Carbon production method, carbon production device, carbon dioxide recovery method, carbon dioxide recovery device

Abstract

The present invention is a carbon-manufacturing method and a carbon-manufacturing device. In the carbon-manufacturing method, a positive electrode 17 and a negative electrode 15 are immersed in a molten salt 13, a voltage is applied to the positive electrode 17 and the negative electrode 15, carbon dioxide is fed, and solid carbon is manufactured by electrolysis. A liquid metal including at least one element selected from the group consisting of tin, bismuth, antimony, zinc, cadmium, lead, indium, and gallium is used as the negative electrode 15, the carbon dioxide being blown into the liquid metal. The present invention can also be used as a carbon dioxide recovery method and a carbon dioxide recovery device for recovering carbon dioxide from a carbon-dioxide-containing gas.

Description

[Technical Field]

[0001] The present invention relates to a carbon production method, a carbon production apparatus, a carbon dioxide recovery method, and a carbon dioxide recovery apparatus for recovering carbon dioxide and producing carbon from the recovered carbon dioxide. [Background technology]

[0002] In recent years, there has been a demand to reduce greenhouse gas emissions in order to prevent global warming. There are various greenhouse gases, such as carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons, but carbon dioxide has the greatest impact on global warming, and therefore, technological development is being carried out to capture carbon dioxide.

[0003] For example, a technique for producing solid carbon by reducing carbon dioxide through an electrochemical process such as electrolysis is known. As such a technique, Patent Documents 1 and 2 and Non-Patent Document 1 disclose a method and apparatus for producing solid carbon at the cathode and oxygen at the anode by placing a solid cathode electrode and a solid anode electrode in a molten salt made of chlorides of alkali metals, alkaline earth metals, or the like, and blowing carbon dioxide into the molten salt at high temperature. The produced solid carbon can be used as a chemical product, a building material, or the like.

[0004] Figures 10 and 11 show the configuration of a conventional carbon production apparatus. There are two types of reaction mechanisms A and B for the technology of reducing carbon dioxide and producing solid carbon as described above. An overview of reaction mechanism A is shown in Figure 10, and an overview of reaction mechanism B is shown in Figure 11. Note that the same components are given the same numbers, and explanations of overlapping parts will be omitted.

[0005] Molten salt 3 is contained within electrolytic cell 1, and cathode 5 and anode 7 are arranged so as to be partially immersed in the molten salt. A power supply 9 for applying a voltage to the cathode 5 and anode 7 is connected between the two electrodes via an electric wire 12. Carbon dioxide is also supplied into the molten salt from a carbon dioxide supply line 11. The carbon production apparatus electrolyzes carbon dioxide into solid carbon and oxygen by passing the carbon dioxide through the molten salt to which a voltage is applied. For example, in reaction mechanisms A and B, a case will be described in which the temperature of the electrolytic bath consisting of molten salt 3 is 650°C and a mixed molten salt of LiCl-Li2O (lithium chloride and lithium oxide) is used as molten salt 3.

[0006] When the voltage during electrolysis is relatively low (for example, 2.0 V), metallic Li (lithium) is not precipitated by the decomposition of the molten salt, and reaction mechanism A (Figure 10) occurs. In reaction mechanism A, oxide ions (O 2- The reaction of the following formula (1) occurs when carbon dioxide is supplied to the molten salt 3 where carbonate ions (CO3 2- When a part of the carbon dioxide (CO₂) is reduced at the cathode 5, the reaction of formula (2) occurs. That is, the carbonate ions are electrochemically decomposed, and the following reactions (1) and (2) occur, and solid carbon (C) is deposited and fixed on the cathode 5. - indicates an electron. Then, by washing the cathode 5, the solid carbon adhering to the cathode 5 can be collected. CO2+ O 2- → CO3 2- (1) CO3 2- + 4e - → C + 3O 2- (2)

[0007] On the other hand, if the voltage during electrolysis is relatively high (for example, 3.0 V), metallic Li (lithium) is precipitated due to the decomposition of the molten salt, and reaction mechanism B (Figure 11) occurs. In reaction mechanism B, the following reaction (3) occurs, and metallic lithium (Li) is precipitated on the cathode. The metallic Li then chemically reduces carbon dioxide. In addition, some metallic Li dissolves and diffuses into the molten salt, so solid carbon is precipitated throughout the container. Note that when the voltage is high, the above equations (1) and (2) also occur simultaneously. Li + + e - → Li (3) 4Li + CO2 → C + 2Li2O (4) In addition, the oxide ions (O 2- ) emits electrons at the anode 7 and becomes oxygen gas, which is then generated from the anode 7.

[0008] Then, carbon deposited on the substrate by reaction mechanism A and carbon deposited throughout the container by reaction mechanism B are collected.

[0009] The present inventors have also developed a technology that can produce solid carbon by using a liquid metal as the cathode (Non-Patent Document 2). In this technology, liquid metal that serves as the cathode is placed at the bottom of an electrolytic cell, molten salt is placed on top of it, and an anode is placed so that it is partially immersed in the molten salt. A voltage is applied and the electrolytic cell is heated, so that the cathode becomes liquid. Note that liquid metal refers to a metal that is in a liquid state at the temperature of use. Furthermore, Non-Patent Document 3 discloses the production of carbon-coated zinc on a liquid zinc cathode. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-53425 [Patent Document 2] Patent No. 6011014 [Non-patent literature]

[0011] [Non-Patent Document 1] Otake et al., CO2 gas decomposition to carbon by electro-reduction in molten salts, Electrochimica Acta, June 30, 2013, 100(2013)293-299 [Non-patent document 2] National Institute of Advanced Industrial Science and Technology et al., "Carbon Recycling Technology Case Studies," Development of CO2 Electrolytic Reduction Technology Using Liquid Metal Cathode, Page 9, [online], [Retrieved October 27, 2021], Internet<URL:enecho.meti.go.jp / category / others / carbon_recycling / df / tech_casebook. pdf> [Non-patent document 3] Teng Lv et al., Adv. Energy Mater. (2020) 2002241 Summary of the Invention [Problem to be solved by the invention]

[0012] It is desirable for these technologies to be able to produce and recover carbon from carbon dioxide as efficiently as possible. An object of the present invention is to provide a carbon production method and carbon production apparatus that can efficiently produce solid carbon from carbon dioxide, and a carbon dioxide recovery method and carbon dioxide recovery apparatus that use such a carbon production method and carbon production apparatus. [Means for solving the problem]

[0013] In order to solve the above problems, the inventors of the present invention have conducted extensive research and found that carbon can be efficiently produced and recovered by blowing carbon dioxide into a liquid metal cathode. Specifically, the following configuration was adopted. In one embodiment, a carbon production method includes immersing an anode and a cathode in a molten salt, applying a voltage to the anode and the cathode, and supplying carbon dioxide to produce solid carbon by electrolysis. The carbon dioxide is blown into the liquid metal, which uses a liquid metal containing at least one element selected from the group consisting of tin (Sn), bismuth (Bi), antimony (Sb), zinc (Zn), cadmium (Cd), lead (Pb), indium (In), and gallium (Ga) as the cathode. Moreover, a method for recovering carbon dioxide in one embodiment is a method for recovering carbon dioxide from a gas containing carbon dioxide by the carbon production method.

[0014] A carbon production apparatus in one embodiment is a carbon production apparatus that produces solid carbon by electrolysis, comprising: an electrolytic cell that contains molten salt; an anode that is immersed in the molten salt; a cathode that is immersed in the molten salt; a power source that applies a voltage to the anode and the cathode; and a carbon dioxide supply line that supplies carbon dioxide, wherein the cathode is a liquid metal containing at least one selected from the group consisting of tin (Sn), bismuth (Bi), antimony (Sb), zinc (Zn), cadmium (Cd), lead (Pb), indium (In), and gallium (Ga), and carbon dioxide is blown into the liquid metal from the carbon dioxide supply line. Moreover, an apparatus for recovering carbon dioxide in one embodiment is an apparatus for recovering carbon dioxide provided with the carbon production apparatus and an introduction part that introduces carbon dioxide from a gas containing carbon dioxide into the carbon dioxide supply line. [Effects of the Invention]

[0015] According to one embodiment, it is possible to provide a carbon production method and a carbon production apparatus that can efficiently produce solid carbon from carbon dioxide. It is also possible to provide a carbon dioxide capture method and a carbon dioxide capture apparatus that use such a carbon production method and carbon production apparatus. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a configuration diagram of a carbon production apparatus according to an embodiment of the present invention. [Figure 2] This is a binary phase diagram of tin and lithium. [Figure 3] FIG. 10 is a configuration diagram of a carbon production apparatus according to a first alternative embodiment. [Figure 4] FIG. 10 is a configuration diagram of a carbon production apparatus according to another embodiment 2. [Figure 5] FIG. 10 is a configuration diagram of a carbon production apparatus according to a third alternative embodiment. [Figure 6] FIG. 1 shows the results of X-ray diffraction of solid carbon. [Figure 7] FIG. 1 shows a Raman spectrum of solid carbon. [Figure 8] FIG. 10 is a diagram showing the relationship between voltage application time and current density. [Figure 9] FIG. 1 is a diagram showing the configuration of a carbon production apparatus used in Example 3. [Figure 10] FIG. 1 is a configuration diagram of a conventional carbon production apparatus for explaining an outline of reaction mechanism A. [Figure 11] FIG. 1 is a configuration diagram of a conventional carbon production apparatus for explaining an outline of reaction mechanism B. [Figure 12] FIG. 1 is a diagram showing the configuration of a carbon production apparatus (using liquid metal as the cathode) of a comparative example. [Figure 13] FIG. 1 is a configuration diagram of a carbon production device according to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0017] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same reference numerals indicate the same or corresponding parts.

[0018] 1(A) shows a configuration diagram of a carbon production apparatus 30 according to an embodiment of the present invention. The carbon production apparatus 30 mainly comprises an electrolytic cell 10, a molten salt 13 contained in the electrolytic cell 10, a cathode 15 placed on the bottom of the electrolytic cell 10, an anode 17 arranged so as to be partially immersed in the molten salt 13, a power supply (DC power supply) 9 that connects the cathode 15 and the anode 17 and applies a voltage to both electrodes, and a carbon dioxide supply line 21 that supplies carbon dioxide into the molten salt. Each component will be described in detail below.

[0019] (electrolytic cell) FIG. 1(B) shows a plan cross-sectional view (schematic diagram) of the electrolytic cell 10, and FIG. 1(C) shows a side view (partial cross-sectional schematic diagram) of the electrolytic cell 10. The electrolytic cell 10 is a container that contains molten salt 13. The interior is divided into a cathode chamber 6 and an anode chamber 8 by a separator 10a, and the molten salt 13 flows through a space 19 below the separator 10a. The presence of the separator 10a effectively prevents solid carbon generated on the cathode 15 side from migrating to the anode 17 side and reacting with oxide ions on the anode 17 side. Furthermore, by preventing the solid carbon from mixing with the molten salt 13 on the anode 17 side, the solid carbon C can be collected on the cathode 15 side, allowing for efficient recovery of the solid carbon C. Furthermore, it is preferable to install the separator 10a so that its lower end is lower than the upper surface of the cathode 15 and the upper end of the partition 23, as this prevents the solid carbon from migrating to the anode 17 side. The electrolytic cell 10 can be made of a heat-resistant ceramic or a refractory material mainly made of alumina. From the viewpoint of durability, it is preferable to use dense magnesia (magnesium oxide, MgO). The electrolytic cell 10 is equipped with a heater (not shown) for heating the electrolytic bath.

[0020] (molten salt) Alkali metal halides, alkaline earth metal halides, and mixtures thereof can be used as the molten salt 13. Examples of the alkali metal halides that can be used include compounds such as LiF, NaF, KF, RbF, CsF, LiCl, NaCl, KCl, RbCl, CsCl, LiBr, NaBr, KBr, RbBr, CsBr, LiI, NaI, KI, RbI, and CsI. Examples of the alkaline earth metal halides that can be used include compounds such as MgF, CaF, SrF, BaF, MgCl, CaCl, SrCl, BaCl, MgBr, CaBr, SrBr, BaBr, MgI, CaI, SrI, and BaI.

[0021] The above compounds can be used alone or in combination of two or more. The combination of these compounds, the number of compounds to be combined, and the mixing ratio are not limited and can be appropriately selected depending on the desired operating temperature range. Other compounds that can be used include halides of relatively less noble metals such as AlCl3 and ZnCl2.

[0022] In particular, the molten salt 13 used in this embodiment is preferably a single molten salt such as LiCl or CaCl, a mixed molten salt of LiCl and KCl, a mixed molten salt of LiCl and CaCl, or a mixed molten salt of LiCl, KCl, and CsCl. Furthermore, a mixed molten salt has a lower melting point than a single molten salt, and is therefore suitable for causing an electrolysis reaction at a low temperature.

[0023] Furthermore, it is preferable to add an oxide ion source to the electrolytic cell 10 together with the molten salt 13 at the beginning of electrolysis, since this allows for smooth generation of carbonate ions by supplying carbon dioxide. Examples of oxide ion sources include alkali metal oxides and alkaline earth metal oxides. For example, Li2O, Na2O, and KO may be used as alkali metal oxides, and MgO, CaO, and BaO may be used as alkaline earth metal oxides. These compounds may be used alone or in combination of two or more. These compounds may also be mixed with the molten salt 13, i.e., integrated with the molten salt 13.

[0024] Furthermore, it is preferable that the molten salt 13 contains carbonate ions, since this facilitates the smooth carbon deposition reaction at the cathode 15 due to the supply of carbon dioxide at the initial stage of the reaction. Examples of carbonate ion sources include alkali metal carbonates and alkaline earth metal carbonates. For example, alkali metal carbonates such as Li2CO3, Na2CO3, and K2CO3 may be used, and alkaline earth metal carbonates such as MgCO3, CaCO3, and BaCO3 may be used. These compounds may be used alone or in combination of two or more. These compounds may also be mixed with the molten salt 13, i.e., integrated with the molten salt. Similar effects can be achieved by supplying a small amount of carbon dioxide to the molten salt at the initial stage of the reaction, rather than using a carbonate as the carbonate ion source.

[0025] Furthermore, as will be described in detail later, since the carbon production apparatus 30 of this embodiment uses a liquid metal as the cathode 15, the type of molten salt should be selected taking into consideration the physical properties of the liquid metal so as to prevent the liquid metal from diffusing in the molten salt 13. The density of these molten salts mentioned above is approximately 1.5-2.5 g / cm 3 That's about it.

[0026] (cathode) The cathode 15 is a liquid metal with a melting point of 700°C or less, as shown in Table 1 below (sources: melting points are calculated based on thermodynamic data from the Physics and Chemistry Dictionary, Third Edition, Iwanami Shoten, and vapor pressures are calculated based on thermodynamic data from HSC Chemistry 9 (chemical equilibrium calculation software)). Examples of suitable metals include tin (Sn), bismuth (Bi), antimony (Sb), zinc (Zn), cadmium (Cd), lead (Pb), indium (In), and gallium (Ga). If the cathode 15 is a liquid metal, the deposited solid carbon separates from the cathode and accumulates above the liquid metal, facilitating recovery of the solid carbon. These metals may be used alone or in combination. Among these metals, tin and bismuth are preferred because they have lower vapor pressures and are easier to handle than antimony and zinc, are less toxic than cadmium and lead, and are more readily available and less expensive than indium and gallium. Furthermore, alloys (mixtures) such as Sn-Bi are preferable because they have a lower melting point than single metals, allowing electrolysis reactions to occur at low temperatures. Furthermore, liquid alloys based on the above metals and mixed with other metals such as nickel (Ni) or copper (Cu) may also be used. By forming an alloy, the melting point is lower than when the above metals are used alone, allowing them to be used as liquids at temperatures of around 600°C or lower. [Table 1]

[0027] A suitable combination of the type of molten salt 13 and the type of liquid metal used in the cathode 15 may be selected from the following conditions. First, it is preferable that the density of the liquid metal is as high as possible compared to the density of the molten salt. By selecting a liquid metal with a high density, it can be stably placed in the molten salt. The density of the liquid metal is approximately 5-10 g / cm. 3 and the density of the molten salt is 1.5-2.5 g / cm as mentioned above. 3Since the density of the liquid metal is about 7-10 g / cm 3 , the liquid metal does not diffuse into the molten salt but sinks to the bottom of the electrolytic cell 10 and remains stable in the molten salt. 3 It is preferable that the thickness of the partition 23 or the container be provided in the electrolytic cell 10 so that the liquid metal can be stably placed in the electrolytic cell 10. The materials and components of the partition 23 and the container may be the same as those of the electrolytic cell 10. The liquid metal is generally a metal that is in a liquid state between its melting point and boiling point, but a metal that is particularly suitable for dissolving the molten salt in the solvent and for liquid state at a temperature suitable for electrolysis is selected.

[0028] Second, it is preferable that the melting points of the molten salt and the liquid metal be as low as possible. The low melting points of both allow the reaction to proceed at a relatively low temperature in the electrolytic cell 10. In particular, the heating temperature during electrolysis is determined taking into account the melting point of the molten salt, which is generally higher than that of the liquid metal. Therefore, a molten salt with a relatively low melting point is preferable. For example, lithium chloride, which has a melting point of approximately 600°C, is a preferable molten salt with a relatively low melting point. Furthermore, if the molten salt is a mixed molten salt (e.g., LiCl and KCl) or the liquid metal is an alloy, the melting points of both the salt and the molten salt are lower. For example, the melting points are preferably 361°C for LiCl (59 mol%) and KCl (41 mol%), and 470°C for LiCl (62 mol%) and CaCl (38 mol%). Note that a higher temperature accelerates the reaction, reduces the decomposition voltage, and increases the amount of CO2 absorbed. On the other hand, as will be described later, a temperature of approximately 600°C is preferable because a temperature that is too high facilitates the generation of CO.

[0029] Third, it is preferable to use a combination that allows for better dissolution of a metal (hereinafter referred to as a molten salt-derived metal; for example, when the molten salt is LiCl, this corresponds to Li (lithium)) produced by reduction of a cation constituting the molten salt into the liquid metal. The dissolution of the molten salt-derived metal into the liquid metal produces an alloy of the liquid metal and the molten salt-derived metal. For example, electrolysis when the molten salt 13 is LiCl and the cathode 15 is a Sn electrode produces a Sn-Li alloy; electrolysis when the molten salt 13 is CaCl2 and the cathode 15 is a Sn electrode produces a Sn-Ca alloy; electrolysis when the molten salt 13 is LiCl-CaCl2 and the cathode 15 is a Sn electrode produces a Sn-Li-Ca alloy; and electrolysis when the molten salt 13 is LiCl and the cathode 15 is a Sn-Bi electrode produces a Sn-Bi-Li alloy.

[0030] Carbon dioxide is then blown into the liquid metal from the carbon dioxide supply line 21. Because the molten salt-derived metal is dissolved in the liquid metal, the metal (e.g., lithium when the molten salt is LiCl) reacts with the carbon dioxide, resulting in the reduction of the carbon dioxide. If the molten salt-derived metal can be more efficiently dissolved in the liquid metal, the reduction reaction of the carbon dioxide will proceed more efficiently. The solubility of the molten salt-derived metal in the liquid metal can be estimated, for example, from a phase diagram of a mixture of the liquid metal and the molten salt-derived metal. In the phase diagram, a wider liquid phase region of the mixture means that the molten salt-derived metal can be more easily dissolved in the liquid metal, and such a combination of the liquid metal and the molten salt-derived metal is suitable. Furthermore, when both are liquid, selecting a mixture with a higher ratio of the molten salt-derived metal allows the molten salt-derived metal to be more easily dissolved in the liquid metal.

[0031] Figure 2 shows an example of a binary phase diagram for Sn and Li. This phase diagram is an excerpt from the Phase diagram web on Fact-Web (https: / / www.crct.polymtl.ca / factweb.php), which provides web-based software for predicting the thermodynamic equilibrium state of multi-component systems. For example, we will explain the case where the cathode 15 is a Sn electrode and the molten salt 13 is a molten salt such as lithium chloride or lithium carbonate, in which the metal derived from the molten salt is lithium. Referring to Figure 2, Sn-Li alloys become liquid (liquid) at relatively low temperatures when the Sn ratio (horizontal axis) is approximately 0.4 or greater. A higher ratio indicates a lower amount of metallic Li dissolved in the liquid metal. Therefore, a lower ratio allows for greater dissolution of the molten salt-derived metal into the liquid metal. For example, it is preferable to select the molten salt and liquid metal so that the liquid metal ratio, where the alloy becomes liquid, is 0.8 or less in the phase diagram. Furthermore, when two or more types of molten salt-derived metals are used, it is recommended that the liquid metal ratio in the phase diagram for each molten salt-derived metal and liquid metal be 0.8 or less. For example, combinations of LiCl, CaCl2, etc. with Sn, Bi, etc. are suitable as molten salt and liquid metal combinations. Among these, using a LiCl-KCl-based molten salt as the molten salt and Sn, Bi, etc. as the liquid metal is preferable because it allows for greater dissolution of metallic Li.

[0032] The cathode 15 does not necessarily have to be placed on the bottom of the electrolytic cell 10, nor does it have to be entirely immersed in the molten salt, as long as the surface of the cathode 15 is in contact with the molten salt 13. Arranging the cathode 15 so that the entire cathode 15 is immersed in the molten salt 13 increases the contact area between the cathode 15 and the molten salt 13, which is preferable, improving the reaction efficiency. A voltage is applied to the cathode 15 via a connecting electric wire 14 that electrically connects the cathode 15 and the electric wire 12. The connecting electric wire 14 can be made of a material that does not react with liquid metals, alkali metals, or alkaline earth metals, such as molybdenum (Mo) or tungsten (W).

[0033] (anode) The anode 17 is made of an anode material typically used in molten salt electrolysis, such as glassy carbon, graphite, conductive ceramics, etc. Among these, an insoluble anode capable of generating oxygen, such as a conductive ceramic electrode made of nickel ferrite oxide or tin oxide, or a conductive diamond electrode, is preferred.

[0034] (carbon dioxide supply line) The carbon dioxide supply line 21 is a supply pipe that supplies carbon dioxide to the cathode 15 of the electrolytic cell 10. For example, the carbon dioxide supply line 21 may be made of a material that does not react with molten salt or liquid metal, or the same ceramic-based pipe as the electrolytic cell 10. The carbon dioxide to be supplied may be any gas containing carbon dioxide, such as factory exhaust gas, biogas (a mixed gas of CO2 and CH4, etc.), or air. These gases may be introduced into the carbon dioxide supply line 21 from an inlet pipe using a pump, blower, or the like. Carbon dioxide can be recovered from exhaust gas or the like. While exhaust gas may contain inert gases such as argon and nitrogen, it is preferable to remove nitrogen oxides (NOx) and sulfur oxides (SOx) from the exhaust gas, as these may dissolve in the dissolved salt and affect carbon production. These gases may also be concentrated or dehydrated before use.

[0035] (Electrolysis conditions) The power supply 9 applies a voltage to the cathode 15 and anode 17 to cause electrolysis. The ratio of alkali metals or alkaline earth metals present in the liquid alloy is determined by the potential, and the appropriate potential for maintaining the liquid alloy in a liquid state varies depending on the combination of metals, the electrolysis temperature, and the type of molten salt. The higher the applied voltage, the more likely the reaction will proceed, but a voltage of approximately 0.5-1.0 V, preferably 0.6-0.9 V, based on the cathode limiting potential is sufficient. From the perspective of power saving, a voltage of approximately 0.7-0.9 V is even more preferable.

[0036] The electrolysis temperature depends on the types of molten salt 13 and cathode 15, but should be set to a temperature at which they melt, generally between 250 and 700°C, and preferably between 500 and 600°C. A higher electrolysis temperature is preferable because it increases the rate of the carbon dioxide reduction reaction. However, temperatures above 700°C facilitate the reaction of solid carbon with carbon dioxide to form carbon monoxide, so the temperature range is preferred. For example, when the temperature of the electrolytic cell 10 is 500 to 600°C and LiCl-KCl-LiO is used as the molten salt 13, electrolysis should be performed at a potential of about 0.6 to 0.9 V relative to the cathode limiting potential, under conditions that maintain the cathode in a liquid state.

[0037] (Reaction mechanism) The reaction mechanism when electrolysis is performed using the apparatus 30 of this embodiment (FIG. 1) will be described. Note that FIG. 12 shows a carbon production apparatus 40 of a comparative example. The carbon production apparatus 30 of this embodiment and the carbon production apparatus 40 of the comparative example have in common the use of liquid metal as the cathode 15, but differ in that the carbon production apparatus 40 of the comparative example injects carbon dioxide into molten salt, whereas the carbon dioxide in the carbon production apparatus 30 of this embodiment injects carbon dioxide into liquid metal.

[0038] As shown in these figures, in both devices, carbon dioxide is supplied into the electrolytic cell 10 from a carbon dioxide supply line 21. For example, a case where a mixed molten salt of LiCl—LiO is used as the molten salt 13 and a Sn electrode is used as the cathode 15 will be described.

[0039] In the carbon production apparatus 30 of this embodiment, first, the electrolytic cell 10 is heated until the temperature reaches a predetermined temperature equal to or higher than the melting points of the molten salt 13 and the cathode 15 (liquid metal), and then a predetermined voltage is applied by the power source 9, and carbon dioxide is blown into the liquid metal (Sn), thereby starting the electrolysis reaction. When carbon dioxide is blown directly into the liquid metal, the following two reactions (5) and (6) occur. Li + + e - → Li(in Sn) (5) 4Li(in Sn) + CO2 → C + 2Li2O (6) Any CO2 gas not used in the reaction is reduced to carbonate ions. CO2+ O 2- → CO3 2- (1) CO3 2- + 4e - → C + 3O 2- (2) or CO3 2- + 4Li → C + 4Li + + 3O 2- (7) In addition, the oxide ions (O 2- ) emits electrons at the anode 17 to become oxygen gas, and oxygen is generated from the anode 7. The above formulas (5) and (6) correspond to the above formulas (3) and (4). Thus, in this embodiment, in addition to the reactions of the above formulas (5) and (6), the reactions of the above formulas (1), (2), and (7) occur. That is, because carbon dioxide (CO2) reacts in two stages, the yield of solid carbon increases. This is not limited to the example of the LiCl-Li2O mixed molten salt and Sn electrode, but can also be said when the above molten salt and liquid metal are used.

[0040] On the other hand, in the carbon production apparatus 40 of the comparative example (FIG. 12), when carbon dioxide is injected into the molten salt, the reactions of formulas (5) and (7) occur, but the reaction of formula (6) does not occur. Furthermore, the reactions of formulas (1), (2), and (7) occur because carbon dioxide first becomes carbonate ions and then diffuses, resulting in a slower overall reaction rate. In contrast, in the carbon production apparatus 30 of the present embodiment (FIG. 1), the liquid metal is stirred by the injection of carbon dioxide, which promotes the reactions of formulas (5), (6), and (7). This increases the current, the reaction rate, and the reaction efficiency. Furthermore, the theoretical decomposition voltage of carbon dioxide gas is approximately 0.2 V lower than the theoretical decomposition voltage of carbonate ions, making it energetically advantageous to promote the reaction of formula (6).

[0041] and low-density solid carbon C (typically 2 g / cm 3 The carbon dioxide (about 100%) floats to the top of the molten salt 13, and the solid carbon C can be easily recovered. Note that the carbon dioxide may be blown toward the liquid metal by arranging the blowing port of the carbon dioxide supply line 21 on or near the surface of the liquid metal, but it is preferable to place the blowing port of the carbon dioxide supply line 21 in the liquid metal, since this increases the bubbling effect of the liquid metal and improves the reaction efficiency.

[0042] In addition, in the above-mentioned reaction mechanism B (FIG. 11), carbon dioxide is chemically reduced by metallic lithium. However, because metallic Li dissolves and diffuses in the molten salt, the reactions of the above formulas (3) and (4) take place throughout the entire vessel of the electrolytic cell, resulting in lower current efficiency compared to the reactions of the above formulas (1) and (2) of reaction mechanism A, in which carbon is deposited on the cathode substrate.

[0043] According to this embodiment, the cathode is a liquid metal, and carbon dioxide is blown into the liquid metal, whereby the carbon dioxide is directly reduced by metallic lithium. Therefore, the reaction efficiency is dramatically higher than in conventional carbon production apparatuses. Furthermore, unreacted carbon dioxide with metallic lithium becomes carbonate ions in the molten salt through the reactions of the above formulas (1) and (2), and is then reduced. Therefore, by going through all of the reactions of the above formulas (1) to (4), the reaction rate is increased and the carbon yield is dramatically improved.

[0044] FIG. 3 shows a configuration diagram of a carbon production apparatus 50 according to Alternative Embodiment 1. The carbon production apparatus 50 according to Alternative Embodiment 1 recovers and reuses unreacted carbon dioxide. A cathode 15 and an anode 17 are disposed on the bottom surface of the electrolytic cell 10, and a power supply 9 is disposed below or below the electrolytic cell. Gas outlets 25 and 27 are provided on the cathode 15 side and the anode 17 side of the top surface (lid) 10b of the electrolytic cell 10, respectively, so that carbon dioxide is discharged from the cathode 15 side and oxygen is discharged from the anode 17 side. The carbon dioxide discharged from the cathode 15 side is then introduced back into the carbon dioxide supply line 21 via a carbon dioxide circulation line (piping) 29 and supplied into the electrolytic cell 10. The carbon dioxide circulation line 29 may be made of the same materials as those used for the carbon dioxide supply line 21. The oxygen gas discharged on the anode side can be used for oxygen roasting in, for example, thermal power plants and steel smelters.

[0045] The carbon dioxide supply line 21 may be arranged vertically as shown in FIG. 1, or horizontally (sideways) as shown in FIG. 3. By providing multiple holes (not shown) on the side of the carbon dioxide supply line 21, carbon dioxide can be blown into the liquid metal through the holes, thereby improving the carbon dioxide reaction efficiency. The holes may also be provided on the side of the lower end of the carbon dioxide supply line 21 shown in FIG. 1. Since the reaction between carbon dioxide and solid carbon (CO2 + C → 2CO) is likely to proceed when the electrolysis temperature exceeds 700°C, it is preferable that the electrolysis temperature be 700°C or lower. Setting the electrolysis temperature to 700°C or lower increases the recycling efficiency of unreacted carbon dioxide, thereby improving the yield of solid carbon.

[0046] In the carbon production apparatus 50 shown in Fig. 3, the anode 17 is entirely immersed in the molten salt 13, but it may be arranged so that it is partially immersed, as shown in Fig. 1. By providing the anode 17 on the bottom surface or lower part of the electrolytic cell 10 as shown in Fig. 3, the length of the electric wire 12 connecting the cathode 15 and the anode 17 can be shortened.

[0047] FIG. 4(A) shows a configuration diagram of a carbon production apparatus 60 according to a second alternative embodiment. FIG. 4(B) shows a cross-sectional plan view (schematic diagram) of the electrolytic cell 10, and FIG. 4(C) shows a side view (partial cross-sectional schematic diagram) of the electrolytic cell 10. This carbon production apparatus 60 is a system constructed to recover solid carbon C that has floated to the top of the molten salt 13. The system includes a molten salt circulation line (piping) 33 that extracts a portion of the molten salt 13 on the cathode 15 side using a pump 31 and supplies it to the anode 17 side. The molten salt circulation line 33 is further provided with a solid-liquid separator 35. The molten salt circulation line 33 may be made of the same material as the carbon dioxide supply line 21. By providing an opening 10c in the separator 10a at the same height as the molten salt circulation line 33, the molten salt 13 can more easily flow through the molten salt circulation line 33. By providing a space below the cathode 15, oxide ions (O 2- ) moves to the anode side.

[0048] As shown in Fig. 4, solid carbon C produced in the molten salt 13 is discharged outside the electrolytic cell 10 by a pump 31 and flows through a molten salt circulation line 33, during which time it is separated by a solid-liquid separator 35 and collected in a collection container 37. The solid-liquid separator 35 can be a filtration type using a filter or the like, a pressure type, a centrifugal separation type (centrifuge), or a reaction type using a flocculant, but the filtration type is preferred because it allows the molten salt to be continuously circulated. By constructing a system for recovering solid carbon C in this way, the recovery of solid carbon C can be performed automatically by controlling the voltage applied to both electrodes of the power source 9 and the pump 31 and the circulation flow rate through the molten salt circulation line 33 using a control device (not shown).

[0049] 5 shows a specific example of a carbon production apparatus according to this embodiment. Fig. 5(A) shows an overall view, and Fig. 5(B) shows a partial enlarged view of the cathode portion. This carbon production apparatus 70 includes an electric furnace 71, a Kanthal (registered trademark) holder 73 housed in the electric furnace 71, and a crucible 75 housed in the holder 73. Furthermore, a supply pipe 77 and an exhaust pipe 79 are provided to supply an atmospheric gas of an inert gas such as argon (helium, neon, or nitrogen) into the holder 73. The atmospheric gas is circulated between the supply pipe 77 and the exhaust pipe 79.

[0050] Molten salt 13 is contained within crucible 75, and thermocouple 81, anode 87, and reference electrode 89 are installed within holder 73 so that they are partially immersed in molten salt 13. The thermocouple 81 measures the temperature of molten salt 13, and the temperature of electric furnace 71 is adjusted to regulate the temperature of molten salt 13. Liquid metal to serve as cathode 85 is contained in a liquid metal container 83 made of dense magnesia (MgO) and placed at the bottom of crucible 75. An electrochemical measurement device 97 is connected to cathode 85, anode 87, and reference electrode 89 to apply voltage to these electrodes. A carbon dioxide supply pipe 91 for blowing carbon dioxide into cathode 85 is installed within molten salt 13 so that it is partially immersed. The carbon dioxide supply pipe 91 is made of aluminum oxide (Al2O3) or dense magnesia (MgO), and a conduit 93 having a tungsten wire W for connection to a power source 9 is arranged adjacent to the supply pipe so that electricity can be applied to the cathode 85 from the power source 9 via the tungsten wire W. The thermocouple 81, anode 87, reference electrode 89, carbon dioxide supply pipe 91, etc. are inserted into and fixed in holes formed in the lid 73a of the holder 73. The above-described embodiments may be combined as appropriate. [Example]

[0051] Example 1 Solid carbon was produced using the carbon production apparatus 70 shown in Figure 5. The method is described below. As the liquid metal to be the cathode 85, 18 g of Sn (Fujifilm Wako Pure Chemical Corporation, special grade reagent) was weighed out and placed in a liquid metal container 83, which was then housed in a crucible 75. Then, a mixture (314 g) consisting of 57 mol % LiCl-39 mol % KCl-1 mol % Li2O-3 mol % Li2CO3 mixed in a eutectic composition was placed in the crucible 75, and the crucible 75 was housed in a holder 73 and covered with a lid 73a. Thereafter, the mixture was heated to 250°C in an electric furnace 71 and vacuum dried overnight.

[0052] After vacuum drying, argon gas was introduced through the supply pipe 77, and the temperature was raised to 550°C in the electric furnace 71 to melt the solution and form a solvent. Then, a thermocouple 81, an anode 87, a reference electrode 89, a carbon dioxide supply pipe 91, and a conduit 93 were inserted through the top of the lid 73a to electrically connect the tungsten wire W to the cathode 85. The cathode 85, the anode 87, and the reference electrode 89 were connected to an electrochemical measurement device (SP-240, manufactured by Bio-Logic Science Instruments). The anode 87 was made of glassy carbon (Glassy Carbon (registered trademark), manufactured by Tokai Carbon Co., Ltd.), and the reference electrode 89 was made of an Ag / Ag electrode (a HB protective tube manufactured by Nikkato Corporation, in which AgCl was added to the molten salt LiCl-KCl at 0.5 mol%, and a silver wire was inserted). Then, while maintaining the temperature at 550°C, Li was charged to the cathode 85. + A potential of 0.7 V was applied relative to / Li, and then electrolysis was carried out for 6.6 hours while blowing carbon dioxide into the liquid metal from the carbon dioxide supply pipe 91 (argon 90 vol%, CO2 10 vol%, total flow rate 10 ml / min).

[0053] After electrolysis, the temperature was lowered to room temperature, and the solidified molten salt 13 containing solid carbon C was washed with distilled water (or hydrochloric acid) and then dried at 80° C. to recover solid carbon. Fig. 6 shows the X-ray diffraction results of the obtained solid carbon C, and Fig. 7 shows the Raman spectrum of the obtained solid carbon C. The measurement conditions for each figure were as follows: X-ray diffraction: X-ray diffractometer (Bruker, D8 Advance diffractometer) Raman spectrum: Raman spectrometer (Renishaw inVia Raman microscope, 514.5 nm laser)

[0054] Generally, solid carbon has a bonding state similar to that of diamond, graphite, carbon nanotubes, and fullerenes. Most practical carbon materials have a structure in which carbon atoms are bonded in a hexagonal mesh pattern, similar to graphite, and the mesh planes are stacked. Referring to Figure 6, the peak intensity corresponding to the (002) diffraction line resulting from the stacked structure is weak and broad, which indicates that the solid carbon C produced is low-crystalline carbon (amorphous carbon). Furthermore, in Raman spectra, solid carbon with a developed graphite structure, highly oriented pyrolytic carbon, etc., have a peak intensity of 1580 cm -1 (G band), but when structural disorder occurs, a peak appears at 1580 cm -1 In addition to the D band (1360cm -1 ) is observed. As the structural disorder increases, the relative intensity to the G band increases, resulting in an overall broader shape. Looking at the Raman spectrum in Figure 7, the same G band and D band as above are confirmed, indicating that the solid carbon C produced is low-crystalline carbon.

[0055] Next, as a comparative example, carbon dioxide was injected into molten salt 13 to produce solid carbon using carbon production apparatus 80 of Fig. 13. Apparatus 80 of Fig. 13 shows a specific example of carbon production apparatus 40 (Fig. 12), and differs from apparatus 70 of Fig. 5 in the position of carbon dioxide supply pipe 91, but the other configurations are the same, so description thereof will be omitted. Moreover, the conditions other than the carbon dioxide injection position were the same as those in Example 1 above.

[0056] FIG. 8 shows the relationship between the voltage application time and the current density. Note that FIG. 8(A) shows the results of measuring the current density in Example 1, and FIG. 8(B) shows the results in Comparative Example. According to FIG. 8, in Example 1, the current density was 100 (mA / cm2 ), but in the comparative example, the current density decreased. The current density represents the reaction rate, and it can be seen that the reaction is promoted by introducing CO2 gas into the liquid metal.

[0057] Example 2 Example 2 differs from Example 1 in that a different type of liquid metal was used for the cathode 85, but solid carbon was otherwise produced under the same conditions as Example 1. A Sn-Bi alloy (Sn 43 mol %, Bi 57 mol %) was used as the liquid metal. 7.5 g of Sn (FUJIFILM Wako Pure Chemical Corporation, special grade reagent) and 17.5 g of Bi (FUJIFILM Wako Pure Chemical Corporation, special grade reagent) were weighed and placed in the liquid metal container 83.

[0058] Example 3 In the above-mentioned Example 1-2, Li2CO3, which is a carbonate, was added to the molten salt 13 so that the carbon deposition reaction at the cathode 85 caused by the injection of carbon dioxide at the initial stage of the reaction would proceed smoothly. However, in this example, Li2CO3 was not added to the molten salt 13, and the same amount of carbon dioxide was supplied to the molten salt 13 at the initial stage of the reaction, and solid carbon was produced under the same conditions as in Example 1. Even with this method, the same effects as in Example 1-2 can be obtained. Specifically, as shown in Figure 9, an apparatus 90 was used, which was the same as the apparatus 70 in Figure 5, but was equipped with an auxiliary carbon dioxide supply pipe 95 for injecting carbon dioxide into the molten salt 13.

[0059] As in Example 1, 18 g of Sn to become cathode 85 was weighed and placed in liquid metal container 83, and liquid metal container 83 was housed in crucible 75. Then, 307 g of a mixture consisting of 57 mol % LiCl-39 mol % KCl-4 mol % LiO mixed in a eutectic composition was placed in crucible 75, and crucible 75 was housed in holder 73 and covered with lid 73a. Thereafter, it was heated to 250°C in electric furnace 71 and vacuum dried overnight.

[0060] After vacuum drying, argon gas was introduced from the supply pipe 77, and the temperature was raised to 550°C in the electric furnace 71 to melt the molten salt 13 to form a solvent. Then, carbon dioxide (argon 50 mol%, CO2 50 mol% concentration, total flow rate 50 ml / min) was blown into the molten salt 13 from the auxiliary carbon dioxide supply pipe 95 for 150 minutes. The amount blown in was equivalent to the amount of 3 mol% Li2CO3 in Example 1. Thereafter, the thermocouple 81, the anode 87, the reference electrode 89, the carbon dioxide supply pipe 91, and the conduit 93 were inserted from above the lid 73a, and the tungsten wire W and the cathode 85 were electrically connected. Then, while maintaining the temperature at 550°C, Li + A potential of 0.7 V relative to / Li was applied, and then electrolysis was carried out for 3.8 hours while blowing carbon dioxide into the liquid metal from the carbon dioxide supply pipe 91 (argon 90 mol %, CO2 10 mol % concentration, total flow rate 10 ml / min). Then, solid carbon C was recovered in the same manner as in Example 1.

[0061] In Examples 1 to 3 and the Comparative Example, the current efficiency, CO2 reaction efficiency, and carbon production rate were measured and the results are shown in Table 2. [Table 2]

[0062] In Table 2, the current efficiency, CO2 reaction efficiency, and carbon production rate are as shown below. Current efficiency: (weight of carbon actually recovered) (mg) / (weight of carbon calculated from the quantity of electricity when assuming a four-electron reaction according to equation (2) above) (mg) CO2 reaction efficiency: (weight of carbon actually recovered) (mg) / (weight of carbon assuming that all CO2 gas injected during electrolysis is converted to carbon) (mg) Carbon production rate: (weight of carbon actually recovered) (mg) / (time for electrolysis) (h)

[0063] From the above results, it was confirmed that Examples 1 and 2 were more effective than the Comparative Example in all aspects, including current efficiency, CO2 reaction efficiency, and carbon production rate. In particular, it was found that the CO2 reaction efficiency and carbon production rate were dramatically improved by directly injecting carbon dioxide into the liquid metal. Furthermore, in Example 3, the current efficiency was slightly lower than in the Comparative Example, but this was within the range of experimental error due to differences in the amount of water contained in the molten salt, and the CO2 reaction efficiency and carbon production rate were confirmed to be about three times higher than in the Comparative Example, so it can be said to be an effective method.

[0064] (Reference example) Also shown are the results of measuring the current efficiency using the carbon production apparatus of Examples 1-3 (Table 3). As a reference example, the current efficiency listed as Yield in Table 1 of the above-mentioned Non-Patent Document 1 is also shown. The theoretical decomposition voltages in Examples 1 to 3 and the reference example are values ​​calculated based on the free energy of formation of CO and LiO at the reaction temperature. [Table 3] It can be seen that in the carbon production apparatus of Examples 1-3, in addition to high current efficiency, the theoretical decomposition voltage is lower than that of the Reference Example. Therefore, it can be said that solid carbon can be produced more efficiently in terms of energy because high current efficiency can be achieved at a low voltage.

[0065] As described above, the carbon production method of one embodiment and the carbon dioxide recovery method of one embodiment are as follows. (1) A carbon production method in which an anode and a cathode are immersed in a molten salt, a voltage is applied to the anode and the cathode, carbon dioxide is supplied, and solid carbon is produced by electrolysis, wherein a liquid metal containing at least one selected from the group consisting of tin (Sn), bismuth (Bi), antimony (Sb), zinc (Zn), cadmium (Cd), lead (Pb), indium (In), and gallium (Ga) is used as the cathode, and the carbon dioxide is blown into the liquid metal. (2) The carbon production method according to (1) above, wherein the molten salt is at least one of an alkali metal halide and an alkaline earth metal halide. (3) The carbon production method according to (2) above, wherein the alkali metal halide is at least one of LiF, NaF, KF, RbF, CsF, LiCl, NaCl, KCl, RbCl, CsCl, LiBr, NaBr, KBr, RbBr, CsBr, LiI, NaI, KI, RbI, and CsI. (4) The carbon production method according to (2) above, wherein the alkaline earth metal halide is at least one of MgF2, CaF2, SrF2, BaF2, MgCl2, CaCl2, SrCl2, BaCl2, MgBr2, CaBr2, SrBr2, BaBr2, MgI2, CaI2, SrI2, and BaI2. (5) The carbon production method according to any one of (1) to (4), wherein an oxide ion source is added to the molten salt. (6) The method for producing carbon according to any one of (1) to (5), wherein a carbonate ion source is added to the molten salt. (7) The carbon production method according to (5) or (6) above, wherein at least one of an alkali metal oxide and an alkaline earth metal oxide is used as the oxide ion source. (8) The method for producing carbon according to (6) or (7) above, wherein at least one of an alkali metal carbonate and an alkaline earth metal carbonate is used as the carbonate ion source. (9) The carbon production method according to any one of (1) to (8), further comprising recovering unreacted carbon dioxide from the carbon dioxide injected into the liquid metal and injecting it again into the liquid metal. (10) A method for recovering carbon dioxide from a gas containing carbon dioxide by the carbon production method according to any one of (1) to (9). Examples of the gas containing carbon dioxide include factory exhaust gas, biogas, and air.

[0066] As described above, the carbon production apparatus according to one embodiment and the carbon dioxide recovery apparatus according to one embodiment are as follows. (11) A carbon production apparatus for producing solid carbon by electrolysis, comprising: an electrolytic cell that contains molten salt; an anode that is immersed in the molten salt; a cathode that is immersed in the molten salt; a power source that applies a voltage to the anode and the cathode; and a carbon dioxide supply line that supplies carbon dioxide, wherein the cathode is a liquid metal containing at least one selected from the group consisting of tin (Sn), bismuth (Bi), antimony (Sb), zinc (Zn), cadmium (Cd), lead (Pb), indium (In), and gallium (Ga), and the carbon dioxide is blown into the liquid metal from the carbon dioxide supply line. (12) The carbon production apparatus according to (11), wherein the molten salt is at least one of an alkali metal halide and an alkaline earth metal halide. (13) In the carbon production apparatus according to (12), the alkali metal halide is at least one of LiF, NaF, KF, RbF, CsF, LiCl, NaCl, KCl, RbCl, CsCl, LiBr, NaBr, KBr, RbBr, CsBr, LiI, NaI, KI, RbI, and CsI. (14) The carbon production apparatus according to (12) above, wherein the alkaline earth metal halide is at least one of MgF2, CaF2, SrF2, BaF2, MgCl2, CaCl2, SrCl2, BaCl2, MgBr2, CaBr2, SrBr2, BaBr2, MgI2, CaI2, SrI2, and BaI2. (15) The carbon production apparatus according to any one of (10) to (14), wherein the molten salt contains an oxide ion source. (16) The carbon production apparatus according to any one of (10) to (15), wherein the molten salt contains a carbonate ion source. (17) The carbon production apparatus according to (15) or (16), wherein the oxide ion source is at least one of an alkali metal oxide and an alkaline earth metal oxide. (18) The carbon production apparatus according to (16) or (17), wherein the carbonate ion source is at least one of an alkali metal carbonate and an alkaline earth metal carbonate. (19) The carbon production apparatus according to any one of (10) to (18), further comprising a carbon dioxide circulation line that recovers unreacted carbon dioxide from the carbon dioxide blown into the liquid metal from the carbon dioxide supply line and introduces the recovered carbon dioxide into the carbon dioxide supply line. (20) The carbon production apparatus according to any one of (10) to (19), further comprising: a pump for extracting molten salt from the electrolytic cell; and a molten salt circulation line having the pump and supplying the extracted molten salt back to the electrolytic cell, wherein the molten salt circulation line is provided with a carbon recovery device for recovering solid carbon in the molten salt in the molten salt circulation line. (21) A carbon dioxide recovery device comprising the carbon production device according to any one of (10) to (20) above and an introduction section for introducing carbon dioxide from a gas containing carbon dioxide into the carbon dioxide supply line. [Explanation of symbols]

[0067] 1, 10 electrolytic cell 3, 13 Molten salt 5, 15, 85 cathode 6 Cathode Chamber 7, 17, 87 Anode 8 Anode chamber 9 Power supply 11, 21 Carbon dioxide supply line 12, 14 Electric wire 19 Space 23 Divider 25, 27 outlet 29 Carbon dioxide circulation line 30, 50, 60, 70, 90 Carbon Production Equipment 31 Pump 33 Molten salt circulation line 35 Solid-liquid separator 37 Collection container 40, 80 Comparative carbon production device 71 Electric Furnace 73 Holder 75 Crucible 77 Supply pipe 79 Discharge pipe 81 Thermocouple 83 Containers for liquid metals 89 Reference electrode 91 Carbon dioxide supply pipe 93 Conduit 95 Auxiliary carbon dioxide supply pipe 97 Electrochemical Measurement Equipment C solid carbon W Wire

Claims

1. 1. A carbon production method comprising immersing an anode and a cathode in a molten salt, applying a voltage to the anode and the cathode, and supplying carbon dioxide to produce solid carbon by electrolysis, the method comprising the steps of: As the cathode, a liquid metal containing at least one selected from the group consisting of tin (Sn), bismuth (Bi), antimony (Sb), zinc (Zn), cadmium (Cd), lead (Pb), indium (In), and gallium (Ga) is used, At least one of an alkali metal halide and an alkaline earth metal halide is used as the molten salt, A method for producing carbon, comprising blowing the carbon dioxide into the liquid metal.

2. 2. The method for producing carbon according to claim 1, wherein the alkali metal halide is at least one of LiF, NaF, KF, RbF, CsF, LiCl, NaCl, KCl, RbCl, CsCl, LiBr, NaBr, KBr, RbBr, CsBr, LiI, NaI, KI, RbI, and CsI.

3. As the alkaline earth metal halide, MgF ２ , CaF ２ , SrF ２ , BaF ２ , MgCl ２ , CaCl ２ , SrCl ２ , BaCl ２ , MgBr ２ , CaBr ２ , SrBr ２ , BaBr ２ , MgI ２ , CaI ２ , SrI ２ , BaI ２ 2. The method for producing carbon according to claim 1, wherein at least one of the following is used.

4. 4. The method for producing carbon according to claim 1, wherein an oxide ion source is added to the molten salt.

5. 4. The method for producing carbon according to claim 1, wherein a carbonate ion source is added to the molten salt.

6. 4. The method for producing carbon according to claim 1, further comprising recovering unreacted carbon dioxide from the carbon dioxide injected into the liquid metal and injecting it again into the liquid metal.

7. A method for recovering carbon dioxide, comprising recovering carbon dioxide from a gas containing carbon dioxide by the carbon production method according to any one of claims 1 to 3.

8. A carbon production apparatus for carrying out the carbon production method according to any one of claims 1 to 3, an electrolytic cell containing the molten salt in which the anode and the cathode are immersed; a power source that applies a voltage to the anode and the cathode; a carbon dioxide supply line that supplies the carbon dioxide to the liquid metal.

9. 9. The carbon manufacturing apparatus according to claim 8, further comprising a carbon dioxide circulation line for recovering unreacted carbon dioxide from the carbon dioxide blown into the liquid metal from the carbon dioxide supply line and introducing the recovered carbon dioxide into the carbon dioxide supply line.

10. a pump for extracting the molten salt from the electrolytic cell; a molten salt circulation line having the pump and supplying the extracted molten salt back to the electrolytic cell.

9. The carbon production apparatus according to claim 8, further comprising a carbon recovery device provided in the molten salt circulation line for recovering solid carbon contained in the molten salt in the molten salt circulation line.

11. 9. A carbon dioxide recovery system comprising: the carbon production system according to claim 8; and an introduction section for introducing carbon dioxide from a gas containing carbon dioxide into the carbon dioxide supply line.

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