Carbon Defect-Induced Reversible CarbonâOxygen Interfaces for

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Energy, Environmental, and Catalysis Applications

Carbon defect induced reversible carbonoxygen interfaces for efficient oxygen reduction Qilong Wu, Qian Liu, Yunjie Zhou, Yue Sun, Jiong-Peng Zhao, Yang Liu, fuchen liu, Manxiu Nie, Fandi Ning, Nianjun Yang, Xin Jiang, Xiaochun Zhou, Jun Zhong, and Zhenhui Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14323 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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ACS Applied Materials & Interfaces

Carbon Defect Induced Reversible Carbon-Oxygen Interfaces for Efficient Oxygen Reduction Qilong Wu1, Qian Liu1, Yunjie Zhou2, Yue Sun2, JiongPeng Zhao1, *, Yang Liu2, Fuchen Liu

1, *,

Manxiu Nie1, Fandi Ning3, Nianjun Yang4, Xin Jiang4, Xiaochun Zhou3, *, Jun Zhong2, *, and Zhenhui Kang2, * 1School

of Chemistry and Chemical Engineering, Tianjin University of Technology Tianjin

300384, China. 2Jiangsu

Key Laboratory for Carbon-based Functional Materials and Devices, Institute of

Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou 215123, China. 3Division

of Advanced Nanomaterials, Suzhou Institute of Nano-tech and Nano-bionics, Chinese

Academy of Sciences (CAS), Suzhou 215123, China. 4Institute

of Materials Engineering, University of Siegen, Siegen 57076, Germany.

KEYWORDS: oxygen reduction, carbon defect, O2-chemisorption, reversible carbon-oxygen interfaces, air-breathing fuel cell

ABSTRACT: It is a great challenge to fabricate a metal-free oxygen reduction reaction (ORR) electro-catalyst in fuel cells due to its insufficient catalytic activity in acidic medium. Here, a metal-free carbon material C-900 with abundant defect sites is fabricated by the self-sacrificed template and solid-state reaction strategy. C-900 shows a superior performance than 20% Pt/C in

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alkaline medium and a performance close to 20% Pt/C in acidic condition. It can thus be applied in air-breathing fuel cell (without extra operation pressure) as the cathode catalyst, which shows a high performance (1160 W L-1; ~62% of 20% Pt/C) with excellent stability. By using oxygen temperature programmed desorption, the strong selective chemisorption of O2 on C-900 has been revealed. The excellent chemisorption property of C-900 may originate from the large amounts of carbon defect sites, which have been confirmed by synchrotron radiation based X-ray absorption spectroscopy. The rich defect sites and excellent chemisorption property can thus induce reversible carbon-oxygen interface for the excellent ORR activity.


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INTRODUCTION Electro-catalytic oxygen reduction reaction (ORR) is a key reaction for renewable/clean energy technologies.1,2 So far, much progress has been achieved in the development of highperformance noble metal-based electro-catalysts for oxygen reduction.3-5 However, these electrocatalysts still cannot meet the demands of large-scale commercialization due to their poor durability and high cost.6 In order to reduce the cost of catalysts, a lot of electro-catalysts with transition metals (Fe, Co etc.) have been prepared with good performance.7 However, the catalytic site with transition metals in these catalysts is usually easy to leak, which will result in serious decrease of the performance and contamination of the reaction system.8 As low-cost and metal-free electro-catalysts, carbon materials have notable merits including wide abundance, high electrical conductivity, good stability and environmental friendship.9,10 Consequently, catalysts based on carbon materials (doped with nitrogen,11,12 boron, phosphorus,13 fluorine,14 etc.) have attracted much attentions in recent years, particularly in improving their ORR activities relative to that of noble metal-based electro-catalysts (platinum or its alloys etc.).15,16 Despite the extensive research efforts in the past decade, the present carbon based catalysts still suffer from low ORR catalytic activity in acidic medium. There is almost no report on efficient proton exchange membrane fuel cell constructed by metal-free carbon electro-catalyst. ORR is an irreversible electrochemical reaction, and the suitable interaction between the O2 and catalyst surface will significantly improve the catalytic performance.17 Enhancing the carbon-oxygen interfacial interaction could be an effective way to improve the electro-catalytic activity of carbon materials to replace the noble metal-based electro-catalysts.7 As demonstrated, heteroatom (N, B, S, O, P, etc.) doping in carbon can alter the charge17 or spin distribution18 to facilitate the oxygen adsorption and then improve the catalytic activity. For example, based on


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DFT calculations, Dai’s group indicated that N doping in carbon could form side-on adsorption of O2 with weak O-O bonds and then lead to high ORR activity.17 However, there are only very limited reports on the direct chemisorption of oxygen on carbon materials without doping for ORR. Recently, defect sites (edge defects or intrinsic defects) in carbon were reported to improve the ORR activity but the performance was not good enough.19 Defect sites typically mean unsaturated carbon, which may favorite the interaction between the catalyst and O2. Therefore, fabricating abundant defect sites in carbon materials with suitable O2 adsorption might be a effective way to design highly efficient metal-free ORR electro-catalyst. In this work a O2-chemisorption dominated metal-free carbon material (C-900) with sufficient defect site for ORR was prepared by combining sacrificial template (ST) method and solid-state reaction (SSR) strategy. ZnO quantum dots (QDs) were used as sacrificial template and carbon expend agent to construct mesoporous structure. Self-assembly of KNO3 was embedded in MAF-4 nanocrystals (Metal Azolate Framework-4; or “ZIF-8”)20 as oxygen-supplier and carbon tailor agent to expand mesopore size and fabricate abundant carbon defect sites. The prepared C900 electro-catalyst exhibits outstanding oxygen adsorption performance by using oxygen temperature programmed desorption (O2-TPD). As a result, the ORR catalytic performance of C900 is close to that of commercial 20% Pt/C in acidic medium and better than that of 20% Pt/C in alkaline medium. Meanwhile, C-900 can also be used in air-breathing fuel cell as a cathode material and shows a high specific power density of 1160 W L-1 and a state-of-the-art peak power density up to 62% of the commercial 20% Pt/C.


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RESULT AND DISCUSSION The electro-catalysts were prepared by an in-situ synthesis strategy. ZnO QDs solution was prepared by a colloid chemical method. Then the precursor ZnO@MAF-4 was obtained by the method of “bottle around the ship”, in which zinc nitrate and 2-methylimidazole (2-MeIM) reacted in the ZnO QDs solution (Figure S1). It is worth noticing that the generated insoluble KNO3 (Figure S2) was also embedded in MIF-4 with the ZnO QDs. For comparison, the ZnO@MAF-4 precursor was also washed by using EtOH:H2O=1:1 to remove KNO3 molecules and labeled as ZnO@MAF4-W (“W” means wash). Finally, ZnO@MAF-4 and ZnO@MAF4-W were transferred to a tube furnace and annealed in different temperatures for 2 h under nitrogen gas flow, resulting in the formation of hierarchical porous carbons C-X and CX-W (X means the temperature), respectively. As shown in Fig.1B and Fig.1C, transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) images exhibit a well-dispersion of ZnO QDs in MAF-4. The particle size distribution of ZnO QDs is about 3-5nm (Figure S3 and S4). Moreover, the lattice fringe images are in good agreement with the (002) face of ZnO, further confirming the existence of ZnO quantum dot in MAF-4. The powder X-ray diffraction pattern (PXRD) and Raman spectrum (Figure 1F, Figure S5) indicate that C-900 is amorphous carbon with many defects. Scanning electron microscopy (SEM) and TEM images suggest that C-900 has a porous structure (Figure 1D, Figure S6 and S7). The chemical mapping of C-900 exhibits the homogeneous elemental distribution over the entire sample (Fig. S7E). X-ray photoelectron spectroscopy (XPS) survey scan of C-900 (Figure 1E) shows a high C-content of 98.06 at.% and ultra-low N-content of 0.71 at.%, in good agreement with the energy disperse X-ray absorption (EDS) and the X-ray absorption spectroscopy (XAS) at N K-edge (Figure S8 and S9, Tables S1-


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3). The contents and existing forms of all elements in C-900 are further confirmed by combustion elemental analysis, ICP elemental analysis, C1s and N1s spectra and Fouriertransform infrared spectroscopy (Figure S10-S11 and Tables S4-S5). N2 absorption isotherms at 77 K and pore diameter distribution datum are shown in Figure 1G. The curve for C-900 shows typical IV isotherm and pronounced hysteresis loop, indicating a typical hierarchical porous structure. The Brunauer-Emmett-Teller (BET) specific surface of C-900 is 2814 m2 g-1. The pore diameter distribution reveals that C-900 has three kinds of pore size around 1.2 nm, 3.0 nm and 25.1 nm, respectively. All these results prove that the C-900 catalyst is a metal-free carbon material with hierarchical porous structure. The electrochemical performance of C-900 for ORR is evaluated by cyclic voltammetry (CV). Compared with that in Ar-saturated condition, the voltammogram of C-900 shows a pronounced redox peak in O2-saturated condition centered at 0.90V versus RHE electrode (Figure 2A). That highlights prominent electro-catalytic performance of C-900 for oxygen reduction. The performance of 20% commercial Pt/C sample is used for comparison. As show in Figure 2B, the linear sweep voltammogram (LSV) polarization curves in alkaline medium reveal that the C-900 catalyst has an excellent ORR performance with an onset potential of 1.030 V and a half-wave potential of 0.916V, exhibiting a positive shift of 75mV at the current density of 3 mA cm-2 when compared to that of 20% Pt/C (0.841V). Such ORR performance represents one of the best half-wave potential values for metal-free ORR electro-catalysts and even superior to the single atom Fe based electro-catalyst (0.900V vs. RHE).21 The rotating ring-disk electrode (RRED) measurement also suggests that it a four electron ORR path-way with low H2O2 yield similar to the 20% commercial Pt/C samples (Figure 2D). The stability and MeOH durability of C-900 are tested under a constant potential of 0.5 V (vs. RHE) in O2-sturated 0.1 M KOH aqueous solution


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at an electrode rotation rate of 1600 rpm. As observed in Figure 2C, the C-900 catalyst shows almost no negative shift with the addition of methanol in the I-T test, exhibiting much better durability than that of 20% commercial Pt/C. Furthermore, the C-900 catalyst shows an excellent stability with only 1.03% decrease in current density after 10 hours I-T test, while 20% Pt/C decreases to 82.71% (Figure 2F). Meanwhile, C-900 also shows just 12 mV decay at the current density of 3 mA cm-2 (Figure S12A). Such high current retention of the C-900 catalyst is also much better than that of most catalysts reported in the literatures.22, 23 The Tafel slope (Figure 2E) of the C-900 sample is only 47 mV dec-1, lower than the commercial 20% Pt/C (72 mV dec1).

Such low slope value can be ascribed to the coverage of adsorbed oxygen, with a Temkin

adsorption of oxygen at adsorbed oxygen intermediates leading to a coverage-dependent activation barrier for electrochemical reactions. Therefore, instead of first electron transfer (118 mV dec-1) and O2 adsorption, the possible immigration of intermediates could be the ratedetermining step.24 As mentioned in the literatures, ORR electro-catalyst workable in acidic medium meet the basic requirement of PEMFC very well. Thus the ORR performance of C-900 is also investigated in acidic medium. The CV curves indicate a conspicuous reduction process for C900, with a pronounced redox peak at 0.770 V (vs RHE) in O2-saturated 0.1 M HClO4 (Figure 3A). Moreover, as shown in Figure 3A, the LSV polarization curve of C-900 displays a decent onset potential (0.85 V) and half-wave potential (0.755V), which are close to that of 20% commercial Pt/C with only 65 mV difference at the current density of 3 mA cm-2. Such performance is superior than the best metal-free electro-catalyst of 1100-CNS in acidic medium and even very close to single iron atom electro-catalyst.25,26 The RRDE polarization curves (Figure 3B) also suggest that it is a four-electron pathway dominating the ORR process. The low


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H2O2 yield of C-900 (Figure 3C) is also within the standard of United States Department of Energy (DOE). Similar to that in alkaline medium, the stability and MeOH durability of the C900 catalyst are investigated by long term I-T test (Figure 3D and 3F). As shown in Figure 3D, C-900 catalyst has an excellent methanol crossover tolerance. In contrast, the performance of commercial 20% decreases dramatically when adding 1M MeOH. Meanwhile, in the 10 hours IT test (Figure 3F) at 0.5 V potential and 1600 rpm, the C-900 catalyst maintains 86.6% of its initial current after 10 hours, while 20% Pt/C decays to 59.4% under the same test. In the 10000 potential cycles test (Figure S12B), C-900 exhibits a slight decrease in the limit current density (0.3 mA cm-2) and half-wave potential (18 mV). Similar to that in alkaline medium, the Tafel slop value also locates in oxygen adsorption area and shows immigration of intermediates suggesting a rate-determining step (Figure 3E). To further investigate the practicability of C-900, we also fabricated air-breathing proton exchange membrane fuel cells (PEMFCs) to explore the real performance of C-900. Notably, different with conventional H2/Air fuel cells, air-breathing H2/Air fuel cell without equipped any sub-systems for air supply and humidification, it represents a simple, light and relatively lowcost fuel cell (Figure 4A).26-29 Figure 4b shows the polarization curves of the air-breathing fuel cells with 20% Pt/C and C-900 as the cathode catalysts, respectively. Note that, the performance of C-900 reaches up to 1160 W L-1 (at 0.13A, 0.25V), suggesting the performance of C-900 is about 62% of that for 20% Pt/C (peak specific power density of 1860 W L-1 (at 0.15A, 0.34V)). The performance reaches a very high level even comparable to the best values reported for the noble metal-free catalysts in fuel cells.26-29 To measure the stability of C-900 in the fuel cell, we conducted a constant current density (50 mA cm-2) test for 2 hours. Fig. 4C shows that the voltage of fuel cell with 20% Pt/C as cathode declines from 0.47 to 0.3V after constant current


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discharge for 2 h, and the performance reduces by 36%. The behavior is similar with the result in our previous research.28 The drop of voltage is a common phenomenon named "reversible degradation", and usually can recover after a short time of rest.30-32 Meanwhile, the voltage of fuel cell with C-900 declines from 0.46 to 0.25V, and the performance reduces by 46%. Notably, its activity becomes more stable after about 0.6 h. It suggested that the stability of C-900 in fuel cell is comparable to that of 20% Pt/C. Thermogravimetric-mass spectra (TG-MS) reveals the formation process of C-900 catalyst. As shown in Figure S13, ZnO@MAF-4 lose its weight sharply about 400°C, which is the selfdecompose signal of KNO3 molecules. The mass spectra exhibit a heavy CO2 (NO, NO2) gas release and O2 (CO) gas consumption at the same temperature (400°C), which indicates KNO3 molecules intensify the pyrolytic process and consume plenty of precursor components. Meanwhile, the release of CO2 indicates the KNO3 molecules also have decarbonization effect, in good agreement with the high BET surface of C-900. To further identify the effect of KNO3 molecules, C900-W serves as a control sample. SEM and TEM images reveal that C-900 has abundant mesopores but the mesopores drop sharply in C900-W, demonstrating a pore expanding effect of KNO3 (Figure S14-S16). Besides, N2 absorption isotherms and pore diameter distribution datum (Figure S17) also indicate the synergistic effect of ZnO QDs and KNO3 in mesopore fabrication. Compared with C900-W (920 m2 g-1), C-900 (2814 m2 g-1) exhibits more mesopores and higher BET surface. Therefore, by comparing the N2 absorption isotherms of C900 and C900-W, it is clear to identify the effect of self-sacrificial template with ZnO QDs and KNO3 in forming the hierarchically porous structure. ZnO QDs contribute to the mesopore structure, while KNO3 enhances BET surface and expands the mesopore structure remarkably. Notably, Raman Spectrum of C-900 exhibits a high ID/IG ratio of 1.24. For comparison, the ID/IG


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ratio of C900-W is only 0.97 (Figure S18B) suggesting the function of KNO3 not only in expanding pore size but also tuning carbon micro-structure and fabricating the defect sites. The electro-catalytic mechanism of C-900 is also explored. It is widely considered that the ORR activity of carbon nanomaterials comes from the heteroatom-doping.33-35 However, there is no obvious correlation between the ORR performance and the N content in C-900. Compared with C900-W, C-900 shows a positive shift of approximately 124 mV at the current density of 3mA cm-2 (Figure S18A). However, the N content of C900-W is near twice as that in C-900 (Figure S18C). To eliminate the effect of BET surface in electro-catalytic performance, C-800 and C-1000 have served as control samples due to their similar BET surfaces. As shown in Figure S20-23, these samples show similar BET surfaces and mesoporous structure distribution but have different N contents (Figure S18E, Table. S6). Notably, the N-content (pyridinic-N and pyrrolic-N) of C-800 is about ten times of that in C-900 (Figure S23 and Table S7). However, C900 exhibits a higher ID/IG ratio than C-800. As a result, C-800 (Eonset of 1.00 V, E1/2 of 0.91 V, and limit current density of 5.94 mA cm-2) exhibits a slightly lower ORR performance than that of C-900 (Figure S18A). On the other hand, although C-1000 contains almost no N atoms, it still shows a decent ORR performance (Eonset of 0.85 V, E1/2 of 0.81 V, and limit current density of 4.72 mA cm-2). The active sites in C-900 might be related to the carbon defects when removing the N species. ZnO QDs and assembling KNO3 can tune the pyrolytic process and then fabricate different amounts of carbon defect sites. To deeply understand the active center and the eletro-catalytic mechanism, we also study the electro-catalytic process of C-900. According to the kinetics analysis of C-900 above (Tafel slope), C-900 shows a low oxygen adsorption barrier. Thus, we investigate the oxygen adsorption properties of C-900. As shown in Figure 5A and Figure 5B, selective absorption of O2


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over N2 is obviously found in C-900 (while C900-W shows no difference). Meanwhile, the O2 adsorption of C-900 increases linearly at 273K with the increased pressure and achieves 0.55 mmol g-1 at 1.0 bar. For comparison, the N2 adsorption is much lower at all pressures with an adsorption of 0.34 mmol g-1 at 1.0 bar. Moreover, the adsorption heat of O2 at the low absorbance region is also higher than that of N2 (Figure 5C). The increase of N2 adsorption heat at the high absorbance region could be attributed to the interaction energy between adsorbed molecules under high pressure.36 These results suggest C-900 has a better adsorption of O2 than that of N2 in the normal conditions. Typically, porous materials have an ability to selectively adsorb N2 over O2 because the polarizability and quadruple moment of N2 are larger than those of O2.37 However, the present selective adsorption of O2 over N2 might be realized through chemical interaction, in which a charge transfer might occur between the materials and O2 rather than the redox-rigid N2.38 The chemisorption of O2 on C-900 can be further confirmed by the O2TPD curve. As shown in Figure 5D, two pronounced peaks can be clearly observed in two completely different temperature areas in the O2-TPD curve. The former peak locates at 90°C, showing a typical O2-physisorption signal due to its low temperature desorption. However, the latter peak locates at 280 °C, revealing an apparent O2 desorption at this temperature and directly confirming the O2-chemisorption on C-900. The chemisorption of O2 on C-900 may lead to a favorable C-O interface for the ORR reaction. External bias is applied in ORR and the electrons can be easily transferred from carbon in C-900 to the chemically adsorbed oxygen to form water. The adsorption sites can thus be “active sites” of the catalyst to accelerate the ORR reaction. This mechanism is further confirmed by synchrotron radiation based XAS spectra. As shown in Figure 5E, the XAS spectrum (at C K-edge) for the pristine C-900 catalyst shows a typical carbon ring structure with prominent π* and σ* peaks at around 285.5 eV and 290.7 eV,


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respectively. The peak for oxidized groups at 288.7 eV is weak for the pristine sample. However, when the C-900 sample was immersed in the O2-saturated 0.1 M KOH solution, the peak at 288.7 eV increases a lot suggesting the formation of large amounts of oxidized groups such as 39,

COOH or C-OH. It confirms the existence of large amounts of “active sites” (ie. defect sites 40)

in C-900, which can lead to the chemisorption of O2 and form oxidized groups in an O2-

saturated solution. The large amounts of “active sites” are favorable for the high ORR performance. Interestingly, the XAS spectrum can recover when performing the LSV test. In the ORR process, electrons can be transferred to the “active sites” with the external bias and the oxidized groups can be thus reduced, which is in good agreement with the XAS results. More importantly, such carbon-oxygen interface indicates the carbon atom is the electro-catalytic center. Different from previous reports,41 this could be the first time to clearly identify the electro-catalytic center of metal free ORR catalysts. Besides, as shown in Figure S24, C-900 also shows an ultra-low electro-catalytic performance in both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Therefore, according to the DFT calculations in the literatures, many carbon defect models can be excluded, such as 585 model, 7557model and C7 model. Meanwhile, both of Zigzag model and 5+1 model show the highest possibility in C-900.39,

40

Based on the above results and discussion, an ORR mechanism of C-900 can thus be proposed and shown in Figure 5F. In which, the O2 molecules can anchor to the “active sites” by strong O2 chemisorption effect and form actived carbon-oxygen interface. Such activated carbon-oxygen interface induces an easier electron transfer and electro-chemical reduction process at a low overpotential. Following that, the adsorbed O2 molecules are reduced to OH-/H2O by a four electrons process.


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CONCLUSION In summary, a superior metal-free electro-catalyst C-900 with high specific surface area and abundant defect sites was obtained by ST and SSR strategy. Such defect-based carbon material shows a strong O2-chemisorption property to form reversible carbon-oxygen interface for high ORR performance. As a result, it exhibits an excellent ORR performance which is close to 20% Pt/C under pH=1 and superior to 20% Pt/C under pH=13. C-900 based air-breathing fuel cell also shows a high specific power density of 1160 W L-1, which is 62% of commercial 20% Pt/C. Our work may open a new way to design the highly effective metal-free electro-catalysts. METHODS All the reagents and solvents used for the synthesis were commercially available and used without further purification. All reagents were purchased from Sinopharm Chemical Reagent Co. Ltd (China). The Powder X-ray diffraction (PXRD) acquired using a Rigaku Ultima IV. Scanning electron microscope (SEM, ZEISS MERLIN Compact) and transmission electron microscope (TEM, Talos F200X) were used to observe the morphology. N2 sorption analysis was conducted using TriStar 3020 and Micrometritics ASAP 2020 at 77 K, using Barrett–Emmett–Teller (BET) calculations for the surface area. X-ray photoelectron spectroscopic (XPS) spectra were collected on a Thermo Scientific Escalab 250 Xi XPS spectrometer. Raman Scattering spectra were recorded with a laser excitation wavelength of 532 nm. The O2-TPD test was investigated by MicrometriticsAutoChem II 2920 chenisorption instrument. The electro-catalysis reactions were


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tested by a Model CHI 920-C workstation (CH Instruments, Chenhua, Shanghai, China) and RRDE-3A (ALS Co., Ltd). X-ray absorption spectroscopy (XAS) experiments were performed at Beijing Synchrotron Radiation Facility (BSRF, soft Xray beamline) and the National Synchrotron Radiation Laboratory (NSRL, XMCD beamline). Hydrocarbons were tested by a flame ionization detector (FID). A thermal conductivity detector (TCD) was used to detect hydrogen, oxygen and CO with nitrogen as the carrier gas. Liquid products were collected from the cathode chambers after electrolysis and quantified by NMR (Bruker AVANCEAV III 400) spectroscopy. Preparation of MAF-4 nanocrystal: Zn(NO3)2·6H2O (9.9 mmol) and 2-methylimidazole (2MeIM) (39.6 mmol) were dissolved in 100 mL of ethanol. Then the solution was magnetic stirred for 24 hours at room temperature. The product was collected by centrifuge (8000 rpm, 2 min) and washed three times by ethanol. Finally, the product was activated by acetone for two days and dried in vacuum at 50 ºC for 6h. Preparation of ZnO quantum dots-EtOH solution: Zn(CH3COO)2·2H2O (34.9 mmol) were dissolved in 700 ml of ethanol and then magnetic stirred about 30 min at 75 ºC until the zinc solution turn to clarifying, and then 30 ml of KOH ethanol solution (2 mol L-1) was added into the above solution under magnetic stirred at 75 ºC for 10 min. Preparation of ZnO@ MAF-4: Zn(NO3)2·6H2O (9.9 mmol) was dissolved in 200 mL of ethanol, meanwhile 2-methylimidazole (2-MeIM) (39.6 mmol) was dissolved in 100 ml of ethanol. Then 200 ml of Zn(NO3)2·6H2O ethanol solution was added into above ZnO QDs solution firstly with magnetic stirred at the room temperature, and then 100 ml of 2-MeIM ethanol solution was poured into the above solution as well. Finally, the mixture solution was


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magnetic stirred for 24h. The product was collected by centrifuge (8000 rpm, 2 min), then the product was dried in vacuum at 50 ºC for 6h. Preparation of ZnO@ MAF4-W: Zn(NO3)2·6H2O (9.9 mmol) was dissolved in 200 mL of ethanol, meanwhile 2-methylimidazole (2-MeIM) (39.6 mmol) was dissolved in 100 ml of ethanol. Then 200 ml of Zn(NO3)2·6H2O ethanol solution was added into above ZnO QDs solution firstly with magnetic stirred at the room temperature, and then 100 ml of 2-MeIM ethanol solution was poured into the above solution as well. Finally, the mixture solution was magnetic stirred for 24h. The product was collected by centrifuge (8000 rpm, 2 min), the product was washed by EtOH: Deionized Water=1:1solution 3 hour to remove KNO3 and then dried in vacuum at 50 ºC for 6h. Preparation of C-X (X=500, 600, 800, 900 and 1000) and C900-W catalyst: C-900 catalyst was prepared via direct carbonization of 6g ZnO@MAF-4 or ZnO@MAF4-W powders at 900 ºC (800 and 1000) for 2 h under N2 gas flow with heating rate of 5 ºC min-1. And the catalyst for XPS, electrochemical catalyst, N2 sorption and Raman spectrum testing were washed by 10% HF aqueous solution, 10% HCl aqueous solution, 100mL deionized water, 100mL absolute ethanol and 50mL acetone for 6 h, respectively. Finally, the products were dried in vacuum at 60 ºC for 12h. Electrochemical measurements of ORR: Electrochemical measurements were carried out in a three-electrode system at an Electrochemical station (CHI 920C). Electrochemical measurements were performed using a three-electrode configuration at 25 ºC. A modified glassy carbon electrode (GCE, d=3 mm) served as a working electrode. A Ag/AgCl (3.5 M KCl) and a graphite rod were used as a reference electrode and a counter electrode, respectively. Polarization data


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were collected at a scan rate of 10 mV s-1 in electrolytes. All the potentials were converted into the potential versus the reversible hydrogen electrode (RHE) according to ERHE = EAg/AgCl + EOAg/AgCl + 0.05916 pH. Electrochemical stability was measured using I-T test at 0.5V vs. RHE in alkaline and acidic medium. All the electrochemistry measurements are represented with iR compensation. 5 mg C-900 was dispersed in 1 ml of ethanol-distilled water (1:1) and 50 µl of 5% Nafion aqueous solution by sonication. Then 4.24 µl of the catalyst ink was dropped onto the glassy carbon disk electrode (GCE, 3.0 mm diameter CH Instruments) and dried under ambient condition. The final catalyst loading on working electrode are 285 ug cm-2. Before test, an N2/O2 flow was used through the electrolyte in the cell about 30 min to saturate it with N2/O2. N2/O2saturated 0.1 M KOH aqueous solution (vs. RHE) and 0.1 M HClO4 aqueous solution (vs. RHE) were used as alkaline and acid medium, respectively, where a carbon rod was used as the counter electrode. The number of electrons transferred (n) were derived from the following equation 1 and 2:

n

4I d I d  I r / N (1)

H 2O2 (%)  200 

Ir / N I d  I r / N (2)

where Id: disk current; Ir: ring current; N: ring collection efficiency (38.6%). MEA Fabrication and PEMFCs Measurements: The MEA with an active area of 1 cm2 was prepared and used in this study as follows. The catalyst layer was spread on the composite electrode by screen printing. The catalyst ink was prepared by dropping Nafion solution (5%)


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into mixed solution of deionized water and isopropanol (1:1 V/V), and with 20% Pt/C or C-900 catalyst in an ultrasonic bath for 15 min. The content of Nafion is 20% in the Pt/C ink. While we set a series of values Nafion in the C-900 ink to discover the optimal content of Nafion. The Pt loading was 0.5 mg cm-2 for both cathode and anode. And the loading of C-900 on cathode is 0.3 mg cm-2. After coating the catalyst to the electrode, the electrode is placed for 2 hours under 105 ºC. The thickness of carbon electrode is 190 μm and 20% wet proof which purchased form Toray. Then, Nafion 115 membrane were hot pressed with two electrodes at a pressure of 40 kgf cm-2 and temperature of 130 °C for 2 min. The electrochemical performance of as-perapared PEMFCs was then measured by an electrochemical workstation. The experiments were performed with pure hydrogen at 20 ºC and atmospheric pressure. The pure hydrogen is prepared from a hydrogen generator. The flow rate of hydrogen is 30 mL min-1. The polarization curve of fuel cell is measured by linear scanning at the rate of 2 mV/s. O2-TPD Measurement: First, 30 mL/min high purity He as the carrier gas, 100mg catalyst was pretreated for 60 min with He carrier gas under 250 ºC, and then cooled down to room temperature. Second, 10% O2/He was supplied for 60 min until the catalyst achieved saturated state. Then, above sample was purged by carrier gas until the baseline stable. Finally, the treatment temperature was slowly increased to 400 ºC with the heating rate of 5 ºC min-1, and the Chemisorption signal was detected by thermal conductivity cell detector. (Attention: before O2TPD test, the background line was measured under pure He gas until without any obvious peaks.) The fabrication of XAS samples (ORR) Pristine Sample: C-900 sample without any extra treatment.


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O2-saturated: At first, 10mg C-900 was dispersed in 4 ml of ethanol-distilled water (1:1) aqueous solution by sonication. Then above 4 ml catalyst ink was dropped onto the carbon fiber paper (0.7 cm × 0.7 cm, double side) and dried under ambient condition. Then, the above carbon fiber paper was immersed into O2-saturated 0.1M KOH solution and washed by deionized water carefully. Finally, the electro-catalyst sample was collected by centrifuge (8000 rpm, 1 min) and dried at 25 ºC for 24h. ORR Middle (LSV): The carbon fiber paper electrode served as working electrode. Linear sweep voltammetry experiments were performed using a standard three-electrode configuration. Notably, the LSV test was stopped at the half-wave potential (0.91V vs. RHE) and then the working electrode was taken out. The working electrode was washed by deionized water carefully. Finally, the electro-catalyst sample was collected by centrifuge (8000 rpm, 1 min) and dried at 25 ºC for 24h. ORR Finished (LSV): Linear sweep voltammetry experiments were performed using a standard three-electrode configuration. The LSV test was stopped at the finished potential (0.2 V vs. RHE) and then the working electrode was taken out. The working electrode was washed by deionized water carefully. Finally, the electro-catalyst sample was collected by centrifuge (8000 rpm, 1 min) and dried at 25 ºC for 24h. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge.


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The following files are available free of charge.Details of XRD, SEM, TEM, XPS, Raman, TG-MS, O2-TPD results of the as-synthesized samples. AUTHOR INFORMATION Corresponding

Authors:

*[email protected];

*[email protected];

*[email protected]; *[email protected]; *[email protected].

Author Contributions The manuscript was written through the contributions from all the authors. All the authors have approved the submission of the final version of this manuscript. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (51725204, 21471112, 21571139, 51422207, 51132006, 51572179, 21471106, 21501126, 11404359, 21373264, 21573275 and 11227902), Collaborative Innovation Center of Suzhou Nano Science and Technology, Ministry of Science and Technology of China (No. 2016YFA0200700, No. 2016YFE0105700), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Special Program of Talents Development for Excellent Youth Scholars in Tianjin (TJTZJH-QNBJRC-2-3). REFERENCES (1) Goodenough, J. B. Evolution of Strategies for Modern Rechargeable Batteries. Acc. Chem. Res. 2009, 46, 1053-1061. (2) Service, R. F. Clean Revolution. Science2015,350, 1020-1023.


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Figures:

Figure 1. Physical and chemical property of the C-900. (A), Synthetic process of C-900. (B), TEM image of ZnO@MAF-4. (C), HRTEM image of a representative ZnO QD encapsulated by MAF-4. (D), TEM of C-900. (E), XPS survey scan of C-900. (F), Raman spectrum of C-900. (G), N2 sorption isotherms of hierarchical porous carbon materials of C-900 at 77 K. The inset shows the pore size distribution of C-900 by DFT (for microporous structure) and BJH (for mesoporous structure) model.


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Figure 2. ORR performance of the C-900 in alkaline medium. (A), CV of C-900 in O2saturated (red) and N2-saturated (blank) 0.1 M KOH solution. (B), LSV curves of C-900, 20% Pt/C in O2-saturated 0.1 M KOH at 1600 rpm (catalyst loading of 285 μg cm-2). (C), I-T curves of C-900 and 20%Pt/C by adding 1 M MeOH at 2500 seconds. (D), Electron transfer numbers and H2O2 yields of C-900 and 20%Pt/C, respectively. (E), Tafel slops of C-900 and 20% Pt/C. (F), The 10 hours I-T curves of C-900 and 20% Pt/C.


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Figure 3. ORR performance of the C-900 in acidic medium.(A), LSV curves of C-900 and 20% Pt/C at the 1600 rpm in 0.1 M HClO4 medium. The inset figure shows the CV curves of C-900 in O2-saturated (red) and N2-saturated (blank) 0.1 M HClO4 solutions. (B), Electron transfer numbers of C-900 and 20%Pt/C, respectively. (C), H2O2 yields of C-900 and 20%Pt/C, respectively. (D), I-T curves of C-900 and 20%Pt/C by adding 1 M MeOH at 2500 seconds. (E), Tafel slope curves of C-900 and 20%Pt/C in 0.1 M HClO4 medium. (F), The 10 hours I-T curves of C-900 and 20%Pt/C in O2-saturated 0.1 M HClO4 medium.


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Figure 4.Performance of the catalysts in air-breathing fuel cells. (A), The structure and photograph of the flexible air-breathing PEMFC used in this work. Air naturally diffuses into the PEMFC. The experiments were performed with pure hydrogen at 20 °C and atmospheric pressure. Pt loading in anode side is 0.5 mg cm-2. (B), The polarization curves of the fuel cells with 20% Pt/C and C-900 as cathode catalysts. (C), Constant current (50 mA cm-2) discharges of the fuel cells with 20% Pt/C and C-900 as cathode catalysts.


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Figure 5. Mechanism of ORR activity. (A), O2/N2 sorption isotherms of C900-W at 273K. (B), O2/N2 sorption isotherms of C-900 at 273K. (C), Heats of O2/N2 adsorption (ΔH) calculated using isotherms at 273 K. (D), O2-TPD curve of C-900 (The background signal was deducted). (E), C K-edge XAS curves of C-900 with different treatment modes. (F) Schematic diagram of the ORR process.


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Carbon Defect-Induced Reversible CarbonâOxygen Interfaces for

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