Method for manufacturing thin plate-like graphite structure and method for manufacturing exfoliated graphite

Abstract

The objective of the present invention is to provide a rational method for producing a high-quality graphite thin plate-shaped structure or exfoliated graphite in view of the current state of there being demand to establish a graphite thin plate-shaped structure production technique suited to large-scale and economical production in order to use exfoliated graphite as an industrial material, etc.　The present invention relates to a graphite thin plate-shaped structure production method, or the like, including: the step of preparing an electrochemical reaction system comprising an anode constituted by a long graphite sheet, an electrolyte solution containing tetrafluoroboric acid or hexafluorophosphoric acid as an electrolyte, and a cathode immersed in the electrolyte solution; the step of unrolling one end of the long graphite sheet for continuous immersion in the electrolyte solution while causing a current to flow into the electrochemical reaction system to convert the graphite sheet into a graphite thin plate-shaped structure; and the step of continuously removing the graphite thin plate-shaped structure from the electrolyte solution.

Description

[Technical field]

[0001] The present invention relates to a method for producing a thin plate-like graphite structure and a method for producing exfoliated graphite. [Background technology]

[0002] In the description of this specification, "graphene" refers to a sheet-shaped material composed of sp2-bonded carbon atoms and having a thickness equivalent to one atom. "Graphite thin plate structure" refers to graphite in which an interlayer material is inserted between the layers of raw graphite having a layer structure, thereby expanding the distance between the layers of graphite (the distance between graphenes). "Exfoliated graphite" refers to a graphite laminate in which the number of graphene layers is reduced compared to the raw graphite by subjecting the graphite plate structure to an exfoliation operation.

[0003] Graphene is a unique material that combines physical properties such as high carrier mobility, high thermal conductivity, and transparency in one material. In addition, because its structure is the ultimate nanosheet, it is easy to enlarge the area of ​​devices, and because it is highly stable both thermally and chemically, it is a nanocarbon material that is expected to be applied to advanced industrial materials, including in the electronics field.

[0004] Graphite is a multilayer structure composed of many graphene stacks, and is abundant on earth. Therefore, graphite is considered to be an ideal raw material for graphene production, and various attempts have been proposed to produce graphene or exfoliated graphite, which has far fewer graphene stacks than graphite, by peeling off the layers of this multilayer structure.

[0005] Known main methods for exfoliating graphite layers include a method of applying a mechanical or physical external force thereto, a method of chemically modifying graphite with an oxidizing agent, exfoliating the layers, and then reducing the modified graphene, and an electrochemical method of immersing graphite as a working electrode in an electrolyte solution and passing an electric current through the solution to intercalate electrolyte ions between the layers of the graphite to obtain a thin graphite structure, and then exfoliating the layers of the thin graphite structure.

[0006] The electrochemical approach of intercalating electrolyte ions between graphite layers and then exfoliating the layers does not require chemicals such as oxidizing or reducing agents, and is an approach to obtain thin-plate structures of graphite using easily controllable electrical energy and mild reaction conditions, which may lead to the process being scaled up.

[0007] In this electrochemical method, many attempts have been made using graphite as the working electrode. The most widely tried method is to use an aqueous solution of an acidic substance such as sulfuric acid, nitric acid, or perchloric acid as an electrolyte solution, pass electricity through the electrolyte, and then intercalate the acidic substance between the layers of graphite, which is the working electrode (anode), and then perform delamination (see Non-Patent Document 1). Among them, sulfuric acid is a substance that is often used as an electrolyte because it is easy to obtain and can easily form an intercalation compound with graphite. However, when these known acids are used as electrolytes, defects are likely to occur in the structure of the product during the process of delamination, and further, destruction and detachment of the graphite structure due to the generation of decomposition gas derived from the electrolyte have been a major barrier to expanding the application of the technology. In addition, there are problems due to the occurrence of undesirable side reactions such as oxidation of water.

[0008] In order to avoid these problems, there have been attempts to carry out intercalation under as mild an electrolysis condition as possible (see Patent Document 1). However, as a result, the electrochemical treatment takes a long time, and a complex electrolysis device for potential control is required, resulting in low production efficiency for large-scale implementation, and is not satisfactory in terms of cost.

[0009] In contrast, there is an example that claims to have attempted electrolysis in a short time. In this example, it is said that graphene oxide can be synthesized using 50% sulfuric acid water as an electrolyte solution (see Non-Patent Document 2). However, this method requires a step prior to the oxidation step in which raw graphite is converted into expanded graphite using concentrated sulfuric acid, and this step takes longer than the oxidation step. In short, it is not a sophisticated method because it is necessary to adopt a complicated two-stage process.

[0010] On the other hand, in order to avoid the above-mentioned drawbacks of electrochemical treatment in an aqueous system, there have been attempts to use non-aqueous electrolytes (see Non-Patent Document 3). In particular, the use of ionic liquids as electrolytes has been actively studied in recent years, but the ionic liquids themselves are extremely expensive and therefore not suitable for large-scale production from an economical standpoint.

[0011] In all of the above-mentioned conventional techniques, when a graphite thin plate structure is produced, the conditions such as the weight of the raw material graphite to be supplied to a reaction tank (reaction system), the amount of electricity, and the reaction time are preset, and only examples of a so-called batch reaction type are shown in which one cycle is performed from the start of the feeding to the end when the target product is obtained. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] JP 2012-131691 A [Non-patent literature]

[0013] [Non-Patent Document 1] CARBON,54,1-21(2013). [Non-Patent Document 2] Nature Commun.,9:145(2018). [Non-Patent Document 3] Adv.Funct.Mater.,18(10),1518-25(2008). Summary of the Invention [Problem to be solved by the invention]

[0014] As described above, many methods for producing exfoliated graphite by subjecting a graphite thin plate structure to interlayer peeling have been reported. However, the known methods for producing a graphite thin plate structure are limited to the so-called batch reaction type described above. The technology for producing a graphite thin plate structure on a large scale and economically is not yet at a satisfactory level. In order to utilize exfoliated graphite as an industrial material, it is necessary to establish a technology for producing a graphite thin plate structure suitable for large-scale and economical production.

[0015] In view of the above-mentioned current situation, an object of the present invention is to provide a rational method for producing a high-quality thin-plate-like graphite structure or exfoliated graphite. [Means for solving the problem]

[0016] As a result of extensive research, the inventors have discovered a method in which, by using tetrafluoroboric acid or hexafluorophosphoric acid as the electrolyte and a so-called long graphite sheet having a sufficient length relative to the width of the sheet as the anode, the raw material graphite can be continuously supplied to a reaction system and, in synchronization with this, high-quality thin graphite structures can be continuously extracted as a product, thereby completing the present invention.

[0017] That is, the present invention provides a continuous reaction system suitable for commercial production, which is capable of continuously supplying graphite as a raw material and continuously removing a high-quality graphite thin plate structure as a product from the system. The present invention relates to an anode made of a long graphite sheet, An electrolyte solution containing tetrafluoroboric acid or hexafluorophosphoric acid as an electrolyte; and providing an electrochemical reaction system including a cathode immersed in the electrolyte solution; a step of converting the graphite sheet into a thin graphite structure by applying a current to the electrochemical reaction system while continuously immersing one end of the long graphite sheet in the electrolyte solution; and and removing the graphite lamellar structure continuously from the electrolyte solution. Preferably, the graphite sheet is a heat-treated sheet composed of a condensation polymerization polymer compound, and more preferably, the condensation polymerization polymer compound is an aromatic polyimide. The method for producing the graphite thin plate structure may further include a step of removing or recovering the electrolyte solution attached to or contained in the graphite thin plate structure taken out of the electrolyte solution. The cathode may be placed opposite each of the two sides of the graphite sheet immersed in the electrolyte solution. The graphite sheet may be transported in a state where it is laminated on a support film. The present invention also provides a method for producing a graphite thin plate structure, comprising the steps of: obtaining a graphite thin plate structure by the method for producing a graphite thin plate structure; and a step of subjecting the thin plate-like structure to an exfoliation operation to obtain exfoliated graphite. Effect of the Invention

[0018] According to the present invention, a method for producing a high quality graphite lamellar structure or exfoliated graphite can be provided. Also, according to the present invention, a method for producing a graphite lamellar structure or exfoliated graphite having a high oxygen content by a continuous process can be provided. [Brief description of the drawings]

[0019] [Figure 1] Schematic diagram showing one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Specific embodiments of the present invention will be described in detail below.

[0021] In this embodiment, a graphite sheet used as an anode is converted into a graphite lamellar structure by a continuous electrochemical reaction using a specific electrolyte.

[0022] In this embodiment, the anode is made of a long graphite sheet. In one preferred embodiment, the graphite sheet is wound in a roll so that one end of the graphite sheet can be continuously fed from the roll into the reaction system.

[0023] The graphite constituting the graphite sheet is not particularly limited as long as it can form an intercalation compound (intercalate) with the electrolyte described below. Examples of the graphite include natural graphite, synthetic graphite, graphite obtained by heat-treating a condensation polymerization polymer compound, highly oriented pyrolytic graphite (HOPG), etc.

[0024] The condensation polymerization polymer compound is not particularly limited, and examples thereof include aromatic polyimide, aromatic polyamide, polyoxadiazole, polyparaphenylenevinylene, etc. Among these, aromatic polyimide is preferable.

[0025] A specific example of the graphite is graphite obtained by heat-treating aromatic polyimide. Such graphite has a structure in which planar graphite crystals are stacked in layers, and intercalation of tetrafluoroborate anions or hexafluorophosphate anions between the graphite layers is particularly likely to proceed. In addition, when the intercalation proceeds, peeling or detachment of small pieces from the graphite is particularly unlikely to occur, and the overall shape of the anode is easily maintained. This makes it possible to more efficiently produce a graphite thin plate structure or exfoliated graphite of higher quality.

[0026] The anode may be one obtained by immersing natural graphite in a strong acid such as concentrated sulfuric acid or nitric acid, followed by a heat treatment process in an expansion furnace, and then molding the expanded graphite by a high-pressure press. By using this as the anode, a high-quality graphite thin plate structure or exfoliated graphite can also be efficiently produced.

[0027] The thickness of the graphite sheet is not particularly limited as long as it provides flexibility and strength that allows the graphite sheet to be continuously supplied to the electrochemical reaction system, and may be, for example, 1 to 200 μm, preferably 4 to 100 μm, and more preferably 10 to 50 μm. The appropriate length of the graphite sheet in the longitudinal direction may vary depending on the scale of the electrochemical reaction, and may be, for example, 0.3 to 1000 m, and preferably 10 to 300 m.

[0028] The cathode used in the method for producing a graphite thin plate structure of this embodiment is an electrode placed opposite a graphite sheet in an electrolyte solution, but is not directly involved in the production of a graphite thin plate structure. Therefore, as long as it has a function of donating electrons to cations generated as a result of an anode reaction and can construct an electrochemically stable system, it is not particularly limited and can be appropriately selected from a wide range of materials. For example, it can be selected from metals such as platinum, stainless steel, copper, zinc, and lead, or carbonaceous materials such as graphite. In addition, the shape of the cathode can be appropriately selected from wire-shaped, plate-shaped, or mesh-shaped.

[0029] When gas is generated in the cathode reaction, the area of ​​the cathode may be made as large as possible in order not to impair the efficiency of the cathode reaction or to avoid an unnecessary increase in the electrical resistance of the electrolysis system.

[0030] In the manufacturing method of this embodiment, an ion exchange membrane or a spacer may be provided between the anode and / or cathode to prevent undesirable reactions from occurring in the anode and / or cathode or to prevent short-circuiting between the anode and cathode.

[0031] The electrode system in the manufacturing method of this embodiment may be the simplest electrode system consisting of only the anode and cathode described above. However, if more precise potential control is required, a reference electrode may be used in addition to the anode and cathode. The reference electrode may be a commonly used one such as Ag / AgCl.

[0032] In this embodiment, tetrafluoroboric acid or hexafluorophosphoric acid is used as the electrolyte. The anions of these electrolytes are intercalated very quickly between the layers of graphite, so that the current efficiency and time efficiency in the electrochemical reaction are very high, and a high-quality thin plate-like graphite structure can be efficiently produced. The above electrolytes may be commercially available as 40 to 50% aqueous solutions, or may be diluted with an appropriate solvent as necessary.

[0033] The electrolyte solution is a solution in which the electrolyte is dissolved in a solvent. Usable solvents can be appropriately selected from among solvents that are miscible with the electrolyte or an aqueous solution thereof and that are electrochemically stable when producing a graphite thin plate structure.

[0034] Preferred solvents are protic polar solvents such as water, lower alcohols such as methanol, ethanol, propanol, etc., and aprotic polar solvents such as acetonitrile, dimethylformamide, dimethoxyethane, dimethyl carbonate, propylene carbonate, dimethyl sulfoxide, etc. Among these, one kind may be used, or two or more kinds may be used in combination.

[0035] The solvent preferably contains water. It is more preferable to contain water and a protic polar solvent, or to contain water and an aprotic polar solvent. It is particularly preferable to contain water and an aprotic solvent. By using a protic polar solvent or an aprotic polar solvent, it is expected that the anions of tetrafluoroborate or hexafluorophosphate, which are considered to have a relatively high lipophilicity, will be supported to penetrate between the graphite layers. In addition, the range of electrolysis conditions that are advantageous for efficiently producing a thin plate-like structure of graphite will be expanded by increasing the options for the solvents that constitute the electrolyte solution.

[0036] The concentration of the electrolyte in the electrolyte solution may be any concentration at which the electrical resistance of the electrochemical reaction system is sufficiently low and at which tetrafluoroborate anions or hexafluorophosphate anions are rapidly supplied to the graphite of the anode to obtain a thin plate-like graphite structure, and is preferably 1.0 to 50 mass %, and more preferably 5.0 to 50 mass %.

[0037] One aspect of this embodiment will be described with reference to FIG. 1. A long graphite sheet 1 having a predetermined width and length is wound into the shape of a roll 2. One end of the graphite sheet 1 is unwound from the roll 2 and transported by a plurality of transport rollers 3 through a predetermined path. The transport speed when transporting the graphite sheet 1 is determined by comprehensively considering factors of the implementation scale of this embodiment, i.e., factors of the reaction scale such as the thickness and width of the graphite sheet, and factors related to the current flow conditions such as the amount of electricity supplied and the current density that govern the desired oxidation degree in the thin graphite structure or exfoliated graphite. Thus, the transport speed of the graphite sheet is not limited to a specific value, but is usually 10 to 2000 cm / hr, preferably 20 to 1000 cm / hr, and more preferably 200 to 500 cm / hr.

[0038] Meanwhile, electrolytic cell 4 is filled with electrolyte solution 5. The transport path of graphite sheet 1 is set so that graphite sheet 1 unwound from roll 2 enters electrolyte solution 5, is transported a predetermined distance (electrochemical reaction zone) within electrolyte solution 5, undergoes a desired electrochemical reaction in the process, and then proceeds out of electrolyte solution 5.

[0039] In electrolyte solution 5, cathode 6 is placed so as to face graphite sheet 1. In FIG. 1, cathode 6 is arranged so as to face only one side (the upward side) of graphite sheet 1, but it may be placed so as to face both the top and bottom sides of graphite sheet 1. By facing both sides, a thin graphite structure can be produced more efficiently.

[0040] In FIG. 1, graphite sheet 1 in electrolyte solution 5 is transported horizontally at the portion facing cathode 6, but the transport direction is not limited to this and can be selected arbitrarily as long as the desired electrolysis results are obtained.

[0041] At least one of the transport rolls 3 and the cathode 6 are each connected to a power source 7. The transport roll connected to the power source 7 is made of an electrically conductive material. A positive voltage is applied to the graphite sheet 1 via the transport roll. This causes the graphite sheet 1 to become an anode.

[0042] The applied voltage should be at least a potential necessary for intercalating tetrafluoroborate anions or hexafluorophosphate anions between the graphite layers of the graphite sheet 1 and for causing a desired electrochemical reaction, but an overvoltage may be applied in order to rapidly obtain a graphite thin plate structure. A practical applied voltage is determined by comprehensively considering factors such as the scale of implementation of this embodiment, i.e., factors of the reaction scale such as the thickness and width of the graphite sheet supplied to the system per unit time, and factors related to the current flow conditions such as the amount of electricity supplied and current density that govern the desired oxidation degree in the graphite thin plate structure or exfoliated graphite. Therefore, the voltage applied to the graphite sheet is not uniquely limited, but is preferably set so as to overcome voltage drop factors governed by the electrical resistance of the electrolysis system, such as the desired current value, electrolyte concentration, solvent composition of the electrolyte solution, distance between the positive and negative electrodes, and electrolysis temperature. Specifically, the preferred range of the applied voltage is 1.5 to 50V, and the more preferred range is 2.0 to 25V.

[0043] The value of the current passed through the electrochemical reaction system of this embodiment is determined by comprehensively considering factors such as the scale of implementation of this embodiment, i.e., the weight scale of the raw material supplied to the system per unit time, such as the thickness and width of the graphite sheet, and factors related to the current flow conditions, such as the amount of electricity supplied and current density, which govern the desired oxidation degree in the thin graphite structure or exfoliated graphite. Thus, the value of the current passed through the graphite sheet is not uniquely limited, but should be at least a value that allows tetrafluoroborate anions or hexafluorophosphate anions to intercalate between the graphite layers of the graphite sheet 1 and allows the desired electrochemical reaction to be completed within the desired reaction time.

[0044] The density of the current supplied to the graphite sheet 1 is controlled by the applied voltage and the surface area of ​​the anode that is in the electrolyte and faces the cathode to undergo electrolysis. According to this embodiment, the anions of tetrafluoroborate or hexafluorophosphate intercalate extremely quickly between the graphite layers, making it possible to uniformly expand the gap between the graphene layers. Therefore, the current density can be set over a wide range, from very low to very high, and a thin graphite structure can be obtained regardless of the magnitude of the current density. The current density is preferably 1 to 2,000 mA / cm. 2 and more preferably, 10 to 1,000 mA / cm 2 It is.

[0045] In one embodiment of the present invention, it is preferable to set the current supplied to the electrochemical reaction system to a constant value. In this case, the preferable set current value is set to be within the preferable current density range described above. In this case, the voltage applied to the electrochemical reaction system may vary depending on the resistance value of the reaction system, but the preferable range of the applied voltage is the same as the above-mentioned range of the applied voltage.

[0046] The amount of electricity (F / mol, F: Faraday constant) supplied to the electrochemical reaction system is preferably 0.2 F / mol or more, more preferably 0.8 to 3.0 F / mol, and even more preferably 1.0 to 2.0 F / mol, relative to the number of moles of carbon atoms in graphite subjected to the electrolytic reaction. By supplying this amount of electricity, a graphite lamellar structure or exfoliated graphite can be effectively obtained.

[0047] The temperature of the electrolyte solution when applying a voltage to the electrochemical reaction system may vary depending on the type of solvent in which the electrolyte is dissolved and the concentration of the electrolyte solution, but in practice, the lower limit is the temperature at which the electrolyte solution does not freeze, and the upper limit is the boiling point of the electrolyte solution. It is preferably carried out in the range of 0 to 100°C. More preferably, it is carried out in the range of 0 to 80°C.

[0048] As described above, in the electrochemical reaction system, graphite sheet 1 to which a positive voltage has been applied is transported through electrolyte solution 5, and as a result, tetrafluoroborate anions or hexafluorophosphate anions intercalate between the graphite layers, resulting in conversion to a crude reaction product sheet 8 containing thin graphite structures. The crude reaction product sheet 8 is continuously removed from the electrochemical reaction system in synchronization with the previous supply of the graphite sheet. This completes a series of flow reaction processes according to this embodiment, which include continuous supply of raw graphite and continuous removal of the product.

[0049] In the graphite lamellar structure in the obtained reaction crude product sheet 8, the graphite is oxidized and tetrafluoroboric acid or hexafluorophosphoric acid is intercalated between the graphite layers, so that the thickness is increased compared to the graphite sheet 1. According to the present embodiment, it is possible to continuously obtain a long reaction crude product sheet 8 having such an increased thickness.

[0050] The long reaction crude product sheet 8 taken out from the electrolyte solution 6 may be continuously wound up on a winding roll (not shown), or may be continuously stored in a container (not shown) without being wound up.

[0051] In this embodiment, the graphite sheet may be transported while being laminated on a support film, and immersed in the electrolyte solution. This allows the graphite sheet and the graphite sheet to be taken out of the electrolyte solution while being laminated on the support film. By laminating the graphite sheet and the graphite sheet, the transportation of the graphite sheet and the graphite sheet can be more reliably performed. The material of the support film may be selected to be stable and not adversely affect the electrochemical reaction, and examples of the material include polyolefin resins such as polyethylene and polypropylene, and fluorine-based resins that are polyfluoroethylene and related substances. The support film may be in the form of a mesh.

[0052] In the present embodiment, the electrolyte solution used in the production of the graphite thin plate structure can be repeatedly reused, and the electrolyte that has been reduced due to adhesion to the graphite thin plate structure removed from the electrolyte solution may be replenished to the reaction system as necessary.

[0053] In addition, the graphite thin plate structure immediately after the reaction according to this embodiment has an electrolyte solution containing tetrafluoroboric acid or hexafluorophosphoric acid entrapped and attached thereto. The electrolyte solution components accompanying such graphite thin plate structures can be removed or recovered. Since the recovered electrolyte solution can be returned to the reaction system as necessary, this recovery method becomes more effective as the scale of production of graphite thin plate structures increases. Specific removal or recovery methods include, for example, a method of subjecting the graphite thin plate structure containing the electrolyte solution to a centrifuge or centrifugal filtration separator, a method of subjecting it to pressure press filtration, or a method of squeezing and recovering the electrolyte solution continuously on a belt press.

[0054] Regardless of whether the graphite sheet structure removed from the electrolyte solution has been subjected to the removal or recovery process, the electrolyte solution components can be removed from the structure by washing the structure with an excess of deionized water until the washing liquid becomes neutral.

[0055] The graphite lamellar structure obtained by the above steps can be supplied in a wet state to the subsequent step of producing exfoliated graphite, or may be subjected to a drying step as necessary before being supplied to the step of producing exfoliated graphite. The specific drying method is not particularly limited, but it may be, for example, dried in a thermostatic dryer or vacuum dryer at a temperature of 80° C. or less.

[0056] As described above, in this embodiment, an anode containing graphite is used, and a current is passed through an electrochemical reaction system using an electrolyte solution containing tetrafluoroboric acid or hexafluorophosphoric acid as an electrolyte, whereby anions of tetrafluoroboric acid or anions of hexafluorophosphoric acid are quickly and uniformly intercalated between the layers of graphite, and as a result, a thin plate-like graphite structure can be obtained in which the interlayer distance between each graphene constituting the graphite is uniformly expanded.

[0057] By subjecting the thin graphite structure obtained by this embodiment to an exfoliation operation, exfoliated graphite preferably having a thickness of 100 nm or less can be obtained.

[0058] The peeling operation is not particularly limited, and examples thereof include a peeling operation by ultrasonic irradiation, a peeling operation by applying a mechanical peeling force, a peeling operation by heating, etc. More specifically, examples include a method of dispersing the graphite lamellar structure in an appropriate amount of deionized water and subjecting the dispersion to an ultrasonic irradiation device, and a method of treating the dispersion with a mixer or a device capable of applying a shear force. The treated product after the peeling operation may be freeze-dried, or the cake obtained by filtering or centrifuging may be subjected to a drying treatment similar to that for the graphite lamellar structure described above.

[0059] As a result, exfoliated graphite having a thickness of 100 nm or less can be advantageously obtained. The thickness of the exfoliated graphite is more preferably 50 nm or less, and even more preferably 10 nm or less. Exfoliated graphite having a thickness of 1 nm or less is particularly preferred. The average particle size of the exfoliated graphite can vary from nanometers to millimeters, but is preferably 30 nm to 1 mm, more preferably 50 nm to 100 μm, and even more preferably 100 nm to 50 μm. The obtained exfoliated graphite is preferably composed of graphene oxide (graphene containing oxygen), and more preferably composed of fluorinated graphene oxide (graphene containing fluorine and oxygen).

[0060] The exfoliated graphite preferably produced by the present embodiment is characterized by high purity and low impurity content due to the characteristics of the production method of the present embodiment. In particular, it is characterized by low contents of heavy metal components and sulfur components. Specifically, in the exfoliated graphite containing fluorine, the manganese content is preferably 0.002 mass% or less and the sulfur content is preferably 0.1 mass% or less, and more preferably the manganese content is 0.001 mass% or less and the sulfur content is 0.01 mass% or less.

[0061] The exfoliated graphite preferably produced in this embodiment contains oxygen atoms and fluorine atoms in a molecular skeleton composed of carbon atoms. It is preferable that the fluorine content is 0.5% by mass or more and 40% by mass or less, the carbon content is 40% by mass or more and 80% by mass or less, and the oxygen content is 1.0% by mass or more and 50% by mass or less, and it is more preferable that the fluorine content is 1.0% by mass or more and 15% by mass or less, the carbon content is 45% by mass or more and 75% by mass or less, and the oxygen content is 15% by mass or more and 45% by mass or less. EXAMPLES

[0062] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0063] <Method for measuring the mass ratio of carbon to oxygen (C / O) in exfoliated graphite> The carbon to oxygen mass ratio (C / O) of exfoliated graphite was measured using the principle of energy dispersive X-ray spectrometry (EDX). The specific measurement method was to evenly attach the dried powder of exfoliated graphite obtained by a specified process to a carbon tape, and measure it with JEOL's JSM IT-100.

[0064] <Method for measuring the minimum thickness of exfoliated graphite> The minimum thickness of exfoliated graphite was measured using an atomic force microscope (AFM). Specifically, a dilute dispersion of exfoliated graphite was applied to a mica substrate, and measurements were made using a Shimadzu SPM-9700HT in tapping mode.

[0065] Example 1 In the electrolytic cell 4 of the electrochemical reaction device shown in FIG. 1, about 1.5 L of a 20% aqueous solution of tetrafluoroboric acid was charged as the electrolyte solution 5. A roll of graphite sheet (a graphite sheet obtained by heat-treating aromatic polyimide, a condensation polymerization polymer compound) with a thickness of about 20 μm, a width of 30 mm, and a length of about 10 m was prepared separately. One end of the roll was passed through a conveying roller 3 (a stainless steel cylindrical rotating roll) that was connected to the anode side of a DC power source and also had a function of sending out the raw graphite sheet. Next, two stainless steel meshes were placed in the electrolyte solution 5 as the cathodes 6 so as to face both the upper and lower surfaces of the graphite sheet with a gap of about 1 cm from the graphite sheet. The cathodes 6 were connected to a DC power source 7, the conveying speed of the graphite sheet was set to 12 cm / hr, and electrolysis was performed at room temperature for 2 hours under a constant current condition of 2.0 A. The total amount of graphite sheets used in the reaction was 0.58 g. During this process, in the anode, a continuous and smooth increase in the thickness of the graphite sheet and slight browning of the surface were observed in the portion immersed in the electrolyte solution 5 (the length of the effective electrochemical reaction zone facing the cathode was 6 cm), forming a crude reaction product sheet 8. The crude reaction product sheet 8 was discharged into the product receiving tank at a speed synchronized with the supply of the graphite sheet, without remaining in the reaction system. After the reaction was completed, the anode hardly peeled off or fell off into the electrolyte solution, and retained its sheet shape, but its thickness was clearly increased compared to before the reaction. This was confirmed to be the result of sufficient intercalation of tetrafluoroboric acid into the graphite sheet, resulting in the reliable formation of a thin graphite structure.

[0066] The reaction crude product sheet 8 was washed with deionized water until the washing liquid became neutral, and a wet, black-brown, thin-plate structure of graphite was obtained. A small amount of deionized water was added to this, and the structure was subjected to ultrasonic irradiation for 15 minutes, followed by freeze-drying to obtain 0.78 g of exfoliated graphite. From the results of EDX analysis, this structure was composed of exfoliated graphite containing oxygen atoms with a carbon to oxygen mass ratio (C / O) of 1.4 and containing 3 mass% fluorine. In addition, from the results of AFM analysis, the minimum thickness of the exfoliated graphite was 1.0 nm.

[0067] Example 2 An electrolytic reaction was carried out under the same conditions as in Example 1, except that about 1.5 L of a 20% aqueous solution of hexafluorophosphoric acid was used as the electrolyte solution 5 instead of the 20% aqueous solution of tetrafluoroboric acid. The total amount of the graphite sheet used in the reaction was 0.57 g. The post-reaction treatment was carried out in the same manner as in Example 1 to obtain 0.72 g of exfoliated graphite. From the results of EDX analysis, this was found to be composed of exfoliated graphite containing oxygen atoms with a carbon to oxygen mass ratio (C / O) of 1.5 and containing 5 mass% fluorine. In addition, from the results of AFM analysis, the minimum thickness of the exfoliated graphite was 1.0 nm.

[0068] Example 3 An electrolytic reaction was carried out under the same conditions as in Example 1, except that about 1.5 L of a 42% aqueous solution of tetrafluoroboric acid was used as the electrolyte solution 5 instead of the 20% aqueous solution of tetrafluoroboric acid, the conveying speed of the graphite sheet was set to 36 cm / hr, and electricity was passed for 1 hour at room temperature under a constant current condition of 3.0 A. The total amount of the graphite sheet used in the reaction was 0.86 g. After the reaction was completed, the anode hardly peeled off or fell off into the electrolyte solution, and the sheet shape was maintained. The thickness had clearly increased compared to before the reaction, but a small amount of unreacted layer was found to remain in the central layer portion in the thickness direction of the sheet. The post-reaction treatment was carried out in the same manner as in Example 1 to obtain 0.80 g of exfoliated graphite. From the results of EDX analysis, this was found to be composed of exfoliated graphite containing oxygen atoms with a carbon to oxygen mass ratio (C / O) of 1.3 and containing 3 mass% fluorine. In addition, from the results of AFM analysis, the minimum thickness of the exfoliated graphite was 1.0 nm.

[0069] Example 4 An electrolytic reaction was carried out under the same conditions as in Example 3, except that the current value supplied to the graphite sheet was changed from 3.0 A to 3.6 A. The total amount of the graphite sheet subjected to the reaction was 0.86 g. After the reaction was completed, the anode hardly peeled off or fell off into the electrolyte solution, and retained its sheet shape, but its thickness was clearly increased compared to before the reaction. This confirmed that the graphite sheet was sufficiently intercalated with tetrafluoroboric acid, resulting in the definite formation of a thin graphite structure. The post-reaction treatment was carried out in the same manner as in Example 1 to obtain 1.20 g of exfoliated graphite. From the results of EDX analysis, this was found to be composed of exfoliated graphite containing oxygen atoms with a carbon to oxygen mass ratio (C / O) of 1.2 and containing 3 mass% fluorine. In addition, from the results of AFM analysis, the minimum thickness of the exfoliated graphite was 1.0 nm.

[0070] [Table 1] [Explanation of symbols]

[0071] 1 Graphite sheet 2 rolls of graphite sheets 3 Conveyor roller 4 Electrolytic cell 5 Electrolyte solution 6 cathode 7 DC power supply 8. Reaction Crude Product Sheet

Claims

1. The anode consists of a long graphite sheet. Electrolyte solutions containing tetrafluoroboric acid or hexafluorophosphoric acid as an electrolyte (excluding cases where an ionic liquid having tetrafluoroboric acid is included), and providing an electrochemical reaction system including a cathode immersed in the electrolyte solution; a step of converting the graphite sheet into a thin graphite structure by applying a current to the electrochemical reaction system while continuously immersing one end of the long graphite sheet in the electrolyte solution; and continuously removing said graphite lamellar structure from said electrolyte solution.

2. 2. The method for producing a graphite thin plate structure according to claim 1, wherein the graphite sheet is a heat-treated sheet made of a condensation polymerization polymer compound.

3. 3. The method for producing a graphite thin plate structure according to claim 2, wherein the condensation polymerization polymer compound is an aromatic polyimide.

4. 4. The method for producing a graphite thin plate-like structure according to claim 1, further comprising a step of removing or recovering the electrolyte solution attached to or contained in the graphite thin plate-like structure taken out from the electrolyte solution.

5. 5. The method for producing a graphite thin plate structure according to claim 1, wherein the cathode is disposed opposite each of the two surfaces of the graphite sheet immersed in the electrolyte solution.

6. 6. The method for producing a graphite thin plate-like structure according to claim 1, wherein the graphite sheet is transported in a state of being laminated on a support film.

7. A step of obtaining a graphite thin plate structure by the manufacturing method according to any one of claims 1 to 6, and and a step of obtaining exfoliated graphite by subjecting the thin plate-like structure to an exfoliation operation.

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