GAS DIFFUSION ELECTRODE AND ELECTROCHEMICAL REACTION DEVICE

Description

This application is based on and claims the benefit of priority from Japanese Patent Application No. 2024-048504, filed on Mar. 25, 2024, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a gas diffusion electrode and an electrochemical reaction device.

Related Art

In recent years, there has been a growing movement to reduce the emission of greenhouse gases such as carbon dioxide (CO₂) and carbon monoxide (CO) on a global scale. An attention-drawing solution to reducing greenhouse gas emissions includes the electrochemical reduction reaction of CO₂ and/or CO (hereinafter also referred to as “CO₂/CO”) to C2 compounds (compounds having two carbon atoms), for which gas diffusion electrodes have been developed, including catalysts for promoting the electrochemical reduction reaction of CO₂/CO (hereinafter also referred to as the “CO₂/CO reduction reaction”).

Copper (Cu) is known to be suitable as a catalyst material for use in gas diffusion electrodes for promoting the CO₂/CO reduction reaction. For example, Patent Document 1 discloses an electrode catalyst including a substrate and copper-containing particles supported on the substrate. Patent Document 2 discloses an electrode catalyst including an electrically-conductive substrate and an alloy catalyst that is supported on the substrate and includes Cu and Ni components.

• Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2015-147990
• Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2020-89878

SUMMARY OF THE INVENTION

CO₂/CO reduction reaction products include C2 compounds such as ethylene (C₂H₄), which are highly demanded and useful in the chemical industry. Conventional gas diffusion electrodes, however, are susceptible to improvement for efficient production of C₂ compounds such as ethylene. Specifically, conventional technologies have a difficulty in efficiently producing desired C₂ compounds at high current density and have a problem in that C₂ compounds are produced with low Faraday efficiency. This means that it is difficult to increase the scale of production of C₂ compounds to an industrial level.

In light of these conventional problems, the inventors have made active investigations. As a result, the inventors have found that C₂ compounds can be efficiently produced at high current density using a gas diffusion electrode including a gas diffusion layer and a catalyst layer that includes a controlled amount of catalyst particles of a specific material and a controlled amount of hydrophobic particles.

The present invention has been completed based on the findings, and an object of the present invention is to provide a gas diffusion electrode that enables efficient production of C₂ compounds at high current density and to provide an electrochemical reaction device including such a gas diffusion electrode.

The present invention encompasses aspects (1) to (9) below. As used herein, a phrase consisting of numerical values and “to” therebetween means the range between the numerical values (inclusive). In other words, the phrase “X to Y” is interchangeable with “X or more and Y or less”.

(1) A gas diffusion electrode for electrochemically reducing one or both of carbon dioxide and carbon monoxide, the gas diffusion electrode including: a gas diffusion layer; and a catalyst layer provided on a surface of the gas diffusion layer, the catalyst layer including: catalyst particles; and hydrophobic particles, the catalyst particles including a copper (Cu) component, the hydrophobic particles including a fluororesin, the catalyst particles having a mass per unit area (M₁) of 0.70 mg/cm² or more in the catalyst layer, the hydrophobic particles having a mass per unit area (M₂) with a ratio (M₂/M₁) of M₂ to M₁ being 0.10 or more and 1.70 or less in the catalyst layer.

(2) The gas diffusion electrode according to aspect (1), wherein the mass per unit area (M₂) of the hydrophobic particles in the catalyst layer is 0.10 mg/cm² or more.

(3) The gas diffusion electrode according to aspect (1) or (2), wherein the catalyst layer includes a mixture of the catalyst particles and the hydrophobic particles.

(4) The gas diffusion electrode according to any one of aspects (1) to (3), wherein the catalyst layer has a thickness of 200 μm or less.

(5) The gas diffusion electrode according to any one of aspects (1) to (4), wherein the copper (Cu) component is metallic copper.

(6) The gas diffusion electrode according to any one of aspects (1) to (5), wherein the fluororesin includes polytetrafluoroethylene.

(7) The gas diffusion electrode according to any one of aspects (1) to (6), wherein the gas diffusion electrode is a cathode electrode.

(8) An electrochemical reaction device for electrochemically reducing one or both of carbon dioxide and carbon monoxide, the electrochemical reaction device including: a cathode; an anode; an anion exchange membrane provided between the cathode and the anode; a cathode-side liquid flow channel provided between the cathode and the anion exchange membrane; and an anode-side liquid flow channel provided between the anode and the anion exchange membrane, the cathode including the gas diffusion electrode according to any one of aspects (1) to (7).

The present invention provides a gas diffusion electrode that enables efficient production of C₂ compounds at high current density and provides an electrochemical reaction device including such a gas diffusion electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a gas diffusion electrode according to an embodiment;

FIG. 2 is a schematic cross-sectional view of an electrolysis cell of an electrochemical reaction device according to an embodiment; and

FIG. 3 is a schematic diagram showing material flows in an electrolysis cell of an electrochemical reaction device.

DETAILED DESCRIPTION OF THE INVENTION

1. Gas Diffusion Electrode

The gas diffusion electrode according to an embodiment is for use in electrochemical reduction of one or both of carbon dioxide (CO₂) and carbon monoxide (CO) (CO₂/CO). Specifically, the gas diffusion electrode according to an embodiment is for use as a cathode in an electrochemical reaction device for electrochemically reducing CO₂/CO. The gas diffusion electrode is preferably for use as a cathode in an electrochemical reaction for electrochemically reducing carbon dioxide (CO₂). In other words, the gas diffusion electrode is preferably a cathode electrode.

The electrochemical reduction of CO₂/CO may be performed using a raw material gas including CO₂/CO. The raw material gas may include one or both of CO₂ and CO. The phrase “electrochemically reducing CO₂/CO” is intended to include electrochemically reducing one of CO₂ and CO and electrochemically reducing both CO₂ and CO.

The CO₂/CO reduction reaction may include reduction of CO₂/CO to carbon compounds and reduction of water to hydrogen. The carbon compound product may be liquid or gaseous, and the hydrogen product may be gaseous.

Examples of such carbon compounds may include C1 compounds (compounds having one carbon atom) and C₂ compounds (compounds having two carbon atoms).

Examples of C₂ compounds that can be produced by the reduction of CO₂/CO include acetic acid (CH₃COOH), acetic acid salts (e.g., alkali metal acetates such as sodium acetate and potassium acetate), acetaldehyde (CH₃CHO), ethanol (C₂H₅OH), and ethylene (C₂H₄). Of these compounds, ethylene is preferred for its usefulness in the chemical industry. In other words, the C₂ compound product of the CO₂/CO reduction preferably includes ethylene. The C₂ compound product of the CO₂/CO reduction may include ethylene and one, two, or more additional compounds. The C₂ compound product may include ethylene in a gaseous form and ethanol and acetic acid each in a liquid form. The type of acetic acid salts produced depends on the type of the electrolytic solution used. For example, sodium acetate can be produced in a case where an electrolytic solution containing sodium ions is used, and potassium acetate can be produced in a case where an electrolytic solution containing potassium ions is used.

Examples of C1 compounds that can be produced by the reduction of carbon dioxide (CO₂) include carbon monoxide (CO), formic acid (HCOOH), formic acid salts (e.g., alkali metal formates such as sodium formate and potassium formate), formaldehyde (HCHO), methanol (CH₃OH), and methane (CH₄). The C1 compound product of the CO₂ reduction may include one, two, or more compounds. For example, the C1 compound product may include CO and methane each in a gaseous form and formic acid, methanol, and formaldehyde each in a liquid form. The type of formic acid salts produced depends on the type of the electrolytic solution used. For example, sodium formate can be produced in a case where an electrolytic solution containing sodium ions is used, and potassium formate can be produced in a case where an electrolytic solution containing potassium ions is used.

Examples of C1 compounds that can be produced by the reduction of carbon monoxide (CO) include methanol (CH₃OH), formaldehyde (HCHO), and methane (CH₄). The C1 compound product of the CO reduction may include one, two, or more compounds. For example, the C1 compound product may include methane in a gaseous form and methanol and formaldehyde each in a liquid form.

Formulae (A), (B), and (C) below represent the electrochemical reduction reaction of carbon dioxide (CO₂) to carbon monoxide (CO), the electrochemical reduction reaction of carbon monoxide (CO) to ethylene (C₂H₄), and the electrochemical reduction reaction of water (H₂O) to hydrogen (H₂), respectively.

CO₂+2H⁺+2e⁻→CO+H₂O  (A)

2CO+8H⁺+8e³¹ →C₂H₄+2H₂O  (B)

2H₂O+2e⁻→H₂+2OH⁻  (C)

The reduction reaction of CO₂/CO can be carried out under known conditions using, as a novel cathode, the gas diffusion electrode according an embodiment.

Hereinafter, a gas diffusion electrode according to an embodiment will be described with reference to drawings. FIG. 1 shows a gas diffusion electrode 10 including: a gas diffusion layer 11; and a catalyst layer 12 provided on a surface of the gas diffusion layer 11. The catalyst layer 12 includes catalyst particles 12a and hydrophobic particles 12b.

Gas Diffusion Layer

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. The substrate 11a in the form of a sheet typically has a thickness of 10 μm or more and 1, 000 μm or less, preferably 100 μm or more and 500 μm or less, more preferably 150 μm or more and 350 μm or less. The substrate 11a preferably has a minimum thickness and a maximum thickness each within the range shown above.

The substrate 11a is gas-permeable. The gas-permeable substrate 11a allows efficient supply of a raw material gas including CO₂/CO to the catalyst layer 12. The gas-permeable substrate 11a also allows efficient collection of a gaseous product produced by the CO₂/CO reduction reaction and allows efficient collection of hydrogen produced by the reduction of water.

For effective gas permeability, the substrate 11a is preferably a porous material. Examples of such a porous material include non-woven fabrics (including paper) and woven fabrics. The porous material typically has a most frequent pore diameter of 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, even more preferably 25 μm or more and 200 μm or less. The most frequent pore diameter can be determined, for example, by mercury intrusion technique.

The substrate 11a may have electrical conductivity. The substrate with electrical conductivity contributes to efficient CO₂/CO reduction reaction.

For effective electrical conductivity, the substrate 11a preferably contains an electrically conductive material. The substrate 11a may contain one, two, or more electrically conductive materials. Examples of electrically conductive materials include carbon materials. Examples of carbon materials include carbon fibers, carbon black, graphite, black lead, activated carbon, carbon nanotubes, carbon nanofibers, fullerenes, and amorphous carbon. The carbon material is typically a material including 50 mass % or more of carbon. Carbon fibers are fibers composed mostly of carbon atoms (e. g., including 90 mass % or more of carbon).

For example, the substrate 11a may be an electrically conductive porous material. The electrically conductive porous material may be a porous material including a carbon material. The porous material including a carbon material may include carbon fibers. The porous material including carbon fibers may be, for example, a non-woven fabric including carbon fibers (including paper) or a woven fabric including carbon fibers. The non-woven fabric including carbon fibers (including paper) is also referred to as carbon paper. The woven fabric including carbon fibers is also referred to as carbon cloth.

The substrate 11a may be a metal or alloy mesh material, a punched metal or alloy material, a sintered metal fiber material, or any other porous material. 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 permeable layer 11b provided on the surface of the substrate 11a. The permeable layer 11b may be provided at least part of the surface of the substrate 11a. The permeable layer 11b is highly permeable. The permeable layer 11b has pores with an average pore diameter smaller than that of the substrate 11a and has a surface area greater than that of the substrate 11a. Thus, a larger amount of the catalyst layer 12 can be supported on the permeable layer 11b.

The substrate 11a has inner and outer surfaces, which may be collectively referred to as “the surface of the substrate 11a”. The inner surface of the substrate 11a is intended to include inner surfaces of pores existing in the substrate 11a (in other words, not exposed on the outer surface of the substrate 11a). The outer surface of the substrate 11a is intended to include inner surfaces of pores exposed on the outer surface of the substrate 11a.

The permeable layer 11b preferably has a part provided on at least part of the outer surface of the substrate 11a. The permeable layer 11b may have a part provided in at least part of the inner surface of the substrate 11a in addition to its part provided on at least part of the outer surface of the substrate 11a.

In a case where the substrate 11a is in the form of a sheet, the permeable layer 11b is preferably provided on one surface of the substrate 11a, more preferably provided on at least part of one outer surface of the substrate 11a. The permeable layer 11b only has to be provided on at least part of one surface of the substrate 11a.

The portion that forms the permeable layer 11b and is provided on the outer surface of the substrate 11a typically has a thickness of 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, even more preferably 70 μm or more and 150 μm or less. The portion that forms the permeable layer 11b and is provided on the outer surface of the substrate 11a preferably has a minimum thickness and a maximum thickness each within the range shown above.

The permeable layer 11b is gas-permeable. The gas-permeable layer 11b allows efficient supply of a raw material gas including CO₂/CO to the catalyst layer 12. The gas-permeable layer 11b also allows efficient collection of a gaseous product produced by the CO₂/CO reduction reaction and allows efficient collection of hydrogen produced by the reduction of water.

For effective gas permeability, the permeable layer 11b is preferably porous and is also preferably a micro-porous layer

(MPL). The microporous layer typically has a most frequent pore diameter of 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, even more preferably 15 nm or more and 70 nm or less. The most frequent pore diameter can be determined by mercury intrusion technique. The microporous layer usually has an average pore diameter smaller than that of the substrate 11a and has a density higher than that of the substrate 11a.

The permeable layer 11b is provided, for example, for the purpose of improving the water repellency of the gas diffusion layer 11. The gas diffusion layer 11 with improved water repellency can suppress the water reduction reaction to produce hydrogen and make more dominant the CO₂/CO reduction reaction to produce carbon compounds.

When provided to improve the water repellency of the gas diffusion layer 11, the permeable layer 11b preferably includes a fluororesin.

Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymers, tetrafluoroethylene-ethylene copolymers, and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers. In particular, the fluororesin is preferably polytetrafluoroethylene.

For effectively improved water repellency, the permeable layer 11b preferably has a fluorine mass percentage of 5 mass % or more and 30 mass % or less, more preferably 8 mass % or more and 25 mass % or less, even more preferably 10 mass % or more and 20 mass % or less, based on the mass of the gas diffusion electrode 10. The mass of fluorine in the permeable layer 11b can be quantified, for example, by alkali fusion-ion selective electrode method. The alkali fusion-ion selective electrode method may be performed under the conditions shown in the Examples section.

The permeable layer 11b may have electrical conductivity. The permeable layer 11b with electrical conductivity contributes to efficient CO₂/CO reduction reaction. For effective electrical conductivity, the permeable layer 11b preferably contains an electrically conductive material. The permeable layer 11b may contain one, two, or more electrically conductive materials. Examples of electrically conductive materials include carbon materials. The above description also applies to the carbon materials in the permeable layer 11b.

For effective electrical conductivity, the permeable layer 11b preferably has a carbon material mass percentage of 70 mass % or more and 95 mass % or less, more preferably 75 mass or more and 92 mass % or less, even more preferably 80 mass % or more and 90 mass % or less, based on the mass of the permeable layer 11b. The mass of the carbon material in the permeable layer 11b can be quantified, for example, by combustion method.

The permeable layer 11b can be formed by coating the surface of the substrate 11a (one surface of the substrate 11a in a case where the substrate 11a is in the form of a sheet) with a solution or dispersion (preferably an emulsion) containing a fluororesin. This process may be performed using a known coating method. Examples of such a coating method include bar coating, blade coating, screen printing, spray coating, curtain coating, and roll coating. The fluororesin-containing solution or dispersion may be applied so as to form a stack of two or more layers with an adjusted amount of the fluororesin coating. The fluororesin-containing solution or dispersion can be prepared by a common method including mixing the fluororesin, a solvent or dispersion medium (e.g., water), a surfactant (e.g., a nonionic surfactant), and any optional material. Alternatively, the fluororesin-containing solution or dispersion may be a commercially available solution or dispersion containing a fluororesin. The types of the fluororesin, the solvent, the dispersion medium, the surfactant, and any optional material may be selected as appropriate. The fluororesin-containing solution or dispersion typically has a fluororesin concentration of 1 mass % or more and 70 mass % or less, preferably 3 mass % or more and 60 mass % or less, based on the mass of the solution or dispersion. The layer formed by the application of the fluororesin-containing solution or dispersion may be dried as necessary. The layer is typically dried at a temperature of 10° C. or more and 120° C. or less, preferably at a temperature of 20° C. or more and 100° C. or less, typically for a time period of 0.5 hours or more and 48 hours or less, preferably 1 hour or more and 24 hours or less.

Catalyst Layer

As shown in FIG. 1, the catalyst layer 12 is provided on the surface of the gas diffusion layer 11. As used herein, the phrase “the surface of the gas diffusion layer 11” means the surface of the permeable layer 11b, if the permeable layer 11b is provided, or means the surface of the substrate 11a, if the permeable layer 11b is not provided.

The permeable layer 11b has inner and outer surfaces, which may be collectively referred to as “the surface of the permeable layer 11b”. The above interpretation of the term “the surface of the substrate 11a” may apply to the interpretation of the term “the surface of the permeable layer 11b”.

In the case where the permeable layer 11b is provided, the catalyst layer 12 preferably has a part supported on at least part of the outer surface of the permeable layer 11b. The catalyst layer 12 may have a part supported on at least part of the inner surface of the permeable layer 11b in addition to its part supported on at least part of the outer surface of the permeable layer 11b.

In the case where the permeable layer 11b is not provided, the catalyst layer 12 preferably has a part supported on at least part of the outer surface of the substrate 11a. The catalyst layer 12 may have a part supported on at least part of the inner surface of the substrate 11a in addition to its part supported on at least part of the outer surface of the substrate 11a.

The catalyst layer 12 includes catalyst particles 12a and hydrophobic particles 12b. The catalyst particles act to promote the electrochemical reduction reaction of CO₂/CO. The catalyst particles include a copper (Cu) component. In the catalyst particles, the copper component may be in a form capable of functioning as a catalytically active component, such as metallic copper, a copper-containing alloy, a copper-containing complex, or a copper-containing compound (e.g., Cu (OH)₂, Cu₂O, CuO).

For efficient CO₂/CO reduction reaction, metallic copper preferably makes up at least part of the copper component in the catalyst particles, more preferably makes up 50 mass % or more of the copper component, and even more preferably makes up 100 mass % of the copper component (i.e., the copper component is even more preferably metallic copper).

In addition to copper, the catalyst particles may include one, two, or more additional metal elements. Examples of such additional metal elements other than copper include gold (Au), platinum (Pt), palladium (Pd), silver (Ag), zinc (Zn), nickel (Ni), cobalt (Co), iron (Fe), aluminum (Al), tin (Sn), manganese (Mn), chromium (Cr), titanium (Ti), cadmium (Cd), indium (In), gallium (Ga), lead (Pb), ruthenium (Ru), and rhenium (Re). In the catalyst particles 12a, the additional metal element other than copper may be in a form capable of functioning as a catalytically active component, such as a metallic form, an alloy, a complex, or a compound (e.g., hydroxide, oxide).

In the catalyst layer, the catalyst particles have a mass per unit area (M₁) of 0.70 mg/cm² or more. With Mi as high as 0.70 mg/cm² or more, the catalyst particles can successfully promote the CO₂/CO reduction reaction and thus contribute to improved efficiency of C₂ compound production. On the other hand, with Mi less than 0.70 mg/cm², the catalyst particles may fail to achieve sufficient CO₂/CO reduction reaction. For high efficiency of C₂ compound production, Mi is preferably 1.00 mg/cm² or more, more preferably 1.50 mg/cm² or more. M₁ may have any specific upper limit. For example, Mi may have an upper limit of 7.00 mg/cm² or less or 6.00 mg/cm² or less.

The mass per unit area (M₁) of the catalyst particles is the amount (mass) of the catalyst particles in a unit area of the catalyst layer, which may also be referred to as the basis weight of the catalyst particles. Similarly, the mass per unit area (M₂) of the hydrophobic particles is the amount (mass) of the hydrophobic particles in a unit area of the catalyst layer, which may also be referred to as the basis weight of the hydrophobic particles. The Examples section below will describe a method for measuring the mass per unit area (M₁ and M₂).

The hydrophobic particles are effective in preventing the submergence of the catalyst layer and increasing the amount of three-phase interface. If the catalyst layer is composed only of the catalyst particles, which are hydrophilic, the electrolytic solution can enter the catalyst layer and submerge the catalyst particles. In such a case, a reduced amount of CO₂/CO (raw material gas) may reach the surface of the catalyst particles. In particular, CO gas, which is not soluble in a strongly alkaline electrolytic solution, may fail to reach the submerged catalyst layer. This may result in suppression of the three-phase (solid-liquid-gas) interface reactions of the CO₂/CO raw material (the reactions of Formulae (A) and (B) above), although the two-phase (solid-liquid) interface reaction of the water (H₂O) raw material (the reaction of Formula (C) above) may proceed to produce hydrogen (H₂) preferentially. With only the catalyst particles, therefore, the production of C₂ compounds, such as ethylene, may decrease.

In contrast, the catalyst layer containing the hydrophobic particles can be highly hydrophobic. This prevents the submergence of the catalyst layer and can increase the area of the three-phase interface. This will result in an increase in the C₂ compound production efficiency at high current density. Specifically, in the presence of the hydrophobic particles, CO₂/CO can exist in the form of bubbles in the catalyst layer. At and around the catalyst particles, CO₂/CO gas can exist at high concentration. This allows CO₂/CO to undergo high-rate reduction reaction and allows an increased amount of CO to adsorb on the catalyst particles. Thus, CO—CO coupling is promoted, which will result in an increase in the efficiency of production of C₂ compounds, such as ethylene.

The hydrophobic particles include a fluororesin. The fluororesin has high water repellency. The hydrophobic particles including a fluororesin with high water repellency can more reliably contribute to the formation of the three-phase interface and the effective promotion of the C₂ compound production. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymers, tetrafluoroethylene-ethylene copolymers, and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers. In particular, the fluororesin preferably includes polytetrafluoroethylene.

In the catalyst layer, the ratio (M₂/M₁) of the mass per unit area (M₂) of the hydrophobic particles to the mass per unit area (M₁) of the catalyst particles is 0.10 or more and 1.70 or less. With the ratio (M₂/M₁) adjusted within the specific range, the catalyst layer can provide desired three-phase interfaces and thus provide increased C₂ compound production efficiency. On the other hand, with a ratio (M₂/M₁) of less than 0.10, the catalyst layer may have too low a hydrophobic particle content and thus may be infiltrated by the electrolytic solution. In such a case, the two-phase (solid-liquid) interface reaction (the reaction of Formula (C) above) may preferentially occur, which may result in lower C₂ compound production efficiency, although hydrogen (H₂) production may occur. With a ratio (M₂/M₁) of more than 1.70, the catalyst layer may have too low a catalyst particle content. In such a case, the contact between the catalyst particles and the electrolytic solution may be insufficient, which may result in reduced H⁺supply from the electrolytic solution and make less likely for the CO₂/CO reduction reactions (the reactions of Formulae (A) and (B) above) to occur. This may also result in lower C₂ compound production efficiency. For high C₂ compound production efficiency, the ratio (M₂/M₁) is preferably 0.20 or more and 1.40 or less, more preferably 0.40 or more and 1.35 or less.

In the catalyst layer, the hydrophobic particles preferably have a mass per unit area (M₂) of 0.10 mg/cm² or more. With M₁ and M₂/M₁ falling within the specified ranges and with a high M₂ value, the catalyst layer can have high water repellency while maintaining the high reduction reaction nature of the catalyst particles. This makes it possible to further increase the efficiency of C₂ compound production. For high C₂ compound production efficiency, M₂ is preferably 0.50 mg/cm² or more, more preferably 0.70 mg/cm² or more. M₂ may have any upper limit. For example, M₂ may be 7.00 mg/cm² or less or 6.00 mg/cm² or less.

The catalyst layer preferably includes a mixture of the catalyst particles and the hydrophobic particles. In such a case, the interior of the catalyst layer can provide relatively uniform three-phase interfaces. In the mixture, hydrophobic particles exit near each catalyst particle. Thus, the three-phase interface forms near each of many catalyst particles, which will allow a further increase in C₂ compound production efficiency. This is also advantageous in that the catalyst layer can be easily formed by a simple method including: applying, to the gas diffusion layer, an ink containing the mixture of the catalyst particles and the hydrophobic particles to form a coating; and then drying the coating.

It should be noted that some voids communicating with the outside exist in at least some of the voids between the catalyst particles and the hydrophobic particles. In other words, the catalyst layer is porous, having pores communicating with the outside, which means having gas permeability. CO₂/CO and the electrolytic solution can enter into and exit from the catalyst layer via the voids communicating with the outside.

The catalyst layer may be composed only of the catalyst particles and the hydrophobic particles or may include an additional component in addition to the catalyst particles and the hydrophobic particles. Examples of the additional component include a binder (a binding agent), a dispersing agent, and a surfactant. However, the additional component content is preferably not too high so that the catalyst particles and the hydrophobic particles can surely act. The content of the additional component in the catalyst layer is preferably 20 mass % or less.

For high CO₂/CO permeability in the thickness direction of the catalyst layer and for high CO₂/CO-electrolyte contact efficiency, the catalyst layer preferably has a thickness of 200 μm or less, more preferably 60 μm or less. The thickness of the catalyst layer may have any lower limit. For example, the thickness of the catalyst layer may be 1 μm or more or 2 μm or more. As used herein, the term “the thickness of the catalyst layer” means the thickness of the part of the catalyst layer on the outer surface of the permeable layer or on the outer surface of the substrate.

The diffusion electrode according to an embodiment has the catalyst layer including: the catalyst particles including a specific material; and the hydrophobic particles including a specific material, in which the amounts of the catalyst particles and the hydrophobic particles are adjusted to fall within specific ranges. Thus, the diffusion electrode is effective for efficient production of C₂ compounds at high current density. In other words, the diffusion electrode can produce C₂ compounds with high Faraday efficiency at high current density (namely under high conversion rate conditions). For example, at a current density of 0.4 A/cm², the diffusion electrode can produce ethylene (C₂H₄) preferably with a Faraday efficiency of 50% or more, more preferably with a Faraday efficiency of 55% or more.

2. Method for Producing Gas Diffusion Electrode

The gas diffusion electrode according to an embodiment may be produced by any method that can satisfy the requirements described above. However, a preferred method for producing the gas diffusion electrode includes the steps of: preparing an ink including the catalyst particles, the hydrophobic particles, and a dispersion medium (ink preparation step); coating a surface of the gas diffusion layer with the ink to form a coating layer (coating step); and drying the coating to form a catalyst layer on the surface of the gas diffusion layer (drying step). Each of the steps will be described in detail below.

Ink Preparation Step

The ink preparation step includes preparing an ink including the catalyst particles, the hydrophobic particles, and a dispersion medium. The details of the catalyst particles and the hydrophobic particles are as described above. Specifically, the catalyst particles include the copper (Cu) component. The hydrophobic particles include the fluororesin. The dispersion medium may be water or an organic solvent. Preferably, an organic solvent, such as an alcohol solvent, is used for the ink to be applied to the surface of the gas diffusion layer having a water-repellent permeable layer. Examples of such an alcohol solvent include methanol, ethanol, propanol, and butanol. The dispersion medium may be a single solvent or a mixture of two or more solvents.

The ink may be prepared using a known technique. For example, the ink may be prepared by adding the catalyst particles and the hydrophobic particles to the dispersion medium; and dispersing them in the dispersion medium. In this process, if necessary, additives, such as a binder (binding agent), a dispersing agent, and a surfactant, may be added to the dispersion medium. The dispersion may be carried out using a known device, such as a homogenizer or an ultrasonic dispersion device.

Coating Step

The coating step includes coating a surface of the gas diffusion layer with the ink to form a coating layer. The ink coating may be carried out using a known method. Examples of such a method include bar coating, blade coating, screen printing, spray coating, curtain coating, and roll coating. The coating may be carried out only once or multiple times to form a coating layer with a desired thickness.

Drying Step

The drying step includes drying the coating layer. During the drying, the dispersion medium is vaporized and removed from the coating layer so that a catalyst layer including the catalyst particles and the hydrophobic particles is formed on the surface of the gas diffusion layer. The drying temperature may be selected depending on the type of the dispersion medium. For example, the drying temperature is 20° C. or more and 120° C. or less. The drying time is, for example, 0.5 hours or more and 24 hours or less.

This process produces the gas diffusion electrode according to an embodiment, which includes the gas diffusion layer; and the catalyst layer provided on the surface of the gas diffusion layer. In this process, the coating step and the drying step may each be carried out once, or a set of the coating step and the drying step may be repeated to form a thicker catalyst layer.

3. Electrochemical Reaction Device

An embodiment is directed to an electrochemical reaction device for use in electrochemical reduction of one or both of carbon dioxide (CO₂) and carbon monoxide (CO). The electrochemical reaction device includes a cathode; an anode; an anion exchange membrane provided between the cathode and the anode; a cathode-side liquid flow channel provided between the cathode and the anion exchange membrane; and an anode-side liquid flow channel provided between the anode and the anion exchange membrane. The cathode includes the gas diffusion electrode described above.

FIG. 2 is a schematic cross-sectional view of the electrochemical reaction device according to an embodiment. As shown in FIG. 2, the electrochemical reaction device 20 includes a cathode 21; an anode 22; an anion exchange membrane 23 provided between the cathode 21 and the anode 22; a liquid flow channel 28a provided between the cathode 21 and the anion exchange membrane 23 to allow a cathode-side electrolytic solution to flow; and a liquid flow channel 29a provided between the anode 22 and the anion exchange membrane 23 to allow an anode-side electrolytic solution to flow.

The electrochemical reaction device 20 may include a liquid flow channel structure 28 that forms the liquid flow channel 28a; and a liquid flow channel structure 29 that forms the liquid flow channel 29a. The electrochemical reaction device 20 may also include a gas flow channel structure 24 that forms a gas flow channel 24a; and a gas flow channel structure 25 that forms a gas flow channel 25a. The electrochemical reaction device 20 may also include power supply terminals 26 and 27.

The liquid flow channel structure 28 has a slit, and the slit space surrounded by the cathode 21, the anion exchange membrane 23, and the liquid flow channel structure 28 corresponds to the liquid flow channel 28a. The liquid flow channel structure 29 has a slit, and the slit space surrounded by the anode 22, the anion exchange membrane 23, and the liquid flow channel structure 29 corresponds to the liquid flow channel 29a.

The gas flow channel structure 24 has a groove on its cathode 21 side, and the groove space surrounded by the gas flow channel structure 24 and the cathode 21 corresponds to the gas flow channel 24a. The gas flow channel structure 25 has a groove on its anode 22 side, and the groove space surrounded by the gas flow channel structure 25 and the anode 22 corresponds to the gas flow channel 25a.

The electrochemical reaction device 20 has the liquid flow channel 28a between the cathode 21 and the anion exchange membrane 23; the liquid flow channel 29a between the anode 22 and the anion exchange membrane 23; the gas flow channel 24a between the cathode 21 and the power supply terminal 26; and the gas flow channel 25a between the anode 22 and the power supply terminal 27.

The power supply terminals 26 and 27 are each connected to a power source (not shown) for supplying power to the electrochemical reaction device 20. The gas flow channel structures 24 and 25 are each electrically conductive and thus can apply a voltage across the cathode 21 and the anode 22 from the power source.

The cathode 21 is an electrode that reduces CO₂/CO to carbon compounds and reduces water to hydrogen. The carbon compound product is liquid or gaseous, and the hydrogen product is gaseous.

The cathode 21 electrochemically reduces CO₂/CO and allows the gaseous carbon compound product and hydrogen to permeate through to the gas flow channel 24a. The liquid carbon compound product flows together with the cathode-side electrolytic solution A through the liquid flow channel 28a and flows out through the outlet of the liquid flow channel 28a and a liquid flow channel 63.

The cathode 21 includes a gas diffusion electrode 10. The cathode 21 may be composed of the gas diffusion electrode 10. The gas diffusion electrode 10 is as described above. The gas diffusion electrode 10 is disposed such that the substrate 11a is located on the gas flow channel 24a side and that the catalyst 12 is located on the liquid flow channel 28a side.

The anode 22 is an electrode that oxidizes hydroxide ions to produce oxygen. The oxygen product is gaseous. The anode 22 electrochemically oxidizes hydroxide ions and allows the oxygen product to permeate through to the gas flow channel 25a.

For example, the anode 22 may be an electrode including: a gas diffusion layer; and a catalyst provided on the liquid flow channel 29a side of the gas diffusion layer (hereinafter also referred to as the “anode catalyst”). 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. The gas diffusion layer may be the same as the gas diffusion layer 11 or may be a known gas diffusion layer different from the gas diffusion layer 11.

The anode catalyst may be a known anode catalyst.

Examples of such an anode catalyst include metals, such as platinum, palladium, and nickel, alloys or intermetallic compounds of these metals, 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 ruthenium complexes and rhenium complexes. The anode catalyst may include one of these materials or a combination of two or more of these materials.

The gas diffusion layer of the anode 22 may be, for example, a carbon paper sheet or a carbon cloth. The gas diffusion layer of the anode 22 may also be a porous material, such as a mesh material, a perforated material, or a metal fiber sintered material. For example, such a porous material may be made of a metal, such as titanium, nickel, or iron, or an alloy, such as stainless steel (SUS).

For example, the liquid flow channel structures 28 and 29 may be made of a fluororesin, such as polytetrafluoroethylene. The gas flow channel structures 24 and 25 may be made of a metal, such as titanium or SUS, or carbon. For example, the power supply terminals 26 and 27 may be made of a metal, such as copper, gold, titanium, or SUS, or carbon. The power supply terminals 26 and 27 may each include a copper member with its surface having undergone plating, such as gold plating. The anion exchange membrane 23 may be a known anion exchange membrane.

The electrochemical reaction device 20 includes a flow cell that allows the cathode-side electrolytic solution A to be supplied through the liquid flow channel 64 and to flow through the liquid flow channel 28a, allows the anode-side electrolytic solution B to be supplied through the liquid flow channel 65 and to flow through the liquid flow channel 29a, and allows the raw material gas G to be supplied through the gas flow channel 76 and to flow through the gas flow channel 24a.

The liquid flow channel 64 has one end connected to the electrochemical reaction device 20 and another end connected to a supply device (not shown) for supplying the cathode-side electrolytic solution A to the electrochemical reaction device 20. The liquid flow channel 65 has one end connected to the electrochemical reaction device 20 and another end connected to a supply device (not shown) for supplying the anode-side electrolytic solution B to the electrochemical reaction device 20. The gas flow channel 76 has one end connected to the electrochemical reaction device 20 and another end connected to a supply device (not shown) for supplying the raw material gas G to the electrochemical reaction device 20.

The cathode-and anode-side electrolytic solutions A and B may each be an alkaline aqueous solution. Examples of such an 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 alkaline aqueous solution is preferably a potassium hydroxide aqueous solution, in which carbon dioxide is highly soluble and which can suppress the reduction reaction of water to hydrogen.

The cathode-and anode-side electrolytic solutions A and B may each have any appropriately adjusted pH. Preferably, the anode-side electrolytic solution B has a pH lower than that of the cathode-side electrolytic solution A. For example, the cathode-side electrolytic solution A has a pH of more than 14, and the anode-side electrolytic solution B has a pH of 14 or less (specifically, 8 or more and 14 or less). The pH of the electrolytic solution may be adjusted by adding an alkali or an alkali aqueous solution to the electrolytic solution to increase pH, dissolving carbon dioxide in the electrolytic solution to reduce pH, or any other method.

The cathode-side electrolytic solution A supplied through the liquid flow channel 64 may be an alkaline aqueous solution (e.g., a potassium hydroxide aqueous solution) with an alkali concentration of 0.1 mol/L or more and 5 mol/L or less, preferably 1 mol/L or more and 3 mol/L or less, for example. The cathode-side electrolytic solution A supplied through the liquid flow channel 64 may have any appropriately adjusted temperature, which is, for example, 10° C. or more and 60° C. or less. The cathode-side electrolytic solution A supplied through the liquid flow channel 64 may have any appropriately adjusted flow rate, which is, for example, 10 mL/min or more and 100 mL/min or less.

The anode-side electrolytic solution B supplied through the liquid flow channel 65 may be an alkaline aqueous solution (e.g., a potassium hydroxide aqueous solution) with an alkali concentration of 0.1 mol/L or more and 5 mol/L or less, preferably 1 mol/L or more and 3 mol/L or less, for example. The anode-side electrolytic solution B supplied through the liquid flow channel 65 may have any appropriately adjusted temperature, which is, for example, 10° C. or more and 60° C. or less. The anode-side electrolytic solution B supplied through the liquid flow channel 65 may have any appropriately adjusted flow rate, which is, for example, 10 mL/min or more and 100 mL/min or less.

The raw material gas G supplied through the gas flow channel 76 includes CO₂/CO. The raw material gas G supplied through the gas flow channel 76 may include either CO₂ or CO or may include both CO₂ and CO. The raw material gas G supplied through the gas flow channel 76 may include CO₂. In such a case, the raw material gas G may have any appropriately adjusted CO₂ concentration, which is, for example, 1% by volume or more and 100% by volume or less. The raw material gas G supplied through the gas flow channel 76 may include CO. In such a case, the raw material gas G may have any appropriately adjusted CO concentration, which is, for example, 1% by volume or more and 100% by volume or less. The raw material gas G supplied through the gas flow channel 76 may have any appropriately adjusted temperature, which is, for example, 10° C. or more and 60° C. or less. The raw material gas G supplied through the gas flow channel 76 may have any appropriately adjusted flow rate, which is, for example, 50 mL/min or more and 500 mL/min or less.

After being supplied through the liquid flow channel 64, the cathode-side electrolytic solution A flows through the liquid flow channel 28a and flows out through the outlet of the liquid flow channel 28a and the liquid flow channel 63. After being supplied through the liquid flow channel 65, the anode-side electrolytic solution B flows through the liquid flow channel 29a and flows out through the outlet of the liquid flow channel 29a and the liquid flow channel 66.

At the cathode 21, CO₂/CO in the raw material gas G and water are reduced to carbon compounds and hydrogen, respectively, when the cathode-side electrolytic solution A, the anode-side electrolytic solution B, and the raw material gas G are allowed to flow through the liquid flow channel 28a, the liquid flow channel 29a, and the gas flow channel 24a, respectively, and a voltage is applied across the cathode 21 and the anode 22. The carbon compound product is liquid or gaseous, and the hydrogen product is gaseous. At the anode 22, the hydroxide ions in the anode-side electrolytic solution B are oxidized to produce oxygen. The gaseous product E including gaseous carbon compounds and hydrogen flows to the gas flow channel 24a through the gas diffusion layer of the cathode 21 (the gas diffusion layer 11 of the gas diffusion electrode 10) and flows out through the outlet of the gas flow channel 24a and the gas flow channel 67. The product E flowing out of the electrochemical reaction device 20 may be fed to a reactor (not shown) and brought into gas-phase contact with an olefin polymerization catalyst in the reactor for polymerization of ethylene. The liquid carbon compound product produced at the cathode 21 flows together with the cathode-side electrolytic solution A through the liquid flow channel 28a and flows out through the outlet of the liquid flow channel 28a and the liquid flow channel 63.

Examples of the carbon compound product resulting from the reduction of CO₂/CO at the cathode 21 include C1 compounds (compounds having one carbon atom) and C₂ compounds (compounds having two carbon atoms).

Examples of the C₂ compound product resulting from the reduction of CO₂/CO at the cathode 21 include acetic acid (CH₃COOH), acetic acid salts (e.g., alkali metal salts of acetic acid, such as sodium acetate and potassium acetate), acetaldehyde (CH₃CHO), ethanol (C₂H₅OH), and ethylene (C₂H₄). Of these compounds, ethylene is preferred for its usefulness in the chemical industry. Thus, the C₂ compound product resulting from the reduction of CO₂/CO preferably includes ethylene. The C₂ compound product resulting from the reduction of CO₂/CO may include one, two, or more additional compounds in addition to ethylene. The C₂ compound product may include ethylene in a gaseous form and ethanol and acetic acid each in a liquid form. The type of the acetic acid salt product depends on the type of the electrolytic solution used. For example, the acetic acid salt product is sodium acetate in a case where the electrolytic solution contains sodium ions, and the acetic acid salt product is potassium acetate in a case where the electrolytic solution contains potassium ions.

Examples of the C1 compound product resulting from the reduction of carbon dioxide (CO₂) at the cathode 21 include carbon monoxide (CO), formic acid (HCOOH), formic acid salts (e.g., alkali metal salts of formic acid, such as sodium formate and potassium formate), formaldehyde (HCHO), methanol (CH₃OH), and methane (CH₄). The C1 compound product resulting from the reduction of CO₂ may include one, two, or more compounds. For example, the C1 compound product may include CO and methane each in a gaseous form and methanol and formaldehyde each in a liquid form. The type of the formic acid salt product depends on the type of the electrolytic solution used. For example, the formic acid salt product is sodium formate in a case where the electrolytic solution contains sodium ions, and the formic acid salt product is potassium formate in a case where the electrolytic solution contains potassium ions.

Examples of the C1 compound product resulting from the reduction of carbon monoxide (CO) at the cathode 21 include formaldehyde (HCHO), methanol (CH₃OH), and methane (CH₄). The C1 compound product resulting from the reduction of CO may include one, two, or more compounds. For example, the C₁ compound product may include methane in a gaseous form and methanol and formaldehyde each in a liquid form.

FIG. 3 shows an example of the material flow in the electrolysis cell of the electrochemical reaction device 20. As shown in FIG. 3, at the cathode 21, CO₂/CO in the raw material gas G is reduced to gaseous and liquid carbon compounds, and hydrogen is produced. After being produced at the cathode 21, the product E including gaseous carbon compounds and hydrogen flows to the gas flow channel 24a through the gas diffusion layer of the cathode 21 (the gas diffusion layer 11 of the gas diffusion electrode 10) and flows out through the outlet of the gas flow channel 24a and the gas flow channel 67. After being produced at the cathode 21, the liquid carbon compound product flows together with the cathode-side electrolytic solution A through the liquid flow channel 28a and flows out through the outlet of the liquid flow channel 28a and the liquid flow channel 63.

After being produced at the cathode 21, hydroxide ions (OH—) move to the anode 22 through the anode-side electrolytic solution B and are oxidized to produce oxygen (O₂) according to the reaction of Formula (D) below. The oxygen product permeates the gas diffusion layer of the anode 22, reaches the gas flow channel 25a, and flows out through the outlet of the gas flow channel 25a.

4OH⁻→O₂+2H₂O  (D)

EXAMPLES

The present invention will be described in more detail with reference to examples below. It should be noted that the examples are not intended to limit the present invention.

(1) Preparation of Gas Diffusion Electrode

Example 1

A gas diffusion layer (Sigracet 39 BB, SGL Carbon) was obtained commercially. The gas diffusion layer (Sigracet 39 BB) includes a carbon paper sheet and a fluorine-containing micro-porous layer (MPL) provided on one surface of the carbon paper sheet, in which the micro-porous layer has a thickness of 315 μm and a fluorine content of 14 mass %. The gas diffusion layer (Sigracet 39 BB) was then cut to form a 37 mm square piece, which was used as a gas diffusion layer for a gas diffusion electrode. Separately, metallic copper particles (Cu particles) and polytetrafluoroethylene particles (PTFE particles) were provided. The Cu particles had an average particle size (average primary particle size) of 25 nm, and the PTFE particles had an average particle size (average primary particle size) of 1,000 nm. Also provided were 2-propanol (dispersion medium) and dissolved Nafion® with a binder content of 5 mass %. The Cu particles (37.6 mg) and the PTFE particles (33.6 mg) were added to 2-propanol (5 mL), and the resulting mixture was subjected to ultrasonic dispersion for 10 minutes. The dissolved Nafion® (274704-100ML manufactured by Sigma-Aldrich) (5 μL) was added to the resulting dispersion mixture, and the resulting mixture was subjected to ultrasonic dispersion for 30 minutes to form an ink.

Subsequently, the gas diffusion layer (Sigracet 39 BB) was placed on a hot plate heated at 80° C. with its MPL surface facing upward, and the ink was applied over the upper surface of the gas diffusion layer by spray coating. The gas diffusion layer with the ink coating was dried in a drying machine at 80° C. for 1 hour. As a result, a gas diffusion electrode was obtained. The thickness of the resulting catalyst layer was 15 μm.

Example 2

A gas diffusion electrode was prepared using the same procedure as in Example 1, except that 16.7 mg of the PTFE particles were used instead when the ink was prepared. The thickness of the resulting catalyst layer was 11 μm.

Example 3

A gas diffusion electrode was prepared using the same procedure as in Example 1, except that 50.2 mg of the PTFE particles and 4 mL of 2-propanol were used instead when the ink was prepared. The thickness of the resulting catalyst layer was 18 μm.

Example 4

A gas diffusion electrode was prepared using the same procedure as in Example 1, except that 75.2 mg of the Cu particles, 66.8 mg of the PTFE particles, and 10 μL of the dissolved Nafion® (274704-100ML manufactured by Sigma-Aldrich) were used instead when the ink was prepared. The thickness of the resulting catalyst layer was 25 μm.

Example 5

A gas diffusion electrode was prepared using the same procedure as in Example 1, except that 85.1 mg of the Cu particles, 75.0 mg of the PTFE particles, and 7 mL of 2-propanol were used instead when the ink was prepared. The thickness of the resulting catalyst layer was 38 μm.

Example 6

A gas diffusion electrode was prepared using the same procedure as in Example 1, except that 102.0 mg of the Cu particles, 90.1 mg of the PTFE particles, and 7 mL of 2-propanol were used instead when the ink was prepared. The thickness of the resulting catalyst layer was 46 μm.

Example 7

A gas diffusion electrode was prepared using the same procedure as in Example 1, except that 32.3 mg of the Cu particles and 3.8 mg of the PTFE particles were used instead when the ink was prepared. The thickness of the resulting catalyst layer was not more than 10 μm.

Example 8

A gas diffusion electrode was prepared using the same procedure as in Example 1, except that 32.3 mg of the Cu particles and 7.3 mg of the PTFE particles were used instead when the ink was prepared. The thickness of the resulting catalyst layer was not more than 10 μm.

Comparative Example 1

A gas diffusion electrode was prepared by sputtering Cu on the MPL surface of the gas diffusion layer (Sigracet 39 BB). The sputtering conditions were as shown below. The thickness of the resulting catalyst layer was not more than 10 μm.

• • Sputtering method: DC magnetron sputtering
  • Exhaust system: Rotary pump+cryopump
  • Target material: Cu
  • Target size: 48 inches
  • Sputtering rate: 0.8 nm/see
  • Pre-sputtering: 5 min
  • Sputtering time: 33 sec
  • Gas diffusion layer temperature: 25° C.
  • Target Cu sputtering film thickness: 25 nm

Comparative Example 2

The ink preparation procedure was changed as follows. Metallic copper particles (Cu particles), polytetrafluoroethylene particles (PTFE particles), and carbon black (Vulcan XC-72 manufactured by CABOT) were provided. The Cu particles had an average particle size (average primary particle size) of 25 nm. The PTFE particles had a particle size distribution (primary particle size distribution) ranging from 25 nm to 50 nm. Also provided were 2-propanol (dispersion medium) and dissolved Nafion® with a binder content of 5 mass %. The Cu particles (6.0 mg), the PTFE particles (5.1 mg), and the carbon black (6.2 mg) were mixed with 2-propanol (3 mL), and the resulting mixture was subjected to ultrasonic dispersion for 10 minutes. The dissolved Nafion® (274704-100ML manufactured by Sigma-Aldrich) (120 μL) was added to the resulting dispersion mixture, and the resulting mixture was subjected to ultrasonic dispersion for 30 minutes to form an ink. A gas diffusion electrode was prepared using the same procedure as in Example 1, except that the resulting ink was used. The thickness of the resulting catalyst layer was not more than 10 μm.

Comparative Example 3

A gas diffusion electrode was prepared using the same procedure as in Example 1, except that 37.6 mg of the Cu particles, 66.9 mg of the PTFE particles, and 4 mL of 2-propanol were used instead when the ink was prepared. The thickness of the resulting catalyst layer was 20 μm.

Comparative Example 4

A gas diffusion electrode was prepared using the same procedure as in Example 1, except that 6.2 mg of the Cu particles, 5.1 mg of the PTFE particles, 1 μL of the dissolved Nafion® (274704-100ML manufactured by Sigma-Aldrich), and 3 mL of 2-propanol were used instead when the ink was prepared.

The thickness of the resulting catalyst layer was 19 μm.

(2) Evaluation

The gas diffusion electrodes obtained in Examples 1 to 8 and Comparative Examples 1 to 4 were evaluated for various characteristics as follows.

Mass Per Unit Area

Equations (1) to (5) below were used to measure the mass per unit area (M₁) of the catalyst particles in the catalyst layer and the mass per unit area (M₂) of the hydrophobic particles in the catalyst layer. The measurement results are shown in Table 1.

Content ⁢ X M ⁢ 1 = ( The ⁢ mass ⁢ of ⁢ the ⁢ catalyst ⁢ particles ⁢ in ⁢ the ⁢ dispersion ⁢ mixture )/( the ⁢ total ⁢ mass ⁢ of ⁢ the ⁢ catalyst ⁢ particles , the ⁢ hydrophobic ⁢ particles , and ⁢ the ⁢ binder ⁢ in ⁢ the ⁢ dispersion ⁢ mixture ) Content ⁢ X M ⁢ 2 = ( The ⁢ mass ⁢ of ⁢ the ⁢ hydrophobic ⁢ particles ⁢ in ⁢ the ⁢ dispersion ⁢ mixture )/( the ⁢ total ⁢ mass ⁢ of ⁢ the ⁢ catalyst ⁢ particles , the ⁢ hydrophobic ⁢ particles , and ⁢ the ⁢ binder ⁢ in ⁢ the ⁢ dispersion ⁢ mixture ) The ⁢ mass ⁢ Y ⁢ of ⁢ the ⁢ catalyst ⁢ layer = ( The ⁢ mass ⁢ of ⁢ the ⁢ gas ⁢ diffusion ⁢ electrode ) - ( the ⁢ mass ⁢ of ⁢ the ⁢ gas ⁢ diffusion ⁢ layer ) ( 3 ) M 1 = Y × X M ⁢ 1 / ( the ⁢ gas ⁢ diffusion ⁢ electrode ⁢ area ) ( 4 ) M 2 = Y × X M ⁢ 2 / ( the ⁢ gas ⁢ diffusion ⁢ electrode ⁢ area ) ( 5 )

Catalyst Layer Thickness

The thickness of the catalyst layer was calculated by subtracting the thickness of the gas diffusion layer from the total thickness of the gas diffusion electrode. The thickness of each layer was measured using a micrometer (MDC-25PX manufactured by Mitutoyo Corporation). The measurement result is shown above in each of the examples and the comparative examples. It should be noted that values less than 10 μm, which are blow the detection limit, are expressed as “not more than 10 μm”.

CO₂ Electrolysis Test

A CO electrolysis test was conducted using the electrochemical reaction device 20 shown in FIG. 2, equipped, as the cathode, with the gas diffusion electrode of each of Examples 1 to 8 and Comparative Examples 1 to 4. A 1 mol/L potassium hydroxide (KOH) aqueous solution was supplied as the anode-side electrolytic solution B at a flow rate of 50 mL/min through the liquid flow channel 65. The cathode 21 was the gas diffusion electrode of each of Examples 1 to 8 and Comparative Examples 1 to 4. The anode 22 was a Ni foam (EQ-bcnf-03 manufactured by MTI Corporation), which was a commercially available porous metal member. The anion exchange membrane 23 was Sustainion X37-50 Grade RT (manufactured by Dioxide Materials). Carbon monoxide gas was supplied as the raw material gas G at a flow rate of 300 mL/min through the gas flow channel 76. The CO electrolysis test was conducted under the conditions below.

• • Electrolysis conditions: Constant current electrolysis
  • Current density: 0.4 A/cm²
  • Electrolysis time: 30 minutes

During the CO electrolysis test, the in-line measurement of the concentration of the gaseous product through the gas flow channel 67 was performed at a time point 20 minutes after the start of the test using gas column chromatography (990 Micro GC manufactured by Agilent). At this time point, hydrogen (H₂), carbon monoxide (CO), and methane (CH₄) were quantified using a Molsieve 5A column (manufactured by GL Sciences Inc.) and using argon as a carrier gas. Ethylene (C₂H₄) was quantified using a PoraPLOT Q column (manufactured by GL Sciences Inc.) and using helium as a carrier gas. The measured concentrations were each used to determine the amount (mol) of each product produced at the time point 20 minutes after the start of the test.

The Faraday efficiency (%) for each product was determined according to Equation (6) below. In Equation (6) below, a “predetermined period” is 20 minutes.

Faraday ⁢ efficiency ⁢ ( % ) ⁢ for ⁢ each ⁢ product = ( ( The ⁢ amount ⁢ ( mol ) ⁢ of ⁢ each ⁢ product ⁢ produced ⁢ at ⁢ ⁢ the ⁢ time ⁢ point ⁢ when ⁢ a ⁢ predetermined ⁢ period ⁢ has ⁢ elapsed ) × n / ( the ⁢ number ⁢ ( mol ) ⁢ of ⁢ electrons ⁢ consumed ⁢ at ⁢ the ⁢ time ⁢ ⁢ point ⁢ when ⁢ a ⁢ predetermined ⁢ period ⁢ has ⁢ elapsed ) ) × 100 ( 6 )

In Equation (6), n is the number of electrons required to produce each product. Specifically, n is the coefficient attached to e—in the reaction formula for the production of each product. Ethylene and other products are produced according to the reaction formulae below.

2H⁺+2e⁻→H₂

CO₂+2H⁺+2e⁻→CO+H₂O

CO₂+8H⁺+8e⁻→CH₄+2H₂O

2CO₂+12H⁺+12e⁻→C₂H₄+4H₂O

2CO₂+14H⁺+14e⁻→C₂H₆+4H₂O

CO+6H⁺+6e⁻→CH₄+H₂O

2CO+8H⁺+8e⁻→C₂H₄+2H₂O

2CO+10H⁺+10e⁻→C₂H₆+2H₂O

(3) Evaluation Results

The evaluation results obtained for Examples 1 to 8 and Comparative Examples 1 to 4 are summarized in Table 1 below. Table 1 shows the masses per unit area of the catalyst particles (Cu particles) and the hydrophobic particles (PTFE particles) and the ratio between the masses per unit area. Table 1 also shows the Faraday efficiency for ethylene (C₂H₄) production at a current density of 0.4 A/cm^2.

A relatively high Faraday efficiency of at least 45% was achieved for Examples 1 to 8 where the mass per unit area of the catalyst particles fell within the range according to an embodiment (0.70 mg/cm² or more) and the ratio of the mass per unit area of the hydrophobic particles to the mass per unit area of the catalyst particles fell within the range according to an embodiment (0.10 or more and 1.70 or less).

On the other hand, a relatively low Faraday efficiency of at most 38% was obtained for Comparative Examples 1 to 4 where the mass per unit area of the catalyst particles and the ratio between the masses per unit area did not fall within the specified ranges according to embodiments.

TABLE 1
Gas Diffusion Electrode Preparation Conditions and Characteristics

	Mass per unit	Mass per unit	Ratio	Faraday
	area M1 of Cu	area M2 of	between	efficiency
	particles	PTFE particles	masses per	at 0.4 A/
	(mg/cm2)	(mg/cm2)	unit area1)	cm2 (%)

Example 1	1.96	1.72	0.88	50
Example 2	2.00	0.88	0.44	56
Example 3	2.20	2.86	1.30	56
Example 4	3.58	3.18	0.89	62
Example 5	4.90	4.30	0.88	56
Example 6	5.80	5.10	0.88	55
Example 7	2.00	0.22	0.11	46
Example 8	2.00	0.44	0.22	48
Comparative	2.10	0.00	0.00	25
Example 1
Comparative	0.55	0.47	0.80	2
Example 2
Comparative	2.10	3.78	1.80	38
Example 3
Comparative	0.38	0.34	0.90	14
Example 4
Note1):
The ratio between masses per unit area is the ratio of the mass per unit area of PTEF particles to the mass per unit area of Cu particles (M2/M1)

The results shown above suggest that the gas diffusion electrode and the electrochemical reaction device according to embodiments enable efficient production of C₂ compounds at high current density.

EXPLANATION OF REFERENCE NUMERALS

• • 10: Gas diffusion electrode
  • 11: Gas diffusion layer
  • 11a: Substrate
  • 11b: Permeable layer
  • 12: Catalyst layer
  • 12a: Catalyst particles
  • 12b: Hydrophobic particles
  • 20: Electrochemical reaction device
  • 21: Cathode
  • 22: Anode
  • 23: Anion exchange membrane
  • 24: Gas flow channel structure
  • 24a: Gas flow channel
  • 25: Gas flow channel structure
  • 25a: Gas flow channel
  • 26: Power supply terminal
  • 27: Power supply terminal
  • 28: Liquid flow channel structure
  • 28a: Liquid flow channel
  • 29: Liquid flow channel structure
  • 29a: Liquid flow channel
  • A: Cathode-side electrolytic solution
  • B: Anode-side electrolytic solution
  • G: Raw material gas including CO₂/CO