Synthesis of conjugated covalent organic frameworks/graphene composite for supercapacitor electrodes

Abstract

A novel composite with imine-linked covalent organic frameworks on graphene was synthesized in one step with the amine functionalized reduced graphene oxide as the support. The excellent electrochemical properties of this composite can be ascribed to the synergistic effect of the introduction of covalent organic frameworks as well-dispersed nanoscale deposits on the conductive graphene surfaces.

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Driven by environmental problems and the energy crisis, the development of clean and renewable energy storage materials as well as their devices is urgently needed.¹ Among various energy storage devices, rechargeable batteries and electrochemical capacitors (ECs) are regarded typically as a suitable choice to store energy by transforming chemical energy into electrical energy.² Electrochemical capacitors, also called supercapacitors, which store energy using either ion adsorption or fast surface redox reactions, are supposed to be promising candidates for alternative electrical energy storage devices due to their high power density, exceptional cycle life, and low maintenance cost.³ The performance of supercapacitors is highly dependent on the properties of electrode materials. Many materials have been investigated as the electrode materials in supercapacitors, including carbonaceous materials, transition metal oxides and conducting polymers.⁴ Each type of electrode material has its pros and cons. The novel electrode materials with terrific performance should be developed.

Covalent organic frameworks (COFs) are a novel class of crystalline organic materials with well-defined and predictable two-or-three-dimensional pore structures assembled from molecular building blocks.⁵ Their crystal structures are entirely held by strong bonds solely from light elements (B, C, N, H, and O) to form rigid porous architectures. These materials have exceptional thermal stabilities, and low densities, and permanent porosity with high specific surface areas. Their extraordinary and versatile properties have offered the COF materials superior potential in diverse applications, such as gas storage,⁶ adsorption,⁷ optoelectricity,⁸ catalysis,⁹ and biosensor.¹⁰ As we know, many COFs have an extended π-conjugated system and inherent nanopores, which may hold promise for fundamental advances in supercapacitor electrode materials.¹¹ While, COFs has been scarcely reported as electrode materials for supercapacitors, which may be due to the limited chemical and oxidative stability of established COFs linkages precludes this applications.

Since the first report of graphene synthesized in 2004, it has emerged as one of the most active research fields.¹² Due to its fascinating properties, graphene is considered as a highly desirable support for many applications.¹³ Here, we present a simple method to fabricate the COFs/graphene composite through the reaction of aldehyde group in 1,3,5-triformylbenzene and amine group in NH₂–graphene and 1,4-diaminobenzene in one step on the surface of amino functionalized graphene. The synthesized conjugated covalent organic frameworks/graphene composite showed improved supercapacitance over COFs and graphene.

Amine modified reduced graphene oxide (NH₂–rGO) was weighed into a small vial and disperse in 1,4-dioxane. After being stirred for 0.5 h, 1,3,5-triformylbenzene, 1,4-diaminobenzene and acetic acid aqueous was added to the above dispersion successively under mild stirring. All the above dispersion transferred into a Teflon-lined autoclave and heated at 120 °C for 2 days. Afterwards, the obtained dark green solid was isolated by centrifugation, washed with N,N-dimethylformamide and tetrahydrofuran, and dried to yield a new composite COFs/NH₂–rGO (Scheme 1). In order to compare the energy storage properties of COFs/NH₂–rGO, COFs was prepared with the same method for fabricating COFs/graphene without adding NH₂–rGO into the reaction. NH₂–rGO–CHO was also prepared by NH₂–rGO reacted with 1,3,5-triformylbenzene in the presence of acetic acid to confirm the formation of imine bonds between graphene and COFs.

Scheme 1 Synthesis of COFs and COFs/NH₂–rGO.

Electrochemical studies were carried out in a three-electrode system with a Na₂SO₄ electrolyte solution (1 mol L⁻¹). Freshly prepared material on nickel mesh, a platinum electrode, and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. The working electrode was fabricated by mixing 80 wt% prepared material, 10 wt% poly(tetrafluoroethylene) binder, and 10 wt% Ketjenblack (EC-600JD) in an agate mortar. An appropriate amount of ethanol was then added to this mixture to make slurry, which was subsequently coated on Ni foam (1 × 1 cm²) and dried in an oven at 110 °C for 2 h. The Ni foams with active materials were finally pressed under 10 MPa to obtain the working electrodes. Cyclic voltammetry (CV) was carried out on a CHI 660E electrochemical workstation. Galvanostatic charge–discharge cycle tests were performed on a CT2001A-LAND cell test system.

GO and NH₂–rGO were successively prepared via modified Hummers method and one-pot solvothermal process. The morphologies of synthesized materials were investigated by using SEM (Fig. 1). Both GO and NH₂–rGO show chiffon sheets like morphologies. Fig. 2 shows a typical XRD pattern of the GO sample with a single broad peak at 10.5°, which corresponds to an interlayer distance of 0.84 nm. The XRD profile of NH₂–graphene has only one peak at 25.4° corresponding to d-spacing of 0.35 nm, and the peak at 10.5° disappeared. Clearly, the GO has been reduced to graphene during the process of GO modificated with amine. This observation is similar to the reduction of GO with NaBH₄ or N₂H₄·H₂O.¹⁴ After amine modification the FT-IR peaks of GO at 1384, 1630 and 1726 cm⁻¹ almost disappear while the band at 3438 cm⁻¹ decays significantly (Fig. 3). A new band at 1580 cm⁻¹ can be observed, which is corresponding to N–H in-plane stretching. The band at 3300–3600 cm⁻¹ with symmetric peak shape may be attributed to the N–H stretching of the amine groups. The characteristic absorption bands of oxide groups (O–H, C=O, and C–O) decreased dramatically, indicating that GO has been reduced to the graphene. XPS results show that the nitrogen concentration of NH₂–rGO is higher than its counterpart parent material GO and the oxygen concentration of NH₂–rGO is lower than its counterpart parent material GO (Fig. 4). Peaks 1348 and 1594 cm⁻¹ can be observed in the Raman spectra of synthesized GO and NH₂–rGO (Fig. S1†). The I_D/I_G for NH₂–rGO is higher than that of GO. All the above results indicate that GO can be modified with amino group and reduced simultaneously in solvothermal process in the presence of ammonia.

Fig. 1 SEM images of GO (a), NH₂–rGO (b), COFs (c) and COFs/NH₂–rGO (d).

Fig. 2 XRD patterns of synthesized GO, NH₂–rGO, COFs/NH₂–rGO and COFs.

Fig. 3 FT-IR spectra of graphite, GO, NH₂–rGO, COFs/NH₂–rGO and COFs.

Fig. 4 XPS of GO, NH₂–rGO and COFs/NH₂–rGO.

As we all know that functional amine group can reacted with formyl group to form the imine-linked materials. Here the amine functionalized reduced graphene oxide was used as the support for the fabrication of imine-linked COFs to enhance the electrochemical property of COFs. The SEM images of synthesized materials were shown in the Fig. 1. The layered NH₂–rGO and the spherical COFs can be observed in the SEM images of synthesized COFs/NH₂–rGO. The spherical COFs was anchored tightly on the graphene sheets, which is ascribed to the reaction of aldehyde group in 1,3,5-triformylbenzene and amine group in NH₂–rGO. Strong coupling between the surface COF and graphene was also helpful for the formation of the structure.¹⁵ The anchored spherical COFs will hinder the stacking or aggregation between graphene sheets, which may increase the effective electrode surface area. The XRD pattern of COFs/NH₂–rGO shows two well-resolved diffraction peaks (Fig. 2). The peak at around 25.4° can be ascribed to (002) diffraction of graphene and the peak centered at 4.7° can be ascribed to (100) diffraction of COFs, which is consistent with pure COFs.

The FT-IR spectrum of COFs/NH₂–rGO shows a strong C=N stretch at 1618 cm⁻¹ of (Fig. 3), which is indicating the formation of imine bonds. The C=N stretching vibrations of COFs overlaps with the vibrations of C=N linkage between COFs and NH₂–rGO (Fig. S2†). The residual signals at these 1695 cm⁻¹ and 3423 cm⁻¹ correspond to the terminal aldehyde and amino groups at the edges of the COFs/graphene, respectively. Raman spectrum of synthesized COFs/NH₂–rGO shows two peaks at 1342 and 1584 cm⁻¹, which can be ascribed to the D and G band of graphene (Fig. S1†). The composition of synthesized material was also verified by solid-state NMR spectroscopy. Fig. 5 shows the ¹³C solid-state NMR spectrum recorded for COFs/NH₂–rGO. The ¹³C NMR peak at ∼157 ppm corresponds to the carbon atom of the C=N bond, which is characteristic for the condensation reaction of 1,3,5-triformylbenzene and 1,4-diaminobenzene. The carbon atoms of the phenyl groups can be observed at ∼122, 129, 136 and 147 ppm. The minor peak at ∼191 ppm is ascribed to the carbon atoms of terminal aldehyde groups in the COFs/NH₂–rGO composite, which is in accordance with the FT IR results. From the XPS spectrum of COFs/NH₂–rGO we can see the increase of the N1s peak and decrease of O1s peak, which confirm the formation of COFs on NH₂–rGO (Fig. 4). All of the above results conjugated covalent organic frameworks/graphene composite can be obtained with the amine functionalized reduced graphene oxide as the support.

Fig. 5 ¹³C CP/MAS NMR spectra of COFs/NH₂–rGO.

Considering the synthesized COFs/NH₂–rGO with extended π-conjugated system and inherent nanopores of COFs and graphene with excellent electro-conductivity, this may hold promising for fundamental advances in electrode materials for supercapacitors. Fig. 6a shows the CV curves of COFs/NH₂–rGO in 1 M Na₂SO₄ aqueous solution under a potential in the range from 0 to 0.5 V with scan rates of 10–100 mV s⁻¹. COFs/NH₂–rGO shows the not nearly rectangular feature of CV curves indicates the combination of double layer and pseudocapacitance behaviors which are resulted from the synergistic effect between COFs and NH₂–rGO. The galvanostatic charge–discharge curves of synthesized materials are shown in Fig. 6b. The charging curve is nearly symmetric to its corresponding discharging counterpart, pointing to a good capacitive behavior and a highly reversible faradic reaction. It was found that COFs/NH₂–rGO possesses specific capacitance of 533 F g⁻¹ at current density of 0.2 A g⁻¹ in 1.0 M Na₂SO₄ electrolyte, which is higher than that of COFs (226 F g⁻¹) and NH₂–rGO (190 F g⁻¹). The specific capacitances of COFs/graphene composite are 533, 506, 493 and 480 F g⁻¹ at 0.2, 0.5, 1 and 2 A g⁻¹ in 1.0 M Na₂SO₄ electrolyte, respectively. The introduction of COFs as well-dispersed nanoscale deposits on the conductive graphene surfaces could effectively promote the electrochemical performance of the synthesized material due to the large pseudocapacity provided by the COFs nanoparticles. The real reason for COFs/NH₂–rGO with higher specific capacitance is not very clear, but we think it related with the increased effective electrode surface area and strong coupling between the surface COF and graphene.¹⁵ The cycle stability of supercapacitors is a crucial parameter for their practical applications, and the cycle performance is shown in Fig. 6d. After 1000 consecutive cycles, the discharge capacitance remains at 79% of its initial value. This result highlights excellent, high-rate capability and long-cycle lifetime of the COFs/NH₂–rGO electrode material.

Fig. 6 CV curves of COFs/NH₂–G (a), galvanostatic charge–discharge curves of synthesized materials at current density of 0.2 A g⁻¹ (b), galvanostatic charge–discharge curves of at various current density (c), and cycle stability of COFs/NH₂–G (d).

Conclusions

A novel composite was synthesized with the amine functionalized reduced grapheme oxide as the support for the fabrication of imine-linked COFs in one step. The existence of C=N linkage between COFs and amine functionalized reduced graphene oxide extended the π-conjugated systems of material. The obtained material COFs/NH₂–rGO shows improved supercapacitance over COFs and graphene. The specific capacitance of COFs/NH₂–rGO is 533 F g⁻¹ at 0.2 A g⁻¹ in a 1.0 M Na₂SO₄ electrolyte. After 1000 cycles, the specific capacitance retention is approximately 79%. The excellent electrochemical properties of this COFs/NH₂–rGO can be ascribed to the synergistic effect of the introduction of COFs as well-dispersed nanoscale deposits on the conductive graphene surfaces. All these results show that COFs/NH₂–rGO could be alternative electrode materials for supercapacitor.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21104070, 21301159), Natural Science Research Assistance Program from the Education Department of Henan Province (15A150083), Program of Zhengzhou City for Science and Technology (20130775), and the Doctoral Funding from Zhengzhou University of Light Industry (2011BSJJ010, 2011BSJJ011).

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Footnote

† Electronic Supplementary Information (ESI) available: Experimental methods and supplementary figures. See DOI: 10.1039/c5ra02251g

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