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COMMUNICATION Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyen et al. Reactivity differences of Sc3N@C2n (2n = 68 and 80). Synthesis of the first methanofullerene derivatives of Sc3N@D5h-C80
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Pyrolysis of Covalent Organic Frameworks: A General Strategy for Template Converting Conventional Skeletons into Conducting Microporous Carbons for High-Performance Energy Storage a,b
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Qing Xu, Yanping Tang, Lipeng Zhai, Qiuhong Chen and Donglin Jiang *
DOI: 10.1039/x0xx00000x www.rsc.org/
Here we describe a general strategy based on template pyrolysis for converting conventional covalent organic frameworks into highperformance carbons, which combine conductivity, microporosity and heteroatom density, thus casting a distinct contrast to these obtained upon direct pyrolysis. The carbons serve as electrode and exhibit exceptional performance in energy storage. Porous carbon materials are attracting a surge of interests as metal-free electrodes for energy storage. The most widely employed approach to porous carbons is based on pyrolysis that thermally decomposes precursors. Synthetic linear polymers, natural polymers and porous polymer networks have been 1-3 explored for pyrolysis to prepare porous carbon materials. Among various precursors, crystalline porous materials are unique in that they can pre-organize the carbon skeletons, heteroatoms, and /or metal species into well-defined frameworks, which could serve as a scaffold for guiding the carbonization process. For example, metal-organic frameworks (MOFs) have been successfully pyrolyzed into metal 4-8 nanoparticle-doped carbons that are useful as catalysts. Covalent organic frameworks (COFs), a class of crystalline porous polymer, are capable of pre-designing two-dimensional (2D) organic skeletons and ordered porous channels, leading to the exploration of various functions ranging from 9-14 semiconductor to magnetization. Their ordered porous structures are especially attractive for designing carbon 15-18 electrodes for energy storage. COFs are unique in structure because of their ordered pores, carbon-rich layers, and diverse heteroatoms such as B, O and N. These structural features render COFs able to synthesize metal19-21 free yet heteroatom-doped porous carbons. However, the
resulting carbons loss the structural features of COFs and are poor in performance. Developing a new strategy for pyrolysis of COFs is highly desired but it remains a challenge.
Fig. 1 (A) Synthesis of TAPT-DHTA-COF. (B) Synthesis of crosslinked PPZS networks. (C) Schematic of the synthesis of TAPT-DHTA-COFX@PPZS and its pyrolysis to process TAPTDHTA-COFX@PPZS900. TAPT-DHTA-COF was in-situ prepared on the surface of PPZS template and was then pyrolyzed at 900 °C under N2. The lower layer shows the corresponding FE-SEM images.
Herein, we report a general strategy based on template pyrolysis for the preparation of high-performance conducting porous carbons from conventional COFs. One distinct feature of this method is that it no longer needs specific COFs. We used a conventional imine-
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linked TAPT-DHTA-COF (Fig. 1A) as a precursor. We chose polycyclotriphosphazene-co-4,4’sulfonyldiphenol (Fig. 1B, PPZS) spheres as template because it possesses high heteroatom content 22 and can be easily synthesized. We conducted in-situ polymerization of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPT) and 2, 5dihydroxyterephthalaldehyde (DHTA) to coat TAPT-DHTA-COF on the surface of PPZS spheres (Fig. 1C). By controlling the ratio of the TAPT and DHTA building blocks to PPZS, TAPT-DHTA-COFX@PPZS (X = weight ratio of COF to PPZS = 0.05, 0.1, and 0.2) was prepared in 92%96% yields (Supplementary Information). The PPZS template is an amorphous polymer without showing any powder X-ray diffraction (PXRD) peaks (Fig. S1, black curve). By contrast, on coating with the TAPT-DHTA-COF shell, the resulting TAPT-DHTA-COFX@PPZS (X = 0.05; blue, X = 0.1; red, and X = 0.2; green) exhibited clear PXRD patterns with peaks at 2.78° and 26° that were assigned to the (100) and (001) facets of TAPT-DHTA-COF, respectively. Notably, these peaks are the same in position as those of bulky TAPT-DHTA-COF samples (Fig. S2). Thus, TAPT-DHTA-COF forms crystalline shells on the surface of the amorphous PPZS cores. The presence of TAPT-DHTA-COF shell endows TAPT-DHTACOFX@PPZS with inherent porosity. Nitrogen sorption measurements at 77 K revealed that the PPZS spheres were almost nonporous with Brunauer–Emmett–Teller (BET) surface area of only 2 –1 8 m g (Fig. 2A, black dots). By contrast, TAPT-DHTA-COFX@PPZS exhibited type-IV sorption curves (X = 0.05; blue dots, X = 0.1; red dots, and X = 0.2; green dots), indicating mesoporous feature. Clearly, the BET surface area increased as the thickness of TAPT-DHTA-COF 2 – shells was increased and was evaluated to be 57, 168, and 256 m g 1 for TAPT-DHTA-COF0.05@PPZS, TAPT-DHTA-COF0.1@PPZS, and TAPT-DHTA-COF0.2@PPZS, respectively. Meanwhile, the pore volume of TAPT-DHTA-COFX@PPZS increased from 0.028 to 0.065 and 0.147 3 –1 cm g as the X value was increased from 0.05 to 0.1 and 0.2 (Fig. 2B). These results indicate that the TAPT-DHTA-COF shell increase the porosity of TAPT-DHTA-COFX@PPZS as the thickness of the porous COF shell is increased along with the X value. Notably, the pore size distribution profiles calculated using nonlocal density functional theory method show that TAPT-DHTA-COFX@PPZS has only one type of pore with a same size of 3.1 nm (Fig. 2B), irrespective of the thickness of the TAPT-DHTA-COF shell. This pore size is identical to that of TAPT-DHTA-COF (Fig. S3). PPZS is spheres with smooth surface and diameters of 830-1200 nm, as characterized by field emission scanning electron microscopy (FE-SEM, Fig. 2C; Fig. S4). On the other hand, bulky TAPT-DHTA-COF assumes a ribbon shape (Fig. S5). By contrast, in-situ polycondensation of TAPT-DHTA-COF in the presence of PPZS yielded only spheres (Fig. 2D; Fig. S6 and S7); no ribbon-shaped TAPT-DHTACOF objects were observable. Compared to PPZS, the diameter of TAPT-DHTA-COFX@PPZS was increased by 100-200 nm, which can be attributed to the growth of the TAPT-DHTA-COF shell on the PPZS surface. These observations indicate that TAPT-DHTA-COF is grown on the surface of PPZS and form a spherical core-shell TAPT-DHTACOFX@PPZS structure. A closer look at the FE-SEM images revealed that the surface of TAPT-DHTA-COFX@PPZS is different from that of PPZS. Unlike the smooth surface of PPZS, the surface of TAPT-DHTACOFX@PPZS is quite rough (Fig. 2D, Figs. S6 and S7). The increased surface roughness may originate from the 2D extended hexagonal polygon nature of TAPT-DHTA-COF that is less bendable. Infrared
–1
spectroscopy shows that the vibration band at 1490 cm observed View Article Online DOI: 10.1039/C7CC07002K for TAPT-DHTA-COFX@PPZS is much broader than that of the PPZS (Fig. S8). This phenomenon originates from the combination of two –1 bands in TAPT-DHTA-COFX@PPZS; one is at 1490 cm assigned to the –1 phenyl C=C bond of the PPZS, and another one is at 1514 cm assigned to the phenyl C=C bond of the COF skeleton. In addition, the –1 – intensity ratio between bands at 1302 cm (C–O bond) and 1367 cm 1 (C–N bond in triazine) was clearly enhanced for TAPT-DHTACOFX@PPZS.
Fig. 2 (A) N2 adsorption–desorption isotherms and (B) pore size distribution profiles of PPZS (black), TAPT-DHTA-COF0.05@PPZS (blue), TAPT-DHTA-COF0.1@PPZS (red), and TAPT-DHTA-COF0.2@PPZS (green). FE-SEM images of (C) PPZS and (D) TAPT-DHTACOF0.1@PPZS.
To determine the pyrolysis temperature, we conducted thermal gravimetric analysis. A significant decomposition occurs at 400500 °C and after this stage a second clear decomposition happens at around 800-850 °C for TAPT-DHTA-COFX@PPZS (Fig. S9). Thus, we chose the pyrolysis temperature at 900 °C. At 900 °C, TAPT-DHTACOFX@PPZS losses 70%-80% of its original weight. After 3-h-pyrolysis at 900 °C under N2, TAPT-DHTA-COFX@PPZS was transformed into its corresponding carbons, namely DHTA-COFX@PPZS900 in 21% (X = 0.05), 32% (X = 0.1), and 34% (X = 0.2) yields. As controls, direct pyrolysis of PPZS and TAPT-DHTA-COF under otherwise identical conditions generates PPZS900 and TAPT-DHTA-COF900 in 38% and 55% yields, respectively. The core-shell hybrid systems could facilitate the pyrolysis process and lead to decreased carbon contents and increased heteroatom contents (Table S1) PPZS900 is spheres with a decreased diameter of 600-830 nm, compared to PPZS (Fig. 3D; Fig. S10). However, in the core-shell structure, the morphology is highly dependent on the thickness of the COF shell. When the shell is thin, such as in the case of TAPTDHTA-COF0.05@PPZS900, the shells tend to break into fragments (Fig. 3E; Fig. S11), suggesting that a thin COF shell is not strong enough to form a continuous spherical surface during the pyrolysis. Interestingly, TAPT-DHTA-COF0.1@PPZS900 retained a spherical shape but with nuts-like surface (Fig. 3F; Fig. S12). When the COF shell becomes thicker, TAPT-DHTA-COF0.2@PPZS900 forms sphere with much smoother surface (Fig. 3G; Fig. S13). By contrast, upon direct pyrolysis, TAPT-DHTA-COF900 resulted in aggregates and could not remain its original ribbon shape (Fig. S5C and D). Nevertheless, HR TEM measurements of the core-shell structures did not show a clear border between the core and shell (Fig. S14).
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As shown in Raman spectra, TAPT-DHTA-COFX@PPZS900 and –1 PPZS900 exhibited D and G bands at 1347 and 1601 cm , which were assigned to disordered carbons and ordered graphitic carbons, respectively (Fig. 3C). Their intensity ratio ID/IG for TAPT-DHTACOFX@PPZS900 was 1.04 (X = 0.05; blue), 0.98 (X = 0.1; red), and 0.96 (X = 0.2; green), respectively, indicating that the degree of graphitization becomes higher as the TAPT-DHTA-COF shell thickness is increased. Notably, direct pyrolysis yielded a low graphitization as evident by a high ID/IG value (black; 1.10 for PPZS900). These results indicate that the COF shell promotes the formation of graphitic 23-25 carbon that is key to conductivity.
Fig. 3 (A) N2 sorption isotherms, (B) pore size distribution profiles, and (C) Raman spectra of PPZS900 (black), TAPT-DHTA-COF0.05@PPZS900 (blue), TAPT-DHTA-COF0.1@PPZS900 (red), and TAPT-DHTA-COF0.2@PPZS900 (green). FE-SEM images of (D) PPZS900, (E) TAPT-DHTACOF0.05@PPZS900, (F) TAPT-DHTA-COF0.1@PPZS900, and (G) TAPT-DHTA-COF0.2@PPZS900 (insets for the enlarged images).
Direct pyrolysis decreased the porosity drastically and TAPT2 –1 DHTA-COF900 has a BET surface area of only 21 m g (Fig. S3A, black 2 –1 dots). This is in a sharp contrast to that (2170 m g ) of the TAPTDHTA-COF precursor (Fig. S3A, red dots). These results revealed that the direct pyrolysis eventually lose the structural features of the COF precursor. On the other hand, the BET surface area of PPZS900 was 2 –1 609 m g (Fig. 3A, black dots), which was much higher than that of pristine PPZS. The pore size distribution profile revealed that PPZS900 3 –1 has a pore volume of 0.26 cm g (Fig. S15; Table S1) and three different micropores with sizes of 0.77, 1.12, and 1.52 nm (Fig. 3B, black dots). With the PPZS core, TAPT-DHTA-COF0.05@PPZS900 (blue dots), TAPT-DHTA-COF0.1@PPZS900 (red dots), and TAPT-DHTACOF0.2@PPZS900 (green dots) exhibited the BET surface areas of 533, 2 –1 456 and 421 m g respectively, which were more than 20-fold higher than that of TAPT-DHTA-COF900. TAPT-DHTA-COFX@PPZS900 consists of only micropores with size between 0.5 and 1.5 nm (Fig. 3B, X = 0.05; blue, X = 0.1; red, and X = 0.2; green). The pore volumes of TAPT-DHTA-COFX@PPZS900 are 0.25 (X = 0.05), 0.22 (X = 0.1), and 3 –1 0.20 (X = 0.2) cm g (Fig. S15; Table S1). Therefore, a general tendency is that a higher COF content results in a lower BET surface area and pore volume of TAPT-DHTA-COFX@PPZS900.
The chemical nature of TAPT-DHTA-COFX@PPZS were 900 Online View Article DOI: 10.1039/C7CC07002K investigated by X-ray photoelectron spectroscopy (XPS). The C1s, O1s, N1s, S2p, and P2p peaks were observed (Fig. S16). As shown in Table S2, the ratio of heteroatoms (O, N, S, and P) to carbon are 7.37%, 12.21%, and 12.38% for TAPT-DHTA-COF0.05@PPZS900, TAPT-DHTACOF0.1@PPZS900, and TAPT-DHTA-COF0.2@PPZS900, respectively. Interestingly, these heteroatom contents are even threefold as high as those of PPZS900 (5.66%) and TAPT-DHTA-COF900 (4.08%). Thus, the COF shell can protect the release of heteroatoms during the pyrolysis, while the thin yet broken COF shell leads to a low heteroatom density. The energy storage capacity of capacitors was evaluated in a fabricated three-electrode system in an aqueous alkaline solution (6 M KOH, Supplementary Information). Cyclic voltammetry (CV) curves revealed typical double layer capacitive behaviors (Fig. S17). Interestingly, the current density is highly dependent on the core–1 shell structure. For example, at a scan rate of 50 mV s , the current density of TAPT-DHTA-COF900 (Fig. 4A, purple curve) and PPZS900 –1 (black curve) at –1.0 V was only 2.1 and 4.8 A g , respectively. In the core-shell structure, the current density is dependent on the thickness of the COF shell. The current density of TAPT-DHTA–1 COF0.05@PPZS900 (blue curve) is increased to 7.5 A g under otherwise identical conditions. Notably, TAPT-DHTA-COF0.1@PPZS900 –1 (red curve) exhibited the highest current density of 11.1 A g , whereas TAPT-DHTA-COF0.2@PPZS900 (green curve) displayed a –1 current density of 9.7 A g . The greatly increased current density reflects a sharply enhanced capacitive energy storage. In agreement with the tendency of current density, the capacitance is dependent on the core-shell structure (Fig. 4B). TAPT–1 DHTA-COF0.05@PPZS900 at a current density of 1 A g exhibited a –1 capacitance of 178 F g , which is much higher than those of PPZS900 –1 –1 (132 F g ) and TAPT-DHTA-COF900 (43.8 F g ). Surprisingly, TAPTDHTA-COF0.1@PPZS900 exhibited an exceptional capacitance of 287 F –1 g , which is 2.2 and 6.6 times higher than those of PPZS900 and TAPTDHTA-COF900, respectively. Similarly, TAPT-DHTA-COF0.2@PPZS900 –1 exhibited a high capacitance of 255 F g . Moreover, when the –1 current density was decreased to 0.5 A g , the capacitance increased significantly. For example, the capacitance of TAPT-DHTA–1 COF0.1@PPZS900 is as high as 411 F g (Fig. S18). To the best of our knowledge, this capacitance is far superior to the COF-based materials and state-of-the art carbons (Table S3). High-rate performance determines the quickness of charge and discharge process. PPZS900 (Fig. 4C, black triangles) and TAPT-DHTACOF900 (purple triangles) exhibited a sharp decrease in capacitance –1 to only 65 and 13.5 F g , respectively, when they were charged and −1 discharged at a high rate of 10 A g . By contrast, the core-shell structure enables a high capacitance at high rate. For example, TAPT−1 DHTA-COF0.05@PPZS900 (blue dots) at 10 A g could retain a –1 capacitance of 123 F g , which is almost one order of magnitude high than that of TAPT-DHTA-COF900, indicating the importance of being a core-shell structure for energy storage. Notably, TAPT-DHTACOF0.1@PPZS900 (red dots) and TAPT-DHTA-COF0.2@PPZS900 (green −1 – dots) at 10 A g exhibited a capacitance as high as 182 and 175 F g 1 , respectively. Thus, the core-shell TAPT-DHTA-COFX@PPZS900 is far superior in energy storage to the pure counterparts of PPZS900 and TAPT-DHTA-COF900. Especially, TAPT-DHTA-COF0.1@PPZS900 reached an exceptional capacitance that is 13.5-fold that of TAPT-DHTACOF900.
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Journal Name storage. These results suggest a novel platform based on COFs for View Article Online DOI: 10.1039/C7CC07002K energy storage. This work was supported by a Grant in-Aid for Scientific Research (A) (17H01218) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and support from the ENEOS Hydrogen Trust Fund and the Ogasawara Foundation for the Promotion of Science and Engineering.
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Fig. 4 (A) CV curves at 50 mV s , (B) galvanostatic charge/discharge curves at 1 A g , (C) capacitance at different current densities, and (D) Nyquist plots of PPZS900 (black), and TAPT-DHTA-COF900 (purple), TAPT-DHTA-COF0.05@PPZS900 (blue), TAPT-DHTACOF0.1@PPZS900 (red), TAPT-DHTA-COF0.2@PPZS900 (green). Inset in (D) is enlarged semicircles. (E) Cycle of TAPT-DHTA-COF0.1@PPZS900. Insets in (e) are charge-discharge curves.
The cycle performance of TAPT-DHTA-COF0.1@PPZS900 was −1 investigated at 10 A g (Fig. 4E and insets). Remarkably, the −1 capacitance retains at 182 F g even after 10,000 cycles. Figure S19 shows the equivalent circuit and the simulated impedance curves of the capacitors. From the impedance plots (Fig. 4D; Fig. S19), the charge-transfer resistance of TAPT-DHTACOF0.2@PPZS900 (Fig. 4D, green curve) and TAPT-DHTACOF0.1@PPZS900 (red curve) was evaluated to be 25 Ω, which is much smaller than those of TAPT-DHTA-COF0.05@PPZS900 (blue curve, 40 Ω) and PPZS900 (black curve, 46.4 Ω). Therefore, the COF-derived carbon shell increases the conductivity. Moreover, TAPT-DHTACOF0.1@PPZS900 exhibited a slope that is larger than those of TAPTDHTA-COFX@PPZS900 (X = 0.05 and 0.2), PPZS900, and TAPT-DHTACOF900. This result suggests that the ion diffusion in TAPT-DHTACOF0.1@PPZS900 is faster than that in PPZS900, TAPT-DHTA-COF900, and TAPT-DHTA-COFX@PPZS900 (X = 0.05 and 0.2). These mechanistic insights elucidate the significance and a synergistic effect of the coreshell structure on facilitating electron conduction and ion transport – two key processes involved in energy storage. In summary, we have disclosed for the first time that template pyrolysis is essential for processing carbons from crystalline porous COFs. This template strategy is general and facile and it enables the conversion of conventional COFs into conducting porous carbons. The core-shell structure not only triggers a synergistic effect on producing highly conducting porous carbons but also enables a combined positive influence on achieving high performance energy
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Pyrolysis of Covalent Organic Frameworks: A General Strategy for Template Converting Conventional Skeletons into Conducting Microporous Carbons for High-Performance Energy Storage a,b
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Qing Xu, Yanping Tang, Lipeng Zhai, Qiuhong Chen and Donglin Jiang *
DOI: 10.1039/x0xx00000x www.rsc.org/
Here we describe a general strategy based on template pyrolysis for converting conventional covalent organic frameworks into highperformance carbons, which combine conductivity, microporosity and heteroatom density, thus casting a distinct contrast to these obtained upon direct pyrolysis. The carbons serve as electrode and exhibit exceptional performance in energy storage. Porous carbon materials are attracting a surge of interests as metal-free electrodes for energy storage. The most widely employed approach to porous carbons is based on pyrolysis that thermally decomposes precursors. Synthetic linear polymers, natural polymers and porous polymer networks have been 1-3 explored for pyrolysis to prepare porous carbon materials. Among various precursors, crystalline porous materials are unique in that they can pre-organize the carbon skeletons, heteroatoms, and /or metal species into well-defined frameworks, which could serve as a scaffold for guiding the carbonization process. For example, metal-organic frameworks (MOFs) have been successfully pyrolyzed into metal 4-8 nanoparticle-doped carbons that are useful as catalysts. Covalent organic frameworks (COFs), a class of crystalline porous polymer, are capable of pre-designing two-dimensional (2D) organic skeletons and ordered porous channels, leading to the exploration of various functions ranging from 9-14 semiconductor to magnetization. Their ordered porous structures are especially attractive for designing carbon 15-18 electrodes for energy storage. COFs are unique in structure because of their ordered pores, carbon-rich layers, and diverse heteroatoms such as B, O and N. These structural features render COFs able to synthesize metal19-21 free yet heteroatom-doped porous carbons. However, the
resulting carbons loss the structural features of COFs and are poor in performance. Developing a new strategy for pyrolysis of COFs is highly desired but it remains a challenge.
Fig. 1 (A) Synthesis of TAPT-DHTA-COF. (B) Synthesis of crosslinked PPZS networks. (C) Schematic of the synthesis of TAPT-DHTA-COFX@PPZS and its pyrolysis to process TAPTDHTA-COFX@PPZS900. TAPT-DHTA-COF was in-situ prepared on the surface of PPZS template and was then pyrolyzed at 900 °C under N2. The lower layer shows the corresponding FE-SEM images.
Herein, we report a general strategy based on template pyrolysis for the preparation of high-performance conducting porous carbons from conventional COFs. One distinct feature of this method is that it no longer needs specific COFs. We used a conventional imine-
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linked TAPT-DHTA-COF (Fig. 1A) as a precursor. We chose polycyclotriphosphazene-co-4,4’sulfonyldiphenol (Fig. 1B, PPZS) spheres as template because it possesses high heteroatom content 22 and can be easily synthesized. We conducted in-situ polymerization of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPT) and 2, 5dihydroxyterephthalaldehyde (DHTA) to coat TAPT-DHTA-COF on the surface of PPZS spheres (Fig. 1C). By controlling the ratio of the TAPT and DHTA building blocks to PPZS, TAPT-DHTA-COFX@PPZS (X = weight ratio of COF to PPZS = 0.05, 0.1, and 0.2) was prepared in 92%96% yields (Supplementary Information). The PPZS template is an amorphous polymer without showing any powder X-ray diffraction (PXRD) peaks (Fig. S1, black curve). By contrast, on coating with the TAPT-DHTA-COF shell, the resulting TAPT-DHTA-COFX@PPZS (X = 0.05; blue, X = 0.1; red, and X = 0.2; green) exhibited clear PXRD patterns with peaks at 2.78° and 26° that were assigned to the (100) and (001) facets of TAPT-DHTA-COF, respectively. Notably, these peaks are the same in position as those of bulky TAPT-DHTA-COF samples (Fig. S2). Thus, TAPT-DHTA-COF forms crystalline shells on the surface of the amorphous PPZS cores. The presence of TAPT-DHTA-COF shell endows TAPT-DHTACOFX@PPZS with inherent porosity. Nitrogen sorption measurements at 77 K revealed that the PPZS spheres were almost nonporous with Brunauer–Emmett–Teller (BET) surface area of only 2 –1 8 m g (Fig. 2A, black dots). By contrast, TAPT-DHTA-COFX@PPZS exhibited type-IV sorption curves (X = 0.05; blue dots, X = 0.1; red dots, and X = 0.2; green dots), indicating mesoporous feature. Clearly, the BET surface area increased as the thickness of TAPT-DHTA-COF 2 – shells was increased and was evaluated to be 57, 168, and 256 m g 1 for TAPT-DHTA-COF0.05@PPZS, TAPT-DHTA-COF0.1@PPZS, and TAPT-DHTA-COF0.2@PPZS, respectively. Meanwhile, the pore volume of TAPT-DHTA-COFX@PPZS increased from 0.028 to 0.065 and 0.147 3 –1 cm g as the X value was increased from 0.05 to 0.1 and 0.2 (Fig. 2B). These results indicate that the TAPT-DHTA-COF shell increase the porosity of TAPT-DHTA-COFX@PPZS as the thickness of the porous COF shell is increased along with the X value. Notably, the pore size distribution profiles calculated using nonlocal density functional theory method show that TAPT-DHTA-COFX@PPZS has only one type of pore with a same size of 3.1 nm (Fig. 2B), irrespective of the thickness of the TAPT-DHTA-COF shell. This pore size is identical to that of TAPT-DHTA-COF (Fig. S3). PPZS is spheres with smooth surface and diameters of 830-1200 nm, as characterized by field emission scanning electron microscopy (FE-SEM, Fig. 2C; Fig. S4). On the other hand, bulky TAPT-DHTA-COF assumes a ribbon shape (Fig. S5). By contrast, in-situ polycondensation of TAPT-DHTA-COF in the presence of PPZS yielded only spheres (Fig. 2D; Fig. S6 and S7); no ribbon-shaped TAPT-DHTACOF objects were observable. Compared to PPZS, the diameter of TAPT-DHTA-COFX@PPZS was increased by 100-200 nm, which can be attributed to the growth of the TAPT-DHTA-COF shell on the PPZS surface. These observations indicate that TAPT-DHTA-COF is grown on the surface of PPZS and form a spherical core-shell TAPT-DHTACOFX@PPZS structure. A closer look at the FE-SEM images revealed that the surface of TAPT-DHTA-COFX@PPZS is different from that of PPZS. Unlike the smooth surface of PPZS, the surface of TAPT-DHTACOFX@PPZS is quite rough (Fig. 2D, Figs. S6 and S7). The increased surface roughness may originate from the 2D extended hexagonal polygon nature of TAPT-DHTA-COF that is less bendable. Infrared
–1
spectroscopy shows that the vibration band at 1490 cm observed View Article Online DOI: 10.1039/C7CC07002K for TAPT-DHTA-COFX@PPZS is much broader than that of the PPZS (Fig. S8). This phenomenon originates from the combination of two –1 bands in TAPT-DHTA-COFX@PPZS; one is at 1490 cm assigned to the –1 phenyl C=C bond of the PPZS, and another one is at 1514 cm assigned to the phenyl C=C bond of the COF skeleton. In addition, the –1 – intensity ratio between bands at 1302 cm (C–O bond) and 1367 cm 1 (C–N bond in triazine) was clearly enhanced for TAPT-DHTACOFX@PPZS.
Fig. 2 (A) N2 adsorption–desorption isotherms and (B) pore size distribution profiles of PPZS (black), TAPT-DHTA-COF0.05@PPZS (blue), TAPT-DHTA-COF0.1@PPZS (red), and TAPT-DHTA-COF0.2@PPZS (green). FE-SEM images of (C) PPZS and (D) TAPT-DHTACOF0.1@PPZS.
To determine the pyrolysis temperature, we conducted thermal gravimetric analysis. A significant decomposition occurs at 400500 °C and after this stage a second clear decomposition happens at around 800-850 °C for TAPT-DHTA-COFX@PPZS (Fig. S9). Thus, we chose the pyrolysis temperature at 900 °C. At 900 °C, TAPT-DHTACOFX@PPZS losses 70%-80% of its original weight. After 3-h-pyrolysis at 900 °C under N2, TAPT-DHTA-COFX@PPZS was transformed into its corresponding carbons, namely DHTA-COFX@PPZS900 in 21% (X = 0.05), 32% (X = 0.1), and 34% (X = 0.2) yields. As controls, direct pyrolysis of PPZS and TAPT-DHTA-COF under otherwise identical conditions generates PPZS900 and TAPT-DHTA-COF900 in 38% and 55% yields, respectively. The core-shell hybrid systems could facilitate the pyrolysis process and lead to decreased carbon contents and increased heteroatom contents (Table S1) PPZS900 is spheres with a decreased diameter of 600-830 nm, compared to PPZS (Fig. 3D; Fig. S10). However, in the core-shell structure, the morphology is highly dependent on the thickness of the COF shell. When the shell is thin, such as in the case of TAPTDHTA-COF0.05@PPZS900, the shells tend to break into fragments (Fig. 3E; Fig. S11), suggesting that a thin COF shell is not strong enough to form a continuous spherical surface during the pyrolysis. Interestingly, TAPT-DHTA-COF0.1@PPZS900 retained a spherical shape but with nuts-like surface (Fig. 3F; Fig. S12). When the COF shell becomes thicker, TAPT-DHTA-COF0.2@PPZS900 forms sphere with much smoother surface (Fig. 3G; Fig. S13). By contrast, upon direct pyrolysis, TAPT-DHTA-COF900 resulted in aggregates and could not remain its original ribbon shape (Fig. S5C and D). Nevertheless, HR TEM measurements of the core-shell structures did not show a clear border between the core and shell (Fig. S14).
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As shown in Raman spectra, TAPT-DHTA-COFX@PPZS900 and –1 PPZS900 exhibited D and G bands at 1347 and 1601 cm , which were assigned to disordered carbons and ordered graphitic carbons, respectively (Fig. 3C). Their intensity ratio ID/IG for TAPT-DHTACOFX@PPZS900 was 1.04 (X = 0.05; blue), 0.98 (X = 0.1; red), and 0.96 (X = 0.2; green), respectively, indicating that the degree of graphitization becomes higher as the TAPT-DHTA-COF shell thickness is increased. Notably, direct pyrolysis yielded a low graphitization as evident by a high ID/IG value (black; 1.10 for PPZS900). These results indicate that the COF shell promotes the formation of graphitic 23-25 carbon that is key to conductivity.
Fig. 3 (A) N2 sorption isotherms, (B) pore size distribution profiles, and (C) Raman spectra of PPZS900 (black), TAPT-DHTA-COF0.05@PPZS900 (blue), TAPT-DHTA-COF0.1@PPZS900 (red), and TAPT-DHTA-COF0.2@PPZS900 (green). FE-SEM images of (D) PPZS900, (E) TAPT-DHTACOF0.05@PPZS900, (F) TAPT-DHTA-COF0.1@PPZS900, and (G) TAPT-DHTA-COF0.2@PPZS900 (insets for the enlarged images).
Direct pyrolysis decreased the porosity drastically and TAPT2 –1 DHTA-COF900 has a BET surface area of only 21 m g (Fig. S3A, black 2 –1 dots). This is in a sharp contrast to that (2170 m g ) of the TAPTDHTA-COF precursor (Fig. S3A, red dots). These results revealed that the direct pyrolysis eventually lose the structural features of the COF precursor. On the other hand, the BET surface area of PPZS900 was 2 –1 609 m g (Fig. 3A, black dots), which was much higher than that of pristine PPZS. The pore size distribution profile revealed that PPZS900 3 –1 has a pore volume of 0.26 cm g (Fig. S15; Table S1) and three different micropores with sizes of 0.77, 1.12, and 1.52 nm (Fig. 3B, black dots). With the PPZS core, TAPT-DHTA-COF0.05@PPZS900 (blue dots), TAPT-DHTA-COF0.1@PPZS900 (red dots), and TAPT-DHTACOF0.2@PPZS900 (green dots) exhibited the BET surface areas of 533, 2 –1 456 and 421 m g respectively, which were more than 20-fold higher than that of TAPT-DHTA-COF900. TAPT-DHTA-COFX@PPZS900 consists of only micropores with size between 0.5 and 1.5 nm (Fig. 3B, X = 0.05; blue, X = 0.1; red, and X = 0.2; green). The pore volumes of TAPT-DHTA-COFX@PPZS900 are 0.25 (X = 0.05), 0.22 (X = 0.1), and 3 –1 0.20 (X = 0.2) cm g (Fig. S15; Table S1). Therefore, a general tendency is that a higher COF content results in a lower BET surface area and pore volume of TAPT-DHTA-COFX@PPZS900.
The chemical nature of TAPT-DHTA-COFX@PPZS were 900 Online View Article DOI: 10.1039/C7CC07002K investigated by X-ray photoelectron spectroscopy (XPS). The C1s, O1s, N1s, S2p, and P2p peaks were observed (Fig. S16). As shown in Table S2, the ratio of heteroatoms (O, N, S, and P) to carbon are 7.37%, 12.21%, and 12.38% for TAPT-DHTA-COF0.05@PPZS900, TAPT-DHTACOF0.1@PPZS900, and TAPT-DHTA-COF0.2@PPZS900, respectively. Interestingly, these heteroatom contents are even threefold as high as those of PPZS900 (5.66%) and TAPT-DHTA-COF900 (4.08%). Thus, the COF shell can protect the release of heteroatoms during the pyrolysis, while the thin yet broken COF shell leads to a low heteroatom density. The energy storage capacity of capacitors was evaluated in a fabricated three-electrode system in an aqueous alkaline solution (6 M KOH, Supplementary Information). Cyclic voltammetry (CV) curves revealed typical double layer capacitive behaviors (Fig. S17). Interestingly, the current density is highly dependent on the core–1 shell structure. For example, at a scan rate of 50 mV s , the current density of TAPT-DHTA-COF900 (Fig. 4A, purple curve) and PPZS900 –1 (black curve) at –1.0 V was only 2.1 and 4.8 A g , respectively. In the core-shell structure, the current density is dependent on the thickness of the COF shell. The current density of TAPT-DHTA–1 COF0.05@PPZS900 (blue curve) is increased to 7.5 A g under otherwise identical conditions. Notably, TAPT-DHTA-COF0.1@PPZS900 –1 (red curve) exhibited the highest current density of 11.1 A g , whereas TAPT-DHTA-COF0.2@PPZS900 (green curve) displayed a –1 current density of 9.7 A g . The greatly increased current density reflects a sharply enhanced capacitive energy storage. In agreement with the tendency of current density, the capacitance is dependent on the core-shell structure (Fig. 4B). TAPT–1 DHTA-COF0.05@PPZS900 at a current density of 1 A g exhibited a –1 capacitance of 178 F g , which is much higher than those of PPZS900 –1 –1 (132 F g ) and TAPT-DHTA-COF900 (43.8 F g ). Surprisingly, TAPTDHTA-COF0.1@PPZS900 exhibited an exceptional capacitance of 287 F –1 g , which is 2.2 and 6.6 times higher than those of PPZS900 and TAPTDHTA-COF900, respectively. Similarly, TAPT-DHTA-COF0.2@PPZS900 –1 exhibited a high capacitance of 255 F g . Moreover, when the –1 current density was decreased to 0.5 A g , the capacitance increased significantly. For example, the capacitance of TAPT-DHTA–1 COF0.1@PPZS900 is as high as 411 F g (Fig. S18). To the best of our knowledge, this capacitance is far superior to the COF-based materials and state-of-the art carbons (Table S3). High-rate performance determines the quickness of charge and discharge process. PPZS900 (Fig. 4C, black triangles) and TAPT-DHTACOF900 (purple triangles) exhibited a sharp decrease in capacitance –1 to only 65 and 13.5 F g , respectively, when they were charged and −1 discharged at a high rate of 10 A g . By contrast, the core-shell structure enables a high capacitance at high rate. For example, TAPT−1 DHTA-COF0.05@PPZS900 (blue dots) at 10 A g could retain a –1 capacitance of 123 F g , which is almost one order of magnitude high than that of TAPT-DHTA-COF900, indicating the importance of being a core-shell structure for energy storage. Notably, TAPT-DHTACOF0.1@PPZS900 (red dots) and TAPT-DHTA-COF0.2@PPZS900 (green −1 – dots) at 10 A g exhibited a capacitance as high as 182 and 175 F g 1 , respectively. Thus, the core-shell TAPT-DHTA-COFX@PPZS900 is far superior in energy storage to the pure counterparts of PPZS900 and TAPT-DHTA-COF900. Especially, TAPT-DHTA-COF0.1@PPZS900 reached an exceptional capacitance that is 13.5-fold that of TAPT-DHTACOF900.
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Journal Name storage. These results suggest a novel platform based on COFs for View Article Online DOI: 10.1039/C7CC07002K energy storage. This work was supported by a Grant in-Aid for Scientific Research (A) (17H01218) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and support from the ENEOS Hydrogen Trust Fund and the Ogasawara Foundation for the Promotion of Science and Engineering.
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Fig. 4 (A) CV curves at 50 mV s , (B) galvanostatic charge/discharge curves at 1 A g , (C) capacitance at different current densities, and (D) Nyquist plots of PPZS900 (black), and TAPT-DHTA-COF900 (purple), TAPT-DHTA-COF0.05@PPZS900 (blue), TAPT-DHTACOF0.1@PPZS900 (red), TAPT-DHTA-COF0.2@PPZS900 (green). Inset in (D) is enlarged semicircles. (E) Cycle of TAPT-DHTA-COF0.1@PPZS900. Insets in (e) are charge-discharge curves.
The cycle performance of TAPT-DHTA-COF0.1@PPZS900 was −1 investigated at 10 A g (Fig. 4E and insets). Remarkably, the −1 capacitance retains at 182 F g even after 10,000 cycles. Figure S19 shows the equivalent circuit and the simulated impedance curves of the capacitors. From the impedance plots (Fig. 4D; Fig. S19), the charge-transfer resistance of TAPT-DHTACOF0.2@PPZS900 (Fig. 4D, green curve) and TAPT-DHTACOF0.1@PPZS900 (red curve) was evaluated to be 25 Ω, which is much smaller than those of TAPT-DHTA-COF0.05@PPZS900 (blue curve, 40 Ω) and PPZS900 (black curve, 46.4 Ω). Therefore, the COF-derived carbon shell increases the conductivity. Moreover, TAPT-DHTACOF0.1@PPZS900 exhibited a slope that is larger than those of TAPTDHTA-COFX@PPZS900 (X = 0.05 and 0.2), PPZS900, and TAPT-DHTACOF900. This result suggests that the ion diffusion in TAPT-DHTACOF0.1@PPZS900 is faster than that in PPZS900, TAPT-DHTA-COF900, and TAPT-DHTA-COFX@PPZS900 (X = 0.05 and 0.2). These mechanistic insights elucidate the significance and a synergistic effect of the coreshell structure on facilitating electron conduction and ion transport – two key processes involved in energy storage. In summary, we have disclosed for the first time that template pyrolysis is essential for processing carbons from crystalline porous COFs. This template strategy is general and facile and it enables the conversion of conventional COFs into conducting porous carbons. The core-shell structure not only triggers a synergistic effect on producing highly conducting porous carbons but also enables a combined positive influence on achieving high performance energy
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