EP4722419A1

EP4722419A1 - 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

Info

Publication number: EP4722419A1
Application number: EP24815554.1A
Authority: EP
Inventors: Yuri MITSUHASHI; Takahiro Yamamoto; Yu Mimura; Kiyoshi Yamamoto; Masakazu Sugiyama; Tsutomu MINEGISHI; Dai Takeda; Jun Matsumoto
Original assignee: University of Tokyo NUC; Chiyoda Corp
Current assignee: University of Tokyo NUC; Chiyoda Corp
Priority date: 2023-05-30
Filing date: 2024-05-30
Publication date: 2026-04-08
Also published as: CN121152902A; JP2024171439A; WO2024248073A1

Abstract

The present invention aims to provide a cathode electrode which prevents an increase in hydrogen selectivity even if the catalyst layer is immersed in an electrolyte solution and enables a catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction to be stably sustained with high efficiency over a long period.

A cathode electrode which electrically reduces carbon dioxide, the cathode electrode comprising: a first layer with a first thickness, located on a surface layer of the cathode electrode; and a second layer with a second thickness, adjacent to the first layer in a thickness direction, wherein: the first layer contains copper; the first layer is surface-modified with a cation exchange substance substituted with a metal ion; and the second layer is a mixed layer of copper and carbon.

Description

Technical Field

The present invention relates to a cathode electrode capable of electrically reducing carbon dioxide to convert it into carbon monoxide, olefinic hydrocarbons such as ethylene, and/or alcohols, a composite body of the cathode electrode and a base material, an electrolytic reduction device provided with the cathode electrode, and a method for producing the composite body of the cathode electrode and the base material.

Background Art

In recent years, the adverse effects of global warming have brought about changes in the global environment, giving rise to numerous problematic phenomena. One of the causes of these changes in the global environment is considered to be the increase in the concentration of greenhouse gases in the atmosphere, particularly carbon dioxide, which constitutes a large portion of greenhouse gases. To reduce the atmospheric carbon dioxide concentration, not only increasing photosynthesis by new terrestrial afforestation and marine algae but also actively absorbing and recovering atmospheric carbon dioxide are being considered. Furthermore, in addition to merely absorbing and recovering carbon dioxide, utilizing carbon derived from carbon dioxide as a raw material for organic compounds is also being considered.
Specifically, converting carbon dioxide by reduction into, for example, C2 compounds such as ethylene and ethanol, and C1 compounds such as carbon monoxide, methane, methanol, and formic acid, for use in the synthesis of organic compounds is being considered. Among these, ethylene and ethanol, which are C2 compounds, are particularly useful as derivatives for synthesizing various organic compounds and have higher utility value than C1 compounds such as carbon monoxide and methane.
In recent years, catalysts such as photocatalysts and electrode catalysts have been widely used for the carbon dioxide reduction reaction as described above, and the development of catalysts with even better performance is sought. Catalysts used for the reduction of carbon dioxide are required to have not only high reaction efficiency but also selectivity for specific reactions. From this perspective, the selection of the catalyst material is important. For example, gold, silver, and zinc are used as catalyst materials to efficiently produce carbon monoxide by reduction and to increase the proportion of carbon monoxide in the reduced substances. Further, copper is used as a catalyst material to efficiently produce hydrocarbons such as methane, ethane, and ethylene by reduction. In particular, copper has attracted attention as a cathode reduction electrode catalyst for carbon dioxide because it can produce C2 compounds such as ethylene.
As a catalyst layer containing a metal catalyst such as copper used in the above-described carbon dioxide reduction reaction, a catalyst layer has been proposed that contains a metal catalyst supported on a carbon material, an ion-conducting substance, and a hydrophilic polymer, in which the ratio of the BET specific surface area determined by water vapor adsorption of the catalyst layer to the BET specific surface area determined by nitrogen adsorption of the catalyst layer is 0.08 or less (Patent Literature 1). In Patent Literature 1, by controlling the ratio of the aforementioned BET specific surface areas, the wettability of the catalyst layer is reduced to enhance its water repellency, thereby preventing water from stagnating within the catalyst layer and promoting the diffusion of carbon dioxide gas. It is stated that by promoting the diffusion of carbon dioxide gas in the catalyst layer, the conversion efficiency of carbon dioxide is improved.
On the other hand, for the industrial applications of producing organic compounds such as ethylene through the carbon dioxide reduction reaction, it is required that the catalytic reaction for producing organic compounds such as ethylene can stably sustain high selectivity of organic compounds such as ethylene over several hundred hours. In Patent Literature 1, although it is stated that enhancing the water repellency of the catalyst layer prevents water stagnation and promotes carbon dioxide gas diffusion, if the catalyst layer becomes immersed in the electrolyte solution, its water repellency cannot prevent water stagnation. Therefore, there was a need for improvement in stably sustaining a highly efficient catalytic reaction for producing organic compounds such as ethylene over a long period.
Furthermore, in a cathode electrode for carbon dioxide reduction, to obtain excellent synthesis efficiency of organic compounds such as ethylene, it is necessary for the carbon dioxide reduction reaction to be dominant over the hydrogen generation resulting from a side reaction (water decomposition reaction), thereby decreasing the selectivity of hydrogen and increasing the selectivity of carbon dioxide reduction products. However, in Patent Literature 1, when the catalyst layer becomes immersed in the electrolyte solution, not only is the diffusion of carbon dioxide gas hindered, but also the hydrogen generation becomes dominant due to the water molecules contained in the electrolyte solution. This leads to an increase in hydrogen selectivity during the carbon dioxide reduction, preventing the attainment of excellent selectivity of carbon dioxide reduction products, which also necessitated improvement.

Citation List

Patent Literature

Patent Literature 1: JP 2021-147677 A

Summary of Invention

Technical Problem

In view of the above circumstances, the present invention aims to provide a cathode electrode which prevents an increase in hydrogen selectivity even if the catalyst layer is immersed in an electrolyte solution and enables a catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction to be stably sustained with high efficiency over a long period, a composite body of the cathode electrode and a base material, an electrolytic reduction device provided with the cathode electrode, and a method for producing the composite body of the cathode electrode and the base material.

Solution to Problem

The gist of the constitution of the present invention is as follows.

[1] A cathode electrode which electrically reduces carbon dioxide, the cathode electrode comprising: a first layer with a first thickness, located on a surface layer of the cathode electrode; and a second layer with a second thickness, adjacent to the first layer in a thickness direction, wherein: the first layer contains copper; the first layer is surface-modified with a cation exchange substance substituted with a metal ion; and the second layer is a mixed layer of copper and carbon.
[2] The cathode electrode of [1], wherein a ratio of the second thickness of the second layer to the first thickness of the first layer is greater than 0 and 100 or less.
[3] The cathode electrode of [1] or [2], wherein: the second layer has a first region located on a side of the first layer, and a second region adjacent to the first region, with a central portion in a thickness direction of the second layer as a boundary; and a content (vol%) of the copper in the first region is greater than a content of the copper (vol%) in the second region.
[4] The cathode electrode of [1] or [2], wherein: the copper in the first layer comprises divalent copper, and zerovalent copper and/or monovalent copper; and the copper in the second layer comprises divalent copper, and zerovalent copper and/or monovalent copper.
[5] The cathode electrode of [1] or [2], wherein: the copper of the first layer comprises monovalent copper and/or divalent copper for reduction, which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper, which is not reduced to zerovalent copper; and the copper of the second layer comprises monovalent copper and/or divalent copper for reduction, which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper, which is not reduced to zerovalent copper.
[6] The cathode electrode of [1] or [2], wherein: the first layer further comprises at least one additive element selected from a group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon; and the second layer further comprises 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 of [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 with respect 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 with respect to 100 atomic% of the copper contained in the second layer.
[8] The cathode electrode of [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 with respect 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 with respect to 100 atomic% of the copper contained in the second layer.
[9] The cathode electrode of [6], wherein the additive element of the first layer comprises aluminum, and the additive element of the second layer comprises aluminum.
[10] The cathode electrode of [1] or [2], wherein the cation exchange substance comprises a sulfonated tetrafluoroethylene-based polymer.
[11] The cathode electrode of [1] or [2], wherein the metal ion comprises an alkali metal ion and/or an alkaline earth metal ion.
[12] The cathode electrode of [1] or [2], wherein the cation exchange substance substituted with a metal ion is formed as a layer on a surface of the first layer.
[13] The cathode electrode of [1] or [2], wherein the cation exchange substance substituted with a metal ion is mixed with the copper of the first layer.
[14] The cathode electrode of [1] or [2], comprising a porous structure.
[15] A composite body, comprising: a base material; and the cathode electrode of [1] or [2] in which the second layer is arranged on the base material.
[16] The composite body of [15], wherein the base material has a porous structure.
[17] The composite body of [16], wherein a material of the base material having the porous structure is carbon, a fluorine-containing resin, or a metal.
[18] The composite body of [16], wherein: a material of the base material having the porous structure is carbon; and a carbon layer of the carbon is adjacent to the second layer.
[19] An electrolytic reduction device for electrically reducing carbon dioxide to carbon monoxide, olefinic hydrocarbons, and/or alcohols, the electrolytic reduction device comprising the cathode electrode of [1] or [2].
[20] An electrolytic reduction device for electrically reducing carbon dioxide to carbon monoxide, olefinic hydrocarbons, and/or alcohols, the electrolytic reduction device comprising the composite body of [15].
[21] A method for producing a composite body of a cathode electrode and a base material for electrically reducing carbon dioxide, comprising: a step of preparing a base material having a carbon layer and a porous structure; a sputtering layer formation step of forming a sputtering layer containing copper by sputtering on the carbon layer of the base material; and a metal-ion-substituted cation exchange substance application step of applying a cation exchange substance substituted with a metal ion onto the sputtering layer to surface-modify the sputtering layer.
[22] The method for producing a composite body of [21], wherein the copper comprises divalent copper, and zerovalent copper and/or monovalent copper.
[23] The method for producing a composite body of [21], wherein the copper comprises monovalent copper and/or divalent copper for reduction, which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper, which is not reduced to zerovalent copper.
[24] The method for producing a composite body of any one of [21] to [23], wherein the sputtering layer further comprises at least one additive element selected from a group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon.
[25] The method for producing a composite body of [22] or [23], further comprising a copper oxidation treatment step of oxidizing at least a part of the copper in the sputtering layer to monovalent copper and/or divalent copper, after the sputtering layer formation step and before the metal-ion-substituted cation exchange substance application step.
[26] The method for producing a composite body of [25], further comprising a partial reduction step of partially reducing the monovalent copper and/or the divalent copper oxidized in the copper oxidation treatment step to zerovalent copper and/or monovalent copper before the metal-ion-substituted cation exchange substance application step.
[27] The method for producing a composite body of [25], wherein the copper oxidation treatment step is electroless plating.
[28] The method for producing a composite body of [24], wherein the sputtering layer formation step comprises: a copper sputtering layer formation step of forming a copper sputtering layer, which is a sputtering layer containing copper; and an additive element sputtering layer formation step of forming an additive element sputtering layer, which is a sputtering layer containing the additive element.
[29] The method for producing a composite body of [28], wherein the sputtering layer formation step further comprises a copper sputtering layer formation step of forming a copper sputtering layer, which is a sputtering layer containing copper, by sputtering on the additive element sputtering layer.
[30] A method for producing a composite body of a cathode electrode and a base material for electrically reducing carbon dioxide, comprising: a step of preparing a base material having a porous structure; a second layer formation step of applying a second suspension, which is a mixture of copper and carbon, onto the base material to form a second layer, which is a mixed layer of copper and carbon; a first layer formation 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 substance application step of applying a cation exchange substance substituted with a metal ion onto the first layer to surface-modify the first layer.
[31] The method for producing a composite body of [30], wherein: the copper of the first layer comprises divalent copper, and zerovalent copper and/or monovalent copper; and the copper of the second layer comprises divalent copper, and zerovalent copper and/or monovalent copper.
[32] The method for producing a composite body of [30], wherein: the copper of the first layer comprises monovalent copper and/or divalent copper for reduction, which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper, which is not reduced to zerovalent copper; and the copper of the second layer comprises monovalent copper and/or divalent copper for reduction, which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper, which is not reduced to zerovalent copper.
[33] The method for producing a composite body of any one of [30] to [32], wherein: the first suspension further comprises at least one additive element selected from a group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon; and the second suspension further comprises at least one additive element selected from a group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon.

Advantageous Effects of Invention

In the aspect of the cathode electrode of the present invention, the cathode electrode has a first layer with a first thickness, located on the surface layer of the cathode electrode, and a second layer with a second thickness, adjacent to the first layer in its thickness direction. The first layer contains copper, and the second layer is a mixed layer of copper and carbon. With this structure, even if the first layer located on the surface layer of the cathode electrode is immersed in the electrolyte solution and the supply of carbon dioxide gas to the first layer is reduced, a three-phase boundary formed by the carbon dioxide (CO₂) gas, water (H₂O) molecules contained in the electrolyte solution, and copper (Cu) as a catalyst, which functions as a site for reduction reaction, is formed in the second layer. Therefore, according to an aspect of the cathode electrode of the present invention, even if the first layer containing the copper catalyst is immersed in the electrolyte solution, the selectivity of hydrogen, which is a by-product, is reduced. As a result, the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period.
Further, according to the aspect of the cathode electrode of the present invention, since the first layer located on the surface layer of the cathode electrode is surface-modified with a cation exchange substance substituted with a metal ion, the supply of water molecules to the copper constituting the cathode electrode is controlled by the cation exchange substance substituted with a metal ion. Therefore, the selectivity of hydrogen is reduced, and the catalytic reaction for producing olefin-based hydrocarbons such as ethylene or alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period. The cation exchange substance substituted with a metal ion has a structure in which the hydrogen ions (H⁺) of the cation exchange substance are substituted with a metal ion, and therefore has a structure with a reduced content of hydrogen ions (H⁺).
Further, according to the 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 100 or less. This more reliably ensures the boundary (three-phase boundary) formed by the carbon dioxide gas, the water molecules contained in the electrolyte solution, and the copper catalyst in the second layer. Therefore, even if the first layer is immersed in the electrolyte solution, the catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency for an even longer period.
Further, according to the aspect of the cathode electrode of the present invention, the second layer has a first region located on the side of the first layer and a second region adjacent to the first region, with a central portion in the thickness direction of the second layer as a boundary, and the content (vol%) of the copper in the first region is greater than the content of the copper (vol%) in the second region. This allows for a more reliable ensuring of the boundary (three-phase boundary) formed by the carbon dioxide gas, the water molecules contained in the electrolyte solution, and the copper catalyst in the second layer. As a result, even if the first layer is immersed in the electrolyte solution, the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency for an even longer period.
In the carbon dioxide reduction reaction at a cathode electrode in which copper is used as a catalyst material, it is considered that the boundary between copper of different valences present in the cathode electrode (e.g., the boundary between zerovalent copper and monovalent copper) mainly functions as a site in which C-C bonds are formed from carbon dioxide-derived carbons and in which the C-C bonds are stabilized. That is, it is considered that the boundary between copper of different valences (e.g., the boundary between zerovalent copper and monovalent copper) is the main active site for the carbon dioxide reduction reaction. From the above, according to the aspect of the cathode electrode of the present invention, the copper in the first layer includes divalent copper, and zerovalent copper and/or monovalent copper, and the copper in the second layer includes divalent copper, and zerovalent copper and/or monovalent copper, or the copper in the first layer includes monovalent copper and/or divalent copper for reduction which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper which is not reduced to zerovalent copper, and the copper in the second layer includes monovalent copper and/or divalent copper for reduction which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper which is not reduced to zerovalent copper. As a result, the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be more reliably and stably sustained with high efficiency over a long period.
In a cathode electrode in which copper is used as a catalyst material, during the carbon dioxide reduction reaction, monovalent copper is reduced to zerovalent copper, and divalent copper is reduced to zerovalent or monovalent copper. When the carbon dioxide reduction reaction is sustained for a long period, it is considered that divalent copper is reduced to zerovalent or monovalent copper, and monovalent copper is reduced to zerovalent copper, leading to a tendency for the boundary between copper of different valences (e.g., the boundary between zerovalent copper and monovalent copper) to decrease. From the above, according to the 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, 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, and the additive element has a function of retaining oxygen in the cathode electrode, thereby appropriately suppressing the reduction of monovalent copper to zerovalent copper. From the above, even if the carbon dioxide reduction reaction is continued for a long period, the abundance ratio of zerovalent copper to monovalent copper is reliably optimized, thereby maintaining the formation and stabilization of C-C bonds, and more reliably, the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the carbon dioxide reduction reaction can be stably sustained over a long period
It should be noted that both ethylene and ethanol are C2 compounds, and the formation of C-C bonds on the catalyst material is an intermediate step in the reaction pathways. Therefore, since the active sites on the catalyst material for ethylene production and ethanol production are identical or very close, the stability of ethylene production and the stability of ethanol production show similar trends, and the carbon dioxide reduction reaction proceeds in the same manner for ethylene production and ethanol production.
Further, according to the aspect of the cathode electrode of the present invention, the additive element of the first layer is contained in an amount of 0.10 atomic% or more and 1.0 atomic% or less with respect to 100 atomic% of copper contained in the first layer, and the additive element of the second layer is contained in an amount of 0.10 atomic% or more and 1.0 atomic% or less with respect to 100 atomic% of copper contained in the second layer, whereby the abundance ratio of zerovalent copper to monovalent copper is reliably optimized even when the carbon dioxide reduction reaction is continued for a long period. Therefore, the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be sustained more stably over a long period.
Further, according to the aspect of the cathode electrode of the present invention, the additive element of the first layer is contained in an amount of 0.25 atomic% or more and 0.70 atomic% or less with respect to 100 atomic% of the copper contained in the first layer, and the additive element of the second layer is contained in an amount of 0.25 atomic% or more and 0.70 atomic% or less with respect to 100 atomic% of the copper contained in the second layer. This allows the abundance ratio of zerovalent copper to monovalent copper to be maintained within a more appropriate range even when the carbon dioxide reduction reaction is continued for a long period. Therefore, the catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be sustained more stably over a longer period.
Further, according to the aspect of the cathode electrode of the present invention, since the additive element of the first layer contains aluminum and the additive element of the second layer contains aluminum, the function of appropriately retaining oxygen within the cathode electrode is reliably obtained. Therefore, even if the carbon dioxide reduction reaction is continued for a long period, the abundance ratio of zerovalent copper to monovalent copper is reliably optimized. Consequently, the catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be sustained more stably over a long period.
Further, according to the aspect of the cathode electrode of the present invention, the cation exchange substance includes a sulfonated tetrafluoroethylene-based polymer, and the metal ion includes an alkali metal ion and/or an alkaline earth metal ion, whereby the supply of water molecules to the copper constituting the cathode electrode is reliably controlled. Therefore, the selectivity of hydrogen, which is a by-product, is more reliably reduced, and the catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol by the carbon dioxide reduction reaction can be stably and more reliably sustained with high efficiency over a long period.
Further, according to the aspect of the cathode electrode of the present invention, the cation exchange substance substituted with a metal ion is formed as a layer on the surface of the first layer, or the cation exchange substance substituted with a metal ion is mixed with the copper of the first layer, whereby the supply amount of water molecules to the copper is more appropriately controlled by the presence of the cation exchange substance substituted with a metal ion.
Further, according to the aspect of the cathode electrode of the present invention, by the cathode electrode comprising a porous structure, the contact between water and carbon dioxide is facilitated at the site of the carbon dioxide reduction reaction of the cathode electrode, so that the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol can be more stably sustained over a long period.
In the aspect of the composite body of the cathode electrode and the base material of the present invention, by comprising the base material and the cathode electrode of the present invention in which the second layer is arranged on the base material, carbon dioxide gas can smoothly contact the second layer of the cathode electrode even if the first layer located on the surface layer of the cathode electrode is immersed in the electrolyte solution. Therefore, according to the aspect of the composite body of the cathode electrode and the base material of the present invention, even if the first layer located on the surface layer of the cathode electrode is immersed in the electrolyte solution, the selectivity of hydrogen as a side reaction product is reduced, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period.
According to the method for producing the composite body of the cathode electrode and the base material of the present invention, by comprising a step of preparing a base material 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 base material by sputtering, and a metal-ion-substituted cation exchange substance application step of applying a cation exchange substance substituted with a metal ion onto the sputtering layer to surface-modify the sputtering layer, it is possible to produce a composite body in which, even if the surface layer of the cathode electrode is immersed in an electrolyte solution, the selectivity of hydrogen as a side reaction product is reduced, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period. Further, according to the method for producing the composite body of the cathode electrode and the base material of the present invention, by comprising the metal-ion-substituted cation exchange substance application step, the supply of water molecules to the copper constituting the cathode electrode can be controlled by the cation exchange substance substituted with a metal ion. Therefore, it is possible to produce a composite body in which the selectivity of hydrogen is reduced, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period.
According to the method for producing the composite body of the cathode electrode and the base material of the present invention, by comprising a step of preparing a base material having a porous structure, a second layer formation step of applying a second suspension, which is a mixture of copper and carbon, onto the base material to form a second layer, which is a mixed layer of copper and carbon, a first layer formation 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 substance application step of applying a cation exchange substance substituted with a metal ion to the first layer to surface-modify the first layer, it is possible to produce a composite body in which, even if the first layer located on the surface layer of the cathode electrode is immersed in an electrolyte solution, the selectivity of hydrogen as a side reaction product is reduced, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period. Further, according to the method for producing the composite body of the cathode electrode and the base material of the present invention, by comprising the metal-ion-substituted cation exchange substance application step, the supply of water molecules to the copper constituting the cathode electrode can be controlled by the cation exchange substance substituted with a metal ion. Therefore, it is possible to produce a composite body in which the selectivity of hydrogen is reduced, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period.

Brief Description of Drawings

Fig. 1 is an explanatory diagram showing an outline of a cross-section of the composite body of the cathode electrode and the base material of the present invention.
Fig. 2 is a cross-sectional view for explaining the reduction reaction of carbon dioxide in the composite body of the cathode electrode and the base material of the present invention.
Fig. 3 is an explanatory diagram of an electropolishing treatment step in the method for producing the composite body of the cathode electrode and the base material.
Fig. 4 is an explanatory diagram of a sputtering layer formation step and a copper oxidation treatment step in the method for producing the composite body of the cathode electrode and the base material.
Fig. 5 is an explanatory diagram of a partial reduction step in the method for producing the composite body of the cathode electrode and the base material.
Fig. 6 is an explanatory diagram showing a state in which a cation exchange substance substituted with a metal ion is applied in the method for producing the composite body of the cathode electrode and the base material.
Fig. 7 is an explanatory diagram showing an outline of the electrolytic reduction device provided with the cathode electrode of the present invention.
Fig. 8 is an explanatory diagram showing an outline of another electrolytic reduction device provided with the cathode electrode of the present invention.

Description of Embodiments

[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 the composite body of the cathode electrode and the base material of the present invention. Fig. 2 is a cross-sectional view for explaining the reduction reaction of carbon dioxide in the composite body of the cathode electrode and the base material of the present invention.
As shown in Figs. 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 with a first thickness, located on the surface layer of the cathode electrode 100, and a second layer 112 with a second thickness, 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 body 120 described later. On the other hand, the second layer 112 of the cathode electrode 100 is not exposed to the external environment of the composite body 120.
The cathode electrode 100 has a first portion 101 and a second portion 102 opposite to 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 body 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.
On the other hand, on the side of the first portion 101 of the cathode electrode 100, a base material 1 of a composite body 120 described later is provided, so that the first portion 101 of the cathode electrode 100 is not exposed to the external environment of the composite body 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.
The first layer 111 of the cathode electrode 100 contains copper (Cu), and the first layer 111 is surface-modified with a cation exchange substance substituted with a metal ion. From the above, the first layer 111 of the cathode electrode 100 contains, as essential components, copper as a catalyst material and a cation exchange substance substituted with a metal ion that modifies the surface of the first layer 111. The cation exchange substance substituted with a metal ion has a chemical structure in which a hydrogen ion (H⁺) of the cation exchange substance is substituted with a metal ion, and is a cation exchange substance with a reduced content of hydrogen ions (H⁺). On the other hand, the first layer 111 does not contain carbon.
The second layer 112 of the cathode electrode 100 is a mixed layer in which copper (Cu) and carbon (C) are mixed. From the above, the second layer 112 of the cathode electrode 100 contains, as essential components, copper as a catalyst material and carbon as a material for uniformizing the diffusion of carbon dioxide gas. As the carbon constituting the second layer 112, for example, particulate carbon can be mentioned, and specifically, for example, carbon black can be mentioned.
Therefore, the cathode electrode 100 has a two-layer structure comprising a first layer 111 containing copper but not carbon, and a second layer 112 containing copper and carbon.
The cathode electrode 100 has the first layer 111 with the first thickness, located on the surface layer of the cathode electrode 100, and the second layer 112 with a second thickness, adjacent to the first layer 111 in the thickness direction. Since the first layer 111 contains copper and the second layer 112 is a mixed layer of copper and carbon, even if the supply of carbon dioxide gas to the first layer 111 decreases due to the first layer 111 located on the surface layer of the cathode electrode 100 being immersed in the electrolyte solution 130, the supply of carbon dioxide gas to the second layer 112 is maintained because the second layer 112 contains carbon. Therefore, even if the first layer 111 is immersed in the electrolyte solution 130, in the second layer 112, the boundary (three-phase boundary) that functions as a site for the reduction reaction is formed by the carbon dioxide (CO₂) gas, the water (H₂O) molecules contained in the electrolyte solution 130, and the copper (Cu) catalyst.
From the above, in the cathode electrode 100, even if the first layer 111 containing copper as a catalyst is immersed in the electrolyte solution 130, the selectivity of hydrogen, which is a by-product of the carbon dioxide reduction reaction, decreases, and the catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period.
Furthermore, in the cathode electrode 100, since the first layer 111 located on the surface layer of the cathode electrode 100 is surface-modified with a cation exchange substance substituted with a metal ion (a cation exchange substance in which the hydrogen ion (H⁺) of the cation exchange substance is substituted with a metal ion), the supply of water molecules to the copper constituting the cathode electrode 100, that is, the supply of hydrogen ions (H⁺), is controlled by the cation exchange substance substituted with a metal ion. Furthermore, since the first layer 111 is surface-modified with a cation exchange substance substituted with a metal ion, it is considered that the action of the metal ions promotes the penetration of copper from the first layer 111 into the underlying carbon, greatly contributing to the formation of the second layer 112, which is a mixed layer of copper and carbon. From the above, the surface modification of the first layer 111 with a cation exchange substance substituted with a metal ion greatly contributes to the formation of the cathode electrode 100 having a two-layer structure comprising a first layer 111 containing copper but not carbon and a second layer 112 containing copper and carbon. Furthermore, since the cation exchange substance is substituted with a metal ion and the content of hydrogen ions is reduced, the supply of hydrogen ions from the cation exchange substance to the copper is also prevented. Therefore, the cathode electrode 100 has a structure that can prevent an excessive supply of hydrogen ions to the copper, which is the catalyst material. In addition, since the state in which the hydrogen ions of the cation exchange substance are substituted with a metal ion is more stable than the state in which the hydrogen ions are not substituted with a metal ion, the state in which the cation exchange substance is substituted with a metal ion is maintained in the cathode electrode 100.
Therefore, in the cathode electrode 100, the selectivity of hydrogen, which is a by-product of the carbon dioxide reduction reaction, is reduced, and the catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period.
The ratio of the second thickness of the second layer 112 to the first thickness of the first layer 111 is not particularly limited, and its lower limit is preferably greater than 0, because this ensures the boundary (three-phase boundary) formed by the carbon dioxide gas, the water molecules contained in the electrolyte 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 electrolyte solution 130, the catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period. Furthermore, the lower limit of the above ratio is preferably 0.01, more preferably 0.05, even more preferably 0.1, and particularly preferably 0.5, because this more reliably ensures the boundary (three-phase boundary) formed by the carbon dioxide gas, the water molecules contained in the electrolyte 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 electrolyte solution 130, the catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be sustained even more stably with high efficiency for an even longer period. 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 allows the catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction to proceed more reliably with high efficiency.
The distribution of the copper content in the second layer 112 is not particularly limited. From the viewpoint that the boundary (three-phase boundary) formed by the carbon dioxide gas, the water molecules contained in the electrolyte solution 130, and the copper catalyst can be ensured more reliably in the second layer 112, and as a result, even if the first layer 111 is immersed in the electrolyte solution 130, the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency, it is preferable that the second layer 112 has a first region located on the side of the first layer 111 and a second region adjacent to the first region, with the central portion in the thickness direction of the second layer 112 as a boundary, and the copper content (vol%) in the first region is larger than the copper content (vol%) in the second region.
Examples of the cation exchange substance constituting the cation exchange substance substituted with a metal ion, that is, the cation exchange substance before being substituted with a metal ion, include cation exchange resins. Examples of the cation exchange resin include sulfonated tetrafluoroethylene-based polymers (trade name "Nafion"), perfluoroalkyl compound (PFAS)-based polymers, and polyethylenedioxythiophene (PEDOT)-based polymers. Among these, sulfonated tetrafluoroethylene-based polymers are preferred from the viewpoint of easy surface modification of the first layer 111 containing copper.
The metal ions constituting the cation exchange substance substituted with a metal ion are not particularly limited as long as the hydrogen ion of the cation exchange substance is substituted with a metal ion. Alkali metal ions and alkaline earth metal ions are preferable from the viewpoint of reliably controlling the supply amount of water molecules to the copper constituting the cathode electrode 100, more reliably reducing the selectivity of hydrogen which is a side reaction product, and more reliably sustaining the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol with high efficiency over a long period. Examples of the alkali metal include lithium ion (Li⁺), sodium ion (Na⁺), potassium ion (K⁺), rubidium ion (Rb⁺), cesium ion (Cs⁺), and francium ion (Fr⁺). Examples of the alkaline earth metal ion include beryllium ion (Be²⁺), magnesium ion (Mg²⁺), calcium ion (Ca²⁺), strontium ion (Sr²⁺), barium ion (Ba²⁺), and radium ion (Ra²⁺). These metal ions may be used alone or in combination of two or more.
Among these metal ions, alkali metal ions are more preferable, and lithium ion (Li⁺), sodium ion (Na⁺), potassium ion (K⁺), and rubidium ion (Rb⁺) are even more preferable from the viewpoint of more reliably controlling the supply amount of water molecules to the copper constituting the cathode electrode 100, and more reliably reducing the selectivity of hydrogen which is a side reaction product. From the viewpoint of further controlling the supply amount of water molecules to the copper and further reducing the selectivity of hydrogen which is a side reaction product, potassium ion (K⁺) is particularly preferable.
The aspect in which the first layer 111 is surface-modified with a cation exchange substance substituted with a metal ion is not particularly limited. From the viewpoint that the supply amount of water molecules to copper is more appropriately controlled by the presence of the cation exchange substance substituted with a metal ion, an aspect in which the cation exchange substance substituted with a metal ion is formed as a layer on the surface of the first layer 111, and an aspect in which the cation exchange substance substituted with a metal ion is mixed with the copper of the first layer 111 are preferable, and the aspect in which the cation exchange substance substituted with a metal ion is formed as a layer on the surface of the first layer 111 is particularly preferable. As the aspect in which the cation exchange substance substituted with a metal ion is formed as a layer on the surface of the first layer 111, for example, an aspect in which the cation exchange substance substituted with a metal ion is formed as a layer on the surface of copper contained in the first layer 111 can be mentioned.
The content of the cation exchange substance substituted with a metal ion is not particularly limited. Its lower limit is preferably such that the deposition amount of the cation exchange substance is 125 g or more and 24333 g or less per 100 g of the total of the copper element in the first layer 111 and the copper element in the second layer 112, from the viewpoint of reliably controlling the supply of water molecules to the copper and reliably preventing an excessive supply of hydrogen ions to the copper.
For example, a method for preparing the cation exchange substance substituted with a metal ion is as follows: a solution of the cation exchange substance (e.g., an aqueous solution) and a metal ion-containing solution (e.g., an aqueous solution) are prepared, and by mixing the cation exchange substance solution and the metal ion-containing solution, the hydrogen ion of the cation exchange substance is substituted with a metal ion, whereby the cation exchange substance substituted with a metal ion can be obtained.
In the cathode electrode 100 of the present invention, as a first aspect of the cathode electrode, the copper as a catalyst material of the first layer 111 includes divalent copper, and zerovalent copper and/or monovalent copper, and the copper as a catalyst material of the second layer 112 includes divalent copper, and zerovalent copper and/or monovalent copper. In the first aspect of the cathode electrode, as essential components of the first layer 111 and the second layer 112, divalent copper, and zerovalent copper and/or monovalent copper are included. Examples of monovalent copper include cuprous oxide (Cu₂O), and examples of divalent copper include cupric oxide (CuO). Examples of zerovalent copper include a simple substance of copper (Cu).
In the carbon dioxide reduction reaction at the cathode electrode 100 using copper as a catalyst material, it is considered that the boundary between copper with different valences (e.g., the boundary between zerovalent copper and monovalent copper) present in the cathode electrode 100 mainly functions as a site in which C-C bonds are formed from carbon dioxide-derived carbons and in which the C-C bonds are stabilized. That is, it is considered that the boundary between copper with different valences (e.g., the boundary between zerovalent copper and monovalent copper) is the main active site for the carbon dioxide reduction reaction. From the above, when the copper of the first layer 111 contains divalent copper, and zerovalent copper and/or monovalent copper, and the copper of the second layer 112 contains divalent copper, and zerovalent copper and/or monovalent copper, the catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be more reliably sustained with high efficiency over a long period.
In the cathode electrode 100 of the present invention, as a second aspect of the cathode electrode, the copper of the first layer 111 includes monovalent copper and/or divalent copper for reduction, which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper, which is not reduced to zerovalent copper, and the copper of the second layer 112 includes monovalent copper and/or divalent copper for reduction, which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper, which is not reduced to zerovalent copper. In the second aspect of the cathode electrode, a part of the monovalent copper and/or divalent copper of the first layer 111 is reduced to zerovalent copper, and a part of the monovalent copper and/or divalent copper of the second layer 112 is reduced to zerovalent copper. From the above, in the second aspect of the cathode electrode, as copper (Cu), there exist monovalent copper and/or divalent copper for reduction, and monovalent copper and/or divalent copper which is not reduced to zerovalent copper. The second aspect of the cathode electrode described above includes monovalent copper and/or divalent copper as essential components of the first layer 111 and the second layer 112. In the second aspect of the cathode electrode, by being subjected to a reduction treatment, the divalent copper for reduction is reduced to zerovalent copper or monovalent copper, and the monovalent copper for reduction is reduced to zerovalent copper. Therefore, in the second aspect of the cathode electrode, by being subjected to a reduction treatment, it becomes a cathode electrode containing zerovalent copper, and monovalent copper and/or divalent copper.
Examples of the monovalent copper include cuprous oxide (Cu₂O), and examples of the divalent copper include cupric oxide (CuO). Further, examples of the zerovalent copper include a simple substance of copper (Cu).
By the copper of the first layer 111 containing monovalent copper and/or divalent copper for reduction that is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper that is not reduced to zerovalent copper, and the copper of the second layer 112 containing monovalent copper and/or divalent copper for reduction that is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper that is not reduced to zerovalent copper, the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be more reliably sustained with high efficiency over a long period.
In the cathode electrode 100 of the present invention (including the first and second aspects of the cathode electrode), the catalyst material may further comprise, 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.
In the cathode electrode 100 using copper as a catalyst material, during the carbon dioxide reduction reaction, monovalent copper is reduced to zerovalent copper, and divalent copper is reduced to zerovalent copper or monovalent copper. When the carbon dioxide reduction reaction is continued for a long period, it is considered that the boundary between copper with different valences (e.g., the boundary between zerovalent copper and monovalent copper) tends to decrease as divalent copper is reduced to zerovalent copper or monovalent copper and monovalent copper is reduced to zerovalent copper. Therefore, when 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 in the cathode electrode 100, so that the reduction of monovalent copper to zerovalent copper is appropriately suppressed. From the above, even when the carbon dioxide reduction reaction is sustained for a long period, the abundance ratio of zerovalent copper to monovalent copper is reliably optimized, thereby maintaining the formation and stabilization of C-C bonds, and more reliably sustaining the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction with high efficiency over a long period.
The aspect of 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 additive element (M) itself (simple substance of the additive element (M)). In addition to the aspect of the additive element (M) itself, the aspects of hydroxides and oxides can be mentioned. The aspect of the additive element (M) may also be a mixture of the aspect of the additive element (M) itself, the aspect of hydroxide, and the aspect of oxide. These additive elements (M) may be used alone or in combination of two or more.
The content of the additive element (M1) of the first layer 111 is not particularly limited, and its lower limit is preferably 0.10 atomic%, and more preferably 0.20 atomic% with respect to 100 atomic% of the total copper element including zerovalent copper, monovalent copper, divalent copper and the like contained in the first layer 111. This is because even when the carbon dioxide reduction reaction is sustained for a long period, the abundance ratio of zerovalent copper to monovalent copper is reliably optimized, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be more stably sustained. The lower limit is particularly preferably 0.25 atomic% because even when the carbon dioxide reduction reaction is sustained for a long period, the abundance ratio of zerovalent copper to monovalent copper can be maintained within a more appropriate range, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be even more stably sustained for a longer period. On the other hand, the upper limit of the content of the additive element (M1) of the first layer 111 is preferably 1.0 atomic%, and more preferably 0.85 atomic% with respect to 100 atomic% of the total copper element including zerovalent copper, monovalent copper, divalent copper and the like contained in the first layer 111. This is because even when the carbon dioxide reduction reaction is sustained for a long period, the abundance ratio of zerovalent copper to monovalent copper is reliably optimized, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be more stably sustained for a long period. The upper limit is particularly preferably 0.70 atomic% because even when the carbon dioxide reduction reaction is sustained for a long period, the abundance ratio of zerovalent copper to monovalent copper can be maintained within a more appropriate range, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be even more stably sustained for a longer period.
The content of the additive element (M2) of the second layer 112 is not particularly limited, and its lower limit is preferably 0.10 atomic%, and more preferably 0.20 atomic% with respect to 100 atomic% of the total copper element including zerovalent copper, monovalent copper, divalent copper and the like contained in the second layer 112. This is because even when the carbon dioxide reduction reaction is sustained for a long period, the abundance ratio of zerovalent copper to monovalent copper is reliably optimized, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be more stably sustained. The lower limit is particularly preferably 0.25 atomic% because even when the carbon dioxide reduction reaction is sustained for a long period, the abundance ratio of zerovalent copper to monovalent copper can be maintained within a more appropriate range, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be even more stably sustained for a longer period. On the other hand, the upper limit of the content of the additive element (M2) of the second layer 112 is preferably 1.0 atomic%, and more preferably 0.85 atomic% with respect to 100 atomic% of the total copper element including zerovalent copper, monovalent copper, divalent copper and the like contained in the second layer 112. This is because even when the carbon dioxide reduction reaction is sustained for a long period, the abundance ratio of zerovalent copper to monovalent copper is reliably optimized, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be more stably sustained for a long period. The upper limit is particularly preferably 0.70 atomic% because even when the carbon dioxide reduction reaction is sustained for a long period, the abundance ratio of zerovalent copper to monovalent copper can be maintained within a more appropriate range, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be even more stably sustained for a longer period.
As the additive element (M1) of the first layer 111 and the additive element (M2) of the second layer 112, any of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon can be used, since they are more easily oxidized than copper and have a higher affinity for oxygen than copper. On the other hand, among the additive element (M1) of the first layer 111 and the additive element (M2) of the second layer 112, aluminum is preferable from the viewpoint that the function of appropriately retaining oxygen in the cathode electrode 100 is reliably obtained, and therefore, even if the carbon dioxide reduction reaction is continued for a long period, the abundance ratio of zerovalent copper to monovalent copper is reliably optimized, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be more stably sustained over a long period.
The structure of the cathode electrode 100 is not particularly limited and may be solid or porous, and a porous structure is preferable from the viewpoint that the contact between water and carbon dioxide is facilitated at the site of the carbon dioxide reduction reaction of the cathode electrode 100, thereby allowing the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol to be more stably sustained over a long period. The proportion of voids (porosity) in the porous structure is not particularly limited, and its lower limit is preferably 1 vol%, and particularly preferably 10 vol%, from the viewpoint that the penetration of carbon dioxide into the cathode electrode 100 is facilitated, 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 particularly preferably 90 vol%, from the viewpoint of maintaining the surface area contributing to the catalytic reaction of the cathode electrode 100, 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 described later.
The cathode electrode 100 of the present invention can produce olefin-based hydrocarbons such as ethylene and alcohols such as ethanol, for example, by supplying carbon dioxide gas from the side of the first portion 101 of the cathode electrode 100 and supplying liquid-phase water from the side of the second portion 102 of the cathode electrode 100, causing carbon dioxide and water to react through the catalytic action of the cathode electrode 100 to electrically reduce carbon dioxide.

[Composite Body of Cathode Electrode and Base Material]

The cathode electrode 100 of the present invention may be used as the cathode electrode 100 alone, or it may be used in a state in which a composite body 120 of the cathode electrode and a base material 1 is formed, as described below.
As shown in Figs. 1 and 2, the composite body 120 of the cathode electrode 100 and the base material 1 comprises the base material 1 and the cathode electrode 100 of the present invention in which a second layer 112 is arranged on the base material 1. The base material 1 may have a carbon layer 113 on its surface. As the carbon layer 113, for example, a carbon layer 113 formed of particulate carbon can be mentioned, and specifically, for example, a carbon layer 113 formed of carbon black can be mentioned.
The base material 1 is provided on the side of the first portion 101 of the cathode electrode 100. That is, the side of the first portion 101 of the cathode electrode 100 is arranged on the base material 1. In the composite body 120, the carbon layer 113 of the base material 1 is adjacent to the second layer 112 of the cathode electrode 100. From the above, the composite body 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 base material 1 are laminated. By supplying carbon dioxide gas from the side of the base material 1 of the composite body 120 (that is, the side of the first portion 101 of the cathode electrode 100), the diffusion of carbon dioxide gas in the second layer 112 of the cathode electrode 100 can be made uniform, so that the supply of carbon dioxide to the cathode electrode 100 is facilitated.
A base material is not provided on the side of the second portion 102 of the cathode electrode 100, and the second portion 102 is exposed to the external environment of the cathode electrode 100 and the composite body 120. The cathode electrode 100 is a coating film that covers the surface of the base material 1. In the composite body 120 of the cathode electrode 100 and the base material 1, even if the first layer 111 located on the surface layer of the cathode electrode 100 is immersed in the electrolyte solution 130, carbon dioxide gas can smoothly contact the second layer 112 of the cathode electrode 100. Therefore, in the composite body 120 of the present invention, even if the first layer 111 located on the surface layer of the cathode electrode 100 is immersed in the electrolyte solution 130, the selectivity of hydrogen, which is a side reaction product, is reduced, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period.
The base material 1 may be solid or porous. A porous structure having gas permeability is preferable from the viewpoint that carbon dioxide gas can smoothly contact the cathode electrode 100, thereby allowing the catalytic reaction for producing olefin-based hydrocarbons such as ethylene and alcohols such as ethanol to be sustained more stably over a long period.
The material of the porous base material 1 is not particularly limited. For example, carbon, fluorine-containing resins, and metals are preferable from the viewpoint of being able to obtain the composite body 120 of the cathode electrode 100 and the base material 1, by which the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be more stably sustained over a long period.
As carbon, for example, a composite body of carbon black and carbon fiber can be mentioned. As shown in Figs. 1 and 2, in the composite body 120, the base material 1 is a carbon material that is a composite body of a carbon layer 113 formed of carbon black and carbon fibers 114, and has a structure in which the carbon layer 113 is laminated on the carbon fibers 114. In the composite body 120, the carbon layer 113 provided on the carbon fibers 114 is adjacent to the second layer 112.
Examples of the fluorine-containing resin include polytetrafluoroethylene, polyvinylidene fluoride, perfluoroalkoxyalkane, perfluoroethylene propylene copolymer, ethylene tetrafluoroethylene copolymer, polychlorotrifluoroethylene, and ethylene chlorotrifluoroethylene copolymer. Examples of the metal include porous metals made of copper (Cu), niobium (Nb), aluminum (Al), titanium (Ti), alloys containing one or more kinds of the above metals, stainless steel and the like. When the material of the porous base material 1 is a metal and the metal is copper (Cu), examples of the porous metal include a sintered body of copper particles.
The average thickness of the base material 1 is not particularly limited, and for example, a plate-shaped member with a thickness of 0.2 mm or more and 1.5 mm or less can be mentioned.
The cathode electrode 100 in the composite body 120 of the cathode electrode 100 and the base material 1 is, for example, a sputtering layer formed on the base material 1 by sputtering, which has copper and, if necessary, the additive element (M).

[Method for Producing Composite Body of Cathode Electrode and Base Material]

A method for producing the composite body of the cathode electrode and the base material that electrically reduces carbon dioxide (first production method) will be described below. Fig. 3 is an explanatory diagram of an electropolishing step in the method for producing the composite body of the cathode electrode and the base material. Fig. 4 is an explanatory diagram of a sputtering layer formation step and a copper oxidation treatment step in the method for producing the composite body of the cathode electrode and the base material. Fig. 5 is an explanatory diagram of a partial reduction step in the method for producing the composite body of the cathode electrode and the base material. Fig. 6 is an explanatory diagram showing a state in which a cation exchange substance substituted with a metal ion is applied in the method for producing the composite body of the cathode electrode and the base material.
The first production method for the composite body of the cathode electrode and the base material comprises: (1) a step of preparing a base material having a carbon layer and a porous structure; (2) an electropolishing treatment step of performing an electropolishing treatment on the prepared base material as necessary; (3) a sputtering layer formation step of forming a sputtering layer containing copper by sputtering on the carbon layer of the base material, which has been optionally subjected to the electropolishing treatment; (4) a copper oxidation treatment step of oxidizing at least a part of the copper in the sputtering layer to monovalent copper and/or divalent copper as necessary after the sputtering layer formation step; (5) a partial reduction step of partially reducing the monovalent copper and/or divalent copper oxidized in the copper oxidation treatment step to zerovalent copper and/or monovalent copper as necessary; and (6) a metal-ion-substituted cation exchange substance application step of applying a cation exchange substance substituted with a metal ion onto the sputtering layer, which has undergone the copper oxidation treatment step and the partial reduction step as necessary, to surface-modify the sputtering layer. Among the above steps, steps (1), (3), and (6) are essential, and steps (2), (4), and (5) are optional.
By the above-described method for producing a composite body of the cathode electrode and the base material, the cathode electrode in which the first layer is surface-modified with a cation exchange substance substituted with a metal ion can be formed on the base material. By surface-modifying the first layer of the cathode electrode with a cation exchange substance substituted with a metal ion, it is considered that the action of the metal ions promotes the penetration of copper from the first layer into the carbon layer of the underlying base material, thereby greatly contributing to the formation of a second layer which is a mixed layer of copper and carbon. From the above, in the step of surface-modifying the first layer with a cation exchange substance substituted with a metal ion (that is, the metal-ion-substituted cation exchange substance application step), the cathode electrode having a two-layer structure comprising the first layer containing copper but not carbon and the second layer containing copper and carbon can be produced.
By the above-described method for producing the composite body of the cathode electrode and the base material, it is possible to produce the composite body in which, even if the surface layer of the cathode electrode is immersed in an electrolyte solution, the selectivity of hydrogen, which is a side reaction product is reduced, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period. Furthermore, the above-described method for producing the composite body of the cathode electrode and the base material includes the metal-ion-substituted cation exchange substance application step, by which the supply of water molecules to the copper constituting the cathode electrode can be controlled by the cation exchange substance substituted with a metal ion. Therefore, the selectivity of hydrogen is reduced, and it is possible to produce the composite body in which the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period.
Examples of the cathode electrode formed by the above-described method for producing the composite body of the cathode electrode and the base material include cathode electrodes in which copper contains divalent copper, and zerovalent copper and/or monovalent copper, and cathode electrodes in which copper contains monovalent copper, and/or divalent copper for reduction that is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper that is not reduced to zerovalent copper.

(1) Step of Preparing Base Material Having Carbon Layer and Porous Structure

The step of preparing a base material having a carbon layer and a porous structure is a step of preparing the above-mentioned base material. The material of the base material having a carbon layer and the porosity of the porous structure can be appropriately selected according to the characteristics required for the composite body of the cathode electrode and the base material.

(2) Electropolishing Treatment Step

The electropolishing treatment step is a step performed as necessary, for example, when a metal is used as the material for the base material. In the electropolishing treatment step, after degreasing the surface of the base material with an organic solvent such as hexane, washing and drying it, as shown in Fig. 3, a mixed acid solution 11 is placed in a container 10, the base material 1 which is the anode is immersed in the mixed acid solution 11, a cathode 2 is immersed at a position sandwiching the base material 1, and an electrolytic potential is applied to the base material 1, which is the anode, and the cathode 2. By applying an electrolytic potential to the base material 1, which is the anode, and the cathode 2, the surface of the base material 1 is electropolished. By electropolishing the surface of the base material 1, the affected layer on the surface of the base material 1 is reduced or removed. As the mixed acid solution 11, for example, a mixed aqueous solution of phosphoric acid and sulfuric acid can be mentioned. As the cathode 2, for example, titanium and the like can be mentioned. It should be noted that, as shown in Figs. 1 and 2, when the base material 1 of the composite body 120 is a carbon material, the electropolishing treatment may not be performed.

(3) Sputtering Layer Formation Step

As shown in Fig. 4, in the sputtering layer formation step, a sputtering layer 20 containing copper is formed on the base material 1 by sputtering. When the sputtering layer 20 contains, in addition to copper (Cu), at least one additive 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 formation step comprises a copper (Cu) sputtering layer formation step of forming a sputtering layer of copper (Cu), which is a sputtering layer containing copper (Cu), on the base material 1 by sputtering, and an additive element (M) sputtering layer formation step of forming a sputtering layer of the additive element (M), which is a sputtering layer containing the additive element (M), on the copper (Cu) sputtering layer by sputtering. Furthermore, it comprises a copper (Cu) sputtering layer formation step of further forming a sputtering layer of copper (Cu), which is a sputtering layer containing copper (Cu), on the formed additive element (M) sputtering layer by sputtering. By adjusting the amount of power from the power source and the sputtering time, the content of the additive element (M) with respect to 100 atomic% of copper (Cu) can be adjusted. Further, if necessary, to make the presence of copper (Cu) and the additive element (M) uniform throughout the sputtering layer 20, the copper (Cu) sputtering layer formation step and the additive element (M) sputtering layer formation step may be performed alternately multiple times. Through the sputtering layer formation step, the cathode electrode containing divalent copper and zerovalent and/or monovalent copper, and optionally an additive element, or a cathode electrode containing monovalent and/or divalent copper for reduction which is reduced to zerovalent copper by a reduction treatment, and monovalent and/or divalent copper which is not reduced to zerovalent copper, and optionally an additive element, can be formed on the base material 1.

(4) Copper Oxidation Treatment Step

The copper oxidation treatment step is performed as necessary when adjusting the abundance ratio of monovalent copper (Cu₂O) and/or divalent copper (CuO) to a predetermined amount. As shown in Fig. 4, in the copper oxidation treatment step, an oxidation treatment by electroless plating is performed on the formed sputtering layer 20 to oxidize at least a part of the zerovalent copper (Cu) contained in the sputtering layer 20 into monovalent copper (Cu₂O) and/or divalent copper (CuO), thereby forming an oxidized portion 22. As the oxidation treatment by electroless plating, for example, an oxidation treatment method of immersing the sputtering layer 20 in a copper sulfate aqueous solution can be mentioned.

(5) Partial Reduction Step

The partial reduction step is a step performed as necessary to further optimize the abundance ratio of zerovalent copper to monovalent copper by reducing the monovalent copper (Cu₂O) and/or divalent copper (CuO) oxidized in the copper oxidation treatment step to zerovalent copper (Cu) and/or monovalent copper (Cu₂O). In the partial reduction step, as shown in Fig. 5, a composite body 1' obtained by forming the sputtering layer 20 on the base material 1, and an anode electrode 33 are immersed in a partial reduction aqueous solution 32 housed in a two-chamber electrolytic cell 30 having a diaphragm 31, and a partial reduction treatment is performed by applying an electrolytic potential from a power source 34 to the two-chamber electrolytic cell 30. Further, by performing the partial reduction treatment, the sputtering layer 20 can be made porous. As the anode electrode 33, for example, platinum can be mentioned. As the partial reduction aqueous solution 32, for example, a potassium bicarbonate aqueous solution can be mentioned for both the side of the composite 1' and the side of the anode electrode.

(6) Metal-Ion-Substituted Cation Exchange Substance Application Step

As shown in Fig. 6, the metal-ion-substituted cation exchange substance application step is a step of applying a cation exchange substance substituted with a metal ion 21 onto the sputtering layer 20, which has undergone the copper oxidation treatment step and the partial reduction step as necessary, to modify the sputtering layer 20 with the cation exchange substance substituted with a metal ion 21. When the oxidized portion 22 is formed by the copper oxidation treatment step, the cation exchange substance substituted with a metal ion 21 is also applied to the oxidized portion 22. As the method for applying the cation exchange substance substituted with a metal ion 21 onto the sputtering layer 20, for example, a method for spraying a solution of the cation exchange substance substituted with a metal ion 21 can be mentioned. By applying the cation exchange substance substituted with a metal ion 21 onto the sputtering layer 20, at least a part of the monovalent copper (Cu₂O) is oxidized to divalent copper (CuO).
By applying the cation exchange substance substituted with a metal ion 21 onto the sputtering layer 20, a structure in which the first layer located on the surface of the cathode electrode is surface-modified with the cation exchange substance substituted with a metal ion can be obtained. Furthermore, by applying the cation exchange substance substituted with a metal ion 21 onto the sputtering layer 20, the action of the metal ions promotes the penetration of copper from the first layer located on the surface layer of the cathode electrode into the carbon layer of the base material, thereby forming the second layer which is a mixed layer of copper and carbon. From the above, in the metal-ion-substituted cation exchange substance application step, the sputtering layer 20 can be formed into a two-layer structure comprising a first layer containing copper but not carbon, and a second layer containing copper and carbon.
Next, another method for producing the composite body of the cathode electrode and the base material that electrically reduces carbon dioxide (second production method) will be described below.
The second production method for the composite body of the cathode electrode and the base material comprises: (1) a step of preparing a base material having a porous structure; (2) a second layer formation step of applying a second suspension, which is a mixture of copper and carbon, onto the base material to form a second layer, which is a mixed layer of copper and carbon; (3) a first layer formation 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 substance application step of applying a cation exchange substance substituted with a metal ion onto the first layer to surface-modify the first layer. In the second production method as well, after the first layer formation step and before the metal-ion-substituted cation exchange substance application step, the above-mentioned copper oxidation treatment step and/or partial reduction step may be optionally performed.
By the above-described method for producing the composite body of the cathode electrode and the base material, a cathode electrode having a two-layer structure comprising a second layer containing copper and carbon, and a first layer containing copper but not carbon and being surface-modified with a cation exchange substance substituted with a metal ion can be formed on the base material. Furthermore, by surface-modifying the first layer of the cathode electrode with a cation exchange substance substituted with a metal ion, the action of the metal ions is considered to promote the 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.
By the above-described method for producing the composite body of the cathode electrode and the base material, it is possible to produce a composite body in which, even if the first layer located on the surface layer of the cathode electrode is immersed in an electrolyte solution, the selectivity of hydrogen as a side reaction product is reduced, and the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period. Furthermore, in the above-described method for producing the composite body of the cathode electrode and the base material, by comprising the metal-ion-substituted cation exchange substance application step, the supply of water molecules to the copper constituting the cathode electrode can be controlled by the cation exchange substance substituted with a metal ion. Therefore, the selectivity of hydrogen is reduced, and it is possible to produce a composite body in which the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained with high efficiency over a long period.
Examples of the cathode electrode formed by the above-described method for producing the composite body of the cathode electrode and the base material include cathode electrodes in which the copper of the first layer contains divalent copper, and zerovalent copper and/or monovalent copper, and the copper of the second layer contains divalent copper and zerovalent copper and/or monovalent copper, and cathode electrodes in which the copper of the first layer contains monovalent copper and/or divalent copper for reduction that is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper that is not reduced to zerovalent copper, and the copper of the second layer contains monovalent copper and/or divalent copper for reduction that is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper that is not reduced to zerovalent copper.

(1) Step of Preparing Base Material Having Porous Structure

The step of preparing a base material having a porous structure is the step of preparing the base material described above. The material of the base material and the porosity of the porous structure can be appropriately selected according to the properties required for the composite body of the cathode electrode and the base material.

(2) Second Layer Formation Step

The second layer formation step is a step of forming a second layer, which is a mixed layer of copper and carbon, by applying a second suspension, which is a mixture of copper and carbon, onto the prepared base material. The second suspension can be prepared, for example, by adding copper, carbon such as carbon black, and as 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 onto the base material is not particularly limited, and for example, spray coating, a bar coater, and the like can be mentioned. The second layer can be formed by drying the second suspension applied onto the base material.

(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 the first layer containing copper on the second layer. The first suspension can be prepared, for example, by adding copper and, as necessary, further adding 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 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 for example, spray coating, a bar coater, or the like can be mentioned. The first layer can be formed by drying the first suspension applied onto the second layer.

(4) Metal-ion-substituted Cation Exchange Substance Application Step

The metal-ion-substituted cation exchange substance application step is a step of applying the cation exchange substance substituted with a metal ion onto the first layer to surface-modify the first layer with the cation exchange substance substituted with a metal ion. As the method for applying the cation exchange substance substituted with a metal ion onto the first layer, for example, a method of spraying a solution of the cation exchange substance substituted with a metal ion can be mentioned. By applying the cation exchange substance substituted with a metal ion onto the first layer, at least a part of the monovalent copper (Cu₂O) is oxidized to divalent copper (CuO).
Of the first production method and the second production method described above, the first production method, in which the first layer and the second layer are formed by applying the cation exchange substance substituted with a metal ion 21 onto the sputtering layer 20, is preferable because the density of the first layer containing copper and the second layer, which is a mixed layer of copper and carbon, is improved, whereby the volume reduction of the first layer and the second layer, which are catalyst layers for the carbon dioxide reduction reaction, can be reliably suppressed, and in turn, the invasion of moisture contained in the electrolyte solution into the catalyst layers can be prevented, thereby preventing an increase in the selectivity of hydrogen as a side reaction product.

[Electrolytic Reduction Device]

Next, an electrolytic reduction device provided with the cathode electrode of the present invention for electrically reducing carbon dioxide to carbon monoxide, olefinic hydrocarbons and/or alcohols, and an electrolytic reduction device provided with the composite body of the cathode electrode and the base material of the present invention for electrically reducing carbon dioxide to carbon monoxide, olefinic hydrocarbons and/or alcohols will be described. Fig. 7 is an explanatory diagram showing an outline of an electrolytic reduction device provided with the cathode electrode of the present invention. Fig. 8 is an explanatory diagram showing an outline of another electrolytic reduction device provided with the cathode electrode of the present invention.
Examples of the electrolytic reduction device 210 include a three-chamber electrolytic reduction device, as shown in Fig. 7. Specifically, the electrolytic reduction device 210 has, for example, an electrolytic cell 214 provided with a cathode gas chamber 211, a catholyte chamber 212, and an anolyte chamber 213, which are partitioned from each other. The cathode gas chamber 211 and the catholyte chamber 212 are partitioned by a cathode 216 serving as a gas diffusion electrode. The catholyte chamber 212 and the anolyte chamber 213 are partitioned by a diaphragm 217 having ion conductivity. An anode electrode 218 is arranged in the anolyte chamber 213. Carbon dioxide gas is supplied to the cathode gas chamber 211. A catholyte is supplied to the catholyte chamber 212. An anolyte is supplied to the anolyte chamber 213. The anode electrode 218 and the cathode 216 are connected to a DC power source 219.
The anolyte and the catholyte are aqueous solutions in which an electrolyte is dissolved. The electrolyte includes, for example, at least one of potassium, sodium, lithium, and a compound thereof. The electrolyte includes, for example, at least one compound selected from the group consisting of LiOH, NaOH, KOH, Li₂CO₃, Na₂CO₃, K₂CO₃, LiHCO₃, NaHCO₃, and KHCO₃.
The cathode 216 is a gas diffusion electrode and has a gas diffusion layer 221 and a microporous layer 222. In the electrolytic reduction device 210, the composite body of the cathode electrode and the base material, which is the present invention and has the first layer and the second layer, in which the first layer is surface-modified with a cation exchange substance substituted with a metal ion, is used as the cathode 216, and the microporous layer 222 corresponds to the base material of the composite body. The gas diffusion layer 221 allows gas containing carbon dioxide to permeate but suppresses the permeation of the aqueous solution containing the catholyte. The microporous layer 222 allows both the gas containing carbon dioxide and the aqueous solution containing the catholyte to permeate. The gas diffusion layer 221 and the microporous layer 222 are each formed in a planar shape. The gas diffusion layer 221 is arranged on the side of the cathode gas chamber 211, and the microporous layer 222 is arranged on the side of the catholyte chamber 212.
As the gas diffusion layer 221, for example, those in which a water-repellent film such as polytetrafluoroethylene is formed on the surface of a porous conductive base material such as carbon paper, carbon felt, and carbon cloth can be mentioned. The conductive base material is connected to the negative electrode of the DC power source 219 and receives a supply of electrons. The microporous layer 222 is formed on the surface of the gas diffusion layer 221 using carbon, fluorine-containing resin, metal, or the like, and supports a catalyst. In the electrolytic reduction device 210, the cathode electrode according to the present invention, which has a first layer and a second layer, in which the first layer is surface-modified with a cation exchange substance substituted with a metal ion, is used as the catalyst supported by the microporous layer 222. Further, by using renewable energy as the DC power source 219 of the electrolytic reduction device 210, it is possible to produce olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction while reducing the environmental load. It should be noted that, 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.
Further, examples of another electrolytic reduction device 210 include an electrolytic reduction device 210 with an MEA type electrolytic cell structure, as shown in Fig. 8. In another electrolytic reduction device 210 shown in Fig. 8, the same components as 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 catholyte, and thus does not have a catholyte chamber 212. Therefore, the electrolytic reduction device 210 with an MEA type electrolytic cell structure has an electrolytic cell 214 provided with a cathode gas chamber 211 and an anolyte chamber 213, which are partitioned from each other, instead of an electrolytic cell 214 provided with a cathode gas chamber 211, a catholyte chamber 212, and an anolyte chamber 213, which are partitioned from each other. The cathode gas chamber 211 and the anolyte chamber 213 are partitioned by a diaphragm 217 sandwiched between the cathode electrode and the anode electrode 218. The anode electrode 218 is arranged in the anolyte chamber 213. Carbon dioxide gas is supplied to the cathode gas chamber 211. An anolyte is supplied to the anolyte chamber 213. The anode electrode 218 and the cathode electrode are connected to a DC power source 219. The electrolytic reduction device 210 with an MEA type electrolytic cell structure is suitable for integration because it does not use a catholyte, as the diaphragm 217 itself is used as the electrolyte.
In the electrolytic reduction device 210 with an MEA type electrolytic cell structure as well, the cathode electrode is a gas diffusion electrode and has a gas diffusion layer 221 and a microporous layer 222. Further, in the electrolytic reduction device 210 with an MEA type electrolytic cell structure, the composite body of the cathode electrode and the base material according to the present invention, which has a first layer and a second layer, in which the first layer is surface-modified with a cation exchange substance substituted with a metal ion, is used as the cathode electrode, and the microporous layer 222 corresponds to the base material of the composite body.

Examples

Hereinafter, examples of the present invention will be described. It should be noted that the present invention is not limited to the following examples.

[Example 1]

Preparation of Cathode Electrode

Sputtering Layer Formation Step

On a base material (porous carbon, which is a composite body of a carbon layer formed of carbon black and carbon fibers), a copper layer was deposited by sputtering under the conditions of DC 100 W, argon gas 6.0 sccm, for 5 minutes, to form a first copper sputtering layer. Then, on the formed first copper sputtering layer, an aluminum layer was deposited by sputtering under the conditions of DC 50 W, argon gas 6.0 sccm, for 30 seconds, to form a first aluminum sputtering layer. Then, on the formed first aluminum sputtering layer, a copper layer was deposited by sputtering under the conditions of DC 100 W, argon gas 6.0 sccm, for 5 minutes, to form a second copper sputtering layer. Then, on the formed second copper sputtering layer, an aluminum layer was deposited by sputtering under the conditions of DC 50 W, argon gas 6.0 sccm, for 30 seconds, to form a second aluminum sputtering layer. Then, on the formed second aluminum sputtering layer, a copper layer was deposited by sputtering under the 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 which is a laminated body of five layers.

Copper Oxidation Treatment Step

The sputtering layer, which is a laminated body, obtained as described above was immersed in 100 mL of an aqueous solution containing copper sulfate and potassium sulfate (9.7 mM copper ion, 0.5 M sulfate ion) and subjected to electroless plating at 20°C for 20 minutes to oxidize a part of the zerovalent copper contained in the copper sputtering layers into monovalent copper and/or divalent copper.

Metal-Ion-Substituted Cation Exchange Substance Application Step

An aqueous solution of a cation exchange resin (sulfonated tetrafluoroethylene-based polymer, "Nafion" (registered trademark), DuPont) (concentration: 5 mass%) and an aqueous solution of potassium hydroxide (concentration: 8M) were mixed to prepare an aqueous solution of potassium-ion-substituted cation exchange resin (an aqueous solution of a cation exchange resin in which hydrogen ions of the cation exchange resin were substituted with potassium ions). The obtained aqueous solution of potassium-ion-substituted cation exchange resin was spray-coated (coating amount of the aqueous solution of potassium-ion-substituted cation exchange resin: 650 µL) onto the surface of the sputtering layer, which is the five-layered laminated body and had undergone the copper oxidation treatment step by electroless plating, and then dried by heating. From the weight difference before and after coating, the sputtering layer was surface-modified with the cation exchange resin substituted with potassium ions so that the deposition amount of the cation exchange resin (Nafion) was 125 g to 24333 g per 100 g of copper element. By surface-modifying the sputtering layer with the cation exchange resin substituted with potassium ions, a first layer surface-modified with the cation exchange substance substituted with a metal ion, and a second layer, which is a mixed layer of copper and carbon formed by promoting the penetration of copper of the first layer into the underlying carbon layer by the action of the metal ions, were formed.
As described above, the cathode electrode which is a sputtering layer was prepared on a base material, and the composite body of the cathode electrode and the base material was produced.

[Comparative Example 1]

The composite body of the cathode electrode and the base material was produced in the same manner as in Example 1, except that the metal-ion-substituted cation exchange substance application step was not performed. From the above, in Comparative Example 1, it was taken as the composite body of the cathode electrode and the base material in which a second layer, which is a mixed layer of copper and carbon, was not formed.

Evaluation Items

[Stability Test]

The ethylene gas selectivity (E24, unit: %) was measured 24 hours after the start of the carbon dioxide reduction reaction. E24 of 50% or more was evaluated as excellent in catalytic reaction stability, indicating that the catalytic reaction for producing ethylene through the carbon dioxide reduction reaction can be stably sustained over a long period. The ethylene gas selectivity was evaluated as follows.

[Ethylene Gas Selectivity (%)]

From the concentration of ethylene contained in the outlet gas of the electrolytic cell (electrolyte solution: 1 M KHCO₃ aqueous solution, electrolyte solution flow rate: 50 mL/min) and the gas flow rate, the number of moles of ethylene per unit time and the required number of moles of electrons were calculated (carbon dioxide gas flow rate: 40 mL/min). On the other hand, from the set current value (-530 mA) of the potential application device, the number of moles of electrons that passed through the electrolytic cell per unit time was calculated. 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.
The measurement results for Examples and Comparative Examples are shown in Table 1 below. [Table 1]

Example 1 Comparative Example 1

E24(unit: %) 57 45
As shown in Table 1 above, in Example 1, in which the sputtering layer was surface-modified with a potassium-ion-substituted cation exchange resin to form a first layer surface-modified with a cation exchange substance substituted with a metal ion and a second layer in which copper and carbon are mixed, the ethylene gas selectivity (E24) after 24 hours from the start of the carbon dioxide reduction reaction was 50% or more. This indicates that even when the catalyst layer was immersed in the electrolyte solution, the increase in hydrogen selectivity was suppressed, and the catalytic reaction for producing ethylene through the carbon dioxide reduction reaction could be stably sustained over a long period, showing excellent stability of the catalytic reaction.
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) after 24 hours from the start of the carbon dioxide reduction reaction was 45%. It could not be evaluated that the catalytic reaction for producing ethylene through the carbon dioxide reduction reaction could be stably sustained over a long period even when the catalyst layer was immersed in the electrolyte solution.

Industrial Applicability

The cathode electrode of the present invention is highly valuable for industrial use in the field of absorbing and recovering atmospheric carbon dioxide to produce industrially useful organic compounds from carbon dioxide, because the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol through the carbon dioxide reduction reaction can be stably sustained over a long period even when the catalyst layer is immersed in an electrolyte solution.

Reference Signs List

1: Base material
20: Sputtering layer
21: Cation exchange substance substituted with a metal ion
100: Cathode electrode
111: First layer
112: Second layer
120: Composite body

Claims

A cathode electrode which electrically reduces carbon dioxide, the cathode electrode comprising: a first layer with a first thickness, located on a surface layer of the cathode electrode; and a second layer with a second thickness, adjacent to the first layer in a thickness direction, wherein:
the first layer contains copper;

the first layer is surface-modified with a cation exchange substance substituted with a metal ion; and

the second layer is a mixed layer of copper and carbon.
The cathode electrode of Claim 1, wherein a ratio of the second thickness of the second layer to the first thickness of the first layer is greater than 0 and 100 or less.
The cathode electrode of Claim 1 or 2, wherein:
the second layer has a first region located on a side of the first layer, and a second region adjacent to the first region, with a central portion in a thickness direction of the second layer as a boundary; and

a content (vol%) of the copper in the first region is greater than a content of the copper (vol%) in the second region.
The cathode electrode of Claim 1 or 2, wherein:
the copper in the first layer comprises divalent copper, and zerovalent copper and/or monovalent copper; and

the copper in the second layer comprises divalent copper, and zerovalent copper and/or monovalent copper.
The cathode electrode of Claim 1 or 2, wherein:
the copper of the first layer comprises monovalent copper and/or divalent copper for reduction, which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper, which is not reduced to zerovalent copper; and

the copper of the second layer comprises monovalent copper and/or divalent copper for reduction, which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper, which is not reduced to zerovalent copper.
The cathode electrode of Claim 1 or 2, wherein:
the first layer further comprises at least one additive element selected from a group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon; and

the second layer further comprises at least one additive element selected from the group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon.
The cathode electrode of 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 with respect 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 with respect to 100 atomic% of the copper contained in the second layer.
The cathode electrode of Claim 1 or 2, wherein the cation exchange substance comprises a sulfonated tetrafluoroethylene-based polymer.
The cathode electrode of Claim 1 or 2, wherein the metal ion comprises an alkali metal ion and/or an alkaline earth metal ion.
The cathode electrode of Claim 1 or 2, wherein the cation exchange substance substituted with a metal ion is formed as a layer on a surface of the first layer.
The cathode electrode of Claim 1 or 2, wherein the cation exchange substance substituted with a metal ion is mixed with the copper of the first layer.
The cathode electrode of Claim 1 or 2, comprising a porous structure.
A composite body, comprising: a base material; and the cathode electrode of Claim 1 or 2 in which the second layer is arranged on the base material.
An electrolytic reduction device for electrically reducing carbon dioxide to carbon monoxide, olefinic hydrocarbons, and/or alcohols, the electrolytic reduction device comprising the cathode electrode of Claim 1 or 2.
An electrolytic reduction device for electrically reducing carbon dioxide to carbon monoxide, olefinic hydrocarbons, and/or alcohols, the electrolytic reduction device comprising the composite body of Claim 13.
A method for producing a composite body of a cathode electrode and a base material for electrically reducing carbon dioxide, comprising:
a step of preparing a base material having a porous structure;

a second layer formation step of applying a second suspension, which is a mixture of copper and carbon, onto the base material to form a second layer, which is a mixed layer of copper and carbon;

a first layer formation 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 substance application step of applying a cation exchange substance substituted with a metal ion onto the first layer to surface-modify the first layer.
The method for producing a composite body of Claim 16, wherein:
the copper of the first layer comprises divalent copper, and zerovalent copper and/or monovalent copper; and

the copper of the second layer comprises divalent copper, and zerovalent copper and/or monovalent copper.
The method for producing a composite body of Claim 16, wherein:
the copper of the first layer comprises monovalent copper and/or divalent copper for reduction, which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper, which is not reduced to zerovalent copper; and

the copper of the second layer comprises monovalent copper and/or divalent copper for reduction, which is reduced to zerovalent copper by a reduction treatment, and monovalent copper and/or divalent copper, which is not reduced to zerovalent copper.
The method for producing a composite body of any one of Claims 16 to 18, wherein:
the first suspension further comprises at least one additive element selected from a group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon; and

the second suspension further comprises at least one additive element selected from a group consisting of silver, gold, cadmium, tin, aluminum, boron, gallium, zinc, titanium, and silicon.