Method for removing salt, and carbon dioxide electrolytic reduction apparatus

Abstract

[Problem] To provide a method for removing salt, by which salt deposited on a cathode can be efficiently removed, and a carbon dioxide electrolytic reduction apparatus. [Solution] Provided is a method for removing salt deposited on the cathode of an electrolysis cell used for a carbon dioxide reduction reaction, wherein the cathode includes a catalyst and a cathode electrode provided with a first layer composed of a material including a conductive substance (A), the first layer has a void part, one surface of the first layer is arranged so as to be in contact with carbon dioxide in the cathode, and a gas-liquid mixed phase fluid including a carrier gas and a cleaning liquid capable of dissolving salt is supplied to the cathode.

Description

Method for removing salt and carbon dioxide electrolysis reduction apparatus

[0001] This invention relates to a method for removing salts precipitated in a carbon dioxide electrolytic reduction reaction and to a carbon dioxide electrolytic reduction apparatus.

[0002] In recent years, research and development have been widely conducted to produce carbon monoxide, methane, ethylene, and other substances using carbon dioxide as a starting material and electrical energy. In electrolytic devices used in the reduction reaction of carbon dioxide, carbon dioxide is supplied to the cathode side, and an aqueous solution containing an electrolyte is supplied to the anode side. A separator is also provided between the anode and the cathode to allow ions generated on the anode side to pass through to the cathode side. However, due to its structure, the separator is known to also allow electrolytes such as ions to pass through. Therefore, electrolytes in the aqueous solution supplied to the anode side may permeate the separator, causing salt to precipitate on the cathode side. If precipitated salt is present on the cathode, the reduction reaction of carbon dioxide is hindered, resulting in a decrease in production efficiency. Methods for removing the precipitated salt have been developed, such as a method in which the operation of the electrolytic cell is stopped and a rinsing solution is introduced into the cathode to directly wash away the salt (Patent Documents 1-4), and a method in which the electrolyte supplied to the anode side is diluted or replaced with a specific recovery solution to remove the salt precipitated on the cathode (Patent Document 5).

[0003] Japanese Patent Publication No. 2018-154901, Japanese Patent Publication No. 2019-056135, Japanese Patent Publication No. 2019-167556, Japanese Patent Publication No. 2019-167557, Japanese Patent Publication No. 2022-145228

[0004] However, in the methods disclosed in Patent Documents 1 to 4, the operation must be stopped each time precipitated salt is removed, and the reaction stops during the removal process, making it difficult to maintain a constant production volume in commercial operation. One way to maintain a constant production volume is to install a separate electrolytic cell that operates during the precipitated salt removal process, but this would not only increase capital investment but also require securing installation space depending on the scale of production. Furthermore, while the method disclosed in Patent Document 5 can remove salt without stopping the operation, it utilizes the concentration gradient of the electrolyte solution, so it is expected that removing precipitated salt will take time, and it will be difficult to remove salts that precipitate at a distance from the solid electrolyte membrane. On the other hand, during their investigation, the inventors discovered a new problem: when a rinse solution such as pure water is supplied directly to the cathode without stopping operation, if there are microstructures (e.g., irregularities or voids) on the contact surface between the cathode and carbon dioxide, the surface tension of the cleaning solution prevents it from reaching the inside of the cathode sufficiently, making it impossible to completely remove the precipitated salt, or requiring a large amount of cleaning solution to achieve sufficient cleaning.

[0005] As a result of further diligent research, the inventors have discovered a method for efficiently removing salt precipitated on the cathode without stopping operation, and have provided a carbon dioxide electrolytic reduction apparatus employing this method.

[0006] To solve the above problems, one aspect of the present invention is a method for removing salt precipitated on the cathode of an electrolytic cell used in a carbon dioxide reduction reaction, wherein a gas-liquid multiphase fluid containing a washing solution capable of dissolving the salt and a carrier gas is supplied to the cathode.

[0007] In this embodiment, since the cleaning solution is supplied to the cathode in the form of a gas-liquid multiphase fluid, the cleaning solution can thoroughly reach the cathode. As a result, even if salt has precipitated in the voids of the cathode, it can be efficiently removed.

[0008] Another aspect of the present invention is a carbon dioxide electrolytic reduction apparatus (1) comprising: a separator (5) that partitions a cathode chamber (3) and an anode chamber (4); a cathode (7) provided on the cathode chamber side of the separator and comprising a catalyst (6) and made of a material having gas diffusion function and conductivity; and an anode (8) provided on the anode chamber side of the separator, wherein an anode solution containing an electrolyte is supplied to the anode chamber and a cathode gas is supplied to the cathode chamber; a cathode gas supply line (31) that supplies the cathode gas to be reduced in the cathode to the cathode chamber; and a gas-liquid multiphase fluid supply unit (26) that supplies a gas-liquid multiphase fluid to the cathode gas supply line, in which a cleaning solution capable of dissolving the salt deposited on the cathode is dispersed in a carrier gas which is at least one selected from the group consisting of air, carbon dioxide, oxygen, nitrogen, helium, and argon.

[0009] According to the above embodiment, since the cleaning solution is supplied to the cathode in the state of a gas-liquid multiphase fluid, the cleaning solution can sufficiently reach the cathode, and salts precipitated in the voids of the cathode can be efficiently removed.

[0010] Diagram of the configuration of the carbon dioxide electrolytic reduction apparatus according to the embodiment. Diagram of the electrolytic cell. Graph showing the differential pressure ratio between the cathode inlet and cathode outlet in the first embodiment. Graph showing the differential pressure ratio between the cathode inlet and cathode outlet in the second embodiment. Graph showing the differential pressure ratio between the cathode inlet and cathode outlet in the third embodiment.

[0011] The salt removal method according to the present invention will be described below.

[0012] One embodiment of the salt removal method according to the present invention is a method for removing salt deposited on the cathode of an electrolytic cell used in a carbon dioxide reduction reaction, wherein the cathode includes a cathode electrode having a first layer made of a material containing a conductive substance, and a catalyst, the first layer having voids, and one surface of the first layer being arranged to be in contact with carbon dioxide at the cathode, and a gas-liquid multiphase fluid containing a salt-dissolving cleaning solution and a gas is supplied to the cathode. The salt removal method and embodiments of a carbon dioxide electrolytic reduction apparatus to which the salt removal method is applied will be described below.

[0013] As shown in Figure 1, the carbon dioxide electrolytic reduction apparatus 1 according to this embodiment has an electrolytic cell 2. As shown in Figure 2, the electrolytic cell 2 has a separator 5 that separates a cathode chamber 3 and an anode chamber 4, a cathode 7 which is a gas diffusion electrode containing a catalyst 6 and is provided on the cathode chamber 3 side of the separator 5, and an anode 8 provided on the anode chamber 4 side of the separator 5. The cathode 7 and anode 8 are arranged in pairs so as to sandwich the separator 5, and the distance between the cathode 7 and the separator 5, and the distance between the separator 5 and the anode 8 are not particularly limited, but from the viewpoint of energy consumption (overvoltage, etc.), it is preferable to have a structure in which the cathode 7 and the separator 5, and the separator 5 and the anode 8 are in contact with each other (zero-gap structure). Carbon dioxide, which is the starting material for the reduction reaction (and is therefore sometimes referred to as "cathode gas" in this specification), is supplied in gaseous form to the cathode chamber 3. Furthermore, carbon dioxide only needs to be in gaseous form when supplied to the cathode chamber 3; its prior form is not particularly limited and may be gas, liquid, or solid.

[0014] The separator 5 is preferably an electrolyte membrane having selective permeability that allows ions to pass through. The separator 5 transports ions between the anode 8 and the cathode 7. The separator 5 may be an anion exchange membrane that selectively allows hydroxide ions to pass through, or a proton exchange membrane that selectively allows protons to pass through. The separator 5, cathode 7, and anode 8 may be formed integrally to constitute a membrane electrode assembly 10 (MEA).

[0015] The membrane electrode assembly 10 is sandwiched between a cathode channel plate 11 and an anode channel plate 12. Multiple cathode channel grooves 14 constituting a cathode chamber 3 are formed on the surface of the cathode channel plate 11 facing the membrane electrode assembly 10. Multiple anode channel grooves 15 constituting an anode chamber 4 are formed on the surface of the anode channel plate 12 facing the membrane electrode assembly 10.

[0016] The cathode 7 includes a cathode electrode 7C having a first layer 7A made of a material containing a conductive substance (A), and a catalyst 6. The cathode electrode 7C may further have a second layer 7B made of a material containing a conductive substance (B). In this specification, the first layer 7A may be referred to as a gas diffusion layer, and the second layer 7B as a porous layer.

[0017] The first layer 7A has voids. When voids are present on the surface of the first layer 7A, they may form microstructures such as irregularities or a network structure. The presence of voids in the first layer 7A ensures gas permeability, i.e., gas diffusion function, allowing gaseous carbon dioxide to be efficiently taken into the cathode and increasing reaction efficiency.

[0018] Furthermore, the first layer 7A is positioned so as to be in contact with carbon dioxide at the cathode 7 and cathode electrode 7C. This arrangement allows gaseous carbon dioxide to diffuse efficiently within the cathode. The first layer 7A may be a single layer or multiple layers may be stacked (multi-layered). From the viewpoint of carbon dioxide diffusion effect, multiple layers are preferable.

[0019] A material containing a conductive substance (A) preferably contains the conductive substance (A) on its surface. The form of the material containing the conductive substance (A) is not particularly limited, but examples include cloth, paper, and felt, although paper is preferred from the viewpoint of availability.

[0020] The material containing the conductive substance (A) may also contain resins such as water-repellent resins and additives such as water-repellent agents. Examples of water-repellent resins and water-repellent agents include polytetrafluoroethylene (PTFE), 4-fluoroethylene-6-fluoropropylene copolymer (FEP), perfluoroalkoxyfluorinated resin (PFA), ethylene-4-fluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), and polyvinyl fluoride (PVF). Other examples of resins include polyolefins (PO), and other examples of additives include dispersants, adhesives, and binders.

[0021] The conductive material (A) can be any material that has conductivity, such as metals and carbons, but carbons are preferred from the viewpoint of corrosion such as electrolytic corrosion. Examples of carbons include amorphous carbon, graphene, carbon nanotubes, carbon black, Ketjenblack, fullerene, and graphite. The form of the conductive material (A) is preferably particulate or fibrous, and more preferably fibrous. When the material containing the conductive material (A) takes the form of particulates or fibrous material, it can form voids of various sizes, so that gaseous carbon dioxide can be diffused efficiently.

[0022] The second layer 7B is preferably positioned between the separator 5 and the first layer 7A. More preferably, the second layer 7B and the first layer 7A are in close contact, from the viewpoint of efficiently diffusing gaseous carbon dioxide from the first layer 7A on the opposite side of the contact surface between the first layer 7A and the second layer 7B. Including the second layer 7B allows for even more efficient diffusion of gaseous carbon dioxide.

[0023] The second layer 7B preferably contains a conductive material (B), and more preferably is composed of a conductive material (B). The conductive material (B) can be any material that possesses conductivity, such as carbons and metals, and preferably carbons or a mixture of carbons and metals. Examples of carbons include amorphous carbon, graphene, carbon nanotubes, carbon black, Ketjenblack, fullerene, and graphite, and one of these may be used, or two or more may be used in combination. Examples of metals include gold, silver, copper, lead, zinc, tin, and indium, and one of these may be used, or two or more metal elements or compounds thereof may be used in combination.

[0024] Examples of the form of the second layer 7B include cloth, paper, and felt, and there are no particular limitations, but paper is preferred from the viewpoint of availability. The second layer 7B may be a single layer, or multiple layers of the second layer 7B may be laminated. From the viewpoint of electrolyte permeability, it is preferable that multiple layers of the second layer 7B are laminated. Multiple laminates may be combined using laminates of the first layer 7A and the second layer 7B as constituent units.

[0025] The cathode 7 may be in close contact with the separator 5 in the second layer 7B. The first layer 7A may be provided on the side of the second layer 7B opposite to the separator 5. The first layer 7A may be in contact with the cathode flow path plate 11. The first layer 7A may also face a plurality of cathode flow path grooves 14.

[0026] A catalyst 6 is supported on the cathode 7. The catalyst 6 may be a known carbon dioxide reduction catalyst and may include, for example, at least one of the following: a group 11 element such as copper, a group 12 element such as zinc, a group 13 element such as gallium, a group 14 element such as germanium, or a metal compound thereof. The metal compound may include at least one of an oxide, sulfide, or phosphide. The catalyst 6 is preferably suitable for the reduction reaction of carbon dioxide, and it is preferable to use a material that combines copper or a copper compound with metals of group 11, group 12, group 13, and group 14 elements, and metal compounds thereof.

[0027] The form of the catalyst 6 is not particularly limited, but from the viewpoint of fully exhibiting its function as a carbon dioxide reduction catalyst, a form that can secure a large surface area, such as particulate matter, is preferred. When the catalyst 6 is in particulate matter (catalyst particles), it may be arranged in layers, or the catalyst particles may be scattered within the cathode electrode, or both arrangements may be used. When the catalyst particles are arranged in layers, it is assumed that the layer consisting of the catalyst particles is placed between the separator 5 and the cathode electrode 7C.

[0028] The anode 8 is formed of a porous conductor. The anode 8 is made of, for example, a metallic material such as titanium, nickel, molybdenum, platinum, gold, silver, copper, iron, or lead, or an alloy of these metals, a carbon-based material such as carbon, or a conductive ceramic. The anode 8 may also be made of, for example, indium oxide. The shape of the anode 8 may be a flat plate with multiple openings, a mesh, etc. An oxygen-evolving catalyst such as platinum or iridium is supported on the anode 8. The anode 8 is in contact with the anode channel plate 12. The anode 8 also faces multiple anode channel grooves 15.

[0029] Multiple electrolytic cells 2 may be stacked on top of each other to form a cell stack. In this case, the multiple electrolytic cells 2 may be connected in series or in parallel. When multiple electrolytic cells 2 are connected in series, the cathode flow channel plate 11 and anode flow channel plate 12 of adjacent electrolytic cells 2 may be in contact with each other and electrically connected.

[0030] The cathode 7 and anode 8 are connected to a DC power supply 20. The DC power supply 20 may be an AC-DC converter, battery, or DC generator that converts electricity obtained from thermal power generation, nuclear power generation, solar power generation, wind power generation, or hydroelectric power generation into DC. From the viewpoint of reducing carbon dioxide emissions, electricity obtained from renewable energy sources such as solar power generation, wind power generation, or hydroelectric power generation may be used as the DC power supply 20. The DC power supply 20 applies a voltage to the anode 8 such that the cathode 7 is at a negative potential. The DC power supply 20 may use a reference electrode to obtain the potential of the cathode 7 and control the applied voltage so that the potential of the cathode 7 is within a predetermined range.

[0031] As shown in FIG. 1, the electrolytic cell 2 has a cathode inlet 21 and a cathode outlet 22 connected to the cathode chamber 3, and an anode inlet 23 and an anode outlet 24 connected to the anode chamber 4. A cathode gas is supplied to the cathode chamber 3. The cathode gas is supplied from the cathode inlet 21 to the cathode chamber 3 and discharged from the cathode outlet 22. An anode liquid is supplied to the anode chamber 4. The anode liquid is supplied from the anode inlet 23 to the anode chamber 4 and discharged from the anode outlet 24.

[0032] The cathode gas is carbon dioxide. The cathode gas is a gas when supplied to the cathode chamber 3, but in the previous process, it does not need to be a gas and may be, for example, a liquid or the like. The anode liquid is an aqueous solution in which an electrolyte is dissolved. The electrolyte contains at least one of potassium, sodium, lithium, or a compound thereof. The electrolyte is, for example, LiOH, NaOH, KOH, Li ２ CO ３ , Na ２ CO ３ , K ２ CO ３ , LiHCO ３ , NaHCO ３ , and KHCO ３ and may contain at least one selected from the group consisting of.

[0033] At the cathode 7 of the electrolytic cell 2, carbon dioxide is reduced and at least one of hydrocarbon, carbon monoxide, and hydrogen is generated. From the cathode outlet 22, a cathode product gas containing hydrocarbon, carbon monoxide, hydrogen, etc. generated by the electrolytic reduction reaction and unreacted carbon dioxide is discharged. Further, the cathode product gas contains water that has permeated through the separator 5 and a cleaning liquid resulting from the gas-liquid mixed-phase fluid supplied by the gas-liquid mixed-phase fluid supply unit 26 described later.

[0034] At the anode 8 of the electrolytic reduction cell, water is oxidized to generate oxygen. The oxygen generated at the anode 8 forms a gas-liquid mixed-phase fluid together with the anode liquid and is discharged from the anode outlet 24.

[0035] As shown in FIG. 1, the carbon dioxide electrolytic reduction device 1 includes a cathode gas supply line 31 that supplies cathode gas to the cathode chamber 3 of the electrolytic cell 2, a cathode generated gas discharge line 32 that discharges cathode generated gas from the cathode chamber 3, and an anode liquid circulation line 33 that circulates anode liquid in the anode chamber 4.

[0036] The cathode gas supply line 31 connects the cathode gas storage unit 35 and the cathode inlet 21 of the electrolytic cell 2. The cathode gas storage unit 35 is a container capable of storing cathode gas. In the present embodiment, the cathode gas storage unit 35 stores carbon dioxide gas as the cathode gas.

[0037] The cathode gas supply line 31 is provided with a first flow rate control unit 36 and a humidifying unit 37 in order from the cathode gas storage unit 35 side. The first flow rate control unit 36 controls the flow rate of the cathode gas flowing through the cathode gas supply line 31 from the cathode gas storage unit 35. The first flow rate control unit 36 may be a control valve.

[0038] The humidifying unit 37 humidifies the cathode gas flowing through the cathode gas supply line 31. The humidifying unit 37 may be, for example, a gas-liquid contact type humidifier that humidifies by directly contacting liquid water and the cathode gas, a steam spray humidifier that sprays water vapor onto the cathode gas, or a membrane humidifier that allows only water vapor to pass through the cathode gas using a semipermeable membrane.

[0039] A temperature sensor T1 and a pressure sensor P1 are provided near the cathode inlet 21 in the cathode gas supply line 31. The temperature sensor T1 detects the temperature of the cathode gas at the cathode inlet 21. The pressure sensor P1 detects the pressure of the cathode gas at the cathode inlet 21.

[0040] The cathode generated gas discharge line 32 is connected to the cathode outlet 22 and the cathode generated gas storage unit 39. The cathode generated gas storage unit 39 is a container capable of storing cathode generated gas. The cathode generated gas discharge line 32 is provided with a first pressure control unit 41, a gas-liquid separation unit 42, a cathode generated gas analysis unit 43, and a first dehumidifying unit 44 in order from the cathode outlet 22 side.

[0041] The first pressure control unit 41 adjusts the pressure at the cathode outlet 22. The first pressure control unit 41 may be a regulating valve. The gas-liquid separation unit 42 removes the liquid in the cathode-generated gas. The gas-liquid separation unit 42 may be, for example, a knockout drum or a mist eliminator.

[0042] The cathode-generated gas analysis unit 43 collects a part of the cathode-generated gas from the cathode-generated gas discharge line 32 and analyzes the composition of the cathode-generated gas. The composition analysis of the cathode-generated gas may be performed by, for example, a gas chromatograph, a laser analyzer, or an infrared gas analyzer.

[0043] The first dehumidification unit 44 dehumidifies the cathode-generated gas. The first dehumidification unit 44 may be a desiccant dehumidifier, a compressor dehumidifier (refrigeration dehumidifier), or a membrane dehumidifier.

[0044] A temperature sensor T2 and a pressure sensor P2 are provided near the cathode outlet 22 in the cathode-generated gas discharge line 32. The temperature sensor T2 and the pressure sensor P2 are provided between the cathode outlet 22 and the first pressure control unit 41 in the cathode-generated gas discharge line 32. The temperature sensor T2 detects the temperature of the cathode-generated gas at the cathode outlet 22. The pressure sensor P2 detects the pressure of the cathode-generated gas at the cathode outlet 22.

[0045] A first flow rate sensor F1 is provided between the first dehumidification unit 44 and the cathode-generated gas storage unit 39 in the cathode-generated gas discharge line 32. The first flow rate sensor F1 detects the flow rate of the cathode-generated gas flowing from the cathode-generated gas discharge line 32 to the cathode-generated gas storage unit 39. The cathode-generated gas stored in the cathode-generated gas storage unit 39 may be supplied to a subsequent process.

[0046] The anode liquid circulation line 33 is connected to the anode outlet 24 and the anode inlet 23. The anode liquid circulation line 33 circulates the anode liquid discharged from the anode outlet 24 to the anode inlet 23. The anode liquid circulation line 33 is provided with a second pressure control unit 51, an anode liquid storage unit 52, a second flow rate control unit 53, and an anode liquid cooling unit 54 in order from the anode outlet 24 side.

[0047] The second pressure control unit 51 adjusts the pressure of the anode liquid at the anode outlet 24. The second pressure control unit 51 may be a control valve. The anode liquid storage unit 52 is a container that temporarily stores the anode liquid flowing through the anode liquid circulation line 33.

[0048] The anode liquid reservoir 52 is provided with a conductivity adjustment unit 56. The conductivity adjustment unit 56 takes in a portion of the anode liquid from the anode liquid reservoir 52, adjusts the conductivity of the anode liquid, and returns it to the anode liquid reservoir 52. The conductivity adjustment unit 56 may include, for example, a circulation line that circulates the anode liquid with the anode liquid reservoir 52, a conductivity sensor that detects the conductivity of the anode liquid in the circulation line, and an input device that introduces electrolyte or water into the circulation line. The conductivity adjustment unit 56 measures the conductivity of the anode liquid flowing through the circulation line, and if the conductivity is lower than the appropriate range, it adds electrolyte to the anode liquid; if the conductivity is higher than the appropriate range, it adds water to the anode liquid; and if the conductivity is within the appropriate range, it does not add anything to the anode liquid. The conductivity adjustment unit 56 then returns the anode liquid with adjusted conductivity to the anode liquid reservoir 52.

[0049] Gas accumulates in the upper part of the anode liquid reservoir 52. The gas accumulated in the upper part of the anode liquid reservoir 52 is called the anode-generating gas. The main component of the anode-generating gas is oxygen produced in the anode 8. The anode-generating gas also contains carbon dioxide gas that has permeated from the cathode chamber 3 through the separator 5 and mixed into the anode liquid, as well as water.

[0050] The upper part of the anode liquid storage section 52 is connected to the anode gas storage section 62 by an anode gas discharge line 61. The anode gas storage section 62 is a container capable of storing anode gas. The anode gas accumulated in the upper part of the anode liquid storage section 52 flows through the anode gas discharge line 61 to the anode gas storage section 62. The anode gas stored in the anode gas storage section 62 may be supplied to a subsequent process. A second dehumidification section 63 is provided in the anode gas discharge line 61. The second dehumidification section 63 dehumidifies the anode gas. The second dehumidification section 63 may be a desiccant dehumidifier, a compressor dehumidifier (refrigeration dehumidifier), or a membrane dehumidifier.

[0051] A second flow sensor F2 is provided between the second dehumidification unit 63 and the anode gas storage unit 62 in the anode gas discharge line 61. The second flow sensor F2 detects the flow rate of anode gas flowing from the anode gas discharge line 61 to the anode gas storage unit 62.

[0052] The anode liquid in the anode liquid reservoir 52 is sent to the anode inlet 23 through the second flow control unit 53 and the anode liquid cooling unit 54. The second flow control unit 53 may be a pump or a control valve. The anode liquid cooling unit 54 cools the anode liquid flowing to the anode inlet 23. The anode liquid cooling unit 54 is a heat exchanger. Heat is generated in the electrolytic cell 2 due to the electrolytic reduction reaction. The electrolytic cell 2 is cooled by the supply of cooled anode liquid to it. The temperature of the electrolytic cell 2 is regulated by the anode liquid cooling unit 54 controlling the temperature of the anode liquid.

[0053] In the anode fluid circulation line 33, a temperature sensor T3 and a pressure sensor P3 are provided in the portion between the anode fluid cooling unit 54 and the anode inlet 23. The temperature sensor T3 detects the temperature of the anode fluid at the anode inlet 23. The pressure sensor P3 detects the pressure of the anode fluid at the anode inlet 23.

[0054] The carbon dioxide electrolytic reduction apparatus 1 has a gas-liquid multiphase fluid supply unit 26 for supplying a gas-liquid multiphase fluid of cleaning solution to the cathode 7. The cleaning solution is a liquid capable of dissolving the salt precipitated on the cathode 7. The cleaning solution is a liquid that achieves a cleaning effect by physically flushing away or chemically dissolving the salt precipitated on the cathode 7. The cleaning solution is, for example, water. The conductivity of the water used as the cleaning solution may be 2 mS / m or less, preferably 1 mS / m or less, and more preferably 0.1 mS / m or less. In other embodiments, the cleaning solution may also contain a low concentration of hydrochloric acid, sulfuric acid, or nitric acid. The pH value of the cleaning solution is preferably 5 to 9, and more preferably 5 to 7. If the pH value of the cleaning solution is less than 5, the cleaning solution remaining in the cathode cell may affect the diffusion resistance of carbon dioxide in the cathode electrode. The salt includes carbonates formed by the reaction of the electrolyte that permeates through the separator 5 with carbon dioxide. Furthermore, the salt may also contain precipitates of electrolytes that have permeated through the separator 5. Note that the salt is not limited to carbonates or electrolyte precipitates.

[0055] The gas-liquid multiphase fluid supply unit 26 generates a gas-liquid multiphase fluid of the cleaning solution and a carrier gas, and supplies the generated gas-liquid multiphase fluid to the cathode 7 via the cathode gas supply line 31 and the cathode chamber 3. The gas-liquid multiphase fluid may contain unavoidable impurities in addition to the cleaning solution and carrier gas. The carrier gas is not particularly limited as long as it is a gas that does not participate in the carbon dioxide reduction reaction, but preferably it is at least one selected from the group consisting of air, carbon dioxide, oxygen, nitrogen, helium, and argon. This makes it possible to generate a gas-liquid multiphase fluid of the cleaning solution using a general-purpose carrier gas. Furthermore, carbon dioxide is more preferable from the viewpoint of being used in the electrolytic reduction reaction of carbon dioxide and not inhibiting the electrolytic reduction reaction.

[0056] The gas-liquid multiphase fluid supply unit 26 includes a cleaning liquid storage unit 71, a carrier gas storage unit 72, and a multiphase conversion unit 73. The cleaning liquid storage unit 71 is a container capable of storing liquid cleaning liquid. The cleaning liquid flows from the cleaning liquid storage unit 71 to the multiphase conversion unit 73 through the cleaning liquid line 75. The cleaning liquid line 75 is provided with a third flow control unit 76 that controls the flow rate of the cleaning liquid supplied to the multiphase conversion unit 73. The third flow control unit 76 may be a pump or a control valve.

[0057] The carrier gas storage section 72 is a container capable of storing carrier gas. The carrier gas flows from the carrier gas storage section 72 to the multiphase conversion section 73 through the carrier gas line 78. The carrier gas line 78 is provided with a fourth flow control unit 79 that controls the flow rate of carrier gas supplied to the multiphase conversion section 73. If the carrier gas is carbon dioxide gas, carbon dioxide gas may be supplied from the cathode gas storage section 35 to the carrier gas storage section 72. Alternatively, the cathode gas storage section 35 may also serve as the carrier gas storage section 72.

[0058] In the carrier gas line 78, a temperature sensor T4 and a pressure sensor P4 are provided in the portion between the fourth flow control unit 79 and the multiphase unit 73. The temperature sensor T4 detects the temperature of the carrier gas supplied to the multiphase unit 73. The pressure sensor P4 detects the pressure of the carrier gas supplied to the multiphase unit 73.

[0059] The multiphase unit 73 mixes the cleaning solution and the carrier gas to generate a gas-liquid multiphase fluid. The multiphase unit 73 may, for example, generate the gas-liquid multiphase fluid by blowing the carrier gas into a tank containing the cleaning solution, pressurizing the tank, and then opening the valve.

[0060] In a gas-liquid multiphase fluid, the ratio of carrier gas to the gas-liquid multiphase fluid (volume percentage; vol%) is preferably 0.1 vol% or more, more preferably 0.5 vol% or more, even more preferably 1 vol% or more, and even more preferably 5 vol% or more. By using a gas-liquid multiphase system containing gas in a liquid, a higher cleaning effect can be obtained than using the cleaning solution alone. Furthermore, from the viewpoint of further improving the cleaning effect, it may be 10 vol% or more, 15 vol% or more, or 20 vol% or more. In addition, the ratio of carrier gas to the gas-liquid multiphase fluid (volume percentage; vol%) is not particularly limited and may be less than 100 vol%, but from the viewpoint of not reducing the time efficiency of cleaning, it is preferably 96 vol% or less, more preferably 93 vol% or less, and even more preferably 90 vol% or less. In other words, the proportion of carrier gas to gas-liquid multiphase fluid (volume percentage; vol%) is as follows: 0.1 vol% or more and less than 100 vol%, 0.1 vol% or more and 96 vol% or less, 0.1 vol% or more and 93 vol% or less, 0.1 vol% or more and 90 vol% or less, 0.5 vol% or more and less than 100 vol%, 0.5 vol% or more and 96 vol% or less, 0.5 vol% or more and 93 vol% or less, 0.5 vol% or more and 90 vol% or less, 1 vol% or more and less than 100 vol%, 1 vol% or more and 96 vol% or less, 1 vol% or more and 93 vol% or less, 1 vol% or more and 90 vol% or less, 5 vol% or more and less than 100 vol%, 5 vol It is preferable that the value falls within one of the following ranges: % or more and 96 vol% or less, 5 vol% or more and 93 vol% or less, 5 vol% or more and 90 vol% or less, 10 vol% or more and less than 100 vol%, 10 vol% or more and 96 vol% or less, 10 vol% or more and 93 vol% or less, 10 vol% or more and 90 vol% or less, 15 vol% or more and less than 100 vol%, 15 vol% or more and 96 vol% or less, 15 vol% or more and 93 vol% or less, 15 vol% or more and 90 vol% or less, 20 vol% or more and less than 100 vol%, 20 vol% or more and 96 vol% or less, 20 vol% or more and 93 vol% or less, and 20 vol% or more and 90 vol% or less. The particle size of the cleaning solution in a gas-liquid multiphase fluid may be several micrometers to several tens of micrometers.For example, the particle size of the cleaning solution in a gas-liquid multiphase fluid may be 0.1 μm or more and 1000 μm or less, preferably 0.5 μm or more and 500 μm or less, and more preferably 1.0 μm or more and 100 μm or less.

[0061] The gas-liquid multiphase fluid generated in the multiphase formation section 73 is supplied to the cathode gas supply line 31 via the gas-liquid multiphase fluid line 81. The gas-liquid multiphase fluid line 81 is connected to the cathode inlet 21 side of the humidification section 37 in the cathode gas supply line 31. That is, in the cathode gas supply line 31, the humidification section 37 is located upstream of the section to which the gas-liquid multiphase fluid is supplied. The gas-liquid multiphase fluid is mixed with the cathode gas humidified in the humidification section 37. A third pressure control unit 82 is provided in the gas-liquid multiphase fluid line 81. The third pressure control unit 82 may be a control valve capable of adjusting the pressure of the gas-liquid multiphase fluid supplied from the gas-liquid multiphase fluid line 81 to the cathode gas supply line 31.

[0062] The carbon dioxide electrolytic reduction apparatus 1 has a liquid recovery unit 85. The liquid recovery unit 85 has a recovered liquid storage unit 86, a cathode-side recovery line 87, a cathode-side return line 88, and an anode-side return line 89. The cathode-side recovery line 87 sends the liquid separated from the cathode-generated gas in the gas-liquid separation unit 42 and the first dehumidification unit 44 to the recovered liquid storage unit 86. The recovered liquid storage unit 86 stores the liquid separated in the gas-liquid separation unit 42 and the first dehumidification unit 44 as recovered liquid. The main component of the recovered liquid is water. The recovered liquid contains the anode liquid and electrolyte that have permeated through the separator 5, and salts precipitated on the cathode 7.

[0063] The cathode-side return line 88 connects the recovered liquid storage section 86 and the cleaning liquid storage section 71. The recovered liquid in the recovered liquid storage section 86 is sent to the cleaning liquid storage section 71 via the cathode-side return line 88. A fifth flow rate control unit 92 is provided in the cathode-side return line 88. The fifth flow rate control unit 92 controls the flow rate of the recovered liquid returned from the recovered liquid storage section 86 to the cleaning liquid storage section 71. The fifth flow rate control unit 92 may be a pump or a control valve. A measurement line 91 for measuring the conductivity of the recovered liquid is connected downstream of the fifth flow rate control unit 92 in the cathode-side return line 88. The other end of the measurement line 91 is connected to the recovered liquid storage section 86. The measurement line 91 is provided with a conductivity sensor C1 for detecting the conductivity of the recovered liquid.

[0064] The anode-side return line 89 connects the recovered liquid storage unit 86 and the conductivity adjustment unit 56. The recovered liquid in the recovered liquid storage unit 86 is sent to the conductivity adjustment unit 56 through the anode-side return line 89. The recovered liquid is mixed with the anode liquid in the conductivity adjustment unit 56 and sent to the anode liquid storage unit 52. A sixth flow rate control unit 93 is provided in the anode-side return line 89. The sixth flow rate control unit 93 controls the flow rate of the recovered liquid returned from the recovered liquid storage unit 86 to the conductivity adjustment unit 56. The sixth flow rate control unit 93 may be a pump or a control valve.

[0065] The liquid separated from the anode-generated gas in the second dehumidification unit 63 is sent to the anode-side return line 89 and returned to the conductivity adjustment unit 56 together with the recovered liquid.

[0066] The carbon dioxide electrolysis reduction apparatus 1 has a control device 95. The control device 95 is a computer having a processor 96 and a memory 97 that is communicatively connected to the processor 96. The processor 96 may include at least one of the following as its core: a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), or a RISC (Reduced Instruction Set Computer). The memory 97 stores control programs executed by the processor 96 and various data. The memory 97 may include at least one of volatile memory and non-volatile memory. The volatile memory may be, for example, DRAM (Dynamic Random Access Memory) or SRAM (Static Random Access Memory). The non-volatile memory may be an SSD (Solid State Drive), flash memory, magnetic disk storage device, or optical disk storage device. At least a part of the control device 95 may be implemented by hardware such as an LSI (Large Scale Integration), ASIC (application specific integrated circuit), FPGA (field-programmable gate array), or by a combination of software and hardware. The control device 95 may be composed of a single piece of hardware, or it may be composed of multiple pieces of hardware that can communicate with each other.

[0067] The processor 96 implements various applications by executing programs stored in memory 97. The programs may be stored on removable recordable media such as DVDs or CD-ROMs, and installed in memory 97 when the recordable media is read by a reader. Alternatively, programs may be downloaded to and installed in memory 97 via a communication network such as the Internet.

[0068] The control device 95 receives signals from temperature sensors T1 to T4, pressure sensors P1 to P4, flow sensors F1 and F2, conductivity sensor C1, conductivity adjustment unit 56, and cathode generated gas analysis unit 43, and acquires the temperature, pressure, flow rate, conductivity, and cathode generated gas components of each unit. The control device 95 also controls the DC power supply 20, humidification unit 37, gas-liquid separation unit 42, first dehumidification unit 44, anode liquid cooling unit 54, second dehumidification unit 63, multiphase unit 73, and first to sixth flow control units 36, 53, 76, 79, 92, and 93.

[0069] The control device 95 controls the timing, flow rate, volume ratio of carrier gas to cleaning liquid, and particle size of cleaning liquid in the gas-liquid multiphase fluid supplied from the gas-liquid multiphase fluid supply unit 26 to the cathode gas supply line 31 by controlling the third flow control unit 76, the fourth flow control unit 79, the multiphase unit 73, and the third pressure control unit 82.

[0070] The operation and effects of the carbon dioxide electrolytic reduction apparatus 1 are described below. In the carbon dioxide electrolytic reduction apparatus 1, cathode gas, which is carbon dioxide, is supplied to the cathode chamber 3, anode liquid is supplied to the anode chamber 4, and a voltage is applied between the cathode 7 and anode 8 by the DC power supply 20. As a result, the reduction reaction of chemical formulas (1) to (4) above occurs in the cathode 7, and cathode-generating gas containing products such as ethylene, methane, carbon monoxide, and hydrogen is generated. After the water is removed from the cathode-generating gas by passing through the gas-liquid separation unit 42 and the first dehumidification unit 44, it is stored in the cathode-generating gas storage unit 39. At the anode 8, the oxidation reaction of chemical formula (5) above occurs, and anode-generating gas containing oxygen is generated. The anode-generating gas is separated from the anode liquid in the anode liquid storage unit 52, and after the water is removed by passing through the second dehumidification unit 63, it is stored in the anode-generating gas storage unit 62.

[0071] If the electrolytic reduction reaction in electrolytic cell 2 continues, salt will precipitate on cathode 7. When salt precipitates, the flow resistance of the cathode chamber 3 and the first layer 7A of cathode 7 increases, making it difficult for cathode gas to reach the reaction field of cathode 7. The carbon dioxide electrolytic reduction apparatus 1 performs a salt removal method. In the salt removal method performed by the carbon dioxide electrolytic reduction apparatus 1, a gas-liquid multiphase fluid containing a cleaning solution is supplied to cathode 7 from the gas-liquid multiphase fluid supply unit 26, so that the salt precipitated on cathode 7 is dissolved by the gas-liquid multiphase fluid. The particles of the cleaning solution contained in the gas-liquid multiphase fluid can easily penetrate into the interior of the first layer 7A, and the salt in the first layer 7A can be efficiently dissolved. Specifically, if cathode 7 is made of a material that has fine voids, gas (including carbon dioxide, which is the starting material) enters the voids and forms a layer of gas, and even if cleaning solution is flowed, the layer of gas is retained, so the cleaning solution cannot enter the voids. However, by creating a gas-liquid multiphase fluid of cleaning solution and carrier gas, the carrier gas contained in the gas-liquid multiphase fluid merges with the gas layer formed in the voids of the first layer 7A and is pushed out together. As a result, the liquid cleaning solution flows into the voids, ensuring that the cleaning solution reaches all parts of the void.

[0072] Since the gas-liquid multiphase fluid of the cleaning solution can be supplied to the cathode 7 together with the cathode gas, the electrolytic reduction reaction of the cathode gas and the salt removal operation by the gas-liquid multiphase fluid can be performed simultaneously. The control device 95 may also control the gas-liquid multiphase fluid supply unit 26 to supply the gas-liquid multiphase fluid to the cathode 7 while a voltage is applied between the cathode 7 and the anode 8. This allows the salt removal operation to be performed without stopping the electrolytic reduction reaction. In this case, the carrier gas is preferably a gas with the same composition as the cathode gas. In another embodiment, the control device 95 may also control the gas-liquid multiphase fluid supply unit 26 to supply the gas-liquid multiphase fluid to the cathode 7 while the voltage application between the cathode 7 and the anode 8 is stopped.

[0073] The control device 95 may supply a gas-liquid multiphase fluid to the cathode 7 based on the difference between the pressure at the cathode inlet 21 and the pressure at the cathode outlet 22. When salt precipitates on the cathode 7, the flow path cross-sectional area of ​​the cathode chamber 3 decreases, and the difference between the pressure at the cathode inlet 21 and the pressure at the cathode outlet 22 increases. Therefore, the state of salt precipitation on the cathode 7 can be estimated based on the difference between the pressure at the cathode inlet 21 and the pressure at the cathode outlet 22. The control device 95 calculates the difference between the pressure at the cathode inlet 21 and the pressure at the cathode outlet 22 based on the pressure at the cathode inlet 21 detected by the pressure sensor P1 and the pressure at the cathode outlet 22 detected by the pressure sensor P2. The control device 95 then controls the third flow control unit 76, the fourth flow control unit 79, the multiphase unit 73, and the third pressure control unit 82 when the difference between the pressure at the cathode inlet 21 and the pressure at the cathode outlet 22 is greater than or equal to a predetermined threshold value, thereby supplying a gas-liquid multiphase fluid from the gas-liquid multiphase fluid supply unit 26 to the cathode gas supply line 31. This ensures that the gas-liquid multiphase fluid of the cleaning solution is supplied when salt deposition is expected on the cathode 7. Conversely, when the amount of salt deposition is expected to be small, the supply of the gas-liquid multiphase fluid of the cleaning solution is stopped.

[0074] Since the gas-liquid multiphase fluid of the cleaning solution is supplied to the cathode gas humidified by the humidification unit 37, evaporation of the gas-liquid multiphase fluid of the cleaning solution in the cathode gas is suppressed. As a result, the cleaning solution can reach the cathode 7 in the form of a gas-liquid multiphase fluid, and salts precipitated inside the cathode 7 can be removed.

[0075] The liquid separated from the cathode-generating gas in the gas-liquid separation unit 42 and the first dehumidification unit 44 is recovered in the recovered liquid storage unit 86. The recovered liquid stored in the recovered liquid storage unit 86 is sent to the washing liquid storage unit 71 or the conductivity adjustment unit 56 based on the conductivity of the recovered liquid. Specifically, the control device 95 controls the fifth flow rate control unit 92 to send the recovered liquid to the washing liquid storage unit 71 when the conductivity of the recovered liquid detected by the conductivity sensor C1 is below a predetermined judgment value. On the other hand, the control device 95 controls the sixth flow rate control unit 93 to send the recovered liquid to the conductivity adjustment unit 56 when the conductivity of the recovered liquid detected by the conductivity sensor C1 is higher than a predetermined judgment value. As a result, if the concentration of impurities in the recovered liquid is relatively low, the recovered liquid is reused as a washing liquid, and if the concentration of impurities in the recovered liquid is relatively high, the recovered liquid is reused as an anode liquid. The liquid separated from the anode-generating gas in the second dehumidification unit 63 is returned to the conductivity adjustment unit 56 via the anode-side return line 89.

[0076] The first and second embodiments of the carbon dioxide electrolytic reduction apparatus 1 are described below. In the first and second embodiments, the electrolytic reduction reaction in the electrolytic cell 2 is continued, and when the differential pressure ratio between the cathode inlet 21 and the cathode outlet 22 is greater than or equal to a predetermined value, the gas-liquid mixed fluid of the cleaning solution is supplied to the cathode 7. Here, the differential pressure ratio between the cathode inlet 21 and the cathode outlet 22 is calculated by the following formula: Differential pressure ratio = (Pressure at cathode inlet [kPa] - Pressure at cathode outlet [kPa]) / Set pressure difference [kPa]

[0077] In the first embodiment, the cleaning solution was water with a conductivity of 2 μS / m, the carrier gas was carbon dioxide gas, and the volume ratio of the carrier gas to the cleaning solution in the gas-liquid multiphase fluid was 2 (gas volume 33.3 vol%). The gas-liquid multiphase fluid was supplied to the cathode 7 when the differential pressure ratio was 5 or more.

[0078] In the second embodiment, the cleaning solution was water with a conductivity of 1 μS / m, the carrier gas was carbon dioxide gas, and the volume ratio of the carrier gas to the cleaning solution in the gas-liquid multiphase fluid was 1000 (gas volume 0.1 vol%). The gas-liquid multiphase fluid was supplied to cathode 7 when the differential pressure ratio was 3 or more.

[0079] Figure 3 is a graph showing the results of the first embodiment, and Figure 4 is a graph showing the results of the second embodiment. As shown in Figures 3 and 4, the differential pressure ratio increases as the electrolytic reduction reaction in the electrolytic cell 2 continues. This is because salt precipitates on the cathode 7 as a result of the electrolytic reduction reaction, and the flow path cross-sectional area of ​​the cathode chamber 3 decreases. When the differential pressure ratio exceeds a predetermined value, a gas-liquid multiphase fluid of the cleaning solution is supplied, and thereafter the differential pressure ratio decreases. From these results, it can be said that the gas-liquid multiphase fluid of the cleaning solution has the effect of lowering the differential pressure ratio between the cathode inlet 21 and the cathode outlet 22, that is, it has the effect of dissolving the salt and restoring the flow path cross-sectional area of ​​the cathode chamber 3.

[0080] In the third example, a comparison was made between cleaning the cathode 7 with a gas-liquid multiphase fluid containing a cleaning solution and a carrier gas, and cleaning the cathode 7 with the cleaning solution alone. The composition of the cathode 7 was the same in both the example and the comparative example, with carbon paper (commercially available) used as the first layer 7A, carbon black (commercially available) as the second layer 7B, and Cu nanoparticles (commercially available) as the catalyst 6. The composition of the gas-liquid multiphase fluid in both the example and the comparative example was as shown in Table 1 below. The cleaning time in both the example and the comparative example was 1 minute.

[0081]

[0082] Figure 5 is a graph showing the differential pressure ratio between the cathode inlet 21 and the cathode outlet 22 in the example and comparative example, with the completion of the first cathode 7 cleaning operation set to 0. As shown in Figure 5, it was confirmed that the differential pressure ratio increased earlier in the comparative example than in the example. In the comparative example, the time required for the differential pressure ratio to reach 10 after the first cleaning was 3.5 hours, while in the example, the time required for the differential pressure ratio to reach 10 after the first cleaning was 6.2 hours. From this, it was confirmed that a higher cleaning effect can be obtained when cleaning is performed using a gas-liquid multiphase fluid compared to when cleaning is performed using only cleaning solution. Furthermore, even though the amount of cleaning solution required for cleaning was the same, a higher cleaning effect was obtained in the example using a gas-liquid multiphase fluid, confirming that the net amount of cleaning solution required for cleaning can be saved in the example.

[0083] This concludes the description of the specific embodiments, but the present invention is not limited to the above embodiments and can be broadly modified and implemented. For example, the humidification unit 37 may be omitted. In this case, the gas-liquid multiphase fluid supply unit 26 may set the flow rate of the gas-liquid multiphase fluid and the volume ratio of the carrier gas to the cleaning liquid in the gas-liquid multiphase fluid, taking into consideration the amount of evaporation of the gas-liquid multiphase fluid in the cathode gas. That is, even if a portion of the gas-liquid multiphase fluid evaporates in the cathode gas, it is preferable to ensure that a sufficient amount of gas-liquid multiphase fluid for salt removal reaches the cathode 7.

[0084] The control device 95 may control the gas-liquid multiphase fluid supply unit 26 based on the differential pressure ratio between the cathode inlet 21 and the cathode outlet 22 to change the volume ratio of the carrier gas to the cleaning liquid and the flow rate in the gas-liquid multiphase fluid. For example, the control device 95 may control the gas-liquid multiphase fluid supply unit 26 so that the flow rate of the gas-liquid multiphase fluid supplied to the cathode gas is a first flow rate when the differential pressure ratio between the cathode inlet 21 and the cathode outlet 22 is less than or equal to a predetermined determination value, and control the gas-liquid multiphase fluid supply unit 26 so that the flow rate of the gas-liquid multiphase fluid supplied to the cathode gas is a second flow rate greater than the first flow rate when the differential pressure ratio is greater than the determination value.

[0085] If the stack includes multiple electrolytic cells 2, salt removal may be performed in some of the electrolytic cells 2, while electrolytic reduction may be performed in the remaining electrolytic cells 2. In this case, electrolytic reduction may be stopped in the electrolytic cells 2 in which the salt removal operation is performed.

[0086] 1: Carbon dioxide electrolytic reduction apparatus 2: Electrolytic cell 3: Cathode chamber 4: Anode chamber 5: Separator 6: Catalyst 7: Cathode 7A: First layer 7B: Second layer 7C: Cathode electrode 8: Anode 20: DC power supply 26: Gas-liquid multiphase fluid supply unit 31: Cathode gas supply line 37: Humidification unit

Claims

1. A method for removing salt deposited on the cathode of an electrolytic cell used in a carbon dioxide reduction reaction, wherein the cathode comprises a cathode electrode having a first layer made of a material containing a conductive substance (A), a catalyst, the first layer having voids, one surface of the first layer being arranged to be in contact with carbon dioxide on the cathode, and a gas-liquid multiphase fluid containing a cleaning solution capable of dissolving the salt and a carrier gas is supplied to the cathode.

2. The method for removing salt according to claim 1, wherein the form of the conductive substance (A) is particulate or fibrous, or at least one of the latter.

3. The method for removing a salt according to claim 1, wherein the conductive substance (A) is a carbon compound.

4. The method for removing salt according to claim 1, wherein the carrier gas is at least one selected from the group consisting of air, carbon dioxide, oxygen, nitrogen, helium, and argon.

5. The method for removing salt according to claim 1, wherein the proportion of the gas in the gas-liquid multiphase fluid is 0.1 vol% or more and 96 vol% or less.

6. The salt removal method according to claim 1, wherein the cathode electrode further comprises a second layer containing a conductive material (B).

7. The method for removing a salt according to claim 6, wherein the conductive substance (B) is carbon compounds or a mixture of carbon compounds and metal compounds.

8. A carbon dioxide electrolytic reduction apparatus comprising: a separator that partitions a cathode chamber and an anode chamber; a cathode provided on the cathode chamber side of the separator and comprising a cathode electrode made of a material containing a catalyst and having gas diffusion function and conductivity; and an anode provided on the anode chamber side of the separator, wherein an anode solution containing an electrolyte is supplied to the anode chamber and a cathode gas is supplied to the cathode chamber; a cathode gas supply line that supplies the cathode gas reduced in the cathode to the cathode chamber; and a gas-liquid multiphase fluid supply unit that supplies a gas-liquid multiphase fluid dispersed in a carrier gas in which a cleaning solution capable of dissolving the salt deposited on the cathode is at least one selected from the group consisting of air, carbon dioxide, oxygen, nitrogen, helium, and argon to the cathode gas supply line.

9. The carbon dioxide electrolytic reduction apparatus according to claim 8, wherein a humidifying section for humidifying the cathode gas is provided upstream of the section to which the gas-liquid multiphase fluid is supplied in the cathode gas supply line.

10. The carbon dioxide electrolytic reduction apparatus according to claim 8, wherein, while a voltage is applied between the cathode and the anode, the gas-liquid multiphase fluid supply unit supplies the gas-liquid multiphase fluid to the cathode gas supply line.

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