Co9S8 nanoparticles embedded in multiple doped and electrospun hollow carbon nanofibers as bifunctional oxygen electrocatalysts for rechargeable zinc-air battery

Journal Pre-proof Co9 S8 Nanoparticles Embedded in Multiple Doped and Electrospun Hollow Carbon Nanofibers as Bifunctional Oxygen Electrocatalysts for rechargeable zinc-air battery Wei Peng, Yang Wang, Xiaoxiao Yang, Linchang Mao, Junhong Jin, Shenglin Yang, Kang Fu, Guang Li

PII:

S0926-3373(19)31183-X

DOI:

https://doi.org/10.1016/j.apcatb.2019.118437

Reference:

APCATB 118437

To appear in:

Applied Catalysis B: Environmental

Received Date:

15 April 2019

Revised Date:

14 October 2019

Accepted Date:

14 November 2019

Please cite this article as: Peng W, Wang Y, Yang X, Mao L, Jin J, Yang S, Fu K, Li G, Co9 S8 Nanoparticles Embedded in Multiple Doped and Electrospun Hollow Carbon Nanofibers as Bifunctional Oxygen Electrocatalysts for rechargeable zinc-air battery, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118437

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Co9S8 Nanoparticles Embedded in Multiple Doped and Electrospun Hollow Carbon Nanofibers as Bifunctional Oxygen Electrocatalysts for rechargeable zinc-air battery

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Wei Peng, Yang Wang, Xiaoxiao Yang, Linchang Mao, Junhong jin, Shenglin Yang, Kang Fu*, Guang Li*

State Key Laboratory for Modification of Chemical Fibers

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and Polymer Materials,

College of Materials Science and Engineering, Donghua

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University, Shanghai 201620, PR China

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*Corresponding authors, Emails: [email protected] (G. Li), [email protected] (K. Fu)

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Graphical Abstract

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Highlights  

The bifunctional catalysts is synthesized by the continuous coaxial electrospinning technology followed by pre-oxidation and annealing process. Co9S8-NSHPCNF exhibits both outstanding ORR and OER activity.

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 Excellent cycling performance and stability are afforded by Co9S8-NSHPCNF

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based liquid and all solid-state zinc-air battery.

Abstract: The development of low-cost electrocatalysts with highly efficient

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catalytic activity and strong durability toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for rechargeable zinc-air battery. In this work, Co9S8 nanoparticles embedded in N/S dual-doped hollow and porous carbon

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nanofibers (Co9S8-NSHPCNF) are prepared via the continuous coaxial electrospinning technology followed by pre-oxidation and carbonization treatment. Attributed to

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synergistic effects between the Co9S8 nanoparticles and NSHPCNF, higher specific surface area, hollow and porous structure, the doping of N and S and better electrical conductivity, the Co9S8-NSHPCNF bifunctional catalysts exhibit preferable performance toward ORR (E1/2 = 0.82 V) and OER (E j = 10 mA cm−2 = 1.58 V) compared to precious metal-based catalysts. In addition, when being used as a cathode catalyst in liquid zinc-air battery, Co9S8-NSHPCNF displays excellent durability. An all solid-state zinc-air battery with Co9S8-NSHPCNF is also fabricated and presents superb charge

and discharge cycling performance. Keywords: Co9S8 nanoparticles; hollow and porous carbon nanofibers (HPCNF); rechargeable zinc-air battery; bifunctional catalysts

Introduction

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The continuously increasing energy demand in modern society necessitates the development and exploration of advanced energy storage and conversion technologies, which are more sustainable, environmentally friendly as well as competitive in price [1-3]. With the high theoretical energy density, environmental benignity and safe

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operation, metal–air batteries (especially the zinc-air battery) are hailed as a green

technology that meets the energy requirements to power future electrical vehicles,

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portable electronics and other energy-consuming devices. Nevertheless, their main technology challenges are the sluggish kinetics of ORR during discharging and OER

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during charging at the air electrode: O2 + 2H2O + 4e– ↔ 4OH–. Although Pt-based and Ir/Ru-based compounds are considered as the benchmark catalysts toward ORR and

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OER, respectively, the large-scale commercial applications are significantly handicapped by their single catalytic activity, unsatisfactory stability, scarcity and high price. In this scenario, the ideally bifunctional oxygen electrocatalysts are highly

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expected to catalyze both ORR and OER efficiently. Many research endeavors, in response, have been made to design efficient

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electrocatalysts with feature of cheap and earth-abundant elements nature toward ORR/OER, such as heteroatom-doped carbon, transition metal based metal oxides, sulfides, nitrides and hydroxides etc. Particularly, the cobalt chalcogenides with different variants such as Co1-xS, CoS, Co3S4, Co9S8 [4], have been widely investigated as highly active catalytic materials for both ORR and OER in alkaline solutions. On the one hand, metal-sulfur covalent bonds strikingly provide ORR activities because of the

local charge density and surface charge states [5, 6]. On the other hand, Co-ions are the main active site for accelerating OER kinetics [7-9]. Albeit the intrinsic activity of cobalt chalcogenides, relatively poor electronic conductivity and severe aggregation should be circumvented if used as the electrode materials. As it is well-documented that the catalytic activities of cobalt chalcogenides could be conspicuously improved when coupling them with heteroatom-doped carbon materials. Zhong-Jie Jiang and coworkers prepared N, S dual-doped graphene supported CoS2 nanoparticles, demonstrating that the catalytic activity of CoS2/NSG is much higher than that of pure

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CoS2 [10]. In addition, John Wang synthesized hollow Co3O4 nanospheres embedded in carbon arrays, also proving that with the presence of carbon arrays, the activity performance of NC-Co3O4-30 outperforms pure Co3O4 nanospheres [11]. Therefore,

combining the cobalt chalcogenides with the carbonaceous materials can effectively

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enhance the catalytic activities of catalysts, which permit electron to transfer fast during the catalytic process [12-14].

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Among the carbon materials, hollow and porous carbon nanofibers (HPCNF) with their well-defined inner channels, high surface areas, larger aspect ratio, reasonable

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pore structures and excellent electrical conductivity [15] can provide more active sites and shorten electron and mass transport length from the exterior into the interior to

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accelerate O2 and electrolyte diffusion [16-18]. Inspired by above, in this study, Co9S8 nanoparticles embedded in N and S dualdoped and electrospun hollow porous carbon nanofibers (Co9S8-NSHPCNF) were

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synthesized by the continuous coaxial electrospinning technology followed by preoxidation and annealing process at 800 ℃ with a ramp rate of 5 ℃ min-1. During the

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annealing process, polymethyl methacrylate (PMMA) from the precursor solution became gaseous and evaporated, and its original position became porosity, resulting in a hollow and porous structure. Thiourea in the reaction mixture acted as the sulphur and nitrogen sources. Besides, sulphur from thiourea can react with Co2+ to form the Co9S8 nanoparticles. Attributed to synergistic effects between the Co9S8 nanoparticles and NSHPCNF, higher specific surface area, porous and hollow structure, the doping of N and S and better electrical conductivity, the as-prepared Co9S8-NSHPCNF exhibits

improved catalytic performance and high durability toward the ORR and OER. Further, zinc-air battery assembled with Co9S8-NSHPCNF as air cathode displayed a discharge peak power density (113 mW cm-2), higher than that of the Pt/C (80 mW cm-2) and excellent cycling stability. An all solid-state zinc-air battery based Co9S8-NSHPCNF was also prepared and showed outstanding flexibility and preferable cycling stability performance under different bending angles.

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2. Experimental section 2.1. Synthesis of samples

Synthesis of Co9S8-NSHPCNF: the carbon nanofibers were fabricated by a

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continuous coaxial electrospinning/annealing technique, which was illustrated

in Fig. 1. In a typical experiment, 1.05 g polyacrylonitrile (PAN, Mw=130000), 0.45 g

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polymethyl methacrylate (PMMA), 2.0 g cobalt(II) acetate tetrahydrate (Co (Ac)2·4H2O) and 0.8 g thiourea were first dissolved in the solvent of N,N-

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dimethylformamide (DMF) (10 mL), which was used as the shell layer precursor solution. While the sacrificial core layer precursor solution was obtained by dissolving

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2.2 g PMMA into 6 mL DMF. Both of the abovementioned mixing solutions were under magnetic stirring for 12 h at the room temperature till a homogeneous precursor solution was formed and then loaded into two plastic syringes (10mL), respectively. The

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electrospinning was conducted at ambient temperature (20 ± 1℃) and relative humidity(30±2%). The spinneret that connected with the plastic syringes was made up

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of two coaxial stainless-steel needles with diameters of 18-gauge and14-gauge (outer diameter ~ 1.6 mm, inner diameter ~ 0.86 mm). The two feeding solutions were pumped by a syringe pump and the constant flow rates of outer and inner solutions were maintained as 0.5 and 0.3 mL h-1, respectively. The distance between the needle tip and grounded collection board was fixed as 15 cm and the high stable voltage was set to 18 kV. Thereafter, the as-prepared electrospun nanofibers were

detached

from the

collector and subjected to stabilization at 280 ℃ for 0.5 h at air atmosphere. The pre-

oxidized fibers were then placed in the porcelain boat to be annealed at 800 ℃ for 2 h with the heating rate of 5 ℃ min-1 under argon atmosphere. After cooled down to room temperature, the obtained nanofiber material was labeled as Co9S8-NSHPCNF. As reference, Co-NHPCNF were also prepared under the identical process in the presence of urea instead of thiourea and N-SHPCNF were prepared in the absence of Co source. And the third sample named Co9S8-NSPCNF without hollow structure was also obtained by replacing PMMA from the core layer precursor solution with PAN.

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2.2. Structure characterization The structures and morphologies of the as-synthesized catalysts were observed by

field emission scanning electron microscope at 5 kV with a beam current of 10 μA

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(FESEM, Hitachi SU 8000) and transmission electron microscope (TEM, JEOL-2100).

And elemental mapping was conducted to differentiate the element dispersion in as-

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prepared samples. The crystalline structures of all samples were analyzed by the powder X-ray diffraction (XRD) patterns using the Cu Kα radiation source. X-ray photoelectron

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spectroscopy (XPS) was used to analyze the surface valence states of samples. The nitrogen adsorption/desorption isotherms (ASAP2020 volumetric adsorption analyzer, USA) were tested to measure the surface area of the samples. Via-Reflex microscopic

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confocal Raman spectrometer (Renishaw, UK), whose laser excitation wavelength was 532 nm, was used to measure Raman spectra. Inductive Coupled Plasma Emission

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Spectrometer (ICP) (Prodigy-ICP) was used to analyze to the element content of

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samples.

2.3. Electrochemical measurements The electrochemical tests of all samples were conducted by means of a CHI 760E

electrochemical workstation as well as a rotation speed controller (Pine Instrument Co, AFMSRCE 3699). As for electrochemical tests, firstly, the catalysts after carbonization were grounded into powders. Then the mixed solution of catalysts powders (2 mg), ethanol (1 ml) and 5wt % Nafion (50 μL) was subjected to ultrasonic irradiation for

about 30 minutes so as to form a homogenous catalysts ink. Finally, the ink (~ 30 μL) was dropped on a glass carbon electrode whose area was 0.196 cm2 to achieve a catalyst loading of ~ 0.3 mg cm-2. During the testing process, three electrodes system (saturated calomel electrode as reference electrode, glass carbon electrode as working electrode and platinum foil as counter electrode) was employed in 0.1 M aqueous electrolyte which was bubbled with oxygen or nitrogen according to different testing requirements. Cyclic voltammetry (CV) was performed at a scan rate of 50 mV s-1 in saturated oxygen or nitrogen environments. With respect to the ORR tests, linear sweep voltammetry

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(LSV) was carried out at 5 mV s-1 with different rotating speeds ranging from 400 to 2500 rpm with the potential scope of 0.2 V to 1.1 V in O2-saturated electrolyte. While the OER measurements were conducted in the N2-saturated electrolyte. The 20% Pt/C and IrO2 as the benchmarks were tested via the above same methods. Electrochemical

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impedance spectroscopy (EIS) measurements were performed in the frequency range

from 0.1 Hz to 106 Hz with an alternating current voltage amplitude of 5 mV. All

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potentials in this work were referenced to the reversible hydrogen electrode (RHE).

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2.4. Liquid zinc-air battery assembly and testing The rechargeable liquid zinc–air battery, as shown in Fig. S14, was made up of

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anode material (a polished zinc plate), current collector (stainless steel net for cathode and copper foil for anode), the electrolyte (6 M KOH and 0.2 M zinc acetate solution)

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and the cathode material. The cathode material was a catalyst layer (carbon paper) whose one side was coated with gas diffusion layer and another side was sprayed with

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the electrocatalyst ink made by mixing a certain amount of ethanol, 5% Nafion solution and our catalysts powders to obtain a catalyst loading of ~ 1.0 mg cm-2. Charge and discharge curves were tested by a galvanodynamic method with the current density ranging from 0 to 90 mA cm-2 and the charge and discharge cycling performance was measured with the battery testing system (Neware) at the current density of 2 and 10 mA cm-2, respectively. Each cycle period was set to 10 (5 min discharge and 5 min charge for 2 mA cm-2) and 20 min (10 min discharge and 10 min charge for 10 mA cm-

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), respectively.

2.5. All solid-state zinc-air battery assembly The fabrication process of all solid-state zinc-air battery was displayed in Fig. S15. The whole battery was composed of four parts including zinc foil as anode, carbon cloth as cathode, PVA gel as the solid electrolyte and Ni foam as cathode current collector. The air electrode was formed by spraying the electrocatalyst ink (the mixed solution of catalyst powder, ethanol and 5% Nafion solution) onto one side of carbon cloth.

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Furthermore, the PVA gel was synthesized according to the article [19]. Charge and

discharge curves were tested by a galvanodynamic method with the current density ranging from 0 to 120 mA cm-2 and the charge and discharge cycling performance was

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measured with the battery testing system (Neware) at the current density of 5 mA cm-2.

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Each cycle period was set to 10 (5 min discharge and 5 min charge).

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3. Results and discussion

3.1. structural and morphological characterization

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As illustrated in Fig. 1, the main synthetic process to prepare Co9S8-NSHPCNF is involved with a continuous coaxial electrospinning and following annealing technique.

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Briefly, PAN, PMMA, Co (Ac)2·4H2O and thiourea are firstly dissolved in DMF, which is the shell layer precursor solution. While the core layer precursor solution is made by

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dissolving PMMA into DMF. After electrospinning, the as-prepared carbon nanofibers are subjected to annealing process, during which the PMMA becomes gaseous and evaporated, and its original position becomes porosity, resulting in a porous and hollow structure. In addition, thiourea is selected as the all-in-one N- and S-containing precursor to synthesize NSHPCNF while simultaneously supplying a sulfur source reacting with the metal precursors in the nanofibers to form Co9S8 nanoparticles on the surface of nanofibers. Finally, the Co9S8-NSHPCNF is obtained after the product

pyrolyzed cool down to room temperature. SEM and TEM are used to reveal the surface and cross-sectional morphology of the as-prepared samples. As shown in Fig. 2a, it is apparently revealed that long fibrous morphology with hollow structure of Co9S8-NSHPCNF can be observed. The outer and inner diameter of the nanofibers are ~ 750 and 420 nm, respectively, which are less than the outer and inner diameter of the electrostatic spinning needle. This is attributed to the shrinkage of as-prepared nanofibers during the annealing process. In addition, the surface of Co9S8-NSHPCNF are filled with uniform white nanoparticles marked in Fig.

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2b. This is in consistent with TEM image in Fig. 2c, which presents a hollow structure comprising many small black dots dispersed on the semi-transparent nanofibers.

Because of well crystallized, the lattice fringes of the Co9S8 nanoparticles is obviously observable, which is shown in High-resolution TEM (HRTEM) image (Fig. 2d). The

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interlayer spacing of nanoparticles is determined to be 0.3 nm, well corresponding to the (311) plane of Co9S8, indicating that these small black dots are assignable to the

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Co9S8 nanoparticles. The selected area electron diffraction (SEAD) pattern (Fig. 2d inset) of Co9S8-NSHPCNF suggests that it is polycrystalline, in good agreement with

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the HRTEM observation (Fig. 2d). In particular, a great number of pores (marked in Fig. 2c), ascribed to decomposition of the PMMA from the surface of nanofibers during

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the annealing process, can be seen, resulting in the porous structure. The SEM images of Co9S8-NSPCNF, Co-NHPCNF and N-SHPCNF are presented in Fig. S1, Supporting Information. Because of without PMMA in the core layer precursor solution, structure

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of nanofibers is only obtained from Co9S8-SNPCNF. Co-NHPCNF and N-SHPCNF own typical hollow structure stemming from the pyrolysis of PMMA. The high-angle

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annular dark field scanning transmission electron microscope and energy dispersive Xray spectroscopy mapping (HAADF-STEM-EDS) are employed to observe the compositions of Co9S8-NSHPCNF. Clearly, Co, S, N, O and C are distributed uniformly on the entire nanofibers, which is displayed in Fig. 2e. The XRD patterns of Co9S8-NSHPCNF, Co9S8-NSPCNF, Co-NHPCNF and NSHPCNF are compared in Fig. 3a. In the pattern of Co9S8-NSHPCNF, it is observed

that the three noticeable peaks located at 29.7o, 31.1o and 51.9o are indexed to the (311), (222) and (440) reflections of Co9S8 (JCPDS No.02-1459), respectively, which is indicative of the existence of Co9S8 nanoparticles on the surface of carbon nanofibers, matching well with the results of the HRTEM (Fig. 2d). Moreover, there is a weak diffraction peak at about 26.2o that is assigned to the (002) plane of graphitic carbon, implying the existence of graphitic materials in the Co9S8-NSHPCNF. It is worth noteworty that that XRD patterns of Co9S8-NSHPCNF and Co9S8-NSPCNF have same diffraction peak position, indicating that this preparation method can lead to the

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formation of Co9S8 nanoparticles. In addition, the characteristic peaks of Co-NHPCNF (2θ = 44.3o, 51.5o and 75.9o, respectively) are ascribed to the reflections of metallic Co particles [20] (JCPDS No.15-0806), disclosing that Co2+ are reduced to metallic cobalt

during the process of calcination. Due to without metal precursor, N-SHPCNF has only

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typical diffraction peak at about 26.2o, attributable to the (002) plane of graphitic carbon. The N2 adsorption/desorption isotherm of Co9S8-NSHPCNF and Co9S8-NSPCNF

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is used to evaluate Brunauer-Emmett-Teller (BET) specific surface area. As shown in Fig. 3b, Co9S8-NSHPCNF exhibits Type Ⅳ isotherms with hysteretic loops at high-

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pressure region, which is attributed to the presence of micropores and mesopores [10]. These pores may originate from the decomposition of PMMA during the annealing

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process and the accumulation of Co9S8 nanoparticles on the entire nanofibers. The inset in Fig. 3b presents the pore size distribution of Co9S8-NSHPCNF, suggesting that this sample own the pore form of micropores and mesopores, which can improve the high

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surface area and expose more active sites to the reactants [21]. The Co9S8-NSHPCNF is possessed of a high surface area of 375.5 m2 g-1, much higher than that of the Co9S8-

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NSPCNF without hollow structure (271.8 m2 g-1) (Fig. S2, Supporting Information). Raman spectroscopy is a formidable technique to provide evidence of the extent

of graphitization of carbonaceous materials. Fig. 3c clearly presents the Raman spectra of Co9S8-NSHPCNF, Co-NHPCNF, N-SHPCNF and Co9S8-NSPCNF. Notably, there are two remarkable peaks located at ~ 1335 (D) and 1590 cm-1 (G) in all composites, indicating the presence of the graphitic structure, which is in accordance with XRD results. The existence of G band reveals the presence of nanocrystalline carbon and a

higher content of sp2-hybridized carbon atoms that is caused by the high temperature treatment of samples, whereas the D band is a clear sign of disordered carbon [22, 23]. In addition, it is well established that the lower the value of ID/IG is, the improved graphitization degree the catalysts are equipped with [24]. Clearly, the value of ID/IG of Co9S8-NSHPCNF (0.95) is lower than that of other samples (1.05 for Co9S8-NSPCNF, 1.18 for N-SHPCNF and 0.98 for Co-NHPCNF), indicating more defects and improved graphitization degree of Co9S8-NSHPCNF, which is beneficial to lead to an enhanced electrical conductivity [25].

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To further gain insight into the chemical status and bonding configurations of assynthesized catalysts, XPS measurements are conducted. The wide-scan survey spectra

of Co9S8-NSHPCNF, Co-NHPCNF, N-SHPCNF and Co9S8-NSPCNF are exhibited in Fig. S3, Supporting Information. As expected, the observed C, N, O, Co and S elements

exist in Co9S8-NSHPCNF, which is in accordance with element mapping results of

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TEM (Fig. 2e). The high-resolution spectrum of Co 2p (Fig. 3d) from Co9S8-NSHPCNF and Co9S8-NSPCNF reveal that there are four kinds of Co species whose peaks are

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located at 782.5, 781.2, 779.3, 787.7 and 785.1 eV. The peaks at 782.5 eV is attributable to CoCxNy [26] and the peaks at 780.3 eV is ascribed to Co-Nx [27] which may be

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induced by Co with N from thiourea in the carbon nanofibers, confirming that N is doped into Co9S8-NSPCNF. The minor peaks at 787.7 and 785.1eV are indexed to the shake-up satellite. The domination of peak at the low binding energy of 779.3 eV is

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assigned to CoSx, suggesting that the Co element in the Co9S8-NSHPCNF and Co9S8NSPCNF mainly exists in the form of Co9S8. By contrast, a new peak at 778.8 eV

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corresponding to Coo can be observed in Co-NHPCNF, agreeing well with XRD results (Fig. 3a). The spectrum of S 2p is presented in Fig. 3f. The peak at 163.9 eV is ascribed to Co-S of S 2p from Co9S8-NSHPCNF [27], which is consistent with Co 2p results of

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Co9S8-NSHPCNF at the binding energy of 779.3 eV, while a peak at 164.9 eV, corresponding to the -C=S- bond, can be observed [27]. This bond may be stemming from excess S in the thiourea doped into the carbon nanofibers scallfold during the annealing process of Co9S8-NSHPCNF, corroborating that in addition to the reaction between S and Co, S has been doped into the nanofibers structure. Except for these two peaks, the deconvoluted S 2p spectrum also displays minor peaks at 168.5 and 169.8 eV, which is ascribed to SOx- [26]. The N 1s spectrum is shown in Fig. 3e and the peak

at 399.2 eV is corresponded to Co-N, which is in accordance with Co 2p results of Co9S8-NSHPCNF at the binding energy of 780.3 eV. Except for this peak, the N 1s spectrum can be further deconvoluted into four peaks, which are pyridinic-N (398.6 eV), pyrrolic-N (400.6 eV), graphitic-N (401.3 eV) and oxide-Nx (404.2 eV), respectively [20, 26]. It is well established that graphitic-N acts as the significant role to boost the electrocatalytic reaction toward ORR, pyridinic-N takes responsibility for the formation of C-N sites [28, 29] and the Co-N sites can enhance ORR activity too [30].

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3.2. Electrochemical catalytic property In order to evaluate the electrochemical activity for the ORR, CV were first

conducted under O2-saturated or N2-saturated in 0.1 M KOH solutions with a threeelectrode system at scan rate of 50 mV s-1. As illustrated in Fig. 4a, the CV curve of

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Co9S8-NSHPCNF exhibits a well-defined peak (0.76 V) in O2-saturated solution, ascribed to the reduction of oxygen, while that in N2-saturated solution exhibits

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featureless voltammetric currents, reflecting the excellent electrochemical activity of Co9S8-NSHPCNF for the ORR. Similar results can be obtained from the LSV curves,

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which is displayed in Fig. 4b. Obviously, Co9S8-NSHPCNF catalysts own outstanding ORR activities in terms of half-wave potential (E1/2 = 0.82 V), which is only 10 mV lower than Pt/C (E1/2 = 0.83 V). Furthermore, the limiting current of Co9S8-NSHPCNF

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was -4.81 mA cm-2 at 0.2 V, close to that of Pt/C (-5.27 mA cm-2), demonstrating that Co9S8-NSHPCNF has comparable mass transfer and catalytic performance of oxygen during the ORR process. All results distinctly testify the enhanced ORR performance

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of Co9S8-NSHPCNF.

To find out the reasons that Co9S8-NSHPCNF is possessed of high catalytic

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activity, the electrochemical activity for Co9S8-NSPCNF, Co-NHPCNF, and NSHPCNF toward ORR is also investigated and compared with Co9S8-NSHPCNF. As showed in Fig. S4, Supporting Information and Fig. 4b, clearly, though N-SHPCNF is active for the ORR, its catalytic performance is much lower than Co9S8-NSHPCNF, which is demonstrated by half-wave potentials and onset potentials (Table S1). This suggests that synergistic effects between the Co9S8 nanoparticles and NSHPCNF is vital to enhance the catalytic activity of Co9S8-NSHPCNF. In addition, high specific surface area may be another important factor accounting for the improved the electrochemical

activity for Co9S8-NSHPCNF toward ORR. This can be verified by the ORR activity between and Co9S8-NSHPCNF and Co9S8-NSPCNF which are prepared with the same procedure. Although they have similar composition component including Co9S8 nanoparticles and N, S co-doped CNF, larger specific surface area and appropriate pore size distribution are also crucial to the mass transfer for further moderating the ORR kinetics [31, 32]. Besides, Co-NX also plays important role in improving the catalytic activity, confirmed by the ORR performance of Co-NHPCNF and N-SHPCNF. The XPS spectra in Fig. 3e and Fig. 3f, exhibit that although N-SHPCNF is doped with N and S, its catalytic performance is much lower than Co-NHPCNF in term of onset

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potential, the limiting currents and half-wave potentials (Table S1), implying that CoNX in the Co9S8-NSHPCNF can also promote its catalytic activity to some extent.

Additionally, The doping of S and N can improve the hydrophilicity of Co9S8NSHPCNF which makes the Co9S8-NSHPCNF more accessible to the electrolyte [10] and promote the O2 reduction in alkaline medium [33]. The electrochemical

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impendence spectra (EIS), which is displayed in Fig. S5, exhibit that Co9S8-NSHPCNF has faster electron-transfer kinetics (Rct = 29.5 ohms) than that of Pt/C (Rct = 31 ohms)

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and other samples verified from the smaller size of semicircle in high frequency. Such remarkable catalytic performance of Co9S8-NSHPCNF is believed to be

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governed by the following reasons. (1) synergistic effects between the Co9S8 nanoparticles and NSHPCNF; (2) higher specific surface area; (3) porous and hollow structure; (4) the doping of S and N; (5) better electrical conductivity.

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To find out the ORR pathway for the catalysts, rotating disk electrode (RDE) measurements were tested by adjusting the rotating speeds ranging from 400 to 2500 rpm. With the increment of rotating speeds, the current density also shows a linear

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increase (Fig. 4c). Furthermore, Koutecky–Levich (K–L) plots (Fig. 4c inset) are drawn from the relationship between J−1 and ω−1/2. In the potential range from 0.4 V to 0.6 V,

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the electron transfer number for Co9S8-NSHPCNF is approximately 3.88, which clearly clarify the essential reason in terms of reaction kinetics that Co9S8-NSHPCNF has excellent ORR properties. The electron transfer number of other as-prepared samples and Pt/C are shown in Fig. S6, Supporting Information. Furthermore, the mass transfer corrected Tafel curves are drawn to obtain further information about the ORR mechanism [34]. As shown in Fig. 4d，the Tafel slopes for Co9S8-NSHPCNF, CoNHPCNF, N-SHPCNF, Co9S8-NSPCNF and Pt/C are 65, 78, 180, 115 and 70 mV/dec,

respectively. Closed values between Co9S8-NSHPCNF and Pt/C mean our synthesized catalyst has similar rate-determining step like Pt/C [35]. The excellent performance also imply that the Co9S8-NSHPCNF is a promising ORR electrocatalyst, and comparable to some recently reported papers [26, 27, 36-38] including Co9S8 nanoparticles. The electron transfer number (n) and peroxide yield (HO2− %) are calculated according to LSV curves of rotating ring disk electrode (RRDE) to reveal the reaction pathway. As shown in Fig. S7, a, Co9S8-NSHPCNF possesses comparable values of n and peroxide yield (n＞3.84，H2O2%＜7.86%) to 20 wt% Pt/C (n＞3.85，H2O2%＜7.40%）， indicting preferable selectivity during ORR process. In addition, the value of n (Co9S8-

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NSHPCNF) is in good agreement with K-L results. The electron transfer number and peroxide yield of other three samples are displayed in Fig. S7, b. It can be seen that the electron transfer numbers for Co-NHPCNF, N-SHPCNF and Co9S8-NSPCNF are approximately 3.78, 2.10 and 3.55, respectively.

Other than the catalytic performance, the ORR durability of as-prepared catalysts,

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which is also an effective indicator of ORR activity, is firstly performed before and after

CV cycling for 2000 times at 50 mV s-1 within the potential ranging from 0.56 to 0.96

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V, which is displayed in Fig. 4e, f. The value of E1/2 for Co9S8-NSHPCNF negatively shifts by ~ 8 mV, compared to 20 wt% Pt/C (~ 30 mV) before and after the CV cycling.

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Secondly, Fig. S8, a displays only 15.1% current reduction for the Co9S8-NSHPCNF catalyst after a 10000 s chronoamperometric test. However, more than 34.6 % of its original catalytic current is lost for the Pt/C catalyst under the same condition, reflecting

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that Co9S8-NSHPCNF possesses the remarkable durability during ORR processes. Furthermore, the methanol tolerant ability is also another crucial indicator to evaluate the ORR catalysts, which is displayed in Fig. S8, b. Mostly impressively, with the

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addition of 3 M methanol into the 0.1 M KOH, Co9S8-NSHPCNF exhibits better resistance to methanol than Pt/C, confirming the excellent methanol tolerant ability of

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Co9S8-NSHPCNF.

The OER activities of Co9S8-NSHPCNF, Co-NHPCNF, N-SHPCNF, Co9S8-

NSPCNF and IrO2 were also tested by using LSV obtained from the RDE at 1600 rpm in O2-saturated 0.1 M KOH solutions. Particularly, Fig. 5a presents that an overpotential of 350 mV at 10 mA cm-2 is afforded by Co9S8-NSHPCNF, which is much smaller than Co-NHPCNF (470 mV), Co9S8-NSPCNF (530 mV) and IrO2 (430 mV). The better OER activity can be confirmed by the Tafel slope. As showed in Fig. 5b, Co9S8-

NSHPCNF displays a smaller Tafel slope (103 mV dec-1) compared to IrO2 (190 mV dec-1), Co-NHPCNF (126 mV dec-1) and Co9S8-NSPCNF (207 mV dec-1). In addition, to estimate the electrochemical surface area (ECSA), the value of the double layer capacitance (Cdl) was obtained by cyclic voltammetry (CV) conducted at different scan rates due to proportional correlation between Cdl and ECSA [34, 39]. As shown in Fig. S9, with the scan rates ranging from 20 to 180 mV s-1 and potential ranging from 0.97 to 1.17 vs. RHE, the Cdl of Co9S8-NSHPCNF is 30.0 mF cm-2, which is larger than that of Co9S8-NSPCNF (16.0 mF cm-2), Co-NHPCNF (15.3 mF cm-2) and N-SHPCNF (2.2

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mF cm-2), disclosing that Co9S8-NSHPCNF possesses more active sites for OER (Fig. S10). Moreover, the turnover frequencies (TOFs) was also performed to probe the catalytic kinetic towards the OER, assuming that all the metal ions in the catalysts were active sites [39, 40]. As displayed in Fig. S11, with the help of ICP to analyze to the

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content of Co (Table S2), the TOFs of Co9S8-NSHPCNF is 15.6 s-1 at an overpotential

of 1.55 V, larger than that of Co9S8-NSPCNF (2.8 s-1) and Co-NHPCNF (10.2 s-1),

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demonstrating that Co9S8-NSHPCNF owns fastest rate among all as-prepared samples for OER catalysis.

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The discrepant activity among Co9S8-NSHPCNF, Co-NHPCNF and N-SHPCNF manifests that the better catalytic performance of Co9S8-NSHPCNF may benefit from

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N dopant and the introduced cobalt in precursor which facilitates the generation of graphitic domains to promote the charge transfer and causes OH− absorbed by cobalt atom with a lower impede [41].

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Likewise, the OER durability (Fig. 5c, d) of Co9S8-NSHPCNF and IrO2 were firstly tested before and after CV cycling for 2000 times at 50 mV s-1 within the potential

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ranging from 1.46 to 1.86 V. Clearly, The OER LSV of Co9S8-NSHPCNF exhibits a positive shift of ~ 13 mV for Ej = 10 mA cm-2, smaller than the value of IrO2 (~ 65 mV), manifesting that Co9S8-NSHPCNF owns promising durability during OER processes. Then the OER chronoamperometric measurement of Co9S8-NSHPCNF also show that Co9S8-NSHPCNF presents a slower degradation rate with 82.5% retention of the initial current density after 12000 s (Fig. S8, c), once suggesting Co9S8-NSHPCNF owns promising durability during OER processes.

From the above, the ORR/OER bifunctional activity of all samples are further ORR evaluated via ∆E (∆E = EOER j = 10 mA cm -E1/2 ) between the potential of OER at current density −2

of 10 mA cm−2 and potential of ORR (E1/2). A lower ∆E value reflects better catalytic activity and high efficiency [42] (Fig. 5e). Clearly, Co9S8-NSHPCNF (Fig. 5f) has comparable ∆E value of 0.76 V compared to Pt/C + IrO2 (0.85 V), Co9S8-NSPCNF (1.04 V) and Co-NHPCNF (0.96 V) and many recently reported bifunctional electrocatalysts (Table S3), indicting an excellent bifunctional electrocatalytic activity,

3.3. Liquid zinc-air battery performance

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and may be applied to rechargeable zinc–air batteries and all solid-state zinc-air battery.

Co-based materials are promising candidates for zinc-air battery [43]. To further

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testify the real performance of our catalysts, based on the high bifunctional activity of

Co9S8-NSHPCNF, a home-made rechargeable zinc–air battery with polished zinc plate

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as anode, carbon paper (hydrophobic-treated) coated by catalysts as air cathode and 6 M KOH solutions containing 0.2 M zinc acetate as electrolyte was fabricated according

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to Fig. 6a. Fig. S12 exhibits that the Co9S8-NSHPCNF-based zinc-air battery has an open circuit voltage of 1.44V. Fig. 6b shows the discharge polarization curves and corresponding power density curves of zinc-air battery that used Co9S8-NSHPCNF and

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Pt/C as the air electrodes, respectively. Apparently, the battery made of Co9S8NSHPCNF has a peak power density of 113 mW cm-2, 30 mW cm-2 higher than Pt/C.

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We further investigate the charging and discharging property of rechargeable zinc-air batteries based on 20% Pt/C + IrO2 and Co9S8-NSHPCNF as the air cathode in alkaline

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medium. As illustrated in Fig. 6c, compared to Pt/C+IrO2-based battery, the battery with Co9S8-NSHPCNF as air cathode has almost the same discharge curve when the current density ranges from 0 to 40 mA cm-2. Nevertheless, the discharge performance of Co9S8-NSHPCNF is better than Pt/C+IrO2 with the continued increase of the current densities. With respect to the charging process, the battery based on Co9S8-NSHPCNF exhibits lower voltage compared to its rival (20% Pt/C+IrO2), especially in high current density. For instance, at the current density of 80 mA cm-2, the voltage gaps between

charge and discharge voltage for battery (Co9S8-NSHPCNF) is about 1.62 V, however, it is 1.884 V for the battery (20% Pt/C+IrO2), demonstrating better performance of charging and discharging. Additionally, at the current density of 20 mA cm-2, the specific capacity of the zinc-air battery with Co9S8-NSHPCNF as air cathode is about 823.5 mAh g-1 after normalized to the mass of consumed zinc plate (Fig. 6d), which is comparable to its counterpart 20% Pt/C (781.7 mAh g-1). In order to verify the durability and stability of Co9S8-NSHPCNF catalyst, the tests at the current density of 2 mA cm-2 (5 min charge and 5 min discharge) and10 mA cm2

(10 min charge and 10 min discharge) were conducted, respectively. As shown in Fig.

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6e, when cycled at the constant current density of 2 mA cm-2, the zinc-air battery made

of Co9S8-NSHPCNF as air cathode is cycled 1000 times. When the current density is 10 mA cm-2, the battery based on Co9S8-NSHPCNF produces an initial charge potential

of 2.00 V and discharge potential of 1.16 V, and it was cycled for more than 200 times

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with small voltage losses. By contrast, the Pt/C and IrO2 is only cycled for 75 times. In

addition, as presented in Fig. 6f (2 mA cm-2), the voltaic efficiency is 62.7% at the first

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cycle and maintains at 54.7% after 1000 cycles (166h). At the current density of 10 mA cm-2, the voltaic efficiency (Fig. 6g) is 58.1% (1st cycle) and 55.7% (200th cycle),

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proving the outstanding rechargeability of Co9S8-NSHPCNF. The excellent cycling durability mainly originates from its special structure of porous and hollow structure, synergistic effects between the Co9S8 nanoparticles and NSHPCNF as well as S and N

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doping. However, after several hundred circulations, the activity decay of Co9S8NSHPCNF-based battery is ascribed to the degradation of zinc anode and damage from positive potential during OER , which can induce the loss of active sites and catalysts

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oxidization. The Pt/C + IrO2 -based battery is stuck with not only the subsequent detachment of Pt nanoparticles from the carbon support but also the carbon support

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corrosion [44]. Fig. S13 displays the SEM and XRD results after cycling. Clearly, the Co9S8-NSHPCNF catalyst maintains its original hollow and porous morphology after long time cycling. The XRD result also confirms that phase structure of Co9S8NSHPCNF changes little. Therefore, the battery with Co9S8-NSHPCNF as air cathode is in possession of much better stability and durability. Fig. 6h presents a light-emitting diode (LED) with DHU (Dong Hua University) characters is driven by three zinc-air battery in series.

3.4. All solid-state zinc-air battery performance In recent years, with the advent of wearable devices and artificial skins, all solidstate zinc batteries which owns the advantages of safety and flexibility [45-49] have attracted much interests as promising next-generation energy storage devices [50]. Here an all solid-state zinc-air battery was constructed, whose configuration was displayed in Fig. 7a. The discharge polarization curves and corresponding power density curves are showed in Fig. 7b. Obviously, the power density of the battery whose air cathode is

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Co9S8-NSHPCNF catalyst is higher than the battery with 20% Pt/C+IrO2 when the Current density is under high current density (40-100 mA cm-2).

Likewise, the durability and stability of Co9S8-NSHPCNF were tested at the current density of 5 mA cm-2 (5 min charge and 5 min discharge). As displayed in Fig.

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7c, the battery can cycle for about 30 cycles (5 h) under different bending angles,

confirming that our catalyst has excellent endurance property when it is used in proper

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situation. The photograph of Fig. 7d shows that the open-circuit voltage is measured to be 1.338 V. As a demonstration, a small fan using the battery with Co9S8-NSHPCNF as

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power source is working, disclosing its potential application in real environment.

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4. Conclusions

Conclusively, Co9S8 embedded in sulfide and nitrogen co-doped hollow and

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porous carbon fibers were synthesized by means of the continuous coaxial electrospinning technology followed by pre-oxidation and annealing process. The

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obtained Co9S8-NSHPCNF catalyst owns good ORR and OER property, which is used as excellent bifunctional catalyst further applied to zinc-air batteries. The liquid zincair battery based on Co9S8-NSHPCNF presents better stability and durability than its counterpart battery (Pt/C + IrO2). In addition, a solid-state zinc-air battery is also fabricated and show stable performance under different bending angles. The good performances of Co9S8-NSHPCNF catalyst are ascribed to synergistic effects between the Co9S8 nanoparticles and NSHPCNF, higher specific surface area, porous and hollow

structure, the doping of N and S and better electrical conductivity. Therefore, this study provides a facile and cost-efficient method for the synthesis of composite materials of HPCNF and metallic compound applied in metal-air batteries.

Notes The authors declare no competing financial interest.

Acknowledgments

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This work was financially supported by China Petrochemical Corporation through the Research Projects of 219009-1.

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Figure captions Fig.1. Schematic diagram for the preparation of Co9S8-NSHPCNF. Fig. 2. (a, b) SEM images of Co9S8-NSHPCNF; (c) TEM image of Co9S8-NSHPCNF; (d) HRTEM image of Co9S8-NSHPCNF; the inset image displays SAED pattern of Co9S8-NSHPCNF; (e) the corresponding element mapping images of N, Co, S, O and C. Fig.3. (a) XRD patterns of all prepared samples; (b) N2 adsorption/desorption isotherms

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of Co9S8-NSHPCNF, the inset image displays the corresponding pore size distribution curve; (c) Raman spectra of all synthesized samples; (d) Co 2p XPS; (e) N 1s XPS; (f) S 2p XPS.

Fig.4. (a)CV curves of Co9S8-NSHPCNF and Pt/C in N2- and O2-saturated 0.1M KOH

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at the scan rate of 50 mV s-1; (b) LSV curves of all the prepared samples and Pt/C

toward ORR at 5mV s-1 with the rotation speed of 1600 rpm; (c) LSV curves of Co9S8-

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NSHPCNF at different rotation speeds, the inset shows the K-L plots of Co9S8NSHPCNF at various potentials; (d) Mass transfer corrected Tafel plots of all as-

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prepared catalysts. (e, f) the ORR durability of Co9S8-NSHPCNF and Pt/C, respectively. Fig.5. (a) LSV curves of all the prepared samples and IrO2 toward OER in 0.1M KOH

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at the rotation speed of 1600rpm; (b) The corresponding Tafel plots of all as-prepared catalysts; (c, d) the OER durability of Co9S8-NSHPCNF and IrO2, respectively; (e) OER and ORR polarization of the five samples; (f) Potential differences between the E -2

of OER and E1/2 of ORR for all the catalysts except for N-SHPCNF.

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j=10 mA cm

Fig.6. (a) Schematic illustration of liquid zinc-air battery; (b) Discharge polarization

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curves and corresponding power densities of Co9S8-NSHPCNF and Pt/C+IrO2; (c) Charge and discharge polarization curves of Co9S8-NSHPCNF and Pt/C+IrO2; (d) Specific capacities of Zinc-air batteries based on Co9S8-NSHPCNF and Pt/C air cathodes at 20 mA cm−2; (e) Galvanostatic charge and discharge cycling curve at 2 and 10 mA cm-2 for Co9S8-NSHPCNF and Pt/C+IrO2, respectively. (f, g) cycle voltage profiles of zinc–air battery with the Co9S8-NSHPCNF at 2 and 10 mA cm-2, respectively; (h) A lighted LED with DHU was powered by three liquid zinc-air batteries in series.

Fig.7. (a) Schematic illustration of all solid-state zinc-air battery; (b) Discharge polarization curves and corresponding power densities of Co9S8-NSHPCNF and Pt/C+IrO2; (c) Galvanostatic charge and discharge cycling curve at 5 mA cm-2 for Co9S8-NSHPCNF; (d) a small fan was driven by the battery with Co9S8-NSHPCNF and

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photograph of open circuit voltage of 1.338V for Co9S8-NSHPCNF.

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Fig.1. Schematic diagram for the preparation of Co9S8-NSHPCNF.

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Fig. 2. (a, b) SEM images of Co9S8-NSHPCNF; (c) TEM image of Co9S8-NSHPCNF;

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(d) HRTEM image of Co9S8-NSHPCNF; the inset image displays SAED pattern of Co9S8-NSHPCNF; (e) the corresponding element mapping images of N, Co, S, O and

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C.

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Fig.3. (a) XRD patterns of all prepared samples; (b) N2 adsorption/desorption isotherms of Co9S8-NSHPCNF, the inset image displays the corresponding pore size distribution

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curve; (c) Raman spectra of all synthesized samples; (d) Co 2p XPS; (e) N 1s XPS; (f)

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S 2p XPS.

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Fig.4. (a)CV curves of Co9S8-NSHPCNF and Pt/C in N2- and O2-saturated 0.1M KOH

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at the scan rate of 50 mV s-1; (b) LSV curves of all the prepared samples and Pt/C toward ORR at 5mV s-1 with the rotation speed of 1600 rpm; (c) LSV curves of Co9S8-

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NSHPCNF at different rotation speeds, the inset shows the K-L plots of Co9S8NSHPCNF at various potentials; (d) Mass transfer corrected Tafel plots of all as-

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prepared catalysts. (e, f) the ORR durability of Co9S8-NSHPCNF and Pt/C, respectively.

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Fig.5. (a) LSV curves of all the prepared samples and IrO2 toward OER in 0.1M KOH

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at the rotation speed of 1600rpm; (b) The corresponding Tafel plots of all as-prepared catalysts; (c, d) the OER durability of Co9S8-NSHPCNF and IrO2, respectively; (e)

of OER and E1/2 of ORR for all the catalysts except for N-SHPCNF.

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j=10 mA cm-2

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OER and ORR polarization of the five samples; (f) Potential differences between the E

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Fig.6. (a) Schematic illustration of liquid zinc-air battery; (b) Discharge polarization curves and corresponding power densities of Co9S8-NSHPCNF and Pt/C+IrO2; (c) Charge and discharge polarization curves of Co9S8-NSHPCNF and Pt/C+IrO2; (d)

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Specific capacities of Zinc-air batteries based on Co9S8-NSHPCNF and Pt/C air cathodes at 20 mA cm−2; (e) Galvanostatic charge and discharge cycling curve at 2 and

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10 mA cm-2 for Co9S8-NSHPCNF and Pt/C+IrO2, respectively. (f, g) cycle voltage profiles of zinc–air battery with the Co9S8-NSHPCNF at 2 and 10 mA cm-2, respectively;

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(h) A lighted LED with DHU was powered by three liquid zinc-air batteries in series.

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Fig.7. (a) Schematic illustration of all solid-state zinc-air battery; (b) Discharge polarization curves and corresponding power densities of Co9S8-NSHPCNF and Pt/C+IrO2; (c) Galvanostatic charge and discharge cycling curve at 5 mA cm-2 for

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Co9S8-NSHPCNF; (d) a small fan was driven by the battery with Co9S8-NSHPCNF and

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photograph of open circuit voltage of 1.338V for Co9S8-NSHPCNF.