Edge Defects Engineering of Nitrogen-Doped Carbon for Oxygen

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Energy, Environmental, and Catalysis Applications

Edge Defects Engineering of Nitrogen-Doped Carbon for Oxygen Electrocatalysts in Zn-air Batteries Qichen Wang, Yongpeng Lei, Yinggang Zhu, Hong Wang, Junzong Feng, Guangying Ma, Yingde Wang, youji li, Bo Nan, qingguo feng, Zhouguang Lu, and Hao Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07863 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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ACS Applied Materials & Interfaces

Edge Defects Engineering of Nitrogen-Doped Carbon for Oxygen Electrocatalysts in Zn-air Batteries Qichen Wangad+, Yongpeng Lei*bc+, Yinggang Zhue, Hong Wangcf, Junzong Fengg, Guangying Mac, Yingde Wangg, Youji Lih, Bo Nane, Qingguo Feng*i,j, Zhouguang Lu*e and Hao Yu*a a

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials，Donghua

University, Shanghai 201620 China E-mail: [email protected] (H. Yu) b

State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, China

E-mail: [email protected] (Y. Lei) c

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

China d

Hunan Provincial Key Laboratory of Chemical Power Sources, College of Chemistry and

Chemical Engineering, Central South University, Changsha, 410083 China e

Department of Materials Science and Engineering, Southern University of Science and

Technology, Shenzhen 518000 China E-mail: [email protected] (Z. Lu) f

Bejing Super Star Count Figure Informantion Technology Co., Ltd, Beijing, China

g

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

University of Defense Technology, Changsha 410073 China h

Key laboratory of mineral cleaner production and exploit of green functional materials in

Hunan province, Jishou University, 416000 China i

Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest

Jiaotong University, Chengdu, Sichuan 610031 China; The Peac Institute of Multiscale Sciences, Chengdu, Sichuan 610031 China j

National Joint Engineering Laboratory of Power Grid with Electric Vehicles (Shandong

University), Jinan, Shandong, 250061 China E-mail: [email protected]

Keywords: Metal-free; nitrogen-doped carbon; edge defects; bifunctional oxygen electrocatalysts; Zn-air batteries 1

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Abstract: Metal-free bifunctional oxygen electrocatalysts are extremely critical to the advanced energy conversion devices, such as high energy metal-air batteries. Effective tuning of edge defects and electronic density on carbon materials via simple methods is especially attractive. In this work, a facile alkali activation method has been proposed to prepare carbon with large specific surface area and optimized porosity. And subsequent nitrogen-doping leads to high pyridinic-N & graphitic-N content and abundant edge defects, further enhancing electrochemical activities. Theoretical modeling via first principles calculations has been conducted to correlate the electrocatalytic activities with their fundamental chemical structure of N doping and edge defects engineering. The metal-free product (NKCNPs-900) shows a high half-wave potential of 0.79 V (ORR). Furthermore, the assembled Zn-air batteries display

excellent

performance

among

carbon-based

metal-free

oxygen

electrocatalysts, such as large peak power density up to 131.4 mW cm−2, energy density as high as 889.0 Wh kg-1 at 4.5 mA cm-2, and remarkable discharge-charge cycles up to 575 times. Preliminarily, the rechargeable nonaqueous Li-air batteries were also investigated. Therefore, our work provides a low-cost, metal-free and high-performance bifunctional carbon-based electrocatalyst for metal-air batteries.

1. INTRODUCTION The demand to replace fossil fuel forces us to develop clean, renewable, reliable and affordable energy storage/conversion systems [1]. Rechargeable metal-air batteries have received great attention as promising alternatives depending on the

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large theoretical specific energy density, superior efficiency and good environmental benignity. However, O2 reduction reaction (ORR) & O2 evolution reaction (OER) in cathode are major efficiency-limiting factors, limited by the sluggish kinetic rate and high overpotential [2,3]. Precious metal Pt-based electrocatalysts are generally recognized as the best catalysts for ORR, and IrO2, RuO2 et al. are efficient for OER. But the scarcity and inefficient stability have consequently uplifted the cost and thus limited the large-scale implementation of metal-air batteries [4-6]. To accomplish the demand in practical applications, rational design of reversible O2 catalysts can simultaneously reduce polarization during charging and discharging. Hence, the development of economical and robust non-noble metal and even metal-free bifunctional electrocatalysts towards both ORR and OER is urgently desired. Previous studies demonstrate that the huge differences in reaction mechanism of ORR＆OER pose great challenge in synthesizing bifunctional catalysts [7]. Clear understanding the structure-property relationship of nanocarbon-based bifunctional catalysts and mastering multi-scale design principles are significant and pressing. The electronic engineering (e.g., introducing defects, heteroatom-doping) and geometric structuring are effective strategies to regulate the electrocatalytic performance [8-10]. As known, O2 electrode catalysis requires the huge consumption or evolution of O2 while the low solubility of O2 in electrolyte usually results in the insufficient supply of reactants or intermediates at the electrochemical interfaces. Meanwhile, maximizing electroactive surface area and shortening diffusion distance is also important [11]. Hence, construction of open three-dimensional (3D) conducting architectures featured

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by hierarchical porous structure to achieve fast reactant diffusion and enhance utilization of accessible electroactive sites is necessary [12]. Jin et al. reported that necklace-like mesoporous N-doped carbon nanoparticles facilitated fast electron transport & mass transport [13]. The typical sp2 hybrid materials (e.g. CNTs and graphene) have been widely identified as potential O2 electrocatalysts. It is attractive to further simplify the preparation procedure, improve yield and lower cost [14]. Wang et al. functionalized commercial VXC72 with only a small quantity of zeolitic imidazolate framework (ZIF-67), achieving superior ORR activity and stability [15]. If the metal-free bifunctional electrocatalyst can attain competitive activity or even surpass noble-metal-based ones, that will be particularly attractive. Carbon aerogel (CA) is a defects-rich carbon nanomaterial consisted of numerous linked carbon nanoparticles. The 3D framework and pore texture are attractive in gas-involving electrocatalysis [16, 17]. More importantly, tuning of edge defects and electronic density on CA provide broad room to improve the performance of carbon based non-metal electrocatalysts. Herein, based on CA, alkali activation and subsequent N-doping strategy were adopted to synthesize N-doped carbon nanoparticles rich in edge defects. Possessing plentiful small mesopores, high surface area (841 m2 g-1), striking enrichment of pyridinic-N & graphitic-N, the optimal sample (NKCNPs-900) with abundant edge defects exhibit excellent ORR&OER performance, emerging as a promising metal-free bifunctional electrocatalyst. As an illustration, the according Zn-air batteries show the maximum power density as high as 131.4 mW cm−2, energy density

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up to 889.0 Wh kg-1 at 4.5 mA cm-2, and remarkable discharge-charge cycling stability (575 cycles), among the top level. Furthermore, the Li-air batteries were also fabricated. This work provides an efficient metal-free bifunctional catalyst, showing great potential in rechargeable meatl-air batteries.

2. EXPERIMENTAL SECTION 2.1 Materials CA was prepared by our group [16]. KOH (AR, 99.5%, Tianjin Hengxing Chemical Reagent Co. Ltd), Melamine (AR, 99.5%, Tianjin Guangfu Fine Chemical Research Institute), Nafion solution (5 wt. %, Dupont D520) and Pt/C (Pt: 20 wt.%, Alfa Aesar, Johnson Matthey, Hispec 3000) were taken from Shanghai Hesen Electric Co. Isopropyl alcohol (AR, 99.7%, Tianjin Damao Chemical Reagent Factory), G -C3N4) was obtained by treating melamine at 550 °C for 4 h in air. Ltd. All reagents were used as received. 2.2 Synthesis of NKCNPs-x The CA (0.2 g) and KOH (0.3 g) were mixed thoroughly with an agate mortar, and then heated up to 800 ºC (3 °C min–1) in N2 to obtain KOH-activated CA (donated as KCA). The obtained sample was continuously washed by HCl, deionized water and ethanol. Next, the KCA, g-C3N4 and melamine (mass ratio=1:5:5) was finely grounded and then heated to 600 ºC (1 °C min–1) under N2. After 1 h treatment, the sample was continually heated to an elevated temperature (700~1000 ºC, 3 °C min–1) and remained for 2 h. For simplicity, the obtained samples were called as NKCNPs-x, where x means the final treated temperature. 5



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2.3 Characterization Morphology observation was performed on Hitachi S-4800 Scanning electron microscope (SEM) and TecnaiTF200 transmission electron microscopy (TEM). X-ray diffraction (XRD) patterns were recorded on the X-ray diffractometer (Siemens D-5005, CuKα radiation). Raman spectra (HR800, Horiba Jobin Yvon) were collected upon 512 nm laser excitation. X-ray photoelectron spectroscopy (XPS) was collected on VG ESCALAB MKΠ instrument (Al Kα excitation). N2 adsorption-desorption isotherms were given by a N2 adsorption apparatus (Micromeritics ASAP 2020). X-ray absorption near-edge structure (XANES) spectra were measured in the beamline BL12B-a in National Synchrotron Radiation Laboratory (NSRL). The electrochemical tests, the assembling and performance of primary/rechargeable Zn-air batteries were given in Supporting Information. 2.4 Calculation Methods All the calculations were performed based on the density functional theory implemented in the Vienna Ab-initio Simulation Package (VASP)[18-21], and with PBE functional [22] applied. In energy calculations and geometry relaxations, the energy is converged to 1E-8 eV and force is relaxed to less than 0.001 eV/Å. As known, the graphene has two types of edge states in the way of connecting of C rings, the so-called zigzag and armchair types (Fig. S1-3). We modeled the zigzag edge system with a 11x3 supercell with graphene hexagon as unit, while armchair one with a 7x5 supercell. The periodicity in x direction is being hold and vacuum is applied in y and z directions to present edge state in y direction. Due to the large size of 6


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supercell, the gamma point is used to sample the Brillouin zone and the cutoff energy is taken as 450 eV. For the theory to achieve the core level shift (CLS), one can consult reference [23].

3. RESULTAS AND DISCUSSION The samples were synthesized by two-step thermal treatment under N2 atmosphere, as illustrated in Figure 1a. During alkali activation process, KOH etching proceeds as 2C + 6 KOH → 2 K + 2 K2CO3 + 3 H2 above 400 °C. Then the nanopores generated from K2CO3 decomposition above 700 °C and the intercalation between K vapor and carbon happened around 800 °C [24]. SEM images (Figure 1b and S4) reveals that CA, KCA and NKCNPs-x samples have similar 3D nanoarchitecture made of irregular and rough carbon nanoparticles. Furthermore, the pore characteristics were investigated by N2 adsorption/desorption analysis (Figure 1c). KCA shows a typical type-IV curve, suggesting the existence of mesopores [25]. At low pressure, KCA shows much stronger N2 adsorption capability than CA, which is attributed to the existence of more micropores. And KCA also displays an improved SBET of 751 m2 g-1 and pore volume (PV) of 1.14 cm3 g-1 compared to CA (488 m2 g-1 and 0.42 cm3 g-1). The pore size distribution shown in Figure 1d indicates KOH activation mainly resulted in smaller nanopores about 3.21 nm. The result suggests KOH activation greatly enhanced the SBET and PV. The SBET for NKCNPs-700, NKCNPs-800, NKCNPs-900, and NKCNPs-1000 were determined to be 768, 845, 841 and 807 m2 g-1 (Figure S5 and Table S1), respectively. But the PV of NKCNPs-900 decreases to about 0.68 cm3 g-1, likely owing to partial collapse of the 7



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carbon skeleton at high temperature during N-doping [17,26]. More structure information was examined by Raman spectra (Figure S6a). The typical D-band (1340 cm-1) and G-band (1593 cm-1) are arising from defects and graphitization, respectively [27], respectively. The Raman spectra were further fitted in Figure 1e and Figure S6 b-d.[2,28] Specifically, the value of ID1/IG for NKCNPs-900 is 3.35, higher than that of KCA (2.58) and CA (2.68), suggesting that more defects existed in NKCNPs-900 and thus would exert a positive contribution to boost the electrocatalytic performance.[29,30]

Figure 1. (a) Schematic illustration of the synthesis of NKCNPs-x. (b) SEM image of NKCNPs-900. (c) N2 adsorption/desorption isotherm curves and (d) pore size distribution of CA, KCA and NKCNPs-900. (e) Curve fitting result for the Raman spectrum of NKCNPs-900.

TEM image (Figure 2a) of NKCNPs-900 suggests that the 3D structure is consisted of disordered but interconnected carbon nanoparticles. As known, in a typical gas-related catalysis, 3D robust architecture with hierarchical pores

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undoubtedly facilitate fast mass transfer through the entire structure, promote the interfacial catalytic reaction and improve the mechanical stability. In Figure 2b, the high-resolution TEM (HRTEM) image reveals an amorphous carbon structure with abundant edge defects, which is believed to play a positive effect on ORR/OER activity [31,32]. The corresponding FFT pattern in the inset also confirms the amorphous nature. XRD pattern in Figure S7 display a weak and broad (002) diffraction peak of disordered carbon over 23.1°.

Figure 2. (a) TEM and (b) HRTEM images of NKCNPs-900. The arrows reflect edge defects and the inset in (b) displays the corresponding FFT pattern.

In Figure 3a, XPS spectrum displays the signal of C 1s, N 1s and O 1s for all NKCNPs-x samples. The content of N species (2.53~5.21 at.%) varies with the treatment temperature. In C 1s spectra (Figure S8), the C=C, C-N, C=O and O-C=O configuration appeared at binding energies of 284.4, 285.8, 287.5 and 288.9 eV [33], respectively, which verify the successful N-doping. High-resolution N 1s spectra (Figure 3b) of NKCNPs-900 could be fitted into four peaks according to previous literature [34], including pyridinic-N (397.9 eV), pyrrolic-N (399.1 eV), graphitic-N 9



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(400.9) and oxygenated-N (402.8 eV), respectively. In Figure 3c, it is noted that NKCNPs-900 possesses the highest content of the sum of pyridinic-N and graphitic-N. Generally, for N-doped carbon catalysts, the electron-donating pyridinic-N and graphitic-N could act as ORR active sites [35]. In addition, the elemental distribution (Figure S9) reveals a homogeneous distribution of C, N, O species. Furthermore, the soft X-ray absorption near-edge structure (XANES) measurement was also applied to investigate the electronic structure. Figure 3d displays the weak peak around 284.1 eV, indicating the structure defects of NKCNPs-900 [36]. A sharp absorption peak A at 285.6 eV responds to π* peak of C=C bonds and peak D at 292.5 eV responds to σ* resonance peak of C-C bonds, respectively. Besides, we also noticed the peaks (denoted as peak B) between π * and σ*, which are attributed to various functional groups (e.g. C=O and C-O (COOH)) and C-N bond [37]. These results coincided well with Raman and XPS conclusions.

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Figure 3. (a) XPS survey spectra for different samples. (b) High-resolution N 1s spectra for NKCNPs-900, E. G. represent “edge defect”. (c) The relative contents of different N species for different samples. (d) Normalized C K-edge XANES spectra for NKCNPs-900.

As known, the CLS can characterize the chemical composition and local atomic bonding of solids and molecules because it is unique for each element and also sensitive to the surrounding environment. Experimentally the CLS can be measured by the XPS spectroscopy. Therefore, to investigate the influence of N doping along with the edge defects and compare with experimental results, we calculated the N1s CLS for pyridinic-N, dehydrogenated pyrrolic-N and pyrrolic-N at the presence of various edge defects, as shown in Figure 4 and Figure S1~3 in SI. To our knowledge, these have not been investigated yet in literature. The calculated CLSs are listed in Table S2. In Ref.[23] the hydrogenated pyridinic-N and pyrrolic-N was found to have similar CLS, while we have observed that the pyridinic-N and dehydrogenated pyrrolic-N displayed large difference and the latter one has a larger binding energy than the pyridinic-N on zigzag edge. Thus the edge defects and dehydrogenated pyrrolic-N has broadened the XPS peak of pyridinic-N (with bulk defect or on zigzag edge) at nearly 398 eV towards higher energy since CLSs show that the edge defect associated pyridinic-N and dehydrogenated pyrrolic-N give a larger binding energy. The peak around 399 eV should be owing to the pyridinic-N and dehydrogenated pyrrolic-N with edge defects. On the other hand, since the samples were synthesized at 800 ºC, hydrogen was almost removed. The hydrogenated pyridinic-N and pyrrolic-N will give less contribution than the pyridinic-N and dehydrogenated

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pyrrolic-N. If there is large amount of hydrogen, the hydrogenated pyridinic-N and pyrrolic-N with bulk defect should have given one high peak at the energy 2.3 eV, higher than that of pyridinic-N [23] but here no this peak. In order to compare with the case with hydrogen, we computed the CLSs for pyrrolic-N with edge defects, which shows a shift of 3~6 eV comparing to pyridinic-N and will contribute to the peaks at 401 and 403 eV together with graphitic-N and oxgenated-N. To know the influence of the edge defect, we calculated the charge density difference for pyridinic-N with edge defects in Figure 4b. In Fig. 4b(1), one can observe that by introducing one N atom at zigzag edge, it does generate one negative charge on this doped N and a net positive charge on neighboring C sites and then will strengthen the ORR on the neighboring C-C bonds [38]. While Fig. 4b(2-3) show that, by adding one additional N on the system with several N yet doped on edge defects, the charge density will be redistributed. Thus the edge defects may increase the length of edge, the concentration of N and the number of neighboring active reaction sites with net positive charge. At the same time the charge distribution induced by N doping with edge defects can give the charge difference which may enable the charge transfer and hence enhance the possibility of occurrence of ORR in electrocatalysis.

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Figure 4 (a) Some DFT optimized pyridinic-N structure with zigzag edge defects and the corresponding calculated CLS of N1s, where the equivalent N sites are neglected. (b) The charge density difference caused by inducing N atoms in red circle: (1) single pyridinic-N at zigzag edge (2) two pyridinic-N near zigzag edge on one structure yet with three pyridinic-N and defect. (3) single pyridinic-N at the deepest C site in one four-pyridinic-N defective structure (4）single pyridinic-N at a side C site in one four-pyridinic-N defective structure.

The ORR activity was tested through CV and LSV technique. Figure 5a shows an obviously cathodic peak (0.77 V) for NKCNPs-900 in O2-saturated electrolyte (Pt/C, 0.80 V). The LSV curve (Figure 5b) of CA exhibits poor ORR activity with onset potential (Eonset) of 0.81 V and half-wave potential (Ehalf-wave) of 0.65 V. The Ehalf-wave (0.69 V) of KCA is slightly 40 mV positive than that of CA. In sharp contrast,

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the ORR activity of NKCNPs-x samples was enhanced after incorporation of N species (Table S3). Especially, the NKCNPs-900 sample delivers the most positive Eonset of 0.92 V and Ehalf-wave of 0.79 V (20% Pt/C, Eonset：0.97 V；；Ehalf-wave: 0.82 V). Such high catalytic capability is also evidenced by the diffusion current density as high as 5.31 mA cm-2. The fitted K-L plots display good linearity and near coincidence from 0.40 to 0.60 V (Figure 5c). The electron transfer number (n) is found as ~4.0, implying a 4e- pathway. In other word, the dissolved O2 molecular is straightly reduced into OH- instead of HO2- and then OH-.

Figure 5. (a) CV and (b) LSVs curves of different samples. (c) LSV curves for NKCNPs-900 at various rotation speed. The inset in (c) gives the fitted K-L plots and corresponding n value. (d) Kinetic current density.

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The kinetic current density (Jk) obtained from the intercept of the fitted K-L plots. (13.96 mA cm-2) is 1.61 folds higher than that of Pt/C (41.84 mA cm-2 vs 25.95 mA cm-2) at 0.70 V (Figure 5d). The efficient ORR activity of NKCNPs-900 mainly attributes to rich pyridinic-N, graphitic-N species and edge defects, high SBET as well as specific porosity. Furthermore, the value of Tafel slope for NKCNPs-900 is only about 62 mV dec-1 (Figure S10), almost the same to that of Pt/C (~ 60 mV dec-1), revealing that the transfer of the first electron during ORR process is the rate-controlling step [39]. The EIS result demonstrate that NKCNPs-900 have a small charge transfer resistance (Rct) of 48.4 Ω (Figure S11), exhibiting a good electron transport ability. Good durability of electrocatalyst is also significant for electrochemical energy conversion system. In Figure S12, after test at 0.79 V for 22500 s, the chronopotentiometry response demonstrates superior stability of NKCNPs-900, which shows current loss of only 15.0%. By contrast, Pt/C suffers from irreversible decay as high as 24.1%, owing to the agglomeration of Pt nanoparticles [40]. Besides, the OER activity was also assessed. In Figure 6a, the NKCNPs-900 shows the lowest overpotential of ~480 mV @ 10 mA cm-2 among different NKCNPs-x samples. Despite that the value is ~150 mV higher than that of RuO2/C. In Figure 6b, the low Tafel slope (68 mV dec-1) also verifies the favorable OER kinetic process. The chronoamperometry response at 1.71 V illustrates that the NKCNPs-900 displays a loss of OER activity (22.2 %) (Figure S13). Besides, the electrochemical active area was estimated by the double layer capacitance (Cdl) by CV method in Figure S14

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[41-43]. The Cdl of NKCNPs-900 was assessed to be 13.78 mF cm-2 (Figure 6c), higher than other nearly 5.9 and 8.0 times higher than those of KCA (2.35 mF cm-2) and CA (1.72 mF cm-2), respectively, confirming more efficient exposure and accessible catalytic sites. Generally, the smaller △E (△E=EOER:j=10 - EORR: half-wave) reflects the better catalytic bifunctional activity [44,45]. As a result, the value for NKCNPs-900 was estimated as 0.92 V (Figure 6d), comparable to those of metal-free catalysts (Table S4). Indeed, the N-doping and rich edge defects in coupled with 3D robust nanocarbon network make NKCNPs-900 potential in electrocatalysis.

Figure 6. (a) LSV curves and (b) the corresponding Tafel plots of NKCNPs-x and RuO2/C. (c) The capacitive current measured at 1.19 V versus scan rate. The inset shows CV curves of NKCNPs-900 at various scan rates. (d) The overall polarization curves of NKCNPs-900 towards both ORR and OER.

Excitingly, the assembled primary Zn-air batteries (Figure 7a and b) present the maximum power density of 131.4 mW cm−2 for NKCNPs-900 (Pt/C: 98.2 mW cm−2),

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even outperforming those metal-based catalysts (e.g., N-GCNT/FeCo-3, 97.6 mW cm−2 [46]; Fe@C-NG/NCNTs, 101.3 mW cm−2 [47]; S,N-Fe/N/C-CNT, 102.7 mW cm−2 [48]; NC@Co-NGC DSNC, 109.0 mW cm−2 [49]; Co/Co3O4@PGS, 118.3 mW cm−2 [50]; Co-N,B-CSs, 100.4 mW cm−2 [51]). The batteries with NKCNPs-900 could delivers a stable galvanodynamic discharge voltage platform at 4.5 and 6.5 mA cm-2, implying the excellent stability of the NKCNPs-900 (Figure 7c). Meanwhile, the corresponding energy density, normalized to amount of Zn electrode consumed, is up to 889.0 Wh kg-1 at 4.5 mA cm-2. Furthermore, the charge and discharge polarization curves display efficient reversibility (Figure 7d). In Figure 7e, after 575 continuous cycles (10 mA cm-2, 20 min / cycle), only ~0.10 V voltage gap is observed in both charge and discharge voltage plateaus, confirming the remarkable cycling stability (Table S5). However, Pt/C + Ir/C electrode exhibits an increase in voltage gap of 0.08 V after merely 190 cycles (Figure S15). The increased charge voltage using NKCNPs-900 may be due to the oxidation/corrosion of carbon substrates at high potential during OER.

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Figure 7. (a) Schematic representation of rechargeable Zn-air batteries. (b) Galvanodynamic discharge polarization and power density curves of the primary Zn-air batteries. (c) Galvanostatic discharge curves of the primary Zn-air batteries with NKCNPs-900 as catalyst. Inset of (c) shows a photograph of red LED (≈ 3.0 V) derived by two Zn-air batteries connected in series. (d) Charge-discharge curves of the rechargeable Zn-air batteries for NKCNPs-900 and Pt/C + Ir/C (1:1 by weight). (e) Cycling performance of NKCNPs-900 electrode at 10 mA cm-2.

In addition, the rechargeable nonaqueous Li-air batteries have received 18


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unprecedented attention due to outstanding theoretical energy density [52, 53]. But the catalytic reaction mechanisms of electrochemical oxygen electrode catalysis (ORR＆OER) are typically very different in aqueous and nonaqueous electrolytes. A neglected but important fact associated with bifunctional electrocatalyst that the considerable catalytic performance could share similarities in these two electrolytes, which proven by many research groups [4, 54-58]. As an illustration, we explored the application of NKCNPs-900 in rechargeable Li-air batteries. In Figure S16, the batteries delivers a higher capacity (13791 mA h g-1) and good cycling stability at 100 mA g−1 with a restricting capacity of 600 mA h g-1. The performance is very attractive compared to literatures. (Table S6). We believe that the excellent bifunctional ORR/OER activity of NKCNPs-900 mainly originate from the following aspects: (1) the high pyridinic-N & graphitic-N content combined with abundant edge defect provide plentiful active sites; (2) the large SBET and suitable pore size distribution are favorable to enhance mass transport and then promote to react with accessible active sites; (3) the robust framework structure consisting of interconnected carbon nanoparticles is of importance to enhanced electrocatalytic activity and stability. More importantly, the cost is much cheaper than that of Pt/C (calculated in Supporting), favoring for its potential application. Based on systematic analysis, the metal-free NKCNPs-900 with excellent bifunctional electrocatalytic performace make itself promising in Zn-air batteries.

4. CONCLUSION In conclusion, effective tuning of edge defects and electronic density were 19



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realized on CA via simple alkali activation and subsequent nitrogen-doping. The integration of optimized porosity, rich active sites and optimized charge density provides the sample with a potential difference of 0.92 V and good stability towards both ORR＆OER. Furthermore, the rechargeable Zn-air batteries exhibit a high energy density of 889.0 Wh kg-1 and remarkable cycling stability up to 575 cycles. Together with good performance, facile synthesis and low-cost merits, the bifunctional electrocatalyst here holds a great potential in metal-air batteries and related energy conversion systems.

Acknowledgements Q.C. Wang and Y.P. Lei contributed equally to this work. This work was financially supported by the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (LK1605), Research Project of NUDT (ZK16-03-32), National University Student Innovation Program, Hunan Provincial Science and Technology Plan Project (No. 2017TP1001). Z.G. Lu acknowledges the support from the National Natural Science Foundation of China (No. 21671096), and the Basic Research Fund of the Science and Technology Innovation Commission of Shenzhen (No. JCYJ20170412153139454). Q. F would like to acknowledge the partial support from the Scientific Challenge Project of China (No. TZ2018001), the NSFC (No. 11627901) and from the National Joint Engineering Laboratory of Power Grid with Electric Vehicles (Shandong University). The computations are performed on the supercomputers supported by the PIMS supercomputing center. We thank the

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Hefei Synchrotron Radiation for XAFS measurements.

Supporting Information Li-air batteries, calculation details, SEM, BET, Raman, XRD, XPS, EDS, Tafel, EIS, i-t curves, CV curves, cycling performance of Pt/C + Ir/C electrodes and performance of Li-air batteries. The data of N2 adsorption/desorption isotherms and pore size distribution and pore volume for all samples (Table S1); The calculated N1s Core-Level shift within pyridinic-N and dehydrogenated pyrrolic-N with various edge defects. (Table S2); ORR and OER activities for NKCNPs-x catalysts (Table S3); The comparison of bifunctional ORR/OER performance of metal-free catalysts previously reported (Table S4); The performance of rechargeable Zn-air batteries with various electrocatalysts (Table S5); The comparison of discharge capacity of oxygen electrode for Li-O2 batteries previously reported (Table S6).

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Author Contributions Yongpeng Lei, Zhouguang Lu and Hao Yu initiated the research, and designed the experiments; Junzong Feng synthesized carbon aerogel, Qichen Wang, Hong Wang, Guangying Ma and Dongchuan Zhang performed the ORR, OER tests, Zn-air batteries measurements and analyzed the data; Yinggang Zhu and Bo Nan carried out Li-air batteries measurements; Yingde Wang analysed Ranamn data; Qiangguo Feng did the calculation, wrote the according part and discussed the whole manuscript, Qichen Wang, Yongpeng Lei, Youji Li and Hao Yu wrote the manuscript; Zhouguang Lu analyzed the Li-air batteries data.

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TOC

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Edge Defects Engineering of Nitrogen-Doped Carbon for Oxygen

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