Graphene dispersion process

Abstract

A process is disclosed for preparing a dispersion of graphene in a fluid medium, the process comprising: providing graphene, said graphene obtained from operation of a plasma reactor system configured

Description

The present invention relates to processes and systems for preparing a dispersion of graphene in a fluid medium. In particular, the present invention relates to dispersion in a liquid medium of graphene by the application of a high shear rate, the graphene being obtained from operation of a plasma reactor system configured to produce graphene from a carbon-containing process gas. Background Graphene is considered to be a valuable material with great potential due to its many advantageous properties including exceptional mechanical properties (high strength and elasticity), high electrical and thermal conductivity, impermeability to gases and transparency to light. As a result, graphene has found use across a wide range of applications. There are multiple ways that graphene can be produced. One method is to convert carbon-containing gas into graphene in a plasma reactor. For example, plasma-based system is disclosed in WO 2015 / 189643 A1 and was demonstrated to produce graphene from hydrocarbon gases including methane. Other methods include exfoliation and chemical vapour deposition. When used in an application where graphene is added into a fluid product, it is necessary to mix the graphene into a fluid medium. Many ways for mixing a particulate material into a fluid are known in the art, such as mechanical mixers using rotors or other moving physical mixing means. In order for graphene to provide improved functionality when added to an existing product as an additive, or when used to produce a newly formulated graphene-based fluid, the graphene should be mixed sufficiently to avoid it having a negative impact on the properties of the product. For example, the processing time may be increased to increase dispersion, or chemical dispersant additives may be used to enhance mixing. However, long processing times may not be suitable or efficient, and chemical additives may not be desirable or compatible with downstream applications. Accordingly, there is a need to address the above-described problems. Summary An aspect of the invention provides a process for preparing a dispersion of graphene in a fluid medium, the process comprising: providing graphene, said graphene obtained from operation of a plasma reactor system configured to produce graphene from a carbon-containing process gas; mixing said graphene with a fluid medium to provide a graphene suspension; and directing said graphene suspension into one or more microchannels of a high-pressure homogeniser at a pressure of at least 15 M Pa so as to subject said graphene suspension to a shear rate of greater than 105 / s. High pressure homogenisers are an established technology that has been widely used in the food processing industry, for example for producing uniform emulsions such as milk and other dairy products. It has been found that by dispersing graphene produced by a plasma reactor system using such high-pressure homogenisers, the products produced are of surprisingly improved quality compared to other mixing techniques. For example, it has been found that by providing a graphene dispersion according to the present process it may efficiently inhibit separation problems in the product, improve visible appearance, lower viscosity and improve stability. Without wishing to be bound by any particular theory, it is believed that the high forces applied to the graphene suspension by use of a high-pressure homogeniser (due to the high energy dissipated over a small volume) results in effective deagglomeration and dispersion of individual particles at the nanoscale. This can achieve surprisingly improved dispersions not possible in a conventional mixing system without the use of excessive processing times or additives, which may not be compatible with downstream applications or processing. The shear rate of greater than 105 / s, and up to around 108 / s obtainable in a high pressure homogeniser is higher than typical high shear mixers such as rotor-stator mixers. Techniques such as ultrasonication can provide high localised shear rates but the energy rapidly dissipates with distance, leaving large volumes unmixed. In this way, despite being already formed as single layer graphene sheets (or may include two layer or few layer graphene or more, i.e. from 3 to 30 layers or from 3 to 10 layers), the present process provides a surprisingly effective way for dispersing such graphene materials in a fluid. This is believed to be due to the fact that agglomeration of single or few layer graphene can produce larger particulates that are difficult to break apart despite the nanoscale structure of individual sheets. Gas phase plasma reactor systems configured to produce graphene from a carbon-containing process gas typically produce graphene that can agglomerate during the production process or during mixing. Thus, the advantages granted by use of a high-pressure homogeniser provides a certain synergy when applied to graphene obtained from a plasma reactor system. Without wishing to be bound by nay particular theory, it is believed that such graphene can contain polycyclic aromatic hydrocarbons and polyacetylene chains adsorbed or bonded to / on its surface, which can increase the tendency of the graphene to form agglomerates as it is formed in the plasma reactor system. By way of example, polycyclic aromatic hydrocarbons may be identified by gas chromatography-mass spectroscopy (GC-MS), and polyacetylenes may be identified by Raman spectroscopy. It will be appreciated that the shear rate applied to the graphene suspension in the high pressure homogeniser will vary depending on the exact conditions used, which may be varied according to need depending on the desired application and fluids used in the process. The graphene suspension is suitably directed into the one or more microchannels of the high-pressure homogeniser so as to subject said graphene suspension to a shear rate of greater than 105 / s. In preferred embodiments, the graphene suspension is directed into the one or more microchannels of the high-pressure homogeniser so as to subject said graphene suspension to a shear rate of greater than 106 / s, for example greater than 107 / s. In embodiments, the graphene suspension may be subjected to a shear rate of 108 / s or higher. It will also be appreciated that the pressure at which the graphene suspension is directed into the microchannels of the high-pressure homogeniser may be varied according to need. The graphene suspension may be pressurised in any suitable way and any suitable pump may be used. In particular, hydraulic piston pumps may be used, which can be well suited for cleanliness, high pressures, and are compatible with many liquid media, however it will be appreciated that other pumps may be used, Suitably, the graphene suspension is directed into said one or more microchannels of the high-pressure homogeniser at a pressure of at least 15 MPa. Preferably, the graphene suspension is directed into said one or more microchannels of the high-pressure homogeniser at a pressure of at least 20 Mpa, preferably at least 50 Mpa, for example at least 100 Mpa. The graphene suspension is directed into said one or more microchannels of the high-pressure homogeniser at a pressure of 500 Mpa or less, preferably 400 Mpa or less, for example 300 Mpa or less. For example, in embodiments the graphene suspension may be directed into said one or more microchannels of the high-pressure homogeniser at a pressure of from 15 to 500 Mpa, from 20 to 400 Mpa, from 50 to 300 Mpa or from 100 to 300 Mpa. The high pressure used in the present process can provide additional benefits beyond providing high shear mixing. For example, by applying high pressures, it may be possible to allow more viscous substances to be used and processed effectively in the system. For instance, a viscous fluid such as coal tar pitch or a fluid formed from or containing polymers may more effectively be provided with graphene dispersed therein. High pressure homogenisers are an established technology that has been widely used in the food processing industry. As referred to herein and as will be appreciated by those skill in the art, a high-pressure homogeniser will be understood to refer to a system in which a fluid at high pressure (for example at least 15 mPa) is directed into a microchannel system configured to apply a high shear rate to the fluid, for example by utilising one or more microchannels that change direction so as to provide impact surfaces for the fluid passing through the microchannels. Accordingly, in the present process the one or microchannels may suitably be configured to provide one or more changes in the direction of flow of the graphene suspension through the one or more microchannels of the high-pressure homogeniser. The portion of the microchannel system configured to apply high-shear rate is typically referred to as the interaction chamber, and there are various different designs known in the art that can be used. For example, the microchannel may comprise a change of direction, the one or more microchannel may comprise a single microchannel and / or may be configured to split and recombine the flow of said graphene suspension. Two commonly used designs are Z type or Y type interaction chambers. Z type interaction chambers typically comprises a flow path where one or more microchannels provide a change in direction that induces a high-shear rate on a fluid. Z type interaction chambers may comprise a single microchannel flow path, or may comprise multiple microchannels. Where a Z type flow path comprises multiple microchannels these are typically parallel to one another and recombine the flow without directing the flow though different microchannels in opposite directions. Y type interaction chambers typically comprise a microchannel arrangement where the flow is split into at least two separate microchannels, wherein the separate microchannels are recombined in a way that directs the flow from one microchannel in substantially an opposite direction to the other to provide an impact region where the two flows recombine. In an embodiment, the high pressure homogeniser comprises a Z type interaction chamber or a Y type interaction chamber, preferably a Z type interaction chamber. As will be appreciated, where a microchannel is referred to herein, this generally refers to a channel having a hydraulic diameter below 1 mm. The one or more microchannels of the high-pressure homogeniser of the present process may suitably have a hydraulic diameter or width of from 10 pm to 800 pm, preferably from 20 pm to 500 pm, for example from 50 pm to 300 pm, such as from 80 pm to 250 pm. It will be appreciated that the flow velocity through the one or more microchannels will vary depending on the size of the channels and the pressure applied. Suitably, the flow velocity of the graphene suspension through the one or more microchannels is from 200 m / s to 2000 m / s, for example from 400 m / s to 1500 m / s or from 500 m / s to 1200 m / s. The fluid medium used to provide the graphene suspension according to the present process may be any suitable fluid medium into which it is desired to disperse graphene. Preferably, the fluid medium comprises a functional product into which graphene can be dispersed in order to improve its properties or impart new properties on the fluid. For example, where the fluid medium comprises a solvent, the fluid medium will preferably comprise the solvent and one or more additional components in addition to the graphene (e.g. the graphene suspension will comprise not only a solvent and graphene or other graphitic carbon). In some embodiments, the fluid medium will comprise various precursor components and the dispersion process will form a functional product simultaneously with dispersion of the graphene. In preferred embodiments, the fluid medium may comprise a paint, a lacquer, a varnish, a polymer or a polymer precursor such as a resin, a wax, a silane, biopolymers or bio-derived polymers such as cellulose, lignosulfonates or derivatives thereof, coal tarpitch, a battery electrode slurry. The fluid medium may comprise a fluid suspension or solution of various non-graphene components into which graphene may be dispersed. For example, in embodiments the fluid medium further comprises one or more carbon allotropes such as carbon nanotubes, graphene oxide, reduced graphene oxide, fullerenes, carbon black, graphite or diamond, boron nitride allotropes such as nanotubes or flakes, one or more metal oxide salts such as titanates, zirconates or aluminates, surfactants, elemental or oxide-based microparticles or nanoparticles such as elemental or oxide-based microparticles or nanoparticles of S, Au, Ag, Pd, Fe, Si, Ti, Mo, or combinations thereof. In some embodiments, the fluid medium does not comprise graphite. In some embodiments the fluid medium may comprise a material having a melting point above ambient temperature. Accordingly, the process may comprise heating the fluid medium and / or the graphene suspension before and / or after directing the graphene suspension into the one or more microchannels, for example so as to melt one or more components of the fluid medium. Thus, in embodiments, the fluid medium is melted before and / or after mixing with the graphene in order to provide the graphene suspension. The heating may be conducted by any suitable means. The heating may be performed by a heater at the inlet of the high-pressure homogeniser, on a reservoir from which the fluid medium or a precursor thereof is taken. It will be appreciated that where the fluid medium or graphene suspension requires heating, the heating may be continued throughout the system to an outlet. The system after the high-pressure homogeniser (e.g. after the high-shear microchannel portion) may be actively or passively heated, for example by thermal fluid jackets or insulation materials. By way of example, materials that may be fluidised by heating to form the fluid medium may comprise: thermoplastic polymers, waxes, thermoset polymers such as epoxys or polyesters, elastomeric precursors, fluids comprising silanes, titanates, zirconates or aluminates, viscous solutions or suspensions of materials such as soluble polymers including cellulose, soluble thermoplastics, surfactants, non-soluble polymers including rubber compounds such as latex, battery mixtures such as mixtures of binders and active and passive components, battery slurries, solvent or aqueous masterbatches, or resin masterbatches. In embodiments, a chemical reaction can be induced by the homogeniser, this may include heating the graphene suspension as discussed previously or may not require heating. In embodiments, the graphene suspension is heated before and / or after directing the graphene suspension into said one or more microchannels in order to induce a chemical reaction with one or more other components of the graphene suspension. Accordingly, in embodiments, the fluid medium may comprise one or more components selected to provide a chemical reaction during the process. With or without active heating of the fluid, the high pressure and increase in temperature within the interaction chamber can effect chemical reactions, allowing the process to be configured not only for effective dispersion and mixing, but also for particle treatments and functionalization. It has been surprisingly found that when using materials to effect functional chemistry, the effectiveness of surface treatments and chemical reactions can be enhanced. Without wishing to be bound by any particular theory, it is believed that as a result of the process, increased particle surfaces become available for chemical interactions that may otherwise be hidden due to agglomeration, which may, for example, be beneficial for use with nanomaterials, for example when adapting a nanoparticle for integration into other systems. In embodiments, the process is conducted under inert atmosphere or vacuum conditions. It will be appreciated that this refers to the conditions under which the starting materials are stored and combined, and consequently the conditions throughout the process (e.g. vacuum conditions may be applied before and / or after the process, but pressure is nonetheless elevated during the process). This can allow mixtures of sensitive components or reactants to be processed without degradation (for example by reaction with oxygen or water vapour in the air). The process comprises providing graphene obtained from operation of a plasma reactor system configured to produce graphene from a carbon-containing process gas. By way of example, plasma reactor systems are described in WO2015 / 189643 and WO2024 / 013488. Preferably, the process comprises providing said graphene by providing a carbon-containing process gas, for example a process gas comprising hydrocarbons, to a plasma reactor system configured to produce hydrogen and graphene from the process gas. The hydrocarbons may be any suitable materials, in preferred embodiments the process gas comprises or consists essentially of methane, for example the process gas comprises or consists essentially of natural gas. As will be appreciated, natural gas is well known in the art and typically comprises more than about 80 %v / v of methane, and smaller amounts of other hydrocarbons such as ethane, propane and butane, and may also contain small amounts of non-hydrocarbon gases such as nitrogen, CO2, argon or other noble gases, and sulfur-containing gases such as H2S, where in some instances the nonhydrocarbon and / or the non-methane hydrocarbons may have been removed or reduced by refining prior to being provided to the apparatus in the feed stream. In some embodiments, undesired gases or particulates such as aerosols in the feed stream may be removed by one or more filters arranged to filter the carbon-containing gas. Preferably, the plasma reactor system comprises a reaction chamber, a plasma nozzle coupled to the reaction chamber, and means for supplying the process gas to the plasma nozzle. The plasma reactor system may comprise means for providing radio frequency radiation, preferably microwave radiation, to the process gas within the plasma nozzle so as to produce a plasma within the plasma nozzle, and thereby cause cracking of hydrocarbons in the process gas within the plasma nozzle to provide cracked hydrocarbon species, wherein the plasma nozzle is arranged such that an afterglow of the plasma extends into the reaction chamber, the cracked hydrocarbon species also pass into the reaction chamber and recombine within the afterglow to provide graphene and hydrogen in the reaction chamber. The plasma nozzle configuration is described in more detail elsewhere in the disclosure, and an example of a plasma reactor comprising a plasma nozzle coupled to a reaction chamber that may be used to produce graphene and hydrogen is described in WO 2015 / 189643 A1. The plasma nozzle is preferably shaped and configured so as to cause at least one vortex to be formed in the process gas within the plasma nozzle, said vortex being subjected to said radio frequency radiation. Preferably, the plasma nozzle is shaped and configured so as to cause multiple vortices to be formed in the process gas within the plasma nozzle, said multiple vortices being subjected to said radio frequency radiation. The use of multiple vortices in this manner increases the time for which the process gas is exposed to the radiation, thereby increasing the efficiency of the plasma cracking process. It can also allow better plasma stability by generating an area of lower pressure inside a third vortex where the plasma is more confined. Thus, in a preferred embodiment, three vortices are formed within the plasma nozzle. The multiple vortices may suitably be concentric vortices. Preferably, the plasma is generated at substantially atmospheric pressure. For example, the process gas may be provided to the plasma nozzle at atmospheric pressure or at a slight positive pressure, such as a pressure from about 1 to 1.5 bar absolute. It will nonetheless be appreciated that the process gas may be provided to the plasma nozzle at pressures higher than atmospheric pressure, for example up to about 3 bar absolute or up to about 10 bar absolute. For the sake of completeness, we note here that some parts of the plasma may not be at atmospheric pressure when it is formed at the core of a vortex where its pressure is likely to be lower than atmospheric pressure. However, there may suitably be no further system used to change the pressure of the plasma in the nozzle other than the fluid mechanics induced by the nozzle design. The plasma nozzle may comprise: one or more inlets to receive a stream of the process gas, that forms a first vortex in use; an open end in communication with the reaction chamber; and a vortex-reflecting end opposite the open end; wherein the nozzle is internally tapered towards the open end; such that, in use, a second vortex is created by the vortex-reflecting end, and a third vortex is produced by reflection of the second vortex from the vortex-reflecting end. Advantageously, the second vortex is created by the vortexreflecting end which “sucks” the first vortex (by virtue of the Coanda effect) and then reflects it to form the third vortex. It will be appreciated that an open end of the plasma nozzle in communication with the reaction chamber will have a cross-section that is smaller than the dimensions of the reaction chamber, for example such that the nozzle provides an opening through a wall of the reaction chamber. Preferably, the exit of one or more plasma nozzles coupled to the reaction chamber (which may include means for actively cooling the afterglow) is flush with the wall of the chamber, so as to provide a flat surface that may for example be cleaned by a scraper system. The plasma nozzle may be coupled to the reaction chamber so as to direct flow from the plasma nozzle towards the centre of the reaction chamber, for example wherein the plasma nozzle provides a flow (and afterglow) radially inwards into the reaction chamber. In other embodiments, the plasma nozzle may be disposed at least partially at an angle from the perpendicular to the wall of the reaction chamber to which it is coupled. For example, the plasma nozzle is disposed so as to provide a flow into the reaction chamber at an angle of less than 90 degrees from the wall of the reaction chamber (e.g. a tangent to the wall at the point at which the plasma nozzle is disposed). The means for supplying radio frequency radiation may comprise a microwave generator (for example operating at 2.45 GHz, although other frequencies are also usable). A waveguide may be arranged to direct the radio frequency, for example microwave, radiation to the nozzle, for example to coincide with the vortex(es) of the process gas. As those skilled in the art will appreciate, the expression “radio frequency radiation” encompasses the full extent of microwave frequencies, together with a range of nonmicrowave frequencies. In some embodiments, the radio frequency radiation is terahertz radiation, for example radio frequency radiation in the range of from 0.3 THz to 3 THz. The radio frequency radiation may be suitably provided at various power levels according to the requirements of a particular system. As a general rule, higher power of the plasma system gives better cracking efficiency and allows processing of higher process gas flow rates. Scaling and increasing the power of the plasma system may be achieved by combining several nozzles around a reaction chamber or by increasing the power of the microwave generator and providing a larger nozzle. The power of the microwave generator may be between 1 and 30 kW, for example between 1 and 20 kW. However, there is no restriction of scale on the nozzle and so the power may be varied accordingly. For example, in some embodiments the power may be up to 100 kW or up to 1 MW. Process gas flow through the plasma nozzle may for example be in the range of about 20 L / min to 150 L / min depending on the precise scale of the nozzle, however it will be appreciated that the flow rate may suitably be varied according to a particular nozzle and reactor setup and desired conversion efficiency of hydrocarbons in the process gas. For example, when the power provided for producing plasma in the plasma nozzle is increased then the flow rate may also be increased whilst maintaining the conversion efficiency of the process gas. The pressure at which the process gas is provided to the plasma nozzle will suitably depend on the desired pressure in the reaction chamber, and may suitably for example be higher than the reaction chamber pressure, for example about 0.1 bar to 0.5 bar higher than the pressure in the reaction chamber, such as 0.2 to 0.4 bar higher than the pressure in the reaction chamber. As described, the plasma reactor system may in some embodiments comprise a plurality of plasma nozzles coupled to the reaction chamber. A plurality of plasma nozzles may be distributed in any suitable way around the reaction chamber and in preferred embodiments the plasma nozzles are distributed so as to minimise interference between the respective afterglow exiting each plasma nozzle, for example the plasma nozzles may be distributed evenly around the periphery of the reaction chamber, for example circumferentially around the reaction chamber (e.g. 180 degrees apart for 2 nozzles, 120 degrees apart for 3 nozzles, 90 degrees apart for 4 nozzles and so on). The plasma nozzles may be distributed vertically within the reaction chamber, for example plasma nozzles may be separated vertically on the walls of the reaction chamber (for example separated by at least about 5 cm, such as at least about 8 cm or at least about 10 cm), and may also be distributed around the periphery of the chamber as previously described (e.g. circumferentially) or may be disposed vertically above or below another plasma nozzle. The number of plasma nozzles coupled to the reaction chamber may be selected based on the reaction chamber volume, so as to limit the temperature within the reaction chamber to avoid degradation of graphene in the reaction chamber (for example to limit the temperature of gases in the reaction chamber to no more than about 200 °C). Each plasma nozzle may be orientated radially towards the centre of the reaction chamber or may be disposed at an angle to the wall of the reaction chamber as described previously. Due to the increase in volume when passing from the plasma nozzle into the reaction chamber, the afterglow from the plasma and the cracked species present in the afterglow may be passively cooled upon entering the reaction chamber. In some preferred embodiments, the reaction chamber incorporates means for actively cooling the afterglow on exiting the plasma nozzle, such as direct cooling by introducing a flow of gas (at a lower temperature than the afterglow) to mix with species in the afterglow, or indirect cooling by a heat exchanger carrying a refrigerant or a heat pipe system configured to draw heat away from the afterglow exiting the plasma nozzle. Preferably, in use, the afterglow within the reaction chamber has an operating temperature lower than 3500°C. More preferably, in use, the afterglow with in the reaction chamber has an operating temperature lower than 1000°C. For instance, the operating temperature of the afterglow within the reaction chamber can be as low as 300°C. Preferably, in use, the temperature just outside the nozzle, at the carbon formation point within the afterglow, is in the range of 800°C to 1200°C. Particularly preferably this temperature is in the range of 900°C to 1000°C. Nonetheless, it will be appreciated that the gas temperature in the reaction chamber will be significantly lower than this outside of the afterglow. The plasma generated in the nozzle is preferably a non-equilibrium plasma (also referred to as a non-thermal plasma) such as is known in the art, for example a plasma in which the electron temperature is greater than the temperature of heavier species (ions and neutral species) in the plasma. In some embodiments, the process gas may comprise a buffer gas, such as argon, nitrogen or helium, that is blended with the feed stream to provide the process gas. For example, the apparatus, e.g. the regulator system, may be configured to blend a buffer gas with a hydrocarbon feed to form the process gas. It will be appreciated in this context that a buffer gas refers to an inert gas, such as argon or nitrogen, added to dilute the hydrocarbons in the process gas and not to inert gases present in the feed stream itself that is provided to the apparatus (for example small amounts of nitrogen, argon and so on that may be present in natural gas) or to blending of hydrocarbon gases with the feed stream. If a buffer gas is used, the ratio of hydrocarbon species to buffer gas in the process gas is preferably 50:50 or less, for example around 20:80 or less. Preferably, no buffer gas is provided in the process gas. Graphene produced by a plasma reactor system as described herein can have specific properties that are different to graphene produced by other methods, such as by exfoliation of graphite. For example, the graphene used in the present process may have a relatively small lateral flake size. Preferably, at least 50 %, preferably at least 70 %, more preferably at least 90%, of the graphene provided for mixing with the fluid medium has a lateral flake size of less than 500 nm, for example less than 400 nm or less than 300 nm. Preferably, at least 50 %, preferably at least 70 %, more preferably at least 90%, of the graphene provided for mixing with the fluid medium has a lateral flake size of 100 nm or more. It will be appreciated that lateral flake size will be a distribution of different sizes and can be characterised by that distribution. The lateral flake size suitably relates to the largest measurement of a graphene flake as measured by SEM (scanning electron microscopy) and measuring the size across a large sample of flakes. The specific surface area of the graphene provided for mixing with the fluid medium may be from 80 m2 / g to 500 m2 / g, preferably from 100 m2 / g to 350 m2 / g. Specific surface area may suitably be measured according to ISO 9277:2022. The Raman spectrum of the graphene provided for mixing with the fluid medium may suitably exhibit one or more of: a D peak intensity to G peak intensity ratio, l(D) / l(G), of less than 1; a D’ peak intensity to G peak intensity ratio, l(D’) / l(G), of less than 0.5; a G peak having a full width at half maximum of from 25 to 50 cm-1; a 2D peak intensity to G peak intensity ratio, l(2D) / l(G), of greater than 0.65; and a single-component 2D peak. Raman spectra can be suitably measured using an excitation laser wavelength of 532nm at a power <0.25mW under a 50x objective lens. As will be appreciated, the peaks referred to are known in the art in relation to Raman analysis of graphene and refer to D peak (around 1350 cm-1), G peak (around 1580 cm-1), 2D peak (around 2690 cm-1), D’ peak (around 1620 cm-1). The graphene provided for mixing with the fluid medium suitably comprises at least 90 % carbon by weight, preferably at least 95 % carbon by weight, for example at least 98 % carbon by weight. The hydrogen and graphene may suitably be separated prior to mixing the graphene with the fluid medium, for example a hydrogen-graphene mixture can be purged with inert gas to provide a mixture of graphene and inert gas for mixing with the fluid medium. In embodiments where an inert atmosphere is used prior to and / or during the present process, it will be appreciated that providing the graphene under inert atmosphere directly from the reactor system may be beneficial for efficiency. The plasma reactor system may be a containerised system configured to produce the graphene in the plasma reactor system within the container, for example wherein the container comprises a shipping container. The container with which the apparatus is provided may be any suitable container for allowing the apparatus to be transported between locations. For example, the apparatus may comprise an intermodal container such as are commonly used for transporting goods, such as a shipping container. For example, the container may comprise an intermodal container conforming to an international standard size for shipping, such as ISO standard 668:2020. Where the plasma reactor system is disposed inside a container, the steps of mixing the graphene with the fluid medium and / or directing the graphene suspension into one or more microchannels of a high-pressure homogeniser may be conducted within the container. For example, a high-pressure homogeniser system and a source of the fluid medium may be provided inside the container, such that graphene from the plasma reactor system may be provided to an inlet of the high-pressure homogeniser with the fluid medium. In some embodiments, the high-pressure homogeniser may be disposed in a further container that can be coupled with a container having plasma reactor system. For example, the plasma reactor system may be disposed in a first container, and the steps of mixing the graphene with the fluid medium and / or directing the graphene suspension into one or more microchannels of a high-pressure homogeniser are conducted within a second container configured to couple to and receive the graphene from the first container. The graphene may be mixed with the fluid medium in the first container and the graphene suspension provided from the first container to the second container for processing with the high-pressure homogeniser. The graphene dispersion prepared by the present process may be provided as a product itself, or may undergo further processing to provide a product. Thus, in some embodiments the process further comprises the step of processing the graphene dispersion to modify one or more components of the dispersion, for example by heating the graphene dispersion, providing the graphene dispersion to a chemical reactor (for example in which the conditions or added reagents induce chemical reactions), or providing the graphene dispersion to a microwave reactor to induce a chemical reaction of one or more components of the graphene dispersion. The graphene dispersion may also be processed to provide a solid product, for example by the removal of solvent from the dispersion. Therefore, the process may further comprise the step of drying the graphene dispersion, such as by spray drying, to provide a modified graphene product, for example a chemically functionalised graphene product. In some embodiments, the graphene dispersion may be processed into a granulate or pellets before drying. For example, a graphene / solvent mixture may be mixed with removal of solvent to form a granulate, and the dispersion of the present process may provide an advantageous graphene solvent mixture to form a granulate from. In addition, where the fluid medium has a melting point above ambient temperature (i.e., where the fluid medium is melted before and / or after mixing with the graphene in order to provide the graphene suspension) the graphene dispersion may be granulated as it cools to provide a solid graphene-containing granulate, for example by mechanical mixing or by extrusion. In particular, where the fluid medium comprises a tar such as coal tar pitch or petroleum pitch, this may provide a graphene-containing granulate that is particularly suited for inclusion into carbon electrodes for aluminium smelting, which are typically formed by baking a mixture of dry aggregate such as coke and a binder such a coal tar pitch. Similarly, a melted polymer or polymer precursor as the fluid medium may form a solid product on cooling the graphene dispersion, which can be a pelleted or granulated masterbatch material. This not only provides a compatibilized graphene material for use in other processes, but can avoid problems posed by the use of dry graphene powder, which is typically incredibly fine and can pose safety and handling problems due to the dust produced. The graphene and the fluid medium may be combined in any suitable way. The graphene is suitably in the form of dry graphene powder, and the graphene powder mixed with the fluid medium fluid prior to providing the graphene suspension to the high-pressure homogeniser. The amount of graphene that is used relative to the fluid medium may be any suitable proportion, and it will be appreciated that the amount of graphene used may be suitably varied depending on the specific dispersion being formed and its intended use. The graphene suspension may comprise at least 0.5 wt.% graphene, for example at least 1 wt.% graphene, such as at least 5 wt.% graphene. It has also been found that the present process can form stable dispersions of graphene at higher graphene loadings than can be achieved by conventional mixing techniques. For example, in some preferred embodiments, the graphene suspension may comprise 10 wt.% or more graphene, for example 15 wt.% or more graphene, such as 20 wt.% or more graphene or 25 wt.% or more graphene. In embodiments, the graphene suspension may comprise up to 40 wt.% graphene, for example up to 30 wt.% graphene, such as up to 25 wt.% graphene. The graphene suspension may for example comprise from 0.5 wt.% to 40 wt.% graphene, for example from 1 wt.% to 30 wt.% graphene, such as from 5 wt.% to 25 wt.% graphene or from 10 wt.% to 20 wt.% graphene. It will be appreciated that while the relative proportion of graphene is discussed in relation to the graphene suspension, these values can equally be applied to the quantity of graphene present in the graphene dispersion obtained from the high-pressure homogeniser. High relative loadings of graphene with the fluid medium may be particularly useful for providing graphene in a fluid medium as a masterbatch or a concentrate for later dilution, which may for example provide benefits to handling safety and transportation as compared to graphene powder. For example, the fluid medium may comprise or consist essentially of a solvent, such as water, that can be used as a graphene source in solvent-based systems (such as water-based systems) or may comprise or consist essentially of a resin that can subsequently be formulated into resin-based systems, such as coatings or composites. While the present process using high-pressure homogenisation has been found to form stable dispersions of graphene at higher graphene loadings than can be achieved by conventional mixing techniques, it will be appreciated that the precise amount of graphene that might be used in each case may generally vary depending on the overall formulation, for example the presence of additives such as surfactants and the like. Without wishing to be bound by any particular theory, it is believed that the agglomerates of graphene particles present in the graphene powder require high shear stress to pull apart the particles and expose the surface area. The present process is believed to allow this, providing an improved dispersion that may, for example, provide a stable, relatively lower viscosity dispersion compared to conventional mixing techniques. As will be appreciated, the dispersion obtained from the high-pressure homogeniser may be recycled back to the inlet so as to pass the mixture through the homogeniser more than once. However, this is not required in order to obtain effective dispersions in the present process, which may be due to the properties of the graphene obtained from the plasma reactor system. Thus, the process may comprise mixing the graphene with the fluid medium to provide the graphene suspension, then directing said graphene suspension through the one or more microchannels of the high-pressure homogeniser less than 15 times, preferably less than 10 times, for example wherein the graphene suspension is directed through the one or more microchannels of the high-pressure homogeniser no more than 5 times, or only a single time, to form the graphene dispersion. A further aspect provides a system for preparing a dispersion of graphene in a fluid medium, the system comprising: a plasma reactor system configured to produce graphene from a carbon-containing process gas; means for mixing said graphene with a fluid medium, wherein the system is configured to mix graphene produced by the plasma reactor system with the fluid medium to provide a graphene suspension; and a high-pressure homogeniser comprising one or more microchannels, wherein the system is configured to direct the graphene suspension into one or more microchannels of a high-pressure homogeniser at a pressure of at least 15 MPa so as to subject said graphene suspension to a shear rate of greater than 105 / s. As will be appreciated, the system may be as defined previously herein in relation to the process and features described in relation to the system apply equally to the process. The system may be configured to direct the graphene suspension into one or more microchannels of a high-pressure homogeniser under the conditions and using the apparatus defined previously herein. For example, the system may be configured to subject the graphene suspension to a shear rate of greater than 106 / s, for example greater than 107 / s. The system may be configured to direct the graphene suspension into the one or more microchannels of the high-pressure homogeniser at a pressure at a pressure of at least 20 MPa, preferably at least 50 MPa, for example at least 100 MPa, and / or at a pressure of 500 MPa or less, preferably 400 MPa or less, for example 300 MPa or less. The high-pressure homogeniser may comprise any suitable apparatus and may suitably be as defined previously herein. For example, the one or microchannels may be configured to provide one or more changes in the direction of flow of said graphene suspension through the one or more microchannels and / or to split and recombine the flow of said graphene suspension. The one or more microchannels may have a hydraulic diameter or width of from 10 pm to 800 pm, preferably from 20 to 500 pm, for example from 50 pm to 300 pm, such as from 80 to 250 pm or from 100 to 200 pm. As discussed previously herein, the system may be configured to heat the fluid medium and / or the graphene suspension prior to providing the graphene suspension to the high-pressure homogeniser. Thus, the system may comprise a heater configured to melt a precursor material to provide the fluid medium. It will be appreciated that the fluid medium may be provided to the system in any suitable way, for example the fluid medium may be stored in the system or may be received via an input stream such as a pipeline. Thus, the system may comprise a source of the fluid medium, for example a reservoir comprising the fluid medium or a precursor of the fluid medium. A precursor of the fluid medium may for example be a solid material that, when heated, is melted to form a fluid for mixing with the graphene. As will be appreciated, the fluid medium will be in a liquid form when it is directed into the one or more microchannels of the high-pressure homogeniser. The system may be configured to provide the graphene for mixing with the fluid medium under an inert atmosphere. For example, the graphene may be conveyed from storage under inert gas, and may use an inert gas as a carrier gas to transport the graphene for mixing with the fluid medium. Graphene may for example be produced under inert atmosphere in the plasma reactor system, and stored or conveyed under inert atmosphere for mixing with the fluid medium. In this way, graphene can be produced and dispersed in a fluid medium efficiently without being exposed to air. The plasma reactor system may be as defined previously herein. The plasma reactor system may for example be configured to produce hydrogen and graphene from a process gas comprising hydrocarbons, preferably methane. The system may comprise a separation system configured to separate the hydrogen and graphene prior to mixing the graphene with the fluid medium, for example wherein the system is configured to purge a hydrogen-graphene mixture with inert gas to provide a mixture of graphene and inert gas for mixing with the fluid medium. The plasma reactor system may be a containerised system configured to produce the graphene in the plasma reactor system within the container, for example wherein the container comprises a shipping container. The means for mixing and / or the high-pressure homogeniser may be disposed inside the container with the plasma reactor system. Alternatively, the plasma reactor system may be disposed in a first container, and the means for mixing and / or the high-pressure homogeniser may be disposed within a second container configured to couple to and receive the graphene from the first container. In this way, a modular containerised system configured to disperse graphene in a fluid medium may be efficiently transportable to a location at which graphene is being produced and operable without removing and constructing apparatus from the container. The system may comprise a processing system configured to modify one or more components of the dispersion, for example to induce a chemical reaction of one or more components of the graphene dispersion, for example wherein the processing system comprises a heater configured to heat the graphene dispersion, a plasma reactor, and / or a microwave reactor. The system may comprise a dryer configured to dry the graphene dispersion to provide a modified graphene product, for example wherein the dryer comprises a spray dryer. Where the system is a containerised system, the further processing apparatus, such as chemical or physical modification or drying, may be disposed in the container with the high-pressure homogeniser, or may in some instances be disposed in a further container configured to receive the graphene suspension from a container comprising the high-pressure homogeniser system. A further aspect provides a process for preparing a dispersion of graphene in a fluid medium, the process comprising: providing graphene powder; mixing said graphene powder with a fluid medium to provide a graphene suspension; and directing said graphene suspension into one or more microchannels of a high-pressure homogeniser at a pressure of at least 15 MPa so as to subject said graphene suspension to a shear rate of greater than 105 / s. As will be appreciated, the graphene powder may be as defined previously herein and may be obtained from operation of a plasma reactor system as described. Other features of the process of this aspect may be as defined previously herein. Brief description of Figures Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which: Figure 1 shows schematically an arrangement of a system for producing and dispersing graphene in a fluid medium; Figures 2a and 2b show graphs of lateral flake size as measured by SEM for samples of graphene produced by a plasma reactor system according to the present disclosure; and Figures 3a and 3b show graphs of Raman spectra measured for samples of graphene produced by a plasma reactor system according to the present disclosure. Detailed Description Figure 1 shows a schematic example of a system configured to produce graphene and to direct the graphene for dispersion in a fluid medium using a high-pressure homogeniser. Specifically, Figure 1 shows a containerised system 100 that is configured to receive a flow of a carbon-containing process gas 102, which may for example be methane or natural gas. The process gas flow 102 is directed to a plasma reactor comprising a reaction chamber 106 via a plasma nozzle (not shown) at an entrance to the reaction chamber 106. The process gas is converted to a mixture of graphene and hydrogen in the plasma nozzle and reaction chamber 106. As will be appreciated, the system may in some instances not be containerised, or the different elements of the system may be disposed in a combination of separate containers, linked so as to form a process flowthrough each container. At 108, the mixture from the reaction chamber 106 is separated into a graphene stream 112 and a hydrogen stream 110. While separation means 108 is shown after the plasma reactor reaction chamber 106, hydrogen and graphene separation may be effected directly from the reaction chamber 106, for example where graphene exits the reaction chamber 106 from a separate outlet to the hydrogen gas (e.g. where hydrogen flow 110 exits at the upper end of reaction chamber 106 and graphene flow 112 exits at the lower end of the reaction chamber 106). The graphene flow 112, in the form of a powder, is conveyed to a high-pressure homogeniser system 114, where it is mixed with a flow of a fluid medium 116. In some instances, the fluid medium may comprise a mixture of components that are provided separately to the system, for example via a secondary input stream 118 (and further input streams where necessary). A further gas input flow 120 may be provided so as to provide an inert gas, for example to purge any remaining hydrogen and provide an inert atmosphere, ora reactive gas where desired to react with or functionalise the graphene or fluid medium. As will be appreciated, where an inert gas is provided via 120, this may be at any stage, for example prior to mixing the graphene with the fluid medium. At high-pressure homogenisation system 114, the graphene flow 112 and fluid medium 116 (and optionally further fluid flows 118 etc) are mixed to provide a graphene suspension that is directed into one or more microchannels of the high-pressure homogeniser. As described herein, where desired, the fluid medium 116 (and / or further flows 118 etc.) may be heated, for example in order to melt a precursor to provide the fluid medium for mixing with the graphene (or to provide the graphene suspension after an unmelted or viscous material is mixed with the graphene before heating). The homogeniser applies high shear forces to the graphene suspension by passage through the one or more microchannels to provide a graphene dispersion 122. The graphene dispersion 122 may be provided as product directly from stream 122 or may be further processed to change its chemical or physical nature. As shown in Figure 1, the graphene suspension 122 is passed to a further processing system 124. The further processing system 124 may for example comprise a heating system configured to heat the graphene suspension, a plasma reactor system, or a microwave reactor system. As will be appreciated the further processing system 124 may be selected based on the requirements of the product being produced, for example based on the nature of the fluid medium in which the graphene is dispersed. The further processing system 124 may chemically alter the graphene suspension, for example wherein heating, applying microwave radiation, or chemical reagents, induces chemical reaction between components of the fluid medium, chemical changes, such as surface modification, of the graphene, and / or reactions between the fluid medium and the graphene. The modified graphene suspension from 124 is passed via pathway 126 to a dryer 128, such as a spray dryer, and the dried graphene-containing product stored in container 130. As will be appreciated, the further processing system 124 may be omitted and a graphene dispersion 122 from the homogeniser 114 may be passed directly to the dryer 128. Alternatively, the dryer 128 may be omitted and the modified graphene suspension may be passed directly as a fluid product to storage 130. Storage container 130 may store the product inside the containerised system 100 or may instead form an outlet where the graphene-containing product may be provided from the container as an output stream to an external system or storage means. It will also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and / or supplied and / or used independently. Other examples and variations will be apparent to the skilled addressee in the context of the present disclosure. Examples The graphene powder used in the following examples was provided by operation of a plasma reactor system as described previously herein. The process gas fed to the system was methane and graphene samples were separated from hydrogen by filtration and collected. Figures 2 and 3 show SEM and Raman spectroscopy analysis of graphene for use in a process according to the present disclosure and produced by conversion of methane in a plasma reactor system as described previously. Figures 2a and 3a correspond to a first sample and Figures 2b and 3b correspond to a second, different sample. Both samples were produced by a plasma reactor system, but differed in the scale and dimensions of the system. As can be seen, in Figures 2a and 2b, the graphene produced and measured by SEM shows a lateral flake size distribution with no substantial presence of flakes larger than 500 nm, and where the majority of flakes are larger than 100 nm. The form of the graphene produced in this way, along with the gas phase formation of the graphene, may contribute to the agglomeration in the powder produced, which makes dispersion of this graphene with a high-pressure homogeniser surprisingly effective. Figures 3a and 3b show Raman spectra of the graphene samples corresponding to those measured in Figures 2a and 2b. The Raman spectra were measured using an excitation laser wavelength of 532nm at a power <0.25mW under a 50x objective lens. The strong 2D peak in the spectra indicates a high crystalline structure in both samples and low defect density is confirmed by the D peak. Mixtures of the graphene powder with a fluid medium were fed to a commercial high-pressure homogeniser system (designed for use in food processing to form emulsions), pressurised to around 200 MPa and passed through the microchannels of a reaction chamber with a flow velocity of around 700 m / s and channels having hydraulic diameters of 125 pm and 200 pm. The system also uses an active cooling fluid heat exchanger on its outlet. In different procedures, graphene powder was mixed with a red oxide coating, an ink, a polyester or epoxy resin, and additives and liquid masterbatches for treatment of battery materials. In each case, in addition to excellent uniformity, the products produced were generally found to have improved stability in terms of separation, a more glossy finish and lower viscosity than an equivalent mixture formed by mixing without the high-pressure homogeniser. In particular, comparison of the hydrogen permeation properties of the red oxide coating prepared using typical shear mixing techniques (rotor stator) against the present high-pressure homogenisation process showed that the present high-pressure homogenisation process had superior performance. In addition, the present high-pressure homogenisation process was able to effectively deagglomerate and disperse the graphene which reduced the viscosity when compared to the typical shear mixing method, allowing more graphene to be added to the formulation. A water based conductive ink was also formulated with graphene, a cellulose based binder, and water. When mixed with typical techniques including sonication or high shear mixing (rotor stator), the hydrophobic nature of the graphene led to a non-uniform dispersion of the particles, as well as unwetted particles settling to the top of the suspension. The same formulation when applied with the present high-pressure homogenisation process was immediately dispersed, showing no separation or floating surface particles. The surface finish and conductivity of the preparation made by typical methods was also worse compared with the sample produced by the present high-pressure homogenisation process. Li-ion battery cathodes based on active materials NMC (nickel manganese cobalt) and LFP (lithium iron phosphate) were formulated into a slurry (composed of the active material, binders, conductive additive, and solvent). The solvent was NMP (N-methyl-2-pyrrolidone) and the binder was PVDF (polyvinylidene fluoride or polyvinylidene difluoride), while the conductive additive was a mixture of carbon black and graphene. The solids composition was 96:2:2 (96 wt.% active material, 2 wt. % PVDF, 2 wt.% conductive carbons). In each case materials were processed using 3 techniques: planetary centrifugal mixing (speed mixer), rotor-stator high shear mixing, or high-pressure homogenization in accordance with the present disclosure. The coatings formed from the processed slurries were observed for defects visually and by SEM imaging. It was found the both the speed mixer and the rotor-stator mixer showed significant coating defects visually as well as under SEM imaging. In addition, significant agglomerations and cracking were present which led to instability in the battery performance or unusable regions of the coating, as well as high batch to batch variability. The samples prepared by the high-pressure homogenization process had consistent, uniform coatings and showed no agglomerations or defects / cracking under SEM analysis. Dispersions of graphene in water were formed with the present high-pressure homogenisation process. Dispersions at graphene loadings as high as 25 wt.% were produced and found to provide a high-quality and stable dispersion, superior to that produced by other mixing techniques. The high loading of graphene allows this graphenewater formulation to be used effectively as a masterbatch or concentrate for easy dilution into water-based systems, such as coatings, inks, or as an admixture into cementitious systems such as mortar or concrete. The formulation has been found to enhance mechanical and electrical properties when used in these applications. Similarly, dispersions of 25 wt.% graphene in resin, such as epoxy resin were formed with the present high-pressure homogenisation process. This formulation has also been found to provide a high-quality and stable dispersion, superior to that produced by other mixing techniques. The high loading of graphene allows this formulation to be used effectively as a masterbatch or concentrate for easy dilution into resin-based systems, such as coatings or composites. The formulation has shown favourable mechanical, electrical, thermal, and barrier properties in the respective applications.

Claims

1. A process for preparing a dispersion of graphene in a fluid medium, the process comprising:providing graphene, said graphene obtained from operation of a plasma reactor system configured to produce graphene from a carbon-containing process gas;mixing said graphene with a fluid medium to provide a graphene suspension; anddirecting said graphene suspension into one or more microchannels of a high-pressure homogeniser at a pressure of at least 15 MPa so as to subject said graphene suspension to a shear rate of greater than 105 / s.

2. A process according to claim 1, wherein the graphene suspension is directed into said one or more microchannels of the high-pressure homogeniser so as to subject said graphene suspension to a shear rate of greater than 106 / s, for example greater than 107 / s.

3. A process according to claim 1 or claim 2, wherein the graphene suspension is directed into said one or more microchannels of the high-pressure homogeniser at a pressure of at least 20 MPa, preferably at least 50 MPa, for example at least 100 MPa.

4. A process according to any one of the preceding claims, wherein the graphene suspension is directed into said one or more microchannels of the high-pressure homogeniser at a pressure of 500 MPa or less, preferably 400 MPa or less, for example 300 MPa or less.

5. A process according to any one of the preceding claims, wherein the one or microchannels are configured to provide one or more changes in the direction of flow of said graphene suspension through the one or more microchannels and / or to split and recombine the flow of said graphene suspension.

6. A process according to any one of the preceding claims, wherein the one or more microchannels have a diameter or width of from 10 pm to 800 pm, preferably from 20 pm to 500 pm, for example from 50 pm to 300 pm such as from 80 pm to 250 pm.

7. A process according to any one of the preceding claims, wherein the fluid medium comprises a paint, a lacquer, a varnish, a polymer or a polymer precursor such as a resin, a wax, a silane, biopolymers or bio-derived polymers such as cellulose, lignosulfonates or derivatives thereof, coal tar pitch, a battery electrode slurry, and / or wherein the fluid medium further comprises carbon allotropes such as carbon nanotubes, fullerenes, carbon black, graphite or diamond, boron nitride allotropes such as nanotubes or flakes, one or more metal oxide salts such as titanates, zirconates or aluminates, surfactants, elemental or oxide-based microparticles or nanoparticles such as elemental or oxide-based microparticles or nanoparticles of S, Au, Ag, Pd, Fe, Si, Ti, Mo, or combinations thereof.

8. A process according to any one of the preceding claims, wherein the process comprises heating the fluid medium and / or the graphene suspension before and / or after directing the graphene suspension into said one or more microchannels, for example wherein the fluid medium is melted in order to provide the graphene suspension, or wherein the graphene suspension is heated so as to induce a chemical reaction of one or more components of the graphene suspension.

9. A process according to any one of the preceding claims, wherein the process is conducted under vacuum or an inert atmosphere.

10. A process according to any one of the preceding claims, wherein the process comprises providing said graphene by providing a carbon-containing process gas, for example a process gas comprising hydrocarbons, to a plasma reactor system configured to produce hydrogen and graphene from the process gas.

11. A process according to claim 10, wherein the process gas comprises or consists essentially of methane, for example wherein the process gas comprises or consists essentially of natural gas.

12. A process according to claim 10 or claim 11, wherein the hydrogen and graphene are separated prior to mixing the graphene with the fluid medium, for example wherein a hydrogen-graphene mixture is purged with inert gas to provide a mixture of graphene and inert gas for mixing with the fluid medium.

13. A process according to any one of claims 10 to 12, wherein the plasma reactor system is a containerised system configured to produce the graphene in the plasma reactor system within the container, for example wherein the container comprises a shipping container.

14. A process according to claim 13, wherein the steps of mixing the graphene with the fluid medium and / or directing the graphene suspension into one or more microchannels of a high-pressure homogeniser are conducted within the container.

15. A process according to claim 13, wherein the plasma reactor system is disposed in a first container, and the steps of mixing the graphene with the fluid medium and / or directing the graphene suspension into one or more microchannels of a high-pressure homogeniser are conducted within a second container configured to couple to and receive the graphene from the first container.

16. A process according to any one of the preceding claims, comprising the step of processing the graphene dispersion to modify one or more components of the dispersion, for example by heating the graphene dispersion, providing the graphene dispersion to a chemical reactor, or providing the graphene dispersion to a microwave reactor to induce a chemical reaction of one or more components of the graphene dispersion.

17. A process according to any one of the preceding claims, comprising the step of drying the graphene dispersion, such as by spray drying, to provide a modified graphene product, for example a chemically functionalised graphene product.

18. A process according to any one of the preceding claims, wherein at least 50 %, preferably at least 70 %, more preferably at least 90%, of the graphene provided for mixing with the fluid medium has a lateral flake size of less than 500 nm.

19. A process according to any one of the preceding claims, wherein the specific surface area of the graphene provided for mixing with the fluid medium is from 80 m2 / g to 500 m2 / g, preferably from 100 m2 / g to 350 m2 / g.

20. A process according to any one of the preceding claims, wherein the Raman spectrum of the graphene provided for mixing with the fluid medium exhibits one or more of: a D peak intensity to G peak intensity ratio, I(D) / l(G), of less than 1; a D’ peak intensity to G peak intensity ratio, l(D’) / l(G), of less than 0.5; a G peak having a full width at half maximum of from 25 to 50 cm-1; a 2D peak intensity to G peak intensity ratio, I(2D) / I(G), of greater than 0.65; and a single-component 2D peak.

21. A process according to any one of the preceding claims, wherein the graphene provided for mixing with the fluid medium comprises at least 90 % carbon, preferably at least 95 % carbon, for example at least 98 % carbon.

22. A system for preparing a dispersion of graphene in a fluid medium, the system comprising:a plasma reactor system configured to produce graphene from a carbon-containing process gas;means for mixing said graphene with a fluid medium, wherein the system is configured to mix graphene produced by the plasma reactor system with the fluid medium to provide a graphene suspension; anda high-pressure homogeniser comprising one or more microchannels, wherein the system is configured to direct the graphene suspension into one or more microchannels of a high-pressure homogeniser at a pressure of at least 15 MPa so as to subject said graphene suspension to a shear rate of greater than 105 / s.

23. A system according to claim 22, wherein the system is configured to subject the graphene suspension to a shear rate of greater than 106 / s, for example greater than 107 / s.

24. A system according to claim 22 or claim 23, wherein the system is configured to direct the graphene suspension into the one or more microchannels of the high-pressure homogeniser at a pressure at a pressure of at least 20 MPa, preferably at least 50 MPa, for example at least 100 MPa, and / or at a pressure of 500 MPa or less, preferably 400 MPa or less, for example 300 MPa or less.

25. A system according to any one of claims 22 to 24, wherein the one ormicrochannels are configured to provide one or more changes in the direction of flow of said graphene suspension through the one or more microchannels and / or to split and recombine the flow of said graphene suspension.

26. A system according to any one of claims 22 to 25, wherein the one or more microchannels have a diameter or width of from 10 to 800 pm, preferably from 20 pm to 500 pm, for example from 50 pm to 300 pm, such as from 80 pm to 250 pm.

27. A system according to any one of claims 22 to 26, wherein the system comprises a heater configured to melt a precursor material to provide the fluid medium.

28. A system according to any one of claims 22 to 27, further comprising a source of the fluid medium, for example a reservoir comprising the fluid medium or a precursor of the fluid medium.

29. A system according to any one of claims 22 to 28, wherein the system is configured to provide the graphene for mixing with the fluid medium under an inert atmosphere.

30. A system according to any one of claims 22 to 29, wherein the plasma reactor system is configured to produce hydrogen and graphene from a process gas comprising hydrocarbons.

31. A system according to claim 29, wherein the system comprises a separation system configured to separate the hydrogen and graphene prior to mixing the graphene with the fluid medium, for example wherein the system is configured to purge a hydrogen-graphene mixture with inert gas to provide a mixture of graphene and inert gas for mixing with the fluid medium.

32. A system according to any one of claims 22 to 31, wherein the plasma reactor system is a containerised system configured to produce the graphene in the plasma reactor system within the container, for example wherein the container comprises a shipping container.

33. A system according to claim 32, wherein the means for mixing and / or the high-pressure homogeniser are disposed inside the container.

34. A system according to claim 32, wherein the plasma reactor system is disposed in a first container, and the means for mixing and / or the high-pressure homogeniser are disposed within a second container configured to couple to and receive the graphene from the first container.

35. A system according to any one of claims 22 to 34, comprising a processing system configured to modify one or more components of the dispersion, for example to induce a chemical reaction of one or more components of the graphene dispersion, for example wherein the processing system comprises a heater configured to heat the graphene dispersion, a chemical reactor, and / or a microwave reactor.

36. A system according to any one of claims 22 to 34, comprising a dryer configured to dry the graphene dispersion to provide a modified graphene product, for example wherein the dryer comprises a spray dryer.

37. A process for preparing a dispersion of graphene in a fluid medium, the process comprising:providing graphene powder;mixing said graphene powder with a fluid medium to provide a graphene suspension; anddirecting said graphene suspension into one or more microchannels of a high-pressure homogeniser at a pressure of at least 15 MPa so as to subject said graphene suspension to a shear rate of greater than 105 / s.

38. A process according to any one of claims 1 to 21 or 37, wherein, the process comprises mixing the graphene with the fluid medium to provide the graphene suspension, then directing said graphene suspension through the one or more microchannels of the high-pressure homogeniser less than 15 times, preferably less than 10 times, for example wherein the graphene suspension is directed through the one or more microchannels of the high-pressure homogeniser no more than 5 times, or only a single time, to form thegraphene dispersion.

39. A process according to claim 37 or claim 38, wherein the graphene provided for mixing with the fluid medium is as defined in any of claims 18 to 21.A

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