Soluble Covalent Organic Polymer for the Flexible Electrode of Supercapacitors

Covalent organic frameworks (COFs) have emerged promising potentials for the fabrication of flexible supercapacitors. Numerous efforts are devoting to the promotion of processability of COFs and broaden their applications in practical devices. Herein we have developed a fully conjugated two-dimensional manganese based COF material (termed as COFBTC-Mn) with nitrogen coordinated metal centers. The unique structures of COFBTC-Mn endow it with high solubility via an in-situ exfoliation pathway and the capability to adsorb onto carbon-based matrix by electrostatic force. COFBTC-Mn demonstrated superior processability, reliability and repeatability for the fabrication of flexible electrodes of supercapacitors. Besides, superior to the electrodes based on traditional insoluble COFs, our as-prepared flexible electrodes exhibited a specific capacitance as high as 325 F g-1 with the current density of 0.2 A g-1 and excellent stability after charge/discharge cycles.


INTRODUCTION
Flexible supercapacitors are vital for the development of portable energy storage devices such as wearable electronics, bendable displays, and electronic papers (Lu et al., 2014;Chen et al., 2015). Recently, numerous efforts have been paid to explore the flexible supercapacitors with high power density, fast charge/discharge rate, and long lifetime (Peng et al., 2016;Wu et al., 2018). As the crucial part of flexible supercapacitors, the flexible electrodes determine the energy storage capability, working performance, and the fabrication of flexible devices (Chen and Dai, 2016). During the past research, various kinds of materials have been used to prepare the flexible electrode through methods such as in-situ growth method, tablet method, smear method etc. Yu et al., 2014;Choi et al., 2016;Zhou et al., 2018). Despite the extensive efforts in the fabrication of electrodes, the rational design of supercapacitive electrodes material that meet large capacitance, high energy density, outstanding stability, and simple efficient processing method remains a substantial challenge (Kou et al., 2011;Chen et al., , 2017Li et al., 2017).
Covalent organic polymer (COPs) are a class of porous frameworks which have been identified as promising electrode materials for supercapacitors (DeBlase et al., 2015;Diercks and Yaghi, 2017;Ma et al., 2017;Wang et al., 2017). The long range ordered arrangement with high accessible surface area and tunable elemental compositions of COPs are beneficial for ion transport between the interface of electrode and electrolyte solution. Besides, the regular network structures of COPs lead to explicit incorporation of redox-active functionalities, holding great potentials for pseudocapacitive energy storage (Kandambeth et al., 2019). The common usage of COPs materials during the fabrication of flexible electrodes is daubing or in-situ growing active material on the support (Xu et al., 2015). During these years, COPs materials have enriched the relevant literature as promising supercapacitors, their superior processability, reliability, and repeatability for the fabrication of flexible electrodes are still limited (Halder et al., 2018). Therefore, a new kind COP show high performance electrode material with sizable electrolyte-accessible surface areas remains a huge challenge (Wang et al., 2019).
In our previous work, we have developed a fully conjugated two-dimensional COP material with rigid structure and uneven charged coordination single-atomic centers which demonstrated great solubility in many solutions through an in-situ charged exfoliation pathway (Peng et al., 2019a). Based on our previous outcomes, we synthesized through the assembly of benzene-1,2,4,5-tetracarbonitrile (BTC) with Mn riveted in the center and finally constructed into a fully closed conjugated planar COP system (termed as COP BTC -Mn). As expected, the COP BTC -Mn could be exfoliated into thin layers with positive charged metal centers. Driven by the intermolecular interactions, the COP BTC -Mn was able to be riveted on the carbon fiber through electrostatic adsorption with uniform distribution which could be directly used as flexible electrodes in supercapacitors (Peng et al., 2019b). Our approach to preparation flexible electrode much more convenient than the commonly used spincasting method, in-situ synthesis and squash technique, holding promising potentials for industrial preparation (Guo et al., 2018). Moreover, the as-prepared flexible electrodes demonstrated outstanding capacitive performance (325 F g −1 ) and considerable galvanostatic charge/discharge (GCD) cyclic stability, exceeding 3,000 cycles without compromising the capacitance.

Materials and Methods
All used chemicals in this work are analytical reagents without further treatment. Carbon fiber is supplied by Shanghai Hesen Company. Manganese(II) chloride (MnCl 2 ) is bought from Saan chemical technology (Shanghai) Co., Ltd. Benzene-1,2,4,5-tetracarbonitrile (BTC) is got from Ark Pharm, Inc. N,N-Dimethylformamide (DMF), Ethylene glycol, and Sodium sulfate (Na 2 SO 4 ) are both bought from Shanghai Macklin Biochemical Co., Ltd. Elemental analysis is performed on CARLO ERBA 1106. ICP-OES is performed on ICAP7400. Xray photoelectron spectroscopy (XPS) analysis is performed on Thermo VG ESCALAB 250 equipped with an A1 Kα Xray source. Scanning electron microscopic (SEM) images are obtained from a S4700 SEM instrument equipped with an energy dispersive spectrometer (EDS) analyzer. Fourier Transform Infrared Spectroscopy (FTIR) is tested by a Bruker AC-80 MHZ. Solid State Nuclear Magnetic Resonance (SSNMR) is measured on a Bruker AV300 spectrometer operating at 75.5 MHz for 13 C.

Electrochemical Measurements
The electrochemical tests are performed in the standard threeelectrode system using a CHI 660e electrochemistry workstation at room temperature. All potentials are calibrated. The obtained carbon felt and COP BTC -Mn/CF are directly used as working electrode. Platinum (Pt) loop is used as the counter electrode and silver chloride electrode is used as the reference electrode. The electrolyte is 1 M Na 2 SO 4 (pH = 7). During the test, working electrode is performed by Cyclic Voltammetry (CV) with the potential set between −1 and 0 V at different sweep rate (10,20,50,100,200, and 400 mV/s, respectively). Galvanostatic charge/discharge (GCD) is used to evaluate the electrochemical performance of the developed flexible electrodes at different current density (0.2, 0.5, 1.0, 2.0, 5.0, and 10 A g −1 , respectively).

Synthesis of Flexible Electrode COP BTC -Mn/CF
Commercial carbon felt (CF) is trimmed into small pieces with the size of 1.5 × 1.0 cm. Then the CF pieces are immersed in ethyl alcohol under ultrasonication for three times. The cleaned CF pieces are dried at 75 • C overnight. During the process of the electrodes, the CF was soaked into the DMF solution with COP BTC -Mn dissolved at different content. The flexible electrodes (termed as COP BTC -Mn/CF) were then obtained after evaporating the solvent (Figure 1A).

RESULTS AND DISCUSSION
Material Characterization of the COP BTC -Mn and COP BTC -Mn/CF COP BTC -Mn was synthesized according our previous work with nitrogen coordinated Mn in the center (Figure 1A). The structure of the COP BTC -Mn was confirmed by the 13 C Solidstate NMR (Figure S1), which demonstrated four peaks at 44. 43, 110.7, 137.52, and 168.11 ppm, attributing to the phthalocyanine macrocycles in the phthalocyanine-based conjugated polymer network. Fourier Transform Infrared Spectroscopy (FTIR) was also performed to further investigate the structure of the assynthesized COP BTC -Mn (Figure S2), while the BTC monomer was used as the controlled sample. The peaks at 1,315, 1,616, and 1,760 cm −1 were the stretching vibration peaks of C=C and C=N on the framework of aromatic ring. Compared with the BTC monomer, new peaks at 742, 1,060, 1,125 cm −1 were found in the FTIR spectra of COP BTC -Mn, indicating the formation of phthalocyanine ring. Besides, cyano as the end group, the related characteristic peaks were found at 2,242 cm −1 in both BTC monomer and COP BTC -Mn, which further proved the synthetic route of COP BTC -Mn.
According to the previous reports, the abundant nitrogen coordinated Mn centers endowed the COP BTC -Mn with the capability to be dissolved and form intermolecular interactions with the carbon matrix. As analyzed by the X-ray photoelectron spectroscopy (XPS), the peaks for C, N, and Mn were generated during the synthesis (Figures 1B-E). The fitting curves in C 1 s spectrum were deconvoluted into three peaks  corresponding to the typical "C-C/C=C" (283.63 eV), "C=N/C-N/C-O" (285.29 eV), and "C=O" (287.07.8 eV) bonds. The fitted peaks of N 1 s at 398.04 and 399 eV were attributable to "N-C" and "N-Mn, " respectively. The Mn 2p spectra demonstrated obvious peaks at 641.39, 645.25, and 652.92 eV, assigned to Mn 2p 3/2 , satellite state Mn, and Mn 2p 1/2 , which indicating the formation of nitrogen coordinated Mn as the centers. As controlled examinations, the chemical valence state of COP BTC -Mn/CF and CF were also performed (Figures S3, S4). The thermal stability of COP BTC -Mn was performed in Figure S5. The survey spectra revealed that CF only contains three elements of C, N, O while the C, N, O, Mn were found in COP BTC -Mn/CF,  suggesting that the COP BTC -Mn was successfully introduce to the system. Elemental analysis (EA) and inductively coupled plasma (ICP) were performed to specifically explore the contents of CF and COP BTC -Mn/CF (Table S1). Consistently, the results proved that the COP BTC -Mn as the active sites were successfully riveted on the CF matrix and the COP BTC -Mn/CF electrodes were thus prepared.
The internal morphology and structure of the COP BTC -Mn/CF were revealed by SEM images. It was clear that the commercial carbon felt was composed of interconnected fibers, demonstrating a skeleton with smooth surface (Figure 2a). During the fabrication of electrodes, the COP BTC -Mn was adsorbed on CF through intermolecular interactions. Thus, the COP BTC -Mn/CF demonstrated a rough surface with uniformly distributed absorptions (Figure 2b). According to the energy-dispersive X-ray spectroscopy (EDS) tests, COP BTC -Mn/CF electrodes presented a homogeneous dispersion of N and Mn elements (Figure 2c), suggesting the efficiency of our developed method for the fabrication of flexible electrodes.

Electrochemical Performance of Flexible Electrode COP BTC -Mn/CF
To evaluate the electrochemical performance of the developed flexible electrodes, we performed the cyclic voltammetry (CV) and GCD. The as-prepared COP BTC -Mn/CF were directly used as the working flexible electrodes. Electrodes with different mass ratio of COP BTC -Mn were tested as comparison (marked as COP BTC -Mn/CF-α, where α was mass of COP BTC -Mn and equaled to 0, 1, 2, 3, and 4 mg, respectively). The CV curves of COP BTC -Mn/CF-α and CF in the voltage range of −1 to 0 V with different scanning rates were presented in Figure 3A and Figure S6. It could be found that the current of CF barely changed during the test, suggesting that CF was inactive for capacitive energy storage. Another feature was that none of obvious redox peaks were observed in the CV circles, indicating that the pseudocapacitance effect was small. The GCD test was performed at various current density from 0.2 to 10 A g −1 to further illuminate the electrochemical properties of the flexible COP BTC -Mn/CF-α electrodes. In Figure 3B and Figure S7, the as-prepared electrodes displayed typical charge-discharge curves. The voltage linearly increased with time in the charging process, while linearly decreased with time in the discharging process. Thus, the electrodes demonstrated symmetrical charge and discharge curves. Besides, with the increase of current density, the charge-discharge time shortened obviously. It should be noted that all of the COP BTC -Mn/CF-α electrodes was capable to be used at high current density of 10 A g −1 . We further evaluated the specific capacitance from the discharge curve and found that the performance highly dependent on the mass of COP BTC -Mn, suggesting that COP BTC -Mn was the necessary active material ( Figure 3C). Moreover, with COP BTC -Mn/CF-α as the electrodes, the capacitors demonstrated a superior specific capacitance to the state-of-art electrodes made up of COP and MOF (Xu et al., 2015;Khattak et al., 2016;Han et al., 2017;Sheberla et al., 2017;Sun et al., 2017;Halder et al., 2018;Liu et al., 2018; Table S2). The impedance plot of COP BTC -Mn/CF-1 was provided in Figure S8. The COP BTC -Mn/CF-α electrodes also demonstrated superiorly long stability. As shown in Figure 4A, during the cycling experiments, the COP BTC -Mn/CF-1 kept about 60% capacitance after 5,000 charge/discharge cycles with the current density as high as 10 A g −1 . Meanwhile, the linearity and symmetry of the charge/discharge curves were well-retained ( Figure 4B).

CONCLUSION
In this work, we have developed a soluble COP BTC -Mn for supercapacitors. Benefited from the abundant nitrogen coordinated Mn centers, the COP BTC -Mn could be dissolved in DMF through an in-situ charge exfoliation pathway and form intermolecular interactions with the CF matrix. Combining the activity of COP BTC -Mn with the high surface area of carbon fiber skeleton, we developed a COP BTC -Mn/CF flexible electrode with considerable capacitance and superiorly long charge/discharge lifetime. Our findings paved a notable avenue to simply and repeatable prepare flexible electrodes for not only high-performance supercapacitors, but also many other flexible energy storage devices.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher.