JP7765003B2 - Graphene electrode, its manufacturing method and power storage device using the same - Google Patents

Description

The present invention relates to graphene electrodes, their manufacturing methods, and electricity storage devices using them.

Electric double-layer capacitors (supercapacitors) and lithium-ion batteries, which offer large capacities, are attracting attention. Furthermore, with the rapid development of portable electronic devices in recent years, there is a growing demand for flexible energy storage systems.

Graphene is known to be used as an electrode material for electric double layer capacitors. Previously, attempts have been made to combine graphene with carbon nanotubes, polymers, etc. to create flexible electrodes.

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. Furthermore, Non-Patent Document 2 describes how free-standing graphene paper (GP) synthesized by inkjet printing using a graphene oxide (GO) dispersion and a graphene hydrogel (GH) and polyaniline (PANI) dispersion has a structure supported by a porous GH-PANI nanocomposite and can be used as an electrode for an electric double layer capacitor.

However, it has been reported that materials combining graphene with substances (nanostructures) or metal oxides, such as those described in Non-Patent Documents 1 and 2, often have performance problems in electric double layer capacitors using these materials as electrodes, such as shortened cycle life, reduced frequency characteristics, and deterioration in rate characteristics (see, for example, Non-Patent Document 3). Furthermore, the process for producing composite materials is generally complex, and the raw materials can be expensive, making them unsuitable for mass production.

In addition, when actually constructing an electric double layer capacitor, conventional electrode materials are mixed with conductive materials and binders and processed into a film. When mixed with such conductive materials and binders, these materials 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.

X. Li et al., Carbon. 129 (2018) 236-244. K. Chi et al., ACS Appl. Mater. Interfaces. 6 (2014) 16312-16319. X. Xiao et al., Energy Storage Mater. 1 (2015) 1-8.

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 an electricity storage device using the graphene electrode.

The graphene electrode according to the present invention does not contain any conductive material or binder, consists essentially of graphene, has a density in the range of 0.2 g /cm ³ to 0.7 g /cm ³ , and is flexible and self-supporting, thereby solving the above-mentioned problems.
The graphene electrode may have a specific surface area measured by a BET method in the range of 200 m ² /g or more and 1000 ^{m 2} /g or less.
The graphene may be thermally reduced graphene oxide.
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, the graphene electrode is bent so that opposing ends of the test piece are in contact with each other, and when 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 overall appearance of the test piece and in SEM observation of an approximate center of the test piece, and the approximate center may have a smooth arch shape with a predetermined radius of curvature.

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 1 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 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.

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.

The graphene electrode of the present invention is an electrode that does not contain a conductive material or binder, and is not a composite combined with substances other than graphene; it is an electrode that consists essentially of graphene alone, has a predetermined density, and is flexible and self-supporting. Therefore, the graphene electrode of the present invention can effectively utilize the electrical properties inherent to graphene, and has excellent properties as an electrode for power storage devices such as electric double layer capacitors and lithium ion batteries. By using the graphene electrode of the present invention, it is possible to provide power storage devices such as electric double layer capacitors and lithium ion batteries that have excellent performance in terms of capacitance, energy density, etc.

The graphene electrode manufacturing method of the present invention uses a graphene oxide dispersion and a thermally reduced graphene oxide dispersion, and can produce a film-shaped molded body from these mixed dispersions using a simple method. When manufacturing the electrode, no composite with substances other than graphene is performed, and no conductive materials or binders are 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.

Flowchart showing the manufacturing process of the graphene electrode of the present invention Schematic diagram showing an electric double layer capacitor of the present invention. (a) to (c) SEM images of Membrane B of Example 1, Membrane B of Example 5, and Membrane B of Example 3. FIG. 1 shows (a) X-ray diffraction patterns, (b) Raman spectra, (c) C1s spectra by XPS, and (d) FTIR spectra of Film B of Example 1, Film B of Example 3, and Film B of Example 3. (a) to (e) are cross-sectional SEM images 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) is a graph comparing the density of the film B of Examples 1 to 5 with the density of the films of Examples 1 to 5. (a) A diagram showing nitrogen adsorption/desorption isotherms of membranes B of Examples 1 to 5, (b) A diagram showing pore size distributions of membranes B of Examples 1 to 5, (c) A diagram showing nitrogen adsorption/desorption isotherms of membranes of Examples 1 to 5, and (d) A diagram showing pore size distributions of membranes of Examples 1 to 5. (a) to (c) are diagrams showing the test results of the flexibility and mechanical strength of the membrane of Example 3. 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). 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 Examples 3 and 5 are used and the electrolyte is ionic liquid (EMI-BF 4 ).

Embodiments of the present invention will be described below with reference to the drawings. Note that like elements are numbered likewise and their description will be omitted.

(Embodiment 1)
In the first embodiment, a graphene electrode and a method for manufacturing the same according to the present invention will be described.

The graphene electrode of the present invention does not contain any conductive material or binder and consists essentially of graphene alone.

In this specification, the phrase "consisting essentially of graphene" means that the electrode does not contain any other materials than graphene as constituents 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 that are inevitably mixed or remain during the manufacturing process is acceptable. However, since such mixed materials or residues may impair the energy characteristics of the 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 situations where a manufacturing method for an electrode is known, an electrode is considered to be "consisting essentially of graphene" if, during that process, graphene (or graphene oxide) is not mixed or composited with other materials (except for the dispersion medium) and is not mixed with conductive materials, binders, or other additives. The manufacturing method for the graphene electrode of the present invention will be described later.

The graphene electrode of the present invention has a density in the range of 0.2 g/cm3 or ^more and 0.7 g/cm3 or ^less . As a result, the graphene electrode of the present invention has the desired flexibility and self-supporting properties, and is 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 in the range of 0.4 g/cm3 or ^more and 0.6 g/cm3 or ^less .

The graphene electrode of the present invention is substantially composed of graphene and has a density within the above-mentioned range, and thus preferably has a specific surface area measured by the BET method in the range of 200 m ² /g or more and 1000 ^{m 2} /g or less. This allows the graphene electrode of the present invention to have high conductivity and adsorb 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 in the range of 240 m ² /g or more and 400 m ² /g or less. This further ensures the adsorption and migration of electrolyte ions. Even more preferably, the graphene electrode of the present invention has a specific surface area measured by the BET method in the range of 240 m ² /g or more and 350 m ² /g or less.

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 current collectors, and achieves high energy density and power density when applied to various electricity storage devices.

The terms "flexibility" and "self-supporting" are commonly used in the technical field, but the flexibility and self-supporting properties of the graphene electrodes of the present invention can be confirmed, for example, by the flexibility and mechanical strength tests described below.

[Test method for flexibility and mechanical strength]
The test specimen used is a film-like structure having a thickness in the range of 50 μm to 80 μm.
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 specimen, which was kept in the curved state, was observed using a scanning electron microscope (SEM). Here, the approximate center of the test specimen refers to the portion of the test specimen that is most heavily loaded due to the curve of the test specimen. The graphene electrode of the present invention was observed using the SEM, and no structural damage or destruction was observed in the test specimen. It was confirmed that the approximate center of the test specimen had a smooth arch shape with a predetermined radius of curvature.

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.

Step S110: Prepare a graphene oxide dispersion and a thermally reduced graphene oxide dispersion.

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.

Graphene oxide may be produced by known production methods. For example, graphene oxide prepared from natural graphite using a modified Hummers method can be used.

Thermally reduced graphene oxide is produced by heat-treating graphene oxide. There are no particular restrictions on the conditions for heat-treating graphene oxide, but a shorter heat treatment time is preferable from the perspectives 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 two 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 less than one minute and quickly removed, yielding the desired thermally reduced graphene oxide.

The dispersion medium used to prepare the graphene oxide dispersion is not particularly limited, and examples include water and ethanol. From the perspective of preparing a mixed dispersion with the thermally reduced graphene oxide dispersion in step S120, which will be described later, 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 it. 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.

The concentration of the graphene oxide dispersion is not particularly limited, and can be selected appropriately so that 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.

The dispersion medium used to prepare thermally reduced graphene oxide is not particularly limited, and examples include water, ethanol, etc. From the perspective of preparing a mixed dispersion with the graphene oxide dispersion in step S120, which will be described later, it is preferable that the dispersion medium be the same as the dispersion medium used in the graphene oxide dispersion or be a dispersion medium that is highly miscible with it.

There are no particular restrictions on the concentration of the thermally reduced graphene oxide dispersion liquid, and it is sufficient to select an appropriate concentration that allows the thermally reduced graphene oxide to be well dispersed in the dispersion medium. Specifically, for example, the concentration of the thermally reduced graphene oxide dispersion liquid may be in the range of 0.1 mg/mL to 1.0 mg/mL.

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

The mixing ratio of the graphene oxide dispersion liquid and the thermally reduced graphene oxide dispersion liquid is not particularly limited, and can be adjusted so that the ratio (mass ratio) of graphene oxide to thermally reduced graphene oxide in the mixed dispersion liquid is the desired value, taking into account the concentrations of the graphene oxide dispersion liquid prepared in step S110 and the thermally reduced graphene oxide dispersion liquid.

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

The method for mixing the graphene oxide dispersion and the thermally reduced graphene oxide dispersion is not particularly limited, but from the perspective of achieving a uniform dispersion state of the 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.

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

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

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

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

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

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

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

The conditions for the heat treatment used to reduce 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 perspectives of production efficiency and reducing environmental impact. Specifically, for example, the self-supporting structure can be heated in a muffle furnace at 300°C to 700°C for less than one 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 desired thermally reduced graphene oxide and thermally reduced graphene oxide. The thermally reduced-GO/TRGO film obtained in this manner is a structure composed entirely of thermally reduced graphene oxide, i.e., graphene, and is the graphene electrode of the present invention.

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-treating the graphene oxide to reduce or delaminate it. However, rapid heat-treating reduction is difficult to directly fabricate a self-supporting graphene film. This is because rapid heating of a graphene oxide film can destroy the film's structure. It has been reported that the decomposition of many oxygen-containing functional groups generates large amounts of carbon dioxide gas, which generates significant pressure inside the film. However, because graphene oxide films form a very compact structure between their layers due to hydrogen bonds resulting from the oxygen-containing functional groups, the pressure generated within 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.

In contrast, in the graphene electrode manufacturing method of the present invention described above, when graphene oxide is used as the 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 below, when graphene oxide is used as the starting material, the total time for two thermal reduction treatments is within two minutes.

Furthermore, the graphene electrode of the present invention obtained in this manner has the flexibility and self-supporting properties required for electrodes in electricity storage devices such as electric double layer capacitors and lithium ion batteries, and as will be explained in the examples below, it exhibits excellent electrical properties in both aqueous and non-aqueous electrolytes (ionic liquids).

In the graphene electrode manufacturing method of the present invention, the intermediate structure produced using a mixed dispersion of a graphene oxide dispersion and a thermally reduced graphene oxide dispersion has a structure in which porous thermally reduced graphene oxide is introduced between the graphene oxide layers, ensuring that the graphene oxide is reduced by a short heat treatment. Furthermore, the introduction of porous thermally reduced graphene oxide ensures that channels (pathways) are provided for the release of gas and pressure generated during the reduction of graphene oxide to the outside of the film, thereby preventing damage to the structure. From this perspective, the ratio of graphene oxide to thermally reduced graphene oxide in the mixed dispersion is preferably in the range of 3:1 to 1:3 by mass, 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.

Furthermore, in the graphene electrode manufacturing method 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 ratios (volume ratios) of the graphene oxide dispersion and the thermally reduced graphene oxide dispersion, thereby making it possible to easily manufacture flexible, self-supporting graphene electrodes that are more suitable (optimized) for the desired application of electricity storage devices.

(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.

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 embodiment 1. 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-BF ₄ ), and 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide (MPPp-TFSI), or M'OH (M' is an alkali metal).

The electric double layer capacitor 200 further has a separator 240 between the positive electrode 210 and the negative electrode 220, isolating the positive electrode 210 and the negative electrode 220.

The material of the separator 240 is selected from, for example, 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 their derivatives, cellulose, paper, and nonwoven fabric.

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 negative electrode 220 each have an existing current collector.

Such an electric double layer capacitor 200 may be a chip type, coin type, molded type, pouch type, laminate type, cylindrical type, square type, etc. capacitor, and may also be used in a module in which multiple of these are connected together.

Next, the operation of the electric double layer capacitor 200 in Figure 2 will be explained.

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. The positive electrode 210 and the negative electrode 220 are formed from the graphene electrode described in embodiment 1 and are electrodes consisting essentially of graphene. This facilitates the adsorption and diffusion of cations and anions by the graphene, achieving high rate characteristics. Furthermore, the positive electrode 210 and the negative electrode 220 are formed from the graphene electrode described in embodiment 1 and are electrodes consisting essentially of graphene. Therefore, many electrolyte ions are adsorbed not only on the surface of the graphene but also inside it, forming an electric double layer. As a result, the exchange of electrons between the graphene and the electrolyte ions is increased, achieving high energy density.

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 negative electrode 220, respectively, are desorbed and discharged. Again, the positive electrode 210 and negative electrode 220 are formed from the graphene electrodes described in embodiment 1, which facilitates desorption and diffusion of electrolyte ions, achieving high rate characteristics and energy density. Furthermore, because of their excellent conductivity, the ease of desorption and diffusion can also lead to improved power density.

In this way, 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, because it utilizes the formation of an electric double layer for charging and discharging, it is highly suitable for repeated use. The electric double layer capacitor 200 of the present invention can be used in wind power generation, electric vehicles, etc.

Note that while the explanation here has been limited to electric double layer capacitors, it goes without saying that the graphene electrodes of the present invention can also be applied to electricity storage devices such as lithium ion batteries in addition to electric double layer capacitors.

The present invention will now be described in detail using specific examples, but please note that the present invention is not limited to these examples.

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

Graphene oxide dispersion (0.5 mg/mL) was prepared by replacing the dispersion medium from a graphene oxide aqueous dispersion prepared by dispersing graphene oxide in water with ethanol.

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 for 1 minute or less in a muffle furnace and quickly removed. In this way, thermally reduced graphene oxide (TRGO) was obtained as a cotton-like black solid.
The obtained thermally reduced graphene oxide was dispersed in ethanol to prepare a thermally reduced graphene oxide dispersion (0.5 mg/mL) (this corresponds to step S110 in FIG. 1).

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).

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

The GO/TRGO membrane formed on the PTFE filter paper was then dried in an oven and peeled off from the filter paper to obtain a self-supporting membrane (step S140 in Figure 1).

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

For comparison, a graphene oxide dispersion (0.5 mg/mL) alone was vacuum filtered through PTFE filter paper to prepare a membrane (GO membrane) as an intermediate structure consisting only of graphene oxide. This GO membrane was dried in an oven and peeled off from the filter paper to obtain a free-standing membrane as a self-supporting structure. The above-mentioned heat treatment was then performed to obtain a membrane consisting only of thermally reduced graphene oxide (thermally reduced-GO membrane).
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, and the membrane formed on the PTFE filter paper was dried in an oven to obtain a membrane consisting only of thermally reduced graphene oxide (TRGO membrane).

Table 1 below summarizes the ratio (mass ratio) of graphene oxide to thermally reduced graphene oxide in the above mixed dispersion.

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

[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.

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

The density of each film was calculated from the SEM images of FIGS. 3(a) to 3(c), and was found to be 0.95 g/cm ³ for Film B in Example 1, 0.05 g/cm ³ for Film B in Example 5, and 0.76 g/cm ³ for Film B in Example 3.

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

The instruments and measurement conditions used for each analysis are as follows:
・X-ray diffraction device (XRD): SmartLab manufactured by Rigaku, X-ray source: Cu-Kα ray (λ = 1.5418 Å), scanning range: 2θ = 5 to 60°
Raman spectrometer: Nanophoton Raman Plus, excitation wavelength: λ = 532 nm
X-ray photoelectron spectrometer (XPS): PHI Quantera SXM manufactured by ULVAC-PHI, Inc. 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

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 panel of Figure 4(a)).
In the case of Film B (GO/TRGO film) in Example 3, the peak near 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 near 2θ = 24.8° is identified as a 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 near 2θ = 10° disappeared due to the reduction of GO, and a clear peak appeared near 2θ = 24.3°. This suggests that graphite-like domains were formed due to the reduction process of GO during the heat treatment, and that the release of gas during this process increased the disorder of the TRGO film (Figure 4(a) top).

According to the Raman spectra shown in FIG. 4(b), the film B of Example 1 (GO film), the film B of Example 3 (GO/TRGO film), and the film of Example 3 (thermally reduced-GO/TRGO film) all exhibit typical D and G bands at around 1360 cm ⁻¹ and 1580 cm ⁻¹ , respectively.

Here, from the D band in the Raman spectrum of the carbonaceous material, information regarding the presence or absence of crystal structural defects and the order (disorder) of ^sp3 carbon can be obtained, and from the area ratio (Area Ratio) or intensity ratio (Central Peak Ratio) of I _D /I _G by Gaussian fitting, the lattice defects in the carbonaceous material to be analyzed can be evaluated.

As shown in Table 2 below, in the film of Example 3, during the GO reduction process accompanying the heat treatment, exfoliation of GO occurs, and cleavage of C—O and C═O occurs in the production of CO ₂ and H ₂ O, resulting in a structure with more lattice defects.
Table 2 also shows the peak index and Raman shift value (cm ⁻¹ ).

The evolution of the oxygen-containing functional groups in the carbonaceous material during the above-described process can be quantitatively evaluated from the measurement results by XPS (FIG. 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 FIG. 4(c).

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 the film B (GO/TRGO film) of Example 3, the TRGO had some of its functional groups removed, so the oxygen content was lower (22.0%) than in the film B of Example 1 (see the middle panel of Figure 4(c)).
In addition, in the film of Example 3 (thermally reduced-GO/TRGO film) obtained through heat treatment, the C/O value significantly increased (C/O = 5.8) (see the upper part of Figure 4(c)).

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

Furthermore, according to the FTIR spectrum shown in FIG. 4(d), characteristic absorptions corresponding to the vibrations of —OH, C═O, C═C, and C—O groups were detected in Film B (GO film) of Example 1, indicating that it contains many oxygen-derived functional groups (lower panel of FIG. 4(d)).
In contrast, in the 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 producing a graphene electrode (film) by the production method of the present invention is effective in improving the structural stability of the graphene electrode.
On the other hand, the FTIR spectrum shown in Figure 4(d) shows the characteristic absorption of C=O groups (carbonyl bonds), 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.

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 are density (g/cm ³ ).

FIG. 5(a) shows that Membrane B (GO membrane) of Example 1 is composed of densely stacked sheets.
In contrast, in the membranes B of Examples 3, 4, and 5, which contain a predetermined proportion of TRGO, TRGO becomes porous during the thermal reduction process, and as the TRGO content increases, the porosity of the entire membrane increases. In other words, as the TRGO content increases, the density of the membrane decreases, and the pores present in the membrane can be connected to each other and function as channels (pathways) for releasing gas generated during the heat treatment of GO to the outside of the membrane. In addition, if the TRGO content in the membrane is high, the GO content is relatively low, and therefore the amount of gas generated during the heat treatment of GO is also reduced.

The change in film density before and after the heat treatment of GO is shown in Fig. 5(f), which is a graph comparing the density of Film B of Examples 1 to 5 with that of 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.

FIG. 5(f) shows that Film B (GO film) of Example 1 has the highest density (about 1.0 g/cm ³ ) and is a dense (compact) structure.
Furthermore, as the TRGO content in the film increases from Example 2 to Example 5, the density of the film decreases, and in Film B (TRGO film) of Example 5, the density is less than 0.1 g/ ^cm3 (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 described above, since Membrane B (TRGO membrane) of Example 5 originally contained zero GO, it was not heat-treated, and in Fig. 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 as a membrane on the substrate (PTFE filter paper), it does not have enough strength to be used as a free-standing membrane.

Furthermore, according to FIG. 5(f), among the films produced 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 film of Example 2 had the lowest density (approximately 0.05 g/cm ³ ).

6(a) and (b) are diagrams showing the nitrogen adsorption/desorption isotherms and pore distributions of Membrane B of Examples 1 to 5, respectively. In Figures 6(a) and (b), "GO film,""GO/TRGO=3:1film,""GO/TRGO=1:1film,""GO/TRGO=1:3film," 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.
6(c) and (d) are graphs showing the nitrogen adsorption/desorption isotherms and pore distributions of the films of Examples 1 to 5, respectively. In FIGS. 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.

Table 4 below shows the specific surface areas (m ² /g) of Membrane B of Examples 1 to 5 and the membranes of Examples 1 to 5.

According to FIGS. 6(a) and (b) and Table 4, in Membrane B (GO membrane) of Example 1, there were some pores with a pore diameter of less than 2 nm, and the specific surface area was less than 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 FIG. 5( a).
Furthermore, in Membrane B of Examples 2 to 5, the specific surface area increased to 357 m ² /g as the TRGO content increased, which also indicates that the pores in the GO/TRGO membrane can function as channels (pathways) to release gases generated during the heat treatment of GO and pressure applied to the membrane.

On the other hand, according to FIGS. 6(c) and 6(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 in the range of 260 to 360 m ² /g.
The pore distributions of 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.

Considering the results of these structural analyses comprehensively, the film of Example 3, which was produced 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.

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 x 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 portion was observed using an SEM.
The results are shown in Figures 7(a) to (c).

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

7(a) shows that the test specimen maintained its curved state with a smooth curve, and visual observation revealed no structural damage or destruction of the test specimen due to the above test.
According to the SEM image shown in FIG. 7(b), even the portion of the test piece subjected to the greatest load due to the curvature (approximately the center of the test piece) had a smooth arch shape, and the radius of curvature R was 3 mm.
Furthermore, the SEM image shown in FIG. 7(c) reveals that the test specimen did not suffer any structural damage or destruction in its microstructure.

The above test results demonstrate that the membrane of Example 3 has excellent flexibility and mechanical strength.

[Analysis of Electrical Characteristics]
Next, in order to evaluate the electrical properties of the fabricated graphene electrodes, electric double layer capacitors (CR2032 type coin cells) were fabricated using these as electrodes.

Specifically, the membrane of Example 3 (thermally reduced-GO/TRGO membrane) and the membrane of Example 5 (TRGO membrane) were cut into circles with a diameter of 15 mm, which were used as electrodes. Next, a separator was placed between these electrodes in a stainless steel cell, and an ionic liquid (EMI-BF ₄ ) or an aqueous electrolyte (aqueous sulfuric acid solution) was filled as the electrolyte to prepare a coin cell. Here, glass fiber was used for the separator, and aluminum foil coated with carbon (Exopack ^™ 0.5 mil double-sided coating) was used as the current collector (conductive material). The coin cell was assembled in a glove box filled with Ar gas.

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 galvanostat 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.

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 _Ecell (Wh/kg) was calculated according to the formula _Ecell = ^CsV2 /8.

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 when the electrolyte is an aqueous electrolyte solution (sulfuric acid aqueous solution). In 8(a) to 8(d), the solid line represents the membrane of Example 3, and the dashed line represents the membrane of Example 5.
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 FIG. 8(b) was measured at a current density of 0.2 A/g.
The EIS shown in FIG. 8(c) was fitted using an equivalent circuit model, and the spectrum shown inside is an expanded range of values on the horizontal axis from 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.

Figure 8(a) shows that the membrane of Example 3 exhibits a rectangular CV curve, representing an ideal electric double layer capacitor. Furthermore, the membrane of Example 3 has a larger CV curve area and a larger capacitance than the membrane 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, significantly higher than the 165 F/g for the membrane of Example 5.

Figure 8(c) shows that 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).

9(a) to (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 when the electrolyte is an ionic liquid (EMI-BF 4 ). In FIG. 9(a) to (d), the solid line represents the membrane of Example 3, and the dashed line represents the membrane of Example 5.
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 FIG. 9(b) was measured at a current density of 0.2 A/g.
The EIS shown in FIG. 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.

Figure 9(a) shows that the membrane of Example 3 exhibits a rectangular CV curve representing an ideal electric double layer capacitor. Furthermore, since the electrolyte is a non-aqueous electrolyte and the differential voltage is extended to 3.7 V, the energy density reaches 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 described above is used, and 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.

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 membrane of Example 5 exhibited an equivalent resistance value of 5.1 Ω, which was 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, indicating that it is suitable for practical use (data not shown).

In practical applications, volumetric capacitance may be more important than gravimetric capacitance in portable devices. Therefore, coin cells similar to those described above were fabricated using the films of Examples 2 to 5, and electrochemical measurements were performed. As a result, the film of Example 3 exhibited the highest volumetric capacitance (101 F/cm ³ ), an energy density of 45 Wh/cm ³ , and excellent rate characteristics (data not shown). This is thought to be due to the fact that the film of Example 3 had a density of 0.5 g /cm ³ , the highest among the films of Examples 2 to 5. Furthermore, as described with reference to FIG. 7 , the film of Example 3 also exhibited excellent flexibility and mechanical strength, suggesting that the film of Example 3 was a superior graphene electrode among the films fabricated in this example. Furthermore, by adjusting the ratio of graphene oxide and thermally reduced graphene oxide in the mixed dispersion during the graphene electrode manufacturing process, the electrical and mechanical properties of the target graphene electrode could be optimized, suggesting that a flexible and self-supporting graphene electrode more suitable for the desired application could be obtained.

The graphene electrode of the present invention is a so-called binder-free electrode consisting essentially of graphene alone. It is flexible and self-supporting, making it suitable for use as an electrode for power storage devices such as electric double-layer capacitors and lithium-ion batteries. When graphene oxide is used as the 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 effectively utilizes the electrical properties inherent to graphene and is advantageous as an electrode for power storage devices (particularly for electric double-layer capacitors), and is expected to be used as an electrode that meets the demand for flexible power storage systems.

200 Electric double layer capacitor 210 Positive electrode 220 Negative electrode 230 Electrolyte 240 Separator 250 Cell

Claims (10)

A graphene electrode that does not contain a conductive material or a binder and is essentially made of graphene alone, has a density in the range of 0.2 g /cm ³ to 0.7 g /cm ³ , is flexible and self-supporting, and has a specific surface area measured by a BET method in the range of 240 m ² /g or more and 400 m ² /g or less. The graphene electrode according to claim 1, wherein the average pore diameter of the pores is in the range of 4 nm to 5 nm . The graphene electrode described in claim 1 or 2, wherein the graphene is thermally reduced graphene oxide. The graphene electrode according to any one of claims 1 to 3, wherein, in a flexibility and mechanical strength test using a membrane-shaped test piece having a thickness in the range of 50 μm to 80 μm, the test piece is bent so that the opposing ends are in contact and maintained in this state for a certain period of time. Visual observation of the entire test piece and SEM observation of the approximate center of the test piece reveal no structural damage or destruction of the test piece, and the approximate center has a smooth arch shape with a predetermined radius of curvature. 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 comprising 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 graphene oxide in the self-supporting structure. 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;
6. The method of claim 5, further comprising heating the obtained graphene oxide solid in a muffle furnace at 300°C to 700°C for 1 minute or less. The method of claim 5 or 6, 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 less than 1 minute to reduce the graphene oxide in the self-supporting structure. The method according to any one of claims 5 to 7, wherein, in the step of preparing the mixed dispersion, the graphene oxide dispersion and the thermally reduced graphene oxide dispersion are mixed so that the ratio of graphene oxide to thermally reduced graphene oxide in the mixed dispersion satisfies a mass ratio range of 3:1 to 1:3. 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. The power storage device according to claim 9, wherein the power storage device is an electric double layer capacitor.