Gas Diffusion Electrode and Electrochemical Reaction Device

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.

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

In recent years, there has been a growing movement to reduce the emission of greenhouse gases such as carbon dioxide (CO) and carbon monoxide (CO) on a global scale. An attention-drawing solution to reducing greenhouse gas emissions includes the electrochemical reduction reaction of COand/or CO (hereinafter also referred to as “CO/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 CO/CO (hereinafter also referred to as the “CO/CO reduction reaction”).

Copper (Cu) is known to be suitable as a catalyst material for use in gas diffusion electrodes for promoting the CO/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.

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

In light of these conventional problems, the inventors have made active investigations. As a result, the inventors have found that Ccompounds 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 Ccompounds 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 (M) of 0.70 mg/cmor more in the catalyst layer, the hydrophobic particles having a mass per unit area (M) with a ratio (M/M) of Mto Mbeing 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 (M) of the hydrophobic particles in the catalyst layer is 0.10 mg/cmor 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 Ccompounds at high current density and provides an electrochemical reaction device including such a gas diffusion electrode.

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

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

The CO/CO reduction reaction may include reduction of CO/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 Ccompounds (compounds having one carbon atom) and Ccompounds (compounds having two carbon atoms).

Examples of Ccompounds that can be produced by the reduction of CO/CO include acetic acid (CHCOOH), acetic acid salts (e.g., alkali metal acetates such as sodium acetate and potassium acetate), acetaldehyde (CHCHO), ethanol (CHOH), and ethylene (CH). Of these compounds, ethylene is preferred for its usefulness in the chemical industry. In other words, the Ccompound product of the CO/CO reduction preferably includes ethylene. The Ccompound product of the CO/CO reduction may include ethylene and one, two, or more additional compounds. The Ccompound 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 Ccompounds that can be produced by the reduction of carbon dioxide (CO) 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 (CHOH), and methane (CH). The Ccompound product of the COreduction may include one, two, or more compounds. For example, the Ccompound 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 Ccompounds that can be produced by the reduction of carbon monoxide (CO) include methanol (CHOH), formaldehyde (HCHO), and methane (CH). The Ccompound product of the CO reduction may include one, two, or more compounds. For example, the Ccompound 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 (CO) to carbon monoxide (CO), the electrochemical reduction reaction of carbon monoxide (CO) to ethylene (CH), and the electrochemical reduction reaction of water (HO) to hydrogen (H), respectively.

CO+2H+2e→CO+HO  (A)

2CO+8H+8e→CH+2HO  (B)

2HO+2e→H+2OH  (C)

The reduction reaction of CO/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.shows a gas diffusion electrodeincluding: a gas diffusion layer; and a catalyst layerprovided on a surface of the gas diffusion layer. The catalyst layerincludes catalyst particlesand hydrophobic particles

As shown in, the gas diffusion layerincludes a substrate. The substrateis, for example, in the form of a sheet. The substratein 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 substratepreferably has a minimum thickness and a maximum thickness each within the range shown above.

The substrateis gas-permeable. The gas-permeable substrateallows efficient supply of a raw material gas including CO/CO to the catalyst layer. The gas-permeable substratealso allows efficient collection of a gaseous product produced by the CO/CO reduction reaction and allows efficient collection of hydrogen produced by the reduction of water.

For effective gas permeability, the substrateis 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 substratemay have electrical conductivity. The substrate with electrical conductivity contributes to efficient CO/CO reduction reaction.

For effective electrical conductivity, the substratepreferably contains an electrically conductive material. The substratemay 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 substratemay 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 substratemay 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, the gas diffusion layerpreferably has a permeable layerprovided on the surface of the substrate. The permeable layermay be provided at least part of the surface of the substrate. The permeable layeris highly permeable. The permeable layerhas pores with an average pore diameter smaller than that of the substrateand has a surface area greater than that of the substrate. Thus, a larger amount of the catalyst layercan be supported on the permeable layer

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

The permeable layerpreferably has a part provided on at least part of the outer surface of the substrate. The permeable layermay have a part provided in at least part of the inner surface of the substratein addition to its part provided on at least part of the outer surface of the substrate

In a case where the substrateis in the form of a sheet, the permeable layeris preferably provided on one surface of the substrate, more preferably provided on at least part of one outer surface of the substrate. The permeable layeronly has to be provided on at least part of one surface of the substrate

The portion that forms the permeable layerand is provided on the outer surface of the substratetypically 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 layerand is provided on the outer surface of the substratepreferably has a minimum thickness and a maximum thickness each within the range shown above.

The permeable layeris gas-permeable. The gas-permeable layerallows efficient supply of a raw material gas including CO/CO to the catalyst layer. The gas-permeable layeralso allows efficient collection of a gaseous product produced by the CO/CO reduction reaction and allows efficient collection of hydrogen produced by the reduction of water.

For effective gas permeability, the permeable layeris 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 substrateand has a density higher than that of the substrate

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

When provided to improve the water repellency of the gas diffusion layer, the permeable layerpreferably 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 layerpreferably 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. The mass of fluorine in the permeable layercan 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.