Cathode electrode, composite of cathode electrode and base material, electrolytic reduction device provided with cathode electrode, and method for producing composite of cathode electrode and base material

Abstract

Provided is a cathode electrode with which it is possible to prevent an improvement in hydrogen selectivity even if a catalyst layer is immersed in an electrolyte, and with which a catalytic reaction that produces, by a reduction reaction of carbon dioxide, an olefin-based hydrocarbon such as ethylene or an alcohol such as an ethanol can stably maintain high efficiency over a long period of time.　This cathode electrode electrically reduces carbon dioxide, the cathode electrode including a first layer positioned on a surface layer of the cathode electrode and having a first thickness, and a second layer adjacent to the first layer in the thickness direction and having a second thickness. The first layer includes copper, the first layer is surface-modified with a cation exchange material substituted with metal ions, and the second layer is a mixed layer of copper and carbon.

Description

Cathode electrode, composite of cathode electrode and substrate, electrolytic reduction device equipped with cathode electrode, and method for manufacturing composite of cathode electrode and substrate

[0001] The present invention relates to a cathode electrode capable of electrically reducing carbon dioxide to convert it into carbon monoxide, an olefinic hydrocarbon such as ethylene, and / or an alcohol, a composite of the cathode electrode and a substrate, an electrolytic reduction device including the cathode electrode, and a method for producing the composite of the cathode electrode and a substrate.

[0002] In recent years, the adverse effects of global warming have brought about changes in the global environment, resulting in many problematic phenomena. One of the causes of these changes is believed to be the rise in the concentration of greenhouse gases in the atmosphere, particularly the rise in the concentration of carbon dioxide, which accounts for the majority of greenhouse gases. In order to reduce the concentration of carbon dioxide in the atmosphere, not only are new afforestation on land and an increase in photosynthesis by marine algae being considered, but active absorption and capture of carbon dioxide from the atmosphere is also being considered. Furthermore, in addition to absorbing and capturing carbon dioxide, the use of carbon derived from carbon dioxide as a raw material for organic compounds is also being considered.

[0003] Specifically, studies are being conducted to reduce carbon dioxide and convert it into C2 compounds such as ethylene and ethanol, or C1 compounds such as carbon monoxide, methane, methanol, and formic acid, and to utilize these compounds for the synthesis of organic compounds. Of these, the C2 compounds ethylene and ethanol are particularly useful as derivatives in the synthesis of various organic compounds, and are more useful than C1 compounds such as carbon monoxide and methane.

[0004] In recent years, catalysts such as photocatalysts and electrocatalysts have been widely used in the carbon dioxide reduction reaction described above, and there is a demand for the development of catalysts with even better performance. Catalysts used in carbon dioxide reduction reactions are required to have not only reaction efficiency but also selectivity for specific reactions, and from this perspective, the selection of catalytic materials is important. For example, gold, silver, and zinc are used as catalytic materials to efficiently reduce and generate carbon monoxide and increase the proportion of carbon monoxide in the reduced materials. Copper is also used as a catalytic material to efficiently reduce and generate hydrocarbons such as methane, ethane, and ethylene. In particular, copper has attracted attention as an electrocatalyst for the cathode reduction of carbon dioxide because it can generate C2 compounds such as ethylene.

[0005] As a catalyst layer containing a metal catalyst such as copper used in the carbon dioxide reduction reaction described above, a catalyst layer has been proposed that includes a metal catalyst supported on a carbon material, an ion-conductive material, and a hydrophilic polymer, and that the ratio of the BET specific surface area of ​​the catalyst layer determined by water vapor adsorption to the BET specific surface area of ​​the catalyst layer determined by nitrogen adsorption is 0.08 or less (Patent Document 1). Patent Document 1 proposes that by controlling the BET specific surface area ratio, the wettability of the catalyst layer is reduced, improving the water repellency of the catalyst layer, preventing water from accumulating within the catalyst layer and promoting the diffusion of carbon dioxide gas. It is claimed that promoting the diffusion of carbon dioxide gas in the catalyst layer improves the carbon dioxide conversion efficiency.

[0006] On the other hand, in order to industrially put the production of organic compounds such as ethylene by the reduction reaction of carbon dioxide into practical use, it is required that the catalytic reaction that produces organic compounds such as ethylene stably maintains a high selectivity for organic compounds such as ethylene for a long period of time, such as several hundred hours or more. Patent Document 1 discloses that the water repellency of the catalyst layer is improved to prevent water from remaining in the catalyst layer and promote the diffusion of carbon dioxide gas, but once the catalyst layer is immersed in the electrolyte solution, the water repellency of the catalyst layer is not enough to prevent water from remaining in the catalyst layer. Therefore, there was a need for improvement in terms of maintaining a high efficiency and stability of the catalytic reaction that produces organic compounds such as ethylene for a long period of time.

[0007] Furthermore, in order to obtain excellent synthesis efficiency of organic compounds such as ethylene in a cathode electrode for carbon dioxide reduction, it is necessary that the carbon dioxide reduction reaction dominates over the generation of hydrogen resulting from a side reaction (water decomposition reaction) during carbon dioxide reduction, reducing the selectivity for hydrogen and improving the selectivity for carbon dioxide reduction products. However, in Patent Document 1, when the catalyst layer is immersed in the electrolyte, not only is the diffusion of carbon dioxide gas hindered, but the water molecules contained in the electrolyte favor the generation of hydrogen, which increases the selectivity for hydrogen during carbon dioxide reduction, preventing excellent selectivity for carbon dioxide reduction products. This required improvement.

[0008] Japanese Patent Application Laid-Open No. 2021-147677

[0009] In view of the above circumstances, an object of the present invention is to provide a cathode electrode that prevents an improvement in hydrogen selectivity even when the catalyst layer is immersed in an electrolytic solution, and that can stably maintain high efficiency over a long period of time in a catalytic reaction that produces olefinic hydrocarbons such as ethylene and alcohols such as ethanol by a reduction reaction of carbon dioxide; a composite of a cathode electrode and a substrate; an electrolytic reduction device including the cathode electrode; and a method for manufacturing the composite of a cathode electrode and a substrate.

[0010] The gist of the configuration of the present invention is as follows. [1] A cathode electrode that electrically reduces carbon dioxide, the cathode electrode comprising: a first layer having a first thickness located on a surface of the cathode electrode; and a second layer having a second thickness adjacent to the first layer in a thickness direction, the first layer containing copper and being surface-modified with a cation exchange material substituted with metal ions, and the second layer being a mixed layer of copper and carbon. [2] The cathode electrode according to [1], wherein a ratio of a second thickness of the second layer to a first thickness of the first layer is greater than 0 and not greater than 100. [3] The cathode electrode according to [1] or [2], wherein the second layer comprises a first region located on the first layer side and a second region adjacent to the first region, with a central portion of the second layer in the thickness direction as a boundary, the first region having a copper content (volume %) greater than the copper content (volume %) in the second region. [4] The cathode electrode according to [1] or [2], wherein the copper in the first layer comprises divalent copper, zero-valent copper, and / or monovalent copper, and the copper in the second layer comprises divalent copper, zero-valent copper, and / or monovalent copper. [5] The cathode electrode according to [1] or [2], wherein the copper in the first layer comprises monovalent copper for reduction and / or divalent copper for reduction that are reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that are not reduced to zero-valent copper, and the copper in the second layer comprises monovalent copper for reduction and / or divalent copper for reduction that are reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that are not reduced to zero-valent copper. [6] The cathode electrode according to [1] or [2], wherein the first layer further contains at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon, and the second layer further contains at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon.[7] The cathode electrode according to [6], wherein the additive element in the first layer is contained in an amount of 0.10 atomic % or more and 1.0 atomic % or less relative to 100 atomic % of the copper contained in the first layer, and the additive element in the second layer is contained in an amount of 0.10 atomic % or more and 1.0 atomic % or less relative to 100 atomic % of the copper contained in the second layer. [8] The cathode electrode according to [6], wherein the additive element in the first layer is contained in an amount of 0.25 atomic % or more and 0.70 atomic % or less relative to 100 atomic % of the copper contained in the first layer, and the additive element in the second layer is contained in an amount of 0.25 atomic % or more and 0.70 atomic % or less relative to 100 atomic % of the copper contained in the second layer. [9] The cathode electrode according to [6], wherein the additive element in the first layer includes aluminum, and the additive element in the second layer includes aluminum.

[10] The cathode electrode according to [1] or [2], wherein the cation exchange material includes a sulfonated tetrafluoroethylene-based polymer.

[11] The cathode electrode according to [1] or [2], wherein the metal ions include alkali metal ions and / or alkaline earth metal ions.

[12] The cathode electrode according to [1] or [2], wherein the cation exchange material substituted with the metal ions is formed in the form of a layer on the surface of the first layer.

[13] The cathode electrode according to [1] or [2], wherein the cation exchange material substituted with the metal ions is mixed with the copper of the first layer.

[14] The cathode electrode according to [1] or [2], wherein the cathode electrode has a porous structure.

[15] A composite of a cathode electrode and a substrate, comprising: a substrate; and the cathode electrode according to [1] or [2], wherein the second layer is disposed on the substrate.

[16] The composite according to

[15] , wherein the substrate has a porous structure.

[17] The composite according to

[16] , wherein the substrate having a porous structure is made of carbon, a fluorine-containing resin, or a metal.

[18] The composite according to

[16] , wherein the substrate having a porous structure is made of carbon, and a carbon layer of the carbon is adjacent to the second layer.

[19] An electrolytic reduction device comprising the cathode electrode according to [1] or [2], for electrically reducing carbon dioxide to carbon monoxide, olefinic hydrocarbons and / or alcohols.

[20] An electrolytic reduction device for electrically reducing carbon dioxide to carbon monoxide, an olefinic hydrocarbon, and / or an alcohol, comprising the composite according to

[15] .

[21] A method for producing a composite of a cathode electrode and a substrate that electrically reduces carbon dioxide, the method comprising: a step of preparing a substrate having a carbon layer and a porous structure; a sputtering layer forming step of forming a sputtering layer containing copper on the carbon layer of the substrate by sputtering; and a metal ion-substituted cation exchange material applying step of applying a cation exchange material substituted with a metal ion onto the sputtering layer to surface-modify the sputtering layer.

[22] A method for producing the composite according to

[21] , wherein the copper comprises divalent copper, zero-valent copper, and / or monovalent copper.

[23] A method for producing the composite according to

[21] , wherein the copper comprises monovalent copper for reduction and / or divalent copper for reduction that is reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper.

[24] The method for producing a composite according to any one of

[21] to

[23] , wherein the sputtering layer further contains at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon.

[25] The method for producing a composite according to

[22] or

[23] , further comprising a copper oxidation treatment step, after the sputtering layer formation step and before the metal ion-substituted cation exchange material application step, of oxidizing at least a portion of the copper in the sputtering layer to the monovalent copper and / or divalent copper.

[26] The method for producing a composite according to

[25] , further comprising a partial reduction step, before the metal ion-substituted cation exchange material application step, of partially reducing the monovalent copper and / or divalent copper oxidized in the copper oxidation treatment step to zero-valent copper and / or monovalent copper.

[27] The method for producing a composite according to

[25] , wherein the copper oxidation treatment step is electroless plating.

[28] The method for producing a composite according to

[24] , wherein the sputtering layer forming step includes a copper sputtering layer forming step of forming a copper sputtering layer that is a sputtering layer containing copper by sputtering, and an additional element sputtering layer forming step of forming an additional element sputtering layer that is a sputtering layer containing the additional element on the copper sputtering layer by sputtering.

[29] The method for producing a composite according to

[28] , wherein the sputtering layer forming step includes a copper sputtering layer forming step of further forming a copper sputtering layer that is a sputtering layer containing copper on the additional element sputtering layer by sputtering.

[30] A method for producing a composite of a cathode electrode and a substrate that electrically reduces carbon dioxide, comprising: a step of preparing a substrate having a porous structure; a second layer forming step of applying a second suspension that is a mixture of copper and carbon onto the substrate to form a second layer that is a mixed layer of copper and carbon; a first layer forming step of applying a first suspension containing copper onto the second layer to form a first layer that contains copper; and a metal ion-substituted cation exchange material applying step of applying a cation exchange material substituted with metal ions to the first layer to surface-modify the first layer.

[31] The method for producing the composite according to

[30] , wherein the copper in the first layer contains divalent copper, zero-valent copper and / or monovalent copper, and the copper in the second layer contains divalent copper, zero-valent copper and / or monovalent copper.

[32] The method for producing a composite according to

[30] , wherein the copper in the first layer comprises monovalent copper for reduction and / or divalent copper for reduction, which are reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper, and the copper in the second layer comprises monovalent copper for reduction and / or divalent copper for reduction, which are reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper.

[33] The method for producing a composite according to any one of

[30] to

[32] , wherein the first suspension further contains at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon, and the second suspension further contains at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon.

[0011] In an aspect of the cathode electrode of the present invention, the cathode electrode comprises a first layer having a first thickness located on the surface of the cathode electrode, and a second layer having a second thickness adjacent to the first layer in the thickness direction, wherein the first layer contains copper and the second layer is a mixed layer of copper and carbon. Therefore, even if the first layer located on the surface of the cathode electrode is immersed in the electrolytic solution and the amount of carbon dioxide gas supplied to the first layer is reduced, the second layer, which is a mixed layer of copper and carbon, functions as a site for a reduction reaction of carbon dioxide (CO ２ ) gas and water (H ２ Therefore, according to this aspect of the cathode electrode of the present invention, even if the first layer containing the copper catalyst is immersed in the electrolyte, the selectivity for hydrogen, a by-reaction product, decreases, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction of carbon dioxide can be stably maintained at high efficiency for a long period of time.

[0012] Furthermore, according to an embodiment of the cathode electrode of the present invention, the first layer located on the surface of the cathode electrode is surface-modified with a cation exchange material substituted with metal ions. This allows the amount of water molecules supplied to the copper constituting the cathode electrode to be controlled by the cation exchange material substituted with metal ions, thereby reducing the hydrogen selectivity and enabling the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide to be stably maintained at a high efficiency for a long period of time. The cation exchange material substituted with metal ions allows the hydrogen ions (H ＋ ) is substituted with a metal ion, and therefore, hydrogen ions (H ＋) content has been reduced.

[0013] Furthermore, according to an aspect of the cathode electrode of the present invention, the ratio of the second thickness of the second layer to the first thickness of the first layer is greater than 0 and not greater than 100. This makes it possible to more reliably ensure an interface (three-phase interface) between carbon dioxide gas, water molecules contained in the electrolyte, and the copper catalyst in the second layer. Therefore, even if the first layer is immersed in the electrolyte, the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through a reduction reaction of carbon dioxide can be maintained stably and with high efficiency for an extended period of time.

[0014] Furthermore, according to an aspect of the cathode electrode of the present invention, the second layer has a first region located on the first layer side and a second region adjacent to the first region, with the second layer separated by a central portion in the thickness direction of the second layer, and the copper content (volume %) in the first region is greater than the copper content (volume %) in the second region. This more reliably ensures an interface (three-phase interface) between carbon dioxide gas, water molecules contained in the electrolyte, and the copper catalyst in the second layer. Therefore, even if the first layer is immersed in the electrolyte, the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through a reduction reaction of carbon dioxide can be maintained stably and with high efficiency for an extended period of time.

[0015] In the reduction reaction of carbon dioxide at a cathode electrode using copper as a catalytic material, it is believed that the boundary between copper atoms of different valences present in the cathode electrode (for example, the boundary between zero-valent copper and monovalent copper) functions mainly as a site where carbon derived from carbon dioxide forms a C-C bond and stabilizes the C-C bond. In other words, it is believed that the boundary between copper atoms of different valences (for example, the boundary between zero-valent copper and monovalent copper) is the main active site for the reduction reaction of carbon dioxide. From the above, according to an aspect of the cathode electrode of the present invention, the copper in the first layer contains divalent copper, zero-valent copper, and / or monovalent copper; the copper in the second layer contains divalent copper, zero-valent copper, and / or monovalent copper; or the copper in the first layer contains monovalent copper for reduction and / or divalent copper for reduction that is reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper; and the copper in the second layer contains monovalent copper for reduction and / or divalent copper for reduction that is reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper. This more reliably enables the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction reaction to be maintained at high and stable efficiency for a long period of time.

[0016] In a cathode electrode using copper as a catalytic material, monovalent copper is reduced to zero-valent copper during the carbon dioxide reduction reaction, and divalent copper is reduced to zero-valent copper or monovalent copper. It is thought that if the carbon dioxide reduction reaction is continued for a long period of time, divalent copper is reduced to zero-valent copper or monovalent copper, and monovalent copper is reduced to zero-valent copper, which tends to reduce the boundary between copper with different valences (for example, the boundary between zero-valent copper and monovalent copper). According to the above, in this aspect of the cathode electrode of the present invention, the first layer further contains at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon, and the second layer further contains at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon, whereby the additive element has the function of retaining oxygen within the cathode electrode, thereby appropriately suppressing the reduction of monovalent copper to zero-valent copper. As a result, even if the reduction reaction of carbon dioxide is continued for a long period of time, the abundance ratio of zero-valent copper to monovalent copper is reliably optimized, thereby maintaining the formation and stabilization of C-C bonds and further reliably maintaining the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide stably for a long period of time.

[0017] Both ethylene and ethanol are C2 compounds, and the formation of a C-C bond on the catalyst material is intermediate in the reaction pathway. Therefore, the active sites on the catalyst material for ethylene production and ethanol production are the same or very close to each other, so the stability of ethylene production and the stability of ethanol production tend to be similar, and the carbon dioxide reduction reaction proceeds in the same way in both ethylene and ethanol production.

[0018] Furthermore, according to an aspect of the cathode electrode of the present invention, the additive element in the first layer is contained in an amount of 0.10 atomic % to 1.0 atomic % relative to 100 atomic % of copper contained in the first layer, and the additive element in the second layer is contained in an amount of 0.10 atomic % to 1.0 atomic % relative to 100 atomic % of copper contained in the second layer, thereby ensuring an optimal abundance ratio of zero-valent copper to monovalent copper even when the reduction reaction of carbon dioxide is continued for a long period of time. Therefore, the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be continued more stably for a long period of time.

[0019] Furthermore, according to an aspect of the cathode electrode of the present invention, the additive element in the first layer is contained in an amount of 0.25 atomic % to 0.70 atomic % relative to 100 atomic % of copper contained in the first layer, and the additive element in the second layer is contained in an amount of 0.25 atomic % to 0.70 atomic % relative to 100 atomic % of copper contained in the second layer, so that the abundance ratio of zero-valent copper to monovalent copper can be maintained within a more appropriate range even when the reduction reaction of carbon dioxide is continued for a long period of time. Therefore, the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be continued for a longer period of time and more stably.

[0020] Furthermore, according to this aspect of the cathode electrode of the present invention, the additive element in the first layer includes aluminum, and the additive element in the second layer includes aluminum, thereby reliably achieving the function of retaining an appropriate amount of oxygen within the cathode electrode, and therefore the ratio of zero-valent copper to monovalent copper is reliably optimized even when the reduction reaction of carbon dioxide is continued for a long period of time. Therefore, the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be continued more stably for a long period of time.

[0021] Furthermore, in accordance with an embodiment of the cathode electrode of the present invention, the cation exchange material contains a sulfonated tetrafluoroethylene polymer, and the metal ions contain alkali metal ions and / or alkaline earth metal ions, thereby reliably controlling the amount of water molecules supplied to the copper constituting the cathode electrode. This more reliably reduces the selectivity for hydrogen, a by-product, and more reliably maintains high efficiency over a long period of time in the catalytic reaction that produces olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the reduction of carbon dioxide.

[0022] Furthermore, according to an aspect of the cathode electrode of the present invention, the cation exchange material substituted with metal ions is formed in the form of a layer on the surface of the first layer, or the cation exchange material substituted with metal ions is mixed with the copper of the first layer. As a result, the presence of the cation exchange material substituted with metal ions more appropriately controls the amount of water molecules supplied to copper.

[0023] Furthermore, according to an aspect of the cathode electrode of the present invention, the cathode electrode has a porous structure, which facilitates contact between water and carbon dioxide at the site of the carbon dioxide reduction reaction at the cathode electrode, thereby enabling the catalytic reaction to produce olefinic hydrocarbons such as ethylene and alcohols such as ethanol to be maintained more stably for a long period of time.

[0024] In one embodiment of the composite of a cathode electrode and a substrate of the present invention, the composite includes a substrate and a cathode electrode of the present invention having a second layer disposed on the substrate, so that even if the first layer located on the surface of the cathode electrode becomes immersed in the electrolytic solution, carbon dioxide gas can smoothly come into contact with the second layer of the cathode electrode. Therefore, in this embodiment of the composite of a cathode electrode and a substrate of the present invention, even if the first layer located on the surface of the cathode electrode becomes immersed in the electrolytic solution, the selectivity for hydrogen, a by-product, decreases, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction of carbon dioxide can be maintained at a high and stable efficiency for a long period of time.

[0025] The method for producing a composite of a cathode electrode and a substrate of the present invention includes a step of preparing a substrate having a carbon layer and a porous structure, a step of forming a sputtering layer containing copper on the carbon layer of the substrate by sputtering, and a step of applying a cation exchange material substituted with metal ions to the sputtering layer to surface modify the sputtering layer. This reduces the selectivity of hydrogen, a by-product, even if the surface of the cathode electrode is immersed in an electrolyte, allowing the production of a composite that can stably maintain high efficiency over a long period of time in the catalytic reaction that produces olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction of carbon dioxide. Furthermore, the method for producing a composite of a cathode electrode and a substrate of the present invention includes the step of applying a cation exchange material substituted with metal ions, allowing the metal ion-substituted cation exchange material to control the amount of water molecules supplied to the copper constituting the cathode electrode, thereby reducing the selectivity of hydrogen, allowing the production of a composite that can stably maintain high efficiency over a long period of time in the catalytic reaction that produces olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction of carbon dioxide.

[0026] The method for producing a composite of a cathode electrode and a substrate of the present invention includes the steps of: preparing a substrate having a porous structure; a second layer forming step of applying a second suspension, which is a mixture of copper and carbon, onto the substrate to form a second layer, which is a mixed layer of copper and carbon; a first layer forming step of applying a first suspension containing copper onto the second layer to form a first layer containing copper; and a metal ion-substituted cation exchange material applying step of applying a cation exchange material substituted with metal ions to the first layer to surface-modify the first layer. This makes it possible to produce a composite in which, even if the first layer located on the surface of the cathode electrode is immersed in the electrolyte, the selectivity for hydrogen, a by-product, is reduced, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by a reduction reaction of carbon dioxide can be maintained at a high and stable efficiency for a long period of time. Furthermore, according to the method for producing a composite of a cathode electrode and a substrate of the present invention, by including the step of applying a cation exchange material substituted with metal ions, the amount of water molecules supplied to the copper constituting the cathode electrode can be controlled by the cation exchange material substituted with metal ions. This reduces the hydrogen selectivity, making it possible to produce a composite in which the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction of carbon dioxide can be maintained at a high and stable efficiency for a long period of time.

[0027] FIG. 1 is an explanatory diagram showing an outline of a cross section of a composite of a cathode electrode and a substrate of the present invention. FIG. 2 is a cross-sectional view illustrating a carbon dioxide reduction reaction in a composite of a cathode electrode and a substrate of the present invention. FIG. 3 is an explanatory diagram of an electrolytic polishing treatment step in a method for manufacturing a composite of a cathode electrode and a substrate. FIG. 4 is an explanatory diagram of a sputtering layer formation step and a copper oxidation treatment step in a method for manufacturing a composite of a cathode electrode and a substrate. FIG. 5 is an explanatory diagram of a partial reduction step in a method for manufacturing a composite of a cathode electrode and a substrate. FIG. 6 is an explanatory diagram showing a state in which a cation exchange material substituted with metal ions has been applied in a method for manufacturing a composite of a cathode electrode and a substrate. FIG. 7 is an explanatory diagram showing an outline of an electrolytic reduction device comprising the cathode electrode of the present invention. FIG. 8 is an explanatory diagram showing an outline of another electrolytic reduction device comprising the cathode electrode of the present invention.

[0028] [Cathode Electrode] The cathode electrode of the present invention will be described below. Fig. 1 is an explanatory diagram showing an outline of a cross section of a composite of a cathode electrode and a substrate of the present invention. Fig. 2 is a cross-sectional view illustrating a carbon dioxide reduction reaction in the composite of a cathode electrode and a substrate of the present invention.

[0029] 1 and 2 , the cathode electrode 100 of the present invention is a cathode electrode that electrically reduces carbon dioxide. The cathode electrode 100 has a first layer 111 having a first thickness located on the surface of the cathode electrode 100, and a second layer 112 having a second thickness that is adjacent to the first layer 111 in the thickness direction. The outer surface of the first layer 111 of the cathode electrode 100 is exposed to the external environment of the cathode electrode 100 and the composite 120 described below. On the other hand, the second layer 112 of the cathode electrode 100 is not exposed to the external environment of the composite 120.

[0030] The cathode electrode 100 has a first portion 101 and a second portion 102 facing the first portion 101. The second portion 102 of the cathode electrode 100 is exposed to the external environment of the cathode electrode 100 and the composite 120. The second portion 102 of the cathode electrode 100 is in contact with an electrolyte solution 130, which is an aqueous solution in which an electrolyte is dissolved. The outer surface of the first layer 111 of the cathode electrode 100 corresponds to the outer surface of the second portion 102 of the cathode electrode 100. From the above, the first layer 111 of the cathode electrode 100 is in contact with the electrolyte solution 130.

[0031] On the other hand, since the substrate 1 of the composite 120 described below is provided on the first portion 101 side of the cathode electrode 100, the first portion 101 of the cathode electrode 100 is not exposed to the external environment of the composite 120. The surface of the second layer 112 of the cathode electrode 100 corresponds to the surface of the first portion 101 of the cathode electrode 100.

[0032] The first layer 111 of the cathode electrode 100 contains copper (Cu), and the first layer 111 is surface-modified with a cation exchange material substituted with metal ions. As described above, the first layer 111 of the cathode electrode 100 contains, as essential components, copper, which is a catalytic material, and the cation exchange material substituted with metal ions that modifies the surface of the first layer 111. The cation exchange material substituted with metal ions converts hydrogen ions (H ＋ ) is substituted with a metal ion, and hydrogen ions (H ＋ The first layer 111 is a cation exchange material having a reduced content of cations such as methyl cellulose, methyl cellulose, methyl cellulose, methyl cellulose ester ...

[0033] The second layer 112 of the cathode electrode 100 is a mixed layer of copper (Cu) and carbon (C). As described above, the second layer 112 of the cathode electrode 100 contains, as essential components, copper, which is a catalytic material, and carbon, which is a material that uniformly diffuses carbon dioxide gas. Examples of the carbon that constitutes the second layer 112 include particulate carbon, and specifically, carbon black.

[0034] Therefore, the cathode electrode 100 has a two-layer structure including a first layer 111 containing copper but not carbon, and a second layer 112 containing copper and carbon.

[0035] The cathode electrode 100 has a first layer 111 having a first thickness and located on the surface of the cathode electrode 100, and a second layer 112 having a second thickness and adjacent to the first layer 111 in the thickness direction, where the first layer 111 contains copper and the second layer 112 is a mixed layer of copper and carbon. Therefore, even if the first layer 111 located on the surface of the cathode electrode 100 is immersed in the electrolytic solution 130, and the amount of carbon dioxide gas supplied to the first layer 111 decreases, the second layer 112, which is a mixed layer of copper and carbon, contains carbon, and therefore the amount of carbon dioxide gas supplied to the second layer 112 is maintained. Therefore, even if the first layer 111 is immersed in the electrolytic solution 130, the second layer 112 functions as a site for a reduction reaction, and carbon dioxide (CO ２ ) gas and water (H ２An interface (three-phase interface) is formed between the O molecules and the copper (Cu) catalyst.

[0036] As described above, in the cathode electrode 100, even if the first layer 111 containing copper as a catalyst is immersed in the electrolytic solution 130, the selectivity of hydrogen, which is a by-product of the reduction reaction of carbon dioxide, decreases, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be stably maintained at a high efficiency for a long period of time.

[0037] In addition, in the cathode electrode 100, the first layer 111 located on the surface of the cathode electrode 100 is made of a cation exchange material (hydrogen ions (H ＋ Since the surface of the cathode electrode 100 is modified with a cation exchange material in which the copper ions are substituted with metal ions, the amount of water molecules supplied to the copper constituting the cathode electrode 100, i.e., the amount of hydrogen ions (H ＋ The amount of cation exchange material substituted with metal ions is controlled by the metal ion-substituted cation exchange material. Furthermore, since the first layer 111 is surface-modified with the metal ion-substituted cation exchange material, the action of the metal ions promotes the penetration of copper from the first layer 111 into the underlying carbon, which is thought to significantly contribute to the formation of the second layer 112, a mixed layer of copper and carbon. From the above, the surface modification of the first layer 111 with the metal ion-substituted cation exchange material significantly contributes to the formation of a two-layer cathode electrode 100 including the first layer 111 containing copper but not carbon and the second layer 112 containing copper and carbon. Furthermore, since the cation exchange material is substituted with metal ions and the hydrogen ion content is reduced, the supply of hydrogen ions from the cation exchange material to copper is also prevented. Therefore, the cathode electrode 100 has a structure that can prevent excessive supply of hydrogen ions to the copper, which is a catalytic material. In addition, since the state in which the hydrogen ions of the cation exchange material are substituted with metal ions is more stable than the state in which the hydrogen ions are not substituted with metal ions, the cation exchange material in the cathode electrode 100 is maintained in a state in which it is substituted with metal ions.

[0038] Therefore, in the cathode electrode 100, the selectivity of hydrogen, a by-product of the carbon dioxide reduction reaction, decreases, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably maintained at high efficiency for a long period of time.

[0039] The ratio of the second thickness of the second layer 112 to the first thickness of the first layer 111 is not particularly limited, but the lower limit thereof is preferably greater than 0, because this ensures an interface (three-phase interface) in the second layer 112 between the carbon dioxide gas, the water molecules contained in the electrolytic solution 130, and the copper catalyst, and as a result, even if the first layer 111 is immersed in the electrolytic solution 130, the catalytic reaction of producing olefin hydrocarbons such as ethylene and alcohols such as ethanol through a reduction reaction of carbon dioxide can be stably and efficiently sustained for a long period of time. Furthermore, the lower limit of the ratio is preferably 0.01, more preferably 0.05, even more preferably 0.1, and particularly preferably 0.5, because this more reliably ensures an interface (three-phase interface) between the carbon dioxide gas, the water molecules contained in the electrolytic solution 130, and the copper catalyst in the second layer 112, and as a result, even if the first layer 111 is immersed in the electrolytic solution 130, the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through a reduction reaction of carbon dioxide can be sustained with high efficiency and stability for a longer period of time. On the other hand, the upper limit of the ratio of the second thickness of the second layer 112 to the first thickness of the first layer 111 is preferably 100, more preferably 50, even more preferably 10, and particularly preferably 5.0, because this more reliably ensures an interface (three-phase interface) between the carbon dioxide gas, the water molecules contained in the electrolytic solution 130, and the copper catalyst.

[0040] The distribution of the copper content in the second layer 112 is not particularly limited. However, in the second layer 112, an interface (three-phase interface) between the carbon dioxide gas, the water molecules contained in the electrolytic solution 130, and the copper catalyst can be more reliably secured. As a result, even if the first layer 111 is immersed in the electrolytic solution 130, the catalytic reaction of producing olefin hydrocarbons such as ethylene and alcohols such as ethanol through a reduction reaction of carbon dioxide can be continued stably with high efficiency for an even longer period of time. Therefore, it is preferable that the second layer 112 has, with the central part in the thickness direction of the second layer 112 as the boundary, a first region located on the first layer 111 side and a second region adjacent to the first region, and that the copper content (volume %) in the first region is greater than the copper content (volume %) in the second region.

[0041] The cation exchange material constituting the cation exchange material substituted with metal ions, i.e., the cation exchange material before being substituted with metal ions, can be, for example, a cation exchange resin, and examples of the cation exchange resin include sulfonated tetrafluoroethylene-based polymers (trade name "Nafion"), perfluoroalkyl compound (PFAS)-based polymers, polyethylenedioxythiophene (PEDOT)-based polymers, etc. Among these, sulfonated tetrafluoroethylene-based polymers are preferred because they facilitate surface modification of the copper-containing first layer 111.

[0042] As long as the hydrogen ions of the cation exchange material are substituted with metal ions, the metal ions constituting the cation exchange material substituted with metal ions are not particularly limited. However, alkali metal ions and alkaline earth metal ions are preferred because they reliably control the amount of water molecules supplied to the copper constituting the cathode electrode 100, more reliably reduce the selectivity of hydrogen, a by-product, and more reliably maintain high efficiency over a long period of time in the catalytic reaction that produces olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the reduction reaction of carbon dioxide. As the alkali metal, lithium ions (Li ＋ ), sodium ions (Na ＋ ), potassium ions (K ＋ ), rubidium ion (Rb ＋), cesium ions (Cs ＋ ), francium ion (Fr ＋ ) as alkaline earth metal ions. ２＋ ), magnesium ions (Mg ２＋ ), calcium ions (Ca ２＋ ), strontium ions (Sr ２＋ ), barium ions (Ba ２＋ ), radium ions (Ra ２＋ These metal ions may be used alone or in combination of two or more.

[0043] Among these metal ions, alkali metal ions are more preferred because they more reliably control the amount of water molecules supplied to the copper constituting the cathode electrode 100 and more reliably reduce the selectivity for hydrogen, which is a by-product of the reaction. Lithium ions (Li ＋ ), sodium ions (Na ＋ ), potassium ions (K ＋ ), rubidium ion (Rb ＋ ) is more preferred, and potassium ions (K) are preferred because they can further control the amount of water molecules supplied to copper and further reduce the selectivity for hydrogen, which is a by-product of the reaction. ＋ ) is particularly preferred.

[0044] The form in which the first layer 111 is surface-modified with a cation exchange material substituted with metal ions is not particularly limited, but from the viewpoint that the presence of the cation exchange material substituted with metal ions more appropriately controls the amount of water molecules supplied to copper, preferred are a form in which the cation exchange material substituted with metal ions is formed in the form of a layer on the surface of the first layer 111, and a form in which the cation exchange material substituted with metal ions is mixed with copper in the first layer 111, and particularly preferred is a form in which the cation exchange material substituted with metal ions is formed in the form of a layer on the surface of the first layer 111. An example of a form in which the cation exchange material substituted with metal ions is formed in the form of a layer on the surface of copper contained in the first layer 111 is one in which the cation exchange material substituted with metal ions is formed in the form of a layer on the surface of copper contained in the first layer 111.

[0045] The content of the cation exchange material substituted with metal ions is not particularly limited, but the lower limit is preferably 125 g or more and 24,333 g or less of the deposited amount of cation exchange material per 100 g of copper element in the first layer 111 and copper element in the second layer 112, in order to reliably control the amount of water molecules supplied to copper and reliably prevent excessive supply of hydrogen ions to copper.

[0046] A method for preparing a cation exchange material substituted with metal ions includes, for example, preparing a solution of the cation exchange material (e.g., an aqueous solution) and a solution containing metal ions (e.g., an aqueous solution), and mixing the solution of the cation exchange material with the solution containing metal ions, thereby substituting the hydrogen ions of the cation exchange material with the metal ions to obtain a cation exchange material substituted with metal ions.

[0047] In the cathode electrode 100 of the present invention, as a first embodiment of the cathode electrode, the copper serving as the catalytic material of the first layer 111 contains divalent copper, zero-valent copper, and / or monovalent copper, and the copper serving as the catalytic material of the second layer 112 contains divalent copper, zero-valent copper, and / or monovalent copper. In the first embodiment of the cathode electrode, the first layer 111 and the second layer 112 contain divalent copper, zero-valent copper, and / or monovalent copper as essential components. The monovalent copper may be cuprous oxide (Cu ２ Divalent copper includes copper oxide (CuO), and zero-valent copper includes simple copper (Cu).

[0048] In the carbon dioxide reduction reaction in the cathode electrode 100 using copper as a catalytic material, it is believed that the boundary between copper of different valences present in the cathode electrode 100 (e.g., the boundary between zero-valent copper and monovalent copper) functions primarily as a site where carbon derived from carbon dioxide forms a C-C bond and stabilizes the C-C bond. In other words, it is believed that the boundary between copper of different valences (e.g., the boundary between zero-valent copper and monovalent copper) is the main active site for the carbon dioxide reduction reaction. From the above, by having the copper of the first layer 111 contain divalent copper, zero-valent copper, and / or monovalent copper, and the copper of the second layer 112 contain divalent copper, zero-valent copper, and / or monovalent copper, the catalytic reaction to produce olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be more reliably maintained at high efficiency over a long period of time.

[0049] In the cathode electrode 100 of the present invention, as a second cathode electrode embodiment, the copper in the first layer 111 includes monovalent copper for reduction and / or divalent copper for reduction that are reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that are not reduced to zero-valent copper, and the copper in the second layer 112 includes monovalent copper for reduction and / or divalent copper for reduction that are reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that are not reduced to zero-valent copper. In the second cathode electrode embodiment, part of the monovalent copper and / or divalent copper in the first layer 111 is reduced to zero-valent copper, and part of the monovalent copper and / or divalent copper in the second layer 112 is reduced to zero-valent copper. From the above, in the second cathode electrode embodiment, copper (Cu) exists as monovalent copper for reduction and / or divalent copper for reduction, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper. The second cathode electrode embodiment described above contains monovalent copper and / or divalent copper as essential components of the first layer 111 and the second layer 112. In the second cathode electrode embodiment, by performing a reduction treatment, the divalent copper for reduction is reduced to zero-valent copper or monovalent copper, and the monovalent copper for reduction is reduced to zero-valent copper. Therefore, in the second cathode electrode embodiment, by performing a reduction treatment, the cathode electrode becomes one that contains zero-valent copper and monovalent copper and / or divalent copper.

[0050] As monovalent copper, cuprous oxide (Cu ２ Divalent copper includes copper oxide (CuO), and zero-valent copper includes simple copper (Cu).

[0051] The copper in the first layer 111 contains monovalent copper for reduction and / or divalent copper for reduction that is reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper, and the copper in the second layer 112 contains monovalent copper for reduction and / or divalent copper for reduction that is reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper. This makes it possible to more reliably maintain high efficiency in the catalytic reaction that produces olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the reduction reaction of carbon dioxide over a long period of time.

[0052] The cathode electrode 100 of the present invention (including the first and second cathode electrode embodiments) may contain, in addition to copper (Cu), at least one additive element (M) selected from the group consisting of silver (Ag), gold (Au), cadmium (Cd), tin (Sn), aluminum (Al), boron (B), gallium (Ga), zinc (Zn), titanium (Ti), and silicon (Si) as an optional component.

[0053] In the cathode electrode 100 in which copper is used as a catalytic material, monovalent copper is reduced to zero-valent copper during the carbon dioxide reduction reaction, and divalent copper is reduced to zero-valent copper or monovalent copper. It is thought that if the carbon dioxide reduction reaction is continued for a long period of time, divalent copper is reduced to zero-valent copper or monovalent copper, and monovalent copper is reduced to zero-valent copper, which tends to reduce the boundary between copper with different valences (for example, the boundary between zero-valent copper and monovalent copper). Therefore, the first layer 111 contains, in addition to copper, at least one additive element (M1) selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon, and the second layer 112 contains, in addition to copper, at least one additive element (M2) selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon. The additive elements (M1) and (M2) have the function of retaining oxygen within the cathode electrode 100, thereby appropriately suppressing the reduction of monovalent copper to zero-valent copper. As described above, even if the reduction reaction of carbon dioxide is continued for a long period of time, the abundance ratio of zero-valent copper and monovalent copper is reliably optimized, thereby maintaining the generation and stabilization of C-C bonds and further reliably maintaining the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide stably for a long period of time.

[0054] The form of the at least one additive element (M) selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon is not particularly limited, and examples thereof include the form of the additive element (M) itself (single additive element (M)). In addition to the form of the additive element (M) itself, examples thereof include the form of a hydroxide and the form of an oxide. Furthermore, the additive element (M) may be a mixture of the form of the additive element (M) itself, the form of a hydroxide, and the form of an oxide. These additive elements (M) may be used alone, or two or more types may be used in combination.

[0055] The content of the additional element (M1) in the first layer 111 is not particularly limited, but the lower limit thereof is preferably 0.10 atomic %, more preferably 0.20 atomic %, relative to 100 atomic % of copper element including all copper, such as zero-valent copper, monovalent copper, and divalent copper, contained in the first layer 111, because the abundance ratio of zero-valent copper to monovalent copper is reliably optimized even when the reduction reaction of carbon dioxide is continued for a long period of time, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be continued for a long period of time. A proportion of 0.25 atomic % is particularly preferable because the abundance ratio of zero-valent copper to monovalent copper can be maintained within an appropriate range even when the reduction reaction of carbon dioxide is continued for a long period of time, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be continued for an even longer period of time. On the other hand, the upper limit of the content of the additional element (M1) in the first layer 111, relative to 100 atomic % of copper element including all copper, such as zero-valent copper, monovalent copper, and divalent copper, contained in the first layer 111, is preferably 1.0 atomic %, more preferably 0.85 atomic %, because the abundance ratio of zero-valent copper to monovalent copper is reliably optimized even if the reduction reaction of carbon dioxide is continued for a long period of time, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be continued for a long period of time. A proportion of 0.70 atomic % is particularly preferable because the abundance ratio of zero-valent copper to monovalent copper can be maintained within an appropriate range even if the reduction reaction of carbon dioxide is continued for a long period of time, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be continued for an even longer period of time.

[0056] The content of the additional element (M2) in the second layer 112 is not particularly limited, but the lower limit thereof is preferably 0.10 atomic %, more preferably 0.20 atomic %, relative to 100 atomic % of copper element, which includes all copper contained in the second layer 112, such as zero-valent copper, monovalent copper, and divalent copper, because the abundance ratio of zero-valent copper to monovalent copper is reliably optimized even when the reduction reaction of carbon dioxide is continued for a long period of time, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be continued for an even longer period of time. A proportion of 0.25 atomic % is particularly preferred because the abundance ratio of zero-valent copper to monovalent copper can be maintained within an even more appropriate range even when the reduction reaction of carbon dioxide is continued for a long period of time, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be continued for an even longer period of time. On the other hand, the upper limit of the content of the additional element (M2) in the second layer 112 is preferably 1.0 atomic %, more preferably 0.85 atomic %, relative to 100 atomic % of copper element, which includes all copper, such as zero-valent copper, monovalent copper, and divalent copper, contained in the second layer 112, because the abundance ratio of zero-valent copper to monovalent copper is reliably optimized even if the reduction reaction of carbon dioxide is continued for a long period of time, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be continued for a long period of time. A proportion of 0.70 atomic % is particularly preferable because the abundance ratio of zero-valent copper to monovalent copper can be maintained within an appropriate range even if the reduction reaction of carbon dioxide is continued for a long period of time, and the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide can be continued for an even longer period of time.

[0057] Any of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon can be used as the additive element (M1) of the first layer 111 and the additive element (M2) of the second layer 112, because they tend to be more easily oxidized than copper and have a higher affinity for oxygen than copper. On the other hand, of the additive element (M1) of the first layer 111 and the additive element (M2) of the second layer 112, aluminum is preferred because it reliably provides the function of retaining an appropriate amount of oxygen within the cathode electrode 100, and therefore the ratio of zero-valent copper to monovalent copper is reliably optimized even if the reduction reaction of carbon dioxide is continued for a long period of time, allowing the catalytic reaction that produces olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the reduction reaction of carbon dioxide to be continued more stably for a long period of time.

[0058] The structure of the cathode electrode 100 is not particularly limited and may be solid or porous. However, a porous structure is preferred because it facilitates contact between water and carbon dioxide during the carbon dioxide reduction reaction at the cathode electrode 100, thereby enabling the catalytic reaction to produce olefinic hydrocarbons such as ethylene and alcohols such as ethanol to continue more stably over a long period of time. The proportion of voids in the porous structure (porosity) is not particularly limited, but the lower limit is preferably 1 vol%, and more preferably 10 vol%, because this facilitates the permeation of carbon dioxide into the cathode electrode 100, thereby further improving the production efficiency of olefinic hydrocarbons such as ethylene and alcohols such as ethanol. On the other hand, the upper limit of the porosity of the porous structure is preferably 99 vol%, and more preferably 90 vol%, because this maintains the surface area of ​​the cathode electrode 100 that contributes to the catalytic reaction, thereby further improving the production efficiency of olefinic hydrocarbons such as ethylene and alcohols such as ethanol. The porous structure of the cathode electrode 100 can be formed, for example, by performing a partial reduction treatment, as described below.

[0059] The cathode electrode 100 of the present invention can produce olefinic hydrocarbons such as ethylene, and alcohols such as ethanol by, for example, supplying carbon dioxide gas from the first portion 101 side of the cathode electrode 100 and supplying liquid-phase water from the second portion 102 side of the cathode electrode 100, causing the carbon dioxide and water to react with each other through the catalytic action of the cathode electrode 100 and electrically reducing the carbon dioxide.

[0060] [Composite of Cathode Electrode and Substrate] The cathode electrode 100 of the present invention may be used in the state of the cathode electrode 100 alone, or may be used in the state of forming a composite 120 with a substrate 1 as described below.

[0061] 1 and 2 , a composite 120 of a cathode electrode 100 and a substrate 1 includes the substrate 1 and the cathode electrode 100 of the present invention, in which a second layer 112 is disposed on the substrate 1. The substrate 1 may have a carbon layer 113 on its surface. Examples of the carbon layer 113 include a carbon layer 113 formed from particulate carbon, and specific examples include a carbon layer 113 formed from carbon black.

[0062] The substrate 1 is provided on the first portion 101 side of the cathode electrode 100. That is, the first portion 101 side of the cathode electrode 100 is disposed on the substrate 1. Furthermore, in the composite 120, the carbon layer 113 of the substrate 1 is adjacent to the second layer 112 of the cathode electrode 100. From the above, the composite 120 has a three-layer structure in which the first layer 111 of the cathode electrode 100, the second layer 112 of the cathode electrode 100, and the carbon layer 113 of the substrate 1 are laminated. By supplying carbon dioxide gas from the substrate 1 side of the composite 120 (i.e., the first portion 101 side of the cathode electrode 100), it is possible to uniformly diffuse the carbon dioxide gas in the second layer 112 of the cathode electrode 100, thereby facilitating the supply of carbon dioxide to the cathode electrode 100.

[0063] No substrate is provided on the second portion 102 side of the cathode electrode 100, and the second portion 102 is exposed to the external environment of the cathode electrode 100 and the composite 120. The cathode electrode 100 has a coating film that covers the surface of the substrate 1. In the composite 120 of the cathode electrode 100 and the substrate 1, even if the first layer 111 located on the surface of the cathode electrode 100 is immersed in the electrolytic solution 130, carbon dioxide gas can smoothly come into contact with the second layer 112 of the cathode electrode 100. Therefore, in the composite 120 of the present invention, even if the first layer 111 located on the surface of the cathode electrode 100 is immersed in the electrolytic solution 130, the selectivity of hydrogen, a by-product, decreases, and the catalytic reaction that produces olefinic hydrocarbons such as ethylene and alcohols such as ethanol by a reduction reaction of carbon dioxide can be stably maintained at high efficiency for a long period of time.

[0064] The substrate 1 may be solid or porous, but a gas-permeable porous structure is preferred because it allows carbon dioxide gas to smoothly come into contact with the cathode electrode 100 and allows the catalytic reaction that produces olefinic hydrocarbons such as ethylene and alcohols such as ethanol to continue more stably for a long period of time.

[0065] The material of the porous substrate 1 is not particularly limited, but is preferably, for example, carbon, a fluorine-containing resin, or a metal, from the viewpoint of being able to obtain a composite 120 of the cathode electrode 100 and the substrate 1, in which the catalytic reaction that produces olefin hydrocarbons such as ethylene or alcohols such as ethanol by a reduction reaction of carbon dioxide can be maintained more stably for a long period of time.

[0066] Examples of carbon include a composite of carbon black and carbon fiber. As shown in Figures 1 and 2, in composite 120, substrate 1 is a carbon material that is a composite of carbon layer 113 formed of carbon black and carbon fiber 114, and has a structure in which carbon layer 113 is laminated on carbon fiber 114. In composite 120, carbon layer 113 provided on carbon fiber 114 is adjacent to second layer 112.

[0067] Examples of fluorine-containing resins include polytetrafluoroethylene, polyvinylidene fluoride, perfluoroalkoxyalkane, perfluoroethylenepropylene copolymer, ethylenetetrafluoroethylene copolymer, polychlorotrifluoroethylene, ethylenechlorotrifluoroethylene copolymer, etc. Examples of metals include porous metals made of metals such as copper (Cu), niobium (Nb), aluminum (Al), titanium (Ti), alloys containing one or more of the above metals, and stainless steel. When the material of the porous substrate 1 is metal and the metal is copper (Cu), examples of the porous metal include sintered bodies of copper particles.

[0068] The average thickness of the substrate 1 is not particularly limited, but may be, for example, a plate-like member having an average thickness of 0.2 mm or more and 1.5 mm or less.

[0069] The cathode electrode 100 in the composite 120 of the cathode electrode 100 and the substrate 1 is, for example, a sputtering layer formed on the substrate 1 by sputtering, and containing copper and, if necessary, an additive element (M).

[0070] [Method for manufacturing a composite of a cathode electrode and a substrate] A method for manufacturing a composite of a cathode electrode and a substrate (first manufacturing method) in which carbon dioxide is electrically reduced will be described below. Fig. 3 is an explanatory diagram of the electrolytic polishing treatment step in the method for manufacturing a composite of a cathode electrode and a substrate. Fig. 4 is an explanatory diagram of the sputtering layer formation step and the copper oxidation treatment step in the method for manufacturing a composite of a cathode electrode and a substrate. Fig. 5 is an explanatory diagram of the partial reduction step in the method for manufacturing a composite of a cathode electrode and a substrate. Fig. 6 is an explanatory diagram showing the state in which a cation exchange material substituted with metal ions has been applied in the method for manufacturing a composite of a cathode electrode and a substrate.

[0071] The first method for producing a composite of a cathode electrode and a substrate includes: (1) preparing a substrate having a carbon layer and a porous structure; (2) an electrolytic polishing treatment step in which the prepared substrate is subjected to electrolytic polishing treatment as necessary; (3) a sputtering layer formation step in which a sputtering layer having copper is formed by sputtering on the carbon layer of the substrate on which electrolytic polishing treatment has been performed as necessary; (4) if necessary, after the sputtering layer formation step, a copper oxidation treatment step in which at least a portion of the copper in the sputtering layer is oxidized to monovalent copper and / or divalent copper; (5) if necessary, a partial reduction step in which the monovalent copper and / or divalent copper oxidized in the copper oxidation treatment step is partially reduced to zero-valent copper and / or monovalent copper; and (6) if necessary, a metal ion-substituted cation exchange material application step in which a cation exchange material substituted with a metal ion is applied to the sputtering layer on which the copper oxidation treatment step and partial reduction step have been performed to surface-modify the sputtering layer. Of the above steps, steps (1), (3), and (6) are essential, and steps (2), (4), and (5) are optional.

[0072] The above-described method for producing a composite of a cathode electrode and a substrate can form a cathode electrode on a substrate, in which a first layer is surface-modified with a cation exchange material substituted with metal ions. Surface-modifying the first layer of the cathode electrode with a cation exchange material substituted with metal ions is believed to promote penetration of copper from the first layer into the underlying carbon layer of the substrate by the action of the metal ions, significantly contributing to the formation of a second layer, which is a mixed layer of copper and carbon. From the above, the step of surface-modifying the first layer with a cation exchange material substituted with metal ions (i.e., the metal ion-substituted cation exchange material application step) can produce a two-layer cathode electrode having a first layer containing copper but not carbon, and a second layer containing copper and carbon.

[0073] The above-described method for producing a composite of a cathode electrode and a substrate reduces the selectivity of hydrogen, a by-reaction product, even if the surface layer of the cathode electrode is immersed in the electrolyte, making it possible to produce a composite in which the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction of carbon dioxide can be stably maintained at high efficiency for a long period of time. Furthermore, the above-described method for producing a composite of a cathode electrode and a substrate includes a step of applying a cation exchange material substituted with metal ions, which allows the cation exchange material substituted with metal ions to control the amount of water molecules supplied to the copper constituting the cathode electrode. This reduces the selectivity of hydrogen, making it possible to produce a composite in which the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction of carbon dioxide can be stably maintained at high efficiency for a long period of time.

[0074] Examples of the cathode electrode formed by the above-described method for producing a composite of a cathode electrode and a substrate include a cathode electrode containing divalent copper, zero-valent copper and / or monovalent copper, and a cathode electrode containing monovalent copper for reduction and / or divalent copper for reduction that is reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper.

[0075] (1) Step of preparing a substrate having a carbon layer and a porous structure The step of preparing a substrate having a carbon layer and a porous structure is the step of preparing the substrate described above. The material of the substrate having a carbon layer and the porosity of the porous structure can be appropriately selected depending on the properties required of the composite of the cathode electrode and the substrate.

[0076] (2) Electrolytic Polishing Process The electrolytic polishing process is a process that is performed as necessary, for example, when a metal is used as the substrate material. In the electrolytic polishing process, the substrate surface is degreased with an organic solvent such as hexane, washed, and dried. After that, as shown in FIG. 3, a mixed acid solution 11 is placed in a container 10, an anode substrate 1 is immersed in the mixed acid solution 11, a cathode 2 is immersed in a position sandwiching the substrate 1, and an electrolytic potential is applied to the anode substrate 1 and the cathode 2. By applying an electrolytic potential to the anode substrate 1 and the cathode 2, the surface of the substrate 1 is electropolished. By electropolishing the surface of the substrate 1, the work-affected layer on the surface of the substrate 1 is reduced and removed. Examples of the mixed acid solution 11 include an aqueous mixed acid solution of phosphoric acid and sulfuric acid. Examples of the cathode 2 include titanium. Note that, as shown in FIGS. 1 and 2, when the substrate 1 of the composite 120 is made of a carbon material, electrolytic polishing does not need to be performed.

[0077] (3) Sputtering Layer Forming Step As shown in Fig. 4, the sputtering layer forming step forms a sputtering layer 20 having copper by sputtering on the substrate 1. When the sputtering layer 20 contains, in addition to copper (Cu), at least one additional element (M) selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon as an optional component, as shown in Fig. 4, the sputtering layer forming step includes a copper (Cu) sputtering layer forming step of forming a copper (Cu) sputtering layer, which is a sputtering layer having copper (Cu), on the substrate 1 by sputtering, and an additional element (M) sputtering layer forming step of forming an additional element (M) sputtering layer, which is a sputtering layer having the additional element (M), on the copper (Cu) sputtering layer by sputtering. The method also includes a copper (Cu) sputtering layer formation step in which a copper (Cu) sputtering layer containing copper (Cu) is formed on the formed additional element (M) sputtering layer by sputtering. The content of the additional element (M) relative to 100 atomic % of copper (Cu) can be adjusted by adjusting the amount of power from the power source and the sputtering time. Furthermore, if necessary, the copper (Cu) sputtering layer formation step and the additional element (M) sputtering layer formation step may be alternately performed multiple times to ensure uniform distribution of copper (Cu) and the additional element (M) throughout the entire sputtering layer 20. Through the sputtering layer formation step, it is possible to form on the substrate 1 a cathode electrode containing divalent copper and zero-valent copper and / or monovalent copper, and optionally containing an additive element, or a cathode electrode containing monovalent copper for reduction and / or divalent copper for reduction that is reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper, and optionally containing an additive element.

[0078] (4) Copper Oxidation Treatment Step The copper oxidation treatment step is a step of adding monovalent copper (Cu ２4, the copper oxidation treatment step is a step that is carried out as necessary when adjusting the abundance ratio of zero-valent copper (Cu) and / or divalent copper (CuO) to a predetermined amount. As shown in FIG. 4, the copper oxidation treatment step is carried out by subjecting the formed sputtering layer 20 to an oxidation treatment by electroless plating, thereby oxidizing at least a portion of the zero-valent copper (Cu) contained in the sputtering layer 20 to monovalent copper (Cu ２ The oxidation treatment portion 22 is formed using copper (II) and / or divalent copper (CuO). As an example of the oxidation treatment by electroless plating, there is an oxidation treatment method in which the sputtering layer 20 is immersed in an aqueous copper sulfate solution.

[0079] (5) Partial Reduction Step The partial reduction step is a step of reducing the monovalent copper (Cu ２ O) and / or divalent copper (CuO), and optionally zero-valent copper (Cu) and / or monovalent copper (Cu ２ 0), thereby further optimizing the ratio of zero-valent copper to monovalent copper. As shown in FIG. 5 , the partial reduction step involves immersing a composite 1′ obtained by forming a sputtering layer 20 on a substrate 1 and an anode 33 in an aqueous solution for partial reduction 32 contained in a two-chamber electrolytic cell 30 having a diaphragm 31, and applying an electrolytic potential from a power source 34 to the two-chamber electrolytic cell 30, thereby performing partial reduction treatment. Furthermore, by performing partial reduction treatment, the sputtering layer 20 can be made porous. Examples of the anode 33 include platinum. Examples of the aqueous solution for partial reduction 32 include an aqueous potassium bicarbonate solution on both the composite 1′ side and the anode side.

[0080] (6) Step of Applying a Cation Exchange Material Substituted with Metal Ions As shown in FIG. 6, the step of applying a cation exchange material substituted with metal ions is a step of applying a cation exchange material 21 substituted with metal ions onto a sputtering layer 20 that has been subjected to the copper oxidation treatment step and partial reduction step as necessary, thereby surface-modifying the sputtering layer 20 with the cation exchange material 21 substituted with metal ions. When an oxidation-treated portion 22 is formed by the copper oxidation treatment step, the cation exchange material 21 substituted with metal ions is also applied to the oxidation-treated portion 22. As a method of applying the cation exchange material 21 substituted with metal ions onto the sputtering layer 20, for example, a method of spraying a solution of the cation exchange material 21 substituted with metal ions can be mentioned. By applying the cation exchange material 21 substituted with metal ions onto the sputtering layer 20, monovalent copper (Cu ２ At least a portion of the copper (CuO) is oxidized to divalent copper (CuO).

[0081] By applying cation exchange material 21 substituted with metal ions onto sputtering layer 20, a structure can be obtained in which a first layer located on the surface of the cathode electrode is surface-modified with the cation exchange material substituted with metal ions. Furthermore, by applying cation exchange material 21 substituted with metal ions onto sputtering layer 20, the action of the metal ions promotes penetration of copper from the first layer located on the surface of the cathode electrode into the carbon layer of the substrate, forming a second layer that is a mixed layer of copper and carbon. From the above, the step of applying cation exchange material substituted with metal ions allows sputtering layer 20 to have a two-layer structure including a first layer containing copper but not carbon, and a second layer containing copper and carbon.

[0082] Next, another method (second manufacturing method) for manufacturing a composite of a cathode electrode and a substrate, in which carbon dioxide is electrically reduced, will be described below.

[0083] A second method for producing a composite of a cathode electrode and a substrate includes: (1) a step of preparing a substrate having a porous structure; (2) a second layer forming step of applying a second suspension, which is a mixture of copper and carbon, onto the substrate to form a second layer, which is a mixed layer of copper and carbon; (3) a first layer forming step of applying a first suspension containing copper onto the second layer to form a first layer containing copper; and (4) a metal ion-substituted cation exchange material applying step of applying a cation exchange material substituted with metal ions to the first layer to surface modify the first layer. Furthermore, in the second production method, the copper oxidation treatment step and / or partial reduction step may also be performed, as necessary, after the first layer forming step and before the metal ion-substituted cation exchange material applying step.

[0084] The above-described method for producing a composite of a cathode electrode and a substrate allows the formation of a two-layer cathode electrode on a substrate, the cathode electrode having a second layer containing copper and carbon and a first layer containing copper but not carbon, the first layer being surface-modified with a cation exchange material substituted with metal ions. Furthermore, by surface-modifying the first layer of the cathode electrode with a cation exchange material substituted with metal ions, the action of the metal ions is thought to promote penetration of copper from the first layer into the underlying second layer, thereby contributing to an increase in the thickness of the second layer, which is a mixed layer of copper and carbon.

[0085] By the above-described method for producing a composite of a cathode electrode and a substrate, even if the first layer located on the surface of the cathode electrode is immersed in the electrolyte, the selectivity of hydrogen, a by-reaction product, decreases, making it possible to produce a composite in which the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction of carbon dioxide can be stably maintained at high efficiency for a long period of time. Furthermore, by including the step of applying a cation exchange material substituted with metal ions, the amount of water molecules supplied to the copper constituting the cathode electrode can be controlled by the cation exchange material substituted with metal ions, making it possible to produce a composite in which the selectivity of hydrogen decreases, making it possible to stably maintain high efficiency for a long period of time in the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction of carbon dioxide.

[0086] Examples of the cathode electrode formed by the above-described method for producing a composite of a cathode electrode and a substrate include a cathode electrode in which the copper of the first layer contains divalent copper, zero-valent copper, and / or monovalent copper, and the copper of the second layer contains divalent copper, zero-valent copper, and / or monovalent copper; or a cathode electrode in which the copper of the first layer contains monovalent copper for reduction and / or divalent copper for reduction that is reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper, and the copper of the second layer contains monovalent copper for reduction and / or divalent copper for reduction that is reduced to zero-valent copper by reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper.

[0087] (1) Step of preparing a substrate having a porous structure The step of preparing a substrate having a porous structure is the step of preparing the substrate described above. The material of the substrate and the porosity of the porous structure can be appropriately selected depending on the properties required of the composite of the cathode electrode and the substrate.

[0088] (2) Second Layer Formation Step The second layer formation step is a step of applying a second suspension, which is a mixture of copper and carbon, to a prepared substrate to form a second layer, which is a mixed layer of copper and carbon. The second suspension can be prepared, for example, by adding copper, carbon such as carbon black, and, if necessary, at least one additive element (M2) selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon to a dispersion medium such as an organic solvent, and stirring the mixture. The method for applying the second suspension to the substrate is not particularly limited, and examples include spray coating, bar coating, etc. The second layer can be formed by drying the second suspension applied to the substrate.

[0089] (3) First Layer Formation Step The first layer formation step is a step of further applying a first suspension containing copper onto the second layer to form a first layer containing copper on the second layer. The first suspension can be prepared, for example, by adding copper and, if necessary, at least one additive element (M1) selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon to a dispersion medium such as an organic solvent, and stirring the mixture. The first suspension does not contain carbon such as carbon black. The method for applying the first suspension onto the second layer is not particularly limited, and examples include spray coating, bar coating, etc. The first layer can be formed by drying the first suspension applied onto the second layer.

[0090] (4) Step of applying a cation exchange material substituted with metal ions The step of applying a cation exchange material substituted with metal ions is a step of applying a cation exchange material substituted with metal ions onto the first layer to surface-modify the first layer with the cation exchange material substituted with metal ions. An example of a method of applying a cation exchange material substituted with metal ions onto the first layer is a method of spraying a solution of the cation exchange material substituted with metal ions with a spray. By applying a cation exchange material substituted with metal ions onto the first layer, monovalent copper (Cu ２ At least a portion of the copper (CuO) is oxidized to divalent copper (CuO).

[0091] Of the first and second manufacturing methods described above, the first manufacturing method in which cation exchange material 21 substituted with metal ions is applied onto sputtering layer 20 to form the first and second layers is preferred because the density of the first layer containing copper and the second layer, which is a mixed layer of copper and carbon, is improved, thereby reliably suppressing the volumetric reduction of the first and second catalytic layers due to the reduction reaction of carbon dioxide, and ultimately preventing moisture contained in the electrolytic solution from penetrating into the catalytic layers and preventing an increase in the selectivity of hydrogen, a by-product.

[0092] [Electrolytic reduction device] Next, an electrolytic reduction device equipped with the cathode electrode of the present invention and electrically reducing carbon dioxide to carbon monoxide, olefinic hydrocarbons, and / or alcohols, and an electrolytic reduction device equipped with the composite of the cathode electrode and substrate of the present invention and electrically reducing carbon dioxide to carbon monoxide, olefinic hydrocarbons, and / or alcohols will be described. Fig. 7 is an explanatory diagram showing an overview of an electrolytic reduction device equipped with the cathode electrode of the present invention. Fig. 8 is an explanatory diagram showing an overview of another electrolytic reduction device equipped with the cathode electrode of the present invention.

[0093] As shown in FIG. 7 , an example of the electrolytic reduction device 210 is a three-chamber electrolytic reduction device. Specifically, the electrolytic reduction device 210 has an electrolytic cell 214 including a cathode gas chamber 211, a cathode fluid chamber 212, and an anode fluid chamber 213, which are separated from one another. The cathode gas chamber 211 and the cathode fluid chamber 212 are separated by a cathode 216 serving as a gas diffusion electrode. The cathode fluid chamber 212 and the anode fluid chamber 213 are separated by an ionically conductive diaphragm 217. An anode electrode 218 is disposed in the anode fluid chamber 213. Carbon dioxide gas is supplied to the cathode gas chamber 211. A cathode fluid is supplied to the cathode fluid chamber 212. Anode fluid is supplied to the anode fluid chamber 213. The anode electrode 218 and the cathode 216 are connected to a DC power supply 219.

[0094] The anolyte and catholyte are aqueous solutions containing dissolved electrolytes, such as potassium, sodium, lithium, or at least one of these compounds. ２ CO ３ , Na ２ CO ３ , K. ２ CO ３ , LiHCO ３ , NaHCO ３ , and KHCO ３ The compound comprises at least one compound from the group consisting of:

[0095] The cathode 216 is a gas diffusion electrode and includes a gas diffusion layer 221 and a microporous layer 222. In the electrolytic reduction device 210, the cathode 216 is a composite of a substrate and a cathode electrode having a first layer and a second layer, the first layer being surface-modified with a cation exchange material substituted with metal ions, according to the present invention, and the microporous layer 222 corresponds to the substrate of the composite. The gas diffusion layer 221 is permeable to a gas containing carbon dioxide but inhibits permeation of an aqueous solution containing a cathode electrolyte. The microporous layer 222 is permeable to both a gas containing carbon dioxide and an aqueous solution containing a cathode electrolyte. The gas diffusion layer 221 and the microporous layer 222 are each formed in a planar shape. The gas diffusion layer 221 is disposed on the cathode gas chamber 211 side, and the microporous layer 222 is disposed on the cathode electrolyte chamber 212 side.

[0096] Examples of the gas diffusion layer 221 include a porous conductive substrate, such as carbon paper, carbon felt, or carbon cloth, on which a water-repellent coating such as polytetrafluoroethylene is formed. The conductive substrate is connected to the negative electrode of the DC power supply 219 and receives a supply of electrons. The microporous layer 222 is formed on the surface of the gas diffusion layer 221 using carbon, a fluorine-containing resin, a metal, or the like, and supports a catalyst. In the electrolytic reduction device 210, the catalyst supported by the microporous layer 222 is a cathode electrode according to the present invention, which has a first layer and a second layer and is surface-modified with a cation exchange material substituted with metal ions. Furthermore, by using renewable energy as the DC power supply 219 of the electrolytic reduction device 210, olefinic hydrocarbons such as ethylene and alcohols such as ethanol can be produced by a reduction reaction of carbon dioxide while reducing environmental impact. Instead of the gas diffusion electrode having the gas diffusion layer 221 and the microporous layer 222 , a gas diffusion electrode in which a part of the microporous layer 222 functions as the gas diffusion layer 221 may be provided.

[0097] As shown in FIG. 8 , another example of the electrolytic reduction device 210 is an electrolytic reduction device 210 with an MEA-type electrolytic cell structure. In the electrolytic reduction device 210 shown in FIG. 8 , the same components as those in the electrolytic reduction device 210 shown in FIG. 7 are denoted by the same reference numerals. The electrolytic reduction device 210 with an MEA-type electrolytic cell structure does not use a cathode fluid and therefore does not have a cathode fluid chamber 212. Therefore, instead of the electrolytic cell 214 including a cathode gas chamber 211, a cathode fluid chamber 212, and an anode fluid chamber 213 that are separated from each other, the electrolytic reduction device 210 with an MEA-type electrolytic cell structure has an electrolytic cell 214 including a cathode gas chamber 211 and an anode fluid chamber 213 that are separated from each other. The cathode gas chamber 211 and the anode fluid chamber 213 are separated from each other by a diaphragm 217 sandwiched between a cathode electrode and an anode electrode 218. The anode electrode 218 is disposed in the anode fluid chamber 213. Carbon dioxide gas is supplied to the cathode gas chamber 211. Anode fluid is supplied to the anode fluid chamber 213. The anode electrode 218 and the cathode electrode are connected to a DC power supply 219. The electrolytic reduction device 210 with an MEA electrolytic cell structure uses the diaphragm 217 itself as the electrolyte and does not use a cathode fluid, making it suitable for integration.

[0098] In the electrolytic reduction device 210 with an MEA-type electrolytic cell structure, the cathode electrode is also a gas diffusion electrode, and includes a gas diffusion layer 221 and a microporous layer 222. Furthermore, in the electrolytic reduction device 210 with an MEA-type electrolytic cell structure, a composite of a cathode electrode and a substrate according to the present invention is used as the cathode electrode, the composite having a first layer and a second layer, the first layer being surface-modified with a cation exchange material substituted with metal ions, and the microporous layer 222 corresponds to the substrate of the composite.

[0099] Next, examples of the present invention will be described, but the present invention is not limited to the following examples.

[0100] [Example 1] Preparation of Cathode Electrode Sputtering Layer Formation Step A copper layer was formed on a substrate (porous carbon, a composite of a carbon layer formed of carbon black and carbon fiber) by sputtering under conditions of DC 100 W, argon gas 6.0 sccm, and 5 minutes to form a first copper sputtering layer. Then, an aluminum layer was formed on the formed first copper sputtering layer by sputtering under conditions of DC 50 W, argon gas 6.0 sccm, and 30 seconds to form a first aluminum sputtering layer. Then, a copper layer was formed on the formed first aluminum sputtering layer by sputtering under conditions of DC 100 W, argon gas 6.0 sccm, and 5 minutes to form a second copper sputtering layer. Thereafter, an aluminum layer was deposited on the second copper sputtering layer by sputtering under conditions of DC 50 W, argon gas 6.0 sccm for 30 seconds to form a second aluminum sputtering layer, and thereafter, a copper layer was deposited on the second aluminum sputtering layer by sputtering under conditions of DC 100 W, argon gas 6.0 sccm for 5 minutes to form a third copper sputtering layer, resulting in a sputtering layer that was a five-layer laminate.

[0101] Copper Oxidation Treatment Step The sputtering layer, which is the laminate obtained as described above, was immersed in 100 mL of an aqueous solution containing copper sulfate and potassium sulfate (9.7 mM copper ions, 0.5 M sulfate ions), and electroless plating was performed at 20°C for 20 minutes, thereby oxidizing a portion of the zero-valent copper contained in the copper sputtering layer to monovalent copper and / or divalent copper.

[0102] Metal Ion-Substituted Cation Exchange Material Application Step: An aqueous solution of a cation exchange resin (sulfonated tetrafluoroethylene polymer, "Nafion" (registered trademark), DuPont) (concentration: 5% by mass) was mixed with an aqueous potassium hydroxide solution (concentration: 8 M) to prepare an aqueous solution of a potassium ion-substituted cation exchange resin (an aqueous solution of a cation exchange resin in which the hydrogen ions of the cation exchange resin were substituted with potassium ions). The resulting aqueous solution of the potassium ion-substituted cation exchange resin was spray-coated (amount of the aqueous solution of the potassium ion-substituted cation exchange resin: 650 μL) onto the surface of a five-layer laminate that had been subjected to a copper oxidation treatment step by electroless plating, followed by heating and drying. The sputtering layer was surface-modified with the potassium ion-substituted cation exchange resin so that the amount of deposited cation exchange resin (Nafion) per 100 g of elemental copper was 125 g to 24,333 g, based on the weight difference before and after coating. The sputtering layer was surface-modified with a cation exchange resin substituted with potassium ions, thereby forming a first layer surface-modified with a cation exchange material substituted with metal ions, and a second layer which was a mixed layer of copper and carbon, in which the copper in the first layer was promoted to penetrate into the underlying carbon layer by the action of the metal ions.

[0103] As described above, a cathode electrode, which is a sputtering layer, was prepared on a substrate, and a composite of the cathode electrode and the substrate was produced.

[0104] [Comparative Example 1] A composite of a cathode electrode and a substrate was produced in the same manner as in Example 1, except that the step of applying a cation exchange material substituted with metal ions was not carried out. From the above, in Comparative Example 1, a composite of a cathode electrode and a substrate was produced in which the second layer, which was a mixed layer of copper and carbon, was not formed.

[0105] Evaluation Items [Stability Test] The ethylene gas selectivity (E24, unit: %) was measured 24 hours after the start of the carbon dioxide reduction reaction. An E24 of 50% or more was evaluated as indicating that the catalytic reaction of producing ethylene by the reduction reaction of carbon dioxide could be stably continued for a long period of time and that the stability of the catalytic reaction was excellent. The ethylene gas selectivity was evaluated as follows:

[0106] [Ethylene gas selectivity (%)] Electrolytic cell (electrolyte: 1M KHCO ３ The number of moles of ethylene per unit time and the number of moles of electrons required were calculated from the concentration of ethylene contained in the outlet gas of the electrolytic cell (aqueous solution, electrolyte flow rate: 50 mL / min) and the gas flow rate (carbon dioxide gas flow rate: 40 mL / min). Meanwhile, the number of moles of electrons passing through the electrolytic cell per unit time was calculated from the set current value (-530 mA) of the potential application device. The ratio of the former to the latter was evaluated as the ethylene selectivity (%). The concentration of ethylene contained in the outlet gas was measured using a gas chromatograph (model number: Agilent 990 Micro GC). The gas flow rate was measured using a mass flow meter.

[0107] The measurement results of the examples and comparative examples are shown in Table 1 below.

[0108]

[0109] As shown in Table 1 above, in Example 1, in which the sputtering layer was surface-modified with a cation exchange resin substituted with potassium ions to form a first layer surface-modified with a cation exchange material substituted with metal ions and a second layer in which copper and carbon were mixed, the ethylene gas selectivity (E24) 24 hours after the start of the carbon dioxide reduction reaction was 50% or more, and even when the catalytic layer was immersed in an electrolytic solution, an improvement in hydrogen selectivity was prevented, and the catalytic reaction of producing ethylene by the reduction of carbon dioxide was able to be stably maintained for a long period of time, demonstrating excellent stability of the catalytic reaction.

[0110] On the other hand, in Comparative Example 1 in which the second layer, which is a mixed layer of copper and carbon, was not formed, the ethylene gas selectivity (E24) 24 hours after the start of the carbon dioxide reduction reaction was 45%, and it could not be evaluated that the catalytic reaction of producing ethylene by the reduction reaction of carbon dioxide could be stably maintained for a long period of time even when the catalyst layer was immersed in the electrolytic solution.

[0111] The cathode electrode of the present invention can stably maintain the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the reduction of carbon dioxide for a long period of time, even when the catalyst layer is immersed in an electrolyte solution. Therefore, the cathode electrode of the present invention is highly useful in fields where carbon dioxide is absorbed and recovered from the atmosphere and used to produce industrially useful organic compounds from the carbon dioxide.

[0112] REFERENCE SIGNS LIST 1 substrate 20 sputtering layer 21 cation exchange material substituted with metal ions 100 cathode electrode 111 first layer 112 second layer 120 composite

Claims

1. A cathode electrode that electrically reduces carbon dioxide, the cathode electrode comprising: a first layer having a first thickness located on a surface of the cathode electrode; and a second layer having a second thickness adjacent to the first layer in a thickness direction, the first layer containing copper, the first layer being surface-modified with a cation exchange material substituted with metal ions, and the second layer being a mixed layer of copper and carbon.

2. The cathode electrode according to claim 1, wherein a ratio of the second thickness of said second layer to the first thickness of said first layer is greater than 0 and not greater than 100.

3. A cathode electrode as described in claim 1 or 2, wherein the second layer has a first region located on the first layer side and a second region adjacent to the first region, with a center portion in the thickness direction of the second layer as a boundary, and the copper content (volume %) of the first region is greater than the copper content (volume %) of the second region.

4. A cathode electrode according to claim 1 or 2, wherein the copper in the first layer contains divalent copper, zero valent copper and / or monovalent copper, and the copper in the second layer contains divalent copper, zero valent copper and / or monovalent copper.

5. The cathode electrode according to claim 1 or 2, wherein the copper in the first layer comprises monovalent copper for reduction and / or divalent copper for reduction, which are reduced to zero-valent copper by a reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper, and the copper in the second layer comprises monovalent copper for reduction and / or divalent copper for reduction, which are reduced to zero-valent copper by a reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper.

6. A cathode electrode according to claim 1 or 2, wherein the first layer further contains at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium and silicon, and the second layer further contains at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium and silicon.

7. A cathode electrode as described in claim 6, wherein the additive element in the first layer is contained in an amount of 0.10 atomic % or more and 1.0 atomic % or less relative to 100 atomic % of the copper contained in the first layer, and the additive element in the second layer is contained in an amount of 0.10 atomic % or more and 1.0 atomic % or less relative to 100 atomic % of the copper contained in the second layer.

8. The cathode electrode of claim 1 or 2, wherein the cation exchange material comprises a sulfonated tetrafluoroethylene-based polymer.

9. The cathode electrode according to claim 1 or 2, wherein the metal ions include alkali metal ions and / or alkaline earth metal ions.

10. The cathode electrode according to claim 1 or 2, wherein the cation exchange material substituted with the metal ions is formed in the form of a layer on the surface of the first layer.

11. The cathode electrode of claim 1 or 2, wherein said metal ion substituted cation exchange material is mixed with said copper of said first layer.

12. The cathode electrode according to claim 1 or 2, which has a porous structure.

13. A composite of a cathode electrode and a substrate, comprising: a substrate; and the cathode electrode according to claim 1 or 2, having the second layer disposed on the substrate.

14. An electrolytic reduction device comprising the cathode electrode according to claim 1 or 2, for electrically reducing carbon dioxide to carbon monoxide, olefinic hydrocarbons and / or alcohols.

15. An electrolytic reduction device comprising the composite according to claim 13, for electrically reducing carbon dioxide to carbon monoxide, olefinic hydrocarbons and / or alcohols.

16. A method for producing a composite of a substrate and a cathode electrode which electrically reduces carbon dioxide, comprising: a step of preparing a substrate having a porous structure; a second layer forming step of applying a second suspension which is a mixture of copper and carbon onto the substrate to form a second layer which is a mixed layer of copper and carbon; a first layer forming step of applying a first suspension containing copper onto the second layer to form a first layer containing copper; and a metal ion-substituted cation exchange material applying step of applying a cation exchange material substituted with metal ions to the first layer to modify the surface of the first layer.

17. A method for producing a composite as described in claim 16, wherein the copper in the first layer contains divalent copper, zero valent copper and / or monovalent copper, and the copper in the second layer contains divalent copper, zero valent copper and / or monovalent copper.

18. A method for producing a composite according to claim 16, wherein the copper of the first layer comprises monovalent copper for reduction and / or divalent copper for reduction, which are reduced to zero-valent copper by a reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper, and the copper of the second layer comprises monovalent copper for reduction and / or divalent copper for reduction, which are reduced to zero-valent copper by a reduction treatment, and monovalent copper and / or divalent copper that is not reduced to zero-valent copper.

19. A method for producing a composite according to any one of claims 16 to 18, wherein the first suspension further contains at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium and silicon, and the second suspension further contains at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium and silicon.

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