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May 10, 2017 - synthesize evenly dispersed fine Co nanoparticles (CoNPs). (particle size of ..... that of Co@NPCM/CNF-850), demonstrating again the su...
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Fine Co Nanoparticles Encapsulated in a N-Doped Porous Carbon Matrix with Superficial N-Doped Porous Carbon Nanofibers for Efficient Oxygen Reduction Xiao Ma,†,‡ Xue Zhao,†,‡ Jianshe Huang,† Litai Sun,†,‡ Qun Li,†,‡ and Xiurong Yang*,† †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China ‡ University of the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Herein, we develop a novel method to synthesize evenly dispersed fine Co nanoparticles (CoNPs) (particle size of ∼42 nm) encapsulated in a N-doped porous carbon matrix (NPCM) with superficial N-doped porous carbon nanofibers (NPCNF) (denoted as Co@NPCM/CNF850) as an oxygen reduction reaction (ORR) electrocatalyst. Such an electrocatalyst is the direct pyrolysis product of the novel pine needle-like ZIF-67-based metal-organic framework nanowire array (MOFNWA) prepared using an inorganic cobalt carbonate hydroxide (Co(CO3)0.5(OH)·0.11H2O) nanowire array as a linear sacrificial template, which is totally different from the traditional method, that is, using inorganic salts to synthesize MOF particles. Because of the high dispersibility of the effective fine N-doped carbon-wrapped CoNPs (rather than the overlarge CoNP aggregates); the unique linear MOF-derived assemblies, which are beneficial to electronic transmission; the high degree of graphitization, which is attributed to the superficial NPCNF and carbon layers wrapping the CoNPs; as well as the high porosity, our catalyst showed remarkable ORR activity (Eonset of 1.033 V vs the reversible hydrogen electrode) in alkaline solution. Besides, our catalyst revealed excellent stability and tolerance of methanol. Furthermore, on the basis of the X-ray absorption near-edge structure, extended X-ray absorption fine structure, and linear sweep voltammetry data, we first provided proof that a catalyst devoid of obvious Co−Nx can have superior ORR activity. KEYWORDS: cobalt carbonate hydroxide, linear sacrificial template, metal-organic framework, nonprecious metal catalyst, oxygen reduction reaction

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INTRODUCTION Oxygen reduction reactions (ORRs) are paramount electrochemical reactions of renewable-energy technologies, such as fuel cells and metal−air batteries. Until now, platinum (Pt)based materials have been widely identified as the most competent catalysts for ORR. However, their application is restricted by prohibitive costs and scarcities.1 In 1964, Jasinski’s group reported that cobalt phthalocyanine could be used as a catalyst for ORRs in alkaline electrolytes. 2 Thereafter, considerable efforts have been devoted to developing catalysts based on transition metal (Fe and Co, etc.)−nitrogencoordinated complexes (M−N4), which were later developed into M−N−C materials, which can be prepared with separate precursors of metal, nitrogen, and carbon. Nevertheless, insufficient doping and structural inhomogeneity make these catalysts still inadequate for commercial applications.3 Over the past 2 decades, because of customizable coordination sites with different metal centers and hetero atoms, long-range order, as well as ultrahigh porosity,4 metalorganic frameworks (MOFs) have been increasingly selected as precursors to prepare M−N−C materials or other nanoarchitectures.5−8 Generally, the pyrolysis products of the selected MOFs are composed of transition metal and/or © XXXX American Chemical Society

metal oxide (M and/or MxOy) nanoparticles (NPs) encapsulated in a N-doped porous carbon matrix (NPCM).9−11 This microstructure of N-doped carbon-wrapped M and/or MxOy NPs has proven to be outstandingly contributive to ORRs according to both electrochemical tests and theoretical calculations.12,13 However, most preparation methods involve pyrolysis of only MOF crystals or of MOF crystals with carbon, such as graphene,14 carbon nanotubes (CNTs),15 or a carbon matrix,16 which can increase the conductivity of the catalyst system. Nevertheless, overlarge and heterogeneous metal aggregates are inevitably formed during the pyrolysis process, which restrict the electrochemical exertion of the microstructures. Besides, although the conductivity can be improved in this manner, the ORR activity is largely limited by the exotic carbon with inert efficiency (without doped hetero atoms or wrapping M and/or MxOy NPs). Therefore, it is quite necessary to develop MOF-derived catalysts whose structures are uniformly dispersed N-doped carbon-wrapped M and/or MxOy NPs and that simultaneously have a high conductivity Received: February 20, 2017 Accepted: May 10, 2017 Published: May 10, 2017 A

DOI: 10.1021/acsami.7b02490 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Illustration of the synthesis of Co@NPCM/CNF-850; SEM images of (b) CoCHNWA, (c) MOFNWA (inset: corresponding enlarged SEM image), (d) Co@NPCM/CNF-850, and (e) enlarged SEM image of Co@NPCM/CNF-850.

Figure 2. TEM images of (a) the nanowires separated from CoCHNWA by ultrasonication, (b) MOFNWA, and (c) Co@NPCM/CNF-850; (d) particle size distribution of Co@NPCM/CNF-850; (e) STEM image of Co@NPCM/CNF-850 (white) and corresponding elemental mapping images of Co, N, and C; (f) HRTEM image, (g) SAED pattern, and (h) X-ray diffraction (XRD) pattern of Co@NPCM/CNF-850.

evenly dispersed fine CoNPs (particle size of ∼42 nm) encapsulated in a NPCM with superficial N-doped porous carbon nanofibers (NPCNF) (denoted as Co@NPCM/CNF850, where 850 represents the pyrolysis temperature) were first obtained, which can be utilized as an ORR electrocatalyst. Different from the traditional method for preparing ZIF-67 crystals (using Co2+), this ZIF-67-based MOFNWA was prepared using an inorganic cobalt carbonate hydroxide (Co(CO3)0.5(OH)·0.11H2O) nanowire array (CoCHNWA) as the source of Co(II) and a linear sacrificial template,21 which is water-insoluble and can provide uniform reaction sites to produce uniformly dispersed CoNPs in the derived pyrolysis product. Because of the high dispersibility of fine N-doped carbon-wrapped CoNPs, excellent conductivity resulting from the superficial NPCNF and N-doped carbon layers wrapping the CoNPs, and the linear MOF-derived assemblies and high porosity, our catalyst showed outstanding ORR activity (Eonset of 1.033 V) as well as excellent stability and tolerance to methanol in alkaline solution.

devoid of any exotic inert carbon. Recently, Duan’s group utilized a CoAl-layered double hydroxide template to grow MOF crystals for improving the dispersibility of Co nanoparticles (Co NPs) in the carbon shell,17 and Sun et al. substituted a portion of Co2+ with Zn2+ to inhibit the formation of overlarge CoNPs.18 Fischer’s group fabricated a certain amount of CNTs grafted on a NPCM with wrapped Co@ Co3O4 NPs by the H2 reduction method to improve the conductivity without exotic inert carbon.19 Generally, nanostructures composed of nanowires and nanotubes outperform NPs in electrochemical reactions due to their satisfying dispersibility, easier charge transfer, and high aspect ratio.20 However, the granular-like shape has been the dominant morphology of MOFs over the past two decades.4 Actually, the lack of diversity in morphology greatly restricts their application. In this work, we developed a novel strategy to fabricate a pine needle-like MOF (ZIF-67, a commonly used Co-based MOF) nanowire array (MOFNWA). After pyrolysis at 850 °C, B

DOI: 10.1021/acsami.7b02490 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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that of simulated ZIF-67,18 demonstrating that CoCHNWA and ZIF-67-based MOFNWAs were successfully synthesized, and it is feasible to replace Co2+ using Co(CO3)0.5(OH)· 0.11H2O to prepare ZIF-67-based MOFNWAs. The XRD pattern of our catalyst (Figure 2h) confirms the presence of fcc cubic Co, and the sharp peak at ∼26° reveals the high degree of graphitization. As shown in Figure 3, the N2 adsorption/

RESULTS AND DISCUSSION The synthesis process of Co@NPCM/CNF-850 is specifically illustrated in the Experimental Section. In brief, Co(NO3)2 and CO(NH2)2 were heated through hydrothermal reaction to obtain CoCHNWA (Figure 1a). The scanning electron microscopy (SEM) image shows that CoCHNWA (Figure 1b) is composed of nanowires with the stuck ends, just like sea urchins. The transmission electron microscopy (TEM) image of the individual nanowire separated from the CoCHNWA by ultrasonication reveals a rodlike crystalline structure with a diameter of ∼40 nm (Figure 2a). The CoCHNWA was gradually transformed into a MOFNWA with the addition of 2methylimidazole (2-MeIM) and triethylamine (TEA). The speculated reaction steps are shown in Table S1 of the Supporting Information (SI). SEM images of the MOFNWA (Figures 1c and S1a) display a pine needle structure as the overall shape, and each “pine needle” is a crystalline cuboid with a length ranging from ∼5 to 7 μm. Actually, the corresponding TEM image (Figure 2b) reveals that these “pine needles” are composed of anchored particle chains as building blocks. It is worth mentioning that the morphology of the formed ZIF-67-based MOFNWA is completely different from the traditional granular shape of ZIF-67, and the anchored particle formed in this manner is much smaller than the traditional ZIF-67 (Figure S2a), which can positively contribute to enhancing the derivative’s ORR activity.22 Upon pyrolysis, the overall “pine needle” outline could still be observed, yet the crystal face had obviously contracted and the smooth surface had become rough (Figures 1d and S1b). In addition, a layer of fluffy long or short carbon nanofibers had formed on the surface of the NPCM, as shown in the enlarged SEM image (Figure 1e). The corresponding TEM image (Figure 2c) shows that evenly dispersed fine CoNPs with particle size mainly centered at ∼42 nm (Figure 2d) are encapsulated in the NPCM. In contrast, the pyrolyzed product of traditional granular ZIF-67 at 850 °C (denoted as carbonized ZIF-67) (Figure S2b) is composed of overlarge and heterogeneous CoNPs in the carbon matrix without the superficial nanofiber layer. We believe that these uniformly dispersed CoNPs result from the much smaller space occupied by every MOF particle19 and the internal uniform reaction sites provided by the CoCHNWA. Moreover, according to the previous literature,23 the size of the transition metal (Fe, Co) particles plays a decisive role in the growth process of tubular carbon, and we suppose that it is the uniformly dispersed CoNPs of such a diameter that force the formation of surface carbon nanofibers. The high uniform dispersity of CoNPs is also identified from the scanning TEM (STEM) image, and elemental mapping images indicate that Co@NPCM/CNF-850 is homogeneously doped with N (Figure 2e). A high-resolution TEM image (Figure 2f) combined with a selected area electron diffraction (SAED) pattern (Figure 2g) shows an encapsulated CoNP lattice fringe of 0.174 nm and the lattice fringe of graphite-like carbon layers to be 0.338 nm, corresponding to the (200) plane of face-centered (fcc) cubic Co (PDF 15-0806) and (002) plane of graphitic carbon (PDF 41-1487), respectively.15 The marked lattice fringes were enlarged in Figure S3. Furthermore, the annealing steps were also performed at 800 and 900 °C, and the resultant products are denoted as Co@NPCM/CNF-X (X represents the pyrolysis temperature). The XRD patterns in Figure S4 match well with that of Co(CO3)0.5(OH)·0.11H2O (PDF 48-0083)24 as well as with

Figure 3. (a) Nitrogen adsorption/desorption isotherms and (b) PSDs of (1) Co@NPCM/CNF-800, (2) Co@NPCM/CNF-850, and (3) Co@NPCM/CNF-900.

desorption isotherms of different Co@NPCM/CNF samples show type-IV isotherms, with a pronounced hysteresis loop, revealing the formation of a mesoporous structure,25 and the pore size distributions (PSDs) are mainly centered at ∼3.8 nm. The specific surface areas (SSAs) of the Co@NPCM/CNF samples obtained at 800, 850, and 900 °C are 736.2, 466.2, and 443.2 m2 g−1, respectively, which are significantly higher than those of carbonized ZIF-67 samples (denoted as ZIF-67-X, where X represents the pyrolysis temperature) obtained at the corresponding temperatures (Figure S5), which probably benefit from the unique configuration of the MOF precursor. A large SSA has been the goal pursued by electrochemists for a long time because it can enhance the density of the active sites and accessibility to the electrolyte, resulting in increased ORR activity.26 Raman spectra (Figure 4) show that the IG/ID values of our samples increase with an increase in the pyrolysis

Figure 4. Raman spectra of (1) Co@NPCM/CNF-800, (2) Co@ NPCM/CNF-850, and (3) Co@NPCM/CNF-900. C

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Figure 5. (a) Rotating ring-disk electrode (RRDE) voltammograms of different catalysts in O2-saturated KOH (0.1 M) at 1600 rpm; (b) electron transfer numbers and percentages of peroxide on the basis of RRDE data; LSV curves of Co@NPCM/CNF-850 in O2-saturated KOH (0.1 M) (c) before and after 3000 cycles and (d) without and with MeOH (1 M); LSV curves of Pt/C in O2-saturated KOH (0.1 M) (e) before and after 3000 cycles and (f) with and without the addition of MeOH (1 M).

(E1/2) of Co@NPCM/CNF-850 is slightly negative compared to that of 20 wt % Pt/C, its onset potential (Eonset) is almost the same as that of 20 wt % Pt/C (1.033 V vs the reversible hydrogen electrode, RHE), which is more positive than those of the most recently reported excellent Co−N−C catalysts (Table S2). By contrast, the Eonset of carbonized ZIF-67 is only ∼0.916 V, revealing a negative shift of ∼117 mV compared to that of Co@NPCM/CNF-850. The current density of Co@NPCM/ CNF-850 is also almost the same as that of Pt/C (5.7 mA cm−2 at 0.17 V), distinctly higher than that of carbonized ZIF-67 (5.05 mA cm−2). This indicates that the linear MOF-derived assemblies and high dispersion of N-doped carbon-wrapped

temperature, indicating enhancement of the graphitization degree. In addition, a two-dimensional peak that represents a high graphitization degree (usually appears in monolayer graphene27) emerges, which is attributed to the superficial NPCNF and carbon layers wrapping highly dispersed CoNPs, and its strength increases with an increase in the pyrolysis temperature. Such a high degree of graphitization can enhance the conductivity of the catalyst so as to improve its ORR activity.26,28 The ORR activity of Co@NPCM/CNF-850 was investigated by linear sweep voltammetry (LSV) in O2-saturated KOH (0.1 M). As shown in Figure 5a, although the half-wave potential D

DOI: 10.1021/acsami.7b02490 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. (a) Rotating disk electrode voltammograms of different catalysts in O2-saturated KOH (0.1 M) at 1600 rpm; (b) rotating ring electrode voltammograms of different catalysts in O2-saturated KOH (0.1 M) at 1600 rpm; (c) electron transfer numbers and percentage of peroxide on the basis of RRDE data.

CoNPs do play significant roles in enhancing the ORR performance. We have also tested different mass loadings of the catalyst on the electrode surface. As shown in Figure S6, the Eonset of the catalyst with the lower loading amount shows no obvious change; yet, the current density declines to some extent, confirming that current density is affected by the catalyst loading amount. As shown in Figure 5b, the calculated transfer electron numbers of Co@NPCM/CNF-850 between 0.17 and 0.77 V are ∼3.8, demonstrating a 4e− ORR process, and the measured HO2− yields are 5.9−9.4%, which remain in a low range. Additionally, after 3000 potential cycles in O2-saturated KOH (0.1 M) solution, the E1/2 of Co@NPCM/CNF-850 exhibited a negative shift of ∼5 mV (Figure 5c). Furthermore, after the addition of MeOH (1 M), no evident variation in the LSV curve was observed for Co@NPCM/CNF-850 (Figure 5d), demonstrating that our catalyst has superior stability and tolerance to methanol compared to those of Pt/C, which showed a negative shift of ∼25 mV after 3000 potential cycles (Figure 5e) and a distinct interfering peak after the addition of MeOH (1 M) (Figure 5f). The ORR activities of Co@NPCM/CNF-800 and Co@ NPCM/CNF-900 were also investigated by LSV. As shown in Figure 6, the Eonset of both Co@NPCM/CNF-800 and Co@ NPCM/CNF-900 are a little more negative than that of Co@ NPCM/CNF-850. To obtain deeper insights into the ORR active ingredients in the bulk phase of the Co@NPCM/CNF samples, hard X-ray absorption near-edge structure (XANES) spectra (Figure 7) were recorded,29 which can sensitively detect M−Nx. In the Co K-edge XANES spectra, the pre-edge peak at 7709 eV is assigned to the 1s → 3d transition of Co2+, such as cobalt phthalocyanine (CoPc), and the white line around 7727 eV is assigned to the 1s → 4p transition of Co0, such as Co foil.30,31 Figure 7a shows that the Co K-edges of Co@NPCM/ CNF-800 and Co@NPCM/CNF-850 almost coincide with the Co foil, indicating that the existing form of the cobalt species is cobalt metal. The calculated Fourier transforms of k3-weighted Co K-edge X-ray fine structure (EXAFS) spectra (Figure 7b,c and Table S3) show that the local environment of Co in Co@ NPCM/CNF-800 and Co@NPCM/CNF-850 can be well described by a Co−Co shell (good fit on the left, ∼2.2 Å, and R-factors in Table S3), which is consistent with the XANES results. The results above show that there is no obvious Co−Nx structure in Co@NPCM/CNF-800 and Co@NPCM/CNF850 (even if there really are very few Co−Nx, we believe that the small content could be ignored). For Co@NPCM/CNF900, the calculations of the EXAFS spectra reveal that the local

Figure 7. (a) Co K-edge XANES spectra of different samples; (b) observed and (c) calculated Fourier transforms of the k3-weighted Co K-edge EXAFS spectra of different samples.

environment of Co in Co@NPCM/CNF-900 can be better described by a three-shell model (Co−Co, Co−N/O, and Co− N/O−Co) (R-factor in Table S3), showing that Co−N/O possibly exists in Co@NPCM/CNF-900. Because the N and O atoms are not distinguishable by XANES spectra, the Raman spectra of our catalysts were recorded to further determine the attribution of the Co−N/O structure. As shown in Figure 4, for Co@NPCM/CNF-800 and Co@NPCM/CNF-850, there are no obvious peaks assigned to cobalt oxides. However, for Co@ NPCM/CNF-900, weak peaks located at around 200, 478, 523, and 604 cm−1 as well as an obvious peak located at around 682 cm−1 can be observed, which corresponded to the F2g, Eg, F2g, F2g, and A1g modes of the Co3O4 phase, respectively,32,33 indicating that a small content of Co was oxidized to Co3O4, which is consistent with the results of XANES and EXAFS. It is possible that a few CoNPs were covered by the defects/voids in the graphite-like layer, exposed to air, and further oxidized to Co3O4.34 Therefore, Co−N/O that exists in Co@NPCM/ CNF-900 could be caused by Co3O4 rather than by Co−Nx. Despite three decades of studies on M−N−C (predominantly Fe or Co) electrocatalysts, the ORR active sites of M−N−C catalysts remain controversial due to the broad range of materials and complexity of the structures. According to previous reports, it is mostly believed that M−Nx (M = Fe or E

DOI: 10.1021/acsami.7b02490 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Co) plays a key role in improving the ORR activity.35,36 However, from the ORR activities of Co@NPCM/CNF samples and the calculation results of the EXAFS spectra, we could at least determine that Co@NPCM/CNF-800 and Co@ NPCM/CNF-850, which have better ORR performances, are devoid of obvious Co−Nx moieties (as for Co@NPCM/CNF900, the presence of Co−Nx is not very certain). In fact, there is an increasing consensus that “metal-free” active sites, namely, the structures of N-doped carbon, exist in non-noble metal catalysts.37−39 However, even when leached by acid (e.g., HCl, HF, etc.), or coupled with calcination, the metal components cannot be completely removed.40 Therefore, to further determine the active site of our catalyst, Zn(MeIM)2 (ZIF-8) is utilized to prepare the contrast N-doped carbon ORR catalyst without any Co species under the same condition. Here, ZIF-8 is an isologue of ZIF-67, except for a different central ion,41 and zinc ions are easy to remove and inactive to ORR. As shown in Figure S7, the Eonset of carbonized ZIF-8 shows a negative shift of ∼100 mV compared to that of Co@NPCM/CNF-850. This reveals that without any cobalt the electrocatalytic performance of ORR obviously declined. Therefore, we can conclude that although cobalt does not bond with nitrogen to form Co−Nx sites it plays a certain role in enhancing the ORR activity. Recently, Mukerjee’s group reported an excellent N-doped porous electrocatalyst with active sites devoid of any direct M− Nx.9 They proposed that the transition metal NPs could stabilize the peroxide so as to contribute to the full 4e− reduction of O2 to H2O. Similarly, Bao’s group synthesized an effective electrocatalyst of transition metal NPs encapsulated in CNTs, which also has no obvious M−Nx.12 They proposed that the electron could transfer from transition metal NPs to the carbon layer, resulting in a decreased local work function on the carbon surface. Besides, N-doping could increase the density of states near the Fermi level, which could also decrease the local work function. The lower local work function could activate O2 so as to enhance the ORR activity. Hence, according the above results of XNAES, EXAFS, and ORR tests, coupled with the previous literatures,9,12,13,42 we owe our catalyst’s major ORR activity to the microstructure of N-doped carbon, which wraps CoNPs finally. The structure of N-doped carbon could improve the absorption of O2,37−39 and CoNPs could probably have a synergistic effect, which stabilizes the peroxide and capacitates the full 4e− reduction of O2 to H2O, both of which could decrease the local work function so as to enhance the ORR activity. So far, macroscopic supports were utilized increasingly by electrochemists to disperse the hydrogen evolution reaction and oxygen evolution reaction catalysts because it can greatly increase the contact area of the catalyst and electrolyte and allow the catalyst to exhibit a better performance.43 However, to the best of our knowledge, there are no related reports on the ORR test using macroscopic supports. Here, we first dispersed Co@NPCM/CNF-850 on a carbon cloth (CC) (denoted as Co@NPCM/CNF-850-CC) to directly conduct the LSV test. Beyond our expectation, Co@NPCM/CNF-850CC exhibited a slightly more positive Eonset (1.05 V, Figure S8a) than that of Co@NPCM/CNF-850 and a very high current density of ∼27.47 mA cm−2 (at 1.7 V, 4.82 times higher than that of Co@NPCM/CNF-850), demonstrating again the superior ORR activity of our catalyst and also revealing that using a macroscopic support to test ORR is feasible and CC is a satisfying selection (see Figures S8b−e and S9 for more details).

CONCLUSIONS We developed a novel method to produce evenly dispersed fine CoNPs (particle size of ∼42 nm) encapsulated in a NPCM with superficial NPCNF as an ORR electrocatalyst by directly pyrolyzing a ZIF-67-based MOFNWA. Different from the traditional strategy (using Co2+ to synthesize MOF crystals), this ZIF-67-based MOFNWA was fabricated using inorganic water-insoluble Co(CO3)0.5(OH)·0.11H2O as the linear sacrificial template. Owing to the evenly dispersed fine microstructures of N-doped carbon-wrapped CoNPs; the unique linear MOF-derived assemblies; the high degree of graphitization, which is attributed to the superficial NPCNF and the carbon layers that wrap the CoNPs; as well as the high porosity, our catalyst shows prominent ORR activity (Eonset of 1.033 V) and excellent stability, as well as tolerance to methanol in alkaline solution. Additionally, XANES, EXAFS, and LSV data show that the catalyst that lacks obvious Co−Nx can have the outstanding ORR activity, and we believe that ORR probably occurs on the microstructures of N-doped carbon, which wraps CoNPs. The strategy of using inorganic carbonate hydroxide salts as the sacrificial template can be extended to prepare MOFs of other morphologies and structures for fuel cells, supercapacitors, and batteries. Furthermore, utilization of CC to disperse catalyst ink opens up the possibility for developing other 3D macroscopic support to conduct the ORR test.

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MATERIALS

Pt/C (20 wt % Pt on carbon black) was purchased from Alfa Aesar. Co(NO3)2·6H2O, Zn(NO3)2·6H2O, 2-MeIM, and CO(NH2)2 were purchased from Aladdin Ltd. KOH, HCl, methanol, ethanol, acetone, and TEA were purchased from Beijing Chemical Reagent Company. CC was purchased from CeTech (Taiwan). Double-distilled (DI) water was obtained from the Milli-Q ultra-purify water system (18.25 MΩ).

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MATERIAL CHARACTERIZATION SEM was performed on JEOL JSM-6700F and Philips XL 30 microscopes. TEM was performed on JEM-2000 FX (JEOL Ltd., Japan), at an accelerating voltage of 200 kV. XRD analysis was conducted on a D/Max 2500 V/PC X-ray diffractometer (Cu Kα radiation; Bruker). Nitrogen adsorption/desorption isotherms were obtained on a Quadra chrome adsorption instrument at 77 K. Before testing, the samples were deaired at 110 °C for 24 h. The SSAs were computed using the Brunauer−Emmett−Teller mode according to the adsorption data in the relative pressure (P/P0) range, which is from 0.06 to 0.14. The total pore volumes were identified from the nitrogen adsorbance at a P/P0 of 0.99. The PSDs were confirmed by a nonlocal density functional theory approach utilizing N2 adsorption data. XANES and EXAFS spectroscopy experiments were carried out at the 1W2B end station, Beijing Synchrotron Radiation Facility (BSRF). The Co K-edge spectra were collected at room temperature in the fluorescence mode.

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ELECTROCHEMICAL MEASUREMENTS Electrochemical measurements were performed with a conventional three-electrode system. A glassy carbon disk and a platinum RRDE were used as the working electrodes (Pine Instruments Co.). A Pt plate was utilized as the counter electrode, and an Ag/AgCl (3.5 M KCl) electrode was used as the reference electrode. The measured potentials were converted to RHE potentials on the basis of the standard F

DOI: 10.1021/acsami.7b02490 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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The as-prepared solution was left to stand for 1 h at 25 °C; after washing several times with methanol and drying at 50 °C, white ZIF-8 powder was obtained. The as-prepared ZIF-8 powder was annealed at 1000 °C (to remove Zn more thoroughly) in an Ar atmosphere for 1 h. The resultant powder was immersed in HCl (2 M) at 80 °C for 8 h and washed with DI water. Thereafter, it was annealed at 1000 °C in an Ar atmosphere for 3 h again.

calculation method: E(RHE) = E(Ag/AgCl) + 0.059 × pH + 0.205. The catalyst solution of carbon material samples were prepared using 6 mg of catalysts dispersed in 0.02 mL of 5 wt % Nafion solution and 0.98 mL of ethanol. Thereafter, the catalyst ink was treated by sonication for 30 min. The catalyst solution of Pt/C was prepared using 3 mg of catalyst dispersed in 0.02 mL of 5 wt % Nafion solution and 0.98 mL of ethanol, followed by sonication for 30 min. The working electrodes were made by loading 30 μL of catalyst ink onto the surface of a glassy carbon disk of S = 0.2475 cm2 for all samples. Then, all working electrodes were dried at room temperature. The CC was cut out to match the area of the glassy carbon disk, and it was attached to the glassy carbon disk using carbon paint cautiously. Thereafter, 30 μL of catalyst ink was deposited on the CCmodified electrode, followed by drying at room temperature; it was then directly used as the working electrode (Scheme S1). After purging using an N2/O2 flow for 30 min at least, KOH solution (0.1 M) was used as the electrolyte. RRDE measurements were carried out on a CHI 760e electrochemical workstation (CH Instruments) with an MSRX speed controller (Pine Instruments Co.) at 1600 rpm for all tests. The scan rates of CV and LSV were 50 and 10 mV s−1, respectively, unless otherwise stated. All CV and LSV curves were normalized by geometric area unless otherwise stated. The electron transfer numbers (n) and HO2− yields were calculated using eqs 1 and 2 n=

4 × ID ID + IR /N

HO2−% =

200 × IR ID × N + IR

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02490. Calculation method of the electrochemical surface area (ECSA); speculated steps toward the formation of a MOFNWA; large-field SEM images of MOFNWA and Co@NPCM/CNF-850; TEM images of traditional granular ZIF-67 and carbonized traditional granular ZIF-67; enlarged lattice fringes; XRD patterns of CoCHNWA and MOFNWA; nitrogen adsorption/ desorption isotherms and PSDs of carbonized ZIF-67 samples; comparison of electrocatalytic ORR activities of various Co−N−C catalysts; structural parameters extracted from quantitative EXAFS curve fitting; schematic illustration for the preparation of the CCmodified electrode; LSV curves of Co@NPCM/CNF850 with different loading amounts in O2-saturated KOH (0.1 M); LSV curves of carbonized ZIF-8 in O2-saturated KOH (0.1 M); LSV curves of acidized CC, Co@NPCM/ CNF-850, and Co@NPCM/CNF-850-CC in O2-saturated KOH (0.1 M); electron transfer numbers and percentages of peroxide based on the RRDE data; CV curve of Co@NPCM/CNF-850-CC in N2-saturated KOH (0.1 M); LSV curves of Co@NPCM/CNF-850 and Co@NPCM/CNF-850-CC in O2-saturated KOH (0.1 M) (normalized by ECSA); CV curves of Co@ NPCM/CNF-850-CC, Co@NPCM/CNF-850, and bare GCE in N2-saturated KOH (0.1 M); capacitive currents of Co@NPCM/CNF-850-CC, Co@NPCM/CNF-850, and bare GCE (PDF)

(1)

(2)

ID is the disk current and IR is the ring current; N is the collection efficiency of 0.37.

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ASSOCIATED CONTENT

EXPERIMENTAL SECTION

Synthesis of CoCHNWA. CO(NH2)2 (25 mmol) and 5 mmol of Co(NO3)2·6H2O were dissolved in DI water (50 mL), and the solution was transferred into a Teflon-lined stainless steel autoclave. The autoclave was heated at 90 °C for 12 h; after washing several times with water and freeze drying, pink CoCHNWA powder was obtained.44 Synthesis of a ZIF-67-Based MOFNWA. 2-MeIM (5 g) was dissolved in DI water (20 mL) with 5 mL of TEA. Here, TEA is a deprotonation agent that can accelerate the reaction.45 The asprepared CoCHNWA powder was added to the solution, followed by heating at 75 °C for 2 h. After washing several times with MeOH, the product was dried by freeze-drying, which can maintain the porous structure better.46 Then, purple ZIF-67-based MOFNWA powder was obtained. Synthesis of Co@NPCM/CNF-850. The as-prepared MOFNWA powder was annealed at 850 °C in an Ar atmosphere for 1 h. To get rid of some unstable cobalt, the resultant powder was immersed in HCl (2 M) at 80 °C for 8 h and washed with DI water. Thereafter, it was annealed at 850 °C in an Ar atmosphere for 3 h again.47 Black Co@NPCM/CNF-850 powder was synthesized. Annealing steps were also performed, at 800 and 900 °C, respectively. Synthesis of Traditional Granular ZIF-67 and Carbonized Traditional Granular ZIF-67. Co(NO3)2·6H2O (0.45 g) and 2MeIM (5.5 g) were dissolved in 3 and 20 mL of DI water, respectively. The two solutions were mixed, followed by stirring for 6 h at 25 °C.48 After washing several times with ethanol and drying at 60 °C, purple traditional granular ZIF-67 powder was synthesized. The annealing process is the same as that for Co@NPCM/CNF-850. The annealing steps were also performed at 800 and 900 °C, respectively. Synthesis of ZIF-8 and Carbonized ZIF-8. Zn(NO3)2·6H2O (0.74 g) and 2-MeIM (1.62 g) were dissolved in 70 mL of methanol.49

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 431 85689278. ORCID

Xiurong Yang: 0000-0002-1289-1248 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 21435005 and 21627808) and Key Research Program of Frontier Sciences, Chinese Academy of Sciences (No. QYZDY-SSW-SLH019).

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ABBREVIATIONS NPCM, N-doped porous carbon matrix NPCNF, N-doped porous carbon nanofibers CoCHNWA, cobalt carbonate hydroxide nanowire array MOFNWA, MOF nanowire array Co@NPCM/CNF-850, Co nanoparticles encapsulated in a N-doped porous carbon matrix with superficial N-doped porous carbon nanofibers, obtained at 850 °C DOI: 10.1021/acsami.7b02490 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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Co@NPCM/CNF-850-CC, Co nanoparticles encapsulated in a N-doped porous carbon matrix with superficial N-doped porous carbon nanofibers, obtained at 850 °C, dispersed on a CC

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DOI: 10.1021/acsami.7b02490 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX