Cathode electrode, composite of cathode electrode and substrate, and method for producing composite of cathode electrode and substrate

Abstract

The present invention provides a cathode electrode on which a catalytic reaction for producing an olefin-type hydrocarbon such as ethylene or an alcohol such as ethanol through a carbon dioxide reduction reaction can be sustained stably for a long period.　A cathode electrode for reducing carbon dioxide electrically, comprising cuprous oxide, copper and at least one other metal element selected from the group consisting of silver, gold, zinc and cadmium.

Description

[Technical Field]

[0001] The present invention relates to a cathode electrode capable of electrically reducing carbon dioxide to convert the carbon dioxide into an olefin such as ethylene, a composite of the cathode electrode and a substrate, and a method for producing the composite of the cathode electrode and a substrate. [Background technology]

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

[0003] Specifically, carbon dioxide can be reduced and converted into, for example, ethylene, ethanol, carbon monoxide, methane, methanol, formic acid, etc., and used in the synthesis of organic compounds. Among these, the C2 compounds ethylene and ethanol are very useful as derivatives in the synthesis of various organic compounds, and are more useful than C1 compounds such as carbon monoxide and methane.

[0004] In recent years, catalysts such as photocatalysts and electrocatalysts have been widely used in the carbon dioxide reduction reaction described above, and there is a demand for the development of catalysts with even better catalytic performance. Catalysts used in carbon dioxide reduction reactions are required to have not only reaction efficiency but also selectivity for specific reactions, and from this perspective, the selection of materials is important (Non-Patent Document 1). For example, gold, silver, and zinc are used as catalytic materials to efficiently reduce and generate carbon monoxide and increase the proportion of carbon monoxide in the reduced materials. Copper is also used as a catalytic material to efficiently reduce and generate hydrocarbons such as methane, ethane, and ethylene. Of these, copper, in particular, is attracting attention as an electrode catalyst for the cathode reduction of carbon dioxide because it can generate C2 compounds such as ethylene.

[0005] As an example of a copper-based cathode reduction electrode catalyst for carbon dioxide, a carbon dioxide reduction cathode electrode has been proposed (Patent Document 1). This electrode comprises a copper-based substrate and a diffusion barrier layer made of an organic substance. A catalytic layer made primarily of metal clusters is then formed on the substrate. This prevents diffusion of metal elements between the catalytic layer and the substrate and side reactions of the metal, thereby preventing a decrease in catalytic efficiency. Patent Document 1 discloses a carbon dioxide reduction cathode electrode that prevents diffusion of metal elements between the catalytic layer and the substrate and side reactions of the metal, thereby preventing a decrease in catalytic efficiency, by forming a diffusion barrier layer made of an organic substance on the copper-based substrate and then forming a catalytic layer made primarily of metal clusters on the diffusion barrier layer. However, Patent Document 1 only evaluates the faradaic efficiency of each product, such as ethylene, in the carbon dioxide reduction reaction in its examples. Patent Document 1 does not verify whether the catalytic reaction that produces ethylene and other products continues stably over a long period of time.

[0006] In order to commercialize the production of ethylene and the like through the reduction reaction of carbon dioxide on an industrial scale, the catalytic reaction for producing ethylene and the like must be able to continue stably for a long period of time, such as several hundred hours or more. The cathode electrode for carbon dioxide reduction in Patent Document 1 leaves room for improvement in terms of stably continuing the catalytic reaction for producing ethylene and the like for a long period of time. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-168410 [Non-patent literature]

[0008] [Non-Patent Document 1] Y Hori “Electrochemical reduction of CO at a Copper Electrode.” J. Phys. Chem. B. 101(36). 7075-7081 (1997) Summary of the Invention [Problem to be solved by the invention]

[0009] In view of the above circumstances, an object of the present invention is to provide a cathode electrode capable of stably maintaining a catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by a reduction reaction of carbon dioxide over a long period of time, a composite of the cathode electrode and a substrate, and a method for producing the composite. [Means for solving the problem]

[0010] The gist of the configuration of the present invention is as follows. [1] A cathode electrode that electrically reduces carbon dioxide. A cathode electrode comprising cuprous oxide, copper, and at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium. [2] A cathode electrode that electrically reduces carbon dioxide. A cathode electrode comprising cuprous oxide that is not reduced to copper, at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, and reducing cuprous oxide that is reduced to copper by reduction treatment. [3] A cathode electrode that electrically reduces carbon dioxide in an electrolyte solution containing carbon dioxide. A cathode electrode comprising cuprous oxide, copper, and at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium. [4] A cathode electrode that electrically reduces carbon dioxide in an electrolyte solution containing carbon dioxide. A cathode electrode comprising cuprous oxide that is not reduced to copper, at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, and reducing cuprous oxide that is reduced to copper by reduction treatment. [5] The cathode electrode according to any one of [1] to [4], wherein the at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium is a hydroxide or an oxide. [6] The cathode electrode according to any one of [1] to [5], wherein the ratio of the maximum peak intensity among the peak intensities in the XRD patterns of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, the hydroxide of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, and the oxide of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, to the peak intensity in the XRD pattern in X-ray diffraction measurement using CuKα radiation of cuprous oxide, is 0.20 or less. [7] The cathode electrode according to any one of [1] to [6], wherein metallic copper and monovalent copper are present on the surface when a potential in the range of +0.2 V to −1.4 V relative to a reversible hydrogen electrode is applied in an electrolyte solution containing carbon dioxide. [8] The cathode electrode according to any one of [1] to [7], wherein the value of the number of moles of copper / the number of moles of cuprous oxide is in the range of 2.5 to 80. [9] The cathode electrode according to [1] or [3], which has a porous structure.

[10] A composite of a cathode electrode and a substrate, comprising a conductive substrate and the cathode electrode according to any one of [1] to [9] formed on the conductive substrate.

[11] The composite according to

[10] , wherein the conductive substrate is a copper substrate.

[12] The composite according to

[10] or

[11] , wherein the copper base material is polycrystalline copper having a copper purity of 99.9 mol% or more, and the copper base material is a plate material having an average thickness of a processing-affected layer of 1.0 μm or less.

[13] The composite according to any one of

[10] to

[12] , wherein the cathode electrode is a codeposited layer.

[14] Providing a conductive substrate; a codeposited layer forming step of codepositing cuprous oxide and at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium on the conductive substrate to form a codeposited layer; A method for producing a composite of a cathode electrode and a substrate, comprising:

[15] The method according to

[14] , further comprising an electrolytic polishing step of electrolytically polishing the conductive substrate, wherein the codeposited layer forming step is carried out after the electrolytic polishing step.

[16] The method according to

[14] or

[15] , further comprising a partial reduction step of partially reducing the codeposited layer after the codeposited layer formation step.

[17] An electrolysis device for electrically reducing carbon dioxide to olefinic hydrocarbons and / or alcohols, comprising the cathode electrode according to any one of [1] to [9]. [Effects of the Invention]

[0011] According to an embodiment of the cathode electrode of the present invention, the electrode contains cuprous oxide, copper, and at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, or cuprous oxide that cannot be reduced to copper, at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, and reducing cuprous oxide that can be reduced to copper by reduction treatment. This allows the catalytic reaction of carbon dioxide reduction to produce olefinic hydrocarbons such as ethylene and alcohols such as ethanol to be maintained stably for a long period of time. Furthermore, both ethylene and ethanol are C2 compounds, and the formation of a C-C bond on the catalyst is intermediate in the reaction pathway. Therefore, the active sites for ethylene and ethanol production are the same or very close to each other, and therefore show similar stability trends. The carbon dioxide reduction reaction proceeds in the same way for both ethylene and ethanol production.

[0012] According to an embodiment of the cathode electrode of the present invention, the ratio of the maximum peak intensity among the peak intensities in the XRD pattern of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, the hydroxide of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, and the oxide of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, measured using CuKα radiation, to the peak intensity in the XRD pattern of cuprous oxide measured using CuKα radiation, is 0.20 or less. This not only enables the catalytic reaction to produce olefinic hydrocarbons such as ethylene and alcohols such as ethanol to be stably maintained for a long period of time, but also improves the faradaic efficiency for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol.

[0013] According to an embodiment of the cathode electrode of the present invention, when a potential in the range of +0.2 V to −1.4 V relative to a reversible hydrogen electrode is applied in an electrolyte solution containing carbon dioxide, the presence of metallic copper and monovalent copper on the surface allows the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction of carbon dioxide to be maintained stably for an even longer period of time.

[0014] According to an embodiment of the cathode electrode of the present invention, the value of the ratio of the number of moles of copper to the number of moles of cuprous oxide is in the range of 2.5 to 80, which not only enables the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol to be stably continued for a long period of time, but also improves the faradaic efficiency for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol.

[0015] According to an aspect of the cathode electrode of the present invention, the cathode electrode has a porous structure, which not only enables the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol to be stably continued for a long period of time, but also improves the faradaic efficiency for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol.

[0016] According to an embodiment of the composite of the cathode electrode and the substrate of the present invention, by including the cathode electrode of the present invention, it is possible to obtain a composite that can stably maintain, for a long period of time, a catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by a reduction reaction of carbon dioxide.

[0017] According to an embodiment of the composite of a cathode electrode and a substrate of the present invention, the substrate is a plate material made of polycrystalline copper with a copper purity of 99.9 mol % or more and with an average thickness of a work-affected layer of 1.0 μm or less. This not only enables the catalytic reaction to produce olefinic hydrocarbons such as ethylene and alcohols such as ethanol to be stably maintained for a long period of time, but also improves the faradaic efficiency of the production of olefinic hydrocarbons such as ethylene and alcohols such as ethanol.

[0018] The method for producing a composite of a cathode electrode and a substrate of the present invention includes a codeposited layer formation step of codepositing cuprous oxide and at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium on a conductive substrate to form a codeposited layer. This makes it possible to produce a composite in which the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol by reduction of carbon dioxide can be stably maintained for a long period of time. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is an explanatory diagram showing an outline of a cross section of a composite of a cathode electrode and a conductive substrate according to the present invention. [Figure 2] FIG. 2 is an explanatory diagram illustrating an outline of a process-affected layer of a conductive substrate. [Figure 3] FIG. 2 is an explanatory diagram of an electrolytic polishing treatment step in a method for producing a composite of a cathode electrode and a substrate. [Figure 4] FIG. 2 is an explanatory diagram of a codeposited layer forming step in the method for producing a composite of a cathode electrode and a substrate. [Figure 5] FIG. 2 is an explanatory diagram of a partial reduction step in the method for producing a composite of a cathode electrode and a substrate. [Figure 6] FIG. 1 is an explanatory diagram of a continuous electrolysis test device used in a continuous electrolysis test. [Figure 7] 1 is a graph showing the results of continuous electrolysis tests in Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0020] [Cathode electrode] The cathode electrode of the present invention will be described below. The first cathode electrode of the present invention is a cathode electrode that electrically reduces carbon dioxide and contains cuprous oxide (CuO), copper (Cu), and at least one other metal element (M) selected from the group consisting of silver (Ag), gold (Au), zinc (Zn), and cadmium (Cd). The above-mentioned first cathode electrode of the present invention contains cuprous oxide (CuO), copper (Cu), and the other metal element (M) as essential components.

[0021] The first cathode electrode of the present invention contains cuprous oxide (CuO), copper (Cu), and the other metal element (M) as essential components, thereby enabling the catalytic reaction of producing C2 compounds such as ethylene through the reduction of carbon dioxide to be stably maintained for a long period of time.Furthermore, the first cathode electrode of the present invention contains cuprous oxide (CuO), copper (Cu), and the other metal element (M) as essential components, thereby enabling the catalytic reaction of producing olefinic hydrocarbons such as ethylene and propylene, and alcohols such as ethanol, propanol, and allyl alcohol through the reduction of carbon dioxide to be stably maintained for a long period of time.

[0022] The second cathode electrode of the present invention is a cathode electrode that electrically reduces carbon dioxide and includes cuprous oxide (CuO) that is not reduced to copper, at least one other metal element (M) selected from the group consisting of silver (Ag), gold (Au), zinc (Zn), and cadmium (Cd), and cuprous oxide (CuO) for reduction that is reduced to copper (Cu) by reduction treatment. In the second cathode electrode, a portion of the cuprous oxide (CuO) is reduced to copper (Cu). The second cathode electrode of the present invention described above includes cuprous oxide (CuO) and the other metal element (M) as essential components. The second cathode electrode of the present invention is subjected to a reduction treatment, whereby the cuprous oxide (CuO) for reduction is reduced to copper (Cu), resulting in a cathode electrode containing cuprous oxide (CuO), copper (Cu), and at least one other metal element (M) selected from the group consisting of silver (Ag), gold (Au), zinc (Zn), and cadmium (Cd).

[0023] The form of the other metal element (M) in the cathode electrode is not particularly limited, and examples include the metal itself, as well as hydroxides and oxides. The other metal element (M) may be a mixture of the metal itself, hydroxides, and oxides. Silver, gold, zinc, and cadmium can all be used as the other metal element (M). However, zinc and silver are preferred, with zinc being particularly preferred, because they enable the catalytic reaction to produce olefinic hydrocarbons such as ethylene and alcohols such as ethanol to continue stably for a longer period of time. These other metal elements (M) may be used alone or in combination of two or more. The beneficial effects of the other metal element (M) include improved stability of the ethylene or ethanol production reaction and the ability to reduce CO to CO. When the content of the other metal element (M) in the cathode electrode reaches a predetermined level, CO produced on the other metal element (M) is released into the electrolyte and further reduced to ethylene or ethanol. In other words, a new reaction pathway that facilitates the production of ethylene or ethanol is believed to be provided. The other metal elements include metal elements added as raw materials and metal elements deposited by electrodeposition or the like.

[0024] When silver, gold, zinc, or cadmium is used as the other metal element (M), the ratio of the peak intensity of the XRD pattern (hereinafter sometimes simply referred to as the "XRD pattern") in X-ray diffraction measurement of cuprous oxide using CuKα radiation to the peak intensity of the XRD pattern of the other metal element (M) is not particularly limited. However, the upper limit of the ratio of the maximum peak intensity among the peak intensities of the XRD patterns of the other metal element (M) itself, the hydroxide of the other metal element (M), and the oxide of the other metal element (M) to the peak intensity of the XRD pattern of cuprous oxide (hereinafter sometimes simply referred to as the "peak intensity ratio of the XRD pattern") is preferably 0.20, more preferably 0.15, and particularly preferably 0.10, from the viewpoints of not only ensuring that the catalytic reaction producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol can be stably continued for a long period of time, but also improving the Faradaic efficiency for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol. On the other hand, the lower limit of the peak intensity ratio in the XRD pattern is preferably 0.005, and particularly preferably 0.0075, in order to reliably improve the faradaic efficiency of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol.

[0025] In this specification, "peak intensity of an XRD pattern" refers to the product of the diffraction peak height of each compound phase measured by X-ray diffraction and the half-width of that diffraction peak. Furthermore, in this specification, "maximum XRD peak intensity" refers to the maximum peak intensity in the XRD pattern for each compound phase. When the cathode electrode is a thin film, a measurement method suitable for measuring thin films, such as the "D8 DISCOVER with VANTEC2000" micro-X-ray diffractometer manufactured by Bruker AXS, is used. When the cathode electrode is a bulk body and has a thickness sufficient to be greater than the penetration depth of X-rays, a conventional X-ray diffraction method may be used.

[0026] The cathode electrode may also be in an embodiment containing cuprous oxide, zero-valent copper, and at least one other metal element (M) selected from the group consisting of silver, gold, zinc, and cadmium. In this case, the value of the moles of copper / moles of cuprous oxide in the cathode electrode, i.e., the ratio of the moles of copper to the moles of cuprous oxide, is not particularly limited. However, the upper limit is preferably 80, more preferably 65, and particularly preferably 50, from the viewpoints of not only enabling the catalytic reaction to produce olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol to be stably continued for a long period of time, but also improving the faradaic efficiency for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol. On the other hand, the lower limit of the ratio of moles of copper to moles of cuprous oxide is preferably 2.5, and particularly preferably 3.0, because this ratio not only ensures stable long-term catalytic reactions for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol, but also improves the faradaic efficiency of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol. When the moles of zero-valent copper / moles of cuprous oxide ratio at the cathode electrode is within this range, the adjacent Cu and monovalent Cu (copper in cuprous oxide) distribute negative and positive charges to the C of the CO molecule, which is thought to be a reaction intermediate adsorbed on the cathode electrode. This is thought to reduce the activation energy for C-C bond formation and improve ethylene selectivity.

[0027] Furthermore, it is preferable that metallic copper and monovalent copper are present on the surface of the cathode electrode when a potential in the range of +0.2 V to -1.4 V relative to the reversible hydrogen electrode is applied in an electrolyte solution containing carbon dioxide. When the above potential is applied, the presence of monovalent copper on the surface of the cathode electrode allows the catalytic reaction of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol by the reduction reaction of carbon dioxide to continue stably for an even longer period of time. When an electrolysis device equipped with a cathode electrode performs a reduction reaction of carbon dioxide for a long period of time under certain operating conditions (current value), the potential of the cathode electrode shifts in the negative direction. When the potential of the cathode electrode shifts to the negative direction, monovalent copper (Cu + ) disappears, the active sites of olefin hydrocarbons such as ethylene and alcohols such as ethanol disappear, and the stability of these hydrocarbons and alcohols tends to decrease. However, even if the potential of the cathode electrode shifts to the negative, the monovalent copper (Cu + The presence of ethylene and other olefinic hydrocarbons and alcohols such as ethanol maintains their active sites, improving the stability of ethylene and other olefinic hydrocarbons and alcohols such as ethanol.

[0028] The cathode electrode may be solid or porous, but a porous structure is preferred because it not only allows the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol to continue stably for a long period of time, but also improves the faradaic efficiency of the production of olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol. The porosity of the porous structure is not particularly limited, but its lower limit is preferably 1% because it facilitates the permeation of carbon dioxide into the cathode electrode, thereby further improving the faradaic efficiency of the production of olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol. On the other hand, the upper limit of the porosity of the porous structure is preferably 99% because it maintains the surface area that contributes to the catalytic reaction of the cathode electrode, thereby further improving the faradaic efficiency of the production of olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol.

[0029] The cathode electrode of the present invention can electrically reduce carbon dioxide to produce olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol by applying an electrolytic potential from a power source while immersed in a cathode-side electrolyte solution containing carbon dioxide.

[0030] [Complex of cathode electrode and substrate] The cathode electrode of the present invention may be used in the form of a single cathode electrode, or may be used in the form of a composite with a substrate, as described below. Fig. 1 is an explanatory diagram showing an outline of a cross section of the composite of the cathode electrode and substrate of the present invention. Fig. 2 is an explanatory diagram showing an outline of a process-affected layer of a conductive substrate.

[0031] As shown in FIG. 1 , the composite of a cathode electrode and a substrate includes a substrate and the cathode electrode of the present invention formed on the substrate. The composite of a cathode electrode and a substrate may be solid, porous, or a combination of porous and solid. For example, a gas diffusion layer may be sandwiched between the substrate and the cathode electrode. The cathode electrode serves as a coating film that covers the surface of the substrate. By including the cathode electrode of the present invention, the composite of a cathode electrode and a substrate of the present invention can be obtained, which can stably maintain a catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol by a carbon dioxide reduction reaction for a long period of time. The structure of the cathode electrode formed on the substrate may be solid or porous. However, a porous structure is preferred because, as described above, not only can the catalytic reaction for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol be stably maintained for a long period of time, but also the faradaic efficiency for producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol is improved. The porous structure of the cathode electrode can be formed by performing a partial reduction treatment, which will be described later, on a solid cathode electrode.

[0032] When carbon dioxide is electrically reduced by electrolysis, current is passed from a power source to a cathode electrode via the substrate, and therefore the substrate is conductive. Examples of conductive substrates include copper (Cu), niobium (Nb), aluminum (Al), titanium (Ti), alloys containing one or more of the above metals, and stainless steel (SUS). The substrate may have a solid or porous structure, but a porous structure is preferred from the viewpoint of improving gas diffusion. Of these, a copper substrate is preferred because it allows the catalytic reaction that produces olefinic hydrocarbons such as ethylene to continue stably for a longer period of time. The average thickness of the substrate is not particularly limited, but examples thereof include a plate material of 0.2 mm to 1.5 mm.

[0033] The copper substrate may be polycrystalline copper having a copper purity of 99.9 mol% or more (i.e., unavoidable impurities of less than 0.1 mol%). The average thickness of the process-affected layer of the copper substrate is not particularly limited, but is preferably 1.0 μm or less, more preferably 0.5 μm or less, and particularly preferably 0 μm, in order to ensure that the catalytic reaction producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol can be stably continued for a long period of time, and also to improve the Faraday efficiency of producing olefinic hydrocarbons such as ethylene and alcohols such as ethanol, propanol, and allyl alcohol. The process-affected layer can be reduced or removed, for example, by electrolytic polishing the copper substrate, as described below.

[0034] A process-affected layer is a layer of a material whose near-surface structure is altered by heat or mechanical forces during rolling or machining of metals, resulting in a change in its structure compared to the bulk structure. This layer typically becomes amorphous or has finer crystal grains than the bulk. When a cross section of a substrate is analyzed using electron backscatter diffraction (EBSD), the process-affected layer can be identified using the circle-equivalent diameter d of a region (crystal grain) consisting of a specific crystal plane that appears monochromatically in a crystal orientation mapping image. In other words, in this specification, a "process-affected layer" is defined as a region within 5 μm of the material surface in EBSD crystal orientation mapping, where there are at least two amorphous regions or crystal grains with d≦0.2 μm per square μm. The "average thickness of the process-affected layer" refers to the average of the thicknesses measured at the thickest points of the process-affected layer within a magnified observation field, measured at five different points.

[0035] The cathode electrode of the composite of the cathode electrode and the substrate is a codeposited layer formed by, for example, immersing the substrate in a codepositing solution containing copper ions, which are raw materials for cuprous oxide, and ions of another metal element (M), and codepositing cuprous oxide and the other metal element (M) on the substrate.

[0036] [Method for producing a composite of a cathode electrode and a substrate] An example of a method for producing a composite of a cathode electrode and a substrate will be described below. Fig. 3 is an explanatory diagram of an electrolytic polishing treatment step in the method for producing a composite of a cathode electrode and a substrate. Fig. 4 is an explanatory diagram of a codeposited layer formation step in the method for producing a composite of a cathode electrode and a substrate. Fig. 5 is an explanatory diagram of a partial reduction step in the method for producing a composite of a cathode electrode and a substrate.

[0037] A method for producing a composite of a cathode electrode and a substrate includes, for example, (1) a step of preparing a conductive substrate, (2) an electrolytic polishing step of, if necessary, electropolishing the prepared conductive substrate, (3) a codeposited layer formation step of codepositing cuprous oxide and at least one other metal element (M) selected from the group consisting of silver, gold, zinc, and cadmium on the electropolished conductive substrate to form a codeposited layer, and (4) a partial reduction step of, if necessary, partially reducing the codeposited layer. Of these steps, steps (1) and (3) are essential, and steps (2) and (4) are optional.

[0038] (1) A process for preparing a conductive substrate The step of preparing a conductive substrate is a step of preparing the substrate described above, and the type of conductive substrate can be appropriately selected depending on the properties required for the composite of the cathode electrode and the substrate.

[0039] (2) Electrolytic polishing process In the electrolytic polishing process, the substrate surface is degreased with an organic solvent such as hexane, washed, and dried. Then, as shown in FIG. 3, a mixed acid solution 11 is placed in a container 10, and the substrate 1 (anode) is immersed in the mixed acid solution 11, with a cathode 2 immersed in a position sandwiching the substrate 1. An electrolytic potential is applied to the substrate 1 (anode) and the cathode 2. By applying an electrolytic potential to the substrate 1 (anode) and the cathode 2, the surface of the substrate 1 is electropolished. By electropolishing the surface of the substrate 1, the process-affected layer on the surface of the substrate 1 is reduced and removed. An example of the mixed acid solution 11 is an aqueous solution of mixed acid of phosphoric acid and sulfuric acid. An example of the cathode 2 is titanium.

[0040] (3)Co-electrodeposition layer formation process As shown in FIG. 4 , a codeposition aqueous solution 21 containing copper ions, another metal element (M), and an organic acid at a predetermined molar ratio is placed in a container 20, and the pH of the codeposition aqueous solution 21 is adjusted to a predetermined range using an alkaline aqueous solution. The temperature of a medium 23, such as water, in which the outer surface of the container 20 is immersed is controlled using a temperature control device 22, thereby adjusting the temperature of the codeposition aqueous solution 21 to 50–60°C. Thereafter, a substrate 1, a reference electrode (Ag / AgCl) 24, and a counter electrode (platinum electrode) 25 are immersed in the codeposition aqueous solution 21. Next, the current density supplied from the power source is controlled to codeposit cuprous oxide and another metal element (M) on the substrate 1, thereby forming a cathode electrode, which is a codeposited layer. The amounts and component ratios of the cuprous oxide and other metal elements (M) to be codeposited can be adjusted by controlling the concentration, component ratio, codeposition time, current density, and pH of the codeposition aqueous solution 21. Examples of the alkaline aqueous solution include a sodium hydroxide aqueous solution and a potassium hydroxide aqueous solution. The pH can be set within a range of, for example, 9.0 to 11. Examples of the organic acid include oxalic acid, acetic acid, lactic acid, and citric acid.

[0041] (4) Partial reduction process As shown in FIG. 5, a composite 1′ obtained by forming a cathode electrode, which is a codeposited layer, on a substrate 1 and an anode 33 are immersed in a partial reduction aqueous solution 32 contained in a two-chamber electrolytic cell 30 having a diaphragm 31, and an electrolytic potential is applied to the two-chamber electrolytic cell 30 from a power source 34, thereby performing a partial reduction treatment. By performing the partial reduction treatment, the cathode electrode can be made porous, as shown in FIG. 1. An example of the anode 33 is platinum. An example of the partial reduction aqueous solution 32 is a potassium bicarbonate aqueous solution on both the cathode and anode sides.

[0042] [Electrolyzer] Next, an electrolysis device equipped with the cathode electrode of the present invention, which electrically reduces carbon dioxide to olefinic hydrocarbons and / or alcohols, will be described below. The electrolysis device for electrochemically reducing carbon dioxide mainly comprises an electrolytic cell, a gas recovery device, an electrolyte circulation device, a carbon dioxide supply unit, a power source, etc.

[0043] The electrolytic cell is a portion where a target substance is reduced, and is also a portion that includes the cathode electrode of the present invention, and is a portion where carbon dioxide (including the case where carbon dioxide is dissolved in a solution as well as bicarbonate ions) is reduced. Electrolytic power is supplied to the electrolytic cell from a power source.

[0044] The electrolyte circulation device is a component that circulates the cathode-side electrolyte to the cathode electrode of the electrolytic cell. The electrolyte circulation device is, for example, a tank and a pump, and is capable of supplying carbon dioxide from a carbon dioxide supply unit into the electrolyte to achieve a predetermined carbon dioxide concentration, and circulating the electrolyte between the electrolyte and the electrolytic cell.

[0045] The cathode-side electrolyte of the electrolytic cell is preferably an electrolyte capable of dissolving a large amount of carbon dioxide, and examples thereof include alkaline solutions such as aqueous sodium hydroxide solution, aqueous potassium hydroxide solution, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate, monomethanolamine, methylamine, and other liquid amines, and mixtures of these liquid amines with aqueous electrolyte solutions. Also usable as the cathode-side electrolyte are acetonitrile, benzonitrile, methylene chloride, tetrahydrofuran, propylene carbonate, dimethylformamide, dimethyl sulfoxide, methanol, and ethanol. Also usable as the anode-side electrolyte of the electrolytic cell are, for example, the same electrolytes as the cathode electrolyte.

[0046] The gas recovery device is a component that recovers gases generated by reduction in the electrolytic cell. The gas recovery device can capture gases such as olefinic hydrocarbons and alcohols generated at the cathode electrode immersed in the electrolytic solution of the electrolytic cell. The gas recovery device may be configured to separate and recover different gases.

[0047] The function of the electrolysis device is as follows: An electrolytic potential is applied to the electrolytic cell from a power source. An electrolyte circulation device supplies an electrolyte to the cathode electrode of the electrolytic cell. Carbon dioxide in the supplied electrolyte is reduced at the cathode electrode of the electrolytic cell. The reduction of carbon dioxide produces carbon-containing substances such as olefinic hydrocarbons such as ethylene and alcohols such as ethanol. The carbon-containing substances produced at the cathode electrode are recovered by a gas recovery device. The gas recovery device can separate and store the gas as needed. [Example]

[0048] Next, examples of the present invention will be described, but the present invention is not limited to the following examples.

[0049] [Example 1] Preparation of the cathode electrode Electrolytic polishing process The surface of commercially available oxygen-free polycrystalline copper with a purity of 99.9 mol% or higher was degreased with hexane, washed, and dried, and then electrolytically polished using a mixed acid solution of phosphoric acid and sulfuric acid in the electrolytic polishing apparatus shown in Figure 3. Titanium was placed as the cathode, sandwiching the copper substrate as the anode, to remove the damage layer from the copper substrate surface. The electron backscatter diffraction (EBSD) measurement device used was a TSL OIM5.0 HIKARI.

[0050] Co-electrodeposition layer formation process In the co-electrodeposition apparatus shown in Figure 4, a co-electrodeposition aqueous solution containing copper sulfate and zinc sulfate as its main components, the pH of which was adjusted to 9.5 to 10 using a sodium hydroxide aqueous solution, was prepared. The temperature of the water medium was then adjusted to 50 to 60°C using a temperature control device. A copper substrate that had been subjected to an electropolishing treatment, a reference electrode (Ag / AgCl), and a counter electrode (platinum electrode) were then placed in the co-electrodeposition aqueous solution, and the current density was controlled to co-electrodeposit copper, cuprous oxide, and zinc (in the form of hydroxide and / or oxide) onto the copper substrate, thereby preparing a cathode electrode as a co-electrodeposited layer on the copper substrate, and a composite of the cathode electrode and substrate was produced.

[0051] Partial reduction process The cathode electrode formed on the copper substrate was subjected to partial reduction treatment by electrolysis in a two-chamber electrolysis cell with a diaphragm as shown in Figure 5, using platinum as the anode and potassium bicarbonate aqueous solution as the partial reduction aqueous solution on both the cathode and anode sides, thereby making the cathode electrode porous.

[0052] [Continuous electrolysis test] As shown in Figure 6, a continuous electrolysis test was conducted using a composite 41 of a porous cathode electrode and a substrate, in which a potassium bicarbonate aqueous solution supplied with CO gas was used as the electrolyte 42. An electrolytic potential was applied from a power source 46 to the porous cathode electrode and anode 44. A pump 45 supplied the electrolyte 42 to a two-chamber continuous electrolysis device 41 equipped with a diaphragm 43. That is, a potassium bicarbonate aqueous solution was used as the electrolyte 42 on both the cathode electrode side and the anode 44 side. A platinum electrode was used as the anode 44. Continuous electrolysis was performed for 700 hours, and gas G generated from the cathode electrode was continuously introduced into a gas analyzer for gas composition analysis. The Faraday efficiency of ethylene gas obtained by gas composition analysis is shown in Figure 7.

[0053] Table 1 also shows the peak intensity ratio of the XRD pattern, the molar ratio of Cu / CuO after partial reduction treatment, the average thickness of the process-affected layer, the faradaic efficiency of ethylene gas after 30 hours, the time until the faradaic efficiency of ethylene gas decreases to 90% of the value at the start of the continuous electrolysis test (ethylene stability), the faradaic efficiency of ethanol after 30 hours, the faradaic efficiency of propanol after 30 hours, and the faradaic efficiency of allyl alcohol after 30 hours.

[0054] X-ray diffraction measurements were performed using a Bruker AXS micro-X-ray diffractometer, "D8 DISCOVER with VANTEC2000." The Cu / CuO molar ratio was determined by measuring and separating the Cu-LMM peak (Auger electron peak) using an ULVAC-PHI XPS (X-ray photoelectron spectroscopy) instrument, "PHI Quantes." The measurement source was Al Kα radiation (hv = 1486.6 eV) with an escape angle of 90 degrees. The peak separation of the Cu-LMM peak obtained by the measurement was determined by the least-squares method using metallic Cu, CuO, and CuO as standard substances. The Faraday efficiency was calculated from the ratio of the total amount of electrons flowing during the electrolysis test to the amount of gas produced, as determined by gas chromatography.

[0055] Monovalent Cu(Cu + ) measurement In the electrolysis test using the continuous electrolysis device 41 shown in Figure 6, a 0.1 M potassium bicarbonate aqueous solution saturated with CO2 gas was used as the electrolyte 42, and a potential was applied to the cathode electrode in the range of +0.2 V to -1.4 V relative to the reversible hydrogen electrode (RHE). Microscopic Raman observation of the cathode electrode surface species was performed using a 785 nm excitation laser beam (10 mW). The change in the electrode potential when applying the potential was 0.2 V. Cu + The Raman band (Raman peak) assigned to -1 ~400cm -1 The presence of monovalent copper was confirmed by observing the presence of Cu. The above measurement conditions were designed to simulate the in-situ CO2 reduction reaction that occurs at the cathode electrode. +The results of the measurement of the presence or absence of Raman peaks are shown in Table 1. + When a Raman peak of Cu was observed, it was marked with a circle. + When no Raman peak was observed, the result was marked as "x."

[0056] [Examples 2 and 3] A composite of a cathode electrode and a substrate was produced by the same procedure as in Example 1, except that the co-electrodeposition aqueous solution and co-electrodeposition time were varied, and the zinc content of the cathode electrode was changed. Cathode electrodes were prepared in which the ratio of the maximum XRD peak intensity of zinc metal, zinc oxide, and zinc hydroxide in the co-electrodeposited layer to the XRD peak intensity of CuO (i.e., the peak intensity ratio in the XRD pattern) was 0.10 or less. Using these electrodes, a continuous electrolysis test similar to that in Example 1 was performed, and the Faraday efficiency of ethylene gas after 30 hours, ethylene stability, the Faraday efficiency of ethanol after 30 hours, the Faraday efficiency of propanol after 30 hours, and the Faraday efficiency of allyl alcohol after 30 hours were measured. Furthermore, the peak intensity ratio in the XRD pattern, the Cu / CuO molar ratio after partial reduction, and the average thickness of the work-affected layer were measured in the same manner as in Example 1. The measurement results are shown in Table 1.

[0057] Furthermore, the presence or absence of monovalent Cu was measured by microscopic Raman observation in the same manner as in Example 1. The measurement results are shown in Table 1.

[0058] [Examples 4 and 5] A composite of a cathode electrode and a substrate was produced by the same procedure as in Example 1, except that the co-electrodeposition aqueous solution and co-electrodeposition time were changed, the zinc in the cathode electrode was replaced with silver, and the silver content of the cathode electrode was also changed. Cathode electrodes with peak intensity ratios of 0.10 or less in the XRD pattern were prepared. Furthermore, a continuous electrolysis test similar to that in Example 1 was performed using the resulting electrodes, and the Faraday efficiency of ethylene gas after 30 hours, ethylene stability, the Faraday efficiency of ethanol after 30 hours, the Faraday efficiency of propanol after 30 hours, and the Faraday efficiency of allyl alcohol after 30 hours were measured. Furthermore, the peak intensity ratio of the XRD pattern, the Cu / CuO molar ratio after partial reduction treatment, and the average thickness of the work-affected layer were measured in the same manner as in Example 1. The measurement results are shown in Table 1.

[0059] [Examples 6, 7, and 8] A composite of a cathode electrode and a substrate was produced by the same procedure as in Example 1, except that the partial reduction conditions in Example 1 were changed to change the Cu / CuO molar ratio contained in the cathode electrode. Cathode electrodes with a Cu / CuO molar ratio of 3.0 to 50 were prepared. Furthermore, a continuous electrolysis test similar to that in Example 1 was performed using the electrodes, and the Faraday efficiency of ethylene gas after 30 hours, ethylene stability, the Faraday efficiency of ethanol after 30 hours, the Faraday efficiency of propanol after 30 hours, and the Faraday efficiency of allyl alcohol after 30 hours were measured. Furthermore, the peak intensity ratio of the XRD pattern, the Cu / CuO molar ratio after partial reduction, and the average thickness of the work-affected layer were measured in the same manner as in Example 1. The measurement results are shown in Table 1.

[0060] [Examples 9 and 10] A composite of a cathode electrode and a substrate was produced by the same procedure as in Example 1, except that the electropolishing time in Example 1 was shortened to leave the process-affected layer on the substrate while reducing the average thickness of the process-affected layer to 1.0 μm or less. A cathode electrode formed on a substrate having a process-affected layer was prepared. Furthermore, a continuous electrolysis test similar to that in Example 1 was performed using the electrode, and the Faraday efficiency of ethylene gas after 30 hours, ethylene stability, the Faraday efficiency of ethanol after 30 hours, the Faraday efficiency of propanol after 30 hours, and the Faraday efficiency of allyl alcohol after 30 hours were measured. Furthermore, the peak intensity ratio of the XRD pattern, the Cu / CuO molar ratio after partial reduction treatment, and the average thickness of the process-affected layer were measured in the same manner as in Example 1. The measurement results are shown in Table 1.

[0061] [Examples 11 and 12] A composite of a cathode electrode and a substrate was produced by the same procedure as in Example 1, except that the co-electrodeposition aqueous solution and co-electrodeposition time were changed in Examples 1 and 4, and the zinc or silver content of the cathode electrode was changed. A cathode electrode with a peak intensity ratio of 0.20 in the XRD pattern was prepared. A continuous electrolysis test similar to that in Example 1 was performed using this electrode, and the Faraday efficiency of ethylene gas after 30 hours, ethylene stability, the Faraday efficiency of ethanol after 30 hours, the Faraday efficiency of propanol after 30 hours, and the Faraday efficiency of allyl alcohol after 30 hours were measured. The peak intensity ratio of the XRD pattern, the Cu / CuO molar ratio after partial reduction, and the average thickness of the work-affected layer were also measured in the same manner as in Example 1. The measurement results are shown in Table 1.

[0062] [Examples 13 and 14] A composite of a cathode electrode and a substrate was produced by the same procedure as in Example 1, except that the partial reduction conditions in Example 1 were changed to vary the molar ratio of Cu to CuO contained in the cathode electrode. Cathode electrodes with Cu / CuO molar ratios of 2.0 and 100 were prepared. A continuous electrolysis test similar to that in Example 1 was performed using the electrodes, and the Faraday efficiency of ethylene gas after 30 hours, ethylene stability, the Faraday efficiency of ethanol after 30 hours, the Faraday efficiency of propanol after 30 hours, and the Faraday efficiency of allyl alcohol after 30 hours were measured. Furthermore, the peak intensity ratio of the XRD pattern, the Cu / CuO molar ratio after partial reduction, and the average thickness of the process-affected layer were measured in the same manner as in Example 1. The measurement results are shown in Table 1.

[0063] [Example 15] A composite of a cathode electrode and a substrate was produced by the same procedure as in Example 1, except that the average thickness of the process-affected layer on the substrate was 1.5 μm by shortening the electropolishing time in Example 1. A cathode electrode formed on a substrate having a process-affected layer was prepared. Furthermore, using this electrode, a continuous electrolysis test similar to that in Example 1 was performed, and the Faraday efficiency of ethylene gas after 30 hours, ethylene stability, the Faraday efficiency of ethanol after 30 hours, the Faraday efficiency of propanol after 30 hours, and the Faraday efficiency of allyl alcohol after 30 hours were measured. Furthermore, similar to Example 1, the peak intensity ratio of the XRD pattern, the Cu / CuO molar ratio after partial reduction treatment, and the average thickness of the process-affected layer were measured. The measurement results are shown in Table 1.

[0064] [Examples 16 to 20] Composites of cathode electrodes and substrates were produced by the same procedure as in Example 1, except that the co-electrodeposition aqueous solution and co-electrodeposition time were varied in Examples 1 and 4, and the zinc or silver content of the cathode electrode was varied. Cathode electrodes with peak intensity ratios of 0.50 and 1.0 in the XRD patterns were prepared. A continuous electrolysis test similar to that in Example 1 was performed using these electrodes, and the Faraday efficiency of ethylene gas after 30 hours, ethylene stability, the Faraday efficiency of ethanol after 30 hours, the Faraday efficiency of propanol after 30 hours, and the Faraday efficiency of allyl alcohol after 30 hours were measured. Furthermore, the peak intensity ratios of the XRD patterns, the Cu / CuO molar ratio after partial reduction, and the average thickness of the work-affected layer were measured in the same manner as in Example 1. The measurement results are shown in Table 1.

[0065] [Comparative Example 1] A composite of a cathode electrode and a substrate was produced by the same procedure as in Example 1, except that the co-electrodeposition aqueous solution in Example 1 did not contain zinc sulfate, and a cathode electrode containing no other metal elements was prepared. Furthermore, a continuous electrolysis test similar to that in Example 1 was performed using this electrode, and the Faraday efficiency of ethylene gas after 30 hours, ethylene stability, the Faraday efficiency of ethanol after 30 hours, the Faraday efficiency of propanol after 30 hours, and the Faraday efficiency of allyl alcohol after 30 hours were measured. Furthermore, the peak intensity ratio of the XRD pattern, the Cu / CuO molar ratio after partial reduction treatment, and the average thickness of the work-affected layer were measured in the same manner as in Example 1. The measurement results are shown in Table 1. The measurement results of the Faraday efficiency of ethylene gas obtained by gas composition analysis in the continuous electrolysis test are shown in Figure 7.

[0066] Furthermore, the presence or absence of monovalent Cu was measured by microscopic Raman observation in the same manner as in Example 1. The measurement results are shown in Table 1.

[0067] [Table 1]

[0068] As shown in Table 1, the cathode electrodes of Examples 1 to 19, which contained cuprous oxide and zinc or silver as the other metal element (M), exhibited ethylene stability of over 500 hours, meaning that the catalytic reaction of producing ethylene by carbon dioxide reduction was maintained stably for a long period of time. Furthermore, when the other metal element (M) was zinc, a comparison of Examples 1 to 3 and 11 with Examples 16 and 17 revealed that the faradaic efficiency of ethylene gas was further improved for cathode electrodes with a peak intensity ratio of 0.20 or less in the XRD pattern. In particular, a comparison of Examples 1 to 5 with Examples 11 and 12 revealed that the faradaic efficiency of ethylene gas, the faradaic efficiency of ethanol, the faradaic efficiency of propanol, and the faradaic efficiency of allyl alcohol were also improved for cathode electrodes with a peak intensity ratio of 0.10 or less in the XRD pattern. Furthermore, comparing Examples 1 to 8 with Examples 13 and 14, it was found that the faradaic efficiency of ethylene gas, ethanol, propanol, and allyl alcohol was improved for cathode electrodes with a Cu / CuO molar ratio of 3.0 to 50. Comparing Examples 1, 9, and 10 with Example 15, it was found that the faradaic efficiency of ethylene gas, ethanol, propanol, and allyl alcohol was improved for cathode electrodes with an average thickness of a process-affected layer of 1.0 μm or less. Comparing Example 1 with Examples 2 and 3, it was found that the cathode electrodes in which monovalent Cu was detected even when a potential was applied to the cathode electrode in the range of +0.2 V to -1.4 V versus the reversible hydrogen electrode (RHE) showed ethylene stability of over 1,000 hours, meaning that the catalytic reaction of producing ethylene by the reduction of carbon dioxide was maintained stably for an even longer period.

[0069] On the other hand, in the cathode electrode of Comparative Example 1, which did not contain other metal elements (M), ethylene stability was limited to 250 hours, and the catalytic reaction to produce ethylene could not be sustained for a long period of time. In addition, in the cathode electrode of Comparative Example 1, no monovalent Cu was detected when a potential in the range of +0.2 V to -1.4 V was applied to the reversible hydrogen electrode (RHE). [Industrial Applicability]

[0070] The cathode electrode of the present invention can stably maintain a catalytic reaction for a long period of time to produce olefinic hydrocarbons such as ethylene and alcohols such as ethanol through a reduction reaction of carbon dioxide, and is therefore highly useful in fields where carbon dioxide is absorbed and recovered from the atmosphere and used to produce industrially useful organic compounds. [Explanation of symbols]

[0071] 1 Base material 1' complex 2 cathode 10 containers 11 Mixed acid solution 20 containers 21 Co-deposition aqueous solution 22 Temperature control device 23 Medium 24 Reference electrode (Ag / AgCl) 25 Counter electrode (platinum electrode) 30 electrolysis cells 31 Diaphragm 32 Partial reduction aqueous solution 33 Anode 34 Power supply

Claims

1. A cathode electrode that electrically reduces carbon dioxide. cuprous oxide, copper, and at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium; A cathode electrode in which the ratio of the maximum peak intensity among the peak intensities in the XRD patterns of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, a hydroxide of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, and an oxide of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, to the peak intensity in the XRD pattern in X-ray diffraction measurement using CuKα radiation of cuprous oxide is 0.20 or less.

2. a cathode electrode that electrically reduces carbon dioxide in an electrolyte solution containing carbon dioxide; cuprous oxide, copper, and at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium; A cathode electrode in which the ratio of the maximum peak intensity among the peak intensities in the XRD patterns of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, a hydroxide of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, and an oxide of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, to the peak intensity in the XRD pattern in X-ray diffraction measurement using CuKα radiation of cuprous oxide is 0.20 or less.

3. 3. The cathode electrode according to claim 1, wherein the at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium is in the form of a hydroxide or an oxide.

4. 4. The cathode electrode according to claim 1, wherein metallic copper and monovalent copper are present on the surface when a potential in the range of +0.2 V to −1.4 V relative to a reversible hydrogen electrode is applied in an electrolyte solution containing carbon dioxide.

5. 5. The cathode electrode according to claim 1, wherein the value of moles of copper / moles of cuprous oxide is in the range of 2.5 to 80.

6. 3. The cathode electrode according to claim 1, which has a porous structure.

7. A composite of a cathode electrode and a substrate, comprising: a conductive substrate; and the cathode electrode according to claim 1 formed on the conductive substrate.

8. 8. The composite of claim 7, wherein the conductive substrate is a copper substrate.

9. 9. The composite according to claim 8, wherein the copper base material is polycrystalline copper having a copper purity of 99.9 mol% or more, and the copper base material is a plate material having a processing-affected layer with an average thickness of 1.0 μm or less.

10. 10. The composite according to claim 7, wherein the cathode electrode is a codeposited layer.

11. providing a conductive substrate; a codeposited layer forming step of codepositing cuprous oxide and at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium on the conductive substrate to form a codeposited layer; A method for producing a composite of a cathode electrode and a substrate, comprising: The above-described manufacturing method, wherein the ratio of the maximum peak intensity among the peak intensities in the XRD patterns of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, the hydroxide of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, and the oxide of at least one other metal element selected from the group consisting of silver, gold, zinc, and cadmium, to the peak intensity in the XRD pattern in X-ray diffraction measurement using CuKα radiation of cuprous oxide, is 0.20 or less.

12. The method according to claim 11, further comprising an electrolytic polishing step of electrolytically polishing the conductive substrate, wherein the codeposited layer forming step is carried out after the electrolytic polishing step.

13. The production method according to claim 11 or 12, further comprising a partial reduction step of partially reducing the codeposited layer after the codeposited layer formation step.

14. An electrolysis device for electrically reducing carbon dioxide to olefinic hydrocarbons and / or alcohols, comprising the cathode electrode according to any one of claims 1 to 6.

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