Electrolysis system, reduction electrode, and laminated body

Abstract

An electrolysis system 10 comprises: a cathode 13 that is constituted by an electroconductive porous material; a reduction catalyst that is contained in the cathode 13 and that promotes reduction of matter to be reduced; and a liquid supply means that supplies to the cathode 13 a liquid which contains the matter to be reduced, wherein a flow path through which the liquid flows is formed inside the cathode 13, and, in the cathode 13, the reduction catalyst is dispersed in the thickness direction of the cathode.

Description

Electrolysis System, Reduction Electrode, and Laminate

[0001] The present invention relates to an electrolysis system, a reduction electrode, and a laminate, and more particularly, to an electrolysis system, a reduction electrode, and a laminate used for carbon dioxide reduction, for example.

[0002] Carbon dioxide reduction has attracted attention as a method for reducing CO2 emissions and storing natural energy, and research and development have been carried out. In recent years, a method using an electrolysis system that electrochemically reduces carbon dioxide at the cathode to produce valuable substances such as carbon monoxide has been studied. Conventionally, in the method of performing carbon dioxide reduction using an electrolysis system, a method of bringing carbon dioxide gas into contact with a cathode containing a reduction catalyst has been generally used. In the method of reducing carbon dioxide using an electrolysis system, it is required to improve the reduction efficiency of carbon dioxide.

[0003] Conventionally, in a carbon dioxide electrolyzer including a cathode that reduces carbon dioxide to produce a carbon compound and an anode that oxidizes water to produce oxygen, in order to suppress a decrease in electrolysis efficiency, a cathode gas flow path that faces the cathode and supplies a gas containing carbon dioxide, and an anode solution flow path that faces the anode and supplies an electrolytic solution containing water are provided (see, for example, Patent Document 1). Further, conventionally, as disclosed in Non-Patent Document 1, it is also known to form a flow path at a position facing the cathode, supply an electrolytic solution containing carbon dioxide to the flow path, and perform a reduction reaction by a liquid-phase reaction.

[0004] Japanese Patent Application Laid-Open No. 2022-143987

[0005] Varela, A. S. and co-workers Energy Fuels 2023, 37, 15283-15308

[0006] In Patent Document 1, as the current density increases, the reaction causes the vicinity of the cathode electrode to become basic, and thus carbonate deposits on the cathode electrode, covering the active sites on the electrode, resulting in a problem of a decrease in the Faraday efficiency. In order to prevent this problem, it is necessary to periodically stop the electrolysis and rinse with water or the like to remove the carbonate deposited on the cathode electrode.

[0007] On the other hand, in order to prevent the precipitation of carbonates, as shown in Non-Patent Document 1, there is a method of supplying an electrolytic solution containing carbon dioxide to the cathode side to carry out a liquid-phase reaction. By supplying it with an electrolytic solution, it becomes difficult for carbonates to precipitate. However, since the amount of carbon dioxide that can be dissolved in the electrolytic solution is small, there is a problem that the current density cannot be increased because the amount of reaction substrate is smaller than in the case of supplying it with gas.

[0008] Therefore, an object of the present invention is to provide an electrolysis system in which the reaction on the cathode side is a liquid-phase reaction and the reduction efficiency can be improved even when the current density is increased.

[0009] The present invention provides the following [1] to

[15] . [1] An electrolysis system comprising a cathode composed of a porous material having conductivity, a reduction catalyst contained in the cathode for promoting the reduction of a reducible substance, and liquid supply means for supplying a liquid containing the reducible substance to the cathode, wherein a flow path through which the liquid flows is formed inside the cathode, and the reduction catalyst is dispersed in the thickness direction of the cathode in the cathode. [2] The electrolysis system according to [1] above, wherein the reducible substance is carbon dioxide and the reduction catalyst is a catalyst for reducing carbon dioxide to carbon monoxide. [3] The electrolysis system according to [1] or [2] above, wherein the current density in the cathode is 100 mA / cm ２ or more. [4] The electrolysis system according to any one of [1] to [3] above, wherein the cathode has a porosity of 50% or more and the thickness of the cathode is 1.0 mm or more. [5] The electrolysis system according to any one of [1] to [4] above, wherein the porous material is selected from the group consisting of porous carbon, fibrous carbon, and carbon felt. [6] The amount of the reduction catalyst per unit area in the cathode is 0.1 mg / cm ２The electrolytic system according to any one of [1] to [5] above, wherein the reduction catalyst comprises powdered carbon and metal particles supported on the powdered carbon, the electrolytic system according to any one of [1] to [6] above, the metal particles being at least one of silver particles and gold particles, or the metal particles being cobalt particles and the reduction catalyst further comprising components derived from organic compounds, the electrolytic system according to [7] above, the specific surface area of ​​the powdered carbon being 200 to 1500 m² ２ The electrolytic system according to [7] or [8] above, wherein the value is / g.

[10] The electrolytic system according to any one of [1] to [9] above, wherein the surface roughness of the cathode is 35 μm or more in Sz.

[11] The electrolytic system according to any one of [1] to

[10] above, wherein the contact angle of at least a part of the porous material is 90° or less.

[12] The electrolytic system according to any one of [1] to

[11] above, wherein the cathode is made up of two or more cathodes having different porosities stacked on top of each other.

[13] The electrolytic system according to any one of [1] to

[12] above, further comprising a cathode current collector, a diaphragm, an anode, and an anode current collector, wherein the cathode current collector, the cathode, the diaphragm, the anode, and the anode current collector are stacked in this order to form a laminate.

[14] A reduction electrode comprising a cathode made of a conductive porous material and a reduction catalyst contained in the cathode that promotes the reduction of a substance to be reduced, wherein a channel is formed inside the cathode through which a liquid containing the substance to be reduced flows, and the reduction catalyst is dispersed in the cathode in the thickness direction of the cathode.

[15] A laminate comprising a cathode, a cathode current collector, a diaphragm, an anode, and an anode current collector, wherein the cathode current collector, the cathode, the diaphragm, the anode, and the anode current collector are stacked in this order, the cathode is made of a conductive porous material and contains a reduction catalyst that promotes the reduction of a substance to be reduced, a channel is formed inside the cathode through which a liquid containing the substance to be reduced flows, and the reduction catalyst is dispersed in the cathode in the thickness direction of the cathode.

[0010] In this invention, the reaction on the cathode side of the electrolytic system is a liquid-phase reaction, and good reduction efficiency can be achieved even with a high current density.

[0011] This is a schematic diagram showing an electrolytic system according to the first embodiment of the present invention. This is a schematic exploded perspective view showing a laminate according to the first embodiment of the present invention. This is a schematic diagram showing an electrolytic system according to the second embodiment of the present invention.

[0012] The present invention will be described in more detail below with reference to embodiments. [First Embodiment] Figure 1 shows an electrolytic system according to the first embodiment of the present invention. As shown in Figure 1, the electrolytic system 10 according to the first embodiment includes an electrolytic cell 11, the electrolytic cell 11 having a cathode 13, a cathode current collector 14, a diaphragm 19, an anode 23, and an anode current collector 24 inside. The inside of the electrolytic cell 11 is divided into a cathode chamber 16 and an anode chamber 26 by the diaphragm 19, and the cathode 13 and cathode current collector 14 are arranged inside the cathode chamber 16. The anode 23 and anode current collector 24 are arranged inside the anode chamber 26. The cathode 13 contains a reduction catalyst as described later and becomes a reduction electrode. The anode 23 becomes an oxidation electrode.

[0013] The cathode chamber 16 contains cathode electrolyte, and it is preferable that the cathode electrolyte flows in a constant direction, as will be described later. Similarly, the anode chamber 26 contains anode electrolyte, and it is preferable that the anode electrolyte flows in a constant direction, as will be described later. The cathode electrolyte and anode electrolyte penetrate into the interior of the cathode 13 and anode 23, respectively, and flow through the interior of the cathode 13 and anode 23.

[0014] Current flows from the power supply 25 between the cathode 13 and the anode 23, thereby causing electrochemical reactions to occur in the cathode 13 and the anode 23, respectively. In this embodiment, the power supply 25 is connected to the cathode current collector 14 and the anode current collector 24, respectively, and current flows from the cathode current collector 14 and the anode current collector 24 to the cathode 13 and the anode 23, respectively.

[0015] In the cathode chamber 16, a first supply line 17 as a liquid supply means and a first discharge line 18 as a liquid discharge means are connected. The first supply line 17 supplies a liquid containing the reducible substance X1 (that is, a cathode electrolyte containing the reducible substance X1) to a cathode 13 disposed inside the cathode chamber 16. The reducible substance X1 is reduced at the cathode 13 to generate a reduced substance Y1. Then, the reduced substance Y1 generated in the cathode chamber 16 is discharged to the outside of the cathode chamber 16 by the first discharge line 18 together with the cathode electrolyte.

[0016] In the present embodiment, it is preferable that the reducible substance X1 is carbon dioxide. In that case, the reduced substance Y1 may be a reduced product of carbon dioxide, but it is preferably carbon monoxide or formic acid. In that case, typically, one of the reactions represented by the following formulas (i), (ii), (i-1) or formula (ii-1) occurs at the cathode 13. Further, as the reduced substance Y1, carbon monoxide is more preferable. CO ２ + 2H<​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​The cathode-side storage unit 31 is connected to a reduction material supply line 12, and the reduction material X1 is supplied to the cathode-side storage unit 31 from a reduction material supply source (not shown) via the reduction material supply line 12. In the cathode-side storage unit 31, the reduction material X1 is mixed with the cathode electrolyte, thereby obtaining a cathode electrolyte containing the reduction material X1. The cathode-side storage unit 31 is connected to a cathode-side supply line, a cathode-side discharge line (not shown), etc., and the cathode electrolyte containing the electrolyte is supplied to the cathode side via the cathode-side supply line, and the cathode electrolyte may also be discharged to the outside from the cathode-side discharge line. However, the components constituting the cathode electrolyte (electrolyte, solvent, etc.) may be supplied separately to the cathode-side storage unit 31.

[0019] The substance to be reduced X1 can be any substance that can be reduced at the cathode, such as carbon dioxide, but it is preferable that it be blown into the cathode electrolyte stored in the cathode-side storage section 31 as a gas. The source of the substance to be reduced is not particularly limited, but may be a gas cylinder or the like. Also, if the substance to be reduced X1 is carbon dioxide, the carbon dioxide may be obtained from exhaust gas emitted from a power plant, steel mill, cement factory, or waste incineration plant, and any of these facilities may be used as the carbon dioxide source.

[0020] The cathode-side storage section 31 preferably has a space (i.e., a gas phase) at its top. With this configuration, when the substance to be reduced X1 is carbon dioxide, the reducing substance Y1 is carbon monoxide, and the solvent of the cathode electrolyte is water or alcohol, and the solubility of the reducing substance Y1 in the cathode electrolyte is lower than the solubility of the substance to be reduced X1 in the cathode electrolyte, the reducing substance Y1 that cannot dissolve in the cathode electrolyte is more easily released into the gas phase of the cathode-side storage section 31. Therefore, the content of the reducing substance Y1 in the cathode electrolyte is reduced, making it easier for the reduction reaction in the cathode 13 to occur. Furthermore, a cathode-side gas discharge line is preferably connected to the cathode-side storage section 31, so that the gas in the gas phase of the cathode-side storage section 31 is discharged to the outside as appropriate. However, a cathode-side gas discharge line is not necessarily required; in that case, the reduced substance Y1 may be discharged to the outside from the cathode-side discharge line together with the cathode electrolyte.

[0021] The first supply line 17 connects the cathode chamber 16 and the cathode-side storage section 31. Therefore, the cathode electrolyte containing the reduced substance X1 stored in the cathode-side storage section 31 is supplied to the cathode chamber 16 via the first supply line 17 as described above. Furthermore, in this embodiment, the first discharge line 18 also connects the cathode chamber 16 and the cathode-side storage section 31. As a result, the cathode electrolyte circulates between the cathode chamber 16 and the cathode-side storage section 31. That is, the cathode electrolyte is discharged to the outside from the cathode chamber 16 and then circulates back to the cathode chamber 16. Therefore, the cathode electrolyte flows along the circulation path in a certain direction and is reduced to the reduced substance Y1 in the cathode 13, so that the reduction reaction in the cathode 13 is carried out efficiently. In this embodiment, the cathode-side circulation path consists of a cathode chamber 16, a first discharge line 18, a cathode-side storage section 31, and a first supply line 17.

[0022] A second supply line 27 and a second discharge line 28 are connected to the anode chamber 26. The second supply line 27 supplies a liquid containing the material to be oxidized X2 (i.e., an anode electrolyte containing the material to be oxidized X2) to the anode chamber 26 (and further, to the inside of the anode 23). The material to be oxidized X2 is oxidized in the anode 23, and oxide Y2 is produced. The oxide Y2 produced in the anode chamber 26 is then discharged to the outside of the anode chamber 26 along with the anode electrolyte through the second discharge line 28.

[0023] In this embodiment, the electrolytic system 10 further includes an anode-side storage section 32. The anode-side storage section 32 stores an anode electrolyte containing the material to be oxidized X2. The configuration of the anode-side storage section 32 is not particularly limited as long as it can store the anode electrolyte, but a stirring device or the like may be provided as appropriate, and the stored anode electrolyte may be mixed by the stirring device. An anode-side supply line, an anode-side discharge line, etc. (not shown) are connected to the anode-side storage section 32, and the anode electrolyte may be supplied to the anode side via the anode-side supply line, and the anode electrolyte may be discharged from the anode side via the anode-side discharge line. In addition, each component constituting the anode electrolyte (solvent, electrolyte, material to be oxidized, etc.) may be supplied to the anode-side storage section 32 separately.

[0024] The second supply line 27 connects the anode chamber 26 and the anode-side storage section 32. Therefore, the anode electrolyte containing the oxide X2 stored in the anode-side storage section 32 is supplied to the anode chamber 26 via the second supply line 27 as described above. Furthermore, in this embodiment, the second discharge line 28 also connects the anode chamber 26 and the anode-side storage section 32. As a result, the anode electrolyte circulates between the anode chamber 26 and the anode-side storage section 32. That is, the anode electrolyte is discharged to the outside from the anode chamber 26 and then circulates back to the anode chamber 26. As a result, the anode electrolyte flows along the circulation path in a constant direction and is reduced to oxide Y2 at the anode 23, so that the oxidation reaction at the anode 23 is carried out efficiently. In this embodiment, the circulation path on the anode side consists of an anode chamber 26, a second discharge line 28, an anode-side storage section 32, and a second supply line 27.

[0025] The oxide X2 is not particularly limited, but for example, water in the anode electrolyte, hydroxide ions (OH) － Examples include the following. When water is the oxide X2, the oxide is oxygen (O ２ ) and hydrogen ions (H ＋ ) are examples. Also, the oxide X2 contains hydroxide ions (OH － In the case of (OH), examples of oxide Y2 include oxygen and water. － The halogen ions (X) may be supplied from the cathode side via the diaphragm 19. Also, the oxide X2 is a halogen ion (X － (Note that X is a halogen atom)) may also be the case, in which case the oxide Y2 is halogen (X ２ It would be good if it were a halogen ion (X － The oxide Y2 is preferably derived from an electrolyte contained in the anode electrolyte, as described later. The oxide X2 may also be an electrolyte component other than a halogen ion, such as an organic redox species, as described later. Among these, it is preferable that the oxide X2 is a halogen ion and the oxide Y2 is a halogen, from the viewpoint that the oxide Y2 can be used in other reactions.

[0026] In the circulation paths on the cathode and anode sides, the cathode electrolyte and anode electrolyte are preferably flowed in a certain direction by a pump 36. The pump 36 is not particularly limited, but examples include a diaphragm pump, syringe pump, and peristaltic pump. Furthermore, the flow of the cathode electrolyte and anode electrolyte may be formed by known means other than pumps, for example, by using gravity.

[0027] In the electrolysis system 10, the reaction may be carried out in a batch or continuous manner. When carried out continuously, the substance to be reduced, X1 such as carbon dioxide, is continuously or intermittently supplied to the cathode side via the substance to be reduced supply line 12, and the gas containing the reduced substance Y1 is continuously or intermittently discharged from the cathode side gas discharge line. However, if the cathode side gas discharge line is omitted, the reduced substance Y1 may be discharged together with the cathode electrolyte from the cathode side discharge line. Also, when carried out continuously, the cathode electrolyte may be partially withdrawn to the outside as needed, for example via the cathode side discharge line, while new cathode electrolyte is supplied from the outside via the cathode side supply line. This prevents byproducts from accumulating in the cathode electrolyte as reduction progresses. Similarly, in a continuous operation, a portion of the anode electrolyte may be removed to the outside via, for example, an anode-side discharge line, while new anode electrolyte is supplied from the outside via an anode-side supply line, as needed. The generated oxide Y2 is usually contained in the anode electrolyte, so it is preferable that it be discharged together with the anode electrolyte through the anode-side discharge line. The temperature inside the anode chamber 26 and cathode chamber 16 in the electrolysis system 10 is not particularly limited, but is preferably around room temperature, for example, about 0 to 60°C, preferably about 10 to 40°C.

[0028] As shown in Figures 1 and 2, the cathode 13 is laminated in contact with one surface of the cathode current collector 14 and is sandwiched between the diaphragm 19 and the cathode current collector 14. Similarly, the anode 23 is laminated in contact with one surface of the anode current collector 24 and is sandwiched between the diaphragm 19 and the anode current collector 24. With this configuration, a laminate 35 is formed inside the electrolytic cell 11, consisting of the cathode current collector 14, cathode 13, diaphragm 19, anode 23, and anode current collector 24 laminated in this order. However, in the laminate 35, the cathode 13 may be laminated on one surface of the cathode current collector 14 via a conductive member, and similarly, the anode 23 may be laminated on one surface of the anode current collector 24 via a conductive member. Furthermore, while the cathode 13 may be directly laminated so as to be in contact with the diaphragm 19, it is not necessarily required to be laminated directly onto the diaphragm 19; it may also be laminated onto the diaphragm 19 via another material such as a fluororesin sheet. The same applies to the anode 23.

[0029] In this embodiment, the cathode current collector 14 is preferably provided with a supply port 17A for supplying cathode electrolyte from the first supply line 17 to the cathode 13, as shown in Figures 1 and 2. The supply port 17A is a hole that penetrates the cathode current collector 14 along the height direction and is connected to the first supply line 17 on the side of the cathode current collector 14 opposite to the side facing the cathode 13. As a result, the cathode electrolyte supplied from the first supply line 17 is supplied through the supply port 17A to the interior of the cathode 13 from the surface 13A of the cathode 13. The height direction referred to here is the height direction of the cathode 13, which is the same direction as the stacking direction of the laminate 35 described above, and is also the same direction as the perpendicular direction of the side of the cathode current collector 14 facing the cathode 13, or the perpendicular direction of the surface 13A of the cathode 13. Furthermore, the surface 13A of the cathode 13 is a surface perpendicular to the height direction of the cathode 13, and is the so-called main surface. By supplying the cathode electrolyte from the surface 13A side of the cathode 13, it can easily penetrate into the interior of the cathode 13, allowing the reduction reaction to be carried out efficiently.

[0030] The holes constituting the supply port 17A are preferably formed parallel to the height direction, so that the supply angle of the cathode electrolyte through the supply port 17A is 90° with respect to the surface 13A of the cathode 13. However, the holes constituting the supply port 17A may be inclined with respect to the height direction, for example, they may be inclined so that the supply angle of the cathode electrolyte is about 45 to 135°. As described above, by inclining the supply angle perpendicular to the surface 13A or within a certain range of angles, the cathode electrolyte can penetrate more easily into the interior of the cathode 13.

[0031] The cathode electrolyte supplied to the inside of the cathode 13 is then carried along a certain direction (for example, a direction perpendicular to the height direction, or the up and down direction in Figures 1 and 2) to the outlet 18A, which will be described later. The outlet 18A is connected to the first discharge line 18 and, in this embodiment, is preferably positioned to face the side surface of the cathode 13. Furthermore, the outlet 18A and the supply port 17A are preferably positioned so as to sandwich the center of the cathode 13 in a direction perpendicular to the height direction of the cathode 13 (the up and down direction in Figure 1). With the outlet 18A and the supply port 17A positioned in this way, the cathode electrolyte is discharged from the outlet 18A after it has flowed sufficiently inside the cathode 13. The outlet 18A is preferably provided in the electrolytic cell 11.

[0032] In this embodiment, similarly, the anode current collector 24 is provided with a supply port 27A connected to the second supply line 27 on the anode side. The discharge port 28A (see Figure 2) connected to the second discharge line 28 is also preferably positioned to face the side surface of the anode 23. The configuration of the supply port 27A and discharge port 28A on the anode side is the same as that of the supply port 17A and discharge port 18A on the cathode side, except that the electrode is changed from cathode to anode.

[0033] Next, each component of the electrolytic system in this embodiment will be described in more detail. (Cathode) In the present invention, the cathode 13 is made of a conductive porous material. The pores inside the porous material of the cathode 13 become channels through which liquid flows, thus forming channels through which liquid (cathode electrolyte) flows inside the cathode 13. Therefore, even if the current density in the cathode 13 is increased, the Faraday efficiency can be maintained at a high level, resulting in good reduction efficiency.

[0034] The current density at cathode 13 is 100 mA / cm². ２ Preferably, the current density is 100 mA / cm². ２ With these specifications, the reduction reaction in cathode 13 is sufficiently carried out, improving practicality. The current density is 130 mA / cm². ２ The above is more preferable, with a current of 150 mA / cm². ２ The above is even more preferable, with a current of 200 mA / cm². ２ The above is even more preferable. While a higher current density is generally better, 500 mA / cm² is preferable from the viewpoint of preventing a decrease in Faraday efficiency. ２ The following is preferable: 400 mA / cm ２ The following is more preferable: 350 mA / cm ２ The following are even more preferable.

[0035] Furthermore, when the current density is above the lower limit mentioned above, the Faraday efficiency in the electrolysis system 10 should be, for example, 45% or more, but preferably 50% or more, more preferably 70% or more, even more preferably 75% or more, and even more preferably 90% or more. Even when the current density is set above a certain level as described above, the practicality can be improved by increasing the Faraday efficiency. The Faraday efficiency may be, for example, the Faraday efficiency when producing carbon monoxide from carbon dioxide.

[0036] The porosity of the cathode 13 may be, for example, 40% or more, but is preferably 50% or more. A porosity of 50% or more allows for the formation of a flow channel that allows sufficient liquid to be impregnated into the interior of the cathode 13, thereby appropriately increasing the Faraday efficiency. From the viewpoint of further increasing the Faraday efficiency, a porosity of 55% or more is more preferable, and 65% or more is even more preferable. The porosity is not particularly limited, but from the viewpoint of easily maintaining the cathode 13 in a certain shape, it is preferably 90% or less, and even more preferably 80% or less.

[0037] The porous material constituting the cathode contains other materials such as catalysts, but the porosity of the cathode is the porosity of the porous material including the catalysts and other materials that constitute the cathode. Furthermore, the porosity of the cathode can be calculated by subtracting the proportion of other materials (calculated from their weight and specific gravity) within the porous material from the porosity of the porous material. Additionally, if the cathode is compressed when set in the electrolytic cell, the porosity of the cathode is the porosity after compression. The porosity after compression can also be calculated using the porosity of the uncompressed cathode before being set in the electrolytic cell and the compression ratio at the time of setting.

[0038] The thickness of the cathode 13 is preferably 1.0 mm or more, more preferably 1.5 mm or more, even more preferably 1.8 mm or more, and even more preferably 2.0 mm or more. A thickness of 1.0 mm or more of the cathode 13 makes it three-dimensional, which facilitates a higher current density and, consequently, improves the Faraday efficiency in the cathode 13. While the thickness of the cathode 13 is not particularly limited, from the viewpoint of appropriately flowing the cathode electrolyte into the cathode 13 and facilitating a higher Faraday efficiency, it is preferably 20 mm or less, more preferably 10 mm or less, even more preferably 5 mm or less, and even more preferably 4 mm or less. Note that the cathode thickness refers to the thickness in the height direction of the cathode 13. That is, the thickness direction of the cathode 13 coincides with the height direction of the cathode 13. Furthermore, if the porous material is compressed and set, the thickness refers to the thickness after compression.

[0039] The porous material constituting the cathode 13 can be in the shape of a sheet or plate. The porous material constituting the cathode 13 can be any conductive material, but specifically, examples include metals, metal oxides, carbon materials, or composites thereof, with carbon materials being preferred among these. By using carbon materials, it is possible to form appropriate channels in the cathode 13 while providing conductivity. Furthermore, when carbon dioxide is used as the substance to be reduced X1, side reactions such as hydrogen production reactions become less likely to occur.

[0040] The porous material is preferably one of porous carbon, fibrous carbon, or carbon felt. Porous carbon is formed by the bonding of multiple carbon powders together. Porous carbon can be produced by kneading carbon powder and a resin binder and firing the mixture in an inert gas atmosphere. Examples of carbon powder include at least one selected from graphite, pitch obtained by further carbonization of tar-like substances, carbon fibers, carbon nanotubes, and mesocarbon microbeads. Examples of resin binders include at least one selected from thermoplastic resins such as polyvinyl chloride, chlorinated polyvinyl chloride resin, polyacrylonitrile, polyvinyl alcohol, and polyvinyl chloride-polyvinyl acetate copolymer; thermosetting resins such as phenolic resin, furan resin, imide resin, and epoxy resin; and natural polymer substances such as cellulose and gum arabic. Commercially available porous carbon may be used; for example, a porous carbon material manufactured by Mitsubishi Pencil Co., Ltd. may be used.

[0041] Fibrous carbon may be composed of various carbon fibers, such as long carbon fibers and short carbon fibers, intertwined to form the base material. Carbon felt is made of a felt-like base material. The felt-like base material is a cloth-like fibrous carbon, and may be made by laminating multiple layers of cloth-like fibrous carbon. Examples of raw materials for the fibrous carbon used in the felt-like base material include polyacrylonitrile (PAN), pitch material made from viscous liquids obtained from petroleum products or coal tar, and rayon. Commercially available carbon felt may be used as the carbon felt; for example, carbon felt manufactured by Nippon Carbon Co., Ltd. may be used.

[0042] When the porous material is a metal or metal oxide, the porous material may be, for example, a fibrous substrate. The fibrous substrate is preferably composed of metal fibers or metal oxide fibers intertwined in an appropriate manner. Examples of metals used for porous materials include gold, silver, platinum, nickel, titanium, chromium, and stainless steel (SUS), with titanium, nickel, and SUS being preferred among these. Examples of metal oxides include indium oxide, tin oxide, tin-doped indium oxide, and fluorine-doped tin oxide (FTO).

[0043] The surface roughness of the cathode 13 is preferably 35 μm or more in terms of Sz. When the surface roughness (Sz) of the cathode 13 is 35 μm or more, the cathode electrolyte penetrates more easily from the surface to the interior of the cathode 13, and the Faraday efficiency tends to improve. Generally, the surface of the cathode 13 is often made smooth, but in this invention, surprisingly, the Faraday efficiency tends to improve by making the surface roughness of the cathode 13 relatively large. The surface roughness (Sz) of the cathode 13 is more preferably 45 μm or more, even more preferably 50 μm or more, and even more preferably 60 μm or more. The surface roughness (Sz) of the cathode 13 is not particularly limited, but for example, it is 250 μm or less, preferably 200 μm or less, and more preferably 150 μm or less. The surface roughness (Sz) can be determined by observing the surface shape with a digital microscope and based on ISO 25178. Furthermore, the cathode 13 also contains materials other than porous materials such as reduction catalysts, but the surface roughness (Sz) referred to here is the surface roughness (Sz) of the cathode 13 containing the other materials.

[0044] The contact angle of the porous material constituting the cathode 13 is preferably 90° or less. A contact angle of 90° or less makes it easier to impregnate the porous material with the dilution solution containing the catalyst (described later) when supporting the reduction catalyst on the porous material. Therefore, it becomes easier to uniformly disperse the reduction catalyst in the cathode 13, thereby improving catalytic activity. A contact angle of 85° or less is more preferable. While it is sufficient for only a portion of the porous material's surface to have the above contact angle, it is even more preferable for its entire surface to have the above contact angle. The contact angle is the contact angle with respect to pure water and can be measured using a known contact angle meter; for example, it can be measured using a contact angle meter manufactured by Excimer.

[0045] The cathode 13 may consist of one cathode or two or more cathodes. When it consists of two or more cathodes, it is preferable that the two or more cathodes are stacked in the height direction. In addition, each cathode may have the same porosity or different porosity. When the porosity of the two or more cathodes are different, it is preferable that the porosity of the cathode on the cathode current collector side is higher than the porosity of the diaphragm side. Furthermore, by having different porosity values ​​for the cathodes as described above, the gaseous reductant can escape more easily, making it possible to reduce the voltage.

[0046] (Reduction Catalyst) As described above, cathode 13 contains a reduction catalyst. The reduction catalyst is not particularly limited as long as it promotes the reduction of the substance to be reduced X1, but it may contain, for example, a metal element. The metal element may be the metal itself or a metal compound. The metal element may be contained in the reduction catalyst as, for example, metal particles. The reduction catalyst is preferably a carbon dioxide reduction catalyst that reduces carbon dioxide to carbon monoxide. The reduction catalyst will be described in more detail below, using the case where the reduction catalyst is a carbon dioxide reduction catalyst as an example. When the reduction catalyst is a carbon dioxide reduction catalyst, the above metal element is not particularly limited, but it may be the metal itself or a metal compound.

[0047] The metal elements in the above metals are not particularly limited, but include V, Cr, Mn, Fe, Co, Ni, Cu, Sn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, Au, Hg, Al, Si, In, Sn, Tl, Pb, Bi, Sb, Te, U, Sm, Tb, La, Ce, Zn, Os, and Nd. The above metal compounds can be inorganic metal compounds and organometallic compounds of these metals, and specifically include metal halides, metal oxides, metal hydroxides, metal nitrates, metal sulfates, metal acetates, metal phosphates, metal carbonyls, and metal acetylacetonates. Among these, metal oxides are preferred.

[0048] The metal elements used in the reduction catalyst are preferably those belonging to Groups 7 through 12. Preferred specific examples include Mn, Fe, Ni, Ru, Co, Rh, Cu, Zn, Ag, Au, Pd, Ir, Pt, and Os, with Co, Fe, Ni, Au, and Ag being more preferred. Using these metal elements increases catalytic activity and makes it easier to increase the conversion efficiency from carbon dioxide to carbon monoxide. The metal elements may be used individually or in combination of two or more.

[0049] The reduction catalyst preferably contains a carbon compound, and more preferably contains powdered carbon. A conductive carbon compound is preferred as the carbon compound. More specifically, examples of carbon compounds include mesoporous carbon, activated carbon, carbon black such as Ketjenblack and acetylene black, graphite, carbon fiber, graphene, and carbon nanotubes. Among these, carbon black is preferred, and conductive carbon black is even more preferred. Furthermore, the carbon compound is preferably a porous carbon powder.

[0050] When a reduction catalyst contains powdered carbon, it is preferable to support the metal element on the powdered carbon. Furthermore, in a reduction catalyst, the metal element (i.e., metal or metal compound) may be included in the reduction catalyst in particulate form (i.e., metal particles). Therefore, it is preferable that the metal particles are supported on the powdered carbon in the reduction catalyst, and it is also preferable that the reduction catalyst itself is in powder form. It is particularly preferable that the reduction catalyst contains gold particles or silver particles, where the metal element is gold or silver. In this case, the reduction catalyst can achieve sufficiently good reduction efficiency even without containing nitrogen elements (i.e., components derived from nitrogen-containing compounds described later).

[0051] The powdered carbon used in reduction catalysts has a specific surface area of, for example, 10 to 3000 m². ２ The amount is / g, preferably 100 to 2000m ２ / g, more preferably 200 to 1500m ２ The value is / g. For powdered carbon, having a specific surface area above a certain level improves the dispersibility of the reduction catalyst, making it easier to improve the activity of the catalyst itself. Furthermore, having a specific surface area within a certain range makes it easier to appropriately support metal elements (metal particles) on the powdered carbon. The specific surface area is the BET specific surface area and can be measured by gas adsorption analysis.

[0052] When using powdered carbon, for example, it is preferable to obtain a reduction catalyst by mixing powdered carbon with a metal derivative containing the above-mentioned metal element to obtain a mixture (hereinafter also referred to as the "reduction catalyst raw material mixture"), and then heat-treating the mixture. Furthermore, it is more preferable to obtain a reduction catalyst by mixing powdered carbon, a metal derivative containing the above-mentioned metal element, and an organic compound to obtain a mixture (reduction catalyst raw material mixture), and then heat-treating the reduction catalyst raw material mixture. Therefore, it is more preferable that the reduction catalyst contains components derived from an organic compound in addition to the powdered carbon and metal elements (metal particles) derived from the metal derivative. The organic compound is preferably a polymer compound that is an oligomer or polymer, as will be described later.

[0053] Furthermore, the reduction catalyst is preferably a catalyst containing a nitrogen element and a metal element (also called a "nitrogen-containing metal catalyst"). Here, the nitrogen element used in the nitrogen-containing metal catalyst is preferably derived from the organic compound described above. Therefore, a nitrogen-containing compound is preferred as the organic compound. In addition, the nitrogen-containing metal catalyst is preferably contained in powdered carbon. The nitrogen-containing metal catalyst is preferably a catalyst obtained by heat-treating a mixture containing a metal derivative and a nitrogen-containing compound (reduction catalyst raw material mixture), and more preferably a catalyst obtained by heat-treating a reduction catalyst raw material mixture containing a metal derivative, a nitrogen-containing compound, and powdered carbon. When the reduction catalyst contains components derived from an organic compound or is a nitrogen-containing metal catalyst, from the viewpoint of reduction efficiency, it is preferable that the metal particles contain iron particles or cobalt particles, where the metal element is iron or cobalt, and more preferably cobalt particles. The cobalt is preferably present in the form of monatomic cobalt, for example.

[0054] Specific examples of the nitrogen-containing compounds mentioned above include pyridine derivatives, imidazole derivatives, pyrazole derivatives, and triazole derivatives. These compounds may be used individually or in combination of two or more. Therefore, nitrogen-containing metal catalysts may contain components derived from these nitrogen-containing compounds, and more specifically, they may contain at least one of a pyridine ring structure, an imidazole ring structure, a pyrazole ring structure, and a triazole ring structure. Among these, from the viewpoint of conversion efficiency and other factors, it is preferable to select one of pyridine derivatives, imidazole derivatives, and triazole derivatives, and particularly preferable to be a pyridine derivative.

[0055] Pyridine derivatives are compounds having a pyridine ring. A pyridine derivative may have one pyridine ring in one molecule, two pyridine rings, three pyridine rings, or four or more pyridine rings. Examples of compounds having one pyridine ring in one molecule (pyridine monomers) include alkylpyridines such as methylpyridine, ethylpyridine, butylpyridine, and pentylpyridine (amylpyridine), alkoxypyridines such as methoxypyridine and butoxypyridine, and aminopyridines such as 4-aminopyridine. Examples of compounds having two pyridine rings in one molecule include compounds having a bipyridine skeleton in which two pyridine rings are directly bonded by a carbon-carbon single bond (bipyridine derivatives), specifically diaminobipyridines such as 2,2'-bipyridine and 4,4'-diamino-2,2'-bipyridine. Compounds having three pyridine rings in one molecule include compounds having a terpyridine skeleton in which the three pyridine rings are directly bonded by carbon-carbon single bonds (terpyridine derivatives). Examples of terpyridine derivatives include terpyridine. Compounds having four or more pyridine rings in one molecule include pyridine oligomers having four or more pyridine rings and a weight-average molecular weight of less than 10,000. Examples of pyridine oligomers include compounds having a polypyridine skeleton in which the pyridine rings are directly bonded to each other by carbon-carbon single bonds. A preferred specific example is polypyridine. Examples of polypyridine include poly(2,5-pyridine) and poly(3,5-pyridine), with poly(2,5-pyridine) being more preferred. The molecular weight of polypyridines such as poly(2,5-pyridine) is not particularly limited, but is preferably 500 to 8,000, preferably 1,000 to 6,000, and more preferably 1,500 to 5,000 in weight-average molecular weight.

[0056] Furthermore, as pyridine derivatives, polymers having multiple pyridine rings in one molecule and a weight-average molecular weight of 10,000 or more are also mentioned. In this case, it is preferable that the pyridine derivative has four or more pyridine rings in one molecule. As specific compounds, polyvinylpyridine, which is a polymer of vinylpyridine, is also preferred, and among these, poly(4-vinylpyridine) is more preferred. Polyvinylpyridines such as poly(4-vinylpyridine) preferably have a molecular weight above a certain level from the viewpoint of reduction efficiency, for example, a weight-average molecular weight of 1,000 or more, preferably 10,000 or more, more preferably 30,000 or more, and even more preferably 50,000 or more. Also, from the viewpoint of availability, the weight-average molecular weight is, for example, 200,000 or less, preferably 100,000 or less. The weight-average molecular weight is a value measured by gel permeation chromatography (GPC), and polystyrene may be used as the standard substance.

[0057] The metal elements in the metal derivative used as a raw material for the reduction catalyst are as described above. The metal derivative preferably contains metal ions. Furthermore, the metal derivative may be used, for example, in the form of a metal salt. Examples of metal salts include metal nitrates, metal sulfates, metal chlorides, metal bromides, metal iodides, and metal acetates, among which metal nitrates are preferred.

[0058] The heat treatment used to obtain a reduction catalyst is typically calcination. The heat treatment is preferably carried out at a temperature of 150 to 550°C, more preferably 200 to 470°C. By performing the heat treatment at such a low temperature, ring structures derived from nitrogen-containing compounds, such as pyridine ring structures, imidazole ring structures, pyrazole ring structures, and triazole ring structures, can be retained in the nitrogen-containing metal catalyst. This allows for the formation of metal-nitrogen element bonds in the nitrogen-containing metal catalyst. Furthermore, the metal derivative can become the metal itself or a metal oxide through the above heat treatment.

[0059] In addition to the above, the reduction catalyst may also be a carbon compound containing at least one heteroatom such as nitrogen, or a metal or a metal compound. Examples of such carbon compounds include nitrogen-containing graphite, nitrogen-containing carbon nanotubes, nitrogen-containing graphene, Ni and nitrogen-containing graphite, Ni and nitrogen-containing carbon nanotubes, Ni and nitrogen-containing graphene, Cu and nitrogen-containing graphite, Cu and nitrogen-containing carbon nanotubes, Cu and nitrogen-containing graphene, Co and nitrogen-containing graphite, Co and nitrogen-containing carbon nanotubes, and Co and nitrogen-containing graphene. The reduction catalyst may be used alone or in combination of two or more types.

[0060] The reduction catalyst may be incorporated into the cathode 13 by supporting it on the porous material constituting the cathode 13. The reduction catalyst may be present both inside the cathode 13 and on the flow path. The method of supporting the reduction catalyst on the porous material is not particularly limited, and the reduction catalyst may be attached to the porous material by known methods. Here, attachment refers to a state in which the catalyst is physically fixed to the porous material, and the atoms constituting the porous material do not necessarily have to be chemically bonded to the atoms constituting the reduction catalyst.

[0061] The reduction catalyst is dispersed in the thickness direction of the cathode 13 and contained within the cathode 13. Here, dispersion in the thickness direction means that the reduction catalyst is not merely present in a layer on the surface of the cathode 13, but is contained not only on the surface of the cathode 13 but also in its interior center. Therefore, it is preferable that the reduction catalyst is contained on one surface 13A, the center in the thickness direction, and the other surface 13B of the cathode 13. Furthermore, it is preferable that the reduction catalyst is present throughout the entire thickness direction of the cathode 13, from one surface 13A to the other surface 13B. By dispersing the reduction catalyst in the thickness direction of the cathode 13, the electrolyte flowing through the internal channels of the cathode 13 comes into sufficient contact with the reduction catalyst, thereby achieving a sufficiently high Faraday efficiency.

[0062] Furthermore, the reduction catalyst may be supported on the cathode 13 together with the catalyst additive. The catalyst additive also functions as a binder when supporting the catalyst on the electrode substrate. In addition, it also functions as an ion conductor, improving the efficiency of the electrochemical reaction. Examples of catalyst additives include cationic conductive compounds, anionic conductive compounds, and fluorine compounds other than cationic conductive compounds and anionic conductive compounds. The catalyst additive may be present inside the cathode 13 together with the reduction catalyst and also on the flow path. Furthermore, the catalyst additive may be dispersed in the thickness direction of the cathode 13 together with the reduction catalyst. Therefore, both the reduction catalyst and the catalyst additive may be contained in one surface 13A, the center in the thickness direction, and the other surface 13B, and may be contained throughout the entire thickness direction of the cathode 13.

[0063] Cationic conductive compounds used as catalyst additives include, for example, functional groups having Brønsted acids or salts thereof, and compounds having functional groups that become anionic when hydrogen ions are released. Examples of cationic conductive compounds include compounds having at least one functional group such as a sulfonyl group, a phosphate group, a hydroxyl group, and a silicate group. Specifically, a commercially available product is "Nafion" (a trademark of DuPont). Examples of anionic conductive compounds include, for example, compounds having functional groups having Brønsted bases or salts thereof, and compounds having functional groups that become cationized when protons are added. Anionic conductive compounds have excellent conductivity for anions such as hydroxide ions. Specific examples of anionic conductive compounds include compounds having functional groups such as a pyridinium group, an imidazolium group, an amino group, and an ammonium group. As anion-conducting compounds, commercially available products such as FuMA-Tech GmbH's "Fumion FAA-3-SOLUT-10" and Resintech's "PowerMax NXS125 OH" can also be used. Examples of fluorine compounds include polytetrafluoroethylene (PTFE), tetrafluoroethylene oligomer (TFEO), graphite fluoride ((CF)n), and fluorinated pitch (FP).

[0064] The catalyst additive may, for example, be in the form of a powder or particulate matter. When using a catalyst additive, the reducing catalyst may be mixed with the catalyst additive and supported on the electrode substrate in the form of a mixture (catalyst composition). When the catalyst is supported on the electrode substrate together with the catalyst additive, the content of the catalyst additive relative to the total amount of the catalyst additive and catalyst is preferably 10% to 50% by mass, more preferably 15% to 45% by mass, and even more preferably 20% to 40% by mass, from the viewpoint of improving the conversion efficiency of the catalyst.

[0065] The method for supporting the reduction catalyst on a porous material is not particularly limited, but examples include applying a diluted solution, obtained by diluting the reduction catalyst and other components such as catalyst additives as needed, with a diluting solvent to the porous material using various coating devices or by spray coating and then drying it, or immersing the porous material in the above-mentioned diluted solution and drying it. Among these, the method using immersion is preferred from the viewpoint of easily dispersing the reduction catalyst in the thickness direction of the cathode 13 and easily supporting the reduction catalyst evenly inside the cathode 13.

[0066] The concentration of the reduction catalyst in the diluent is not particularly limited, but from the viewpoint of being able to appropriately disperse the reduction catalyst in the diluent without using more diluent solvent than necessary, and from the viewpoint of easily dispersing the reduction catalyst in the thickness direction of the cathode 13, it is, for example, 0.1 to 20 g / L, preferably 0.2 to 8 g / L, and more preferably 0.4 to 4 g / L. The reduction catalyst, or the reduction catalyst and catalyst additive, is preferably dispersed in the diluent, and dispersibility may be improved by using ultrasonic vibration or the like. The solvent used for dilution is water or an organic solvent, of which organic solvents are preferred, and alcohol-based solvents are more preferred. Examples of alcohol-based solvents include methanol, ethanol, phenol, 1-propanol, isopropanol, ethylene glycol, and propylene glycol, of which methanol, ethanol, 1-propanol, and isopropanol are preferred.

[0067] The amount of reducing catalyst per unit area in cathode 13 is, for example, 0.1 mg / cm². ２That concludes the report. The amount of reducing catalyst is 0.1 mg / cm³. ２ With the above conditions, the Faraday efficiency can be sufficiently high even at high current densities. From the viewpoint of Faraday efficiency, the amount of reducing catalyst per unit area in the porous material is preferably 0.5 mg / cm². ２ More preferably 2 mg / cm³ ２ More preferably 4 mg / cm³ ２ More preferably 8 mg / cm² ２ That concludes the explanation. Furthermore, the amount of reducing catalyst per unit area in cathode 13 is, for example, 50 mg / cm². ２ Preferably, 30 mg / cm³ ２ More preferably 27 mg / cm³ ２ More preferably, 23 mg / cm³ ２ More preferably, 20 mg / cm³ ２ The following applies: By keeping the amount of reduction catalyst below a certain level, it becomes easier to maintain a high Faraday efficiency by preventing the voids in the porous material from being blocked more than necessary by the reduction catalyst. Note that the amount per unit area referred to here is the amount per unit area when the cathode is viewed in a plan view along the height direction mentioned above.

[0068] (Cathode Current Collector) The cathode current collector 14 can be any material used as a current collector in an electrochemical reaction without any particular limitations. The cathode current collector 14 can be made of a conductive material, and materials that can be used for the cathode current collector 14 include metals, metal oxides, carbon materials, or composites thereof. Examples of metals that can be used for the cathode current collector 14 include gold, silver, platinum, nickel, titanium, chromium, and stainless steel (SUS), with titanium, nickel, and SUS being preferred among these. Examples of metal oxides include indium oxide, tin oxide, tin-doped indium oxide, and fluorine-doped tin oxide (FTO). Among these, when carbon dioxide is the material to be reduced X1, a carbon material is preferred from the viewpoint of reducing the occurrence of side reactions such as hydrogen production reactions. Conductive carbon can be used as the carbon material.

[0069] The cathode current collector 14 may be one that has voids, but it is generally preferable to use one that does not have voids. That is, it is preferable for the cathode current collector 14 to have a void ratio of 0%, but as long as the effects of the present invention are achieved, it may be used that has voids, for example, one with a void ratio of less than 40%, preferably less than 10%. The cathode current collector 14 is usually a plate-shaped or sheet-shaped member. The cathode current collector 14 may be attached to a holder, for example, and then attached to the electrolytic cell 11 via the holder.

[0070] (Cathode Electrolyte) The cathode electrolyte is preferably a solvent that can contain a reduceable substance X1 such as carbon dioxide, and examples include water and organic solvents. As organic solvents, appropriate solvents can be selected from those commonly used in electrochemical reactions, and examples include alcohol-based solvents such as methanol, ethanol, phenol, 1-propanol, ethylene glycol, and propylene glycol; nitrile-based solvents such as acetonitrile; carbonate-based solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactone-based solvents such as γ-butyrolactone; ether-based solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; phosphate ester solvents; phosphoric acid compounds; sulforane-based solvents; pyrrolidone compounds, etc. These organic solvents may be used individually or in combination of two or more. In this specification, the term "alcohol-based" includes aromatic hydroxy compounds, such as phenol, in which a hydroxyl group is directly bonded to an aromatic ring such as a benzene ring. As a solvent for the cathode electrolyte, at least one of an alcohol-based solvent or water is preferred from the viewpoint of being able to dissolve a certain amount of carbon dioxide, and at least one of methanol or water is more preferred, with water being even more preferred.

[0071] The cathode electrolyte preferably contains an electrolyte in the solvent. The electrolyte preferably dissolves in the solvent. By containing an electrolyte, the cathode electrolyte allows the electrochemical reaction to proceed appropriately, and the reduced substance X1 is appropriately reduced to the reduced substance Y1. Examples of electrolytes include halogenated salts, perchlorates, sulfates, and bicarbonates. Examples of halogenated salts include metal halide salts, specifically lithium halide salts such as lithium chloride and lithium bromide, sodium halide salts such as sodium chloride and sodium bromide, potassium halide salts such as potassium chloride and potassium bromide, and cesium halide salts such as cesium chloride and cesium bromide. Ammonium halides such as ammonium chloride and ammonium bromide are also examples. The halogenated salt is preferably a metal halide salt, and more preferably an alkali metal halide salt. Specifically, lithium chloride, lithium bromide, sodium chloride, sodium bromide, potassium chloride, and potassium bromide are preferred.

[0072] Examples of perchlorates include sodium perchlorate (NaClO4), zinc perchlorate, barium perchlorate, lithium perchlorate, and ammonium perchlorate. Examples of sulfates include sodium sulfate, potassium sulfate, and ammonium sulfate. Examples of bicarbonates include alkali metal bicarbonates such as potassium bicarbonate, sodium bicarbonate, and lithium bicarbonate, and alkaline earth metal bicarbonates such as magnesium bicarbonate, calcium bicarbonate, and barium bicarbonate. Among these, halogenated salts and bicarbonates are preferred, with alkali metal halide salts and alkali metal bicarbonates being more preferred, alkali metal bicarbonates being even more preferred, and sodium bicarbonate being even more preferred. In the cathode electrolyte, one electrolyte may be used alone, or two or more may be used in combination. The concentration of the electrolyte in the cathode electrolyte is not particularly limited, but is often, for example, 0.001 to 10 M, preferably 0.01 to 3 M, and more preferably 0.05 to 2 M.

[0073] (Anode) In this embodiment, the anode 23 is preferably made of a conductive porous material. In the anode 23, the pores of the porous material become channels through which liquid flows, thus forming channels through which liquid (anode electrolyte) flows inside the anode 23. Therefore, even if the current density in the anode 23 is high, the Faraday efficiency can be maintained at a high level, and the oxidation efficiency is good. Details of the porous material constituting the anode 23 (e.g., porosity, thickness, shape, material, etc.) are as described in detail for the cathode 13, and a detailed explanation is omitted here. The porous material used in the anode 23 may be the same as or different from the porous material used in the cathode 13. Also, the porosity and thickness of the porous material in the anode 23 may be the same as or different from the porosity and thickness of the porous material in the cathode 13.

[0074] Anode 23 may or may not contain a catalyst (oxidation catalyst). The presence or absence of a catalyst in anode 23, and the type of catalyst used, should be appropriately selected depending on the type of oxide X2 and oxide Y2. For example, if the oxide X2 is water or hydroxide ions, a known oxidation catalyst that promotes the oxidation of water or hydroxide ions should be used. Also, for example, if the oxide X2 is a halogen ion, anode 23 may or may not contain a catalyst. The oxidation catalyst can be supported on the porous material constituting anode 23, and the method of support is as described in the cathode section. Alternatively, the oxidation catalyst may be supported on the porous material together with a catalyst additive as described in the cathode section. If the anode 23 contains an oxidation catalyst, the oxidation catalyst, or the oxidation catalyst and catalyst additive, may be dispersed in the thickness direction of the anode 23. Therefore, the oxidation catalyst, or the oxidation catalyst and catalyst additive, may be contained in one surface 23A, the center in the thickness direction, and the other surface 23B, and may also be contained throughout the entire thickness direction.

[0075] (Anode current collector) The anode current collector 24 can be any current collector used in electrochemical reactions without any particular restrictions, and its details are as described in the section on the cathode current collector.

[0076] (Anode Electrolyte) The anode electrolyte is often made using solvents commonly used in electrolytes, such as water or organic solvents. Specific examples of organic solvents are as illustrated in the section on the cathode electrolyte, and one organic solvent may be used alone or two or more may be used in combination. As the solvent for the anode electrolyte, at least one of an alcohol-based solvent or water is preferred, of which at least one of methanol or water is more preferred, and of which water is even more preferred.

[0077] The anode electrolyte preferably contains an electrolyte in the solvent. The electrolyte preferably dissolves in the solvent. By containing an electrolyte in the anode electrolyte, the electrochemical reaction proceeds appropriately, and the oxide X2 can be appropriately oxidized to oxide Y2. Examples of electrolytes include halogenated salts, perchlorates, sulfates, and bicarbonates. Specific examples of halogenated salts, perchlorates, sulfates, and bicarbonates are as described above for the cathode electrolyte. Organic compounds and metal complexes may also be used as electrolytes. Examples of organic compounds include those that can be used as organic redox species, such as quinone derivatives and anthraquinone derivatives. The electrolyte used in the anode electrolyte is preferably a halogenated salt, preferably a metal halide salt, and more preferably an alkali metal halide salt, with specific examples as described above. By using a halogenated salt as the electrolyte in the anode electrolyte, the oxide X2 can be converted into a halogen ion on the anode side, and a halogen can be produced as oxide Y2. In the anode electrolyte, one type of electrolyte may be used alone, or two or more types may be used in combination. The concentration of the electrolyte in the anode electrolyte is not particularly limited, but is often, for example, 0.001 to 10 M, preferably 0.01 to 3 M, and more preferably 0.05 to 2 M.

[0078] <Membrane> The membrane 19 is permeable to ions but preferably not to the substance to be reduced X1, the reduced substance Y1, the solvent, or other substances other than ions, such as the oxides X2 and Y2. The membrane 19 may be an ion exchange membrane or a porous membrane other than an ion exchange membrane. As an ion exchange membrane, a solid membrane is used, and examples include a cation exchange membrane that can permeate cations such as protons, and an anion exchange membrane that can permeate anions such as hydroxide ions, but a cation exchange membrane is preferred.

[0079] If the diaphragm 19 is a cation exchange membrane, cations such as protons move from the anode chamber 26 to the cathode chamber 16, and the reaction of formula (i) described above can easily occur in the cathode chamber 16. Also, if the diaphragm 19 is an anion exchange membrane, anions such as hydroxide ions move from the cathode chamber 16 side to the anode chamber 26 side, and the reaction of formula (ii) described above can easily occur in the cathode chamber 16.

[0080] Examples of cation exchange membranes include those having at least one of the following functional groups: sulfonyl group, carboxyl group, phosphate group, and silicate group. Examples of cation exchange membranes having a sulfonyl group as a functional group include hydrocarbon resin-based polysulfonic acids such as polyethylene sulfonic acid and fullerene crosslinked polysulfonic acid, and fluororesin-based sulfonic acids such as perfluoroethylene sulfonic acid. Examples of perfluoroethylene sulfonic acid include copolymers of tetrafluoroethylene and perfluoro[2-(fluorosulfonylethoxy)propyl vinyl ether], and a commercially available product is "Nafion" (a trademark of DuPont). Examples of membranes having a carboxyl group as a functional group include polycarboxylic acids such as polyacrylic acid. Examples of membranes having a phosphate group or silicate group as a functional group include heteropoly acids such as tetratungstic acid and phosphotungstic acid. Furthermore, as a cation exchange membrane, SiO ２ -P ２ O ５ Ceramics such as phosphated glass and perovskite oxides can also be used.

[0081] Furthermore, examples of anion exchange membranes include resins and polyethers containing quaternary ammonium salts such as poly(styrylmethyltrimethylammonium chloride), and polymers containing imidazolium groups. Examples of resins containing ammonium salts include "FAA-3-50" from FuMA-Tech GmbH and "TM1 Durion Grade" from Orion. Examples of polymers containing imidazolium groups include styrene-based polymers containing imidazolium groups, specifically copolymers of styrene and 1-(p-vinylbenzyl)-3-methylimidazolium (PSMIM), copolymers of styrene and 1-(p-vinylbenzyl)-tetramethylimidazolium (PSTMIM), and copolymers of styrene and 1-(p-vinylbenzyl)-2,3-dimethylimidazolium (PSDMIM).

[0082] Furthermore, the diaphragm 19 may be a porous membrane other than an ion exchange membrane, and it is also preferable that it be a porous membrane that allows both anions and cations to pass through. Examples of porous membranes that allow both anions and cations to pass through include polyolefin porous membranes such as polypropylene porous membranes, and fluorine resin membranes such as polyvinylidene fluoride porous membranes.

[0083] Furthermore, in this embodiment, the diaphragm 19 may move cations, anions, etc. other than protons and hydroxide ions from the cathode chamber 16 to the anode chamber 26, or vice versa. For example, if the electrolytes contained in the anode electrolyte and the cathode electrolyte are metal halide salts, cations constituting the halide salt, such as metal ions, can be moved from the anode chamber 26 to the cathode chamber 16, and halogen ions can be moved from the cathode chamber 16 to the anode chamber 26. As a result, halogens are more easily generated at the anode, and the cations and anions of the electrolyte can be more effectively utilized.

[0084] As described above, in this embodiment, the cathode is constructed of a porous material and a channel for the cathode electrolyte is formed inside it, so even at high current densities, reduced products such as carbon monoxide can be produced with high Faraday efficiency, resulting in good reduction efficiency. Furthermore, by flowing the cathode electrolyte through the cathode to cause a liquid-phase reaction, the deposition of by-products such as carbonates on the cathode can be prevented. Moreover, in this embodiment, the anode is also constructed of a porous material, so that oxides can be produced at the anode with high Faraday efficiency even at high current densities.

[0085] [Second Embodiment] In the first embodiment described above, the product from the cathode side (reduced product Y1) was not supplied to the anode side. However, the product from the cathode side (reduced product Y1) may be supplied to the anode side and used in the reaction carried out at the anode side. Hereinafter, a second embodiment will be described with reference to Figure 3, in which the reduced product Y1 is supplied to the anode side and the reaction product is generated using the reduced product Y1 and oxide Y2. In the second embodiment (Figure 3), the same reference numerals will be used for the same components as in the first embodiment. In the following description, the differences from the first embodiment will be explained, and parts that are omitted from the explanation will be the same as in the first embodiment.

[0086] The electrolytic system 50 according to the second embodiment further comprises a reaction unit 40, which may be provided in the circulation path on the anode side. Specifically, first and second connecting lines 41 and 42 are provided to connect the reaction unit 40 to the anode-side storage unit 32, and a circulation path is further provided through which the anode electrolyte is circulated between the anode-side storage unit 32 and the reaction unit 40. In this circulation path, the anode electrolyte is preferably circulated in the order of anode-side storage unit 32, first connecting line 41, reaction unit 40, and second connecting line 42. Furthermore, the anode electrolyte in the circulation path may be flowed in a constant direction by a pump 36 or the like, similar to other circulation paths.

[0087] Therefore, in this embodiment, the anode electrolyte circulates between the anode chamber 26 and the anode-side storage section 32, and also between the anode-side storage section 32 and the reaction section 40, thereby enabling the anode electrolyte to circulate between the anode chamber 26, the anode-side storage section 32, and the reaction section 40. The circulation path on the anode side can be formed in any way as long as the anode electrolyte circulates between the anode chamber 26, the anode-side storage section 32, and the reaction section 40. For example, the reaction section 40 may be provided on a second supply line 27 between the anode chamber 26 and the anode-side storage section 32, thereby circulating the anode electrolyte in the order of anode chamber 26, anode-side storage section 32, and reaction section 40.

[0088] Furthermore, in this embodiment, the cathode-side gas discharge line 39 is connected to the reaction section 40, and the reduced product Y1 (e.g., carbon monoxide) generated on the cathode side is supplied to the reaction section 40 via the cathode-side gas discharge line 39. However, as long as the reduced product Y1 is supplied to the reaction section 40, the cathode-side gas discharge line 39 may be connected to any position in the circulation path on the anode side, for example, it may be connected to the anode-side storage section 32, or to the first connection line 41, etc. Also, the reduced product Y1 supplied from the cathode side to the anode side via the cathode-side gas discharge line 39 may contain the reduced substance X1 such as carbon dioxide, or by-products (e.g., hydrogen) or water vapor generated in the cathode 13, etc.

[0089] As described above, the oxide Y2 produced at the anode 23 and the reduced product Y1 supplied from the cathode side are supplied to the reaction section 40, where the oxide Y2 and reduced product Y1 are used to carry out a reaction and produce a reaction product. The reaction product may be produced from the reduced product Y1 and the oxide Y2, but it is preferable that the anode electrolyte contains a reaction substrate and the reaction product is produced from the reaction substrate, the reduced product Y1 and the oxide Y2. The reaction substrate is preferably one that acts as a solvent for the anode electrolyte. Therefore, when the anode electrolyte contains a reaction substrate, the anode electrolyte may contain the reaction substrate, an electrolyte and a solvent, but if the reaction substrate functions as a solvent, it is not necessary to include a separate solvent in addition to the reaction substrate.

[0090] In this embodiment, it is preferable that the reduced product Y1 is carbon monoxide, the oxide Y2 is a halogen, and the reaction substrate is an alcohol-based compound, from which a carbonyl compound is produced. Organic carbonates are preferred as the carbonyl compound. In this case, a halogenated salt is often used as the electrolyte, and the halogenated salt or a compound derived from a halogenated salt functions as a redox species. The reaction section 40 preferably contains a third catalyst that produces a carbonyl compound from carbon monoxide and the reaction substrate in the presence of a redox species.

[0091] As described above, when the oxide X2 is a halide salt, the oxide Y2 is a halogen, and the reaction substrate is an alcohol compound, the reaction shown in formula (iii) below typically takes place at the anode 23 to produce a halogen. The produced halogen is then supplied to the reaction section 40, and in the presence of the third catalyst described later, the reduced product Y3 (halogen ion X) is produced. － ) is reduced to, and then a reaction is carried out to produce an organic carbonate as shown in the following formula (iv) or (iv-1). However, in reaction section 40, substances other than organic carbonates may be produced, such as organic oxalates. 2X － →X ２ +2e － (iii) CO+2ROH+2X － → (RO) ２CO+2HX (iv)

[0092] (Note that in equations (iv) and (iv-1), X is a halogen atom. R, R １１ (This will be explained later.)

[0093] The alcohol-based compound used as the reaction substrate is a reaction substrate that reacts with carbon monoxide to produce an organic carbonate, an organic oxalate, or both. The alcohol-based compound may be a solid, liquid, or gas under the temperature conditions of the reaction carried out in the reaction section 40, but it is preferably a liquid. A liquid alcohol-based compound is even more preferable because it can also function as a solvent for dissolving the electrolyte in the anode electrolyte.

[0094] Alcohol compounds are compounds having at least one hydroxyl group, and more specifically, compounds represented by the following general formula (1): ROH (1) (R represents an organic group having 1 to 15 carbon atoms.) Examples of organic groups having 1 to 15 carbon atoms represented by R in the above general formula (1) include hydrocarbon groups having 1 to 15 carbon atoms. Examples of hydrocarbon groups include alkyl groups having 1 to 15 carbon atoms, alkenyl groups having 2 to 15 carbon atoms, and aryl groups having 6 to 15 carbon atoms. Examples of alkyl groups having 1 to 15 carbon atoms include methyl groups, ethyl groups, various propyl groups, various butyl groups, various pentyl groups, various hexyl groups, various heptyl groups, various octyl groups, various nonyl groups, various decyl groups, various dodecyl groups, and various pentadecyl groups. Examples of C2-C15 alkenyl groups include vinyl groups, various propynyl groups, various butynyl groups, various pentynyl groups, various hexenyl groups, various heptenyl groups, various octenyl groups, various nonenyl groups, various decenyl groups, various dodecenyl groups, and various pentadecenyl groups. "Various" refers to various isomers including n-, sec-, tert-, and iso-. Furthermore, alkyl or alkenyl groups may be linear, branched, or cyclic. Examples of C6-C15 aryl groups include phenyl groups and naphthyl groups. The above-mentioned hydrocarbon groups may have substituents, in which case the total number of carbon atoms, including the substituents, is 1-15.

[0095] Furthermore, the organic group having 1 to 15 carbon atoms in general formula (1) may contain heteroatoms such as nitrogen atoms, oxygen atoms, sulfur atoms, phosphorus atoms, and halogen atoms. Among these, oxygen atoms are preferred. If an oxygen atom is present, it is preferable that the oxygen atom is either a hydroxyl group or an ether bond oxygen atom. Therefore, R is preferably a hydrocarbon group having at least one of a hydroxyl group and an ether bond. Also, it is preferable that there is one hydroxyl group in R. That is, the alcohol compound may have two hydroxyl groups. More specifically, the alcohol compound having two hydroxyl groups is preferably the group represented by the following formula (1-1): HO-R 11 -OH (1-1) Note that in equation (1-1), R 11 R is a divalent saturated hydrocarbon group having 2 to 15 carbon atoms, 11 The number of carbon atoms is preferably 2 to 4, more preferably 2 to 3.

[0096] Among the compounds represented by the above general formula (1), R is preferably an alkyl group having 1 to 8 carbon atoms, an alkenyl group having 2 to 8 carbon atoms, or an aryl group having 6 to 8 carbon atoms. Also, the compound represented by the general formula (1-1), R 11 Compounds with 2 to 4 carbon atoms are also preferred. Among these, compounds in which R is an alkyl group or an aryl group are more preferred, and compounds in which R is an alkyl group are even more preferred. Furthermore, alkyl groups with 1 to 3 carbon atoms are more preferred, with 1 or 2 carbon atoms being even more preferred, and 1 carbon atom being the most preferred. Specifically, from the viewpoint of reactivity and production efficiency, methanol, ethanol, phenol, 1-propanol, ethylene glycol, propylene glycol, etc. are preferred as alcohol compounds, and among these, methanol is more preferred. Note that one alcohol compound may be used alone, or two or more may be used in combination.

[0097] In this embodiment, a known reactor may be used for the reaction section 40. The reaction section 40 may contain the third catalyst as described above. The third catalyst may be dispersed or dissolved in the anode electrolyte in the reaction section 40. The third catalyst does not need to be dispersed or dissolved in the anode electrolyte as long as it is in contact with the anode electrolyte in the reaction section 40, and may be in the form of a fixed bed in the reaction section 40. Furthermore, a stirring device for stirring the anode electrolyte may be attached to the reaction section 40. In addition, filters or the like may be provided at the connection ports to each line of the reaction section 40 as needed to prevent the third catalyst from flowing out of the reaction section 40. However, filters or the like may not be provided, and the third catalyst may be flowed together with the anode electrolyte and circulated between the anode chamber 26 and the reaction section 40. The temperature inside the reaction section 40 is not particularly limited, but is preferably around room temperature, for example, about 0 to 60°C, preferably about 10 to 40°C.

[0098] As the third catalyst, a catalyst is used that promotes a chemical reaction to synthesize a carbonyl compound from carbon monoxide and a reaction substrate (alcohol-based compound) in the presence of redox species. The third catalyst may contain a metal element, and more preferably contains a metal element selected from Group 8 to Group 11 elements. Specifically, Group 8 to Group 11 elements used in the third catalyst include Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au. Among these, Co, Ni, Cu, Rh, Pd, Ag, Ir, Au, and Pt are preferred, with Pd, Au, Ag, and Ir being more preferred, Pd and Au being even more preferred, and Pd being even more preferred. The metal element used in the third catalyst may be used alone or in combination of two or more elements. When using two or more elements in combination, two or more metallic elements selected from Group 8 to Group 11 may be used in combination, or a metallic element from Group 8 to Group 11 may be used in combination with a metallic element from another Group. Preferably, the metallic elements from the fourth period are used, but other metallic elements from the fourth period are also acceptable.

[0099] The third catalyst may be a catalyst containing active particles, a metal salt, or a combination of both. By using a catalyst containing active particles or a metal salt, carbonyl compounds, particularly organic carbonates, can be produced from carbon monoxide with high conversion efficiency. Among these, the use of a catalyst containing active particles is preferred.

[0100] In a catalyst containing active particles, the active particles have catalytic activity that promotes the reaction when synthesizing carbonyl compounds from carbon monoxide. Active particles containing metal elements are not particularly limited as long as they contain metal elements, and may be composed of metal oxides, metal itself, or both metal oxides and metal. The metal elements used for the active particles are as described above. In the third catalyst, the active particles are, for example, particulate matter and are not particularly limited, but are preferably nano-order particles (diameter of 1000 nm or less).

[0101] Furthermore, it is preferable that the active particle-containing catalyst further contains a support, with the active particles supported on the support. The support used in the active particle-containing catalyst is not particularly limited, but examples include carbon compounds, silica, aluminum oxide, and zirconium oxide, with carbon compounds, silica, and aluminum oxide being preferred. The support may be used alone or in combination of two or more. The support may be in powder or particulate form. The carbon compound is preferably a conductive carbon compound, and porous carbon is also preferred. Details of the carbon compound are as described in the section on reduction catalysts above. The active particle-containing catalyst may also contain components other than metal particles and support, such as components derived from nitrogen-containing compounds.

[0102] Catalysts containing active particles having a support such as porous carbon can be produced by mixing a metal precursor with a support and heat-treating the mixture. The metal precursor becomes active particles upon heat treatment, and these active particles are supported on the support. The metal precursor contains the above-mentioned metal atoms, and preferably contains metal ions. The metal precursor may also be used in the form of a metal salt, for example. Examples of metal salts include metal nitrates, metal sulfates, metal chlorides, metal bromides, metal iodides, and metal acetates, among which metal chlorides and metal nitrates are preferred. The temperature during heat treatment is preferably 150°C to 800°C, more preferably 180°C to 550°C, and even more preferably 200°C to 380°C. Furthermore, catalysts containing active particles may also be produced by methods such as those described in WO2024 / 150772.

[0103] Examples of metal salts used as the third catalyst include metal nitrates, metal sulfates, metal chlorides, metal bromides, metal iodides, and metal acetates, with metal chlorides and metal nitrates being preferred among these. The metal salt used as the third catalyst may be supported on a support. The support is as described above. The metal salt may be supported on the support, for example, by dispersing the support and the metal salt in a solvent and drying the resulting dispersion. In addition, known catalysts can be used as the third catalyst, such as the catalysts described in WO2024 / 150772 and WO2023 / 153489. The third catalyst may be used alone or in combination of two or more types.

[0104] In the electrolysis system 50, the reaction may be carried out in batches or continuously. When carried out continuously, the reducible substance X1, such as carbon dioxide, is continuously or intermittently supplied to the cathode-side storage unit 31, and in conjunction with this, a gas containing the reduced substance Y1 is continuously or intermittently discharged from the cathode-side gas discharge line 39. Furthermore, the cathode electrolyte may, if necessary, be partially withdrawn to the outside, for example, at the cathode-side discharge line, while new cathode electrolyte is supplied from the outside via the cathode-side supply line. Also, when carried out continuously, the anode electrolyte may, if necessary, be partially withdrawn to the outside from the anode-side discharge line along with the reaction products generated in the reaction unit 40, while new anode electrolyte is supplied from the outside via the anode-side supply line.

[0105] In the second embodiment, by using a specific cathode and making the cathode side a liquid-phase reaction, similar to the first embodiment, reduced products such as carbon monoxide can be produced with high Faraday efficiency even at high current densities, resulting in good reduction efficiency and preventing the precipitation of by-products such as carbonates on the cathode. In this embodiment as well, by configuring the anode with a porous material, oxides can be produced with high Faraday efficiency even at high current densities in the anode. Furthermore, in this embodiment, by supplying the reduced product Y1 to the anode side, organic substances such as carbonyl compounds can also be produced on the anode side, thus enabling the production of more valuable substances from carbon dioxide.

[0106] <Other Embodiments> The first and second embodiments described above are merely examples of the present invention, and various modifications or improvements are possible as long as the gist of the invention is not altered. For example, in the above embodiments, the anode was made of a porous material in addition to the cathode, but the anode does not need to be made of a porous material, and an electrode substrate used in conventional electrochemical reactions may be used. In that case, the anode current collector may be omitted as appropriate.

[0107] In each of the above embodiments, a supply port 17A is provided on the cathode current collector 14, and the cathode electrolyte is supplied from the surface 13A side of the cathode 13. However, it is not necessary for the cathode current collector 14 to have a supply port 17A, and the cathode electrolyte may be supplied from the surface 13A side by means other than the supply port 17A of the cathode current collector 14. Furthermore, the cathode electrolyte does not need to be supplied from the surface 13A side of the cathode 13, and the supply port 17A may be positioned opposite the side surface of the cathode 13, similar to the discharge port 18A. In this case, the cathode electrolyte may be supplied from the side surface of the cathode 13. Moreover, the discharge port 18A may be formed on the cathode current collector 14, and in this case, the discharge port 18A may be configured as a through hole, similar to the supply port 17A. Furthermore, although the cathode chamber 16 is shown to have one supply port 17A and one discharge port 18A, there may be two or more supply ports 17A and two or more discharge ports 18A. In addition, although the above has described a modified embodiment of the supply port 17A and discharge port 18A on the cathode side, the supply port 27A and discharge port 28A on the anode side may be similarly modified.

[0108] In the second embodiment described above, the reduced product Y1 was supplied in a gaseous state from the cathode side to the anode side. However, it is not necessary to supply it in a gaseous state; it may be supplied to the anode side together with the cathode electrolyte. In this case, it is preferable to use a cathode electrolyte that can be used as the anode electrolyte on the anode side. Furthermore, although a reaction section 40 was provided in the second embodiment, the reaction section 40 may be omitted. In that case, the third catalyst may be contained inside the anode chamber 26, and the reaction to obtain the reaction product from the reaction substrate and the reduced product Y1 in the presence of redox species may be carried out inside the anode chamber 26. In such embodiments, connection lines 41 and 42 are omitted, while the second supply line 27 and the second discharge line 28 may both connect the anode chamber 26 and the anode-side storage section 32. As a result, the anode electrolyte flows in the order of anode chamber 26, second discharge line 28, anode-side storage section 32, second supply line 27, and anode chamber 26, circulating on the anode side. When a third catalyst is used, the third catalyst may be circulated together with the anode electrolyte, or it may be designed to remain inside the anode chamber 26 with a filter or the like. Furthermore, although the second embodiment specifically described the case where the oxide to be oxidized X2 is a halide salt, other than halide salts can also be used for the oxide to be oxidized X2. For example, it may be an organic redox species such as a quinone derivative or anthraquinone derivative. Inorganic redox species other than halide salts may also be used. In addition, it may be an organic-inorganic composite redox species such as a metal complex.

[0109] In each embodiment described above, a cathode-side storage section 31 is provided, but the cathode-side storage section 31 may be omitted. In this case, the cathode electrolyte may circulate through a circulation path without the cathode-side storage section 31. However, the cathode electrolyte does not need to circulate through a circulation path. As long as a constant flow is formed in which the cathode electrolyte containing the reduced substance X1 is supplied to the cathode chamber 16 (i.e., the cathode 13 inside the cathode chamber 16) and the cathode electrolyte supplied to the cathode chamber 16 is discharged from the cathode chamber 16, the cathode electrolyte does not need to circulate. Furthermore, as long as the cathode electrolyte flows inside the cathode 13, a constant flow does not need to be formed. Also, the anode-side storage section 32 may be omitted. In this case, the anode electrolyte may circulate through a circulation path without the anode-side storage section 32. However, the anode electrolyte does not need to circulate through a circulation path. As long as a constant flow is formed in which the anode electrolyte is supplied to the anode chamber 26 (i.e., the anode 23 inside the anode chamber 26) and discharged from the anode chamber 26, the anode electrolyte does not need to circulate. Furthermore, similar to the cathode electrolyte, a constant flow does not need to be formed in the anode electrolyte.

[0110] The present invention will be described in more detail by reference to examples, but the present invention is not limited in any way by these examples.

[0111] <Preparation of reduction catalyst> [Co / C] 100 mg of Co(NO ３ ) ２ 6H ２ O (Aldrich), 200 mg of 4-polyvinylpyridine (Aldrich, weight-average molecular weight ~60000) and 100 mg of Ketjenbrak (product name "EC-300J" (Lion Chemicals)), BET specific surface area 800 m² ２ The ions ( / g) were dispersed in 200 mL of ethanol, dried in an evaporator, and heated at 350°C for 2 hours to obtain catalyst powder (Co / C).

[0112] [Ag / C] 100 mg of Ag(NO) ３ ) ２(Aldrich) and 100 mg of Ketjenblack (product name "EC-300J" (Lion Chemicals), BET specific surface area 800 m²) ２ The mixture (Ag / g) was dispersed in 200 mL of ethanol, dried in an evaporator, and heated at 350°C for 2 hours to obtain catalyst powder (Ag / C).

[0113] (Example 1) <Preparation of Cathode> 10 mg of catalyst powder (Co / C) and 25 mg of Nafion 20 wt% solution (manufactured by Wako Co., Ltd.) as a catalyst additive were added to 10 mL of ethanol, and ultrasonic vibration was performed for 30 minutes to obtain a catalyst dispersion, into which carbon felt (manufactured by Nippon Carbon Co., Ltd.) as a porous material was immersed. The carbon felt was removed from the dispersion, and the ethanol was evaporated at room temperature and atmospheric pressure. Furthermore, it was dried in an oven at 120°C for several hours to obtain a cathode. In the cathode, the catalyst and catalyst additive were dispersed in the thickness direction of the cathode and contained throughout the thickness direction from one surface 13A to the other surface 13B of the cathode. In the following examples as well, the reducing catalyst and catalyst additive were dispersed in the thickness direction of the cathode, similar to Example 1, and contained throughout the thickness direction from one surface 13A to the other surface 13B of the cathode.

[0114] <Preparation and Evaluation Method of Electrolytic Cell> An electrolytic cell 11, as shown in Figure 1, was prepared. The cathode 13 had a thickness of 2.0 mm, a size of 1 cm x 2 cm, a porosity of 70%, and a catalyst amount of 11 mg / cm³ per unit area. ２ The amount of catalyst per unit area was calculated by measuring the weight of the porous material (carbon felt) before and after the catalyst was supported, and also considering the mixing ratio of the catalyst and catalyst additive. The diaphragm 19 was made of "Nafion N117" (manufactured by Aldrich), the anode 23 was made of porous carbon with a void ratio of 80% (Mitsubishi Pencil Co., Ltd., carbon porous material "3A"), and the cathode current collector 14 and anode current collector 24 were made of carbon material. Each current collector used had a carbon current collector embedded in the surface that contacts the electrode of a PEEK holder. In addition, 0.1 M NaHCO3 was used as the cathode electrolyte. ３100 mL of aqueous solution and 100 mL of 0.2 M NaBr aqueous solution were prepared as the anode electrolyte and stored in the cathode-side storage section 31 and the anode-side storage section 32, respectively.

[0115] Next, 80 sccm of CO2 is added to the cathode electrolyte in the cathode-side reservoir 31. ２ Flow the gas for more than 30 minutes, and use CO2 as the electrolyte. ２ The cell was saturated with gas. Then, the Takumina pump 36 was set to 75 mL / min and started to supply CO to the cathode chamber 16 of the electrolytic cell 11. ２ A cathode electrolyte containing gas was supplied. Similarly, anode electrolyte was supplied to the anode chamber 26 of the electrolytic cell 11. After 3 minutes of pump circulation, the current was 150 mA / cm². ２ Constant current electrolysis was started. The product was supplied to the cathode-side storage section 31 from the outlet 18A of the cathode chamber 16, and the Faraday efficiency of CO generation was calculated based on the results of measuring the components of the gas phase in the cathode-side storage section 31 by gas chromatography (GC) 10 minutes after the start of constant current electrolysis. The results are shown in Table 1. A Faraday efficiency of 75% or more was evaluated as "A", 50% or more but less than 75% as "B", 45% or more but less than 50% as "C", and less than 45% as "D".

[0116] (Example 2) The procedure was carried out in the same manner as in Example 1, except that porous carbon was used instead of carbon felt as the porous material. As the porous carbon, carbon porous material "3A" manufactured by Mitsubishi Pencil Co., Ltd. was used.

[0117] (Comparative Example 1) A catalyst dispersion similar to that in Example 1 was spray-coated onto one side of carbon paper (product name "H23", manufactured by Freudenberg, thickness 0.21 mm) and dried to prepare a catalyst-containing carbon paper with a thickness of 0.25 mm. Eight sheets of catalyst-containing carbon paper were stacked to the same height as the carbon felt in Example 1 and evaluated as a cathode.

[0118] (Examples 3 and 4, Comparative Example 2) The same procedures as in Examples 1 and 2 and Comparative Example 1 were followed, except that Ag / C was used instead of Co / C as the catalyst.

[0119] (Comparative Example 3) 30 mg of catalyst powder (Co / C) and 75 mg of a 20 wt% Nafion solution (manufactured by Wako Co., Ltd.) as a catalyst additive were added to 1 mL of ethanol, and ultrasonic vibration was performed for 30 minutes to obtain a catalyst dispersion. Using this catalyst dispersion, a catalyst film was formed by coating carbon felt (manufactured by Nippon Carbon Co., Ltd.) with a bar coater. After film formation, the film was dried in an oven at 120°C for several hours to obtain a cathode. The catalyst was present only on the surface of the cathode and in its vicinity in a film-like manner. The obtained cathode was evaluated in the same manner as in Example 1. In the electrolytic cell, the cathode was installed so that the surface containing the catalyst faced the diaphragm side.

[0120] (Comparative Example 4) Carbon paper (product name "H23", manufactured by FREUDENBERG, thickness 0.21 mm) was coated on one side with the same catalyst dispersion as in Comparative Example 3 using a bar coater and dried to prepare carbon paper with a catalyst film formed on the other side. The prepared carbon paper with the catalyst was used as a cathode and evaluated in the same manner as in Comparative Example 3.

[0121]

[0122] As shown in Examples 1 to 4 above, by constructing the cathode from a conductive porous material, dispersing the reduction catalyst in the thickness direction, and forming a channel for the cathode electrolyte to flow inside the cathode, it was possible to sufficiently increase the Faraday efficiency even at high current densities. In contrast, in Comparative Examples 1, 2, and 4, a cathode without voids was used, making it impossible to form a channel for the cathode electrolyte to flow inside the cathode. Therefore, it was not possible to increase the Faraday efficiency at high current densities. In Comparative Example 3, a cathode with voids and a channel formed inside was used, but the reduction catalyst was not dispersed in the thickness direction of the cathode and was only present on the surface and its vicinity, so it was not possible to sufficiently increase the Faraday efficiency.

[0123] (Examples 5-8) Examples 5-8 were conducted to confirm the relationship between the porosity of a porous material and the Faraday efficiency. Examples 5-8 were carried out in the same manner as Example 1, except that porous carbon with different porosities was used compared to Example 2, as shown in Table 2. The porous carbon used in Examples 5-8 was "TSK" (Example 5), "3AN" (Examples 6 and 7), and "3A" (Example 8), all manufactured by Mitsubishi Pencil Co., Ltd. Examples 6 and 7 used porous carbon with the same product name but different porosities. Similarly, Example 8 also used porous carbon with the same product name but different porosities compared to Example 2. The results of Examples 5-8 are shown in Table 2. For reference, the results of Example 2 are also shown in Table 2.

[0124]

[0125] (Examples 9-11) Examples 9-11 were conducted to confirm the relationship between the thickness of the porous material and the Faraday efficiency. Examples 9-11 were carried out in the same manner as Example 1, except that carbon felt of a different thickness was used, as shown in Table 3. In Examples 9-11, PAN material was used as the carbon felt, and the thickness was controlled by changing the number of layers of carbon fiber cloth that were laminated. The results of Examples 9-11 are shown in Table 3. For reference, the results of Example 1 are also shown in Table 3.

[0126]

[0127] (Examples 12-17) Examples 12-17 were conducted to confirm the relationship between the amount of catalyst per unit area, the current density, and the Faraday efficiency. In Examples 12-17, catalyst powder (Co / C) and a 20 wt% Nafion solution (manufactured by Wako Co., Ltd.) were added to 10 mL of ethanol, and ultrasonic vibration was performed for 30 minutes to obtain a catalyst dispersion. Carbon felt (manufactured by Nippon Carbon Co., Ltd.) was then immersed in the dispersion as a porous material. The carbon felt was removed from the dispersion, the ethanol was evaporated at room temperature and atmospheric pressure, and then it was dried in an oven at 120°C for several hours to obtain a cathode. In each of Examples 12-17, the weight ratio of catalyst to 20 wt% Nafion solution was fixed at 1:2.5, and the amount added was varied to adjust the amount of catalyst per unit area as shown in Table 4, and a cathode was prepared. Otherwise, the procedure was carried out in the same manner as in Example 1. In Examples 12-17, the current density was 300 mA / cm². ２ Except for that, the current density is 150 mA / cm². ２ The evaluation was carried out in the same manner as in the previous case, and the current density was 300 mA / cm². ２ The Faraday efficiency of CO production was also calculated under these conditions. The results are shown in Table 4.

[0128]

[0129] (Examples 18-21) Examples 18-21 were conducted to confirm the relationship between the surface roughness (Sz) of the cathode and the Faraday efficiency. In Examples 18-21, a catalyst dispersion was prepared in the same manner as in Example 1. The prepared catalyst dispersion was applied using an airbrush manufactured by Tamiya. Specifically, the catalyst dispersion was poured into the cup of the airbrush, and carbon felt was placed on a hot plate adjusted to 120°C. Spray coating was performed from a position where the distance of the airbrush nozzle to the carbon felt was 5 cm. After spraying, the cathode was dried in an oven at 150°C to obtain the cathode. The amount of coating was adjusted so that the amount of catalyst per unit area in each example was as shown in Table 5. The obtained cathode was evaluated in the same manner as in Example 1, and the Faraday efficiency was calculated. Furthermore, the surface properties of the obtained cathodes were observed using a digital microscope (Keyence, product name "VHX-X1"), and the surface roughness (Sz) was calculated according to ISO 25178. The results are shown in Table 5.

[0130]

[0131] 10, 50 Electrolytic System 11 Electrolytic Cell 12 Reduced Material Supply Line 13 Cathode 14 Cathode Current Collector 16 Cathode Chamber 17 First Supply Line 17A, 27A Supply Port 18 First Discharge Line 18A, 28A Discharge Port 19 Diaphragm 25 Power Supply 23 Anode 24 Anode Current Collector 26 Anode Chamber 27 Second Supply Line 28 Second Discharge Line 31 Cathode Side Storage Section 32 Anode Side Storage Section 36 Pump 40 Reaction Section X1 Reduced Material Y1 Reduced Material X2 Oxide Y2 Oxide

Claims

1. An electrolytic system comprising a cathode made of a conductive porous material, a reduction catalyst contained in the cathode that promotes the reduction of a substance to be reduced, and a liquid supply means for supplying a liquid containing the substance to be reduced to the cathode, wherein a channel for the liquid to flow is formed inside the cathode, and the reduction catalyst is dispersed in the thickness direction of the cathode.

2. The electrolytic system according to claim 1, wherein the product to be reduced is carbon dioxide, and the reduction catalyst is a catalyst that reduces carbon dioxide to carbon monoxide.

3. The current density at the cathode is 100 mA / cm². ２ The electrolytic system according to claim 1 or 2, wherein the above conditions apply.

4. The electrolytic system according to claim 1 or 2, wherein the cathode has a porosity of 50% or more and the thickness of the cathode is 1.0 mm or more.

5. The electrolytic system according to claim 1 or 2, wherein the porous material is selected from the group consisting of porous carbon, fibrous carbon, and carbon felt.

6. The amount of the reducing catalyst per unit area in the cathode is 0.1 mg / cm². ２ The electrolytic system according to claim 1 or 2, wherein the above conditions apply.

7. The electrolytic system according to claim 1 or 2, wherein the reduction catalyst comprises powdered carbon and metal particles supported on the powdered carbon.

8. The electrolytic system according to claim 7, wherein the metal particles are at least one of silver particles and gold particles, or the metal particles are cobalt particles and the reduction catalyst further comprises a component derived from an organic compound.

9. The specific surface area of ​​the powdered carbon is 200 to 1500 m². ２ The electrolytic system according to claim 7, wherein the value is / g.

10. The electrolytic system according to claim 1 or 2, wherein the surface roughness of the cathode is 35 μm or more in Sz.

11. The electrolytic system according to claim 1 or 2, wherein the contact angle of at least a portion of the porous material is 90° or less.

12. The electrolytic system according to claim 1 or 2, wherein the cathode is made up of two or more cathodes having different porosities stacked on top of each other.

13. The electrolytic system according to claim 1 or 2, further comprising a cathode current collector, a diaphragm, an anode, and an anode current collector, wherein the cathode current collector, the cathode, the diaphragm, the anode, and the anode current collector are stacked in this order to form a laminate.

14. A reduction electrode comprising a cathode made of a conductive porous material and a reduction catalyst contained in the cathode that promotes the reduction of a substance to be reduced, wherein a channel is formed inside the cathode through which a liquid containing the substance to be reduced flows, and the reduction catalyst is dispersed in the thickness direction of the cathode.

15. A laminate comprising a cathode, a cathode current collector, a diaphragm, an anode, and an anode current collector, wherein the cathode current collector, the cathode, the diaphragm, the anode, and the anode current collector are stacked in this order, the cathode is made of a conductive porous material and contains a reduction catalyst that promotes the reduction of the substance to be reduced, a channel is formed inside the cathode through which a liquid containing the substance to be reduced flows, and the reduction catalyst is dispersed in the cathode in the thickness direction.

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