EP4678790A1

EP4678790A1 - Electrode catalyst, electrochemical reaction device, and method for producing electrode catalyst

Info

Publication number: EP4678790A1
Application number: EP24767236.3A
Authority: EP
Inventors: Satoshi Kuramochi; Tasuku Komanoya; Daisuke Kurashina; Teruyuki Ushijima; Phuongtu Mai; Kenji Fujiki
Original assignee: Honda Motor Co Ltd; Mitsui Kinzoku Co Ltd
Current assignee: Honda Motor Co Ltd; Mitsui Kinzoku Co Ltd
Priority date: 2023-03-09
Filing date: 2024-03-08
Publication date: 2026-01-14
Also published as: CN120826498A; WO2024185870A1; JPWO2024185870A1

Abstract

An object of the present invention is to provide: an electrode catalyst capable of achieving a significantly higher generation efficiency of a C2 compound than that of a C1 compound; an electrochemical reaction apparatus including the electrode catalyst; and a method for producing the electrode catalyst, and the present invention provides: an electrode catalyst (10) for electrochemically reducing carbon dioxide and/or carbon monoxide, the electrode catalyst (10) including a gas diffusion layer (11), a catalyst (12) present on a surface of the gas diffusion layer (11), and a coating layer (13) covering at least part of a surface of the catalyst (12), wherein the catalyst (12) contains a Cu element, and wherein the coating layer (13) contains a fluororesin; an electrochemical reaction apparatus including the electrode catalyst; and a method for producing the electrode catalyst.

Description

Technical Field

The present invention relates to an electrode catalyst, an electrochemical reaction apparatus including the electrode catalyst, and a method for producing the electrode catalyst.

Background Art

In recent years, movement toward reduction of emission of greenhouse effect gases such as carbon dioxide and carbon monoxide has been increasing worldwide. As one solution for reduction of greenhouse effect gases, an electrochemical reduction reaction of carbon dioxide and/or carbon monoxide to convert them into a C2 compound (compound having 2 carbon atoms) has attracted attention, and there have been developed various electrode catalysts for promoting such an electrochemical reduction reaction of carbon dioxide and/or carbon monoxide (which may hereinafter be referred to as "the reduction reaction of carbon dioxide and/or carbon monoxide").
It is known that copper is suitable as a catalyst material contained in an electrode catalyst promoting the reduction reaction of carbon dioxide and/or carbon monoxide. For example, Patent Document 1 discloses an electrode catalyst in which copper-containing particles are supported on a substrate. Patent Document 2 discloses an electrode catalyst in which an alloy catalyst containing a Cu component and a Ni component is supported on a conductive substrate.

Citation List

Patent Documents

Patent Document 1: Japanese Patent Laid-Open No. 2015-147990
Patent Document 2: Japanese Patent Laid-Open No. 2020-89878

Summary of Invention

Technical Problem

Among products generated through the reduction reaction of carbon dioxide and/or carbon monoxide, a C2 compound such as ethylene (C₂H₄) is useful in terms of chemical industry and therefore is much in demand. However, the reduction reaction of carbon dioxide and/or carbon monoxide generates not only the C2 compound, but also hydrogen and a C1 compound as by-products. Therefore, a step of separating the C2 compound from products is required, which poses a problem in both energy consumption and cost.
Accordingly, an object of the present invention is to provide an electrode catalyst capable of achieving a significantly higher generation efficiency of a C2 compound than that of a C1 compound, an electrochemical reaction apparatus including the electrode catalyst, and a method for producing the electrode catalyst.

Solution To Problem

The present invention provides the following invention.

[1] An electrode catalyst for electrochemically reducing carbon dioxide and/or carbon monoxide,
- the electrode catalyst including: a gas diffusion layer; a catalyst present on a surface of the gas diffusion layer; and a coating layer covering at least part of a surface of the catalyst,
- wherein the catalyst contains a Cu element, and
- wherein the coating layer contains a fluororesin.
[2] The electrode catalyst according to [1], wherein the catalyst is in the form of a catalyst layer.
[3] The electrode catalyst according to [2], wherein a thickness of the catalyst layer falls within ±10% of an average thickness of the catalyst layer.
[4] The electrode catalyst according to [3], wherein the average thickness of the catalyst layer is 5 nm or more and 1000 nm or less.
[5] The electrode catalyst according to any one of [1] to [4], wherein a mass percentage of a fluorine element in the coating layer based on a mass of the electrode catalyst is 0.2% by mass or more and 15.0% by mass or less.
[6] The electrode catalyst according to any one of [1] to [5], wherein the catalyst is metal Cu.
[7] The electrode catalyst according to [1] to [6], wherein the fluororesin contains polytetrafluoroethylene.
[8] An electrochemical reaction apparatus for electrochemically reducing carbon dioxide and/or carbon monoxide,
- the apparatus including: a cathode; an anode; an anion exchange membrane provided between the cathode and the anode; a liquid flow path which is provided between the cathode and the anion exchange membrane and through which a cathode side electrolytic solution flows; and a liquid flow path which is provided between the anode and the anion exchange membrane and through which an anode side electrolytic solution flows,
- wherein the cathode includes the electrode catalyst according to any one of [1] to [7].
[9] A method for producing an electrode catalyst for electrochemically reducing carbon dioxide and/or carbon monoxide,
the method including the following steps of:
1. (a) forming a catalyst layer containing a Cu element, on a surface of a gas diffusion layer; and
2. (b) forming a coating layer containing a fluororesin, on at least part of a surface of the catalyst layer.
[10] The method according to [9], wherein, in step (b), the coating layer is formed by spray-coating at least part of the surface of the catalyst layer with a fluororesin-containing solution or dispersion liquid.

Advantageous Effect of Invention

According to the present invention, there are provided an electrode catalyst capable of achieving a significantly higher generation efficiency of a C2 compound than that of a C1 compound, an electrolysis reaction apparatus including the electrode catalyst, and a method for producing the electrode catalyst.

Brief Description of Drawings

[FIG. 1] FIG. 1 is a schematic cross-sectional view of an electrode catalyst according to one embodiment of the present invention.
[FIG. 2] FIG. 2 is a schematic cross-sectional view of an electrolytic cell of an electrochemical reaction apparatus according to one embodiment of the present invention.
[FIG. 3] FIG. 3 is a schematic diagram showing a substance flow in an electrolytic cell of an electrochemical reaction apparatus according to one embodiment of the present invention.

Detailed Description of Invention

<<Electrode catalyst>>

The electrode catalyst of the present invention is used for electrochemically reducing carbon dioxide and/or carbon monoxide. Specifically, the electrode catalyst of the present invention is used in a cathode in an electrochemical reaction apparatus for electrochemically reducing carbon dioxide and/or carbon monoxide. The electrode catalyst of the present invention is preferably used in a cathode in an electrochemical reaction apparatus for electrochemically reducing carbon dioxide.
When electrochemically reducing carbon dioxide and/or carbon monoxide, a raw material gas containing carbon dioxide and/or carbon monoxide is used. The raw material gas may contain either one of carbon dioxide or carbon monoxide, or may contain both carbon dioxide and carbon monoxide. The phrase "electrochemically reducing carbon dioxide and/or carbon monoxide" encompasses electrochemical reducing either one of carbon dioxide or carbon monoxide, and electrochemical reducing both carbon dioxide and carbon monoxide.
In the reduction reaction of carbon dioxide and/or carbon monoxide, a carbon compound is generated through the reduction of carbon dioxide and/or carbon monoxide, and hydrogen is generated through the reduction of water. The carbon compound generated is liquid or gaseous, and the hydrogen generated is gaseous.
Examples of the carbon compound include a C1 compound (compound having 1 carbon atom) and a C2 compound (compound having 2 carbon atoms).
Examples of the C2 compound generated through the reduction of carbon dioxide and/or carbon monoxide include acetic acid (CH₃COOH), acetic acid salts (for example, alkali metal acetates such as sodium acetate and potassium acetate), acetaldehyde (CH₃CHO), ethanol (C₂H₅OH), and ethylene (C₂H₄). Among these compounds, ethylene is preferred because it is useful in terms of chemical industry. In other words, the C2 compound generated through the reduction of carbon dioxide and/or carbon monoxide preferably includes ethylene. The C2 compound generated through the reduction of carbon dioxide and/or carbon monoxide can include, in addition to ethylene, one or more other compounds. In the C2 compound generated, ethylene is gaseous, and ethanol and acetic acid are each liquid. The type of an acetic acid salt generated depends on the type of an electrolytic solution used. For example, when the electrolytic solution contains sodium ions, sodium acetate is generated, and when the electrolytic solution contains potassium ions, potassium acetate is generated.
Examples of the C1 compound generated through the reduction of carbon dioxide include carbon monoxide (CO), formic acid (HCOOH), formic acid salts (for example, alkali metal formates such as sodium formate and potassium formate), formaldehyde (HCHO), methanol (CH₃OH), and methane (CH₄). The C1 compound generated through the reduction of carbon dioxide can include one or more compounds. In the C1 compound generated, for example, carbon monoxide and methane are each gaseous, and formic acid, methanol and formaldehyde are each liquid. The type of a formic acid salt generated depends on the type of an electrolytic solution used. For example, when the electrolytic solution contains sodium ions, sodium formate is generated, and when the electrolytic solution contains potassium ions, potassium formate is generated.
Examples of the C1 compound generated through the reduction of carbon monoxide include methanol (CH₃OH), formaldehyde (HCHO), and methane (CH₄). The C1 compound generated through the reduction of carbon monoxide can include one or more compounds. In the C1 compound generated, for example, methane is gaseous, and methanol and formaldehyde are each liquid.
The reaction in which carbon dioxide (CO₂) is electrochemically reduced to generate carbon monoxide (CO), the reaction in which carbon monoxide (CO) is electrochemically reduced to generate ethylene (C₂H₄), and the reaction in which water (H₂O) is electrochemically reduced to generate hydrogen (H₂) are as follows.

CO₂ + 2H⁺ + 2e^- -> CO + H₂O

2CO + 8H⁺ + 8e^- -> C₂H₄ + 2H₂O

2H₂O + 2e^- -> H₂ + 2OH^-
The reduction reaction of carbon dioxide and/or carbon monoxide can be performed according to known conditions except that the electrode catalyst of the present invention is used in a cathode.
Hereinafter, an embodiment of the electrode catalyst of the present invention will be described based on the drawing.
As shown in FIG. 1, an electrode catalyst 10 according to one embodiment of the present invention includes a gas diffusion layer 11, a catalyst 12, and a coating layer 13.
As shown in FIG. 1, the electrode catalyst 10 is, for example, in the form of an electrode catalyst layer.

The gas diffusion layer 11 can be configured in the same manner as a known gas diffusion layer. A commercially available gas diffusion layer may be used as the gas diffusion layer 11. Examples of a suitable commercially available product include a gas diffusion layer (GDL) manufactured by SGL Carbon SE (for example, Sigracet GDL 39 series).
Hereinafter, an example of an embodiment of the gas diffusion layer 11 will be described.
As shown in FIG. 1, the gas diffusion layer 11 includes a substrate 11a.
The substrate 11a is, for example, in the form of a sheet. When the substrate 11a is in the form of a sheet, the thickness of the substrate 11a is, for example, 10 µm or more and 1000 µm or less, preferably 100 µm or more and 500 µm or less, more preferably 150 µm or more and 350 µm or less. Both the minimum thickness and the maximum thickness of the substrate 11a preferably fall within the above range.
The substrate 11a has gas permeability. Thus, a raw material gas containing carbon dioxide and/or carbon monoxide can be efficiently supplied to the catalyst 12. In addition, a gaseous product generated through the reduction reaction of carbon dioxide and/or carbon monoxide, and hydrogen generated through the reduction of water can be efficiently recovered.
The substrate 11a is preferably a porous body from the viewpoint of effectively achieving gas permeability. Examples of the porous body include a non-woven fabric (including paper) and a woven fabric. The porous body has a most frequent pore size of, for example, 1 µm or more and 500 µm or less, preferably 10 µm or more and 300 µm or less, more preferably 20 µm or more and 250 µm or less, still more preferably 25 µm or more and 200 µm or less. The most frequent pore size can be determined by, for example, a mercury intrusion technique.
The substrate 11a has conductivity. Thus, the reduction reaction of carbon dioxide and/or carbon monoxide can be efficiently performed.
The substrate 11a preferably contains a conductive material from the viewpoint of effectively achieving conductivity. The substrate 11a may contain one conductive material, or may contain two or more conductive materials. Examples of the conductive material include a carbon material. Examples of the carbon material include carbon fiber, carbon black, graphite, black lead, activated carbon, carbon nanotube, carbon nanofiber, fullerene, and amorphous carbon. The carbon material is, for example, a material in which 50% by mass or more of the material is composed of carbon. The carbon fiber is a fiber in which most (for example, 90% by mass or more) of the fiber is composed of carbon.
As the substrate 11a, for example, a porous body having conductivity can be used. As the porous body having conductivity, for example, a porous body containing a carbon material can be used. As the porous body containing a carbon material, a porous body composed of a carbon fiber is preferably used. Examples of the porous body composed of a carbon fiber include a non-woven fabric (including paper) composed of a carbon fiber, and a woven fabric composed of a carbon fiber. The non-woven fabric (including paper) composed of a carbon fiber is also called carbon paper. The woven fabric composed of a carbon fiber is also called carbon cloth.
As the substrate 11a, a porous body such as a mesh material made of a metal or an alloy, a punching material made of a metal or an alloy, or a metal fiber sintered body may be used. Examples of the metal include titanium, nickel, and iron. Examples of the alloy include stainless steel (SUS).
As shown in FIG. 1, the gas diffusion layer 11 preferably has a permeating layer 11b formed on a surface of the substrate 11a. It is sufficient for the permeating layer 11b to be formed on at least part of the surface of the substrate 11a. The permeating layer 11b is excellent in permeability. The permeating layer 11b has pores whose average pore size is smaller than that of the substrate 11a, and is larger in surface area than the substrate 11a. Therefore, a greater amount of the catalyst 12 can be supported by the permeating layer 11b.
The "surface of the substrate 11a" encompasses an inner surface and an outer surface of the substrate 11a. The inner surface of the substrate 11a encompasses inner surfaces of pores present inside the substrate 11a (namely, not exposed to the outer surface of the substrate 11a). The outer surface of the substrate 11a encompasses inner surfaces of pores exposed to the outer surface of the substrate 11a.
The permeating layer 11b preferably has a portion formed on at least part of the outer surface of the substrate 11a. The permeating layer 11b may have, in addition to the portion formed on at least part of the outer surface of the substrate 11a, a portion formed on at least part of the inner surface of the substrate 11a.
When the substrate 11a is in the form of a sheet, the permeating layer 11b is preferably formed on one side of the substrate 11a, more preferably formed on at least part of the outer surface of one side of the substrate 11a. It is sufficient for the permeating layer 11b to be formed on at least part of one side of the substrate 11a.
The thickness of the portion of the permeating layer 11b, present on the outer surface of the substrate 11a, is, for example, 1 µm or more and 500 µm or less, preferably 20 µm or more and 300 µm or less, more preferably 50 µm or more and 200 µm or less, still more preferably 70 µm or more and 150 µm or less. Both the minimum thickness and the maximum thickness of the portion of the permeating layer 11b, present on the outer surface of the substrate 11a, preferably fall within the above range.
The permeating layer 11b has gas permeability. Thus, a raw material gas containing carbon dioxide and/or carbon monoxide can be efficiently supplied to the catalyst 12. In addition, a gaseous product generated through the reduction reaction of carbon dioxide and/or carbon monoxide, and hydrogen generated through the reduction of water can be efficiently recovered.
The permeating layer 11b is preferably porous, and is preferably a micro-porous layer (MPL) from the viewpoint of effectively achieving gas permeability. The micro-porous layer has a most frequent pore size of, for example, 5 nm or more and 500 nm or less, preferably 10 nm or more and 300 nm or less, more preferably 15 nm or more and 100 nm or less, still more preferably 15 nm or more and 70 nm or less. The most frequent pore size can be determined by a mercury intrusion technique. The micro-porous layer is usually smaller in average pore size than the substrate 11a and higher in density than the substrate 11a.
The permeating layer 11b is formed, for example, to enhance the water-repellent properties of the gas diffusion layer 11. By enhancing the water-repellent properties of the gas diffusion layer 11, it is possible to suppress the hydrogen generation reaction through the reduction of water and promote the preferential generation of the carbon compound through the reduction of carbon dioxide and/or carbon monoxide.
When the permeating layer 11b is formed to enhance the water-repellent properties of the gas diffusion layer 11, the permeating layer 11b preferably contains a fluororesin. The permeating layer 11b may contain one fluororesin, or may contain two or more fluororesins. The molecular weight of such a fluororesin can be appropriately adjusted to achieve desired water-repellent properties.
Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer. Among them, polytetrafluoroethylene is preferred.
The mass percentage of the fluorine element in the permeating layer 11b based on the mass of the electrode catalyst 10 is preferably 5% by mass or more and 30% by mass or less, more preferably 8% by mass or more and 25% by mass or less, still more preferably 10% by mass or more and 20% by mass or less from the viewpoint of effectively achieving enhanced water-repellent properties.
The mass of the fluorine element in the permeating layer 11b can be quantitatively determined by, for example, an alkali fusion-ion selective electrode method. The alkali fusion-ion selective electrode method can be performed according to conditions described in Examples.
The permeating layer 11b has conductivity. Thus, the reduction reaction of carbon dioxide and/or carbon monoxide can be efficiently performed.
The permeating layer 11b preferably contains a conductive material from the viewpoint of effectively achieving conductivity. The permeating layer 11b may contain one conductive material, or may contain two or more conductive materials. Examples of the conductive material include a carbon material. The description with respect to the carbon material is the same as described above.
The mass percentage of the carbon material in the permeating layer 11b based on the mass of the permeating layer 11b is preferably 70% by mass or more and 95% by mass or less, more preferably 75% by mass or more and 92% by mass or less, still more preferably 80% by mass or more and 90% by mass or less from the viewpoint of effectively achieving conductivity.
The mass of the carbon material in the permeating layer 11b can be quantitatively determined by, for example, a combustion method.
The permeating layer 11b can be formed by coating a surface of the substrate 11a (one side of the substrate 11a when the substrate 11a is in the form of a sheet) with a fluororesin-containing solution or dispersion liquid (preferably emulsion). In this case, a known coating method can be used. Examples of the coating method include a bar coating method, a blade coating method, a screen printing method, a spray-coating method, a curtain coating method, and a roll coating method. The fluororesin-containing solution or dispersion liquid may be applied in a stacked manner so as to form a plurality of layers, thereby adjusting the amount of a fluororesin applied. The fluororesin-containing solution or dispersion liquid can be prepared with a fluororesin, a solvent or a dispersion medium (for example, water), a surfactant (for example, non-ionic surfactant), and the like, by an ordinary method. The types of the fluororesin, the solvent, the dispersion medium, the surfactant, and the like can be appropriately selected. The concentration of the fluororesin is, for example, 1% by mass or more and 70% by mass or less, preferably 3% by mass or more and 60% by mass or less based on the mass of the fluororesin-containing solution or dispersion liquid. A commercially available fluororesin-containing solution or dispersion liquid may be used. A layer formed by coating using the fluororesin-containing solution or dispersion liquid may be, if necessary, dried. The drying temperature is, for example, 10°C or more and 120°C or less, preferably 20°C or more and 100°C or less, and the drying time is, for example, 0.5 hours or more and 48 hours or less, preferably 1 hour or more and 24 hours or less.

Hereinafter, an example of an embodiment of the catalyst 12 will be described.
As shown in FIG. 1, the catalyst 12 is present on a surface of the gas diffusion layer 11. In other words, the catalyst 12 is supported on the surface of the gas diffusion layer 11.
The "surface of the gas diffusion layer 11" means the surface of the permeating layer 11b when the permeating layer 11b is formed on the surface of the substrate 11a, or means the surface of the substrate 11a when the permeating layer 11b is not formed on the surface of the substrate 11a.
The "surface of the permeating layer 11b" encompasses an inner surface and an outer surface of the permeating layer 11b. The inner surface of the permeating layer 11b encompasses inner surfaces of pores present inside the permeating layer 11b (namely, not exposed to the outer surface of the permeating layer 11b). The outer surface of the permeating layer 11b encompasses inner surfaces of pores exposed to the outer surface of the permeating layer 11b.
The "surface of the substrate 11a" has the same meaning as described above.
When the permeating layer 11b is formed on the surface of the substrate 11a, the catalyst 12 preferably has a portion supported on at least part of the outer surface of the permeating layer 11b. The catalyst 12 may have, in addition to the portion supported on at least part of the outer surface of the permeating layer 11b, a portion supported on at least part of the inner surface of the permeating layer 11b.
When the permeating layer 11b is not formed on the surface of the substrate 11a, the catalyst 12 preferably has a portion supported on at least part of the outer surface of the substrate 11a. The catalyst 12 may have, in addition to the portion supported on at least part of the outer surface of the substrate 11a, a portion supported on at least part of the inner surface of the substrate 11a.
The catalyst 12 promotes the electrochemical reduction reaction of carbon dioxide and/or carbon monoxide.
The catalyst 12 contains a Cu element. The Cu element contained in the catalyst 12 is in the form of being capable of functioning as a catalytically active component, for example, in the form of metal Cu, an alloy containing the Cu element, a complex containing the Cu element, a compound containing the Cu element (for example, Cu(OH)₂, Cu₂O, or CuO), or the like.
At least part of the Cu element contained in the catalyst 12 is preferably present as metal Cu, and 50% by mass or more, in particular, all of the Cu element contained in the catalyst 12 is more preferably present as metal Cu (namely, the catalyst 12 is composed of metal Cu) from the viewpoint of efficiently performing the reduction reaction of carbon dioxide and/or carbon monoxide.
The catalyst 12 may contain one or more metal elements other than the Cu element. Examples of such a metal element other than the Cu element include Au, Pt, Pd, Ag, Zn, Ni, Co, Fe, Al, Sn, Mn, Cr, Ti, Cd, In, Ga, Pb, Ru, and Re. Such a metal element other than the Cu element contained in the catalyst 12 is in the form of being capable of functioning as a catalytically active component, for example, in the form of metal, an alloy, a complex, a compound (for example, hydroxide or oxide), or the like.
The catalyst 12 is in the form of, for example, catalyst particles or a catalyst layer.
The average particle size of catalyst particles is, for example, 1 nm or more and 100 nm or less, preferably 3 nm or more and 50 nm or less, more preferably 5 nm or more and 30 nm or less. The average particle size of catalyst particles can be determined as the average value of the Feret diameters of 100 arbitrarily selected catalyst particles in an image observed with a scanning electron microscope (SEM).
The catalyst 12 is preferably in the form of a catalyst layer from the viewpoint of efficiently performing the reduction reaction of carbon dioxide and/or carbon monoxide. The catalyst layer may be a catalyst layer formed by binding catalyst particles. The catalyst layer may be formed from one continuous layer, or may be formed from a plurality of discontinuous portions (for example, a plurality of island-like portions), and is preferably formed from one continuous layer from the viewpoint of suppressing the hydrogen generation reaction through the reduction of water.
The thickness of the catalyst layer is preferably uniform. The "thickness of the catalyst layer being uniform" means that the thickness of any portion of the catalyst layer falls within ±10% of the average thickness of the catalyst layer. The average thickness of the catalyst layer can be determined as the average value of the thicknesses of 50 arbitrarily selected portions of the catalyst layer. The average thickness of the catalyst layer is, for example, 5 nm or more and 1000 nm or less, preferably 10 nm or more and 500 nm or less, more preferably 20 nm or more and 200 nm or less.
The thickness of the portion of the catalyst layer, present on the outer surface of the permeating layer 11b or on the outer surface of the substrate 11a, is, for example, 5 nm or more and 1000 nm or less, preferably 10 nm or more and 500 nm or less, more preferably 20 nm or more and 200 nm or less. Both the minimum thickness and the maximum thickness of the portion of the catalyst layer, present on the outer surface of the permeating layer 11b or on the outer surface of the substrate 11a, preferably fall within the above range.
The catalyst 12 is preferably formed by a sputtering method from the viewpoint of forming a catalyst layer having a uniform thickness. The catalyst 12 may be formed by any other method, for example, a vapor deposition method such as an arc plasma vapor deposition method, an electron beam vapor deposition method, a heating vapor deposition method, or a pulse laser vapor deposition method, or a plating method such as an electrolytic plating method, a non-electrolytic plating method, or a displacement plating method.
The mass percentage of the catalyst 12 based on the mass of the electrode catalyst 10 is, for example, 0.10% by mass or more and 2.0% by mass or less, preferably 0.15% by mass or more and 1.5% by mass or less, more preferably 0.20% by mass or more and 1.0% by mass or less.

Hereinafter, an example of an embodiment of the coating layer 13 will be described.
The coating layer 13 covers at least part of a surface of the catalyst 12. The coating layer 13 may cover part of the surface of the catalyst 12, or may cover the whole of the surface of the catalyst 12, and preferably covers the whole of the surface of the catalyst 12 from the viewpoint of more effectively exerting the effects of the coating layer 13 described later. The coating layer 13 may have, in addition to a portion formed on at least part of the surface of the catalyst 12, a portion formed on the surface of the gas diffusion layer 11.
The thickness of the portion of the coating layer 13, present on the surface of the catalyst 12, is, for example, 0.10 µm or more and 100 µm or less, preferably 0.15 µm or more and 50 µm or less, more preferably 0.25 µm or more and 10 µm or less. Both the minimum thickness and the maximum thickness of the portion of the coating layer 13, present on the surface of the catalyst 12, preferably fall within the above range.
The coating layer 13 contains a fluororesin. The coating layer 13 may contain one fluororesin, or may contain two or more fluororesins. The molecular weight of such a fluororesin can be appropriately adjusted to achieve desired water-repellent properties. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer. Among them, polytetrafluoroethylene is preferred.
The coating layer 13 exerts the following effects.
The fluororesin is a water-repelling resin. Accordingly, the coating layer 13 has water-repellent properties. By having at least part of the surface of the catalyst 12 covered with the coating layer 13 having water-repellent properties, the penetration of an electrolyte used in the reduction reaction of carbon dioxide and/or carbon monoxide into the interface between the catalyst 12 and the coating layer 13 is suppressed. Thus, a raw material gas containing carbon dioxide and/or carbon monoxide can be present, for example, as a bubble form, between the catalyst 12 and the coating layer 13. In other words, the raw material gas can be present near the catalyst 12, and the concentration of the raw material gas near the catalyst 12 is increased. The increase in the concentration of the raw material gas near the catalyst 12 enhances the reduction reaction rate of carbon dioxide and/or carbon monoxide, raises the concentration of carbon monoxide adsorbed on the catalyst 12, promotes COCO coupling, and improves the generation efficiency of the C2 compound such as ethylene. As a result of the above effects of the coating layer 13, the generation efficiency of the C1 compound is decreased, thereby making it possible to achieve a significantly higher generation efficiency of the C2 compound compared to that of a C1 compound.
The coating layer 13 can also exert the following effects.
By having at least part of the surface of the catalyst 12 covered with the coating layer 13 having water-repellent properties, the penetration of an electrolyte used in the reduction reaction of carbon dioxide and/or carbon monoxide into the interface between the catalyst 12 and the coating layer 13 is suppressed. Thus, the generation of hydrogen through the reduction of water is suppressed, and the generation efficiency of hydrogen is decreased. As a result of the above effects of the coating layer 13, it is possible to achieve a decrease in generation efficiency of hydrogen.
It is preferable that the generation efficiency of the C1 compound is decreased, thereby achieving a significantly higher generation efficiency of the C2 compound compared to that of a C1 compound, and a decrease in generation efficiency of hydrogen is also achieved.
The fact that the generation efficiency of the C2 compound is significantly higher than that of the C1 compound can be evaluated based on the ratio of the Faraday efficiency of the C2 compound to that of the C1 compound (Faraday efficiency of the C2 compound/Faraday efficiency of the C1 compound). Specifically, when the ratio is 3.0 or more, the generation efficiency of the C2 compound can be evaluated to be significantly higher than that of the C1 compound. The ratio is preferably 3.3 or more, more preferably 5.0 or more, still more preferably 8.0 or more. The upper limit is, for example, 8.5, 9.0 or 10.0. Such upper limits may be each combined with any of the above lower limits. The Faraday efficiency of the C1 compound and the Faraday efficiency of the C2 compound can be each determined by a method described in Examples. The Faraday efficiency of the C1 compound generated through the reduction of carbon dioxide can be determined as the total Faraday efficiency of CO, CH₄, CH₃OH and HCOOH. The Faraday efficiency of the C1 compound generated through the reduction of carbon monoxide can be determined as the total Faraday efficiency of CH₄, CH₃OH and HCOOH. The Faraday efficiency of the C2 compound generated through the reduction of carbon dioxide and/or carbon monoxide can be determined as the total Faraday efficiency of C₂H₄, C₂H₅OH and CH₃COOH.
A decrease in generation efficiency of hydrogen can be evaluated based on the Faraday efficiency of hydrogen. Specifically, when the Faraday efficiency of hydrogen is 27.0% or less, the generation efficiency of hydrogen can be evaluated to be decreased. The Faraday efficiency of hydrogen is preferably 20.0% or less, more preferably 15.0% or less, still more preferably 13.0% or less, still more preferably 12.0% or less. The lower limit is not particularly limited, and is, for example, 1.0%, 5.0% or 10.0%. Such lower limits can be each combined with any of the above upper limits. The Faraday efficiency of hydrogen can be determined by a method described in Examples.
A decrease in generation efficiency of the C1 compound can be evaluated based on the Faraday efficiency of the C1 compound. Specifically, when the Faraday efficiency of the C1 compound is 30.0% or less, the generation efficiency of a C1 compound can be evaluated to be decreased. The Faraday efficiency of the C1 compound is preferably 25.0% or less, more preferably 20.0% or less, still more preferably 15.0% or less, still more preferably 10.0% or less. The lower limit is not particularly limited, and is, for example, 1.0%, 5.0% or 8.0%. Such lower limits can be each combined with any of the above upper limits. The Faraday efficiency of the C1 compound can be determined by a method described in Examples. The Faraday efficiency of the C1 compound generated through the reduction of carbon dioxide can be determined as the total Faraday efficiency of CO, CH₄, CH₃OH and HCOOH. The Faraday efficiency of the C1 compound generated through the reduction of carbon monoxide can be determined as the total Faraday efficiency of CH₄, CH₃OH and HCOOH.
The mass percentage of the fluorine element in the coating layer 13 based on the mass of the electrode catalyst 10 is preferably 0.2% by mass or more and 15.0% by mass or less, more preferably 0.5% by mass or more and 11.0% by mass or less, still more preferably 0.5% by mass or more and 10.0% by mass or less, still more preferably 1.0% by mass or more and 7.0% by mass or less, still more preferably 1.5% by mass or more and 7.0% by mass or less from the viewpoint of more effectively achieving the above effects of the coating layer 13.
The proportion of a fluororesin-derived fluorine element in the fluorine element in the coating layer 13 is preferably 20.0% by mass or more, more preferably 30.0% by mass or more, still more preferably 40.0% by mass or more from the viewpoint of more effectively achieving the above effects of the coating layer 13. The upper limit of the proportion is 100% by mass (namely, the fluorine element contained in the coating layer 13 is fully a fluororesin-derived fluorine element).
The mass percentage of the fluorine element in the coating layer 13 based on the mass of the electrode catalyst 10 can be determined from the following expression.
Mass percentage of fluorine element in coating layer 13 based on mass of electrode catalyst 10 =((Mass of fluorine element in electrode catalyst 10) - (Mass of fluorine element in gas diffusion layer 11)) / (Mass of electrode catalyst 10) × 100
The mass of the fluorine element in the electrode catalyst 10 and the mass of the fluorine element in the gas diffusion layer 11 can be each quantitatively determined by, for example, an alkali fusion-ion selective electrode method. The alkali fusion-ion selective electrode method can be performed according to conditions described in Examples.
The mass of the coating layer 13 per unit geometric area of the surface on which the coating layer 13 is formed is preferably 0.02 mg/cm² or more and 2.0 mg/cm² or less, more preferably 0.05 mg/cm² or more and 1.6 mg/cm² or less, still more preferably 0.05 mg/cm² or more and 1.0 mg/cm² or less, still more preferably 0.1 mg/cm² or more and 0.8 mg/cm² or less, still more preferably 0.2 mg/cm² or more and 0.8 mg/cm² or less from the viewpoint of more efficiently achieving the above effects of the coating layer 13. The mass of the coating layer 13 per unit geometric area of the surface on which the coating layer 13 is formed is determined from the expression: (Mass of coating layer 13) / (Geometric area of surface on which coating layer 13 is formed). When the coating layer 13 is flat, the geometric area of the surface on which the coating layer 13 is formed is equal to the area of the main surface of the coating layer 13 as viewed in plan from the normal direction.
The coating layer 13 can be formed by coating at least part of a surface of the catalyst 12 with a fluororesin-containing solution or dispersion liquid (preferably emulsion). In this case, a known coating method can be used. Examples of the coating method include a bar coating method, a blade coating method, a screen printing method, a spray-coating method, a curtain coating method, and a roll coating method. Among them, a spray-coating method is preferred from the viewpoint of uniformly covering the surface of the catalyst 12 with the coating layer 13. The fluororesin-containing solution or dispersion liquid may be applied so as to form a plurality of layers, thereby adjusting the amount of fluororesin application. The fluororesin-containing solution or dispersion liquid can be prepared by an ordinary method with a fluororesin, a solvent or a dispersion medium (for example, water), a surfactant (for example, non-ionic surfactant), and the like. The types of the fluororesin, the solvent, the dispersion medium, the surfactant, and the like can be appropriately selected. The concentration of the fluororesin in the fluororesin-containing solution or dispersion liquid is, for example, 1% by mass or more and 70% by mass or less, preferably 3% by mass or more and 60% by mass or less. A commercially available fluororesin-containing solution or dispersion liquid may be used. The layer formed by coating using the fluororesin-containing solution or dispersion liquid may be, if necessary, dried. The drying temperature is, for example, 20°C or more and 120°C or less, preferably 50°C or more and 100°C or less, and the drying time is, for example, 0.5 hours or more and 24 hours or less, preferably 1 hour or more and 12 hours or less.

<<Method for producing electrode catalyst>>

Hereinafter, an embodiment of the method for producing the electrode catalyst 10 will be described.
A method for producing the electrode catalyst 10 according to one embodiment of the present invention includes the following steps of:

(a) forming a catalyst layer containing a Cu element, on a surface of the gas diffusion layer 11; and
(b) forming the coating layer 13 containing a fluororesin, on at least part of a surface of the catalyst layer formed.

Step (a) is a step of forming a catalyst layer containing a Cu element, on a surface of the gas diffusion layer 11.
In step (a), the catalyst layer containing a Cu element is preferably formed by a sputtering method. Thus, a sputtering film of metal, containing a Cu element, is formed on the surface of the gas diffusion layer 11. The sputtering film is one example of the catalyst 12 which is in the form of a catalyst layer. Such sputtering can be performed according to an ordinary method.
When the permeating layer 11b is formed on a surface of the substrate 11a, the catalyst layer preferably has a portion supported on at least part of an outer surface of the permeating layer 11b. The catalyst layer may have, in addition to the portion supported on at least part of the outer surface of the permeating layer 11b, a portion supported on at least part of an inner surface of the permeating layer 11b.
When the permeating layer 11b is not formed on a surface of the substrate 11a, the catalyst layer preferably has a portion supported on at least part of an outer surface of the substrate 11a. The catalyst layer may have, in addition to the portion supported on at least part of the outer surface of the substrate 11a, a portion supported on at least part of an inner surface of the substrate 11a.
The "surface of the gas diffusion layer 11", the "surface of the permeating layer 11b" and the "surface of the substrate 11a" have the same meanings respectively as described above.
The catalyst layer contains a Cu element. The Cu element contained in the catalyst layer is in the form of, for example, metal Cu, an alloy containing the Cu element, a complex containing the Cu element, a compound containing the Cu element (for example, Cu(OH)₂, Cu₂O, or CuO),or the like.
The catalyst layer may contain one or more metal elements other than the Cu element. Examples of such a metal element other than the Cu element include Au, Pt, Pd, Ag, Zn, Ni, Co, Fe, Al, Sn, Mn, Cr, Ti, Cd, In, Ga, Pb, Ru, and Re. Such a metal element other than the Cu element contained in in the catalyst layer is in the form of, for example, metal, an alloy, a complex, a compound (for example, hydroxide or oxide), or the like.
The catalyst layer is preferably composed of a Cu element (namely, composed of metal Cu) from the viewpoint of efficiently performing the reduction reaction of carbon dioxide and/or carbon monoxide.

Step (b) is a step of forming the coating layer 13 containing a fluororesin, on at least part of a surface of the catalyst layer.
The coating layer 13 formed in step (b) covers at least part of the surface of the catalyst layer. The coating layer 13 may cover part of the surface of the catalyst layer or may cover the whole of the surface of the catalyst layer, and preferably covers the whole of the surface of the catalyst layer from the viewpoint of more effectively achieving the above effects of the coating layer 13.
The coating layer 13 contains a fluororesin. The description with respect to the fluororesin is the same as described above.
The coating layer 13 can be formed by coating at least part of the surface of the catalyst layer with a fluororesin-containing solution or dispersion liquid (preferably emulsion). In this case, a known coating method can be used. The description with respect to the coating method is the same as described above.
In a preferred embodiment of step (b), the coating layer 13 is formed by spray-coating at least part of the surface of the catalyst layer with a fluororesin-containing solution or dispersion liquid.
In step (b), a structure is obtained which includes the gas diffusion layer 11, the catalyst layer formed on the surface of the gas diffusion layer 11, and the coating layer 13 containing a fluororesin and covering of at least part of the surface of the catalyst layer. The structure obtained may be, if necessary, calcined. Such calcination can allow for an enhancement in water-repellent properties of the coating layer 13.
The calcination temperature is, for example, 150°C or more and 450°C or less, preferably 170°C or more and 350°C or less, more preferably 200°C or more and 300°C or less, and the calcination time is, for example, 10 minutes or more and 240 minutes or less, preferably 20 minutes or more and 180 minutes or less, more preferably 30 minutes or more and 150 minutes or less. The rate of temperature rise during calcination is, for example, 1°C/min or more and 30°C/min or less, preferably 3°C/min or more and 20°C/min or less, more preferably 5°C/min or more and 15°C/min or less. The calcination can be performed under an atmosphere of an inert gas such as a nitrogen gas or an argon gas.

<<Electrochemical reaction apparatus>>

The electrochemical reaction apparatus of the present invention is used for electrochemically reducing carbon dioxide and/or carbon monoxide. The electrochemical reaction apparatus of the present invention is preferably used for electrochemically reducing carbon dioxide.
When electrochemically reducing carbon dioxide and/or carbon monoxide, a raw material gas containing carbon dioxide and/or carbon monoxide is used. The raw material gas may contain either one of carbon dioxide or carbon monoxide, or may contain both carbon dioxide and carbon monoxide. The phrase "electrochemically reducing carbon dioxide and/or carbon monoxide" encompasses electrochemical reducing either one of carbon dioxide or carbon monoxide, and electrochemical reducing both carbon dioxide and carbon monoxide.
In the reduction reaction of carbon dioxide and/or carbon monoxide, not only a carbon compound is generated through the reduction of carbon dioxide and/or carbon monoxide, but also hydrogen is generated through the reduction of water. The carbon compound generated is liquid or gaseous, and the hydrogen generated is gaseous.
The description with respect to the carbon compound is the same as described above.
The electrochemical reaction apparatus of the present invention can be configured in the same manner as a known electrochemical reaction apparatus (for example, electrochemical reaction apparatus 2 described in Japanese Patent No. 7145264 ), except that the electrode catalyst of the present invention is used in a cathode.
Hereinafter, an embodiment of the electrochemical reaction apparatus of the present invention will be described based on the drawing.
As shown in FIG. 2, an electrochemical reaction apparatus 2 according to one embodiment of the present invention includes a cathode 21, an anode 22, an anion exchange membrane 23 provided between the cathode 21 and the anode 22, a liquid flow path 28a which is provided between the cathode 21 and the anion exchange membrane 23 and through which a cathode side electrolytic solution flows, and a liquid flow path 29a which is provided between the anode 22 and the anion exchange membrane 23 and through which an anode side electrolytic solution flows.
As shown in FIG. 2, the electrochemical reaction apparatus 2 may include a liquid flow path structure 28 for forming the liquid flow path 28a and a liquid flow path structure 29 for forming the liquid flow path 29a.
As shown in FIG. 2, the electrochemical reaction apparatus 2 may include a gas flow path structure 24 in which a gas flow path 24a is formed and a gas flow path structure 25 in which a gas flow path 25a is formed.
As shown in FIG. 2, the electrochemical reaction apparatus 2 may include a power supply body 26 and a power supply body 27.
When the electrochemical reaction apparatus 2 includes the cathode 21, the anode 22, the anion exchange membrane 23, the liquid flow path structure 28, the liquid flow path structure 29, the gas flow path structure 24, the gas flow path structure 25, the power supply body 26 and the power supply body 27, the power supply body 26, the gas flow path structure 24, the cathode 21, the liquid flow path structure 28, the anion exchange membrane 23, the liquid flow path structure 29, the anode 22, the gas flow path structure 25 and the power supply body 27 are laminated in this order.
A slit is formed in the liquid flow path structure 28, and a region surrounded by the cathode 21, the anion exchange membrane 23 and the liquid flow path structure 28 in the slit corresponds to the liquid flow path 28a.
A slit is formed in the liquid flow path structure 29, and a region surrounded by the anode 22, the anion exchange membrane 23 and the liquid flow path structure 29 in the slit corresponds to the liquid flow path 29a.
A groove is formed on the cathode 21 side of the gas flow path structure 24, and a region surrounded by the gas flow path structure 24 and the cathode 21 in the groove corresponds to the gas flow path 24a.
A groove is formed on the anode 22 side of the gas flow path structure 25, and a region surrounded by the gas flow path structure 25 and the anode 22 in the groove corresponds to the gas flow path 25a.
In the electrochemical reaction apparatus 2, the liquid flow path 28a is formed between the cathode 21 and the anion exchange membrane 23, the liquid flow path 29a is formed between the anode 22 and the anion exchange membrane 23, the gas flow path 24a is formed between the cathode 21 and the power supply body 26, and the gas flow path 25a is formed between the anode 22 and the power supply body 27.
The power supply body 26 and the power supply body 27 are each electrically connected to a power source (not shown) configured to supply electric power to the electrochemical reaction apparatus 2.
The gas flow path structure 24 and the gas flow path structure 25 are each a conductor, and a voltage can be applied between the cathode 21 and the anode 22 by electric power supplied from the power source.
The cathode 21 is an electrode that not only reduces carbon dioxide and/or carbon monoxide to generate a carbon compound, but also reduces water to generate hydrogen. The carbon compound generated is liquid or gaseous, and the hydrogen generated is gaseous.
The cathode 21 not only electrochemically reduces carbon dioxide and/or carbon monoxide, but also allows the generated gaseous carbon compound and hydrogen to permeate to the gas flow path 24a. The liquid carbon compound generated flows in the liquid flow path 28a, together with a cathode side electrolytic solution A, and flows out of an outlet of the liquid flow path 28a through a liquid flow path 63.
The cathode 21 includes the electrode catalyst 10. The cathode 21 may be composed of the electrode catalyst 10. The description with respect to the electrode catalyst 10 is the same as described above. The electrode catalyst 10 is disposed so that the substrate 11a is located on the gas flow path 24a side and the catalyst 12 is located on the liquid flow path 28a side.
The anode 22 is an electrode that oxidizes hydroxide ions to generate oxygen. The oxygen generated is gaseous. The anode 22 not only electrochemically oxidizes hydroxide ions, but also allows the generated oxygen to permeate to the gas flow path 25a.
As the anode 22, for example, an electrode including a gas diffusion layer, and a catalyst formed on the liquid flow path 29a side of the gas diffusion layer (which may hereinafter be referred to as "the anode catalyst") can be used. The anode catalyst is in the form of, for example, catalyst particles or a catalyst layer. The anode catalyst is preferably in the form of a catalyst layer. As the gas diffusion layer, the gas diffusion layer 11 may be used, or a known gas diffusion layer other than the gas diffusion layer 11 may be used.
As the anode catalyst, a known anode catalyst can be used. Examples of the anode catalyst include metals such as platinum, palladium, and nickel, alloys or intermetallic compounds thereof, metal oxides such as manganese oxide, iridium oxide, nickel oxide, cobalt oxide, iron oxide, tin oxide, indium oxide, ruthenium oxide, lithium oxide, and lanthanum oxide, and metal complexes such as a ruthenium complex and a rhenium complex. One of such anode catalysts may be used singly, or two or more of such anode catalysts may be used in combination.
As the gas diffusion layer of the anode 22, for example, carbon paper, carbon cloth, or the like can be used. As the gas diffusion layer of the anode 22, a porous body such as a mesh material, a punching material, or a metal fiber sintered body may be used. Examples of a material of the porous body include metals such as titanium, nickel, and iron, and alloys such as stainless steel (SUS).
Examples of materials of the liquid flow path structures 28 and 29 include a fluororesin such as polytetrafluoroethylene.
Examples of materials of the gas flow path structures 24 and 25 include metals such as titanium and SUS, and carbon.
Examples of materials of the power supply bodies 26 and 27 include metals such as copper, gold, titanium, and SUS, and carbon. As the power supply bodies 26 and 27, those having a surface of a copper substrate plated with gold or the like may be used.
As the anion exchange membrane 23, a known anion exchange membrane can be used.
The electrochemical reaction apparatus 2 includes a flow cell in which a cathode side electrolytic solution A supplied through a liquid flow path 64 flows through the liquid flow path 28a, an anode side electrolytic solution B supplied through a liquid flow path 65 flows through the liquid flow path 29a, and a raw material gas G supplied through a gas flow path 76 flows through the gas flow path 24a.
One end of the liquid flow path 64 is connected to the electrochemical reaction apparatus 2, and the other end of the liquid flow path 64 is connected to a supplying apparatus (not shown) for supplying the cathode side electrolytic solution A to the electrochemical reaction apparatus 2.
One end of the liquid flow path 65 is connected to the electrochemical reaction apparatus 2, and the other end of the liquid flow path 65 is connected to a supplying apparatus (not shown) for supplying the anode side electrolytic solution B to the electrochemical reaction apparatus 2.
One end of the gas flow path 76 is connected to the electrochemical reaction apparatus 2, and the other end of the gas flow path 76 is connected to a supplying apparatus (not shown) for supplying the raw material gas G to the electrochemical reaction apparatus 2.
As each of the cathode side electrolytic solution A and the anode side electrolytic solution B, an alkaline aqueous solution can be used. Examples of the alkaline aqueous solution include a potassium hydroxide aqueous solution, a sodium hydroxide aqueous solution, a potassium carbonate aqueous solution, and a sodium carbonate aqueous solution. The potassium hydroxide aqueous solution is preferred because the solution has excellent solubility of carbon dioxide and suppress the generation of hydrogen through the reduction reaction of water.
The pH of each of the cathode side electrolytic solution A and the anode side electrolytic solution B can be appropriately adjusted, and the pH of the anode side electrolytic solution B is preferably lower than the pH of the cathode side electrolytic solution A. The pH of the cathode side electrolytic solution A is, for example, more than 14, and the pH of the anode side electrolytic solution B is, for example, 14 or less (specifically 8 or more and 14 or less). Examples of the method for adjusting the pH of such an electrolytic solution include a method including adding an alkali or alkali aqueous solution to such an electrolytic solution to increase the pH, and a method including dissolving carbon dioxide in such an electrolytic solution to decrease the pH.
As the cathode side electrolytic solution A supplied through the liquid flow path 64, an alkaline aqueous solution (for example, potassium hydroxide aqueous solution) having an alkali concentration of, for example, 4 mol/L or more and 12 mol/L or less, preferably 5 mol/L or more and 11 mol/L or less, more preferably 6 mol/L or more and 10 mol/L or less can be used. The temperature of the cathode side electrolytic solution A supplied through the liquid flow path 64 can be appropriately adjusted, and is, for example, 10°C or more and 60°C or less. The flow rate of the cathode side electrolytic solution A supplied through the liquid flow path 64 can be appropriately adjusted, and is, for example, 0.5 mL/min or more and 5 mL/min or less.
As the anode side electrolytic solution B supplied through the liquid flow path 65, an alkaline aqueous solution (for example, potassium hydroxide aqueous solution) having an alkali concentration of, for example, 0.1 mol/L or more and 3 mol/L or less, preferably 0.2 mol/L or more and 2.5 mol/L or less, more preferably 0.5 mol/L or more and 2 mol/L or less can be used. The temperature of the anode side electrolytic solution B supplied through the liquid flow path 65 can be appropriately adjusted, and is, for example, 10°C or more and 60°C or less. The flow rate of the anode side electrolytic solution B supplied through the liquid flow path 65 can be appropriately adjusted, and is, for example, 0.5 mL/min or more and 5 mL/min or less.
The raw material gas G supplied through the gas flow path 76 contains carbon dioxide and/or carbon monoxide. The raw material gas G supplied through the gas flow path 76 may contain either one of carbon dioxide or carbon monoxide, or may contain both carbon dioxide and carbon monoxide. When the raw material gas G supplied through the gas flow path 76 contains carbon dioxide, the concentration of the carbon dioxide in the raw material gas G can be appropriately adjusted, and is, for example, 1% by volume or more and 100% by volume or less. When the raw material gas G supplied through the gas flow path 76 contains carbon monoxide, the concentration of the carbon monoxide in the raw material gas G can be appropriately adjusted, and is, for example, 1% by volume or more and 100% by volume or less. The temperature of the raw material gas G supplied through the gas flow path 76 can be appropriately adjusted, and is, for example, 10°C or more and 60°C or less. The flow rate of the raw material gas G supplied through the gas flow path 76 can be appropriately adjusted, and is, for example, 5 mL/min or more and 50 mL/min or less.
The cathode side electrolytic solution A supplied through the liquid flow path 64 flows in the liquid flow path 28a, and flows out of an outlet of the liquid flow path 28a through the liquid flow path 63.
The anode side electrolytic solution B supplied through the liquid flow path 65 flows in the liquid flow path 29a, and flows out of an outlet of the liquid flow path 29a through the liquid flow path 66.
When the cathode side electrolytic solution A flows in the liquid flow path 28a, the anode side electrolytic solution B flows in the liquid flow path 29a, the raw material gas G flows in the gas flow path 24a, and a voltage is applied between the cathode 21 and the anode 22, at the cathode 21, a carbon compound is generated through the reduction of carbon dioxide and/or carbon monoxide in the raw material gas G, and hydrogen is generated through the reduction of water. The carbon compound generated is liquid or gaseous, and the hydrogen generated is gaseous. At the anode 22, hydroxide ions in the anode side electrolytic solution B are oxidized to generate oxygen. A gaseous product E containing a gaseous carbon compound and hydrogen passes through the gas diffusion layer of the cathode 21 (the gas diffusion layer 11 of the electrode catalyst 10) and reaches the gas flow path 24a, and flows out of an outlet of the gas flow path 24a through the gas flow path 67. The product E flowing out of the electrochemical reaction apparatus 2 may be sent to a reactor (not shown) and brought into gas phase contact with an olefin multimerization catalyst in the reactor to perform ethylene multimerization. A liquid carbon compound generated at the cathode 21 flows in the liquid flow path 28a together with the cathode side electrolytic solution A, and flows out of an outlet of the liquid flow path 28a through the liquid flow path 63.
Examples of the carbon compound generated through the reduction of carbon dioxide and/or carbon monoxide at the cathode 21 include a C1 compound (compound having 1 carbon atom) and a C2 compound (compound having 2 carbon atoms).
Examples of the C2 compound generated through the reduction of carbon dioxide and/or carbon monoxide at the cathode 21 include acetic acid (CH₃COOH), acetic acid salts (for example, alkali metal acetates such as sodium acetate and potassium acetate), acetaldehyde (CH₃CHO), ethanol (C₂H₅OH), and ethylene (C₂H₄). Among these compounds, ethylene is preferred because it is useful in terms of chemical industry. In other words, the C2 compound generated through the reduction of carbon dioxide and/or carbon monoxide preferably includes ethylene. The C2 compound generated through the reduction of carbon dioxide and/or carbon monoxide can include, in addition to ethylene, one or more other compounds. In the C2 compound generated, ethylene is gaseous, and ethanol and acetic acid are each liquid. The type of an acetic acid salt generated depends on the type of an electrolytic solution used. For example, when the electrolytic solution contains sodium ions, sodium acetate is generated, and when the electrolytic solution contains potassium ions, potassium acetate is generated.
Examples of the C1 compound generated through the reduction of carbon dioxide at the cathode 21 include carbon monoxide (CO), formic acid (HCOOH), formic acid salts (for example, alkali metal formates such as sodium formate and potassium formate), formaldehyde (HCHO), methanol (CH₃OH), and methane (CH₄). The C1 compound generated through the reduction of carbon dioxide can include one or more compounds. In the generated C1 compound, for example, carbon monoxide and methane are each gaseous, and formic acid, methanol and formaldehyde are each liquid. The type of a formic acid salt generated depends on the type of an electrolytic solution used. For example, when the electrolytic solution contains sodium ions, sodium formate is generated, and when the electrolytic solution contains potassium ions, potassium formate is generated.
Examples of the C1 compound generated through the reduction of carbon monoxide at the cathode 21 include formaldehyde (HCHO), methanol (CH₃OH), and methane (CH₄). The C1 compound generated through the reduction of carbon monoxide can include one or more compounds. In the generated C1 compound, for example, methane is gaseous, and methanol and formaldehyde are each liquid.
FIG. 3 shows one example of a substance flow in an electrolytic cell of the electrochemical reaction apparatus 2. As shown in FIG. 3, at the cathode 21, not only a gaseous carbon compound and a liquid carbon compound are generated through the reduction of carbon dioxide and/or carbon monoxide in the raw material gas G, but also hydrogen is generated. The product E containing a gaseous carbon compound and hydrogen generated at the cathode 21 passes through the gas diffusion layer of the cathode 21 (the gas diffusion layer 11 of the electrode catalyst 10) and reaches the gas flow path 24a, and flows out of an outlet of the gas flow path 24a through the gas flow path 67. A liquid carbon compound generated at the cathode 21 flows in the liquid flow path 28a together with the cathode side electrolytic solution A, and flows out of an outlet of the liquid flow path 28a through the liquid flow path 63.
Hydroxide ions (OH^-) generated at the cathode 21 move to the anode 22 in the anode side electrolytic solution B and are oxidized by the following reaction to generate oxygen (O₂). The generated oxygen passes through the gas diffusion layer of the anode 22 and reaches the gas flow path 25a, and flows out of an outlet of the gas flow path 25a.

4OH^- -> O₂ + 2H₂O

Examples

[Example 1]

(1) Film formation of Cu by sputtering

As a gas diffusion layer, a gas diffusion layer manufactured by SGL Carbon SE, Sigracet 39 BB was used. Sigracet 39 BB is composed of carbon paper and a micro-porous layer (MPL) containing a fluorine element, formed on one side of the carbon paper. Sigracet 39 BB has a total thickness of 315 µm and contains a fluorine element at a mass percentage of 14% by mass.
Cu was sputtered on a surface on the MPL side of Sigracet 39 BB. Sputtering conditions are as follows.

Sputtering system: DC magnetron sputtering
Exhauster: rotary pump + cryopump
Target material: Cu
Target size: φ8 inches
Sputtering rate: 0.8 nm/sec
Pre-sputtering: 5 minutes
Sputtering time: 33 seconds
Temperature of gas diffusion layer: 25°C
Target thickness of Cu-sputtered film: 25 nm

Thus, an electrode (hereinafter, referred to as "Cu/C (BB) electrode") including a gas diffusion layer (Sigracet 39 BB) and a Cu-sputtered film formed on a surface on the MPL side of the gas diffusion layer was obtained.

(2) Formation of coating layer

As an aqueous dispersion liquid containing polytetrafluoroethylene (PTFE), Polyflon PTFE DC210-C manufactured by DAIKIN Industries, Ltd. was used. Polyflon PTFE DC210-C is a lacteous aqueous dispersion liquid in which no perfluorooctanoic acid (PFOA) is used and polytetrafluoroethylene polymer particles are stabilized by a non-ionic surfactant. Typical values of physical properties of Polyflon PTFE DC210-C (cited from the catalog) are as follows.

Solid content: 60% by mass (test method: ASTM D 4441)
Specific gravity of liquid (25°C): 1.52 (test method: ASTM D 4441)
Surfactant: 6.0% by mass/PTFE (test method: ASTM D 4441)
Average particle size: 0.25 µm (test method: method according to DAIKIN Industries, Ltd.)
pH (25°C): 9.7 (test method: JIS K 6893)
Viscosity (25°C): 26cP (test method: ASTM D 4441)

A diluted liquid was prepared by diluting Polyflon PTFE DC210-C 10-fold with distilled water. A surface (Cu-sputtered surface) of the Cu/C (BB) electrode cut into a 30-mm square by a Thomson cutter (press cutter MB type, manufactured by Aichi Tec) was coated with the diluted liquid using a spray-coating method. In this case, the spray-coating was performed on a hot plate warmed to 60°C, and a 2-layer coating was formed using the diluted liquid. The electrode subjected to 2-layer coating using the diluted liquid was dried in a drier at 80°C for 1 hour. Thus, an electrode (hereinafter, referred to as "PF/Cu/C (BB) electrode") including the Cu/C (BB) electrode and a coating layer covering the whole surface of the Cu-sputtered film of the Cu/C (BB) electrode was obtained.
The mass (mg) of the Cu/C (BB) electrode was subtracted from the mass (mg) of the PF/Cu/C (BB) electrode, and the difference was divided by the geometric area (cm²) of the surface on which the coating layer was formed (namely, ((Mass of PF/Cu/C (BB) electrode) - (Mass of Cu/C (BB) electrode)) / (Geometric area of surface on which coating layer was formed)), thereby calculating the mass (mg/cm²) of the coating layer per unit geometric area of the surface on which the coating layer was formed. The calculation results are shown in Table 1. In Table 1, "Mass of coating layer" represents the mass of the coating layer per unit geometric area of the surface on which the coating layer was formed. As shown in Table 1, the mass of the coating layer per unit geometric area of the surface on which the coating layer was formed was 0.1 mg/cm².

(3) Calcination

The PF/Cu/C (BB) electrode was calcined in a tubular furnace under a N₂ atmosphere in conditions of a rate of temperature rise of 10°C/min, a retention temperature of 200°C, and a retention time of 2 hours. Thus, an electrode catalyst of Example 1 was obtained.

(4) Quantitative determination of fluorine element

The content of the fluorine element in the coating layer based on the mass of the electrode catalyst was determined based on the following expression. Content of fluorine element in coating layer based on mass of electrode catalyst = ((Mass of fluorine element in electrode catalyst) - (Mass of fluorine element in gas diffusion layer)) / (Mass of electrode catalyst) × 100
The mass of the fluorine element in the electrode catalyst and the mass of the fluorine element in the gas diffusion layer were quantitatively determined by an alkali fusion-ion selective electrode method. As the alkali, a mixture of sodium carbonate (Na₂CO₃) and sodium peroxide (Na₂O₂) was used. As the ion electrode, a composite fluoride ion selective electrode (6561S-10C, manufactured by Horiba Ltd.) was used.
The results of quantitative determination are shown in Table 1. In Table 1, "F in coating layer (% by mass)" represents the mass percentage of the fluorine element in the coating layer based on the mass of the electrode catalyst. As shown in Table 1, the mass percentage of the fluorine element in the coating layer based on the mass of the electrode catalyst was 1.1% by mass.

(5) Evaluation

A CO₂ electrolysis test was performed using the electrochemical reaction apparatus 2 shown in FIG. 2.
As the cathode side electrolytic solution A supplied through the liquid flow path 64, a 7 mol/L potassium hydroxide (KOH) aqueous solution (flow rate: 1 mL/min) was used.
As the anode side electrolytic solution B supplied through the liquid flow path 65, a 1 mol/L potassium hydroxide (KOH) aqueous solution (flow rate: 1 mL/min) was used.
As the cathode 21, the electrode catalyst of Example 1 obtained in (3) described above was used.
As the anode 22, commercially available porous metal body Ni foam (EQ-bcnf-03, MTI corporation) was used.
As the anion exchange membrane 23, Fumasep FAB-PK-130 (manufactured by FuMA-Tech) was used.
As the raw material gas G supplied through the gas flow path 76, a carbon dioxide gas (flow rate: 20 mL/min) was used.
The CO₂ electrolysis test was performed in the following conditions.

Pre-treatment conditions: cyclic voltammetry
- Atmosphere: under nitrogen atmosphere
- Sweep range: -0.85 V to -0.10 V
- Number of sweep cycles: 10 cycles
Electrolysis conditions: constant potential electrolysis
- Potential: -2.5 V
- Electrolysis time: 30 minutes

In the CO₂ electrolysis test, a gaseous product was collected from the gas flow path 67 by use of a sampling bag (Smart Bag PA, manufactured by GL Sciences Inc.) after a lapse of 30 minutes from the start of test, and the concentration of each product was measured with gas chromatograph (CP-4900 Micro GC, manufactured by Varian). In this case, hydrogen (H₂), carbon monoxide (CO) and methane (CH₄) were quantitatively determined by use of a Molsieve 5A column (manufactured by GL Sciences Inc.) with argon as a carrier gas. Ethylene (C₂H₄) was quantitatively determined by use of a PoraPLOT Q column (manufactured by GL Sciences Inc.) with helium as a carrier gas. These were each converted to determine the molar number (mol) of each product generated after a lapse of 30 minutes from the start of test.
The cathode side electrolytic solution A containing a liquid product was collected from the liquid flow path 63 after a lapse of 30 minutes from the start of test, and subjected to a neutralization treatment with concentrated hydrochloric acid, and then the concentration of each product was measured with HPLC (high-performance liquid chromatography). This was converted to determine the molar number (mol) of each product generated after a lapse of 30 minutes from the start of test. The HPLC method was performed in the following conditions.

Apparatus: Prominence (manufactured by Shimadzu Corporation)
Dissociation liquid: sulfuric acid (0.010 mol/L)
Column: Shodex SUGAR SC1821, Shodex Rspak DE 13L (manufactured by Resonac Corporation)
Column temperature: 50°C

The Faraday efficiency (%) of each product was determined based on the following expression. In the following expression, the "predetermined time" is 30 minutes. Faraday efficiency (%) of each product = Molar number (mol) of each product generated after lapse of predetermined time × n / Number of electrons (mol) consumed after lapse of predetermined time × 100
In the expression, "n" is the number of electrons necessary for generation of each product. Specifically, "n" is the coefficient of e^- in reaction formulae in which each product is generated. Such reaction formulae in which each product is generated are as follows.

2H⁺+2e^-->H₂

CO₂+2H⁺+2e^-->CO+H₂O

CO₂+2H⁺+2e^-->HCOOH

CO₂+6H⁺+6e^-->CH₃OH+H₂O

CO₂+8H⁺+8e^-->CH₄+2H₂O

2CO₂+8H⁺+8e^-->CH₃COOH+2H₂O

2CO₂+10H⁺+10e^-->CH₃CHO+3H₂O

2CO₂+12H⁺+12e^-->CH₃CH₂OH+3H₂O

2CO₂+12H⁺+12e^-->C₂H₄+4H₂O

3CO₂+18H⁺+18e^-->CH₃CH₂CH₂OH+5H₂O
The Faraday efficiencies (%) of CO, CH₄, CH₃OH and HCOOH were summed, and the total was defined as the Faraday efficiency (%) of the C1 compound. The Faraday efficiencies (%) of C₂H₄, C₂H₅OH and CH₃COOH were summed, and the total was defined as the Faraday efficiency (%) of the C2 compound.
The respective Faraday efficiencies (%) of the C2 compound, the C1 compound and hydrogen are shown in Table 1. In Table 1, "C2/C1" represents the ratio of the Faraday efficiency of the C2 compound to the Faraday efficiency of the C1 compound (Faraday efficiency of C2 compound/Faraday efficiency of C1 compound).

[Example 2]

The same operation as in Example 1 was performed except that the coating formed by the diluted liquid using a spray-coating method was changed to a 4-layer coating, thereby changing the mass percentage of the fluorine element in the coating layer based on the mass of the electrode catalyst to 1.6% by mass, and the mass of the coating layer per unit geometric area of the surface on which the coating layer was formed to 0.2 mg/cm². The respective Faraday efficiencies (%) of the C2 compound, the C1 compound and hydrogen are shown in Table 1.

[Example 3]

The same operation as in Example 1 was performed except that the coating formed by the diluted liquid using a spray-coating method was changed to an 8-layer coating, thereby changing the mass percentage of the fluorine element in the coating layer based on the mass of the electrode catalyst to 2.8% by mass, and the mass of the coating layer per unit geometric area of the surface on which the coating layer was formed to 0.4 mg/cm². The respective Faraday efficiencies (%) of the C2 compound, the C1 compound and hydrogen are shown in Table 1.

[Example 4]

The same operation as in Example 1 was performed except that the coating formed by the diluted liquid using a spray-coating method was changed to a 10-layer coating, thereby changing the mass percentage of the fluorine element in the coating layer based on the mass of the electrode catalyst to 6.2% by mass, and the mass of the coating layer per unit geometric area of the surface on which the coating layer was formed to 0.8 mg/cm². The respective Faraday efficiencies (%) of the C2 compound, the C1 compound and hydrogen are shown in Table 1.

[Example 5]

The same operation as in Example 1 was performed except that a diluted liquid was prepared by diluting Polyflon PTFE DC210-C 100-fold with distilled water, and the coating formed by the diluted liquid using a spray-coating method was changed to an 8-layer coating, thereby changing the mass percentage of the fluorine element in the coating layer based on the mass of the electrode catalyst to 0.7% by mass, and the mass of the coating layer per unit geometric area of the surface on which the coating layer was formed to 0.09 mg/cm². The respective Faraday efficiencies (%) of the C2 compound, the C1 compound and hydrogen are shown in Table 1.

[Example 6]

The same operation as in Example 1 was performed except that a diluted liquid was prepared by diluting Polyflon PTFE DC210-C 100-fold with distilled water, and the coating formed by the diluted liquid using a spray-coating method was changed to a 10-layer coating, thereby changing the mass percentage of the fluorine element in the coating layer based on the mass of the electrode catalyst to 0.9% by mass, and the mass of the coating layer per unit geometric area of the surface on which the coating layer was formed to 0.1 mg/cm². The respective Faraday efficiencies (%) of the C2 compound, the C1 compound and hydrogen are shown in Table 1.

[Example 7]

The same operation as in Example 1 was performed except that a diluted liquid was prepared by diluting Polyflon PTFE DC210-C 5-fold with distilled water, and the coating formed by the diluted liquid using a spray-coating method was changed to an 8-layer coating, thereby changing the mass percentage of the fluorine element in the coating layer based on the mass of the electrode catalyst to 11.0% by mass, and the mass of the coating layer per unit geometric area of the surface on which the coating layer was formed to 1.6 mg/cm². The respective Faraday efficiencies (%) of the C2 compound, the C1 compound and hydrogen are shown in Table 1.

[Comparative Example 1]

The same operation as in Example 1 was performed except that the Cu/C (BB) electrode was used instead of the electrode catalyst of Example 1. The respective Faraday efficiencies (%) of the C2 compound, the C1 compound and hydrogen are shown in Table 1.

[Comparative Example 2]

The same operation as in Example 1 was performed except that 4 mg of graphite dispersed in 0.75 mL of 2-propanol and ultrasonically diffused for 1 hour was used instead of the diluted liquid of Polyflon PTFE DC210-C, and the mass of the coating layer per unit geometric area of the surface on which the coating layer was formed was changed to 0.4 mg/cm². The ultrasonic diffusion was performed using an ultrasonic cleaner ASU-10M manufactured by AS ONE Corporation for 1 hour. The respective Faraday efficiencies (%) of the C2 compound, the C1 compound and hydrogen are shown in Table 1.

[Comparative Example 3]

The same operation as in Example 1 was performed except that a mixed dispersion liquid in which 0.909 g of SYLGARD 184 UL94 V-0 (manufactured by Dow Toray Co., Ltd.) being a silicon resin containing polydimethylsiloxane and 0.091 g of a SYLGARD (manufactured by Dow Toray Co., Ltd.) 184 potting material were dispersed in 5 mL of 2-propanol was used instead of the diluted liquid of Polyflon PTFE DC210-C, and the mass of the coating layer per unit geometric area of the surface on which the coating layer was formed was changed to 0.4 mg/cm². The respective Faraday efficiencies (%) of the C2 compound, the C1 compound and hydrogen are shown in Table 1.

[Table 1]

Table 1

	F in coating layer (% by mass)	Mass of coating layer (mg/cm²)	Faraday efficiency (%)			C2/C1
	F in coating layer (% by mass)	Mass of coating layer (mg/cm²)	C2 compound	C1 compound	Hydrogen	C2/C1
Example 1	1.1	0.1	65.0	18.2	11.9	3.6
Example 2	1.6	0.2	75.8	14.8	9.5	5.1
Example 3	2.8	0.4	47.7	9.3	13.8	5.1
Example 4	6.2	0.8	49.2	5.9	18.1	8.3
Example 5	0.7	0.09	62.9	19.7	16.3	3.2
Example 6	0.9	0.1	63.4	20.6	16.5	3.1
Example 7	11.0	1.6	46.9	14.8	26.1	3.2
Comparative Example 1	-	-	59.5	20.3	12.7	2.9
Comparative Example 2		0.4	23.1	51.5	27.4	0.4
Comparative Example 3	-	0.4	36.0	35.7	27.5	1.0

Description of Reference Signs

10...: Electrode catalyst 10
11...: Gas diffusion layer 11
11a... Substrate
11b... Permeating layer
12...: Catalyst
13...: Coating layer
2...: Electrochemical reaction apparatus
21... Cathode
22... Anode
23... Anion exchange membrane
24... Gas flow path structure
24a... Gas flow path
25... Gas flow path structure
25a... Gas flow path
26... Power supply body
27... Power supply body
28... Liquid flow path structure
28a... Liquid flow path
29... Liquid flow path structure
29a... Liquid flow path

A...: Cathode side electrolytic solution
B...: Anode side electrolytic solution
G...: Raw material gas containing carbon dioxide and/or carbon monoxide

Claims

An electrode catalyst for electrochemically reducing carbon dioxide and/or carbon monoxide, the electrode catalyst comprising: a gas diffusion layer; a catalyst present on a surface of the gas diffusion layer; and a coating layer covering at least part of a surface of the catalyst,
wherein the catalyst comprises a Cu element, and

wherein the coating layer comprises a fluororesin.
The electrode catalyst according to claim 1, wherein the catalyst is in the form of a catalyst layer.
The electrode catalyst according to claim 2, wherein a thickness of the catalyst layer falls within ±10% of an average thickness of the catalyst layer.
The electrode catalyst according to claim 3, wherein the average thickness of the catalyst layer is 5 nm or more and 1000 nm or less.
The electrode catalyst according to claim 1 or 2, wherein a mass percentage of a fluorine element in the coating layer based on a mass of the electrode catalyst is 0.2% by mass or more and 15.0% by mass or less.
The electrode catalyst according to claim 1 or 2, wherein the catalyst is metal Cu.
The electrode catalyst according to claim 1 or 2, wherein the fluororesin comprises polytetrafluoroethylene.
An electrochemical reaction apparatus for electrochemically reducing carbon dioxide and/or carbon monoxide, the apparatus comprising: a cathode; an anode; an anion exchange membrane provided between the cathode and the anode; a liquid flow path which is provided between the cathode and the anion exchange membrane and through which a cathode side electrolytic solution flows; and a liquid flow path which is provided between the anode and the anion exchange membrane and through which an anode side electrolytic solution flows,
wherein the cathode comprises the electrode catalyst according to claim 1 or 2.
A method for producing an electrode catalyst for electrochemically reducing carbon dioxide and/or carbon monoxide, the method comprising the following steps of:
(a) forming a catalyst layer comprising a Cu element, on a surface of a gas diffusion layer; and

(b) forming a coating layer comprising a fluororesin, on at least part of a surface of the catalyst layer.
The method according to claim 9, wherein, in step (b), the coating layer is formed by spray-coating at least part of the surface of the catalyst layer with a fluororesin-containing solution or dispersion liquid.