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Deutsche Ausgabe: DOI: 10.1002/ange.201708385 Internationale Ausgabe: DOI: 10.1002/anie.201708385
Trifunctional Electrocatalysts
Three-Dimensional Hierarchical Architectures Derived from Surface-Mounted Metal–Organic Framework Membranes for Enhanced Electrocatalysis Gan Jia+, Wen Zhang+, Guozheng Fan, Zhaosheng Li,* Degang Fu,* Weichang Hao, Chunwei Yuan, and Zhigang Zou Abstract: Inspired by the rapid development of metal–organicframework-derived materials in various applications, a facile synthetic strategy was developed for fabrication of 3D hierarchical nanoarchitectures. A surface-mounted metal– organic framework membrane was pyrolyzed at a range of temperatures to produce catalysts with excellent trifunctional electrocatalytic efficiencies for the oxygen reduction, hydrogen evolution, and oxygen evolution reactions.
The intensifying energy crisis and corresponding environ-
mental impacts have stimulated enormous interest in studying various unconventional energy conversion and storage technologies (for example, solar cells, water splitting devices, fuel cells and metal–air batteries). Several basic electrochemical reactions, such as the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are crucial components of these devices.[1] The exploration of efficient catalysts for the above mentioned electrochemical reactions is of paramount importance. To date, platinum catalysts remain the best catalysts for HER and ORR, showing high cathodic current densities at ideal overpotentials. Similarly, less abundant materials, such as ruthenium- and iridium-based compounds, are highly active OER catalysts. However, the prohibitive costs of precious metals and their scarcity pose limitations to the widespread use of these catalysts. Accordingly, various cost-effective and robust substitutes composed of earth-abundant elements[2] have been developed to effectively boost reaction kinetics [*] G. Jia,[+] G. Fan, Prof. Z. Li, Prof. Z. Zou Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University Nanjing 210093 (P. R. China) E-mail:
[email protected] W. Zhang,[+] Prof. D. Fu, Prof. C. Yuan State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University Nanjing 210096 (P. R. China) E-mail:
[email protected] Prof. W. Hao Department of Physics and Key Laboratory of Micro-nano Measurement, Manipulation and Physics, Beihang University Beijing 100191 (P. R. China) [+] These authors contributed equally to this work. Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/anie.201708385. Angew. Chem. 2017, 129, 13969 –13973
and further reduce energy consumption through accelerated electrochemical processes. However, exploiting trifunctional electrocatalysts to simultaneously satisfy the three aforementioned reactions remains an enormous challenge. Efficient multifunctional catalysts with respectable electrochemical performances suitable for broad application are undoubtedly needed, and would simplify operation and reduce manufacturing costs. Metal–organic frameworks (MOFs) are the most rapidly growing class of crystalline porous materials because it is possible to control their porous properties (pore volumes and pore size), chemical composition, and functionalities.[3] These structural features have motivated investigations of MOFs as ideal platforms for a variety of applications in gas storage, chemical separations, sensing, biomedicine, and catalysis.[4] Nevertheless, studies on typical bulky MOF powders may have limited importance for practical applications. Coating the desired MOF crystals onto functionalized/pretreated substrate surfaces can provide an alternative avenue for further development of MOF materials. The deposition of MOF coatings onto various solid substrates (including semiconductors, metals/metal oxides, and polymer surfaces) and the resulting thin-film MOFs (referred to as surface-mounted MOFs or SURMOFs), have been extensively studied and further developed for many advanced applications.[5] More recently, MOFs have been employed as effective sacrificial templates or starting materials for constructing nanoarchitectures with versatile properties;[6] such strategies are effective for enhancing the charge-transport abilities of the obtained materials, and capitalize on the innate properties of MOFs. A cobalt-based MOF (ZIF-67) was converted into a highly graphitic nitrogen-doped carbon structure with catalytic Co-Nx moieties and a uniform nitrogen distribution.[7] Unfortunately, a lack of rational nanostructure design of the MOF-derived catalyst resulted in undesired aggregations of the metal nanoparticles and carbon frameworks during the pyrolysis processes at an elevated temperature, leading to steric blockage of active sites and impeding ion diffusion into the porous structure. To alleviate this problem, SURMOF-derived materials were also developed,[8] which combined the advantages of the MOF-derived electrocatalytically active material with the desired geometrical architecture and enabled rapid charge transfer to the current collector. Herein, we present a method for the scalable fabrication of 3D hierarchical architectures simply by pyrolysis of a ZIF-67 thin film on a 3D macroporous polymeric substrate (Figure 1). Using this approach, we produced vertically
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Figure 1. Illustration of the synthesis process for the MSZIF-T electrocatalysts.
aligned nitrogen-doped carbon nanotubes (NCNTs) derived from a SURMOF as active and stable trifunctional electrocatalysts for ORR and overall water splitting (OER and HER). The new strategy introduced herein offers new prospects for the rational design and synthesis of SURMOF-derived materials with unprecedented electrocatalytic activities and stabilities, which will broadly impact energy-related research. Melamine sponge (MS) is a well-known thermosetting resin produced by a two-stage reaction of melamine and formaldehyde (Supporting Information, Figures S1 and S2).[9] A MS with a 3D macroporous structure as the supporting scaffold provides an ideal platform to promote the nucleation and growth of MOF crystals. Irrespective of the deposition method, the key factor for the growth of a homogeneous and high-quality SURMOF membrane is the rational modification of the surface properties of the platform, which dominate the interactions between the MOF structure and the substrate surface. A suitable surface roughing functionalization was required to achieve successful grafting of ZIF-67 onto the surface of a MS. We determined that a NaOH-treated MS (Supporting Information, Figure S3) acted to promote ZIF-67 growth by stabilizing cobalt ions on the hydroxyl-groupterminated organic surface. After growth of the nucleated crystals, the surface of the functionalized MS support was completely covered with well-intergrown ZIF-67 crystals, and no obvious large-scale vacancies were visible (Supporting Information, Figures S4 and S5). Subsequently, SURMOF nanocomposites were prepared by thermal treatment of the ZIF-67 thin film on MS at different temperatures and under a nitrogen atmosphere; the resulting samples were named MSZIF-T (T = thermal treatment temperature). In the field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images (Figure 2; Supporting Information, Figures S6–S9), a substantial transformation of the surface morphology was observed upon increasing the calcination temperature (700, 800, 900, and 1000 8C). TEM images of MSZIF-700
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Figure 2. a–d) FESEM images of MSZIF-900 with various magnifications; e) FESEM energy-dispersive X-ray mapping of MSZIF-900; TEM images of f) MSZIF-700 (scale bar: 50 nm), g) MSZIF-800 (scale bar: 100 nm), h) MSZIF-900 (scale bar: 200 nm), and i) MSZIF-1000 (scale bar: 200 nm); j–m) amplified views of the morphologies (scale bar: 5 nm).
and MSZIF-800 show the formed cobalt species homogenously embedded in the graphitic carbon matrix, whereas MSZIF-900 is characterized by finely distributed cobalt metal centers within the apex of ordered NCNTs. Upon increasing the pyrolysis temperature to 1000 8C for MSZIF-1000, a tubular NCNTs structure grew in abundance, and smaller metal nanoparticles were grown. The tubular structure tended to intertwine to form larger aggregates. The light weight of the platform allowed a flower to support a 6 cm3 MSZIF-900 sample without obvious deformation (Supporting Information, Figure S10). The X-ray diffraction (XRD) patterns in Figure S11 (Supporting Information) revealed that the structures of the SURMOF thin films agreed well with the simulated patterns of the corresponding bulk materials. The pyrolysis temperature also played an important role in enhancing the crystallinity of the carbon framework. Graphitization was confirmed by intense XRD peaks upon increasing the temperature from MSZIF-700 to MSZIF-1000 (Supporting Information, Figure S12). After pyrolysis, the typical diffraction peaks at about 4488 and 5188 correspond to the (1 1 1) and (2 0 0) planes, respectively, of face-centered cubic cobalt (JCPDS No. 15-806), and the peak at approximately 2688 is indexed to the (0 0 2) crystal plane of graphitic carbon.[10] The Raman spectrum further provided support for the formation of graphitized carbon, where D and G bands were located at 1359 and 1590 cm@1, respectively (Supporting Information, Figure S13).[11a] The G band corresponds to the first-order scattering of the E2g vibrational mode of graphite, while the D band is attributed to the A1g mode. Specifically, the intensity ratio of D and G bands (ID/IG) decreased as the pyrolysis temperature increased, indicating that a higher
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operating temperature facilitates the formation of larger as well as a larger diffusion-limited current density (5.0 mA cm@2) than the other MS-based materials (Figdomains of the ordered graphitic structure.[12] Furthermore, the microscale roughness of the surface and the unique ure 3 b). Although the onset potential of MSZIF-900 was chemical characteristics led to a hydrophobic surface for slightly more negative than that of commercial Pt/C (0.93 V MSZIF-900 (Supporting Information, Figure S14, Movie S2). vs. RHE), the half-wave potential was identical to that of Pt/C X-ray photoelectron spectroscopy (XPS) was performed to (0.84 V vs. RHE). Additionally, the ORR activity of MSZIFprobe the features of the nitrogen species in these materials. 900 was also comparable to the best previously reported The spectra in Figure S15a (Supporting Information) demonresults of non-noble metal ORR catalysts in alkaline solutions strate that nitrogen was successfully incorporated into the as(Supporting Information, Table S1). ORR polarization curves prepared samples. The high-resolution N 1s spectrum of each at different rotation rates are plotted in Figure 3 c to extract fabricated MSZIF-T sample was further deconvoluted into the pertinent kinetic parameters. The corresponding Koufour peaks corresponding to oxidized N (ca. 404.1 eV), pyridinic N (ca. 398.4 eV), pyrrolic N (ca. 400.1 eV), and graphitic N (ca. 401.1 eV) (Supporting Information, Figure S15b). Generally, pyridinic N is regarded as the active component to enhance the electron-donating capability; it is believed that pyridinic N facilitates ORR, in particular by improving the onset potential. Additionally, graphitic N is believed to increase the limiting current density.[11] The relative contents of the various nitrogen species were calculated based on the XPS peak areas (Supporting Information, Figure S15c), and indicated that MSZIF-900 possesses a relatively balanced proportion of pyridinic N and graphitic N. Evolution of Co 2p is observed by XPS for MSZIF samples undergoing pyrolysis under temperature-dependent measurement (700–1000 8C; Supporting Information, Figure S16). The well-graphitized carbon layers intimately interact with entrapped cobalt species, leading to the occurrence of interfacial electron transfer between the NCNTs and cobalt species (Supporting Information, Figure S17).[13] In an attempt to further probe the chemical and structural characteristics in MSZIF900, X-ray absorption fine structure (XAFS) measurements were performed (Supporting Information, Figure S18). Subsequently, we examined the electrocatalytic performances of the as-mentioned catalysts. Comparing the clearly enhanced cathodic peak in the O2-saturated solution with that in the N2-saturated solution indicates the substantial ORR activity of this material (Figure 3 a). Linear sweep voltammetry (LSV) was conducted in an Figure 3. a) Cyclic voltammograms of MSZIF-900 in N2- and O2-saturated 0.1 m KOH. O2-saturated 0.1m KOH solution using b) ORR polarization curves for the various electrocatalysts at a rotation speed of 1600 rpm. a rotating disk electrode (RDE) to further c) LSVs of MSZIF-900 under different rotating rates of the disk electrode. d) The correspondevaluate the ORR kinetics of the various ing Koutecky–Levich plots at different potentials. e) RRDE voltammograms and f) plots of the hydrogen peroxide yields and electron-transfer numbers for MSZIF-900 and Pt/C. materials. MSZIF-900 displays a more posg) Chronoamperometric responses of the MSZIF-900 and Pt/C catalysts over 18 h in O2itive onset potential (0.91 V vs. RHE; saturated 0.1 m KOH at a rotation rate of 1600 rpm. Unless otherwise noted, all measureRHE = reversible hydrogen electrode) ments were conducted in O2-saturated 0.1 m KOH solution at a scan rate of 5 mVs@1; glassy and half-wave potential (0.84 V vs. RHE) carbon (GC). Angew. Chem. 2017, 129, 13969 –13973
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Zuschriften tecky–Levich (K-L) plots derived from RDE measurements are shown in Figure 3 d from which the electron-transfer number (n) was calculated to be approximately 3.5–3.9 for MSZIF-900, suggesting a predominant 4-electron pathway for ORR. Consistent results were obtained in the rotating ring disk electrode (RRDE) measurement (Figure 3 e). The H2O2 yield was below about 10 % over the potential range of 0.2 to 0.9 V, suggesting n & 3.83 (Figure 3 f). As well as high ORR performance, the MSZIF-900 catalyst also exhibited longterm stability (Figure 3 g) and a superior methanol tolerance compared to commercial Pt/C for ORR (Supporting Information, Figure S19). Together with the remarkable performance in alkaline media, MSZIF-900 demonstrated decent ORR activity in acidic media (Supporting Information, Figures S20 and S21, and Table S2). The satisfactory performance of the MSZIF-900 catalyst may be attributed to its unique hierarchical structure. The NCNTs enhanced electrolyte-ion transport and the cobalt species on top of the nanotubes captured additional ORR-related species. Nevertheless, a further elevated pyrolysis temperature (MSZIF1000) resulted in reduced ORR activity because of insufficient active sites on the agglomerated NCNTs and the undersized metal species. Based on the encouraging performance of the MSZIF-900 electrocatalyst in ORR, we logically examined the performances of the above materials in water splitting. The HER activities of the as-fabricated electrocatalysts were evaluated in 0.5 m H2SO4 electrolyte using a three-electrode system (Figure 4 a). MSZIF-900 exhibited an excellent HER activity, as noted by the rapid increase of the cathodic current density with an increasing overpotential, reaching a geometric current density of 10 mA cm@2 at an overpotential of only about 233 mV (Figure 4 b). To assess the durability of MSZIF-900, a galvanostatic polarization measurement at a cathodic current density of 10 mA cm@2 was also performed (Supporting Information, Figure S22). The results revealed that the as-
Figure 4. Electrochemical performance of the MSZIF-900 catalyst for a) HER and c) OER with iR compensation. b, d) Corresponding overpotentials at a geometric current density of 10 mA cm@2.
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prepared MSZIF-900 was a stable electrocatalyst for HER in acidic electrolyte. In addition to the ORR and HER electrocatalytic activities observed above, the MSZIF-900 catalyst also displayed an impressive performance for OER (Figure 4 c). In 1m KOH, the MSZIF-900 electrocatalyst exhibited an overpotential of 337 mV to reach a current density of 10 mA cm@2 with iR compensation, representing a higher activity than the other three electrocatalysts (Figure 4 d). The stability of MSZIF-900 was also investigated (Supporting Information, Figure S23). To investigate the electrode kinetics during the OER process, electrochemical impedance measurements were performed from 100 MHz to 0.1 Hz at a fixed overpotential (Supporting Information, Figure S24). Unsurprisingly, a smaller charge-transfer resistance was displayed for MSZIF-900 than the other three catalysts, in accordance with the lower overpotential of MSZIF-900. For MSZIF-900 and MSZIF-1000, the depressed semicircles, which deviated from ideal semicircles, are attributed to the rough surfaces of the as-fabricated catalysts. To better understand the reason behind the remarkably enhanced electrocatalytic activity of the MSZIF-900 catalysts for OER, the electrochemically active surface area (ECSA) was obtained from cyclic voltammetry (CV) measurements in the non-Faradaic region and assessed by the double-layer capacitance. The capacitance values were estimated by plotting DJ = (Ja@Jc ; where Ja = anodic current density and Jc = cathodic current density) at 0.20 V versus Hg/HgO as a function of the scan rate, and were determined to be approximately half of the linear slope. The results in Figure S25 (Supporting Information) indicate that MSZIF-900 possesses an approximately three-fold greater slope than those of the other catalysts. The notable increase in the active surface area at a similar catalyst loading is likely due to the unique structural design of MSZIF-900 relative to that of the other three SURMOFderived catalysts. To understand the catalytically active sites and the contribution of the embedded cobalt species in the OER, control experiments were performed (Supporting Information, Figure S26). The results of density functional theory calculations for the OER mechanism of the MSZIF900 (Supporting Information, Figures S27–S29) further highlight the synergetic coupling between the cobalt and nitrogendoped carbon in generating a favorable surface electronic environment and consequently promoting the electrochemical performance. In summary, we successfully demonstrated a novel synthetic protocol for the fabrication of SURMOF-derived nanoarchitectures, in which the cobalt-incorporated NCNT structures were aligned vertically on 3D macroporous foams resulting from pyrolysis at a certain temperature. To the best of our knowledge, this study is the first report on the utilization of this approach for the controllable growth of cobalt species embedded in vertical NCNTs. The hybrid electrocatalysts exhibited excellent trifunctional electrocatalytic activity for ORR, HER, and OER because of the unique nanomaterial architectures with abundant active sites and effective mass transport. This general synthetic protocol can be extended to the design and synthesis of other SURMOF/MOF-derived systems for various applications.
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Zuschriften Acknowledgements This work was supported by the National Basic Research of China (973 Program, 2013CB632404) and the National Natural Science Foundation of China (Grant Nos. 21473090 and U1663228).
[5]
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Manuscript received: August 15, 2017 Accepted manuscript online: September 4, 2017 Version of record online: September 25, 2017
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