Cube-shaped metal-nitrogen–carbon derived from metal-ammonia complex-impregnated metal-organic framework for highly efficient oxygen reduction reaction

Journal Pre-proof Cube-shaped metal-nitrogen–carbon derived from metal-ammonia compleximpregnated metal-organic framework for highly efficient oxygen reduction reaction Lulu Chai, Linjie Zhang, Xian Wang, Zhuoyi Hu, Yuwei Xu, Ting-Ting Li, Yue Hu, Jinjie Qian, Shaoming Huang PII:

S0008-6223(19)31178-9

DOI:

https://doi.org/10.1016/j.carbon.2019.11.046

Reference:

CARBON 14803

To appear in:

Carbon

Received Date: 6 September 2019 Revised Date:

1 November 2019

Accepted Date: 14 November 2019

Please cite this article as: L. Chai, L. Zhang, X. Wang, Z. Hu, Y. Xu, T.-T. Li, Y. Hu, J. Qian, S. Huang, Cube-shaped metal-nitrogen–carbon derived from metal-ammonia complex-impregnated metal-organic framework for highly efficient oxygen reduction reaction, Carbon (2019), doi: https://doi.org/10.1016/ j.carbon.2019.11.046. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

Table of Content Cube-shaped Metal-Nitrogen-Carbon Derived from Metal-Ammonia Complex-Impregnated Metal-Organic Framework for Highly Efficient Oxygen Reduction Reaction Lulu Chai, Linjie Zhang, Xian Wang, Zhuoyi Hu, Yuwei Xu, Ting-Ting Li, Yue Hu, Jinjie Qian*, and Shaoming Huang*

Herein, a series of high-performance non-precious TM-based M-NC catalyst for ORR are successfully obtained by direct pyrolysis of a MAC-impregnated coreshell structure synthesized from the self-sacrificing cube-shaped MOF-5 precursors. The as-pyrolyzed M-NC (M= Co, Ag, Cu, and Ni) electrocatalysts showcase a high electrocatalytic ORR activity in an alkaline medium due to metal-based nanoparticles and active M-N sites in the carbonaceous matrix.

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Cube-shaped Metal-Nitrogen-Carbon Derived from Metal-Ammonia ComplexImpregnated Metal-Organic Framework for Highly Efficient Oxygen Reduction Reaction Lulu Chaia, Linjie Zhangc, Xian Wanga, Zhuoyi Hua, Yuwei Xua, Ting-Ting Lid, Yue Hua, Jinjie Qiana,*, and Shaoming Huanga,b,* a

College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325000,

China b

School of Materials and Energy, Guangdong University of Technology, Guangzhou,

Guangdong, 510006, China c

Chimie du solide et de l'énergie-Collège de France 11 Place Marcelin Berthelot, Paris, 75005,

France d

School of Materials Science and Chemical Engineering, Ningbo University, Ningbo, 315211,

China

ABSTRACT: The non-platinum metal-nitrogen-carbon (M-NC) system is a class of highly active oxygen reduction reaction (ORR) electrocatalysts that are well known and widely used in fuel cell applications. Herein, one simple synthesizing method of efficient M-NC electrocatalysts is proposed by the direct pyrolysis of a pretreated core-shell structure of ZnO@ZIF-8 containing metal-ammonia complex (MAC, [M(NH3 ) x]n+ ) from the polydisperse selfsacrificing MOF-5 cubes. The as-pyrolyzed M-NC (M= Co, Ag, Cu, and Ni) electrocatalysts showcase high electrocatalytic ORR activity in an alkaline medium. Among them, Co-NC catalyst exhibits an excellent ORR performance where its E1/2 =

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0.80 V, J L = 5.88 mA cm -2 , as well as Tafel slope of 67.0 mV dec -1 are close to commercial Pt/C and Ag-NC, better than the other two catalysts of Cu-NC and Ni-NC. Impressively, zinc-O2 batteries assembled with M-NC materials exhibit the better discharge performance, and have great potential in the practical energy conversion and storage. The experimental demonstration of metal-based nanoparticles and active M-N sites in the carbonaceous matrix can be efficiently used to promote the ORR activity, which is derived from a synergistic contribution of its particular hollow structure, large specific surface area, rich M-N active sites, and high degree of graphitization. This attractive preparation approach provides us a powerful contribution to the construction of high-performance carbon-based ORR electrocatalysts. Keywords: Metal-organic framework; Metal-ammonia complex; Metal-nitrogen-carbon system; Oxygen reduction reaction; Zinc-O2 battery; 1. Introduction Metal-organic frameworks (MOFs), also called porous coordination polymers, are a class of crystalline materials that are self-assembled by metal cations or its polynuclear clusters and organic linkers through relatively flexible and diverse coordination bonds [12]. MOFs are currently a rapidly emerging type of new frontier materials because of its adaptability to the rational design of different functions for a variety of important applications, which have been carried out extensive exploration at the molecular level [3,4]. Generally speaking, the key is that MOFs have good advantages, including high surface area, porous structure and flexible framework, which can not only stand out in conventional materials in gas adsorption and separation but also show good prospects in catalytic applications [5-7]. Recently, MOFs materials have been extensively served as a

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novel kind of self-sacrificing templates to prepare nanostructured materials such as porous carbon and metal carbides/oxides/sulfides [8-11]. This viability option makes sense because the hierarchically porous carbons or metal derivatives have a multi-porous structure, which can be directly preserved from MOFs precursors without an indispensable part of additional templates. On the other hand, oxygen reduction reaction (ORR) is one of the vital processes in the cathodic reaction for fuel cells and metal-air batteries [12,13], and is also an important research object in basic electrochemical research. Among them, clean energy devices urgently require the efficient and low-cost electrocatalysts to eliminate their dynamic and/or kinetic limitations, which further highlights the integrally electrochemical properties of the devices [14,15]. As we all know, platinum-based materials (metallic Pt and its alloys) have been reported to be the most effective catalysts in ORR performance [16]. However, Pt-based materials simultaneously own many deficiencies, such as low natural abundance, high price, and poor stability due to the coalescence of Pt nanoparticles (NPs), etc., which severely hinder the practical applications [17,18]. In recent years, multiple research reports have placed the design and syntheses of new low-platinum catalysts at the forefront [19,20], the development of non-precious metal catalysts [21,22], and their in-depth ORR electrocatalytic mechanism [23]. In terms of the rich elements in the earth crust, the development of highly active transition metalnitrogen-carbon (M-NC) catalysts as the highly efficient ORR catalysts for the replacement of Pt-based materials [24,25], which are typically pyrolyzed from microporous MOFs precursors normally made up of nitrogen, carbon and transition metals. The rapid entry and diffusion of electrolytes (such as OH - and H2 O) and gas

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molecules (H2 and O2 ) can be highly accessed through open pores and/or channels in these hierarchically porous MOF-derived M-NC materials. Due to its advantages in the following factors, including the amount of active M-N sites, ultra-fine metallic NPs and the enhanced transport properties, these M-NC materials have been anticipated in the commercialization of clean energy technologies nowadays. However, a facile preparation of the M-NC system is still remaining a huge challenge. In order to further investigate the effect of M-NC on the catalysis of ORR, we report the simple and efficient synthesis method by the stepwise in-situ conversion of

Scheme 1. Schematic synthesizing process of M-NC cubes by the stepwise in-situ conversion of MOF-5 cubes.

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polydisperse MOF-5 cubes. The as-prepared M-NC (M = Co, Ag, Cu and Ni) cubeshaped catalysts are a type of meso/macroporous structure with high specific surface area and many active M-N sites, resulting in an excellent ORR performance comparable to the commercial Pt/C catalyst in the 0.1 M KOH. Among them, Co-NC catalyst exhibits an excellent ORR performance where its E1/2 = 0.80 V, J L = 5.88 mA cm -2 , as well as Tafel slope of 67.0 mV dec -1 are close to representational Pt/C. The experimental demonstration of metal-based NPs as well as active M-N sites in the carbonaceous matrix can be efficiently used to promote the ORR activity, which is derived from a collaborative contribution of hollow structure, active surface area, rich active sites, and the degree of high graphitization. 2. Experimental 2.1 Synthesis of core-shell MAC/ZnO@ZIF-8 (metal-ammonia complex (MAC) = [Co(NH3)6]Cl3, [Ag(NH3)2]OH, [Ni(NH3)6]Cl2, [Cu(NH3)4]SO4) cubes. The mixed solution of ZnO@ZIF-8 (100 mg), different MAC solution (MAC: H2O = 20 mg: 10 mL), 8 M NH3⋅H2O (2 mL) is reacted at 40 oC, 900 rpm for 12 h, cooled and centrifuged with distilled water until the pH is neutral. Most of the ZnO can be etched by the action of 8 M NH3⋅H2O, the products dry at 85 oC in the oven (Fig. S1). The obtained materials are named AgAC/ZnO@ZIF-8 (AgAC = Ag(NH3)2]OH), CoAC/ZnO@ZIF-8 (CoAC = [Co(NH3)6]Cl3), NiAC/ZnO@ZIF-8 (NiAC = [Ni(NH3)6]Cl2), and CuAC/ZnO@ZIF-8 (CuAC = [Cu(NH3)4]SO4), respectively. 2.2 Synthesis of M-NC with dispersed M-N Sites. The MAC/ZnO@ZIF-8 samples are placed in the middle of a CVD tube furnace and maintain under an Ar (150 sccm) atmosphere. The setup procedure is to raise the temperature to 800 oC at a heating rate of 20 oC min-1. The

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temperature is kept at a constant temperature for 2 h. After the temperature of the furnace is lowered to room temperature, the samples are collected and labeled as Co-NC, Ag-NC, Cu-NC, and Ni-NC, respectively. 3. Results and discussion The synthetic method of the M-NC-based catalysts is well illustrated in Scheme 1. MOF-5 is one of the most widely studied MOF materials, which possesses a 3dimensional cubic skeleton consisting of Zn 4 O-based secondary building units bridged by linear carboxylate BDC 2- deprotonated from 1,4-benzenedicarboxylatic acid (Fig. 1a) [26]. The morphology and structure of MOF-5 can be collected by scanning electron microscopy (SEM) and powder X-ray diffraction (PXRD) analysis (Fig. 1b, 1c), where it is exhibited that the as-synthesized MOF-5 sample has a cubic morphology with an average size of 3 µm and its high crystallinity by the narrow diffraction peaks in the range of 5-20 degree. In Fig. 1d, the N2 sorption isotherms reveal that the as-obtained material has a relatively large specific surface area (BET surface area: 2528 m2 g -1 ; Langmuir surface area: 2785 m2 g-1 ) and a regular pore structure (pore size: 0.78 nm). In thermogravimetric analysis (Fig. 1e), MOF-5 exhibits a high decomposition temperature above 500 oC with an additional slight weight loss of ~6.1% over the temperature range of 400-480 o C due to the good thermal stability, while pristine MOF-5 cubes are calcined at 500 oC under the oxygen flow that is conveniently converted into zinc oxide cubes. Its morphology and structure are depicted in Fig. 1f, S2, the surface of as-pyrolyzed ZnO cubes is relatively rough but maintains a cubic morphology. After that these ZnO@ZIF-8 cubes can be successfully fabricated by the self-assembly of a nano-sized ZIF-8 shell with in-situ released zinc cations together with an addition of 2-methylimidazole ligands

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under the solvothermal condition grown on the surface of ZnO (Fig. 1g, more details please see supplementary information, SI). Moreover, a typical TEM image shows a clearly distinguishable boundary, which is between the ZnO core and the ZIF-8 shell (Fig. S4a, S4b). The resulting ZnO@ZIF-8 cubes are perfectly matched with the simulated patterns of pure ZnO and single crystal of ZIF-8 (Fig. S5a). Then ZnO@ZIF-8 is immersed in different metal-ammonia complex (MAC, AgAC = [Ag(NH3 )2 ]OH, CoAC = [Co(NH3 )6 ]Cl 3 , NiAC = [Ni(NH3 )6 ]Cl 2 , CuAC = [Cu(NH3 )4 ]SO4 , Fig. S3) solution for 12 h, thus we have pre-filled the micropores of the ZIF-8 shell with

Fig. 1. MOF-5: (a) Cell unit; (b) SEM image; (c) PXRD patterns; (d) N2 sorption isotherms at 77 K (inset: pore size distribution); and (e) TGA image under O2 atmosphere; SEM images of (f) ZnO and (g) ZnO@ZIF-8.

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different MAC cations to obtain MAC/ZnO@ZIF-8. The SEM images of the core-shell cubes after impregnation show that the morphology of the cubic precursors is well preserved with a slight deterioration (Fig. S4c-f). In addition, the PXRD patterns of MAC/ZnO@ZIF-8 display that the shell portion of the nanoscale ZIF-8 structure is also well maintained and the original structure is not damaged even after the MAC adsorption process (Fig. S5b). As shown in Fig. S6, all the N2 adsorption isotherms as well as pore size distribution

(PSD)

analyses

of

CoAC/ZnO@ZIF-8, CuAC/ZnO@ZIF-8, and

NiAC/ZnO@ZIF-8 cubes exhibit a type-IV pattern indicating the existence of micropores and macropores, inferring the microporous features of crystalline ZIF-8 and macroporous characteristics of ZnO. However, the adsorption isotherms of AgAC/ZnO@ZIF-8 show the type-IV isotherm characteristic with a low capacity, which indicates the existence of macropores from the encapsulated ZnO cubes. In this case, these larger silver ions are encapsulated into this hybrid composite with a stronger interaction [27]. Therefore, the as-obtained adsorption isotherms exhibit a similar adsorption behaviour to that of the initial ZnO template. Four types of as-obtained core-shell MAC/ZnO@ZIF-8 cubes are subjected to pyrolysis process at 800 oC in an argon flow to synthesize the M-NC catalyst co-doped with M and N atoms, wherein Co-NC is used as a representative example to illustrate, and three remaining catalysts are also interpreted in Fig. S7-S10. The SEM image of CoNC reveals a 3-dimensional cubic porous structure with a slightly shrunk diameter of about 2-3 µm compared to that of ZnO@ZIF-8 and MAC/ZnO@ZIF-8 (Fig. 2a). Co NPs aggregation is embedded in the as-derived carbonaceous matrix, whose size is clearly characterized by atomic force microscopy and the corresponding HR-TEM images to be

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Fig. 2. (a) SEM, (b) TEM, (c,d) HR-TEM images of the Co-NC cubes, inset in (d) shows the corresponding SAED pattern; (e) HAADF-STEM images of a single Co-NC cube and the corresponding C-, N-, O-, Co-, and Zn-elemental mappings; (f) EDX spectrum, (g) line scan and (h) PXRD pattern of Co-NC. approximately 16±1 nm (Fig. S11, S12). As shown in Fig. 2b, 2c, cobalt clusters are uniformly dispersed into the carbon support, which can be expected to promote the ORR reactivity due to the enhanced conductivity [28]. To gain deep insight into Co NPs, the HR-TEM image of the Co-NC catalyst shows that the entire porous carbon skeleton is successfully covered by the graphitized carbon where the characteristic spacings of 0.203 and 0.212 nm belong to the (111) and (100) planes of Co and graphite, respectively (Fig.

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2d). Elemental mapping (Fig. 2e) and energy dispersive X-ray spectrum (EDX) (Fig. 2f, S13) confirm the presence of well distributed C, N, O, Co and Zn in the Co-NC cubes. Under the line scan mode, the cobalt element is mainly present in the shell portion of the cube, while the zinc element mainly exists in the core portion (Fig. 2g). Noteworthily, a broad peak at around 14.6 o is observed in the M-NC materials, which indicates its amorphous carbon properties. The PXRD pattern of the Co-NC catalyst shows that the three peaks are located at 44.2, 51.5, and 75.8o (2θ), which are very consistent with the diffraction peaks of the crystalline cubic metallic Co phase (PDF#15-0806), and two broadened peaks center at 28.4 o and 43.1 o, corresponding to the (002) and (100) faces of the typical carbon material with a degree of graphitization in Fig. 2h. The D band (~1350 cm -1 , sp3 ) and the G band (~1580 cm -1 , sp2 ) mainly reflect the disorder and crystallization of the material, respectively [29]. The intensity ratios of ID/I G for all products are similar where the smaller the ratio, the higher the degree of graphitization. As displayed in Fig. 3a, the I D /IG value of Ag-NC cube (1.04) is smaller than that of Co-NC (1.05), Cu-NC (1.06) and Ni-NC (1.08), which indicates the highest degree of graphitization for Ag-NC. This result demonstrates that the high degree of

Fig. 3. (a) Raman spectroscopy of M-NC cubes; (b) N2 sorption isotherms (inset: an enlarged view in the range of P/P0 ≤0.1); and (c) PSD curves of M-NC and NCCs cubes (inset: an enlarged view in the range of pore size≤2 nm).

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graphitization can boost the electrical conductivity and thus enhance the ORR performance. The FT-IR spectra of the M-NC cubes showcased in Fig. S14 also confirm the presence of the strong C=O peak with the characteristic vibration of 1726 cm -1 , the OH deformation peak at 1410 cm -1 , and the C-OH peak at 1240 cm -1 , while the bands located at 1170 cm -1 and 1367 cm -1 are assigned to the C-C and C-O stretching vibrations, respectively [30]. What’s more, the N2 adsorption-desorption isotherms including specific surface area and PSD of all M-NC samples are shown to be the similar type-III/IV characteristics of the mesoporous materials (Fig. 3b-c). For the as-prepared Co-NC, CuNC, Ni-NC, and NCCs (N-doped carbon cubes, more details please see SI), the adsorption volume is large at P/P0 <0.1, while the hysteresis loops in the P/P0 range of 0.45-1.0 indicates the presence of micropores and mesopores. The Brunauer-EmmettTeller surface area (S BET ) and total pore volume (VTotal) characteristics of NCCs support (SBET =742.8 m2 g-1 and VTotal=2.61 cm3 g -1 ), and Co-NC (SBET =678.8 m2 g-1 and VTotal=1.68 cm3 g -1 ) exhibit a larger BET surface area and a higher active total pore volume than that of conventional Ag-NC (S BET=33.1 m2 g-1 and VTotal=0.15 cm3 g -1 ), CuNC (SBET =198.4 m2 g-1 and VTotal=0.61 cm3 g-1 ), and Ni-NC (SBET=183.5 m2 g-1 and VTotal=0.55 cm3 g-1 ). In view of the relatively porous structure with high active specific surface area and rich pore volume, these features make M-NC materials favorable to expose active site and rapidly transport relevant species and/or intermediates for ORR [31]. The chemical elements, including C, N, O, Zn and MAC-based metals, are more indepth characterized by X-ray photoelectron spectroscopy (XPS) of these M-NC cubes (Fig. 4, S15). Moreover, the C 1s peak of ~285 eV and the N 1s peak of ~400 eV are

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Fig. 4. Deconvoluted XPS spectra of the Co-NC, Ag-NC, Cu-NC, Ni-NC (a) C 1s and (b) N 1s; Deconvoluted XPS spectra of the Co-NC (c) Co 2p and (d) Zn 2p.

present for all the M-NC materials in the full XPS survey spectra (Fig. S15a) [32]. Fig. 4a highlights the deconvoluted C 1s spectra of the M-NC materials, which can be fitted to three sub-peaks, corresponding to the C-C peak of 284.7 eV, the C-N peak of 285.9 eV, and the C=O peak of 288.2 eV [33]. The four sub-peaks can also be obtained by the deconvolution into N peaks, namely pyridinic N (~398.5 eV), pyrrolic N (~399.6 eV), graphitic N (~400.8 eV) and oxidized N (~402.4 eV) (Fig. 4b) [34]. In this context, some studies have suggested that pyridinic N owns a major contribution on ORR performance because it provides the suitable Lewis base sites chemically adsorbing the O2 molecules [35]. Meanwhile, the structural stability and the limiting current in the ORR are mainly affected by the graphitic N sites [36]. Specifically, Fig. S16 shows the relative C-N as well as N pyridine+graphite percentage of these M-NC catalysts, where the C-N bond is

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cleaved and preferentially transformed to active nitrogen (pyridinic N and graphitic N). The deconvoluted Co 2p spectrum is displayed in Fig. 4c, in which Co 3+/2+ 2p3/2 (~781.0 eV) and Co3+/2+ 2p 1/2 (~795.8 eV) are mainly derived from the oxidized Co 2+ and Co3+ species and/or the catalytically active M-N species on the surface. In addition, the Co-N bond is attributed to the two main peaks at 784.3 and 800.3 eV, and the metallic cobalt peaks of Co 0 2p 3/2 and Co0 2p1/2 locate at 778.7 and 782.4 eV [37,38], with two typical satellite peaks at 787.8 and 793.2 eV, respectively. In this case, the XPS results analyzed from the cube-shaped carbonaceous framework further demonstrate the effective integration of Co and N elements after the CoAC impregnation. It may be due to the presence of M-N catalytic active sites to achieve efficient mass transport. XPS analyses of other metals (Ag 3d, Cu 2p, Ni 2p) are clearly interpreted in SI (Fig. S15b-d). The high-resolution Zn 2p spectrum (Fig. 4d) can also be easily deconvoluted into two peaks of Zn 2p1/2 (~1044.9 eV) and Zn 2p3/2 (~1021.9 eV). In order to investigate the catalytic ORR performance of the unique metal- and nitrogen-doped carbon-based structure, the detailed characterization is conducted by utilizing the electrochemical methods in this experiment. First of all, all of the catalysts’ ORR activity is measured using the cyclic voltammetry (CV) profiles with an electrolyte of saturated 0.1 M KOH by the use of N2 and O2 , respectively. The CV profile (Fig. 5a, S17) reveals that no cathodic peak is appeared in the N2 saturated electrolyte, while a significant cathodic peak of all catalysts is clearly observed in the O2 saturated solution. Obviously, the Co-NC catalyst has a cathodic peak at 0.84 V (vs RHE) which is close to the commercial Pt/C (0.86 V) and is higher than Ag-NC (0.83 V), Cu-NC (0.76 V), and Ni-NC (0.62 V), which signs that the Co-NC sample owns a better ORR electrocatalytic

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Fig. 5. (a) CV profiles in N2 - (wine) and O2 - (royal) saturated solutions of M-NC materials at 10 mV s -1 ; (b) LSV profiles at 5 mV s -1 in 0.1 M O2 -saturated KOH solution of M-NC materials and 20 wt% Pt/C at 1600 rpm; (c) LSV profiles obtained from different rotational speeds for the Co-NC (the inset: the K-L diagram from 0.3 V-0.7 V); (d) the histogram of electron transfer numbers; (e) The corresponding Tafel plots; (f) EIS spectra of M-NC materials; (g) The stability evaluation of Co-NC and 20% Pt/C; (h) The Current-time chronoamperometricon Co-NC and 20% Pt/C followed by the introduction of 3.0 M methanol at 400 s in O2 -saturated 0.1 M KOH at 0.6 V. activity. Therewith, the linear sweep voltammogram (LSV) curve is used to further evaluate the ORR activity in which the catalyst of Ni-NC manifests a relatively poor catalytic activity for ORR compared to the other catalysts, with the onset potential (E onset) of 0.83 V, the half-wave potential (E1/2 ) of 0.61 V, and the diffusion limited current density (J L) of 3.86 mA cm -2 , which are consistent with the CV results (Fig. 5b, S18). Most importantly, the Co-NC (Eonset= 0.91 V, E1/2 = 0.80 V, and J L= 5.88 mA cm -2 ), AgNC (E onset= 0.93 V, E 1/2 = 0.82 V, and J L= 5.93 mA cm-2 ), and Cu-NC (E onset= 0.92 V, E1/2 = 0.72 V, and J L= 4.41 mA cm-2 ) catalyst have an excellent ORR activity when the active metal NPs are dispersedly loaded and/or single metal sites coordinated into the

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carbon layer compared to the commercial Pt/C catalyst and other electrocatalysts (Table S1). The rotating (ring) disk electrode (R(R)DE) system is operated to test the LSV profiles at the rotate speed of 100-2500 rpm by clarifying the kinetics and catalytic pathway of ORR (Fig. 5c), where the Co-NC catalyst also improves the limiting current density with the increase in the rotation rate. Meanwhile, the corresponding KouteckyLevich (K-L) plots, fitted to be approximately parallel, show an almost linear relationship at different potentials and indicate a kinetics of first-order reaction for the concentration of dissolved O2 as well as the similar electron transfer numbers (n, inset of Fig. 5c, Fig. S19) [39]. Applied at the potential of 0.3-0.7 V, the n values are theoretically calculated to be 3.9-4.0 (Co-NC), 3.8-3.9 (Ag-NC), 3.5-3.9 (Cu-NC), and 3.4-3.8 (Ni-NC) from the fitting slope of the K-L curves (Fig. 5d), indicating that all these M-NC catalysts favor a complete 4e ORR process in 0.1 M KOH electrolyte. Simultaneously, the estimated yield of hydrogen peroxide is further characterized to confirm an almost 4e ORR process for these M-NC catalysts (Fig. S20). In addition, the Tafel slopes of the Co-NC and Ag-NC catalysts are 67.0 and 66.3 mV dec -1 , respectively, which are smaller than that of those other catalysts (Cu-NC: 102.5 mV dec -1 , Ni-NC: 111.6 mV dec -1 ) and slightly higher than that of the commercial 20 wt% Pt/C (62.3 mV dec-1 ), demonstrating that the catalyst has a fast electron transfer rate (Fig. 5e). The electron transfer properties of M-NCs are compared by electrochemical impedance spectroscopy (EIS) as shown in Fig. 5f, where the fitted Nyquist diagram shows the R ct (charge transfer resistance) of Ag-NC (25.1 Ω) is lower than Co-NC (31.0 Ω), Cu-NC (32.7 Ω), Ni-NC (41.8 Ω). Moreover, Ag-NC has a larger double-layer capacitance (C dl=7.24 mF cm -2 ) than other catalysts (Co-NC: 3.24 mF cm -2 , Cu-NC: 3.37

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mF cm -2 , Ni-NC: 0.70 mF cm -2 ), indicating a high electrochemically active surface area (Fig. S21-22). This high electrochemically active surface area is derived from the hierarchically porous structure with good pore distribution and high specific surface area, which is beneficial to the effective exposure of active sites for an improved catalytic activity [40,41]. Another important criterion for judging the practical application of electrocatalyst is its stability. In order to determine the prospective application of M-NC materials in actual fuel cells, we further evaluate long-term durability and methanol tolerance by chronoamperometry method. In Fig. 5g, S23, after 10 hours of continuous operation at 0.6 V vs RHE in O2 -saturated 0.1 M KOH at a rotation rate of 1600 rpm, the current of Co-NC is maintained at 97.4% of the initial value; and Ag-NC (96.1%), Cu-NC (95.9%), and Ni -NC (95.3%) current retention rate is also higher than Pt/C (90.2%). This confirms that M-NC materials have higher stability than the commercial Pt/C. More importantly, when 3 M methanol is added to the electrolyte of 0.1 M KOH solution, it is found that the current of Pt/C exhibits a sharp drop with the current retention rate of 73.8% after 1200 s. However, there is a small current density decay for the M-NC materials (Co-NC: 98.7%). This shows that the M-NC materials have an excellent methanol tolerance ability (Fig. 5h, inset of Fig. S23). Based on the above-mentioned promising ORR electrocatalytic activity and stability, we further study the practical applicability of M-NC materials in assembling a self-made Zn-O2 battery, in which 6.0 M KOH solution is used as the electrolyte. In Fig. 6a, Ag-NC catalyst provides an open circuit voltage (OCV) of 1.47 V, close to the OCV of Pt/C (1.49 V), that is higher than Co-NC (1.43 V), Cu-NC (1.41 V), and Ni-NC (1.39 V), which indicates a low loss of

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Fig. 6. The Zn-O2 battery performance in the 6.0 M KOH electrolyte with M-NC materials acting as the O2 cathode. (a) Open circuit voltage curves of Zn-O2 batteries with different M-NC materials and Pt/C; (b) Polarization and power density curves of MNC materials and Pt/C based Zn-O2 batteries; (c) Galvanostatic discharge curves of the Zn-O2 batteries at 10 mA cm -2 ; (d) The photograph of 25 light-emitting diode (LED) lights powered by two Zn-O2 batteries connected in series with the Co-NC as the O2 cathode. catalytic activity. Most importantly, the polarization and power density curves in Fig. 6b display that the the maximum power density of a zinc-O2 battery assembled from Ag-NC at 217 mA cm2

is 129 mW cm-2, which is close to Pt/C (134 mW cm-2 at 229 mA cm-2 ) and higher than Co-NC

(120 mW cm-2 at 195 mA cm-2), Cu-NC (105 mW cm-2 at 173 mA cm-2), and Ni-NC (97 mW cm-2 at 144 mA cm-2). Furthermore, the primary battery of Ag-NC generates a higher specific capacity of 643.8 mA h g-1 than Pt/C (590.6 mAh g-1), Co-NC (567.1 mAh g-1), Cu-NC (544.7 mAh g-1), and Ni-NC (471.2 mAh g-1) in Fig. 6c. The potential gaps between M-NC and Pt/C

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catalysts at different discharge currents are also calculated to give an insight into the origin of high activity of the M-NC catalysts (Fig. S24). As a result of the excellent kinetics, M-NC materials are capable of delivering narrow average discharge voltage gap even when the current rate is increased from 0 to 200 mA cm-2. As shown in Fig. 6d and S25, the better stability of the Co-NC can also be continuously illuminated for more than 90 minutes without no significant brightness degradation by continuously lighting 25 LED lights with two Co-NC based integrated Zn-O2 batteries. The better electrocatalytic performance of the M-NC materials for ORR mainly attributed to the following reasons: i) the highly dispersed metal nanoparticles provide a large number of catalytically active sites. Meanwhile, the coordination between TM nanoparticles and N-doped carbon supports facilitate the outstanding activity and catalytic durability; ii) the meso/macroporous structure and large specific surface area in the hollow carbon matrix expose more active site that accelerates the diffusion of O2 and electrolyte; iii) the carbon framework of M-NC has good graphitization properties for enhancing conductivity and accelerating electron transfer. These above-mentioned results reflect the excellent ORR performance from these M-NC materials. 4. Conclusion In summary, a series of high-performance non-precious TM-based M-NC catalyst for ORR are successfully obtained by direct pyrolysis of a MAC-impregnated core-shell structure synthesized from the self-sacrificing cube-shaped MOF-5 precursors. Physical characterization exhibits the as-made cubic M-NC catalyst consists of M- and N-doped carbon material and a small amount of metal-based NPs, which shows a high specific

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surface

area

and

a

meso/macroporous

structure.

Meanwhile,

electrochemical

measurements display that M-NC cubes demonstrate an excellent ORR activity, wherein the ORR activity of Co-NC is comparable to the representational Pt/C in an alkaline medium, in which we learn that both the metal-based NPs, the M-N active sites and Ndoped carbon can be used to efficiently promote the ORR activity. Furthermore, the MNC catalysts can replace Pt/C when served as an O2 cathode in a home-built Zn-O2 batteries. The results of this work clearly motivate a new approach to rationally design and prepare the low-cost but high-activity M-NC electrocatalysts for practical fuel cells applications and Zn-O2 batteries. Acknowledgements This work was financially supported by the Natural Science Foundation of China (21601137, 51672193, 21601187), the State Key Laboratory from Structural Chemistry, Chinese Academy of Sciences (20190008), and also funded by the Graduate Scientific Research Foundation of Wenzhou University (3162018030, 2019R429048). Appendix A. Supplementary data Supplementary

material

is

available

in

http://dx.doi.org/***-****-*.)

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the

online

version

of

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article

at

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The authors declare no completing financial interest.