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Xiaodong Zhuang, Dominik Gehrig, Nina Forler, Haiwei Liang, Manfred Wagner, Michael Ryan Hansen, Frédéric Laquai, Fan Zhang,* and Xinliang Feng* Porous polymers,[1] a type of polymeric material containing numerous pores, possess high specific surface areas and tailorable chemical compositions, and are easily available through many classical polymerization methods and from many organic monomers. Covalent organic networks,[2] polymer aromatic frameworks,[3] polymers with intrinsic microporosity,[4] covalent organic frameworks,[5] and conjugated microporous polymers (CMPs)[6] and ionic polymer networks[7,8] are typical porous polymeric materials featuring high porosity, light weight, and structural variations. However, due to the lack of proper synthetic protocols, most of the reported porous polymers present nanosphere or monolith architectures due to their rapid precipitation from reaction solvents. Mini-emulsion polymerization[9] and silica-based hard-template methods[10] have been used for the construction of microporous polymer networks with quasi0D structures. However, these protocols produced the desired morphology for porous polymers at the price of a significantly decreased specific surface area.[11] Porous polymers with a large specific surface area and a high carbon yield[12] can serve as precursors for preparing heteroatom-doped porous carbons, which hold great promise as metal-free catalysts for the oxygen reduction reaction (ORR),[13] and as advanced electrodes for supercapacitors[13] and other clean-energy devices.[14] Clearly, the morphologies and microstructures of porous polymers and the resultant porous carbons exert significant impact on the performance of energy devices.[15] Among porous polymers, CMPs are a class of amorphous materials[16] that permit the linking of building blocks in Dr. X. Zhuang, Prof. F. Zhang, Prof. X. Feng School of Chemistry and Chemical Engineering Shanghai Jiao Tong University 200240 Shanghai, P. R. China E-mail: [email protected]; [email protected] D. Gehrig, Dr. N. Forler, Dr. H. Liang, Dr. M. Wagner, Dr. M. R. Hansen, Dr. F. Laquai Max-Planck-Institute for Polymer Research Ackermannweg 10, 55128 Mainz, Germany Dr. M. R. Hansen Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry Aarhus University Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark Prof. X. Feng Center for Advancing Electronics Dresden (cfaed) and Department of Chemistry and Food Chemistry Technische Universität Dresden Mommsenstrasse 4, 01062 Dresden, Germany
DOI: 10.1002/adma.201501786
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a π-conjugated fashion and possess continuous networks. Such structural features are unique and usually not available in other porous materials. CMPs have attracted significant research interests in optoelectronics because numerous photoelectrical building blocks can easily be made available. Most of the reported works have focused on p-type building blocks, such as carbazole and tetrakisphenylethene, and corresponding CMPs’ photoluminescence properties.[17–19] The introduction of electron-accepting (n-type) species will result in charge/energy transfer between p/n components, which will greatly expand the application of CMPs, such as for light-emitting and photovoltaic devices and nonlinear optics. To realize this, one efficient way is to physically fill or chemically bind an n-type molecule, such as fullerene, into p-type-conjugated polymer networks. The resulting covalent/conjugated organic frameworks-based on donor–acceptor heterostructures have shown very promising application for charge separation and photoenergy conversion.[20,21] However, the acceptor molecules in these systems are all present in pore channels, resulting in a significant decrease in the specific surface area. In this study, we demonstrate a series of unprecedented p/n heterostructures comprising CMPs (acting as donors, and thus denoted as D) and nanocarbons (acting as acceptors, and denoted as A) with controlled dimensionalities of 0D, 1D, or 2D, employing bromo-functionalized carbon nanospheres (0D CSs), single-walled carbon nanotubes (1D SWNTs), and reduced graphene oxide (2D RGO) as structurally directing templates. Halogenated BODIPY (M1: 2,6-diiodo-1,3,5,7-tetramethyl8-phenyl-4,4-difluoroboradiazaindacene) was chosen as the key monomer for polymerization with 1,3,5-triethynylbenzene (M2) through the metal-catalyzed Sonogashira–Hagihara coupling reactions on the template surface. The resulting CMPs had well-defined nanosphere, nanotube, and nanosheet morphologies, inheriting the original architectures of the nanocarbons. The specific surface areas of these dimensionality controlled CMPs (593–622 m2 g−1) were higher than the CMP (574 m2 g−1) obtained without use of a template. The H2 isosteric heats for these CMPs (Qst = 7.7 ± 0.2 kJ mol−1) at zero coverage were unaffected by the nanocarbon templates. Importantly, an effective inverse decay rate (τeff) reduction of 38–46% for nanocarbon-directed CMPs in comparison with that of amorphous CMP was observed, indicating a strong electronic interaction between the nanocarbons and the polymer networks, with the most pronounced interaction observed for CMP-2D, followed by CMP-1D, then CMP-0D (CMP-2D > CMP-1D > CMP-0D). Further, the CMP-nD were readily converted into boron/nitrogen (B/N) co-doped porous carbons with maintained 0D/1D/2D morphologies by direct pyrolysis. The porous
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Conjugated Microporous Polymers with DimensionalityControlled Heterostructures for Green Energy Devices
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carbons obtained exhibited high specific surface areas (up to 573 m2 g−1), and high B/N doping contents (N = 5.1–8.6 wt%, B = 0.6–0.9 wt%), leading to an efficient catalytic performance (half-wave-potential = 0.71 V vs the reversible hydrogen electrode (RHE), diffusion limiting current density ≈6.0 mA cm−2, four-electron transfer mechanism) for the ORR. The reliable structure–property relationship 2D > 1D > 0D was established between the electrochemically catalyzed ORR and the nanostructures. As an air electrode, these nanocarbon-based zinc–air batteries manifested strong relationship between morphology and performance (2D > 1D > 0D), and the current density and peak power density for 2D porous carbon reached 23.9 and 14.6 mW cm−2 at 0.61 V, respectively. Thereby, these unique structural features of porous polymers with controlled dimensions provide a new insight into porous materials for multiple functions. The strategy for the synthesis of CMPs with different dimensionalities is presented in Figure 1, using CSs, SWNTs, and RGO as the 0D, 1D, and 2D templates, respectively. The preparation of CSs (≈7 nm) is demonstrated in Scheme S1
(Supporting Information). First, the nanocarbon templates were functionalized by treatment with a p-bromobenzene diazonium salt in water. The obtained bromo-functionalized carbon templates (CS-Br, SWNT-Br, and RGBr) were highly dispersible in various organic solvents, such as dimethylformamide (DMF) (≈1.0 mg mL−1 for all samples). Next, halogenated BODIPY (M1) was mixed with 1,3,5-triethynylbenzene (M2) and the functionalized nanocarbon in anhydrous DMF along with catalytic amounts of [Pd(PPh3)4] and CuI. The reaction mixture was sealed and stirred under Sonogashira–Hagihara reaction conditions. After 72 h, the precipitated black solids were collected and purified by Soxhlet extraction. Finally, 0D ball-like, 1D stick-like, and 2D sandwich-like CMPs, denoted as CMP-0D, CMP-1D, and CMP-2D for the CS-, SWNT-, and RGO-containing polymers, respectively, were obtained in high yields (≈90 wt%) after vacuum drying. For comparison, amorphous-type CMP (CMP-Amor) was also prepared by the same procedure without the use of a nanocarbon template. As polymerization on the template surfaces proceeds, polymer networks generally precipitate from the solvent;
Figure 1. Dimensionally controlled synthesis of BODIPY-based CMPs and the corresponding B/N co-doped porous carbons employing nanocarbons as templates. i) Argon, Pd(PPh3)4, CuI, Et3N, DMF, 80 °C, 3 d; ii) 4-bromobenzenediazonium tetrafluoroborate, 0 °C to room temperature, 2 h; iii) argon, M1, M2, Pd(PPh3)4, CuI, Et3N, DMF, 80 °C, 3 d; iv) argon, pyrolyzed at X °C (X = 700, 800, and 900) for 2 h. BN, boron, and nitrogen co-doped porous carbon.
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COMMUNICATION Figure 2. Structural and morphology characterization of CMP-nD. a–d) Solid-state 13C{1H} CP/MAS NMR spectra of: CMP-2D (a), CMP-1D (b), CMP-0D (c), CMP-Amor (d), and M1 (e), recorded using 25.0 kHz MAS and high-power 1H and 19F decoupling during acquisition. Assignments follow the chemical structures shown in (a) and (e). f–k) SEM (left) and TEM (right) images of: CMP-0D (f,i), CMP-1D (g,j), and CMP-2D (h,k).
therefore, the degree of polymerization and the presence of terminal groups may affect the porosity of the resulting materials. Initially, solid-state 13C{1H} cross-polarization/magicangle spinning nuclear magnetic resonance (CP/MAS NMR) spectroscopy was employed to analyze the molecular structure and the influence of the carbon template on the resulting CMPs (Figure 2). For the pure monomer M1 (Figure 2e), the spectrum includes narrow and sharp 13C signals, suggesting a crystalline structure with well-defined geometries.[22] Remarkably, the spectrum only includes about half the number of 13C resonances expected from M1 (see the inset in Figure 2d and Figures S1, S2, Supporting Information).[23] This is attributed to one distinct molecular conformation of the phenylene moiety, leading to a highly symmetric, extended aromatic system.[24] The reaction of M1 with M2 leads to a clear highfield shift of ≈4–7 ppm for the methyl groups at positions 14 and 15 (cf. Figure 2a–d). Moreover, cross-linking with the nanocarbons resulted in a signal decrease accompanied by an increase in the 13C line width. This is a consequence of the tightly cross-linked CMP materials, where free radicals from the nanocarbons induce signal loss and line broadening for the 13C nuclei of the polymer network (more effective T2 relaxation mechanism).[25] However, all CMP-nD materials (Figure 2a–d) show similar structural information in comparison with that of CMP-Amor (Figure 2e), indicating that the incorporation of
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nanocarbon templates did not affect the polymerization reaction significantly. By comparing the Fourier transform infrared (FTIR) spectra of CMP-Amor and CMP-nD (n = 0, 1, 2), a peak with a very low intensity at 2110 cm−1 that corresponds to the alkyne C C stretch was observed. The intense alkynyl C H stretching near 3300 cm−1 almost disappeared in the spectra (Figure S3, Supporting Information). These results are suggestive of a high degree of polymerization in the carbon templatemediated synthesis of CMPs. The chemical nature of the CMPs was further investigated by X-ray photoelectron spectroscopy (XPS). All four CMPs showed almost the same heteroatom components. Typically, the N1s, B1s, and F1s spectra of CMP-2D are presented in Figure S4 (Supporting Information). The peaks at the binding energies of 399.5, 191.6, and 685.6 eV can each be fitted by only one component, which can easily be attributed to the N and B species of the N B bond and the F species of the B F bond, respectively (atomic ratio for N/B/F: 2.00/0.43/2.07). The morphologies of the as-prepared CMPs were inspected by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 2f–k, many freestanding ball/stick/sheet-like nanomaterials with morphologies similar to those of nanocarbon templates can be recognized. Based on the same ratio of nanocarbons (65 mg) to monomers (M1, 800 mg, 1.5 mmol; M2, 200 mg, 1.5 mmol), the obtained
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nanospheres (CMP-0D) had a diameter of 20 ± 4 nm (Figure 2f,i); in contrast, CMP-1D had a diameter and length of 150 ± 30 nm and 0.6–2 µm, respectively (Figure 2g,j); and the nanosheet (CMP-2D) had a lateral size of between 1 × 1 and 4 × 4 µm2 and a thickness of about 80 ± 10 nm. No free porous polymer particles or naked carbon templates can be observed in either the TEM or SEM visualizations, suggesting that most of the monomers have been polymerized on the nanocarbon surface. The control sample CMP-Amor prepared without use of a template exhibited the amorphous nanoparticle (≈10–20 nm) structure (Figure S5, Supporting Information). Notably, the thickness of the porous polymer shells on nanocarbons could easily be adjusted by controlling the template/ monomer ratio. As shown in Figure S6 (Supporting Information), CMP-1D with three different diameters, 35 ± 5, 100 ± 15, and 210 ± 20 nm, were obtained by adjusting the template/monomer ratio of 65 mg/0.5 mmol, Figure 3. Time-resolved photoluminescence. a) Emission spectra and b) concentration profiles 65 mg/1.0 mmol, and 65 mg/1.5 mmol, of component 1 (filled symbols) and component 2 (open symbols) from MCR-ALS analysis of emission data. respectively. Consequently, these results highlight the crucial role of the carbon template as a substrate for directing the morphology of the dynamics of components 1 and 2 of CMP-nD and CMP-Amor resultant CMP in a 0D, 1D, and 2D manner. were described with a biexponential decay function. The emisThe porous nature of CMP-nD (n = 0, 1, 2) was confirmed sion of component 1 is attributed to a monomer-like emission, by nitrogen physisorption measurements. It was found that which is in agreement with the previously published diethynylthe isotherms of all the CMP-nD variants and CMP-Amor are BODIPY dyes with additional electron-donating groups exhibtype I (Figures S7–10, Supporting Information). The Brunauer– iting emission maxima up to 659 nm.[26] Moreover, the emisEmmet–Teller (BET) surface areas (SBET) of these polymer netsion of component 2, which appears strongly redshifted with respect to component 1, is consequently assigned to polymeric works varied between 574 and 622 m2 g−1 (Table S1, Supporting states distributed over a higher number of chromophores. Information) and had a similar pore diameter of 1.6 nm, which is a typical microporous ( 4.94 > 3.98 mA cm−2) and the electron transfer number (3.85 > 3.69 > 3.03) for 2DBN-800, 1DBN-800, and 0DBN-800 exhibit the strong relationship between dimensionality and electrochemical catalyzed ORR performance. The improved catalytic performance for 2D porous carbons is believed to originate from the large lateral and layer-to-layer conductive feature (several micrometers) in comparison with the dot-to-dot and line-to-line conductive pathway of 0D and 1D porous carbons, respectively (Figure 5). This result is also consistent with the above time-resolved photoluminescence studies of the dimensionality controlled CMP precursors. Although the HWP for 2DBN-800 is ≈39 mV lower than for Pt/C, the mainly four-electron-transfer pathway (n ≈ 3.85 at 0.5 V, see Figure 4c) calculated from the rotating ring–disk electrode (RRDE, Figure S15, Supporting Information) curve and Equation S3 (Supporting Information), together with the ultrahigh JDL, makes 2DBN-800 promising as an electrocatalyst for the ORR. The mean number of transferred electrons per O2 molecule involved in the ORR, which can be determined from the K–L relation (Equation S1, Supporting Information), was approximately 4.0 between 0 and 0.5 V. These results highlight that the ORR proceeds via a primary four-electron pathway. In addition to their excellent ORR performance, nDBN-800 and AmorBN-800 were further employed as the air cathode in Zn–air batteries. To construct primary Zn–air batteries, the ORR electrocatalysts discussed above were loaded onto carbonfiber paper as the air cathode, and paired with a Zn foil in 6 M KOH. The battery had an open-circuit voltage of ≈1.40 V. At a voltage of 0.8 V, AmorBN-800, 0DBN-800, 1DBN-800, and 2DBN-800 showed current densities of approximately 7, 5, 15, and 17 mA cm−2, respectively. The peak power densities for AmorBN-800, 0DBN-800, 1DBN-800, and 2DBN-800 at 9.7, 6.7, 21.2, and 23.9 mA cm−2 reached 6.9, 4.8, 13.2, and 14.6 mW cm−2, respectively (2D > 1D > 0D, Figure 4d). The current densities at 0.8 V and peak power densities for the 1D and 2D (but not 0D) porous carbons were significantly higher than the porous carbon prepared without use of a template. This result indicates that 1D and 2D porous carbons capable of longdistance charge transport contribute more to oxygen reduction on an air-diffusion layer than porous carbons with confined 0D morphologies. Thus, this dimensionality-related catalytic performance offers a new platform for developing metal-free air cathodes for Zn–air batteries. The as-prepared porous carbons were next investigated as air catalysts for rechargeable Zn–air batteries. First, these materials were mixed and loaded onto a single cathode for charge and discharge cycling experiments (Figure 4e and Figure S16, Supporting Information). The electrolyte was 6 M KOH with 0.2 M zinc acetate (dissolved in KOH to form zincate, Zn(OH)42−) to
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hand, long-distance transport in graphene (which exceeds the transport in SWNT and CS) is another factor determining electronic communication in CMPs. In line with this argument is the steady redshift of the emission maximum for the series CMP-0D, CMP-1D, and CMP-2D with the strongest redshift for CMP-2D. This result (0D < 1D < 2D) is most likely due to a gradually increasing charge-transporting distance and electronaccepting properties for CS, SWNT, and graphene. Various clean energy devices, such as Li-ion batteries, supercapacitors, and fuel cells, have been developed for portable electronics and vehicle electrification. However, developing novel devices with higher energy/power densities, such as metal–air batteries,[27–29] Li–sulfur batteries,[30,31] etc.[32,33] is a major challenge. From a materials perspective, carbon is low cost, precursor selectable, component controllable, and nanostructure processable; therefore, varieties of carbon-based nanomaterials have been developed for application in these new energy devices, including hollow carbon spheres, carbon nanotubes, and graphene-based sheet-like or sponge-like carbons.[34] Dimension-controlled porous carbon materials may provide new insights into the mechanism of these applications, and help in the development of high-throughput energy devices. Given that CMPs are carbon-rich precursors, the asprepared CMPs (CMP-Amor and CMP-nD) were pyrolyzed at X °C (X = 700, 800, 900, 1000) for 2 h under an argon atmosphere, affording boron and nitrogen co-doped porous carbons (denoted AmorBN-X and nDBN-X, n = 0, 1, 2, respectively, see Figure 1). The chemical nature of AmorBN-X and nDBN-X was investigated by XPS. Owing to the similar results for these four porous carbons, only 2DBN-700/800/900 are illustrated here (Figure S4, Supporting Information). For 2DBN-X, the major contributions at binding energies of around 398.4 and 400.8 eV can be attributed to the nitrogen species of the C B N and C N C bonds[35–37] (which should be pyrrolic N), respectively. The B1s spectra can be deconvoluted into two different peaks at binding energies of 190.5 and 191.4 eV, corresponding to the B species in B N and B C N bonds, respectively. Another minor contribution at the higher binding energy of around 184.0 eV can be assigned to the double-nitrogen-bonded boron (N B N) moieties in which the nitrogen originates from the BODIPY blocks. This strategy thus provides a feasible way to build up 0D-, 1D-, and 2D-porous carbons with a high level of boron and nitrogen doping content (Figure S4, Supporting Information: B = 0.6–0.9 wt%, N = 5.1–8.6 wt%). B/N co-doped carbon materials[35–37] have demonstrated promising electrocatalytic performance and showed advantages over other single-heteroatom-doped carbon materials due to the synergistic effects from boron and nitrogen. The electrocatalytic activities of the B/N co-doped porous carbons prepared in this study in the ORR under alkaline conditions (0.1 M KOH) were examined. The ORR catalytic activity of 2DBN-X (X = 700, 800, 900, 1000) was first evaluated by cyclic voltammetry (CV, Figure S13, Supporting Information). It was found that the ORR peak potential of 0.69 V versus RHE for 2DBN-800 was slightly higher than the results for the other three porous carbons. Koutecky–Levich plots (K–L plots, Figure S14e,f, Supporting Information) with a good linear relationship for the 2DBN-X electrodes were calculated from linear sweep voltammetry
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Figure 4. The ORR and Zn–air battery performance of the catalysts. a) LSV curves of 2DBN-X (X = 700, 800, 900, 1000) in O2-saturated 0.1 M KOH solution at 1600 rpm at a scan rate of 5 mV s−1. b) LSV curves of AmorBN-800, 0DBN-800, 1DBN-800, 2DBN-800, and Pt/C in O2-saturated 0.1 M KOH solution at 1600 rpm at a scan rate of 5 mV s−1. c) Electron-transfer number for AmorBN-800, 0DBN-800, 1DBN-800, and 2DBN-800 as a function of the electrode potential. d) Polarization curve (V–i) and the corresponding power density plot of the battery using AmorBN-800, 0DBN-800, 1DBN-800, and 2DBN-800 as the cathode catalysts. e) The primary Zn–air battery. f) Discharge and charge cycling of 2DBN-800. g) A real Zn–air device after 48 h charge and discharge cycles. h) An LED (3.4 V rated voltage) powered by two Zn–air devices using 2DBN-800.
The battery eventually ceased functioning when all the Zn metal was consumed. The as-prepared metal-free ORR catalyst is ideally suited for Zn–air batteries by replacing the electrolyte and offering fresh Zn, owing to its exceptional ORR activity and durability.[39] In summary, a novel nanocarbon (carbon nanosphere, carbon nanotube, and graphene)-inspired strategy for the synthesis of 0D, 1D, and 2D CMPs and B/N co-doped porous carbons with high specific surface areas has been demonstrated. Dimensionality has been shown to have a significant influence on CMPs in respect of photoluminescence. An effective inverse decay rate (τeff) reduction of 38–46% for nanocarbondirected CMPs in comparison with that of amorphous CMP was observed, indicating electronic interactions between n-type nanocarbons and p-type polymer networks. The porous carbons obtained herein exhibited high B/N doping contents and efficient catalytic performance for the ORR, superior to state-of-the-art noble-metal catalysts. Remarkably, a reliable structure–property relationship (2D > 1D > 0D) was established between the electrochemical catalyzed ORR and the dimensionality. It is expected that the synthetic strategy as well as the structure– property relationship in this work will offer opportunities for the templated synthesis Figure 5. Conductive architectures for nanocarbon-directed 0D, 1D, and 2D porous carbons.
ensure reversible Zn-involved electrochemical redox reactions at the anode.[29,38] Such batteries exhibit cycling stability when charged and discharged galvanostatically at low current densities (5–20 mA cm−2) and short cycle times (300 s per charge or discharge period). As an example, the charge–discharge curve for 2DBN-800 is presented in Figure 4f. Importantly, when repeatedly charged or discharged at 20 mA cm−2 for a total of 11 h with a 5 min cycling period, the battery showed outstanding cycling stability, superior to rechargeable batteries using a commercial Pt/C cathode (Figure S17, Supporting Information). As charging–discharging continued, the Zn foil was gradually thinned, and the electrolyte accumulated more and more soluble Zn salts (in the right-hand chamber of the device in Figure 4g; see also the SEM image in Figure S16, Supporting Information).
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[13]
Supporting Information is available from the Wiley Online Library or from the author. [14]
Acknowledgements X.Z. and D.G. contributed equally to this work. The authors thank the financial support from 973 Programs of China (2013CBA01602, 2012CB933400), Natural Science Foundation of China (51403126), and ERC Grant on 2DMATER and EU Graphene Flagship. M.R.H. acknowledges financial support from the Villum Foundation under the Young Investigator Programme (VKR023122). F.L. thanks the Max Planck Society for funding the Max Planck Research Group of Organic Optoelectronics and the Deutsche Forschungsgemeinschaft (DFG) for funding in the framework of the priority program SPP1355 “Elementary Processes in Organic Photovoltaics.” D.G. acknowledges a Kekulé scholarship of the Fonds der Chemischen Industrie (FCI). Received: April 15, 2015 Revised: April 29, 2015 Published online: May 20, 2015
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of porous polymers and heteroatom-enriched porous carbons with controlled morphologies and functions that will provide an important platform for developing a variety of functional devices, such as batteries, fuel cells, electronics, and sensors.
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Xiaodong Zhuang, Dominik Gehrig, Nina Forler, Haiwei Liang, Manfred Wagner, Michael Ryan Hansen, Frédéric Laquai, Fan Zhang,* and Xinliang Feng* Porous polymers,[1] a type of polymeric material containing numerous pores, possess high specific surface areas and tailorable chemical compositions, and are easily available through many classical polymerization methods and from many organic monomers. Covalent organic networks,[2] polymer aromatic frameworks,[3] polymers with intrinsic microporosity,[4] covalent organic frameworks,[5] and conjugated microporous polymers (CMPs)[6] and ionic polymer networks[7,8] are typical porous polymeric materials featuring high porosity, light weight, and structural variations. However, due to the lack of proper synthetic protocols, most of the reported porous polymers present nanosphere or monolith architectures due to their rapid precipitation from reaction solvents. Mini-emulsion polymerization[9] and silica-based hard-template methods[10] have been used for the construction of microporous polymer networks with quasi0D structures. However, these protocols produced the desired morphology for porous polymers at the price of a significantly decreased specific surface area.[11] Porous polymers with a large specific surface area and a high carbon yield[12] can serve as precursors for preparing heteroatom-doped porous carbons, which hold great promise as metal-free catalysts for the oxygen reduction reaction (ORR),[13] and as advanced electrodes for supercapacitors[13] and other clean-energy devices.[14] Clearly, the morphologies and microstructures of porous polymers and the resultant porous carbons exert significant impact on the performance of energy devices.[15] Among porous polymers, CMPs are a class of amorphous materials[16] that permit the linking of building blocks in Dr. X. Zhuang, Prof. F. Zhang, Prof. X. Feng School of Chemistry and Chemical Engineering Shanghai Jiao Tong University 200240 Shanghai, P. R. China E-mail: [email protected]; [email protected] D. Gehrig, Dr. N. Forler, Dr. H. Liang, Dr. M. Wagner, Dr. M. R. Hansen, Dr. F. Laquai Max-Planck-Institute for Polymer Research Ackermannweg 10, 55128 Mainz, Germany Dr. M. R. Hansen Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry Aarhus University Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark Prof. X. Feng Center for Advancing Electronics Dresden (cfaed) and Department of Chemistry and Food Chemistry Technische Universität Dresden Mommsenstrasse 4, 01062 Dresden, Germany
DOI: 10.1002/adma.201501786
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a π-conjugated fashion and possess continuous networks. Such structural features are unique and usually not available in other porous materials. CMPs have attracted significant research interests in optoelectronics because numerous photoelectrical building blocks can easily be made available. Most of the reported works have focused on p-type building blocks, such as carbazole and tetrakisphenylethene, and corresponding CMPs’ photoluminescence properties.[17–19] The introduction of electron-accepting (n-type) species will result in charge/energy transfer between p/n components, which will greatly expand the application of CMPs, such as for light-emitting and photovoltaic devices and nonlinear optics. To realize this, one efficient way is to physically fill or chemically bind an n-type molecule, such as fullerene, into p-type-conjugated polymer networks. The resulting covalent/conjugated organic frameworks-based on donor–acceptor heterostructures have shown very promising application for charge separation and photoenergy conversion.[20,21] However, the acceptor molecules in these systems are all present in pore channels, resulting in a significant decrease in the specific surface area. In this study, we demonstrate a series of unprecedented p/n heterostructures comprising CMPs (acting as donors, and thus denoted as D) and nanocarbons (acting as acceptors, and denoted as A) with controlled dimensionalities of 0D, 1D, or 2D, employing bromo-functionalized carbon nanospheres (0D CSs), single-walled carbon nanotubes (1D SWNTs), and reduced graphene oxide (2D RGO) as structurally directing templates. Halogenated BODIPY (M1: 2,6-diiodo-1,3,5,7-tetramethyl8-phenyl-4,4-difluoroboradiazaindacene) was chosen as the key monomer for polymerization with 1,3,5-triethynylbenzene (M2) through the metal-catalyzed Sonogashira–Hagihara coupling reactions on the template surface. The resulting CMPs had well-defined nanosphere, nanotube, and nanosheet morphologies, inheriting the original architectures of the nanocarbons. The specific surface areas of these dimensionality controlled CMPs (593–622 m2 g−1) were higher than the CMP (574 m2 g−1) obtained without use of a template. The H2 isosteric heats for these CMPs (Qst = 7.7 ± 0.2 kJ mol−1) at zero coverage were unaffected by the nanocarbon templates. Importantly, an effective inverse decay rate (τeff) reduction of 38–46% for nanocarbon-directed CMPs in comparison with that of amorphous CMP was observed, indicating a strong electronic interaction between the nanocarbons and the polymer networks, with the most pronounced interaction observed for CMP-2D, followed by CMP-1D, then CMP-0D (CMP-2D > CMP-1D > CMP-0D). Further, the CMP-nD were readily converted into boron/nitrogen (B/N) co-doped porous carbons with maintained 0D/1D/2D morphologies by direct pyrolysis. The porous
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Conjugated Microporous Polymers with DimensionalityControlled Heterostructures for Green Energy Devices
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carbons obtained exhibited high specific surface areas (up to 573 m2 g−1), and high B/N doping contents (N = 5.1–8.6 wt%, B = 0.6–0.9 wt%), leading to an efficient catalytic performance (half-wave-potential = 0.71 V vs the reversible hydrogen electrode (RHE), diffusion limiting current density ≈6.0 mA cm−2, four-electron transfer mechanism) for the ORR. The reliable structure–property relationship 2D > 1D > 0D was established between the electrochemically catalyzed ORR and the nanostructures. As an air electrode, these nanocarbon-based zinc–air batteries manifested strong relationship between morphology and performance (2D > 1D > 0D), and the current density and peak power density for 2D porous carbon reached 23.9 and 14.6 mW cm−2 at 0.61 V, respectively. Thereby, these unique structural features of porous polymers with controlled dimensions provide a new insight into porous materials for multiple functions. The strategy for the synthesis of CMPs with different dimensionalities is presented in Figure 1, using CSs, SWNTs, and RGO as the 0D, 1D, and 2D templates, respectively. The preparation of CSs (≈7 nm) is demonstrated in Scheme S1
(Supporting Information). First, the nanocarbon templates were functionalized by treatment with a p-bromobenzene diazonium salt in water. The obtained bromo-functionalized carbon templates (CS-Br, SWNT-Br, and RGBr) were highly dispersible in various organic solvents, such as dimethylformamide (DMF) (≈1.0 mg mL−1 for all samples). Next, halogenated BODIPY (M1) was mixed with 1,3,5-triethynylbenzene (M2) and the functionalized nanocarbon in anhydrous DMF along with catalytic amounts of [Pd(PPh3)4] and CuI. The reaction mixture was sealed and stirred under Sonogashira–Hagihara reaction conditions. After 72 h, the precipitated black solids were collected and purified by Soxhlet extraction. Finally, 0D ball-like, 1D stick-like, and 2D sandwich-like CMPs, denoted as CMP-0D, CMP-1D, and CMP-2D for the CS-, SWNT-, and RGO-containing polymers, respectively, were obtained in high yields (≈90 wt%) after vacuum drying. For comparison, amorphous-type CMP (CMP-Amor) was also prepared by the same procedure without the use of a nanocarbon template. As polymerization on the template surfaces proceeds, polymer networks generally precipitate from the solvent;
Figure 1. Dimensionally controlled synthesis of BODIPY-based CMPs and the corresponding B/N co-doped porous carbons employing nanocarbons as templates. i) Argon, Pd(PPh3)4, CuI, Et3N, DMF, 80 °C, 3 d; ii) 4-bromobenzenediazonium tetrafluoroborate, 0 °C to room temperature, 2 h; iii) argon, M1, M2, Pd(PPh3)4, CuI, Et3N, DMF, 80 °C, 3 d; iv) argon, pyrolyzed at X °C (X = 700, 800, and 900) for 2 h. BN, boron, and nitrogen co-doped porous carbon.
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therefore, the degree of polymerization and the presence of terminal groups may affect the porosity of the resulting materials. Initially, solid-state 13C{1H} cross-polarization/magicangle spinning nuclear magnetic resonance (CP/MAS NMR) spectroscopy was employed to analyze the molecular structure and the influence of the carbon template on the resulting CMPs (Figure 2). For the pure monomer M1 (Figure 2e), the spectrum includes narrow and sharp 13C signals, suggesting a crystalline structure with well-defined geometries.[22] Remarkably, the spectrum only includes about half the number of 13C resonances expected from M1 (see the inset in Figure 2d and Figures S1, S2, Supporting Information).[23] This is attributed to one distinct molecular conformation of the phenylene moiety, leading to a highly symmetric, extended aromatic system.[24] The reaction of M1 with M2 leads to a clear highfield shift of ≈4–7 ppm for the methyl groups at positions 14 and 15 (cf. Figure 2a–d). Moreover, cross-linking with the nanocarbons resulted in a signal decrease accompanied by an increase in the 13C line width. This is a consequence of the tightly cross-linked CMP materials, where free radicals from the nanocarbons induce signal loss and line broadening for the 13C nuclei of the polymer network (more effective T2 relaxation mechanism).[25] However, all CMP-nD materials (Figure 2a–d) show similar structural information in comparison with that of CMP-Amor (Figure 2e), indicating that the incorporation of
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nanocarbon templates did not affect the polymerization reaction significantly. By comparing the Fourier transform infrared (FTIR) spectra of CMP-Amor and CMP-nD (n = 0, 1, 2), a peak with a very low intensity at 2110 cm−1 that corresponds to the alkyne C C stretch was observed. The intense alkynyl C H stretching near 3300 cm−1 almost disappeared in the spectra (Figure S3, Supporting Information). These results are suggestive of a high degree of polymerization in the carbon templatemediated synthesis of CMPs. The chemical nature of the CMPs was further investigated by X-ray photoelectron spectroscopy (XPS). All four CMPs showed almost the same heteroatom components. Typically, the N1s, B1s, and F1s spectra of CMP-2D are presented in Figure S4 (Supporting Information). The peaks at the binding energies of 399.5, 191.6, and 685.6 eV can each be fitted by only one component, which can easily be attributed to the N and B species of the N B bond and the F species of the B F bond, respectively (atomic ratio for N/B/F: 2.00/0.43/2.07). The morphologies of the as-prepared CMPs were inspected by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 2f–k, many freestanding ball/stick/sheet-like nanomaterials with morphologies similar to those of nanocarbon templates can be recognized. Based on the same ratio of nanocarbons (65 mg) to monomers (M1, 800 mg, 1.5 mmol; M2, 200 mg, 1.5 mmol), the obtained
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nanospheres (CMP-0D) had a diameter of 20 ± 4 nm (Figure 2f,i); in contrast, CMP-1D had a diameter and length of 150 ± 30 nm and 0.6–2 µm, respectively (Figure 2g,j); and the nanosheet (CMP-2D) had a lateral size of between 1 × 1 and 4 × 4 µm2 and a thickness of about 80 ± 10 nm. No free porous polymer particles or naked carbon templates can be observed in either the TEM or SEM visualizations, suggesting that most of the monomers have been polymerized on the nanocarbon surface. The control sample CMP-Amor prepared without use of a template exhibited the amorphous nanoparticle (≈10–20 nm) structure (Figure S5, Supporting Information). Notably, the thickness of the porous polymer shells on nanocarbons could easily be adjusted by controlling the template/ monomer ratio. As shown in Figure S6 (Supporting Information), CMP-1D with three different diameters, 35 ± 5, 100 ± 15, and 210 ± 20 nm, were obtained by adjusting the template/monomer ratio of 65 mg/0.5 mmol, Figure 3. Time-resolved photoluminescence. a) Emission spectra and b) concentration profiles 65 mg/1.0 mmol, and 65 mg/1.5 mmol, of component 1 (filled symbols) and component 2 (open symbols) from MCR-ALS analysis of emission data. respectively. Consequently, these results highlight the crucial role of the carbon template as a substrate for directing the morphology of the dynamics of components 1 and 2 of CMP-nD and CMP-Amor resultant CMP in a 0D, 1D, and 2D manner. were described with a biexponential decay function. The emisThe porous nature of CMP-nD (n = 0, 1, 2) was confirmed sion of component 1 is attributed to a monomer-like emission, by nitrogen physisorption measurements. It was found that which is in agreement with the previously published diethynylthe isotherms of all the CMP-nD variants and CMP-Amor are BODIPY dyes with additional electron-donating groups exhibtype I (Figures S7–10, Supporting Information). The Brunauer– iting emission maxima up to 659 nm.[26] Moreover, the emisEmmet–Teller (BET) surface areas (SBET) of these polymer netsion of component 2, which appears strongly redshifted with respect to component 1, is consequently assigned to polymeric works varied between 574 and 622 m2 g−1 (Table S1, Supporting states distributed over a higher number of chromophores. Information) and had a similar pore diameter of 1.6 nm, which is a typical microporous ( 4.94 > 3.98 mA cm−2) and the electron transfer number (3.85 > 3.69 > 3.03) for 2DBN-800, 1DBN-800, and 0DBN-800 exhibit the strong relationship between dimensionality and electrochemical catalyzed ORR performance. The improved catalytic performance for 2D porous carbons is believed to originate from the large lateral and layer-to-layer conductive feature (several micrometers) in comparison with the dot-to-dot and line-to-line conductive pathway of 0D and 1D porous carbons, respectively (Figure 5). This result is also consistent with the above time-resolved photoluminescence studies of the dimensionality controlled CMP precursors. Although the HWP for 2DBN-800 is ≈39 mV lower than for Pt/C, the mainly four-electron-transfer pathway (n ≈ 3.85 at 0.5 V, see Figure 4c) calculated from the rotating ring–disk electrode (RRDE, Figure S15, Supporting Information) curve and Equation S3 (Supporting Information), together with the ultrahigh JDL, makes 2DBN-800 promising as an electrocatalyst for the ORR. The mean number of transferred electrons per O2 molecule involved in the ORR, which can be determined from the K–L relation (Equation S1, Supporting Information), was approximately 4.0 between 0 and 0.5 V. These results highlight that the ORR proceeds via a primary four-electron pathway. In addition to their excellent ORR performance, nDBN-800 and AmorBN-800 were further employed as the air cathode in Zn–air batteries. To construct primary Zn–air batteries, the ORR electrocatalysts discussed above were loaded onto carbonfiber paper as the air cathode, and paired with a Zn foil in 6 M KOH. The battery had an open-circuit voltage of ≈1.40 V. At a voltage of 0.8 V, AmorBN-800, 0DBN-800, 1DBN-800, and 2DBN-800 showed current densities of approximately 7, 5, 15, and 17 mA cm−2, respectively. The peak power densities for AmorBN-800, 0DBN-800, 1DBN-800, and 2DBN-800 at 9.7, 6.7, 21.2, and 23.9 mA cm−2 reached 6.9, 4.8, 13.2, and 14.6 mW cm−2, respectively (2D > 1D > 0D, Figure 4d). The current densities at 0.8 V and peak power densities for the 1D and 2D (but not 0D) porous carbons were significantly higher than the porous carbon prepared without use of a template. This result indicates that 1D and 2D porous carbons capable of longdistance charge transport contribute more to oxygen reduction on an air-diffusion layer than porous carbons with confined 0D morphologies. Thus, this dimensionality-related catalytic performance offers a new platform for developing metal-free air cathodes for Zn–air batteries. The as-prepared porous carbons were next investigated as air catalysts for rechargeable Zn–air batteries. First, these materials were mixed and loaded onto a single cathode for charge and discharge cycling experiments (Figure 4e and Figure S16, Supporting Information). The electrolyte was 6 M KOH with 0.2 M zinc acetate (dissolved in KOH to form zincate, Zn(OH)42−) to
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hand, long-distance transport in graphene (which exceeds the transport in SWNT and CS) is another factor determining electronic communication in CMPs. In line with this argument is the steady redshift of the emission maximum for the series CMP-0D, CMP-1D, and CMP-2D with the strongest redshift for CMP-2D. This result (0D < 1D < 2D) is most likely due to a gradually increasing charge-transporting distance and electronaccepting properties for CS, SWNT, and graphene. Various clean energy devices, such as Li-ion batteries, supercapacitors, and fuel cells, have been developed for portable electronics and vehicle electrification. However, developing novel devices with higher energy/power densities, such as metal–air batteries,[27–29] Li–sulfur batteries,[30,31] etc.[32,33] is a major challenge. From a materials perspective, carbon is low cost, precursor selectable, component controllable, and nanostructure processable; therefore, varieties of carbon-based nanomaterials have been developed for application in these new energy devices, including hollow carbon spheres, carbon nanotubes, and graphene-based sheet-like or sponge-like carbons.[34] Dimension-controlled porous carbon materials may provide new insights into the mechanism of these applications, and help in the development of high-throughput energy devices. Given that CMPs are carbon-rich precursors, the asprepared CMPs (CMP-Amor and CMP-nD) were pyrolyzed at X °C (X = 700, 800, 900, 1000) for 2 h under an argon atmosphere, affording boron and nitrogen co-doped porous carbons (denoted AmorBN-X and nDBN-X, n = 0, 1, 2, respectively, see Figure 1). The chemical nature of AmorBN-X and nDBN-X was investigated by XPS. Owing to the similar results for these four porous carbons, only 2DBN-700/800/900 are illustrated here (Figure S4, Supporting Information). For 2DBN-X, the major contributions at binding energies of around 398.4 and 400.8 eV can be attributed to the nitrogen species of the C B N and C N C bonds[35–37] (which should be pyrrolic N), respectively. The B1s spectra can be deconvoluted into two different peaks at binding energies of 190.5 and 191.4 eV, corresponding to the B species in B N and B C N bonds, respectively. Another minor contribution at the higher binding energy of around 184.0 eV can be assigned to the double-nitrogen-bonded boron (N B N) moieties in which the nitrogen originates from the BODIPY blocks. This strategy thus provides a feasible way to build up 0D-, 1D-, and 2D-porous carbons with a high level of boron and nitrogen doping content (Figure S4, Supporting Information: B = 0.6–0.9 wt%, N = 5.1–8.6 wt%). B/N co-doped carbon materials[35–37] have demonstrated promising electrocatalytic performance and showed advantages over other single-heteroatom-doped carbon materials due to the synergistic effects from boron and nitrogen. The electrocatalytic activities of the B/N co-doped porous carbons prepared in this study in the ORR under alkaline conditions (0.1 M KOH) were examined. The ORR catalytic activity of 2DBN-X (X = 700, 800, 900, 1000) was first evaluated by cyclic voltammetry (CV, Figure S13, Supporting Information). It was found that the ORR peak potential of 0.69 V versus RHE for 2DBN-800 was slightly higher than the results for the other three porous carbons. Koutecky–Levich plots (K–L plots, Figure S14e,f, Supporting Information) with a good linear relationship for the 2DBN-X electrodes were calculated from linear sweep voltammetry
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Figure 4. The ORR and Zn–air battery performance of the catalysts. a) LSV curves of 2DBN-X (X = 700, 800, 900, 1000) in O2-saturated 0.1 M KOH solution at 1600 rpm at a scan rate of 5 mV s−1. b) LSV curves of AmorBN-800, 0DBN-800, 1DBN-800, 2DBN-800, and Pt/C in O2-saturated 0.1 M KOH solution at 1600 rpm at a scan rate of 5 mV s−1. c) Electron-transfer number for AmorBN-800, 0DBN-800, 1DBN-800, and 2DBN-800 as a function of the electrode potential. d) Polarization curve (V–i) and the corresponding power density plot of the battery using AmorBN-800, 0DBN-800, 1DBN-800, and 2DBN-800 as the cathode catalysts. e) The primary Zn–air battery. f) Discharge and charge cycling of 2DBN-800. g) A real Zn–air device after 48 h charge and discharge cycles. h) An LED (3.4 V rated voltage) powered by two Zn–air devices using 2DBN-800.
The battery eventually ceased functioning when all the Zn metal was consumed. The as-prepared metal-free ORR catalyst is ideally suited for Zn–air batteries by replacing the electrolyte and offering fresh Zn, owing to its exceptional ORR activity and durability.[39] In summary, a novel nanocarbon (carbon nanosphere, carbon nanotube, and graphene)-inspired strategy for the synthesis of 0D, 1D, and 2D CMPs and B/N co-doped porous carbons with high specific surface areas has been demonstrated. Dimensionality has been shown to have a significant influence on CMPs in respect of photoluminescence. An effective inverse decay rate (τeff) reduction of 38–46% for nanocarbondirected CMPs in comparison with that of amorphous CMP was observed, indicating electronic interactions between n-type nanocarbons and p-type polymer networks. The porous carbons obtained herein exhibited high B/N doping contents and efficient catalytic performance for the ORR, superior to state-of-the-art noble-metal catalysts. Remarkably, a reliable structure–property relationship (2D > 1D > 0D) was established between the electrochemical catalyzed ORR and the dimensionality. It is expected that the synthetic strategy as well as the structure– property relationship in this work will offer opportunities for the templated synthesis Figure 5. Conductive architectures for nanocarbon-directed 0D, 1D, and 2D porous carbons.
ensure reversible Zn-involved electrochemical redox reactions at the anode.[29,38] Such batteries exhibit cycling stability when charged and discharged galvanostatically at low current densities (5–20 mA cm−2) and short cycle times (300 s per charge or discharge period). As an example, the charge–discharge curve for 2DBN-800 is presented in Figure 4f. Importantly, when repeatedly charged or discharged at 20 mA cm−2 for a total of 11 h with a 5 min cycling period, the battery showed outstanding cycling stability, superior to rechargeable batteries using a commercial Pt/C cathode (Figure S17, Supporting Information). As charging–discharging continued, the Zn foil was gradually thinned, and the electrolyte accumulated more and more soluble Zn salts (in the right-hand chamber of the device in Figure 4g; see also the SEM image in Figure S16, Supporting Information).
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Supporting Information
[13]
Supporting Information is available from the Wiley Online Library or from the author. [14]
Acknowledgements X.Z. and D.G. contributed equally to this work. The authors thank the financial support from 973 Programs of China (2013CBA01602, 2012CB933400), Natural Science Foundation of China (51403126), and ERC Grant on 2DMATER and EU Graphene Flagship. M.R.H. acknowledges financial support from the Villum Foundation under the Young Investigator Programme (VKR023122). F.L. thanks the Max Planck Society for funding the Max Planck Research Group of Organic Optoelectronics and the Deutsche Forschungsgemeinschaft (DFG) for funding in the framework of the priority program SPP1355 “Elementary Processes in Organic Photovoltaics.” D.G. acknowledges a Kekulé scholarship of the Fonds der Chemischen Industrie (FCI). Received: April 15, 2015 Revised: April 29, 2015 Published online: May 20, 2015
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