ Cathodes. Nano Energy 2017, 40, 382-390. [5] Chen, Z. Y.; Wang, Q. C.; Zhang, X. B.; Lei, Y. P.; Hu, W.; Luo, Y.; Wang, Y. B. N-doped Defective Carbon with Trace Co for Efficient Rechargeable Liquid/all-solid-state Zn-air Batteries, Sci. Bull., 2018, 63, 548-555. [6] Wang, Q. C.; Lei, Y. P.; Ji Y. J.; Wang S. Y. Pyridinic-N-dominated Doped Graphene with Abundant Defects as Superior Oxygen Electrocatalyst for UltrahighEnergy-Density Zn-Air Batteries. ACS Energy Lett. 2018, 3, 1183-1191. [7] 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. [8] Zhu, Y. P.; Guo, C.; Zheng, Y.; Qiao, S. Z. Surface and Interface Engineering of Noble-Metal-Free Electrocatalysts for Efficient Energy Conversion Processes. Acc. Chem. Res. 2017, 50, 915-923. [9] Su, C. Y.; Cheng, H.; Li, W.; Liu, Z. Q.; Li, N.; Hou Z.; Bai, F. Q.; Zhang, H. X.; Ma, T. Y. Zinc-Air Batteries: Atomic Modulation of FeCo-Nitrogen-Carbon Bifunctional Oxygen Electrodes for Rechargeable and Flexible All-Solid-State Zinc-Air Battery. Adv. Energy Mater. 2017, 7, 1602420-1602431. [10] Wang, Z.; Peng, S.; Hu, Y.; Li, L.; Yan, T.; Yang, G.; Ji, D.; Srinivasan, M.; Pan, Z.; Ramakrishna, S. Cobalt Nanoparticles Encapsulated in Carbon Nanotube-Grafted 18


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Combined Electron and Structure Manipulation on Fe-Containing N

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