CO2 ELECTROLYSIS DEVICE, AND METHOD FOR MANUFACTURING CO2 ELECTROLYSIS PRODUCT

Abstract

Provided are a technique related to a CO2 electrolysis device that is more excellent in production efficiency for producing a reduction product (CO or the like) from CO2, and a technique related to a CO2 electrolysis device that can suppress a decrease in CO2 reduction efficiency and be stably operated. An aspect of the present invention is a CO2 electrolysis device including an electrode material. The CO2 electrolysis device includes: a support including a conductive carrier and a catalyst supported on the conductive carrier, the catalyst including any one or more of particles of a metal complex, a metal, or an inorganic compound; and an anion exchange resin covering a part or the whole of the surface of the support and including an ionomer of the following formula (1): where m and n represent natural numbers of 1 to 200.

Description

TECHNICAL FIELD

The present disclosure relates to a CO₂ electrolysis device and a method for producing a CO₂ electrolysis product.

BACKGROUND ART

Fetal fuels (oil, coal, natural gas) support a modem energy consuming society. The extraction of energy from fossil fuels involves the emission of CO₂ (carbon dioxide). An increase in the concentration of carbon dioxide in the atmosphere is said to be one of causes of global warming, and reduction of the concentration is required. CO₂ is an extremely stable substance, and thus it is difficult to reuse CO₂ by decomposition or the like, and there is a need for a new technology for converting CO₂ into another substance and recycling it again.

As one of the technologies, research on CO₂ using electric energy has been widely conducted all over the world. A CO₂ reduction device having a polymer electrolyte electrolytic cell has been found to be superior to other devices in that movement resistance of ions can be sufficiently lowered by using a thin film polymer electrolyte (Patent Literature 1). In general, a negative electrode for CO₂ reduction used in a polymer electrolyte electrolytic cell contains catalyst fine particles and a conductive carrier.

In CO₂ reduction, the CO₂ adsorption amount in the vicinity of the CO₂ reduction catalyst strongly contributes to the production efficiency of reduction products such as CO (carbon monoxide), and it is desirable to develop an electrode catalyst that can adsorb a large amount of CO₂. For example, a method has been devised in which a compound having a property of interacting with CO₂, such as adsorption, is co-supported on an electrode with a catalyst to increase the adsorption amount of weakly acidic CO₂ and to improve the production efficiency (Patent Literatures 2 and 3 and Non-Patent Literature 1).

CITATION LIST

Patent Literature

• Patent Literature 1: JP 2019-515142 A
• Patent Literature 2: JP 2019-011492 A
• Patent Literature 3: JPWO 2019/065258 A1

Non Patent Literature

• Non Patent Literature 1: S. Ren, D. Joulie, D. Salvatore, K. Torbensen, M. Wang, M. Robert, C. P. Berlinguette, Science, 2019, 365. 367-369.

SUMMARY OF INVENTION

Technical Problem

In the invention disclosed in Patent Literature 1, it is proposed to use various ion exchange membranes as a thin-film polymer electrolyte. An ion exchange membrane for such an application is required to have high ion conductivity, high toughness, high chemical resistance, heat resistance, an appropriate water content, and the like, but there is concern that the ion exchange membrane proposed in Patent Literature 1 is insufficient. For example, low ion conductivity decreases the transport efficiency of ions consumed or generated in association with the CO₂ electrolytic reduction reaction, and the reaction rate is likely to decrease. In addition, insufficient toughness, chemical resistance, or heat resistance is likely to cause the electrode not to withstand the reaction conditions of CO₂ reduction. Furthermore, low selective permeability of the ion exchange membrane or high water content may arise a problem in that the electrolytic solution permeates from the positive electrode side and overflows to the negative electrode side. Furthermore, there is a possibility that the electrolyte is precipitated as a salt. These phenomena hinder the supply of CO₂ and cause a decrease in CO₂ reduction efficiency, and thus cause hindrance to stable operation of the CO₂ electrolysis device.

In the inventions disclosed in Patent Literatures 2 and 3, it is difficult to maintain a large number of low concentration CO₂ in the vicinity of the reduction catalyst under the condition that the CO₂ partial pressure (that is, CO₂ concentration) to be supplied is reduced, and the production efficiency of the reduction product and there is a possibility that the CO₂ conversion rate of the CO₂ electrolysis device is reduced.

In addition, when a carbon dioxide reduction membrane containing a proton permeable polymer which is a cation exchange resin is used for the electrode on the negative electrode side as in the carbon dioxide reduction device disclosed in Patent Literature 2, the cation exchange resin is acidic, and therefore a side reaction (hydrogen generation reaction, it easily progresses in something acid) may easily occur. Furthermore, the cation exchange resin is acidic, and thus there is no CO₂ adsorption capability, and there is a possibility that both the ion conductivity and the CO₂ adsorption capability are not achieved. This is apparent from the data that the addition of Nafion, which is a cation exchange resin, decreases the amount of adsorbed CO₂ in Reference Example 5 (paragraph 0061) of Patent Literature 2.

In addition, metal ions permeate the cation exchange resin, and thus deposition of an electrolyte salt easily occurs, and accumulation of the deposited salt may decrease the carbon dioxide electrolysis efficiency.

Therefore, an object of the present disclosure is to provide a technology related to a CO₂ electrolysis device that is more excellent in production efficiency of producing a reduction product (CO or the like) from CO₂. Furthermore, an object thereof is to provide a technology related to a CO₂ electrolysis device that can suppress a decrease in CO₂ reduction efficiency and be stably operated.

The anion exchange resin is basic, and thus the side reaction is less likely to occur. The present inventors have performed intensive investigations for achieving the above object, and as a result, have found that the above problem can be solved by using a support containing a catalyst and a conductive carrier, the support coated with a specific anion exchange resin, for an electrode material. In addition, the present inventors have found that the above problem can be solved by using an anion exchange membrane using the same anion exchange resin as a thin-film polymer electrolyte (solid electrolyte) to satisfy the requirements of a thin-film polymer electrolyte such as high ion conductivity, high selective permeability, high toughness, high chemical resistance and heat resistance, and an appropriate water content. Thus, the present disclosure technology has been completed. That is, the present disclosure technology is as follows.

A CO₂ electrolysis device including an electrode material according to one aspect of the present disclosure, including:

• • a support including a conductive carrier and a catalyst supported on the conductive carrier, the catalyst including any one or more of particles of a metal complex, a metal, or an inorganic compound; and
  • an anion exchange resin covering a part or the whole of the surface of the support and including an ionomer of the following formula (1):

• • where m and n represent natural numbers of 1 to 200.

Advantageous Effects of Invention

The present disclosure can provide a technology relating to an electrode material having excellent production efficiency for producing a reduction product (CO or the like) from CO₂, and a membrane electrode assembly and a CO₂ electrolysis device that can suppress a decrease in CO₂ reduction efficiency and be stably operated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an example of a schematic view illustrating an example of a CO₂ electrolysis device suitably used in the present disclosure.

FIG. 2 is a schematic view for explaining an electrode material in the present disclosure.

FIG. 3 is an example of a schematic view for explaining a membrane electrode assembly suitably used in the present disclosure.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the CO₂ electrolysis device in the present disclosure will be specifically described. The invention according to the present disclosure is not limited to the embodiment described below.

The CO₂ electrolysis device of the present disclosure includes an electrode material. Using the electrode material according to the present disclosure as a negative electrode can provide a CO₂ electrolysis device that is excellent in CO₂ reduction efficiency and is more effective particularly when the CO₂ concentration to be supplied is low. The CO₂ electrolysis device can be used, for example, in a method for producing a CO₂ electrolysis product such as CO.

An example of the CO₂ electrolysis device of the present disclosure will be described with reference to FIG. 1. The CO₂ electrolysis device including:

• • a negative electrode 101;
  • a positive electrode 102 constituting a pair of electrodes with the negative electrode 101;
  • a solid electrolyte 103 interposed between the negative electrode 101 and the positive electrode 102 in a state where at least a part of the solid electrolyte is in contact with both electrodes;
  • a current collector 104 in contact with a surface 101-2 opposite to a surface 101-1 in contact with the solid electrolyte 103 of the negative electrode 101:
  • a support plate 105 in contact with a surface 102-1 opposite to a surface 102-2 in contact with the solid electrolyte 103 of the positive electrode 102; and
  • a voltage application portion 106 that applies a voltage between the current collector 104 and the support plate 105 (that is, between the negative electrode and the positive electrode). In addition. CO₂ in a gas phase state and an aqueous electrolyte solution such as H₂O or KHCO₃ as a supporting electrolyte are supplied by a supply source and a supply device, which is not illustrated. The CO₂ electrolysis device 100 illustrated in FIG. 1 is illustrated in a state where the components such as the negative electrode 101 and the positive electrode 102 are separated for explanation, but actually, the current collector 104, the negative electrode 101, the solid electrolyte 103, the positive electrode 102, and the support plate 105 are each bonded by a predetermined method and integrated together. Each component can be detachably configured to constitute one CO₂ electrolysis device 100.

Herein, the electrode material according to the present disclosure is used for the negative electrode 101.

In addition, the membrane electrode assembly according to the present disclosure serves as the current collector 104, the negative electrode 101, and the solid electrolyte 103 in FIG. 1. That is, the current collector constituting the membrane electrode assembly becomes the current collector 104, the electrode material according to the present disclosure becomes the negative electrode 101, and the anion exchange membrane constitutes the solid electrolyte 103, thereby forming an integrated negative electrode.

Hereinafter, the electrode material according to the present disclosure will be described.

1. Electrode Material

An electrode material according to the present disclosure is an electrode material including a catalyst, a conductive carrier, and an anion exchange resin. The catalyst is supported on a conductive carrier to form a support. The anion exchange resin covers a part or the whole of the surface of the support (refer to FIG. 2).

1-1. Conductive carrier A conductive carrier according to the present disclosure includes a carbon material, titanium, tantalum, gold, silver, or copper. These can be used singly or in combination of two or more. These can be selected in consideration of corrosion resistance.

Herein, the carbon material is not particularly limited as long as it has conductivity and does not inhibit the effect of the present disclosure technology. As the carbon material, those known to be used for an electrode material can be used, and for example, graphite carbon, glassy carbon, carbon black, graphene, carbon nanotube, and the like can be used.

The conductive carrier is in the form of particles or short fibers. The conductive carrier may be an aggregate in which particles (primary particles) or short fibers are aggregated. Herein, the term “form of particles or form of short fibers” is determined to be form of particles or form of short fibers based on general technical common knowledge. In the present disclosure, aggregates formed by aggregation of short fibers are also included in the secondary particles.

The average particle diameter of the particles or the average fiber length of the short fibers of the conductive carrier is not particularly limited as long as the effects of the present disclosure technology are not impaired, and can be set to, for example, 10 nm to 1000 μm. The average particle diameter and the average fiber length of the conductive carrier can be freely selected in consideration of the surface area and the porosity of the conductive carrier. Herein, the average particle diameter is an average particle diameter for primary particles or short fibers and secondary particles. Herein, when the conductive carrier is in the form of short fibers, the average particle diameter is a value obtained by averaging the primary particle diameter obtained by considering the fiber length of the short fibers as the primary particle diameter, and the particle diameter of the secondary particles of the short fibers. The average particle diameter can be measured by measuring diameters of 50 pieces of randomly selected particles with a known observation means such as an optical microscope, a scanning electron microscope, or a transmission electron microscope, and calculating an average value. The observation means can be selected according to the average particle diameter.

1-2. Catalyst

The catalyst according to the present disclosure is supported on a conductive carrier to form a support.

The catalyst is a known catalyst that can reduce carbon dioxide, and includes: particles of a metal or an inorganic compound such as gold, silver, copper, nickel, iron, cobalt, zinc, chromium, palladium, tin, manganese, aluminum, indium, bismuth, molybdenum, tin oxide, copper oxide, or carbon nitride; or particles of a metal complex of copper, nickel, iron, cobalt, zinc, manganese, molybdenum, rhenium, tin, indium, lead, ruthenium, or aluminum. These can be used singly or in combination of two or more. These can be selected in consideration of corrosion resistance.

The catalyst is mainly in the form of particles. In addition, the catalyst may be secondary particles in which particles (primary particles) are aggregated. Herein, the term “form of particles” is not limited to those determined to be in the form of particles based on general technical common knowledge, and includes those in which a catalyst is very small and a coordinate-bonded metal is highly dispersed at an atomic level, which is called “monatomic catalyst”.

The average particle diameter of the catalyst particles is not particularly limited as long as the effect of the present disclosure technology is not impaired, and can be set to, for example, 0.001 to 100 μm, preferably 0.001 to 1 μm, and more preferably 0.001 to 0.1 μm. The particle diameter of the catalyst can be freely selected in consideration of the surface area of the catalyst and the size effect of the catalyst. The surface area of the catalyst increases as the particle diameter of the catalyst increases, thus providing an effect of increasing the number of active sites (sites) where the catalyst contributes to the reaction. Whereas, in addition to the effect of the surface area, the particle diameter of the catalyst also has an effect of greatly changing the activity and selectivity called size effect. Therefore, the activity of the catalyst only needs to be confirmed depending on the device to be used, and the particle diameter of the catalyst may be selected. The average primary particle diameter of the catalyst for the reduction reaction of carbon dioxide that is smaller due to the size effect is more effective, and in the present disclosure technology, the average primary particle diameter of the catalyst is preferably 50 nm or less, and more preferably 20 nm or less. In addition, the catalyst that is not aggregated but dispersed, that is, has more primary particles contained is preferable because the effect of the catalyst is high. Herein, the average particle diameter of the catalyst is an average particle diameter including primary particles and secondary particles of the catalyst. The average primary particle is an average particle diameter of only the primary particle diameter. These average (or primary) particle diameters can be measured by measuring diameters of randomly selected 50 pieces of particles (or primary particles) with a known observation means such as an optical microscope, a scanning electron microscope, or a transmission electron microscope, and calculating an average value. The observation means can be selected according to the average (or primary) particle diameter.

The amount of the catalyst supported on the conductive carrier is not particularly limited as long as the effect of the present disclosure technology is not impaired. For example, when the total amount of the support is 100% by mass, the amount of the catalyst in the support can be set to 10% by mass or more, 20% by mass or more, 30% by mass or more, or 40% by mass or more, and 70% by mass or less, 60% by mass or less, or 50% by mass or less. When the amount of the catalyst supported is in such a range, aggregation of the catalyst can be suppressed, thereby maintaining high catalytic activity.

1-3. Anion Exchange Resin

The anion exchange resin according to the present disclosure covers a part or the whole of the surface of a support including a catalyst and a conductive carrier.

The anion exchange resin according to the present disclosure is an ionomer of the following formula (1):

where m and n represent natural numbers of 1 to 200.

The ion exchange capacity of the anion exchange resin (ionomer) has 0.5 mmol/g or more and 3.5 mmol/g or less, and preferably 1.0 mmol/g or more and less than 2.5 mmol/g. When the ion exchange capacity of the anion exchange resin is in such a range, an electrode material having excellent CO₂ reduction efficiency can be obtained.

Although the support with the catalyst supported on the conductive carrier is not coated with the anion exchange resin or although the support is coated with the anion exchange resin, low ion exchange capacity of the anion exchange resin reduces the abundance and mobility of ions to be generated or consumed by the CO₂ reduction reaction, thereby reducing the reduction reaction rate. In addition, CO₂ supplied to the electrode material is gas, and thus CO₂ can freely move, a chance for CO₂ to be adsorbed to the active site of the catalyst is limited, and CO₂ reduction efficiency is also limited.

Whereas, when the support is coated with the anion exchange resin of the formula (1), preferably, when the ion exchange capacity of the anion exchange resin of the formula 1 is higher than a certain level (when the ion exchange capacity is 1.0 mmol/g or more), the abundance and the conductivity of ions to be generated or consumed by the CO₂ reduction reaction are improved, thereby improving the reduction reaction rate. In addition, CO₂ that is a weak acid incorporated into the coating is neutralized by the base point of the anion exchange resin, and can remain in the anion exchange resin mainly as bicarbonate (HCO₃⁻) or carbamate ester (carbamate). As a result, hydrogen carbonate ions are abundantly stored in the vicinity of the catalyst supported on the support, and the hydrogen carbonate ions become CO₂, by equilibrium reaction, whereby CO₂ can be efficiently adsorbed to the active site of the catalyst. This can improve the CO₂ reduction efficiency of the electrode material. This effect is also effective when the CO₂ concentration to be supplied is high, but is more effective when the CO₂ concentration to be supplied is low.

In addition, when the ion exchange capacity is more than 3.5 mmol/g, increased hydrophilicity promotes swelling due to water (H₂O) generated during the neutralization reaction described above, inhibiting the supply of CO₂, and thus there is a possibility that H₂ generation of a side reaction proceeds, or mechanical properties of CO₂ as an electrode material is deteriorated.

The ion exchange capacity of the anion exchange resin can be adjusted by the ratio between the hydrophobic structure and the hydrophilic structure in the molecular structure of the ionomer.

Herein, the hydrophobic structure is a moiety represented by the following formula (2) in the formula (1).

In addition, the hydrophilic structure is a moiety represented by the following formula (3) in the formula (1).

Furthermore, the ratio of hydrophobic and hydrophilic structures in the molecular structure of the ionomer can be represented as n/m using m and n of the formula 1. Adjusting this ratio can adjust the ion exchange capacity of the ionomer.

In each of the above formulae, the repeating structural portion of each of the hydrophobic structure and the hydrophilic structure is represented in parentheses, but the repeating structure is not limited to a structure in which the repeating structure is polymerized in a block shape (block polymer), and may be a structure in which the repeating structures are bonded to each other in a regular manner such as random or alternating.

Therefore, as a method for adjusting the ion exchange capacity of the anion exchange resin, the ion exchange capacity can be adjusted by copolymerizing a monomer having a hydrophobic structure or a polymer obtained by previously polymerizing the monomer with a monomer having a hydrophilic structure or a polymer obtained by previously polymerizing the monomer with adjusting the blending ratio (m/n) of each of them.

The ion exchange capacity of the anion exchange resin is obtained from the integrated value of signals of a quaternary ammonium group and other functional groups serving as base points by 1H-NMR measurement.

The anion exchange resin covers a part or the whole of the surface of the support, and the coverage, which is the ratio of the coverage area to the surface area of the support (coverage area/support surface area×100), can be set to 70% or more, 80% or more, 90% or more, 95% or more, or 100%. The coverage is preferably high from the viewpoint of the effect of accumulating a large amount of CO₂ in the vicinity of the catalyst. Herein, the coverage is an average value of the coverages calculated from the above-described formula by observing the surfaces of randomly selected 50 pieces of particles of the electrode material with a transmission electron microscope.

The average coating thickness of the anion exchange resin is not particularly limited as long as the effect of the present disclosure technology is not impaired, and can be set to, for example, 0.01 to 100 μm.

When the average coating thickness of the anion exchange resin is 0.01 μm or more, a channel for ion conduction is sufficiently formed, hydroxide ions (OH⁻) generated by the reaction can be more efficiently transported to the ion exchange membrane, and the amount of basic points becomes sufficient, thereby providing a sufficient amount of carbonic acid species retained such as CO₂ and bicarbonate ions.

In addition, when the average coating thickness of the anion exchange resin is 100 μm or less, the distance by which ions need to move becomes appropriate, whereby the resistance to movement of ions becomes appropriate, and an increase in voltage (reduction in efficiency) can be suppressed. Furthermore, the distance by which CO₂ needs to diffuse to reach the catalyst is not too large, and thus the movement of CO₂ becomes easy, and an increase in voltage (reduction in efficiency) can be suppressed.

As described above, when the average coating thickness of the anion exchange resin is in such a range, there can be obtained the electrode material that is excellent in the production efficiency of producing a reduction product (CO or the like) from CO₂, and is more excellent in the production efficiency of the reduction product particularly when the supply concentration of CO₂ is low.

2. Method for Producing Electrode Material

2-1. Method for Synthesizing Anion Exchange Resin (Ionomer)

A preferred synthesis method example of the anion exchange resin (ionomer) according to the present disclosure technology will be described. The present disclosure technology is not limited to the following synthesis method at all.

2-1-1. Synthesis of Monomer 1

To a round-bottom three-necked flask equipped with a nitrogen inlet and condenser, are added 1,6-diiodoperfluorohexane (for example, 10.0 mmol), 3-chloroiodobenzene (for example, 50 mmol), and N,N-dimethylsulfoxide (for example, 60 mL). This mixture is stirred to form a homogeneous solution, then a copper powder (for example, 150 mmol) is added, and the mixture is reacted at 120° C. for 48 hours. The reaction solution is dropped into a 0.1 M nitric acid aqueous solution to stop the reaction. The precipitate collected from the mixture by filtration is washed with methanol, and the filtrate is collected. The same operation is repeated, and then pure water is added to the combined filtrates to precipitate white solid, which is then collected by filtration, and the white solid is washed with a mixed solution of pure water and methanol (pure water/methanol=1/1), and then vacuum-dried (60° C.) overnight, thereby synthesizing the monomer 1 (white solid) represented by the following formula (4).

2-1-2. Synthesis of Monomer 2

To a round-bottom three-necked flask, are added fluorene (for example, 0.50 mol), N-chlorosuccinimide (for example, 1.25 mol), and acetonitrile (for example, 166 mL). This mixture is stirred to form a homogeneous solution, then 12 M hydrochloric acid (for example, 16.6 mL) was added, and the mixture was reacted at room temperature for 24 hours. The precipitate collected by filtration from the reaction solution is washed with methanol and pure water, and then vacuum-dried (60° C.) overnight to obtain the monomer 2 (white solid) represented by the following formula (5).

2-1-3. Synthesis of Monomer 3

To a round-bottom three-necked flask, are added the monomer 2 (for example, 35.0 mmol) and 1,6-dibromohexane (for example, 53 mL). This mixture is stirred to form a homogeneous solution, then a mixed solution of tetrabutylammonium (for example, 7.00 mmol), potassium hydroxide (for example, 35.0 g), and pure water (for example, 35 mL) is added, and the mixture is reacted at 80° C. for 1 hour. Pure water is added to the reaction solution to stop the reaction. The target product is extracted from the aqueous layer with dichloromethane, and the combined organic layers are washed with pure water and saline, then water, dichloromethane, and 1,6-dibromohexane are distilled off. The crude product is purified by silica gel column chromatography (developing solvent:dichloromethane/hexane=1/4), then subjected to vacuum drying (60° C.) overnight, thereby providing a monomer 3 (pale yellow solid) represented by the following formula (6).

2-1-4. Synthesis of Monomer 4

To a round-bottom three-necked flask, are added the monomer 3 (for example, 23.4 mol) and tetrahydrofuran (for example, 117 mL). This mixture is stirred to form a homogeneous solution, and then a 40 wt % aqueous dimethylamine solution (for example, 58.6 mL) is added, and the mixture is reacted at room temperature for 24 hours. A saturated aqueous sodium bicarbonate solution is added to the reaction solution to stop the reaction. After tetrahydrofuran is removed, hexane is added to extract the target component. The organic layer is washed with saline, and then water and hexane are distilled off. After vacuum drying at 40° C. overnight, a monomer 4 (pale yellow solid) represented by the following formula (7) can be obtained.

2-1-5. Polymerization Reaction

To a round-bottom three-necked flask equipped with a nitrogen inlet and condenser, are added the monomer 1 (for example, 2.91 mmol), the monomer 4 (for example, 1.67 mmol), 2,2′-bipyridine (for example, 10.9 mmol), and N,N-dimethylacetamide (for example, 11 mL). The mixture is stirred to form a homogeneous solution, then bis(1,5-cyclooctadiene)nickel (0) (for example, 10.9 mmol) is added, and the mixture is reacted at 80° C. for 3 hours. The reaction mixture is added dropwise to a mixed solution of methanol and 12 M hydrochloric acid (methanol/12 M hydrochloric acid=1/1) to stop the reaction. The precipitate collected by filtration from the mixture is washed with 12 M hydrochloric acid, a 0.2 M potassium carbonate aqueous solution, and pure water, and then vacuum-dried (60° C.) overnight, thereby providing an anion exchange resin precursor polymer (yellow solid) represented by the following formula (8).

2-1-6. Quaternization Reaction

To a round-bottom three-necked flask, are added the anion exchange resin precursor polymer (for example, 1.70 g) and N,N-dimethylacetamide (for example, 9.6 mL). The mixture is stirred to form a homogeneous solution, then methyl iodide (for example, 7.22 mmol) is added, and the mixture is reacted at room temperature for 48 hours. The reaction solution to which N,N-dimethylacetamide (for example, 10 mL) has been added is filtered. The filtrate is poured onto a glass plate bordered with silicone rubber and dried on a horizontally adjusted hot plate (50° C.). This film is washed with pure water (for example, 2 L) and then vacuum-dried (60° C.) overnight to provide a light brown transparent film. Furthermore, the film is immersed in a 1 M aqueous potassium hydroxide solution for 48 hours and then washed with the solution with degassed pure water, thereby converting the counter ion of the ion exchange group (quaternary ammonium group) from an iodide ion to a hydroxide ion. This can provide an anion exchange resin (ionomer) (for example, when m/n=1/0.60, the ion exchange capacity=1.47 mmol/g) represented by the following formula (1). In addition, forming a film of the obtained anion exchange resin can provide an anion exchange membrane. As the film formation method, a known method such as a casting method using an applicator can be used.

2-2. Method for Producing Electrode Material

The predetermined amounts of a conductive carrier and a catalyst are mixed using a known mixer to prepare a support. The mixing time can be 3 to 60 minutes. Examples of another method for preparing the support includes a method of precipitating a catalyst on a conductive carrier by a reduction reaction. More specifically, the catalytic metal can be supported on the conductive carrier by mixing the predetermined amounts of the conductive carrier, the catalytic metal, and a reducing agent, and then reducing cations. The mixing time in this method can be 1 to 48 hours.

An organic solvent is put in a container, and an anion exchange resin (ionomer) is put therein, and dissolved to prepare an ionomer solution. The ionomer concentration of the ionomer solution is 0.1 to 50% by mass with respect to the total amount of the ionomer solution, and the coating thickness and the coverage can be adjusted by the ionomer concentration of the ionomer solution. The organic solvent used for the ionomer solution is not particularly limited as long as the ionomer can be dissolved, and can be freely selected in consideration of the solubility of the ionomer.

The obtained support is put into the prepared ionomer solution and mixed with a mixer or the like to prepare a support dispersion solution. The mixing time can be 5 to 60 minutes.

The obtained support dispersion solution is blown onto an electrode support such as carbon paper using a known blowing device such as a spray and dried to prepare an electrode material attached to the carbon paper or the like. As the drying, natural drying or a drying furnace or the like can be used as necessary.

3. Application of Electrode Material

The electrode material of the present disclosure can be used for a CO₂ electrolysis device, for example, as an electrode or a membrane electrode assembly.

3-1. Membrane Electrode Assembly

The electrode material of the present disclosure is used to form a membrane electrode assembly, allowing to provide a membrane electrode assembly having high CO₂ reduction efficiency.

The membrane electrode assembly of the present disclosure includes the electrode material, the solid electrolyte, and the current collector of the present disclosure, and the electrode material of the present disclosure is provided and used between the solid electrolyte and the current collector. The electrode material is molded or attached to a base material, to form an electrode having a desired shape. As the solid electrolyte, an anion exchange membrane is used.

3-1-1. Solid Electrolyte

The solid electrolyte of the present disclosure is not particularly limited as long as the effect of the present disclosure technology is not impaired, and examples thereof include cation exchange membranes such as Nafion (registered trademark) and Aquivion (registered trademark) and anion exchange membranes such as Sustainion (registered trademark) and Fumasep (registered trademark), and the anion exchange membranes are preferably used. In the membrane electrode assembly of the present disclosure, it is particularly preferable to use an anion exchange membrane in which a primary amino group, a secondary amino group, a tertiary amino group, a quaternary ammonium group, and a plurality of these ion exchange groups are mixed. Specific examples thereof include: Neosepta (registered trademark); ASE, AHA, AMX, ACS, AFN, AFX (manufactured by Tokuyama Corporation); Selemion (registered trademark); and AMV, AMT, DSV, AAV, ASV, AHO, AHT, APS4 (manufactured by Asahi Glass Co., Ltd.). Furthermore, an ion exchange membrane or the like formed with an ionomer of the following formula (1) can be used.

The material of the anion exchange membrane may be the same as or non-same as the material of the anion exchange resin covering the electrode material of the present disclosure. When the material of the anion exchange membrane is the same as the material of the anion exchange resin covering the electrode material of the present disclosure, it is possible to provide a membrane electrode assembly and a CO₂ electrolysis device that can suppress a decrease in CO₂ reduction efficiency and be stably operated. In addition, when the anion exchange resin and the anion exchange membrane are combined, it is possible to avoid deterioration of the interface between the anion exchange resin and the anion exchange membrane, and it is also possible to achieve an effect of smoothly moving (conducting) ions by avoiding phase separation of the interface between the anion exchange resin and the anion exchange membrane.

The ion exchange capacity of the anion exchange membrane is 0.3 mmol/g or more and 3.5 mmol/g or less, and preferably 0.5 mmol/g or more and 2.5 mmol/g or less. When the ion exchange capacity of the anion exchange membrane is in such a range, it is possible to provide a membrane electrode assembly and a CO₂ electrolysis device that can suppress a decrease in CO₂ reduction efficiency and be stably operated at a lower voltage.

3-1-2. Current Collector

Examples of the current collector according to the present disclosure include metal materials such as copper (Cu), nickel (Ni), stainless steel (SUS), nickel-plated steel, and brass, and among them, copper is preferable from the viewpoints of ease of processing and cost. When the current collector is made of a metal material, examples of the shape of the negative electrode current collector include a metal foil, a metal plate, a metal thin film, an expanded metal, a punching metal, and a foamed metal.

Herein, the current collector is provided with vent holes for supplying and collecting gas (raw material gas or generated gas) to and from the electrode (or an electrode material). Through these vent holes, the raw material gas can be uniformly and efficiently supplied to the electrode (or the electrode material) and the product gas (including the unreacted raw material gas) can be discharged. The number, location, and size of the vent holes are not limited, and are appropriately set. In addition, when the current collector has air permeability, vent holes are unnecessary. FIG. 3 illustrates an explanatory view of the membrane electrode assembly, and the current collector in FIG. 3 is illustrated to be made of a porous material having air permeability.

EXAMPLES

Then, the present disclosure technology will be described in detail with reference to Examples and Comparative Examples, but the present disclosure technology is not limited thereto at all.

<<Preparation of Electrode Material>>

<Synthesis of Anion Exchange Resin (Ionomer)>

Ionomers of the above formula (1) having different ion exchange capacities were synthesized by the above-described method to provide ionomers of Examples 1 and 2.

<Preparation of Electrode>

As a catalyst, powder obtained by precipitating 2 mg of Ag nanoparticles (particle size of 1 to 100 nm) on 10 mg of conductive carbon black (average particle size of 30 nm) by reduction of Ag⁺ ions was mixed using a mixer (device name and treatment conditions) to prepare a support. The support was dispersed in an ionomer solution prepared by dissolving 6 mg of an ionomer having different ion exchange capacities in an organic solvent in a container as described below, and the dispersion was applied onto a carbon paper (area of 2.25 cm²) using a spray to provide electrodes of Examples 1 and 2. In addition, for the ionomers of Comparative Examples 1 and 2, electrodes were prepared in the same manner as described above, and used as the electrodes of Comparative Examples 1 and 2. The coverage of the electrode material of each of Examples and Comparative Examples was 100%.

(Raw Materials)

Ionomer of Example 1: ionomer of formula (1) with ion exchange capacity of 2.1 mmol/g

Ionomer of Example 2: ionomer of formula (1) with ion exchange capacity of 1.5 mmol/g

Ionomer of Comparative Example 1: FAA-3-50 manufactured by FUMATECH BWT GmbH with ion exchange capacity of 2.0 mmol/g

Ionomer of Comparative Example 2: XA-9 manufactured by Dioxide Materials. Inc. with ion exchange capacity of 1.1 mmol/g

<CO₂ Electrolysis Device>

The obtained electrode of each of Examples and Comparative Examples was used as a negative electrode, and a titanium mesh supporting iridium oxide was used as a positive electrode. In addition, an anion exchange membrane having an ion exchange capacity of 1.5 mmol/g and a thickness of 30 to 35 μm was used as a solid electrolyte. An electrolytic solution tank (0.5 M KHCO₃ aqueous solution) was used as a solution on the positive electrode side. A negative electrode, a solid electrolyte, a positive electrode, and an electrolytic solution tank were disposed in this order, and the negative electrode and the electrolytic solution tank sandwiched the ion exchange membrane and the positive electrode. The evaluation was performed by supplying the gas mixed at a volume ratio of CO₂:N₂=3:1 to the negative electrode, and setting the applied potential of the negative electrode to −1.8 V with respect to the silver/silver chloride reference electrode.

<<Evaluation>>

<Electrolysis Performance Evaluation>

A CO generation current density J_CO [mA/cm²] at the time of electrolysis of CO₂ to form CO was measured using a CO₂ electrolysis device incorporating the electrodes of Examples and Comparative Examples.

                      TABLE 1
                      CO Generation Current Density [mA/cm2]
Example 1             100
Example 2             65
Comparative Example 1 50
Comparative Example 2 35

<Stability Evaluation>

The solid electrolyte of the CO₂ electrolysis device incorporating the electrode of Example 1 was changed to the ion exchange membrane shown below, and stability was evaluated as the respective CO₂ electrolysis device of Examples A to D. In the stability evaluation, an operation was performed continuously for 20 hours under the same conditions as in the above-described electrolysis performance evaluation, and a CO generation current density J_CO [mA/cm²] at the time of electrolysis of CO₂ to form CO was measured. The stability was evaluated according to the following evaluation criteria. The results are shown in Table 2.

(Solid Electrolyte)

Example A: ionomer of formula (1) with ion exchange capacity of 1.0 mmol/g

Example B: ionomer of formula (1) with ion exchange capacity of 1.5 mmol/g

Example C: ionomer of formula (1) with ion exchange capacity of 2.1 mmol/g

Example D: FAS-30 manufactured by FUMATECH BWT GmbH with ion exchange capacity of 1.8 mmol/g

(Evaluation Criteria)

⊙: J_CO after 20 hours from the start of electrolysis is 50 mA/cm² or more, and CO selectivity after 20 hours from the start of electrolysis is 90% or more.

◯: J_CO after 20 hours from the start of electrolysis is 50 mA/cm² or more, and CO selectivity after 20 hours from the start of electrolysis is less than 90%.

Δ: J_CO after 20 hours from the start of electrolysis is less than 50 mA/cm².

	TABLE 2
	Ion Exchange	CO Selectivity [%]	JCO [mA/cm2]
	Capacity	Immediately After	After 20	Immediately After	After 20	Stability
	[mmol/g]	Electrolysis	Hours	Electrolysis	Hours	Evaluation
Example A	1.0	95%	90%	121	51	⊚
Example B	1.5	95%	70%	117	56	◯
Example C	2.1	95%	55%	90	56	◯
Example D	1.8	95%	70%	60	35	Δ

Reference Signs List
1     Electrode material
10    Conductive carrier
11    Catalyst (active site)
12    Ion exchange resin
100   CO2 electrolysis device
101   Negative electrode (cathode)
101-1 Surface in contact with solid electrolyte of negative electrode
101-2 Surface in contact with current collector of negative electrode
102   Positive electrode (anode)
102-1 Surface in contact with support plate of positive electrode
102-2 Surface in contact with solid electrolyte of positive electrode
103   Solid electrolyte
104   Current collector
104-1 Gas supply hole of current collector
104-2 Gas collection hole of current collector
105   Support plate
105-1 Gas flow path of support plate
106   Voltage application portion

Claims

1. A CO2 electrolysis device including an electrode material, comprising:

a support including a conductive carrier and a catalyst supported on the conductive carrier, the catalyst including any one or more of particles of a metal complex, a metal, or an inorganic compound; and

an anion exchange resin covering a part or a whole of a surface of the support and including an ionomer of the following formula (1):

where m and n represent natural numbers of 1 to 200.

2. The CO2 electrolysis device according to claim 1, wherein an ion exchange capacity of the anion exchange resin is 0.5 mmol/g or more and less than 3.5 mmol/g.

3. The CO2 electrolysis device according to claim 1 or 2, comprising:

a membrane electrode assembly including the electrode material, a solid electrolyte, and a current collector,

wherein the electrode material is provided between the solid electrolyte and the current collector.

4. The CO2 electrolysis device according to claim 3, wherein the solid electrolyte is an anion exchange membrane.

5. The CO2 electrolysis device according to claim 4, wherein the anion exchange membrane includes an ionomer of the following formula (1):

where m and n represent natural numbers of 1 to 200.

6. The CO2 electrolysis device according to claim 4 or 5, wherein an ion exchange capacity of the anion exchange membrane is 0.3 mmol/g or more and less than 3.5 mmol/g.