Method and device for producing carbon materials incorporating additives

Abstract

A method of producing carbon material from one or more carbon containing feedstock gases is described. The method comprises the following steps: providing an electrolysis cell 120 comprising an anode

Description

The present disclosure relates generally to the production of carbon materials incorporating additives, using carbon-containing gases as feedstock gases. Carbon materials in different forms such as powders and dispersions are becoming increasingly important in the production of energy storage devices (e.g., supercapacitors, batteries, fuel cells and other energy storage systems) due to their electrical and mechanical properties. For example, transition metal nitrogen-doped carbon materials (M-N-C) have gained attention as promising candidates for replacing Pt catalysts in polymer electrolyte membrane fuel cells (PEMFCs), due to the low cost and high electrocatalytic activity of the doped carbon materials. In recent years, many reports have shown copper-nitrogen codoped carbon nanomaterials to be efficient and durable electrocatalysts. Catalysts containing Cu-Nx moieties supported on a carbon matrix, also referred to as single atom catalysts (SACs), reduce the oxygen through the 4-electron pathway. Cu-Nx sites consist of copper atoms surrounded by x nitrogen atoms (mainly x = 2, 3, 4). Transition metal macrocycles, such as metal phthalocyanines and porphyrins, are often used for fabrication of SACs. These molecules have metal atoms bonded to 4 nitrogen atoms and therefore serve as active sites in M-N-C catalysts. Copper particles encapsulated by N-doped carbon (Cu@NC) present another group of Cu-N-C catalysts. Carbon shells protect the metal particles from corrosion and oxidation and regulate the electronic interactions with adsorbates in the oxygen reduction reaction (ORR). Moreover, the activity of M-N-C materials is partly attributed to the nitrogen moieties in the carbon lattice, pyridinic-N and graphitic-N being the most favourable species for effective oxygen reduction. Molten salt CO2 capture and electrochemical transformation (MSCC-ET) has recently gained attention as an attractive method for producing carbon-based materials. In this process carbonate-containing molten salts are split into solid carbon and O2 ions that absorb the CO2 from the atmosphere. Carbon nanotubes, carbon spheres, and graphite have been successfully synthesized using MSCC-ET. The morphologies of the CO2-derived carbon materials can be varied according to the electrolysis conditions. For example, carbon nanotubes, carbon spheres, and graphite have been successfully synthesized using MSCC-ET. Liu et al in “Controlled Growth of Unusual Nanocarbon Allotropes by Molten Electrolysis of CO2” (Catalysts 2022, 12, 125) disclose a method of synthesising carbon materials via electrolysis using molten carbonate electrolytes and discusses the effect of different metal and metal oxide additives in the molten electrolyte on the resulting nanocarbon morphology. Remmel et al. in “Nickel and Nitrogen-Doped Bifunctional ORR and HER Electrocatalysts Derived from CO2” (ACS Sustainable Chem. Eng. 2022, 10, 1, 134 145) disclose the synthesis of nickel and nitrogen co-doped carbon materials via electrolysis using molten carbonate electrolytes containing NiO as an additive and subsequent N-doping using DCDA and PVP. The present invention seeks to provide a method and device for producing carbon materials incorporating additives, using carbon-containing gases as a feedstock gas. According to an aspect of the present invention there is provided a method of producing carbon material from one or more carbon-containing feedstock gases, the method comprising the following steps: providing an electrolysis cell comprising an anode, a cathode and a reaction vessel arranged to contain a molten electrolyte, wherein the cathode is arranged at least at a base of the reaction vessel; providing a molten electrolyte within the reaction vessel so as to permit an electric current to pass between the anode and the cathode through the electrolyte, wherein an additive is arranged as a layer coating a surface of the cathode at the base of the reaction vessel so as to be positioned on the surface of the cathode and in contact with the molten electrolyte; and applying a voltage across the anode and the cathode, whilst providing one or more carbon-containing feedstock gases to the molten electrolyte, thereby producing the carbon material. The method enables the production of carbon materials incorporating an additive into the carbon network during the formation of carbon from a carbon-containing feedstock gas, such as CO2. By arranging the additive as a layer coating a surface of the cathode, the carbon material is formed through the layer of additive, allowing the additive to be embedded in the carbon network of the carbon material as it is being formed. This results in the additive being well dispersed in the carbon material formed at the cathode. The method also allows for the incorporation of an additive into the carbon matrix without requiring an additional doping step subsequent to the synthesis of the carbon material in order to incorporate the additive into the carbon network. The electrolysis cell comprises a reaction vessel arranged to contain the molten electrolyte, wherein the cathode is arranged at least at a base of the reaction vessel, such that, when the molten electrolyte is accommodated within the reaction vessel, the surface of the cathode coated with the additive is arranged within the molten electrolyte. The method thereby provides a suitable arrangement by which a wide range of additive materials can be coated on the surface of the cathode. For example, an additive material in the form of a powder can be spread across the surface of the cathode so as to coat said surface and allow for incorporation of the additive into the carbon material formed at the cathode surface during electrolysis. It will be appreciated that the reaction vessel may have any appropriate shape that is suitable for containing the molten electrolyte. The reaction vessel may be any three-dimensional structure defining a cavity in which the molten electrolyte can be accommodated. The reaction vessel may be substantially cuboidal or substantially cylindrical, for example. The reaction vessel preferably comprises a substantially flat internal base surface at which the cathode is arranged. The cathode may define at least a portion of an internal base surface of the reaction vessel, or may define the whole of an internal base surface of the reaction vessel. The cathode may form at least a base of the reaction vessel (in particular, a substantially flat internal base surface of the reaction vessel) and in some embodiments may form the entire reaction vessel (that is, the reaction vessel is formed of a cathode material such that the reaction vessel acts as the cathode during the electrolysis process). In other words, the cathode may be shaped to form the reaction vessel (that is, the cathode is the reaction vessel). The molten electrolyte can thus be accommodated within the cathode, and the additive is arranged as a layer coating an internal base surface of the cathode. This provides a suitable arrangement by which the additive can be provided as a layer coating a surface of the cathode, so as to be positioned between the cathode and the molten electrolyte. The additive may have a density that is greater than a density of the molten electrolyte, at the reaction temperature. Accordingly, the additive can be coated on the cathode by simply placing the additive on the cathode located at the base of the reaction vessel. Due to the relative densities of the additive and molten electrolyte, the additive remains coated on the cathode by gravity during the synthesis step. The additive is thereby kept adjacent the cathode during the synthesis step, allowing for effective incorporation of the additive into the carbon material being formed at the cathode. The additive may be placed into the reaction vessel, so as to coat the cathode surface (for example, by spreading the additive material across the surface of the cathode), prior to adding the electrolyte to the reaction vessel. The molten electrolyte may be placed into the reaction vessel first (either by melting the electrolyte and subsequent addition to the reaction vessel, or by melting the electrolyte within the reaction vessel), and the additive subsequently added to the molten electrolyte and allowed to settle at the base of the reaction vessel by gravity, so as to coat the cathode surface, prior to electrolysis taking place. The additive may comprise a metal, in particular at least one of copper, cobalt, nickel, aluminium or titanium, which may be elemental metal or a metal compound. The additive may include a metal complex, such as a transition metal complex. The metal complex may be a macrocyclic metal complex. The metal complex may include a M-Nx moiety, where M is the metal and optionally where x = 2, 3 or 4. That is, the MNX moiety consists of a metal atom surrounded by x nitrogen atoms. The additive may include a metal phthalocyanine or metal porphyrin complex. The additive may include a metal complex selected from the group consisting of cooper phthalocyanine, cobalt phthalocyanine, nickel phthalocyanine, aluminium phthalocyanine and titanium phthalocyanine, or any combination thereof. In embodiments, a loading of the additive on the surface of the cathode is in the range 1 to 20 mg cm'2. In one embodiment, the additive is copper phthalocyanine, and a loading of the copper phthalocyanine on the surface of the cathode is in the range 2 to 8 mg cm'2, and in particular may be 4 mg cm'2. The electrolyte may include a carbonate-group. The electrolyte may include Na2COs, Li2CO3, K2CO3, BaCOs, CaCOs or any combination thereof. In one embodiment, the electrolyte includes a eutectic mixture of Na2CO3, Li2COs and K2CO3. The provision of a eutectic mixture of salts lowers the melting point of the electrolyte and therefore allows the synthesis to be carried out at a lower temperature. This leads to an increase in the specific surface area and enhanced microporosity of the synthesised materials, which in turn improves the electrocatalytic performance of the carbon materials. The electrolyte may be heated to a temperature of from 400 °C to 900 °C for melting the electrolyte and maintaining the electrolyte in the molten state. Application of a voltage across the anode and the cathode may be effected by a power source adapted to provide to the device 12,000 to 24,000 ampere (A) current, for example 4,000 A. According to embodiments of the present disclosure, the applied direct current density is in the range from 100 A m'2 up to 20,000 (preferably 6,000) A m'2 For example, when using a direct current (DC) power supply or rectifier the positive (+) terminal is connected to the one or more anode(s) and negative (-) terminal connected to the one or more cathode(s). The high current density and low voltage enables slower growth of carbon material and thus to achieve better structure of the carbon material. As a power source, renewable energy such as solar, wind, hydro etc. power can be used. The carbon-containing feedstock gas may comprise one or a combination of CO2, CH4 and CO. The carbon containing feedstock gas preferably includes CO2. The feedstock gas may be, for example, at least one of an exhaust gas or industrial gas comprising one or more of CO, CO2 or CH4. Using feedstock gases such as exhaust gases and industrial flue gases or other waste gases comprising CO2 enables environmental damage to be reduced. For example, the CO2 may be industrial waste CO2 or exhaust CO2 from heavy industry emitters, which according to the present disclosure is used as the feedstock for producing carbon materials. The carbon-containing feedstock gas may be provided to the electrolysis cell by any appropriate means. The feedstock gas may be present in an atmosphere of the electrolysis cell, such that the electrolyte is in fluid communication with the atmosphere containing the feedstock gas. For example, where the carbon-containing feedstock gas is CCh, the electrolyte may be open to the air such that CO2 naturally present in the air is captured into the molten electrolyte during the electrolysis process. The atmosphere of the electrolysis cell may alternatively be controlled to contain a concentration of CO2 that is greater than the natural concentration of CO2 present in air. For example, an exhaust gas, industrial gas or other carbon-containing waste gas may be introduced into the atmosphere of the electrolysis cell, such that the electrolyte is in fluid communication with the waste gas so as to be capable of capturing CO2, CH4 and / or CO into the molten electrolyte during the electrolysis process. Other feedstock gases may be provided in the same way. Alternatively or additionally, the carbon-containing feedstock gas may be introduced directly into the molten electrolyte during the electrolysis process, for example by feeding the gas into the molten electrolyte via a tube. The feedstock gas may be bubbled into the molten electrolyte. Additional dopants can be incorporated into the carbon material by selection of an appropriate cathode material. For example, the cathode may comprise at least one metal selected from iron, nickel and chromium or any combination thereof. Incorporation of such dopant metals during the synthesis allows for further optimisation and enhancement of the electrocatalytic properties of the material, without requiring additional doping steps. In some embodiments, the cathode may comprise stainless steel. Alternatively, the cathode may be formed of an inert cathode material, such that the only dopants incorporated into the carbon material are derived from the additive. The cathode may comprise at least one of graphite or silicon carbide. The method may further include removing the carbon material from the cathode. Subsequent steps can then be carried out on the recovered carbon materials. For example, the method may comprise a subsequent nitrogen-doping step. This is of benefit, in particular, where the additive includes a metal complex containing M-Nx moi eties. This increases the basicity of the carbon support, which modifies the electronic properties of the carbon support and enhances the activity of the oxygen reduction reaction activity of the metal-nitrogen active sites in the material. The electrocatalytic properties of the carbon materials are thereby enhanced. The nitrogen doping step may comprise pyrolyzing a mixture of the carbon material recovered from the cathode with a nitrogen-containing doping agent. The nitrogencontaining doping agent may include at least one of polyvinylpyrrolidone and dicyandiamide. According to a further aspect of the present invention, there is provided a carbon material obtainable by a method as described above. The carbon material may comprise graphite as a carbon allotrope. According to a further aspect of the present invention, there is provided a device for producing carbon material from one or more carbon-containing feedstock gases, the device comprising: a reaction vessel arranged to contain a molten electrolytes; an anode; a cathode arranged at least at a base of the reaction vessel; a power source for applying a voltage across the anode and the cathode; and a heating unit arranged to heat the reaction vessel to maintain the electrolyte in a molten state. With this arrangement, the device can be set up to provide a layer of additive coating a surface of the cathode, by arranging the additive material at the base of the reaction vessel so as to cover the cathode. Formation of a carbon material by means of electrolysis using a molten electrolyte and a carbon-containing feedstock gas, as described above in respect of the method of the present disclosure, allows the carbon material to be formed through the layer of additive material, such that the additive material is embedded in the carbon network of the carbon material as it is being formed. This results in the additive being well dispersed in the carbon material formed at the cathode. The device also facilitates the incorporation of an additive into the carbon matrix without a requiring an additional doping step subsequent to the synthesis of the carbon material. The device provides a suitable arrangement by which a wide range of additive materials can be coated on the surface of the cathode. As described above in relation to the method of the present disclosure, it will be appreciated that the reaction vessel may have any appropriate shape that is suitable for containing the molten electrolyte. The reaction vessel may be any three-dimensional structure definition a cavity in which the molten electrolyte can be accommodated. The reaction vessel may be substantially cuboidal or cylindrical, for example. The reaction vessel preferably comprises a substantially flat internal base surface at which the cathode is arranged. The cathode may define at least a portion of an internal base surface of the reaction vessel. The cathode may form at least a base of the reaction vessel and in some embodiments may form the entire reaction vessel (that is, the reaction vessel is formed of a cathode material such that the reaction vessel is the cathode during the electrolysis process). In particular embodiments, the cathode may be shaped to form the reaction vessel (that is, the cathode is the reaction vessel). The molten electrolyte can thus be accommodated within the cathode, and the additive can be arranged as a layer coating an internal base surface of the cathode. This provides a suitable arrangement by which the additive can be arranged as a layer coating a surface of the cathode, so as to be positioned between the cathode and the molten electrolyte. This arrangement is particularly beneficial, for example, where the additive has a density that is greater than a density of the molten electrolyte, as described above. The device may comprise an additive arranged as a layer coating a surface of the cathode at the base of the reaction vessel. The device may comprise a gas supply unit arranged to provide one or more feedstock gases into the molten electrolyte in the reaction vessel. The gas supply unit may be configured to provide the gas continuously when the device is running. The gas supply unit may comprise means to diffuse the feedstock gases evenly into the electrolyte. Such means to diffuse the feedstock gases may be piping of the feedstock gases arranged in the reaction vessel. Optionally, the gas supply unit comprises electrolyte mixing means to diffuse the CO2 gas evenly into the electrolyte. The electrolyte mixing means enable the gas to be dissolved and distributed in the electrolyte evenly. Slow mass transport in the rector creates a concentration gradient for oxides, leading to faster corrosion of the anode. The electrolyte mixing means may be an S-shaped tube comprising holes and arranged in the electrolysis cell chamber inside the electrolyte. If the feedstock gas is pumped through the S-shaped tube inside the molten electrolyte it generates feedstock gas bubbling in the electrolyte and enables the electrolyte to be mixed. Other suitable means for introducing the feedstock gas directly into the molten electrolyte will be conceivable to the skilled person. Non-limiting embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a block scheme illustrating a device for producing carbon materials from one or more feedstock gases; Figure 2 is a schematic illustration of an electrolysis cell of the device of Figure 1; Figure 3 is a flow chart illustrating an example method of producing carbon material; Figures 4A to 4C are SEM images are carbon materials produced according to the method of Figure 3; Figures 4D and 4E are bright-field TEM images of a carbon material produced according to the method of Figure 3; Figures 4F and 5A are HAADF-STEM images of a carbon material produced according to the method of Figure 3; Figures 5B, 5C and 5D are STEM-EDX images of a carbon material produced according to the method of Figure 3; Figures 6A, 7A and 8A are HAADF-STEM images of carbon materials produced according to the method of Figure 3; Figures 6B to 6E, 7B to 7E and 8B to 8E, are STEM-EDX images of carbon materials produced according to the method of Figure 3; Figure 9 shows X-ray diffraction patterns of carbon materials produced according to the method of Figure 3; Figure 10A is a graph showing polarization curves of carbon materials produced according to the method of Figure 3; Figure 1 OB is a graph showing Tafel slopes of carbon materials produced according to the method of Figure 3; Figures 11A to 1 IC are graphs showing Koutecky^Levich plots for oxygen reduction on carbon materials produced according to the method of Figure 3; Figure 12 shows X-ray diffraction patterns of further carbon materials produced according to the method of Figure 3; Figures 13 and 14 are graphs showing polarization curves for the further carbon materials produced according to the method of Figure 3; and Figures 15A to 15D are SEM images of the further carbon materials. Referring to Figure 1, there is shown a block scheme illustrating a device 100 for producing carbon materials from one or more feedstock gases according to the present disclosure. The device 100 comprises an electrolysis cell 120, a power source 170, a heating unit 180 and an optional gas supply unit 190. Referring to Figure 2, there is shown a schematic illustration of the electrolysis cell 120 of Figure 1. The electrolysis cell 120 comprises an anode 130 and a cathode 140. The cathode 140 is shaped to form a reaction vessel for accommodating a molten electrolyte 150 and a layer of additive 160. The anode 130 is insertable into an internal space defined by the cathode 140, so as to be placed into the molten electrolyte 150 contained within the cathode 140. The anode 130 may alternatively be in a fixed position relative to the cathode 140, said fixed position permitting the anode 130 to be immersed within the molten electrolyte 150, in use. The anode 130 and cathode 140 are connected via the power source 170 to permit the application of a voltage across the anode 140 and cathode 150. It will be appreciated that the reaction vessel may be formed in any appropriate shape capable of accommodating the molten electrolyte 150. In the illustrated embodiment, the reaction vessel defined by the cathode 140 has a substantially cuboidal or rectangular cuboidal form, so as to adopt a ‘basket’ shape, however other shapes such as cylinders may be used. Various other suitable forms of cathode will be conceivable to the skilled person. Though not illustrated in Figure 2, a gas supply unit 180 (where one is provided) can be arranged within the internal space defined by the cathode 140, either in a fixed position or insertable thereinto. The gas supply unit 180 may take the form of one or more tubes arranged to direct a gas directly into the electrolyte 150 accommodated within the reaction vessel defined by the cathode 140. Alternatively or additionally, the carbon-containing feedstock gas may be introduced into an atmosphere 200 of the electrolysis cell, so as to be in fluid communication with the molten electrolyte 150. In this way, the feedstock gas can be captured into the molten electrolyte during the electrolysis process. Figure 3 provides a flow chart illustrating an example method of producing carbon material in accordance with the present disclosure, which will now be described with further reference to the device illustrated schematically in Figures 1 and 2. The example method includes the following steps: SI: providing an electrolysis cell 120 comprising an anode 130 and a cathode 140, the cathode 140 being shaped to form a reaction vessel for containing a molten electrolyte 150. S2: arranging a layer of additive 160 at the base of the reaction vessel defined by the cathode 140, so as to coat an internal base surface 142 of the cathode 140. S3: providing an electrolyte 150 within the reaction vessel defined by the cathode 140 and bringing the electrolyte 150 into a molten state by operation of the heating unit 180, wherein the additive 160 is selected such that it has a density greater than a density of the molten electrolyte 150 at the temperature of reaction, thereby maintaining the additive 160 at the internal base surface 142 of the cathode 140. S4: applying a voltage across the anode 130 and cathode 140 by operation of the power source 170, whilst maintaining the molten electrolyte 150 in equilibrium with an atmosphere 200 containing CO2, thereby generating a carbon material at the internal base surface 142 of the cathode 140. S5: removing the carbon material from the cathode 140. S6: performing a further doping step on the synthesised carbon material. In the described example, the electrolyte 150 is melted within the reaction vessel defined by the cathode 140 in step S3. However, the electrolyte 150 may alternatively be brought into a molten state outside of the reaction vessel and subsequently transferred into the reaction vessel by appropriate means. In either case, the heating unit 180 is arranged to heat the electrolyte 150 within the reaction vessel defined by the cathode 140 to a sufficient temperature to maintain the electrolyte 150 in the molten state during electrolysis. In the described example, the additive 160 is selected such that it has a density greater than a density of the molten electrolyte 150 at the temperature of reaction, thereby maintaining the additive 160 at the internal base surface 142 of the cathode 140 by gravity. This provides a suitable arrangement for maintaining the layer of additive 160 at the surface of the cathode 140 during the electrolysis reaction. However, it will be appreciated that the surface of the cathode 140 may be coated by additive 160 by other means conceivable to the skilled person. In at least the described example, steps S2 and S3 may be performed in any order. In particular, where the additive 160 is selected to have a higher density than the molten electrolyte 150, as in the described example, the additive 160 may alternatively be placed into the reaction vessel after the molten electrolyte and allowed to settle to the internal base surface 142 of the cathode 140 prior to the electrolysis reaction. In step S4, for the described example, the carbon-containing feedstock gas (in this case CO2) is provided to the molten electrolyte by maintaining the molten electrolyte 150 in equilibrium with an atmosphere 200 containing CO2. However, the carbon-containing feedstock gas may alternatively or additionally be provided by the optional gas supply unit 190, which may take any appropriate form for introducing a gas to the molten electrolyte 150, as described above. In the described example, step S6 may be, in particular, a nitrogen-doping step. In particular, a mixture of the carbon material recovered from the cathode with a nitrogencontaining doping agent (e.g. polyvinylpyrrolidone and / or dicyandiamide) may be subjected to pyrolysis. It will be appreciated that step S6 will be optional, depending on the carbon material produced and the desired application. The following describes worked examples of the method of producing carbon materials according to the present disclosure. EXAMPLE I Synthesis of carbon materials A DC01 steel ‘basket’ reaction vessel was used as a cathode and a layer of copper phthalocyanine (CuPc) was used as an additive. CuPc was evenly spread at the bottom of the cathode to coat an internal base surface of the cathode, after which cathode was filled with a eutectic mixture of Li2CO3, NaaCOs and K2CO3 (>99.5%, Lach-Ner) as the electrolyte. The basket was then placed into a heating unit and the electrolyte mixture was heated to a temperature of 450 °C. A graphite rod was used as an anode and placed into the molten electrolyte. The electrodes were then connected to a power supply and a 4 V voltage was applied across the electrodes for 2 hours. Three different materials were synthesized with 50 mg, 100 mg, and 200 mg of CuPc coated onto the internal base surface of the cathode basket, corresponding to a loading of 2, 4 and 8 mg cm 2 relative to the surface area of the internal base surface of the cathode (the synthesised materials hereafter being referred to as CO2-CU2.0, CO2-CU4.0 and CO2-CU8.0, respectively). After the synthesis was complete, 5 M HC1 was added, the mixture was sonicated for 30 minutes and left at room temperature for 3 days to remove any carbonate and soluble impurities. Afterwards, the carbon material was filtered and dried. Nitrogen-doping of carbon materials The synthesised materials were subsequently doped with additional nitrogen. 50 mg of each sample of carbon material was first sonicated in ethanol along with 5 mg of polyvinylpyrrolidone (PVP) and 1 g of dicyandiamide (DCDA) for 2 hours. Afterwards, the mixture was dried and placed into a ceramic boat. The boat was then placed into a quartz tube which in turn was placed into a furnace. The furnace was purged with Ar and heated to 800 °C. The material was pyrolyzed for 1 hour, cooled down and removed from the furnace. Results During the synthesis, carbon material is produced by electrolysis of the carbonate salt, creating lithium oxide and solid carbon: CO32 + 4e C + 302 The carbon is deposited onto the insides of the steel reaction vessel that works as a cathode during the electrolysis process, capturing the CuPc (which is known to be stable up to 500 °C) into the carbon network. The steel reaction vessel also acts as a source for Fe doping as iron is dissolved into the carbonate salt during the deposition of carbon. The low synthesis temperature (and high deposition voltage) also serves to create a porous carbon network to allow for better access to the active sites for both CO2 and oxygen reduction. Since the carbonate salt is in equilibrium with the surrounding atmosphere, CO2 is captured into the electrolyte, thereby regenerating Li2CO3 and allowing for a continuous process: CO2+ O2 ^CO32 To incorporate the CuPc into the carbon lattice more strongly, increase the basicity of the carbon material and its nitrogen doping level and homogenize the carbon material, it was then suspended in ethanol and pyrolyzed in the presence of DCDA and PVP (see the nitrogen-doping step described above). Figures 4A to 4C are SEM images of the CO2-CU2.0, CO2-CU4.0 and CO2-CU8.0 carbon materials, respectively, produced as described above. The images reveal the highly porous nature of the materials. Flakes of graphitic carbon are more prominent where less CuPc was incorporated (Figure 4A). A higher CuPc content leads to a more amorphous and porous network (Figures 4B and 4C). At low levels of CuPc (Figure 4A), there are large flakes (d >1 pm) of graphitic carbon visible. Increasing the CuPc content in the reaction vessel decreased the size of these flakes. Figures 4D and 4E are bright-field TEM images of the CO2-CU4.0 material, confirming the co-existence of graphitic flakes with porous carbon material. Figure 4F is an HAADF-STEM image of the C02-Cul00 material, illustrating the well-dispersed copper on the surface of the catalyst (visible as bright dots). Most of the copper is atomically dispersed over the surface of the material, however, Figure 4F also shows the presence of small metal particles. STEM-EDX analysis was carried out in order to elucidate whether or not the origin of the particulate phases was the CuPc or the steel cathode. Figure 5A is a further HAADF-STEM image of the C02-Cu4.o material, showing the presence of two nanoparticles within the carbon network. Figures 5B, 5C and 5D are STEM-EDX images showing the chemical distribution of nitrogen (Figure 5B), copper (Figure 5C) and iron (Figure 5D). As can be seen, the distribution of Cu and N is uniform over all of the material, showing that the homogenous incorporation of Cu from the CuPc and N from the CuPc and DCDA was successful. The Fe STEM-EDX mapping image (Figure 5D) shows Fe being concentrated at the nanoparticles, showing that the particulate phases in the material originate from the steel cathode, rather than the decomposition of CuPc and the formation of copper particles. The results therefore demonstrate the successful incorporation of CuPc as an additive into the carbon network without decomposition. It will be appreciated that utilizing cathodes of different metals can introduce other metal dopants, or utilizing inert cathodes can simply keep the central metal in the additive (in the present example, Cu) as the sole metal dopant. Figures 6A to 8E provide further STEM-EDX analysis of the CO2-Cu2 0 (Figures 6A to 6E), C02-Cu4.o (Figures 7A to 7E) and C02-Cu8.o (Figures 8A to 8E) carbon materials synthesised according to Example 1. Figures 6A, 7A and 8A are HAADF-STEM images of the C02-Cu2.o, C02-Cu4.o and C02-Cug.o materials, respectively. Corresponding STEM-EDX elemental mapping images show the chemical distribution of carbon (Figures 6B, 7B, 8B), nitrogen (Figures 6C, 7C, 8C), copper (Figures 6D, 7D, 8D) and iron (Figures 6E, 7E, 8E) in the carbon materials. For all materials, homogenous incorporation of Cu from the CuPc and N from the CuPc and DCDA was observed, indicating successful incorporation of CuPc as an additive into the carbon network without decomposition. All materials also show incorporation of Fe, with particulate phases of Fe originating from the steel cathode. The C02-Cu2.o, CO2-Cii4 0 and CO2-Cus 0 materials were further analysed by X-ray photoelectron spectroscopy. The surface elemental composition of each material is shown in Table 1. At a loading level of 2 mg cm 2 of basket (C02-Cu2.o), copper was not detected at the surface of the material, indicating the copper content of the material at the surface was insufficient for detection by XPS, despite the confirmation of copper in the material in the STEM-EDX mapping analysis described above. At higher loadings of 4 and 8 mg cm 2, the copper content at the surface of the material was 0.3 and 0.2 at.%, respectively. This indicates that increasing the CuPc loading above a threshold leads to incorporation of copper into the bulk of the material, without increasing the surface content (which is relevant for electrocatalysis). The effect of the Cu on nitrogen content is also indicated by the XPS data: a loading of 4 mg cm’2 CuPc allows for the nitrogen content of CO2-CU4.0 to 5 be 14.5 at.%, much higher than the other samples, resulting in increased ORR activity, as discussed below. A higher nitrogen content is related to the basicity of the catalyst material, known to be an important descriptor for electrocatalytic activity. A higher degree of iron incorporation from the steel basket is also visible, which in turn increases the activity even further. The Fe content was much higher for the CO2-CU4.0 sample than for the other 10 materials, indicating an optimal CuPc loading for incorporating Fe from the steel cathode. Table 1 C at.% N at.% O at.% Cu at.% Fe at.% C02-Cu2.o 88.0 4.4 7.0 nd. 0.6 CO2-CU4.0 77.2 14.5 6.3 0.3 1.7 CO2-CU8.0 87.1 4.8 7.3 0.2 0.7 Figure 9 shows powder X-ray diffraction patterns of the CO2-CU2.0, CO2-CU4.0 and CO2-CU8.0 materials. Graphitic carbon is observed in all three patterns by the (002) 15 diffraction peak at 26.8° 20. The X-ray diffraction patterns support the findings from microscopy imaging: up to an optimal CuPc content, the catalyst material becomes more amorphous (as witnessed by the lower intensity (002) peak of graphite at 26.8° 20 for CO2-Cu4.o relative to CO2-CU2.0), after which it once again graphitizes to a higher degree during the pyrolysis process (as witnessed by the higher intensity (002) peak of graphite at 26.8° 20 29 for CCh-Cug.o relative to CO2-CU4.0). Fe impurities from the steel basket used as the cathode can also be seen in the diffraction pattern, which can also affect the materials’ degree of graphitization and porosity. In order to measure the electrocatalytic activity of synthesized materials, a rotating disk electrode (RDE) method was employed. Firstly, a 1 ml suspension was made containing 2 mg of material and 3 pl of a QAPS-Sm ionomer solution per mg of carbon. The suspension was then pipetted onto glassy carbon disk electrodes that were polished prior to the experiment with 1 pl and 0.3 pl alumina slurries and sonicated consecutively in isopropanol and Milli-Q water to remove the residue. The catalyst loading was 0.2 mg cm 2. The measurements were performed in a 0.1 M KOH solution saturated with oxygen in a five-neck electrochemical cell. A graphite rod was used as a counter electrode and silver / silver chloride electrode (Ag / AgCl) was utilized as a reference electrode. Polarization curves were measured from 0.05 V to -1.15 V vs Ag / AgCl using Gamry potentiostat 1010E with a sweep rate 1.667 mV s'. Rotation speeds of the working electrode were: 400, 800, 1200, 1600, 1900, 2400, 3600 rpm. The speed was controlled via an OrigaTrod apparatus. Recorded potential values were later converted to reversible hydrogen electrode (RHE). Table 2 provides kinetic current densities at 0.8 and 0.9 V vs RHE, Tafel slopes and mass activities of the CO2-derived catalysts. Figure 10A shows polarization curves measured in 0.1 M KOH using the CO2-CU2.0, CO2-CU4.0 and CO2-CU8.0 materials as electrocatalysts via the RDE method at 1600 rpm. Among the three different copper loadings in the reaction vessel, the CO2-CU4.0 material performed the best, with an onset potential of 0.927 V vs RHE and mass activity at 0.8 V vs RHE of 31.2 A g-1. A lower copper loading resulted in a more positive onset potential and a lower mass activity, while a higher loading similarly decreased these values. Figure 10B shows Tafel slopes for the CO2-CU2.0, CO2-CU4.0 and CO2-Cu8.omaterials. While being the most active catalyst for ORR, CO2-CU4.0 material had a Tafel slope of 76.1 mV dec-1 that increased to 115.8 mV dec-1 with the increasing potential. The Tafel slope value for the COs-Cus.0 catalyst was 67.9 mV dec-1, which is the most similar to the average Tafel slope of Pt / C (60 mV dec-1). Similar Tafel slope values among catalysts indicate same rate-determining steps and reaction mechanisms. Table 2 Catalyst W (mA cm-2) at 0.8 V vs RHE ^k\ (mA cm-2) at 0.9 V vs RHE Tafel slope A (mV dec-1) Tafel slope B (mV dec-1) Mass activity (A g-1) at 0.8 V vs RHE CO2-CU2.0 4.63 0.67 81.8 121.8 23.2 CO2-CU4.0 6.24 0.81 76.1 115.8 31.2 C02-Cu8.o 4.00 0.40 67.9 102.6 20.0 Figures 11A to 1 IC show Koutecky-Levich plots for oxygen reduction on CO2-Cu2.o (Figure HA), CO2-CU4.0 (Figure 11B) and CO2-CU8.0 (Figure 11C) materials. The plots were derived on the obtained ORR data at different rotation rates with the insets in 5 each of Figures 11A to 1 IC showing the number of electrons transferred per O2 molecule. The number of transferred electrons n was calculated using the equation: 1111 1 j id nFkcb02 +Q^nFD^v-F^F^^ where F is the Faradaic constant (96485 C mol '); k is the rate constant for O2 reduction; Co2 is the oxygen concentration (1.2 x 10 6 mol cm1) in 0.1 M KOH; co is the rotation rate 10 (rad s'); jk and jd represent the kinetic and diffusion-limited current densities; Dq2 is the diffusion coefficient of O2 (1.9 x IO5 cm2 s'); and u is the kinematic viscosity of the solution (0.01 cm2 s'). For all materials the value n was around 4 at more negative potentials, indicating a 4-electron oxygen reduction process. At more positive potentials, however, the calculated 15 number of transferred electrons was approximately 3.4 to 3.5, which points to a certain amount of H2O2 produced during the reduction. H2O2 is an undesirable product since it causes the degradation of fuel cell components and decreases current efficiencies, which hinders the overall performance of fuel cells. Nevertheless, the electrochemical testing demonstrated high electrocatalytic activity of the materials synthesised according to a method in accordance with the present disclosure. EXAMPLE 2 Six further carbon materials were synthesized by the same electrolysis method described above for Example 1, except that CuPc was replaced by zinc phthalocyanine (ZnPc), cobalt (II) phthalocyanine (Co(II)Pc), nickel phthalocyanine (NiPc), aluminium phthalocyanine chloride (AlPcCl) and titanium (IV) phthalocyanine dichloride (Ti(IV)PcCh), respectively. Each additive was provided at a loading of 4 mg cm’2, relative to the surface area of the internal base surface of the reaction vessel defined by the cathode. The carbon materials produced are referred to hereafter as CCh-Zm.o, CO2-CO4.0, C02-Ni4.o, CO2-AI4.0 and C02-Ti4.o. Figure 12 shows powder X-ray diffraction patterns of the C02-Zn4.o, CO2-CO4.0, C02-Ni4.o, CO2-AI4.0 and C02-Ti4.o carbon materials synthesised as described above. XRD patterns showed peaks corresponding to graphite planes (002) and (004) at 20 equal to 26.8 and 54.6, respectively. Other peaks were attributed to the presence of iron oxides in the material that appeared due to the corrosion of the cathode. Figure 13 shows ORR polarization curves for the C02-Zn4 o. CO2-CO4.0, C02-Ni4.o. CO2-AI4.0 and C02-Ti4.o materials, measured using the same rotating disk electrode method as described above for Example 1. Small differences in the electrocatalytic activity of the synthesised materials were observed. Analysis of the polarisation curves using the Koutecky-Levich equation revealed that all materials catalysed ORR through the indirect 2+2 electron pathway. The C02-Zn4.o, CO2-CO4.0, C02-Ni4.o and CO2-AI4.0 materials were subsequently doped with nitrogen by pyrolysis using DCDA and PVP, by the same process described above in relation to the carbon materials of Example 1. The N-doped materials are denoted hereafter as C02-N-Zn4.o, CO2-N-CO4.0, C02-N-Ni4.o and CO2-N-AI4.0. Figure 14 shows a comparison of the polarization curves for the carbon materials before and after nitrogen doping at a rotation speed of 1600 rpm for Co (Fig. 14A), Zn (Fig. 14B), Al (Fig. 14C) and Ni (Fig. 14E). As can be seen, a notable increase in ORR activity was observed for all materials after doping with nitrogen. Figure 15 shows SEM images demonstrating the morphological differences of the CO2-CO4.0 (Figure 15A), CO-Ako (Figure 15B), C02-Ni4.o (Figure 15C) and C02-Zn4.o 5 (Figure 15D). All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and 10 interchangeable with one another.

Claims

1. A method of producing carbon material from one or more carbon-containingfeedstock gases, the method comprising the following steps:providing an electrolysis cell comprising an anode, a cathode and a reaction vessel arranged to contain a molten electrolyte, wherein the cathode is arranged at least at a base of the reaction vessel;providing a molten electrolyte within the reaction vessel so as to permit an electric current to pass between the anode and the cathode through the electrolyte, wherein an additive is arranged as a layer coating a surface of the cathode at the base of the reaction vessel so as to be positioned on the surface of the cathode and in contact with the molten electrolyte; andapplying a voltage across the anode and the cathode, whilst providing one or more carbon-containing feedstock gases to the molten electrolyte, thereby producing the carbon material.

2. A method as claimed in claim 1, wherein the cathode defines at least a portion of an internal base surface of the reaction vessel.

3. A method as claimed in claim 2, wherein the cathode is shaped to form the reaction vessel, the molten electrolyte is contained within the cathode, and the additive is arranged as a layer coating an internal base surface of the cathode.

4. A method as claimed in any preceding claim, wherein the additive has a density greater than a density of the molten electrolyte.

5. A method as claimed in 4, wherein the additive is placed in the reaction vessel so as to coat a surface of the cathode, prior to adding the electrolyte to the reaction vessel.

6. A method as claimed in claim 4, wherein the electrolyte is placed in the reaction vessel prior to the additive and / or wherein the electrolyte is melted prior to addition of the additive.

7. A method as claimed in any preceding claim, wherein the additive comprises at least one of copper, cobalt, nickel, aluminium, titanium, or iron.

8. A method as claimed in any preceding claim, wherein the additive includes a metal complex.

9. A method as claimed in claim 8, wherein the metal complex includes a M-Nx moiety, where M is the metal and x = 2, 3 or 4.

10. A method as claimed in claim 8 or 9, wherein the additive includes a metal phthalocyanine or metal porphyrin complex.

11. A method as claimed in claim 10, wherein the additive includes a metal complex selected from the group consisting of cooper phthalocyanine, cobalt phthalocyanine, nickel phthalocyanine, aluminium phthalocyanine and titanium phthalocyanine or any combination thereof.

12. A method as claimed in any preceding claim, wherein a loading of the additive on the surface of the cathode is in the range 1 to 20 mg cm’2.

13. A method as claimed in any preceding claim, wherein the additive is copper phthalocyanine, and wherein a loading of the copper phthalocyanine on the surface of the cathode is in the range 2 to 8 mg cm’2.

14. A method as claimed in any preceding claim wherein the electrolyte includes a carbonate-group.

15. A method as claimed in any preceding claim wherein the electrolyte includes Na2COs, Li2COs, K2CO3, BaCOs, CaCOs or any combination thereof.

16. A method as claimed in any preceding claim wherein the electrolyte includes a mixture of Na2COs, Li2CO3 and K2CO3.

17. A method as claimed in any preceding claim, wherein the electrolyte is heated to a temperature from 400 °C to 900 °C.

18. A method as claimed in any preceding claim, wherein the feedstock gas comprises one or a combination of CO2, CH4 and CO.

19. A method as claimed in any preceding claim, wherein the feedstock gas is present in an atmosphere of the electrolysis cell and / or wherein the one or more feedstock gases are introduced directly into the molten electrolyte.

20. A method as claimed in any preceding claim, wherein the cathode comprises at least one metal selected from iron, nickel and chromium or any combination thereof.

21. A method as claimed in any preceding claim, further comprising removing the carbon material from the cathode and subsequently doping the carbon material with nitrogen.

22. A method as claimed in claim 21, wherein the nitrogen doping step comprises pyrolyzing a mixture of the carbon material with a nitrogen-containing doping agent.

23. A method as claimed in claim 22, wherein the nitrogen-containing doping agent includes at least one of polyvinylpyrrolidone and dicyandiamide.

24. A carbon material obtainable by a method as claimed in any preceding claim.

25. A carbon material as claimed in claim 24, comprising graphite as a carbon allotrope.

26. A device for producing carbon material from one or more carbon-containing feedstock gases, the device comprising:a reaction vessel arranged to contain a molten electrolyte;an anode;a cathode arranged at least at a base of the reaction vessel;a power source for applying a voltage across the anode and the cathode; anda heating unit arranged to heat the reaction vessel to maintain the electrolyte in a molten state.

27. A device as claimed in claim 26, wherein the cathode defines at least a portion of an internal base surface of the reaction vessel.

28. A device as claimed in claim 27, wherein the reaction vessel is formed of a cathode material.

29. A device as claimed in any one of claims 26 to 28, further comprising an additive arranged as a layer coating a surface of the cathode at the base of the reaction vessel.

30. A device as claimed in any one of claims 26 to 29, further comprising a gas supply unit arranged to provide one or more feedstock gases into the molten electrolyte in the5 reaction vessel.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:11 03 25Claims1. A method of producing carbon material from one or more carbon-containing feedstock gases, the method comprising the following steps:providing an electrolysis cell comprising an anode, a cathode and a reaction vessel arranged to contain a molten electrolyte, wherein the cathode is arranged at least at a base of the reaction vessel;providing a molten electrolyte within the reaction vessel so as to permit an electric current to pass between the anode and the cathode through the electrolyte, wherein an additive is arranged as a layer coating a surface of the cathode at the base of the reaction vessel so as to be positioned on the surface of the cathode and in contact with the molten electrolyte; andapplying a voltage across the anode and the cathode, whilst providing one or more carbon-containing feedstock gases to the molten electrolyte, thereby producing the carbon material,wherein the additive has a density greater than a density of the molten electrolyte, andwherein the additive is placed in the reaction vessel so as to coat a surface of the cathode, prior to adding the electrolyte to the reaction vessel.

2. A method as claimed in claim 1, wherein the cathode defines at least a portion of an internal base surface of the reaction vessel.

3. A method as claimed in claim 2, wherein the cathode is shaped to form the reaction vessel, the molten electrolyte is contained within the cathode, and the additive is arranged as a layer coating an internal base surface of the cathode.

4. A method as claimed in any preceding claim, wherein the additive comprises at least one of copper, cobalt, nickel, aluminium, titanium, or iron.

5. A method as claimed in any preceding claim, wherein the additive includes a metal complex.11 03 256. A method as claimed in claim 5, wherein the metal complex includes a M-Nx moiety, where M is the metal and x = 2, 3 or 4.

7. A method as claimed in claim 5 or 6, wherein the additive includes a metal phthalocyanine or metal porphyrin complex.5 8. A method as claimed in claim 7, wherein the additive includes a metal complexselected from the group consisting of cooper phthalocyanine, cobalt phthalocyanine, nickel phthalocyanine, aluminium phthalocyanine and titanium phthalocyanine or any combination thereof.

9. A method as claimed in any preceding claim, wherein a loading of the additive on 10 the surface of the cathode is in the range 1 to 20 mg cm'2.

10. A method as claimed in any preceding claim, wherein the additive is copper phthalocyanine, and wherein a loading of the copper phthalocyanine on the surface of the cathode is in the range 2 to 8 mg cm’2.

11. A method as claimed in any preceding claim wherein the electrolyte includes a15 carbonate-group.

12. A method as claimed in any preceding claim wherein the electrolyte includes NazCOa, LhCCh, K2CO3, BaCO?, CaCOs or any combination thereof.

13. A method as claimed in any preceding claim wherein the electrolyte includes a mixture of NajCCh, Li2CO3 and K2CO3.20 14. A method as claimed in any preceding claim, wherein the electrolyte is heated to atemperature from 400 °C to 900 °C.

15. A method as claimed in any preceding claim, wherein the feedstock gas comprises one or a combination of CO2, CH4 and CO.

16. A method as claimed in any preceding claim, wherein the feedstock gas is present in 25 an atmosphere of the electrolysis cell and / or wherein the one or more feedstock gases are introduced directly into the molten electrolyte.11 03 2517. A method as claimed in any preceding claim, wherein the cathode comprises at least one metal selected from iron, nickel and chromium or any combination thereof.

18. A method as claimed in any preceding claim, further comprising removing the carbon material from the cathode and subsequently doping the carbon material with nitrogen.

19. A method as claimed in claim 18, wherein the nitrogen doping step comprises pyrolyzing a mixture of the carbon material with a nitrogen-containing doping agent.

20. A method as claimed in claim 19, wherein the nitrogen-containing doping agent includes at least one of polyvinylpyrrolidone and dicyandiamide.

21. A device for producing carbon material from one or more carbon-containing feedstock gases, the device comprising:a reaction vessel arranged to contain a molten electrolyte;an anode;a cathode arranged at least at a base of the reaction vessel;a power source for applying a voltage across the anode and the cathode; anda heating unit arranged to heat the reaction vessel to maintain the electrolyte in a molten state,further comprising an additive arranged as a layer coating a surface of the cathode at the base of the reaction vessel.

22. A device as claimed in claim 21, wherein the cathode defines at least a portion of an internal base surface of the reaction vessel.

23. A device as claimed in claim 22, wherein the reaction vessel is formed of a cathode material.

24. A device as claimed in any one of claims 21 to 23, further comprising a gas supply unit arranged to provide one or more feedstock gases into the molten electrolyte in the reaction vessel.

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