Pyridinic nitrogen exclusively doped carbon materials as efficient oxygen reduction electrocatalysts for Zn-air batteries

Journal Pre-proof Pyridinic Nitrogen Exclusively Doped Carbon Materials as Efficient Oxygen Reduction Electrocatalysts for Zn-air Batteries Qing Lv, Ning Wang, Wenyan Si, Zhufeng Hou, Xiaodong Li, Xin Wang, Fuhua Zhao, Ze Yang, Yanliang Zhang, Changshui Huang

PII:

S0926-3373(19)30981-6

DOI:

https://doi.org/10.1016/j.apcatb.2019.118234

Reference:

APCATB 118234

To appear in:

Applied Catalysis B: Environmental

Received Date:

2 August 2019

Revised Date:

16 September 2019

Accepted Date:

24 September 2019

Please cite this article as: Lv Q, Wang N, Si W, Hou Z, Li X, Wang X, Zhao F, Yang Z, Zhang Y, Huang C, Pyridinic Nitrogen Exclusively Doped Carbon Materials as Efficient Oxygen Reduction Electrocatalysts for Zn-air Batteries, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118234

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Pyridinic Nitrogen Exclusively Doped Carbon Materials as Efficient Oxygen Reduction Electrocatalysts for Zn-air Batteries

Qing Lv,a Ning Wang,b Wenyan Si,a Zhufeng Hou,c Xiaodong Li,a Xin Wang,a Fuhua Zhao,a Ze Yang,a Yanliang Zhangd and Changshui Huanga,*

Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences

(CAS). No. 189 Songling Road, 266101, Qingdao, China b

School of Chemistry and Chemical Engineering, Shandong University, 250100, Jinan, China

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure

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c

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a

Thermo Fisher Scientific Ltd, 201206, Shanghai, China

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E-mail: [email protected]

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d

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of Matter, Chinese Academy of Sciences, 350002, Fuzhou, China

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Graphical Abstract

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An exclusively pyridinic nitrogen doped carbon material is rationally designed and precisely synthesized with a bottom-up method, which exhibits excellent performance for oxygen reduction reaction and superior stability for Zn-air battery.

Highlights: 

A pyridinic N exclusively doped carbon material, named as PyN-GDY, is prepared by an ingenious cross-coupling reaction. The PyN-GDY exhibits remarkable activity for oxygen reduction reaction with higher

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

current density than commercial Pt/C throughout the whole test potential window in alkaline medium.

The Zn-air battery with PyN-GDY as cathode exhibits superior stability and a higher

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

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activity than Pt/C-based battery.

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Abstract

Rational design a metal-free catalyst with well-defined structure as alternative of noble metal

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is highly desirable but challenging to catalyze oxygen reaction for metal−air batteries. In this report, nitrogen with a specific configuration is selectively doped into the carbon skeleton to

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prepare a graphdiyne-like carbon material, in which one carbon atom in every benzene ring of graphdiyne (GDY) is substituted by pyridinc N (PyN-GDY). Composed by pyridine ring and

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acetylenic linkers, the PyN-GDY is prepared through a bottom-up strategy using pentaethynylpyridine as the monomer. The as-synthesized PyN-GDY with “defined” molecular structure is an ideal model for addressing the intrinsic activity of active sites at molecular level. It exhibits excellent performance in both alkaline and acidic media as electrochemical catalyst for oxygen reduction reaction (ORR). The PyN-GDY-based Zn-air battery is demonstrated more active and stable than commercial Pt/C-based battery. Density 2

functional theory calculations are used to analyze and determine the possible active sites of PyN-GDY in ORR. The precise construction of specific nitrogen doped carbon material is an effective method to produce efficient catalysts for electrocatalysis.

Keywords: pyridinic nitrogen doped carbon materials, metal-free electrocatalysts, oxygen reduction reaction, Zn-air batteries

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Keywords: pyridinic nitrogen, carbon materials, metal-free electrocatalysts, electrochemistry, oxygen reduction reaction

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1. Introduction

Metal-air batteries (e.g. Zn-air and Li-air) are potential energy storage devices, because of

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their large theoretical energy density and unlimited oxygen supply.[1-3] Commonly, noble

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metals or metal oxides, such as Pt and IrO2, are used in air electrodes for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Considering the scarcity of these

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metals, an alternative method is to develop stable and efficient metal-free electrocatalysts. Nowadays, carbon-based metal-free catalysts have been extensively studied.

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Nitrogen (N) doped carbon is a kind of potential oxygen electrocatalyst that has attracted much attention, on which the carbon materials could be tailored through the doping of N. The

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common bonding configurations of the doped N are pyridinic N, pyrrolic N and graphitic N.[4, 5] The doping styles of N display much effect on the characteristic properties of the carbon materials. For example, many literatures have reported that pyridinic N may be more active than pyrrolic and graphitic N in carbon materials as electrocatalysts of ORR.[6-8] Therefore, to explore efficient catalysts for metal-air batteries, it is important to controllably dope N with specific styles. Besides, carbon materials with single N doping style are also of significance 3

for the mechanism study. However, to get an N doped carbon catalyst with only one type of N is extremely challenging. The general approaches to dope N into carbon skeleton are post-treatment of carbon materials with nitrogen sources,[9-11] pyrolysis[12, 13] and chemical vapor deposition of precursors with nitrogen and carbon,[14] which lacks capability to dope N at a specific position. To achieve a breakthrough in preparing carbon materials with a given N doping style, some special methods were reported.[6, 10, 15] For example, Kondo and Nakamura et al.

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obtained pyridinic N doped carbon with edges patterned highly oriented pyrolytic graphite which was prepared by bombarding the pyrolytic graphite with an Ar+ ion beam through a thin metallic mask.[6] However, these methods are not capable of exclusion other kinds of N

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completely and complicated for large-scale applications. Recently, we succeeded in designing a unique carbon material to incorporate only one type of nitrogen into the carbon skeleton, in

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which pyridinic N can be exclusively doped under certain post-treatment conditions with a

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undemanding method.[16, 17]

Graphdiyne (GDY) is a new kind of carbon allotrope, composing of sp and sp2 hybridized

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carbon atoms. Comparing to sp2 and/or sp3 hybridized carbon materials, GDY possesses unique electronic structure, due to the special sp hybridized carbon atoms. Large triangular

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molecular pores in GDY with diameter of ca. 2.5 Å are conducive to the mass transfer in the electrocatalytic reaction. Besides, the GDY-based materials are synthesized by a cross-

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coupling reaction. By tuning the monomer, abundant GDY derivatives with varied chemical and electronic structures can be obtained. Herein, a new method was developed to dope a particular species of N and construct an N-C framework with defined structure for carbon materials, which did not require post-treatment. A delocalized π-conjugated pyridinic nitrogen composed GDY-like material (PyN-GDY) was wisely designed, and in-situ prepared through a facile cross-coupling synthesis method with pentaethynylpyridine as monomer. The as4

designed monomer pentaethynylpyridine determined that pyridine must be the only N contained repeat units in the carbon material, PyN-GDY. Therefore, the as-synthesized PyNGDY maintains the unique exclusive pyridinic N configuration and “definite” molecular structure, which is also demonstrated by multi-characterization methods. Applied as metalfree electrochemical catalyst for ORR, the PyN-GDY exhibits excellent performance in both alkaline and acidic media. Especially in alkaline medium, the PyN-GDY expresses higher activity than commercial Pt/C (Johnson Matthey, JM). The Zn-air battery with PyN-GDY as

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cathode reveals superior stability and high activity. The possible active sites of PyN-GDY for ORR are confirmed by the density functional theory (DFT) calculations. It indicates that the high activity of PyN-GDY derives from the pyridinic N unit, which demonstrates that

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pyridinic N is an efficient N doping style in carbon materials to improve their electrocatalytic performance for ORR.

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2. Experimental

Tetrahydrofuran

(THF)

and

and

pyridine

KOH,

were

pretreated

respectively.

under

reflux

with

Trimethylsilylacetylene

and

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sodium/benzophenone

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2.1. Materials

tetrabutylamonium fluride (TBAF) were purchased from Acos Organics. Hexane, 2,3,4,5,6-

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pentachlorinepyridine, Tetrakis(triphenylphosphine) palladium (Pd(PPh3)4), n-butyllithium (nBuLi), ZnCl2 and Na2SO4 are purchased from Sigma-Aldrich. Copper sheets were got from

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Sinopharm Chemical Reagent Co., Ltd and sonicated with 3 M hydrochloric acid, water, ethanol and acetone, successively, dried in argon atmosphere and used immediately. 2.2. Synthesis of [(trimethylsilyl)ethynyl]zinc chloride 27.4 g (280 mmol) of trimethylsilylacetylene dissolved in 200 mL distilled THF. Then 280

mmol

of

n-BuLi

in

112

mL

hexane

was

added

into

the

obtained

trimethylsilylacetylene/THF solution slowly at -78 °C, keeping stirring for 30 minutes. After 5

that, 38 g (280 mmol) ZnCl2 was added into the above mixture. The temperature of the new mixture rose to room temperature gradually. The mixture was stirred for another 2 hours to get [(trimethylsilyl)ethynyl]zinc chloride. 2.3. Synthesis of pentaethynylpyridine 400 mL toluene with 8.8 g (35 mmol) 2,3,4,5,6-pentachlorinepyridine and 2.0 g (1.75 mmol) Pd(PPh3)4 was added into the as-synthesized [(trimethylsilyl)ethynyl]zinc chloride solution, stirring for 3 days at 80 °C. The obtained solution was evaporated to remove the

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solvent. Then the residue was purified by column chromatography and white solid was got as 2,3,4,5,6-penta((trimethylsilyl)ethynyl)pyridine (15.6 g, 82%). Pentachloropyridine (Figure S1) was selected as the starting material for preparing the monomer, which was due to the

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modest reaction activity of the substitional chorine on the pyridine ring. In the reaction

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process, to guarantee the well solubility of starting materials pentachloropyridine, a mixture solvent of toluene and tetrahydrofuran (THF) (volume ratio = 2:1) was selected to reduce the

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reaction time and get the high preparing yield of compound 1.[18] 1.5 mL of 1 M TBAF /THF solution was added into 15 mL THF with 120 mg (0.21 mmol)

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2,3,4,5,6-penta((trimethylsilyl)ethynyl)pyridine. Notably, to avoid the creation of oligomer by-product in the pyridine solvent, it is necessary to perform the subsequent deprotected

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reaction of trimethyl trimethylsilicon group at a low temperature in the presence of tetrabutylammonium fluoride (TBAF), because the existence of electron-donating nitrogen

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atom in the pyridine ring can activate the connected alkynyl group on the ortho- and paraposition. The mixture was stirred for 15 minutes at -78 °C and then diluted with ethyl acetate, washed with deionized water and dried with anhydrous Na2SO4. After that, vacuum rotary evaporation was used to remove solvent in the above solution to yield pentaethynylpyridine as monomer. 2.4. Synthesis of PyN-GDY 6

The monomer was dissolved in pyridine (25 mL), which was added slowly (over 8 hours) to a solution with 120 mL pyridine and several copper sheets at 60 °C. The solution was stirred for 3 days in nitrogen atmosphere. After reaction, polypentaethynylpyridine, a kind of pyridinic nitrogen substituted conjugated carbon material, was grown on the surface of copper sheets. Acetone and N-methyl pyrrolidone were used to wash the copper sheets which were then immersed in concentrated hydrochloric acid for 2 days to obtain dark brown powder. After that, the powder was washed with acetone, HCl (3 M), NaOH (3 M) and water,

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successively, and dried in a vacuum oven at 60 °C for 12 hours. Finally, the powder was heattreated at 900 ºC in Ar atmosphere to remove the oligomers and improve the order degree, obtaining black PyN-GDY powder.

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GDY was synthesized for comparison with the similar method, using hexaethynylbenzene

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as monomer. 2.5. Physical Characterization

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SEM images were obtained with a HITACHI S-4800 field emission scanning electron microscope. TEM measurements were conducted on the HITACHI H-7650 electron

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microscope. Raman spectra were collected on a Thermo Scientific DXRxi Raman spectrometer with 532 nm wavelength incident laser light. Fourier transform infrared spectra

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were measured with the Thermo-Fisher Nicolet iN10. XPS were carried out on a VG Scientific ESCALab220i-XL X-

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excitation sources. Nitrogen adsorption/desorption measurements were performed at 77 K using a Micromeritics ASAP2020 gas-sorption system. 1H NMR, 13C NMR were recorded on Bruker AVANCE-III 600 (600 MHz for 1H, 150 MHz for tetramethylsilane as an internal standard. 2.6. Electrochemical measurements.

7

13

C) instrument in CDCl3 with

The electrochemical measurements are conducted with CH Instruments 760E Bipotentiostat. 5 mg of the catalyst was dispersed in a solution with 50 μL Nafion (5 wt %) and 950 μL ethanol ultrasonically to get catalyst ink. Then proper amount of the catalyst ink was dripped on the glassy carbon part of the rotating ring-disk electrode (RRDE) and dried at room temperature. The catalyst loading was 400 μg cm-2 for PyN-GDY catalysts. A conventional three-electrode system was used for the electrochemical tests with a saturated calomel electrode (SCE) as reference electrode, the catalyst coated RRDE as working

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electrode and a Pt foil as counter electrode. 0.1 M HClO4 and 0.1 M KOH solution were used as acidic and alkaline electrolytes, respectively, in the electrochemical measurements. The potentials in this work were all converted into values versus the reversible hydrogen electrode

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(RHE). The conversion from SCE to RHE was 0.998 V for 0.1 M KOH solution and 0.304 V for 0. 1 M HClO4 solution. The catalytic activities of the catalysts were measured by liner

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sweep voltammetry (LSV) at rotating speed of 1600 rpm. The disk potential was changed

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between 0.2 and 1.1 V with scan rate of 5mV s-1, while the ring electrode was held at 1.3 V. The LSV curves shown in the figures are LSV curves measured in O2-saturated solution deducting the background LSV curves which is tested in the Ar-saturated solution. The

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stabilities were investigated by comparing the polarization curves before and after 5000 continuous cyclic voltammetry cycles between 0.6 and 1.2 V with the scan rate of 100 mV s-1.

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Methanol tolerance experiments were conducted by comparing the LSV curves in 0.1 M KOH

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(0.1 M HClO4) and in 0.1 M KOH + 5 mmol L-1 methanol (0.1 M HClO4 + 5 mmol L-1 methanol).

The onset potential (Eonset) is got from the intersection of the baseline (j = 0 mA cm-2) and

the tangents of the rising current in the LSV. Half-wave potential (E1/2) is the potential at which the value of current is half of the limited-current. The electron transfer number (n) and

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H2O2 yield were calculated according to the RRDE results. The H2O2 collection coefficient (N) at the ring of RRDE was 0.37. n=

4|𝑗𝐷 | |𝑗𝐷 |+(𝑗𝑅 /𝑁)

(1) 2𝑗𝑅 /𝑁 𝐷 |+(𝑗𝑅 /𝑁)

%𝐻2 𝑂2 = 100 |𝑗

(2)

where jD and jR are the current densities at the disk and at the ring, respectively.

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2.7. Zn-Air Battery The Zn-air battery tests were conducted in a home-made electrochemical cell. First, 5 mg of PyN-GDY or commercial Pt/C catalyst was dispersed in a mixed solution containing 50 μL

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Nafion (0.5 wt%) and 950 μL ethanol to obtain a catalyst ink. Then the ink was sprayed on the hydrophobic carbon paper substrate as cathode. The mass loading of the catalyst was 1.0

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mg cm-2. The commercial Zn foil was used as anode and 6 M KOH was used as electrolyte. All the measurements were performed with CHI 660D electrochemical workstation or LAND

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3. Results and Discussion

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testing system at room temperature.

3.1. Catalyst Morphologies and Structure

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The synthesis process of PyN-GDY was shown in Figure 1a. The chemical structure of compound 1 was confirmed by 1H NMR and

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C NMR (Figure S2 and S3). The

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polymerization of the monomer was occurred via a modified Glaser-Hay coupling reaction on the surface of copper foil in pyridine to obtain PyN-GDY film. Here the copper foils can not only supply copper ion as catalyst for the cross-coupling reaction, but also act as the substrate for the growth of PyN-GDY film. Figure S4 shows four possible structures of PyN-GDY containing triangular, quadrangular and hexagonal pores. The triangular pores are constituted of only C atoms, while the quadrangular and hexagonal pores are composed of C and N atoms. 9

The calculated relative energy differences of nine PyN-GDY structural fragments are small (Table S1). Thus, the actual structure of PyN-GDY should be multiple. The morphology of the as-prepared PyN-GDY and GDY powder was investigated with scanning electron microscope (SEM) and transmission electron microscope (TEM). The SEM images reveal that the PyN-GDY powder is composed by nano-granules aggregated together with similar sizes (Figure 1b and 1c). Elemental mappings show that the nitrogen atoms are distributed uniformly in the PyN-GDY (Figure 1d), consistent with the structure of PyN-GDY

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(Figure 1a). The microporous structure of PyN-GDY can be observed in the TEM image (Figure 1e), which is advantageous to improve its surface area and mass transfer in ORR. The clear interlayer strippers can be seen in the HRTEM image; the interlayer spacing was

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measured to be ca. 0.40 nm (Figure 1f). The GDY exhibits similar morphology to PyN-GDY (Figure S6a-b). The Brunauer–Emmett–Teller (BET) surface area and pore size distribution of

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PyN-GDY powder were investigated by N2 adsorption–desorption measurements (Figure 1g

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and Figure S7a). The BET surface area was calculated as 689.7 m2 g-1. Many micropores around 0.58 nm can be seen in the pore-size distribution curve (Figure S6a), similar to the

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pore size of the quadrangular pores (~0.6 nm) in the PyN-GDY molecule (Figure S4), indicating that many quadrangular molecular pores should exist in the PyN-GDY. 13

C solid-state nuclear magnetic

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The structure of PyN-GDY was characterized by

resonance (NMR), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge

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structure (XANES), Fourier transform infrared spectroscopy (FT-IR) and Raman spectra (Figure 2). In the solid state

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C-NMR spectrum (Figure 2a), the two groups of peaks are

consistent well with all the two kinds of carbons in PyN-GDY. The peak at 135 ppm is corresponding to the carbon atoms in the pyridine rings, which are connected to the butadiyne groups. The peaks below 100 ppm could be ascribed to the carbon atoms of butadiyne.[19, 20] The multiple and broaden peaks in this range might be due to the coexistence of multifold 10

configurations in PyN-GDY. The possible structures of PyN-GDY are complex due to the low molecular symmetry of the monomer. However, benefiting from our bottom-up strategy, a synthetic route of constructing nanostructures from small repetitive structural, all the N in the PyN-GDY should be pyridinic N. The XPS survey spectrum reveals that C, N and O elements exist in the PyN-GDY (Figure S7b). The atomic ratio of C and N is ca. 14.86, approximate to the theoretical value of 15 (Figure 1a). In the high-resolution XPS spectra, the C 1s peak of PyN-GDY was deconvoluted into five subpeaks, corresponding to C=C (284.5 eV), C≡C

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(285.1 eV), C=N (286.4 eV), C-O (286.9 eV) and C=O (288.2 eV), respectively (Figure 2b). The subpeaks area ratio of C=C, C≡C and C=N is close to 3:10:2, consistent with the structure of PyN-GDY shown in Figure 1a, while the subpeaks area ratil of C=C and C≡C in GDY is

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1:1. As for the N 1s spectrum, only one peak at 399.9 eV can be seen, which is assigned to pyridinic N (Figure 2c).[21, 22] Therefore, pyridinic N is the only configuration for N

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element in the PyN-GDY material. To further confirm the structure of PyN-GDY, the N K-

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edge XANES was measured and simulated (Figure 2d). The two peaks at 400.3 and 402.4 eV are the N 1s-π* transition of the pyridinic N. The broad peak above 405 eV is the N 1s-σ* transition. The experimental and calculated results are well consistent, confirming the

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structure of PyN-GDY. The functional groups in PyN-GDY and monomer were further investigated by FT-IR spectra (Figure 2e). In the spectrum of monomer, the peak appeared at

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1628 cm-1 is ascribed to the C=N stretching vibration. The band located at 1503 cm-1, 1378

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cm-1 and 1246 cm-1 is assigned to the skeletal vibrations of pyridinic ring. The band at 2163 cm-1 is the typical C≡C stretching vibration. Compared to the FT-IR spectrum of the monomer, the peak intensity of C≡C stretching vibration in PyN-GDY decreases, while the skeletal vibrations of aromatic ring shift to higher wavenumber region, which indicates the conjugated carbon framework increases in the PyN-GDY. The Raman spectra (Figure 2f) of the sample exhibit three main peaks. The peak at 1530 cm-1 is assigned to G band, suggesting the samples possess abundant aromatic rings. The peak of 1356 cm-1 is D band, representing 11

defects and edges. The ratio of ID/IG is calculated as 0.77. The relative high value might derive from the multiple possible configurations of PyN-GDY. Moreover, a peak at 2185 cm-1 can also be observed, which is ascribed to acetylenic bond.[23-25] All the above characterization results verify the structure of PyN-GDY. 3.2. Electrocatalytic Performance and Zn-air battery. According to some recently reported literatures, pyridinic N atoms in carbon materials should be effective to generate active sites for the ORR, which can decrease the energy barrier

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for oxygen adsorption on their adjacent carbon atoms, and accelerate the electron transfer of rate-limiting step during the ORR.[6-8] The as-prepared PyN-GDY contains only pyridinic N with content as high as 7.2 wt% theoretically and 7.3 wt% calculated from the survey of XPS

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(Figure S7b). Therefore, the PyN-GDY has much potentiality to be an excellent

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electrocatalyst for ORR. The PyN-GDY was firstly tested as catalyst for ORR in alkaline medium. Cyclic voltammography (CV) curves of PyN-GDY were measured in both N2- and

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O2-saturated 0.1 M KOH solution (Figure S8). A cathodic ORR peak at 0.82 V can be seen for PyN-GDY, whereas the peak is at 0.79 V for Pt/C, indicating that the electrocatalytic

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activity of PyN-GDY for ORR is higher than that of Pt/C in alkaline medium. Rotating disk electrode (RDE) tests were further used to verify the catalytic activity of PyN-GDY. As

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shown in Figure 3a, the onset potential (Eonset), half-wave potential (E1/2) and limited current density (jL) of PyN-GDY in the polarization curve are 1.0 V, 0.84 V (versus RHE) and 5.5

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mA cm-2, better than most reported metal-free catalysts (Table S2).[26-28] The current densities of PyN-GDY are larger than those of commercial Pt/C (JM) during the whole test potential range. The kinetic current densities at different potentials were calculated by correcting the diffusion-limited current using Koutecky-Levich equation, which demonstrated the intrinsic activity of PyN-GDY was much higher than the state-of-the-art commercial Pt/C (Figure 3b). Moreover, the catalytic activity of PyN-GDY is much higher than that of GDY, 12

demonstrating the improvement effect of pyridinic N (Figure S9a). The catalytic activities of PyN-GDY with a series of loadings (100 to 600 μg cm-2) were also tested (Figure S11a). PyNGDY with loadings of 400 or 500 μg cm-2 exhibits the best activity. Higher PyN-GDY loading (600 μg cm-2) will affect the mass transfer of reactant. The tafel slope of PyN-GDY (73.2 mV dec-1) is close to that of Pt/C (72.6 mV dec-1), indicating that the ORR mechanism of PyN-GDY is similar to Pt/C (Figure 3c). The electron transfer number was calculated as 3.75–3.95 according to the rotating ring-disk electrode (RRDE) results with H2O2 yield below

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13%, which shows a four-electron pathway dominates the ORR (Figure S12a). The methanol tolerance of PyN-GDY was measured by testing its catalytic activity in a methanol contained 0.1 M KOH solution. The LSV curves of PyN-GDY tested in the media with and without

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methanol are almost overlapped, indicating that methanol has little effect on the catalytic activity of PyN-GDY for ORR (Figure 3d). However, there is an obvious methanol oxidation

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peak in the LSV curve of Pt/C tested in the methanol contained solution (Figure S12c). The

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results demonstrate that the methanol tolerance of PyN-GDY is better than Pt/C (JM). The stability of the PyN-GDY was determined by comparing its catalytic activity before and after 5000 potential cycles between 0.6 and 1.2 V (versus RHE) in O2-saturated 0.1 M KOH

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solution. The liner sweep voltammetry (LSV) curves shows negligible E1/2 shift (~7 mV) after the potential cycles for PyN-GDY (Figure 3e). As for Pt/C, the LSV curve exhibits a negative

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shift of 35 mV in E1/2 after the same treatment (Figure 3f). Therefore, both the stability and

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methanol tolerance of PyN-GDY are much better than that of Pt/C (JM). The catalytic performances of PyN-GDY in acidic medium were also measured in 0.1 M

HClO4 solution. The PyN-GDY also expresses good catalytic activity as a metal-free catalyst, though it is poorer than Pt/C (Figure 4a). The Eonset, E1/2 and jL of PyN-GDY are 0.81 V, 0.55 V (versus RHE) and 4.9 mA cm-2, respectively, which is much higher than those of GDY (Figure S9b) and among the best metal-free catalysts in acidic media (Table S3). The 13

polarization curves of PyN-GDY with different loadings (100 to 600 μg cm-2) were shown in Figure S11b. The tafel slope of PyN-GDY is similar to that of Pt/C (Figure 4b). Moreover, the methanol tolerance and stability of PyN-GDY are much better than those of Pt/C. The LSV curves of PyN-GDY reveal ignorable changes after mixing methanol into the electrolyte solution (Figure 4c) or the accelerated durability test (Figure 4e). As for Pt/C (JM), the negative shift (Figure 4d) and methanol oxidation peak (Figure 4f) in the LSV curves are obvious after the accelerated durability test and mixture of methanol in the 0.1 M HClO4

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solution, respectively. The electron transfer number of PyN-GDY was calculated to be 3.55– 3.84 according to the RRDE results, demonstrating that the ORR mainly proceeded with a 4epathway (Figure S12b).

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To further comprehensively evaluate the performance of PyN-GDY as a catalyst for ORR, Eonset (Figure 5a), E1/2 (Figure 5b) and jL (Figure 5c) of PyN-GDY tested in 0.1 M KOH

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solution were compared with other reported N doped carbon-based metal-free catalysts.[8, 29-

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36] It can be seen that the PyN-GDY exhibits excellent comprehensive performance (Figure 5d). The Eonset, E1/2 and jL of PyN-GDY are all among the best of the advanced N doped

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carbon materials. The ORR process on PyN-GDY has been shown in Figure 5e. There are three reasons that endow PyN-GDY high activity for ORR. First, a large proportion of

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pyridinic N atoms exist in the PyN-GDY material. Theoretical combined with experimental results have demonstrated that carbon atoms adjacent to pyridinic N hold a local density of

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state in the occupied region near the Fermi level. These carbon atoms possess Lewis basicity, which can be adsorbed by oxygen, act as active sites, and be benefit to the ORR.[6, 37-39] Many literatures have reported that pyridinic N may be the most effective N doping configuration to improve the activity of carbon-based metal-free catalysts for ORR,[6-8, 40, 41] which should be the most important reason for the high activity of PyN-GDY. Second, the large π-conjugated structure of PyN-GDY is beneficial for the electronic transmission of the 14

ORR,[42-44] which is an essential requirement for the electrocatalysts to obtain high activity. Third, the large molecular pores (hexagonal, tetragon and triangle) in the PyN-GDY are conducive to the mass transfer of O2 and H2O (Figure S4). However, as shown in the poresize distribution curve (Figure S7a), the PyN-GDY material only holds small amount of mesopores, which should be caused by its less-than-ideal three-dimensional morphology and lead to its lower activity than some other N doped porous carbon catalysts (Figure 5d). Those also suggest that obvious improvement can be made to further promote the activity of PyN-

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GDY as ORR catalyst. To evaluate the potential application of PyN-GDY, the as-synthesized PyN-GDY was used as cathode for a Zn-air battery.[45, 46] It can be seen that two connected Zn-air batteries

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could power a light-emitting diode (LED, 3 V, Figure 5f). For comparison, a similar Zn-air battery with commercial Pt/C of equal mass as air cathode was also made (Figure 5g). The

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open-circuit voltage (OCV) of the Zn-air battery with PyN-GDY is as high as 1.51 V (Figure

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S13a), a little higher than that with Pt/C (1.50 V). The maximum power density of PyN-GDYbased battery is 130 mW cm-2 (Figure S13b), comparable to those of Pt/C-based battey (136

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mW cm-2). The specific discharging capacity of the PyN-GDY-based battery is 647 mA h g-1, normalized to the mass of the consumed Zn foil, at a current density of 20 mA cm-2 (Figure S13c), a little higher than that of Pt/C-based battery (641 mA h g-1). The galvanostatic

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discharge and charge profiles at a current density of 2 mA cm-2 is further employed to

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investigate the cycling performance (Figure S13d). The intial discharging potential of PyNGDY-based battery is 1.14 V (1.10 V for Pt/C), while the onset charging potential is 2.06 V (2.06 V for Pt/C) at current density of 2 mA cm-2. More importantly, the discharging and charging potentials of PyN-GDY-based battery remain unchanged after 150 hours (225 cycles), whereas the discharging and charging potentials of Pt/C-based battery change from 1.10 and 2.06 V to 0.84 and 2.63 V, repectively, which demonstrates the remarkable stability 15

of PyN-GDY. The cycling performance at current density of 5 mA cm-2 was also tested, confiming the stability of PyN-GDY. 3.3. Density Functional Theory Calculations. Furthermore, to elucidate the possible active sites of PyN-GDY, we performed density functional theory (DFT) calculations to predict the Gibbs free energy changes of the ORR process on PyN-GDY. We choose the periodic structure of Figure S4a as a prototype structure to simulate the ORR process, considering that too large computational resources are needed

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for other periodic and the non-periodic structure of PyN-GDY. The details of computational method are given in the Supporting Information. To better understand the beneficial effect of pyridinic N in PyN-GDY, graphdiyne (GDY) with similar structure but without pyridinic N is

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used as a control. The possible active sites were labeled with Arabic numerals as shown in

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Figure 6a for GDY and Figure 6b for PyN-GDY. As the experiment results showed the number of transferred electrons of tested catalyst in the ORR, we concentrated on the

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discussion on the standard four-electron pathway. The whole ORR cycle for the standard

(3)

*OOH + H+ + e- → *O + H2 O(l)

(4)

*O + H+ + e- → *OH

(5)

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O2 (g) + H+ + e- + * → *OOH

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four-electron pathway under acidic condition can be divided into four elementary steps:

*OH + H+ + e- → H2 O(l) + *

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(6)

The standard four-electron pathway in alkaline media is summarized using the following four elementary steps: O2 (g) + H2 O(l) + e- + * → *OOH+OH*OOH + e- → *O + OH-

(7) (8)

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*O + H2 O(l) + e- → *OH + OH-

(9)

*OH + e- → * + OH-

(10)

The calculated Gibbs free energy changes of the ORR process on GDY and PyN-GDY in the acidic condition are shown in Figure 6c-f (at 0 V vs. standard hydrogen electrode, SHE) and Figure S14 (at 1.23 V vs. SHE), while the atomic configurations of ORR intermediate states on these reaction sites are displayed in the insets of Figure 6c-f and Figures S15-S18. The Gibbs free energy diagrams indicate that the hydrogenation of adsorbed O2 molecule (i.e.,

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the formation of *OOH intermediate) is the rate-limiting step in ORR for all the studied sites (Figure 6c-f and Figure S14). Moreover, it can be seen that the acetylenic carbon atoms linked to benzene ring (i.e., sites 2, 5 and 7) possess more negative free energy changes of rate-

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limiting step than other carbon sites for both GDY and PyN-GDY, which is caused by the non-uniform charge density distribution of GDY-based materials due to the existence of

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acetylenic carbon.[47] The rate-limiting step (i.e., step 1) is uphill for the sites 1, 3, 4, 6 and 8

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at 0 V (vs. SHE) for PyN-GDY, indicating that these sites would be inactive toward ORR. As for the sites 2, 5 and 7, the four elementary steps in the the ORR processes are all completely

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downhill at 0 V (vs.SHE), although the Gibbs free energy changes for the rate-limiting step are different at these three sites. This suggests that the sites 2, 5 and 7 could be active toward

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ORR. The corresponding Gibbs free energy changes in rate-limiting step are -0.24, -0.20 and 0.36 eV for the site 2, 5 and 7, respectively, at 0 V (vs.SHE). The free energy changes of the

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site 2 and 5 in PyN-GDY are similar with the site 2' (-0.23 eV) in GDY, which indicates that pyridinic N has less influence on sites 2 and 5 for PyN-GDY. Site 7, the acetylenic carbon nearest to the N atom, expresses the lowest free energy change for the rate-limiting step. Therefore, the site 7 would be the most active site toward ORR for PyN-GDY. The schematic profile of free energy change for the standard four-electron pathway of ORR on the site 7 of PyN-GDY in alkaline medium is given in Figure S14e. We can notice that OOH formation is 17

the rate-determining step of ORR and is endothermic in alkaline media under voltage of 0.402 V. 4. Conclusion In conclusion, a new carbon-based material, PyN-GDY, with relatively defined molecular structure and merely pyridinic N was rationally designed and synthesized. It is the first time that a facile cross-coupling reaction is used to control the bonding configuration of N in the carbon-based ORR catalysts accurately. The PyN-GDY was used as electrocatalyst for ORR

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in both alkaline and acidic media, which exhibited excellent catalytic activity, stability and methanol tolerance. With PyN-GDY as cathode, the Zn-air battery revealed high activity and superior stability. The results verify that pyridinc N is an effective N doping style for ORR.

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DFT calculations confirm that the acetylenic carbon atom nearest to the N atom is the most

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possible active site for PyN-GDY. This work will open a new avenue to controllably prepare carbon materials with specific N style through bottom-up strategy like chemical synthesis

Acknowledgements

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methods.

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Q.L. and N.W. contributed equally to this work. This study was supported by the National Natural Science Foundation of China (21790051, 51822208, 21771187, 21603250,

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21790050), the Frontier Science Research Project (QYZDB-SSW-JSC052) of the Chinese

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Academy of Sciences, the Natural Science Foundation of Shandong Province (China) for Distinguished Young Scholars (JQ201610), the Natural Science Foundation of Shandong Province (ZR2016BB24), and Project funded by China Postdoctoral Science Foundation (2016M592261). Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at 18

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http://dx.doi.org/10.1016/j.apcatb.xxxxxxx

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(2012) 20472-20479.

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Figure 1. (a) Schematic illustration of the synthetic route of PyN-GDY. (b, c) SEM images of

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PyN-GDY; inserts are model images for particle-cumulated PyN-GDY and one PyN-GDY particle. (d) The elemental mapping analysis of PyN-GDY. (e) TEM and (f) HRTEM images

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of PyN-GDY. (g) N2-adsorption/desorption isotherm of PyN-GDY.

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Figure 2. (a) Solid-state 13C NMR of PyN-GDY. High-resolution (b) C 1s and (c) N 1s XPS

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spectra of PyN-GDY. (d) The experimental and calculated curves of N K-edge X-ray

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absorption near-edge structure (XANES) spectra for PyN-GDY. (e) FT-IR spectra of the

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monomer and PyN-GDY. (f) Raman spectrum of PyN-GDY.

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Figure 3. (a) RDE polarization curves of PyN-GDY and Pt/C (JM) in O2-saturated 0.1 M

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KOH solution at a rotating speed of 1600 rpm with a scan rate of 5mV s-1. (b) Kinetic current density comparision and (c) tafel plots of PyN-GDY and Pt/C (JM). (d) Methanol tolerance

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curves of PyN-GDY. Stability curves of (e) PyN-GDY and (f) Pt/C.

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Figure 4. (a) RDE polarization curves of PyN-GDY and Pt/C (JM) in O2-saturated 0.1 M

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HClO4 solution at a rotating speed of 1600 rpm with a scan rate of 5mV s-1. (b) tafel plots of PyN-GDY and Pt/C (JM). Methanol tolerance curves of (c) PyN-GDY and (d) Pt/C (JM).

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Stability curves of (e) PyN-GDY and f) Pt/C.

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Figure 5. Performance of (a) onset potential (Eonset), (b) half-wave potential (E1/2), (c) limited

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current density (jL) and (d) comprehensive comparisons of PyN-GDY with reported representative N doped carbon-based metal-free catalysts for ORR in 0.1 M KOH. (e)

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Schematic diagram for the process of ORR on PyN-GDY, (f) Photograph of a LED powered by two Zn-air batteries in series with PyN-GDY as cathode. (g) rechargeability cycling tests

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of the Zn-air batteries using PyN-GDY or Pt/C as cathode at current density of 2 mA cm-2.

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Figure 6. (a) A fragment of graphdiyne (GDY), three possible C active sites for ORR were labeled with Arabic numerals. (b) A fragment of PyN-GDY, eight possible C active sites for

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ORR were labeled with Arabic numerals. (c) The free energy diagrams for ORR on GDY catalyst at 0 V (vs. standard hydrogen electrode, SHE) at the three possible C active sites in (a), inset is the ORR intermediate states of site 2'. (d-f) The free energy diagrams for ORR on PyN-GDY catalyst at 0 V (vs. SHE) at the eight possible C active sites in (b), insets are the ORR intermediate states of site 2 (d), site 5 (e) and site 7 (f), respectively.

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