N-Doped Hierarchical Carbon Architectures

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Letter

Atomically Dispersed Fe/N-Doped Hierarchical Carbon Architectures Derived from a Metal–Organic Framework Composite for Extremely Efficient Electrocatalysis Qi-Long Zhu, Wei Xia, Li Rong Zheng, Ruqiang Zou, Zheng Liu, and Qiang Xu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00686 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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Atomically Dispersed Fe/N-Doped Hierarchical Carbon Architectures Derived from a Metal–Organic Framework Composite for Extremely Efficient Electrocatalysis Qi-Long Zhu†,§, Wei Xia‡,§, Li-Rong Zheng#, Ruqiang Zou‡,*, Zheng Liu¶, and Qiang Xu†,* †

Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science

and Technology (AIST), Ikeda, Osaka 563-8577, Japan ‡

Beijing Key Lab of Theory and Technology for Advanced Battery Materials, Department of

Material Science and Engineering, College of Engineering, Peking University, Beijing 100871, China #

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Sciences (CAS), Beijing 100049, China ¶

Inorganic Functional Materials Research Institute, AIST, Nagoya, 463-8560, Japan

Corresponding Author *E-mail: [email protected] (Q. Xu) and [email protected] (R. Zou)

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ABSTRACT: The hierarchical graphitic porous carbon architectures with atomically dispersed Fe, N doping have been fabricated from a metal–organic framework (MOF) composite by using a facile strategy, which show high specific surface areas, hierarchical pore structures with macro/meso/micro multimodal pore size distributions, abundant surface functionality with single-atom dispersed N and Fe doping, and improved hydrophilicity. The detailed analyses unambiguously disclosed the main active sites of doped N atoms and FeNx species in the catalyst. The resultant catalyst affords high catalytic performance for oxygen reduction, outperforming the benchmark Pt catalyst and many state-of-the-art noble-metal-free catalysts in alkaline media, particularly in terms of the onset and half-wave potentials and durability. Such catalytic performance demonstrates the significant advantages of the unique hierarchical porous structure with efficient atomic doping, which provides a high density of accessible active sites at much improved mass and charge transports.

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Metal–organic frameworks (MOFs) are a new class of crystalline porous materials possessing high internal surface areas, adjusted pore metrics and abundant metal/organic species in their scaffolds.1-8 By taking advantage of these features, MOFs can act as outstanding templates and precursors to create porous carbons.9-14 Particularly, many MOFs can be facilely converted to highly porous carbons without additional carbon sources by direct carbonization.10-12 The inherent high surface areas and controllable pore textures of MOF-derived porous carbons have motivated researchers to explore their diverse applications in energy storage and conversion.15-20 To date, although extensive research effort has been devoted to the fabrication of MOF-derived carbons, it lacks rational design for morphology control of the resultant structures. Due to the high pyrolysis temperature, the MOF crystals tend to convert into bulk carbons dominated by the micropores, which would lead to reduced effective surface area for electrochemical applications. On the other hand, the incorporation of heteroatoms (e.g. N, S, metals, etc.) to carbon structures would provide the opportunity to harvest more active sites for enhancing their electrochemical performance.21-25 Recently, some N-doped carbons have been prepared through direct carbonization of the N-rich MOFs, such as ZIF-8, ZIF-67, etc.26-32 Nevertheless, the limitation of available heteroatom-rich MOFs severely hinders the growth of this new research field. Consequently, the development of general methods to synthesize hierarchically porous carbon nanostructures with controllable doping and morphology, eventually endowing enhanced functionality, from common MOFs is highly desired. Oxygen reduction reaction (ORR) is one of the most technologically important electrochemical reactions for fuel cells and metal-air batteries.33-40 The widespread implementation of these technologies is greatly dependent on the exploration of non-precious catalysts with high ORR activity and stability to replace the costly Pt catalysts. Nanostructured


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carbons doped with heteroatoms, especially N, S, Fe and Co, represent promising candidates as non-precious catalysts for ORR owing to their unique electronic properties and structural features.41-47 Several MOF-derived N-doped carbons as ORR catalysts have been assessed, and significant achievements have been obtained.26-32,48-53 However, their ORR performances are still inferior to the benchmark Pt/C catalyst, in particular, in terms of half-wave potential. To further enhance the catalytic activities of MOF-derived nanostructures for ORR, the introduction of hierarchical pores and incorporation of more active species into the carbon frameworks with enhanced synergetic effect are desperately needed. Herein, we present the construction of a new type of atomically dispersed Fe/N-doped hierarchical carbon architectures from a MOF composite, achieving simultaneous optimization of both porous structures and surface functionalities of the catalysts. The hierarchical pore structures with on-demand macro/mesopore-connected micropores and atomic Fe and N doping would be greatly significant to provide more accessible active surface sites, facilitate the mass diffusion, and thus improve the catalytic performance (Scheme 1).19,29 Consequently, the novel structural and doping properties and the synergetic effect between the active species endow the resultant catalysts an extremely high catalytic activity for ORR in alkaline media, outperforming the Pt/C catalyst (20 wt%, Johnson Matthey). Scheme 1. Schematic representation for (a) the preparation of the Fe/N-GPC catalyst and (b) the advantages of the hierarchically porous architecture for promoting the ORR.


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Due to its high surface area and large pore size (2–3 nm), MIL-101-NH254 was selected as a prototypical MOF host to accommodate dicyandiamide (DCD) and FeCl3, which are used as the nitrogen source with a high nitrogen content and iron source most readily available, respectively. The resulted MOF composite was subsequently used as the precursor for catalyst synthesis under pyrolysis conditions. The quantitative encapsulation of DCD and FeCl3 was conducted by using a double solvent method (DSM) to avoid the deposition of the precursors on the out surface of MIL-101-NH2 crystals, which is based on the immiscibility of two liquid phases.55 When the hydrophilic DMF solution of DCD and FeCl3, whose volume is slightly less than the pore volume of the adsorbent, was added slowly into the hydrophobic n-hexane suspension of MIL101-NH2, the dispersed droplets containing precursor molecules could uniformly and completely diffuse into the hydrophilic pores. The weight ratio of DCD, FeCl3 and MIL-101-NH2 in the resulted FeD@MIL-101-NH2 composite was kept to be 1:1:5. The scanning electron microscopy (SEM) and powder X-ray diffraction (PXRD) measurements demonstrate that the integrity of the


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MOF framework was maintained well during the impregnation process (Figures S1 and S2). The accommodation of DCD and FeCl3 in the pores resulted in a marked decrease in the surface area of FeD@MIL-101-NH2 (Figure S3 and Table S1). The FeD@MIL-101-NH2 composite was then subjected to heat treatment at a temperature of 800 °C for 5 h under flowing argon and final acid etching, affording the hierarchically structured Fe/N-doped graphitic porous carbon nanospheres (denoted as Fe/N-GPC) with 1.1 wt% of Fe and 3.3 wt% of N incorporation. The uniform encapsulation of DCD and FeCl3 within the MOF framework could not only be favorable for promoting the homogeneous doping of the Fe and N active sites, but also lead to an intriguing thermally guest-induced morphology control process, where the rapid expanding gas released from the decomposition of the fillers produces substantial internal stresses and strains in the MOF-derived carbon framework, resulting in the unique hierarchically porous architecture. For comparison, N-PC, Fe-PC and PC were also prepared from corresponding composites and pure MOF.


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Figure 1. (a) PXRD patterns and (c) Raman spectra of Fe/N-GPC, Fe-PC, N-PC and PC. (b) N2 sorption isotherms at 77 K of Fe/N-GPC. (d) Photographic images of the catalyst suspensions in toluene/water mixture showing the different hydrocompatibility of Pt/C (20 wt%, JM) and Fe/NGPC. Inset in (b): corresponding pore size distribution for Fe/N-GPC calculated using NL-DFT method. In contrast to the amorphous carbon structures of the reference samples, the PXRD patterns of Fe/N-GPC exhibit a sharp peak at 26.5° assigned to the (002) planes of graphitic carbon, indicating a high degree of graphitization of the resulted carbon nanospheres (Figure 1a), which could be catalytically induced by the in situ formed metal nanoparticles (NPs) during carbonization. The relative weak diffraction peaks of Fe3C (JCPDC no. 89-2867) also can be identified due to the presence of few aggregated Fe3C particles, which are protected against acid corrosion by the graphitic layers on the particle surface (Figures S4 and S17). N2 sorption of Fe/N-GPC displays a type-IV curve with a hysteresis loop, giving a high BET surface area of 1207 m2 g–1 and a pore volume of 1.973 cm3 g–1 (Figures 1b and S5, Table S2). The pore size distribution (PSD) suggests the hierarchical pore structure of Fe/N-GPC with both micro- (1–2 nm) and mesopores (10–30 nm) (inset in Figure 1b). Such a pore structure would be able to provide more effective active sites and be favorable for the accessibility of the electrolyte and oxygen. As compared to the reference samples, the relative low intensity ratio (ID/IG) of D band (~1345 cm–1) to G band (~1580 cm–1) for Fe/N-GPC in Raman spectra further confirms its high degree of graphitization (Figure 1c), which is favorable for improving the electrical conductivity for ORR. X-ray photoelectron spectroscopic (XPS) analyses clearly demonstrate the successful doping of N atoms in the carbon matrix with a surface content of 3.5 at% and the complete leaching of aluminum (Figure S6). The high-resolution N 1s spectrum reveals the presence of


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three types of N species with the domination of pyrrolic N and graphitic N. Only weak signal of Fe species can be detected, which should be attributed to its low surface content. Particularly noteworthy is that in contrast to the hydrophobicity of the commercial Pt/C (20 wt%, JM) catalyst, the decoration of N atoms greatly enhances the hydrophilicity of Fe/N-GPC probably ascribed to the defects from nitrogen doping (Figure 1d),56,57 which would benefit the diffusion of the electrolyte toward the active sites.

Figure 2. (a) SEM, (b) TEM and (c, d) HAADF-STEM images of Fe/N-GPC. (e) Elemental mapping revealing the elemental distribution. SEM reveals that Fe/N-GPC architecture consists of highly interconnected carbon nanosheets with the thickness less than 10 nm, leading to a large number of macropores with an average


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diameter of 50 ± 20 nm (Figures 2a and S7). In spite of the differences in the doping concentration and the graphitization degree of the resulted carbons, the similar architectures were also found in Fe/N-GPCT derived at different pyrolysis temperatures (T) and N-PC (Figures S8– S12). By contrast, the MOF framework without DCD for N doping would collapse after carbonization, leading to the featureless carbon morphology of Fe-PC and PC (Figures S13–S15). These results suggest that the guest organic molecules filled in the MOF composite not only play a significant role in the generation of such unique morphology, but also provide additional carbon and nitrogen sources to avoid the severe shrinkage and aggregation of the derived particles. The TEM and high-annular dark-field scanning TEM (HAADF-STEM) images of Fe/N-GPC confirm the formation of the porous architecture (Figures 2 and S16–S19). Elemental mapping and selected-area energy dispersive X-ray (EDX) analyses clearly demonstrate the uniform distribution of N and Fe elements in the carbon frameworks (Figures 2e, S18 and S19), which may exist as FeNx, the well-known electrocatalytic sites for ORR.23,36 The existence of few Fe3C particles with the sizes of 100–150 nm also has been revealed, which are tightly confined by the graphitic carbon shells with about ten layers (Figure S17). As an ORR catalyst, the unique hierarchical porous structure of Fe/N-GPC combining macro-, meso- and micropores, which can serve as the interior buffering reservoirs of electrolyte in the interior of the catalyst, channels for accelerating the rapid transport of ions and oxygen molecules, and the locations for catalytic electroreduction, respectively, would be greatly favorable for the ORR process.


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Figure 3. (a) Normalized Fe K-edge XANES spectra and (b) Fourier-transform k3-weighted Fe K-edge EXAFS of the Fe/N-GPC catalyst and the Fe foil and FePc references. The highly dispersed iron atoms can be unambiguously identified and mapped out by the highresolution elemental distribution analysis (Figure S19). In order to further identify the status of the iron species dispersed in the Fe/N-GPC catalyst, X-ray absorption spectroscopy at Fe k-edge was performed to analyze the coordination environment of iron, which is a well-established and powerful technique to determine the chemical state and coordination environment of the center atoms in the sample. Figure 3a shows the XANES spectra at the Fe K-edge of Fe/N-GPC and the commercial iron foil and iron phthalocyanine (FePc) references. Fe/N-GPC features a weak preedge peak at 7114 eV similar to that of FePc, which was regarded as a fingerprint of FeN4


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square-planar structure.58,59 The experimental Fourier transforms at the Fe K-edge of extended X-ray absorption fine structure (EXAFS) data of the samples were further analyzed (Figure 3b). The radial structure functions for Fe/N-GPC appear to reflect a combined contribution of the Ncoordinated and metallic carbide Fe. The signal of Fe/N-GPC at 1.5 Å should be ascribed to Fe– N distance due to a nitrogen shell surrounding Fe in reference to that of FePc, corroborating the existence of the FeNx configurations in the graphitic carbon framework.58-60 The signals at 2.0– 2.6 Å and 4.4 Å could be assigned to Fe–Fe and Fe–C interactions.61,62 These results are consistent with the TEM observation. The further information on the local structures has been obtained by fitting the EXAFS data, suggesting the Fe–N shell with a coordination number of 2.7 at a distance of 2.01 Å (Figure S20 and Table S4). Recent studies by Wan and coworkers have demonstrated that the interactions between metallic iron and FeNx coordination structure would favor the adsorption of oxygen molecule, highly improving the ORR activities of the catalysts.60

Figure 4. (a) CVs of Fe/N-GPC and commercial Pt/C in (solid lines) O2- and (dash lines) N2saturated 0.1 M aqueous KOH at 50 mV s–1. (b) LSV curves of Fe/N-GPC, the comparative


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catalysts and Pt/C catalysts at 10 mV s–1 and a rotating speed of 1600 rpm. (c) LSV curves at various rotating speeds and (d) corresponding K–L plots at various potentials of Fe/N-GPC. (e) Kinetic-limiting current density and electron-transfer number of ORR on the different catalysts at –0.45 V. (f) Chronoamperometric responses of Fe/N-GPC and Pt/C in O2-saturated 0.1 M KOH at –0.2 V. The ORR electrocatalytic performance of Fe/N-GPCT and the reference catalysts as well as the commercial Pt/C catalyst was evaluated in 0.1 M aqueous KOH using a three-electrode system. The cyclic voltammograms (CVs) of the catalysts in N2-saturated electrolyte are virtually featureless, while their well-defined cathodic peaks for ORR emerged in O2-saturated electrolyte (Figures 4a, S21 and S22). Notably, Fe/N-GPC afforded the most positive peak potential at –0.16 V versus the Ag/AgCl electrode among all samples, which is even slightly higher than that of Pt/C (–0.17 V), suggesting a superior ORR catalytic activity of Fe/N-GPC. The linear sweep voltammogram (LSV) polarization curves recorded on a rotating disk electrode (RDE) further demonstrate the ORR activities of these catalysts (Figures 4b and S23, Table S5). In agreement with the CV observations, Fe/N-GPC shows an exceedingly high ORR activity with the most positive onset potential of –0.01 V and the highest half-wave potential of –0.13 V, which exceed the values measured with the Pt/C reference by ca. 30 mV and 16 mV, respectively (Figure 4b). By contrast, PC exhibits very poor ORR activity with the onset potential of –0.14 V, while N-PC and Fe-PC with the introduction of additional active sites exhibit moderate improvement in ORR activity with the onset potentials of –0.13 and –0.09 V, respectively. As the N-PC and Fe/N-GPC possess similar texture and N-doping properties, their obvious performance gap validates the important roles of the metal centers for ORR. As compared to Fe-PC, the substantial improvement in activity of Fe/N-GPC should be partly


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attributed to its unique hierarchically porous structure. The difference in ORR activity between the Fe/N-GPCT catalysts may be associated with their difference in porosity, the content of active sites, as well as the degree of graphitization (Figure S23). Furthermore, the sample, which is directly pyrolyzed from a mixture of 2-aminoterephthalic acid, DCD and FeCl3, exhibits the worst ORR performance, demonstrating the significant importance of the MOF for the catalyst preparation (Figure S24). The excellent ORR catalytic performance makes Fe/N-GPC one of the few non-precious catalysts exhibiting superior ORR activities compared to the commercial Pt/C catalysts. More remarkably, as compared with these state-of-the-art noble-metal-free catalysts, larger positive shifts relative to the commercial Pt/C in both onset and half-wave potentials (30 and 16 mV, respectively) have been observed for Fe/N-GPC (Table S6). For further clarify the pathway of ORR for all the catalysts, the RDE measurements were conducted at various rotating speeds under an O2-saturated system (Figures 4c, S25 and S26). Typically, the LSV curves of Fe/N-GPC exhibit the rotating-speed-dependent current densities with a constant onset potential. The corresponding Koutechy–Levich (K–L) plots possess excellent linearity and almost the same slopes over the potential varying from –0.4 to –0.6 V, suggesting the similar electron transfer number per oxygen molecule in ORR at different potentials (Figure 4d). Calculations from the K–L equations reveal an electron transfer number of ~4.0 at –0.4 to –0.6 V for Fe/N-GPC, similar to that of the Pt/C catalyst and much higher than those of other reference catalysts (Figures 4e and S27), suggesting that the ORR for Fe/N-GPC supported electrode was a four-electron transfer process. Meanwhile, Fe/N-GPC achieved the highest kinetic-limiting current density of 19.5 mA cm–2 at –0.45 V among the tested catalysts, which is even higher than that of Pt/C (16.6 mA cm–2). Reasonably, the improved conductivity, abundant and highly active sites and their synergetic interactions should account for the observed


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high catalytic activity of Fe/N-GPC. More importantly, the hierarchical porous structure further significantly increases the accessibility of the active sites and shorten the diffusion path lengths of oxygen and electrolyte to facilitate ORR process. In addition, the improved hydrophilicity of Fe/N-GPC can strengthen the interactions between the aqueous electrolyte and the catalyst surface, and thus facilitate the electrolyte to enter the pores. By contrast, the raw sample of Fe/NGPC without acid etching, of which the mesopores and macropores are blocked severely by the aluminium oxide nanoparticles in situ formed from the MOF as shown in the pore size distribution, shows much lower ORR catalytic activity, confirming the importance of the hierarchical pore structure of the catalyst for ORR (Figure S28). Except for the excellent ORR activity of Fe/N-GPC, it exhibits extraordinary stability as well as methanol tolerance. In contrast to the rapid activity decrease of Pt/C over time probably due to nanoparticle migration, coalescence, and even detaching from carbon support during continuous operation, especially in the alkaline electrolytes,35 only a slight current loss with a high retention of more than 95% after 20 000 s was observed for Fe/N-GPC at a high constant voltage of –0.2 V (Figure 4f). Moreover, Fe/N-GPC shows perfect ORR selectivity in methanol containing electrolyte, whereas a dramatic change in the current density due to the methanol oxidation was detected for Pt/C under the same conditions (Figure S29). These results demonstrate that the Fe/N-GPC exhibits better catalytic performance than the benchmark Pt/C catalyst and even many state-of-the-art noble-metal-free catalysts for ORR, particularly in terms of onset and half-wave potentials and durability (Table S6). Consequently, Fe/N-GPC would be a promising candidate for replacing the precious Pt as a cathode catalyst employed in alkaline fuel cell. Furthermore, the ORR performance of Fe/N-GPC in 0.5 M H2SO4 was assessed (Figure S30). A high ORR catalytic activity with an onset potential of 0.85 V and a half-wave potential of 0.63


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V versus RHE was observed, which is at the same level as the recently reported good results.47,6365

The electron transfer number of Fe/N-GPC was calculated to be 3.62–3.73 at 0.4 to 0.2 V,

suggesting that the Fe/N-GPC electrode also mainly favored an efficient four-electron transfer process in acidic media. Till now, only few noble-metal-free catalysts have shown high ORR performance in both alkaline and acidic media. Taking into account the structures and compositions of the catalysts, the superior ORR performance of Fe/N-GPC is suggested to be cooperatively governed by the following key aspects. (1) The macro- and mesopores in the hierarchically porous textures can serve as a buffering reservoir where the electrolyte can shorten the diffusion distances, while the meso- and micropores with high surface area (1207 m2 g–1) within the ultrathin graphitic carbon layers (< 10 nm) allow the closed interactions between oxygen and fully accessible active sites and provide large electrode–electrolyte interfaces for enhancing the catalytic activity.29 (2) The abundant and uniformly distributed atomic active species and their synergistic effect significantly improve the ORR activity. (3) The high degree of graphitization of the porous carbon nanospheres enhances the electrical conductivity, guaranteeing the fast electron transport through the catalyst during the ORR process. (4) The unique morphology can offer more graphene-like edges, thereby introducing massive active sites for ORR.34 In summary, we demonstrated a strategy for the construction of the hierarchically structured graphitic porous carbon architectures with abundant Fe and N atomic doping from a MOF composite. Such unique open structures possess hierarchical pores with high-density accessible active sites exposed to electrochemical interface, identifying an optimal catalyst for ORR. The unique morphology and atomically dispersed active sites with a strong synergetic effect enable the final material to afford a high activity superior to those of the commercial Pt/C and state-of-


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the-art noble-metal-free catalysts toward ORR in alkaline media. This study also opens up a new avenue for the development of high-performance electrocatalysts with advanced nanostructures from MOFs. ASSOCIATED CONTENT Supporting Information. Experimental procedures, characterization of the MOF composites and catalysts, and electrocatalytic performance of the catalysts. Author Contributions §

These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Q.-L.Z. and W.X. contributed equally to this work. The authors are thankful to the reviewers for valuable suggestions, Dr. Takeyuki Uchida for SEM and TEM measurements, the staff of Beijing Synchrotron Radiation Facility (BSRF) for XANES and EXAFS measurements, and AIST and JSPS (KAKENHI no. 26289379) for financial support. Q.L.Z. thanks JSPS for postdoctoral fellowship. R.Z. thanks the National Natural Science Foundation of China 51322205 and 21371014 for financial support. Z.L. acknowledges Grant-in-Aid for Scientific Research (C) (25390023).

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