JP2023016064A - Graphene electrode, manufacturing method for the same, and power storage device using the same - Google Patents

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 The present invention relates to a graphene electrode, a method for manufacturing the same, and an electric storage device using the same.

Electricity storage devices such as electric double layer capacitors (supercapacitors) and lithium ion batteries have large capacities and are attracting attention. In recent years, with the rapid development of portable electronic devices, there is an increasing demand for a flexible power storage system.

It is known to use graphene as an electrode material for electric double layer capacitors. Attempts have been made to fabricate flexible electrodes by combining graphene with carbon nanotubes, polymers, or the like.

For example, Non-Patent Document 1 describes a flexible self-supporting film (AC/CNT/rGO film) that is a composite of activated carbon (AC), carbon nanotubes (CNT), and reduced graphene oxide (rGO). is proposed to be used as an electrode of an electric double layer capacitor. In addition, in Non-Patent Document 2, a self-supporting graphene paper (GP) synthesized by an inkjet printing method using a dispersion of graphene oxide (GO) and a dispersion of graphene hydrogel (GH) and polyaniline (PANI) has a structure supported by a porous GH-PANI nanocomposite and can be used as an electrode for an electric double layer capacitor.

However, regarding materials such as materials (nanostructures) and metal oxides combined with graphene as described in Non-Patent Document 1 and Non-Patent Document 2, the performance of electric double layer capacitors using this as an electrode is poor. It has been reported that problems such as shortening of cycle life, degradation of frequency characteristics, and deterioration of rate characteristics may often occur (see, for example, Non-Patent Document 3). In addition, there is also the problem that the production of composite materials is generally unsuitable for mass production because the process is generally complicated and the raw materials are often expensive.

In addition, conventional electrode materials can be mixed with a conductive material or a binder and processed into a film when actually constructing an electric double layer capacitor. When mixed with such a conductive material or binder, they may adsorb to the graphene surface, affect the infiltration and diffusion of electrolyte ions, and reduce 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 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 in an efficient and environmentally friendly manner, and an electricity storage device using the graphene electrode.

The graphene electrode according to the present invention does not contain a conductive material and a binder, consists essentially of graphene, has a density in the range of 0.2 mg/cm ³ to 0.7 mg/cm ³ , and exhibits flexibility and self-supporting properties. to solve the above problems.
The graphene electrode may have a BET specific surface area 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-shaped test piece with a thickness in the range of 50 μm to 80 μm, the graphene electrode is bent so that the opposite ends of the test piece are in contact, and the state is When held 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 SEM observation of the approximately central portion of the test piece, and the approximately central portion is the predetermined It may have a smooth arcuate shape with a radius of curvature.

The method for producing the above graphene electrode according to the present invention comprises 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, Preparing a mixed dispersion in which graphene oxide and thermally reduced graphene oxide are dispersed; vacuum filtering the mixed dispersion on 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 remove graphene oxide in the self-supporting structure. and the step of reducing, thereby solving the above problem.
The steps of preparing the graphene oxide dispersion and the thermally reduced graphene oxide dispersion include the steps of: freeze-drying a graphene oxide aqueous dispersion in which a predetermined amount of graphene oxide is dispersed in water; heating the solid in a muffle furnace at 300° C. to 700° C. within 1 minute.
The step of heat-treating the self-supporting structure includes heating the self-supporting structure at 300° C. to 700° C. within 1 minute in a muffle furnace to reduce graphene oxide in the self-supporting structure. good.
In the step of preparing the mixed dispersion, the graphene oxide dispersion and the thermally-reduced graphene oxide dispersion are such that the ratio of graphene oxide and thermally-reduced graphene oxide in the mixed dispersion is 3:1 by mass. It may be mixed to satisfy the range of ~1:3.

A power storage device according to the present invention includes an electrode and an electrolyte, and the electrode is composed of the graphene electrode described above, thereby solving the above problems.
The electric storage device may be an electric double layer capacitor.

The graphene electrode of the present invention is an electrode consisting essentially of graphene, which does not contain a conductive material or a binder, is not a composite combined with a substance other than graphene, has a predetermined density, is flexible and self-contained. Supportive. Therefore, the graphene electrode of the present invention can effectively exhibit the inherent electrical properties of graphene, and has excellent properties as an electrode for electric 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 electric storage devices such as electric double layer capacitors and lithium ion batteries that are excellent in performance such as capacitance and energy density.

In the method for producing a graphene electrode of the present invention, a film-like compact can be produced from a mixed dispersion by a simple method using a graphene oxide dispersion and a thermally reduced graphene oxide dispersion. In manufacturing the electrodes, no materials other than graphene are combined, and no conductive materials or binders are used. In addition, reduction of graphene oxide can be performed in a short time by thermal reduction. Such a method does not require skilled techniques or expensive equipment, is inexpensive and efficient, and has little environmental impact, and is therefore suitable for mass production.

Flowchart showing the manufacturing process of the graphene electrode of the present invention Schematic diagram showing the electric double layer capacitor of the present invention (a)-(c) SEM images of Film B of Example 1, Film B of Example 5 and Film B of Example 3. 1 shows (a) X-ray diffraction patterns, (b) Raman spectra, (c) XPS C1s spectra, and (d) FTIR spectra of Example 1 film B, Example 3 film B, and Example 3 film. (a) to (e) are cross-sectional SEM images of Film B of Example 1, Film B of Example 2, Film B of Example 3, Film B of Example 4, and Film B of Example 5. FIG. (f) A graph comparing the densities of the films B of Examples 1-5 and the films of Examples 1-5. (a) Diagram showing nitrogen adsorption and desorption isotherms of membranes B of Examples 1 to 5, (b) Diagram showing pore distribution of membranes B of Examples 1 to 5, (c) membranes of Examples 1 to 5 (d) A diagram showing the pore distribution of the membranes of Examples 1 to 5 (a)-(c) Diagrams showing the test results of flexibility and mechanical strength of the membrane of Example 3 (a) specific capacity-voltage curve (CV curve), (b) constant current charge-discharge curve (GCD curve ), (c) electrochemical impedance spectra (EIS), and (d) rate characteristics. (a) specific capacity-voltage curve (CV curve), (b) constant current charge _- discharge curve (GCD curves), (c) electrochemical impedance spectra (EIS), and (d) rate characteristics.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, the same number is given to the same element, and the description is omitted.

(Embodiment 1)
Embodiment 1 describes a graphene electrode of the present invention and a method for manufacturing the same.

The graphene electrode of the present invention does not contain a conductive material and a binder, and consists essentially of graphene.

In the present specification, the phrase “consisting essentially of graphene” means that the electrode does not contain any substance other than graphene. Therefore, the presence of substances other than graphene is not confirmed at least in the SEM image of the graphene electrode of the present invention. However, the presence of unavoidably mixed or residual substances in the manufacturing process is permissible. On the other hand, it is desirable not to include such contaminants or residues as much as possible, since such contaminants or residues may impair the energy characteristics of the electricity storage device. From such a point of view, in the graphene electrode of the present invention, the graphene is preferably thermally reduced graphene oxide. In addition, in the aspect where the manufacturing method of the electrode is known, in the process, the graphene (or graphene oxide) is not mixed or composited with other substances (excluding the dispersion medium), and the conductive material, Unless mixed with a binder and other additives, the electrode shall be treated as "consisting essentially of graphene." A method for manufacturing the graphene electrode of the present invention will be described later.

The graphene electrode of the present invention has a density ranging from 0.2 g/cm ³ to 0.7 g/cm ³ . As a result, the graphene electrode of the present invention has desired flexibility and self-supporting properties and is suitable for use as an electrode for a power storage device. In addition, if the density is within this range, the electrolyte ions can easily reach and migrate into the graphene. The graphene electrode of the present invention preferably has a density in the range of 0.4 g/cm ³ to 0.6 g/cm ³ .

The graphene electrode of the present invention consists essentially of graphene and has a density within the range described above, so that the specific surface area according to the BET method preferably satisfies the range of 200 m ² /g or more and 1000 m ² /g or less. Thereby, the graphene electrode of the present invention enables not only high conductivity but also adsorption of electrolyte ions while maintaining the required flexibility and self-supporting properties. More preferably, the graphene electrode of the present invention has a BET specific surface area in the range of 240 m ² /g to 400 m ² /g. Thereby, the graphene electrode of the present invention further ensures the adsorption and migration of electrolyte ions. More preferably, the graphene electrode of the present invention has a specific surface area according to the BET method that satisfies 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 thickness of the membrane is preferably between 10 μm and 100 μm. Within this range, the graphene electrode of the present invention has excellent handleability, is easy to apply to a current collector, and achieves high energy density and power density when applied to various electrical storage devices.

Although the terms "flexibility" and "self-support" are commonly used in the technical field, the flexibility and self-support of the graphene electrode of the present invention are shown below, for example. It can be confirmed by such flexibility and mechanical strength tests.

[Test method for flexibility and mechanical strength]
A film-like structure having a thickness in the range of 50 μm to 80 μm is used as the test piece.
The opposite ends of the test piece are bent so that they are in contact and held in that state for a certain period of time using any fixing means. Here, when the test piece is substantially rectangular in plan view, it is curved so that both ends in the longitudinal direction are in contact with each other. For example, tweezers or the like can be used as fixing means, although not limited thereto.
Next, the entire state of the test piece that is held in the curved state is visually observed. The graphene electrode of the present invention has a smooth curve and retains its curved state upon visual observation, and no structural damage or destruction of the specimen is observed.
Further, a scanning electron microscope (SEM) is used to observe the substantially central portion of the test piece that is in the curved state. Here, the substantially central portion of the test piece is intended to be the portion where the load is most applied due to the curvature of the test piece. In the graphene electrode of the present invention, no structural damage or destruction of the test piece was observed even in the SEM observation, and the substantially central portion of the test piece had a smooth arch shape with a predetermined radius of curvature. The situation is confirmed.

Next, a method for manufacturing the graphene electrode of the present invention will be described.
FIG. 1 is a flow chart showing the manufacturing process of the graphene electrode of the present invention.

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

Commonly available graphene oxide (GO) and thermally-reduced graphene oxide (TRGO) may be used, or in step S110 graphene oxide and/or thermal It may further comprise preparing a reduced graphene oxide.

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

Thermally reduced graphene oxide is produced by heat-treating graphene oxide. The conditions for heat-treating the graphene oxide are not particularly limited, but a shorter time is preferable from the viewpoint of production efficiency and reduction of environmental load. Specifically, for example, a graphene oxide aqueous dispersion obtained by dispersing a predetermined amount of graphene oxide in water is freeze-dried for several days (about two days) in a freeze dryer, and then the obtained graphene oxide is The desired thermally reduced graphene oxide can be obtained by heating the solid in a muffle furnace at 300° C.-700° C. within 1 minute and removing it quickly.

A dispersion medium for preparing a graphene oxide dispersion is not particularly limited, and examples thereof include water and ethanol. The dispersion medium is the same as the dispersion medium of the thermally-reduced graphene oxide dispersion or has excellent miscibility therewith, from the viewpoint of preparing a mixed dispersion with the thermally-reduced graphene oxide dispersion in step S120 described later. is preferably Specifically, for example, a graphene oxide dispersion can be prepared by replacing a dispersion medium with ethanol from an aqueous graphene oxide 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 may be appropriately selected so that the graphene oxide can be well dispersed in the dispersion medium. Specifically, for example, the concentration of the graphene oxide dispersion may range from 0.1 mg/mL to 1.0 mg/mL.

A dispersion medium for preparing the thermally reduced graphene oxide is not particularly limited, and examples thereof include water and ethanol. From the viewpoint of preparing a mixed dispersion with the graphene oxide dispersion in step S120, which will be described later, the dispersion medium is the same as the dispersion medium of the graphene oxide dispersion or has excellent miscibility therewith. preferable.

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, for example, the concentration of the thermally reduced graphene oxide dispersion may range from 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 the graphene oxide and the thermally reduced graphene oxide are dispersed.

The mixing ratio of the graphene oxide dispersion and the thermally-reduced graphene oxide dispersion is not particularly limited. The proportion (mass ratio) of graphene oxide and thermally reduced graphene oxide in the dispersion may be adjusted to a desired value.

For example, if the concentration of the graphene oxide dispersion and the concentration of the thermally reduced graphene oxide dispersion are the same, the amount (volume ratio) of each dispersion is the same as that of the graphene oxide in the mixed dispersion. is the same as the ratio (mass ratio) of Therefore, by changing the amount (volume ratio) of each dispersion, the ratio (mass ratio) of graphene oxide and thermally reduced graphene oxide in the mixed dispersion can be easily adjusted. On the other hand, when the ratio (mass ratio) of the graphene oxide and the thermally-reduced graphene oxide in the mixed dispersion is predetermined, the concentration of the graphene oxide dispersion and the concentration of the thermally-reduced graphene oxide dispersion are determined in advance. Both dispersions may be mixed in the same volume ratio by adjusting the ratio to be the same.

The method for mixing the graphene oxide dispersion and the thermally-reduced graphene oxide dispersion is not particularly limited. It is preferable to use a mixing/dispersing device of

Step S130: Vacuum filtering the mixed dispersion on the substrate to produce an intermediate structure composed of graphene oxide and thermally reduced graphene oxide.

The substrate is not particularly limited, and known substrates such as filter paper made of various resins can be used. Specifically, for example, filter paper made of tetrafluoroethylene resin (PTFE) can be used. Since the intermediate structural body is peeled off from the base material in step S140, which will be described later, it is preferable to select the base material in consideration of the easiness of the peeling operation.

The method of vacuum-filtrating the mixed dispersion on the substrate is not particularly limited, and a known technique can be employed.

Step S140: Drying the intermediate structure and peeling it 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, and it may be dried in an air atmosphere, or may be dried using a drying device such as an oven.

The method for peeling the dried intermediate structure from the substrate is not particularly limited, and a known method can be adopted in consideration of the material of the substrate, the size (diameter) of the intermediate structure, and the like.

Step S150: Heat-treating the self-supporting structure to reduce the graphene oxide in the self-supporting structure.

The heat treatment conditions for reducing the graphene oxide in the self-supporting structure (intermediate structure separated from the substrate) obtained in step S140 are not particularly limited, but the shorter the time, the better. It is preferable from the viewpoint of production efficiency and reduction of load on the environment. 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 taken out quickly, thereby reducing the graphene oxide in the self-supporting structure. A film composed of thermally reduced graphene oxide and thermally reduced graphene oxide (thermally reduced-GO/TRGO film) can be obtained. The thermally reduced-GO/TRGO film obtained in this manner is a graphene electrode of the present invention, which is a structure composed entirely of the thermally reduced form of graphene oxide, that is, graphene.

Conventionally, it is known that oxygen-containing functional groups in graphene oxide can be removed in a short period of time (for example, in seconds) by heat-treating graphene oxide for reduction or delamination. However, it is difficult to directly fabricate self-supporting graphene films by reduction by rapid heat treatment. This is because if the graphene oxide film is subjected to rapid heating and reduction treatment, the structure of the film may be destroyed. In fact, it has been reported that when many oxygen-containing functional groups decompose, a large amount of carbon dioxide gas is produced in the process, which creates a large pressure inside the membrane. On the other hand, the graphene oxide film has a very compact structure formed between the layers by hydrogen bonds derived from oxygen-containing functional groups. pressure cannot be released outside the membrane. Due to such problems, various attempts have been made so far to produce graphene electrodes having flexibility by improving the manufacturing process or manufacturing equipment. In terms of mechanical strength, it has been difficult to obtain a material that can be used for practical purposes.

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

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

In the method for producing a graphene electrode of the present invention, in an intermediate structure produced using a mixed dispersion of a graphene oxide dispersion and a thermally reduced graphene oxide dispersion, porous thermally reduced graphene is formed between graphene oxide layers. By having a structure in which an oxide is introduced, the graphene oxide is reliably reduced by heat treatment in a short time, and by introducing the porous thermally reduced graphene oxide, the graphene oxide is reduced. A channel (passage) is secured for releasing the gas and pressure generated at the time of heating to the outside of the membrane, and destruction of the structure is suppressed. From such a point of view, the ratio of graphene oxide and thermally reduced graphene oxide in the mixed dispersion is preferably in the range of 3:1 to 1:3 by mass, and 2.5:1 to 1:1. A range of 2.5 is more preferred, a range of 2:1 to 1:2 is more preferred, and a range of 1.5:1 to 1:1.5 is even more preferred.

Furthermore, in the method for producing a graphene electrode of the present invention, by variously adjusting the concentration and mixing ratio (volume ratio) of the graphene oxide dispersion and the thermally reduced graphene oxide dispersion, the graphene oxide in the mixed dispersion and The ratio (mass ratio) of the thermally reduced graphene oxide can be adjusted to a desired range, thereby making the graphene having flexibility and self-supporting properties more suitable (optimized) for the desired application of the electricity storage device. Electrodes can be manufactured simply.

(Embodiment 2)
In Embodiment 2, an electric double layer capacitor will be described as an electricity storage device using graphene electrodes 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 comprises at least electrodes and an electrolyte. In the electric double layer capacitor 200 of FIG. 2, a positive electrode 210 and a negative electrode 220 are immersed in an electrolyte 230 as electrodes. These positive electrode 210 and negative electrode 220 are made of the graphene electrodes described in the first embodiment. Electrolyte 230 is, for example, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI), 1-ethyl-3-methylimidazolium borofluoride (EMI-BF ₄ ) and 1-methyl- An ionic liquid selected from the group consisting of 1-propylpiperidinium bis(trifluoromethylsulfonyl)imide (MPPp-TFSI) or M'OH (M' is an alkali metal).

Electric double layer capacitor 200 further has separator 240 between positive electrode 210 and negative electrode 220 to separate positive electrode 210 and negative electrode 220 .

Materials for the separator 240 include, 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, and polymethacrylonitrile. A material selected from nitrile, polyvinyl acetate, polyvinylpyrrolidone, polyethyleneimine, polybutadiene, polystyrene, polyisoprene, polyurethane polymers and their derivatives, cellulose, paper, and non-woven fabric.

In electric double layer capacitor 200 , positive electrode 210 , negative electrode 220 , electrolyte 230 and separator 240 described above are accommodated in cell 250 . Also, the positive electrode 210 and the negative electrode 220 each have an existing current collector.

Such an electric double layer capacitor 200 may be a chip-type, coin-type, mold-type, pouch-type, laminate-type, cylindrical-type, rectangular-type capacitor, etc., and is used in a module in which a plurality of these are connected. may

Next, the operation of the electric double layer capacitor 200 of FIG. 2 will be described.

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, respectively. As a result, an electric double layer is formed on each of the positive electrode 210 and the negative electrode 220 and charged. Here, the positive electrode 210 and the negative electrode 220 are formed from the graphene electrodes described in Embodiment 1, and are electrodes substantially made only of graphene. High rate performance can be achieved. In addition, the positive electrode 210 and the negative electrode 220 are formed of the graphene electrodes described in Embodiment 1, and are electrodes substantially made of only graphene. It adsorbs and forms an electric double layer. As a result, exchange of electrons between graphene and electrolyte ions increases, and high energy density can be achieved.

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 are desorbed and discharged. Again, since the positive electrode 210 and the negative electrode 220 are formed of the graphene electrodes described in Embodiment 1, electrolyte ions can be easily desorbed and diffused, and high rate characteristics and energy density can be achieved. Moreover, since it is excellent in conductivity, the power density can be improved with the easiness of desorption/diffusion.

As described above, the electric double layer capacitor 200 of the present invention can fully exhibit the properties of graphene in the electrodes, so that quick charging is possible and high energy density and high power density can be achieved. In addition, since the formation of an electric double layer is used for charging and discharging, it is excellent in repeated use. The electric double layer capacitor 200 of the present invention can be used for wind power generation, electric vehicles, and the like.

Although the explanation is limited to the electric double layer capacitor here, it goes without saying that the graphene electrode of the present invention can be applied to an electric storage device such as a lithium ion battery in addition to the electric double layer capacitor.

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.

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

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

A thermally reduced graphene oxide dispersion was prepared as follows.
First, the graphene oxide aqueous dispersion was freeze-dried in a freeze dryer for 2 days, and then the resulting sponge-like graphene oxide solid was heated in a muffle furnace at 500° C. within 1 minute. , quickly took out. Thermally reduced graphene oxide (TRGO) was thus 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). (The above is step S110 in FIG. 1)

The mixing ratio (volume ratio) of the graphene oxide dispersion and the thermally reduced graphene oxide dispersion is changed from 3:1 to 1:3, and mixed with an ultrasonic homogenizer, and the graphene oxide and the thermally reduced graphene oxide are mixed by mass. A mixed dispersion was prepared by dispersing at a ratio of 3:1 to 1:3 (Step S120 in FIG. 1).

Next, the obtained mixed dispersion is vacuum-filtered on a tetrafluoroethylene resin (PTFE) filter paper, and a membrane (GO/TRGO membrane), which is an intermediate structure composed of graphene oxide and thermally reduced graphene oxide, is obtained. was produced (step S130 in FIG. 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, which is a self-supporting structure (step S140 in FIG. 1).

This free-standing film (GO/TRGO film) was then heated (heat treated) in a muffle furnace at 500° C. for less than 1 minute and quickly taken out. In this way, the graphene oxide in the GO/TRGO film was thermally reduced to obtain a film (thermally reduced-GO/TRGO film) composed of thermally reduced graphene oxide and thermally reduced graphene oxide (FIG. 1). step S150). The thickness of the obtained film was in the range of about 60 μm to about 70 μm.

For comparison, only the graphene oxide dispersion (0.5 mg/mL) was vacuum-filtered on PTFE filter paper to prepare a membrane (GO membrane), which is an intermediate structure composed only of graphene oxide. The GO film is dried in an oven, peeled off from the filter paper to obtain a self-supporting film, which is a self-supporting structure, and subjected to the heat treatment described above to obtain a film consisting only of thermally reduced graphene oxide (thermally reduced-GO film). rice field.
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. , a film (TRGO film) consisting only of thermally reduced graphene oxide was obtained.

Table 1 below summarizes the proportions (mass ratios) of graphene oxide and thermally reduced graphene oxide in the mixed dispersion.

In the following description, in order to distinguish between the film that is the final product and the film that is the intermediate structure, the former may be simply referred to as "membrane" and the latter may be referred to as "membrane B". For example, in the case of Example 3, "the film of Example 3" means the "thermally reduced-GO/TRGO film" obtained through the above heat treatment, and "the film of Example 3 B" means It means the "GO/TRGO film" before the heat treatment. However, for the sake of comparison with other examples, Example 5 may be referred to as "film of Example 5" or "film of Example 5" for convenience. However, since no heat treatment was performed for the purpose of reducing GO, it should be noted that all of them refer to "TRGO films".

[Analysis of fine structure]
3(a)-(c) show the surface morphologies of film B of Example 1, film B of Example 5 and film B of Example 3, respectively, under a scanning electron microscope (SEM, JSM-6500F manufactured by JEOL Ltd.). ) is a diagram showing the observation results. The scale bar in each figure is 10 μm.

As can be seen from FIG. 3(a), the membrane B (GO membrane) of Example 1 has few pores and a dense surface.
On the other hand, according to FIG. 3(b), the film B (TRGO film) of Example 5 has flake-like structures randomly distributed and has high porosity.
Further, according to FIG. 3(c), the membrane B (GO/TRGO membrane) of Example 3 exhibits a porous surface morphology, suggesting its usefulness as a self-supporting electrode membrane. rice field.

The density of each film was calculated from the SEM images of FIGS. ³ (a) to ³ (c). Membrane B of Example 3 was 0.76 g/cm ³ .

4(a)-(d) show X-ray diffraction patterns, Raman spectra, XPS C1s spectra, and FTIR spectra of Example 1 film B, Example 3 film B, and Example 3 film, respectively. It is a diagram.
In each figure, "GO film", "GO/TRGO film", and "reduced-GO/TRGO film" mean the film B of Example 1, the film B of Example 3, and the film of Example 3, respectively. .

The apparatus and measurement conditions used for each analysis are as follows.
・ X-ray diffraction device (XRD): Rigaku SmartLab, X-ray source: Cu-Kα ray (λ = 1.5418 Å), scanning range: 2θ = 5 to 60 °
・ Raman spectrometer: Raman plus manufactured by Nanophoton, excitation wavelength: λ = 532 nm
・ X-ray photoelectron spectrometer (XPS): PHI Quantera SXM manufactured by ULVAC-Phi, X-ray source: Al Kα, Analyzer: Hemispherical analyzer (energy value is 284.5 eV for C1s peak of aliphatic carbon corrected.)
・ Fourier transform infrared spectrophotometer (FTIR): JASCO FT / IR-6100

According to the X-ray diffraction pattern shown in FIG. 4A, in film B (GO film) of Example 1, a sharp peak centered at 2θ=10.6° ((001) plane of graphene oxide) according to Bragg's law ), which indicates that the stacking spacing of the GO sheets is 0.84 nm (Fig. 4(a), bottom).
In film B (GO/TRGO film) of Example 3, the peak near 2θ=10° is broadened with the peak at 2θ=11°, which indicates that the regularity of stacking of GO flakes is reduced. It is shown that. On the other hand, the gentle peak seen near 2θ = 24.8° was identified as a diffraction peak from the (002) plane of thermally reduced graphene oxide, suggesting the existence of a TRGO sheet (Fig. 4(a) middle row).
On the other hand, in the film of Example 3 (thermally reduced-GO/TRGO film), the peak near 2θ=10° disappears due to the reduction of GO, and a clear peak appears near 2θ=24.3°. suggests that graphite-like domains are formed due to the reduction process of GO that accompanies the heat treatment, and that the outgassing in this process enhances the disorder of the TRGO film (Fig. 4(a) top).

According to the Raman spectrum shown in FIG. 4B, 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) are Both have typical D and G bands near 1360 cm ⁻¹ and 1580 cm ⁻¹ , respectively.

Here, from the _D band in the Raman spectrum of the _carbonaceous material, information on the presence or absence of crystal structural defects and the regularity (irregularity) of sp ³ carbon can be obtained. , the area ratio (Area Ratio) by Gaussian fitting, or the intensity ratio (Central Peak Ratio), the lattice defect 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 that accompanies heat treatment, GO exfoliation occurs and C _{—O and C═O cleavage occurs in the production of CO and H 2 O.} occurs, resulting in a structure with more lattice defects.
Table 2 also shows peak assignments (Peak Index) and Raman shift values (cm ⁻¹ ).

The evolution of the oxygen-containing functional groups in the carbonaceous material in the process described above can be quantitatively evaluated from the XPS measurement results (FIG. 4(c)).
Table 3 below shows the carbon content (C%), oxygen content (O%) and carbon to oxygen ratio ( C/O).

According to Table 3, film B (GO film) of Example 1 has a small C/O value (C/O=2.4), which indicates that the graphite is completely oxidized and its oxygen content is is very high (29.3%) (see lower part of FIG. 4(c)).
Film B of Example 3 (GO/TRGO film) has a lower oxygen content (22.0%) than Film B of Example 1 because TRGO has some functional groups removed. (See middle of FIG. 4(c)).
Then, in the film of Example 3 (thermally reduced-GO/TRGO film) obtained through heat treatment, the value of C/O is significantly increased (C/O=5.8) (Fig. 4(c) see above).

Thus, in the three C1s spectra shown in FIG. 4(c), it can be seen that the relative intensities of C=O and C—O with respect to the C=C domain clearly decrease in the order of the lower, middle and upper rows. .

Furthermore, according to the FTIR spectrum shown in FIG. 4(d), in film B (GO film) of Example 1, characteristics corresponding to the vibrations of —OH, C═O, C═C, and CO groups Absorption was detected, indicating that many oxygen-derived functional groups were contained (Fig. 4(d), bottom).
On the other hand, in the film B (GO/TRGO film) of Example 3 and the film of Example 3 (thermally reduced-GO/TRGO film), the characteristic absorption of —OH groups decreased with the reduction of GO. In particular, in the film of Example 3, the characteristic absorption of —OH group is remarkably reduced (FIG. 4(d) middle and upper). This indicates that fabricating a graphene electrode (film) by the production method according to the present invention is effective in improving the structural stability of the graphene electrode.
On the other hand, in the FTIR spectrum shown in FIG. 4(d), the characteristic absorption of the C=O group (carbonyl bond) is observed, suggesting the existence of functional groups that remain without being removed during the heat treatment process. rice field. Oxygen-containing functional groups can facilitate reversible adsorption and desorption of ions, especially in aqueous electrolytes, but can impair cycling performance.

5(a)-(e) are cross-sectional SEM images of Example 1 Film B, Example 2 Film B, Example 3 Film B, Example 4 Film B, and Example 5 Film B, respectively. is. The scale bar in each figure is 5 μm in FIGS. 5( a ), ( c ), and ( d ), 2 μm in FIG. 5( b ), and 20 μm in FIG. 5( e ). Also, the upper right numerical value in each figure is the density (g/cm ³ ).

It can be seen from FIG. 5(a) that the film B (GO film) of Example 1 is composed of densely laminated sheets.
In contrast, in films B of Example 3, Film B of Example 4, and Film B of Example 5, which contain TRGO in a predetermined proportion, the TRGO content is It can be seen that the porosity of the film as a whole increases as the σ increases. In other words, as the TRGO content increases, the density of the film decreases, and the pores present in the film communicate with each other to create channels ( It can be said that it can function as a route). Also, if the content of TRGO in the film is large, the content of GO is relatively small, so the amount of gas generated during the heat treatment of GO is also small.

FIG. 5(f) shows changes in film density before and after the above heat treatment of GO. That is, FIG. 5(f) is a graph comparing the densities of the films B of Examples 1-5 and the films of Examples 1-5.
The numerical value shown on the horizontal axis of FIG. 5(f) is the ratio (mass ratio) of GO and TRGO in the mixed dispersion (see Table 1). , and the right side shows the density after thermal treatment. The arrows are for making it easier to visually understand the degree of density change before and after the heat treatment.

According to FIG. 5(f), film B (GO film) of Example 1 has the highest density (approximately 1.0 g/cm ³ ) and is a dense (compact) structure.
In addition, as the TRGO content in the film increases in order from Example 2 to Example 5, the density of the film decreases, and in film B (TRGO film) of Example 5, it is less than 0.1 g/ ^cm3 . (See FIG. 5(e)).
Here, even in the film of Example 1 (thermally reduced-GO film), the density value is greatly reduced by the heat treatment of GO, but the structure of the film is destroyed and it cannot function as a self-supporting film. rice field.
As described above, since the film B (TRGO film) of Example 5 originally had zero GO content, no heat treatment was performed, and in FIG. Treated as being the same as membrane B of Example 5. However, the membrane of Example 5, that is, the TRGO membrane obtained by drying the membrane B of Example 5 in an oven, can retain its shape as a membrane on the substrate (PTFE filter paper), but is not sufficient to be used as a free-standing membrane. has no strength.

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

6(a) and (b) are diagrams showing nitrogen adsorption/desorption isotherms and pore size distributions of membranes B of Examples 1 to 5, respectively. 6(a) and (b), "GO film", "GO/TRGO=3:1 film", "GO/TRGO=1:1 film", "GO/TRGO=1:3 film", ""TRGOfilms" refer to Film B of Example 1, Film B of Example 2, Film B of Example 3, Film B of Example 4, and Film B of Example 5, respectively. These plot figures are squares, circles, triangles, rhombuses, and stars, respectively.
6(c) and (d) are diagrams showing nitrogen adsorption/desorption isotherms and pore size distributions of the membranes of Examples 1 to 5, respectively. In FIGS. 6(c) and (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” are the film of Example 1, the film of Example 2, and the film of Example 3, respectively. Membrane, Membrane of Example 4, Membrane of Example 5. These plot figures are squares, circles, triangles, rhombuses, and stars, respectively.

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

According to FIGS. 6(a), (b) and Table 4, membrane B (GO membrane) of Example 1 has some pores with a pore size of less than 2 nm and a specific surface area of less than 10 m ² /g. there were.
This is because, as described above with reference to the SEM image of FIG. 5(a), the density of the film is increased by the formation of hydrogen bonds between the functional groups in the plane of the layers and/or of each layer. It can be understood that the result reflects the
In addition, in the membranes B of Examples 2 to 5, as the TRGO content increased, the specific surface area increased to 357 m ² /g, which also indicates that the pores in the GO/TRGO membranes are more likely to be affected by the heat treatment of GO. It can be seen that it can act as a channel to release the gases that are generated during the process and the pressure that is applied to the membrane.

On the other hand, according to FIGS. 6(c), (d) and Table 4, the specific surface areas of the films of Examples 1 to 5 increased compared to before the heat treatment (film B), and ranged from 260 to 360 m ² /g. It has become.
Moreover, the pore distributions of the membranes of Examples 1 to 5 showed generally similar tendencies, and the average pore diameter was in the range of 4 to 5 nm.

Comprehensively considering the results of these structural analyzes, the membrane of Example 3 prepared using a mixed dispersion in which GO and TRGO are dispersed at a mass ratio of 1:1 has a more compact structure. , was expected to function as a graphene electrode with better electrical properties.

The membrane of Example 3 was now tested for flexibility and mechanical strength.
Specifically, a substantially rectangular test piece of 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 with each other, and held in this state for a certain period of time with tweezers. A visual observation of the entire state of the bent portion and an SEM observation of the state of the curved portion were performed.
The results are shown in FIGS. 7(a)-(c).

FIG. 7(a) is a photograph showing a state in which the test piece is held in a curved state, and FIG. 7(b) is an SEM image of the portion surrounded by the dotted line in FIG. 7(a). (c) is an enlarged SEM image of the portion surrounded by the dotted line in FIG. 7(b). Scale bars in FIGS. 7(b) and (c) are 500 μm and 1 μm, respectively.

It can be seen from FIG. 7(a) that the test piece has a smooth curve and retains its curved state. Further, according to visual observation, no structural damage or destruction of the test piece was observed in the above test.
According to the SEM image shown in FIG. 7(b), even the portion (substantially central portion of the test piece) where the load is most applied due to the curvature of the test piece has a smooth arch shape, and the radius of curvature R is 3 mm. Met.
Furthermore, according to the SEM image shown in FIG. 7(c), it can be seen that no structural damage or breakage of the test piece due to the above test occurred in the fine structure.

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

[Analysis of electrical characteristics]
Next, in order to evaluate the electrical properties of the produced graphene electrodes, an electric double layer capacitor (CR2032 type coin cell) was produced using these as electrodes.

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

Electrochemical measurements of coin cells were performed using a multi-channel potentiostat galvanostat (Bio-Logic, VMP-300). Specific capacitance-voltage measurements (CV measurements), galvanostat charge-discharge measurements were performed at room temperature in a potential range of 0V to 3.7V or a potential range of 0V to 1.0V. Also, an electrochemical impedance measurement was performed.

Specific capacity 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 Vmax (the voltage at the start of discharge) plus 1/2 Vmax. is the slope obtained by linear fitting of the discharge curve between The energy density E _cell (Wh/kg) was calculated according to the formula E _cell =CsV ² /8.

8 (a) to (d) respectively show specific capacity-voltage curve (CV curve), constant current charge-discharge curve (GCD curve), and electrochemical impedance when the electrolyte is an aqueous electrolyte (sulfuric acid aqueous solution). FIG. 3 is a diagram showing spectrum (EIS) and rate characteristics; In FIGS. 8(a)-(d), the solid line is the film of Example 3, and the dashed line is the film of Example 5. FIG.
Here, the CV curve shown in FIG. 8(a) was measured at a sweep rate of 100 mV/s in the potential range of 0V to 1.0V.
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) is obtained by fitting using an equivalent circuit model, and the inside shows the spectrum with the value of the horizontal axis expanded in the range of 0Ω to 1.5Ω.
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.

According to FIG. 8(a), the film of Example 3 showed a rectangular CV curve representing an ideal electric double layer capacitor. It can also be seen that the film of Example 3 has a larger area of the CV curve and a larger capacitance than the film of Example 5. In fact, the specific capacity at a current density of 0.2 A/g calculated from the GCD curve in FIG.

According to FIG. 8(c), the film of Example 3 showed an equivalent resistance as low as 0.33Ω. Also, the charge transfer resistance is as small as 0.2Ω, which can contribute to exhibiting high rate characteristics. In addition, the membrane of Example 3 exhibited a specific capacity of 171 F/g at 0.5 A/g and retained a specific capacity of 111 F/g even at an even higher 20 A/g in charge-discharge measurements at higher current densities. (data not shown). These results are believed to be due to the compact structure of the Example 3 membrane compared to the Example 5 membrane. Furthermore, the film of Example 3 exhibited a capacitance retention of over 99% even after 10,000 cycles, demonstrating excellent longevity (data not shown).

FIGS. 9(a) to (d) respectively show a specific capacity-voltage curve (CV curve), constant current charge/discharge curve (GCD curve), and electrochemical when the electrolyte is an ionic liquid (EMI-BF ₄ ). FIG. 4 is a diagram showing an impedance spectrum (EIS) and rate characteristics; In FIGS. 9(a)-(d), the solid line is the film of Example 3 and the dashed line is the film of Example 5. FIG.
Here, the CV curve shown in FIG. 9(a) was measured at a sweep rate of 100 mV/s in the potential range of 0V to 3.7V.
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) is obtained by fitting 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.

According to FIG. 9(a), the film of Example 3 showed a rectangular CV curve representing an ideal electric double layer capacitor. Moreover, the electrolyte was a non-aqueous electrolyte, and the energy density reached 92 Wh/kg at a current density of 0.1 A/g as the differential voltage was increased to 3.7 V. This is more than 10 times the value (6.4 Wh / kg) when using the aqueous electrolyte described above, and using a conventional graphene-containing composite material such as Non-Patent Document 1 It can be said that the performance is equal to or better than that of conventional electrodes.

According to FIG. 9(c), the film of Example 3 showed an equivalent resistance of 3.2Ω. This value is higher than the value ₍ 0.33 Ω) explained with reference to FIG. be. It should be noted that the equivalent resistance value of the film of Example 5 was 5.1Ω, which is higher than that of the film of Example 3. In addition, the film of Example 3 exhibited a capacitance retention rate of 80% or more even after 10,000 cycles, indicating that it can withstand practical use (data not shown).

Practically, in a portable device, there are cases where the value of volumetric capacity is more important than weight specific capacity. Therefore, using the membranes of Examples 2 to 5, coin cells similar to those described above were produced, and electrochemical measurements were performed. As a result, the film of Example 3 exhibited the highest volumetric capacity value (101 F/cm ³ ), energy density of 45 Wh/cm ³ , and excellent rate characteristics (data not shown). This is believed to be due to the fact that the film of Example 3 had a density of 0.5 mg/cm ³ , which was the highest value among the films of Examples 2-5. In addition, as described with reference to FIG. 7, the film of Example 3 is superior in flexibility and mechanical strength, and therefore, among the films produced in this example, the film of Example 3 is superior. It was suggested that the graphene electrode was Furthermore, in the manufacturing process of the graphene electrode, by adjusting the ratio of graphene oxide and thermally reduced graphene oxide in the mixed dispersion, the electrical properties and mechanical properties of the target graphene electrode are optimized. It was suggested that it is possible to obtain flexible and self-supporting graphene electrodes that are more suitable for the desired application.

The graphene electrode of the present invention is a so-called binder-free electrode consisting essentially of graphene, has flexibility and self-supporting properties, and is applicable to electrodes for electric storage devices such as electric double layer capacitors and lithium ion batteries. It is suitable for When graphene oxide is used as a starting material, the graphene electrode of the present invention can be produced by performing thermal reduction treatment twice, which facilitates scale-up for mass production. The graphene electrode of the present invention can effectively exhibit the electrical properties that graphene originally has, is advantageous as an electrode for an electrical storage device (especially for an electric double layer capacitor), and has flexibility. It is expected to be used as an electrode that meets the demand for

200 electric double layer capacitor 210 positive electrode 220 negative electrode 230 electrolyte 240 separator 250 cell

Claims (10)

A graphene electrode containing no conductive material and binder, consisting essentially of graphene, having a density in the range of 0.2 mg/cm ³ to 0.7 mg/cm ³ , and having flexibility and self-supporting properties. The graphene electrode according to claim 1, wherein the specific surface area by the BET method is in the range of 200 m ² /g or more and 1000 m ² /g or less. 3. The graphene electrode according to claim 1 or 2, wherein the graphene is thermally reduced graphene oxide. In a flexibility and mechanical strength test using a film-shaped test piece with a thickness in the range of 50 μm to 80 μm, when the opposite ends of the test piece are bent so that they are in contact and this state is maintained for a certain period of time Furthermore, no structural damage or destruction of the test piece is observed by visual observation of the entire state of the test piece and SEM observation of the approximately central portion of the test piece, and the approximately central portion is smooth with a predetermined radius of curvature. The graphene electrode according to any one of claims 1 to 3, which has an arch shape. A method for producing the graphene electrode according to any one of claims 1 to 4,
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 the graphene oxide and the thermally reduced graphene oxide are dispersed;
vacuum filtering the mixed dispersion on 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 graphene oxide in the self-supporting structure. Preparing the graphene oxide dispersion and the thermally reduced graphene oxide dispersion comprises:
freeze-drying a graphene oxide aqueous dispersion in which a predetermined amount of graphene oxide is dispersed in water;
and heating the resulting graphene oxide solid in a muffle furnace at 300° C. to 700° C. within 1 minute. The step of heat-treating the self-supporting structure includes heating the self-supporting structure at 300° C. to 700° C. within 1 minute in a muffle furnace to reduce graphene oxide in the self-supporting structure. 7. The method according to Item 5 or 6. In the step of preparing the mixed dispersion, the graphene oxide dispersion and the thermally-reduced graphene oxide dispersion are such that the ratio of graphene oxide and thermally-reduced graphene oxide in the mixed dispersion is 3:1 by mass. A method according to any one of claims 5 to 7, mixed to satisfy the range of ~1:3. An electricity storage device comprising an electrode and an electrolyte,
An electricity storage device, wherein the electrode comprises the graphene electrode according to any one of claims 1 to 4. 10. The electricity storage device according to claim 9, wherein said electricity storage device is an electric double layer capacitor.

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