Convergent Covalent Organic Framework Thin Sheets as Flexible

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Convergent Covalent Organic Framework Thin Sheets as Flexible Supercapacitor Electrodes Abdul Khayum M., Vidyanand Vijayakumar, Suvendu Karak, Sharath Kandambeth, Mohitosh Bhadra, Karthika Suresh, Nikhil Acharambath, Sreekumar Kurungot, and Rahul Banerjee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10486 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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ACS Applied Materials & Interfaces

Convergent Covalent Organic Framework Thin Sheets as Flexible Supercapacitor Electrodes Abdul Khayum M,a,b Vidyanand Vijayakumar,a,b Suvendu Karak,a,b Sharath Kandambeth,a Mohitosh Bhadra,a,b Karthika Suresh,a,b Nikhil Acharambath,a Sreekumar Kurungot,a,b

*

Rahul

Banerjee a, c* a

Physical/ Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr.Homi Bhabha

Road, Pune-411008, India b

c

Academy of Scientific and Innovative Research, New Delhi, India.

Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata,

Mohanpur-741246, India. KEYWORDS: covalent organic frameworks, free-standing electrodes, flexible supercapacitor, redox chemistry, mechanical properties.

ABSTRACT

The flexible supercapacitors in modern electronic equipment require light-weight electrodes which have a high surface area, precisely integrated redox moieties, and mechanically strong flexible free-standing nature. However, the incorporation of the aforementioned properties into a

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single electrode remains a great task. Herein, we could overcome these challenges by a facile and scalable synthesis of the convergent covalent organic framework (COF) free-standing flexible thin-sheets through solid-state molecular baking strategy. Here, redox-active anthraquinone (Dq) and π-electron rich anthracene (Da) are judiciously selected as two different linkers in a βketoenamine linked 2D-COF. As a result of precisely integrated anthraquinone moieties, COF thin-sheet exhibits redox activity. Meanwhile, π electron rich anthracene linker assists to improve the mechanical property of the free-standing thin-sheet through the enhancement of non-covalent interaction between crystallites. This binder-free strategy offers the togetherness of crystallinity and flexibility in 2D-COF thin-sheets.Also, the synthesized porous crystalline convergent COF thin-sheets are benefited with free of cracking; uniform surface and light-weight nature. Further, to demonstrate the practical utility of the material as an electrode in the energy storage system, we fabricated a solid-state symmetrical flexible COF supercapacitor device using a grafoil peeled carbon tape as the current collector.

Introduction Induction of redox-active units into a porous host matrix could distinctly alter its electronic properties.1-4 However, in practice, this rational modification has often been compromised due to a) the poor understanding of their structure and b) tedious and hectic reaction conditions for the incorporation of such redox-active units.5-7 In this regard, the development of two-dimensional covalent organic frameworks (COFs)8-17 could facilitate the chemical construction of the redoxactive materials with long-range periodicity and porosity.18 These covalently stitched organic redox-active building units could showcase an excellent electrochemical energy storage in supercapacitors.19-23 Since these crystalline frameworks do contain well-defined nanopores,24-25


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they can boost the electrode-electrolyte contact by the lucid ion transport through the ordered pore channels. It is noteworthy that the cutting-edge wearable supercapacitor26-31 demands flexible and free-standing electrodes.32-34 However, the granular form of these COFs culminates an inadequacy towards their development as a stand-alone material to be used in such wearable technology. Thus, the quest for a unique strategy that will turn an ordered crystalline granular material into a flexible and mechanically strong electrode is still on. This made us interested in challenging this formidable task of fabricating a granular polymeric material into a porous, crystalline, flexible and free-standing electrode. Amplifying the non-covalent interactions35-37 often proved to be a major factor to induce the mechanical robustness within a 2D layered material.38-40 keeping this in perspective, we surmise that the π-electron rich moieties could play a pivotal role to provide the desired mechanical strength into the COF matrix. This led us to the judicious selection of π electron rich 2,6diaminoanthracene41-43 as the organic building unit to pave the construction of a mechanically robust COF. However, the anthracene moieties do not possess any redox-activity, which creates the need for an alternative linker in the framework to serve that purpose. To converge these two properties, i.e. mechanical robustness as well as redox activity into one framework matrix, 2,6diaminoanthracene (Da) and redox-active 2,6-diaminoanthraquinone (Dq) were purposefully chosen for the construction of the hetero-linked44-46 COFs. This molecular-level functionality control converges the individual properties of the distinct linkers into the COF matrix without compromising the crystallinity and porosity. Herein, we have reported an in-situ insertion of both Dq and Da linkers into a β-ketoenamine linked47 COF in 1:2, 1:1 and 2:1 ratio (Dq1Da2Tp, Dq1Da1Tp, and Dq2Da1Tp) through a solid-state molecular mixing procedure. Also, these COFs could be fabricated as highly desirable thin structured free-standing sheets (thickness: 25-100


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µm). Among all, the Dq1Da1Tp COF thin-sheet exhibits good mechanical strength (5% breaking strain) as well as specific capacitance (111 Fg-1). Furthermore, we have fabricated flexible and mechanically robust symmetrical solid-state supercapacitor device by taking grafoil peeled carbon tape (C.T) as a current collector and COF thin-sheet as the electrode (C.T-COF supercapacitor). As a proof of concept, a 3.5 V light emitting diode (LED) is enkindled by connecting four flexible C.T-COF supercapacitor devices in series. To the best of our knowledge, this is the first report on a COF based self-standing flexible supercapacitor device.

Experimental Section Synthesis of COF-thin-sheets: 0.30 mmol of diamine [2,6-diaminoanthracene (Da) and 2,6diaminoanthraquinone (Dq) in different ratio- 1:0, 0:1, 1:1, 2:1 and 1:2] and 1.5 mmol ptoluenesulphonic acid (PTSA) were mixed thoroughly with the addition of 50 µL distilled water, 0.20 mmol of 1,3,5-triformylphloroglucinol (Tp) was then added to the mixture, and the mixing was continued for another 10 minutes. The obtained paste was uniformly coated on a 2.5 × 8 cm2 glass slide and kept at 120°C for 24 h in a closed condition. The thin-sheets were sequentially washed with water, N,N-dimethylacetamide, water and acetone (Figure 1; S-2, Figure S1). Three-electrode assembly: The 1M H2SO4 activated 0.16 cm2 COF thin-sheet, which has been served as working electrode, was pasted on a grafoil current collector by means of an adhesive carbon tape. Pt wire and Hg/Hg2SO4 act as a counter and reference electrodes respectively, whereas 1M H2SO4 has been used as an electrolyte (S-10; Figure S37 & S38). Device fabrication: 1M H2SO4 activated COF thin-sheet (0.8×0.8 cm2) was pasted on a grafoil peeled carbon adhesive tape. Here, the COF thin-sheet and grafoil peeled carbon tape act as an electrode and current collector respectively. Subsequently, two such electrodes were coated with


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PVA-H2SO4 gel electrolyte. Finally, a polycarbonate separator was used for the disjunction of the positive and negative electrodes in the device (S-10; Figure S39).

Results and Discussion The β-ketoenamine linked 2D-COF thin-sheets have been synthesized by following the solidstate molecular mixing process48 in the presence of PTSA (Figure 1a; S-2, Figure S1). Herein, we have synthesized two different single-linker DqTp (Dq:Da=1:0) and DaTp (Dq:Da=0:1) COFs,

and

three

different

convergent

COFs

Dq1Da2Tp

(Dq:Da=1:2),

Dq1Da1Tp

(Dq:Da=1:1),and Dq2Da1Tp (Dq:Da=2:1). The single-linker DqTp COF has been previously reported as DAAQ-TFP COF by Dichtel et al.19 The thickness of the COF sheets can be tuned (~25-100 µm) by adjusting the amount of COF precursor paste and the area of casting on the glass surface (S-2; Figure S1). Although the as-synthesized 2,6-diaminoanthraquinone linked COF (DqTp) exhibits good electrochemical performance (154 Fg-1), it shows poor mechanical strength (2 % of breaking strain) during the physical fluctuations. In contrast, the π-electron rich 2,6-diaminoanthracene linked COF (DaTp) possess robust mechanical strength (9 % of breaking strain). But, the poor performance of DaTp (< 10 Fg-1) in electrochemical energy storage makes it inappropriate for the supercapacitor application. The rational fabrication of the convergent COFs (Dq:Da- 1:2, 1:1 and 2:1 ratio) logically correlates their properties with the quantitative insertion of the functional linkers. The superior mechanical response of the Dq1Da2Tp and Dq1Da1Tp COFs compared to Dq2Da1Tp COF toward the physical fluctuations can be attributed to the higher content of the anthracene moiety into the COF matrix. Similarly, Dq1Da1Tp and Dq2Da1Tp display better electrochemical performance than Dq1Da2Tp due to the higher loading


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of anthraquinone linker. It must be accounted that along with their mechanical robustness and redox activity, these convergent COF thin-sheets conserve crystallinity as well as porosity. The crystallinity of the COF thin-sheets reported in this paper is inspected by the powder Xray diffraction (PXRD) analysis. The diffraction patterns indicate their crystalline nature. The

Figure 1: a) Schematic representation of the synthesis of COFs (Dq-2,6-diaminoanthraquinone, Da-2,6-diaminoanthracene, Tp-1,3,5-triformylpholoroglucinol) and b) Powder X-ray diffraction patterns of COF thin-sheets.

distinctive PXRD profiles of the COFs with respect to the precursors demonstrate the bulk phase purity of the material (Figure S19). The first two peaks in DqTp (3.6 & 6.2), DaTp (3.6 & 6.2), Dq1Da2Tp (3.7 & 6.0), Dq1Da1Tp (3.5 & 5.8) and Dq2Da1Tp (3.5 & 5.9) correspond to 100 and 110 planes respectively (Figure 1b). The most plausible eclipsed stacking models3,

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were

constructed in material studio-6 (MS-6) software to decipher the molecular structure of these COFs. The PXRD pattern of all the as-synthesized COFs match well with their simulated


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patterns. Additionally, we have performed the Pawley refinement on all models with the corresponding experimental PXRD of each COFs. [DqTp (Rp = 5.1%, Rwp= 6.5%); DaTp (Rp = 4.6%, Rwp= 2.9%); Dq1Da2Tp (Rp = 2.2%, Rwp= 2.8%); Dq1Da1Tp (Rp = 9.96%, Rwp= 12.62%); Dq2Da1Tp (Rp = 7.45%, Rwp= 9.07%)] (Figure 1b, Figure S3-S18). The absence of the N–H and the aldehyde carbonyl stretching vibration peaks in the FT-IR spectra rules out the possibility of the presence of the diamine and aldehyde precursors as impurities (S-4; Figure S21 - S24). The absence of intense and sharp stretching vibration peak at 815 cm-1 signifies the complete removal of PTSA during washing (Figure S24). The FT-IR spectrum of DqTp shows stretching vibrations at 1670, 1611, 1560, 1227 cm-1 corresponding to the ketone (C=O) of anthraquinone moiety, ketone (C=O) of the keto-enol tautomeric center, C=C and C–N functional groups respectively (Figure S21). Similarly, DaTp exhibits its characteristics stretching vibrations of C=O and C–N at 1582 and 1270 cm-1 respectively (Figure S22). The distinguishable variation of stretching vibrations ensures the formation of the convergent COFs (Figure S23). The vibration frequency due to the C=O functionality of the anthraquinone moiety could be assigned at 1674, 1674 and 1669 cm-1 for Dq1Da2Tp, Dq1Da1Tp, and Dq2Da1Tp COFs respectively. Additionally, the stretching vibrations at 1660-1670 cm-1 (C=O) and 1237-1242 cm-1 (C–N) region could be attributed to the presence of β-ketoenamine linkage in these convergent COF thin-sheets. The atomic level construction mapping of single-linker and convergent COFs is further corroborated by

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C CP-MAS solid-state NMR spectra (Figure 2a; Figure S26). DqTp exhibits

anthraquinone ketone resonance at 180 ppm and the carbonyl group of typical keto-enol tautomeric COF at 184 ppm. Similarly, enamine carbon (=C-NH-) and α-enamine carbon show peaks at 147 and 118 ppm respectively. The DaTp also exhibits its characteristic

13

C peaks at


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182 and 144 ppm representing the keto and enamine carbons. The Dq1Da1Tp COF also shows similar characteristic peaks except for a slight chemical shift (145 ppm) in enamine carbon position compared to the single-linker COFs, which indicates the formation of the convergent COF with Da and Dq linkers (Figure 2a). Likewise, a slight chemical shift of α-enamine carbon has been

Figure 2: a)13C CP MASsolid-stateNMR. b) Gas adsorption isotherms and c) Dynamic mechanic analysis of COF thin-sheets


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observed in Dq1Da2Tp (146.8 ppm) and Dq1Da2Tp (147.1ppm). It is noteworthy that the physical mixture (1:1) of DqTp & DaTp COFs features individual solid-state NMR peaks at the identical locations. This result could be an indication of the poor possibility of physical mixture formation during the synthesis. Furthermore, we figured out the linker ratio (Dq:Da) in convergent COFs by the quantitative integration of 13C solid-state NMR peaks of anthraquinone C=O peak (179-182 ppm) and keto-enol tautomeric C=O peak (183-185 ppm) (S-5, Figure S27S30, Table S10). All COF thin-sheets reported in this paper exhibit excellent thermal stability up to 430 °C, which is revealed by thermo gravimetric analysis (TGA) (Figure S33). The N2 adsorption isotherms have confirmed the well-organized nature of pores in crystalline COF thin-sheets. The obtained type IV adsorption isotherms confirm the microporous nature of the material (Figure 2b). DqTp and DaTp COFs exhibit Brunner Emmet Teller (BET) surface area of 940 and 577 m2g-1 respectively. Dq1Da2Tp, Dq1Da1Tp, and Dq2Da1Tp COFs maintain ordered porous nature with a surface area as high as 1400, 804 and 1004 m2g-1 respectively. This microporous feature with a high crystallinity points out the regular arrangement of the building blocks even after the incorporation of two distinct linkers into the same framework. Also, the non-local density functional theorem (NLDFT) explains the pore size distribution ranging from 2-2.3 nm for all the as-synthesized COFs (Figure S34). Unlike amorphous carbon or polymer based free-standing electrodes in flexible supercapacitors,26-34 these COF thin-sheets possess precisely built open porous structure with intrinsically ordered pore channels in nano-domains. The scanning electron microscope (SEM) images of the as-synthesized COF thin-sheets provide the critical information regarding their continuous, uniform and crack-free nature. The cross-sectional inspection further reveals the dense packing of the crystallites, which is also


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responsible for such strong mechanical properties as well as the sustainability of the freestanding thin-sheets (Figure S31). It is important to note that, we could maintain the thin-sheet thickness within 25-100 µm along with large areal (≥ 15 cm2) scalability (Figure 3b, c & d). The nanolevel morphology of the COFs has been revealed through the transmission electron microscopy (TEM) by taking the finely ground COF particles. DqTp, DaTp and Dq2Da1Tp COFs exhibit the agglomerated sheet-like morphology, which is distinguishable from the ribbonlike morphology of Dq1Da2Tp and Dq1Da1Tp COFs (Inset: Figure 3d; Figure S32).

Figure 3: a) Diagrammatic representation of the fabrication of COF the thin-sheets (precursor paste is uniformly cast on a glass slide and then heated in a closed condition).b)Cross-sectional


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SEM image of Dq1Da1Tp having a thickness of 25 µm. c) Digital image of the 16 cm2 area Dq1Da1Tp COF.d) Top view SEM image of Dq1Da1Tp COF thin-sheet and e) TEM image of the finely ground Dq1Da1Tp particles.

All the as-synthesized COF thin-sheets are chemically stable in H2O, 1M H2SO4 as well as organic solvents (N,N-dimethylacetamide (DMA), acetone, acetonitrile) due to the keto-enol tautomeric nature. The retention of structural integrity even after 24 hours in 1M H2SO4 has further been confirmed by their PXRD patterns (Figure S20). The construction of free-standing COF thin-sheet could be similar to the interlocked tile arrangement mechanism in graphene oxide thin-sheet formation.40 Although good crystallinity and flexibility are mutually exclusive in most 2D thin-sheet materials, here we could overcome this intriguing issue through the regulation of electronic environment of the COF crystallites. Unlike the layer by layer packing in graphene oxide, the crystallite-crystallite interlocking could be a possible reason behind the mechanical strength of the free-standing COF thin-sheets. The interlocked crystallites with additional non-covalent interaction sites create higher mechanical robustness among these convergent COFs. The π-electron rich anthracene can lift the efficiency of the inter-locked packing of the crystallites through the π-π stacking resulting in higher mechanical strength into the COF thin-sheets. We have characterized all COFs with the dynamic mechanical analyzer (DMA) to assess their mechanical properties (Figure 2c, Figure S35). The stress-strain measurement of DqTp COF thin-sheet provides the percentage of breaking strain as 2% only. Meanwhile, the π-electron rich DaTp COF thin-sheet exhibits remarkably high breaking strain (9%) compares to DqTp COF thin-sheets. The mechanical robustness of DaTp could be attributed to the strong inter-crystallite interaction through π-π stacking of the anthracene moieties in the framework. As expected, the convergent Dq1Da1Tp, and Dq1Da2Tp


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COF thin-sheets exhibit improved breaking strain of 5 and 7% compared to the DqTp COF. However, the DMA measurement reveals only 2.3% of the breaking strain of Dq2Da1Tp COF thin-sheets. The quantitative inclusion of the anthracene units and subsequent enhancement of the mechanical strength of the thin-sheets proves its critical role in improving the strength of the COF thin-sheets (Figure 4a, b; Figure S35). The

Figure 4: a) The hybrid property of Dq1Da1Tp COF thin-sheet [the Da (cyan linker) moiety is responsible for the strong interlocked crystallite interaction in a thin-sheet, and it keeps the crystallites together without breaking during the mechanical disturbance of the COF thin-sheet. Meanwhile, Dq (Green linker) could serve as an active redox unit in the framework and provide the pseudo-capacitance in the electro-chemical energy storage. b) DMA of Dq1Da1Tp COF thinsheet. c) Digital image of bending of Dq1Da1Tp and d) Redox behavior of Dq1Da1Tp in CV at 5 mVSec-1.


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mechanical disturbances such as bending or stretching are unable to detach the crystallites of antharcene rich COF thin-sheets because of the aforementioned π interaction that makes them flexible (Figure 4c; Figure S36; Video 1). However, a minute mechanical force is sufficient enough to rupture the DqTp COF thin-sheet due to the inadequacy of such non-covalent interaction sites within the COF matrix. The electrochemical analysis of the COF thin-sheets reported in this paper is performed in a three-electrode assembly (S-10, Figure S37-S43). Electrochemical impedance spectroscopy (EIS) analysis is carried out to determine the electrochemical series resistance (ESR) value corresponds to each COF thin-sheets (Fig S42). We have set a relatively higher potential window of 1 V (-0.7 to 0.3 V) for all COFs in the cyclic voltammetry (CV) analysis (Figure 5b; Figure S40). The reversible redox peaks in the CV profiles of Dq1Da1Tp and Dq2Da1Tp COFs are attributed to the reversible switching of C=O to C–OH of the anthraquinone linker in the framework (Figure 4a & Figure 5b). The reversible redox activity of the anthraquinone moiety is further supported by the CV analysis of the DqTp COF thin-sheet (Figure 5b). In contrast, the modest result of the Dq1Da2Tp COF denotes the significance of rational incorporation of the redox-active linker. Further, we could observe that the increasing scanning rate from 10 to 500 mVSec-1 causes the enhancement of current during CV analysis (Figure S40). The specific capacitance values of each COF thin-sheets were evaluated through the galvano static chargedischarge experiment (GCDC) in the same potential window (-0.7 to 0.3V) at various current densities ranging from 1.56 mAcm-2 to 6.25 mAcm-2 (Figure 5b, S41). The Dq2Da1Tp COF thinsheet spectacles with the specific capacitance of 122 Fg-1 at 1.56 mAcm-2 current density which is almost close to the specific capacitance of DqTp COF thin-sheet (154 Fg-1).The mechanically robust Dq1Da1Tp COF thin-sheet also exhibits good specific capacitance value at the same


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conditions (111 Fg-1 at 1.56 mAcm-2). These obtained specific capacitances are higher than the previously reported granular β-ketoenamine anthraquinone COF (Table S11).19 It should be noted that the poor electrochemical performance of DaTp COF is due to the lack of redox-active moieties in the framework. After the electrochemical experiment, the COF thin-sheets were subjected to the IR analysis, which substantiates the intact nature of keto-enol C=O and C–N bonds in the

Figure 5: a) Diagrammatic representation of the fabrication of C.T-COF supercapacitor device. b) Three-electrode characterization using CV, CD and the plot represents current density vs specific capacitance and c) Device characterization: CV (inset: 3.5V LED lighted up by the


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series connection of 4 flexible devices),CD and impedance analysis C.T-DqTp and C.TDq1Da1Tp COF supercapacitor devices. framework (Figure S25). Notably, all COF thins sheets maintain their continuous, crack-free and uniform nature even after the electrochemical analysis which has been verified through the SEM imaging (Figure S31). To demonstrate the practical utility of the mechanically robust and redox-active convergent COF thin-sheets, we have fabricated highly flexible symmetric solid-state COF supercapacitor devices with grafoil peeled adhesive carbon tape (C.T) as a current collector (S-10; Figure 5a; figure S39). Herein, we have chosen Dq1Da1Tp COF as the electrode in the device (C.TDq1Da1Tp COF) by virtue of the simultaneous good performance in mechanical strength and redox activity of the thin-sheet as compared to other two convergent COFs. We have also fabricated a device out of the DqTp COF electrode to compare the relative performance of the supercapacitor (C.T-DqTp COF). However, the later device is more fragile towards the external mechanical interruptions. Both the devices exhibit appreciable charge storage property as it is evident from the corresponding CV profiles (Figure 5c; Figure S44). The GCDC experiments reveal that the areal capacitances obtained corresponding to a single electrode from C.TDq1Da1Tp and C.T-DqTp COF supercapacitor devices are 8.5 mFcm-2 and 12 mFcm-2 respectively at 0.39 mAcm-2 (Figure 5c). Further, the Ragone plot communicates that the highest energy and power density of the C.T-DqTp COF is 0.43 µWhcm-2 and 980 µWcm-2 respectively. At the same time, C.T-Dq1Da1Tp COF exhibits the energy density and power density values of 0.30 µWhcm-2 and 960 µWcm-2 respectively (Figure S47). The long-term cyclic stability of the C.T-Dq1Da1Tp COF exhibits 90% capacitance retention after 2500 cycles. Further, the same device could hold the 78 % capacitance of the initial value even after the 7000 charge-discharge


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cycle (Figure S46). Meanwhile, C.T-DqTp supercapacitor device exhibits the retention of only 80% capacitance after the 2500 charge-discharge cycle. The comparable performance of C.TDq1Da1Tp with C.T-DqTp points out the significant applicability of the convergent COF thinsheet as mechanically strong flexible supercapacitor electrodes. We have assessed the performance through the connection of four C.T-Dq1Da1Tp COF supercapacitor devices in series and then lighted up a 3.5V LED for ∼15 seconds by charging the device for 20 seconds (Video 2, Figure 4c, and Figure S48). Conclusion In summary, a novel convergent synthetic strategy has been demonstrated for the successful synthesis and fabrication of mechanically robust redox-active 2D-COF into flexible solid-state supercapacitor. This facile and scalable method uses simple mechano-chemical mixing strategy of reactants and avoids the use of binders or additives. The redox activity of the convergent COF thin sheet arises from the Dq linker and mechanical robustness was originated from the πelectron rich Da moieties. The convergent COFs maintain its high crystallinity and porosity while achieving the desired mechanical strength and flexibility required for the solid-state supercapacitors. We believe that this unique synthetic strategy will have the potential relevance for the rational designing of the redox-active material used in flexible supercapacitor devices.

ASSOCIATED CONTENT Supporting Information. Full experimental procedures and characterization data are available as a separate file (PDF).


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AUTHOR INFORMATION Corresponding Author * [email protected]*[email protected] ORCID Abdul khayum M: 0000-0001-8626-1751 Vidyanand Vijayakumar: 0000-0001-6061-5902 Suvendu Karak: 0000-0001-6567-2789 Sharath Kandambeth: 0000-0003-2045-8062 Karthika Suresh: 0000-0003-4890-2778 Sreekumar Kurungot: 0000-0001-5446-7923 Rahul Banerjee: 0000-0002-3547-4746

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT A.K, V.V & S.K acknowledge UGC. M.B. acknowledges CSIR, Govt. of India for Senior Research Fellowship. R.B. acknowledges DST Indo-Singapore Project (INT/SIN/P-05) and DST Nano-mission Project (SR/NM/NS-1179/2012G) for funding. We acknowledge Dr. T. G. Ajithkumar for providing NMR and Dr. G. Kumaraswamy for PXRD facility. REFERENCES 1. Bekyarova, E.; Itkis, M. E.; Ramesh, P.; Berger, C.; Sprinkle, M.; de Heer, W. A.; Haddon, R. C. Chemical Modification of Epitaxial Graphene: Spontaneous Grafting of Aryl Groups. J. Am. Chem. Soc. 2009, 131, 1336-1337.


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Table of Contents (TOC)


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Figure 1 248x151mm (96 x 96 DPI)





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Figure_TOC 96x59mm (96 x 96 DPI)


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Convergent Covalent Organic Framework Thin Sheets as Flexible

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