Reduction reaction electrode and method for manufacturing the same

Abstract

To provide a reduction reaction electrode capable of maintaining high current density for a long time.SOLUTION: A reduction reaction electrode 10 comprises a substrate 12, a catalyst layer 16 including a reduction catalyst and a carbon material, and a conductive adhesive layer 14 provided between the substrate 12 and the catalyst layer 16, and the conductive adhesive layer 14 comprises a fluororesin with a vinylidene skeleton and graphite flakes with particle sizes ranging from 1 μm to 100 μm.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a reduction reaction electrode.

Background Art

[0002] In recent years, research on reaction devices used for artificial photosynthesis that synthesize hydrogen (H2) from water (H2O), carbon monoxide (CO), formic acid (HCOOH), methanol (CH3OH), etc. from water (H2O) and carbon dioxide (CO2) using solar energy has been actively conducted.

[0003] As a reduction reaction electrode used in a reaction device, for example, Patent Documents 1 to 3 and Non-Patent Document 1 disclose a reduction reaction electrode in which a catalyst layer containing a reduction catalyst and a substrate are adhered with a conductive adhesive.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0005]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Incidentally, in order to put CO2 reduction by artificial photosynthesis into practical use, a reduction reaction electrode that can stably obtain a high current density is desired in order to maintain an efficient CO2 reduction reaction. However, conventional conductive adhesives have low chemical resistance and conductivity, so when the reduction reaction electrode is brought into contact with (immersed in) the electrolyte and operated for a long time, the adhesive strength and conductivity between the catalyst layer and the substrate decrease, making it difficult to maintain a high current density for a long time.

[0007] Therefore, the present invention aims to provide a reduction reaction electrode capable of maintaining a high current density for a long period of time during constant voltage control. [Means for solving the problem]

[0008] The reduction reaction electrode of the present invention comprises a substrate, a catalyst layer containing a reduction catalyst and a carbon material, and a conductive adhesive layer provided between the substrate and the catalyst layer, wherein the conductive adhesive layer contains a fluororesin having a vinylidene skeleton and graphite flakes having a particle size in the range of 1 μm to 100 μm.

[0009] Furthermore, in the reduction reaction electrode, it is preferable that the conductive adhesive layer contains mesocarbon microbeads having a particle size in the range of 5 μm to 30 μm.

[0010] Furthermore, in the reduction reaction electrode, it is preferable that the conductive adhesive layer contains a reduction catalyst.

[0011] Furthermore, in the reduction reaction electrode, it is preferable that the carbon material of the catalyst layer is a carbon sheet containing carbon nanotubes, and that the reduction catalyst is supported on the carbon sheet.

[0012] Furthermore, in the reduction reaction electrode, it is preferable that the reduction catalyst includes a Ru complex polymer.

[0013] Furthermore, in the reduction reaction electrode, it is preferable that the fluororesin having a vinylidene skeleton includes polyvinylidene fluoride (PVDF).

[0014] Further, the method for manufacturing a reduction reaction electrode of the present invention includes a first step of providing a conductive adhesive containing a fluororesin having a vinylidene skeleton and graphite flakes with a particle size in the range of 1 μm to 100 μm between a sheet containing a carbon material and a substrate, and bonding the sheet containing the carbon material and the substrate, and a second step of applying a reduction catalyst-containing solution to the sheet containing the carbon material and drying it.

Effects of the Invention

[0015] According to the present invention, it is possible to provide a reduction reaction electrode capable of maintaining a high current density for a long time during constant voltage control.

Brief Description of the Drawings

[0016] [Figure 1] It is a schematic configuration diagram showing an example of the configuration of a reduction reaction electrode according to an embodiment of the present invention. [Figure 2] It is a schematic configuration diagram showing an example of the configuration of a carbon dioxide reduction device using the reduction reaction electrode of the present embodiment. [Figure 3] It is a SEM photograph of the surface of a conductive adhesive layer formed by the conductive adhesive prepared in Example 1. Y [Figure 4] It is a SEM photograph of the surface of a conductive adhesive layer formed by the conductive adhesive used in Comparative Example 1. [Figure 5] It is a diagram showing the change in current density when the reduction reaction electrodes of Examples 1 to 3 are operated for 100 hours. [Figure 6] It is a diagram showing the change in current density when the reduction reaction electrodes of Example 2 and Comparative Examples 1 to 2 are operated for a predetermined time. [Figure 7] It is a diagram showing the results of the current density maintenance rate when the reduction reaction electrodes of Example 1, Example 4, Example 5, and Comparative Example 1 are operated for 100 hours. [Figure 8] It is a diagram showing the change in current density when the reduction reaction electrodes of Example 2, Example 3, Example 6, and Comparative Example 1 are operated for a predetermined time.

Modes for Carrying Out the Invention

[0017] Hereinafter, embodiments of the present invention will be described based on the drawings. Note that the present invention is not limited to the embodiments described herein.

[0018] [Reduction reaction electrode] FIG. 1 is a schematic configuration diagram showing an example of the configuration of a reduction reaction electrode according to an embodiment of the present invention. The reduction reaction electrode 10 in FIG. 1 includes a base material 12, a catalyst layer 16, and a conductive adhesive layer 14 provided between the base material 12 and the catalyst layer 16. The base material 12 and the catalyst layer 16 are adhered by the conductive adhesive layer 14. When a voltage is applied to the reduction reaction electrode 10, the charges generated in the base material 12 are passed to the catalyst layer 16 and used for a reduction catalyst reaction in the catalyst layer 16, for example, a reaction in which carbon dioxide (CO2) is reduced to formic acid (HCOOH) or the like.

[0019] The base material 12 is preferably a member that structurally supports the electrode and has electrical current collecting properties. Examples thereof include plate-like members and mesh-like members containing metals or semiconductors. The metal used as the base material is not particularly limited, and examples thereof include titanium (Ti), silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), lead (Pb), and the like. The semiconductor used as the base material is not particularly limited, and examples thereof include titanium oxide (TiO2), tin oxide (SnO2), silicon (Si), strontium titanate (SrTiO3), zinc oxide (ZnO), tantalum oxide (Ta2O5), and the like.

[0020] The catalyst layer 16 includes a reduction catalyst and a carbon material. The carbon material is preferably used as a carrier on which the reduction catalyst is supported. As the carbon material, for example, a carbon sheet containing carbon and carbon fibers that have been heat-treated at high temperatures is preferred. Examples of carbon sheets include carbon paper and carbon cloth. Carbon paper is made by impregnating organic fibers such as polyacrylonitrile (PAN) fibers in a dispersion of polyvinyl alcohol and an aqueous medium, carbonizing them at about 2000°C, and binding them together to form a sheet. Carbon paper may also contain about 25% by mass of Teflon®-based material. Carbon cloth is made by spinning carbon fibers obtained by firing and carbonizing organic fibers. Carbon paper and carbon cloth are porous materials and have countless pores of several tens of micrometers (about 10 μm to 100 μm). The thickness of carbon paper and carbon cloth is, for example, in the range of 0.1 mm to 0.4 mm per sheet.

[0021] The carbon material may contain carbon nanotubes such as multi-walled carbon nanotubes (MWCNTs). A suitable example of a carbon material is a carbon sheet containing carbon nanotubes. For example, by including carbon nanotubes such as multi-walled carbon nanotubes (MWCNTs), a three-dimensional network structure of nanocarbon can be formed. Preferably, the MWCNTs contain at least 1 nm to 100 nm in diameter. A carbon sheet containing carbon nanotubes can be formed, for example, by preparing an ink in which carbon nanotubes are highly dispersed in a solvent such as ethanol, applying it to a carbon sheet by methods such as dip coating or impregnation coating, and then drying it.

[0022] The reduction catalyst is not particularly limited as long as it is a material that has a reduction catalytic function for reducing carbon dioxide, but a complex catalyst is preferred, for example. The complex catalyst preferably contains a Ru complex polymer.

[0023] Examples of ruthenium complexes (Ru complex monomers) used to obtain Ru complex polymers include [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(MeCN)Cl2], [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2Cl2], and [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2]. n Examples include [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(CH3CN)Cl2]. Ru complex polymers are, for example, Ru complex polymers obtained by polymerizing (polymerizing) a Ru complex monomer (e.g., [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2Cl2]), a polymerization agent (e.g., pyrrole), and a polymerization catalyst (e.g., iron chloride).

[0024] The Ru complex polymer can be supported, for example, by dissolving the Ru complex monomer, polymerization agent (e.g., pyrrole), and polymerization catalyst (e.g., iron chloride) in a solvent such as acetonitrile (MeCN) (Ru complex polymer solution), coating it onto a carbon material such as a carbon sheet, and then drying it.

[0025] Examples of polymerization agents (radical initiators) include heterocyclic aromatic compounds having a five-membered ring structure to a nine-membered ring structure containing at least one of N, S, or B, such as pyrrole, pyridine, thiophene, borepin, and azonine, as well as azo compounds and organic peroxides. Pyrrole is preferred because the reaction is easily initiated by oxidation.

[0026] Examples of polymerization catalysts (chemical oxidation polymerization catalysts) include iron salts such as iron chloride (FeCl3), FeCl3·O2, or FeCl3·O2-ClO4, copper salts such as copper chloride, and aluminum salts such as aluminum chloride. Iron chloride (FeCl3), FeCl3·O2, or FeCl3·O2-ClO4 are preferred due to their high catalytic activity.

[0027] Examples of solvents include acetonitrile, diethyl ether, N,N-dimethylformamide (DMF), and tetrahydrofuran (THF), with acetonitrile being preferred due to its ability to highly disperse the Ru complex polymer.

[0028] The conductive adhesive layer 14 comprises a fluororesin having a vinylidene skeleton and graphite flakes. The thickness of the conductive adhesive layer 14 is, for example, in the range of 1 μm to 100 μm.

[0029] Examples of fluororesins having a vinylidene skeleton include polyvinylidene fluoride (PVDF), derivatives of polyvinylidene fluoride (PVDF), and copolymers containing units derived from vinylidene fluoride (VDF). The copolymer may be, for example, a block copolymer or a random copolymer. Among these, polyvinylidene fluoride (PVDF) is preferred in terms of having high adhesive strength and high chemical resistance. The content of the fluororesin having a vinylidene skeleton may be, for example, in the range of 1% to 20% by mass relative to the total mass of the conductive adhesive layer 14.

[0030] Graphite flakes are, for example, flat, plate-like particles with angular edges. The size of the graphite flakes can be defined by two values: in-plane length (particle size) and thickness. The particle size of the graphite flakes may be in the range of 1 μm to 100 μm, but preferably in the range of 5 μm to 50 μm, in order to impart high electrical conductivity to the conductive adhesive layer 14. The thickness of the graphite flakes is not particularly limited, but may be in the range of 100 nm to 1000 nm, for example. The content of the graphite flakes may be in the range of 68% to 87% by mass relative to the total mass of the conductive adhesive layer 14, for example.

[0031] The method for measuring the particle size of graphite flakes is as follows: For example, graphite flakes taken from the conductive adhesive layer 14 are imaged using a scanning electron microscope (SEM). Then, 30 graphite flakes are randomly selected from these SEM images. After identifying the outlines of the 30 selected graphite flakes, the in-plane length (longest length) of each of the 30 graphite flakes is determined, and the average value of these lengths is taken as the particle size of the graphite flakes. The thickness of the graphite flakes is the average thickness of the 30 graphite flakes randomly selected from the SEM images.

[0032] The fluororesin having a vinylidene skeleton contained in the conductive adhesive layer 14 has high chemical resistance. Therefore, even if the reduction reaction electrode 10 of this embodiment is in contact with or immersed in the electrolyte used for carbon dioxide reduction for a long time, the adhesive strength of the conductive adhesive layer 14 is maintained, and the decrease in adhesive strength between the substrate 12 and the catalyst layer 16 is suppressed. Furthermore, because the conductive adhesive layer 14 contains graphite flakes with a particle size in the range of 1 μm to 100 μm, the electrical conductivity of the conductive adhesive layer 14 is increased, and thus the electrical conductivity between the substrate 12 and the catalyst layer 16 is also increased. As a result, with the reduction reaction electrode 10 of this embodiment, it is possible to maintain a high current density for a long time when a constant voltage is applied during carbon dioxide reduction. Moreover, with the reduction reaction electrode 10 of this embodiment, it is also possible to maintain a constant cell voltage when constant current control is used during carbon dioxide reduction.

[0033] The conductive adhesive layer 14 preferably contains mesocarbon microbeads having a particle size of 5 μm to 30 μm, for example, in order to enhance the electrical conductivity of the conductive adhesive layer 14. The particle size of the mesocarbon microbeads is the average of the longest diameters of 30 mesocarbon microbeads randomly selected from SEM images of the mesocarbon microbeads.

[0034] The conductive adhesive layer 14 preferably contains a reduction catalyst. This increases the reaction area for carbon dioxide reduction, making it possible to maintain a higher current density for a longer period of time. The reduction catalyst is the same catalyst as the reduction catalyst contained in the catalyst layer 16.

[0035] An example of a method for fabricating the reduction reaction electrode of this embodiment will be described. For example, a reduction catalyst-containing solution, such as the Ru complex polymer solution mentioned above, is applied to a sheet containing a carbon material, such as a carbon sheet, and dried to form a catalyst layer on which the reduction catalyst is supported on the carbon material. Then, a conductive adhesive prepared by adding and mixing a fluororesin having a vinylidene skeleton, such as PVDF, and graphite flakes with a particle size of 1 μm to 100 μm to a solvent such as N-methyl-2-pyrrolidone (NMP) is applied to the substrate or the catalyst layer, and the substrate and the catalyst layer are bonded together via the conductive adhesive, and then dried. In this way, a conductive adhesive layer is formed between the substrate and the catalyst layer, and the substrate and the catalyst layer are bonded together. Note that as the solvent evaporates when the conductive adhesive is dried, the formed conductive adhesive layer becomes porous.

[0036] Another example of the method for manufacturing the reduction reaction electrode of this embodiment will be described. First, a conductive adhesive layer is formed between a sheet containing carbon material and a substrate, thereby bonding the sheet containing carbon material and the substrate (first step). Specifically, a conductive adhesive prepared by adding and mixing a fluororesin having a vinylidene skeleton such as PVDF and graphite flakes with a particle size of 1 μm to 100 μm to a solvent such as N-methyl-2-pyrrolidone (NMP) is applied to the sheet containing carbon material or the substrate, and the sheet containing carbon material and the substrate are bonded together via the conductive adhesive, and then dried. Note that as the conductive adhesive dries, the solvent evaporates, so the formed conductive adhesive layer becomes porous. Next, a reduction catalyst-containing solution is applied to the sheet containing carbon material (i.e., the surface of the sheet containing carbon material opposite to the substrate) and dried (second step). According to this manufacturing method, not only can a reduction catalyst-containing solution coated on a carbon material-containing sheet be impregnated into the carbon material-containing sheet, but a portion of the reduction catalyst-containing solution within the sheet can also be impregnated into the conductive adhesive layer. Therefore, a reduction reaction electrode containing the reduction catalyst in both the carbon material-containing sheet (catalyst layer) and the conductive adhesive layer can be produced.

[0037] [Responding device] The reaction device according to the embodiment of the present invention is a reaction device composed of a reduction reaction electrode and an oxidation reaction electrode. The reaction device can be, for example, a carbon dioxide reduction device, or it can be an artificial photosynthesis device that combines the carbon dioxide reduction device with a solar cell.

[0038] Figure 2 is a schematic diagram showing an example of the configuration of a carbon dioxide reduction device using the reduction reaction electrode described above. In the carbon dioxide reduction device 1, the reduction reaction electrode 10 that carries out the reduction reaction of carbon dioxide and the oxidation reaction electrode 18 that carries out the oxidation reaction of water are arranged in a containment section 28 at a distance from each other and facing each other. The containment section 28 has an inlet 22 and an outlet 24. An electrolyte containing a reaction substrate such as CO2 flows in from the inlet, and an electrolyte containing organic matter obtained by the reduction of CO2, such as formic acid, flows out from the outlet 24.

[0039] A DC voltage is applied from a power supply 30 between the reduction electrode 10 and the oxidation electrode 18, thereby promoting the reduction reaction at the reduction electrode 10 and the oxidation reaction at the oxidation electrode 18. At the oxidation electrode 18, water (H2O) is oxidized to obtain oxygen (1 / 2O2) and electrons are generated. At the reduction electrode 10, by receiving the electrons generated by the oxidation reaction, for example, carbon dioxide is reduced to produce formic acid (HCOOH). By using a solar cell as the power supply 30, organic substances such as formic acid can be obtained by reducing CO2 with solar energy.

[0040] The reduction reaction electrode 10 and the oxidation reaction electrode 18 may each have a configuration in which multiple electrodes are stacked, and a separator that separates liquid and gas and allows proton movement may be provided between the reduction reaction electrode 10 and the oxidation reaction electrode 18. The separator can be made of any material that separates liquid and gas and allows proton movement, and is not particularly limited, but for example, a solid polymer electrolytic membrane such as Nafion® can be used. Alternatively, a porous hydrophilic resin (for example, porous polyethylene) that is not an ion exchange membrane can also be used.

[0041] The oxidation reaction electrode 18 is an electrode used to oxidize water through an oxidation reaction. The oxidation reaction electrode 18 is composed of, for example, a substrate on which a conductive layer is formed and an oxidation catalyst layer formed on the substrate.

[0042] The reaction substrate contained in the electrolyte can be, for example, a carbon compound, such as carbon dioxide (CO2). Furthermore, the electrolyte is preferably a phosphate buffer solution or a borate buffer solution. In a specific example configuration, for instance, a tank of CO2-saturated phosphate buffer solution is provided, and this solution is supplied between the reduction reaction electrode 10 and the oxidation reaction electrode 18 by a pump. The formic acid (HCOOH) and other substances produced by the reduction reaction can then be collected in an external tank. "Structure of the present invention" Configuration 1: Substrate and A catalyst layer containing a reduction catalyst and a carbon material, The system comprises a conductive adhesive layer provided between the substrate and the catalyst layer, The conductive adhesive layer is characterized by comprising a fluororesin having a vinylidene skeleton and graphite flakes having a particle size in the range of 1 μm to 100 μm, thus forming a reduction reaction electrode. Configuration 2: The reduction reaction electrode according to configuration 1, characterized in that the conductive adhesive layer contains mesocarbon microbeads having a particle size in the range of 5 μm to 30 μm. Configuration 3: The reduction reaction electrode according to configuration 1 or 2, characterized in that the conductive adhesive layer contains a reduction catalyst. Configuration 4: The carbon material of the catalyst layer is a carbon sheet containing carbon nanotubes. The reduction reaction electrode according to any one of configurations 1 to 3, characterized in that the reduction catalyst is supported on the carbon sheet. Configuration 5: The reduction reaction electrode according to any one of configurations 1 to 4, characterized in that the reduction catalyst contains a Ru complex polymer. Configuration 6: The reduction reaction electrode according to any one of configurations 1 to 5, characterized in that the fluororesin having a vinylidene skeleton contains polyvinylidene fluoride (PVDF). Composition 7: A first step involves bonding the carbon material sheet and the substrate by providing a conductive adhesive between the carbon material sheet and the substrate, the adhesive containing a fluororesin having a vinylidene skeleton and graphite flakes with particle sizes ranging from 1 μm to 100 μm. A method for producing a reduction reaction electrode, characterized by comprising a second step of applying a reduction catalyst-containing solution to a sheet containing the carbon material and drying it. [Examples]

[0043] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0044] <Example 1> A carbon sheet containing carbon nanotubes (hereinafter referred to as a CP / MWCNTs sheet) was prepared by using an ink in which 5 parts by mass of multi-walled carbon nanotubes (MWCNTs) were dispersed in a solvent (95 parts by mass of ethanol), dipping the ink onto carbon paper (CP), and then drying it at 350°C.

[0045] A Ru complex polymer solution was prepared by mixing 0.0414 g of Ru complex monomer ([Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2Cl2]), 1.65 mL of 0.22 M iron chloride (FeCl3) ethanol solution, 0.33 mL of 0.72 mM pyrrole acetonitrile solution, and 6.33 mL of acetonitrile (total volume 8.31 mL). This solution was applied to a CP / MWCNTs sheet and dried at 30°C for 3 hours to prepare a catalyst layer (hereinafter referred to as CP / MWCNTs / RuCP sheet) on which the Ru complex polymer was supported.

[0046] A conductive adhesive (ratio of graphite flakes to total mass of PVDF and graphite flakes: 68% by mass) was prepared by mixing 2g of PVDF binder (Kureha Corporation, KF Polymer L #1120 (88% NMP, 12% PVDF)) with 0.5g of graphite flakes (Lonza Corporation, KS44, particle size: 40-50 μm) and letting it stand for 24 hours. After applying this conductive adhesive to a Ti substrate, a CP / MWCNTs / RuCP sheet was placed on top of it, and the mixture was heated and dried at 100°C for 3 hours to fabricate a reduction reaction electrode with a conductive adhesive layer between the Ti substrate and the CP / MWCNTs / RuCP sheet.

[0047] Figure 3 is an SEM image of the surface of the conductive adhesive layer formed by the conductive adhesive prepared in Example 1. Specifically, the conductive adhesive layer shown in Figure 3 was formed by applying the conductive adhesive prepared in Example 1 onto a Ti substrate and then heating and drying it at 100°C for 3 hours. As can be seen from Figure 3, the conductive adhesive layer has a porous structure in which gaps exist partially between the graphite flakes.

[0048] <Example 2> A conductive adhesive (ratio of graphite flakes to the total mass of PVDF and graphite flakes: 80% by mass) was prepared in the same manner as in Example 1, except that the amount of graphite flakes (Lonza, KS44, particle size: 40-50 μm) added was 1 g. Then, a reduction reaction electrode was fabricated in the same manner as in Example 1, except that this conductive adhesive was used.

[0049] <Example 3> A conductive adhesive (ratio of graphite flakes to total mass of PVDF and graphite flakes: 87% by mass) was prepared in the same manner as in Example 1, except that the amount of graphite flakes (Lonza, KS44, particle size: 40-50 μm) added was 1.5 g. Then, a reduction reaction electrode was fabricated in the same manner as in Example 1, except that this conductive adhesive was used.

[0050] <Comparative Example 1> The reduction reaction electrode was prepared in the same manner as in Example 1, except that a commercially available #15-1137 graphite paste (described in Non-Patent Literature 1, containing graphite powder with a particle size of 100 nm or less and an acrylic polymer binder) was used as the conductive adhesive.

[0051] Figure 4 is an SEM image of the surface of the conductive adhesive layer formed with the conductive adhesive used in Comparative Example 1. Specifically, the conductive adhesive layer shown in Figure 4 was formed by applying the graphite paste used in Comparative Example 1 onto a Ti substrate and then heating and drying it at 100°C for 3 hours. As can be seen from Figure 4, the conductive adhesive layer formed with commercially available graphite paste had aggregated graphite powder and did not have a porous structure like the conductive adhesive layer formed with the conductive adhesive in Example 1.

[0052] <Comparative Example 2> A conductive adhesive was prepared by mixing 0.5 g of graphite powder with a particle size of 100 nm or less with 2 g of PVDF binder (Kureha Corporation, KF Polymer L #1120 (88% NMP, 12% PVDF)) and letting it stand for 24 hours. A reduction reaction electrode was prepared in the same manner as in Example 1, except that this conductive adhesive was used.

[0053] [Electrochemical evaluation] Using the reduction reaction electrodes from Examples 1-3, a three-electrode cell (counter electrode: Pt, reference electrode: saturated calomel electrode (SCE)) was constructed, and the current-time (it) characteristics were evaluated using a potentiostat / galvanostat while bubbling carbon dioxide gas (CO2) into the electrolyte in the three-electrode cell. The current-time (it) characteristics were measured by operating the cell for 100 hours with a constant voltage of -1.2V (vs Hg / Hg2SO4). A 0.4M aqueous phosphate buffer solution (K2HPO4 + KH2PO4) was used as the electrolyte.

[0054] Figure 5 shows the change in current density when the reduction reaction electrodes of Examples 1 to 3 were operated for 100 hours. The current density in the reduction reaction electrode at the start of operation was 2.9 mA / cm² for Example 1. 2 Example 2 showed a reading of 4.0 mA / cm². 2 Example 3 showed a reading of 4.2 mA / cm². 2In other words, the higher the content of graphite flakes, the higher the current density obtained. Also, the ratio of the current density after 100 hours to the current density at the start of operation (current density maintenance rate) was 59% for Example 1, 53% for Example 2, and 58% for Example 3.

[0055] Next, the current-time (it) characteristics were evaluated using the reduction reaction electrodes of Example 2 and Comparative Examples 1 and 2, in the same manner as described above. However, the operating time of the reduction reaction electrodes in Example 2 and Comparative Example 1 was set to 500 hours, and the operating time of the reduction reaction electrode in Comparative Example 2 was set to 200 hours.

[0056] Figure 6 shows the current density changes when the reduction reaction electrodes of Example 2 and Comparative Examples 1-2 were operated for a predetermined time. In Figure 6, the current density is plotted every 100 hours. As shown in Figure 6, the reduction reaction electrode of Example 2 had a higher current density than Comparative Examples 1-2, and the ratio of the current density after 500 hours to the current density at the start of operation was higher than that of Comparative Example 1. From these results, it can be said that by using a reduction reaction electrode in which a conductive adhesive layer containing large-particle-sized graphite flakes and PVDF is formed between the substrate and the catalyst layer, it is possible to maintain a high current density for a long time. In Comparative Example 1, the substrate and catalyst layer (CP / MWCNTs / RuCP sheet) peeled off after 500 hours. In Example 2, after the same test, the substrate and catalyst layer did not peel off, and the adhesive strength was improved.

[0057] <Example 4> A conductive adhesive was prepared by mixing slurry A, which consisted of 2 g of PVDF binder (Kureha Corporation, KF Polymer L #1120 (88% NMP, 12% PVDF)) and 0.5 g of graphite flakes (Lonza Corporation, KS44, particle size: 40-50 μm), with slurry B, which consisted of 10 g of PVDF binder (Kureha Corporation, KF Polymer L #1120 (88% NMP, 12% PVDF)) and 2 g of mesocarbon microbeads (Osaka Gas Chemical Co., Ltd., particle size: 25-28 μm). After mixing these two slurry mixtures, the mixtures were left to stand for 24 hours. A reduction reaction electrode was then prepared in the same manner as in Example 1, except that this conductive adhesive was used.

[0058] <Example 5> A conductive adhesive was prepared by mixing slurry A, which consisted of 2 g of PVDF binder (Kureha Corporation, KF Polymer L #1120 (88% NMP, 12% PVDF)) and 0.5 g of graphite flakes (Lonza Corporation, KS44, particle size: 40-50 μm), with slurry B, which consisted of 10 g of PVDF binder (Kureha Corporation, KF Polymer L #1120 (88% NMP, 12% PVDF)) and 2 g of mesocarbon microbeads (Osaka Gas Chemical Co., Ltd., particle size: 6-28 μm). After mixing these two slurry mixtures, the mixtures were left to stand for 24 hours. A reduction reaction electrode was then prepared in the same manner as in Example 1, except that this conductive adhesive was used.

[0059] The current-time (it) characteristics were evaluated using the reduction reaction electrodes of Examples 4 and 5, in the same manner as described above.

[0060] Figure 7 shows the current density retention rate results when the reduction reaction electrodes of Examples 1, 4, 5, and Comparative Example 1 were operated for 100 hours. The current density retention rate shown in Figure 7 is the ratio of the current density after 100 hours to the current density at the start of operation. As shown in Figure 7, the current density retention rates of the reduction reaction electrodes of Examples 1, 4, and 5 were 59%, 67%, and 58%, respectively. On the other hand, the current density retention rate of Comparative Example 1 was 38%, which was significantly lower than that of the examples.

[0061] <Example 6> After applying the conductive adhesive used in Example 2 onto a Ti substrate, a CP / MWCNTs sheet was placed on top of it and heated and dried at 100°C for 3 hours to form a conductive adhesive layer between the Ti substrate and the CP / MWCNTs sheet. Then, the aforementioned Ru complex polymer solution was applied onto the CP / MWCNTs sheet and dried at 60°C for 1 hour to support the Ru complex polymer. In this way, a reduction reaction electrode containing the Ru complex polymer in the CP / MWCNTs sheet and conductive adhesive layer was fabricated.

[0062] The current-time (it) characteristics were evaluated using the reduction reaction electrodes of Examples 2, 3, and 6 and Comparative Example 1, in the same manner as described above. However, the operating times of the reduction reaction electrodes were 1000 hours for Comparative Example 1, 800 hours for Example 2, 600 hours for Example 3, and 300 hours for Example 6.

[0063] Figure 8 shows the current density changes when the reduction reaction electrodes of Examples 2, 3, 6, and Comparative Example 1 were operated for a predetermined time. As shown in Figure 8, Example 6 showed the highest current density maintenance rate up to 300 hours of operation. By using a reduction reaction electrode in which the Ru complex polymer is included not only in the catalyst layer but also in the conductive adhesive layer, as in Example 6, it is possible to maintain a higher current density for a longer period of time. [Explanation of Symbols]

[0064] 1 carbon dioxide reduction device, 10 reduction reaction electrode, 12 substrate, 14 conductive adhesive layer, 16 catalyst layer, 18 oxidation reaction electrode, 22 inlet, 24 outlet, 28 housing, 30 power supply.

Claims

1. Substrate and A catalyst layer containing a reduction catalyst and a carbon material, The system comprises a conductive adhesive layer provided between the substrate and the catalyst layer, The conductive adhesive layer comprises a fluororesin having a vinylidene skeleton, graphite pieces having a particle size in the range of 1 μm to 100 μm, and a reduction catalyst. The content of the fluororesin having a vinylidene skeleton is in the range of 1% to 20% by mass relative to the total mass of the conductive adhesive layer. The content of the graphite pieces is in the range of 68% to 87% by mass with respect to the total mass of the conductive adhesive layer. A reduction reaction electrode characterized in that the graphite pieces are plate-shaped particles, and the particle size represents the longest length of the plate-shaped particles in the in-plane direction.

2. The reduction reaction electrode according to claim 1, characterized in that the conductive adhesive layer contains mesocarbon microbeads having a particle size in the range of 5 μm to 30 μm.

3. The carbon material of the catalyst layer is a carbon sheet containing carbon nanotubes. The reduction reaction electrode according to claim 1 or 2, characterized in that the reduction catalyst is supported on the carbon sheet.

4. The reduction reaction electrode according to claim 1 or 2, characterized in that the reduction catalyst contains a Ru complex polymer.

5. The reduction reaction electrode according to claim 1 or 2, characterized in that the fluororesin having a vinylidene skeleton contains polyvinylidene fluoride (PVDF).

6. A first step involves forming a conductive adhesive layer between a sheet containing carbon material and a substrate by providing a conductive adhesive containing a fluororesin having a vinylidene skeleton and graphite flakes with particle sizes ranging from 1 μm to 100 μm, thereby bonding the sheet containing carbon material and the substrate. The process comprises a second step of applying a reduction catalyst-containing solution to a sheet containing the carbon material and drying it, The content of the fluororesin having a vinylidene skeleton is in the range of 1% to 20% by mass relative to the total mass of the conductive adhesive layer. The content of the graphite pieces is in the range of 68% to 87% by mass with respect to the total mass of the conductive adhesive layer. A method for manufacturing a reduction reaction electrode, characterized in that the graphite pieces are plate-shaped particles, and the particle size represents the longest length in the in-plane direction of the plate-shaped particles.

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