Electrode Catalyst Layer for Electrolytic Cell, Electrode for Electrolytic Cell, and Carbon Dioxide Electrolysis Device

Abstract

To provide an electrode catalyst layer for an electrolysis cell that improves a catalytic reaction on a reduction electrode (cathode) in a carbon dioxide electrolytic apparatus to make it efficient, and can maintain the performance of the electrolysis cell for a long period of time.SOLUTION: An electrode catalyst layer 3 for an electrolysis cell of an embodiment of the present invention comprises: a carbon material; a metal catalyst carried by the carbon material; and a water repellent organic substance. The water repellent organic substance includes a sulfur-containing organic substance in the electrode catalyst layer for the electrolysis cell of the embodiment. The sulfur-containing organic substance forms a metal-sulfur bond with the metal catalyst. The mass ratio (S / M) between a sulfur element (S) and a metal element (M) in the metal catalyst of the catalyst layer is 0.03 or more and 0.1 or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present invention relate to an electrode catalyst layer for an electrolytic cell, an electrode for an electrolytic cell, and a carbon dioxide electrolyzer.

Background Art

[0002] In recent years, from the viewpoints of both energy problems and environmental problems, it has been desired not only to convert renewable energy such as sunlight into electrical energy and use it, but also to store it and convert it into a state where it can be transported. In response to such demands, research and development of power-to-chemical technology for producing chemical substances using sunlight, such as photosynthesis by plants, has been promoted. By such technology, renewable energy can be stored as storable fuels and the like, and it is expected to create value by producing chemical substances as industrial raw materials.

[0003] As a device for producing chemical substances using renewable energy such as sunlight, for example, an electrolyzer for reducing carbon dioxide (CO2) generated from a power plant, a waste incinerator, or the like is known. A CO2 electrolyzer includes a cathode (reduction electrode) that reduces CO2 to produce a carbon compound such as carbon monoxide (CO), and an anode (oxidation electrode) that oxidizes water (H2O) or hydroxide ions (OH - -). In such a CO2 electrolyzer, it is effective to apply a cell form (electrolytic cell) in which the cathode and the anode are stacked via a diaphragm such as an ion exchange membrane, and by directly supplying CO2 to the cathode catalyst layer of the electrolytic cell, the reduction reaction of CO2 can be advanced rapidly. In such an electrolytic cell, it is required to appropriately control the structure of the cathode catalyst layer to improve and enhance the catalytic reaction at the cathode and maintain the performance of the electrolytic cell over a long period of time.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] [Non-Patent Document 1] Effect of surface ligands on gold nanocatalysts for CO2 reduction, Chem. Sci., 2020, 11, p.12298-12308 [Summary of the Invention] [Problems to be Solved by the Invention]

[0006] The problem to be solved by the present invention is to provide an electrode catalyst layer for an electrolytic cell, an electrode for an electrolytic cell, and a carbon dioxide electrolytic cell that improve and enhance the catalytic reaction at the reduction electrode (cathode) in a carbon dioxide electrolytic cell and enable the performance of the electrolytic cell to be maintained over a long period of time. [Means for Solving the Problems]

[0007] The electrode catalyst layer for an electrolytic cell according to an embodiment includes a carbon material, a metal catalyst supported on the carbon material, and a water-repellent organic substance. In the electrode catalyst layer for an electrolytic cell according to the embodiment, the water-repellent organic substance includes an organic substance containing sulfur. The organic substance containing sulfur forms a metal-sulfur bond with the metal catalyst. The mass ratio (S / M) of sulfur element (S) to metal element (M) in the metal catalyst in the catalyst layer is 0.03 or more and 0.1 or less. [Brief Description of the Drawings]

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

MODE FOR CARRYING OUT THE INVENTION

[0009] Hereinafter, an electrode catalyst layer and an electrode for an electrolysis cell and a carbon dioxide electrolysis device according to an embodiment will be described with reference to the drawings. In each embodiment, substantially the same component parts are denoted by the same reference numerals, and the description thereof may be partially omitted. The drawings are schematic, and the relationship between the thickness and the planar dimensions of each part, the ratio of the thicknesses of each part, etc. may be different from the actual ones.

[0010] FIG. 1 is a diagram showing an electrode 1 for an electrolysis cell according to an embodiment. The electrode 1 is a reduction electrode (cathode) used for reducing carbon dioxide (CO2) in a carbon dioxide electrolysis device, and includes a conductive base material 2 and a catalyst layer (electrode catalyst layer) 3 provided on the conductive base material 2. The catalyst layer 3 is a cathode catalyst layer. The conductive base material 2 includes a material containing at least one selected from the group consisting of, for example, titanium, nickel, iron, and carbon. However, the constituent material of the conductive base material 2 is not limited to these. The conductive base material 2 preferably has a porous structure such as a mesh material, a punching material, a porous body, or a sintered metal fiber body, like the catalyst layer 3 described later, so that gas diffusion and water discharge are likely to occur.

[0011] As will be described later, a carbon dioxide electrolysis device using the electrode 1 for an electrolysis cell according to an embodiment reduces carbon dioxide (CO2) at the cathode (reduction electrode) to produce a carbon compound, and water (H2O) or a hydroxide ion (OH -) oxidizes to produce oxygen (O2). Examples of the carbon compounds produced at the cathode include carbon monoxide (CO), methane (CH4), ethane (C2H6), ethylene (C2H4), methanol (CH3OH), ethanol (C2H5OH), ethylene glycol (C2H6O2), and the like.

[0012] The catalyst layer 3 includes a carbon material, a metal catalyst supported on the carbon material, and a water-repellent organic substance. In such a catalyst layer 3, the water-repellent organic substance includes an organic substance containing sulfur, and the organic substance containing sulfur forms a metal-sulfur bond with the metal catalyst. In the catalyst layer 3, the mass ratio (S / M) of sulfur element (S) to metal element (M) in the metal catalyst is 0.03 or more and 0.1 or less. Such a catalyst layer 3 can increase the reduction efficiency of CO2 by the reduction electrode (cathode) 1 and the production efficiency of carbon compounds such as carbon monoxide (CO) based thereon, and can maintain the production efficiency of carbon compounds (such as the production efficiency of CO) over a long period of time. Details of the catalyst layer 3 will be described below.

[0013] The carbon material in the catalyst layer 3 functions as a carrier for the metal catalyst and contains at least one selected from the group consisting of, for example, carbon particles, carbon nanotubes, activated carbon, and graphene. The carbon material as the catalyst carrier preferably has a porous structure. Applicable materials include, for example, carbon blacks such as Ketjen black and Vulcan XC-72 (trade name, manufactured by Cabot Corporation), activated carbon, carbon nanotubes, graphene, and the like. Since the carbon material having a porous structure increases the area of the active surface contributing to the redox reaction, the conversion efficiency can be increased.

[0014] Not only the above-described catalyst carrier, but also the catalyst layer 3 itself formed on the conductive substrate 2 preferably has a porous structure and has a large number of relatively large pores. Specifically, in the pore size distribution of the catalyst layer 3 measured by the mercury intrusion method, it is preferable that the distribution frequency of pores is the largest in the range of 5 μm or more and 200 μm or less in diameter. In such a case, gas diffuses quickly throughout the catalyst layer 3, and the reduction products are also easily discharged out of the catalyst layer 3 through this path, so that an efficient electrode 1 can be obtained.

[0015] In order to efficiently supply CO2 to the catalyst layer 3, it is preferable that a gas diffusion layer is provided on the conductive substrate (electrode substrate) 2 that supports the catalyst layer 3. The gas diffusion layer is formed of a porous body having conductivity. If the gas diffusion layer is formed of a water-repellent porous body, the amount of water generated by the reduction reaction and the amount of water that has migrated from the oxidation electrode (anode) side can be reduced, the water can be discharged through the gas flow path, and the proportion of carbon dioxide gas in the porous body can be increased, which is preferable. If the thickness of the gas diffusion layer is extremely small, the uniformity on the cell surface is impaired. On the other hand, if the thickness of the gas diffusion layer is extremely large, not only does the member cost increase, but the efficiency also decreases due to an increase in the diffusion resistance of the gas. In order to further improve the diffusibility, a denser diffusion layer (mesoporous layer) can be provided between the gas diffusion layer and the catalyst layer 3 to change the water repellency and the porosity of the porous body, thereby promoting the gas diffusibility and the discharge of the liquid component.

[0016] For the metal catalyst supported on the above-described carbon material (support), a material that reduces the activation energy for reducing hydrogen ions or CO2 is used. In other words, a metal material that reduces the overvoltage when generating a carbon compound by the reduction reaction of CO2 is used. For example, at least one metal selected from the group consisting of gold (Au), silver (Ag), copper (Cu), platinum (Pt), palladium (Pd), nickel (Ni), cobalt (Co), iron (Fe), manganese (Mn), titanium (Ti), cadmium (Cd), zinc (Zn), indium (In), gallium (Ga), lead (Pb), and tin (Sn), an alloy containing the metal, or an oxide of the metal is preferably used. It is not limited to these, and metal complexes such as Ru complexes or Re complexes can also be used as the reduction catalyst. Further, a plurality of materials may be mixed and used. Various shapes such as particulate, plate-like, mesh-like, wire-like, porous, thin film-like, and island-like can be applied to the metal catalyst. Here, although it is referred to as a metal catalyst, it is not limited to a material composed only of a metal element, and it may be a metal oxide containing the above-described metal element.

[0017] The metal catalyst preferably has at least one structure selected from the group consisting of nanoparticles, nanostructures, and nanowires. When applying metal nanoparticles to the metal catalyst, its average diameter is preferably 1 nm or more and 15 nm or less, more preferably 1 nm or more and 10 nm or less, and even more preferably 1 nm or more and 5 nm or less. By satisfying such conditions, the surface area of the metal per unit mass of the catalyst increases, and it exhibits high activity. The same applies when applying nanostructures or nanowires.

[0018] In the catalyst layer 3, the mass per unit area of the metal catalyst is 0.01 mg / cm 2 or more and 5 mg / cm 2 or less, preferably 0.01 mg / cm 2 or more and 3 mg / cm 2 or less, more preferably 0.01 mg / cm 2 or more and 1 mg / cm 2The following are more preferable. By satisfying such conditions, the inside of the catalyst layer 3 becomes water-repellent, and water and products can be discharged promptly. The thickness of the catalyst layer is preferably 5 μm or more and 200 μm or less. By satisfying such conditions, the reduction reaction of CO2 by the catalyst layer 3 can occur efficiently.

[0019] The organic substance containing sulfur (S) as the water-repellent organic substance is not particularly limited, and the site containing S may be any of sulfide, sulfoxide, sulfone, etc. As such an organic substance containing S, for example, thiol, sulfene, thiophene, tetrahydrothiophene, etc. can be used. As the substituent bonded to the organic substance containing S, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, etc. can be used. Furthermore, it may have a plurality of unsaturated bonds. These substituents may be substituted with halogens such as F and Cl.

[0020] Specific examples of the organic substance containing sulfur (S) include alkanethiols such as methanethiol, ethanethiol, propanethiol, butanethiol, pentanethiol, hexanethiol, heptanethiol, octanethiol, nonanethiol, decanethiol, undecanethiol, dodecanethiol, and alkanesulfenes such as methanesulfene, ethanesulfene, propanesulfene, butanesulfene, pentanesulfene, hexanesulfene, heptanesulfene, octanesulfene, nonanesulfene, decanesulfene, undecanesulfene, dodecanesulfene. However, it is not limited thereto.

[0021] Thiophene is not limited to thiophene (C4H4S) itself, and may also be a thiophene derivative in which a substituent is bonded to the carbon at the 2-position or 3-position of thiophene. The thiophene derivative may have a substituent bonded to the carbon at the 4-position or 5-position. When applying a thiophene derivative, examples of the substituent bonded to the thiophene ring include the above-described substituted or unsubstituted alkyl group, substituted or unsubstituted alkenyl group, substituted or unsubstituted alkynyl group, etc. Further, a part of the substituent may be substituted with a halogen such as F or Cl. However, it is not limited thereto. As the sulfur-containing organic substance, thiophene having a thiophene ring or its derivative is preferably used.

[0022] The above-described sulfur-containing organic substance is likely to be adsorbed or bonded via sulfur on the metal catalyst. The steric hindrance and water repellency of the organic substance present on the metal catalyst suppress the approach of water molecules to the metal catalyst surface and can suppress the generation of hydrogen by hydrolysis, which is a side reaction. Further, at the same time, since the approach of impurity metal elements dissolved in water is suppressed, advantages such as suppressing the poisoning of the catalyst surface can be obtained. Depending on the type, structure, and required properties of the catalyst to which the organic substance is added, it is possible to select one or more of the above-described types of organic substances.

[0023] The sulfur-containing organic substance as described above forms a metal (M)-sulfur (S) bond between the sulfur element (S) contained therein and the metal element (M) in the metal catalyst. Since sulfur has a property of easily bonding to the surface of the metal element, the bonding property of the organic substance (water-repellent organic substance) to the catalyst metal can be enhanced. As a result, as will be described in detail later, water repellency can be imparted to the surface of the catalyst metal. Therefore, it is possible to suppress a decrease in the function as a CO2 reduction catalyst due to the approach of water (H2O) to the catalyst metal surface, specifically, a decrease in the production efficiency of carbon compounds such as CO by the catalyst metal. The M-S bond may be, for example, in a chemically bonded state, or may be adsorption by van der Waals force or adsorption accompanied by a chemical reaction between elements, and the bonding state is not particularly limited.

[0024] In the catalyst layer 3 containing an organic substance in which an M-S bond is formed between the sulfur element (S) and the metal element (M) described above and a metal catalyst, the mass ratio of the sulfur element (S) to the metal element (M) in the metal catalyst (S / M ratio) is 0.03 or more and 0.1 or less. The S / M mass ratio is more preferably 0.05 or more and 0.09 or less. When the S / M mass ratio is less than 0.03, the amount of the organic substance covering the surface of the catalyst metal is insufficient, so that water molecules can easily approach the surface of the catalyst layer 3, resulting in a decrease in water repellency. When the S / M mass ratio is greater than 0.1, the coating of the solid electrolyte component mixed when forming the catalyst layer 3 is not effectively performed, it is difficult to form a triple-phase interface, and the performance of the metal catalyst may decrease.

[0025] The organic substance containing the sulfur element (S) described above is preferably contained in the range of 5% by mass or more and 25% by mass or less, more preferably in the range of 6% by mass or more and 20% by mass, based on the total mass of the carbon material as a carrier, the metal catalyst, and the water-repellent organic substance. If the content of the organic substance containing S is less than 5% by mass, there may be insufficient manifestation of water repellency and steric hindrance by the organic substance. If it exceeds 25% by mass, there may be a risk of inhibiting the arrival of the reaction gas to the metal catalyst. Further, the mass ratio of the organic substance containing S to the metal catalyst is preferably 0.3 or more and 0.9 or less. If the above mass ratio is less than 0.3, there may be a risk of allowing water molecules to approach the surface of the metal catalyst. If the above mass ratio is greater than 0.9, there may be a risk of inhibiting the arrival of the reaction gas to the metal catalyst as described above.

[0026] The S / M mass ratio in the catalyst layer 3 is determined by the following procedure. Regarding the S element, the catalyst powder can be quantified by, for example, a high-frequency induction heating furnace combustion-infrared absorption device (manufactured by Horiba, Ltd., EMIA-920V2 (trade name)) or an equivalent device. Regarding the catalyst metal (M), the powder can be dissolved by, for example, a pressurized acid extraction method, etc., and quantified by an ICP emission spectroscopic device (manufactured by Hitachi High-Technologies Corporation, SPS-3520UVDD (trade name)) or an equivalent device. The S / M mass ratio is calculated from these measured values.

[0027] By causing the water-repellent organic substance containing S described above to be present on the surface of the catalytic metal, the reason why the production efficiency of carbon compounds (such as CO) can be effectively maintained when the CO2 electrolysis cell is operated for a long time can be considered as follows. The cathode catalyst layer 3 is likely to retain water inside the layer due to moisture derived from the electrolyte solution and humidification. This becomes a factor inhibiting the diffusion of CO2 gas. Furthermore, since the electrolyte solution components are likely to move into the layer together with water, the electrolyte solution components solidify and become a factor of clogging. In order to achieve stable long-term operation, it is important to control and maintain the water repellency inside the catalyst layer 3 in order to suppress the movement and retention of water.

[0028] In order to impart water repellency to the surface of the catalytic metal, when a water-repellent organic substance having S that is likely to adsorb and bond to the metal surface is blended, it was found that the water-repellent organic substance having S uniformly covers the surface of the catalytic metal, and when the S / M mass ratio of the catalyst layer 3 is 0.03 or more and 0.1 or less, the approach of water to the surface of the catalytic metal can be effectively suppressed. In addition, since the water-repellent organic substance is present on the metal surface, the solid electrolyte components mixed during the formation of the catalyst layer 3 can obtain a structure advantageous for the formation of the triple-phase interface without overly covering the surface of the catalytic metal. Based on these, it was found that a cathode catalyst layer 3 suitable for long-term operation can be obtained. Furthermore, by mixing carbon fibers into the catalyst layer 3, the retention of water inside the cathode catalyst layer 3 can be controlled, and the effect of suppressing the increase in the cell voltage can be obtained, which is effective for long-term operation.

[0029] In addition to the above-described constituent materials, the catalyst layer 3 may contain an ion-conductive substance and carbon fibers. The ion-conductive substance contained in the catalyst layer 3 functions to transfer ions between the metal catalysts contained in the layer. Therefore, it is present in the catalyst layer 3 in a form that coats a part of the metal catalyst. As such an ion-conductive substance, a cation-exchange resin or an anion-exchange resin is preferably used. These are polymers having ionic modifying groups, and for example, cationic polymers having perfluorosulfonic acid groups are known. More specifically, cation-exchange resins such as Nafion (trade name, manufactured by DuPont), Flemion (trade name, manufactured by AGC, Inc.), and anion-exchange resins such as Diaion (trade name, manufactured by Mitsubishi Chemical Corporation) can be used. The content of the ion-conductive substance is preferably 0.1 times or more and 1 time or less, more preferably 0.1 times or more and 0.8 times or less, based on the total mass of the carbon material and the metal catalyst described above. If the content of the ion-conductive substance is less than 0.1 times the total mass of the carbon material and the metal catalyst, it becomes difficult to maintain the structure of the catalyst layer 3 during long-term operation, and the production efficiency of the carbon compound decreases. On the other hand, if it is more than 1 time, the ion-conductive substance fills the pores in the catalyst layer 3, causing the cell voltage to rise rapidly. The volume of the ion-conductive substance is preferably 0.1 times or more and 1.5 times or less, more preferably 0.16 times or more and 1.2 times or less, based on the total volume of the carbon material and the metal catalyst described above. If the volume of the ion-conductive substance is less than 0.1 times the total volume of the carbon material and the metal catalyst, it becomes difficult to maintain the structure of the catalyst layer 3 during long-term operation, and the production efficiency of the carbon compound decreases. On the other hand, if it is more than 1.5 times, the ion-conductive substance fills the pores in the catalyst layer 3, causing the cell voltage to rise rapidly.

[0030] The carbon fibers mixed into the catalyst layer 3 may be tubular or fibrous, but preferably have an average diameter of 10 nm or more and 100 nm or less, and more preferably 30 nm or more and 100 nm or less. When carbon fibers having an average diameter within the above range are used, voids are effectively formed in the catalyst layer 3. The average length of the carbon fibers is preferably 5 μm or more and 100 μm or less. If the average length is less than 5 μm, it is difficult to form voids in the catalyst layer 3, and the effect of reducing the cell voltage is also low. If the average length exceeds 100 μm, the forming material of the catalyst layer 3 cannot be uniformly coated, and the in-plane thickness uniformity is impaired. The mass ratio of the carbon fibers in the catalyst layer 3 is preferably 10% by mass or more and 70% by mass or less, and more preferably 30% by mass or more and 60% by mass or less. If the mass ratio of the carbon fibers is less than 10% by mass, it is difficult to obtain pores effective for gas diffusion. If the mass ratio of the carbon fibers exceeds 70% by mass, the metal catalyst density decreases, resulting in a decrease in the production efficiency of carbon compounds.

[0031] Next, the carbon dioxide electrolyzer of the embodiment will be described with reference to FIGS. 2 and 3. FIG. 2 is a cross-sectional view showing a carbon dioxide (CO2) electrolyzer 10 of the embodiment using the reduction electrode described above. The CO2 electrolyzer 10 shown in FIG. 2 includes an electrolytic cell 20. The electrolytic cell 20 includes a cathode portion 30, an anode portion 40, and a separator 50 disposed so as to separate these.

[0032] The cathode portion 30 includes a reduction electrode (cathode) 31 having a catalyst layer of the embodiment, a gas flow path 32, and a cathode current collector 33. The gas flow path 32 is a flow path for supplying CO2 gas to the cathode 31, and is constituted by pits (grooves) provided in the first flow path plate 34. The cathode 31 is disposed so as to be in contact with CO2 flowing through the gas flow path 32. The anode portion 40 includes an anode (oxidation electrode) 41, an anode solution flow path 42, and an anode current collector 43. The anode solution flow path 42 is a flow path for supplying an anode solution to the anode 41, and is constituted by pits (grooves) provided in the second flow path plate 44. The anode 41 is disposed so as to be in contact with the anode solution flowing through the anode solution flow path 42.

[0033] The CO2 electrolysis device 10 includes a gas supply unit 60 that supplies CO2 to the electrolytic cell 20 and an anode solution supply unit 70 that supplies an anode solution to the electrolytic cell 20. The gas supply unit 60 includes a CO2 storage unit 61 such as a CO2 gas cylinder and a gas flow control unit 62, and CO2 gas is supplied from the CO2 storage unit 61 to the gas flow path 32 through the gas flow control unit 62 and the gas pipe 63. The CO2 gas supplied to the gas flow path 32 is not limited to a single gas of CO2, and any gas having CO2 as a main component (for example, a gas containing 90% by volume or more of CO2) may be used. The anode solution supply unit 70 includes an anode solution tank 71, a pump 72, and a flow control unit 73, and the anode solution is supplied from the anode solution tank 71 to the anode solution flow path 42 through the pump 72, the flow control unit 73, and the solution pipe 74. The anode solution circulates through the anode solution flow path 24 and the solution pipe 74. The cathode 31 and the anode 41 in the CO2 electrolysis device 10 are connected to a power source 80.

[0034] As shown in FIG. 3, the cathode portion 30 in the electrolytic cell 20 may have a cathode solution flow path 35. The cathode solution flow path 35 is constituted by a pit (recess) provided in the third flow path plate 36. The anode portion 30 shown in FIG. 3 is formed by laminating in order the third flow path plate 36 constituting the cathode solution flow path 35, the cathode 31, the first flow path plate 34 constituting the gas flow path 32, and the cathode current collector plate 33. Such a CO2 electrolysis device 10 includes a cathode solution supply unit 90 that supplies a cathode solution to the electrolytic cell 20. The cathode solution supply unit 90 includes a cathode solution tank 91, a pump 92, and a flow control unit 93, and the cathode solution is supplied from the cathode solution tank 91 to the cathode solution flow path 35 through the pump 92, the flow control unit 93, and the solution pipe 94. The cathode solution circulates through the cathode solution flow path 35 and the solution pipe 94.

[0035] In the cathode 31 of the electrolytic cell 20 shown in FIG. 2, an anode solution and ions are supplied from the separator 50, and CO2 gas is supplied from the gas flow path 32. The CO2 reduction product is mainly discharged from the gas flow path 32. In the cathode 31 of the electrolytic cell 20 shown in FIG. 3, a cathode solution and ions are supplied from the cathode solution flow path 35, and CO2 gas is supplied from the gas flow path 32. The gaseous CO2 reduction product is mainly discharged from the gas flow path 32, and the liquid CO2 reduction product is mainly discharged from the cathode solution flow path 35. Therefore, the electrolytic cell 20 shown in FIG. 3 is preferably used when liquid carbon compounds such as formic acid (HCOOH), methanol (CH3OH), ethanol (C2H5OH), formaldehyde (HCHO), and ethylene glycol (C2H6O2) are generated. The recovery becomes easy because these products dissolve in the cathode solution.

[0036] For the flow path plate 44 constituting the anode solution flow path 42 and the flow path plate 34 constituting the gas flow path 32, it is preferable to use a material with low chemical reactivity and high conductivity. Such materials include metal materials such as Ti and SUS, and carbon. For the flow path plate 36 constituting the cathode solution flow path 35, it is preferable to use a material with low chemical reactivity and no conductivity. Such materials include insulating resin materials such as acrylic resin, polyether ether ketone (PEEK), and fluororesin.

[0037] The electrolytic cell 20 is generally sandwiched between a pair of support plates (not shown) and further tightened with bolts or the like. In FIGS. 2 and 3, reference numeral 80 denotes a power source that supplies current to the anode 41 and the cathode 31. The power source 80 is not limited to a normal commercial power source or a battery, etc., and may be a power source that converts renewable energy into electrical energy and supplies it. Examples of such power sources include power sources that convert kinetic energy or potential energy such as wind power, hydraulic power, geothermal energy, and tidal power into electrical energy, power sources such as solar cells having a photoelectric conversion element that converts light energy into electrical energy, power sources such as fuel cells and storage batteries that convert chemical energy into electrical energy, and power sources such as devices that convert vibration energy such as sound into electrical energy. Using renewable energy is also environmentally preferable in terms of the effective utilization of carbon dioxide.

[0038] The anode 41 causes an oxidation reaction of water (H2O) in the anode solution to generate oxygen (O2) and hydrogen ions (H + ), or causes an oxidation reaction of hydroxide ions (OH - ) generated in the cathode part 30 to generate oxygen and water, and is an electrode (oxidation electrode). The anode 41 is disposed between the separator 50 and the anode solution flow path 42 so as to be in contact with them. More specifically, the anode 41 has a first surface in contact with the separator 50 and a second surface facing the anode solution flow path 42. The first surface of the anode 41 is in close contact with the separator 50. Solution inlets and solution outlets (both not shown) are connected to the second flow path plate 44, and the anode solution is introduced and discharged by the pump 72 through these solution inlets and solution outlets. The anode solution flows through the anode solution flow path 42 so as to be in contact with the anode 41. The anode current collector plate 43 is in electrical contact with the surface of the second flow path plate 44 constituting the anode solution flow path 42, which is opposite to the anode 41.

[0039] The compound generated by the oxidation reaction on the anode 41 varies depending on the type of oxidation catalyst and the like. When an aqueous solution of an electrolyte is used, the anode 41 oxidizes water (H2O) to generate oxygen and hydrogen ions, or- ) can oxidize to produce water and oxygen, and is preferably mainly composed of a catalyst material (anode catalyst material) capable of reducing the overvoltage of such a reaction. Such catalyst materials include metals such as platinum (Pt), palladium (Pd), nickel (Ni), alloys and intermetallic compounds containing these metals, manganese oxide (Mn - O), iridium oxide (Ir - O), nickel oxide (Ni - O), cobalt oxide (Co - O), iron oxide (Fe - O), tin oxide (Sn - O), indium oxide (In - O), ruthenium oxide (Ru - O), lithium oxide (Li - O), lanthanum oxide (La - O) and other binary metal oxides, ternary metal oxides such as Ni - Co - O, Ni - Fe - O, La - Co - O, Ni - La - O, Sr - Fe - O, quaternary metal oxides such as Pb - Ru - Ir - O, La - Sr - Co - O, and metal complexes such as Ru complexes and Fe complexes.

[0040] The anode 41 preferably has a structure capable of moving the anode solution and ions between the separator 50 and the solution flow path 42, for example, a base material having a porous structure such as a mesh material, a punching material, a porous body, etc. The base material having a porous structure includes those with relatively large voids such as a sintered metal fiber body. The base material may be composed of a metal material such as a metal like titanium (Ti), nickel (Ni), iron (Fe), or an alloy (for example, SUS) containing at least one of these metals, or may be composed of the above-described anode catalyst material. When an oxide is used as the anode catalyst material, it is preferable to form a catalyst layer by adhering or laminating the anode catalyst material on the surface of the base material made of the above-described metal material. The anode catalyst material preferably has a shape such as nanoparticles, nanostructures, nanowires, etc. in enhancing the oxidation reaction. A nanostructure is a structure in which nano-scale irregularities are formed on the surface of the catalyst material. Also, it is not necessarily required to provide an oxidation catalyst on the anode (oxidation electrode) 41. An oxidation catalyst layer provided outside the oxidation electrode may be electrically connected to the oxidation electrode.

[0041] As the anode solution and the cathode solution, an aqueous solution containing any electrolyte can be used. Examples of the aqueous solution containing an electrolyte include an aqueous solution containing phosphate ions (PO4 2- ), borate ions (BO3 3- ), sodium ions (Na + ), potassium ions (K + ), calcium ions (Ca 2+ ), lithium ions (Li + ), cesium ions (Cs + ), magnesium ions (Mg 2+ ), chloride ions (Cl - ), hydrogen carbonate ions (HCO3 - ), carbonate ions (CO3 2- ), etc. In addition, an aqueous solution containing LiHCO3, NaHCO3, KHCO3, CsHCO3, phosphoric acid, etc. may also be used.

[0042] The cathode solution consists of a salt of a cation such as imidazolium ion or pyridinium ion and an anion such as BF4 - or PF6 - , and an ionic liquid or an aqueous solution thereof that is in a liquid state in a wide temperature range may be used. Other examples of the cathode solution include amines such as ethanolamine, imidazole, and pyridine or their aqueous solutions. The amine can be any of primary amine, secondary amine, and tertiary amine.

[0043] The separator 50 is composed of an ion exchange membrane or the like that can allow ions to move between the anode 41 and the cathode 31 and can separate the anode portion 40 and the cathode portion 30. Examples of the ion exchange membrane include cation exchange membranes such as Nafion (registered trademark) and Flemion (registered trademark), and anion exchange membranes such as Neosepta (registered trademark), Selemion (registered trademark), and Sustainio (registered trademark). Also, when it is not necessary to control the movement of ions between the two electrolytic solutions, it is not always necessary to provide an ion exchange membrane in the electrolytic solution tank 2. In addition to the ion exchange membrane, the separator 50 may be filled with a filler such as a glass filter or agar, an insulating porous body such as zeolite or an oxide, or a polymer membrane through which water molecules and ions can pass, as long as it is a material that can allow ions to move between the anode and the cathode. However, if gas circulation occurs between the cathode portion 30 and the anode portion 40, a circulation reaction may occur due to the reoxidation of the reduction product. Therefore, it is preferable that there is less gas exchange between the cathode portion 30 and the anode portion 40.

[0044] The CO2 gas to be supplied may be supplied in a dry state, but it is more preferably humidified. Thereby, when an ion exchange membrane is used for the separator 50, drying of the membrane can be prevented. The CO2 concentration of the gas to be supplied does not have to be 100%. Although the efficiency decreases, it is also possible to reduce the gas containing carbon dioxide discharged from various facilities.

[0045] Next, an operation example of the carbon dioxide electrolyzer 10 shown in FIG. 2 will be described. Here, the case of generating carbon monoxide (CO) as a carbon compound will be mainly described. However, the carbon compound as a reduction product of carbon dioxide is not limited to carbon monoxide, and may be methane (CH4), ethane (C2H6), ethylene (C2H4), methanol (CH3OH), ethanol (C2H5OH), ethylene glycol (C2H6O2), etc. Further, the generated carbon monoxide as a reduction product may be further reduced to generate an organic compound as described above. When a solution-like carbon compound is generated as described above, it is preferable to use the electrolytic cell 20 shown in FIG. 3. Further, as the reaction process by the electrolytic cell 20, mainly the case of generating hydrogen ions (H + ) and the case of mainly generating hydroxide ions (OH - ) are conceivable, but it is not limited to any of these reaction processes.

[0046] First, the reaction process in the case of mainly oxidizing water (H2O) to generate hydrogen ions (H + ) will be described. When a current is supplied from the power source 80 between the anode 41 and the cathode 31, an oxidation reaction of water (H2O) occurs at the anode 41 in contact with the anode solution. Specifically, as shown in the following formula (1), the H2O contained in the anode solution is oxidized to generate oxygen (O2) and hydrogen ions (H + ). 2H2O → 4H + +O2+4e - …(1)

[0047] The H + generated at the anode moves through the electrolytic solution or the separator 50 existing in the anode 41 and reaches the vicinity of the cathode 31. Based on the electrons (e - ) supplied from the power source 50 to the cathode 31 and the H + that has moved to the vicinity of the cathode 31, a reduction reaction of carbon dioxide (CO2) occurs. Specifically, as shown in the following formula (2), the CO2 supplied from the CO2 gas flow path 32 to the cathode 41 is reduced to generate CO. Further, as shown in the following formula (3), hydrogen ions (H+ ) generates hydrogen by receiving electrons. At this time, hydrogen may be generated simultaneously with carbon monoxide. 2CO2 + 4H + + 4e - → 2CO + 2H2O …(2) 2H + + 2e - → H2…(3)

[0048] Next, the reaction process when mainly reducing carbon dioxide (CO2) to generate hydroxide ions (OH - ) will be described. When an electric current is supplied from a power source 80 between the anode 41 and the cathode 31, near the cathode 41, as shown in the following formula (4), water (H2O) and carbon dioxide (CO2) are reduced to generate carbon monoxide (CO) and hydroxide ions (OH - ). Also, as shown in the following formula (5), hydrogen is generated when water receives electrons. At this time, hydrogen may be generated simultaneously with carbon monoxide. The hydroxide ions (OH - ) generated by these reactions diffuse near the anode 41 and, as shown in the following formula (6), the hydroxide ions (OH - ) are oxidized to generate oxygen (O2). 2CO2 + 2H2O + 4e - → 2CO + 4OH - …(4) 2H2O + 2e - → H2 + 2OH - …(5) 4OH - → 2H2O + O2 + 4e - …(6)

[0049] The carbon dioxide electrolyzer of such an embodiment is not only specialized in the reduction of carbon dioxide, but can also produce carbon dioxide reduction products and hydrogen in any ratio, for example, producing carbon monoxide and hydrogen in a ratio of 1:2 and then producing methanol in a subsequent chemical reaction. Since hydrogen is an inexpensive and easily available raw material from the electrolysis of water or fossil fuels, it is not necessary for the ratio of hydrogen to be large. From these viewpoints, it is preferable from the viewpoints of economy and environmental friendliness that the ratio of carbon monoxide to hydrogen is at least 1 or more, preferably 1.5 or more.

[0050] In the carbon dioxide electrolyzer 1 of the above-described embodiment, as the reduction catalyst of the cathode 31, it includes the carbon material of the above-described embodiment, the metal catalyst supported on the carbon material, and the water-repellent organic substance containing S, and the mass ratio (S / M) of sulfur element (S) to metal element (M) in the metal catalyst is 0.03 or more and 0.1 or less. Therefore, the approach of water to the surface of the catalyst metal can be suppressed, and a structure advantageous for the formation of a triple-phase interface on the surface of the catalyst metal can be obtained. By these means, the production efficiency of carbon compounds such as CO in the carbon dioxide electrolyzer 1 can be increased, and even when the carbon dioxide electrolyzer 1 is operated for a long time, it is possible to maintain the production efficiency of carbon compounds.

Example

[0051] Next, the examples and their evaluation results will be described.

[0052] (Example 1) First, as the constituent materials of the cathode catalyst layer, catalyst particles in which Au nanoparticles (metal catalyst) with an average diameter of 2 nm are supported on carbon particles, 2-octylthiophene as a water-repellent organic substance (organic substance containing S), and Nafion solution (trade name, manufactured by DuPont) as an ion-conductive substance (ion-exchange resin) were prepared. These, pure water, and isopropanol were mixed at a predetermined ratio to prepare a catalyst coating solution. The catalyst particles and the water-repellent organic substance were mixed so that the ratio of S in the water-repellent organic substance to the metal element M (Au) in the catalyst layer was 0.08. Nafion as a solid electrolyte was mixed so that its solid mass was 0.23 times the total mass of the carrier carbon material and the gold catalyst.

[0053] Carbon paper with a diffusion layer having a microporous layer was prepared as the electrode substrate. The above-prepared catalyst coating solution was filled into a spray nozzle and sprayed onto the carbon paper placed on a heated hot plate to perform spray coating. The spray coating of the catalyst coating solution was performed so that the thickness of the cathode was 60 μm. Also, the mass of the metal catalyst per unit area of the catalyst layer was set to 0.2 mg / cm 2 This was cut out to a size of 4×4 cm to obtain a cathode (electrode area: 16 cm 2 ).

[0054] As the anode, an electrode in which IrO2 nanoparticles serving as a catalyst were coated on a Ti mesh was used. This IrO2 / Ti mesh was cut out to 4×4 cm to obtain an anode.

[0055] Next, the electrolytic cell shown in FIG. 2 was fabricated. The electrolytic cell was fabricated by laminating, in order from above, a cathode current collector plate, a CO2 gas flow path, a cathode, a separator, an anode, an anode solution flow path, and an anode current collector plate, sandwiching them with a support plate (not shown), and further tightening with bolts. As the separator, Sustainion (trade name, manufactured by Dioxide Materials) was used as an anion exchange membrane. The anode current collector plate and the cathode current collector plate were connected to an external power source to fabricate the carbon dioxide electrolyzer shown in FIG. 2.

[0056] (Example 2) In the catalyst coating liquid in Example 1, the catalyst particles and the water-repellent organic substance were mixed so that the ratio of S in the water-repellent organic substance to the metal element M (catalyst metal) in the catalyst layer was 0.07. A catalyst coating liquid was prepared in the same manner as in Example 1, except that Nafion was added as a solid electrolyte to the catalyst coating liquid so that the mass ratio to the total mass of the electrolyte / support carbon material and the gold catalyst was 0.51. Using such a catalyst coating liquid, a cathode was produced in the same manner as in Example 1. Next, an electrolytic cell and an electrolysis apparatus shown in FIG. 2 were produced in the same manner as in Example 1, except that the cathode in Example 1 was changed to the cathode in Example 2 described above.

[0057] (Example 3) In the catalyst coating liquid of Example 1, carbon fibers having an average diameter of 50 nm and an average length of 10 μm were further mixed so that the ratio of carbon fibers in the catalyst layer was 30% by mass. A catalyst coating liquid was prepared in the same manner as in Example 1, except that Nafion was added as a solid electrolyte to the catalyst coating liquid so that the mass ratio to the total mass of the support carbon material and the gold catalyst was 0.25. Using such a catalyst coating liquid, a cathode was produced in the same manner as in Example 1. Next, an electrolytic cell and an electrolysis apparatus shown in FIG. 2 were produced in the same manner as in Example 1, except that the cathode in Example 1 was changed to the cathode in Example 3 described above.

[0058] Next, the electrolysis apparatuses of Examples 1 to 3 were operated under the conditions shown below. A predetermined amount of CO2 was supplied to the CO2 gas flow path, and an aqueous sodium hydrogen carbonate solution (concentration: 0.1 M) was flowed through the anode solution flow path. A constant current of 200 mA / cm 2 was continuously applied between the anode and the cathode for a long time. At that time, the gas generated from the cathode side was collected, and the conversion efficiency of CO was measured. The gases generated by the above operation were CO and hydrogen, and the sum of their partial current densities almost coincided with the value of the above current density. The time change of the generation efficiency of CO under the above conditions for the electrolysis apparatuses of Examples 1 to 3 is shown in FIG. 4.

[0059] (Comparative Example 1) In the catalyst coating liquid of Example 1, a catalyst coating liquid was prepared in the same manner as in Example 1, except that a water-repellent organic substance was not added. Nafion as a solid electrolyte was added to the catalyst coating liquid so that the mass ratio to the total mass of the carrier carbon material and the gold catalyst was 0.20. Using such a catalyst coating liquid, a cathode was produced in the same manner as in Example 1. Then, an electrolytic cell and an electrolysis apparatus shown in FIG. 2 were produced in the same manner as in Example 1, except that the cathode in Example 1 was changed to the cathode of Comparative Example 1 described above.

[0060] (Comparative Example 2) In the catalyst coating liquid of Example 1, a catalyst coating liquid was prepared in the same manner as in Example 1, except that the catalyst particles and the water-repellent organic substance were mixed so that the ratio of S in the water-repellent organic substance to the metal element M (Au) in the catalyst layer was 0.01. Nafion as a solid electrolyte was added to the catalyst coating liquid so that the mass ratio to the total mass of the carrier carbon material and the gold catalyst was 0.03. Using such a catalyst coating liquid, a cathode was produced in the same manner as in Example 1. Then, an electrolytic cell and an electrolysis apparatus shown in FIG. 2 were produced in the same manner as in Example 1, except that the cathode in Example 1 was changed to the cathode of Comparative Example 2 described above.

[0061] The electrolysis apparatuses of Comparative Examples 1 and 2 were subjected to an operation test under the same conditions as in Example 1. The time change of the generation efficiency of CO under the above-described conditions for the electrolysis apparatuses of Comparative Examples 1 and 2 is shown in FIG. 4. Also, the production conditions of the cathode catalyst layers in the Examples and Comparative Examples are shown in Table 1.

[0062]

Table 1

[0063] As is clear from FIG. 4, according to the electrolysis apparatuses of Examples 1 to 3, even when operated for a long time, no decrease in the production efficiency of CO was observed, indicating that they have a catalyst layer suitable for long-term operation. On the other hand, in the electrolysis apparatus using a catalyst layer without sulfur as in Comparative Example 1, the initial production efficiency of CO was low, and a sharp decrease in efficiency was observed as the operation time elapsed. Also, as in Comparative Example 2, when the ratio of S in the water-repellent organic substance to the metal element M (catalyst metal) was low (less than 0.03), the initial production efficiency of CO was equivalent to that of Examples 1 to 3, but the efficiency decreased significantly over time. In addition, in the electrolysis apparatus using a catalyst layer with the mass ratio of S / metal catalyst (Au) set to a value exceeding 0.1, sufficient CO production efficiency could not be obtained. Further, even when the mass ratio of the total mass of the electrolyte / support carbon material and the gold catalyst was too low, as in Comparative Example 2, the CO production efficiency decreased over time. In the electrolysis apparatus using a catalyst layer with the mass ratio of the total mass of the electrolyte / support carbon material and the gold catalyst set to 2, the cell voltage was too high to perform measurements.

[0064] Note that the configurations of the respective embodiments can be combined and applied, and partial replacement is also possible. Here, several embodiments of the present invention have been described, but these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and at the same time, are included in the invention described in the claims and its equivalent scope.

Explanation of Reference Numerals

[0065] 1... Electrode for electrolysis cell, 2... Conductive substrate, 3... Catalyst layer, 10... Carbon dioxide electrolysis apparatus, 20... Electrolysis cell, 30... Cathode portion, 31... Cathode, 32... Gas flow path, 40... Anode portion, 41... Anode, 42... Anode solution flow path, 50... Separator, 60... Gas supply portion, 70 Anode solution supply portion, 80... Power source.

Claims

1. An electrode catalyst layer for an electrolysis cell of carbon dioxide, comprising a carbon material, a metal catalyst supported on the carbon material, and a water-repellent organic substance, The water-repellent organic substance comprises an organic substance containing sulfur, The organic substance containing sulfur forms a metal-sulfur bond with the metal catalyst, An electrode catalyst layer for an electrolysis cell, wherein a mass ratio of sulfur element to metal element in the metal catalyst in the catalyst layer is 0.03 or more and 0.1 or less.

2. The electrode catalyst layer for an electrolysis cell according to claim 1, wherein the organic substance containing sulfur is contained in a range of 5% by mass or more and 25% by mass or less based on the total mass of the carbon material, the metal catalyst, and the water-repellent organic substance.

3. A mass ratio of the organic substance containing sulfur to the metal catalyst is 0.3 or more and 0.9 or less, The electrode catalyst layer for an electrolysis cell according to claim 1 or claim 2.

4. Further comprising an ion conductive substance, The electrode catalyst layer for an electrolysis cell according to any one of claims 1 to 3, wherein a solid mass of the ion conductive substance is 0.1 times or more and 1 time or less based on a total mass of the carbon material and the metal catalyst.

5. The catalyst layer has a porous structure in which the distribution frequency of pores is maximum in the range of 5 μm or more and 200 μm or less in the pore size distribution of the catalyst layer measured by the mercury intrusion method. The electrode catalyst layer for an electrolysis cell according to any one of claims 1 to 4.

6. Further comprising carbon fibers having an average diameter of 10 nm or more and 100 nm or less and an average length of 5 μm or more and 100 μm or less, The electrode catalyst layer for an electrolysis cell according to any one of claims 1 to 5, wherein a ratio of the carbon fibers in the catalyst layer is 10% by mass or more and 70% by mass or less.

7. The organic substance containing sulfur has a thiophene ring, and the electrode catalyst layer for an electrolytic cell according to any one of claims 1 to 6.

8. The electrode catalyst layer for an electrolytic cell according to any one of claims 1 to 7, wherein the thickness of the catalyst layer is 5 μm or more and 200 μm or less.

9. The electrode catalyst layer for an electrolytic cell according to any one of claims 1 to 8, wherein the carbon material contains at least one selected from the group consisting of carbon particles, carbon nanotubes, activated carbon, and graphene.

10. The metal catalyst includes at least one metal selected from the group consisting of gold, silver, copper, platinum, palladium, nickel, cobalt, iron, manganese, titanium, cadmium, zinc, indium, gallium, lead, and tin, an alloy containing the metal, or an oxide of the metal. The electrode catalyst layer for an electrolytic cell according to any one of claims 1 to 9, wherein the metal catalyst has at least one structure selected from the group consisting of nanoparticles, nanostructures, and nanowires.

11. The mass of the metal catalyst per unit area of the catalyst layer is 0.01 mg / cm ２ or more and 5 mg / cm ２ or less, and the electrode catalyst layer for an electrolytic cell according to any one of claims 1 to 10.

12. The electrode catalyst layer for an electrolytic cell according to claim 4, wherein the ion conductive substance includes a cation exchange resin or an anion exchange resin.

13. A conductive substrate, The electrode catalyst layer according to any one of claims 1 to 12 provided on the conductive substrate, and An electrode for an electrolytic cell comprising the same.

14. The electrode for an electrolytic cell according to claim 13, wherein the conductive substrate includes a material containing at least one selected from the group consisting of titanium, nickel, iron, and carbon.

15. The electrode for an electrolytic cell according to claim 13 or claim 14, wherein the conductive substrate has at least one selected from the group consisting of a mesh material, a punching material, a porous body, and a sintered metal fiber body.

16. A cathode portion having a cathode composed of the electrode according to any one of claims 13 to 15, which is arranged to be in contact with carbon dioxide and reduces the carbon dioxide to produce a carbon compound, and a gas flow path for supplying carbon dioxide to the cathode; an anode portion having an anode arranged to be in contact with an anode solution containing water or hydroxide ions and oxidizing the water or hydroxide ions to produce oxygen, and an anode solution flow path for supplying the anode solution to the anode; and a separator separating the cathode portion and the anode portion, an electrolytic cell comprising: A gas supply unit for supplying carbon dioxide to the gas flow path; A solution supply unit for supplying the anode solution to the anode solution flow path, A carbon dioxide electrolyzer, wherein the cathode and the anode are configured to be supplied with current from a power source.

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