Electron Highways into Nanochannels of Covalent Organic

Letter www.acsami.org

Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Electron Highways into Nanochannels of Covalent Organic Frameworks for High Electrical Conductivity and Energy Storage Yang Wu,*,† Dongwan Yan,† Zhongyue Zhang,† Michio M. Matsushita,† and Kunio Awaga*,† †

Department of Chemistry and Integrated Research Consortium on Chemical Sciences (IRCCS), Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by MIAMI UNIV on 02/04/19. For personal use only.

S Supporting Information *

ABSTRACT: To enhance the electron transfer within the covalent organic frameworks (COFs), we obtained a nanocomposite of conductive poly(3,4ethylenedioxythiophene) (PEDOT) and redox-active AQ-COF by performing a facile in situ solid-state polymerization inside the nanochannels of COFs. The PEDOT chains functioned like electron highways within the nanochannels, resulting in a PEDOT@AQ-COF nanocomposite with an excellent electrical conductivity of 1.1 S cm−1 and a remarkably improved performance in faradaic energy storage. The all-organic PEDOT@AQ-COF electrode showed specific capacitance as high as 1663 F g−1 (at 1 A g−1), ultrafast charge/discharge rate performance (998 F g−1 at 500 A g−1), and excellent stability for 10 000 cycles. This research demonstrates a promising strategy for increasing the conductivity of COF-based materials and broadening their applications. KEYWORDS: covalent organic frameworks, nanochannels, electron highways, electrical conductivity, energy storage

C

To address these issues, we introduced conductive poly(3,4ethylenedioxythiophene) (PEDOT) into the nanochannels of redox active AQ-COF (Figure 1A and Figure S1)16−20 in powder form by the in situ solid-state polymerization (SSP)26 method (Figure 1B, C). PEDOT is one of the most widely investigated conductive polymers because of its excellent electronic properties and high stability. PEDOT is usually prepared by chemical or electrochemical polymerization of a monomer in solution. Compared with polymerization in solution, the solvent-free and catalyst-free SSP method is a more promising approach for confining the polymerization to within the nanochannels.26 By building electron “highways” with conductive PEDOT chains inside the nanochannels, it becomes possible to produce a PEDOT@AQ-COF nanocomposite with billion-fold enhanced electrical conductivity and electrochemical performance, including high specific capacitance, ultrafast charge/discharge capability, and excellent stability. AQ-COF was synthesized according to the previously described method.20 The linkage, crystallinity and permanent porosity of AQ-COF were characterized by FT-IR (Figure S2), powder X-ray diffraction (PXRD) (Figure 1D), and nitrogen (N2) sorption measurements (Figure 1E), respectively. The Brunauer−Emmett−Teller (BET) surface area and pore volume were calculated to be 1203 m2 g−1 and 0.78 cm3 g−1,

ovalent organic frameworks (COFs) are a class of porous crystalline organic polymers that can be obtained by combining of proper organic building blocks into periodic structures via template-free polymerization reactions.1−4 The high flexibility in the structural and functional designs, along with the intrinsic properties such as the low skeleton density, large specific surface area, and high chemical stability, make COFs a promising materials platform for gas storage and separation,5,6 catalysis,7−10 sensing,11,12 and other applications.13−15 With their high porosity, tunable functionality and redox activity, COFs are promising candidates for use in electrochemical energy storage.16−22 Nevertheless, the insulating nature of the COFs has limited their electrochemical performance. Recently, several conductive COFs were prepared via a bottom-up strategy from specific building blocks.23−25 However, the highest conductivity was only 4 × 10−4 S cm−1. Additionally, the preparation of highly conductive COF materials via a bottom-up strategy is restricted by the limited number of building blocks.3 An alternate approach for increasing the electrical conductivity of COF materials would be to adopt a postsynthesis strategy. Dichtel and co-workers reported their strategy of introducing conductive polymer into the pores of redox active COF films by electropolymerization.17 The resulting modified COF films showed highly improved electrochemical performance, which demonstrated that a postsynthesis strategy could be an effective way to address the charge transfer limitation in electrochemical energy storage. Nevertheless, the disadvantages of electropolymerization and requirement of COF films as a precursor make it difficult to scale up to high-throughput production lines for practical application.22 © XXXX American Chemical Society

Received: December 11, 2018 Accepted: January 31, 2019 Published: January 31, 2019 A

DOI: 10.1021/acsami.8b21696 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (A) Chemical structure of AQ-COF. (B) SSP of DBrEDOT. (C) Schematic image for the preparation of PEDOT@AQ-COF. (D) PXRD patterns and (E) N2 sorption isotherms of AQ-COF and PEDOT@AQ-COF.

Figure 2. (A) Room-temperature I−V plots of AQ-COF, PEDOT and PEDOT@AQ-COF. (B) Temperature-dependent conductivity profiles of PEDOT and PEDOT@AQ-COF. (C) CV curves of AQ-COF/PEDOT and PEDOT@AQ-COF recorded at a scan rate of 5 mV s−1. Inset, enlarged CV curve of AQ-COF/PEDOT. (D) GCD profiles of PEDOT@AQ-COF at different current densities. Inset, enlarged GCD profile of PEDOT@AQ-COF at a current density of 500 A g−1. (E) discharge profiles of AQ-COF/PEDOT and PEDOT@AQ-COF. Inset, enlarged discharge profile of AQ-COF/PEDOT. (F) Specific capacitance of PEDOT@AQ-COF at different current densities, and cyclic stability over 10 000 cycles at 50 A g−1.

a DBrEDOT solution (see the Supporting Information). The resulting DBrEDOT@AQ-COF showed the same dark red color as AQ-COF. The second step was the in situ SSP of DBrEDOT into PEDOT within the nanochannels of AQ-COF. The PEDOT@AQ-COF nanocomposite was obtained as black powder after washing. The nitrogen/sulfur ratio in the

respectively. In chemical stability experiment (Figure S3), AQCOF showed strong resistance toward 1 M H2SO4. The PEDOT@AQ-COF nanocomposite was prepared in two steps (Figure 1C). In the first step, the 2,5-dibromo-3,4ethylenedioxythiophene (DBrEDOT) monomer was loaded into nanochannels of AQ-COF by evaporation of the solvent of B


Letter


linkages, Figure S1) and the porous structure of the PEDOT@ AQ-COF make this nanocomposite a promising candidate for application to supercapacitors. The electrochemical performances of AQ-COF/PEDOT (a well ground mixture of AQCOF and PEDOT in a mass ratio of 1:1.4) and PEDOT@AQCOF were evaluated by cyclic voltammetry (CV) measurements using three-electrode configurations in 1 M H2SO4 electrolyte under the potential window from −0.2 to 0.6 V vs Ag/AgCl (Figure 2C, Figure S13 and S14). The CV curves of AQ-COF/PEDOT and PEDOT@AQ-COF suggested that the capacitance characteristics of both AQ-COF/PEDOT and PEDOT@AQ-COF consisted of electrical double layer capacitance and pseudocapacitance. The pseudocapacitance was dominated by a reversible faradaic reaction of redox-active groups (Figure S1). At a sweep rate of 5 mV s−1, the peak current density due to the redox of redox-active groups in PEDOT@AQ-COF was 27 A g −1, which was 15 times that of AQ-COF/PEDOT (1.8 A g −1). This result suggests that more redox-active groups were reduced in PEDOT@AQ-COF, which refers to a rapid electronic exchange between the working electrode and redox-active groups in PEDOT@AQCOF, enhanced by highly conductive PEODT chains in the nanochannels of the AQ-COF. However, AQ-COF/PEDOT showed an equivalent series resistance (ESR; Figure S15) of only 4.2 Ω, which was lower than that of PEDOT@AQ-COF (4.4 Ω). However, in the ground AQ-COF/PEDOT mixture, PEDOT chains were located around the COF particles and outside of the channels. Therefore, the PEDOT chains could not directly deliver the electrons to the redox-active groups deep within the nanochannels of the insulating COF. To investigate the electrocapacitive behavior in greater detailed, the galvanostatic charge/discharge (GCD) analysis was performed at various current densities (Figure 2D and S16). The GCD profiles of PEDOT@AQ-COF exhibited voltage plateaus at around −0.03 V vs Ag/AgCl, which agrees with the position of the oxidation and reduction peaks in CV patterns. The GCD profiles are nearly symmetrical, suggesting the excellent electrochemical reversibility. In the discharge profiles, the rapid voltage decay from 0.6 to 0.1 V was responsible for a nonfaradaic process, namely, the electrostatic storage of the electrical energy via the separation of charges at the electrode surface. Subsequently, the slow voltage decay between 0.1 and −0.13 V was dominated by faradaic processes, which were achieved by electrochemical storage of the electrical energy via the redox reaction with charge transfer. Finally, the rapid voltage decay from −0.13 to −0.2 V was again dominated by a nonfaradaic process, indicating that the faradaic reaction was complete. For AQ-COF/PEDOT, the voltage decay was faster than that for PEDOT@AQ-COF in the nonfaradaic range. The voltage plateaus due to the faradaic reaction were shorter than that of PEDOT@AQ-COF. At a current density of 1 A g −1 (Figure 2E), the specific capacitance of PEDOT@AQ-COF was calculated to be 1663 F g−1, which is ∼6 times that of AQ-COF/PEDOT (274 F g−1). In the PEDOT@AQ-COF electrode, the abundant redoxactive groups provided high pseudocapacitance, and the chemical stability ensured long-term cycle stability. Additionally, the conductive PEDOT chains inside the nanochannels improved the charge-transfer by delivering the electron to each redox-active group, while the porous structure improved ion transport by shortening the diffusion distance. To the best of our knowledge, the PEDOT@AQ-COF electrode exhibited the highest specific capacitance among the COF-based

elemental analysis result indicated that the AQ-COF to PEDOT mass ratio was about 1:1.4. The formation of PEDOT in AQ-COF was confirmed using Fourier transform infrared (FT-IR) spectroscopy (Figure S2). The crystallinity of PEDOT@AQ-COF was assessed by PXRD (Figure 1D). AQ-COF shows an intense diffraction peak at 3.5° and a broad peak at 27°, corresponding to the 100 and 001 reflections, respectively. After SSP, the decrease in the relative intensity of the 100 reflection peak can be explained by the presence of disordered guest PEDOT polymer chains in the pores. PEDOT@AQ-COF presented a type I N2 sorption isotherm (Figure 1E), indicating microporosity. The BET surface area was calculated as 131 m2 g−1, and the sharply decreased porosity of the PEDOT@AQ-COF compared with AQ-COF indicated that the pores of AQ-COF contained PEDOT chains. The thermal stability of PEDOT@AQ-COF was evaluated by thermogravimetric analysis (TGA, Figure S4). The weight loss between 200 °C and 400 °C could be assigned to the decomposition of PEDOT. The decomposition of AQ-COF gave rise to the weight loss after 400 °C. SEM images (Figures S5 and S6) showed that the morphologies of AQ-COF and PEDOT@AQ-COF were almost the same. TEM images (Figures S7 and S8) indicated that both AQ-COF and PEDOT@AQ-COF are porous materials. In the energydispersive X-ray (EDX) elemental maps (Figures S9 and S10), the uniform distribution of sulfur element throughout the framework demonstrated the homogeneous distribution of PEDOT in the channels of AQ-COF. The electrical conductivity was evaluated by two-probe current−voltage (I−V) measurement (Figure S11A). Roomtemperature I−V plots of AQ-COF, PEDOT (prepared by SSP, see the Supporting Information) and PEDOT@AQ-COF are shown in Figure 2A. AQ-COF showed a conductivity of only ∼1 × 10−10 S cm−1, which indicates that AQ-COF was an insulator. The conductivities of PEDOT and PEDOT@AQCOF were estimated as 1.9 and 1.1 S cm−1, respectively. From the point of view of electrical conductance, PEDOT and PEDOT@AQ-COF appeared as conducting materials. The conductivity of PEDOT@AQ-COF was ∼10 orders of magnitude higher than that of AQ-COF. PEDOT@AQ-COF showed the highest value among the reported COFs to date (Table S1).23−25 The temperature-dependent conductivity was also studied (Figure 2B). It can be seen that the conductivities of PEDOT@AQ-COF and PEDOT decreased with decreasing temperatures, and the ln σ vs 1000/T plots were linear at all temperatures investigated. The rate of decrease of PEDOT@ AQ-COF was slower than that of PEDOT. At 77 K, the conductivity of PEDOT@AQ-COF was 1.12 × 10−1 S cm−1, whereas the conductivity of PEDOT was only 8.92 × 10−2 S cm−1. The activation energy (Ea) could be calculated from the slope of the ln σ vs 1000/T plot based on the Arrhenius equation. The Ea of PEDOT@AQ-COF (19 meV) was lower than that of PEDOT (32 meV), possibly due to the host−guest charge transfer interaction between the electron-accepting AQ units in the COF skeleton and the PEDOT in the channels. The conductivity of PEDOT@AQ-COF was also evaluated by four-probe method (Figure S11B). PEDOT@AQ-COF showed an electrical conductivity value of 1.0 S cm−1 at room-temperature (Figure S12A, B) and an activation energy of 17 meV (Figure S12C), which are close to those evaluated by two-probe method. The excellent electrical conductivity along with the abundant redox-active groups (AQ units and β-ketoenamine C



■

supercapacitors reported to date (Table S2). When the current density was increased to 500 A g−1, the specific capacitance decreased to 998 F g−1, which was 60% of the capacitance at 1 A g−1 (Figure 2F). This indicates that the PEDOT@AQ-COF electrode exhibited ultrafast charge/discharge capability. The cyclic stability was investigated by measuring the variation of the specific capacitance under 10000 GCD cycles at 50 A g−1 (Figure 2F). An increment of initial capacitance from 1076 to 1267 F g−1 was observed during the cycling process. This may have been attributable to the reorganization of PEDOT chains in nanochannels during the charge/ discharge process, which increased the porosity and thus the availability of H+ ions around the redox-active groups. In summary, the conductivity of a redox active AQ-COF was billion-fold enhanced over that of pristine AQ-COF by endogenous polymerization of PEDOT in the nanochannels. The resulting PEDOT@AQ-COF-based all-organic electrode showed impressive performance, including the high specific capacitance, ultrafast charge/discharge rate performance, and great stability. The approach of introducing conductive PEDOT to form electron “highways” into the nanochannels of COFs by performing catalyst-free SSP in the nanochannels will serve as a promising strategy for increasing the electrical conductivity of COFs and extending the applications of COF materials. This strategy could also be applied for other porous material platforms, including metal−organic frameworks (MOFs), porous aromatic frameworks (PAFs), conjugated microporous polymers (CMPs), etc. Finally, we note that this method would be applicable to large-scale syntheses, which is important for high-throughput production lines in industrial applications.

■

REFERENCES

(1) Diercks, C. S.; Yaghi, O. M. The Atom, the Molecule, and the Covalent Organic Framework. Science 2017, 355 (6328), eaal1585. (2) Huang, N.; Wang, P.; Jiang, D. Covalent Organic Frameworks: A Materials Platform for Structural and Functional Designs. Nat. Rev. Mater. 2016, 1 (10), 16068. (3) Ding, S.-Y.; Wang, W. Covalent Organic Frameworks (COFs): From Design to Applications. Chem. Soc. Rev. 2013, 42 (2), 548−568. (4) Lohse, M. S.; Bein, T. Covalent Organic Frameworks: Structures, Synthesis, and Applications. Adv. Funct. Mater. 2018, 28 (33), 1705553−1705571. (5) Zeng, Y.; Zou, R.; Zhao, Y. Covalent Organic Frameworks for CO2 Capture. Adv. Mater. 2016, 28 (15), 2855−2873. (6) Guan, X.; Ma, Y.; Li, H.; Yusran, Y.; Xue, M.; Fang, Q.; Yan, Y.; Valtchev, V.; Qiu, S. Ambient Temperature and Pressure Ionothermal Synthesis of Three-Dimensional Covalent Organic Frameworks. J. Am. Chem. Soc. 2018, 140 (13), 4494−4498. (7) Wang, X.; Han, X.; Zhang, J.; Wu, X.; Liu, Y.; Cui, Y. Homochiral 2D Porous Covalent Organic Frameworks for Heterogeneous Asymmetric Catalysis. J. Am. Chem. Soc. 2016, 138 (38), 12332−12335. (8) Wei, P.-F.; Qi, M.-Z.; Wang, Z.-P.; Ding, S.-Y.; Yu, W.; Liu, Q.; Wang, L.-K.; Wang, H.-Z.; An, W.-K.; Wang, W. Benzoxazole-Linked Ultrastable Covalent Organic Frameworks for Photocatalysis. J. Am. Chem. Soc. 2018, 140 (13), 4623−4631. (9) Pachfule, P.; Acharjya, A.; Roeser, J.; Langenhahn, T.; Schwarze, M.; Schomäcker, R.; Thomas, A.; Schmidt, J. Diacetylene Functionalized Covalent Organic Framework (COF) for Photocatalytic Hydrogen Generation. J. Am. Chem. Soc. 2018, 140 (4), 1423−1427. (10) Wu, Y.; Xu, H.; Chen, X.; Gao, J.; Jiang, D. A Π-Electronic Covalent Organic Framework Catalyst: Π-Walls as Catalytic Beds for Diels−Alder Reactions Under Ambient Conditions. Chem. Commun. 2015, 51 (50), 10096−10098. (11) Li, Z.; Huang, N.; Lee, K. H.; Feng, Y.; Tao, S.; Jiang, Q.; Nagao, Y.; Irle, S.; Jiang, D. Light-Emitting Covalent Organic Frameworks: Fluorescence Improving via Pinpoint Surgery and Selective Switch-on Sensing of Anions. J. Am. Chem. Soc. 2018, 140 (39), 12374−12377. (12) Lin, G.; Ding, H.; Yuan, D.; Wang, B.; Wang, C. A PyreneBased, Fluorescent Three-Dimensional Covalent Organic Framework. J. Am. Chem. Soc. 2016, 138 (10), 3302−3305. (13) Sun, Q.; Aguila, B.; Perman, J.; Earl, L. D.; Abney, C. W.; Cheng, Y.; Wei, H.; Nguyen, N.; Wojtas, L.; Ma, S. Postsynthetically Modified Covalent Organic Frameworks for Efficient and Effective Mercury Removal. J. Am. Chem. Soc. 2017, 139 (7), 2786−2793. (14) Ma, H.; Liu, B.; Li, B.; Zhang, L.; Li, Y.-G.; Tan, H.-Q.; Zang, H.-Y.; Zhu, G. Cationic Covalent Organic Frameworks: A Simple Platform of Anionic Exchange for Porosity Tuning and Proton Conduction. J. Am. Chem. Soc. 2016, 138 (18), 5897−5903. (15) Hao, Q.; Zhao, C.; Sun, B.; Lu, C.; Liu, J.; Liu, M.; Wan, L.-J.; Wang, D. Confined Synthesis of Two-Dimensional Covalent Organic Framework Thin Films Within Superspreading Water Layer. J. Am. Chem. Soc. 2018, 140 (38), 12152−12158. (16) DeBlase, C. R.; Silberstein, K. E.; Truong, T.-T.; Abruña, H. D.; Dichtel, W. R. β-Ketoenamine-Linked Covalent Organic Frameworks Capable of Pseudocapacitive Energy Storage. J. Am. Chem. Soc. 2013, 135 (45), 16821−16824. (17) Mulzer, C. R.; Shen, L.; Bisbey, R. P.; McKone, J. R.; Zhang, N.; Abruña, H. D.; Dichtel, W. R. Superior Charge Storage and Power Density of a Conducting Polymer-Modified Covalent Organic Framework. ACS Cent. Sci. 2016, 2 (9), 667−673. (18) Halder, A.; Ghosh, M.; Khayum, M. A.; Bera, S.; Addicoat, M.; Sasmal, H. S.; Karak, S.; Kurungot, S.; Banerjee, R. Interlayer Hydrogen-Bonded Covalent Organic Frameworks as High-Performance Supercapacitors. J. Am. Chem. Soc. 2018, 140 (35), 10941− 10945. (19) Han, Y.; Hu, N.; Liu, S.; Hou, Z.; Liu, J.; Hua, X.; Yang, Z.; Wei, L.; Wang, L.; Wei, H. Nanocoating Covalent Organic

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b21696.

■

Letter

Experimental methods, chemical stability experiment, FT-IR, TGA, SEM, TEM, EDX elemental mapping, conductivity measurement, CV, Nyquist plot, summary table of electrical conductivity (room temperature) of different COF-based materials reported to date, summary table of supercapacitor performance of different COF-based materials reported to date (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected]. *Email: [email protected]. ORCID

Yang Wu: 0000-0002-9899-3498 Kunio Awaga: 0000-0002-2193-0747 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge the financial support of a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. This work was partly supported by Nanotechnology Platform Program (Molecule and Material Synthesis) of MEXT, Japan. D


Letter

ACS Applied Materials & Interfaces Frameworks on Nickel Nanowires for Greatly Enhanced-Performance Supercapacitors. Nanotechnology 2017, 28 (33), 33LT01−LT09. (20) Wang, S.; Wang, Q.; Shao, P.; Han, Y.; Gao, X.; Ma, L.; Yuan, S.; Ma, X.; Zhou, J.; Feng, X.; Wang, B. Exfoliation of Covalent Organic Frameworks into Few-Layer Redox-Active Nanosheets as Cathode Materials for Lithium-Ion Batteries. J. Am. Chem. Soc. 2017, 139 (12), 4258−4261. (21) Wu, Y.; Zhang, Z.; Bandow, S.; Awaga, K. A Novel Strategy to Functionalize Covalent Organic Frameworks for High-Energy Rechargeable Lithium Organic Batteries via Graft Polymerization in Nano-Channels. Bull. Chem. Soc. Jpn. 2017, 90 (12), 1382−1387. (22) Xu, Q.; Dalapati, S.; Jiang, D. Charge Up in Wired Covalent Organic Frameworks. ACS Cent. Sci. 2016, 2 (9), 586−587. (23) Jin, E.; Asada, M.; Xu, Q.; Dalapati, S.; Addicoat, M. A.; Brady, M. A.; Xu, H.; Nakamura, T.; Heine, T.; Chen, Q.; Jiang, D. TwoDimensional sp2 Carbon-Conjugated Covalent Organic Frameworks. Science 2017, 357 (6352), 673−676. (24) Wang, L.; Zeng, C.; Xu, H.; Yin, P.; Chen, D.; Deng, J.; Li, M.; Zheng, N.; Gu, C.; Ma, Y. A highly soluble, crystalline, covalent organic framework compatible with device implementation. Chem. Sci. 2019, 10 (4), 1023−1028. (25) Cai, S.-L.; Zhang, Y.-B.; Pun, A. B.; He, B.; Yang, J.; Toma, F. M.; Sharp, I. D.; Yaghi, O. M.; Fan, J.; Zheng, S.-R.; Zhang, W.-G.; Liu, Y. Tunable electrical conductivity in oriented thin films of tetrathiafulvalene-based covalent organic framework. Chem. Sci. 2014, 5 (12), 4693−4700. (26) Meng, H.; Perepichka, D. F.; Wudl, F. Facile Solid-State Synthesis of Highly Conducting Poly(ethylenedioxythiophene). Angew. Chem., Int. Ed. 2003, 42 (6), 658−661.

E


Electron Highways into Nanochannels of Covalent Organic

Recommend Documents