Preparation of graphene sponge with mechanical stability for compressible supercapacitor electrode
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The graphene porous structure has attracted attention as an electrode material of an energy storage device because it has a high specific capacitance due to its excellent electrical properties and high surface area. This research suggests easy-to-fabricate graphene porous structure based on polymer sponge and graphene oxide (GO). The graphene sponge was made by hydrothermal synthesis of a polymer sponge and an aqueous dispersion of GO. The graphene sponge produced by inexpensive and simple method shows good electrochemical performance. Morphology and structural properties of graphene sponge were investigated by field effect scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and Raman spectroscopy. The electrochemical performance was measured in 1 M KOH aqueous solution and showed good specific capacitance (214.06 F g−1) and ideal electric double-layer capacitor characteristics. This graphene sponge is flexible and can be applied to wearable devices and various next-generation energy storage devices.
KeywordsCoating Graphene Hydrothermal Porous structure Sponge Supercapacitor
Graphene is noticeable two-dimensional (2D) sheet of carbon nanomaterial . It exhibits outstanding thermal, chemical stability, extraordinarily high electrical conductivity, large specific surface area and great mechanical strength. Because of these fascinating properties, graphene is the most remarkable candidate for next-generation energy storage devices. Since high surface-to-volume ratio is important for high-performance energy storage devices, a porous structure is required for the electrode material [2, 3, 4]. Thus, the production of electrode materials with excellent electrical properties and a high surface-to-volume ratio structure by forming a porous structure using graphene as a nanoscale material is a key challenge for developing next-generation energy storage devices. And, it is also important to make flexible electrode materials for various applications such as wearable devices .
Herein, there are several methods to fabricate 3D interconnected structures of graphene such as self-aggregation , chemical vapor deposition (CVD)  and polymer-graphene composite  for developing integrated properties. However, the graphene foams fabricated by these methods have a brittle mechanical performance and require several complex stage of fabrication process . These results restrict application of 3D structure of graphene. For overcoming the limitation, developing simple method for fabricating graphene foam with excellent mechanical properties is important.
In this work, we present the method to fabricate ‘graphene sponge’ by coating GO dispersed in DI water on the surface of polymer with porous structure using a simple hydrothermal and freeze dry synthesis method. To maintain excellent electrical properties and to achieve mechanical strength and flexibility, concentration conditions of graphene oxide (GO) aqueous dispersion solution suitable for fabricating graphene sponge will be introduced, and a graphene sponge coated with CNT will be introduced to enhance network bonding between the GO nanosheets. The graphene sponge is able to achieve elastic mechanical performance and exhibits excellent electrochemical performance based on its excellent electrical properties unique to graphene. Also, since it is very easy to fabricate, it is highly commercial. In graphene sponge, a melamine foam was used as a polymer porous structure material for coating GO nanosheet. Melamine foam has a microscale uniform porous structure and good chemical stability and flexibility. The energy storage mechanism of the graphene sponge is based on the electrical double-layer capacitor (EDLC) . The EDLC stores energy using an electrical double layer at the electrode surface by electrostatic ion charge separation. Therefore, to evaluate the electrochemical performance of graphene sponge, the cyclic voltammetry and charge–discharge were performed under KOH 1 M solution.
2 Experimental method
Melamine sponge was purchased from DAIHAN Scientific, Korea. Graphene oxide (GO) aqueous dispersion solution was sonicated during 24 h. GO was synthesized via a Hummers’ method. For Hummers’ method, concentrated H2SO4 was added to a mixture of graphite flakes and NaNO3, and the mixture was cooled to 0 °C. And KMnO4 was added slowly in portions to keep the reaction temperature below 20 °C. The reaction was warmed to 35 °C and stirred for 30 min, and time water was added slowly, producing a large exotherm to 98 °C. External heating was introduced to maintain the reaction temperature at 98 °C for 15 min, then the heat was removed and the reaction was cooled using a water bath for 10 min. Additional water and 30% H2O2 were added, producing another exotherm. After air cooling, the mixture was purified . Carbon Nanotube (CNT) powder was purchased from Hanhwa Chemical Corporation.
2.2 Graphene sponge fabrication process
The morphology of the graphene sponge structure was measured using Field Emission Scanning Electron Microscope (FE-SEM, JEOL-7800F, JEOL Ltd.). The molecular and atomic structure characterization was performed via X-ray diffraction (XRD, Rigaku Ultima diffractometer using Cu-Kα radiation). Chemical bonds were investigated using X-ray photoelectron spectroscopy (XPS, k-alpha, Thermo. U.K.), and lattice vibration and structure of GO were analyzed via Raman spectroscopy (WITeck ALPHA300 M Raman System, excitation at 532 nm, 2.33 eV). And electrochemical measurements were carried out via a conventional three-electrode cell configuration system using Ivium n Stat (HS technology). A three-electrode system was used for the half-cell test with a graphene sponge as the working electrode, a platinum (Pt) wire as a counter electrode, and Ag/AgCl as a reference electrode in 1 M KOH.
3 Result and discussion
The graphene sponge was fabricated using simple two-step process. GO aqueous dispersion was prepared via mixing and sonicating GO with DI water, and melamine sponge was prepared in same size. The GO nanosheet coated the surface of melamine sponge and interconnected with each other compactly via hydrothermal process at 120 °C during 3 h. After freeze dry process, there are two types of graphene sponges, 2 mg ml−1 GO-dipping sponge and CNT/GO-dipping sponge.
3.2 Mechanical properties
3.3 Structural characterizations
3.4 Electrochemical measurement
Graphene can be used as an excellent EDLC material due to its high electrical conductivity, large surface-to-volume ration, and chemical stability. Therefore, the graphene sponge can be a supercapacitor electrode with highly performance. Therefore, electrochemical performance of graphene sponge was evaluated by cyclic voltammetry (CV) measurement, charge–discharge process, and electrochemical impedance measurement. To evaluate the electrochemical performance of a single electrode, a half-cell test was required. In experiment, a graphene sponge was used as the working electrode, a platinum wire was used as the counter electrode, and Ag/AgCl was used as the reference electrode in 1 M KOH.
The average specific capacitance of the sponge graphene calculated from CV curves is 214.06 F g−1.
This value is an improvement over previous studies on other supercapacitors utilizing graphene . And the graphene sponge was composed of GO and did not undergo any reduction process. Since the electrical conductivity of GO is much lower than that of defect-free graphene, specific capacitance values are increased when the electrical conductivity is increased by reducing GO through various methods . Therefore, graphene sponge has a great advantage as a supercapacitor electrode material. Figure 6b presents the galvanostatic charge–discharge (CD) curves of graphene sponge at various current densities form 1 mA cm−2 to 4 mA cm−2. By measuring and analyzing the CD curve, it can be seen that the CD curve shape of the graphene sponge is similar to typical that of EDLC. The electrochemical impedance spectroscopy (EIS) analysis is the principal method for evaluating the electrical conductivity of electrode materials . Fig. 6d shows the Nyquist profile of the graphene sponge in the frequency range 10 mHz–10 MHz under the open circuit potential condition. The shape of the EIS curve shows the ideal EDLC characteristic of the graphene sponge .
In this work, graphene sponge was prepared via simple hydrothermal and freeze dry synthesis of a melamine sponge and a GO aqueous dispersion solution. GO successfully enveloped the surface of the melamine sponge and formed a membrane between the pores to improve the surface-to-volume ration of the porous polymer structure. Through this fabrication process, a porous graphene structure of high quality can be formed very simply and inexpensively compared to other methods. When graphene sponge is made with GO solution of higher concentration than low concentration GO solution, more precise structure can be obtained. This structure has high surface-to-volume and excellent electrical, chemical and mechanical properties of graphene, maximizing the specific capacitance of supercapacitor electrodes. Despite the simple fabrication process, the graphene sponge showed good electrochemical performance: has high surface-to-volume and excellent electrical, chemical and mechanical properties of graphene, maximizing the specific capacitance of supercapacitor electrodes. Despite the simple fabrication process, the graphene sponge showed good electrochemical performance: high specific capacitance (214.06 F g−1) in aqueous 1 M KOH solution. This value is not very high compared to other electrode materials, but the electrical conductivity is greatly improved when the GO is subjected to a reduction process. Therefore, the easy-to-fabricate graphene sponge has a great potential to be used as an electrode material for next-generation energy storage devices.
This subject is supported by Korea Ministry of Environment as “Global Top Project (2016002130005)” and Development of diagnostic system for mild cognitive impairment due to Alzheimer’s disease (2017-11-0951).
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