Fe2O3 heterostructure embedded in N-doped graphene as a bifunctional catalyst for quasi-solid-state zinc–air batteries

Accepted Manuscript Fe3C/Fe2O3 heterostructure embedded in N-doped graphene as a bifunctional catalyst for quasi-solid-state zinc–air batteries Yuhui Tian, Li Xu, Junchao Qian, Jian Bao, Cheng Yan, Henan Li, Huaming Li, Shanqing Zhang PII:

S0008-6223(19)30171-X

DOI:

https://doi.org/10.1016/j.carbon.2019.02.046

Reference:

CARBON 13961

To appear in:

Carbon

Received Date: 19 November 2018 Revised Date:

12 February 2019

Accepted Date: 15 February 2019

Please cite this article as: Y. Tian, L. Xu, J. Qian, J. Bao, C. Yan, H. Li, H. Li, S. Zhang, Fe3C/Fe2O3 heterostructure embedded in N-doped graphene as a bifunctional catalyst for quasi-solid-state zinc–air batteries, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.02.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Fe3C/Fe2O3 heterostructure embedded in N-doped graphene as a bifunctional catalyst for quasi-solid-state zinc–air batteries

Yuhui Tian,a Li Xu,a,b,* Junchao Qian,d Jian Bao,a Cheng Yan,c Henan Li,a,c,*

a

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Huaming Li,a Shanqing Zhanga,b*

Institute for Energy Research, School of Chemistry and Chemical Engineering,

Key Laboratory of Zhenjiang, Jiangsu University, Zhenjiang 212013, P. R. China Centre for Clean Environment and Energy, Griffith School of Environment, Gold

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b

Coast Campus, Griffith University, QLD 4222, Australia. c

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School of Chemistry, Physics and Mechanical Engineering, Queensland

University of Technology, Brisbane, QLD 4001, Australia. d

Jiangsu Key Laboratory for Environment Functional Materials, Suzhou

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University of Science and Technology, Suzhou 215009, P. R. China

∗

Corresponding authors: [email protected] (L. Xu); [email protected] (H. Li);

[email protected] (S.Q. Zhang).

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Graphical abstract

ACCEPTED MANUSCRIPT Abstract The development of nonprecious metal catalysts with highly efficient and durable activities is of great importance for high performances of zinc–air batteries. Herein, the N-doped graphene wrapped Fe3C/Fe2O3 heterostructure has been designed as a

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bifunctional oxygen catalyst for zinc–air batteries. In the synthesis process of the catalyst, graphene oxide (GO) can assemble with graphitic carbon nitride (g-C3N4) and FeOOH nanorods by the π−π stacking and hydrogen bonds, respectively. The

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assembly of GO, g-C3N4 and FeOOH nanorods results in nanostructure of carbon layers coated iron species and leads to outstanding durability in both alkaline and

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acidic media. The synergistic effect of nitrogen doping and Fe3C/Fe2O3 heterostructure allows the catalyst (Fe3C/Fe2O3@NGNs) to display high oxygen reduction and evolution reaction activities. The liquid zinc–air battery assembled with the catalyst presented a remarkable peak power density (139.8 mW cm−2), large

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specific capacity (722 mAh g−1) and excellent charging–discharging cycling performance. The quasi-solid-state zinc–air battery with the catalyst exhibited an impressive open-circuit voltage and a peak power density. Therefore, the

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Fe3C/Fe2O3@NGNs catalyst is expected to have a bright future for practical

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applications in energy conversion devices.

Keywords: Zinc–air battery; ORR; OER; N-doped graphene; Fe3C/Fe2O3 heterostructure

ACCEPTED MANUSCRIPT 1. Introduction Recently, due to the low cost, environmental friendliness, and abundance of Zn raw materials, as well as high theoretical energy density, zinc–air batteries have been considered as a sustainable energy conversion system with

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unlimited potential [1,2]. However, the power density and long-term rechargeability of zinc–air batteries have been severely constrained by the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution

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reaction (OER) [3]. The bifunctional electrocatalysts are critical for enhancing power density, stability, and cycle life of zinc–air batteries. Despite the

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desirable electrocatalytic activities of noble metal catalysts such as Pt and Ir-based catalysts, their high costs preclude zinc–air batteries from large-scale commercial applications [4]. Thus, tremendous efforts have been devoted to exploring low-cost, non-noble metal electrocatalysts with high electrocatalytic

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activities [5,6]. Carbon-based materials have been identified as promising electrocatalysts, which can be attributed to their reasonable balance among the catalytic activity, durability and cost [7]. In particular, N-doped carbon

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materials can achieve an impressive ORR catalytic activity because of the

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incorporation of nitrogen atoms into the graphitic structure, which results in positive charges on the adjacent carbon atoms, and thus facilitates the adsorption and reduction reaction of oxygen molecules [8]. However, it is still challenging to develop efficient N-doped carbon electrocatalysts to achieve a high catalytic activity that is comparable to current commercially available catalysts [9,10]. Currently, various kinds of transition metals (e.g., Fe, Co, Ni) and metal oxides have been introduced to N-doped carbon materials to enhance the

ACCEPTED MANUSCRIPT electrocatalytic activities further [11]. Synergistic effects between transition metals and N-doped carbon materials can enhance the charge transfer and increase the catalytic stability of these hybrid materials for ORR [12,13]. For example, anchoring transition metal Fe onto nitrogen-carbon frameworks

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shows an excellent ORR activity that is comparable to the Pt/C benchmark [14]. In particular, the Fe-Nx or Fe/Fe3C motifs in Fe-based N-doped carbon electrocatalysts have been considered to be the active sites that boost the ORR

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process [15]. For rechargeable zinc–air batteries, OER is another important reaction for charging process. Hence, it is imperative to design Fe-based,

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N-doped carbon electrocatalysts with desirable OER activity to meet the practical application for rechargeable zinc–air batteries.

Generally, the synthesis of Fe–N–C catalysts is achieved by pyrolysis of N-containing macrocycle molecules (e.g., phthalocyanines and porphyrins)

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with carbon precursors and Fe salts [16]. However, the N content in these precursors is typically low, and thus greatly reduce the density of active sites in the carbon support. Recently, graphitic carbon nitride (g-C3N4) with a

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prototypical two-dimensional (2D) layered structure has been widely developed

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for use in efficient OER and ORR electrocatalysts [17-18]. An intrinsically high N content (both pyridinic and graphitic N species) of g-C3N4 can provide abundant active sites for the ORR [19]. Unfortunately, g-C3N4 suffers from inherent low electronic conductivity and low surface area, which severely restricts the application in electrocatalysis [20]. However, g-C3N4 can be applied as a nitrogen-rich precursor for introducing a high N level into carbon materials [21]. Specifically, the pyridine-like nitrogen in g-C3N4 can provide lone-pair electrons to bind Fe species in the ligands [22]. The graphene-like

ACCEPTED MANUSCRIPT structure of g-C3N4 makes it a desirable host material for the formation of Fe–N–C structures [23]. In this work, we designed a facile method to prepare the N-doped graphene wrapped Fe3C/Fe2O3 heterostructure (Fe3C/Fe2O3@NGNs) for rechargeable zinc–air

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batteries. The preparation process is illustrated in Fig. 1. The FeOOH nanorods (Fig. S1) used as the Fe source were electrostatically attracted to the graphene oxide (GO) surface through hydrogen bonds to form FeOOH/GO mixtures. The g-C3N4

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nanosheets (Fig. S2) were applied as the N source and template for producing N-doped graphene (NGNs). At the same time, sodium alginate (SA) can not only

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provide defect-rich amorphous carbon upon pyrolysis to enhance the surface area of the composites [24], but also prevent the weight losses of g-C3N4 (Fig. S3). The as-prepared Fe3C/Fe2O3@NGNs catalyst possessed a high content of pyridinic and graphitic N, as well as a large surface area, and thus presented a comparable ORR Furthermore,

the

primary

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battery

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activity.

Fe3C/Fe2O3@NGNs catalyst displayed a higher power density and better stability than those of mixed Pt/C+IrO2 (with a mass ratio of 1:1) catalysts. Impressively, the

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quasi-solid-state batteries were also assembled. The single prototype battery was able

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to reach a peak power density of 28.4 mW cm−1. It is indicated that Fe3C/Fe2O3@NGNs air cathode has a bright future for practical applications in rechargeable zinc–air batteries.

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Fig. 1. Schematic illustration of the preparation procedures for Fe3C/Fe2O3@NGNs. (Step I: Stirring and drying; Step II: Pyrolysis under Ar atmosphere; Step III: Acid

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etching and activation in NH3.) 2. Experimental section 2.1 Chemicals

All of the chemical reagents were used without further treatment. Iron (III)

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chloride hexahydrate (FeCl3·6H2O), urea, sodium hydroxide (NaOH), sodium alginate (SA), hydrochloric acid (HCl, 36%) and ethanol (99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Synthesis

of

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heterostructure catalyst

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The preparation of Fe3C/Fe2O3@NGNs: 20 mg of GO (Nanjing XFNANO Materials Tech Co., Ltd) and 89 mg of the FeOOH nanorod samples (Supporting Information) were dispersed in 10 mL of deionized water by ultrasonication for 1 h. Then, 0.2 g of g-C3N4 (Supporting Information) and 0.2 g of SA were added into the above mixture and subjected to vigorous magnetic stirring for 6 h. The aerogel could be obtained by drying the resulting mixture at 80 °C for 12 h. The aerogel was heated at 800 °C with a ramp speed of 5 °C min−1 in a tube furnace under a flowing of Ar and maintained for 3 h.

ACCEPTED MANUSCRIPT Subsequently, the heating treated aerogel was immersed in 1 M HCl solution at 60 °C for 12 h to remove inactive species. After being filtrated and washed with deionized water and ethanol to neutral, the obtained black powders were dried in vacuum at 60 °C for 12 h. Then, the acid-treated catalyst was placed into a

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tube furnace with a ramping rate of 10 °C min−1 under Ar atmosphere. When the temperature reached 800 °C, Ar atmosphere was replaced by NH3 with a flow rate of 200 sccm for 20 minutes. The sample was then cooled to the room

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temperature under the Ar atmosphere. The resulting sample was denoted as Fe3C/Fe2O3@NGNs. For comparison, the nitrogen doped graphene nanosheets

FeOOH nanorods. 2.3 Characterization

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(NGNs) were synthesized under the similar procedure without the addition of

Transmission electron microscopy (TEM, JEOL JEM-2100F), field-emission

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scanning electron microscopy (SEM, JEOL JSM-7001F) and X-ray diffraction (Shimadzu, XRD-6000X -ray) with high-intensity Cu Kα (λ=1.54 Å) were utilized to evaluate the morphology and crystal structure of all prepared catalysts.

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Thermogravimetric analysis (TGA) was carried out using a TA SDT 2960

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thermoanalyzer with a heating rate of 5 °C min−1 in Ar atmosphere. X-ray photoelectron spectroscopy (XPS) analysis was performed on X-ray photoelectron spectrometer (PHI5300) with a monochromatic Mg Kα source. The

57

Fe Mössbauer

spectra were recorded on Topologic 500A spectrometer at room temperature using 57

Co source as a radioactive source. The velocity was calibrated by a standard α-Fe

foil. Raman spectra were carried out on Renishaw Invia with a 532 nm excitation laser. Specific surface areas and pore size distributions were obtained from nitrogen sorption isotherms at 77 K (Micromeritics Instrument Corporation, USA) by using

ACCEPTED MANUSCRIPT Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, respectively. 3. Results and discussion The morphologies of as-prepared samples were first analysed by SEM and TEM.

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The SEM images of NGNs and Fe3C/Fe2O3@NGNs show that all samples exhibit a silk-like nanosheet structure with abundant crinkles on the surface (Fig. S4). The TEM image in Fig. 2a reveals a similar laminar structure of Fe3C/Fe2O3@NGNs.

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Nanoparticles with a size of approximately 30 nm are encapsulated in carbon layers. The HRTEM image (Fig. 2b) shows that graphitic carbon with lattice fringes of 0.34

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nm is coated with amorphous carbon layers that are converted from SA. The Fe-based nanoparticles are surrounded by carbon walls. The fingerprint of the Fe-based nanoparticles is difficult to identify due to the deep wrapping by carbon layers and the simultaneous presence of multiphase. High-angle annular dark field scanning

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transmission electron microscope (HAADF-STEM) elemental mappings were performed for Fe3C/Fe2O3@NGNs. As shown in Fig. 2c, strong Fe signals on carbon layers with a uniform distribution of N element can be observed in the selected region.

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The TEM images in Fig. S5 demonstrate a few-layer structure of the NGNs. Typical

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element mapping images of NGNs (Fig. S5c) demonstrate the presence of the elements C and N, which are distributed homogeneously on the nanosheets. The specific surface area and porous features of the NGNs and Fe3C/Fe2O3@NGNs (Fig. 2d, e and Table S1) were investigated by Brunauer–Emmett–Teller (BET) measurements. The N2 adsorption/desorption isotherm curves show type IV isotherms (Fig. 2d) for both samples, revealing the existence of mesopores [25,26]. In particular, the NGNs exhibits a high BET surface area of 751.1 m2 g−1 and a total pore volume of 0.96 cm3 g−1. The BET surface area and the total pore volume of Fe3C/Fe2O3@NGNs

ACCEPTED MANUSCRIPT reduce to 435.6 m2 g−1 and 0.64 cm3 g−1, respectively. The high surface area and the existence of mesopores are beneficial for mass transport and exposure of more accessible active sites for electrochemical reactions [27].

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Fig. 2. (a) TEM and (b) HRTEM images of Fe3C/Fe2O3@NGNs. (c) HAADF-STEM image and corresponding elemental mapping images of Fe3C/Fe2O3@NGNs showing the presence of C, Fe, N elements. (d) N2 adsorption-desorption isotherms and (e) corresponding pore size distributions of NGNs and Fe3C/Fe2O3@NGNs. XRD patterns of the electrocatalysts are presented in Fig. 3a. Both samples display a distinct peak at approximately 26°, which is corresponding to the (002) plane of graphitic carbon. A series of weak peaks ranging from 40° to 50° is observed,

ACCEPTED MANUSCRIPT which can be attributed to the Fe3C in Fe3C/Fe2O3@NGNs [28]. The indistinct Fe-based peaks suggest that Fe species are anchored deeply on the carbon matrix [29]. By introducing Fe, the diffraction peak of graphitic carbon becomes sharper and stronger, indicating a deep degree of graphitization [30]. Raman spectra show the

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variation in the intensity of the D (≈ 1340 cm−1) and G (≈ 1600 cm−1) bands for all prepared samples (Fig. S6). Compared with NGNs, the ratio of the D-band and G-band intensities (ID/IG) shows a decrease for Fe3C/Fe2O3@NGNs, suggesting that

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the more graphitic carbon formed after the incorporation of Fe [31]. The increased graphitic carbon content can enhance the electrical conductivity of catalysts [32]. The

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surface chemical states of all synthesized samples were investigated by X-ray photoelectron spectroscopy (XPS). The wide range of spectra demonstrates the presence of C, N, O for NGNs and Fe3C/Fe2O3@NGNs (Fig. S7). No obvious Fe signal was detected in the survey spectrum of Fe3C/Fe2O3@NGNs, which can be

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attributed to the low content of Fe species that are covered with graphitic layers [33]. The high-resolution spectrum of elemental nitrogen for the metal-free NGNs catalyst (Fig. S8) can be deconvoluted into four peaks, which are assigned to pyridinic N (≈

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398.1 eV), pyrrolic N (≈ 398.9 eV), graphitic N (≈ 400.2 eV) and oxidized N (≈ 402.0

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eV), respectively [34]. The total content of N in metal-free NGNs catalyst is 12.24 at%. The value is relatively high compared with other metal-free N-doped carbon catalysts that have been reported (Table S2). The high proportion of pyridinic N (35.7%) and graphitic N (46.9%) is obtained for NGNs. It has been reported that both the pyridinic N and graphitic N are essential to enhancing ORR performances [35-38]. For Fe3C/Fe2O3@NGNs (Fig. 3b), a new peak centred at approximately 399.1 eV can be observed apart from the above four types of N configurations, which can be assigned to the Fe-coordinated N (Fe-N) [29]. Some studies also indicated that the

ACCEPTED MANUSCRIPT pyridinic and pyrrolic N could associate with Fe to form Fe-Nx moieties [33,39]. The ammonolysis not only creates more edge-N in N-doped graphene, but also enables the formation of more Fe-Nx sites in Fe3C/Fe2O3@NGNs. The high-resolution Fe 2p spectrum for Fe3C/Fe2O3@NGNs (Fig. 3c) shows noticeable peaks centred at 724.6

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eV that can be assigned to Fe 2p1/2 of Fe3+ and Fe2+, while the deconvoluted peaks from approximately 711.0 eV to 715.3 eV correspond to Fe 2p3/2 of Fe2+ and Fe3+ ions. In addition, the peak at 718.8 eV can be assigned to a satellite peak, further verifying

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the co-existence of Fe2+ and Fe3+ [26]. To understand the composition of Fe species in Fe3C/Fe2O3@NGNs more deeply, Mössbauer spectroscopy was carried out. As shown

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in Fig. 3d, the Mössbauer spectra of Fe3C/Fe2O3@NGNs can be fitted with four sextets and two doublets. The two doublets correspond to a square-planar Fe-Nx coordination [40]. The main Fe species are Fe3C and Fe2O3 in Fe3C/Fe2O3@NGNs [41-43]. A small amount of metallic Fe was also detected [44]. Fe3C has been

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considered as an active centre to boost the ORR process [45], while the Fe2O3 has also been demonstrated with both excellent ORR and OER catalytic activities [46,47]. In addition, due to its strong electron-withdrawing effect, the component of Fe2O3 can

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influence the electronic structure of Fe3C [48], and Fe3C in turn can help to overcome

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the low electron mobility of Fe2O3. Hence, the coexistence of Fe3C and Fe2O3 can endow Fe3C/Fe2O3@NGNs with desirable bifunctional electrocatalytic activities.

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Fe3C/Fe2O3@NGNs

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Pyridinic N Fe-N Pyrllolic N Graphitic N Oxidated N

Intensity (a.u.)

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Fig. 3. (a) XRD patterns of NGNs and Fe3C/Fe2O3@NGNs catalysts. High resolution

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XPS spectra of (b) N 1s and (c) Fe 2p for Fe3C/Fe2O3@NGNs. (d)

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Fe Mössbauer

spectrum of Fe3C/Fe2O3@NGNs.

The electrocatalytic activity of as-synthesized samples towards ORR was first

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investigated by cyclic voltammetry (CV) measurements. As shown in the CV curves

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(Fig. S9), all synthesized samples display a clear oxygen reduction peak in O2-saturated 0.1 M KOH. The reduction peak for metal-free N-doped graphene catalyst shifts positively from 0.77 V to 0.81 V after the incorporation of Fe species, suggesting an excellent ORR activity for Fe3C/Fe2O3@NGNs in alkaline medium. Then, the ORR activities of the as-synthesized materials were further investigated by liner sweep voltammetry (LSV) at 1600 rpm in O2-saturated 0.1 M KOH. As shown in Fig. 4a, the NGNs catalyst exhibits a half-wave potential of 0.82 V. Remarkably, Fe3C/Fe2O3@NGNs displays a boosted ORR activity with a halfwave potential of

ACCEPTED MANUSCRIPT 0.86 V, which is equal to that of commercial 20 wt% Pt/C (E1/2 = 0.86 V). Moreover, the Fe3C/Fe2O3@NGNs catalyst presents a higher diffusion-limited current density (5.64 mA cm−2 at 0.4 V) than that of Pt/C (5.59 mA cm−2 at 0.4 V). Tafel slopes of different samples are also calculated on the basis of the LSV curves at 1600 rpm (Fig.

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4b). The Tafel slope of Fe3C/Fe2O3@NGNs is 73 mV dec−1, which is close to that of Pt/C (74 mV dec−1). For the metal-free counterpart, the Tafel slope is 81 mV dec−1. The small Tafel slope of Fe3C/Fe2O3@NGNs catalyst indicates the satisfactory

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kinetics during the ORR process. The electron transfer number (n) of all prepared samples are calculated based on the LSV curves at different rotating speeds according

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to the Koutecky–Levich (K-L) equation (Fig. 4c, d and Fig. S10) and showed in Table S3 with other parameters. The K-L plots of Fe3C/Fe2O3@NGNs (Fig. 4d) display high linearity, suggesting the similar electron transfer number in the potential range of 0.2 to 0.6 V [49]. The electron transfer number is determined to be 4.15, indicating a

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quasi-four-electron pathway for oxygen reduction. Rotating ring disk electrode (RRDE) tests were employed to further investigate the electron transfer number and hydrogen peroxide yields (Fig. S11). The average electron transfer number of

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Fe3C/Fe2O3@NGNs is 3.94 (Fig. S11d), which is consistent with the value obtained

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from K-L equations and is close to that of Pt/C (3.95). For NGNs, the electron transfer number is determined to be 3.77. The hydrogen peroxide yield of Fe3C/Fe2O3@NGNs is lower than 6.5 %, further verifying a direct four-electron ORR process.

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Fig. 4. (a) ORR polarization curves of NGNs, Fe3C/Fe2O3@NGNs and Pt/C catalysts

Fe3C/Fe2O3@NGNs

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in O2-saturated 0.1 M KOH. (b) The corresponding Tafel plots of NGNs, and

Pt/C

catalysts.

(c)

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polarization

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Fe3C/Fe2O3@NGNs at different rotating speeds. (d) The corresponding K–L plots of

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Fe3C/Fe2O3@NGNs at different potentials.

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To further reveal the reasons of the remarkably improved ORR activity for the Fe3C/Fe2O3@NGNs catalyst, Fe3C-domainted Fe3C@NGNs and Fe2O3-domainted Fe2O3@NGNs catalysts were synthesized (Supporting Information). The XRD patterns in Fig. S12 show that several peaks related to Fe3C (PDF #85-1317) and Fe2O3 (PDF #33-0664) are observed in the Fe3C@NGNs and Fe2O3@NGNs catalysts, respectively. The TEM image of Fe3C@NGNs in Fig. S13a displays the lattice spacing of 0.40 nm corresponding to the (001) plane of Fe3C [50]. The lattice fringe with spacing of 0.25 nm in Fe2O3@NGNs (Fig. S13b) can match with (110) plane of

ACCEPTED MANUSCRIPT Fe2O3 [51]. Compared with Fe3C/Fe2O3@NGNs, the Fe3C@NGNs and Fe2O3@NGNs catalysts display a degraded ORR activity. In particular, the half-wave potential shifts negatively to 0.82 V for Fe3C@NGNs and 0.81 V for Fe2O3@NGNs (Fig. S14). The results reveal that the hybrid of Fe3C and Fe2O3 can boost the ORR activity. The

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excellent electrocatalytic activity towards ORR can be attributed to the heterogeneous structure of Fe3C, Fe2O3 and Fe-Nx species. According to previous reports [50,52], the Fe3C can not only deliver a high ORR activity, but also boost the intrinsic activity of

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Fe-Nx. The electronic effect between Fe3C and Fe2O3 results in a decreased local work function on the surface, which facilitates O2 adsorption [48].

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The ORR activity of Fe3C/Fe2O3@NGNs was further evaluated in O2-saturated 0.1 M HClO4. As shown in Fig. 5a, Fe3C/Fe2O3@NGNs achieves a halfwave potential of 0.76 V, which is just 50 mV lower than that of Pt/C (0.81 V). However, the Fe3C/Fe2O3@NGNs shows a higher diffusion-limited current density than that of Pt/C.

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Although the ORR activity in acidic condition is inferior compared to that in alkaline medium, the Fe3C/Fe2O3@NGNs catalyst displays comparable or even better ORR performance than other reported noble-metal-free electrocatalysts in acidic solutions

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(Table S4). Chronoamperometric response measurements were conducted to

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investigate the long-term stability of the Fe3C/Fe2O3@NGNs catalyst in both alkaline and acidic conditions. As shown in Fig. 5b, the Fe3C/Fe2O3@NGNs catalyst displays extraordinary durability with only a 0.83 % decay of the original current density (vs. 25.3 % for Pt/C) after a 15000 s test in O2-saturated 0.1 M KOH. Even in an acid electrolyte (Fig. 5c), the current density of Fe3C/Fe2O3@NGNs shows a 92.41 % residue after 15000 s, which is much better than that of Pt/C (38.34 % residue). The excellent stability of Fe3C/Fe2O3@NGNs catalyst indicates that the inner iron species are protected from being corroded by graphitic layers [26]. Furthermore, the methanol

ACCEPTED MANUSCRIPT tolerant ability was compared for Fe3C/Fe2O3@NGNs and Pt/C by adding 3 mL of methanol into the acidic medium during the chronoamperometric response tests. As depicted in Fig. S15, the current density of Pt/C ascends to positive significantly after the rapid injection of methanol at 300 s. In contrast, there is no significant change in

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the current density of Fe3C/Fe2O3@NGNs regarding methanol injection, suggesting the better methanol tolerance and higher stability of Fe3C/Fe2O3@NGNs.

The OER performances were also tested for the NGNs, Fe3C/Fe2O3@NGNs and

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IrO2 catalysts. The operating potential that delivers a current density of 10 mA cm−2 (Ej= 10) has been used to compare the different samples. As shown in Fig. 5d, the

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Fe3C/Fe2O3@NGNs catalyst exhibits a potential of 1.69 V at 10 mA cm−2. The potential of Fe3C/Fe2O3@NGNs is only 80 mV higher than that of the IrO2 catalyst (1.61 V), while the potential of NGNs at 10 mA cm−2 is 1.78 V and is much higher than that of Fe3C/Fe2O3@NGNs. The Tafel slope of Fe3C/Fe2O3@NGNs (Fig. 5e) is

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calculated to be 163 mV dec−1, lower than that of NGNs (288 mV dec−1). As expected， Fe3C@NGNs and Fe2O3@NGNs samples display inferior OER activity compared with that of Fe3C/Fe2O3@NGNs (Fig. S16). The enhanced OER performance can be

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attributed to the incorporation of Fe3C/Fe2O3 heterostructure, which can provide more

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intrinsic OER active sites, and thus effectively promote the OER performance of the catalyst [15,46]. The oxygen electrode activity parameter ∆E (∆E=Ej=10,OER − E1/2,ORR) of Fe3C/Fe2O3@NGNs is determined to be 0.83 V (Fig. 5f) and is comparable to that of Fe-based bifunctional electrocatalysts that have been reported in recent years (Table S5). The small ∆E implies the ideal reversible oxygen electrode property of Fe3C/Fe2O3@NGNs catalyst, which has high potential for practical applications in rechargeable metal-air batteries [14]. The enhanced bifunctional catalytic activities may result from the following factors: (a) Fe3C/Fe2O3 heterostructure is integrated

ACCEPTED MANUSCRIPT into N-doped carbon frameworks and is enwrapped by graphitic layers, which provide more intrinsic active sites for ORR and OER, and thus facilitate the catalytic performances of the carbon materials. Furthermore, the incorporation of Fe2O3 in Fe3C can effectively tune the electronic structure and promote the synergistic

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interplay, which can be a critical factor for the enhanced bifunctional catalytic activities. (b) The encapsulation structure of the Fe3C/Fe2O3@NGNs can protect inner Fe3C/Fe2O3, therefore enhancing the long-term stability of the catalyst. The inner iron

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species in turn can also affect the peopoerties of the outer graphitic layers. This synergetic interaction between Fe-based nanoparticles and N-doped graphitic layers

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further improves the overall electrocatalytic performance of Fe3C/Fe2O3@NGNs [53-54]. (c) The high degree of graphitization of N-doped graphene endows the catalyst with high electrical conductivity. The high content of pyridinic N and graphitic N is achieved by using g-C3N4 as a self-template, which can act as efficient

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active sites towards ORR in addition to Fe species. (d) The high surface area and porosity of Fe3C/Fe2O3@NGNs are conducive to the mass transport and exposure of more active sites. Therefore, the integration of Fe2O3 into Fe3C are expected to be a

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facile and efficient method to develop monometallic Fe-based catalysts for

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bifunctional electrodes in the zinc–air battery.

ACCEPTED MANUSCRIPT (b)

Fe3C/Fe2O3@NGNs Pt/C

(c)

100

100

99.17%

-1.6

92.41%

-3.2

75

74.7% 50

j/j0 (%)

75

j/j0 (%)

j (mA cm-2)

(a)0.0

Pt/C Fe3C/Fe2O3@NGNs

-4.8

25

50 Pt/C

Fe3C/Fe2O3@NGNs

25

38.34%

-6.4

0 0.6

0.8

0 0

1.0

3000

6000

(d)27

(e)1.8

IrO2

12000 15000

0

(f) 15

Potential (V)

18

9

NGNs

1.7

28

1.6

163

mV

de V/ 8m

10

c

/dec

c 70 mV/de

1.5

1.8

0.0

0.2

Ej=10

Fe3C/Fe2O3@NGNs

5 0

1.23 V

0.4

0.6

Log (j / mA cm-2)

ORR

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1.6

12000 15000

△E = 0.83 V

-5 1.4

Potential (V vs. RHE)

9000

E1/2

0 1.4

6000

OER

Fe3C/Fe2O3@NGNs

NGNs

1.2

3000

Time (s)

IrO2

Fe3C/Fe2O3@NGNs

j (mA cm-2)

9000

Time (s)

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0.4

Potential (V vs. RHE)

j (mA cm-2)

0.2

0.8

0.3

0.6

0.9

1.2

1.5

1.8

Potential (V vs. RHE)

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Fig. 5. (a) ORR polarization curves of Fe3C/Fe2O3@NGNs and Pt/C catalysts in O2-saturated 0.1 M HClO4. Chronoamperometric responses of Fe3C/Fe2O3@NGNs and Pt/C catalysts in O2-saturated (b) 0.1 M KOH and (c) 0.1 M HClO4. (d) OER polarization curves of NGNs, Fe3C/Fe2O3@NGNs and IrO2 catalysts in N2-saturated

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0.1 M KOH. (e) The corresponding Tafel plots obtained from OER polarization curves. (f) The overall polarization curve of Fe3C/Fe2O3@NGNs in the whole ORR and OER region in 0.1 M KOH electrolyte.

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The liquid zinc–air battery was constructed to investigate the practical application of the as-prepared Fe3C/Fe2O3@NGNs. For comparison, the battery with Pt/C+IrO2

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catalysts (with a mass ratio of 1:1) was also tested under the same condition. The zinc–air battery assembled with the Fe3C/Fe2O3@NGNs catalyst displays a higher open-circuit voltage (1.46 V) than that of Pt/C+IrO2 catalysts (1.44 V, Fig. S17). At a discharge current of 5 mA cm−2 for 24 h, the voltage plateau of the zinc–air battery with the Fe3C/Fe2O3@NGNs air electrode is more stable than that of the battery with Pt/C+IrO2 catalysts (Fig. 6a). Moreover, the Fe3C/Fe2O3@NGNs air electrode is able to generate a specific capacity of 722 mAh g−1 (Fig. 6b) corresponding to a gravimetric energy density of 805 Wh kg−1 at a discharge current density of 20 mA

ACCEPTED MANUSCRIPT cm−2. These values outperform the battery with Pt/C+IrO2 air electrode (specific capacity: 709.8 mAh g−1, energy density: 773.7 Wh kg−1). The discharge polarization curves and corresponding power density curves of the assembled zinc–air batteries are displayed in Fig. 6c. The peak power density of the Fe3C/Fe2O3@NGNs air electrode

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is calculated to be 139.8 mW cm−2, which exceeds that of Pt/C+IrO2 (125.1 mW cm−2). Moreover, by replenishing the Zn foil and KOH electrolyte, the output voltage at a discharge current density of 5 mA cm−2 displays no significant decline over 60 h (Fig.

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6d). The stability of the zinc–air battery was further assessed by a cycle discharging–charging test at 10 mA cm−2. The whole cycle discharging–charging

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process was conducted without replenishing the Zn foil and electrolyte. As demonstrated in Fig. 6e, the values of final charging and discharging potential of the battery with Fe3C/Fe2O3@NGNs air electrode after 40 h are 1.10 V and 2.11 V, respectively, which exhibit no obvious change compared with the initial values of

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1.18 V and 2.10 V. In contrast, a gradual increase in the charging voltage and a decrease in the discharging voltage are observed for battery with Pt/C+IrO2 air electrode, revealing the better rechargeability of Fe3C/Fe2O3@NGNs air electrode. 1.2

1.2

Pt/C + IrO2 Fe3C/Fe2O3@NGNs

(c)1.5

150 139.8 mW cm-2

0.4

6

12

20 mA cm-2

0.8

0.4

1.0

125.1 mW cm-2

0.5

0.0

18

24

50

0.0 0

Time (h)

300

600

Specific capacity (mAh g-1)

900

0

200

0 300

Pt/C+IrO2 Fe3C/Fe2O3@NGNs

1.2

j = 10 mA cm-2

Voltage (V)

Voltage (V)

100

Current density (mA cm-2)

(e)2.8

(d)

0.8 mechanical recharge 1 2 3

0.4

100

Pt/C+IrO2 Fe3C/Fe2O3@NGNs

0.0

0

Voltage (V)

Voltage (V)

0.8

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Voltage (V)

5 mA cm-2 Pt/C + IrO2 Fe3C/Fe2O3@NGNs

Power density (mW/cm2)

(b)

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(a)

2.1

1.4

0.7

0.0 0

15

30

Time (h)

45

60

0

10

20

Time (h)

30

40

ACCEPTED MANUSCRIPT Fig. 6. The liquid zinc–air battery performances with Fe3C/Fe2O3@NGNs and Pt/C+IrO2 catalysts as air-cathode: (a) Galvanostatic discharge curves of primary zinc–air batteries at a current density of 5 mA cm−2. (b) Specific capacities of the zinc–air batteries at a discharging current density of 20 mA cm−2. (c) Discharging

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polarization curves and the corresponding power density plots. (d) Long-time durability of the zinc–air battery using Fe3C/Fe2O3@NGNs air electrode at a discharging current density of 5 mA cm–2 for 60 hours. (e) Long-term cycling

The

quasi-solid-state

zinc–air

battery

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performances at a current density of 10 mA cm−2. was

also

constructed

with

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Fe3C/Fe2O3@NGNs catalyst coated carbon cloth as the air electrode. The schematic illustration of the fabrication process is demonstrated in Fig. 7a. The battery with the Fe3C/Fe2O3@NGNs air-cathode exhibits an open-circuit-voltage of 1.329 V (Fig. 7b). A red LED light (3 V) can be powered by two batteries that are connected in series

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(Fig. 7c). A high peak power density of 28.4 mW cm−2 can be reached by a single battery (Fig. 7d). The galvanostatic discharge voltage plateaus (the inset image of Fig. 7e) shows that the decline in voltage with the increasing current densities.

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Remarkably, the quasi-solid-state zinc–air battery exhibits a high voltage plateau of 1.26 V and a long discharge time of more than 8 h at a discharge current density of 1

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mA cm−2 (Fig. 7e). The performances of the quasi-solid-state zinc–air battery indicate that the Fe3C/Fe2O3@NGNs catalyst has great potential in future portable and wearable energy conversion devices.

ACCEPTED MANUSCRIPT (a) Insulating tape

Air cathode

PVA gel

Ni foam

Pure Zn plate

10

0.5

0.0 0

10

20

Current denisty (mA cm-2)

0 30

1.3 -2

1 mA cm

Voltage (V)

20 1.0

1 mA cm-2

1.2

0.8

1.2

3 mA cm-2

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Voltage (V)

(e)

30

28.4 mW cm-2

1.5

Voltage (V)

(d)

Power density (mW cm-2)

(c)

(b)

4 mA cm-2

5 mA cm-2

1.1

0.4

1.0

0

800 1600 Time (s)

2400

0.0

0

2

4

6

8

Time (h)

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Fig. 7. The assembly and performances of the quasi-solid-state zinc–air battery with Fe3C/Fe2O3@NGNs: (a) Schematic illustration of the fabrication of the cathode of the

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quasi-solid-state zinc–air battery. (b) Photograph of the quasi-solid-state zinc–air battery with an open-circuit voltage of 1.329 V. (c) Photograph of a red LED (3 V) lighted by two quasi-solid-state zinc–air batteries connected in series. (d) Discharge polarization curve and corresponding power density of the resultant quasi-solid-state

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zinc–air battery. (e) Galvanostatic discharge curves of quasi-solid-state zinc–air battery at a current density of 1 mA cm−2 (inset: Galvanostatic discharge curves of the resultant quasi-solid-state zinc–air battery at different current densities).

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4. Conclusion

In summary, the N-doped graphene wrapped Fe3C/Fe2O3 heterostructure was

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successfully fabricated by using FeOOH as an iron source and g-C3N4 as a nitrogen source. Owing to the N-doped graphene matrix and the Fe3C/Fe2O3 heterostructure, the as-prepared catalyst demonstrates a high ORR catalytic activity in alkaline media and a comparable OER activity with oxygen electrode activity parameter ∆E = 0.83 V. Based on experimental analyses, the introduction of the heterogeneous components of Fe2O3 and Fe3C is essential to the enhancement of the bifunctional ORR and OER activities. Compared with commercial Pt/C+IrO2 catalysts, the primary zinc–air battery constructed with the Fe3C/Fe2O3@NGNs air electrode presented a high energy

ACCEPTED MANUSCRIPT density of 139.8 mW cm−2, a large specific capacity of 722 mAh g−1 and excellent stability. In addition, the catalyst was fabricated into the homemade quasi-solid-state zinc–air battery that displayed a high open circuit voltage (1.329 V). Therefore, the Fe3C/Fe2O3@NGNs catalyst is a promising candidate to substitute precious metals in

synthesis, high catalytic efficiency and long-term stability. Acknowledgements

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practical applications for zinc-air batteries due to its simple and facile process of

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This work has been financially supported by the National Natural Science Foundation of China (No. 21705058, 21601067), Six Talent Peaks Project of Jiangsu

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Province (XNY-009), the Provincial Natural Science Foundation of Jiangsu (No. BK20170524, BK20160492), Chinese Postdoctoral Foundation (2018T110450), High-tech Research Key laboratory of Zhenjiang (SS2018002) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education

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