Ultrastable Triazine-Based Covalent Organic Framework with an

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

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Ultrastable Triazine-Based Covalent Organic Framework with an Interlayer Hydrogen Bonding for Supercapacitor Applications Li Li,†,‡ Feng Lu,†,‡ Rui Xue,§ Baolong Ma,† Qi Li,† Ning Wu,† Hui Liu,† Wenqin Yao,† Hao Guo,*,† and Wu Yang*,†

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†

Key Lab of Bioelectrochemistry and Environmental Analysis of Gansu Province, Key Lab of Eco-Environment-Related Polymer Materials of MOE, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, Gansu, P. R. China ‡ Department of Metallurgy and Chemical Engineering, Gansu Vocational & Technical College of Nonferrous Metallurgy, Jinchang 737100, Gansu, P. R. China § College of Chemistry and Chemical Engineering, Provincial Key Laboratory of Gansu Higher Education for City Environmental Pollution Control, Lanzhou City University, Lanzhou 730070, Gansu, P. R. China S Supporting Information *

ABSTRACT: Covalent organic frameworks (COFs) with redox-active units are a class of ideal materials for electrochemical-energy-storage devices. A novel two-dimensional (2D) PDC−MA−COF with redox-active triazine units was prepared via aldehyde− amine condensation reaction by using 1,4-piperazinedicarboxaldehyde (PDC) and melamine (MA) as structural units, which possessed high specific surface area (SBET = 748.2 m2 g−1), narrow pore width (1.9 nm), large pore volume (1.21 cm3 g−1), and high nitrogen content (47.87%), for pseudocapacitance application. The interlayer C−H···N hydrogen bonding can “lock” the relative distance between two adjacent layers to avoid an interlayer slip, which is more conducive to maintaining the ordered pore structure of the COF and improving a fast charge transfer between the electrode interface and triazine units. The PDC−MA−COF exhibited an excellent electrochemical performance with the highest specific capacitance of 335 F g−1 along with 19.71% accessibility of the redox-active triazine units in a three-electrode system and 94 F g−1 in a twoelectrode system at 1.0 A g−1 current density. Asymmetric supercapacitor of PDC−MA−COF//AC assembled using PDC− MA−COF and activated carbon (AC) as positive and negative electrode materials, respectively, exhibited a high energy density of 29.2 W h kg−1 with a power density of 750 W kg−1. At the same time, it also showed an excellent cyclic stability and could retain 88% of the initial capacitance after 20 000 charge−discharge cycles, which was better than those of the most of the analogous materials reported previously. This study provided a new strategy for designing redox-active COFs for pseudocapacitive storage. KEYWORDS: covalent organic frameworks, triazine units, microporous material, interlayer hydrogen bonding, conductivity, pseudocapacitors

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INTRODUCTION Due to the continuous consumption of fossil fuels and the gradual deterioration of the environment, more and more attention is being paid to develop renewable energy technologies.1 Electrochemical-energy-storage devices have played an important role in overcoming energy shortage.2 Among them, electrochemical capacitors, also called supercapacitors (SCs), have attracted great interest owing to their higher energy density and power density, faster charge− discharge rate, and longer cycle life compared with those of conventional dielectric capacitors.3 According to the energystorage mechanism, SCs store charges mainly through two different processes: the non-Faradic processes based on the electrochemical double-layer capacitance (EDLC) and Faradic pseudocapacitance processes resulting from the reversible redox reaction of the electrode materials.4 The electrochemical © XXXX American Chemical Society

double-layer capacitors (EDLCs) store charges through ion adsorption at the electrode−electrolyte interface, while the pseudocapacitors store charges through reversible redox reactions occurring on the surface of the electrodes, which can yield higher capacitance than most EDLCs.5,6 Both of the above capacitors require electrode materials to provide a high specific surface area. Normally, the electrode materials for the EDLCs are composed of various carbon-based materials, such as graphene,7,8 activated carbon,9 and carbon nanotubes,10 while pseudocapacitors are mainly made of transition-metal oxides including MnO2, NiO, Fe2O3, etc,11−13 and conducting polymers including polyaniline, polypyrrole, polythiophene, Received: April 19, 2019 Accepted: July 1, 2019 Published: July 1, 2019 A

DOI: 10.1021/acsami.9b06867 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces etc.14−16 However, most of these materials are not well-defined so that their structural characterization and modification are more complex. To make SCs exhibit better capacitance properties, it is especially important to design and develop more structurally adjustable porous materials with a high surface area and pseudocapacitive characteristics. Covalent organic frameworks (COFs) are crystalline, porous, high-specific-surface-area, and light-weight polymer networks with uniform micropores. These materials are often composed of light atoms, such as C, H, N, and O, with a strong covalent linkage.17−21 Although COFs have been applied in many fields like gas storage,22 gas separation,23 catalysis,24 light conversion,25 drug delivery,26 proton conduction,27 and fluorescence sensing,28 their application as SCs electrode materials is still limited due to their poor conductivity in the electrolyte solution.29,30 To improve the conductivity of the electrode materials, COFs usually combine with other conductive materials.31−33 However, due to weak interaction, the performance of COFs is difficult to improve remarkably. It is better to directly introduce some functional units into the COFs to improve the conductivity of the COFs when they are structurally designed, such as porphyrins34 and phthalocyanine.35 However, although porphyrin- and phthalocyaninebased COFs have good conductivity, their synthesis costs are high and the preparation processes are also complicated. In view of this, COFs with low cost, simple synthetic route, and good conductivity have received increasing attention. Introduction of redox-active units into the structure of COFs can give the material excellent pseudocapacitive properties.36 That is because pseudocapacitors can provide higher specific capacitance, energy density, and power density than those of the EDLCs. The chemical stability is also an essential property of electrode materials. Compared with COFs, metal−organic frameworks have better conductivity and considerable specific capacitance, energy density, and power density and have been extensively investigated as supercapacitor electrode materials. But because of weak coordination bonds between metal ions and ligands, their stability is usually poor, showing a short cycle life in electrochemical tests,37 whereas COFs have a better electrochemical stability owing to their strong covalent interactions. In this present study, a triazine-based COF, named PDC− MA−COF, was synthesized by Schiff-base condensation reaction between 1,4-piperazinedicarboxaldehyde (PDC) and melamine (MA). Among which, MA is a common nitrogen dopant, which has been widely used by researchers due to its low cost and high nitrogen content;38 PDC also has a high nitrogen content. The introduction of these two monomers endows PDC−MA−COF with better conductivity than that of the conventional COFs reported previously. The redox transitions of aromatic and quinone structures in triazine units give PDC−MA−COF a pseudocapacitive characteristic. In particular, interlayer hydrogen bonding (H-bonding) was constructed in the PDC−MA−COF skeleton structure. Due to the “locking” effect of H-bonding, the stability was further improved, and excellent electrochemical stability was exhibited by a supercapacitor electrode material. In the cycle life test, the cyclic stability can retain 88% of the initial capacitance after 20 000 charge and discharge cycles, such an excellent cycle stability is rarely reported in analogous materials. Moreover, to the best of our knowledge, PDC−MA−COF is the first COF

reported with both very high nitrogen content and interlayer H-bonding.

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EXPERIMENTAL SECTION

Descriptions of the reagents and materials, instrumentation, synthetic procedures of PDC−MA−COF, electrode preparation, and electrochemical measurements are all provided in the Supporting Information. The synthetic route of PDC−MA−COF is depicted in Scheme 1.

Scheme 1. Synthesis of PDC−MA−COF

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RESULTS AND DISCUSSION The crystalline property of PDC−MA−COF was characterized by powder X-ray diffraction (PXRD) analysis, and theoretical simulations were made by using Materials Studio (MS) version 7.0 software, as illustrated in Figure 1. PDC−MA−COF exhibited relatively obvious diffraction peaks at 2θ = 8.4, 11.3, and 23.0°, respectively, corresponding to the (110), (200), and (001) reflections, which matched well with the simulated PXRD patterns (the eclipsed stacking model with a pore diameter of 2.1 nm; Figure 1b), suggesting that the frameworks possessed a certain degree of ordering. The (001) reflection plane corresponding to two-dimensional hierarchical architecture was ascribed to the π−π stacking between the COF layers. However, the (100) crystal plane diffraction peak in the small angle region did not appear in the experimental pattern and the (001) plane was significantly broadened with a strong background, indicating poor crystallinity, short-range ordering, and long-range disordering structure of the as-obtained materials. Meanwhile, the simulated results of the eclipsed model offered us a hexagonal structure of the P6/m space group with unit cell parameters a = b = 23.21 Å, c = 3.81 Å and α = β = 90°, γ = 120° (Figure 1d). Furthermore, the vertical distance between two adjacent layers was approximately 3.81 Å from the d spacing of the (001) plane according to simulations. Due to the presence of an interlayer C−H···N H-bonding between the adjacent layers of piperazine rings, the relative distance between atoms in two adjacent layers can be “locked” well,39 and it is difficult to cause an interlayer slip,40 which can better maintain the planar conformation and ordered tunnels of the COF and reduce the total energy of the system.41 There are 12 H-bonding in each hexagonal unit between two adjacent piperazine rings, that is, two H-bondings exist between every two piperazine rings in the vertical direction. The calculated Hbonding distances in the eclipsed stacking structures for PDC− MA−COF (d = 3.81 Å, d′ = 2.55 Å, θ′ = 145.8°) indicated the existence of the C−H···N H-bonding in the COF (Figure 1f). B


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Figure 1. (a) Experimental PXRD pattern of PDC−MA−COF (black) compared with simulated eclipsed (b, red) and staggered (c, green). Simulated eclipsed (d) and staggered (e) stacking models for PDC−MA−COF (C: gray, N: blue, and H was omitted for clarity). (f) Interlayer Hbonding structures of PDC−MA−COF. (g) FT-IR spectra of PDC−MA−COF (blue), PDC (red), and MA (black).

Figure 2. XPS survey spectra (a) and the high-resolution spectra of N 1s (b) for PDC−MA−COF.

spectra (Figure 2a). Among them, C 1s and N 1s peaks were attributed to the carbon and nitrogen elements in the COF skeleton, while the O 1s peak was derived from the adsorbed water. The N 1s peak was split into four peaks (Figure 2b), which were located at 398.19, 399.04, 399.52, and 400.22 eV, attributable to the N in the imine bond, CN in the triazine ring, a dangling −NH2 bond, C−N−C in the piperazine ring,45 respectively. By calculating the above peak areas, the content ratio of four different types of N atoms in the COF was 4.44:8.71:1:4.59. The XPS spectrum showed that a certain amount of unreacted terminal amino groups were present at the extremity of the COF framework fragment. Combining all of the above spectral results, it was proved that the PDC− MA−COF had been successfully synthesized. The elemental composition of PDC−MA−COF was determined by elemental analysis. Calculated: C, 50.46%; N, 44.15%; H, 5.26%. Found: C, 44.32%; N, 47.87%; H, 4.98%; S, 0.14%, where sulfur element was derived from the dimethyl sulfoxide (DMSO) solvent. The material had a high nitrogen content and was expected to have good conductivity and a potential application in a supercapacitor.

Similar regional conformations can also be extended to other materials with similar interlayer structures. The Fourier transform infrared (FT-IR) spectra (Figure 1g) shows that as the absorption peak of CO at 1649 cm−1 vanished and a series of peaks of −NH2 between 3468 and 3131 cm−1 disappeared, the characteristic CN stretching band at 1707 cm−1 appeared, which proved the occurrence of the aldehyde−amine condensation reaction.42−44 The imine bonds (CN) in the COF were connected with the triazine rings. Due to the conjugation effect, the breathing vibrations at 1551, 1474, and 1352 cm−1 of the triazine unit were shifted to the lower frequency by about 80 cm−1 compared to that of the monomer MA. Moreover, a broad absorption band around 3422 cm−1 appeared due to the existence of an interlayer Hbonding. The stretching vibrations at 2920 and 2852 cm−1 were ascribed to the saturated C−H and C−N stretching, and the corresponding bending modes were located at 1284 and 811 cm−1 in the fingerprint region, respectively. X-ray photoelectron spectroscopy (XPS) measurement was employed to demonstrate the surface species and elemental chemical states of PDC−MA−COF, as illustrated in Figure 2. The peaks of C 1s, N 1s, and O 1s appeared in the XPS survey C


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Scanning electron microscopy (SEM) images displayed that PDC−MA−COF had a uniform coral-like morphology with a diameter of about 100 nm, accompanied by a certain degree of aggregation (Figure 4a,b). High-resolution transmission electron microscopy (HRTEM) was employed to further investigate in more detail the structure of COF (Figure 4c−e). Since the particle size of PDC−MA−COF was too small, the PXRD spectrum was significantly broadened and the characteristic peak of the (100) crystal plane could not be displayed. Therefore, PXRD could not accurately and intuitively reflect its crystallization; we adopted the selected area electron diffraction (SAED) for further study. The SAED pattern (inset of Figure 5d) revealed that PDC−MA−COF had evident electron-diffraction spots, which supported the PXRD result, that is to say, the product was polycrystalline.41 The well-resolved lattice fringes with an interplanar spacing of 0.381 nm in the HRTEM image could be attributed to the (001) plane of PDC−MA−COF (Figure 4e). The corresponding energy-dispersive X-ray spectroscopy (EDX) elemental area mapping (Figure 4f−j) showed that the carbon and nitrogen elements were distributed homogeneously in PDC− MA−COF, whereas tiny amount of oxygen and sulfur elements were derived from adsorbed water and DMSO solvent. The N and C elements were present in equal quantities and much higher than O and S elements, which was consistent with the elemental analysis results. It is known from the basic laws of thermodynamics that slower crystal growth rates and smaller entropies tend to result in long-range ordering crystals. If the nucleation is too fast, a large number of crystallites are formed, which would make the

The N2 adsorption isotherm at 77 K was measured to estimate the surface area and porosity of PDC−MA−COF. As shown in Figure 3, there was a steep rise below P/P0 = 0.01

Figure 3. N2 absorption−desorption isotherms and pore size distribution (inset) of PDC−MA−COF.

and then turned flat, exhibiting typical type II adsorption isotherm characteristics, which indicated the formation of microporous materials. Brunauer−Emmett−Teller and Langmuir surface areas were 748.2 and 1078.4 m2 g−1, respectively, with the total pore volume of 1.21 cm3 g−1, which further confirmed the porous structure of the COF. The pore width was mainly distributed in 1.9 nm evaluated by the density functional theory model, which matched ideally with the simulated results of the eclipsed stacking model (2.1 nm). It was demonstrated that the H-bonding interaction between adjacent layers allowed the pore structure of the COF to be well-maintained, and the interlayer slippage was less likely to occur.

Figure 4. SEM images (a, b) and HRTEM images (c−e) of PDC−MA−COF (inset of (d): SAED pattern). (f−j) EDX elemental mapping images of C, N, O, and S for PDC−MA−COF. D


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Figure 5. (a) CV curves at different scan rates (10, 20, 30, 50, 75, and 100 mV s−1) with saturated calomel electrode as the reference electrode. (b) GCD curves at various current densities (1.0, 2.0, 3.0, 5.0, 8.0, and 10 A g−1). (c) Specific capacitances at various current densities of 1.0−10 A g−1. (d) The Nyquist plot of PDC−MA−COF electrode; the insets are the magnification of high-frequency region and the fitting equivalent circuit diagram.

indicating typical pseudocapacitive characteristics. The possible redox mechanism is illustrated in Scheme 2, which involved

crystals difficult to grow and easy to aggregate. Excessive growth rates can cause defects in the crystal.46 So, during the experiment, the monomer solution was slowly added to reduce the nucleation rate in the initial stage of the reaction. At the same time, 6 M HAc was also added to slow down the rate of the Schiff-base reaction and to ensure the reaction reversibility. It is expected that dynamic covalent chemistry will facilitate the large-area growth of the frameworks,47 but these efforts do not solve the problem that crystals cannot grow. A great number of crystallites are formed at the beginning of the reaction, which can explain the reason for the unsatisfactory crystallinity and the smaller framework fragments of the material in the previous PXRD and XPS results. In the future work, we will do more research on how to improve the crystallinity of PDC− MA−COF. Owing to the existence of an interlayer H-bonding, TGA of PDC−MA−COF exhibited a good thermal stability with a thermal decomposition temperature close to 400 °C (Figure S2). Only 3% weight loss below 200 °C resulted from the adsorbed water and DMSO solvent removal from the material. Almost no weight loss between 200 and 400 °C indicated that there was no residual monomers in the COF. Approximately 10% weight loss between 400 and 500 °C was attributed to the collapse of the frameworks; the curve showed a steep drop in the range of 500−700 °C (about 68% weight loss), which was attributed to the decomposition and carbonization of the material. The electrochemical performance of PDC−MA−COF as a supercapacitor electrode material was studied using a threeelectrode cell in the 6 M KOH aqueous solution, in which the platinum foil electrode was used as the counter electrode and the saturated calomel electrode was used as the reference electrode. The cyclic voltammetry (CV) curves at different scan rates ranging from 10 to 100 mV s−1 in the potential window of 0−0.5 V (Figure 5a) showed symmetrical redox peaks, which suggested a quasi-reversible redox process,

Scheme 2. Redox Mechanism of the Triazine Unit for PDC−MA−COF

a redox transition of aromatic and quinone structures in the triazine units.48 In addition, both the redox peak current and the corresponding CV profile area increased as the scan rate increased; the CV curves could maintain a good symmetry even at higher scan rate, which revealed a high electrochemical activity of the COF. The oxidation peak potential shifted positively, and the reduction peak potential shifted negatively; the peak−peak potential separation increased with the increasing scan rate, which was mainly ascribed to the internal resistance of the working electrode.49,50 All of the above results could further prove the quasi-reversibility of the redox process.29 Figure S3 shows the relationship of the redox peak currents to the square root of the scan rate. It could be observed that the peak currents were linear with the scan rate square root, suggesting that the redox process of the PDC− MA−COF electrode was mainly controlled by the diffusion of the electrolyte. Galvanostatic charge−discharge (GCD) curves of the PDC−MA−COF electrode at various current densities of 1.0−10 A g−1 showed good symmetry, as shown in Figure 5b. The charge−discharge platforms of the GCD curves corresponded to the redox peaks of the CV curves, and the maximum specific capacitance of 335 F g−1 was acquired at the current density of 1.0 A g−1. As the current density increased to E


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Figure 6. (a) CV curves of PDC−MA−COF and the AC electrodes tested at the scan rate of 10 mV s−1 in a three-electrode mode. (b) CV curves tested at different voltage windows at the scan rate of 50 mV s−1. (c) CV curves at different scan rates (10, 30, 50, 75, and 100 mV s−1). (d) GCD curves at different current densities (1.0, 2.0, 3.0, 5.0, 8.0, and 10 A g−1). (e) Ragone plot of energy density versus power density for the PDC− MA−COF//AC supercapacitor. (f) Cycling performance test at the current density of 5.0 A g−1 (inset: GCD curves of the first eight and last eight cycles).

10 A g−1, PDC−MA−COF still exhibited a relatively high capacitance of 248 F g−1 with the specific capacitance retention of 74% (Figure 5c). Furthermore, the accessible redox-active triazine units of PDC−MA−COF were analyzed by comparing the charge transfer in the GCD test to its theoretical maximum based on the loading weight on the electrode. The calculations suggested that approximately 19.71% triazine units were accessed during the electrochemical process (Section S1). Electrochemical impedance spectroscopy (EIS) measurement was performed in the frequency ranging from 0.01 Hz to 100 kHz at an open circuit potential. Figure 5d shows Nyquist plot of the PDC−MA−COF electrode. A small characteristic semicircle in the high-frequency region indicated a lower charge transfer resistance and a higher conductivity inside the COF electrode. An almost vertical straight line in the lowfrequency region suggested a lower ionic diffusion resistance between the electrode and electrolyte, which indicated that the electrode process was diffusion-controlled.51 The result was consistent with the relation curves in Figure S3. The EIS result was fitted by the software of ZSimpWin using the equivalent circuit (the inset of Figure 5d), where Rs stood for the solution resistance of the electrolyte; Rct was the interfacial charge-transfer resistance between the electrode and electrolyte; while Cdl, Cf, Q, Rf, and R′f represented the doublelayer capacitance, Faradic capacitance, constant phase element,

and passivating film resistance, respectively. According to the equivalent circuit, the values of Rs and Rct were 0.708 and 0.132 Ω, respectively. Rct could be measured from the diameter of the semicircle in the Nyquist plot. It is well known that the small semicircle indicates a low interfacial charge-transfer resistance due to good conductivity of the electrode.52,53 PDC−MA−COF had a high nitrogen content (47.87%); the nitrogen atoms in the material had a higher charge density and a strong affinity to the electron-deficient K+ in the electrolyte solution; at the same time, PDC−MA−COF had a rich-pore structure, so the ions could rapidly transfer at the interface between the electrode and electrolyte solution, and the Rct could be significantly reduced. PDC−MA−COF showed a relatively high electronic conductivity of 3.34 × 10−2 S cm−1 measured by the four-probe tester. In the cycling performance test, the PDC−MA−COF electrode showed a good cyclic stability during 9000 cycles of charge−discharge measurements at a current density of 5.0 A g−1, as displayed in Figure S4. The specific capacitance retained about 78% of the initial value, which indicated that the electrode possessed good capacitive property. The decrease in retention may be caused by a small amount of PDC−MA− COF falling off the working electrode surface. From the above results, it could be confirmed that the high nitrogen content in COF could improve the conductivity of F


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density of 1.0 A g−1 in a three-electrode system, and the specific capacitance retention reached 74% at the current density range of 1.0−10 A g−1. The assembled PDC−MA− COF//AC ASC exhibited a high energy density of 29.2 W h kg−1 with a power density of 750 W kg−1 and an excellent cyclic stability, which retained 88% of the capacitance after 20 000 GCD cycles. In future research studies, we expect more low-cost, highly conductive, redox-active COF materials to be continuously developed as energy-storage devices for tangible commercialization.

the electrode; the high specific surface area could expose more active sites and allow the active material to fully contact with the electrolyte, thereby the triazine units exhibited good redox activity and excellent pseudocapacitive characteristics; the porosity and π−π stacking interactions were favorable for a fast charge transfer between the electrode and electrolyte interface, and the charge-transfer resistance and ionic diffusion resistance were greatly reduced; and the interlayer C−H···N H-bonding could maintain an ordered pore structure and enhance the stability of COF, which provided a good capacitance retention. To further evaluate the capacitor performance of PDC− MA−COF, an asymmetric supercapacitor (ASC) of PDC− MA−COF//AC was assembled in a two-electrode setup using 6 M KOH as an electrolyte, in which PDC−MA−COF was used as the positive electrode material and activated carbon (AC) as the negative electrode material. The electrochemical performance of the AC electrode was measured in a threeelectrode operation, as shown in Figure S5. The specific capacitance of the AC electrode was 331 F g−1 at the current density of 1.0 A g−1. The assembled PDC−MA−COF//AC ASC is shown in Scheme S1. The CV curves of AC and PDC− MA−COF at the scan rate of 10 mV s−1 was investigated to estimate the potential window of the asymmetric supercapacitor (Figure 6a), it showed that the maximum potential window could reach 1.5 V. Figures 6b and S6 show the CV and GCD curves of the PDC−MA−COF//AC ASC at different potential windows from 0.7 to 1.5 V at the scan rate of 50 mV s−1 and the current density of 1.0 A g−1, respectively, indicating a stable electrochemical performance range of 0−1.5 V. The CV profiles of the PDC−MA−COF//AC ASC at different scan rates of 10−100 mV s−1 in Figure 6c exhibited a good electrochemical reversibility. In addition, the GCD curves at different current densities between 1.0 and 10 A g−1 are exhibited in Figure 6d. The specific capacitances of the PDC− MA−COF//AC ASC reached 94 and 43 F g−1 at the current density of 1.0 and 10 A g−1, respectively. The energy density (E) and power density (P) are important factors for evaluating the energy-storage property of a supercapacitor. The Ragone plots showed that the PDC−MA−COF//AC ASC displayed a higher energy density of 29.2 W h kg−1 with a power density of 750 W kg−1 and 13.5 W h kg−1 with a power density of 7500 W kg−1. A comparison with other analogous electrode materials is shown in Figure 6e and Table S1.5,30,31,54−57 The cycling performance test of the PDC−MA−COF//AC ASC was carried out in the potential ranging from 0 to 1.5 V at the current density of 5.0 A g−1, suggesting excellent cyclic stability (Figure 6f). The PDC−MA−COF//AC ASC could still retain approximately 88% of the capacitance after 20 000 cycles of charge−discharge measurements.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06867.

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Reagent and materials, instruments, synthetic procedures of PDC−MA−COF, electrode preparation and electrochemical measurements, pore size distribution, TGA, linear relationship between the anodic and cathodic peak currents and the scan rate square root of the PDC−MA−COF electrode, cycling performance test in a three-electrode system, CV and GCD curves of the AC electrode, GCD curves tested at different voltage windows, comparison of CV and GCD curves of the PDC−MA−COF and Ni foam electrodes, determination of the triazine units accessed, and comparison of the electrochemical performance of analogous electrode materials (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.G.). *E-mail: [email protected] (W.Y.). ORCID

Wu Yang: 0000-0003-1599-4071 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21665024 and 20973101) and the Key Lab of Polymer Materials of Gansu Province.

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REFERENCES

(1) Saha, S.; Samanta, P.; Murmu, N. C.; Kuila, T. A Review on the Heterostructure Nanomaterials for Supercapacitor Application. J. Energy Storage 2018, 17, 181−202. (2) Wang, F.; Wu, X.; Yuan, X.; Liu, Z.; Zhang, Y.; Fu, L.; Zhu, Y.; Zhou, Q.; Wu, Y.; Huang, W. Latest Advances in Supercapacitors: from New Electrode Materials to Novel Device Designs. Chem. Soc. Rev. 2017, 46, 6816−6854. (3) Borenstein, A.; Hanna, O.; Attias, R.; Luski, S.; Brousse, T.; Aurbach, D. Carbon-Based Composite Materials for Supercapacitor Electrodes: a Review. J. Mater. Chem. A 2017, 5, 12653−12672. (4) DeBlase, C. R.; Hernández-Burgos, K.; Silberstein, K. E.; Rodríguez-Calero, G. G.; Bisbey, R. P.; Abruña, H. D.; Dichtel, W. R. Rapid and Efficient Redox Processes within 2D Covalent Organic Framework Thin Films. ACS Nano 2015, 9, 3178−3183. (5) Khattak, A. M.; Ghazi, Z. A.; Liang, B.; Khan, N. A.; Iqbal, A.; Li, L.; Tang, Z. A Redox-Active 2D Covalent Organic Framework with Pyridine Moieties Capable of Faradaic Energy Storage. J. Mater. Chem. A 2016, 4, 16312−16317.

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CONCLUSIONS A novel triazine-based covalent organic framework linked by imine bonds with an interlayer H-bonding was prepared by a Schiff-base condensation reaction using cheap monomers. The new material possessed high specific surface area, abundant pores, high nitrogen content, and large number of the interlayer C−H···N H-bonding, which gave it excellent electrochemical characteristics. Additionally, due to the presence of the redox-active triazine units in PDC−MA− COF, the electrochemical behavior of the COF electrode showed pseudocapacitive characteristics as a supercapacitor and underwent a quasi-reversible redox process. The maximum specific capacitance of 335 F g−1 was obtained at the current G


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Ultrastable Triazine-Based Covalent Organic Framework with an

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