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Cu, N-codoped Hierarchical Porous Carbons as Electrocatalysts for Oxygen Reduction Reaction Haiyan Yu, Adrian C. Fisher, Daojian Cheng, and Dapeng Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04189 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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ACS Applied Materials & Interfaces

Cu, N-codoped Hierarchical Porous Carbons as Electrocatalysts for Oxygen Reduction Reaction Haiyan Yu,† Adrian Fisher,‡ Daojian Cheng *,†,‡ and Dapeng Cao*,†,‡ †

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China



International Research Center for Soft Matter, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

ABSTRACT: It remains a huge challenge to develop non-precious electrocatalysts with high activity to substitute commercial Pt catalysts for oxygen reduction reaction (ORR). Here, the Cu, N-codoped hierarchical porous carbon (Cu-N-C) with high content of pyridinic N was synthesized by carbonizing Cu-containing ZIF-8. Results indicate that Cu-N-C shows excellent ORR electrocatalyst properties. First of all, it nearly follows four-electron route, and its electron transfer number reaches 3.92 at -0.4 V. Second, both the onset potential and limited current density of Cu-N-C are almost equal to those of commercial Pt/C catalyst. Third, it exhibits better half-wave potential (~16mV) than commercial Pt/C catalyst. More importantly, the Cu-N-C displays better stability and methanol tolerance than Pt/C catalyst. All these good properties are attributed to hierarchical structure, high pyridinic N content and the synergism of Cu and N dopants. The metal-N codoping strategy can significantly enhance the activity of electrocatalysts, and it will provide reference for the design of novel N-doped porous carbon ORR catalysts. KEYWORDS: Cu, N-codoped porous carbon, Pyridinic N, Hierarchical pores, ORR.

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INTRODUCTION Facing the severe challenges of global warming, air pollution and resource shortage, scientists are focusing on advanced clean energy conversion technologies including fuel cells and solar cells. Polymer electrolyte membrane fuel cells (PEMFCs) are extensive and fundamental technologies owing to their advantages such as safety, high power density, relative quick start-up, low operation temperature.1 However, in a hydrogen-oxygen fuel cell, the rate of cathodic oxygen reduction reaction (ORR) is much slower than that of the relatively facile anodic hydrogen oxidation reaction (HOR) by six or more orders of magnitude.2 Conventionally, platinum (Pt) has been regarded as the state-of-the-art catalyst for ORR. Nevertheless, the large scale commercialization of Pt catalyst is hindered by its scarcity and high price, let alone low stability and tolerance to methanol. Therefore, it is an urgent need to develop better non-precious catalysts owing high ORR activities and low stability. To cut down the cost of precious metal ORR catalysts, metal-free catalysts have been investigated widely3, among which porous carbons are the most promising alternative because of low price and high stability.4 However, the electrocatalytic activities of pristine carbons (carbon nanotubes, graphene sheets, and nanostructured carbons) are hardly commeasurable with commercial Pt/C.5-9 Different non-metal heteroatoms doping (such as N,2, 8-11 B,12, 13 S,14, 15 P,16 and F17, also including dual-doped18, 19 and trinary-doped20 ) in the carbon frameworks can largely enhance their ORR activities. For example, Zhang et al. used ZIF-7/Glucose composite as a precursor to synthesize ZIF-derived nitrogen-doped porous carbons and the obtained Carbon-L shows excellent electrocatalystic activity for 2

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ORR.2 The metal-free N-containing CNTs obtained by simple plasma-etching technology showed the relatively good ORR activity and good stability.6 Moreover, Vineesh et al. demonstrated the boron-doped graphene (BG) derived from rhombohedral boron carbide (B4C) as a successful ORR catalyst.21 Among these heteroatoms-doped carbons, N-doped porous carbons are extensively studied, and the exact ORR active sites in N-doped carbon materials were announced to be the carbon atoms with Lewis basicity next to pyridinic N.22 However, it remains a huge challenge to promote the catalytic activity of metal-free catalysts. Generally, a good ORR catalyst often holds the following several features, such as sufficient and well-distributed active sites, high electrical conductivity, large surface area and hierarchical porous structure. The high electrical conductivity assures the efficient charge transfer to the active sites, while large surface area and hierarchical porous structure assure the efficient mass transfer to the active sites and evidently facilitates ORR diffusion kinetics. The contribution of the hierarchical porous structure of carbon to Li-O2 batteries was illuminated by Zhang’s group.23 However, it is hard to achieve all the properties at the same time for most reported metal-free catalysts. Doping non-metal heteroatoms into carbon frameworks may increase their active sites, but also generate an intrinsic barrier that hinders charge transfer. In contrast, doping non-precious metal into the metal-free catalysts can not only serve as active sites, but also improve their electrical conductivity. Thus, designing the hierarchical porous metal-containing N-doped carbons may meet the requirement of ORR catalysts. Currently, most metal-containing N-doped carbons24-26 are focused on transition metal doping. A N-doped carbon catalyst containing trace iron 3

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displays similar limited current density with Pt/C catalyst in acid media.24 The sustainable controllable Fe-N-C catalysts with higher ORR activity than precious Pt/C catalysts were synthesized by using the sol-gel chemistry of gelatin and iron.25 N-doped hierarchical porous carbon nanofibers with embedded cobalt nanoparticles were synthesized by single-nozzle electrospinning and post pyrolysis, and the resultant materials exhibit the good performance for ORR activity, stability, and tolerance to methanol.27 In the metal doping process, the aggregation of metal often hinders the catalytic performance. Traditional doping methods can’t ensure the dispersion of active sites, while metal-organic frameworks (MOFs) with uniform metal-ion sites and high specific surface area may achieve their good dispersion. So, MOFs not only can be widely used in the fields of gas adsorption28, 29, separation30, 31, catalysis32-35 and sensing36-38, but also can serve as pyrolysis sacrifice to form MOFs-derived porous carbons with well-distributed metal active sites and stronger electron transfer capability than MOF itself. Further, as a subclass of MOFs, zeolitic imidazolate frameworks (ZIFs) are excellent candidates for generating N-doped porous carbon thanks to the rich nitrogen source provided by imidazolate ligands.39-41 For example, Zhang et al. have synthesized N-doped porous carbon with high degree of graphitization by using ZIF-8 as a self-sacrificing template, and the resulting material shows promising performance for the ORR activity.39 Further, Zhong et al. developed new 2D sheets based on graphene and ZIF-8 derived N-doped porous carbon with high ORR activity.41 And noble metal free catalysts CoNx/C for ORR via pyrolysis of ZIF-67 were studied by varying the pyrolysis temperature and the acid leaching process.42 N-doped mesoporous carbon layer/MWCNT hybrids with embedded Co3O4 nanoparticles was 4

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synthesized using multiwalled carbon nanotube and ZIF-9 hybrids as precursors, exhibiting fine ORR activity.43 High effective non-precious metal electrocatalysts prepared from ZIF-8 with varied ligands were studied by Zhao et al.44 The Co containing COP-derived Co, N-codoped porous carbons show higher ORR activity than the commercial Pt/C catalyst with accurately controlled locations of cobalt and nitrogen heteroatoms.45 Besides directly serving as active sites, metal can tune the structure of materials. For instance, Co species may just help nitrogen to dop into carbon lattice, rather than directly work as the active sites in Co-N-C catalysts prolysis by ethylene diamine or polyaniline.46 Besides Co and Fe, as a kind of cheap and abundant metal, Cu can also act as the ORR electrocatalyst. For example, copper can behave as a good ORR electrocatalyst in lithium-air battery.47 And with copper active sites, a graphene oxide and copper-centered MOF composite shows tri-functional activities for ORR, OER and HER.48 Moreover, Cu has the second highest electrical conductivity (only 6% less than Ag),49 which can enhance the performance of active sites. However, Cu, N-codoped porous carbons were not studied very well. In this work, we used Cu-ZIF-8 as a self-sacrificing template to synthesize a Cu, N-codoped porous carbons (Cu-N-C) with hierarchical pores. For comparison, we also used ZIF-8 as a template to synthesize N-doped porous carbon (N-C). Then we systematically characterized these samples, and tested their electrocatalytic ORR activity, long-term stability and methanol tolerance in 0.1 M KOH. Finally, some discussion is also addressed. EXPERIMENTAL SECTION The synthesis process of Cu-N-C is shown in Scheme 1. First, copper nanowires (Cu NWs) were synthesized through a nonaqueous method.50 Subsequently, Cu-ZIF-8 was 5

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prepared with Cu NWs, zinc acetate and 2-methylimidazole at room temperature, similar to a previous method.51 For comparison, ZIF-8 was also synthesized, but Cu NWs was absent during the synthesis. Then Cu-N-C and N-C samples were obtained by pyrolyzing the self-sacrifice templates of Cu-ZIF-8 and ZIF-8 at 950 for 8 hours, respectively. The systematic characterizations for the two samples were also carried out. The detailed synthesis

process

and

characterization

technologies

including

electrochemical

measurements were described in Supporting Information. RESULTS AND DISCUSSION The scanning electron microscope images of the Cu-ZIF-8 and ZIF-8 exhibit both the uniformly sized particles of ~50 nm in size (Figure 1a and 1c), whereas the Cu-N-C and N-C catalysts show irregular size with partial structure collapse (Figure 1b and 1d). The image of Cu-N-C in Figure 1b exhibits plenty of uniform pores which are significant different with that of N-C. Figure 2a and 2b show the high-resolution transmission electron microscope images of Cu-ZIF-8 and Cu-N-C, in which we do not observe big copper nanoparticles in Cu-ZIF-8 and Cu-N-C. But elemental mapping images of Cu-ZIF-8 and Cu-N-C (Figure 2c and 2d) show uniform dispersions of copper and carbon as well as nitrogen. It suggests that copper is inserted successfully into the material. The results of inductively coupled plasma (ICP) show that the exact content of copper increases from 1.35 wt% (Cu-ZIF-8) to 2.67 wt% (Cu-N-C) after carbonization (Table 1). This is because, during the pyrolysis process, the MOF is pyrolyzed with Zn evaporated and some small molecules released, including H2O and carbon, nitrogen-containing compounds. Thus, the Cu content increases. 6

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We also performed powder X-ray diffraction (PXRD) analyses of the synthesized materials Cu-ZIF-8, ZIF-8, Cu-N-C and N-C to validate their structure features. In Figure 3a, the PXRD patterns of the two precursors are similar, indicating that Cu-ZIF-8 maintains the structure feature of ZIF-8. Cu-N-C and N-C both display two broad peaks at around 23° and 44°, which respectively correspond to the carbon (002) and (101) diffractions, due to the formation of graphitic structure and amorphous carbon during pyrolysis. Specially, the two sharp peaks at around 43.4°, 50.4° and a small peak at 74.3°, respectively correspond to Cu (111), (200) and (220) diffractions, indicating the existence of pure copper. The valence of Cu can usually be defined through XPS spectra. While the content of Cu in Cu-N-C is extremely low (0.04 at% derived from 2.67 wt%), the XPS spectra (Figure S1) cannot offer necessary information to confirm the contents of Cu (0) and Cu (II). Since the peak at ~934 eV in the XPS spectra of Cu of Cu-N-C belongs to neither Cu (0) at ~932 eV nor Cu (II) at ~935 eV,52 it may also contain Cu (II). The Raman spectra in Figure 3b display the characteristic D band (1353 cm-1) and G band(1588 cm-1)of the porous carbons. The D band reflects the defected carbon structure, while the G band suggests the graphitic carbon structure which results from in-plane vibrations of ideal sp2 carbon atoms in graphene. So, the graphitization degree of carbon materials usually is estimated by the ratio of the peaks of G and D bands (IG/ID). As shown in Figure 3b, the IG/ID value of Cu-N-C is 1.08, similar with that (1.06) of N-C, meaning that they have the similar graphitization degree. To explore the porosity properties of the samples, we also measured adsorptiondesorption isotherms of N2 at 77 K (inset of Figure 4). The pore size distribution (PSD) (see 7

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Figure 4) was obtained by using density functional theory. The feature parameters including the BET specific surface area and total pore volume of the four samples are summarized in Table 2. In the inset of Figure 4, the type-IV curve of the isotherm of Cu-N-C porous carbon suggesting both micropores and mesopores exist. The PSDs in Figure 4 indicate that Cu-N-C and N-C have fewer micropores than Cu-ZIF-8 and ZIF-8, which may be caused by partial structure collapse. At the temperature of 950 °C which is higher than the boiling point of metal Zn (908 °C), metal Zn was easily evaporated out through the materials, etching and creating more pores. The high evaporation of metal Zn and gasification of the carbons attributed to the mesopores and micropores in N-C and Cu-N-C, while the destruction of the cubic structure attributed to partial macropores. Cu-N-C shows increased meso- and macro-pores than the matrix, while N-C has fewer meso- and macro-pores. The PSD in Figure 4 and Table 2 show that Cu-N-C is a kind of hierarchically structured porous carbon with micro-, meso- and macro- pores. The 3D hierarchical porous structure, combining the synergic effects of various sizes of pores, can make the mass transport resistance lower and expose more ORR active sites, thus benefit to the ORR activity. X-ray photoelectron spectroscopy (XPS) spectra can illustrate the chemical composition and content in samples. The XPS spectra of the two porous carbons (Figure 5a and 5b) show the N1s, C1s and O1s peaks, and special Cu 2p3 also appears in Cu-N-C. It is hard to confirm whether Cu-N bonds exist or not. Even if Cu-N bonds exist, copper nitrides (XPS peak at ~397 eV) will not influence the contents of other types of nitrogen much. Thus, we did not take copper nitrides into consideration. As shown in Figure 5c and 5d, the high-resolution N1s spectrum can be deconvoluted into four types: 398.4 ± 0.2, 8

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400.0 ± 0.2, 401.0 ± 0.2 and 402.0 ± 0.2 eV, corresponding to pyridinic N, pyrrolic N, graphitic N, and oxidized N, respectively.52,

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The contents of the four type N are

summarized in Table S1 and can be intuitively observed in Figure 5e. Compared with N-C, Cu-N-C has a significantly higher content of pyridinic N species and a similar content of graphitic N. To investigate the electrocatalytic activity of Cu-N-C for the ORR, cyclic voltammetry (CV) measurements was carried out. Figure 6a shows the voltammogram without significant voltammetric current peak within the potential range from -1.0 and 0.2 V (vs. SCE) in the N2-saturated solution. However, the CV curve displays an obvious cathodic peak at -0.2 V in O2-saturated solution. No reduction peak appears at the same potential in N2-saturated curve, so the peak at -0.2 V suggests a high electrocatalytic activity for oxygen reduction.25, 39, 41, 42, 54, 55 The promising ORR activity of Cu-N-C was further proved by the linear sweep voltammetry (LSV) curves at a scan rate of 5 mV s-1 in O2-saturated solution (Figure 6b). For comparison, N-C and commercial 20 wt% Pt/C were also tested under the same conditions. Surprisingly, Cu-N-C porous carbon shows excellent activity proved by onset potential and half-wave potential (E1/2) of -0.067 V and -0.156 V, respectively. The potentials are significant higher than those of N-C (-0.129 and -0.233 V), and are almost equal to those of the commercial 20 wt% Pt/C catalyst (-0.066 V and -0.172 V). The higher ORR activity of Cu-N-C was also reflected by its higher current density of 4.75 mA cm-2 than that of N-C (4.44 mA cm-2), which is also almost equal to that of commercial 20 wt% Pt/C (4.78 mA cm-2). Considering the possible application of ORR catalysts in acid 9

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environment, we also tested the ORR activities of Cu-N-C, N-C and Pt/C in O2-saturated 0.1 M HClO4 (Figure S2). The Cu-N-C is a kind of N-doped carbon substantially because it only contains trace Cu. Like some other nitrogen–doped carbons,6, 46, 52 the Cu-N-C and N-C also show significantly lower ORR activities than Pt/C in acid medium. In alkaline media, the ORR may follow a two-electron pathway with peroxide as intermediate or a four-electron pathway.56 The electron transfer number per O2 (n) and peroxide yield can clarify the electron transfer kinetics and the pathway of oxygen reduction reaction. Rotating ring-disk electrode (RRDE) was used to quantify the ORR pathway. The disk current density and the ring current density of the three samples by RRDE voltammograms are shown in Figure 6b. Figure 6c reveals that the measured peroxide yields vary from -0.8 to -0.4 V, where the peroxide yields for Cu-N-C is much lower than those of N-C at various potentials, and even lower than those of Pt/C at range of -0.8 V to -0.65 V. The exact electron transfer numbers of the samples at different potentials are presented in Figure 6d. Obviously, n values of Cu-N-C (3.92, at -0.4 V) are significantly higher than those of N-C (3.65, at -0.4 V), and are still almost equal to that of commercial 20 wt% Pt/C (3.96, at -0.4 V). The electron transfer number implies that the ORR catalyzed by Cu-N-C is mainly followed four-electron route. This conclusion can be confirmed by Koutecky-Levich (K-L) equation. Rotating disk electrode (RDE) voltammograms of Cu-N-C, N-C and Pt/C at different rotating speeds and the corresponding K-L plots are displayed in Figure S3. For all the three samples, the good linearity of K-L plots demonstrates first-order reaction kinetics towards oxygen.41, 57 The values of n at various potentials are calculated to be around 3.9, reconfirming that the ORR 10

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of Cu-N-C follows a four-electron-transfer route, which is consistent with the results obtained from the RRDE voltammograms. The high-performance ORR electrochemical activity of Cu-N-C is mainly attributed to three reasons. First, Cu-N-C has sufficient and well-distributed active sites. Many researchers approved that pyridinic N and graphitic N are beneficial for electrochemical activity.46 Recently, Guo et al.22 pointed out that O2 molecules can easily adsorb on the carbon atoms next to pyridinic N, because these carbon atoms serve as the active sites for ORR. As listed in Table S1, the contents of graphitic N in Cu-N-C and N-C are 0.97 at% and 1.20 at%, which are almost the same. Thus, we consider that the contribution of graphitic N in the two materials for electrochemical activity may be also similar. However, the content of pyridinic N in Cu-N-C reaches 1.25 at%, which is more than two times that (0.56 at%) of N-C. So, we believe that the higher activity of Cu-N-C is attributed to the higher pyridinic N content, compared to N-C. And owing to sacrificing template Cu-ZIF-8, active sites are well distributed in the material. Second, Cu-N-C has appropriate hierarchical porous structure with high surface area tuned by Cu. More ORR active sites get exposed, and the mass transport resistance decrease greatly due to the appropriate structure. With sufficient exposed ORR active sites, shorter channel length and enhanced mass transfer, Cu-N-C performs high electrochemical activity. Third, with metal Cu inlayed, the enhanced electrical conductivity of Cu-N-C may facilitate the ORR activity. Figure S4 shows the Nyquist plots of Cu-N-C and N-C at open circuit potential, which can reflect the enhanced electrical conductivity and lower diffusion resistence of Cu-N-C. Besides activity, the methanol tolerance and stability are also key parameters for 11

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high-performance ORR electrocatalysts. We use current-time (i-t) technique to judge the tolerance of the three samples toward methanol. Figure 7a shows the i-t curves of the three samples before and after injecting 3 mL methanol into the electrolyte. We can observe an obvious drop for Pt/C caused by the methanol oxidation reaction. On the contrary, the i-t curves of Cu-N-C and N-C are much more stable. Furthermore, the stability of Cu-N-C and N-C is evaluated by i-t chronoamperometric measurement. As shown in Figure 7b, the i-t curves show high stability for Cu-N-C and N-C, with nearly 92% and 90% retention, respectively, much higher than 66% of Pt/C, which is basically in agreement with other literatures.2, 27,

43, 58

The agglomeration and migration of Pt nanoparticles are important

factors due to the degradation of Pt nanoparticles in commercial catalyst.59 The strong covalent bonds between C and N as well as the protection of Cu by the carbon matrix55 can delay the degradation of Cu-N-C catalysts. These results above suggest that Cu-N-C and N-C porous carbons both have better methanol tolerance and stability than commercial 20 wt% Pt/C. CONCLUSIONS In summary, the hierarchically structured Cu, N-codoped porous carbon Cu-N-C has been synthesized through carbonizing Cu-ZIF-8. The as-prepared Cu-N-C catalyst possesses excellent ORR catalytic activity. It exhibits better half-wave potential than commercial Pt/C catalyst by ∼16 mV. The onset potential and limited current density almost are equal to those of commercial Pt/C. The Cu-N-C also shows almost four-electron route, and its electron transfer number is 3.92 at -0.4 V. The activity of Cu-N-C remains well besides the methanol crossover effects as a property of noble metal-free electrocatalyst, 12

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and stability is also better than Pt/C in alkaline media. The improved ORR performance of Cu-N-C could be attributed to the increased pyridinic N, the hierarchical porous structure and enhanced electrical conductivity tuned by Cu, N dopants. The metal-N codoping strategy can enhance the electrocatalytic activity greatly, and this work provides useful reference for the design of novel N-doped porous carbon ORR catalysts.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details on synthesis and characterizations; The detailed XPS results of Cu-N-C and N-C (Table S1) and XPS spectra of Cu in Cu-N-C

(Figure S1); The LSV

curves of Cu-N-C, N-C and Pt/C in acid media (Figure S2); The LSV curves of Cu-N-C, N-C and Pt/C at different rotating speeds in alkaline media and corresponding Koutecky-Levich plots (Figure S3); The Nyquist impedance plots of Cu-N-C and N-C in 0.1 M KOH solution (Figure S4). AUTHOR INFORMATION Corresponding Author * Fax: +8610-64427616. E-mail: [email protected] (D. C). * Tel: +8610-64443254. E-mail: [email protected] (D. C). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Notes The authors declare no competing financial interest. Funding Sources This work is supported by NSF of China (No. 21576008, 91334203), Fundamental Research Funds for the Central Universities (Project No. buctrc201530)

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Synergistic Effect between Metal-Nitrogen-Carbon Sheets and NiO Nanoparticles for Enhanced Electrochemical Water-Oxidation Performance. Angew. Chem. Int. Ed. 2015, 54, 10530-10534. (27) Wang, S. G.; Cui, Z. T.; Cao, M. H. A Template-Free Method for Preparation of Cobalt Nanoparticles Embedded in N-Doped Carbon Nanofibers with a Hierarchical Pore Structure for Oxygen Reduction. Chem. Eur. J. 2015, 21, 2165-2172. (28) Li, J. R.; Yu, J.; Lu, W.; Sun, L. B.; Sculley, J.; Balbuena, P. B.; Zhou, H. C. Porous Materials with Pre-Designed Single-Molecule Traps for CO2 Selective Adsorption. Nat. Commun. 2013, 4, 1538. (29) Xiang, Z. H.; Leng, S. H.; Cao, D. P. Functional Group Modification of Metal–Organic Frameworks for CO2 Capture. J. Phys. Chem. C 2012, 116, 10573-10579. (30) Ge, L.; Zhou, W.; Du, A. J.; Zhu, Z. H. Porous Polyethersulfone-Supported Zeolitic Imidazolate Framework Membranes for Hydrogen Separation. J. Phys. Chem. C 2012, 116, 13264-13270. (31) Xiang, Z. H.; Peng, X.; Cheng, X.; Li, X. J.; Cao, D. P. [email protected](BTC)2 and Metal– Organic Frameworks for Separation of CO2/CH4 Mixture. J. Phys. Chem. C 2011, 115, 19864-19871. (32) Liu, J.; Chen, L.; Cui, H.; Zhang, J.; Zhang, L.; Su, C. Y. Applications of Metal-Organic Frameworks in Heterogeneous Supramolecular Catalysis. Chem. Soc. Rev. 2014, 43, 6011-6061. (33) Hu, P.; Morabito, J. V.; Tsung, C.-K. Core–Shell Catalysts of Metal Nanoparticle Core and Metal–Organic Framework Shell. ACS Catal. 2014, 4, 4409-4419. (34) Dhakshinamoorthy, A.; Garcia, H. Metal-Organic Frameworks as Solid Catalysts for the Synthesis of Nitrogen-Containing Heterocycles. Chem. Soc. Rev. 2014, 43, 5750-5765. (35) Wang, H.; Yin, F. X.; Li, G. R.; Chen, B. H.; Wang, Z. Q. Preparation, Characterization and Bifunctional Catalytic Properties of MOF(Fe/Co) Catalyst for Oxygen Reduction/Evolution Reactions in Alkaline Electrolyte. Int. J. Hydrogen. Energ 2014, 39, 16179-16186. (36) Liu, S.; Xiang, Z. H.; Hu, Z.; Zheng, X. P.; Cao, D. P. Zeolitic Imidazolate Framework-8 as a Luminescent Material for the Sensing of Metal ions and Small Molecules. J. Mater. Chem. 2011, 21, 6649-6653. (37) Xiang, Z. H.; Cao, D. P. Synthesis of Luminescent Covalent-Organic Polymers for Detecting Nitroaromatic Explosives and Small Organic Molecules. Macromol. Rapid. Comm. 2012, 33, 1184-1190. (38) Mottillo, C.; Friščić, T. Carbon Dioxide Sensitivity of Zeolitic Imidazolate Frameworks. Angew. Chem. Int. Ed. 2014, 53, 7471-7474. (39) Zhang, L. J.; Su, Z. X.; Jiang, F. L.; Yang, L. L.; Qian, J. J.; Zhou, Y. F.; Li, W. M.; Hong, M. C. Highly Graphitized Nitrogen-Doped Porous Carbon Nanopolyhedra Derived from ZIF-8 Nanocrystals as Efficient Electrocatalysts for Oxygen Reduction Reactions. 17

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Nanoscale 2014, 6, 6590-6602. (40) Wang, J.; Zhong, H. X.; Qin, Y. L.; Zhang, X. B. An Efficient Three-Dimensional Oxygen Evolution Electrode. Angew. Chem. Int. Ed. 2013, 52, 5248-5253. (41) Zhong, H. X.; Wang, J.; Zhang, Y. W.; Xu, W. L.; Xing, W.; Xu, D.; Zhang, Y. F.; Zhang, X. B. ZIF-8 Derived Graphene-Based Nitrogen-Doped Porous Carbon Sheets as Highly Efficient and Durable Oxygen Reduction Electrocatalysts. Angew. Chem. Int. Ed. 2014, 53, 14235-14239. (42) Wang, X. J.; Zhou, J. W.; Fu, H.; Li, W.; Fan, X. X.; Xin, G. B.; Zheng, J.; Li, X. G. MOF Derived Catalysts for Electrochemical Oxygen Reduction. J. Mater. Chem. A 2014, 2, 14064-14070. (43) Li, X. Z.; Fang, Y. Y.; Lin, X. Q.; Tian, M.; An, X. C.; Fu, Y.; Li, R.; Jin, J.; Ma, J. T. MOF Derived Co3O4 Nanoparticles Embedded in N-Doped Mesoporous Carbon Layer/MWCNT Hybrids: Extraordinary Bi-Functional Electrocatalysts for OER and ORR. J. Mater. Chem. A 2015, 3, 17392-17402. (44) Zhao, D.; Shui, J. L.; Grabstanowicz, L. R.; Chen, C.; Commet, S. M.; Xu, T.; Lu, J.; Liu, D.-J. Highly Efficient Non-Precious Metal Electrocatalysts Prepared from One-Pot Synthesized Zeolitic Imidazolate Frameworks. Adv. Mater. 2014, 26, 1093-1097. (45) Xiang, Z.; Xue, Y.; Cao, D.; Huang, L.; Chen, J. F.; Dai, L. Highly Efficient Electrocatalysts for Oxygen Reduction Based on 2D Covalent Organic Polymers Complexed with Non-Precious Metals. Angew. Chem. Int. Ed. 2014, 53, 2433-2437. (46) Nie, Y.; Li, L.; Wei, Z. D. Recent Advancements in Pt and Pt-Free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168-2201. (47) Wang, Y.; Zhou, H. A Lithium-Air Fuel Cell Using Copper to Catalyze Oxygen-Reduction Based on Copper-Corrosion Mechanism. Chem. Commun. 2010, 46, 6305-6307. (48) Jahan, M.; Liu, Z. L.; Loh, K. P. A Graphene Oxide and Copper-Centered Metal Organic Framework Composite as a Tri-Functional Catalyst for HER, OER, and ORR. Adv. Funct. Mater. 2013, 23, 5363-5372. (49) Wang, R. L.; Ruan, H. B. Synthesis of Copper Nanowires and Its Application to Flexible Transparent Electrode. J. Alloy. Compd. 2016, 656, 936-943. (50) Chen, B.; Cheng, D. J.; Zhu, J. Q. Synthesis of PtCu Nanowires in Nonaqueous Solvent with Enhanced Activity and Stability for Oxygen Reduction Reaction. J. Power Sources 2014, 267, 380-387. (51) Wang, X.; Huang, P. C.; Yu, P.; Yang, L. F.; Mao, L. Q. Rapid and Cost-Effective Synthesis of Nanosized Zeolitic Imidazolate Framework-7 with N,N'-Dimethylformamide as Solvent and Metal Acetate Salt as Metal Source. ChemPlusChem 2014, 79, 907-913. (52) Kundu, S.; Nagaiah, T. C.; Xia, W.; Wang, Y.; Dommele, S. V.; Bitter, J. H.; Santa, M.; Grundmeier, G.; Bron, M.; Schuhmann, W.; Muhler, M. Electrocatalytic Activity and Stability of Nitrogen-Containing Carbon Nanotubes in the Oxygen Reduction Reaction. J. 18

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Table 1. ICP results of Cu-ZIF-8, ZIF-8, Cu-N-C and N-C. Cu (wt%)

Zn (wt%)

1.35 2.67 -

26.73 27.98 0.27 -

Cu-ZIF-8 ZIF-8 Cu-N-C N-C

“-” represents undetectable.

Table 2. Summary of porosity parameters of Cu-ZIF-8, ZIF-8, Cu-N-C and N-C. Sample

a

BET SSA 2

-1

Langmuir SSA 2

Pore Volumea

Pore size distribution

-1

(m g )

(m g )

(cm3 g-1)

(nm)

Cu-ZIF-8

1584

2621

1.60

1.1, 25-52, 60

ZIF-8

1387

1921

0.96

1.1, 25- 49

Cu-N-C

1071

1467

1.90

0.58-1.2; 3- 9.5; 12.8, 29.7- 61

N-C

1331

1773

0.77

0.56-1;7- 9.7; 13, 19-66

Total pore volume. At p/p0=0.991567, 0.99326, 0.991677, 0.99263, respectively.

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Scheme 1. Synthesis strategy of Cu-N-C.

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Figure 1. SEM images of four samples. (a) Cu-ZIF-8, (b) Cu-N-C, (c) ZIF-8 and (d) N-C.

Figure 2. HRTEM images of (a) Cu-ZIF-8 and (b) Cu-N-C; HAADF-STEM image and copper, carbon and nitrogen elemental mapping images of (c) Cu-ZIF-8 and (d) Cu-N-C.

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Figure 3. (a) PXRD graphs of Cu-ZIF-8, ZIF-8, Cu-N-C and N-C, (b) Raman spectra of Cu-N-C and N-C.

Figure 4. Pore size distribution and (inset) nitrogen adsorption-desorption isotherms of Cu-ZIF-8, ZIF-8, Cu-N-C and N-C.

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Figure 5. XPS spectra of (a) Cu-N-C and (b) N-C. And corresponding deconvoluted N1s XPS spectrum of the (c) Cu-N-C and (d) N-C samples. (e) The atomic content of four types of nitrogen in the two samples. (f) The illustration of four types of nitrogen.

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Figure 6. (a) CV curves of Cu-N-C in N2-saturated (black dash line) or O2-saturated (red solid line) 0.1 M KOH solutions at a sweep rate of 50 mV s-1. (b) RRDE voltammograms of Cu-N-C, N-C and commercial 20 wt% Pt/C, under oxygen bubbling at scan rate of 5 mV s-1 and electrode-rotation speed of 1600 rpm. (c) The peroxide yield of the three samples at potential range of -0.8 V to -0.4 V on the basis of RRDE voltammograms. (d) The electron transfer numbers (n) of the three samples at potential range of -0.8 V to -0.4 V on the basis of RRDE voltammograms.

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Figure 7. (a) Current-time (i-t) chronoamperometric response of Cu-N-C, N-C and 20 wt% Pt/C electrodes by adding 3 mL methanol after about 400 s. (b) Current-time (i-t) chronoamperometric response of Cu-N-C, N-C and 20 wt% Pt/C at -0.4 V in O2-saturated 0.1 M KOH at a rotating rate of 1000 rpm.

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Table of Contents Graphic

Cu, N-codoped Hierarchical Porous Carbons as Electrocatalysts for Oxygen Reduction Reaction Haiyan Yu, Adrian Fisher, Daojian Cheng and Dapeng Cao

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