Graphite electrochemical exfoliation for producing graphene sheets

Abstract

A method of producing graphene. The method includes loading particulate graphite in an open-cell porous framework material, immersing at least a portion of the graphite-loaded porous framework material in a solution, and applying a cathodic potential to the graphite-loaded porous framework material, where the cathodic potential is sufficient to exfoliate graphite.

Description

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

[0002] Statement regarding federally funded research or development This invention was made with government support under grant number #2970576300-01 from the National Institute of Standards and Technology (NIST), entitled “Scale-up and commercialization of continuous manufacturing of graphene for advanced respirator and biosensor applications.” The government holds certain rights to this invention. Technical Field

[0003] This invention relates to electrochemical exfoliation of graphite for the production of graphene sheets. Background Technology

[0004] Idealized graphene is an infinitely large, defect-free two-dimensional material with a single-atom thickness and composed of sp... 2 Graphene is a crystal sequence composed of hybrid carbon atoms arranged in a hexagonal lattice. In reality, graphene has a finite size, is not defect-free, and typically contains multiple stacked layers. However, the physical properties of real-world graphene can approach those of idealized graphene.

[0005] Graphite is a three-dimensional structure typically comprising thousands of graphene sheets stacked in an ordered order and held together by weak van der Waals interactions. In some cases, the area of ​​each graphene sheet in a given graphite particle is equivalent to the area defined by two dimensions of the particle, and the third dimension of the particle reflects the number of stacked graphene sheets. In other words, the entire graphite particle can be a single crystal, but this is not always necessary.

[0006] Graphene can be produced using a variety of different techniques, including electrochemical exfoliation. For example, established methods for producing graphene that do not involve electrochemical exfoliation include epitaxial growth on catalytic substrates at high temperatures, chemical vapor deposition (CVD), micromechanical exfoliation, and direct ultrasonication. These methods have significant disadvantages compared to the potential properties of electrochemical exfoliation, including low yields and practical barriers to large-scale production.

[0007] For electrochemical exfoliation, some methods apply oxidizing chemical solutions or anodic potentials to graphite. These methods produce graphene in at least a partially oxidized form, known as graphene oxide (GO). While graphene oxide can be produced in relatively large quantities and is commercially available, it is subject to limitations due to oxygen impurities and sp... 2 and sp 3Hybridized carbons exhibit physical properties that differ from those of idealized graphene. Oxygen can be removed from graphene oxide through chemical reduction or thermal treatment to form graphene with improved physical properties. However, oxygen removal requires additional processing steps, leading to increased cost and energy consumption. Furthermore, oxygen removal is more difficult than hydrogen removal from hydrogenated graphene.

[0008] Other electrochemical exfoliation methods apply a high cathode potential (e.g., -60 V) to graphite in an electrochemical cell. This cell typically contains a liquid electrolyte solution 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. A large ionic radius salt soluble in the electrolyte solvent is inserted between stacked graphite sheets, and the applied potential drives the cations of the electrolyte salt and those generated by the solvent at the anode to insert into the interlayer spaces of the graphite. Gaseous material forms in the interlayer spaces and further expands and exfoliates the graphene sheets. Typically, the exfoliated graphene is hydrogenated, manifested by hydrogen atoms binding 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 the ideal physical properties of graphene.

[0009] Examples of such methods can be found in WO 2021 / 048089 and the publication entitled “High Voltage Electrochemical Exfoliation of Graphite for High-Yield Graphene Production” (RSC Adv. 2019, 9(50), p. 29305–29311, doi.org / 10.1039 / C9RA04795F), the contents of which are incorporated herein by reference. Summary of the Invention

[0010] A system and technology for the electrochemical exfoliation of graphite for the production of graphene sheets are described.

[0011] In one aspect, a method is used to produce graphene. The method includes: loading particulate graphite into an open-cell porous framework material; immersing at least a portion of the graphite-loaded porous framework material in a solution; and applying a cathode potential to the graphite-loaded porous framework material, wherein the cathode potential is sufficient to exfoliate graphene.

[0012] This and other aspects may include one or more of the following features. The method may further include washing and exfoliating graphene from the porous framework material. The pores in the porous framework material can typically be 3 to 25 times larger, or 5 to 10 times larger, than the Sauter mean diameter of the graphite particles. The average maximum size of the pores in the porous framework material can be 0.5-2 mm, and the average diameter of the graphite particles ranges from 0.5 to 500 micrometers. The pore volume of the porous framework material can exceed 50%, for example, exceeding 75%. The mass loading of the particulate graphite in the open-cell porous framework material can be 0.1 to 0.3 g graphite particles per cm³. 3 Porous framework material. The porous framework material may be a mesh-like glassy carbon foam. The solution may contain an organic solvent and a supporting electrolyte salt.

[0013] The organic solvent may be propylene carbonate, ethylene carbonate, or dimethyl carbonate, and the electrolyte salt may be tetrabutylammonium hexafluorophosphate, tetrabutylammonium hexafluoroborate, bis(trifluoromethanesulfonyl)imide tetrabutylammonium, or N-benzyl-N,N,N-trimethylammonium hexafluorophosphate. During the application of the cathode potential, a membrane that allows the transport of the electrolyte salt but impedes or prevents the transport of exfoliated graphene may be disposed between the porous framework material supported on graphite and the anode. The membrane may be self-supporting. The average pore size of the membrane may be 0.5 to 1 µm. The membrane may encapsulate the porous framework material supported on graphite in the solution. The cathode potential may be applied relative to a doped diamond anode electrode that is at least partially immersed in the solution. The cathode potential may exceed -40 volts.

[0014] In another aspect, the composite electrode comprises a reticulated glassy carbon foam and graphite particles loaded within the reticulated glassy carbon foam.

[0015] This aspect and other aspects may include one or more of the following features: The average maximum size of the pores in the reticulated glassy carbon foam may be 0.5-2 mm, and the average diameter of the graphite particles ranges from 0.5 to 500 micrometers. The pores in the reticulated glassy carbon foam can typically be 3 to 25 times larger, or 4 to 10 times larger, than the Sotter average diameter of the graphite particles. The mass loading of the particulate graphite in the reticulated glassy carbon foam can be 0.1 to 0.3 g graphite particles per cm. 3 A reticulated glassy carbon foam. The pore volume of the reticulated glassy carbon foam can exceed 50%, for example, exceed 75%.

[0016] The composite electrode may be included in an electrode assembly comprising a membrane disposed between a graphite-loaded porous framework material and an anode, which allows the transport of electrolytes and supporting electrolyte salts in an organic solvent when a cathode potential is applied relative to the anode at the cathode via different conductors, each of the conductors being connected to the cathode and the anode and to a respective output terminal of a DC power supply.

[0017] Details of one or more embodiments are set forth in the following drawings and description. Other features, objects, and advantages of the invention will be apparent from the description, drawings, and claims. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of an electrode assembly that can be used in the electrochemical exfoliation of graphite for the production of graphene sheets.

[0019] Figure 2 This is a schematic diagram of an exemplary porous framework material that can be used to form composite electrodes.

[0020] Figure 3 This is a schematic diagram of an electrode assembly that can be used in the electrochemical exfoliation of graphite for the production of graphene sheets.

[0021] Figure 4 This is a schematic diagram of an electrode assembly that can be used in the electrochemical exfoliation of graphite for the production of graphene sheets.

[0022] Figure 5 This is a schematic diagram of an electrode assembly that can be used in the electrochemical exfoliation of graphite for the production of graphene sheets.

[0023] Figure 6 Includes images and Raman spectra of graphene.

[0024] Figure 7 To compare the current versus time curves when using two different electrode assemblies.

[0025] The same figure number in various figures indicates the same element. Detailed Implementation

[0026] As described above, applying a high cathode potential to graphite in an electrochemical cell can exfoliate graphene sheets, which can then be processed to have material properties similar to idealized graphene.

[0027] However, these methods rely on a sustained, tight electrical contact between the cathode and the graphite. If insertion and separation occur at the midpoint of a graphite particle (i.e., away from the particle's surface), electrical contact with a portion of the particle can be lost. In practice, a graphite particle can simply split into two particles, which will retain their graphitic physical properties without further peeling.

[0028] This problem can be addressed by mechanically compressing aggregates of graphite particles or graphite flakes to ensure electrical contact is maintained and that exfoliation can be performed on the largest possible portion of the graphite. Typically, because the compressed aggregates or graphite flakes are planar, the compression confines the graphite feedstock to batch processing with a two-dimensional shape factor, thereby limiting scalability.

[0029] Figure 1 This is a schematic diagram of an electrode assembly 100 that can be used in the electrochemical exfoliation of graphite for the production of graphene sheets. Typically, the electrode assembly 100 will act as the cathode in the electrochemical cell, but implementations in which the electrode assembly 100 acts as the anode are also feasible. Typically, the electrochemical cell only needs to be a two-electrode cell, but three-electrode and four-electrode electrochemical cells with a reference electrode and a sensing electrode are feasible. Incidentally, the “electrode” in a two-electrode cell may include multiple instances of the electrode assembly 100. In other words, multiple instances of the electrode assembly 100 may be electrically connected together and act as a single electrode in the two-electrode cell.

[0030] Electrode assembly 100 includes a wire connection 105, a membrane 110, and a composite electrode 115. The wire connection 105 is a site configured for electrical connection to an electrical wire. The illustrated embodiment of the wire connection 105 is embedded within and extends into the composite electrode 115, serving as a robust, low-resistance path for electrons. Other embodiments are possible. For example, the wire connection 105 may be clamped or otherwise mechanically fixed to the outer surface of the composite electrode 115.

[0031] The wire connection 105 is typically metallic, but embodiments with other conductors (such as doped diamond, graphite, carbon fiber, or wax-impregnated graphite) are possible. In some embodiments, the wire connection 105 is made of copper.

[0032] The membrane 110 is a thin material that encapsulates at least a portion of the surface of the composite electrode 115 and allows solvent electrolytes and charged substances to pass through the membrane while reducing or preventing the transport of exfoliated graphene.

[0033] Membrane 110 is typically chemically compatible with and wetted by the electrolyte used in the electrochemical stripping reaction. Membrane 110 is typically able to withstand the potential used in the electrochemical stripping reaction, as well as the products and byproducts generated during the reaction. For example, the potential relative to the opposite anode can be 0 to 100 volts. In some embodiments, membrane 110 is formed of a material suitable for use as a diaphragm in an electrochemical energy storage device. For example, in some embodiments, membrane 110 can be a flexible polymer, carbon fiber, or glass fiber sheet. For example, membrane 110 can be a quartz fiber sheet 200-700 µm thick (e.g., 475 µm thick) with an average pore size of 0.5 to 1 µm, such as about 0.7 µm.

[0034] exist Figure 1 The schematic diagram illustrates the flexibility of the membrane 110 by showing it clustered at the wire connection 105 above the composite electrode 115. This clustering is illustrative of flexibility and is not essential for the functional electrode assembly 100. For example, the composite electrode 115 may only need to be partially immersed in the stripping solution. Alternatively, the composite electrode 115 may be fully immersed in the stripping solution with its top near the surface, allowing even the flexible membrane 110 to enclose the composite electrode 115 in contact with the stripping solution. Furthermore, the membrane 110 may be clamped or otherwise mechanically supported above the stripping solution.

[0035] The composite electrode 115 includes a porous framework material in which particulate graphite is embedded. The porous framework material includes at least a number of open pores with pore sizes that allow the associated particulate graphite to be loaded into the pores and allow the removal of exfoliated graphene (e.g., by washing). Although the pore size of the framework material and the particle size of the graphite can vary (e.g., depending on the different particulate feedstock and the expected size of the exfoliated graphene), the pore size will typically be 3 to 25 times larger, or 5 to 10 times larger, than the souter average diameter of the particles in the particulate graphite feedstock. For example, to obtain relatively large graphene sheets, the porous framework material may have open pores with an average maximum size in or close to the millimeter range (e.g., 0.5–2 mm), and the average diameter of the graphite particles may range from 0.5 to 500 micrometers.

[0036] Although the composite electrode 115 is schematically shown in a generally cylindrical shape, other shapes (such as cuboids, cubes, tubes, and cones) are also feasible and may be preferred in certain electrochemical cell geometries.

[0037] The porous framework material of composite electrode 115 typically exhibits chemical compatibility with and is wetted by the electrolyte used in the electrochemical exfoliation reaction. The porous framework material of composite electrode 115 is also typically inert and capable of withstanding the potentials used in the electrochemical exfoliation reaction, as well as the products and byproducts generated during the reaction. The porous framework material of composite electrode 115 is typically robust enough to withstand reasonable handling after loading particulate graphite. For example, the porous framework material of composite electrode 115 will not only be able to support the weight of reactants and products during the electrochemical exfoliation reaction, but the porous framework material will also typically allow, for example, composite electrode 115 to be transferred into and out of the reaction vessel and washed to remove the exfoliated graphene without breakage.

[0038] Typically, the porous framework material is conductive. During peeling, the conductivity of the porous framework material provides multiple uninterrupted electrical paths through the composite electrode 115 to improve yield.

[0039] Typically, for a given exfoliation, it is desirable to load a reasonably large amount of particulate graphite into the porous framework material of the composite electrode 115. Therefore, porous framework materials with high pore volume are generally preferred. For example, a pore volume exceeding 50% or 75% is preferred. In some embodiments, 0.1 to 0.3 grams of particulate graphite can be loaded up to 1 cm³. 3 In porous framework materials.

[0040] Figure 2 This is a schematic diagram of an exemplary porous framework material 200 that can be used to form the composite electrode 115. As shown, the porous framework material 200 forms a three-dimensional network of interconnected pores, which is suitable for loading particulate graphite and recovering exfoliated graphene. The relatively high pore volume allows for the simultaneous loading and exfoliation of a relatively large amount of particulate graphite.

[0041] In some implementations, compressive forces may be applied to the framework material 200 after loading and during peeling. However, this is not always necessary. Instead, if the loading is sufficiently high, the conductivity of the framework material 200 ensures that even small graphite particles can be properly biased and participate in the peeling reaction.

[0042] In some embodiments, the porous framework material is a mesh glassy carbon (RVC) foam. Glassy carbon is a non-graphitized carbon with physical properties suitable for use as a porous framework material 200 in the composite electrode 115. Table 1 below lists the typical physical properties of mesh glassy carbon foam. Table 1 Figure 3This is a schematic diagram of an electrode assembly 300 that can be used in the electrochemical exfoliation of graphite for the production of graphene sheets. The electrode assembly 300 has multiple electrodes connected to the electrode assembly 100 (…). Figure 1 They share common characteristics. However, unlike electrode assembly 100, membrane 110 is a self-supporting membrane and is able to maintain its shape. Figure 3 The schematic diagram illustrates the self-supporting nature of the membrane 110 by its extension over the composite electrode 115. This is illustrative and not essential for the functional electrode assembly 300.

[0043] The shape and size of the self-supporting membrane 110 and the composite electrode 115 can be customized to achieve the desired spacing between the outer surface of the composite electrode 115 and the inner surface of the self-supporting membrane 110. For example, in the embodiment shown where the composite electrode 115 is generally cylindrical, the inner surface of the self-supporting membrane 110 can define cylindrical pores, the dimensions of which are designed to enclose the composite electrode 115. In some embodiments, the shape and size of the self-supporting membrane 110 can be selected to maintain a relatively small 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 two surfaces can be considered to be "in contact" when at least some parts of the outer surface of the composite electrode 115 and the inner surface of the self-supporting membrane 110 are in contact and the maximum spacing between the uncontacted parts is within a tolerance of 0.5 mm.

[0044] Figure 4 This is a schematic diagram of an electrode assembly 400 that can be used in the electrochemical exfoliation of graphite for the production of graphene sheets. The electrode assembly 400 has multiple electrodes connected to electrode assemblies 100, 300 (…). Figure 1 , 3 They share common features. However, unlike electrode assemblies 100 and 300, electrode assembly 400 includes a self-supporting porous material that encapsulates composite electrode 115 and acts as membrane 110.

[0045] In some embodiments, the porous framework material 405 is a relatively thin mesh glassy carbon foam. For example, in some embodiments, the thickness of the porous framework material 405 may be less than or equal to 20% or less than or equal to 10% of the maximum size of the composite electrode 115. Also, in some embodiments, the porosity of the mesh glassy carbon foam in the porous framework material 405 may be tailored to impede or prevent the transport of exfoliated graphene.

[0046] Using reticulated glassy carbon foam (RVC) as the porous framework material 405 offers several advantages. For example, RVC is chemically compatible with and wetted by the stripping electrolyte and can withstand the stripping reaction. Simultaneously, RVC is generally mechanically robust and can be reused multiple times in graphite electrochemical stripping. An example of a suitable RVC foam is the material marketed as DUOCEL® by ERG Aerospace Corporation (Oakland, CA).

[0047] Figure 5 This is a schematic diagram of an electrode assembly 500 that can be used in the electrochemical exfoliation of graphite for the production of graphene sheets. The electrode assembly 500 has multiple electrodes connected to electrode assemblies 100, 300, and 400 (…). Figure 1 , 3 4) Common features. However, the electrode assembly 500 includes an alternating stacked structure of one or both of the composite electrode 115 and the membrane 110 and the porous framework material 405. In the illustrated embodiment, the composite electrode 115 and the membrane 110 / porous framework material 405 are shown as a stacked structure of uniformly sized cuboid sheets without gaps between them. In other embodiments, the shape and / or size of the composite electrode 115 and the membrane 110 / porous framework material 405 may vary. In some embodiments, the stripping solution can be facilitated to be transported into the interior of the electrode assembly 500 by, for example, by separating or defining channels between or within the composite electrode 115 and the membrane 110 / porous framework material 405 and / or by active transport (pumping).

[0048] In some embodiments, the stripping solution may comprise a salt composed of large ions dissolved in an organic carbonate solvent, such as tetrabutylammonium hexafluorophosphate, tetrabutylammonium hexafluoroborate, bis(trifluoromethanesulfonyl)imide tetrabutylammonium, or N-benzyl-N,N,N-trimethylammonium hexafluorophosphate, wherein the organic carbonate solvent is such as propylene carbonate, ethylene carbonate, or dimethyl carbonate. The concentration of these exemplary salt solutions may vary between 0.01 and 0.5 M.

[0049] Example Results Example 1:The electrochemical cell comprises a freestanding boron-doped diamond anode (2.5 cm × 5 cm) and a cathode formed by an electrode assembly having a cuboid composite electrode and a flexible membrane. The composite electrode comprises a mesh-like glassy carbon porous framework material loaded with 250 mg of graphite sheets. Both the composite electrode and the boron-doped diamond anode are immersed in the electrolyte solution to a depth slightly below their respective wire connections and are separated from each other by a distance of approximately 2 cm. The electrolyte solution comprises a 0.1 M solution of tetrabutylammonium hexafluorophosphate [CH3CH2CH2CH2)4N(PF6), TBA-PF6] in propylene carbonate [C4H6O3, PC]. The electrolyte solution is degassed with argon before use, and the entire electrochemical cell is sealed in a glass container and purged with argon.

[0050] The cathode assembly and boron-doped diamond anode were connected to a 600 W constant current / constant voltage DC power supply via insulated copper wire conductors, with the cathode assembly negatively polarized relative to the boron-doped diamond anode. Using the constant voltage mode of the power supply, the voltage applied to the cathode assembly was increased relative to the boron-doped diamond anode to -60 V at a rate of approximately 3 V / min while monitoring the current. This target voltage was maintained throughout the electrochemical cell for 24 hours.

[0051] During voltage ramp-up, the stripping reaction proceeded rapidly with a significant increase in current and temperature. A cloud formed within the argon-purged reaction vessel of the electrochemical cell. This cloud was a mixture of electrolyte volatiles and hydrogen generated at the cathode assembly. The current throughout the electrochemical cell was observed to reach its maximum within approximately 30 minutes, followed by an exponential decay until the reaction terminated. Simultaneously, the volume of the cathode assembly increased significantly with the formation of the stripping product, hydride graphene.

[0052] After the reaction was terminated, the cathode assembly was removed from the electrochemical cell and the product was recovered from the reticulated glassy carbon porous framework and membrane. The product was washed several times with acetone using a vacuum filter and dried in air. The graphene was then thermally dehydrogenated in a tube furnace under vacuum at 600°C for 1 hour.

[0053] Figure 6 Images include 605 of hydrogenated graphene recovered from a porous glassy carbon framework, 610 of dehydrogenated graphene, 615 of the Raman spectrum of hydrogenated graphene, and 620 of the Raman spectrum of dehydrogenated graphene.

[0054] In Raman spectra 615 and 620, it is located at 1360 cm⁻¹. -1 The nearby phonon mode labeled "D" corresponds to a well-known disorder-induced mode found in graphite microcrystals with short-range crystal order. Located at 1580 cm⁻¹. -1The nearby modes labeled "G" correspond to in-plane displacement phonon modes (E) occurring within the graphene sheet in graphene microcrystals or in individual graphene particles. 2g2 Located at 2730 cm. -1 The nearby pattern labeled "2D" corresponds to the second-order mode of the disorder-induced D peak in graphite microcrystals (i.e., at 1360 cm⁻¹). -1 (overtones at the maximum phonon density of states). This second-order mode is sensitive to the accumulated disorder in stacked graphene sheets of graphite microcrystals, and its shape exhibits asymmetry when the number of disordered stacks is greater than five.

[0055] In the absence of graphite exhibiting a long-range crystal order, the Raman spectral characteristics of graphene are: a weak or absent D peak, and a sharp and minimally broadened G peak; that is, the D / G intensity ratio will be small. Furthermore, if the number of stacked graphene sheets is less than five, the 2D peak will be highly symmetrical; or if the synthesized product contains only a single-atom-thickness graphene, the 2D peak will be absent.

[0056] As shown in spectrum 615, due to the sp... 3 The disorder generated by the carbon centers results in a significant D peak in hydrogenated graphene. Conversely, in the spectrum of dehydrogenated graphene (620), the D peak is almost absent and the 2D peak is highly symmetrical. These spectroscopic features indicate that the final graphene product is very pure and, on average, consists of only a few stacked sheets.

[0057] Example 2: The electrochemical cell comprises a freestanding boron-doped diamond anode (2.5 cm × 5 cm) and a cathode formed by an electrode assembly having a cubic composite electrode and a self-supporting membrane structure shaped like a thimble, open at the top and closed at the bottom. The composite electrode comprises a mesh-like glassy carbon porous framework material loaded with 500 mg of graphite. Both the composite electrode and the boron-doped diamond anode are immersed in an electrolyte solution to a depth slightly below their respective insulated copper wire connections and are separated from each other by a distance of approximately 2 cm. The same electrolyte solution as in Example 1 is used. The electrochemical stripping reaction is also carried out under the same polarization conditions.

[0058] During the voltage increase to -60 V, the reaction proceeded more vigorously than in Example 1, with higher current and greater temperature rise. A cloud was regenerated within the argon-purged electrochemical cell.

[0059] Figure 7A graph 700 shows the current versus time curves for comparison in Examples 1 and 2. In graph 700, the position along the x-axis represents time in hours, while the position along the y-axis represents current in mA. Curve 705 shows the change of reaction current over time during Example 1. Curve 710 shows the change of reaction current over time during Example 2.

[0060] As shown in the figure, despite the presence of a large amount of graphite, the peak current during Example 2 was lower than that during Example 1. However, the total charge transfer in Example 2 (i.e., the integral of the current-time response) was approximately twice that in Example 1, which is commensurate with the twice-higher mass loading of graphite in the composite cathode. This is considered to indicate that the electrochemical stripping reaction is scalable.

[0061] Several embodiments have been described. However, it should be understood that various modifications can be made. Therefore, other embodiments are within the scope of the following claims.

Claims

1. A method of producing graphene, the method comprising: loading particulate graphite into an open-cell porous skeletal material; immersing at least part of the loaded graphite porous skeletal material in a solution; and applying a cathodic potential to the loaded graphite porous skeletal material, wherein the cathodic potential is sufficient to exfoliate graphene.

2. The method of claim 1, further comprising washing exfoliated graphene from the porous skeletal material.

3. The method of any preceding claim, wherein the pores in the porous skeletal material are typically 3 to 25 times larger, or 5 to 10 times larger, than the Sauter mean diameter of the graphite particles.

4. The method of any preceding claim, wherein the average maximum dimension of the pores in the porous skeletal material is 0.5-2 mm, and the average diameter of the graphite particles ranges from 0.5 to 500 microns.

5. The method of any preceding claim, wherein the porous skeletal material has a porosity volume of more than 50%, for example more than 75%.

7. The method of any preceding claim, wherein the porous skeletal material is reticulated vitreous carbon foam.

6. The method of any one of the preceding claims, wherein the mass loading of the particulate graphite in the open-cell porous scaffold material is 0.1 to 0.3 g of graphite particles per cm3 of the open-cell porous scaffold material. 3 porous scaffold material.

8. The method of any preceding claim, wherein the solution comprises an organic solvent and a supporting electrolyte salt.

9. The method of claim 8, wherein: the organic solvent is propylene carbonate, vinyl carbonate or dimethyl carbonate; and the electrolyte salt is tetrabutylammonium hexafluorophosphate, tetrabutylammonium hexafluoroborate, tetrabutylammonium bis(trifluoromethylsulfonyl)imide or N-benzyl-N,N,N-trimethylammonium hexafluorophosphate.

10. The method of claim 8 or 9, wherein, during the application of the cathodic potential, a membrane that allows transport of the electrolyte salt but hinders or prevents transport of exfoliated graphene is disposed between the loaded graphite porous skeletal material and an anode.

11. The method of claim 10, wherein the membrane is self-supporting.

12. The method of claim 10 or 11, wherein the average pore size of the membrane is 0.5 to 1 pm.

13. The method of claim 10, 11 or 12, wherein the membrane encases the loaded graphite porous skeletal material in the solution.

14. The method of any preceding claim, wherein the cathodic potential is applied relative to a doped diamond anode electrode that is at least partially immersed in the solution.

15. The method of any preceding claim, wherein the cathodic potential is more negative than -40 volts.

16. A composite electrode comprising: a reticulated vitreous carbon foam; and graphite particles loaded within the reticulated vitreous carbon foam.

17. The composite electrode of claim 16, wherein the average maximum dimension of the pores in the reticulated vitreous carbon foam is 0.5-2 mm, and the average diameter of the graphite particles ranges from 0.5 to 500 microns.

18. The composite electrode of claim 16 or 17, wherein the pores in the reticulated vitreous carbon foam are typically 3 to 25 times larger, or 4 to 10 times larger, than the Sauter mean diameter of the graphite particles. ​ ​ 19. The composite electrode of any one of claims 16 to 18, wherein the mass loading of the particulate graphite in the reticulated vitreous carbon foam is 0.1 to 0.3 g of graphite particles per cm 3 reticulated vitreous carbon foam.

20. The composite electrode of any one of claims 16 to 19, wherein the reticulated glassy carbon foam has a pore volume in excess of 50%, for example in excess of 75%.

21. The composite electrode of any one of claims 16 to 20, wherein the composite electrode is comprised in an electrode assembly comprising a membrane disposed between a porous skeletal material loaded with graphite and an anode, the membrane allowing transport of electrolyte and supporting electrolyte salt in an organic solvent upon application of a cathodic potential at the cathode relative to the anode by different conductors, each of the conductors being connected to the cathode and anode and to a respective corresponding output terminal of a DC power source.

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