Carbon dioxide electrolysis method, method for producing carbon monoxide or ethylene, carbon dioxide reduction electrolyzer, and carbon dioxide reduction electrolysis device

Abstract

To provide a carbon dioxide reduction electrolyzer, a carbon dioxide reduction device, an electrolytic method using the carbon dioxide reduction device, and a method for producing carbon monoxide or ethylene, which involve a small cell voltage and allow the production of a gas containing carbon dioxide reduction products at a high concentration.SOLUTION: A carbon dioxide electrolysis method using an electrolyzer having an anode, a cathode, and a porous diaphragm arranged between the anode and the cathode includes supplying an electrolytic solution from the anode side, supplying the electrolytic solution to the cathode through the porous diaphragm, and applying a voltage between the anode and the cathode thereby reducing carbon dioxide. The electrolytic solution passing the porous diaphragm contains carbon dioxide.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for electrolyzing carbon dioxide, a method for producing carbon monoxide or ethylene, a carbon dioxide reduction electrolytic cell, and a carbon dioxide reduction electrolytic device. [Background technology]

[0002] In order to realize a sustainable society through carbon neutrality, technology is required to convert carbon dioxide into fuels and chemicals for use. Carbon dioxide electrolytic reduction reaction (CO2RR) is a reaction that uses electricity from renewable energy sources to directly electrochemically reduce carbon dioxide and convert it into valuable materials, and is attracting attention as a potential means to achieve carbon neutrality.

[0003] In addition, by using CO2RR, carbon monoxide and ethylene, which are products of the reduction of carbon dioxide, can be obtained by passing electricity through an electrolytic cell. Since the amount of production can be controlled by the amount and duration of electricity passing through, CO2RR can be used as a simple means of obtaining carbon monoxide and ethylene.

[0004] Patent Documents 1 and 2 describe the configuration of a carbon dioxide reduction electrolytic cell in which carbon dioxide gas is supplied to a cathode gas diffusion electrode.

[0005] Non-Patent Document 1 describes the configuration of a carbon dioxide reduction electrolytic cell in which an electrolyte solution containing carbonate ions is supplied to the cathode, and the cathode, porous diaphragm, and anode are joined together using a bipolar membrane as a diaphragm. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2019-56136 A [Patent Document 2] International Publication No. WO2023 / 038116 [Non-patent literature]

[0007] [Non-Patent Document 1] EH Sargent et al, ACS Energy Lett., 4, 1427-1431, (2019) Summary of the Invention [Problem to be solved by the invention]

[0008] In the case of an electrolysis method in which carbon dioxide gas is directly supplied to the cathode as disclosed in Patent Document 1, the carbon dioxide gas dissolves and diffuses into the electrolytic reaction field of the cathode, and the electrolytic reduction reaction of carbon dioxide proceeds. In order to efficiently proceed with the reduction reaction of carbon dioxide at the cathode, it is desirable to supply a large amount of highly concentrated carbon dioxide gas. For this reason, the gas discharged from the electrolytic cell contains the reduction product of carbon dioxide and a large amount of unreacted carbon dioxide, and the concentration of the reduction product is low.

[0009] In the case of an electrolysis method in which an electrolyte solution containing carbonate ions is supplied to the cathode as disclosed in Non-Patent Document 1, when electrolysis is performed using a bipolar membrane, hydrogen ions are generated on the cathode side and hydroxide ions are generated on the anode side. The generated hydrogen ions react with carbonate ions in the electrolyte, and carbon dioxide is generated around the electrolytic reaction field of the cathode, and the electrolytic reduction reaction of carbon dioxide proceeds. Since carbon dioxide is generated around the electrolytic reaction field of the cathode, there is no need to supply excess carbon dioxide gas from the outside. Therefore, the gas discharged from the electrolytic cell contains a high concentration of reduction products of carbon dioxide. On the other hand, the bipolar membrane has a high resistance, which causes a problem of a large cell voltage.

[0010] Therefore, an object of the present invention is to provide a carbon dioxide reduction electrolytic cell, a carbon dioxide reduction device, an electrolysis method using the carbon dioxide reduction device, and a method for producing carbon monoxide or ethylene, which have a small cell voltage and can produce a gas containing a high concentration of a reduction product of carbon dioxide. [Means for solving the problem]

[0011] As a result of intensive research to solve the above problems, the inventors have found that by containing carbon dioxide in the electrolytic solution in the process of supplying the electrolytic solution to the anode and then supplying the electrolytic solution from the anode to the cathode through a porous diaphragm, the electrochemical reduction reaction of carbon dioxide proceeds efficiently at the cathode and the gas discharged from the electrolytic cell contains a high concentration of the reduction product of carbon dioxide, which led to the completion of the present invention.

[0012] That is, the present invention includes the following embodiments. <1> A method for electrolyzing carbon dioxide using an electrolytic cell having an anode, a cathode, and a porous diaphragm disposed between the anode and the cathode, comprising the steps of: Supplying an electrolyte from the anode side; supplying the electrolyte to the cathode through the porous diaphragm; applying a voltage between the anode and the cathode to reduce carbon dioxide; Including, A method for electrolysis of carbon dioxide, wherein the electrolytic solution passing through the porous diaphragm contains carbon dioxide. <2> the electrolyte contains a carbon dioxide precursor; applying a voltage between the anode and the cathode to generate carbon dioxide by an oxidation reaction of the carbon dioxide precursor at the anode; Further comprising: <1> The method for electrolysis of carbon dioxide according to claim 1. <3> The electrolyte contains a carbonate or a bicarbonate, applying a voltage between the anode and the cathode to generate carbon dioxide through a protonation reaction of the carbonate or hydrogen carbonate at the anode; Further comprising: <1> or <2> The method for electrolysis of carbon dioxide according to claim 1. <4> The electrolyte contains a bicarbonate, heating the electrolyte supplied to the anode to generate carbon dioxide through a decomposition reaction of the hydrogen carbonate; Including, <1> ~ <3> 2. The method for electrolysis of carbon dioxide according to claim 1 . <5> In the step of supplying the electrolytic solution, the electrolytic solution contains carbon dioxide. <1> ~ <4> 2. The method for electrolysis of carbon dioxide according to claim 1 . <6> The amount of electrolyte Q (mL / min) passing through the porous membrane from the anode to the cathode is expressed as a function of the cathode unit area S (cm 2 ) satisfies the following formula (1): <1> ~ <5> 13. The electrolysis method according to claim 12, 0.1< Q / S <1000 (1) <7> providing an electrolyte pressure to the anode that is greater than the pressure of the electrolyte effluent from the cathode; <1> ~ <6> 13. The electrolysis method according to claim 12, <8> The carbon dioxide precursor is at least one selected from the group consisting of formate ions, acetate ions, oxalate ions, and glycolate ions. <2> The electrolysis method according to claim 1. <9> Carbon monoxide or ethylene is produced by reducing carbon dioxide at the cathode. <1> ~ <8> 13. The electrolysis method according to claim 12, <10> A method for producing carbon monoxide or ethylene using an electrolytic cell having an anode, a cathode, and a porous diaphragm disposed between the anode and the cathode, comprising: Supplying an electrolyte from the anode side; supplying the electrolyte to the cathode through the porous diaphragm; applying a voltage between the anode and the cathode and reducing carbon dioxide at the cathode to produce carbon monoxide or ethylene; Including, The method for producing carbon monoxide or ethylene, wherein the electrolytic solution passing through the porous diaphragm contains carbon dioxide. <11> A carbon dioxide reduction electrolytic cell for electrochemically reducing carbon dioxide, comprising: a supply flow path for supplying an electrolyte from the anode side; An anode where the oxidation reaction proceeds when a voltage is applied, A cathode that reduces carbon dioxide by applying a voltage; a porous diaphragm disposed between the anode and the cathode; the supply flow path supplies the electrolytic solution containing carbon dioxide to the cathode by passing the electrolytic solution through the porous diaphragm. <12> The electrolytic solution contains at least one carbon dioxide precursor selected from the group consisting of formate ions, acetate ions, oxalate ions, and glycolate ions. <11> The carbon dioxide reduction electrolysis cell according to claim 1. <13> The permeability of the porous membrane is 5 mL / (min cm 2 )~300mL / (min cm 2 ), <12> The carbon dioxide reduction electrolysis cell according to claim 1. <14> The electrolyte contains at least one selected from a bicarbonate ion and a carbonate ion. <11> The carbon dioxide reduction electrolysis cell according to claim 1. <15> The permeability of the porous membrane is 1 mL / (min cm 2 )~50mL / (min cm 2 ), <14> The carbon dioxide reduction electrolysis cell according to claim 1. <16> The drying shrinkage rate of the porous membrane is 2% or less. <11> ~ <15> 3. The carbon dioxide reduction electrolytic cell according to claim 1 . <17> <11> ~ <16> A carbon dioxide reduction electrolysis device comprising the carbon dioxide reduction electrolysis cell according to any one of claims 1 to 5. <18> an electrolyte supply device for supplying the electrolyte containing carbon dioxide to the cathode by passing the electrolyte through the porous diaphragm; <17> The carbon dioxide reduction electrolysis device according to claim 1. <19> A gas-liquid separator that separates a gas containing a reduction product of carbon dioxide discharged from the cathode from an electrolytic solution; <17> or <18> The carbon dioxide reduction electrolysis device according to claim 1. Effect of the Invention

[0013] According to the present invention, it is possible to provide a carbon dioxide reduction electrolytic cell, a carbon dioxide reduction device, an electrolysis method using the carbon dioxide reduction device, and a method for producing carbon monoxide or ethylene, which have a small cell voltage and can produce a gas containing a high concentration of a reduction product of carbon dioxide. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a schematic diagram of an example of a carbon dioxide reduction electrolytic cell according to the present embodiment. [Diagram 2] FIG. 2 is a schematic diagram of an example of a carbon dioxide reduction electrolytic cell according to the present embodiment. [Diagram 3] 1A and 1B are diagrams showing an example of an anode current collector plate / anode flow path plate and an example of a cathode current collector plate / cathode flow path plate. [Figure 4] 1A and 1B are diagrams showing an example of an anode current collector plate / anode flow path plate and an example of a cathode current collector plate / cathode flow path plate. [Diagram 5] FIG. 1 is a schematic diagram of an example of a carbon dioxide reduction electrolysis device according to an embodiment of the present invention. [Figure 6] FIG. 1 is a schematic diagram of an example of a carbon dioxide reduction electrolysis device according to an embodiment of the present invention. [Figure 7] FIG. 2 is a schematic diagram of a carbon dioxide reduction electrolytic cell of a comparative example. [Figure 8] FIG. 1 is a diagram showing a cell stack in which a plurality of unit cells are connected together, as an example of an electrolytic cell. [Figure 9] FIG. 1 is a diagram showing a bipolar electrode type cell stack as an example of an electrolytic cell. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, an embodiment for carrying out the present invention (hereinafter, simply referred to as "the present embodiment") will be described in detail. The present embodiment is an example for explaining the present invention, and the present invention is not limited to the embodiment. In other words, the present invention can be modified in various ways without departing from the gist of the present invention.

[0016] [Carbon dioxide reduction electrolysis cell] The carbon dioxide reduction electrolytic cell of this embodiment (indicated by symbols A and B in the drawings) has an inlet 12 for supplying an electrolytic solution to the anode 3, the anode 3 in which an oxidation reaction proceeds by applying a voltage, a cathode 7 for reducing carbon dioxide, a porous diaphragm (sometimes referred to as a "hydrophilic porous membrane" in the drawings) 5 arranged between the anode and the cathode and for passing an electrolytic solution containing carbon dioxide from the anode 3 to the cathode 7, and an outlet 13 for discharging the electrolytic solution supplied from the anode 3 to the cathode 7 through the pores of the porous diaphragm 5 to the outside of the cell (see FIG. 1, FIG. 2, FIG. 5, FIG. 6, etc.). The carbon dioxide reduction electrolytic cell of this embodiment electrochemically reduces carbon dioxide. In addition, as shown in FIG. 2, the carbon dioxide reduction electrolytic cell of this embodiment may have an outlet 11 for discharging bubbles from the anode and an outlet 14 for discharging gas components from the cathode. It should be noted that in Figs. 1, 2, etc., in order to clearly show the internal structures of the carbon dioxide reduction electrolytic cells A and B, cross sections of the internal anode 3, cathode 7, porous diaphragm gasket 6, etc. are shown using curved broken lines as appropriate.

[0017] In the carbon dioxide reduction electrolytic cell of this embodiment, at least a part of the electrolytic solution supplied to the anode 3 from the anode electrolyte flow path 40 passes through the pores of the porous diaphragm 5 and reaches the cathode 7. The electrolytic solution passing through the pores of the porous diaphragm 5 preferably contains carbon dioxide exceeding the amount of carbon dioxide that is saturated under the partial pressure of carbon dioxide in the atmosphere. At this time, the mixed state of the electrolytic solution and the carbon dioxide may be a state in which the carbon dioxide is dissolved in the electrolytic solution, or a state in which carbon dioxide bubbles are mixed with the electrolytic solution. The carbon dioxide that reaches the cathode 7 in this way is reduced at the cathode 7 to generate a reduction product of carbon dioxide. The electrolytic solution in which carbon dioxide is consumed at the cathode 7 and the carbon dioxide concentration is reduced is discharged to the outside of the electrolytic cell through the cathode electrolyte flow path 30. In the carbon dioxide reduction electrolytic cell of this embodiment, the carbon dioxide reduction reaction proceeds without supplying an excessive amount of carbon dioxide from the outside of the electrolytic cell, so that the gas discharged from the electrolytic cell contains a high concentration of the reduction product of carbon dioxide.

[0018] In this specification, the term "carbon dioxide reduction electrolytic cell" (or sometimes referred to as "electrolytic cell") is used in the following sense. That is, an "electrolytic cell" is classified into a single cell (denoted by reference symbol 100C in FIG. 8 and other figures) and a cell stack (denoted by reference symbol 100S in FIG. 8 and other figures), and in other words, the "electrolytic cell" in this specification includes both the single cell 100C and the cell stack 100S. A "single cell" refers to an electrolytic cell consisting of a pair of a cathode 7 and an anode 3, and a "cell stack" refers to an electrolytic cell in which a plurality of single cells 100C are connected together. The specific form for connecting the unit cells 100C in the cell stack 100S is not limited, and may be, for example, a form in which the unit cells 100C are electrically connected by conductive wires 102 (see FIG. 8), or a form in which two current collectors of adjacent unit cells 100C (for example, the anode current collector and anode flow path plate 2 and the cathode current collector and cathode flow path plate 9 in the unit cell 100C shown in FIG. 8) are connected together (see FIG. 8). When the current collectors of adjacent unit cells 100C (the anode current collector and anode flow path plate 2 and the cathode current collector and cathode flow path plate 9) are connected together as in the latter case, a bipolar electrode 128 may be used (see FIG. 9). Incidentally, this embodiment is basically described assuming the use of a single cell 100C, but given the characteristics of the carbon dioxide reduction electrolytic cell according to the present invention as will be understood below, it is clear that the scope of application is not limited to either the single cell 100C or the cell stack 100S.

[0019] (anode) An oxidation reaction proceeds when a voltage is applied to the anode 3. The type of oxidation reaction is not particularly limited, and examples include a reaction in which water in the electrolyte or anions such as hydroxide ions, hydrogen carbonate ions, and carbonate ions are oxidized to generate oxygen (oxygen generation reaction), an oxidation reaction of halide ions such as chloride ions, an oxidation reaction of metal cations such as iron ions and vanadium ions, an oxidation reaction of organic acids such as formic acid and acetic acid, and an oxidation reaction of organic substances such as ethanol.

[0020] When the electrolytic solution supplied to the anode 3 contains at least one selected from the group consisting of a carbon dioxide precursor, a carbonate, and a hydrogencarbonate, a reaction accompanied by the generation of carbon dioxide proceeds when the electrolytic solution comes into contact with the anode 3 while a voltage is applied to the anode 3. In this case, even if carbon dioxide is not added to the electrolytic solution supplied to the anode 3 in advance, carbon dioxide can be added to the electrolytic solution during the process of reaching the cathode 7 from the anode 3 through the porous diaphragm 5.

[0021] The reaction accompanied by the generation of carbon dioxide may be, for example, an oxidation reaction in which carbon dioxide is generated as a product by the oxidation of an organic substance, or a protonation reaction in which hydrogen ions are generated by an oxidation reaction in an electrolyte containing carbonate ions or bicarbonate ions, and the pH of the electrolyte is lowered to generate carbon dioxide derived from carbonate ions or bicarbonate ions. The reaction in which hydrogen ions are generated as a product can also be described as a reaction in which hydroxide ions are consumed. Examples of reactions in which carbon dioxide is generated as a product by the oxidation of an organic substance include reactions in which organic acids such as formic acid, acetic acid, and oxalic acid, and organic substances such as methanol, formaldehyde, ethanol, and acetaldehyde are oxidized to carbon dioxide, and decarboxylation reactions from carboxylic acids. Examples of oxidation reactions in which hydrogen ions are generated as a product include a reaction in which water in the electrolyte, or anions such as hydroxide ions, bicarbonate ions, and carbonate ions are oxidized to generate oxygen (oxygen generation reaction), a reaction in which alcohols are oxidized to aldehydes, ketones, and carboxylic acids, a reaction in which amines are oxidized to nitriles, and a reaction in which thiols are oxidized to disulfides.

[0022] The anode 3 is connected to an electrolyte supply device (e.g., electrolyte tank 15) via the inlet 12. This forms a supply flow path for supplying electrolyte from the anode 3 side (see FIG. 5). The anode 3 and the porous diaphragm 5 do not necessarily need to be in contact with each other, but from the viewpoint of reducing ohmic resistance, it is preferable that the anode 3 be in contact with the porous diaphragm 5. In addition, the anode 3 is electrically connected to a power source 28 outside the electrolytic cell via a current collector (e.g., the anode current collector plate / anode flow path plate 2).

[0023] The anode 3 may be configured to discharge gas bubbles and the electrolyte generated at the anode 3 through the outlet 11 (see FIG. 2, etc.). If an excessive amount of gas bubbles remain between the anode 3 and the porous diaphragm 5, the electrolyte becomes difficult to flow from the anode 3 to the cathode 7 through the pores of the porous diaphragm 5. If the electrolyte becomes difficult to supply to the cathode 7, the amount of carbon dioxide supplied to the cathode 7 decreases, and the carbon dioxide reduction reaction becomes difficult to proceed efficiently. In particular, when gases other than carbon dioxide, such as oxygen and chlorine, are generated by the oxidation reaction proceeding at the anode 3, gas bubbles are likely to remain at the anode 3. In this case, if an outlet 11 for discharging the gas bubbles is provided from the anode 3 side, gas bubbles are unlikely to remain in the electrolytic cell, and the carbon dioxide reduction reaction can proceed efficiently at the cathode 7.

[0024] The method of connecting the inlet 12 and outlet 11 to the anode 3 is not particularly limited. The anode 3 may be made of a porous body, and the inlet 12 and outlet 11 may be arranged so that the electrolyte passes only through the structure of the anode 3. Alternatively, the anode 3 may be connected to the inlet 12 and outlet 11 via an anode flow plate (for example, an anode current collector plate / anode flow plate 2) (see FIG. 1, FIG. 2, etc.). When an anode flow plate is used, the arrangement method is not particularly limited. For example, the anode 3 may be arranged so that a first surface of the anode 3 contacts the porous diaphragm 5 and a second surface of the anode 3 located on the back side of the first surface contacts the flow plate. The width and depth of the electrolyte flow path formed in the electrolyte flow path plate and the folding back of the flow path are not particularly limited. Known examples of flow path plate configurations such as parallel flow paths, spiral flow paths, and serpentine flow paths can be used (see FIG. 3, FIG. 4, etc.).

[0025] The anode 3 is preferably in the form of a porous body from the viewpoint of increasing the contact area with the electrolyte and improving the reaction efficiency, for example, in the form of a mesh, a microporous plate, a sponge, a nonwoven fabric, or the like.

[0026] The material of the anode 3 is not particularly limited, and examples thereof include metal materials such as titanium, nickel, and SUS, conductive materials such as carbon materials, and resin materials coated with conductive materials. The anode 3 may contain an anode catalyst that reduces overvoltage depending on the reaction that proceeds in the anode 3. Examples of the anode catalyst include platinum, palladium, rhodium, iridium, and ruthenium. The anode 3 may be composed of only the anode catalyst, or may be supported on a substrate made of a conductive material such as titanium or nickel. When the anode catalyst is supported on the substrate, the catalyst may be supported uniformly on the entire substrate, or may be supported on only one surface of the substrate so that a catalyst layer is formed between the substrate and the porous membrane. In addition, a binder may be mixed with the anode catalyst and supported on the substrate so that the anode catalyst does not easily peel off from the substrate.

[0027] The binder is not particularly limited, and examples of the binder that can be used include polymer compounds such as PTFE and PVDF, cation exchange resins such as fluororesins having sulfonic acid groups (e.g., Nafion (trademark)), and anion exchange resins such as hydrocarbon resins having nitrogen-based cations (e.g., Sustainion (trademark)).

[0028] (Cathode) The cathode 7 reduces carbon dioxide to produce carbon monoxide, formic acid, acetic acid, methane, ethane, ethylene, methanol, ethanol, propanol, and the like. As shown in FIG. 1, the cathode 7 is electrically connected to a power source 28 outside the electrolytic cell via a current collector (for example, a cathode current collector plate / cathode flow path plate 9). The cathode 7 and the porous diaphragm 5 do not necessarily need to be in contact with each other, but from the viewpoint of reducing ohmic resistance, it is preferable that the cathode 7 be in contact with the hydrophilic porous diaphragm 5. In addition, when the cathode 7 is in contact with the porous diaphragm 5, the porous diaphragm 5 can be well supported by the cathode 7 when stress is applied to the porous diaphragm 5 due to the pressure of the electrolyte in the anode 3.

[0029] The reduction reaction of carbon dioxide in the cathode 7 has a reduction potential close to that of the reduction reaction of water (hydrogen generation reaction), and the hydrogen generation reaction is a major side reaction. In order to preferentially advance the reduction reaction of carbon dioxide, it is effective to efficiently supply carbon dioxide to the cathode 7.

[0030] In the carbon dioxide reduction electrolytic cell of the present embodiment, the electrolytic solution containing carbon dioxide passes from the anode 3 through the pores of the porous diaphragm 5 to reach the cathode 7, thereby supplying an electrolytic solution with a high carbon dioxide concentration to the cathode 7, and the carbon dioxide reduction reaction proceeds smoothly. Note that the carbon dioxide supplied to the cathode 7 from the anode 3 through the porous diaphragm 5 does not necessarily need to be dissolved in the electrolytic solution, and may be in a gaseous state such as bubbles.

[0031] The material of the cathode 7 is not particularly limited as long as it contains at least a part of a conductive substance, and may be a metal material such as titanium, nickel, or SUS, a conductive substance such as a carbon material, or a resin material coated with a conductive substance. However, it is preferable that at least the part in contact with the electrolyte is made of a material that has a low ability to promote the reduction reaction of water. Examples of materials that have a low ability to promote the reduction reaction of water include metal materials such as gold, silver, copper, zinc, and titanium, and carbon materials.

[0032] It is preferable to use a porous electrode for the cathode 7 from the viewpoint of efficiently discharging the gas generated by electrolysis and the electrolyte solution with a reduced carbon dioxide concentration as a result of carbon dioxide being consumed by electrolysis from the cathode 7 to the cathode electrolyte flow path 30. Examples of the porous electrode include a sintered metal powder, a metal mesh, a metal felt, carbon paper, carbon felt, and carbon cloth.

[0033] The cathode 7 preferably contains a carbon dioxide reduction catalyst in order to lower the overvoltage of the carbon dioxide reduction reaction and to cause the carbon dioxide reduction reaction to proceed preferentially over the hydrogen evolution reaction.

[0034] Examples of carbon dioxide reduction catalysts include materials containing metals such as gold, silver, copper, zinc, lead, indium, tin, and cadmium; metal complexes in which metals such as iron and cobalt are coordinated to phthalocyanine, porphyrin, or the like; and materials in which metals such as iron, cobalt, and nickel are incorporated into carbon.

[0035] The carbon dioxide reduction catalyst can control the composition of the carbon dioxide reduction product by the composition, structure, etc. of the catalyst material. For example, when a catalyst containing gold, silver, or zinc is used, carbon monoxide is generally selectively generated. When a catalyst containing lead, indium, tin, or cadmium is used, formic acid is generally selectively generated. When a catalyst containing copper is used, products containing one or more of carbon monoxide, formic acid, acetic acid, methane, ethane, ethylene, methanol, ethanol, and propanol are generally generated. However, the relationship between the catalyst and the product selectivity is not limited to these, and a carbon dioxide reduction product can be obtained by using a known carbon dioxide reduction catalyst according to the product selectivity specific to each catalyst.

[0036] When a porous electrode is used as the cathode 7, the method for producing the porous electrode containing the carbon dioxide reduction catalyst is not particularly limited, and the carbon dioxide reduction catalyst may be formed into a porous electrode by forming the carbon dioxide reduction catalyst into a porous shape, or the carbon dioxide reduction catalyst may be formed into a porous electrode by fusing carbon dioxide reduction catalyst particles together through heat treatment or the like. Also, a porous electrode containing the carbon dioxide reduction catalyst may be formed by modifying a porous electrode substrate with the carbon dioxide reduction catalyst.

[0037] The method for combining the carbon dioxide reduction catalyst with the cathode 7 is not particularly limited, and the entire surface of a porous electrode substrate such as a metal mesh or carbon paper may be modified with the carbon dioxide reduction catalyst, or one surface of the porous electrode substrate may be intensively modified to form a catalyst layer. When intensively modifying one surface of the porous electrode substrate, it is preferable to intensively modify the portion that comes into contact with the electrolyte supplied from the porous diaphragm 5, from the viewpoint of ensuring that the electrolyte with a high carbon dioxide concentration supplied from the porous diaphragm 5 is sufficiently distributed throughout the catalyst layer.

[0038] When modifying a porous electrode substrate with a carbon dioxide reduction catalyst, a catalyst particle layer can be formed by coating the porous electrode substrate with a paint containing carbon dioxide reduction catalyst particles. In addition, a catalyst particle layer in which carbon dioxide reduction catalyst particles are fused together by performing a heat treatment or the like after coating can also be used. In addition, a paint containing a mixture of a catalyst and a binder can be coated on the porous electrode substrate so that the carbon dioxide reduction catalyst does not easily peel off from the porous electrode substrate. There are no particular limitations on the binder, and examples of the binder that can be used include polymer compounds such as PTFE and PVDF, cation exchange resins such as fluororesins having sulfonic acid groups (e.g., Nafion (trademark)), and anion exchange resins such as hydrocarbon resins having nitrogen-based cations (e.g., Sustainion (trademark)).

[0039] The cathode 7 is connected to the cathode electrolyte flow path 30 via the outlet 13. The electrolyte supplied from the anode 3 to the cathode 7 via the porous diaphragm 5 is sent from the outlet 13 to the cathode electrolyte flow path 30 and discharged outside the cell (see Figs. 1, 5, etc.). The reduction product of carbon dioxide generated at the cathode 7 may be discharged from the outlet 13 to the cathode electrolyte flow path 30, or an outlet 14 may be provided for discharging only the reduction product (see Fig. 2, etc.). For example, when the reduction product of carbon dioxide generated at the cathode 7 is a gas, the outlet 14 may be provided at an upper portion of the outlet 13 so that the gas component is discharged from the outlet 14 and the liquid component including the electrolyte is discharged from the outlet 13.

[0040] The outlet 13, the outlet 14, and the cathode 7 may be connected such that the outlet 13 and the outlet 14 are arranged so that the electrolyte passes only through the structure of the cathode 7 made of a porous body, or the cathode 7 and the outlet 13 may be connected via a flow path plate (for example, a cathode current collector plate / cathode flow path plate 9) (see Figs. 1, 2, etc.). When a flow path plate is used, the arrangement method is not particularly limited, but for example, the cathode 7 can be arranged so that a first surface of the cathode 7 contacts the porous diaphragm 5 and a second surface of the cathode 7 located on the back side of the first surface contacts the cathode flow path plate. The width and depth of the electrolyte flow path formed in the cathode flow path plate and the folding back method of the flow path are not particularly limited, and known examples of flow path plate configurations such as parallel flow paths, spiral flow paths, and serpentine flow path types can be used (see Figs. 3, 4, etc.).

[0041] (Current collector) The current collecting part electrically connects the electrolytic cell to a power source 28 outside the electrolytic cell. The method of connecting the cathode 7 or anode 3 (hereinafter also referred to as the electrode) to the current collecting part is not particularly limited, and the electrode itself may be processed so as to be directly connected to the power source 28 outside the electrolytic cell to serve as the current collecting part, or the flow path plate may be made of a conductive material to serve as the current collecting part, and the electrode may be connected to the power source 28 outside the electrolytic cell via the flow path plate. Alternatively, a current collecting plate made of a conductive material may be disposed so as to be in contact with the electrode or a conductive flow path plate to serve as the current collecting part, and the electrode may be connected to the power source outside the electrolytic cell via the current collecting plate or the flow path plate. The current collecting plate may have holes through which gas or electrolyte can pass.

[0042] In order to sufficiently fix the cathode 7, the porous diaphragm 5, the anode 3, the flow path plate, the current collector plate, etc., the end plates 1, 10 may be disposed at the outermost part of the electrolytic cell (see Figs. 1, 2, etc.). The end plates 1, 10 preferably have insulating properties at least on their surfaces to prevent the electrolytic cell from being exposed as a conductive material and causing a current leak. The end plates 1, 10 may be provided with an outlet 11, an inlet 12, an outlet 13, an outlet 14, etc. Also, holes may be provided for fastening the electrolytic cell with bolts or the like so that the cathode 7, the porous diaphragm 5, and the anode 3 are sufficiently joined.

[0043] The distance between the cathode 7 and the anode 3 is preferably 1 μm to 1000 μm, more preferably 5 μm to 500 μm, more preferably 10 μm to 300 μm, and more preferably 15 μm to 200 μm. When the distance between the cathode 7 and the anode 3 is within the above-mentioned upper limit value or less, the cell voltage can be reduced. The distance between the cathode 7 and the anode 3 can be controlled by the distance between the cathode 7 and the porous diaphragm 5 (for example, 0 when they are in contact), the distance between the anode 3 and the porous diaphragm 5 (for example, 0 when they are in contact), and the film thickness of the porous diaphragm 5.

[0044] (Porous diaphragm) The porous diaphragm 5 has insulating properties and electrically separates the cathode 7 from the anode 3. The first surface 51 of the porous diaphragm 5 does not necessarily have to be in contact with the cathode 7, and the second surface 52 does not necessarily have to be in contact with the anode 3. However, from the viewpoint of minimizing ohmic resistance and supporting the membrane to which a force is applied due to the difference between the cathode 7 and the anode 3, it is preferable to arrange the porous diaphragm 5 so that the first surface 51 is in contact with the cathode 7 and the second surface 52 is in contact with the anode 3. (See FIG. 1, FIG. 2, etc.) When the first surface 51 of the porous diaphragm 5 is arranged so as to be in contact with the anode, it is sufficient that a part of the first surface 51 is in contact with the cathode 7, and when the second surface 52 is arranged so as to be in contact with the anode, a part of the second surface 52 may be in contact with the anode 3. The porous diaphragm 5 is composed of a porous structure having pores, and can hold an electrolyte in the membrane structure and function as an ion conductor. In addition, the electrolyte passes through the pores of the porous diaphragm 5, allowing the electrolyte to move from the anode 3 to the cathode 7.

[0045] The porous diaphragm 5 is preferably hydrophilic from the viewpoints of favorably retaining the electrolyte supplied from the anode electrolyte flow path 40 and sufficiently contacting the electrolyte with the cathode 7 and the electrolyte with the anode 3. The porous diaphragm 5 only needs to have hydrophilic surfaces in the pores, and may be, for example, a porous membrane made of a hydrophobic material having hydrophilic surfaces in the pores.

[0046] The hydrophilicity of the porous diaphragm means that the contact angle θ with pure water is 140 degrees or less. The contact angle of the porous diaphragm with water on the surface of the porous diaphragm is within the above range, regardless of the contact angle of the material itself that constitutes the porous diaphragm with water, so that the electrolyte can be sufficiently distributed over the porous diaphragm. The contact angle θ can be measured according to the sessile drop method of JIS R3257:1999, by reading the angle formed by the water droplet and the porous diaphragm 1 second after the pure water is dropped. Even if the porous diaphragm surface absorbs the water droplet before the above-mentioned angle is read, making it difficult to measure the contact angle, this means that the porous diaphragm surface is very hydrophilic, so the contact angle of the porous diaphragm surface can be considered to be 140 degrees or less. The contact angle θ may be a value measured on the same or equivalent porous diaphragm as that incorporated in the electrolytic cell.

[0047] From the viewpoint of reducing the distance between the cathode 7 and the anode 3 and reducing the ohmic resistance, the porous diaphragm 5 has a thickness of preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 150 μm or less. Moreover, from the viewpoint of preventing the membrane from breaking, the membrane thickness is preferably 5 μm or more, and more preferably 10 μm or more.

[0048] In order to ensure that the carbon dioxide reduction reaction proceeds satisfactorily at the cathode 7, the porous diaphragm 5 preferably allows the electrolyte containing carbon dioxide generated at the anode 3 to pass through the porous diaphragm 5 to the cathode 7 satisfactorily. If a porous diaphragm that allows electrolyte to pass easily is used, the electrolyte can be efficiently supplied from the anode 3 to the cathode 7 without applying a large pressure between the anode 3 and the cathode 7. From this viewpoint, the water permeability of the porous diaphragm 5 is preferably 0.1 mL / (min cm 2 ) or more, and the water permeability is preferably 1.0 mL / (min cm 2 ) or more, and more preferably 5.0 mL / (min cm 2 ) or more. Note that the water permeability is the amount of pure water passing through a porous membrane per minute under a differential pressure of 69.0 kPa divided by the area of ​​the porous membrane.

[0049] The pore diameter d of the porous membrane 5 is preferably 0.01 μm or more, more preferably 0.1 μm or more, and even more preferably 0.2 μm or more. When the pore diameter d is within the above range, the electrolyte containing carbon dioxide generated at the anode 3 is more easily transferred through the porous membrane 5 to the cathode 7. In addition, the pore diameter d of the porous membrane 5 is preferably 1000 μm or less, more preferably 100 μm or less, even more preferably 50 μm or less, and even more preferably 10 μm or less. When the pore diameter d is within the above range, even if the catalyst or conductive material constituting the electrode at the anode 3 is peeled off, it does not pass through the porous membrane 5, and a short circuit can be prevented. The pore diameter d of the porous membrane can be measured according to the ASTM F838 bacteria challenge test.

[0050] The material of the porous diaphragm 5 is not particularly limited, but it is preferable that the strength is sufficient when wet with the electrolyte in order to prevent the membrane from breaking during assembly or operation of the electrolytic cell. Examples of such materials include polymeric materials such as polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, nylon, polyethersulfone, cellulose acetate, cellulose mixed ester, regenerated cellulose, and polycarbonate, as well as ceramic filters and glass fiber filters made of metal oxides such as silica and alumina. Among the above-mentioned materials, the porous diaphragm preferably contains at least one material selected from the group consisting of polyethersulfone, nylon, polytetrafluoroethylene, polyvinylidene fluoride, and cellulose acetate.

[0051] It is preferable that the porous membrane 5 has a property that the pores are difficult to deform. When the electrolyte passes through the pores, stress is applied to the pores due to the action of surface tension and the like. The shape of the pores is deformed by the stress, which prevents the pore diameter from becoming large locally and prevents large-diameter bubbles composed of carbon dioxide and the like generated at the anode 3 from being captured in the pores, making it easier for the electrolyte to pass through the pores.

[0052] The porous membrane 5 can be produced by a method such as a stretching method, a phase separation method, or an etching method, but from the viewpoint of having pores that are difficult to deform, it is preferable to use one produced by a phase separation method or an etching method. Moreover, from the viewpoint of increasing the porosity and promoting ion conductivity, it is more preferable to use one produced by a phase separation method.

[0053] The resistance of the pores of the porous membrane 5 to deformation can be determined, for example, by the shrinkage rate when the porous membrane is immersed in water and dried (this shrinkage rate may be referred to as the drying shrinkage rate). The drying shrinkage rate α [%] is expressed by the following formula using the diameter L2 [cm] measured after a circular cut-out of the porous membrane with a diameter L1 [cm] is immersed in water for 1 hour, the porous membrane is taken out of the water and dried in a dryer at 40 degrees for 24 hours, and the moisture is removed. α=100×(L1-L2) / L1 However, when the porous membrane shrinks into an ellipse due to drying, L2 is the length of the minor axis. The drying shrinkage rate α is preferably 10% or less, more preferably 5% or less, more preferably 2% or less, more preferably 1% or less, more preferably 0.5% or less, and more preferably 0.1% or less. When the drying shrinkage rate α is within the above range, the pores of the porous membrane 5 are less likely to deform, and it becomes possible to supply the electrolyte solution containing carbon dioxide to the cathode 7 well for a long period of time, and carbon dioxide can be efficiently reduced.

[0054] In the case of using a bipolar membrane as a diaphragm instead of a porous diaphragm as in the configuration disclosed in Non-Patent Document 1 as a conventional technique, the resistance is likely to increase due to the low ionic conductivity of the bipolar membrane. On the other hand, in the case of using a porous diaphragm and performing electrolysis while moving the electrolyte from the anode to the cathode as in the present embodiment, the porous diaphragm 5 containing the electrolyte with high electrical conductivity can be used as an ion conductor, so that the cell voltage is unlikely to increase.

[0055] (electrolyte) The electrolytic solution functions as an ion conductor between the cathode 7 and the anode 3. To achieve good ion conduction, the electrolytic solution preferably contains an electrolyte.

[0056] The electrolyte is not particularly limited, but preferably contains at least one of cations such as hydrogen ions, lithium ions, sodium ions, potassium ions, ammonium ions, imidazolium ions, pyridinium ions, and quaternary alkyl ammonium ions, and at least one of anions such as hydroxide ions, fluoride ions, chloride ions, bromide ions, iodide ions, nitrate ions, perchlorate ions, formate ions, acetate ions, bicarbonate ions, carbonate ions, dihydrogen phosphate ions, monohydrogen phosphate ions, phosphate ions, sulfate ions, oxalate ions, tartrate ions, citrate ions, borate ions, tetrafluoroborate ions, hexafluorophosphate ions, and trifluoromethanesulfonate ions. The electrolyte does not necessarily have to be composed of one type each of cations and anions, and may be a mixture of multiple components.

[0057] From the viewpoint of reducing ohmic resistance, the electrolyte concentration in the electrolytic solution is preferably such that the sum of the concentration of cations multiplied by the charge and the sum of the concentration of anions multiplied by the charge are each 0.001 M or more, more preferably 0.01 M or more, and even more preferably 0.1 M or more.

[0058] In order for the electrolytic solution that reaches the cathode 7 to contain carbon dioxide, the electrolytic solution preferably contains at least one of carbon dioxide, a carbon dioxide precursor, a carbonate, and a hydrogen carbonate when it comes into contact with the anode 3.

[0059] When the electrolytic solution contains carbon dioxide, the carbon dioxide may be dissolved in the electrolytic solution, or the carbon dioxide bubbles may be mixed in the electrolytic solution. For example, the carbon dioxide supply flow path 48 is connected to the electrolytic solution tank 15, and a gas containing carbon dioxide is blown into the electrolytic solution, or the gas containing carbon dioxide is placed in gas-liquid equilibrium, whereby carbon dioxide can be added to the electrolytic solution. The carbon dioxide contained in the electrolytic solution passes through the pores of the porous diaphragm 5 and contacts the cathode 7, whereby the carbon dioxide can be reduced. In this case, from the viewpoint of efficiently adding carbon dioxide to the electrolytic solution, the carbon dioxide concentration in the gas is preferably 5% or more, and more preferably 10% or more. The partial pressure of carbon dioxide in the gas is preferably 0.05 atm or more, more preferably 0.1 atm or more, more preferably 0.3 atm or more, and more preferably 0.5 atm or more.

[0060] The carbon dioxide precursor is a substance for promoting an oxidation reaction accompanied by the generation of carbon dioxide. When the carbon dioxide precursor is contained in the electrolytic solution, the carbon dioxide precursor is oxidized at the anode 3 to generate carbon dioxide. When this carbon dioxide contained in the electrolytic solution passes through the pores of the porous membrane 5 and comes into contact with the cathode 7, the carbon dioxide can be reduced.

[0061] Examples of the carbon dioxide precursor include organic acids such as formic acid, acetic acid, oxalic acid, glycolic acid, and glyoxylic acid, or salts thereof, alcohols such as methanol, ethanol, and ethylene glycol, and carbonyl compounds such as formaldehyde, acetaldehyde, and glyoxal. Examples of the salts of organic acids include ions of the above-mentioned organic acids, that is, formate ions, acetate ions, oxalic acid ions, glycolic acid ions, and glyoxylic acid ions. From the viewpoint of easily progressing the oxidation reaction and efficiently generating carbon dioxide, at least one selected from organic acids such as formic acid, acetic acid, oxalic acid, glycolic acid, and glyoxylic acid, or salts thereof, methanol, ethanol, ethylene glycol, formaldehyde, acetaldehyde, and glyoxal is preferred, and at least one selected from formate ions, acetate ions, oxalic acid ions, and glycolic acid ions is preferred.

[0062] The hydrogen carbonate contains hydrogen carbonate ions. The carbonate contains carbonate ions. When protons are generated by the progress of the oxidation reaction at the anode 3, the hydrogen carbonate and carbonate are protonated and converted into carbon dioxide. The hydrogen carbonate is decomposed into carbon dioxide and carbonate by heating. The hydrogen carbonate can be decomposed into carbon dioxide and carbonate by heating the electrolyte solution containing the hydrogen carbonate to 40° C. or higher, more preferably 50° C. or higher, more preferably 60° C. or higher. In this way, when the electrolyte solution contains hydrogen carbonate and carbonate, carbon dioxide is generated by applying a voltage to the anode 3 or by heating the electrolytic cell. This carbon dioxide, contained in the electrolyte solution, passes through the pores of the porous diaphragm 5 and comes into contact with the cathode 7, whereby the carbon dioxide can be reduced. Note that the heating is performed as long as the decomposition reaction occurs before the electrolyte solution reaches the cathode, so the cell may be heated, the electrolyte piping upstream of the cell may be heated, or the electrolyte tank may be heated.

[0063] The present inventors have come up with the idea that it is effective to increase the carbon dioxide concentration in the ion conductor (e.g., electrolyte or ion exchange resin) near the surface of the cathode 7 in order to efficiently advance the carbon dioxide reduction reaction at the cathode 7 and suppress the progress of the hydrogen generation reaction, which is a side reaction. As described in Patent Documents 1 and 2, when a gas diffusion electrode is used for the cathode and gaseous carbon dioxide is supplied to an electrolytic cell, the dissolution and diffusion rate of carbon dioxide in the ion conductor is low, so that it is necessary to supply an excess amount of carbon dioxide gas compared to the amount of carbon dioxide consumed in the reaction in order to compensate for the dissolution and diffusion rate. Therefore, the gas discharged from the cathode contains a large amount of unreacted carbon dioxide gas, and the concentration of the reduction product of carbon dioxide is low. In contrast, when the electrolytic solution containing carbon dioxide is supplied to the cathode 7 through the pores of the porous diaphragm 5, the carbon dioxide concentration in the electrolytic solution near the surface of the cathode 7 can be supplied more efficiently than when gaseous carbon dioxide is dissolved in the electrolytic solution near the cathode 7, so the amount of carbon dioxide supplied to the cathode 7 can be approximately the same as the amount of carbon dioxide consumed in the cathode 7. Therefore, the gas discharged from the cathode contains a small amount of unreacted carbon dioxide gas and can contain a high concentration of reduction products of carbon dioxide.

[0064] [Carbon dioxide reduction electrolysis device] The carbon dioxide reduction electrolysis device of this embodiment (indicated by reference numerals 200 and 300 in the drawings) is connected to a power source (DC power source) 28 which is connected to a current collector (for example, the anode current collector plate / anode flow path plate 2 or the cathode current collector plate / cathode flow path plate 9) via a conductor (see FIGS. 5 and 6). The carbon dioxide reduction electrolysis device of this embodiment has an anode electrolyte flow path 40 connected to the anode 3 and a cathode electrolyte flow path 30 connected to the cathode 7. The anode electrolyte flow path 40 may be connected to an electrolyte tank 15, and the cathode electrolyte flow path 30 may be connected to a gas-liquid separator 18 for separating the gas generated at the cathode 7 from the electrolyte. The carbon dioxide reduction electrolysis device of this embodiment may also have a gas exhaust flow path 42 for exhausting gas from the anode and a gas exhaust flow path 34 for exhausting gas components from the cathode.

[0065] In the carbon dioxide reduction electrolysis device of this embodiment, the pressure of the anode 3 is preferably made higher than that of the cathode 7 so that the electrolytic solution flows from the anode 3 through the pores of the porous diaphragm 5 to the cathode 7. From this perspective, the carbon dioxide reduction electrolysis device of this embodiment preferably has an anode-cathode pressure difference control device.

[0066] The anode-cathode pressure difference control device can be configured, for example, using at least one of an electrolyte pump 16 connected to the anode electrolyte flow path 40 and a cathode pressure reduction pump 19 connected to the cathode electrolyte flow path 30. The electrolyte pump 16 can be provided, for example, between the electrolyte tank 15 and the inlet 12, and the cathode pressure reduction pump 19 can be provided, for example, between the outlet 13 and the gas-liquid separator 18. When the anode 3 is connected to a gas exhaust flow path 42, a valve 21 may be provided in the gas exhaust flow path 42 so that the pressure of the anode 3 can be controlled.

[0067] The pressure of the anode 3 can be measured using an anode pressure gauge 43 provided between the electrolyte pump 16 and the inlet 12, and the pressure of the cathode 7 can be measured using a cathode pressure gauge 31 provided between the outlet 13 and the cathode pressure reduction pump 19.

[0068] The electrolyte pump 16 can be replaced with a mechanism capable of circulating the electrolyte. For example, a mechanism for making the liquid level higher upstream of the flow path than the liquid level downstream may be used, or a mechanism for pressurizing the upstream of the flow path with pressurized gas or the like may be used.

[0069] The carbon dioxide reduction electrolysis device of this embodiment may be provided with an electrolyte flow path 60 for returning the electrolyte that has flowed out into the cathode electrolyte flow path 30 to the anode electrolyte flow path 40. For example, the gas-liquid separator 18 and the electrolyte tank 15 may be connected by the electrolyte flow path 60, and the electrolyte that has accumulated in the gas-liquid separator 18 may be returned to the electrolyte tank 15 by an electrolyte pump 22 provided in the electrolyte flow path 60. Alternatively, the anode electrolyte flow path 40 may be directly connected to the electrolyte flow path 60 without providing the electrolyte tank 15 or the electrolyte pump 22.

[0070] In the carbon dioxide reduction electrolysis device of this embodiment, a mixture of liquid and gas, consisting of the electrolyte flowing out from the anode 3 to the cathode 7 through the porous diaphragm 5 and gas products such as carbon monoxide and ethylene generated by the reduction of carbon dioxide at the cathode 7, may be discharged to the cathode electrolyte flow path 30. In order to collect only the gas products, it is preferable to connect the cathode electrolyte flow path 30 to a gas-liquid separator 18 for gas-liquid separation. For example, the gas-liquid separator 18 may have a mechanism in which a mixture of liquid and gas flowing in from the cathode electrolyte flow path 30 is introduced into the gas-liquid separator 18, the liquid accumulates in the lower part of the gas-liquid separator 18, and the gas is discharged from the upper part of the gas-liquid separator 18 to the gas discharge flow path 32. In addition, an electrolyte flow path 60 may be connected to the lower part of the gas-liquid separator 18 so that the electrolyte flowing in the gas-liquid separator 18 can be returned to the anode 3. When the electrolytic cell is configured so that the gas product can be discharged separately from the electrolytic solution from outlet 14 provided above outlet 13, the electrolytic cell itself can be regarded as a gas-liquid separator.

[0071] In the electrolysis device 200, the flow rate of the electrolyte solution passing from the anode 3 through the porous diaphragm 5 to the cathode 7 (hereinafter, may be referred to as the membrane-passing electrolyte amount) can be measured. The membrane-passing electrolyte amount Q [mL / min] refers to the flow rate of the electrolyte solution flowing out from the outlet 13 to the cathode electrolyte flow path 30 per minute.

[0072] The amount of electrolyte passing through the membrane Q can be measured, for example, by measuring the increase in liquid level Δh [cm] using a level gauge provided in the gas-liquid separator 18, calculating the amount of electrolyte using the cross-sectional area S of the gas-liquid separator 18, and measuring the amount of electrolyte Qout [mL / min] flowing out from the gas-liquid separator 18 using a flow meter 33 provided in the electrolyte flow path 60, as follows: Q = Δh × S + Qout When the gas-liquid separator 18 is not connected to the electrolyte flow path 60, the value of Qout is set to 0. As will be described later, in the electrolysis method of the present embodiment, when the amount of electrolyte passing through the membrane is within an appropriate range, an electrolyte containing a large amount of carbon dioxide is supplied to the cathode 7, and the carbon dioxide reduction reaction can proceed efficiently.

[0073] The electrolyte pump 16, the cathode pressure reduction pump 19, the electrolyte pump 22, the anode pressure gauge 43, the cathode pressure gauge 31, the valve 21, the anode electrolyte flow path 40, the flow meters (not shown) provided in the cathode electrolyte flow path 30, and the electrolyte flow meter 33 provided in the electrolyte flow path may be electrically connected to a control device including a CPU and a memory so that the discharge rates of the electrolyte pump 16, the cathode pressure reduction pump 19, and the electrolyte pump 22 can be controlled based on the respective values.

[0074] [Carbon dioxide electrolysis method] The carbon dioxide electrolysis method of the present embodiment is a method that uses the electrolytic cell according to the present embodiment, or the carbon dioxide reduction electrolysis device according to the present embodiment. The electrolysis method of the present embodiment is a method for electrolyzing carbon dioxide using an electrolytic cell having an anode 3, a cathode 7, and a porous diaphragm 5 disposed between the anode 3 and the cathode 7, Supplying an electrolyte from the anode 3 side; Applying a voltage to the anode 3 to cause an oxidation reaction to proceed; Supplying an electrolyte solution containing carbon dioxide to the cathode 7 through a porous diaphragm 5; applying a voltage between the anode 3 and the cathode 7 to reduce carbon dioxide at the cathode 7; Includes. Moreover, in the electrolysis method of the present embodiment, the electrolytic solution is supplied to the cathode 7 through the porous diaphragm 5, so it is preferable that the pressure of the electrolytic solution supplied to the anode 3 is made higher than the pressure of the outflow (electrolytic solution and product) from the cathode 7 so that the electrolytic solution flows from the anode 3 through the porous diaphragm 5 to the cathode 7.

[0075] Furthermore, the flow rate of the electrolyte passing through the pores of the porous membrane 5 from the anode 3 to the cathode 7 is expressed as the membrane-passing electrolyte volume Q / S [mL / (min cm 2 )] preferably satisfies the condition of the following formula (1). 0.1 The above-mentioned configuration makes it possible to provide an electrolysis method which has excellent selectivity for the carbon dioxide reduction reaction and a high conversion rate of carbon dioxide at a small cell voltage.

[0076] The amount of electrolyte passing through the membrane per unit area of ​​the cathode, Q / S, is 0.5 mL / (min cm 2 ) or more, and 1.0 mL / (min cm 2 ) or more, and 3.0mL / (min cm 2 ) or more. If the amount of electrolyte passing through the membrane per unit area of ​​the cathode Q / S is small, carbon dioxide may not be sufficiently supplied from the anode 3 side to the cathode 7, and the reduction reaction of carbon dioxide may not proceed efficiently, but if the amount of electrolyte passing through the membrane per unit area of ​​the cathode Q / S is in the above range, electrolyte with a high carbon dioxide concentration can be efficiently supplied from the anode 3 side to the cathode 7, and the efficiency of carbon dioxide reduction can be increased.

[0077] The amount of electrolyte passing through the membrane per unit area of ​​the cathode, Q / S, is 500 mL / (min cm 2 ) or less, and 100 mL / (min cm 2 ) or less, and 50 mL / (min cm 2 ​) or less, and 30mL / (min cm 2 ) or less. If the amount of electrolyte passing through the membrane per unit area of ​​the cathode Q / S is too large, the carbon dioxide supplied from the anode 3 side is excessively diluted by the electrolyte, which may reduce the efficiency of the carbon dioxide reduction reaction in the cathode 7. However, if the amount of electrolyte passing through the membrane per unit area of ​​the cathode Q / S is within the above range, the electrolyte with a high carbon dioxide concentration supplied from the anode 3 side can be efficiently supplied to the cathode 7, and the efficiency of carbon dioxide reduction can be increased.

[0078] The voltage (cell voltage) applied to the single cell in the electrolysis method of the present embodiment is preferably 1.0 V to 10 V, more preferably 1.1 V to 6 V, and further preferably 1.2 V to 4 V. When the cell voltage value is within the above range, energy suitable for the progress of the reduction reaction of carbon dioxide can be supplied, and the reduction reaction of carbon dioxide can be progressed.

[0079] The current density in the electrolysis method of this embodiment is preferably 10 mA / cm 2 ~2000mA / cm 2 and more preferably 50 mA / cm 2 ~1500mA / cm 2 and more preferably 100 mA / cm 2 ~1000mA / cm 2 and more preferably 200 mA / cm 2 ~1000mA / cm 2 and more preferably 300 mA / cm 2 ~1000mA / cm 2 When the current density value is within the above range, the productivity of the product of the carbon dioxide reduction reaction can be sufficiently increased.

[0080] The electrolytic solution containing a carbon dioxide precursor in the electrolysis method of this embodiment is as described above in the carbon dioxide reduction electrolytic cell or carbon dioxide reduction electrolysis device according to this embodiment.

[0081] In the electrolysis method of the present embodiment, CO2 can be reduced to produce CO2-reduced substances such as carbon monoxide, formic acid, acetic acid, methane, ethane, ethylene, methanol, ethanol, and propanol. In the electrolysis method of the present embodiment, it is preferable to produce carbon monoxide or ethylene by reducing carbon dioxide at the cathode 7.

[0082] [Method of producing carbon monoxide or ethylene] In the method for producing carbon monoxide or ethylene according to the present embodiment, carbon monoxide or ethylene is obtained by the electrolysis method described above. The suitable conditions are the same as those for the electrolysis method described above. EXAMPLES

[0083] The present invention will be described in more detail below with reference to examples, but these are merely illustrative and the present invention is not limited thereto. Therefore, those skilled in the art can practice the present invention by making various modifications to the examples shown below.

[0084] Various measurements were performed according to the methods shown below.

[0085] [Contact angle θ] The contact angle θ was measured according to the sessile drop method specified in JIS R3257:1999, replacing the substrate glass with a porous membrane. A droplet of pure water was dropped onto the porous membrane, and the contact angle θ formed between the droplet and the sample was recorded N seconds after the drop. If the water droplet was dropped and soaked by the time the angle was recorded, the contact angle θ with water was determined to be 0 degrees. The time N from dropping to recording the contact angle θ was set to 1 second.

[0086] [Water permeability] The water permeability of the porous membrane was measured by the following method. The porous membrane moistened with pure water was placed in a filter holder, and pure water was supplied from one side of the porous membrane so that it passed through the pores of the porous membrane. The differential pressure applied to both sides of the porous membrane was set to 69.0 kPa, and the amount of pure water passing through the porous membrane per minute was measured, and the amount was divided by the area of ​​the porous membrane through which the pure water passed to obtain the water permeation amount.

[0087] [Drying shrinkage rate] The drying shrinkage rate α of the porous diaphragm was measured by the following method. A circular porous membrane with a diameter of L1 [cm] was cut out and immersed in water for 1 hour. The porous membrane was then removed from the water and dried in a dryer at 40 degrees for 24 hours. The diameter L2 [cm] was measured after removing the moisture and used to calculate the diameter using the following formula. α=100×(L1-L2) / L1

[0088] 〔pressure〕 The pressure of the anode 3 was measured by an anode pressure gauge 43 provided upstream of the inlet 12 , and the pressure of the cathode 7 was measured by a cathode pressure gauge 31 provided downstream of the outlet 13 .

[0089] [Amount of electrolyte passing through the membrane] The amount of electrolyte passing through the membrane Q was measured by the following method. The amount of electrolyte passing through the membrane Q is measured by measuring the increase in the liquid level Δh [cm] using a level gauge installed in the gas-liquid separator 18 and the cross-sectional area S [cm 2 ], and the amount of electrolyte passing through the membrane Qout [mL / min] measured by the flow meter 33 provided in the electrolyte flow path 60, were used to calculate as follows: Q[mL / min]=Δh×S+Qout

[0090] [Indices for evaluating electrolytic performance] The performance of the carbon dioxide electrolysis device was evaluated by applying a constant current to the electrolytic cell. In the carbon dioxide reduction reaction, various substances such as carbon monoxide, ethylene, and hydrogen may be generated. For this reason, the performance of the cathode gas composition, Faraday efficiency, cell voltage, and current density were evaluated. Each index when a certain product (called product A) is generated is explained below.

[0091] [Cathode gas composition] The cathode gas composition is the volume fraction of a certain substance in the gas discharged from the gas discharge flow path 32. The gas discharged from the gas discharge flow path 32 may contain unreacted carbon dioxide in addition to the reduction product of carbon dioxide. The cathode gas composition was calculated by measuring the gas discharged from the gas discharge flow path 32 by gas chromatography.

[0092] [Faraday efficiency] The Faraday efficiency is a value expressed in percentage as a ratio of the amount of A actually observed to the amount of A produced per unit time when all the current applied to the electrolytic cell is used for producing product A. The current value applied to the electrolytic cell was measured by an electrochemical measurement device, and the amount of A actually produced was calculated by measuring the gas discharged from gas exhaust flow path 32 by a gas chromatograph, thereby calculating the Faraday efficiency.

[0093] [Cell voltage] The cell voltage was measured by an electrochemical measurement device, and the voltage applied between the cathode and anode of the single cell was recorded.

[0094] A high Faraday efficiency of A means high selectivity of A. A small cell voltage means efficient use of electrical energy. Therefore, in order to efficiently produce substance A, it is preferable to achieve both high Faraday efficiency and small cell voltage.

[0095] [Current density] The current density was calculated by measuring the current applied to the electrolytic cell using an electrochemical measuring device and dividing the current value by the electrode area. The current density was multiplied by the Faraday efficiency of A to calculate the "partial current density of A."

[0096] Current density is an index of productivity. In particular, a high partial current density of A means that the productivity of product A per unit time is high. In order to increase the partial current density, it is necessary to efficiently supply reactants such as carbon dioxide and water to the reaction site and efficiently discharge the produced substances from the reaction site. For this reason, simply increasing the value of the current density applied to the electrolytic cell does not necessarily increase the partial current density of product A. For example, it may happen that only the partial current density of the by-product hydrogen increases, and the partial current density of product A does not increase.

[0097] [Example 1] (Carbon dioxide reduction electrolysis cell A) A carbon dioxide reduction electrolytic cell A was constructed as shown in Figure 1. The electrolytic cell was assembled by stacking, in this order, an anode current collector plate / anode flow path plate 2, an anode gasket 4 made of PTFE with an anode 3 fitted in it, a PTFE porous diaphragm gasket 6 made of PTFE with a porous diaphragm 5 fitted in it, a cathode gasket 8 made of PTFE with a cathode 7 fitted in it, a cathode current collector plate / cathode flow path plate 9, and an end plate 10 on an end plate 1, and fastening them together with bolts (not shown).

[0098] An inlet 12 was provided on end plate 1 so that it could be connected to an anode electrolyte flow path 40 outside the electrolytic cell. Also, an outlet 13 was provided on end plate 10 so that it could be connected to a cathode electrolyte flow path 30 outside the electrolytic cell. End plates 1 and 10 were provided with through holes having a diameter of 2 mm for supplying electrolyte and carbon dioxide to anode current collector / anode flow path plate 2 and cathode current collector / cathode flow path plate 9.

[0099] As shown in Figure 3, the anode current collector / anode flow path plate 2 and the cathode current collector / cathode flow path plate 9 have a through hole a with a diameter of 2 mm through which the electrolyte is supplied, and a serpentine flow path b on one side of the flow path plate. Flow path b has a groove with a width of 1 mm and a depth of 1 mm engraved on it, and has a structure in which it is folded back four times within an area of ​​1 cm square. The anode current collector / anode flow path plate 2 and the cathode current collector / cathode flow path plate 9 are made of conductive carbon, and are provided with terminals 2', 9' that can be connected to a power source 28 outside the electrolytic cell.

[0100] Anode 3 was made by cutting an anode electrode manufactured by CHEMIX into a 1 cm square. This anode electrode was made by applying a water-repellent treatment to one side of carbon paper and coating the water-repellent surface with a catalyst ink made of platinum particles and Nafion binder (hereafter referred to as anode a). Anode gasket 4 and cathode gasket 8 were made of PTFE sheets with 1 cm square holes, into which cathode 7 and anode 3 were fitted, respectively.

[0101] A commercially available membrane filter for aqueous filtration was cut into a 2 cm piece and used as the porous diaphragm 5. A PTFE sheet of the same thickness as the porous diaphragm 5 with a 2 cm square hole was used as the gasket for the porous diaphragm 6, and the porous diaphragm 5 was fitted into it.

[0102] (Carbon dioxide reduction electrolysis device 200) The carbon dioxide reduction electrolysis cell A was connected to the anode electrolyte flow path 40 and the cathode electrolyte flow path 30 outside the electrolysis cell as shown in FIG. 5 to form a carbon dioxide reduction electrolysis device 200. The anode current collector plate / anode flow path plate 2 and the cathode current collector plate / cathode flow path plate 9 were connected to a power source (DC power source) 28 via a conductor 29. The electrolyte was supplied from the electrolyte tank 15 to the inlet 12 with the flow rate controlled by the electrolyte pump 16. An anode pressure gauge 43 was provided in the anode electrolyte flow path 40 between the electrolyte pump 16 and the electrolyte inlet 12. The electrolyte containing the reduction product discharged from the outlet 13 was separated into the electrolyte and the gas product by the gas-liquid separator 18. A cathode pressure gauge 31 was provided in the cathode electrolyte flow path 30 between the outlet 13 and the gas-liquid separator 18. The gas-liquid separator 18 was connected to an electrolyte flow path 60, and the electrolyte separated by the gas-liquid separator was returned to the electrolyte tank 15 by an electrolyte pump 22 provided in the electrolyte flow path 60. A glass level gauge was provided on the side of the gas-liquid separator 18 so that the height of the liquid level could be grasped. An electrolyte flow meter 33 was provided in the electrolyte flow path 60.

[0103] Carbon dioxide reduction electrolysis cell A was assembled using a hydrophilic PVDF membrane (polyvinylidene fluoride membrane manufactured by Merck) with a pore size of 5.0 μm as the porous diaphragm 5, and a gas diffusion electrode TGP-H-060 manufactured by Toray Co., Ltd. coated with gold particles with a particle size of 70 nm (hereafter referred to as cathode A) cut into 1 cm squares as the cathode 7. The hydrophilic PVDF membrane has a contact angle θ with water of less than 90 degrees and a water permeability of 288 mL / (min cm 2 ), the film thickness was 125 μm, and the drying shrinkage rate α was less than 0.1% (see Table 1).

[0104] The carbon dioxide reduction electrolysis device 200 was constructed using the carbon dioxide reduction electrolysis cell A, and the electrolyte was supplied under the following conditions. An aqueous solution containing a mixture of sodium formate with a concentration of 1.0 M and KHCO3 with a concentration of 2.0 M was used as the electrolyte as the carbon dioxide precursor and electrolyte, and the electrolyte was supplied to the carbon dioxide reduction electrolysis cell A at 15 mL / min using the electrolyte pump 16. At this time, the difference between the values ​​of the anode pressure gauge 43 and the cathode pressure gauge 31 was 0.05 MPa. In addition, while watching the liquid level gauge of the gas-liquid separator 18, the electrolyte pump 22 was operated so that the height of the liquid level did not fluctuate, and the electrolyte discharged from the cathode electrolyte flow path 30 was returned to the electrolyte tank 15. At this time, the value of the electrolyte flow meter 33 was 15 mL / min. From this, the amount of electrolyte passing through the membrane per unit area of ​​the cathode Q / S was 15 mL / [min·cm 2 ] was calculated.

[0105] Next, the carbon dioxide reduction electrolysis device 200 was supplied with 200 mA / cm from a DC power source. 2 The carbon dioxide reduction electrolysis performance was evaluated by supplying a constant current of 1000 kcal / s to the electrochemical cell A and operating it. After 10 minutes had elapsed since the start of energization, the gas composition discharged from the cathode of the carbon dioxide reduction electrolytic cell A and separated in the gas-liquid separator 18 was sampled from the gas exhaust flow path 32 and analyzed using a gas chromatograph. Table 1 shows the Faraday efficiency of hydrogen and carbon monoxide (hereinafter also referred to as H2 FE or CO FE) calculated from the product concentrations observed by the gas chromatograph, the cathode gas composition of hydrogen, carbon monoxide, and carbon dioxide, the cell voltage 10 minutes after the start of energization, and the cell voltage 1 hour after the start of energization.

[0106] [Examples 2 to 6] A carbon dioxide reduction electrolysis device 200 was constructed and carbon dioxide reduction electrolysis performance was evaluated in the same manner as in Example 1, except that porous membranes with different pore sizes, membrane thicknesses, and materials were used as the porous diaphragm 5 as shown in Table 1. The material, membrane thickness, pore size, water permeability, and drying shrinkage rate of the porous diaphragm 5, the amount of electrolyte passing through the membrane per unit area of ​​the cathode Q / S, the anode-cathode pressure difference, and carbon dioxide reduction electrolysis performance are as shown in Table 1, and the contact angles of the porous diaphragm 5 with water were all 90 degrees or less. The porous membranes used are as follows: PES: Polyethersulfone membrane (manufactured by AS ONE) Hydrophilic PTFE: Polytetrafluoroethylene membrane (Merck) Hydrophilic PVDF: Polyvinylidene fluoride membrane (Merck)

[0107] [Examples 7 to 10] A carbon dioxide reduction electrolysis device 200 was constructed and carbon dioxide reduction electrolysis performance was evaluated in the same manner as in Example 2, except that the amount of electrolyte passing through the membrane Q / S per unit area of ​​the cathode was changed by changing the discharge rates of the electrolyte pump 16 and the electrolyte pump 22 as shown in Table 1. The amount of electrolyte passing through the membrane Q / S per unit area of ​​the cathode, the anode-cathode pressure difference, and carbon dioxide reduction electrolysis performance were as shown in Table 1.

[0108] [Examples 11 to 14] A carbon dioxide reduction electrolysis device 200 was constructed and the carbon dioxide reduction electrolysis performance was evaluated in the same manner as in Example 2, except that a different carbon dioxide precursor was used as shown in Table 1. The carbon dioxide reduction electrolysis performance was as shown in Table 1.

[0109] [Example 15] (Carbon dioxide reduction electrolysis cell B) Carbon dioxide reduction electrolytic cell B was constructed as shown in Fig. 2. As shown in Fig. 6, the differences from carbon dioxide reduction electrolytic cell A are that an outlet 11 was added to end plate 1 so that it could be connected to gas exhaust flow path 42 outside the electrolytic cell, an outlet 14 was added to end plate 10 so that it could be connected to gas exhaust flow path 34 outside the electrolytic cell, and a through hole a portion having a diameter of 2 mm through which the electrolyte is supplied was added to anode current collector / anode flow path plate 2 and cathode current collector / cathode flow path plate 9 as shown in Fig. 4 so that it could be connected to the added inlet 11 and outlet 14.

[0110] (Carbon dioxide reduction electrolysis device 300) Carbon dioxide reduction electrolysis cell B was connected to an anode electrolyte flow path 40, a gas exhaust flow path 42, a cathode electrolyte flow path 30, and a gas exhaust flow path 34 outside the electrolytic cell as shown in Fig. 6 to form a carbon dioxide reduction electrolysis device 300. The difference from the carbon dioxide reduction electrolysis device 200 is that the outlet 11 is connected to the gas exhaust flow path 42 so that the electrolyte can be returned to the electrolyte tank 15, and the outlet 14 is connected to the gas exhaust flow path 34 so that gas can be exhausted from the cathode 7. In addition, a valve 21 is provided in the gas exhaust flow path 42, and a vent line 44 and a carbon dioxide supply flow path 48 are provided in the electrolyte tank 15.

[0111] The carbon dioxide reduction electrolysis performance was evaluated in the same manner as in Example 2, except that a carbon dioxide reduction electrolysis device 300 configured using the carbon dioxide reduction electrolysis cell B was used, the electrolytic solution was supplied to the carbon dioxide reduction electrolysis cell B at 30 mL / min using the electrolytic solution pump 16, the aperture of the valve 21 was adjusted so that the difference between the values ​​of the anode pressure gauge 43 and the cathode pressure gauge 31 was 0.10 MPa, and the gas discharged from the gas discharge flow path 34 was analyzed by gas chromatography. The carbon dioxide reduction electrolysis performance was as shown in Table 1.

[0112] [Example 16] The carbon dioxide reduction electrolysis performance was evaluated in the same manner as in Example 15, except that the carbon dioxide reduction electrolysis cell B was heated using a heater to set the temperature of the electrolysis cell to 70° C. The carbon dioxide reduction electrolysis performance was as shown in Table 1.

[0113] [Example 17] The carbon dioxide reduction electrolysis performance was evaluated in the same manner as in Example 15, except that a 1.0 M K2SO4 aqueous solution was used as the electrolyte, and carbon dioxide was bubbled into the electrolyte from a carbon dioxide flow path 48 connected to the electrolyte tank 15 and added to the electrolyte until carbon dioxide was saturated. The carbon dioxide reduction electrolysis performance was as shown in Table 1.

[0114] [Example 18] A carbon dioxide reduction electrolysis device 200 was constructed in the same manner as in Example 2, except that a gas diffusion electrode TGP-H-060 manufactured by Toray Industries, Inc., coated with copper particles having a particle size of 70 nm (hereinafter referred to as cathode B) was cut into a 1 cm square and used as the cathode 7, and the carbon dioxide reduction electrolysis performance was evaluated. The carbon dioxide reduction electrolysis performance was as shown in Table 1.

[0115] [Comparative Example 1] A carbon dioxide reduction electrolysis device 200 was constructed in the same manner as in Example 2, except that no carbon dioxide precursor was added and an electrolytic solution containing 2.0 M KHPO4 was used as the electrolyte. The carbon dioxide reduction electrolysis performance is shown in Table 1.

[0116] [Comparative Example 2] A carbon dioxide reduction electrolysis device 200 was constructed in the same manner as in Example 1, except that a hydrophobic PVDF membrane (hydrophobic Durapore membrane manufactured by Merck) with a pore size of 0.1 μm was used as the porous diaphragm 5. The hydrophobic PVDF membrane had a contact angle with water of 90 degrees or more and was highly hydrophobic, so a water permeability test using water could not be performed. The thickness of the hydrophobic PVDF membrane was 125 μm. The carbon dioxide reduction electrolysis performance is as shown in Table 1.

[0117] [Comparative Example 3] (Carbon dioxide reduction electrolysis device 400) Carbon dioxide reduction electrolysis cell B was connected to an anode electrolyte flow path 40, a gas exhaust flow path 42, a gas flow path 36, and a gas exhaust flow path 34 outside the electrolytic cell as shown in Fig. 7 to form a carbon dioxide reduction electrolysis device 400. The difference with carbon dioxide reduction electrolysis device 300 is that a gas flow path 36 is connected instead of a cathode electrolyte flow path 30 so that carbon dioxide gas can be supplied to the cathode from a carbon dioxide cylinder 45 via a mass flow controller 46, and a pressure control valve 47 is provided in the gas exhaust flow path 34 so that the pressure of the carbon dioxide supplied to the cathode can be controlled.

[0118] The carbon dioxide reduction electrolysis device 400 was constructed, and carbon dioxide and an electrolyte were supplied under the following conditions. Carbon dioxide gas was supplied from the carbon dioxide cylinder 45 to the carbon dioxide reduction electrolysis cell B at a flow rate of 50 mL / min by the mass flow controller 46. An aqueous solution containing a mixture of KHCO3 with a concentration of 2.0 M was used as the electrolyte, and the electrolyte was supplied to the carbon dioxide reduction electrolysis cell B at 15 mL / min using the electrolyte pump 16. At this time, the pressure control valve 47 was operated while checking the cathode pressure gauge 31, and the pressure of the carbon dioxide supplied to the cathode was controlled to be 0.14 MPa higher than the pressure of the electrolyte supplied to the anode. In other words, the difference between the values ​​of the anode pressure gauge 43 and the cathode pressure gauge 31 was -0.14 MPa. At this time, the electrolyte did not flow from the anode 3 to the cathode 7 through the porous diaphragm 5.

[0119] Next, the carbon dioxide reduction electrolysis device 400 was supplied with 200 mA / cm from a DC power source. 2 The carbon dioxide reduction electrolysis performance was evaluated by supplying a constant current of 1000 kcal / s to the electrochemical cell B. After 10 minutes had elapsed since the start of energization, the gas composition discharged from the cathode of the carbon dioxide reduction electrolytic cell B was sampled from the gas exhaust flow path 34 and analyzed using a gas chromatograph. Table 1 shows the Faraday efficiency of hydrogen and carbon monoxide (hereinafter also referred to as H2 FE or CO FE) calculated from the product concentrations observed by the gas chromatograph, the cathode gas composition of hydrogen, carbon monoxide, and carbon dioxide, the cell voltage 10 minutes after the start of energization, and the cell voltage 1 hour after the start of energization.

[0120] [Comparative Example 4] (Carbon dioxide reduction electrolysis device 400) A carbon dioxide reduction electrolysis device 400 was configured in the same manner as in Comparative Example 3, except that the supply rate of carbon dioxide gas supplied to the carbon dioxide reduction electrolysis cell B was changed to 4 mL / min. The carbon dioxide reduction electrolysis performance is as shown in Table 1.

[0121] [Table 1-1]

[0122] [Table 1-2]

[0123] From the results of the Examples and Comparative Examples described above, it can be seen that the electrolytic cell according to this embodiment has a small cell voltage, and the gas discharged from the electrolytic cell contains a high concentration of reduction products of carbon dioxide.

[0124] When the carbon dioxide reduction electrolysis device described in Examples 1 to 18 is used, ohmic resistance is reduced by ion conduction through the electrolyte in the porous membrane, so that a current of 200 mA / cm is obtained at a low cell voltage of 2.7 V or less. 2 It was possible to pass electricity through the

[0125] Moreover, from the results obtained in Examples 1 to 6, it is found that if the anode pressure is higher than the cathode pressure and the electrolyte is supplied from the anode to the cathode through the porous membrane, the reduction reaction of carbon dioxide proceeds and the cathode gas composition of the product carbon monoxide is as high as 45%, regardless of the material or type of the porous membrane. In addition, it is found that when a membrane with a small drying shrinkage rate is used, the cell voltage 1 hour after the start of current application is approximately the same as the cell voltage 10 minutes after the start of current application.

[0126] The results obtained in Examples 7 to 10 show that by changing the anode-cathode pressure difference and controlling the amount of electrolyte passing through the membrane per unit area of ​​the cathode Q / S to an appropriate value, the cathode gas composition of carbon monoxide, which is the product, is increased.

[0127] Furthermore, from the results obtained in Examples 11 to 17, it is understood that, as long as the anode pressure is higher than the cathode pressure and the electrolyte is supplied from the anode to the cathode via a porous diaphragm, the reduction reaction of carbon dioxide proceeds and the cathode gas composition of carbon monoxide, which is the product, is as high as 33% or more, regardless of the type of carbon dioxide source (carbon dioxide, or carbon dioxide precursor, or bicarbonate) contained in the electrolyte.

[0128] The results obtained in Example 18 show that, when the anode pressure is greater than the cathode pressure and the electrolyte is supplied from the anode to the cathode through a porous diaphragm, not only carbon monoxide but also ethylene can be obtained with a high cathode gas composition (18%) by using an appropriate catalyst.

[0129] On the other hand, when the carbon dioxide reduction electrolysis device described in Comparative Example 1 was used, carbon dioxide was not generated at the anode, so only the by-product hydrogen was obtained as the cathode gas, and no reduction product of carbon dioxide was obtained.

[0130] When the carbon dioxide reduction electrolysis device described in Comparative Example 2 was used, the electrolyte did not sufficiently permeate the porous diaphragm, so that the ohmic resistance increased and the cell voltage became 15 V or higher. As a result, it was not possible to pass current for 30 minutes and therefore it was not possible to analyze the products.

[0131] When the carbon dioxide reduction electrolysis device described in Comparative Example 3 was used, carbon dioxide gas was supplied to the cathode at a large flow rate, so the generated carbon monoxide was diluted and the cathode gas composition of carbon monoxide was low at 1.9%.

[0132] When the carbon dioxide reduction electrolysis device described in Comparative Example 4 was used, carbon dioxide was supplied at a small flow rate, so the cathode gas composition of carbon dioxide was small; however, the supply of carbon dioxide to the cathode was insufficient, and hydrogen, a by-product, was generated predominantly, so the cathode gas composition of carbon monoxide was low at 2%. [Industrial Applicability]

[0133] The carbon dioxide reduction electrolytic cell of the present invention is industrially useful because it can produce a high concentration of carbon dioxide reduction product at a small cell voltage. [Explanation of symbols]

[0134] 1...end plate, 2...anode current collector plate / anode flow path plate, 2'...terminal, 3...anode, 4...anode gasket, 5...porous diaphragm, 6...porous diaphragm gasket, 7...cathode, 8...cathode gasket, 9...cathode current collector plate / cathode flow path plate, 9'...terminal, 10...end plate, 11...outlet, 12...inlet, 13...outlet, 14...outlet, 15...electrolyte tank, 16...anode pressure boost pump, 18...gas-liquid separator, 19...cathode pressure reduction pump, 21...valve, 22...electrolyte pump, 28...power source, 30...cathode electrolyte flow path, 31...cathode pressure gauge, 32...gas exhaust flow path, 33...electrolyte flow meter, 34...gas gas exhaust flow path, 36...gas flow path, 40...anode electrolyte flow path, 42...gas exhaust flow path, 43...anode pressure gauge, 44...vent line, 45...carbon dioxide cylinder, 46...mass flow controller, 47...pressure control valve, 48...carbon dioxide supply flow path, 51...first surface of porous diaphragm, 52...second surface of porous diaphragm, 60...electrolyte flow path, 100C...single cell, 100S...cell stack, 102...conductor, 128...bipolar electrode, 200...carbon dioxide reduction electrolysis device, 300...carbon dioxide reduction electrolysis device, 400...carbon dioxide reduction electrolysis device, A...carbon dioxide reduction electrolysis cell, B...carbon dioxide reduction electrolysis cell, a...through hole, b...flow path engraved in plate.

Claims

1. A method for electrolyzing carbon dioxide using an electrolytic cell having an anode, a cathode, and a porous diaphragm disposed between the anode and the cathode, comprising the steps of: Supplying an electrolyte from the anode side; supplying the electrolyte to the cathode through the porous diaphragm; applying a voltage between the anode and the cathode to reduce carbon dioxide at the cathode; Including, A method for electrolysis of carbon dioxide, wherein the electrolytic solution passing through the porous diaphragm contains carbon dioxide.

2. the electrolyte contains a carbon dioxide precursor; applying a voltage between the anode and the cathode to generate carbon dioxide by an oxidation reaction of the carbon dioxide precursor at the anode; The method for electrolysis of carbon dioxide according to claim 1, further comprising:

3. The electrolyte contains a carbonate or a bicarbonate, applying a voltage between the anode and the cathode to generate carbon dioxide through a protonation reaction of the carbonate or hydrogen carbonate at the anode; The method for electrolysis of carbon dioxide according to claim 1, further comprising:

4. The electrolyte contains a bicarbonate, heating the electrolyte supplied to the anode to generate carbon dioxide through a decomposition reaction of the hydrogen carbonate; The method for electrolysis of carbon dioxide according to claim 1, comprising:

5. The method for electrolyzing carbon dioxide according to claim 1 , wherein the electrolytic solution in supplying the electrolytic solution contains carbon dioxide.

6. The amount of electrolyte Q (mL / min) passing through the porous membrane from the anode to the cathode is expressed as a function of the cathode unit area S (cm 2 2. The electrolysis method according to claim 1 , wherein the following formula (1) is satisfied: 0.1<Q / S<1000...(1)

7. 2. The process of claim 1, wherein the pressure of the electrolyte supplied to the anode is greater than the pressure of the effluent from the cathode.

8. The electrolysis method according to claim 2 , wherein the carbon dioxide precursor is at least one selected from the group consisting of formate ions, acetate ions, oxalate ions, and glycolate ions.

9. 2. The electrolysis method according to claim 1, wherein carbon monoxide or ethylene is produced by reducing carbon dioxide at the cathode.

10. A method for producing carbon monoxide or ethylene using an electrolytic cell having an anode, a cathode, and a porous diaphragm disposed between the anode and the cathode, comprising: Supplying an electrolyte from the anode side; supplying the electrolyte to the cathode through the porous diaphragm; applying a voltage between the anode and the cathode and reducing carbon dioxide at the cathode to produce carbon monoxide or ethylene; Including, The method for producing carbon monoxide or ethylene, wherein the electrolytic solution passing through the porous diaphragm contains carbon dioxide.

11. A carbon dioxide reduction electrolytic cell for electrochemically reducing carbon dioxide, comprising: a supply flow path for supplying an electrolyte from the anode side; An anode where the oxidation reaction proceeds when a voltage is applied, A cathode that reduces carbon dioxide by applying a voltage; a porous diaphragm disposed between the anode and the cathode; the supply flow path supplies the electrolytic solution containing carbon dioxide so that the electrolytic solution passes through the porous diaphragm and is supplied to the cathode.

12. 12. The carbon dioxide reduction electrolytic cell according to claim 11, wherein the electrolytic solution contains at least one carbon dioxide precursor selected from the group consisting of formate ions, acetate ions, oxalate ions, and glycolate ions.

13. The porous membrane has a water permeability of 5 mL / (min cm 2 )~300mL / (min・cm 2 13. The carbon dioxide reduction electrolytic cell according to claim 12,

14. 12. The carbon dioxide reduction electrolytic cell according to claim 11, wherein the electrolytic solution contains at least one ion selected from the group consisting of bicarbonate ions and carbonate ions.

15. The water permeability of the porous membrane is 1 mL / (min cm 2 )~50mL / (min・cm 2 15. The carbon dioxide reduction electrolytic cell according to claim 14,

16. The carbon dioxide reduction electrolytic cell according to claim 11, wherein the drying shrinkage rate of the porous diaphragm is 2% or less.

17. A carbon dioxide reduction electrolysis device comprising the carbon dioxide reduction electrolysis cell according to any one of claims 11 to 16.

18. 18. The carbon dioxide reduction electrolysis device according to claim 17, further comprising an electrolyte supply device for supplying the carbon dioxide-containing electrolyte to the cathode through the porous diaphragm.

19. The carbon dioxide reduction electrolysis device according to claim 17, further comprising a gas-liquid separator for separating a gas containing a reduction product of carbon dioxide discharged from the cathode from an electrolytic solution.

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