Electrochemical exfoliation of graphite for the production of graphene flakes

Abstract

A method for producing graphene. The method includes the steps of supporting particulate graphite on an open-cell porous framework material, immersing at least a portion of the graphite-supported porous framework material in a solution, and applying a cathode potential to the graphite-supported porous framework material, wherein the cathode potential is sufficient to exfoliate the graphene.

Description

[Technical Field]

[0001] Cross-reference of related applications This application claims priority to U.S. Provisional Patent Application No. 63 / 461,969, filed on 26 April 2023, the entirety of which is incorporated herein by reference.

[0002] Description of research and development funded by the federal government. This invention was made with government support under the award given by the National Institute of Standards and Technology (NIST) for “Scale-up of Continuous Manufacturing and Productization of Graphene for Advanced Respirator and Biosensor Applications”, #2970576300-01. The government has certain rights to this invention.

[0003] This invention relates to the electrochemical exfoliation of graphite for producing graphene flakes. [Background technology]

[0004] Ideal graphene has sp² arranged in a hexagonal lattice. 2 Graphene is a two-dimensional material composed of hybrid carbon atoms, with a single-atom thickness and infinite crystalline order, and no defects. In practice, real-world graphene has a finite size, is not defect-free, and generally contains multiple layers. Nevertheless, the physical properties of real-world graphene can approach those of idealized graphene.

[0005] Graphite is generally a three-dimensional structure containing thousands of graphene sheets stacked on top of each other in an orderly manner, held in place by weak van der Waals interactions. In some examples, the region of each graphene sheet in a graphite particle corresponds to a region defined in two dimensions of the particle, and the three dimensions of the particle reflect the number of stacked graphene sheets. That is, although not always, the entire graphite particle can be a single crystal.

[0006] Graphene can be manufactured by a variety of different techniques, including electrochemical exfoliation. For example, well-established methods for manufacturing graphene without electrochemical exfoliation include epitaxial growth on a catalytic substrate at high temperatures, chemical vapor deposition (CVD), micromechanical exfoliation, and direct sonication. These methods have significant drawbacks compared to the potential attributes of electrochemical exfoliation, including low product yields and practical barriers to large-scale production.

[0007] Regarding electrochemical exfoliation, several approaches involve applying an oxidizing chemical solution or an anodic potential to graphite. These approaches result in at least a partially oxidized form of graphene, known as graphene oxide (GO). Graphene oxide can be manufactured in relatively large quantities and is commercially available, but its physical properties are affected by oxygen impurities and sp²⁻¹. 2 and sp 3 Due to the hybrid carbon mixture, the physical properties of graphene differ from those of idealized graphene. Oxygen can be removed from graphene oxide by chemical reduction or heat treatment to form graphene with improved physical properties. However, removing oxygen requires additional processing steps, increasing costs and energy consumption. Furthermore, removing oxygen is more difficult than removing hydrogen from graphene hydride.

[0008] Another approach to electrochemical exfoliation involves applying a high cathode potential (e.g., -60V) to the graphite in an electrochemical cell. The cell typically contains a liquid electrolyte of an organic solvent and a supporting electrolyte salt, with the anode at an equally high anodic potential during exfoliation. The electrolyte composition facilitates exfoliation. Salts with large ionic radii that are soluble in the electrolyte solvent are intercalated between the laminated sheets of graphite, and the applied potential causes cations from the electrolyte salt and cations generated from the solvent at the anode to be intercalated into the interlayer space of the graphite. Gas species are formed in the interlayer space, which further expand and exfoliate the graphene sheet. Generally, exfoliated graphene is hydrogenated at points where hydrogen atoms are bound to defects. Thermal dehydrogenation, which occurs under relatively mild conditions, can be used to remove defects and bring the physical properties of the product closer to those of idealized graphene.

[0009] Examples of such approaches can be found in International Publication No. 2021 / 048089 and “High Voltage Electrochemical Exfoliation of Graphite for High-Yield Graphene Production” (RSC Adv.2019, 9(50), pp.29305-29311, doi.org / 10.1039 / C9RA04795F), both of which are incorporated herein by reference. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] International Publication No. 2021 / 048089 [Non-patent literature]

[0011] [Non-Patent Document 1] High Voltage Electrochemical Exfoliation of Graphite for High-Yield Graphene Production(RSC Adv.2019,9(50),p.29305-29311,doi.org / 10.1039 / C9RA04795F) [Overview of the project]

[0012] Systems and techniques for the electrochemical exfoliation of graphite for the production of graphene flakes are described.

[0013] In one embodiment, the method is for the production of graphene. The method includes the steps of supporting particulate graphite on an open-cell porous framework material, immersing at least a portion of the graphite-supported porous framework material in a solution, and applying a cathode potential to the graphite-supported porous framework material, wherein the cathode potential is sufficient to exfoliate the graphene.

[0014] This and other embodiments may include one or more of the following features: The method may further include a step of washing exfoliated graphene from the porous framework material. The pores of the porous framework material may generally be 3 to 25 times larger, or 5 to 10 times larger, than the Sauter mean particle size of the graphite particles. The mean maximum dimension of the pores of the porous framework material may be 0.5 to 2 mm, and the graphite particles may have an mean diameter in the range of 0.5 to 500 micrometers. The porous framework material may have a void volume greater than 50%, for example, greater than 75%. The mass loading of particulate graphite in the open-cell porous framework material is such that the porous framework material is 1 cm 3 The amount may be 0.1 to 0.3 g of graphite particles per unit. The porous framework material may be a network of vitreous carbon foam. The solution may contain an organic solvent and a supporting electrolyte salt.

[0015] The organic solvent may be propylene carbonate, ethylene carbonate, or dimethyl carbonate, and the electrolyte salt may be tetrabutylammonium hexafluorophosphate, tetrabutylammonium hexafluoroborate, tetrabutylammonium bis(trifluoromethanesulfonyl)imide, or N-benzyl-N,N,N-trimethylammonium hexafluorophosphate. A membrane that allows transport of the electrolyte salt but hinders or prevents transport of exfoliated graphene may be placed between the graphite-supported porous framework material and the anode while the cathode potential is applied. The membrane may be self-supporting. The membrane may have an average pore size between 0.5 and 1 μm. The membrane may enclose the graphite-supported porous framework material in solution. The cathode potential may be applied to a doped diamond anode electrode at least partially immersed in the solution. The cathode potential may exceed -40 volts.

[0016] In another embodiment, the composite electrode includes a mesh-like vitreous carbon foam and graphite particles supported within the mesh-like vitreous carbon foam.

[0017] This and other embodiments may include one or more of the following features: The average maximum size of the pores in the vitreous carbon foam may be 0.5 to 2 mm, and the graphite particles may have an average diameter in the range of 0.5 to 500 micrometers. The pores in the vitreous carbon foam can generally be 3 to 25 times larger, or 4 to 10 times larger, than the Sauter mean particle size of the graphite particles. The mass loading of the vitreous carbon foam is 1 cm 3 The amount of graphite particles may be 0.1 to 0.3 g per unit. The vitreous carbon foam may have a void volume exceeding 50%, for example, exceeding 75%.

[0018] The composite electrode may be included in an electrode assembly that includes a membrane that enables transport of an electrolyte of an organic solvent, and the supporting electrolyte salt is disposed between the graphite-supported porous skeletal material and the anode while applying a cathode potential to the cathode with respect to the anode via a separate conductor, and each of the conductors is connected to the respective output terminals of the cathode, the anode, and the DC power supply.

[0019] The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

Brief Description of the Drawings

[0020] [Figure 1] It is a schematic representation of an electrode assembly that can be used for the electrochemical exfoliation of graphite for the production of graphene flakes. [Figure 2] It is a schematic representation of an exemplary porous skeletal material that can be used to form a composite electrode. [Figure 3] It is a schematic representation of an electrode assembly that can be used for the electrochemical exfoliation of graphite for the production of graphene flakes. [Figure 4] It is a schematic representation of an electrode assembly that can be used for the electrochemical exfoliation of graphite for the production of graphene flakes. [Figure 5] It is a schematic representation of an electrode assembly that can be used for the electrochemical exfoliation of graphite for the production of graphene flakes. [Figure 6] It includes an image of graphene and a Raman spectrum. [Figure 7] It is a graph comparing current as a function of time when using two different electrode assemblies.

[0021] Like reference numerals in the various drawings indicate like elements.

Mode for Carrying Out the Invention

[0022] As described above, applying a high cathode potential to the graphite in an electrochemical cell allows for the peeling of a graphene sheet that can be processed to have material properties similar to those of idealized graphene.

[0023] However, these methods rely on continuous, close electrical contact between the cathode and the graphite. If intercalation and separation occur incidentally at intermediate positions within the graphite particles (i.e., away from the particle's surface), electrical contact with a portion of the particle can be lost. In fact, graphite particles can be simply broken down into two particles that retain the physical properties of their graphite without further delamination.

[0024] This problem can be addressed by mechanically compressing aggregates of graphite particles or thin disks of graphite to ensure that electrical contact is maintained and delamination can proceed over as large a proportion of the graphite as possible. Typically, compression restricts the graphite feed material to batch processing with a two-dimensional form factor when planar aggregates or disks of graphite are compressed, thereby limiting scalability.

[0025] Figure 1 is a schematic representation of an electrode assembly 100 that can be used for electrochemical exfoliation of graphite for the production of graphene flakes. Generally, the electrode assembly 100 acts as the cathode in an electrochemical cell, but it is also possible to implement it as the anode. Generally, the electrochemical cell can be as simple as a two-electrode cell, but three-electrode and four-electrode electrochemical cells with a reference electrode and a sensing electrode are also possible. Incidentally, the "electrode" in a two-electrode cell can include multiple examples of electrode assemblies 100. In other words, multiple examples of electrode assemblies 100 can be electrically coupled to each other and act as a single electrode in a two-electrode cell.

[0026] The electrode assembly 100 includes a lead connector 105, a membrane 110, and a composite electrode 115. The lead connector 105 is a portion configured to be electrically connected to an electrical lead wire. In the illustrated embodiment, the lead connector 105 is embedded and extends within the composite electrode 115 and serves as a robust, low-resistance path for electrons. Other embodiments are possible. For example, the lead connector 105 may be clamped to the outer surface of the composite electrode 115 or otherwise mechanically fixed.

[0027] The lead connector 105 is typically made of metal, but embodiments using other conductors (e.g., doped diamond, graphite, carbon fiber, wax-impregnated graphite) are also possible. In some embodiments, the lead connector 105 is made of copper.

[0028] The film 110 is a thin material that covers at least a portion of the surface of the composite electrode 115, allowing for the transport of solvent electrolytes and charged species across the film while reducing or preventing the transport of exfoliated graphene.

[0029] The membrane 110 is generally chemically compatible with the electrolyte used in the electrochemical exfoliation reaction and is wetted by the electrolyte. The membrane 110 is generally able to withstand the potential used in the electrochemical exfoliation reaction as well as the products and by-products generated during the reaction. For example, the potential relative to the opposite anode may be 0 to -100 volts. In some embodiments, the membrane 110 is formed from a material that can be used as a separator membrane in an electrochemical energy storage device. For example, in some embodiments, the membrane 110 may be a flexible polymer, carbon fiber, or glass fiber sheet. For example, the membrane 110 may be a quartz fiber sheet with a thickness of 200 to 700 μm (e.g., 475 μm) and an average pore size between 0.5 and 1 μm, for example, about 0.7 μm.

[0030] In the schematic representation shown in Figure 1, the flexibility of the film 110 is demonstrated by gathering the film 110 at the lead connection portion 105 above the composite electrode 115. This gathering is an example of flexibility and is not essential to the functional electrode assembly 100. For example, the composite electrode 115 only needs to be partially immersed in the stripping solution. As another example, the composite electrode 115 can be fully immersed in the stripping solution by bringing the top of the composite electrode 115 closer to the surface so that the flexible film 110 can wrap around the surface of the composite electrode 115 that is in contact with the stripping solution. As yet another example, the film 110 can be clamped over the stripping solution or otherwise mechanically supported.

[0031] The composite electrode 115 includes a porous framework material embedded with particulate graphite. The porous framework material includes at least several open-cell pores having a pore size that allows the relevant particulate graphite to be supported in the pores and allows the exfoliated graphene to be removed, for example, by washing. The pore size of the framework material and the particle size of the graphite can be varied (for example, depending on different particle feed materials and desired sizes of exfoliated graphene), but the pore size is generally 3 to 25 times larger, or 5 to 10 times larger, than the Sauter mean particle size of the particles of the particulate graphite feed material.

[0032] For example, to obtain relatively large graphene flakes, the porous framework material may have open pores with an average maximum dimension in the millimeter range or close to it (e.g., 0.5 to 2 mm), and the graphite particles may have an average diameter in the range of 0.5 to 500 micrometers.

[0033] Although the composite electrode 115 is schematically shown as having a roughly cylindrical shape, other shapes (e.g., rectangular or square cuboid, tubular, pyramidal) are also possible and may be preferred in the geometric shape of a particular electrochemical cell.

[0034] The porous framework material of the composite electrode 115 generally exhibits chemical compatibility with the electrolyte used in the electrochemical exfoliation reaction and is moistened by the electrolyte. The porous framework material of the composite electrode 115 is also generally inert and can withstand the potential used in the electrochemical exfoliation reaction as well as the products and by-products generated during the reaction. The porous framework material of the composite electrode 115 is generally robust enough to withstand reasonable handling after supporting particulate graphite. For example, the porous framework material of the composite electrode 115 can not only support the weight of the reactants and products during the electrochemical exfoliation reaction, but generally the porous framework material also allows, for example, the composite electrode 115 to be moved in and out of the reaction vessel and the exfoliated graphene to be removed without damage by washing.

[0035] Generally, porous skeletal materials are conductive. During delamination, the conductivity of the porous skeletal material provides multiple continuous electrical paths through the composite electrode 115 to improve yield.

[0036] In general, it is desirable to support a sufficient amount of particulate graphite in the porous framework material within the composite electrode 115 for a given degree of delamination. Therefore, a porous framework material with a high void volume is generally preferred. For example, a void volume exceeding 50% or exceeding 75% is preferred. In some embodiments, particulate graphite between 0.1 and 0.3 grams is supported at a density of 1 cm². 3 It can be filled into the porous skeletal material.

[0037] Figure 2 is a schematic representation of an exemplary porous skeletal material 200 that can be used to form a composite electrode 115. As shown, the porous skeletal material 200 forms a three-dimensional network of connected pores suitable for loading with particulate graphite and recovering exfoliated graphene. The relatively high void volume allows for the simultaneous loading and exfoliation of relatively large amounts of particulate graphite.

[0038] In some embodiments, compressive forces may be applied to the skeletal material 200 after loading and during delamination. However, this is not always the case. Rather, if the load is sufficiently high, the conductivity of the skeletal material 200 can ensure that even small graphite particles are properly biased and participate in the delamination reaction.

[0039] In some embodiments, the porous framework material is a vitreous carbon (RVC) foam. Vitreous carbon is amorphous carbon with physical properties suitable for use as the porous framework material 200 in the composite electrode 115. Table 1 below shows the typical physical properties of the vitreous carbon foam.

[0040] [Table 1]

[0041] Figure 3 is a schematic representation of an electrode assembly 300 that can be used for electrochemical exfoliation of graphite for the production of graphene flakes. Electrode assembly 300 shares many features with electrode assembly 100 (Figure 1). However, in contrast to electrode assembly 100, the film 110 is a self-supporting film that can maintain its shape. In the schematic representation shown in Figure 3, the self-support of the film 110 is indicated by the extension of the film 110 above the composite electrode 115. This is illustrative and not essential to the functional electrode assembly 300.

[0042] The shape and dimensions of the self-supporting membrane 110 and the composite electrode 115 can be adjusted to achieve a desired distance between the outer surface of the composite electrode 115 and the inner surface of the self-supporting membrane 110. For example, in the illustrated embodiment where the composite electrode 115 is substantially cylindrical, the inner surface of the self-supporting membrane 110 can define a cylindrical void dimensioned to enclose the composite electrode 115. In some embodiments, the shape and dimensions of the self-supporting membrane 110 can be selected to maintain a relatively short distance between the outer surface of the composite electrode 115 and the inner surface of the self-supporting membrane 110. For example, a distance of 0 to 5 mm can be maintained. For example, the outer surface of the composite electrode 115 and the inner surface of the self-supporting membrane 110 can be considered "in contact" if at least a portion of the surfaces are in contact and the maximum separation distance between the non-contacting portions is within a tolerance of 0.5 mm.

[0043] Figure 4 is a schematic representation of electrode assembly 400, which can be used for electrochemical exfoliation of graphite for the production of graphene flakes. Electrode assembly 400 shares many features with electrode assemblies 100 and 300 (Figures 1 and 3). However, in contrast to electrode assemblies 100 and 300, electrode assembly 400 includes a self-supporting porous material that encases the composite electrode 115 and acts as a membrane 110.

[0044] In some embodiments, the porous framework material 405 is a relatively thin mesh vitreous carbon foam. For example, in some embodiments, the thickness of the porous framework material 405 may be 20% or less, or 10% or less, of the maximum dimensions of the composite electrode 115. As another example, in some embodiments, the porosity of the mesh vitreous carbon foam in the porous framework material 405 can be adjusted to hinder or prevent the transport of exfoliated graphene.

[0045] The use of mesh vitreous carbon foam as a porous skeletal material 405 offers several advantages. For example, mesh vitreous carbon foam is chemically compatible with exfoliation electrolytes, thereby becoming wettable and able to withstand the exfoliation reaction. Furthermore, mesh vitreous carbon foam is generally mechanically robust and can be reused multiple times in the electrochemical exfoliation of graphite. A suitable example of RVC foam is available from ERG Aerospace Corporation (Oakland, California) under the name DUOCEL®.

[0046] Figure 5 is a schematic representation of an electrode assembly 500 that can be used for electrochemical exfoliation of graphite for the production of graphene flakes. Electrode assembly 500 shares many features with electrode assemblies 100, 300, and 400 (Figures 1, 3, and 4). However, electrode assembly 500 includes a stacked structure of alternating composite electrodes 115 and either / both membranes 110 and porous skeletal material 405. In the illustrated embodiment, the composite electrodes 115 and membranes 110 / porous skeletal material 405 are shown as a stack of uniformly sized rectangular parallelepiped sheets with no space between them. In other embodiments, the shape and / or size of the composite electrodes 115 and membranes 110 / porous skeletal material 405 can be varied. In some embodiments, the transport of exfoliating fluid into the electrode assembly 500 can be facilitated, for example, by separating or defining channels between or within the composite electrodes 115 and membranes 110 / porous skeletal material 405, and / or by active transport (pumping).

[0047] In some embodiments, the stripping solution may include salts composed of large ions such as tetrabutylammonium hexafluorophosphate, tetrabutylammonium hexafluoroborate, tetrabutylammonium bis(trifluoromethanesulfonyl)imide, or N-benzyl-N,N,N-trimethylammonium hexafluorophosphate, dissolved in an organic carbonate solvent such as propylene carbonate, ethylene carbonate, or dimethyl carbonate. The concentrations of these exemplary salt solutions may range from 0.01 M to 0.5 M.

[0048] Exemplary results Example 1: The electrochemical cell contained an independent boron-doped diamond anode (2.5 cm × 5 cm) and a cathode formed by an electrode assembly having a rectangular cuboid composite electrode and a flexible membrane. The composite electrode contained a vitreous carbon porous framework material supported with 250 mg of graphite flakes. Both the composite electrode and the boron-doped diamond anode were immersed in the electrolyte to a depth slightly below the connection points of their respective leads, and separated from each other by a distance of approximately 2 cm. The electrolyte contained 0.1 M tetrabutylammonium hexafluorophosphate [CH3CH2CH2CH2]4N(PF6), TBA-PF6 in propylene carbonate [C4H6O3, PC]. The electrolyte was degassed with argon before use, the entire electrochemical cell was sealed in a glass container, and purged with argon.

[0049] The cathode assembly and boron-doped diamond anode were connected to a 600W constant current / constant voltage DC power supply via insulated copper wire conductors, negatively polarizing the cathode assembly relative to the boron-doped diamond anode. Using the power supply's constant voltage mode, the voltage applied to the cathode assembly was increased to -60V relative to the boron-doped diamond anode at a rate of approximately 3V / min while monitoring the current. This target voltage was maintained across the cell for 24 hours.

[0050] The peeling reaction proceeded vigorously in the voltage lamp, and the current and temperature increased significantly. Clouds were formed in the argon purge reaction vessel of the cell. The clouds are a mixture of electrolyte volatile substances and hydrogen gas generated in the cathode assembly. The current across the cell was observed to reach its maximum value within about 30 minutes and then exponentially decayed until the reaction ended. At the same time, when the exfoliation product, hydrogenated graphene, was formed, the volume of the cathode assembly increased significantly.

[0051] After the reaction ended, the cathode assembly was removed from the electrochemical cell, and the product was recovered from the reticulated vitreous carbon porous skeleton and the separator membrane, washed several times with acetone using a vacuum filtration device, and dried in air. The graphene was thermally dehydrogenated at 600 °C for 1 hour under vacuum in a tubular furnace.

[0052] Figure 6 includes an image 605 of hydrogenated graphene after recovery from the reticulated vitreous carbon porous skeleton, an image 610 of graphene after dehydrogenation, a Raman spectrum 615 of hydrogenated graphene, and a Raman spectrum 620 of dehydrogenated graphene.

[0053] In the Raman spectra 615, 610, the phonon mode labeled "D" near 1360 cm -1 corresponds to a well-known disorder-induced mode that appears in graphite crystallites with short-range crystal order. The mode labeled "G" near 1580 cm -1 corresponds to an in-plane displacement phonon mode (E 2g2 ) that occurs within the graphene sheet in graphite crystallites or graphene alone. The mode labeled "2D" near 2730 cm -1 corresponds to the second harmonic of the disorder-induced D line in graphite crystallites (i.e., the overtone of the maximum value of the phonon density of states at 1360 cm -1 ). This second harmonic mode is sensitive to the cumulative irregularities of the stacked graphene sheets in graphite crystallites and becomes asymmetric when the number of irregular stacked sheets exceeds 5.

[0054] Graphene lacking long-range crystalline order, such as graphite, is characterized by Raman spectra showing small or absent D-lines and sharp G-lines with minimal extent; i.e., a small D / G intensity ratio. Furthermore, the 2D line is particularly symmetrical when the number of stacked graphene sheets is less than five, or absent when the synthetic product contains only graphene with a single atom thickness.

[0055] As shown in spectrum 615, hydrogenated graphene is sp in the exfoliated graphene sheet. 3 Significant D lines are observed due to disorder generated by carbon centers. In contrast, the 620 spectrum of dehydrogenated graphene shows almost no D lines, and the 2D lines are highly symmetrical. These spectral characteristics indicate that the final graphene product is very pure and consists of only a few stacked sheets on average.

[0056] Example 2: The electrochemical cell consisted of an electrode assembly comprising an independent boron-doped diamond anode (2.5 cm × 5 cm), a rectangular composite electrode, and a thimble-shaped self-supporting membrane structure, and included a cathode that was open at the top and closed at the bottom. The composite electrode contained a mesh-like vitreous carbon porous framework material supported with 500 mg of graphite. Both the composite electrode and the boron-doped diamond anode were immersed in the electrolyte to a depth slightly below the connection points of their respective insulated copper wire leads, and separated from each other by a distance of approximately 2 cm. The same electrolyte as in Example 1 was used. The electrochemical exfoliation reaction was also carried out under the same polarization conditions.

[0057] During the voltage increase up to -60V, the reaction proceeded more vigorously than in Example 1, accompanied by higher currents and greater temperature increases. In this case as well, cloud formation occurred within the argon-purged electrochemical cell.

[0058] Figure 7 is a graph 700 comparing the currents in Example 1 and Example 2 as a function of time. In graph 700, the position along the x-axis represents time in mA, and the position along the y-axis represents current in mA. Curve 705 shows the reaction current as a function of time during Example 1. Curve 710 shows the reaction current as a function of time during Example 2.

[0059] As shown, the peak current during Example 2 was lower than during Example 1, despite the large amount of graphite. However, the total charge transfer (i.e., the integral of the current-time response) in Example 2 was approximately twice that of Example 1, corresponding to a doubling of the graphite mass loading in the composite cathode. This suggests that the electrochemical exfoliation reaction is scalable.

[0060] Several embodiments have been described. Nevertheless, it will be understood that various modifications may be made. Therefore, other embodiments are within the scope of the following claims.

Claims

1. A method for producing graphene, wherein the method is The steps include supporting particulate graphite on an open-cell porous skeletal material, The steps include immersing at least a portion of the graphite-supported porous framework material in a solution, A method comprising the steps of applying a cathode potential to the graphite-supported porous skeletal material, wherein the cathode potential is sufficient to exfoliate the graphene.

2. The method according to claim 1, further comprising the step of washing off graphene from the porous skeletal material.

3. The method according to any one of claims 1 to 2, wherein the pores of the porous framework material are generally 3 to 25 times larger, or 5 to 10 times larger, than the Sauter mean particle size of the graphite particles.

4. The method according to any one of claims 1 to 3, wherein the average maximum size of the pores in the porous skeletal material is 0.5 to 2 mm, and the graphite particles have an average diameter in the range of 0.5 to 500 micrometers.

5. The method according to any one of claims 1 to 4, wherein the porous skeletal material has a void volume of more than 50%, for example, more than 75%.

6. The mass load of the particulate graphite in the open-cell porous framework material is 1 cm ３ The method according to any one of claims 1 to 5, wherein the amount of graphite particles is 0.1 to 0.3 g per unit.

7. The method according to any one of claims 1 to 6, wherein the porous skeletal material is a mesh-like vitreous carbon foam.

8. The method according to any one of claims 1 to 7, wherein the solution comprises an organic solvent and a supporting electrolyte salt.

9. The organic solvent is propylene carbonate, ethylene carbonate, or dimethyl carbonate. The method according to claim 8, wherein the electrolyte salt is tetrabutylammonium hexafluorophosphate, tetrabutylammonium hexafluoroborate, tetrabutylammonium bis(trifluoromethanesulfonyl)imide, or N-benzyl-N,N,N-trimethylammonium hexafluorophosphate.

10. The method according to claim 8 or 9, wherein, while the cathode potential is applied, a membrane is placed between the graphite-supported porous skeletal material and the anode that allows transport of the electrolyte salt but obstructs or prevents transport of exfoliated graphene.

11. The method according to claim 10, wherein the film is self-supporting.

12. The method according to claim 10 or 11, wherein the film has an average pore diameter between 0.5 and 1 μm.

13. The method according to claim 10, 11, or 12, wherein the membrane encloses the graphite-supported porous skeletal material in the solution.

14. The method according to any one of claims 1 to 13, wherein the cathode potential is applied to a doped diamond anode electrode that is at least partially immersed in the solution.

15. The method according to any one of claims 1 to 14, wherein the cathode potential exceeds -40 volts.

16. It is a composite electrode, A mesh-like vitreous carbon foam, A composite electrode comprising graphite particles supported within the aforementioned mesh-like vitreous carbon foam.

17. The composite electrode according to claim 16, wherein the average maximum size of the pores in the mesh-like vitreous carbon foam is 0.5 to 2 mm, and the graphite particles have an average diameter in the range of 0.5 to 500 micrometers.

18. The composite electrode according to claim 16 or 17, wherein the pores of the mesh-like vitreous carbon foam are generally 3 to 25 times or 4 to 10 times larger than the Sauter mean particle size of the graphite particles.

19. The mass load of the aforementioned mesh-like vitreous carbon foam is 1 cm ３ A composite electrode according to any one of claims 16 to 18, wherein the graphite particles are 0.1 to 0.3 g per unit.

20. The composite electrode according to any one of claims 16 to 19, wherein the mesh-like vitreous carbon foam has a void volume of more than 50%, for example, more than 75%.

21. The composite electrode according to any one of claims 16 to 20, comprising an electrode assembly including a membrane that enables transport of an electrolyte of an organic solvent, wherein a supporting electrolyte salt is placed between the graphite-supported porous framework material and the anode while applying the cathode potential to the cathode relative to the anode via a separate conductor, and each of the conductors is connected to the cathode and anode and the respective output terminals of a DC power supply.

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