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Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode
Journal Pre-proofs Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode Zejun Xu, Yanan Liu, Zhuoting Wu, Ruitong Wang, Qiufan Wang, Ting Li, Junheng Zhang, Juan Cheng, Zehui Yang, Sufang Chen, Menghe Miao, Daohong Zhang PII: DOI: Reference:
S1385-8947(20)30062-0 https://doi.org/10.1016/j.cej.2020.124071 CEJ 124071
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
3 November 2019 18 December 2019 7 January 2020
Please cite this article as: Z. Xu, Y. Liu, Z. Wu, R. Wang, Q. Wang, T. Li, J. Zhang, J. Cheng, Z. Yang, S. Chen, M. Miao, D. Zhang, Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej. 2020.124071
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Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode Zejun Xu#, a, Yanan Liu#, a, Zhuoting Wua, Ruitong Wanga, Qiufan Wang a, Ting Li a, Junheng Zhanga, Juan Chenga, Zehui Yangb, Sufang Chenc, Menghe Miaod, and Daohong Zhang*, a
a
Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of
Education & Hubei Key Laboratory of Catalysis and Materials Science, Hubei R&D Center of Hyperbranched Polymers Synthesis and Applications, South-Central University for Nationalities, Wuhan 430074, China. b
Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China
University of Geosciences Wuhan, 388 Lumo RD, Wuhan, 430074, P. R. China. c Key
Laboratory for Green Chemical Process of Ministry of Education, Hubei Key
Laboratory of Novel Reactor and Green Chemical Technology, School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan, Hubei 430205, PR China. d CSIRO
Manufacturing, 75 Pigdons Road, Waurn Ponds, Victoria 3216, Australia.
KEYWORDS: Covalent organic framework; carbon nanotube film; hyperbranched polymers; flexible supercapacitor
Abstract Covalent organic frameworks (COFs) have emerged as promising electrode materials in flexible and wearable supercapacitors. However, achieving high electrical conductivity and high mechanical strength of flexible COF composite membranes is still a major challenge. Herein, we first prepared the COF complex by using hydroxyl-ended hyperbranched polymer (OHP) as a template via a simple solid-state mechanical mixing method, and the COF@OHP complex was then impregnated on a microporous carbon nanotube film (CNTF) to construct a composite membrane (CHCM) that can be prepared as CHCM electrodes for extensible and flexible supercapacitors. A large number of cavities, reactive end-groups and flexible polymer chains of the hyperbranched polymer are utilized to stabilize and disperse COFs and to enhance the COF-polymer’s interfacial interaction. CNTF is an attractive material for energy storage due to its excellent conductivity, slippability, flexibility, and high specific surface area. The CHCM electrode showed not only excellent electrochemical performance (high gravimetric capacitance of 249 F g-1 and charging-discharging stability of 80% after 10000 cycles) in the relatively environmentally-friendly phosphoric acid (H3PO4) electrolyte, but also a high tensile strength of 180 MPa and an elongation of 10%. The gravimetric capacitance of the CHCM electrode reached 425 F g-1 in 2 M aq. H2SO4 electrolyte. This work will demonstrate a promising strategy for preparing wearable and flexible supercapacitors with high mechanical strength with potentially wide applications.
Introduction Covalent organic frameworks (COFs) have shown unique design features, tunable porosity and ordered network structures.[1-4] The syntheses routes of COFs include solvothermal,[5, 6] mechanochemical,[7] ionothermal,[8] catalytic-chemical[9] and other methods.[10] The solvothermal synthetic procedure is tedious and requires very long reaction time (72-90 h), which presents hurdles for exploring the diverse potentials of COF. Traditional mechanochemically, ionothermal, and catalyticchemical formed COF powders have poor crystallinity and moderate porosity in comparison to their solvothermal counterparts.[11] Until 2017, Rahul Banerjee’s group[10, 12] prepared a series of self-standing, porous and crystalline COF membranes[13] via the mechanochemical grinding modification method in the presence of a co-reagent [p-toluene sulfonic acid monohydrate (PTSA.H2O)]. This approach is very simple and can be highly cost-effective. COFs are used mostly in microcrystalline powder form with low processability, elongation and capacitive performance in the pure state, so they need to be densified to be suitable for industrial applications.[3, 14] The weak π-π interactions between COF nanosheets result in the poor elongation and mechanical strength of the COF membrane. For example, the breaking strain and tensile strength of 2,6-diaminoanthracene - 1,3,5-triformylphloroglucinol (DqTp) COF thin sheets were only 2% and 1.9 MPa, respectively.[15] 2,4,6-Trimethoxy-1,3,5-benzenetricarbaldehyde - 2,6-diaminoanthraquinone (TpOMe -DAQ) COF thin sheets showed a breaking strain of 3.5% and tensile strength of 7.0 MPa,[12] and DqDaTp- carbon nanofiber (CNF) hybrid exhibited 5.8% breaking
strain and 7.5 MPa tensile strength.[16] While there has been significant progress, further improvement of the mechanical strength of flexible COF composite membranes remains a challenge to meet industrial applications. The components require abundant active sites, internal cavity and slippability to enhance the elongation[17] of COF composite membranes. Hydroxyl -terminated hyperbranched polyesters (OHP, Scheme 1)[18] are highly branched polymers containing a large number of branching units[19, 20] and abundant hydroxyl groups on the surface. A large number of cavities, reactive end-groups, and flexible polymer chains can be utilized to stabilize and disperse COF and to enhance the COF-polymer’s interfacial interaction.[21] Carbon nanotube films (CNTF) possess excellent slippability, flexibility, and adjustable surface area.[22] Hyperbranched polyesters could functionalize CNTF to improve the dispersion of CNTF and to increase the slip between CNTF, leading to improve strength and elongation of CNTF.[17] Therefore, OHP@CNTF complex can potentially serve as an electrode additive in COF composite membrane to improve material mechanical strength by constructing an elastic framework of flexible OHP chains and slip between CNTF. With their high surface area, tunable redox-active functionality and high chemical stability, β-keto-enamine-based COFs are outstanding electrodes for electrochemical energy storage.[7, 23, 24] However, their very low electrical conductivity, poor electrochemical stability, and poor elongation greatly restrict their utilization in supercapacitors.[23] Many physical and chemical modification methods have been researched in order to improve the electrical conductivity of COF. Physical
mechanochemical modifications include doping with carbon black [7, 15] and carbon nanotube (CNT) [16, 25, 26]. For example, the COF@CNT based composites resulted in improved charge storages in batteries owing to enhance the electrical conductivity.[25] Chemical modifications have also been investigated to the enhancement of the electrical conductivity of COF via ex-situ loading of conducting polymers poly(3,4-ethylenedioxythiophene) (PEDOT) in the COF backbone.[27] Both physical and chemical modification approaches could improve the electrochemical performance, but the constructed rigid electrodes are difficult to be used in wearable electronics. An available preparation strategy for flexible COF composite membranes with high electrical conductivity and high mechanical strength is still a large challenge. Herein, we demonstrate a strategy for constructing β-ketoenamine-linked, redox-active COF complex by using OHP as template via a simple solid-state mechanical mixing method,[3, 16] the COF@OHP complex was then impregnated on microporous electrically conducting CNTF to construct a COF@OHP@CNTF composite membrane (CHCM), which can be prepared as CHCM electrodes for extensible and flexible supercapacitors. The CHCM electrode showed outstanding gravimetric capacitance of 249 F g-1 in three-electrode assemblies in the relatively environmentally-friendly
phosphoric
acid
(H3PO4)
electrolyte
and
retains
charging-discharging stability of 80% after 10 000 charge cycles, which was attributed to COF@OHP complex prevent the agglomeration and collapse of COF and CNTF by restriction effect of CNTF, increase the interaction between COF and
CNTF, and improve electron conduction rate between COF and CNTF. The resulting CHCM demonstrated a high tensile strength of 180 MPa and elongation of 10% due to the π-π interaction between CNTF and COF in the framework and slide of the flexible OHP chains between COF and CNTF while maintaining high load transfer efficiency. This work will supply a promising strategy for preparing wearable and flexible supercapacitors with high mechanical strength and wide application.
Experimental Section Materials Carbon nanotube films (CNTF) with 6-12 μm thickness were synthesized by a floating catalyst chemical vapor deposition (CVD) method.[22] Hydroxyl-ended hyperbranched polymers (OHP, Scheme 1) were supplied by Wuhan Hyperbranched Polymers Science & Technology Co. Ltd., China. 1,3,5-triformylphloroglucnol (Tp), 2,6-diaminoanthracene (DAQ) and p-toluenesulfonic acid monohydrate (PTSA.H2O) are purchased from Energy Chemical Reagents Co. Ltd., China.
Scheme 1. Chemical structure of OHP
Synthesis of DAQ-Tp COF membrane PTSA.H2O (1.8 mmol) was mixed with DAQ (0.45 mmol, 108 mg) in a mortar-pestle. The mixture was ground for 3 minutes. Tp (0.3 mmol, 63.0 mg) was added to the mixture and ground for about 15 minutes. A few drops of water (∼ 150 μL) was added to the mixture. The dough was cast on a glass slide (length 5 cm and diameter 2.5 cm) to form a film of uniform thickness, and then heated at 90 °C for 10 h in a closed container under humid condition. The crystalline and porous COF membranes were gained after washed hot water (~80 °C) and N,N-dimethylacetamide (DMF).
Synthesis of CHCM The CHCM synthesis process was described in Fig. 1. PTSA.H2O (1.8 mmol) was mixed with DAQ (0.45 mmol, 108 mg) in a mortar-pestle. The mixture is ground for 3 minutes. Tp (0.3 mmol, 63.0 mg) was added to the mixture and ground for about 15 minutes. Next, OHP (1%, 5%, 10%, 15%, and 20%) was added and thoroughly mixed through grinding. A few drops of water (∼ 200 μL based on requirement) is added to the mixture. The resulting mixture was then ground thoroughly until it became a dough-like material. The dough was cast on a glass slide (length 5 cm and diameter 2.5 cm) to form a film of uniform thickness. The CNTF was placed on the above film and then drops cast on the CNTF with uniform thickness and is additionally allowed to heat at 90 °C for 2-10 h in a closed container under humid condition. After completion, the glass slide coated with CHCM was dipped into hot water (~80 °C) to isolate CHCM and to remove PTSA. It is then washed with DMF to remove unreacted starting materials. The crystalline and porous CHCM (80-85 % isolated yield) is
stored finally in distilled water for use in further studies.
Results and discussion
Fig. 1 Scheme of the construction process for extensible and flexible supercapacitors DAQ-Tp COF membrane synthesis process is described in the Experimental Section.[10] The FT-IR spectra (Fig. 2a-b) of COF show the disappearance of the N-H absorption peak (3330 cm-1) and the appearance of a new C-N absorption peak (1250 cm-1). The C=O absorption peak of Tp at 1645 cm-1 shifted to the β-ketoenamine C=O absorption peak at 1618 cm-1. The resonances of C=O and C-N were attributed to the presence of β-ketoenamine linkage in COF (Fig. 2a-b), which were consistent with the results reported in the literature.[7, 15] The COF shows a strong diffraction peak at 3.5° and a broad peak at 27°, corresponding to the 100 and 001 reflections, respectively (Fig. 2d), which matched well with the simulated tilted-AA structure.[7, 16] The Brunauer-Emmett-Teller (BET) pore volume and surface area of COF were 0.73 cm3g-1and 1486 m2g-1, respectively (Fig. 2f). The cross-section scanning electron microscope (SEM) images of COF show closely
spaced sheets (Fig. S1, ESI). Moreover, the thickness of the COF membrane was calculated to be 30-50 μm from the cross-section images.
Fig. 2 (a&b) FT-IR spectra of Tp, DAQ, OHP, COF and CHCM (5% OHP, 90 oC heating for 10 h). (c) FT-IR spectra of CHCM (5% OHP) at 90 oC heating for 2-12 h. (d) XRD of COF and CHCM (5% OHP, 90 oC heating for 10 h). (e) Layered chemical structures of DAQ-Tp COF. (f) BET of COF and CHCM (5% OHP, 90 oC heating for 10 h).
The CHCM synthesis process is shown schematically in Fig. 1 and described in the Experimental Section. Briefly, PTSA-H2O, DAQ, Tp and OHP (1%, 5%, 10%, 15%, and 20%) were mixed and ground for about 45 minutes. The mixtures in the form of paste were suspended on both sides of CNTF with a uniform thickness and heated at 90 °C for 2-12 h to obtain CHCMs (Fig. 1). FT-IR spectra of the CHCM exhibit in Fig. 2a-b, characteristic bands attributable to C-N stretching at 1265 cm-1 and C=O bond at 1618 cm-1 of the β-ketoenamine in the spectra of pure COF and -OH bond at 3730 cm-1, -CH3 at 2950 cm-1 and COO- bond at 1727 cm-1 for OHP, indicating the
formation of CHCM. The characteristic frequencies at 1265 cm-1 shifted to the higher wavenumbers, demonstrating the presence of strong hydrogen-bond interaction between COF, OHP, and CNTF.[28] The absorption peak area of -OH bond at 3730 cm-1, -CH3 at 2950 cm-1, COO- bond at 1727 cm-1 and C-O-C bond at 1175 cm-1 increased with increase of heating time from 2 h to 10 h and then decreased with further increase of time to 12 h (Fig. 2c), indicating that too long reaction time could decrease the content of OHP in CHCM. The crystallinity of CHCM was assessed by XRD. The appearance of two crystalline XRD peaks of CHCM with 100 and 001 reflections at 3.5 and 25° (2θ), respectively (Fig. 2d), which bears similarity with COF. The decrease of the relative intensity at 100 peak for CHCM was attributed to the presence of OHP@CNTF in the COF pores. The CHCM displays a smooth sheet of an estimated 20 μm in thickness with large areal synthetic scalability and flexibility (Fig. S2). These findings encouraged us to investigate the use of CHCM as free-standing supercapacitor electrodes.[12] The BET surface area of CHCM was calculated to be 167 m2g-1 with a pore volume estimated to be 0.72 cm3g-1 (Fig. 2f). The sharply decreased surface area of CHCM due to the pores of COF contained OHP chains and CNTF. The thermal stability of CHCM was characterized by thermogravimetric analysis (TGA, Fig. S3). The weight loss of pristine CNTF, OHP, COF, and CHCM (5% OHP, 90 oC heating for 10 h) with char yields in Fig. S2 were 98.50%, 2.28, 54.07 and 71.50%, respectively. The TGA profiles exhibit good thermal stability of CHCM up to 365 °C. The morphologies of CHCMs were characterized by the surface and the cross-sectional SEM images, as shown in Fig. 3.
The pristine CNTF network exhibited excellent continuity and uniformity (Fig. S4). Fig. 3 shows that the surface of the CNTF was coated evenly with COF@OHP, causing the bundles of CNTF to become slightly thicker than those of the pristine CNTF. In order to clearly observe the morphological evolution procedures, we heated the CHCMs between 2 and 12 hours. As shown in Fig. 3a and 3e, at 2h, COF@OHP complex was gradually impregnated in CNTF and formed a very thick aggregation of OHP and CNTF. At 6h, COF@OHP complex was further impregnated in CNTF and a distinctive self-assembled layered structure occurred in the densely packed CNTF sandwiched by adjacent COF sheets and flexible OHP chains (Fig. 3b and 3f).[29, 30] However, there was still some aggregation of COF@OHP, which was not infiltrated into CNTF. With further increase of heating time, COF@OHP complex was completely impregnated in CNTF and evenly dispersed, the COF nanosheets and flexible OHP chains composed of closely packed CNTF networks became a contract and densify, as shown in Fig. 3c and 3g for 10h. The cross-section SEM images of CHCM (5% OHP, 90 oC heated for 10 h) are provided in Fig. 3d and 3h, which suggests that COF@OHP penetrated inside the CNTF. SEM images of CHCMs displayed in Fig. 3 show that the CNTF was fully covered by COF@OHP. The above observations indicated that OHP acted as a template during controlled heating time CHCM synthesis.
Fig. 3 SEM images of CHCMs (5% OHP) with different time durations at 90 oC: 2h (a & e), 6h (b & f) and 10h (c & g). The cross-section SEM images of (d & h) CHCM (5% OHP, 90 oC heated for 10 h).
The electrical conductivities of the COF membrane, CNTF and CHCM (5% OHP, 90 oC heated for 10h) were evaluated using an Agilent U1241B multimeter (Fig. S5a).[31] Compared with the pristine COF membrane (48.2 S/cm), neat CNTF (85.4 S/cm) and COF@CNTF hybrid (61.6 S/cm), the electrical conductivity (234.3 S/cm) of CHCM electrode was increased to 2-5 times.[27] The electrochemical impedance spectroscopy (EIS)[32] of the CHCM electrode showed an equivalent series resistance (ESR) of 38 Ω (Fig. S5b). The EIS of CHCM electrode was lower than COF, CNTF, and COF@CNTF, indicating that the CHCM possessed a low charge transport resistance. The electrochemical performances of CHCM electrodes were further investigated in a three-electrode system with an environmentally-friendly H3PO4 (1 M, aq.) electrolyte. Compared with sulfuric acid (H2SO4), H3PO4 is a relatively safe acid without strong oxidation and corrosiveness, easy to store and transport.[33-35] Fig. 4a displays the cyclic voltammetry (CV) curves of 1 M aq. H3PO4 electrode at 30 mV s-1 scan rate for COF, CNTF, COF@CNTF and CHCM, respectively. CHCM electrode
demonstrated a far larger surrounding area than COF, CNTF and COF@CNTF of the CV curve, suggesting that the CHCM possessed a much higher capacitance. Fig. 4b illustrates the reversible switching between hydroquinone (C-OH) and quinone (C=O) transformation due to the π-π interactions between COFs and CNTF.[12, 16] Fig. 4c shows the galvanostatic charge-discharge (GCD) curves of the COF, CNTF, COF@CNTF and CHCM electrodes at the same current of 0.2 mA. The results proved that the performance of the CHCM electrode was far better than that of the COF, CNTF and COF@CNTF electrodes, which was consistent with Fig. 4a.
Fig. 4 (a) CV curves of electrochemical capacitors based on CHCM (5% OHP, 90 oC heating for 10 h) electrode compared with COF, CNTF, and COF@CNTF in 1 M aq. H3PO4 electrolyte. (b) Redox behavior of CHCM through reversible quinine to hydroquinone transformation. (c) GCD curves of electrochemical capacitors based on CHCM (5% OHP, 90 oC heating for 10 h) compared with COF, CNTF and COF@CNTF in 1 M aq. H3PO4 electrolyte.
We explored the effect of the OHP content and heating time on the electrochemical performance of the CHCM electrode (Fig. 5-6). As shown in Fig. 5a-c, the capacitance of CHCM increased with increasing content of OHP from 0% to 5% and then decreased with a further increase of the content to 20%, indicating that too much content of OHP could decrease the electrode’s capacitance. The capacitance
decreased with increase of heating time from 2 h to 6 h and then started to increase with further increase of heating time to 10 h before it started to decrease sharply with further increase of heating time to 12 h (Fig. 6), indicating that too long time for construction of CHCM could weaken the electrode’s capacitance. Therefore, the optimum content of OHP and heating time were 5 wt % and 10 h, respectively. The highest gravimetric capacitance of 249 F g-1 was achieved by using 1 M aq. H3PO4 electrolyte. All the above analyses demonstrated that OHP played an important role in the electrochemical process. The outstanding capacitive performance mechanism of CHCM by the COF complex is explained. As a template, OHP can effectively disperse and open COF in flexible polymer chains (Fig. 1).[22] COF@OHP complex was gradually impregnated in CNTF and evenly dispersed (Fig. 3c and 3g). Agglomeration and collapse of COF and CNTF were prevented by restricting the effect of CNTF, increasing the interaction between COF and CNTF, and improving the electron conduction between COF and CNTF.[12, 16] These consequently improved the electrochemical performance.
Fig. 5 Electrochemical properties of CHCM (5% OHP, 90 oC heating for 10 h) electrode in three-electrode assembly using 1M aq. H3PO4 electrolyte with different content of OHP (a) CV and (b) GCD at 0.15 mg effective quality. (c) Gravimetric capacitance as a function of the content of OHP.
Fig. 6 Electrochemical properties of CHCM (5% OHP, 90 oC heating for 10 h) electrode in three-electrode assembly using 1M aq. H3PO4 electrolyte at different heating times. (a) CV and (b) GCD using 0.15 mg effective quality. (c) Gravimetric capacitance as a function of reaction time.
Fig. 7a and b show the typical CV and GCD curves of the CHCM electrodes. The CHCM electrodes demonstrated pseudocapacitive characteristics in the potential window, as suggested by the nonrectangular CV curves (Fig. 7a). As the scan rate increased from 10 to 300 mV s-1, the CV curve changed size but maintained almost the same shape. The GCD curves in Fig. 7b were recorded at different currents 0.2, 0.3, 0.5, 0.8, and 1.0 mA, from which capacitance was determined. The GCD curves (Fig. 7b) were nearly symmetric, demonstrating the high reversibility of CHCM electrodes during discharge and charge processes. During 0.2 mA scanning in 1 M aq. H3PO4 electrolyte, the recorded gravimetric capacitance reached 249 F g-1. In order to compare with TpOMe-DAQ COF,[12] the electrochemical performances of CHCM were further investigated with 2 M aq. H2SO4 electrolyte. When tested in 2 M aq. H2SO4 electrolyte, the CHCM displayed an outstanding capacitive performance of 425 F g-1 (Fig. S6). As far as we know, this gravimetric capacitance was the highest value ever reported for pure COF-based electrodes (Table S1).[12, 15] The CHCM
electrode also exhibited high cyclic stability of 80% retention over 10 000 cycles at the current of 0.8 mA (Fig. 7c). The corresponding Coulombic efficiency[12] of the CHCM electrode keeps intact (Fig. 7c).
Fig. 7 Electrochemical properties of CHCM in three-electrode assembly using 1M aq. H3PO4 electrolyte: (a) CV, (b) GCD and (c) Cyclic stability performance (0.8 mA). The mechanical strength of CHCM (5% OHP, 90 oC heating for 10 h) was evaluated by tensile testing (Fig. 8). The CHCM showed lower elongation than neat CNTF because the slip between CNT and COF in the CHCM became more difficult after functionalization. The CHCM exhibited a higher breaking strain (10%, Fig. 8a, c-d) than COF[15] and DqDaTp-CNF hybrid[16], which was attributed to slide of the flexible OHP chains between COF and CNTF while maintaining high load transfer efficiency, leading to improve elongation (Fig. 9).[22] The CHCM showed much higher tensile strength (180 MPa, Fig. 8b) than neat CNTF (~20 MPa) and other COF thin sheets.[12, 15] Compared with neat CNTF and COF, CHCM had much higher tensile strength due to the stronger π-π interaction between CNTF and COF in the framework, the high stretch of hyperbranched OHP chains together with the densification of the CNTF. The excellent mechanical strength and electrochemical performance allowed us to construct extensible and flexible supercapacitors from two
0.5 cm2 CHCM sheet electrodes and 1M H3PO4/PVA gel electrolyte. The supercapacitor achieved a high gravimetric capacitance of 14 F/g (Fig. 10). When the supercapacitor was bent to different bending angles (0°, 45°, 90°, 135°, and 180°), the GCD curves did not change significantly (Fig. 10c), indicating remarkably stable capacitive performance under bending states.[36] To demonstrate the energy storage of flexible supercapacitors, we connected five such solid-state devices in series to light a 1.5 V LED (Fig. 11).
Fig. 8 Mechanical strength and flexible performance of CHCM
Fig. 9 Flexible mechanism of CHCM
Fig. 10 The electrochemical properties of extensible and flexible supercapacitors (a) CV, (b) GCD and (c) GCD under different bending angles using 0.5 cm2 real area.
Fig. 11 Lighting up of 1.5 V LED using five solid-state-devices of supercapacitors
Conclusion Redox-active, chemically stable supercapacitor electrode material was fabricated using a simple solid-state mechanical mixing method. The resulting CHCM-based electrode demonstrated a high tensile strength of 180 MPa and elongation of 10% due to the π-π interaction between CNTF and COF in the framework and slide of the flexible OHP chains between COF and CNTF while maintaining high load transfer efficiency. The three-electrode system of CHCM electrode exhibited an exceptionally high gravimetric capacitance of 249 F g-1 and charging-discharging stability of 80% after 10 000 cycles in the relatively environmentally-friendly H3PO4 electrolyte, which was attributed to the strong π-π interaction between CNTF and COF in the framework and increased the electron conduction between COF and CNTF. The high-performance flexible supercapacitor with high mechanical strength has the potential for a wide range of applications.
ASSOCIATED CONTENT Supporting Information Supplementary data associated with this article can be found in the online version at https://doi.org/10.1016/.
AUTHOR INFORMATION Corresponding Author *(Dr. D. Zhang), Email: [email protected]. Author Contributions #
These authors contributed equally to this work.
Notes The authors declare no competing financial interest.
Acknowledgements We gratefully acknowledge the financial support of the National Natural Science Foundation of China (51703250, 51873233 and 21703212), Hubei Major Projects of Technological Innovation (2017AAA131) and the Hubei Natural Science Foundation (2018CFA023).
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Graphical abstract Title: “Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode”
Redox-active, chemically stable supercapacitor electrode material was constructed by impregnating the COF@OHP complex on microporous electrically conducting CNTF. The CHCM-based electrode showed not only excellent electrochemical performance (high gravimetric capacitance of 249 F g-1 and charging-discharging stability of 80% after 10 000 cycles) in the relatively environmentally-friendly H3PO4 electrolyte, and but also high tensile strength of 180 MPa and elongation of 10%.
Dec. 18th, 2019
Title: “Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode” Chemical Engineering Journal Research Highlights:
The composite membrane (CHCM) electrode showed a high tensile strength of 180 MPa and elongation of 10%.
The gravimetric capacitance of the CHCM electrode reached 425 F g-1 in 2 M aq. H2SO4 electrolyte.
CHCM electrode showed excellent electrochemical performance in the relatively environmentally- friendly H3PO4 electrolyte.
Dec. 18th, 2019
Declaration of Interest Statement Title: “Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode” Chemical Engineering Journal The authors declare no competing financial interest.
S1385-8947(20)30062-0 https://doi.org/10.1016/j.cej.2020.124071 CEJ 124071
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
3 November 2019 18 December 2019 7 January 2020
Please cite this article as: Z. Xu, Y. Liu, Z. Wu, R. Wang, Q. Wang, T. Li, J. Zhang, J. Cheng, Z. Yang, S. Chen, M. Miao, D. Zhang, Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej. 2020.124071
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© 2020 Published by Elsevier B.V.
Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode Zejun Xu#, a, Yanan Liu#, a, Zhuoting Wua, Ruitong Wanga, Qiufan Wang a, Ting Li a, Junheng Zhanga, Juan Chenga, Zehui Yangb, Sufang Chenc, Menghe Miaod, and Daohong Zhang*, a
a
Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of
Education & Hubei Key Laboratory of Catalysis and Materials Science, Hubei R&D Center of Hyperbranched Polymers Synthesis and Applications, South-Central University for Nationalities, Wuhan 430074, China. b
Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China
University of Geosciences Wuhan, 388 Lumo RD, Wuhan, 430074, P. R. China. c Key
Laboratory for Green Chemical Process of Ministry of Education, Hubei Key
Laboratory of Novel Reactor and Green Chemical Technology, School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan, Hubei 430205, PR China. d CSIRO
Manufacturing, 75 Pigdons Road, Waurn Ponds, Victoria 3216, Australia.
KEYWORDS: Covalent organic framework; carbon nanotube film; hyperbranched polymers; flexible supercapacitor
Abstract Covalent organic frameworks (COFs) have emerged as promising electrode materials in flexible and wearable supercapacitors. However, achieving high electrical conductivity and high mechanical strength of flexible COF composite membranes is still a major challenge. Herein, we first prepared the COF complex by using hydroxyl-ended hyperbranched polymer (OHP) as a template via a simple solid-state mechanical mixing method, and the COF@OHP complex was then impregnated on a microporous carbon nanotube film (CNTF) to construct a composite membrane (CHCM) that can be prepared as CHCM electrodes for extensible and flexible supercapacitors. A large number of cavities, reactive end-groups and flexible polymer chains of the hyperbranched polymer are utilized to stabilize and disperse COFs and to enhance the COF-polymer’s interfacial interaction. CNTF is an attractive material for energy storage due to its excellent conductivity, slippability, flexibility, and high specific surface area. The CHCM electrode showed not only excellent electrochemical performance (high gravimetric capacitance of 249 F g-1 and charging-discharging stability of 80% after 10000 cycles) in the relatively environmentally-friendly phosphoric acid (H3PO4) electrolyte, but also a high tensile strength of 180 MPa and an elongation of 10%. The gravimetric capacitance of the CHCM electrode reached 425 F g-1 in 2 M aq. H2SO4 electrolyte. This work will demonstrate a promising strategy for preparing wearable and flexible supercapacitors with high mechanical strength with potentially wide applications.
Introduction Covalent organic frameworks (COFs) have shown unique design features, tunable porosity and ordered network structures.[1-4] The syntheses routes of COFs include solvothermal,[5, 6] mechanochemical,[7] ionothermal,[8] catalytic-chemical[9] and other methods.[10] The solvothermal synthetic procedure is tedious and requires very long reaction time (72-90 h), which presents hurdles for exploring the diverse potentials of COF. Traditional mechanochemically, ionothermal, and catalyticchemical formed COF powders have poor crystallinity and moderate porosity in comparison to their solvothermal counterparts.[11] Until 2017, Rahul Banerjee’s group[10, 12] prepared a series of self-standing, porous and crystalline COF membranes[13] via the mechanochemical grinding modification method in the presence of a co-reagent [p-toluene sulfonic acid monohydrate (PTSA.H2O)]. This approach is very simple and can be highly cost-effective. COFs are used mostly in microcrystalline powder form with low processability, elongation and capacitive performance in the pure state, so they need to be densified to be suitable for industrial applications.[3, 14] The weak π-π interactions between COF nanosheets result in the poor elongation and mechanical strength of the COF membrane. For example, the breaking strain and tensile strength of 2,6-diaminoanthracene - 1,3,5-triformylphloroglucinol (DqTp) COF thin sheets were only 2% and 1.9 MPa, respectively.[15] 2,4,6-Trimethoxy-1,3,5-benzenetricarbaldehyde - 2,6-diaminoanthraquinone (TpOMe -DAQ) COF thin sheets showed a breaking strain of 3.5% and tensile strength of 7.0 MPa,[12] and DqDaTp- carbon nanofiber (CNF) hybrid exhibited 5.8% breaking
strain and 7.5 MPa tensile strength.[16] While there has been significant progress, further improvement of the mechanical strength of flexible COF composite membranes remains a challenge to meet industrial applications. The components require abundant active sites, internal cavity and slippability to enhance the elongation[17] of COF composite membranes. Hydroxyl -terminated hyperbranched polyesters (OHP, Scheme 1)[18] are highly branched polymers containing a large number of branching units[19, 20] and abundant hydroxyl groups on the surface. A large number of cavities, reactive end-groups, and flexible polymer chains can be utilized to stabilize and disperse COF and to enhance the COF-polymer’s interfacial interaction.[21] Carbon nanotube films (CNTF) possess excellent slippability, flexibility, and adjustable surface area.[22] Hyperbranched polyesters could functionalize CNTF to improve the dispersion of CNTF and to increase the slip between CNTF, leading to improve strength and elongation of CNTF.[17] Therefore, OHP@CNTF complex can potentially serve as an electrode additive in COF composite membrane to improve material mechanical strength by constructing an elastic framework of flexible OHP chains and slip between CNTF. With their high surface area, tunable redox-active functionality and high chemical stability, β-keto-enamine-based COFs are outstanding electrodes for electrochemical energy storage.[7, 23, 24] However, their very low electrical conductivity, poor electrochemical stability, and poor elongation greatly restrict their utilization in supercapacitors.[23] Many physical and chemical modification methods have been researched in order to improve the electrical conductivity of COF. Physical
mechanochemical modifications include doping with carbon black [7, 15] and carbon nanotube (CNT) [16, 25, 26]. For example, the COF@CNT based composites resulted in improved charge storages in batteries owing to enhance the electrical conductivity.[25] Chemical modifications have also been investigated to the enhancement of the electrical conductivity of COF via ex-situ loading of conducting polymers poly(3,4-ethylenedioxythiophene) (PEDOT) in the COF backbone.[27] Both physical and chemical modification approaches could improve the electrochemical performance, but the constructed rigid electrodes are difficult to be used in wearable electronics. An available preparation strategy for flexible COF composite membranes with high electrical conductivity and high mechanical strength is still a large challenge. Herein, we demonstrate a strategy for constructing β-ketoenamine-linked, redox-active COF complex by using OHP as template via a simple solid-state mechanical mixing method,[3, 16] the COF@OHP complex was then impregnated on microporous electrically conducting CNTF to construct a COF@OHP@CNTF composite membrane (CHCM), which can be prepared as CHCM electrodes for extensible and flexible supercapacitors. The CHCM electrode showed outstanding gravimetric capacitance of 249 F g-1 in three-electrode assemblies in the relatively environmentally-friendly
phosphoric
acid
(H3PO4)
electrolyte
and
retains
charging-discharging stability of 80% after 10 000 charge cycles, which was attributed to COF@OHP complex prevent the agglomeration and collapse of COF and CNTF by restriction effect of CNTF, increase the interaction between COF and
CNTF, and improve electron conduction rate between COF and CNTF. The resulting CHCM demonstrated a high tensile strength of 180 MPa and elongation of 10% due to the π-π interaction between CNTF and COF in the framework and slide of the flexible OHP chains between COF and CNTF while maintaining high load transfer efficiency. This work will supply a promising strategy for preparing wearable and flexible supercapacitors with high mechanical strength and wide application.
Experimental Section Materials Carbon nanotube films (CNTF) with 6-12 μm thickness were synthesized by a floating catalyst chemical vapor deposition (CVD) method.[22] Hydroxyl-ended hyperbranched polymers (OHP, Scheme 1) were supplied by Wuhan Hyperbranched Polymers Science & Technology Co. Ltd., China. 1,3,5-triformylphloroglucnol (Tp), 2,6-diaminoanthracene (DAQ) and p-toluenesulfonic acid monohydrate (PTSA.H2O) are purchased from Energy Chemical Reagents Co. Ltd., China.
Scheme 1. Chemical structure of OHP
Synthesis of DAQ-Tp COF membrane PTSA.H2O (1.8 mmol) was mixed with DAQ (0.45 mmol, 108 mg) in a mortar-pestle. The mixture was ground for 3 minutes. Tp (0.3 mmol, 63.0 mg) was added to the mixture and ground for about 15 minutes. A few drops of water (∼ 150 μL) was added to the mixture. The dough was cast on a glass slide (length 5 cm and diameter 2.5 cm) to form a film of uniform thickness, and then heated at 90 °C for 10 h in a closed container under humid condition. The crystalline and porous COF membranes were gained after washed hot water (~80 °C) and N,N-dimethylacetamide (DMF).
Synthesis of CHCM The CHCM synthesis process was described in Fig. 1. PTSA.H2O (1.8 mmol) was mixed with DAQ (0.45 mmol, 108 mg) in a mortar-pestle. The mixture is ground for 3 minutes. Tp (0.3 mmol, 63.0 mg) was added to the mixture and ground for about 15 minutes. Next, OHP (1%, 5%, 10%, 15%, and 20%) was added and thoroughly mixed through grinding. A few drops of water (∼ 200 μL based on requirement) is added to the mixture. The resulting mixture was then ground thoroughly until it became a dough-like material. The dough was cast on a glass slide (length 5 cm and diameter 2.5 cm) to form a film of uniform thickness. The CNTF was placed on the above film and then drops cast on the CNTF with uniform thickness and is additionally allowed to heat at 90 °C for 2-10 h in a closed container under humid condition. After completion, the glass slide coated with CHCM was dipped into hot water (~80 °C) to isolate CHCM and to remove PTSA. It is then washed with DMF to remove unreacted starting materials. The crystalline and porous CHCM (80-85 % isolated yield) is
stored finally in distilled water for use in further studies.
Results and discussion
Fig. 1 Scheme of the construction process for extensible and flexible supercapacitors DAQ-Tp COF membrane synthesis process is described in the Experimental Section.[10] The FT-IR spectra (Fig. 2a-b) of COF show the disappearance of the N-H absorption peak (3330 cm-1) and the appearance of a new C-N absorption peak (1250 cm-1). The C=O absorption peak of Tp at 1645 cm-1 shifted to the β-ketoenamine C=O absorption peak at 1618 cm-1. The resonances of C=O and C-N were attributed to the presence of β-ketoenamine linkage in COF (Fig. 2a-b), which were consistent with the results reported in the literature.[7, 15] The COF shows a strong diffraction peak at 3.5° and a broad peak at 27°, corresponding to the 100 and 001 reflections, respectively (Fig. 2d), which matched well with the simulated tilted-AA structure.[7, 16] The Brunauer-Emmett-Teller (BET) pore volume and surface area of COF were 0.73 cm3g-1and 1486 m2g-1, respectively (Fig. 2f). The cross-section scanning electron microscope (SEM) images of COF show closely
spaced sheets (Fig. S1, ESI). Moreover, the thickness of the COF membrane was calculated to be 30-50 μm from the cross-section images.
Fig. 2 (a&b) FT-IR spectra of Tp, DAQ, OHP, COF and CHCM (5% OHP, 90 oC heating for 10 h). (c) FT-IR spectra of CHCM (5% OHP) at 90 oC heating for 2-12 h. (d) XRD of COF and CHCM (5% OHP, 90 oC heating for 10 h). (e) Layered chemical structures of DAQ-Tp COF. (f) BET of COF and CHCM (5% OHP, 90 oC heating for 10 h).
The CHCM synthesis process is shown schematically in Fig. 1 and described in the Experimental Section. Briefly, PTSA-H2O, DAQ, Tp and OHP (1%, 5%, 10%, 15%, and 20%) were mixed and ground for about 45 minutes. The mixtures in the form of paste were suspended on both sides of CNTF with a uniform thickness and heated at 90 °C for 2-12 h to obtain CHCMs (Fig. 1). FT-IR spectra of the CHCM exhibit in Fig. 2a-b, characteristic bands attributable to C-N stretching at 1265 cm-1 and C=O bond at 1618 cm-1 of the β-ketoenamine in the spectra of pure COF and -OH bond at 3730 cm-1, -CH3 at 2950 cm-1 and COO- bond at 1727 cm-1 for OHP, indicating the
formation of CHCM. The characteristic frequencies at 1265 cm-1 shifted to the higher wavenumbers, demonstrating the presence of strong hydrogen-bond interaction between COF, OHP, and CNTF.[28] The absorption peak area of -OH bond at 3730 cm-1, -CH3 at 2950 cm-1, COO- bond at 1727 cm-1 and C-O-C bond at 1175 cm-1 increased with increase of heating time from 2 h to 10 h and then decreased with further increase of time to 12 h (Fig. 2c), indicating that too long reaction time could decrease the content of OHP in CHCM. The crystallinity of CHCM was assessed by XRD. The appearance of two crystalline XRD peaks of CHCM with 100 and 001 reflections at 3.5 and 25° (2θ), respectively (Fig. 2d), which bears similarity with COF. The decrease of the relative intensity at 100 peak for CHCM was attributed to the presence of OHP@CNTF in the COF pores. The CHCM displays a smooth sheet of an estimated 20 μm in thickness with large areal synthetic scalability and flexibility (Fig. S2). These findings encouraged us to investigate the use of CHCM as free-standing supercapacitor electrodes.[12] The BET surface area of CHCM was calculated to be 167 m2g-1 with a pore volume estimated to be 0.72 cm3g-1 (Fig. 2f). The sharply decreased surface area of CHCM due to the pores of COF contained OHP chains and CNTF. The thermal stability of CHCM was characterized by thermogravimetric analysis (TGA, Fig. S3). The weight loss of pristine CNTF, OHP, COF, and CHCM (5% OHP, 90 oC heating for 10 h) with char yields in Fig. S2 were 98.50%, 2.28, 54.07 and 71.50%, respectively. The TGA profiles exhibit good thermal stability of CHCM up to 365 °C. The morphologies of CHCMs were characterized by the surface and the cross-sectional SEM images, as shown in Fig. 3.
The pristine CNTF network exhibited excellent continuity and uniformity (Fig. S4). Fig. 3 shows that the surface of the CNTF was coated evenly with COF@OHP, causing the bundles of CNTF to become slightly thicker than those of the pristine CNTF. In order to clearly observe the morphological evolution procedures, we heated the CHCMs between 2 and 12 hours. As shown in Fig. 3a and 3e, at 2h, COF@OHP complex was gradually impregnated in CNTF and formed a very thick aggregation of OHP and CNTF. At 6h, COF@OHP complex was further impregnated in CNTF and a distinctive self-assembled layered structure occurred in the densely packed CNTF sandwiched by adjacent COF sheets and flexible OHP chains (Fig. 3b and 3f).[29, 30] However, there was still some aggregation of COF@OHP, which was not infiltrated into CNTF. With further increase of heating time, COF@OHP complex was completely impregnated in CNTF and evenly dispersed, the COF nanosheets and flexible OHP chains composed of closely packed CNTF networks became a contract and densify, as shown in Fig. 3c and 3g for 10h. The cross-section SEM images of CHCM (5% OHP, 90 oC heated for 10 h) are provided in Fig. 3d and 3h, which suggests that COF@OHP penetrated inside the CNTF. SEM images of CHCMs displayed in Fig. 3 show that the CNTF was fully covered by COF@OHP. The above observations indicated that OHP acted as a template during controlled heating time CHCM synthesis.
Fig. 3 SEM images of CHCMs (5% OHP) with different time durations at 90 oC: 2h (a & e), 6h (b & f) and 10h (c & g). The cross-section SEM images of (d & h) CHCM (5% OHP, 90 oC heated for 10 h).
The electrical conductivities of the COF membrane, CNTF and CHCM (5% OHP, 90 oC heated for 10h) were evaluated using an Agilent U1241B multimeter (Fig. S5a).[31] Compared with the pristine COF membrane (48.2 S/cm), neat CNTF (85.4 S/cm) and COF@CNTF hybrid (61.6 S/cm), the electrical conductivity (234.3 S/cm) of CHCM electrode was increased to 2-5 times.[27] The electrochemical impedance spectroscopy (EIS)[32] of the CHCM electrode showed an equivalent series resistance (ESR) of 38 Ω (Fig. S5b). The EIS of CHCM electrode was lower than COF, CNTF, and COF@CNTF, indicating that the CHCM possessed a low charge transport resistance. The electrochemical performances of CHCM electrodes were further investigated in a three-electrode system with an environmentally-friendly H3PO4 (1 M, aq.) electrolyte. Compared with sulfuric acid (H2SO4), H3PO4 is a relatively safe acid without strong oxidation and corrosiveness, easy to store and transport.[33-35] Fig. 4a displays the cyclic voltammetry (CV) curves of 1 M aq. H3PO4 electrode at 30 mV s-1 scan rate for COF, CNTF, COF@CNTF and CHCM, respectively. CHCM electrode
demonstrated a far larger surrounding area than COF, CNTF and COF@CNTF of the CV curve, suggesting that the CHCM possessed a much higher capacitance. Fig. 4b illustrates the reversible switching between hydroquinone (C-OH) and quinone (C=O) transformation due to the π-π interactions between COFs and CNTF.[12, 16] Fig. 4c shows the galvanostatic charge-discharge (GCD) curves of the COF, CNTF, COF@CNTF and CHCM electrodes at the same current of 0.2 mA. The results proved that the performance of the CHCM electrode was far better than that of the COF, CNTF and COF@CNTF electrodes, which was consistent with Fig. 4a.
Fig. 4 (a) CV curves of electrochemical capacitors based on CHCM (5% OHP, 90 oC heating for 10 h) electrode compared with COF, CNTF, and COF@CNTF in 1 M aq. H3PO4 electrolyte. (b) Redox behavior of CHCM through reversible quinine to hydroquinone transformation. (c) GCD curves of electrochemical capacitors based on CHCM (5% OHP, 90 oC heating for 10 h) compared with COF, CNTF and COF@CNTF in 1 M aq. H3PO4 electrolyte.
We explored the effect of the OHP content and heating time on the electrochemical performance of the CHCM electrode (Fig. 5-6). As shown in Fig. 5a-c, the capacitance of CHCM increased with increasing content of OHP from 0% to 5% and then decreased with a further increase of the content to 20%, indicating that too much content of OHP could decrease the electrode’s capacitance. The capacitance
decreased with increase of heating time from 2 h to 6 h and then started to increase with further increase of heating time to 10 h before it started to decrease sharply with further increase of heating time to 12 h (Fig. 6), indicating that too long time for construction of CHCM could weaken the electrode’s capacitance. Therefore, the optimum content of OHP and heating time were 5 wt % and 10 h, respectively. The highest gravimetric capacitance of 249 F g-1 was achieved by using 1 M aq. H3PO4 electrolyte. All the above analyses demonstrated that OHP played an important role in the electrochemical process. The outstanding capacitive performance mechanism of CHCM by the COF complex is explained. As a template, OHP can effectively disperse and open COF in flexible polymer chains (Fig. 1).[22] COF@OHP complex was gradually impregnated in CNTF and evenly dispersed (Fig. 3c and 3g). Agglomeration and collapse of COF and CNTF were prevented by restricting the effect of CNTF, increasing the interaction between COF and CNTF, and improving the electron conduction between COF and CNTF.[12, 16] These consequently improved the electrochemical performance.
Fig. 5 Electrochemical properties of CHCM (5% OHP, 90 oC heating for 10 h) electrode in three-electrode assembly using 1M aq. H3PO4 electrolyte with different content of OHP (a) CV and (b) GCD at 0.15 mg effective quality. (c) Gravimetric capacitance as a function of the content of OHP.
Fig. 6 Electrochemical properties of CHCM (5% OHP, 90 oC heating for 10 h) electrode in three-electrode assembly using 1M aq. H3PO4 electrolyte at different heating times. (a) CV and (b) GCD using 0.15 mg effective quality. (c) Gravimetric capacitance as a function of reaction time.
Fig. 7a and b show the typical CV and GCD curves of the CHCM electrodes. The CHCM electrodes demonstrated pseudocapacitive characteristics in the potential window, as suggested by the nonrectangular CV curves (Fig. 7a). As the scan rate increased from 10 to 300 mV s-1, the CV curve changed size but maintained almost the same shape. The GCD curves in Fig. 7b were recorded at different currents 0.2, 0.3, 0.5, 0.8, and 1.0 mA, from which capacitance was determined. The GCD curves (Fig. 7b) were nearly symmetric, demonstrating the high reversibility of CHCM electrodes during discharge and charge processes. During 0.2 mA scanning in 1 M aq. H3PO4 electrolyte, the recorded gravimetric capacitance reached 249 F g-1. In order to compare with TpOMe-DAQ COF,[12] the electrochemical performances of CHCM were further investigated with 2 M aq. H2SO4 electrolyte. When tested in 2 M aq. H2SO4 electrolyte, the CHCM displayed an outstanding capacitive performance of 425 F g-1 (Fig. S6). As far as we know, this gravimetric capacitance was the highest value ever reported for pure COF-based electrodes (Table S1).[12, 15] The CHCM
electrode also exhibited high cyclic stability of 80% retention over 10 000 cycles at the current of 0.8 mA (Fig. 7c). The corresponding Coulombic efficiency[12] of the CHCM electrode keeps intact (Fig. 7c).
Fig. 7 Electrochemical properties of CHCM in three-electrode assembly using 1M aq. H3PO4 electrolyte: (a) CV, (b) GCD and (c) Cyclic stability performance (0.8 mA). The mechanical strength of CHCM (5% OHP, 90 oC heating for 10 h) was evaluated by tensile testing (Fig. 8). The CHCM showed lower elongation than neat CNTF because the slip between CNT and COF in the CHCM became more difficult after functionalization. The CHCM exhibited a higher breaking strain (10%, Fig. 8a, c-d) than COF[15] and DqDaTp-CNF hybrid[16], which was attributed to slide of the flexible OHP chains between COF and CNTF while maintaining high load transfer efficiency, leading to improve elongation (Fig. 9).[22] The CHCM showed much higher tensile strength (180 MPa, Fig. 8b) than neat CNTF (~20 MPa) and other COF thin sheets.[12, 15] Compared with neat CNTF and COF, CHCM had much higher tensile strength due to the stronger π-π interaction between CNTF and COF in the framework, the high stretch of hyperbranched OHP chains together with the densification of the CNTF. The excellent mechanical strength and electrochemical performance allowed us to construct extensible and flexible supercapacitors from two
0.5 cm2 CHCM sheet electrodes and 1M H3PO4/PVA gel electrolyte. The supercapacitor achieved a high gravimetric capacitance of 14 F/g (Fig. 10). When the supercapacitor was bent to different bending angles (0°, 45°, 90°, 135°, and 180°), the GCD curves did not change significantly (Fig. 10c), indicating remarkably stable capacitive performance under bending states.[36] To demonstrate the energy storage of flexible supercapacitors, we connected five such solid-state devices in series to light a 1.5 V LED (Fig. 11).
Fig. 8 Mechanical strength and flexible performance of CHCM
Fig. 9 Flexible mechanism of CHCM
Fig. 10 The electrochemical properties of extensible and flexible supercapacitors (a) CV, (b) GCD and (c) GCD under different bending angles using 0.5 cm2 real area.
Fig. 11 Lighting up of 1.5 V LED using five solid-state-devices of supercapacitors
Conclusion Redox-active, chemically stable supercapacitor electrode material was fabricated using a simple solid-state mechanical mixing method. The resulting CHCM-based electrode demonstrated a high tensile strength of 180 MPa and elongation of 10% due to the π-π interaction between CNTF and COF in the framework and slide of the flexible OHP chains between COF and CNTF while maintaining high load transfer efficiency. The three-electrode system of CHCM electrode exhibited an exceptionally high gravimetric capacitance of 249 F g-1 and charging-discharging stability of 80% after 10 000 cycles in the relatively environmentally-friendly H3PO4 electrolyte, which was attributed to the strong π-π interaction between CNTF and COF in the framework and increased the electron conduction between COF and CNTF. The high-performance flexible supercapacitor with high mechanical strength has the potential for a wide range of applications.
ASSOCIATED CONTENT Supporting Information Supplementary data associated with this article can be found in the online version at https://doi.org/10.1016/.
AUTHOR INFORMATION Corresponding Author *(Dr. D. Zhang), Email: [email protected]. Author Contributions #
These authors contributed equally to this work.
Notes The authors declare no competing financial interest.
Acknowledgements We gratefully acknowledge the financial support of the National Natural Science Foundation of China (51703250, 51873233 and 21703212), Hubei Major Projects of Technological Innovation (2017AAA131) and the Hubei Natural Science Foundation (2018CFA023).
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Graphical abstract Title: “Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode”
Redox-active, chemically stable supercapacitor electrode material was constructed by impregnating the COF@OHP complex on microporous electrically conducting CNTF. The CHCM-based electrode showed not only excellent electrochemical performance (high gravimetric capacitance of 249 F g-1 and charging-discharging stability of 80% after 10 000 cycles) in the relatively environmentally-friendly H3PO4 electrolyte, and but also high tensile strength of 180 MPa and elongation of 10%.
Dec. 18th, 2019
Title: “Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode” Chemical Engineering Journal Research Highlights:
The composite membrane (CHCM) electrode showed a high tensile strength of 180 MPa and elongation of 10%.
The gravimetric capacitance of the CHCM electrode reached 425 F g-1 in 2 M aq. H2SO4 electrolyte.
CHCM electrode showed excellent electrochemical performance in the relatively environmentally- friendly H3PO4 electrolyte.
Dec. 18th, 2019
Declaration of Interest Statement Title: “Construction of extensible and flexible supercapacitors from covalent organic framework composite membrane electrode” Chemical Engineering Journal The authors declare no competing financial interest.