Graphene electrode, its manufacturing method and power storage device using the same

Abstract

To provide a graphene electrode having excellent properties as an electrode for a power storage device (in particular, for an electric double layer capacitor) having flexibility and self-supporting properties without including a conductive material or a binder.SOLUTION: The graphene electrode is free of conductive materials and binders and consists substantially only of graphene. The density is in a range of 0.2 mg / cm3 to 0.7 mg / cm3. The graphene electrode has flexibility and self-supporting properties. Preferably, the graphene is a thermally reduced graphene oxide.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a graphene electrode, a method for producing the same, and an electricity storage device using the same. [Background technology]

[0002] Electric storage devices such as electric double layer capacitors (supercapacitors) and lithium-ion batteries have attracted attention due to their large capacity. In addition, with the rapid development of portable electronic devices in recent years, there is a growing demand for flexible energy storage systems.

[0003] Graphene is known to be used as an electrode material for electric double layer capacitors. To date, attempts have been made to fabricate flexible electrodes by combining graphene with carbon nanotubes, polymers, etc.

[0004] For example, Non-Patent Document 1 proposes the use of a flexible, self-supporting film (AC / CNT / rGO film) made by combining activated carbon (AC), carbon nanotubes (CNT), and reduced graphene oxide (rGO) as an electrode for an electric double layer capacitor. Non-Patent Document 2 also describes the use of free-standing graphene paper (GP) synthesized by inkjet printing using a dispersion of graphene oxide (GO) and a dispersion of graphene hydrogel (GH) and polyaniline (PANI). The GP has a structure supported by a porous GH-PANI nanocomposite and can be used as an electrode for an electric double layer capacitor.

[0005] However, it has been reported that in the case of materials combining graphene with substances (nanostructures) or metal oxides as described in Non-Patent Documents 1 and 2, problems such as shortened cycle life, reduced frequency characteristics, and deterioration in rate characteristics often occur in the performance of electric double layer capacitors using these materials as electrodes (see, for example, Non-Patent Document 3).In addition, there is also the problem that the production process for composite materials is generally complicated and the raw materials can be expensive, making them unsuitable for mass production.

[0006] In addition, when conventional electrode materials are actually constructed into electric double layer capacitors, they are mixed with conductive materials and binders and processed into a film. When mixed with such conductive materials and binders, they may adsorb onto the graphene surface, affecting the infiltration and diffusion of electrolyte ions and potentially reducing the energy characteristics of the electric double layer capacitor. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] X. Li et al., Carbon. 129 (2018) 236-244. [Non-patent document 2] K. Chi et al., ACS Appl. Mater. Interfaces. 6 (2014) 16312-16319. [Non-patent document 3] X. Xiao et al., Energy Storage Mater. 1 (2015) 1-8. Summary of the Invention [Problem to be solved by the invention]

[0008] In view of the above, an object of the present invention is to provide a graphene electrode that does not contain a conductive material or a binder, has flexibility and self-supporting properties, and has excellent properties as an electrode. Another object of the present invention is to provide a method for producing such a graphene electrode efficiently and with a low environmental impact, and to provide an electricity storage device using the graphene electrode. [Means for solving the problem]

[0009] The graphene electrode according to the present invention does not contain a conductive material or a binder, and is essentially composed of graphene alone. g / cm 3 ~0.7 g / cm 3 The present invention has flexibility and self-supporting properties, thereby solving the above-mentioned problems. The graphene electrode has a specific surface area of ​​200 m by the BET method. 2 / g or more 1000m 2 / g or less. The graphene may be thermally reduced graphene oxide. In a flexibility and mechanical strength test using a membrane-like test piece having a thickness in the range of 50 μm to 80 μm, the graphene electrode may be bent so that opposing ends of the test piece are in contact with each other and maintained in this state for a certain period of time. When the entire test piece is visually observed and a substantially central portion of the test piece is observed with an SEM, no structural damage or destruction of the test piece is observed, and the substantially central portion may have a smooth arch shape with a predetermined radius of curvature.

[0010] The above-described graphene electrode manufacturing method according to the present invention includes the steps of preparing a graphene oxide dispersion and a thermally reduced graphene oxide dispersion, mixing the graphene oxide dispersion and the thermally reduced graphene oxide dispersion to prepare a mixed dispersion in which graphene oxide and thermally reduced graphene oxide are dispersed, vacuum filtering the mixed dispersion onto a substrate to produce an intermediate structure composed of graphene oxide and thermally reduced graphene oxide, drying the intermediate structure and peeling it from the substrate to obtain a self-supporting structure, and heat-treating the self-supporting structure to reduce the graphene oxide in the self-supporting structure, thereby solving the above-described problem. The step of preparing the graphene oxide dispersion and the thermally reduced graphene oxide dispersion may further include the steps of freeze-drying a graphene oxide aqueous dispersion in which a predetermined amount of graphene oxide is dispersed in water, and heating the obtained graphene oxide solid in a muffle furnace at 300°C to 700°C for one minute or less. The step of heat-treating the self-supporting structure may involve heating the self-supporting structure in a muffle furnace at 300° C. to 700° C. for one minute or less to reduce graphene oxide in the self-supporting structure. In the step of preparing the mixed dispersion, the graphene oxide dispersion and the thermally reduced graphene oxide dispersion may be mixed so that a mass ratio of graphene oxide to thermally reduced graphene oxide in the mixed dispersion satisfies a range of 3:1 to 1:3.

[0011] The electricity storage device according to the present invention includes an electrode and an electrolyte, and the electrode is made of the graphene electrode described above, thereby solving the above-mentioned problems. The power storage device may be an electric double layer capacitor. [Effects of the Invention]

[0012] The graphene electrode of the present invention is an electrode that does not contain a conductive material or a binder, is not a composite combined with a substance other than graphene, and is composed essentially of graphene alone, has a predetermined density, and is flexible and self-supporting. Therefore, the graphene electrode of the present invention can effectively exhibit the electrical properties inherent to graphene, and has excellent properties as an electrode for electricity storage devices such as electric double layer capacitors and lithium ion batteries. Use of the graphene electrode of the present invention makes it possible to provide electricity storage devices such as electric double layer capacitors and lithium ion batteries that have excellent performance, such as capacitance and energy density.

[0013] The method for producing a graphene electrode of the present invention uses a graphene oxide dispersion and a thermally reduced graphene oxide dispersion, and can produce a film-like molded body from these mixed dispersions in a simple manner. In producing the electrode, no composite with any substance other than graphene is performed, and no conductive material or binder is used. Furthermore, graphene oxide can be reduced by thermal reduction in a short time. This method does not require skilled techniques or expensive equipment, is inexpensive and efficient, and has a low environmental impact, making it suitable for mass production. [Brief explanation of the drawings]

[0014] [Figure 1] Flowchart showing the manufacturing process of the graphene electrode of the present invention [Figure 2] Schematic diagram showing an electric double layer capacitor of the present invention. [Figure 3] (a) to (c) are SEM images of Membrane B of Example 1, Membrane B of Example 5, and Membrane B of Example 3. [Figure 4] FIG. 1 shows (a) X-ray diffraction patterns, (b) Raman spectra, (c) XPS C1s spectra, and (d) FTIR spectra of film B of Example 1, film B of Example 3, and film B of Example 3. [Figure 5] (a) to (e) SEM images of the cross sections of the film B of Example 1, the film B of Example 2, the film B of Example 3, the film B of Example 4, and the film B of Example 5. (f) A graph comparing the density of the film B of Examples 1 to 5 with the films of Examples 1 to 5. [Figure 6] (a) A graph showing nitrogen adsorption / desorption isotherms of membranes B in Examples 1 to 5, (b) A graph showing pore size distributions of membranes B in Examples 1 to 5, (c) A graph showing nitrogen adsorption / desorption isotherms of membranes in Examples 1 to 5, and (d) A graph showing pore size distributions of membranes in Examples 1 to 5. [Figure 7] (a) to (c) are diagrams showing the test results of the flexibility and mechanical strength of the membrane of Example 3. [Figure 8] FIG. 1 shows (a) specific capacity-voltage curves (CV curves), (b) constant current charge / discharge curves (GCD curves), (c) electrochemical impedance spectra (EIS), and (d) rate characteristics when the membranes of Example 3 and Example 5 are used and the electrolyte is an aqueous electrolyte solution (aqueous sulfuric acid solution). [Figure 9] FIG. 1 shows (a) specific capacity-voltage curves (CV curves), (b) constant current charge / discharge curves (GCD curves), (c) electrochemical impedance spectra (EIS), and (d) rate characteristics when the membranes of Example 3 and Example 5 are used and the electrolyte is an ionic liquid (EMI-BF4). DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that like elements are designated by like numbers and their description will be omitted.

[0016] (Embodiment 1) In the first embodiment, a graphene electrode of the present invention and a method for producing the same will be described.

[0017] The graphene electrode of the present invention does not contain a conductive material or a binder and is composed substantially of graphene alone.

[0018] In this specification, the phrase "consisting essentially of graphene" means that the electrode does not contain any other material than graphene as a constituent material of the electrode. Therefore, in the graphene electrode of the present invention, the presence of materials other than graphene is not confirmed, at least in its SEM image. However, the presence of materials unavoidably mixed or remaining in the manufacturing process is acceptable. On the other hand, since such materials may impair the energy characteristics of an energy storage device, it is desirable to minimize their presence. From this perspective, in the graphene electrode of the present invention, the graphene is preferably thermally reduced graphene oxide. Note that, in a case where a manufacturing method for an electrode is known, the electrode is considered to be "consisting essentially of graphene" if, in the process, graphene (or graphene oxide) is not mixed or composited with other materials (except for the dispersion medium) and is not mixed with a conductive material, a binder, or other additives. The manufacturing method for the graphene electrode of the present invention will be described later.

[0019] The graphene electrode of the present invention has a density of 0.2 g / cm 3 More than 0.7g / cm 3 The graphene electrode of the present invention has a density in the range of 0.4 g / cm. This allows the graphene electrode of the present invention to have the desired flexibility and self-supporting properties, making it suitable for use as an electrode for an electricity storage device. Furthermore, a density in this range allows electrolyte ions to easily reach and move within the graphene. The graphene electrode of the present invention preferably has a density of 0.4 g / cm. 3 More than 0.6g / cm 3 It has a density in the following range:

[0020] The graphene electrode of the present invention is substantially composed of graphene and has a density in the above-mentioned range, and therefore preferably has a specific surface area measured by the BET method of 200 m 2 / g or more 1000m 2 / g or less. This allows the graphene electrode of the present invention to have high conductivity and be capable of adsorbing electrolyte ions while maintaining the required flexibility and self-supporting properties. More preferably, the graphene electrode of the present invention has a specific surface area measured by the BET method of 240 m or less. 2 / g or more 400m 2 / g or less. This further ensures the adsorption and migration of electrolyte ions in the graphene electrode of the present invention. More preferably, the graphene electrode of the present invention has a specific surface area measured by the BET method of 240 m 2 / g or more 350m 2 / g or less.

[0021] The graphene electrode of the present invention is generally used in the form of a film. In this case, the film thickness is preferably 10 μm or more and 100 μm or less. Within this range, the graphene electrode of the present invention is easy to handle, can be easily applied to a current collector, and achieves high energy density and power density when applied to various electricity storage devices.

[0022] The terms "flexibility" and "self-supporting" are commonly used in the art, and the flexibility and self-supporting properties of the graphene electrode of the present invention can be confirmed, for example, by the following tests of flexibility and mechanical strength.

[0023] [Test method for flexibility and mechanical strength] As the test piece, a film-like structure having a thickness in the range of 50 μm to 80 μm is used. The test piece is bent so that the opposing ends of the test piece are in contact with each other, and this state is maintained for a certain period of time using any fixing means. Here, if the test piece is approximately rectangular in plan view, it is bent so that both ends in the longitudinal direction are in contact with each other. The fixing means is not limited, but tweezers or the like can be used, for example. Next, the overall appearance of the test piece that has maintained its curved state is visually observed. The graphene electrode of the present invention maintains its curved state with a smooth curve, and no structural damage or destruction of the test piece is observed. Furthermore, the approximate center of the test piece that was kept in the curved state was observed with a scanning electron microscope (SEM). Here, the approximate center of the test piece refers to the part of the test piece that is most heavily loaded due to the curve of the test piece. The graphene electrode of the present invention was observed with the SEM, and no structural damage or destruction was observed in the test piece, and it was confirmed that the approximate center of the test piece had a smooth arch shape with a predetermined radius of curvature.

[0024] Next, a method for producing a graphene electrode of the present invention will be described. FIG. 1 is a flowchart showing the steps of manufacturing a graphene electrode of the present invention.

[0025] Step S110: A graphene oxide dispersion and a thermally reduced graphene oxide dispersion are prepared.

[0026] Graphene oxide (GO) and thermally reduced graphene oxide (TRGO) may be commercially available, or step S110 may further include a step of preparing graphene oxide and / or thermally reduced graphene oxide.

[0027] Graphene oxide may be produced by known production methods, for example, graphene oxide prepared from natural graphite using a modified Hummers method.

[0028] Thermally reduced graphene oxide is produced by heat-treating graphene oxide. The conditions for heat-treating graphene oxide are not particularly limited, but a shorter heat treatment time is preferable from the viewpoints of production efficiency and reducing environmental impact. Specifically, for example, a graphene oxide aqueous dispersion in which a predetermined amount of graphene oxide is dispersed in water is freeze-dried for several days (approximately 2 days) in a freeze dryer, and the resulting graphene oxide solid is then heated in a muffle furnace at 300°C to 700°C for 1 minute or less and quickly removed, thereby obtaining the desired thermally reduced graphene oxide.

[0029] The dispersion medium for preparing the graphene oxide dispersion is not particularly limited, and examples thereof include water, ethanol, etc. From the viewpoint of preparing a mixed dispersion with the thermally reduced graphene oxide dispersion in step S120 described below, the dispersion medium is preferably the same as the dispersion medium of the thermally reduced graphene oxide dispersion or a dispersion medium that is highly miscible with the thermally reduced graphene oxide dispersion. Specifically, for example, the graphene oxide dispersion can be prepared by replacing the dispersion medium with ethanol from a graphene oxide aqueous dispersion in which a predetermined amount of graphene oxide is dispersed in water.

[0030] The concentration of the graphene oxide dispersion is not particularly limited, and may be appropriately selected so that the graphene oxide can be well dispersed in the dispersion medium. Specifically, the concentration of the graphene oxide dispersion may be in the range of 0.1 mg / mL to 1.0 mg / mL, for example.

[0031] The dispersion medium for preparing the thermally reduced graphene oxide is not particularly limited, and examples thereof include water, ethanol, etc. From the viewpoint of preparing a mixed dispersion with the graphene oxide dispersion in step S120 described below, the dispersion medium is preferably the same as the dispersion medium of the graphene oxide dispersion or a dispersion medium that is highly miscible with it.

[0032] The concentration of the thermally reduced graphene oxide dispersion is not particularly limited, and may be appropriately selected so that the thermally reduced graphene oxide can be well dispersed in the dispersion medium. Specifically, the concentration of the thermally reduced graphene oxide dispersion may be in the range of 0.1 mg / mL to 1.0 mg / mL, for example.

[0033] Step S120: The graphene oxide dispersion liquid and the thermally reduced graphene oxide dispersion liquid are mixed to prepare a mixed dispersion liquid in which graphene oxide and thermally reduced graphene oxide are dispersed.

[0034] The mixing ratio of the graphene oxide dispersion and the thermally reduced graphene oxide dispersion is not particularly limited, and may be adjusted so that the ratio (mass ratio) of graphene oxide to thermally reduced graphene oxide in the mixed dispersion is a desired value, taking into consideration the concentrations of the graphene oxide dispersion prepared in step S110 and the thermally reduced graphene oxide dispersion.

[0035] For example, when the concentrations of the graphene oxide dispersion and the thermally reduced graphene oxide dispersion are the same, the amount (volume ratio) of each dispersion is the same as the ratio (mass ratio) of graphene oxide to thermally reduced graphene oxide in the mixed dispersion. Therefore, by changing the amount (volume ratio) of each dispersion, the ratio (mass ratio) of graphene oxide to thermally reduced graphene oxide in the mixed dispersion can be easily adjusted. On the other hand, when the ratio (mass ratio) of graphene oxide to thermally reduced graphene oxide in the mixed dispersion is predetermined, the concentrations of the graphene oxide dispersion and the thermally reduced graphene oxide dispersion may be adjusted in advance to be the same ratio, and the two dispersions may be mixed in equal volumes.

[0036] The method for mixing the graphene oxide dispersion and the thermally reduced graphene oxide dispersion is not particularly limited, but from the viewpoint of achieving a uniform dispersion state of graphene oxide and thermally reduced graphene oxide in the mixed dispersion, it is preferable to use a mixing / dispersing device such as an ultrasonic homogenizer.

[0037] Step S130: The mixed dispersion is vacuum filtered onto the substrate to prepare an intermediate structure consisting of graphene oxide and thermally reduced graphene oxide.

[0038] The substrate is not particularly limited, and known substrates such as filter paper made of various resins can be used. Specific examples include filter paper made of polytetrafluoroethylene (PTFE). Since the intermediate structure is peeled off from the substrate in step S140 described below, it is preferable to select the substrate taking into consideration the ease of this peeling operation.

[0039] The method for vacuum filtering the mixed dispersion on the substrate is not particularly limited, and any known method can be used.

[0040] Step S140: The intermediate structure is dried and peeled off from the substrate to obtain a self-supporting structure.

[0041] The method for drying the intermediate structure formed on the substrate is not particularly limited, and the structure may be dried in the air or using a drying device such as an oven.

[0042] The method for peeling the dried intermediate structure from the substrate is not particularly limited, and any known method can be adopted taking into consideration the material of the substrate, the size (diameter) of the intermediate structure, and the like.

[0043] Step S150: The self-supporting structure is heat-treated to reduce the graphene oxide in the self-supporting structure.

[0044] The conditions for the heat treatment for reducing the graphene oxide in the self-supporting structure (intermediate structure peeled from the substrate) obtained in step S140 are not particularly limited, but a shorter heat treatment time is preferable from the viewpoints of production efficiency and reducing environmental impact. Specifically, for example, the self-supporting structure is heated in a muffle furnace at 300°C to 700°C for less than 1 minute and then quickly removed, thereby reducing the graphene oxide in the self-supporting structure and obtaining a film (thermally reduced-GO / TRGO film) composed of the target thermally reduced graphene oxide and thermally reduced graphene oxide. The thermally reduced-GO / TRGO film obtained in this manner is a structure entirely composed of thermally reduced graphene oxide, i.e., graphene, and is the graphene electrode of the present invention.

[0045] It has been known that oxygen-containing functional groups in graphene oxide can be removed in a short time (e.g., within seconds) by heat treatment to reduce or delaminate the graphene oxide. However, rapid heat treatment to directly fabricate a self-supporting graphene film is difficult because rapid heating of a graphene oxide film can destroy the film structure. It has been reported that the decomposition of many oxygen-containing functional groups generates a large amount of carbon dioxide gas, which generates a large amount of pressure inside the film. However, because graphene oxide films have a very compact structure formed between their layers by hydrogen bonds derived from the oxygen-containing functional groups, the pressure generated inside the film cannot be released. Due to these issues, various attempts have been made to fabricate flexible graphene electrodes by improving the manufacturing process or equipment. However, none of these attempts have yielded a material that is practically viable in terms of electrical properties or mechanical strength.

[0046] In contrast, in the graphene electrode manufacturing method of the present invention described above, when graphene oxide is used as a starting material, the desired graphene electrode can be manufactured by performing thermal reduction treatment twice. Specifically, in the manufacturing process shown in the examples described later, when graphene oxide is used as a starting material, the total time for two thermal reduction treatments is within 2 minutes.

[0047] Furthermore, the graphene electrode of the present invention obtained in this manner has the flexibility and self-supporting properties required for an electrode for an electricity storage device such as an electric double layer capacitor or a lithium ion battery, and as will be described in the examples below, it exhibits excellent electrical properties in both aqueous electrolyte solutions and non-aqueous electrolyte solutions (ionic liquids).

[0048] In the graphene electrode manufacturing method of the present invention, an intermediate structure is produced using a mixed dispersion of a graphene oxide dispersion and a thermally reduced graphene oxide dispersion. This structure, in which porous thermally reduced graphene oxide is introduced between graphene oxide layers, ensures that graphene oxide is reduced reliably by a short heat treatment. Furthermore, the introduction of porous thermally reduced graphene oxide ensures channels for releasing gas and pressure generated during graphene oxide reduction to the outside of the film, thereby preventing damage to the structure. From this perspective, the mass ratio of graphene oxide to thermally reduced graphene oxide in the mixed dispersion is preferably in the range of 3:1 to 1:3, more preferably 2.5:1 to 1:2.5, even more preferably 2:1 to 1:2, and even more preferably 1.5:1 to 1:1.5.

[0049] Furthermore, in the method for producing a graphene electrode of the present invention, the ratio (mass ratio) of graphene oxide to thermally reduced graphene oxide in the mixed dispersion can be adjusted to a desired range by adjusting the concentrations and mixing ratio (volume ratio) of the graphene oxide dispersion and the thermally reduced graphene oxide dispersion. This makes it possible to simply produce a flexible and self-supporting graphene electrode that is more suitable (optimized) for the desired application of an electricity storage device.

[0050] (Embodiment 2) In the second embodiment, an electric double layer capacitor will be described as an electricity storage device using the graphene electrode of the present invention. FIG. 2 is a schematic diagram showing the electric double layer capacitor of the present invention.

[0051] The electric double layer capacitor of the present invention includes at least electrodes and an electrolyte. Electric double layer capacitor 200 in FIG. 2 includes electrodes, i.e., a positive electrode 210 and a negative electrode 220, immersed in electrolyte 230. Positive electrode 210 and negative electrode 220 are made of the graphene electrodes described in the first embodiment. Electrolyte 230 is, for example, an ionic liquid selected from the group consisting of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI), 1-ethyl-3-methylimidazolium borofluoride (EMI-BF4), and 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide (MPPp-TFSI), or M'OH (M' is an alkali metal).

[0052] The electric double layer capacitor 200 further includes a separator 240 between the positive electrode 210 and the negative electrode 220 to separate the positive electrode 210 and the negative electrode 220 .

[0053] The material of separator 240 is, for example, a material selected from fluorine-based polymers, polyethers such as polyethylene oxide and polypropylene oxide, polyolefins such as polyethylene and polypropylene, polyacrylonitrile, polyvinylidene chloride, polymethyl methacrylate, polymethyl acrylate, polyvinyl alcohol, polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone, polyethyleneimine, polybutadiene, polystyrene, polyisoprene, polyurethane-based polymers and derivatives thereof, cellulose, paper, and nonwoven fabric.

[0054] In the electric double layer capacitor 200, the above-mentioned positive electrode 210, negative electrode 220, electrolyte 230, and separator 240 are housed in a cell 250. Furthermore, the positive electrode 210 and the negative electrode 220 each have an existing current collector.

[0055] Such an electric double layer capacitor 200 may be a chip type, coin type, mold type, pouch type, laminate type, cylindrical type, square type, or the like capacitor, and may further be used in a module in which multiple of these are connected together.

[0056] Next, the operation of the electric double layer capacitor 200 shown in FIG. 2 will be described.

[0057] When a voltage is applied to the electric double layer capacitor 200, electrolyte ions (anions) of the electrolyte 230 are adsorbed to the positive electrode 210, and electrolyte ions (cations) of the electrolyte 230 are adsorbed to the negative electrode 220. As a result, an electric double layer is formed in each of the positive electrode 210 and the negative electrode 220, and charging occurs. Here, the positive electrode 210 and the negative electrode 220 are formed from the graphene electrode described in the first embodiment and are electrodes consisting essentially of graphene. This facilitates the adsorption and diffusion of cations and anions by the graphene, thereby achieving high rate characteristics. Furthermore, since the positive electrode 210 and the negative electrode 220 are formed from the graphene electrode described in the first embodiment and are electrodes consisting essentially of graphene, many electrolyte ions are adsorbed not only on the surface of the graphene but also inside the graphene, forming an electric double layer. As a result, the exchange of electrons between the graphene and the electrolyte ions is increased, and high energy density can be achieved.

[0058] When the charged electric double layer capacitor 200 is connected to a circuit such as a resistor, the anions and cations adsorbed to the positive electrode 210 and the negative electrode 220, respectively, are desorbed and discharged. Here again, the positive electrode 210 and the negative electrode 220 are formed from the graphene electrodes described in the first embodiment, which facilitates desorption and diffusion of electrolyte ions, thereby achieving high rate characteristics and energy density. Furthermore, because of their excellent electrical conductivity, the ease of desorption and diffusion can also lead to improved power density.

[0059] As described above, the electric double layer capacitor 200 of the present invention can fully utilize the properties of graphene in the electrodes, enabling rapid charging and achieving high energy density and high power density. Furthermore, since the formation of an electric double layer is utilized for charging and discharging, the electric double layer capacitor 200 of the present invention is excellent for repeated use. The electric double layer capacitor 200 of the present invention can be used in wind power generation, electric vehicles, etc.

[0060] Although the description here is limited to electric double layer capacitors, it goes without saying that the graphene electrode of the present invention can be applied to electricity storage devices such as lithium ion batteries in addition to electric double layer capacitors.

[0061] The present invention will now be described in detail using specific examples, but it should be noted that the present invention is not limited to these examples. [Example]

[0062] [Fabrication of graphene electrodes] (Examples 1 to 5) Graphene oxide (GO) was prepared from natural graphite using a modified Hummers method.

[0063] A graphene oxide dispersion (0.5 mg / mL) was prepared by replacing the dispersion medium of a graphene oxide aqueous dispersion prepared by dispersing graphene oxide in water with ethanol.

[0064] The thermally reduced graphene oxide dispersion was prepared as follows. First, the graphene oxide aqueous dispersion was freeze-dried for 2 days in a freeze dryer, and then the resulting sponge-like graphene oxide solid was heated at 500 °C in a muffle furnace for less than 1 min and quickly removed, yielding thermally reduced graphene oxide (TRGO) as a cotton-like black solid. The thermally reduced graphene oxide thus obtained was dispersed in ethanol to prepare a thermally reduced graphene oxide dispersion (0.5 mg / mL) (Step S110 in FIG. 1).

[0065] The mixing ratio (volume ratio) of the graphene oxide dispersion and the thermally reduced graphene oxide dispersion was changed from 3:1 to 1:3, and they were mixed using an ultrasonic homogenizer to prepare a mixed dispersion in which graphene oxide and thermally reduced graphene oxide were dispersed at a mass ratio of 3:1 to 1:3 (step S120 in Figure 1).

[0066] The resulting mixed dispersion was then vacuum filtered through a polytetrafluoroethylene (PTFE) filter paper to produce a film (GO / TRGO film), which is an intermediate structure consisting of graphene oxide and thermally reduced graphene oxide (step S130 in Figure 1).

[0067] Next, the GO / TRGO film formed on the PTFE filter paper was dried in an oven and peeled off from the filter paper to obtain a free-standing film, which was a self-supporting structure (step S140 in Figure 1).

[0068] Next, this free-standing film (GO / TRGO film) was heated (heat-treated) in a muffle furnace at 500 °C for less than 1 min and then quickly removed. In this way, the graphene oxide in the GO / TRGO film was thermally reduced, and a film consisting of thermally reduced graphene oxide and thermally reduced graphene oxide (thermally reduced-GO / TRGO film) was obtained (step S150 in Figure 1). The thickness of the obtained film was in the range of approximately 60 μm to approximately 70 μm.

[0069] For comparison, a graphene oxide dispersion (0.5 mg / mL) alone was vacuum-filtered onto a PTFE filter paper to produce a film (GO film) consisting of only graphene oxide. This GO film was then dried in an oven and peeled off from the filter paper to obtain a free-standing film, which was then subjected to the heat treatment described above to obtain a film consisting of only thermally reduced graphene oxide (thermally reduced-GO film). In addition, only the thermally reduced graphene oxide dispersion (0.5 mg / mL) was vacuum filtered on a PTFE filter paper to prepare a membrane as an intermediate structure. The membrane formed on the PTFE filter paper was then dried in an oven to obtain a membrane consisting only of thermally reduced graphene oxide (TRGO membrane).

[0070] Table 1 below shows the ratio (mass ratio) of graphene oxide to thermally reduced graphene oxide in the mixed dispersion.

[0071] [Table 1]

[0072] In the following, to distinguish between the final product membrane and the intermediate structure membrane, the former is sometimes simply referred to as the "membrane" and the latter as the "membrane B." For example, in the case of Example 3, the "membrane of Example 3" refers to the "thermally reduced-GO / TRGO membrane" obtained through the heat treatment, and the "membrane B of Example 3" refers to the "GO / TRGO membrane" before the heat treatment. However, for Example 5, for comparison with other examples, the "membrane of Example 5" or "membrane B of Example 5" may be used. However, please note that both terms refer to the "TRGO membrane" because the GO content in the membrane is zero and no heat treatment for GO reduction was performed.

[0073] [Microstructure analysis] 3(a) to 3(c) are diagrams showing the results of observing the surface morphology of Membrane B of Example 1, Membrane B of Example 5, and Membrane B of Example 3, respectively, using a scanning electron microscope (SEM, JSM-6500F manufactured by JEOL Ltd.) The scale bar in each figure is 10 μm.

[0074] FIG. 3(a) shows that Membrane B (GO membrane) in Example 1 has fewer pores and a dense surface. On the other hand, Fig. 3(b) shows that membrane B (TRGO membrane) in Example 5 has randomly distributed flake-like structures and is highly porous. Furthermore, as shown in Figure 3(c), Membrane B (GO / TRGO membrane) in Example 3 exhibited a porous surface morphology, suggesting its usefulness as a membrane for a self-supporting electrode.

[0075] The density of each film was calculated from the SEM images in Figures 3(a) to 3(c). As a result, the density of film B in Example 1 was 0.95 g / cm 3 , and Film B in Example 5 is 0.05 g / cm 3 , and Membrane B in Example 3 is 0.76 g / cm 3 It was.

[0076] 4(a) to 4(d) are diagrams showing the X-ray diffraction patterns, Raman spectra, C1s spectra by XPS, and FTIR spectra of Membrane B of Example 1, Membrane B of Example 3, and Membrane B of Example 3, respectively. In each figure, "GO film," "GO / TRGO film," and "reduced-GO / TRGO film" refer to Membrane B of Example 1, Membrane B of Example 3, and Membrane B of Example 3, respectively.

[0077] The instruments and measurement conditions used for each analysis are as follows: ·X-ray diffraction device (XRD): Rigaku SmartLab, X-ray source: Cu-Kα ray (λ=1.5418Å), scanning range: 2θ=5~60° Raman spectrometer: Nanophoton Raman plus, excitation wavelength: λ = 532 nm X-ray photoelectron spectrometer (XPS): PHI Quantera SXM manufactured by ULVAC-PHI Corporation, X-ray source: Al Kα, analyzer: hemispherical analyzer (energy values ​​were corrected against the C1s peak of aliphatic carbon at 284.5 eV). Fourier transform infrared spectrophotometer (FTIR): JASCO FT / IR-6100

[0078] According to the X-ray diffraction pattern shown in Figure 4(a), in Film B (GO film) of Example 1, a sharp peak (diffraction peak from the (001) plane of graphene oxide) was observed at 2θ = 10.6° according to Bragg's law, indicating that the stacking spacing of the GO sheets was 0.84 nm (bottom of Figure 4(a)). In Film B (GO / TRGO film) in Example 3, the peak around 2θ = 10° is broadened with a peak at 2θ = 11°, indicating a decrease in the regularity of the stacking of GO flakes. On the other hand, the gentle peak around 2θ = 24.8° is identified as the diffraction peak from the (002) plane of thermally reduced graphene oxide, suggesting the presence of TRGO sheets (Fig. 4(a) middle). In contrast, in the film of Example 3 (thermally reduced-GO / TRGO film), the peak around 2θ = 10° disappears due to the reduction of GO, and a clear peak appears around 2θ = 24.3°. This suggests that graphite-like domains are formed due to the reduction process of GO during the thermal treatment, and that the release of gas during this process increases the disorder of the TRGO film (Figure 4(a) top).

[0079] According to the Raman spectra shown in Figure 4(b), all of Film B (GO film) in Example 1, Film B (GO / TRGO film) in Example 3, and Film B (thermally reduced-GO / TRGO film) in Example 3 exhibited a Raman peak at 1360 cm -1 Around 1580cm -1 Typical D and G bands are seen nearby, respectively.

[0080] Here, the presence or absence of crystal structural defects and sp 3 Information about the order (disorder) of carbon can be obtained. D / I G The lattice defects in the carbonaceous material being analyzed can be evaluated from the area ratio (Area Ratio) or intensity ratio (Central Peak Ratio) obtained by Gaussian fitting.

[0081] As shown in Table 2 below, in the film of Example 3, exfoliation of GO occurs during the GO reduction process accompanying heat treatment, and cleavage of CO and C=O occurs in the production of CO2 and HO, resulting in a structure with many lattice defects. Table 2 shows the peak index and Raman shift value (cm -1 ) is also shown.

[0082] [Table 2]

[0083] The evolution of oxygen-containing functional groups in the carbonaceous material during the above-mentioned process can be quantitatively evaluated from the measurement results by XPS (Figure 4(c)). Table 3 below summarizes the carbon content (C%), oxygen content (O%), and carbon to oxygen ratio (C / O) in each sample, calculated using the XPS C1s spectrum shown in Figure 4(c).

[0084] [Table 3]

[0085] According to Table 3, the C / O value of Film B (GO film) in Example 1 was small (C / O = 2.4), which is due to the completely oxidized graphite and its very high oxygen content (29.3%) (see the bottom panel of Figure 4(c)). In Film B (GO / TRGO film) of Example 3, the oxygen content was lower (22.0%) than that of Film B of Example 1 because some of the functional groups of TRGO had been removed (see the middle panel of Figure 4(c)). In addition, the film obtained through the heat treatment in Example 3 (thermally reduced-GO / TRGO film) showed a significant increase in C / O (C / O = 5.8) (see the upper panel of Figure 4(c)).

[0086] As such, in the three C1s spectra shown in Figure 4(c), it can be seen that the relative intensities of C=O and CO to the C=C domain clearly decrease in the order of bottom, middle, and top.

[0087] Furthermore, according to the FTIR spectrum shown in Figure 4(d), characteristic absorptions corresponding to the vibrations of -OH, C=O, C=C, and CO groups were detected in Film B (GO film) of Example 1, indicating that it contains many oxygen-derived functional groups (bottom panel of Figure 4(d)). In contrast, in Film B of Example 3 (GO / TRGO film) and the film of Example 3 (thermally reduced-GO / TRGO film), the characteristic absorption of the -OH group decreased with the reduction of GO. In particular, the characteristic absorption of the -OH group decreased significantly in the film of Example 3 (middle and top rows of Figure 4(d)). This indicates that the fabrication method of the present invention is effective in improving the structural stability of the graphene electrode (film). On the other hand, the FTIR spectrum shown in Figure 4(d) shows the characteristic absorption of the C=O group (carbonyl bond), suggesting the presence of functional groups that were not removed during the heat treatment. Oxygen-containing functional groups can facilitate the reversible adsorption and desorption of ions, especially in aqueous electrolytes, but may impair cycle performance.

[0088] Figures 5(a) to (e) are SEM images of the cross sections of Membrane B of Example 1, Membrane B of Example 2, Membrane B of Example 3, Membrane B of Example 4, and Membrane B of Example 5, respectively. The scale bars in each figure are 5 μm for Figures 5(a), (c), and (d), 2 μm for Figure 5(b), and 20 μm for Figure 5(e). The numbers in the upper right corner of each figure represent the density (g / cm 3 )

[0089] FIG. 5(a) shows that Membrane B (GO membrane) in Example 1 is composed of densely stacked sheets. In contrast, in the films B of Examples 3, 4, and 5, which contain a given percentage of TRGO, the TRGO becomes porous during the thermal reduction process, and as the TRGO content increases, the overall porosity of the film increases. In other words, as the TRGO content increases, the film density decreases, and the interconnected pores within the film can function as channels for releasing gases generated during the thermal treatment of GO. Furthermore, since the TRGO content in the film is relatively low, the amount of gas generated during the thermal treatment of GO also decreases.

[0090] The change in film density before and after the heat treatment of GO described above is shown in Fig. 5(f). That is, Fig. 5(f) is a graph comparing the densities of Film B of Examples 1 to 5 and the films of Examples 1 to 5. The horizontal axis in Figure 5(f) represents the mass ratio of GO to TRGO in the mixed dispersion (see Table 1). For each example, the left side represents the density before thermal treatment, and the right side represents the density after thermal treatment. The arrows are used to visually indicate the degree of change in density before and after thermal treatment.

[0091] According to Fig. 5(f), Membrane B (GO membrane) of Example 1 had the highest density (approximately 1.0 g / cm 3 ) and it can be seen that it is a dense (compact) structure. In addition, from Example 2 to Example 5, as the content of TRGO in the film increases, the density of the film decreases. In Film B (TRGO film) of Example 5, the density was 0.1 g / cm 3 (See Figure 5(e)). Here, in the film of Example 1 (thermally reduced-GO film), although the density value was significantly reduced by the heat treatment of GO, the film structure was destroyed and it could not function as a free-standing film. As mentioned above, since Membrane B (TRGO membrane) of Example 5 originally contained zero GO, it was not heat-treated, and in Figure 5(f) it is treated as having the same density as Membrane B of Example 5. However, although the membrane of Example 5, i.e., the TRGO membrane obtained by drying Membrane B of Example 5 in an oven, can retain its shape on the substrate (PTFE filter paper), it does not have enough strength to be used as a free-standing membrane.

[0092] Furthermore, according to FIG. 5(f), among the films prepared in this example in which the ratio (mass ratio) of GO to TRGO was changed from 3:1 to 1:3, the film of Example 3 had the highest density (approximately 0.5 g / cm 3 ), and the membrane of Example 2 has the lowest density (about 0.05 g / cm 3 )

[0093] 6(a) and 6(b) show the nitrogen adsorption / desorption isotherms and pore size distributions of Membrane B of Examples 1 to 5, respectively. In FIGS. 6(a) and 6(b), "GO film," "GO / TRGO=3:1 film," "GO / TRGO=1:1 film," "GO / TRGO=1:3 film," and "TRGO film" refer to Membrane B of Example 1, Membrane B of Example 2, Membrane B of Example 3, Membrane B of Example 4, and Membrane B of Example 5, respectively. The plots are square, circle, triangle, diamond, and star, respectively. Figures 6(c) and 6(d) show the nitrogen adsorption / desorption isotherms and pore size distributions of the films of Examples 1 to 5, respectively. In Figures 6(c) and 6(d), "Reduced-GO / TRGO(1:0) film," "Reduced-GO / TRGO(3:1) film," "Reduced-GO / TRGO(1:1) film," "Reduced-GO / TRGO(3:1) film," and "Reduced-GO / TRGO(0:1) film" refer to the films of Examples 1, 2, 3, 4, and 5, respectively. The plots are square, circle, triangle, diamond, and star, respectively.

[0094] Table 4 below lists the specific surface areas (m 2 / g) are shown together.

[0095] [Table 4]

[0096] According to Figures 6(a) and (b) and Table 4, in Membrane B (GO membrane) of Example 1, there are some pores with a pore diameter of less than 2 nm, and the specific surface area is 10 m 2 / g. This can be understood as a result of the increased density of the film being caused by the formation of hydrogen bonds between functional groups between layers and / or within the plane of each layer, as described above with reference to the SEM image in Figure 5(a). In addition, in the membranes B of Examples 2 to 5, as the content of TRGO increased, the specific surface area increased from 357 m 2 / g, which also indicates that the pores in the GO / TRGO membrane can function as channels for releasing gases evolved during the thermal treatment of GO and pressure applied to the membrane.

[0097] On the other hand, according to Figures 6(c) and (d) and Table 4, the specific surface areas of the membranes of Examples 1 to 5 increased compared to before the heat treatment (Membrane B), and were 260 to 360 m 2 / g range. The pore distributions in the membranes of Examples 1 to 5 showed roughly similar trends, with the average pore diameter falling within the range of 4 to 5 nm.

[0098] Considering the results of these structural analyses comprehensively, the film of Example 3, which was prepared using a mixed dispersion in which GO and TRGO were dispersed in a mass ratio of 1:1, has a more compact structure and is therefore expected to function as a graphene electrode with superior electrical properties.

[0099] Here, the membrane of Example 3 was used to carry out tests on flexibility and mechanical strength. Specifically, a roughly rectangular test piece measuring 4 cm × 0.5 cm was cut out from the film of Example 3, bent so that both ends in the longitudinal direction were in contact, and held in this state with tweezers for a certain period of time. The overall appearance of the test piece was then visually observed, and the state of the bent part was observed using an SEM. The results are shown in Figures 7(a) to 7(c).

[0100] Figure 7(a) is a photograph showing the specimen held in a curved state, Figure 7(b) is an SEM image of the area enclosed by the dotted line in Figure 7(a), and Figure 7(c) is an enlarged SEM image of the area enclosed by the dotted line in Figure 7(b). The scale bars in Figures 7(b) and (c) are 500 μm and 1 μm, respectively.

[0101] Figure 7(a) shows that the test specimen maintained its curved state with a smooth curve. Visual observation revealed no structural damage or destruction of the test specimen. According to the SEM image shown in FIG. 7(b), even the part of the test piece that was most heavily loaded due to its curvature (approximately the center of the test piece) had a smooth arch shape, with a curvature radius R of 3 mm. Furthermore, the SEM image shown in Figure 7(c) reveals that the microstructure of the specimen did not suffer any structural damage or destruction as a result of the above test.

[0102] From the above test results, it was found that the film of Example 3 had excellent flexibility and mechanical strength.

[0103] [Analysis of electrical characteristics] Next, to evaluate the electrical properties of the graphene electrodes, electric double-layer capacitors (CR2032 coin cells) were fabricated using the electrodes.

[0104] Specifically, the membranes of Example 3 (thermally reduced-GO / TRGO membrane) and Example 5 (TRGO membrane) were cut into circles with a diameter of 15 mm to form electrodes. Next, a separator was placed between these electrodes in a stainless steel cell, and the cells were filled with an ionic liquid (EMI-BF4) or aqueous electrolyte (sulfuric acid solution) to form coin cells. Glass fiber was used for the separator, and carbon-coated aluminum foil (Exopack) was used as the current collector. TM The coin cell was assembled in a glove box filled with Ar gas.

[0105] Electrochemical measurements of the coin cells were performed using a multi-channel potentiostat / galvanostat (VMP-300, manufactured by Bio-Logic). Specific capacitance-voltage measurements (CV measurements) and galvanostatic charge / discharge measurements were performed at room temperature in the potential range of 0 V to 3.7 V or 0 V to 1.0 V. Electrochemical impedance measurements were also performed.

[0106] The specific capacitance Cs (F / g) was calculated according to the formula Cs = 4I / (mdV / dt), where I (A) is the constant current, m (g) is the total mass of the two electrodes, and dV / dt (V / s) is the slope obtained by linear fitting of the discharge curve between Vmax (the voltage at the start of discharge) and 1 / 2Vmax. The energy density E cell (Wh / kg) in the formula E cell =CsV 2 Calculated according to / 8.

[0107] 8(a) to 8(d) are graphs showing the specific capacitance-voltage curve (CV curve), constant current charge / discharge curve (GCD curve), electrochemical impedance spectrum (EIS), and rate characteristics, respectively, when the electrolyte is an aqueous electrolyte solution (sulfuric acid aqueous solution). In Fig. 8(a) to 8(d), the solid line represents the membrane of Example 3, and the dashed line represents the membrane of Example 5. Here, the CV curve shown in FIG. 8(a) was measured in the potential range of 0 V to 1.0 V at a sweep rate of 100 mV / s. The GCD curve shown in Figure 8(b) was measured at a current density of 0.2 A / g. The EIS shown in Figure 8(c) was fitted using an equivalent circuit model, and the inside shows a spectrum with the horizontal axis value expanded in the range of 0 Ω to 1.5 Ω. The rate characteristics shown in FIG. 8(d) are the results obtained in the current density range of 0.2 A / g to 5 A / g.

[0108] As shown in Figure 8(a), the film of Example 3 exhibited a rectangular CV curve, which represents an ideal electric double layer capacitor. Furthermore, the film of Example 3 had a larger CV curve area and a larger capacitance than the film of Example 5. In fact, the specific capacitance at a current density of 0.2 A / g calculated from the GCD curve in Figure 8(b) was 185 F / g, which was significantly higher than the 165 F / g of the film of Example 5.

[0109] As shown in Figure 8(c), the membrane of Example 3 exhibited a low equivalent resistance of approximately 0.33 Ω. Furthermore, the charge transfer resistance was small, at 0.2 Ω, which may contribute to its high rate performance. Additionally, in charge-discharge measurements at higher current densities, the membrane of Example 3 exhibited a specific capacitance of 171 F / g at 0.5 A / g and maintained a specific capacitance of 111 F / g even at a higher current density of 20 A / g (data not shown). These results are believed to be due to the membrane of Example 3 having a more compact structure than the membrane of Example 5. Furthermore, the membrane of Example 3 exhibited a capacitance retention rate of over 99% even after 10,000 cycles, demonstrating its excellent longevity (data not shown).

[0110] 9(a) to 9(d) are graphs showing the specific capacitance-voltage curve (CV curve), constant current charge / discharge curve (GCD curve), electrochemical impedance spectrum (EIS), and rate characteristics, respectively, when the electrolyte is an ionic liquid (EMI-BF4). In Fig. 9(a) to 9(d), the solid line represents the membrane of Example 3, and the dashed line represents the membrane of Example 5. Here, the CV curve shown in FIG. 9(a) was measured in the potential range of 0 V to 3.7 V at a sweep rate of 100 mV / s. The GCD curve shown in Figure 9(b) was measured at a current density of 0.2 A / g. The EIS shown in Figure 9(c) was fitted using an equivalent circuit model. The rate characteristics shown in FIG. 9(d) are the results obtained in the current density range of 0.2 A / g to 5 A / g.

[0111] As shown in Figure 9(a), the membrane of Example 3 exhibited a rectangular CV curve, which represents an ideal electric double layer capacitor. Furthermore, since the electrolyte was a non-aqueous electrolyte and the differential voltage was extended to 3.7 V, the energy density reached 92 Wh / kg at a current density of 0.1 A / g. This is more than 10 times the value (6.4 Wh / kg) obtained when the aqueous electrolyte was used. Furthermore, it can be said that the performance is comparable to or even superior to that of electrodes using conventional graphene-containing composite materials, such as those described in Non-Patent Document 1.

[0112] According to FIG. 9(c), the membrane of Example 3 exhibited an equivalent resistance of 3.2 Ω. This is a higher value than the value (0.33 Ω) described with reference to FIG. 8(c), but this is because ionic liquids such as EMI-BF4 have lower conductivity than aqueous electrolytes. The equivalent resistance value of the membrane of Example 5 was 5.1 Ω, which is higher than that of the membrane of Example 3. In addition, the membrane of Example 3 exhibited a capacitance retention rate of 80% or more even after 10,000 cycles, demonstrating that it is suitable for practical use (data not shown).

[0113] In practice, in portable devices, the volumetric capacitance is sometimes more important than the gravimetric capacitance. Therefore, using the films of Examples 2 to 5, coin cells similar to those described above were fabricated and electrochemical measurements were carried out. As a result, the film of Example 3 had the highest volumetric capacitance (101 F / cm 3 ) and has an energy density of 45Wh / cm 3 The membrane of Example 3 also had excellent rate characteristics (data not shown). g / cm 3This is thought to be because the film of Example 3 had a density of 0.01, which was the largest among the films of Examples 2 to 5. Furthermore, as described with reference to FIG. 7, the film of Example 3 also had excellent flexibility and mechanical strength, suggesting that the film of Example 3 was a superior graphene electrode among the films produced in this example. Furthermore, it was suggested that by adjusting the ratio of graphene oxide and thermally reduced graphene oxide in the mixed dispersion in the graphene electrode production process, the electrical and mechanical properties of the target graphene electrode could be optimized, and that a flexible and self-supporting graphene electrode more suitable for the desired application could be obtained. [Industrial Applicability]

[0114] The graphene electrode of the present invention is a so-called binder-free electrode consisting essentially of graphene, and is flexible and self-supporting, making it suitable for use as an electrode for electricity storage devices such as electric double layer capacitors and lithium ion batteries. When graphene oxide is used as a starting material, the graphene electrode of the present invention can be produced by performing two thermal reduction treatments, facilitating scale-up for mass production. The graphene electrode of the present invention can effectively exhibit the electrical properties inherent to graphene and is advantageous as an electrode for electricity storage devices (particularly for electric double layer capacitors), and is expected to be used as an electrode that meets the demand for flexible electricity storage systems. [Explanation of symbols]

[0115] 200 Electric double layer capacitor 210 Positive electrode 220 Negative electrode 230 Electrolytes 240 Separator 250 cells

Claims

1. It does not contain any conductive material or binder, is composed essentially of graphene, and has a density of 0.2 g / cm ３ ~0.7g / cm ３ The graphene electrode has flexibility and self-supporting properties, and a specific surface area measured by the BET method is in the range of 240 m 2 / g or more and 400 m 2 / g or less.

2. A graphene electrode as described in claim 1, wherein the average pore diameter of the pores is in the range of 4 nm to 5 nm.

3. The graphene electrode according to claim 1 or 2, wherein the graphene is thermally reduced graphene oxide.

4. 4. The graphene electrode according to claim 1, wherein, in a flexibility and mechanical strength test using a film-like test piece having a thickness in the range of 50 μm to 80 μm, when the test piece is bent so that opposing ends are in contact and this state is maintained for a certain period of time, no structural damage or destruction of the test piece is observed in visual observation of the entire state of the test piece and in SEM observation of an approximate center of the test piece, and the approximate center has a smooth arch shape with a predetermined radius of curvature.

5. A method for producing a graphene oxide dispersion and a thermally reduced graphene oxide dispersion, comprising: mixing the graphene oxide dispersion and the thermally reduced graphene oxide dispersion to prepare a mixed dispersion in which graphene oxide and thermally reduced graphene oxide are dispersed; vacuum filtering the mixed dispersion onto a substrate to produce an intermediate structure comprising graphene oxide and thermally reduced graphene oxide; drying the intermediate structure and peeling it from the substrate to obtain a self-supporting structure; heat-treating the self-supporting structure to reduce graphene oxide in the self-supporting structure; A method for producing a graphene electrode, comprising:

6. The steps of preparing the graphene oxide dispersion and the thermally reduced graphene oxide dispersion include: freeze-drying a graphene oxide aqueous dispersion in which a predetermined amount of graphene oxide is dispersed in water; heating the resulting graphene oxide solid in a muffle furnace at 300°C to 700°C for less than 1 minute; The method of claim 5 further comprising:

7. 7. The method according to claim 5, wherein the step of heat-treating the self-supporting structure comprises heating the self-supporting structure in a muffle furnace at 300°C to 700°C for one minute or less to reduce graphene oxide in the self-supporting structure.

8. 8. The method according to claim 5, wherein, in the step of preparing the mixed dispersion, the graphene oxide dispersion and the thermally reduced graphene oxide dispersion are mixed such that a mass ratio of graphene oxide to thermally reduced graphene oxide in the mixed dispersion satisfies a range of 3:1 to 1:

3.

9. An electricity storage device including an electrode and an electrolyte, The electrode is an electricity storage device comprising the graphene electrode according to any one of claims 1 to 4.

10. The power storage device according to claim 9 , wherein the power storage device is an electric double layer capacitor.

Images