Graphene production by electrochemical exfoliation

Abstract

A method for the continuous production of graphene. The method includes the steps of: flowing a suspension of graphite particles through a conductive open-cell porous material placed in a reaction vessel; applying a cathode potential to the conductive open-cell porous material, wherein the cathode potential is sufficient to exfoliate the graphene; and draining a suspension of exfoliated graphene from the conductive open-cell porous material.

Description

Technical Field

[0001] [Cross - Reference to Related Applications] This application claims priority to U.S. Provisional Application No. 63 / 461,977, filed on April 26, 2023, the entire content 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 “Scale - up of Continuous Manufacturing and Productization of Graphene for Advanced Respirator and Biosensor Applications”, #2970576300 - 01, awarded by the National Institute of Standards and Technology (NIST). The government has certain rights in this invention.

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

Background Art

[0004] Ideal graphene is a two - dimensional material composed of sp 2 hybridized carbon atoms arranged in a hexagonal lattice, having 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 Initiative]

[0012] A system for producing graphene flakes by electrochemical exfoliation and techniques for their continuous production are described.

[0013] In one embodiment, a continuous method for producing graphene includes the steps of: flowing a suspension of graphite particles through a conductive open-cell porous material placed in a reaction vessel; applying a cathode potential to the conductive open-cell porous material, wherein the cathode potential is sufficient to exfoliate the graphene; and draining a suspension of exfoliated graphene from the conductive open-cell porous material.

[0014] This and other embodiments may include one or more of the following features: The membrane may be fitted tightly to the outer surface of the conductive open-cell porous material to prevent the establishment of channels for graphite particles that do not pass through the conductive open-cell porous material. The membrane may allow transport of electrolyte salts but inhibit or prevent transport of exfoliated graphene and may be positioned between the conductive open-cell porous material and the anode during the application of a cathode potential. The membrane may have an average pore size between 0.5 and 1 μm. The anode, including multiple members, may be in electrical contact with the fluid of the reaction vessel. The members may be spatially distributed around the conductive open-cell porous material. The residence time of the material in the conductive open-cell porous material may be 1 to 600 minutes. 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 porous framework material may have a void volume greater than 50%, for example, greater than 75%. The porous framework material may be a vitreous carbon foam. The cathode potential can be applied to a doped diamond anode electrode. The cathode potential may exceed -40 volts. The average maximum pore size of the conductive open-cell porous material 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 graphite particles can be suspended in propylene carbonate. The method may further include flowing a stripping solution containing an organic solvent and a supporting electrolyte salt into the reaction vessel. The organic solvent may be propylene carbonate, ethylene carbonate, or dimethyl carbonate. The electrolyte salt may be tetrabutylammonium hexafluorophosphate, tetrabutylammonium hexafluoroborate, tetrabutylammonium bis(trifluoromethanesulfonyl)imide, or N-benzyl-N,N,N-trimethylammonium hexafluorophosphate. The method may further include draining the reaction byproducts from the reaction vessel through an outlet different from the outlet through which the suspension of stripped graphene drains from the reaction vessel.The method may further include the steps of draining the exfoliated graphene from the reaction vessel, flowing the liquid carrying the exfoliated graphene from the reaction vessel into a second conductive open-cell porous material, and applying a cathode potential to the second conductive open-cell porous material, wherein the cathode potential is sufficient to exfoliate the graphene. The method may further include separating the exfoliated graphene from the liquid carrying the exfoliated graphene from the reaction vessel before flowing the liquid into the second conductive open-cell porous material.

[0015] In another embodiment, the electrochemical reactor is for the production of graphene. The reactor includes an anode, a mesh vitreous carbon foam cathode, and a membrane positioned between the mesh vitreous carbon foam cathode and the anode. The membrane is configured to prevent the transport of graphite particles but to allow the transport of electrolytes and organic solvents of a solution for the exfoliation of graphite.

[0016] This and other embodiments may include one or more of the following features: The membrane may be fitted so closely to the outer surface of the cathode to prevent the establishment of a channel for graphite particles that do not pass through the cathode. The membrane may be in contact with the outer surface of the cathode. The electrochemical reactor may further include a graphite suspension inlet that is fluid-coupled to supply suspended graphite particles to the cathode. The electrochemical reactor may further include an active control device configured to control the parameters of the electrochemical exfoliation of graphene.

[0017] The average maximum pore size of the mesh vitreous carbon foam may be 0.5 to 2 mm. The electrochemical reactor may further include a suspension of graphite particles and a pump configured to pump the suspension of graphite particles through the cathode of the mesh vitreous carbon foam. The residence time of the suspension pumped through the mesh vitreous carbon foam may be 1 to 600 minutes.

[0018] Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

Brief Description of the Drawings

[0019] [Figure 1] It is a schematic representation of an electrochemical reactor that can be used for the continuous production of graphene flakes by electrochemical exfoliation. [Figure 2] It is a schematic cross-sectional view along section 2-2 of the electrochemical reactor of FIG. 1. [Figure 3] It is a schematic partial broken view of a part of the reaction vessel of the electrochemical reactor of FIG. 1. [Figure 4] It is a schematic representation of an exemplary porous skeletal material that can be used to form a cathode in an electrochemical reactor. [Figure 5] It is a schematic representation of an electrochemical reactor system that can be used for the continuous production of graphene flakes by electrochemical exfoliation. [Figure 6] It is a schematic representation of a side view of an electrochemical reactor system that can be used for the continuous production of graphene flakes.

[0020] Like reference numerals in the various drawings indicate like elements.

Modes for Carrying Out the Invention

[0021] As described above, applying a high cathode potential to the graphite of an electrochemical cell can exfoliate graphene sheets that can be processed to have material properties similar to those of idealized graphene.

[0022] 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., separation from the particle surface), electrical contact with a portion of the particle may be lost. In fact, graphite particles can be simply broken down into particles that retain the physical properties of their graphite without further delamination.

[0023] 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 and complicating continuous manufacturing.

[0024] Figure 1 is a schematic representation of an electrochemical reactor 100 that can be used for the continuous production of graphene flakes by electrochemical exfoliation. In the illustrated embodiment, the reactor 100 includes a reaction vessel 105, a graphite particle suspension inlet 110, a exfoliation solution inlet 115, a graphene suspension outlet 120, an exfoliation solution outlet 125, and anode and cathode lead connectors 130, 135. During operation, the exfoliation solution and the suspension of graphite particles can be supplied to the reaction vessel 105 of the reactor 100 via the inlets 110, 115. A bias applied to the anode and cathode in the reaction vessel 105 via the anode and cathode lead connectors 130, 135 drives the exfoliation of graphene. The exfoliated graphene, used exfoliation solution, and other by-products exit the reaction vessel 105 through the outlets 120, 125.

[0025] Figure 2 is a schematic cross-sectional view of the electrochemical reactor 100 along section 2-2 of Figure 1. Figure 3 is a schematic partial breakaway view of a portion of the reaction vessel 105. Referring here to Figures 1-3, the reaction vessel 105 includes a vessel wall 205 defining an electrochemical cell for the exfoliation reaction. The electrochemical cell includes a cathode 210 and an anode 215 separated by a membrane 220. A graphite suspension inlet 110 is coupled to supply suspended graphite particles to the cathode 210. As will be discussed further below, the graphite particles and exfoliated graphene are confined by the membrane 220 so as to flow along the cathode 210 from the inlet 110 to the outlet 120. A exfoliating liquid inlet 115 is coupled to supply the exfoliating liquid to the volume between the membrane 220 and the anode 215, either directly (as shown) or indirectly (for example, if the exfoliating liquid passes radially outward through the membrane 220). As described above, the exfoliated graphene can exit the reaction vessel 105 through the graphene suspension outlet 120. The spent exfoliating solution and other by-products can exit the reaction vessel 105 through one or both of the outlets 120, 125. The anode lead connector 130 is connected to the anode 215 (not shown). The cathode lead connector 135 is connected to the cathode 210. The bias applied to the anode 215 and cathode 210 via the anode and cathode lead connectors 130, 135 promotes the exfoliation of graphene as graphite particles are carried through the cathode 210 by the flow from the graphite suspension inlet 110 to the graphene suspension outlet 120.

[0026] The cathode 210 is formed from a conductive porous skeletal material that provides multiple continuous electrical paths throughout the cathode 210 to bias the graphite particles for exfoliation throughout the cathode 210. The porous skeletal material contains pores of at least several open pores having a pore diameter that allows the associated graphite particles and exfoliated graphene to be carried by a liquid flow through the pores. The pore diameter of the skeletal material and the particle size of the graphite can be varied (e.g., depending on different particle feedstocks 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 feedstock particles. For example, to obtain relatively large-sized graphene flakes, the porous skeletal material may have open pores having an average maximum dimension in the millimeter size 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. The porous skeletal material also generally has a relatively large volume of voids. For example, a volume of voids exceeding 50% or 75% is preferred.

[0027] Although the cathode 210 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 certain geometric shapes of the electrochemical reactor 100.

[0028] In the illustrated embodiment, a portion of the cathode lead connector 135 is embedded within the cathode 210 and extends longitudinally within the cathode. This is not necessarily the case. For example, the conductivity of the cathode 210 may be sufficient to eliminate the need for extension of the cathode lead connector 135. In some embodiments, the longitudinally extending portion of the cathode lead connector 135 may be in the shape of a rod. In other embodiments, the longitudinally extending portion of the cathode lead connector 135 may have other shapes, such as a woven wire mesh.

[0029] The electrical contact between the portion of the cathode lead connector 135 visible in Figure 1 and the cross-section visible in Figure 2 can be maintained in a variety of ways. For example, the cathode lead connector 135 may extend through one end of the container wall 205 and be insulated from the stripping solution until it reaches the interior of the cathode 210. The portion of the cathode lead connector 135 exposed to the stripping solution in the cathode 210 is typically metal, but it can also be made of other conductors (e.g., doped diamond, graphite rod, carbon fiber). For example, the lead connector 135 can be made from copper.

[0030] The porous framework material of cathode 210 is generally chemically compatible with the electrolyte used in the electrochemical exfoliation reaction and is wetted by the electrolyte. The porous framework material of cathode 210 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 cathode 210 generally has sufficient mechanical strength and can withstand the pressure generated by flowing a suspension of graphite particles into cathode 210 and flowing exfoliated graphene out of cathode 210 without fracture.

[0031] Figure 4 is a schematic representation of an exemplary porous skeletal material 400 that can be used to form a cathode 210. As illustrated, the porous skeletal material 400 forms a three-dimensional network of interconnected pores large enough to allow graphite particles to enter the liquid flow at least partially and be driven by the liquid flow. Thus, the porous skeletal material 400 has a relatively large volume of voids.

[0032] In some embodiments, the porous framework material 400 is a vitreous carbon (RVC) foam. Vitreous carbon is amorphous carbon with physical properties suitable for use as the porous framework material 400 in the cathode 210. Table 1 below shows typical physical properties of the vitreous carbon foam.

[0033] [Table 1]

[0034] The use of mesh vitreous carbon foam as the porous framework material 400 in cathode 210 offers several advantages. For example, mesh vitreous carbon foam is chemically compatible with the exfoliation electrolyte, 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. Examples of suitable RVC foams are available from ERG Aerospace Corporation (Oakland, California) under the name DUOCEL®.

[0035] Returning to Figures 1-3, the illustrated embodiment of the anode 215 includes three electrically coupled conductive members. In the illustrated embodiment where the cathode 210 is substantially cylindrical, the members are strip-shaped and radially arranged around the cathode 210 in the same longitudinal orientation as the cathode 210. This arrangement acts to distribute the current flow more uniformly between the cathode 210 and the anode 215, ensuring that the peeling proceeds relatively uniformly within the cathode. However, as mentioned above, the cathode 210 can have other shapes, and the shape and orientation of the anode 215 can be changed accordingly.

[0036] The anode 215 can be made of one of several different conductors. For example, doped diamond, diamond-like carbon, and other conductive materials and ceramics are possible. The electrical connection between the anode lead connector 130 and the conductive member of the anode 215 can be made in a variety of different ways. For example, the conductor can contact different members of the anode 215 from the anode lead connector 130 along the outer surface of the container wall 205, and then through the container wall 205. As another example, the conductor can extend through the container wall 205 itself, or through the annular space between the inner surface of the container wall 205 and the outer surface of the film 220 (with appropriate insulation).

[0037] The membrane 220 is a thin material positioned between the cathode 210 and the anode 215, allowing for the transport of solvent electrolytes and charged species across the membrane while reducing or preventing the transport of both graphite particles and exfoliated graphene.

[0038] The membrane 220 is generally chemically compatible with the electrolyte used in the electrochemical exfoliation reaction and is wetted by the electrolyte. The membrane 220 can generally 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.

[0039] In some embodiments, the membrane 220 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 220 may be a flexible polymer, carbon fiber, or glass fiber sheet. For example, the membrane 220 may be a quartz fiber sheet with a thickness of 200 to 700 μm (e.g., 475 μm) having an average pore diameter between 0.5 and 1 μm, for example, about 0.7 μm. In some embodiments, the membrane 220 is a self-supporting membrane that can maintain its shape without internal components such as a cathode 210. For example, the membrane 220 may be formed from a relatively thin mesh vitreous carbon foam having porosity adjusted to hinder or prevent the transport of graphite particles and exfoliated graphene.

[0040] In the illustrated embodiment, the film 220 is tightly fitted to the outer surface of the cathode 210. This arrangement prevents the film 220 from establishing a channel for graphite particles through the reaction vessel 105 that does not pass through the cathode 210. For example, the film 220 can be in contact with the outer surface of the cathode 210. As a result, the graphite particles remain in at least intermittent electrical contact with the cathode 210 as they move through it, increasing the graphene yield. In addition, the film 220 is relatively far from the anode 215, which contributes to a more uniform distribution of the current flow between the cathode 210 and the anode 215.

[0041] The membrane 220 can be fitted to the outer surface of the cathode 210 in different ways. For example, the flexible membrane 220 can be wrapped around the cathode 210 and secured. As another example, the size and shape of the self-supporting membrane 220 can be adjusted to match the size and shape of the cathode 210 into which the flexible membrane 220 is fitted. For example, in the illustrated embodiment, the inner surface of the self-supporting membrane 220 can define a cylindrical void dimensioned to surround the cylindrical cathode 210.

[0042] In some embodiments, the membrane 220 extends between the outlet of the graphite suspension inlet 110 in the reaction vessel 105 and the inlet of the graphene suspension outlet 120 from within the reaction vessel 105. For example, the membrane 220 can be formed in a tubular shape and directly bonded to the tubular inlet 110 and outlet 120 in a variety of different ways. For example, the membrane 220 can be bonded to the inlet 110 / outlet 120 by compression fitting or sealing, or by embedding the fibers of the membrane 220 within a cast polymer inlet 110 / outlet 120. Direct bonding is not always necessary, and in other embodiments, one or more flow channels can be defined within the reaction vessel 105 to transport graphite particles from the inlet 110 to the cathode 210 in the membrane 220, or to transport exfoliated graphene from the cathode 210 in the membrane 220 to the outlet 120.

[0043] In some embodiments, the longitudinal range of the cathode 210 can coincide with the longitudinal range of the film 220. In other embodiments, the cathode 210 can be shorter than the film 220.

[0044] In the illustrated embodiment, the inlets 110, 115 and outlets 120, 125 are separate channels that traverse the container wall 205 separately and can be implemented, for example, using separate partition fittings. This is not necessarily the case. For example, in other embodiments, the graphite suspension may be mixed with the stripping solution away from the container wall 205, and a single channel carrying the mixture to the reactor 100 can act as both the graphite suspension inlet 110 and the stripping solution inlet 115. Similarly, in other embodiments, a single channel traversing the container wall 205 can act as both the graphene suspension outlet 120 and the stripping solution outlet 125.

[0045] In embodiments with separate flow paths, the direction of flow can be changed to achieve meaningful results. For example, in the illustrated embodiment, both inlets 110, 115 are on the left side of reactor 100 in the illustrated orientation, and both outlets 120, 125 are on the right side. Thus, both the graphite suspension and the stripping agent flow in the same direction, i.e., from left to right in the illustrated orientation, traversing reactor 100. However, this is not always the case. For example, in some embodiments, the graphite suspension and the stripping agent can flow in opposite directions and traverse reactor 100. As another example, in some embodiments, the graphite suspension and the stripping agent can flow perpendicular to each other in a cross-flow arrangement and traverse reactor 100. For example, in the illustrated orientation, the graphite suspension can flow from left to right, and the stripping agent can flow from top to bottom between appropriately repositioned inlets 115 and outlets 125.

[0046] Generally, the peeling reaction produces gaseous byproducts. For example, hydrogen gas is commonly produced in the cathode assembly, and electrolytes may form volatile byproducts. The structure and operation of the reactor 100 can be adjusted to facilitate the removal of such gaseous byproducts. For example, in the illustrated embodiment, part of the peeling outlet 125 is located near the top of the reaction vessel 105 to preferentially release gaseous byproducts from the reaction vessel 105. Another part of the peeling outlet 125 may be located at a lower level to preferentially allow liquid peeling (and liquid byproducts) to exit the reaction vessel 105.

[0047] In other embodiments, the shape of the reaction vessel 105 is adjusted to collect gaseous by-products for the outlet from the reaction vessel 105. For example, the top surface of the reaction vessel 105 may be sloped rather than flat so that the gaseous by-products collect at their highest point for the outlet from the reaction vessel 105 by a stripping outlet 125 at that location.

[0048] In yet another embodiment, the gaseous by-product may be discharged from the reaction vessel 105 via one or more unidirectional pressure relief valves.

[0049] During operation, the flow rates of the graphite suspension and exfoliating liquid into and out of reactor 100 can be adjusted to other parameters such as the characteristics of the feedstock, the applied potential, and the temperature and transport rates of graphite particles and graphene through the cathode 210, as well as the rate of gaseous byproducts produced, which leads to pressurization of the reaction vessel 105. However, generally, a high yield per unit time of the exfoliation reaction is desirable. In some embodiments, the mass flow rate of the graphite suspension into the reaction vessel 105 at inlet 110 can be adjusted so that the total time any single graphite particle interacts with the porous framework of the cathode within the reaction zone of reactor 105 is at least equivalent to the total reaction time required to achieve a yield of 60%, 70%, or 80% of the exfoliated product when the product exits the reactor at outlet 120. In some embodiments, the mass flow rate of the graphite suspension into the reaction vessel 105 is adjusted such that the total time any single graphite particle interacts with the porous framework of the cathode within the reaction zone of reactor 105 is greater than or equal to the total reaction time required to achieve the maximum yield of exfoliated graphite. The total elapsed time through the meandering path is hereby referred to as the residence time in the continuous flow reactor for graphite exfoliation. Such a continuous flow reactor residence time is a function of factors such as the mass flow rate, the effective viscosity of the suspension, the temperature, the pressure drop across the reactor inlet and outlet, and, importantly, the path the graphite particles take across the cathode 210. The meandering of the path through the cathode 210 is a function of the geometric properties of the cathode, such as volume, aspect ratio, and porosity. The residence time can be pre-tuned for different geometric shapes of reactor 105 using model simulations, such as those based on computational fluid dynamics (CFD) or the lattice Boltzmann method (LBM).

[0050] In some embodiments, residence time or other parameters are actively controlled during the continuous manufacturing process. For example, dynamic light scattering or other techniques can be used to monitor the solution flowing out of the graphene suspension outlet 120 for graphite particles that have not been completely detached. Other parameters that can be actively controlled include the concentration of graphite particles in the graphite suspension, the concentration of salt in the detachment solution, the residence time or concentration of the detachment solution, the effective viscosity of the suspension, and the potential difference between the anode and cathode.

[0051] Generally, solvents in which graphite particles are suspended are also used as stripping solutions. However, in some embodiments, the graphite particles can be suspended in a solvent different from the solvent of the stripping solution. In some embodiments, the graphite suspension can contain 0.5% to 50% by weight of particles in the solvent. Examples of solvents include propylene carbonate [C4H6O3], ethylene carbonate, or dimethyl carbonate. Generally, the solvent in which the graphite particles are suspended is degassed or passed through a gas bubbler to replace the oxygen with an unreactive gas such as argon or nitrogen.

[0052] 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.

[0053] Figure 5 is a schematic representation of an electrochemical reactor system 200 that can be used for the continuous production of graphene flakes by electrochemical exfoliation. In the illustrated embodiment, the reactor system 200 includes two reactors 100 and 100', which are connected in series such that the graphene suspension outlet 120 of the upstream reactor 100 is connected to the graphite suspension inlet 110' of the downstream reactor 100'.

[0054] In the illustrated embodiment, the upstream graphene suspension outlet 120 is directly coupled to the downstream graphite suspension inlet 110' so that all fluid passing through the upstream graphene suspension outlet 120 enters the downstream graphite suspension inlet 110'. This is not necessarily the case.

[0055] For example, in some embodiments, one or more mechanisms can be used to concentrate exfoliated graphene and graphite particles that are not completely exfoliated in the solution entering the downstream graphite suspension inlet 110'. For example, the solvent can be filtered from the suspension to concentrate the exfoliated graphene and graphite particles.

[0056] As another example, in some embodiments, one or more mechanisms can be used to separate exfoliated graphene and graphite particles that have not been completely exfoliated. For example, a centrifuge can be used to concentrate the graphite particles for exfoliation in the downstream reactor 100'. In another example, a field flow fractionation (FFF) device can be used to separate multiple fractions of the exfoliation product having different densities or particle sizes (or both).

[0057] In some embodiments, the reaction conditions are nearly identical in the upstream and downstream reactors 100, 100'. This is not always the case. For example, in some instances, the residence time, exfoliant, applied voltage, and / or cathode characteristics may differ in reactors 100, 100'. This difference can be adjusted to accommodate the differences in the graphite particles entering reactors 100, 100' through their respective graphite suspension inlets 110, 110'. For example, the cathode of the downstream reactor 100' may have smaller pores than the cathode of the upstream reactor 100.

[0058] Figure 6 is a schematic side view of an electrochemical reactor system 300 that can be used for the continuous production of graphene flakes by electrochemical exfoliation. The reaction vessel 305 of the reactor system 300 defines a vertically oriented but flat internal volume that helps to retain unreacted graphite particles within the vessel 305.

[0059] More specifically, the internal volume of the reaction vessel 305 has a first dimension 605, a second dimension 610, and a third dimension. In the illustrated side view, the first dimension 605 extends left and right, the second dimension 610 extends up and down, and the third dimension extends in and out of the plane of the paper. Dimensions 605 and 610 are larger than the third dimension, and the internal volume of the reaction vessel 305 is flat with respect to the vertical. For example, the third dimension of the reaction vessel 305 can be 5% to 20% of the shorter of dimensions 605 and 610.

[0060] In the illustrated embodiment, the top, bottom, left, and right sides of the reaction vessel 305 are approximately equal in length, linear, and intersect at right angles. These are not necessarily the case. For example, in some embodiments, the sides may have different lengths. The sides may be rounded or deviate from linearity. Also, the sides may intersect at different angles. In some cases, the embodiment may include a combination of such deviations from the illustrated characteristics. For example, the top, bottom, left, and right sides may have different lengths and may be rounded with different curvatures. The intersections of the sides may also be rounded.

[0061] In addition to the reaction vessel 305, the electrochemical reactor system 300 includes composite particle suspension / exfoliation inlets 110, 115 and composite graphene suspension / exfoliation outlets 120, 125. The composite inlets 110, 115 are located at the bottom of the reaction vessel 305, while the composite outlets 120, 125 are located at the top of the reaction vessel 305. The channels for graphite, graphene, exfoliation, and other by-products passing through the reaction vessel 305 run almost perpendicular to the vessel. The perpendicularity of the channels helps to retain unreacted or incompletely reacted graphite particles in the reaction vessel 305 while allowing the exfoliated graphene to exit the vessel 305.

[0062] Furthermore, the combined inlets 110 and 115 include a control valve 625. The control valve 625 can be used to regulate the flow of solution into the reaction vessel 305, and thus the time that the graphite particles interact with the porous framework of the cathode 210 within the reaction zone of the reactor 105.

[0063] In the illustrated embodiment, the cathode 210 also has a flat volume portion where the dimensions 605, 610 are larger than the third dimension. As previously stated, the cathode 210 is formed of a conductive porous skeletal material that provides multiple continuous electrical paths. The cathode 210 is tightly fitted to at least a portion of the inner wall of the reaction vessel 305 so as to prevent the establishment of a channel for graphite particles that do not pass through the cathode 210. In the illustrated embodiment, the cathode 210 is tightly fitted only to a portion of the inner side wall, top wall, and bottom wall of the reaction vessel 305. The cathode 210 is also tightly fitted to the vertical wall that borders the vessel 305 in the third dimension. Since the cathode 210 is closely fitted to only a portion of the inner wall of the reaction vessel 305, the cathode 210 and the wall of the vessel 305 define internal manifold-like volumetric sections 615, 620 within the vessel 305, where the porous skeletal material of the cathode 210 does not obstruct transport. The manifold-like volumetric section 615 is located at the bottom of the vessel 305 and allows particles and solutions entering through the composite inlets 110, 115 to be distributed over a relatively large portion of the cathode 210. The manifold-like volumetric section 620 is located at the top of the vessel 305 and can, for example, collect gaseous byproducts generated by the exfoliation reaction, or distribute particles and / or solutions entering through a second composite inlet (not shown). For example, a nearly cross-flow arrangement can be achieved. In some embodiments, a pressure relief valve can be coupled to the manifold-like volumetric section 620 to discharge gaseous byproducts. Alternatively or additionally, one or more pressure relief valves may be coupled to the combined inlets 110, 115 and / or combined outlets 120, 125.

[0064] In the illustrated embodiment, the anode and cathode lead connectors 130, 135 extend through a vertical wall that borders the vessel 305 in a third dimension. In the illustrated embodiment, the reactor system 300 includes a pair of spaced-apart cathode lead connectors 135, both electrically coupled to the cathode 210. The multiple cathode lead connectors 135 help ensure that the peeling voltage is maintained across the cathode 210.

[0065] The anode connection portion 130 is coupled to the anode 215, which is located near the center of the region of the vertical wall that borders the container 305 in a third dimension. The membrane 220 is sealed to the inner wall of the container 305 around the anode 215 to separate the cathode 210 from the anode 215.

[0066] 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 the continuous production of graphene, The steps include: pouring a suspension of graphite particles onto a conductive open-cell porous material placed in a reaction vessel; A step of applying a cathode potential to the conductive open-cell porous material, wherein the cathode potential is sufficient to exfoliate the graphene, A method comprising the step of draining the suspension of exfoliated graphene from the conductive open-cell porous material.

2. The method according to claim 1, wherein the membrane is fitted to the outer surface of the conductive open-cell porous material so tightly as to prevent the establishment of channels for the graphite particles that do not pass through the conductive open-cell porous material.

3. The method according to claim 2, wherein the membrane enables the transport of the electrolyte salt but inhibits or prevents the transport of the exfoliated graphene and is positioned between the conductive open-cell porous material and the anode during the application of the cathode potential.

4. The method according to claim 2 or 3, wherein the film has an average pore diameter between 0.5 and 1 μm.

5. The method according to any one of claims 1 to 4, wherein an anode including a plurality of members is in electrical contact with the fluid of the reaction vessel, and the members are spatially distributed around the conductive open-cell porous material.

6. The method according to any one of claims 1 to 5, wherein the residence time of the material in the conductive open-cell porous material is 1 to 600 minutes.

7. The method according to any one of claims 1 to 6, 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.

8. The method according to any one of claims 1 to 7, wherein the porous skeletal material has a void volume of more than 50%, for example, more than 75%.

9. The method according to any one of claims 1 to 8, wherein the porous skeletal material is a mesh-like vitreous carbon foam.

10. The method according to any one of claims 1 to 9, wherein the cathode potential is applied to a doped diamond anode electrode.

11. The method according to any one of claims 1 to 10, wherein the cathode potential exceeds -40 volts.

12. The method according to any one of claims 1 to 11, wherein the average maximum size of the pores in the conductive open-cell porous material is 0.5 to 2 mm, and the graphite particles have an average diameter in the range of 0.5 to 500 micrometers.

13. The method according to any one of claims 1 to 12, wherein the graphite particles are suspended in propylene carbonate.

14. The method according to any one of claims 1 to 13, further comprising flowing a stripping solution containing an organic solvent and a supporting electrolyte salt into the reaction vessel.

15. The organic solvent is propylene carbonate, ethylene carbonate, or dimethyl carbonate. The method according to claim 14, wherein the electrolyte salt is tetrabutylammonium hexafluorophosphate, tetrabutylammonium hexafluoroborate, tetrabutylammonium bis(trifluoromethanesulfonyl)imide, or N-benzyl-N,N,N-trimethylammonium hexafluorophosphate.

16. The method according to any one of claims 1 to 15, further comprising draining the reaction byproduct from the reaction vessel through an outlet different from the outlet from which the suspension of exfoliated graphene drains from the reaction vessel.

17. The step of draining the exfoliated graphene from the reaction vessel, The steps include: flowing the liquid carrying the exfoliated graphene from the reaction vessel into a second conductive open-cell porous material; The method according to any one of claims 1 to 16, further comprising the step of applying a cathode potential to the second conductive open-cell porous material, wherein the cathode potential is sufficient to exfoliate the graphene.

18. The method according to any one of claims 1 to 17, further comprising separating the exfoliated graphene from the liquid that carries the exfoliated graphene from the reaction vessel before flowing the liquid into the second conductive open-cell porous material.

19. An electrochemical reactor for producing graphene, A-scatter, A mesh-like vitreous carbon foam cathode, An electrochemical reactor comprising a membrane disposed between the mesh vitreous carbon foam cathode and the anode, wherein the membrane is configured to prevent the transport of graphite particles but to allow the transport of electrolytes and organic solvents of a solution for the exfoliation of graphite.

20. The electrochemical reactor according to claim 19, wherein the membrane is fitted so tightly to the outer surface of the cathode in order to prevent the establishment of a channel for graphite particles that do not pass through the cathode.

21. The electrochemical reactor according to claim 19 or 20, wherein the film is in contact with the outer surface of the cathode.

22. The electrochemical reactor according to any one of claims 19 to 21, further comprising a graphite suspension inlet fluid-coupled to supply suspended graphite particles to the cathode.

23. The electrochemical reactor according to any one of claims 19 to 22, further comprising an active control device configured to control the parameters of the electrochemical exfoliation of graphene.

24. The electrochemical reactor according to any one of claims 19 to 23, wherein the average maximum size of the pores in the mesh-like vitreous carbon foam is 0.5 to 2 mm.

25. An electrochemical reactor according to any one of claims 19 to 24, further comprising a suspension of graphite particles and a pump configured to pump the suspension of graphite particles through the mesh vitreous carbon foam cathode.

26. The electrochemical reactor according to claim 25, wherein the residence time of the suspension delivered by the pump within the mesh-like vitreous carbon foam is 1 to 600 minutes.

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