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A Long-life Rechargeable Zn Air Battery Based on Binary Metal Carbide Armored by Nitrogen-doped Carbon Chao Lin, Xiaopeng Li, Sambhaji S. Shinde, DongHyung Kim, Xiaokai Song, Haojie Zhang, and Jung-Ho Lee ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01865 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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ACS Applied Energy Materials

A Long-life Rechargeable Zn Air Battery Based on Binary Metal Carbide Armored by Nitrogen-doped Carbon Chao Lin,a Xiaopeng Li,* b Sambhaji S. Shinde,a Dong-Hyung Kim,a Xiaokai Song,c Haojie Zhang,b Jung-Ho Lee* a

a Department of Materials Science and Chemical Engineering, Hanyang University, Ansan, Gyeonggi-do, 15588, Korea. b CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute (SARI), Chinese Academy of Sciences (CAS), Shanghai 201210, China. c School of Chemical & Environmental Engineering, Jiangsu University of Technology, Changzhou 213001, China.

KEYWORDS: Metal carbide, Metal organic framework (MOF), Rechargeable Zn air battery, bifunctional electrocatalyst, Armored electrocatalyst

ABSTRACT: Developing low-cost and high-performance bifunctional oxygen electrocatalysts is essential for commercial realization of regenerative fuel cells and rechargeable metal air batteries. Iron carbide (Fe3C) is an ideal electrocatalyst candidate, however its poor oxygen evolution reaction (OER) activity and stability make it only serve as unifunctional oxygen reduction reaction (ORR) electrocatalyst. Here, we report a robust bifunctional electrocatalyst consisting of manganese-iron binary carbide (MnxFe3-xC) nanoparticles armored by nitrogen-doped graphitic carbon (MnxFe3-xC/NC). Synthesis involved facile pyrolysis of a tri-metallic (Fe, Mn, Zn) zeolitic imidazolate framework (ZIF). Incorporation of Mn

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modulated the electronic properties of Fe3C and the surrounding carbon, enhancing ORR and OER activities. MnxFe3-xC, well armored by carbon layers, displayed high resistance to oxidation and corrosion. The assembled Zn-air battery (ZAB) exhibited a large peak power density (160 mW cm-2 at 250 mA cm-2) with an energy density of up to 762 mWh gZn-1, high open-circuit voltage (OCV) of 1.5 V, and impressive long-term stability over 1000 cycles, indicating that MnxFe3-xC is one of the most stable earth abundant (cobalt-free) bifunctional electrocatalysts for rechargeable ZABs currently available.

1. INTRODUCTION Over the past decade, interest in renewable energies (e.g., solar and wind) has rapidly increased, and the generated ‘green’ electricity is expected to power human society in the near future.1 However, ‘green’ electricity in many geographical regions is timely and spatially decoupled from customer needs. Integrating electrical energy storage devices into the current grid infrastructure is one way to address this issue. Nonetheless, the energy stored from a grid is sufficient only for ~30 min of usage, which is 2000 times less than oil storage (~46 days). Bridging this large gap requires breakthroughs in energy storage technologies. Although pumped hydroelectric storage is the most widely used energy storage technology with >99% market share due to its low cost and simplicity, it is limited to specific geographic regions.2 Therefore, there is overwhelming interest in developing new types of batteries (e.g., metal-air batteries, vanadium redox flow batteries) that are low cost and have high energy density.3-6 Among various batteries, Zn air batteries (ZABs), which use oxygen from air as part of their redox chemistry, have attracted interest because of their theoretical energy density (1350 Wh kg-1, three times higher than that of lithium ion batteries (LIBs)).7 More importantly, Zn is cheap, non-toxic, and abundant with billions of tons of mineable Zn reserves present worldwide.8, 9 There is also no risk of explosion of


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ZABs, even in the case of a short-circuit between the anode and cathode, because of their underlying chemistry. Therefore, ZAB is a promising candidate for grid energy storage. Commercialization of ZABs is dependent on the air electrode consisting of a bifunctional electrocatalyst loaded on a porous current collector that can effectively catalyze the oxygen reduction reaction (ORR) during the discharge process and oxygen evolution reaction (OER) during the charge process.10-12 Significant advances have been made in generating bifunctional electrocatalysts for ZABs.13-19 Guan et al.18 reported hollow Co3O4 nanosphere-based air electrodes with excellent cycling stability over 200 h. Jiang et al.19 reported an interpenetrating cobalt based nanocomposite (Co/Co3O4@PGS) with robust coupling between Co-based nanoparticles and a nitrogen-doped carbon shell. The air cathode based on this multiphase nanocomposite had ultrastable cyclability with a record life-time of 4800 cycles over 800 h. We note that the ZABs with the best reported cycling life-times (>200 h) usually contain cobalt.18-20 However, the price of cobalt, which is a byproduct of Ni and Cu mining, has skyrocketed in recent years and reached ~$210 Kg-1, and supply is limited.21 To make ZABs competitive with pumped hydroelectric storage, the material costs of ZABs have to be minimized. Cheaper metals such as Fe ($0.89 Kg-1) and Mn ($1.75 Kg-1) and their derivatives (e.g., oxide, selenides, phosphides) perform well in single electrocatalytic reactions, but show relatively poor performance when simultaneously catalyzing OERs and ORRs.22 Here, we report a bifunctional manganese-iron binary carbide electrocatalyst supported by nitrogen-doped carbon (MnxFe3-xC/NC) synthesized by pyrolysis of tri-metallic ZIF. Although iron carbide has been investigated and is regarded to be an ideal ORR catalyst because of its high electrical conductivity and electronic properties that resemble those of Pt, iron carbide has poor stability in the OER potential region due to transformation to Fe3O4 from the Fe3C phase, rendering it unsuitable for rechargeable ZAB applications.23,

24

Introducing Mn into the ZIF precursor resulted in homogeneous



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incorporation of Mn into the Fe3C phase, endowing the carbide with OER activity.25, 26 Furthermore, intimate coupling between crystalline binary metal carbide nanoparticles and N-doped graphitic carbon framework ensured rapid charge transport to the active phase and enhanced ORR performance. Moreover, nitrogen-doped carbon firmly encapsulated the MnxFe3-xC, which protected the carbide from further oxidation and corrosion, and enhanced electrochemical stability during constant cycling between OER and ORR. As a result, the corresponding ZAB had a high open circuit potential, large power density, outstanding rate performance, and excellent stability over 1000 cycles.

2. EXPERIMENTAL SECTION

2.1 Synthesis of MnxFe3-xC/NC nanocomposite. MOFs derived binary carbide-based catalysts with different proportions of Mn and Fe were synthesized and denoted as MnxFe3-xC/NC. The x values of two MnxFe3-xC/NC samples were determined to be 0.3 and 0.9 using X-ray photoelectron spectroscopy (XPS) analysis, respectively. Typically, FeCl3·6H2O (0.27 mmol),

MnCl2·4H2O (1.08 mmol), and

Zn(CH3COO)2·2H2O (5.4 mmol) were dissolved in ethanol (80 ml), followed by the addition of a 80 ml 2-methylimidazole (45 mmol) solution under vigorous stirring, and then stored at room temperature for 12 h. Products were obtained by centrifugation and then washed with ethanol and deionized water several times, respectively. After drying at room temperature, Zn/Mn/Fe-ZIF was obtained as light-yellow crystals. Finally, Mn0.9Fe2.1C/NC was prepared by pyrolysis of as-prepared Zn/Mn/Fe-ZIF at 900 °C for 2 h under N2 atmosphere. Mn0.3Fe2.7C/NC and Fe3C/NC composites were synthesized under similar conditions with MnCl2·4H2O contents of 0.27 and 0 mmol, respectively. MnxC/NC and nitrogen-doped carbon (NC) composites were synthesized from the precursor as MnCl2·4H2O (1.08 mmol) and without MnCl2·4H2O and FeCl3·6H2O, respectively.


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2.2 Material characterization. As-prepared samples were characterized by powder wide-angle XRD using a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ= 0.154178 nm). SEM (JEOL-6700F) and TEM (JEOL JEM-2010) were used to investigate surface morphology. Porous textures were analyzed by nitrogen adsorption-desorption isotherms (Micromeritics ASAP 2020) using BET and BJH methods. XPS measurements were performed on a VG Scienta R3000 X-ray photoelectron spectroscope using a monochromatic Al Kα X-ray source. All binding energies were calibrated by setting C 1s to 284.8 eV. Raman measurements were carried out on using an RM 1000 (Ranishaw, UK) spectrometer and the 514 nm laser at 10 mW. 2.3 Electrochemical measurements. Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were performed using an electrochemistry workstation (IVIUMSTAT) with Pt wire and Ag/AgCl (3M KCl) as counter and reference electrodes, respectively. The working electrode was prepared as follows: 5 mg of the powder catalyst was dispersed in 1 ml water and isopropyl alcohol solvent (VH2O/Viso = 3:1) with addition of 50 μL of 5 wt% Nafion solution. The mixture was ultra-sonicated for 1 h and then drop-casted onto the GC with mass loading of 40 μg cm-2. 0.1 M KOH was used as the electrolyte with O2 bubbling before ORR measurement (30 min) and 1 M KOH was used for OER measurement. Based on the Nernst equation (ERHE = EAg/AgCl + 0.205 + 0.059×pH), the potential was converted to the reversible hydrogen electrode (RHE). ZAB performance was measured by a homemade Teflon cell. Catalyst ink was drop-casted onto the porous carbon fiber electrode with a mass loading of 2 mg cm-2. A pure Zn plate (thickness: 0.3 mm) was used as the anode and a 6 M KOH solution with or without 0.2 M Zn(CH3COO)2·2H2O was used as the electrolyte. A land CT2001A system was used to record charge-discharge curves and measure battery cycling performance. To exactly determine the specific capacity of the binary carbide catalyst air electrode-based



ZAB, a Zn-powder-based electrode was prepared to replace the Zn plate according to a previous report.12 Typically, a compound including Zn powder (500 mg, DAEJUNG), PVDF (50 mg, Sigma-Aldrich), Vulcan XC-72 (40 mg), multiwalled carbon nanotubes (40 mg, Nanjing XF Nano), and N-methyl pyrrolidone (400 μL, Sigma-Aldrich) was uniformly spread on the surface of carbon cloth by scalpel, followed by impregnation in deionized water to coagulate Zn powder-related composites and then drying in air for 24 h.

3. RESULTS AND DISCUSSION A schematic of the synthetic process is provided in Figure 1a. Bimetallic Zn/Fe-ZIF and trimetallic Zn/Mn/Fe-ZIF were prepared by mixing zinc acetate, 2-methylimidazole, and metal chlorides (e.g., FeCl3 and MnCl2). X-ray diffraction (XRD) patterns of Zn/Mn-ZIF, Zn/Fe-ZIF, and Zn/Mn/Fe-ZIF agreed with that of pure ZIF-8 (Figure S1a).12, 27 Zn/Mn/Fe-ZIF and Zn/Fe-ZIF shared a similar polyhedral morphology to ZIF-8. Energy dispersive spectroscopy (EDS) analysis verified the presence Mn, Fe, N, C, and Zn in Zn/Mn/Fe-ZIF (Figure S1b). The Fe3C/NC, Mn0.9Fe2.1C/NC and Mn0.3Fe2.7C/NC catalysts were obtained by carbonization of Zn/Fe-ZIF and Zn/Mn/Fe-ZIFs at 900oC under N2 atmosphere. Zinc element was evaporated during pyrolysis, and no Zn was detected by XPS in the as-prepared electrocatalysts (Figure S2).28 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Mn0.9Fe2.1C/NC are shown in Fig. 1b-d. Mn0.9Fe2.1C/NC had a polyhedral shape and consisted of carbide nanoparticles with an average particle size of ~15 nm homogeneously embedded in porous carbon framework (Figures. 1c, d and inset of Figure 1f). High-resolution TEM image (Figure 1e) showed that carbide nanoparticles were firmly encapsulated by a few layers of graphitic carbon. The carbide nanoparticles had clear lattice fringes with an interspacing of 0.20 nm, corresponding to the (031) plane of


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Fe3C. EDS mapping shows that N was homogeneously distributed through the carbon polyhedra, and the position of the Mn signal generally fit that of Fe, suggesting homogenous incorporation of Mn into iron carbide nanoparticles.29 The XRD pattern of Mn0.9Fe2.1C/NC indicated the presence of the (002) plane of graphitic carbon, and the characteristic carbide peaks were slightly shifted to higher diffraction angles compared to that of Fe3C/NC, confirming successful incorporation of Mn into the crystalline lattice of Fe3C (Figure 1g, and Figure S3).30 Figure 1h shows the Raman spectra. Bands at 217, 270, 383 cm-1 were ascribed to binary metal carbides and bands at 1324 and 1586 cm-1 were assigned to defects and graphitic carbon (Figure 1h).31 The ratio of the D band to the G band of Mn0.9Fe2.1C/NC (ID/IG = 1.65) was higher than that of Fe3C/NC (1.55), suggesting that more heteroatoms or defects were present in Mn0.9Fe2.1C/NC.22, 31 Figure 1i displays N2 adsorption-desorption curves of Mn0.9Fe2.1C/NC (Figure 1i). The presence of type-IV curves with an H1-type hysteresis loop revealed the co-existence of micropores and mesopores.32 The observed Brunauer-Emmett-Teller surface area was 409.8 m2 g-1, which is higher than that of a previously reported Fe3C based catalyst (~270 m2 g-1).22 The corresponding Barret-Joyner-Halenda (BJH) plots showed a continuous pore size distribution spanning from 2 to 150 nm with a sharp peak centered at 4.2 nm. A large surface area and hierarchical pore structure can effectively facilitate mass transport and active site exposure in electrocatalytic reactions.25, 26, 33



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Figure 1. (a) Schematic of the process used to synthesize MnxFe3-xC/NC catalyst. The magnified view shows the structures of Zn/Mn/Fe-ZIF and MnxFe3-xC/NC. (b) SEM image of Zn/Mn/Fe-ZIFs and (c) SEM, (d) TEM, and (e) high-resolution TEM images of Mn0.9Fe2.1C/NC catalysts. (f) STEM image with its corresponding elemental mappings for C, N, Mn, and Fe of the Mn0.9Fe2.1C/NC catalyst. (g) Magnified XRD patterns of Fe3C/NC and Mn0.9Fe2.1C/NC. (h) Raman spectra of Fe3C/NC and Mn0.9Fe2.1C/NC. (i) Adsorption-desorption isotherms of Mn0.9Fe2.1C/NC; the inset shows the BJH pore size distribution.


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Figure 2. (a) ORR polarization curves of carbide-based catalysts and commercial Pt/C; (b) comparison of Jk at 0.75 V, 0.8 V, and 0.85 V for carbide-based catalysts; (c) Tafel slopes derived from (a); (d) comparison of OER polarization curves of Fe3C/NC, Mn0.3Fe2.7C/NC, and Mn0.9Fe2.1C/NC.

To evaluate the ORR performance of the binary carbide-based catalysts, linear sweep voltammetry (LSV) at various rotating rates was performed in 0.1 M KOH solution. Figure 2a shows LSV curves at a rotating speed of 1600 rpm. The onset potential (E0 = 0.93 V) of the Mn0.9Fe2.1C/NC catalyst was more positive and its half-wave potential was higher (E1/2 = 0.78 V) than those of NC (0.73 V and 0.62 V, respectively) and Fe3C/NC (0.88 V and 0.74 V, respectively). To further explore the ORR mechanism, the electron transfer number (n) was determined from the LSV curves using the Koutecky–Levich (K-L) equation. K-L plots with a good linear relationship (Figure S4) suggested first-order reaction kinetics toward the concentration of dissolved oxygen, from which n for Mn0.9Fe2.1C/NC were determined to be 4.0 at 0.2-0.6 V, indicative of the ideal four-electron reaction pathway for ORR.34, 35 Kinetic current



density (jk) obtained from the slopes of K-L plots for catalysts with different Mn/Fe molar ratios are summarized in Figure 2b. The value of jk for Mn0.9Fe2.1C/NC at 0.75 V was 5.1 mA cm-2, higher than that of Mn0.3Fe2.7C/NC (4.7 mA cm-2) and Fe3C/NC (2.7 mA cm-2), suggesting higher intrinsic activity. Tafel slopes are shown in Figure 2c. The Mn0.9Fe2.1C/NC catalyst had a lower Tafel slope (93.3 mV dec-1) than the other catalysts, highlighting its rapid reaction kinetics (Figure 2c). The OER performances of the MnxFe3-xC/NC catalysts were also evaluated by LSV curves (Figure 2d). Mn0.9Fe2.1C/NC catalysts exhibited a lower overpotential (414 mV at 10 mA cm-2) required to deliver 10 mA cm-2 than Fe3C/NC and Mn0.3Fe2.7C/NC catalysts. We next constructed a two-electrode ZAB using Zn foil and porous carbon fiber paper loaded with catalysts as metal and air electrodes, respectively. The Mn0.9Fe2.1C/NC-based battery showed a high open circuit voltage (OCV) of 1.5 V (Figure 3a), a large power density of 160 mW cm-2 at a current density of 250 mA cm-2, and superior discharge performance in comparison to the batteries using Fe3C/NC, MnxC/NC, and commercial Pt/C (Figure 3b). Rate performance is an important criterion for advanced battery systems that require a dynamic response. The Mn0.9Fe2.1C/NC-based ZAB displayed good current step down and step up responses ranging from 5 to 100 mA cm-2 (Figure 3c). The battery also discharged stably over 100 h (Figure 3d). Specific capacity and the corresponding energy density of the Mn0.9Fe2.1C/NC-based ZAB were determined to be 635 mAh gZn-1 and 762 Wh kgZn-1, respectively (Figure 3e). The ZAB was able to power an electronic clock (rated voltage: 1.5 V) for more than 12 h (Figure 3e, inset). The rechargeability and durability of electrocatalysts are major concerns for practical application. The Mn0.9Fe2.1C/NC-based ZAB showed a narrower discharge-charge voltage gap than that of other carbide-based ZABs (Figure 3f, S5). As shown in Figure 3g, the Mn0.9Fe2.1C/NC-based ZAB demonstrated excellent cycling durability without significant performance decrease over 1000 charge-discharge cycles


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(equivalent to 334 h), while the PtRu/C-based ZAB had a limited durability of 360 cycles (120 h). Table 1 lists the ZAB performance of Mn0.9Fe2.1C/NC and state-of-the-art non-cobalt earth abundant electrocatalysts.13, 15, 26, 34-42 Mn0.9Fe2.1C/NC-based ZAB showed comparable or better performance than recently reported ZABs in terms of OCV, power density, and cycling lifetime. To investigate structural changes upon long-term cycling, the cycled Mn0.9Fe2.1C/NC was ex-situ investigated by TEM. As shown in the inset of Figure 3g, binary metal carbide nanoparticles in MnxFe3-xC remained intact, suggesting that the surrounding carbon layers effectively prevented dissolution and oxidation of carbides.

Figure 3. (a) Open-circuit voltage vs. time curve for a battery based on a Mn0.9Fe2.1C/NC air cathode. (b) Polarization curves and power densities of Fe3C/NC, MnxC/NC, Mn0.9Fe2.1C/NC, and 20% Pt/C based ZABs. (c) Galvanostatic discharge polarization plots of a Mn0.9Fe2.1C/NC air electrode-based ZAB at various current densities. (d) Long-term discharge curve for a Mn0.9Fe2.1C/NC air electrode-based ZAB at a current density of 5 mA cm-2. (e) Specific energy capacity of the Mn0.9Fe2.1C/NC air electrode-based



ZAB at a current density of 10 mA cm-2 and photograph of a smart clock powered by a single Mn0.9Fe2.1C/NC air electrode-based ZAB. (f) Charge and discharge polarization plots of Fe3C/NC, MnxC/NC, Mn0.9Fe2.1C/NC and PtRu/C catalysts as air electrodes of ZABs. (g) Two-electrode rechargeable ZAB galvanostatic charge-discharge cycles with Mn0.9Fe2.1C/NC catalyst as the air electrode at the current density of 5 mA cm-2. HR-TEM image of Mn0.9Fe2.1C/NC catalyst after 1000 galvanostatic charge-discharge cycles.

To understand the enhanced activity upon Mn doping of Fe3C/NC, we carried out detailed XPS studies. Quantitative XPS analysis revealed the Mn0.3Fe2.7C/NC and Mn0.9Fe2.1C/NC for the composition of fabricated catalysts (Table S1). As shown in Figure 4a-d, N 1s spectra of all samples were deconvoluted into five peaks ascribed to pyridinic-N (398.4 ± 0.2 eV), metal-Nx (399.4 ± 0.5 eV), pyrrolic-N (400.8 ± 0.5 eV), graphitic-N (401.6± 0.3 eV), and oxidized-N (403.8 ± 0.4 eV).43 The content of total N species of Fe3C/NC, Mn0.3Fe2.7C/NC, Mn0.9Fe2.1C/NC are 1.49, 3.29, and 2.06 at.%, respectively. The concentration of different nitrogen species including oxidized-N, pyrrolic-N, pyridinic-N, graphitic-N and M-Nx were listed in Table S2 and Figure 4d, based on deconvolution of N 1s spectra. Considering that Mn0.9Fe2.1C/NC had better electrocatalytic performance than Mn0.3Fe2.7C/NC, we speculated that the intrinsic activity of MnxFe3-xC/NC was affected by an electronic effect regulated by Mn doping. We compared high-resolution Fe 2p, Mn 2p, and C 1s spectra of Fe3C/NC, Mn0.3Fe2.7C/NC, and Mn0.9Fe2.1C/NC (Figure 4e-i, Figure S6). The Fe 2p3/2 spectra (Figure 4f-h) were deconvoluted into three characteristic peaks at 707.3 ± 0.3 eV, 710.4 ± 0.1 eV, and 713.6 ± 0.6 eV, which we ascribed to Fe0, Fe2+, and Fe3+ species, respectively.44 The Fe0 content follows the order of Fe3C/NC < Mn0.3Fe2.7C/NC < Mn0.9Fe2.1C/NC (Figure 4i). Analysis of the Mn 2p spectrum confirmed that the Mn species in Mn0.3Fe2.7C/NC showed a high oxidation state.45 The change in the lattice constant of Fe3C by addition of Mn implies a strong electronic bonding between Mn, Fe, and C atoms (Figure 1g). This evidence suggested that the increased content of metallic Fe species in Fe3C by the Mn addition was caused by the electron migration from Mn to adjacent Fe atom because Mn (1.55) is less electronegative than Fe (1.83).46


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Electron enrichment in iron carbide had a further impact on the electronics of the surrounding N-doped carbon layers.47 Figure 4e shows the C 1s spectra, which were deconvoluted into three peaks at 284.8, 285.7, and 287.6 eV, corresponding to C=C, C-N, and O-C=O, respectively. The C 1s spectra (Figure 4e) showed the shifting in C-N peak towards the lower binding energy for MnxFe3-xC/NC compared to Fe3C/NC, suggesting relaxation in the interaction between N and surrounding C atoms 48, 49. Therefore, we ascribed the enhanced catalytic activity of the Mn0.9Fe2.1C/NC catalyst to its electron redistribution behavior caused by extra electron transfer from metallic Fe species to adjacent N-doped carbon layers (Figure 4j), 50-53 which has been shown to be beneficial for optimizing the binding strength of reaction intermediates (e.g. *OOH, *O) during oxygen electrocatalysis. 36, 54-56 As a result, the electrochemical performance is enhanced by a stronger electronic interaction between electron-rich Mn-doped Fe3C and nitrogen functional groups (i.e., pyridinic, graphitic N, and Fe-Nx species).5, 50, 57



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Figure 4. High-resolution N 1s for (a) Fe3C/NC, (b) Mn0.3Fe2.7C/NC, and (c) Mn0.9Fe2.1C/NC. (d) Atomic concentration of different N species in MnxFe3-xC/NC and Fe3C/NC. (e) High-resolution C 1s for Fe3C/NC, Mn0.3Fe2.7C/NC, and Mn0.9Fe2.1C/NC. High-resolution Fe 2p for (f) Fe3C/NC, (g) Mn0.3Fe2.7C/NC, and (h) Mn0.9Fe2.1C/NC. (i) Percentages of different Fe species in MnxFe3-xC/NC and Fe3C/NC. (j) Schematic representation of charge transfer between MnxFe3-xC nanoparticles and the N-doped carbon layer.


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Table 1. Comparisons of the performance of a ZAB containing MnxFe3-xC/NC versus ZABs containing other non-cobalt based electrocatalysts.

Durability, Open-circuit

Cycle time,

Power density,

Bifunctional electrocatalyst

cycle number @j, mA voltage, V

mW cm-2

Reference h

cm-2

Mn0.9Fe2.1C/NC

1.5

160

1000@5

334

This work

FeNx-PNC

1.55

278

300@10

55

13

NiFeOx/NP-C

1.39

82.5

150@5

75

15

Fe/N/C@BMZIF

1.48

235

100@10

17

26

C-Fe-UFR

1.4

142

100@10

34

34

NPMC foam

1.48

55

600@2

100

35

N-GRW

1.46

65

150@2

150

36

BNPC-1000

NA

NA

600@2

100

37

NCNF-1000

1.48

185

500@10

185

38

P,S-CNS

1.51

198

200@2

40

39

Mn3O4/O-CNTs

1.45

86.6

150@2

150

40

Fe@N-C-700

1.4

220

100@10

16.7

41

NiFe@NC

NA

NA

200@10

33

42

4. Conclusions In summary, we developed a feasible and facile strategy for preparing non-cobalt, earth-abundant electrocatalysts via one-step pyrolysis of trimetallic ZIF. Nitrogen-doped carbon layers effectively prevent



the dissolution and oxidation of binary MnxFe3-xC layers in the harsh electrochemical environment, giving rise to outstanding electrochemical stability. The electronic interaction between the Mn-doped Fe3C and N-doped graphitic carbon shell endows MnxFe3-xC/NC catalysts with enhanced bifunctional activity. A Mn0.9Fe2.1C/NC-based ZAB delivered high peak power density (160 mW cm-2), large specific capacity (762 Wh kgZn-1), and long-term cycling durability over 1000 cycles.

ASSOCIATED CONTENT Supporting Information. Additional XRD, EDS data, XPS results are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Jung-Ho Lee) *E-mail: [email protected] (Xiaopeng Li)

Author Contributions All the authors have made contributions to the manuscript and given their approval of the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant (2017R1A2B3006941), and also supported by Creative Materials Discovery Program (2018M3D1A1057844) through the NRF funded by


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Ministry of Science and ICT. Financial support from the National Natural Science Foundation of China (Grant No. 21403280 and 21401083) is also acknowledged.

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Table of content entry

Binary metal carbide armored by nitrogen-doped carbon shows promising activity and outstanding stability in catalyzing oxygen reduction and oxygen evolution reaction (ORR & OER).


A Long-life Rechargeable Zn Air Battery Based on ... - ACS Publications

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