Carbon dioxide electrolyzer

Abstract

To provide a carbon dioxide electrolytic device capable of increasing utilization efficiency of carbon dioxide.SOLUTION: A carbon dioxide electrolytic device 1 of an embodiment comprises: an electrochemical reaction cell 5 including a first accommodation part 2 accommodating a gas containing CO2 or a first electrolytic solution containing CO2, a second accommodation part 3 accommodating a second electrolytic solution containing H2O, a diaphragm 4 provided between the first accommodation part 2 and the second accommodation part 3, a cathode in contact with the gas or the first electrolytic solution, and an anode in contact with the second electrolytic solution; a first supply part 6 for supplying the gas or the first electrolytic solution to the first accommodation part 2; a second supply part 7 for supplying the second electrolytic solution to the second accommodation part 3; and a carbon dioxide separation part 8 connected to a discharge part for discharging emissions containing O2 and CO2 from the second accommodation part 7 and including a cryogenic separation device for separating CO2 from a gas component in the emissions.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION An embodiment of the present invention relates to a carbon dioxide electrolysis device. [Background technology]

[0002] In recent years, concerns about the depletion of fossil fuels such as oil and coal have led to growing expectations for sustainable renewable energy. From the perspective of such energy and environmental issues, development of power-to-chemical technology is underway, which uses renewable energy such as solar power to electrochemically reduce carbon dioxide to create a storable chemical energy source. A carbon dioxide electrolysis device that realizes power-to-chemical technology includes, for example, an anode that oxidizes water (HO) to produce oxygen (O), and a cathode that reduces carbon dioxide (CO) to produce carbon compounds such as carbon monoxide (CO). The anode and cathode of the carbon dioxide electrolysis device are connected to a power source derived from renewable energy sources such as solar power, hydroelectric power, wind power, and geothermal power.

[0003] The cathode of a carbon dioxide electrolysis device is, for example, immersed in water with dissolved CO2 or is arranged so as to be in contact with CO2 flowing through a flow path. The cathode obtains the reduction potential of CO2 from a power source derived from renewable energy, and reduces CO2 to produce carbon compounds such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH3OH), methane (CH4), ethanol (C2H5OH), ethane (C2H6), ethylene (C2H4), and ethylene glycol (C2H6O2). The anode is arranged so as to be in contact with an electrolyte containing water (H2O), and generates oxygen (O2) and hydrogen ions (H + In such carbon dioxide electrolysis devices, there is a demand for improving the utilization efficiency of CO2, as well as the utilization efficiency and utility value of the reduction products of CO2. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2019-506165 [Patent Document 2] Japanese Patent Application Publication No. 2018-070936 Summary of the Invention [Problem to be solved by the invention]

[0005] The problem to be solved by the present invention is to provide a carbon dioxide electrolysis device that makes it possible to increase the utilization efficiency of CO2. [Means for solving the problem]

[0006] a first carbon dioxide electrolysis device connected to a discharge unit that discharges an effluent containing oxygen and carbon dioxide from the second storage unit, the first carbon dioxide electrolysis device including a cryogenic separation device that separates the carbon dioxide from gas components in the effluent; a first storage unit for storing a gas containing carbon dioxide or a first electrolytic solution containing carbon dioxide; a second storage unit for storing a second electrolytic solution containing water; a diaphragm provided between the first storage unit and the second storage unit; a cathode that is arranged to be in contact with the gas or the first electrolytic solution and that reduces carbon dioxide to produce a carbon compound; and an anode that is arranged to be in contact with the second electrolytic solution and that oxidizes water to produce oxygen; a first supply unit that supplies the gas or the first electrolytic solution to the first storage unit; a second supply unit that supplies the second electrolytic solution to the second storage unit; and a first carbon dioxide separation unit that is connected to a discharge unit that discharges an effluent containing oxygen and carbon dioxide from the second storage unit, the first carbon dioxide separation unit including a cryogenic separation device that separates the carbon dioxide from gas components in the effluent. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram showing a carbon dioxide electrolysis device according to an embodiment. [Figure 2] FIG. 2 is a diagram showing a first example of an electrochemical reaction cell in the carbon dioxide electrolysis device shown in FIG. [Figure 3] FIG. 2 is a diagram showing a second example of an electrochemical reaction cell in the carbon dioxide electrolysis device shown in FIG. [Figure 4]FIG. 2 is a diagram showing a cryogenic separation device as a carbon dioxide separation section in the carbon dioxide electrolysis device shown in FIG. [Figure 5] FIG. 1 is a diagram showing a first modified example of a carbon dioxide electrolysis device according to an embodiment. [Figure 6] FIG. 10 is a diagram showing a second modified example of the carbon dioxide electrolysis device of the embodiment. [Figure 7] FIG. 10 is a diagram showing a third modified example of the carbon dioxide electrolysis device of the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, carbon dioxide electrolysis devices according to embodiments will be described with reference to the drawings. In each of the embodiments shown below, substantially identical components are denoted by the same reference numerals, and some of their descriptions may be omitted. The drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each component, etc. may differ from the actual ones.

[0009] FIG. 1 is a diagram illustrating a carbon dioxide electrolysis device 1 according to an embodiment. The carbon dioxide electrolysis device 1 shown in FIG. 1 includes an electrochemical reaction cell 5 having a first storage unit 2 for storing a gas containing carbon dioxide (CO) or a first electrolytic solution containing CO, a second storage unit 3 for storing a second electrolytic solution containing water (HO), and a diaphragm 4; a first supply unit 6 for supplying the gas or the first electrolytic solution to the first storage unit 2; a second supply unit 7 for supplying the second electrolytic solution to the second storage unit 3; and a carbon dioxide separation unit 8 connected to an outlet for discharging an effluent containing oxygen (O) and CO from the second storage unit 3, the carbon dioxide separation unit 8 including a cryogenic separator for separating CO from gas components in the effluent. The gas discharged from the second storage unit 3 will be described in detail below.

[0010] A CO2 generation source 9 is connected to the first supply unit (CO2 supply unit) 6. If the CO2 generation source 9 is equipped with a CO2 separation and capture device 10, the CO2 supply unit 6 is connected to the CO2 separation and capture device 10. The CO2 separation and capture device 10 may be installed separately from the CO2 generation source 9. Examples of the CO2 generation source 9 include thermal power plants, facilities with various incinerators and combustion furnaces such as waste incinerators, steel mills, and facilities with blast furnaces. The CO2 generation source 9 may also be various other factories that generate CO2, and is not particularly limited. A valuable resource production unit 11 is connected to the discharge unit that discharges exhaust gas containing carbon monoxide (CO) and the like from the first storage unit 2. The valuable resource production unit 11 is a chemical synthesis unit that synthesizes valuable resources using the CO and the like discharged from the first storage unit 2 as raw materials. The valuable resource production unit 11 is provided as needed and may instead be a tank or the like that stores exhaust gas containing CO and the like.

[0011] The electrochemical reaction cell 5 has a structure as shown in, for example, FIGS. 2 and 3. The electrochemical reaction cell 5 (5A) shown in FIG. 2 includes a cathode (reduction electrode) 12, an anode (oxidation electrode) 13, a diaphragm 4 disposed between the cathode 12 and the anode 13, a first flow path 14 through which a CO2-containing gas or a CO2-containing first electrolyte solution flows so as to contact the cathode 12, a second flow path 15 through which a water-containing second electrolyte solution flows so as to contact the anode 13, a first current collector 16 electrically connected to the cathode 12, and a second current collector 17 electrically connected to the anode 13. The first flow path 14 functions as a first storage section 2, and the second flow path 15 functions as a second storage section 3. The first and second current collectors 16 and 17 of the electrochemical reaction cell 5A are electrically connected to a power source 18. The electrochemical reaction cell may be formed by integrating a plurality of stacked cells. When a plurality of cells are integrally arranged, the amount of carbon dioxide reacted per unit site area increases, and the amount of treatment can be increased, and it is preferable to stack about 10 to 150 cells.

[0012] The first flow path 14 is connected to a first supply flow path 19 that supplies a CO2-containing gas or a CO2-containing first electrolytic solution and a first discharge flow path 20 that discharges the generated gas. The first flow path 14 is connected to a CO2 supply unit 6. When supplying a CO2-containing gas to the first flow path 14, the CO2 supply unit 6 supplies the CO2 gas sent from the CO2 separation and capture device 10 to the first supply flow path 19 directly or after temporarily storing it. When supplying the CO2-containing first electrolytic solution to the first flow path 14, the CO2 supply unit 6 mixes the CO2 gas sent from the CO2 separation and capture device 10 with the first electrolytic solution and then supplies it to the first supply flow path 19. When supplying the CO2-containing first electrolytic solution to the first flow path 14, a circulation path and a pump similar to those on the anode 13 side may be connected to the first flow path 14. In this case, a gas-liquid separator is connected to the circulation path. The electrochemical reaction cell 5A may have, in addition to the first flow path (gas flow path) 14, a third flow path (liquid flow path) that allows the first electrolytic solution (which may or may not contain CO2) to flow so as to come into contact with the anode 13.

[0013] The second flow path 15 is connected to the electrolytic solution supply unit 7. The electrolytic solution supply unit 7 has a circulation path 21 and a pump 22 through which the second electrolytic solution is circulated via the second flow path 15. A gas-liquid separation unit 23 is connected to the circulation path 21. The gas-liquid separation unit 23 separates a liquid component containing the second electrolytic solution from a gas component containing a product, and the liquid component is circulated by the pump 22 through the circulation path 21, which includes the second flow path 15. The gas component separated by the gas-liquid separation unit 23 contains oxygen (O2), which is a product at the anode 13. Furthermore, as will be described in detail later, since CO2 supplied to the first flow path 14 moves to the second flow path 15, the gas component also contains CO2. A gas component discharge unit of the gas-liquid separation unit 23 is connected to the CO2 separation unit 8.

[0014] The electrochemical reaction cell 5 (5B) shown in FIG. 3 includes a first storage tank 24 that contains a first electrolytic solution containing CO2, a second storage tank 25 that contains a second electrolytic solution containing HO, a partition wall 4 disposed between the first storage tank 24 and the second storage tank 25, a cathode 12 disposed in the first storage tank 24 so as to be in contact with the first electrolytic solution, and an anode 13 disposed in the second storage tank 25 so as to be in contact with the second electrolytic solution. The first storage tank 24 functions as the first storage section 2, and the second storage tank 25 functions as the second storage section 3. The cathode 12 and anode 13 of the electrochemical reaction cell 5B are electrically connected to a power source 18.

[0015] A first supply flow path 19 for supplying a CO2-containing gas or a CO2-containing first electrolytic solution and a first discharge flow path 20 for discharging the generated gas are connected to the first storage tank 24. The first supply flow path 19 is connected to the CO2 supply unit 6. A space for storing the generated gas is provided above the first storage tank 24, and the first discharge flow path 20 is connected to this space. When supplying the CO2-containing first electrolytic solution into the first storage tank 24, the CO2 supply unit 6 mixes the CO2 gas sent from the CO2 separation and capture device 10 with the first electrolytic solution and supplies the resulting mixture to the first storage tank 24. The CO2 supply unit 6 may supply the CO2 gas sent from the CO2 separation and capture device 10 into the first electrolytic solution stored in the first storage tank 24. A circulation path and a pump similar to those of the second storage tank 25 may be connected to the first storage tank 24.

[0016] The CO2 gas flow rate from the CO2 separation and capture device 10 and the CO2 gas flow rate from the CO2 separation device 8 may be adjusted by the CO2 supply unit 6 and introduced into the first storage unit 2 (first flow path 14 or first storage tank 24). A detector for measuring the CO2 gas flow rate from the CO2 separation and capture device 10 and a detector for measuring the CO2 gas flow rate from the CO2 separation device 8 may be provided to adjust the CO2 gas introduced into the first storage unit 2 (first flow path 14 or first storage tank 24). Furthermore, information from the detector for measuring the CO2 gas flow rate from the CO2 separation and capture device 10 and the detector for measuring the CO2 gas flow rate from the CO2 separation device 8 may be adjusted by an adjustment unit for adjusting the gas flow rate. The adjustment unit can adjust the CO2 flow rate to a more appropriate level depending on the operating status (CO2 generation status) of the CO2 generation source 9, the CO2 flow rate from the CO2 separation and capture device 10, the operating status of the electrochemical reaction cell 5, etc.

[0017] The second storage tank 25 is connected to the electrolytic solution supply unit 7. The electrolytic solution supply unit 7 has a circulation path 21 that circulates the second electrolytic solution through the second storage tank 25, and a pump 22. A space for storing the generated gas is provided above the second storage tank 25, and a second discharge flow path 26 is connected to this space. The gas components discharged from the second discharge flow path 26 contain oxygen (O2), which is a product at the anode 13, as described above, and also contain CO2 because CO2 supplied to the first storage tank 24 moves to the second storage tank 25. Therefore, the second discharge flow path 26 is connected to the CO2 separation unit 8.

[0018] The first storage tank 24 and the second storage tank 25 contain hydrogen ions (H + ), hydroxide ion (OH - ), bicarbonate ion (HCO3 - ), carbonate ions (CO3 2-The two chambers (24, 25) are separated by a membrane 4 that allows the movement of ions such as ions, thereby forming a two-chamber reaction vessel. The two-chamber reaction vessels (24, 25) may be made of, for example, quartz glass, polystyrene, polymethacrylate, or the like. A light-transmitting material may be used for a portion of the two-chamber reaction vessels (24, 25), and a resin material may be used for the remaining portion. Examples of resin materials include polyether ether ketone (PEEK), polyamide (PA), polyvinylidene fluoride (PVDF), polyacetal (POM) (copolymer), polyphenylene ether (PPE), acrylonitrile-butadiene-styrene copolymer (ABS), polypropylene (PP), polyethylene (PE), and the like.

[0019] The first electrolytic solution supplied to the first flow path 14 or the first storage tank 24 functions as a cathode solution and contains CO2 as a substance to be reduced. Here, the form of CO2 present in the first electrolytic solution does not need to be gaseous, but may be dissolved CO2 or carbonate ions (CO3 2- ) and bicarbonate ions (HCO3 - ) or other forms. The first electrolytic solution may contain hydrogen ions and is preferably an aqueous solution. The second electrolytic solution supplied to the second flow path 15 or the second storage tank 25 functions as an anode solution and contains H2O as a substance to be oxidized. By changing the amount of water contained in the first and second electrolytic solutions or the components of the electrolytic solutions, the reactivity can be changed, and the selectivity of the reduced substance or the ratio of the generated substance can be changed. The first and second electrolytic solutions may contain a redox couple as needed. The redox couple may be Fe 3+ / Fe 2+ and IO 3- / I - etc.

[0020] The temperature of the electrochemical reaction cell 5 (5A, 5B) is preferably set to a temperature in the range of room temperature (e.g., 25°C) to 150°C at which the electrolyte does not vaporize. A temperature in the range of 60°C to 150°C is more preferable, and a temperature in the range of 80°C to 150°C is even more preferable. A cooling device such as a chiller is required to maintain a temperature below room temperature, which may reduce the overall energy efficiency of the system. At temperatures above 150°C, the water in the electrolyte may turn into vapor, increasing the resistance and reducing the electrolysis efficiency. There are no particular restrictions on the current density of the cathode 12, but a higher current density is preferable in order to increase the amount of reduction product produced per unit area. A current density of 100 mA / cm 2 More than 1.5A / cm 2 Preferably less than 300 mA / cm 2 More than 700mA / cm 2 Less than 100mA / cm 2 Below 1.5A / cm, the amount of reduction product produced per unit area is low, requiring a large area. 2 If the temperature exceeds this value, the side reaction of hydrogen generation increases and the concentration of reduction products decreases. If Joule heat also increases due to an increase in the current density, the temperature will rise above an appropriate level, so a cooling mechanism may be provided in or near the electrochemical reaction cell 5. The cooling mechanism may be water-cooled or air-cooled. Even if the temperature of the electrochemical reaction cell 5 is higher than room temperature, it may be left at that temperature as long as it is 150°C or lower.

[0021] The first and second electrolyte solutions may contain different substances, or may be the same electrolyte solution containing the same substance. When the first and second electrolyte solutions contain the same substance and the same solvent, the first and second electrolyte solutions may be considered to be a single electrolyte solution. The pH of the second electrolyte solution is preferably higher than the pH of the first electrolyte solution. This facilitates the movement of ions such as hydrogen ions, hydroxide ions, bicarbonate ions, and carbonate ions through the diaphragm 4. Furthermore, the liquid junction potential difference caused by the pH difference allows the oxidation-reduction reaction to proceed effectively.

[0022] The first electrolyte solution is preferably a solution with a high CO2 absorption rate. The CO2 does not necessarily exist in a dissolved state in the first electrolyte solution; CO2 may be present in the form of bubbles mixed in the first electrolyte solution. Examples of electrolyte solutions containing CO2 include aqueous solutions containing bicarbonates or carbonates such as lithium bicarbonate (LiHCO3), sodium bicarbonate (NaHCO3), potassium bicarbonate (KHCO3), cesium bicarbonate (CsHCO3), sodium carbonate (Na2CO3), and potassium carbonate (K2CO3), phosphoric acid, boric acid, etc. The CO2-containing electrolyte solution may contain alcohols such as methanol and ethanol, or ketones such as acetone, or may be an alcohol solution or a ketone solution. The first electrolyte solution may also be an electrolyte solution containing a CO2 absorbent that reduces the reduction potential of CO2, has high ionic conductivity, and absorbs CO2.

[0023] The second electrolytic solution can be a solution using water (H2O), for example, an aqueous solution containing any electrolyte. This solution is preferably an aqueous solution that promotes the oxidation reaction of water. The aqueous solution containing an electrolyte can be, for example, a solution containing phosphate ions (PO4 2- ), borate ion (BO3 3- ), sodium ions (Na + ), potassium ions (K + ), calcium ions (Ca 2+ ), lithium ion (Li + ), cesium ions (Cs + ), magnesium ions (Mg 2+ ), chloride ions (Cl - ), bicarbonate ion (HCO3 - ), carbonate ions (CO3 - ), hydroxide ion (OH - ) and the like.

[0024] The electrolyte solution described above is a solution containing a cation such as an imidazolium ion or a pyridinium ion and BF4 - and PF6 -An ionic liquid or an aqueous solution thereof, which is composed of a salt with an anion such as ammonium hydroxide, ammonium nitrate, or the like and remains in a liquid state over a wide temperature range, can be used. Other examples of the electrolyte include solutions of amines such as ethanolamine, imidazole, and pyridine, or aqueous solutions thereof. Examples of amines include primary amines, secondary amines, and tertiary amines. These electrolytes may have high ionic conductivity, the ability to absorb carbon dioxide, and the ability to reduce reduction energy.

[0025] Examples of primary amines include methylamine, ethylamine, propylamine, butylamine, pentylamine, and hexylamine. The hydrocarbon amine may be substituted with an alcohol, a halogen, or the like. Examples of substituted hydrocarbon amines include methanolamine, ethanolamine, and chloromethylamine. An unsaturated bond may also be present. The same applies to secondary and tertiary amines.

[0026] Examples of secondary amines include dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, dimethanolamine, diethanolamine, and dipropanolamine. The substituted hydrocarbon may be different. This also applies to tertiary amines. For example, examples of amines with different hydrocarbons include methylethylamine and methylpropylamine.

[0027] Examples of tertiary amines include trimethylamine, triethylamine, tripropylamine, tributylamine, trihexylamine, trimethanolamine, triethanolamine, tripropanolamine, tributanolamine, trihexanolamine, methyldiethylamine, and methyldipropylamine.

[0028] Examples of cations of the ionic liquid include 1-ethyl-3-methylimidazolium ion, 1-methyl-3-propylimidazolium ion, 1-butyl-3-methylimidazole ion, 1-methyl-3-pentylimidazolium ion, and 1-hexyl-3-methylimidazolium ion.

[0029] The imidazolium ion may be substituted at the 2-position. Examples of cations substituted at the 2-position of the imidazolium ion include a 1-ethyl-2,3-dimethylimidazolium ion, a 1,2-dimethyl-3-propylimidazolium ion, a 1-butyl-2,3-dimethylimidazolium ion, a 1,2-dimethyl-3-pentylimidazolium ion, and a 1-hexyl-2,3-dimethylimidazolium ion.

[0030] Examples of pyridinium ions include methylpyridinium, ethylpyridinium, propylpyridinium, butylpyridinium, pentylpyridinium, hexylpyridinium, etc. Both the imidazolium ion and the pyridinium ion may have a substituted alkyl group and may have an unsaturated bond.

[0031] The anion is fluoride ion (F - ), chloride ions (Cl - ), bromide ion (Br - ), iodide ion (I - ), BF4 - , PF6 - , CF3COO - , CF3SO3 - , NO3 - , SCN - , (CF3SO2)3C - Examples of suitable ionic liquids include bis(trifluoromethoxysulfonyl)imide, bis(trifluoromethoxysulfonyl)imide, and bis(perfluoroethylsulfonyl)imide. Zwitterions in which the cation and anion of an ionic liquid are connected by a hydrocarbon may also be used. A buffer solution such as a potassium phosphate solution may also be supplied to the first and second storage tanks 24 and 25.

[0032] The diaphragm 4 is a membrane or the like that allows selective passage of anions or cations. This allows the electrolyte solutions in contact with the cathode 12 and the anode 13 to contain different substances, and furthermore, differences in ionic strength, pH, etc. can promote reduction reactions and oxidation reactions. The diaphragm 4 can be used to separate the first electrolyte solution from the second electrolyte solution. The diaphragm 4 may have a function of allowing some ions contained in the electrolyte solution in which the cathode 12 and the anode 13 are immersed to pass through, i.e., a function of blocking one or more types of ions contained in the electrolyte solution. This can, for example, make the pH and other parameters different between the two electrolyte solutions. Furthermore, with regard to ion blocking, the diaphragm may not completely block some ions but may have an effect of limiting the amount of migration of certain ion species.

[0033] Examples of ion exchange membranes that can be used as the diaphragm 4 include Neosepta (registered trademark) from Astom Corporation, Selemion (registered trademark) and Aciplex (registered trademark) from Asahi Glass Co., Ltd., Fumasep (registered trademark) and fumapem (registered trademark) from Fumatech Corporation, Nafion (registered trademark), a fluororesin obtained by sulfonating and polymerizing tetrafluoroethylene from DuPont Corporation, Lewabrane (registered trademark) from LANXESS, IONSEP (registered trademark) from IONTECH, Mustang (registered trademark) from PALL Corporation, Ralex (registered trademark) from Mega Corporation, and Gore-Tex (registered trademark) from Gore-Tex Corporation. The ion exchange membrane may also be formed using a membrane with a hydrocarbon skeleton or, for anion exchange, a membrane with an amine group. When there is a pH difference between the first and second electrolyte solutions, a bipolar membrane consisting of a cation exchange membrane and an anion exchange membrane can be used, maintaining the pH of each electrolyte solution stable.

[0034] In addition to an ion exchange membrane, other materials that can be used for the diaphragm 4 include silicone resin, fluororesin such as perfluoroalkoxyalkane (PFA), perfluoroethylenepropene copolymer (FEP), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), and ethylene-chlorotrifluoroethylene copolymer (ECTFE), ceramic porous membranes, glass filters, packings filled with agar, and insulating porous materials such as zeolites and oxides. In particular, hydrophilic porous membranes are preferably used as the diaphragm 4 because they are not clogged with air bubbles.

[0035] The cathode 12 is an electrode that reduces CO2 supplied as a gas or CO2 contained in the first electrolytic solution to produce carbon compounds. In the electrification cell 5A, the cathode 12 is disposed so as to be in contact with the first flow path 14, and in the electrification cell 5B, the cathode 12 is disposed in the first storage tank 24 and immersed in the first electrolytic solution. The cathode 12 contains a reduction catalyst for producing carbon compounds by a reduction reaction of CO2. A material that reduces the activation energy for reducing CO2 is used as the reduction catalyst. In other words, a material that reduces the overvoltage that occurs when carbon compounds are produced by a reduction reaction of CO2 is used.

[0036] The cathode 12 may be made of, for example, a metal material or a carbon material. Metal materials include, for example, gold, aluminum, copper, silver, platinum, palladium, zinc, mercury, indium, nickel, and titanium, as well as alloys containing these metals. Carbon materials include, for example, graphene, carbon nanotubes (CNTs), fullerenes, and Ketjen black. The reduction catalyst is not limited to these, and may also be a metal complex such as a Ru complex or a Re complex, or an organic molecule having an imidazole or pyridine skeleton. The reduction catalyst may also be a mixture of multiple materials. The cathode 12 may have a structure in which a reduction catalyst in the form of a thin film, lattice, particles, wire, or the like is provided on a conductive substrate.

[0037] The carbon compounds produced by the reduction reaction at the cathode 12 vary depending on the type of reduction catalyst, etc., and include, for example, carbon monoxide (CO), formic acid (HCOOH), methane (CH4), methanol (CH3OH), ethane (C2H6), ethylene (C2H4), ethanol (C2H5OH), formaldehyde (HCHO), ethylene glycol (C2H6O2), etc. Furthermore, at the cathode 12, a side reaction of generating hydrogen (H2) by the reduction reaction of H2O may occur simultaneously with the reduction reaction of CO2.

[0038] The anode 13 is an electrode that oxidizes substances to be oxidized, such as substances and ions, in the second electrolyte solution. For example, it oxidizes water (HO) to produce oxygen or hydrogen peroxide solution, or chloride ions (Cl - ) to generate chlorine. In the electrochemical reaction cell 5A, the anode 13 is disposed so as to be in contact with the second flow path 13, and in the electrochemical reaction cell 5B, it is disposed in the second storage tank 25 and immersed in the second electrolyte. The anode 13 contains an oxidation catalyst for the substance to be oxidized, such as HO. As the oxidation catalyst, a material that reduces the activation energy when oxidizing the substance to be oxidized, in other words, a material that reduces the reaction overvoltage, is used.

[0039] Examples of oxidation catalyst materials include metals such as ruthenium, iridium, platinum, cobalt, nickel, iron, and manganese. Binary, ternary, and quaternary metal oxides can also be used. Examples of binary metal oxides include manganese oxide (Mn-O), iridium oxide (Ir-O), nickel oxide (Ni-O), cobalt oxide (Co-O), iron oxide (Fe-O), tin oxide (Sn-O), indium oxide (In-O), and ruthenium oxide (Ru-O). Examples of ternary metal oxides include Ni-Fe-O, Ni-Co-O, La-Co-O, Ni-La-O, and Sr-Fe-O. Examples of quaternary metal oxides include Pb-Ru-Ir-O and La-Sr-Co-O. The oxidation catalyst is not limited to these, and metal hydroxides containing cobalt, nickel, iron, manganese, etc., and metal complexes such as Ru complexes and Fe complexes can also be used. Also, a mixture of multiple materials can be used.

[0040] The anode 13 may be a composite material containing both an oxidation catalyst and a conductive material. Examples of conductive materials include carbon materials such as carbon black, activated carbon, fullerene, carbon nanotubes, graphene, Ketjen black, and diamond; transparent conductive oxides such as indium tin oxide (ITO), zinc oxide (ZnO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), and antimony-doped tin oxide (ATO); metals such as Cu, Al, Ti, Ni, Ag, W, Co, and Au; and alloys containing at least one of these metals. The anode 13 may have a structure in which an oxidation catalyst in the form of a thin film, lattice, particles, or wire is provided on a conductive substrate. Examples of conductive substrates include metal materials such as titanium, titanium alloys, and stainless steel.

[0041] The power source 18 supplies power to the electrochemical reaction cell 5 to cause an oxidation-reduction reaction, and is electrically connected to the cathode 12 and the anode 13. A reduction reaction at the cathode 12 and an oxidation reaction at the anode 13 are carried out using the electrical energy supplied from the power source 18. The power source 18 and the cathode 12, and the power source 18 and the anode 13 are connected, for example, by wiring. Electrical devices such as an inverter, a converter, or a battery may be installed between the electrochemical reaction cell 5 and the power source 18, as needed. The electrochemical reaction cell 5 may be driven by either a constant voltage method or a constant current method.

[0042] The power source 18 may be a conventional commercial power source, a battery, or the like, or may be a power source that converts renewable energy into electrical energy and supplies it. Examples of such power sources include power sources that convert kinetic energy or potential energy, such as wind, hydroelectric, geothermal, and tidal energy, into electrical energy; power sources such as solar cells with photoelectric conversion elements that convert light energy into electrical energy; power sources such as fuel cells and storage batteries that convert chemical energy into electrical energy; and power sources such as devices that convert vibrational energy, such as sound, into electrical energy. The photoelectric conversion element functions to separate charges using the energy of light, such as irradiated sunlight. Examples of photoelectric conversion elements include pin-junction solar cells, pn-junction solar cells, amorphous silicon solar cells, multi-junction solar cells, single-crystalline silicon solar cells, polycrystalline silicon solar cells, dye-sensitized solar cells, and organic thin-film solar cells. The photoelectric conversion element may also be stacked with at least one of the cathode 12 and the anode 13 inside the reaction vessel.

[0043] Next, the operation of the carbon dioxide electrolysis device 1 will be described. Here, we will describe the case where CO2 is reduced to mainly produce carbon monoxide (CO) and H2O is oxidized to produce oxygen. When a voltage equal to or greater than the electrolysis voltage is applied between the cathode 12 and the anode 13, an oxidation reaction of H2O occurs near the anode 13 in contact with the second electrolytic solution. As shown in the following formula (1), an oxidation reaction of H2O contained in the second electrolytic solution occurs, electrons are lost, and oxygen (O2) and hydrogen ions (H +) is generated. The generated hydrogen ions (H + ) moves through the diaphragm 4 into the electrolyte in the first storage portion 2. 2H2O → 4H + +O2+4e - …(1)

[0044] The hydrogen ions (H + ) arrives near the cathode 12, and electrons (e - ) is supplied, a reduction reaction of CO2 occurs. As shown in the following formula (2), hydrogen ions (H + ) and electrons (e - ) reduces the CO2 contained in the electrolyte to produce carbon monoxide (CO). 2CO2+4H + +4e - → 2CO+2H2O …(2) The reduction reaction of CO2 is not limited to the production reaction of CO, but may also be the production reaction of ethanol (C2H5OH), ethylene (C2H4), ethane (C2H6), methane (CH4), methanol (CH3OH), acetic acid (CH3COOH), propanol (C3H7OH), etc.

[0045] As shown in the above formula (1), the gas components discharged from the second flow path 15 or the second storage tank 25 on the anode 13 side have been thought to be mainly oxygen (O2) gas. The exhaust gas on the anode 13 side, which was thought to be O2 gas, has been released into the atmosphere unless it is recycled. In the reaction at the cathode 12 and anode 13 described above, CO2 supplied to the cathode 12 side is reduced at the cathode 12, but a part of it is converted into CO2 or carbonate ions (CO3 2- ) and bicarbonate ions (HCO3 - ) and so on, flow into the anode 13 side. The carbonate ions (CO3 2- ) and bicarbonate ions (HCO3 -When the pH of the anode solution (second electrolyte) becomes, for example, 6 or less, CO2 is present as CO2 through a chemical equilibrium reaction, and some of it is dissolved in the anode solution. Such CO2 gas that does not dissolve in the anode solution is contained together with O2 gas in the gas discharged from the anode 13 side. Under typical operating conditions of the electrochemical reaction cell 5, the ratio of CO2 to O2 in the gas discharged from the anode 13 side may rise to, for example, 2:1. The ratio (volume ratio) of CO2 to O2 varies depending on the operating conditions, but is thought to be approximately CO2:O2 = 4:6 to 8:2.

[0046] If the gas containing CO2 and O2 as described above were to be released into the atmosphere as exhaust gas on the anode 13 side, it would increase the burden on the environment and reduce the utilization efficiency of CO2 and the utilization efficiency and utility value of the reduction products of CO2. Therefore, in the carbon dioxide electrolysis device 1 of this embodiment, a CO2 separation unit 8 that separates and extracts CO2 from the exhaust gas is connected to an outlet that discharges gas components (gas components containing O2 and CO2) from the second storage unit of the electrochemical reaction cell 5. Specifically, in the case of the electrochemical reaction cell 5A, the CO2 separation unit 8 is connected to a gas component discharge unit of the gas-liquid separation unit 23 that is connected to a circulation path 21 that circulates the second electrolytic solution. In the case of the electrochemical reaction cell 5B, the CO2 separation unit 8 is connected to a second discharge flow path 26 that is connected to a storage space for the generated gas provided in the upper part of the second storage tank 25.

[0047] The CO2 separation unit 8 employs a cryogenic cooling method, which separates CO2 from other gas components, including O2, based on the temperature difference (difference in boiling point or melting point) between CO2 and other gas components. As shown in FIG. 4, the CO2 separation unit 8 includes a cryogenic separation device 81 that separates CO2 from the exhaust gas on the anode 13 side. The cryogenic separation device 81 may include a refrigeration device (cooling device), a compressor, a heat exchanger, a rectification device, and other components (not shown). For example, when CO2 is cooled to a low temperature (below -56.7°C) under high pressure (e.g., 0.52 MPa or higher), it becomes liquid CO2. Since O2 has a boiling point of -183°C under atmospheric pressure (0.1 MPa), it remains in a gaseous state even under high pressure and does not become liquid. Therefore, under the high-pressure, low-temperature conditions described above, only CO2 can be liquefied and separated as liquid CO2. Furthermore, CO2 sublimes at -78.5°C under normal pressure (e.g., atmospheric pressure (0.1 MPa)) and becomes a solid (dry ice). O2 remains in a gaseous state even under such conditions, so CO2 can be separated as a solid (dry ice).

[0048] As described above, cryogenic separation is a method for separating gases by utilizing the difference in temperature at which gases become liquid or solid. The cryogenic separation device 81 serving as the CO2 separation unit 8 may include a compressor for compressing the gas. In this case, the CO2 to O2 ratio in the gas discharged from the gas-liquid separator 23 shown in FIG. 2 or the second discharge flow path 26 connected to the upper space of the second storage tank 25 shown in FIG. 3 may increase to, for example, 2:1, as described above. While this varies depending on the operating conditions, the volume ratio is generally considered to be approximately CO2:O2 = 4:6 to 8:2. When separating CO2 from a gas containing gas components other than CO2 in the exhaust gas, if the CO2 concentration in the exhaust gas components is low, the other gases must be compressed and cooled, which requires a lot of power for separation, i.e., cooling energy.

[0049] The exhaust gas from the second storage unit (anode unit) 3 in the carbon dioxide electrolysis device 1 of this embodiment has a higher CO2 concentration than the CO2 concentration in the air or the exhaust gas from a CO2 generation source 9, such as a thermal power plant or a waste incinerator. Therefore, when a cryogenic separation device 81 is installed as the CO2 separation unit 8 in the carbon dioxide electrolysis device 1, the size of the CO2 separation unit (device) 8 can be reduced, and energy consumption can be kept low. On the other hand, for example, in a CO2 separation device using an amine absorbent, the plant size and energy input depend in part on the gas volume, but because the amount of CO2 absorbing solution and the CO2 absorption capacity are related, it is not possible to expect the same level of miniaturization and energy input reduction as a cryogenic separation device. As such, the cryogenic separation device 81 can achieve a higher CO2 concentration than other CO2 separation devices, making it possible to reduce the plant size and energy input. The cryogenic separation device 81 can separate CO2 with a small amount of energy input. The separated CO2 can be used as a gas, and further, as will be described later, it can be used to cool the electrochemical reaction cell 5 and the like as cooled liquid CO2 and solid CO2 (dry ice).

[0050] The CO2 gas separated by the CO2 separation unit 8 may be reused in a device other than the carbon dioxide electrolysis device 1, but is preferably reused in the first chamber 2 of the carbon dioxide electrolysis device 1. Specifically, as shown in FIG. 5 , the CO2 re-transmission flow path 27 connected to the CO2 outlet of the CO2 separation unit 8 can be connected to the CO2 supply unit 6. This allows CO2 discharged from the anode 13 to be reused in the carbon dioxide electrolysis device 1, improving the utilization efficiency of CO2 and the utilization efficiency and value of the CO2 reduction product. The CO2 re-transmission flow path 27 may be directly connected to the first chamber 2 of the electrochemical reaction cell 5, but is preferably connected to the first supply unit 6 that supplies CO2 gas to the first chamber 2. The first supply unit 6 may include a tank that temporarily stores CO2 or a supply device that supplies CO2 from a storage tank to the first chamber 2 while adjusting the supply amount so as to adjust the amount of CO2 input to the electrochemical reaction cell 5. This stabilizes the reduction state of CO2 in the electrochemical reaction cell 5. Furthermore, the CO2 re-transmission passage 27 may be configured to re-transmit the CO2 separated in the CO2 separation unit 8 to the CO2 generation source 9 or the CO2 separation and capture device 10.

[0051] Here, a case has been described in which the CO2 separation unit (first CO2 separation unit) 8 is connected to the discharge part of the second storage unit 3 that stores the second electrolytic solution containing water in the electrochemical reaction cell 5, but a second CO2 separation unit (not shown) may also be connected to the discharge part of the first storage unit 2 that stores CO2. The configuration of the second CO2 separation unit may be the same as that of the first CO2 separation unit 8, or may be a CO2 separation device other than a cryogenic separation device. Furthermore, a hydrogen separation unit (not shown) may be further connected to the discharge part of the first storage unit 2, as will be described later.

[0052] The cold generated when the above-described cryogenic separation device 81 is used can be used, for example, to cool at least a part of the electrochemical reaction cell 5. If cooling is also required in the valuable resource production unit 11, the cold generated by the cryogenic separation device 81 can be used. The cold generated by the cryogenic separation device 81 may be used directly to cool at least a part of the electrochemical reaction cell 5, for example, cooled liquid CO2 and solid CO2 (dry ice), cooled CO2 gas generated therefrom, or the residual gas (separated gas) after CO2 separation, or these cooling media may cool the electrochemical reaction cell 5 etc. via a heat conductive member. Here, the cold generated by the cryogenic separation device 81 is not limited to the electrochemical reaction cell 5, and may be used to cool any part of the valuable resource production unit 11 that requires cooling, if such part is present.

[0053] For example, when cooling the electrochemical reaction cell 5, as shown in FIG. 6, a cooling unit 29 using a medium such as cooling water may be used in the heat exchanger for cooling the cooling water in the cooling unit 29. The cooling medium may be a gas, liquid, or solid primarily composed of cooled CO2 separated in the cryogenic separator 81, or the residual gas (separated gas) after CO2 separation. Furthermore, a metal joint capable of transferring heat, into which gas or liquid is injected, may be cooled with the cooling medium described above, and the electrochemical reaction cell 5 may be cooled by the cooled joint. This further improves the energy efficiency of the carbon dioxide electrolysis device 1. While FIG. 6 illustrates the cooling unit 29 connected to the second housing 3 of the electrochemical reaction cell 5, this is for convenience's sake. The cooling unit 29 is also used to cool the first housing 2 and the entire cell.

[0054] A detector for detecting the gas flow rate and composition input to the CO2 separation unit 8, and a detector for detecting the gas flow rate, CO2 concentration, and composition of the gas discharged from the CO2 separation unit 8 may also be provided. The cooling capacity of the CO2 separation unit 8 can be adjusted based on data from the detector for measuring the CO2 gas flow rate from the CO2 separation and capture device 10, the detector for measuring the CO2 gas flow rate from the CO2 separation unit 8, and the current and voltage detector of the electrochemical cell 5. Temperature detection functions can also be added to the detector for detecting the gas flow rate and composition input to the CO2 separation unit 8, and the detector for detecting the gas flow rate, CO2 concentration, and composition of the gas discharged from the CO2 separation unit 8. This temperature information can be used to adjust the cooling capacity of the CO2 separation unit 8. The operation of the cooling unit 29 can also be controlled based on information from each detector. Each adjustment unit can appropriately adjust the cooling capacity of the CO2 separation unit 8 depending on the operating status (CO2 generation status) of the CO2 generation source 9, the operating status of the electrochemical reaction cell 5, etc. Since the amount of heat generated by the electrochemical reaction cell 5 varies depending on the operating conditions of the electrochemical reaction cell 5, an adjusting device for adjusting the amount of heat introduced may be installed in the pipe leading to the cooling section.

[0055] The valuable resource production unit 11 is a chemical synthesis device that chemically synthesizes valuable resources using CO2 and the like discharged from the first storage unit 2 as raw materials. The product gas discharged from the first storage unit 2 of the electrochemical reaction cell 5 may be used or consumed directly, but by providing a chemical synthesis device downstream of the electrochemical reaction cell 5, valuable resources with high added value can be produced. The valuable resource production unit 11 is connected to a first discharge flow path 28 that discharges the product gas from the first storage unit 2. The first discharge flow path 28 may be connected to a product separator or the like that separates excess CO2 from the exhaust gas or removes moisture from the exhaust gas to separate products such as CO2. The product gas such as CO2 is supplied to the valuable resource production unit 11 from the first storage unit 2 via the first discharge flow path 28. For example, when CO gas is generated in the electrochemical reaction cell 5 according to the above formula (2), the generated CO gas and H gas, a by-product of the reduction reaction, are mixed together to produce a mixed gas, which can be used as a raw material to produce methanol by methanol synthesis, or jet fuel, diesel, etc. by Fischer-Tropsch synthesis.

[0056] The valuable resource production unit 11 is not limited to the above-described chemical synthesis device, and is not particularly limited as long as it can react and synthesize other substances from the reduction products generated in the first storage unit 2. Reactions of the reduction products in the valuable resource production unit 11 include chemical reactions, electrochemical reactions, and biological conversion reactions using organisms such as algae, enzymes, yeast, and bacteria. Chemical reactions, electrochemical reactions, and biological conversion reactions using bacteria may improve at least one parameter, i.e., reaction efficiency and reaction rate, when temperatures are higher than room temperature. The energy conversion efficiency of the carbon dioxide electrolysis device 1 can be improved by introducing the reduction products into the valuable resource production unit 11 at a temperature of 60°C to 150°C. Because biological conversion reactions using bacteria and the like proceed most efficiently around 80°C, introducing the reduction products into the valuable resource production unit 11 at a temperature of 60°C to 100°C further improves efficiency. The valuable resource production unit 11 may be heated or pressurized by applying external energy to improve reaction efficiency.

[0057] The reduction product may contain H2 produced by electrolysis of CO2, CO, and H2O. The H2 concentration can be adjusted as desired depending on the type of use. When H2 is used in the valuable resource production unit 11, CO2 may be separated and used as a mixture of CO and H2. When H2 is not used, only CO is separated. When producing methanol or the like, the number of moles of H2 can be adjusted to approximately twice the number of moles of CO, allowing H2, a by-product of the reaction in the first storage unit 2 of the electrochemical reaction cell 5, to be used as a valuable resource. On the other hand, the side reaction of H2 can be suppressed by adjusting the reaction conditions in the first storage unit 2, and the concentration of H2 in the reduction product can be adjusted to a volume percentage of 0.1% to 5%. This allows the device to be used as a CO2 production device that provides high-concentration CO2.

[0058] The valuable resource production unit 11 can be used in various other ways. For example, depending on the type of CO2 generating source 9, such as a thermal power plant or a blast furnace, the exhaust gas may contain specific ratios of various components, as shown in FIG. 7. For example, gas emitted from a thermal power plant or a blast furnace may contain primarily N2 and CO2, with H2 as a minor component, and the CO2 content may be approximately 15%. When such exhaust gas is directly sent to the first storage unit 2 and a CO2 reduction reaction is carried out in the first storage unit 2, a mixed gas containing the generated CO, H2 as a by-product, and excess CO2 and N2 is discharged from the first storage unit 2. This mixed gas containing CO, CO2, H2, and N2 may be suitable as a feedstock gas for a fermentation reaction device (gas fermentation or anaerobic respiration) used as the valuable resource production unit 11, i.e., a biosynthetic device that produces fuel or chemical substances such as methanol, ethanol, and butanol using anaerobic microorganisms. By supplying the mixed gas discharged from the first storage unit 2 to such a biosynthetic device as a raw material gas, the utilization efficiency and utility value of the CO2 reduction product can be improved. Furthermore, since the CO2 separation and capture device 10 as a pre-stage of the carbon dioxide reaction device 1 is no longer necessary, a cheaper and more efficient system can be provided.

[0059] The configurations of the above-described embodiments can be applied in combination with each other, and some of them can be replaced with other configurations. Although several embodiments of the present invention have been described herein, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are intended to be included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. [Explanation of symbols]

[0060] 1...carbon dioxide electrolysis device, 2...first storage section, 3...second storage section, 4...diaphragm, 5, 5A, 5B...electrochemical reaction cell, 6...carbon dioxide supply section (first supply section), 7...second electrolyte supply section (second supply section), 8...carbon dioxide separation section, 9...carbon dioxide generation source, 11...valuable resource production section, 12...cathode, 13...anode, 14...first flow path, 15...second flow path, 18...power source, 24...first storage tank, 25...second storage tank, 27...carbon dioxide re-transmission flow path, 29...cooling section, 81...cryogenic separation device.

Claims

1. an electrochemical reaction cell including: a first storage section for storing a gas containing carbon dioxide or a first electrolytic solution containing carbon dioxide; a second storage section for storing a second electrolytic solution containing water; a diaphragm provided between the first storage section and the second storage section; a cathode that is disposed so as to be in contact with the gas or the first electrolytic solution and that reduces carbon dioxide to produce a carbon compound; and an anode that is disposed so as to be in contact with the second electrolytic solution and that oxidizes water to produce oxygen; a first supply unit that supplies the gas or the first electrolytic solution to the first container; a second supply unit that supplies the second electrolytic solution to the second container; a first carbon dioxide separation unit connected to an outlet that discharges an effluent containing oxygen and carbon dioxide from the second storage unit, the first carbon dioxide separation unit including a cryogenic separation device that separates the carbon dioxide from gas components in the effluent; A carbon dioxide electrolysis device comprising:

2. 2. The carbon dioxide electrolysis device according to claim 1, wherein the first carbon dioxide separation unit has a carbon dioxide re-transmission flow path that re-transmits the separated carbon dioxide to the first storage unit of the electrochemical reaction cell, the first supply unit, or a carbon dioxide generation source that supplies the carbon dioxide to the first supply unit.

3. 3. The carbon dioxide electrolysis device according to claim 1 or 2, further comprising a cooling unit that cools at least a part of the electrochemical reaction cell using a gas, liquid, or solid mainly containing the carbon dioxide separated in the cryogenic separation device, or a residual gas after the carbon dioxide is separated in the cryogenic separation device, as a cooling medium.

4. 4. The carbon dioxide electrolysis device according to claim 1, further comprising: a second carbon dioxide separation unit connected to an outlet that discharges an effluent from the first storage unit, and that separates carbon dioxide from gas components of the effluent.

5. 5. The carbon dioxide electrolysis device according to claim 1, further comprising: a valuable resource production unit connected to a discharge unit that discharges a effluent containing the carbon compounds from the first storage unit, and that synthesizes a valuable resource using at least a part of the effluent as a raw material.

6. The carbon dioxide electrolysis device according to claim 5 , wherein the valuable resource production unit includes a fermentation reaction device that synthesizes fuel or chemical substances as the valuable resource.

7. 7. The carbon dioxide electrolysis device according to claim 5 or 6, further comprising a cooling unit that cools a part of the valuable resource production unit using, as a cooling medium, a gas, liquid, or solid mainly containing the carbon dioxide separated in the cryogenic separation device, or a residual gas after separation of the carbon dioxide in the cryogenic separation device.

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