Electrode for reduction reaction, method for manufacturing electrode for reduction reaction, reaction device, and artificial photosynthesis apparatus

Abstract

To provide an electrode for a reduction reaction excellent in durability, a method of producing the electrode for a reduction reaction, and a reaction device using the electrode for a reduction reaction.SOLUTION: An electrode 10 for a reduction reaction, comprises a substrate 12; and a catalyst layer 16 including a composite substrate constituted of a carbon fiber including a carbon material and carbon, a heterocyclic aromatic compound of a 5-9 membered ring structure including at least one of an amino group, an alkylamino group, a carboxyl group, and a hydroxyl group and at least one of N, S, and B, and a Ru complex polymer constituted to include a Ru complex monomer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a reduction reaction electrode, a method for manufacturing the reduction reaction electrode, a reaction device using the reduction reaction electrode, and an artificial photosynthesis apparatus. [Background technology]

[0002] Reaction device technology has been disclosed for use in artificial photosynthesis, which uses solar energy to synthesize hydrogen (H2) from water (H2O), and carbon monoxide (CO), formic acid (HCOOH), and methanol (CH3OH) from water (H2O) and carbon dioxide (CO2). To put this technology into practical use, it is necessary to realize a reaction electrode that can cause reactions with high efficiency.

[0003] In a reaction device, a reduction electrode and an oxidation electrode are used in combination. For example, Patent Document 1 discloses a catalyst layer comprising a substrate; a composite substrate of carbon fiber and carbon; and a Ru complex polymer comprising a Ru complex monomer and a heterocyclic aromatic compound having a 5- to 9-membered ring structure containing at least one of N, S, and B; and the content of the Ru complex monomer in the Ru complex polymer in the catalyst layer is 3.35×10 -6 mol / cm 2 Exceeding 6.70 x 10 -6 mol / cm 2 The content of heterocyclic aromatic compounds is in the range of less than 1.32 × 10 -6 mol / cm 2 exceeding 2.64 × 10 -6 mol / cm 2 In the electrode for reduction reaction in Patent Document 1, the component ratio of the Ru complex polymer is optimized to improve the current density and the Faraday efficiency of formic acid production, and it is possible to obtain a reduction reaction electrode using the Ru complex polymer that can be made large in area and high in output.

[0004] However, the reduction reaction electrode prepared according to the method of Patent Document 1 has a problem of low durability during long-term continuous operation. Specifically, although performance can be maintained for, for example, about 6 hours of operation, problems become apparent in that the current density and the Faraday efficiency of formic acid production decrease after several weeks of operation. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-063246 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a reduction reaction electrode having excellent durability, a method for manufacturing the reduction reaction electrode, a reaction device using the reduction reaction electrode, and an artificial photosynthesis apparatus. [Means for solving the problem]

[0007] The present invention provides an electrode for reduction reactions, comprising: a substrate; a composite substrate of carbon and carbon fiber containing a carbon material; and a catalyst layer containing a Ru complex polymer constituted by a Ru complex monomer and a heterocyclic aromatic compound having a 5- to 9-membered ring structure containing at least one of N, S, and B, which has at least one of an amino group, an alkylamino group, a carboxyl group, and a hydroxyl group.

[0008] In the reduction reaction electrode, the heterocyclic aromatic compound is preferably 2-(1H-pyrrol-1-yl)ethanamine or 1H-pyrrol-1-amine.

[0009] In the reduction reaction electrode, the Ru complex monomer is preferably [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2Cl2].

[0010] In the reduction electrode, the content of the heterocyclic aromatic compound in the Ru complex polymer in the catalyst layer is preferably in the range of 0.003 to 2.5 in terms of molar ratio relative to the content of the Ru complex monomer.

[0011] The present invention is a method for producing an electrode for a reduction reaction, the method comprising: a preparation step of preparing a Ru complex polymer solution containing the Ru complex monomer, the heterocyclic aromatic compound, a polymerization catalyst, and a solvent; a treatment step of subjecting a composite substrate of carbon and carbon fiber containing a carbon material to ultraviolet ray ozone treatment; and an application step of applying the prepared Ru complex polymer solution to the composite substrate that has been subjected to the ultraviolet ray ozone treatment, and drying the solution.

[0012] The present invention is a reaction device configured by combining the reduction reaction electrode and the oxidation reaction electrode.

[0013] The present invention is an artificial photosynthesis apparatus including the reaction device and a solar cell that generates electricity to be supplied to the oxidation reaction electrode and the reduction reaction electrode. [Effects of the Invention]

[0014] The present invention can provide a reduction reaction electrode having excellent durability, a method for manufacturing the reduction reaction electrode, and a reaction device and an artificial photosynthesis apparatus that use the reduction reaction electrode. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic diagram showing an example of the configuration of an electrode for a reduction reaction according to an embodiment of the present invention. FIG. [Figure 2] FIG. 1 is a schematic diagram showing an example of the configuration of a reaction device (artificial photosynthesis apparatus) using an electrode for a reduction reaction according to an embodiment of the present invention. [Figure 3]Photographs showing the appearance of the Ru complex polymer solutions prepared in Examples 1 to 6 and Comparative Example 1 ((e) Example 1, (d) Example 2, (f) Example 3, (g) Example 4, (h) Example 5, (i) Example 6, (c) Comparative Example 1), as well as the appearance of a Ru complex monomer solution (without pyrrole and iron chloride) and a solution of Ru complex monomer and iron chloride (without pyrrole and with iron chloride). [Figure 4] 1 is a graph showing current density (mA / cm 2 ) versus operating time (h) in Example 1 and Comparative Example 1. [Figure 5] 1 is a graph showing the Faraday efficiency (%) of formic acid production versus operation time (h) in Example 1, Example 2, and Comparative Example 1. [Figure 6] 1 is a graph showing current density (mA / cm 2 ) versus operating time (h) in Examples 1, 3, 4, 5, and 6. [Figure 7] 1 is a graph showing the Faraday efficiency (FE) (%) of formic acid production versus operation time (h) in Examples 1, 3, 4, 5, and 6. [Figure 8] FIG. 1 shows the results of waveform separation of C1s spectra of (a) untreated, (b) UV ozone treated, (c) pyrrole, and (d) pyrrole derivative 1 samples. [Figure 9] FIG. 1 shows XPS spectra of each element of (A) C1s, (B) N1s, (C) O1s, and (D) Si2p for each sample. DETAILED DESCRIPTION OF THE INVENTION

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes an embodiment of the present invention. The embodiment is an example of implementing the present invention, and the present invention is not limited to the embodiment.

[0017] [Electrode for reduction reaction] FIG. 1 is a schematic diagram illustrating 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 substrate 12; a composite substrate of carbon fiber containing a carbon material and carbon; and a catalyst layer 16 including a Ru complex polymer composed of a Ru complex monomer and a heterocyclic aromatic compound having a 5- to 9-membered ring structure containing at least one of N, S, and B, which has at least one of an amino group, an alkylamino group, a carboxyl group, and a hydroxyl group. The catalyst layer 16 may be bonded to the substrate 12 by an adhesive layer 14. When a voltage is applied to the reduction reaction electrode 10, the charge generated in the substrate 12 is transferred to the catalyst layer 16 and utilized in a reduction catalytic reaction in the catalyst layer 16, for example, a reaction in which carbon dioxide (CO2) is reduced to formic acid (HCOOH).

[0018] The substrate 12 is a member that structurally supports the electrode. The substrate 12 is not particularly limited in material, but examples thereof include a glass substrate. The substrate 12 may include, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but examples thereof include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), lead (Pb), and titanium (Ti). The semiconductor used as the substrate is not particularly limited, but examples thereof include titanium oxide (TiO), tin oxide (SnO), silicon (Si), strontium titanate (SrTiO), zinc oxide (ZnO), and tantalum oxide (TaO). When the substrate includes a metal or a semiconductor, an insulating layer may be formed between the catalyst layer and the substrate. The insulating layer is not particularly limited, but examples thereof include semiconductor oxides, nitrides, and resins. The metal substrate and the catalyst layer may be electrically connected directly to each other. When a metal substrate is used, the conductive layer may not be required.

[0019] The substrate 12 may contain carbon fiber. The substrate 12 is, for example, a composite substrate of carbon and carbon fiber heat-treated at high temperature, such as carbon paper or carbon cloth. Carbon paper (carbon sheet) is, for example, a sheet-shaped substrate obtained by impregnating organic fibers such as polyacrylonitrile (PAN) fibers with a dispersion of polyvinyl alcohol and an aqueous medium, carbonizing the fibers at approximately 2000°C, and bonding the fibers. Carbon paper may also contain approximately 25% by mass of a Teflon-based material. Carbon cloth is woven from carbon fibers obtained by baking and carbonizing organic fibers. Carbon paper and carbon cloth are porous and have countless pores of several tens of μm (approximately 10 μm to 100 μm). The thickness of the carbon paper or carbon cloth is, for example, in the range of 0.1 mm to 0.4 mm per sheet. This composite substrate may be multilayered (for example, 5 to 10 layers) to have a thickness of approximately 10 times, in the range of 1 mm to 4 mm. A multilayered composite substrate can be formed, for example, by laminating multiple pieces of carbon paper, carbon cloth, etc. using a carbon-based adhesive containing a conductive carbon material, such as a polymer (e.g., an acrylic polymer binder) containing graphite or graphene as the adhesive.

[0020] The use of a composite substrate of carbon fiber and carbon as the substrate 12 in the reduction reaction electrode enables multilayering, large area, and high output. Because the substrate has excellent compressibility and is lightweight, it is easy to stack the electrode in the surface direction to create multiple layers (e.g., 10 or more layers). This allows for high output (high current) of the reaction device. Because the composite substrate of carbon fiber and carbon is lightweight, the entire electrode can be made lighter and large area can be easily achieved. Furthermore, the composite substrate of carbon fiber and carbon can ensure high conductivity, and its porous nature allows for good flow of reaction substrates and reaction products, which is thought to suppress side reactions such as hydrogen generation and improve the reduction reaction efficiency of carbon compounds. It also has excellent corrosion resistance, even in an electrolyte with a pH of 6.5 to 7. Furthermore, the surface smoothness can be easily improved by, for example, filling the gaps in the composite substrate with a carbon material such as carbon, resulting in excellent surface smoothness.

[0021] The catalyst layer 16 can be attached to the substrate 12 via an adhesive layer 14. In this case, it is preferable to use a carbon-based adhesive containing a polymer (e.g., an acrylic polymer binder) containing a conductive carbon material, such as graphite or graphene, as the adhesive for the adhesive layer 14. It is preferable that the adhesive layer 14 contains, for example, at least graphite or graphene with a diameter of 1 μm or more and 100 μm or less.

[0022] The catalyst layer 16 includes a composite substrate of carbon fiber and carbon, including a carbon material such as multi-walled carbon nanotubes (MWCNTs), and a Ru complex polymer as a reduction catalyst for reduction reactions such as carbon dioxide reduction. The Ru complex polymer is a catalyst that reduces carbon dioxide and converts it into useful organic substances (such as formic acid). The catalyst layer 16 may be a carbon fiber and carbon composite substrate supported with a reduction catalyst, or a carbon fiber and carbon composite substrate with a reduction catalyst layer formed on the carbon fiber and carbon composite substrate. When a carbon fiber and carbon composite substrate is used as the substrate 12, using a carbon fiber and carbon composite substrate for the catalyst layer 16 is thought to stabilize the electrochemical properties and enable high current because the electrodes are made of the same materials.

[0023] As mentioned above, conventional reduction electrodes made with Ru complex polymers have a problem of low durability during long-term continuous operation. Specifically, while performance can be maintained for approximately six hours of operation, problems of a decrease in current density and the Faraday efficiency for formic acid production become apparent after several weeks of operation. One of the causes of this low durability is thought to be the desorption of the Ru complex polymer from the electrode. Conventional reduction electrodes using Ru complex polymers are polymerized using Ru complex monomers with pyrrole or other compounds and iron chloride as a polymerization catalyst, and then supported on a composite substrate such as porous carbon fiber. However, the Ru complex polymer is physically adsorbed to the composite substrate and has little interaction with the composite substrate, which is thought to lead to the desorption of the Ru complex polymer during long-term continuous operation.

[0024] The inventors have discovered that by using a derivative in which a functional group such as an amino group or an alkylamino group has been introduced into a heterocyclic aromatic compound such as pyrrole, which is a polymerization agent for polymerizing a Ru complex monomer, it is possible to create an interaction through chemical bonding between a composite substrate of carbon fiber and carbon, on which a carbon material such as a multi-walled carbon nanotube (MWCNT) is supported, and the Ru complex, or between the Ru complex polymer and the Ru complex, thereby immobilizing the Ru complex catalyst and suppressing detachment of the Ru complex, thereby improving the durability of the catalytic electrode.

[0025] Furthermore, by using derivatives of heterocyclic aromatic compounds with functional groups such as amino or alkylamino groups, the degree of polymerization of the Ru complex monomer can be controlled to reduce the particle size of the Ru complex polymer, thereby increasing the number of active sites for the electrochemical reaction and improving the current density.

[0026] In this way, by using the reduction reaction electrode according to this embodiment, it is possible to improve the durability of a reaction device that performs a reaction of producing formic acid, etc. by reducing carbon dioxide during long-term operation. Specifically, it is possible to suppress a decrease in current density during continuous operation (e.g., 1000 hours or more) of electrochemical reduction of carbon dioxide under the condition of applying a constant potential, and further to suppress a decrease in the Faraday efficiency (FE) of the production of formic acid, etc.

[0027] The composite substrate of carbon fiber and carbon used in the catalyst layer 16 may be the same as the composite substrate of carbon fiber and carbon used as the substrate 12 .

[0028] The catalyst layer 16 contains a carbon material such as carbon nanotubes, including multi-walled carbon nanotubes (MWCNTs). By incorporating a carbon material such as multi-walled carbon nanotubes (MWCNTs), a three-dimensional nanocarbon network structure can be formed. The multi-walled carbon nanotubes preferably have a diameter of at least 1 nm to 100 nm. Carbon materials such as multi-walled carbon nanotubes (MWCNTs) can be incorporated into the catalyst layer 16 by preparing an ink in which the carbon material is highly dispersed in a solvent, such as ethanol, applying the ink to a carbon fiber and carbon composite substrate by dip coating or impregnation coating, and then drying the ink.

[0029] Examples of ruthenium complexes (Ru complex monomers) for obtaining Ru complex polymers include [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] nand [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(CHCN)Cl]. The Ru complex polymer is a Ru complex polymer that has been polymerized using, for example, a Ru complex monomer, a polymerizing agent (for example, a pyrrole derivative having a functional group) that constitutes the polymer, and a polymerization catalyst (for example, iron chloride).

[0030] The Ru complex polymer can be supported, for example, by applying a solution (Ru complex polymer solution) prepared by dissolving the Ru complex monomer, a polymerizing agent (e.g., a pyrrole derivative having a functional group) that constitutes the polymer, and a polymerization catalyst (e.g., iron chloride) in a solvent such as acetonitrile (MeCN) onto a composite substrate of carbon fiber containing a carbon material and carbon, and then drying the solution.

[0031] The polymerization agent that constitutes the polymer is a heterocyclic aromatic compound having a 5- to 9-membered ring structure containing at least one of N, S, and B, such as pyrrole, pyridine, thiophene, borepin, or azonin, which has at least one of amino, alkylamino, carboxyl, and hydroxyl groups as a functional group. Of these, pyrrole, which has at least one of amino, alkylamino, carboxyl, and hydroxyl groups, is preferred because it can easily initiate a reaction by oxidation.

[0032] Examples of pyrroles having at least one of an amino group, an alkylamino group, a carboxyl group, and a hydroxyl group include pyrroles having the following structural formula: [ka] (In the formula, R 1 represents an amino group, a linear alkylamino group having 1 to 3 carbon atoms, a linear hydroxyalkyl group having 1 to 3 carbon atoms, a linear carboxylalkyl group having 1 to 3 carbon atoms, a hydroxyl group, or a carboxyl group; R 2 ~R 5are independently a hydrogen atom, a linear alkyl group having 1 to 3 carbon atoms, a linear alkylamino group having 1 to 3 carbon atoms, or a linear carboxylalkyl group having 1 to 3 carbon atoms.

[0033] In composite substrates of carbon fiber containing carbon materials such as multi-walled carbon nanotubes (MWCNTs), OH groups, COOH groups, etc. are present at surface defects of the carbon. It is believed that the reaction between at least one functional group of the amino group, alkylamino group, carboxyl group, or hydroxyl group possessed by the heterocyclic aromatic compound contained in the Ru complex polymer and the hydroxyl group (-OH group) or carboxyl group (-COOH group) present at the surface defects of the carbon in the carbon fiber / carbon composite substrate immobilizes the Ru complex polymer containing the heterocyclic aromatic compound on the surface of the composite substrate, thereby preventing the Ru complex polymer from detaching.

[0034] For example, the following are examples of chemical bonds formed by a reaction between the -OH or -COOH groups of a composite substrate of carbon fiber containing a carbon material such as multi-walled carbon nanotubes (MWCNTs) and a pyrrole having an amino group, a carboxyl group, or a hydroxyl group contained in a Ru complex polymer. (amide bond formation) MWCNT-COOH + Ru complex polymer containing a pyrrole derivative with an NH2 group → MWCNT-CONH-pyrrole-Ru complex (ester bond formation) MWCNT-COOH + Ru complex polymer containing pyrrole derivative with OH group → MWCNT-COO-pyrrole-Ru complex MWCNT-OH + Ru complex polymer containing a pyrrole derivative with COOH groups → MWCNT-OOC-pyrrole-Ru complex

[0035] It is preferable that the composite substrate of carbon and carbon fiber containing a carbon material such as multi-walled carbon nanotubes (MWCNTs) is treated with ultraviolet light and ozone. It is believed that the ultraviolet light and ozone treatment forms carbon defects on the surface of the composite substrate containing the carbon material, promoting and increasing the formation of functional groups (-OH, -COOH, etc.) on the carbon surface.

[0036] Examples of polymerization catalysts (chemical oxidation polymerization catalysts) include iron salts such as iron chloride (FeCl3), FeCl3·O2, and FeCl3·O2-ClO4, copper salts such as copper chloride, and aluminum salts such as aluminum chloride, with iron chloride (FeCl3), FeCl3·O2, and FeCl3·O2-ClO4 being preferred due to their high catalytic activity.

[0037] Examples of the solvent include acetonitrile, diethyl ether, N,N-dimethylformamide (DMF), and tetrahydrofuran (THF), and acetonitrile is preferred because it allows the Ru complex polymer to be highly dispersed.

[0038] In the reduction reaction electrode according to this embodiment, the content of the Ru complex monomer in the Ru complex polymer in the catalyst layer is 3.0 × 10 -6 mol / cm 2 ~6.7×10 -6 mol / cm 2 The range is preferably 4.5×10 -6 mol / cm 2 ~5.1×10 -6 mol / cm 2 If the content of the Ru complex monomer in the Ru complex polymer in the catalyst layer is outside this range, the obtained reduction current value may decrease.

[0039] In the reduction reaction electrode according to this embodiment, the content of the heterocyclic aromatic compound having a functional group in the Ru complex polymer in the catalyst layer relative to the content of the Ru complex monomer is preferably in the range of 0.003 to 2.5 in terms of molar ratio, and more preferably in the range of 0.004 to 2.0. If the content of the heterocyclic aromatic compound having a functional group in the Ru complex polymer in the catalyst layer relative to the content of the Ru complex monomer is less than 0.003 in terms of molar ratio, the degree of polymerization may be insufficient, whereas if it exceeds 2.5, the catalytic activity for reducing carbon dioxide may decrease.

[0040] When a Ru complex polymer is supported on a composite substrate / carbon material sheet (e.g., a carbon sheet (CS) / MWCNTs sheet supporting MWCNTs), if the content of the Ru complex polymer, which serves as a reduction catalyst, is low, the number of catalytically active sites is small, which may result in a small current involved in the CO reduction reaction, a small amount of charge, and a low production amount of formic acid and other compounds. Furthermore, if the content of the Ru complex polymer is increased, the current may decrease, the amount of charge may decrease, and the production amount of formic acid and other compounds may also decrease. This is thought to be because the thick deposition of the Ru complex polymer increases charge transfer resistance. In contrast, by setting the content of the Ru complex polymer within a certain range, the number of catalytically active sites is increased, which may result in an increase in current, a large amount of charge, and an increase in the production amount of formic acid and other compounds. Therefore, there is a preferred content of the Ru complex polymer that increases the number of catalytically active sites without increasing charge transfer resistance.

[0041] The method for producing a reduction electrode according to this embodiment includes, for example, a preparation step of preparing a Ru complex polymer solution containing a Ru complex monomer, a heterocyclic aromatic compound having a 5- to 9-membered ring structure containing at least one of N, S, and B, and having at least one of an amino group, an alkylamino group, a carboxyl group, and a hydroxyl group, a polymerization catalyst, and a solvent; a treatment step of subjecting a composite substrate of carbon fiber containing a carbon material and carbon to ultraviolet ozone treatment; and a coating step of applying the prepared Ru complex polymer solution to the ultraviolet ozone-treated composite substrate and drying. After the coating step, the method may further include a cleaning step in which the composite substrate is immersed in, for example, ultrapure water and then dried at, for example, 60 to 120°C. The cleaning step can reduce the amount of remaining polymerization catalyst, etc.

[0042] For example, a UV ozone cleaner is used, and the ultraviolet intensity is 5.2 mW / cm 2 Under the condition of 30 minutes of treatment, UV-ozone treatment can be performed on a composite substrate of carbon fiber and carbon containing carbon materials such as multi-walled carbon nanotubes (MWCNTs). It is thought that UV-ozone treatment forms defects in the carbon of the composite substrate and promotes the formation of functional groups (-OH, -COOH, etc.) on the carbon surface.

[0043] For example, a carbon material dispersion, in which a carbon material such as MWCNTs is dispersed in a solvent, is impregnated and applied to a carbon fiber / carbon composite substrate, followed by drying to obtain a composite substrate / carbon material sheet (e.g., CS / MWCNTs sheet). The carbon fiber / carbon composite substrate carrying the carbon material is treated with ultraviolet light and ozone (treatment step). Meanwhile, a Ru complex polymer solution is prepared (preparation step) containing a Ru complex monomer, a polymerization catalyst (e.g., iron chloride), a polymerizing agent (a heterocyclic aromatic compound having a 5- to 9-membered ring structure containing at least one of N, S, and B, and having at least one of an amino group, an alkylamino group, a carboxyl group, and a hydroxyl group) (e.g., 2-(1H-pyrrol-1-yl)ethanamine, 1H-pyrrol-1-amine, etc.), and a solvent. The prepared Ru complex polymer solution is then applied to the UV- and ozone-treated carbon material-carrying composite substrate / carbon material sheet (e.g., CS / MWCNTs sheet) (application step). The composite substrate / carbon material sheet coated with the Ru complex polymer solution is dried by vacuum drying or the like to obtain a composite substrate / carbon material / catalyst sheet (e.g., CS / MWCNTs / RuCP sheet). This composite substrate / carbon material / catalyst sheet (e.g., CS / MWCNTs / RuCP sheet) is adhesively bonded to a substrate with an adhesive layer containing, for example, a polymer and a carbon material such as graphite, to obtain a reduction reaction electrode 10 having a substrate 12, a composite substrate of carbon fiber containing a carbon material and carbon, and a catalyst layer 16 (e.g., CS / MWCNTs / RuCP) containing the Ru complex polymer.

[0044] A storage step may be provided in which the prepared Ru complex polymer solution is stored, for example, at −15 to 5° C. for 5 to 15 hours, preferably at 0 to 5° C. for 12 to 15 hours. When the storage time and storage temperature are within the above ranges, the reduction current value of the reduction reaction electrode may be improved.

[0045] [Reaction Device] The reaction device according to the embodiment of the present invention is a reaction device configured by combining the above-mentioned reduction reaction electrode and oxidation reaction electrode. The reaction device can be, for example, a carbon dioxide reduction device, or can be an artificial photosynthesis device by combining the carbon dioxide reduction device with a solar cell.

[0046] The carbon dioxide reduction device is, for example, an electrochemical reaction device in which the reduction reaction electrode, which causes the reduction reaction of carbon dioxide, and an oxidation reaction electrode, which is electrically connected to the reduction reaction electrode and causes an oxidation reaction, are arranged at positions facing each other with a gap between them, and which has a flow path through which an electrolyte solution containing a reaction substrate (carbon dioxide) flows between the reduction reaction electrode and the oxidation reaction electrode.The carbon dioxide reduction device may be, for example, a device that has the reduction reaction electrode, which causes the reduction reaction of carbon dioxide, and an oxidation reaction electrode, which is electrically connected to the reduction reaction electrode and causes an oxidation reaction, and in which the reduction reaction electrode and the oxidation reaction electrode are immersed in an electrolyte solution containing a reaction substrate (carbon dioxide).

[0047] The artificial photosynthesis device is a device that includes the carbon dioxide reduction device and a solar cell that generates electricity to be supplied to the reduction reaction electrode and the oxidation reaction electrode of the carbon dioxide reduction device.

[0048] 2 is a schematic diagram showing an example of the configuration of an artificial-photosynthesis apparatus as an example of a reaction device using the reduction reaction electrode. The artificial-photosynthesis apparatus 1 is an apparatus comprising: a carbon dioxide reduction device 3 having a flow path 26 through which an electrolyte solution containing a reaction substrate flows between the reduction reaction electrode 10 and the oxidation reaction electrode 18, the reduction reaction electrode 10 causing a carbon dioxide reduction reaction to proceed and the oxidation reaction electrode 18 in a storage section 28, the carbon dioxide reduction device 3 having a flow path 26 through which an electrolyte solution containing a reaction substrate flows between the reduction reaction electrode 10 and the oxidation reaction electrode 18, and a solar cell 30 that generates electricity to be supplied to the reduction reaction electrode 10 and the oxidation reaction electrode 18 of the carbon dioxide reduction device 3.

[0049] In the example of Fig. 2, the reduction reaction electrode 10 and the oxidation reaction electrode 18 each have a configuration in which a plurality of electrodes are layered (stacked). The reduction reaction electrode 10 and the oxidation reaction electrode 18 may each have a single-layer configuration, or may have a configuration in which only the reduction reaction electrode 10 is layered (stacked), or may have a configuration in which only the oxidation reaction electrode 18 is layered (stacked). When used as an artificial photosynthesis device, it is preferable that the reduction reaction electrode 10 and the oxidation reaction electrode 18 each have a configuration in which a plurality of electrodes are layered (stacked).

[0050] The artificial photosynthesis device 1 functions by introducing an electrolyte solution containing a reaction substrate such as carbon dioxide from a flow path inlet into a flow path 26 between the reduction reaction electrode 10 and the oxidation reaction electrode 18. The electrolyte solution is discharged from a flow path outlet. At the oxidation reaction electrode 18, water (H2O) is oxidized to obtain oxygen (1 / 2O2) and electrons are generated. At the reduction reaction electrode 10, by receiving the electrons generated by the oxidation reaction, carbon dioxide is reduced to generate formic acid (HCOOH), for example.

[0051] (Oxidation reaction electrode) The oxidation reaction electrode 18 is an electrode used to oxidize a substance through an oxidation reaction. The oxidation reaction electrode 18 includes, for example, a base material 20 having a conductive layer formed on a substrate, and an oxidation catalyst layer 22 formed thereon.

[0052] The substrate is a member that structurally supports the oxidation reaction electrode 18. The substrate is not particularly limited in material, but examples thereof include a glass substrate. The substrate may include, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but examples thereof include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), lead (Pb), and titanium (Ti). The semiconductor used as the substrate is not particularly limited, but examples thereof include titanium oxide (TiO), tin oxide (SnO), silicon (Si), strontium titanate (SrTiO), zinc oxide (ZnO), and tantalum oxide (TaO). When the substrate includes a metal or a semiconductor, an insulating layer may be formed between the conductive layer and the substrate. The insulating layer is not particularly limited, but examples thereof include semiconductor oxides, nitrides, and resins. The metal substrate and the conductive layer may be electrically connected directly. In order to make the oxidation reaction electrode 18 translucent, the substrate is preferably made of, for example, a glass substrate or plastic. When a metal substrate is used, the conductive layer may not be required. A composite substrate of carbon fiber and carbon may also be used as the substrate. This has the advantages of ensuring light weight and high conductivity and eliminating the need for a separate conductive layer.

[0053] The conductive layer is provided to effectively collect current at the oxidation reaction electrode 18. The conductive layer is not particularly limited, but examples thereof include transparent conductive layers such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, in consideration of thermal and chemical stability, it is preferable to use fluorine-doped tin oxide (FTO).

[0054] The oxidation catalyst layer 22 is configured to contain a material having an oxidation catalytic function. Examples of the material having an oxidation catalytic function include a material containing iridium oxide (IrOx). Iridium oxide can be supported on the surface of the conductive layer as a nanocolloid solution (see T. Arai et al., Energy Environ. Sci 8, 1998 (2015)).

[0055] For example, nanocolloids of iridium oxide (IrOx) were synthesized. Next, 50 mL of 2 mM potassium chloroiridate (IV) (K2IrCl6) aqueous solution was added with 10 wt% sodium hydroxide (NaOH) aqueous solution to adjust the pH to 13. This yellow solution was heated at 90 °C for 20 minutes using a hot stirrer. The resulting blue solution was then cooled in ice water for 1 hour. Then, 3 M nitric acid (HNO3) was added dropwise to the cooled solution (20 mL) to adjust the pH to 1, and the solution was stirred for 80 minutes to obtain a nanocolloidal solution of iridium oxide (IrOx). This solution was then adjusted to pH 12 by adding 1-2 mL of 1.5 wt% NaOH aqueous solution dropwise. The resulting nanocolloidal solution of iridium oxide (IrOx) was then applied to a conductive layer at pH 12 and dried in a drying oven at 60 °C for 40 minutes. After drying, the precipitated salt can be washed with ultrapure water to form the oxidation reaction electrode 18. Note that the application and drying of the nanocolloidal aqueous solution of iridium oxide (IrOx) may be repeated multiple times.

[0056] When a composite substrate of carbon fiber and carbon is used as the substrate in the oxidation reaction electrode 18, for example, a nanocolloidal aqueous solution of iridium oxide (IrOx) may be used as an oxidation catalyst, and the oxidation catalyst may be supported on a composite substrate of carbon fiber and carbon, such as carbon paper, which may contain carbon materials such as carbon nanotubes, including multi-walled carbon nanotubes (MWCNTs), and the sheet may be bonded to the composite substrate of carbon fiber and carbon, such as carbon paper, with a carbon-based adhesive.

[0057] A current collecting wire may be provided to enhance the current collecting effect of the oxidation reaction electrode 18. That is, when the area of the oxidation reaction electrode 18 is increased, a conductive layer alone cannot ensure sufficient conductivity across the entire surface of the oxidation reaction electrode 18 to promote the reaction. Therefore, a current collecting wire is provided to enhance the conductivity of the oxidation reaction electrode 18. For example, the current collecting wire may be configured by combining linear finger electrodes arranged at intervals in a comb shape with bus electrodes for further current collection from the finger electrodes. The current collecting wire is preferably configured from a conductive portion, a first sealing portion, and a second sealing portion. The conductive portion is preferably configured from a highly conductive material, including a metal. For example, it is preferably configured from a material including silver (Ag), copper (Cu), or the like. Furthermore, the first sealing portion and the second sealing portion are provided to cover at least a portion of the conductive portion to chemically and mechanically protect the conductive portion. The first sealing portion may be made of a low-melting-point glass coating material. In addition, the second sealing portion can be made of resin such as silicone rubber (oxime-free type, low molecular weight siloxane-reducing material, oil-resistant and solvent-resistant fluorosilicone, etc.), polyisobutylene, polypropylene, methacrylate (acrylic), polycarbonate, fluororesin (Teflon (registered trademark)), epoxy resin, etc.

[0058] The reaction substrate contained in the electrolyte solution may be, for example, a carbon compound, such as carbon dioxide (CO2). The electrolyte solution is preferably a phosphate buffer solution or a borate buffer solution. In a specific configuration example, a tank of carbon dioxide (CO2)-saturated phosphate buffer solution is provided, and this solution is supplied by a pump to a flow path 26 provided between the reduction reaction electrode 10 and the oxidation reaction electrode 18. Formic acid (HCOOH), oxygen (O2), and the like produced by the reduction reaction are collected in an external fuel tank.

[0059] The housing 28 is a member that supports the reduction reaction electrode 10 and the oxidation reaction electrode 18 and also constitutes a flow path 26 through which the electrolyte flows. The housing 28 is made of a material that has the mechanical strength required to configure the electrochemical reaction device as a cell. For example, the housing 28 can be made of metal, plastic, etc.

[0060] A separator 24 that separates liquid from gas and allows protons to move may be provided between the reduction reaction electrode 10 and the oxidation reaction electrode 18. The separator 24 may be made of any material that can separate liquid from gas and allow protons to move, and is not particularly limited; for example, Nafion (registered trademark), which is a solid polymer electrolyte membrane, may be used.

[0061] The reduction reaction electrode 10 and the oxidation reaction electrode 18 are electrically connected to each other, and an appropriate bias voltage is applied thereto. The means for applying the bias voltage is not particularly limited, and examples thereof include a chemical battery (including a primary battery, a secondary battery, etc.), a constant voltage source, a solar cell, etc. At this time, the positive electrode is connected to the oxidation reaction electrode 18, and the negative electrode is connected to the reduction reaction electrode 10.

[0062] As shown in Figure 2, by using a solar cell 30 as a means for applying a bias voltage, an artificial photosynthesis device can be created that includes an electrochemical reaction device such as a carbon dioxide reduction device, and a solar cell that generates power to be supplied to the reduction reaction electrode and the oxidation reaction electrode. When a solar cell is used as a means for applying a bias voltage, the solar cell can be placed adjacent to the oxidation reaction electrode and the reduction reaction electrode, for example. For example, the solar cell can be placed behind the reduction reaction electrode, with the positive electrode of the solar cell connected to the oxidation reaction electrode and the negative electrode connected to the reduction reaction electrode.

[0063] When synthesizing formic acid (HCOOH) from carbon dioxide (CO2), water (H2O) is oxidized to provide electrons and protons to carbon dioxide (CO2). At a pH of around 7, the oxidation potential of water (H2O) is 0.82 V, and the reduction potential is -0.41 V (both on the standard hydrogen electrode (NHE)). The reduction potentials of carbon dioxide (CO2) to carbon monoxide (CO), formic acid (HCOOH), and methyl alcohol (CH3OH) are -0.53 V, -0.61 V, and -0.38 V, respectively. Therefore, the potential difference between the oxidation and reduction potentials is 1.20 to 1.43 V. When reducing carbon dioxide (CO2), a carbon compound, a suitable solar cell configuration is one in which four to six crystalline silicon solar cells are connected in series, or an amorphous silicon triple-junction solar cell.

[0064] It is preferable to provide a window material on the light-receiving side of the solar cell. The window material is a member that protects the solar cell. The window material is a member that transmits light of wavelengths that contribute to power generation in the solar cell, and can be, for example, glass, plastic, etc. [Example]

[0065] EXAMPLES The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0066] <Examples 1 to 6, Comparative Example 1> [Preparation of Ru complex polymer solution] The amounts of each material used in preparing the Ru complex polymer solution are shown in Table 1. The Ru complex monomer, pyrrole derivative or pyrrole as a polymerization agent for constituting the polymer, iron chloride (FeCl3) as a polymerization catalyst, and acetonitrile as a solvent were mixed and stirred at 25°C to prepare the Ru complex polymer solution. Various electrodes for reduction reactions were also prepared using the prepared Ru complex polymer solution, and cell characteristics were compared. The amounts of each material shown in Table 1 were calculated for 1.0 cm of CS / MWCNTs sheet. 2 When loaded onto a unit area (cm 2In all of Examples 1 to 6 and Comparative Example 1, the Ru complex monomer is contained in the same amount.

[0067] The materials used are as follows: Ru complex monomer: [Ru{4,4'-di(1-H-1-pyrropropyl carbonate)-2,2'-bipyridine}(CO)2Cl2] Pyrrole Derivatives 1: 2-(1H-pyrrol-1-yl)ethanamine Pyrrole Derivatives 2: 1H-Pyrrol-1-amine Pyrrole (comparison example)

[0068] [ka] Pyrrole derivative 1 [ka] Pyrrole derivative 2 [ka] Pyrrole

[0069] [Table 1]

[0070] Photographs of the appearance of the Ru complex polymer solutions prepared in Examples 1 to 6 and Comparative Example 1 are shown in Figure 3 ((e) Example 1, (d) Example 2, (f) Example 3, (g) Example 4, (h) Example 5, (i) Example 6, (c) Comparative Example 1). Photographs of the appearance of the Ru complex monomer solution (no pyrrole, no iron chloride) and the Ru complex monomer + iron chloride solution (no pyrrole, with iron chloride at 1x concentration) are also shown in Figure 3 ((a) and (b)).

[0071] The solution containing iron chloride was a dark brown solution whether the Ru complex monomer contained pyrrole (Figure 3(c)) or not (Figure 3(b)). This is thought to be because, even when pyrrole was not contained, the pyrrole moiety contained in the Ru complex monomer reacted with the iron chloride catalyst, polymerizing the Ru complex monomer to form a Ru complex polymer.

[0072] As the amount of pyrrole derivative 1 (pyrrole-ethanamine) added increased and the amount of iron chloride catalyst increased, the Ru complex polymer solution became reddish-orange, approaching the appearance of the original Ru complex monomer solution (see Figures 3(e), (f), (g), (h), and (i)). This suggests that pyrrole derivative 1 (ethanamine) acts as an inhibitor of polymerization. In other words, this indicates that the degree of polymerization of the Ru complex monomer can be controlled by the functional groups of the pyrrole derivative.

[0073] [Preparation of CS / MWCNTs sheet] A porous carbon sheet (CS) composed of porous carbon fiber was dip-coated with an ink containing 5% by mass of multi-walled carbon nanotubes (MWCNTs) dispersed in an ethanol solvent. After drying at 25°C, the sheet was heated at 350°C under an argon atmosphere to produce a CS / MWCNTs sheet carrying MWCNTs.

[0074] [UV and ozone treatment of CS / MWCNTs sheet] A UV ozone cleaner (Nippon Laser, NL-UV253 type) was used, and the ultraviolet intensity was 5.2 mW / cm 2 The CS / MWCNTs sheet was subjected to ultraviolet ozone treatment under the conditions of 1000 saturation, 1000 saturation, and 30 minutes of treatment.

[0075] [Preparation of electrode for reduction reaction] The Ru complex polymer solutions prepared in Examples 1 to 6 and Comparative Example 1 were applied to a porous carbon sheet (CS / MWCNTs sheet) composed of porous carbon fibers carrying multi-walled carbon nanotubes (MWCNTs) that had been previously treated with ultraviolet light and ozone. The sheet was then vacuum dried 10 times, immersed in water to remove iron chloride, and dried at 60°C for 2 hours to prepare a reduction electrode (CS / MWCNTs / RuCP sheet) carrying the Ru complex polymer. An electric wire terminal was then attached to the reduction electrode, and the terminal was sealed with silicone to form a working electrode.

[0076] [Evaluation of electrodes for reduction reactions] A platinum wire was used as the counter electrode, and a 0.4M phosphate buffer solution saturated with CO2 was used as the electrolyte. A potentiostat was used to apply a constant potential (-1.2 V vs Hg / Hg2SO4) to the electrochemical cell, and cathodic polarization was used to reduce CO2 to formic acid. The amount of formic acid produced was quantified using an ion chromatograph (Thermo Scientific, Model: Integrion RFICEG). The charge was calculated from the current value, and the Faraday efficiency (FE) of formic acid production was calculated.

[0077] FIG. 4 shows the relationship between the current density (mA / cm ) and the operating time (h) in Example 1 and Comparative Example 1. 2 ) is shown. FIG. 5 shows the Faraday efficiency (%) of formic acid production versus the operating time (h) in Example 1, Example 2, and Comparative Example 1. In Example 1, the decrease in current density is suppressed compared to Comparative Example 1. In Examples 1 and 2, the decrease in the Faraday efficiency (FE) of formic acid production is suppressed compared to Comparative Example 1.

[0078] FIG. 6 shows the current density (mA / cm ) versus operating time (h) in Examples 1, 3, 4, 5, and 6. 2) is shown. Figure 7 shows the Faraday efficiency (%) of formic acid generation versus operating time (h) in Examples 1, 3, 4, 5, and 6. As the amount of pyrrole derivative 1 added and the amount of FeCl3 increased, the current density tended to increase (Figure 6). Furthermore, when the amount of pyrrole derivative 1 (pyrrole-ethanamine) added was 252 times (Examples 4 and 5) and 504 times (Example 6) that of Example 1, the formic acid FE tended to be high (Figure 7).

[0079] When a Ru complex polymer solution containing pyrrole derivative 1 (pyrrole-ethanamine) or pyrrole derivative 2 (pyrrole-amine) was used and supported on a carbon electrode, it was thought that the polymers were chemically bonded and supported in a molecular form more effectively than conventional Ru complex polymers without functional groups.

[0080] [XPS analysis] We investigated the formation of functional groups on the surface of the multi-walled carbon nanotube (MWCNT)-supported carbon sheets (CS / MWCNTs sheets) prepared as described above, both with and without UV ozone treatment. We also investigated whether the state of interaction with the carbon support differed between the UV ozone-treated CS / MWCNTs sheets coated with the pyrrole derivative 1 and those coated with pyrrole alone. The evaluation was performed by examining the elemental composition and chemical state of the sample surfaces using X-ray photoelectron spectroscopy (XPS).

[0081] (sample) Sample (a): A carbon sheet (CS / MWCNTs sheet) carrying multi-walled carbon nanotubes (MWCNTs) that has not been treated with UV or ozone. Sample (b): A carbon sheet (CS / MWCNTs sheet) carrying multi-walled carbon nanotubes (MWCNTs) that has been treated with UV ozone. Sample (c): A carbon sheet (CS / MWCNTs sheet) carrying multi-walled carbon nanotubes (MWCNTs) was treated with UV ozone and then coated with pyrrole. Sample (d): A carbon sheet (CS / MWCNTs sheet) carrying multi-walled carbon nanotubes (MWCNTs) was treated with UV ozone and then coated with pyrrole derivative 1.

[0082] (Analysis method) Table 2 shows the XPS measurement conditions. Following the methodology described in "High resolution XPS characterization of chemically functionalized MWCNTs and SWCNTs" by TIT Okpalugo, P. Papaknstantinou, H. Murphy, and J. McLaughlin, NMD Brown, "Carbon, 43 (2005)," pp. 153-161, waveform component analysis of the XPS carbon spectrum was performed to determine the component ratio of the carbon functional groups. The elemental composition ratios of each sample are shown in Table 3, and the component ratios of the carbon functional groups (each state / total carbon content %) for each sample are shown in Table 4. Figure 8 shows the waveform separation results for the C1s spectrum, and Figure 9 shows the XPS spectrum for each element.

[0083] [Table 2]

[0084] [Table 3]

[0085] [Table 4]

[0086] (result) Comparing samples before and after UV ozone treatment, it was found that the amount of oxygen (O) increased due to UV ozone treatment (see Table 3 and Figure 9). Furthermore, the amount of functional groups CO and -COO increased due to UV ozone treatment (see Table 4 and Figure 8). The amount of N was small both before and after UV ozone treatment (see Table 3), and therefore it is thought that the amount of C-NHx was small (Table 4). This confirmed that the amount of surface functional groups containing oxygen (O) on the carbon surface increased due to UV ozone treatment.

[0087] Comparing the samples coated with pyrrole and those coated with pyrrole derivative 1, the sample coated with pyrrole derivative 1 had four times the amount of N as the sample coated with pyrrole (see Table 3 and Figure 9). The sample coated with pyrrole derivative 1 had an increased amount of functional groups CO and C-NHx compared to the sample coated with pyrrole (see Table 4 and Figure 8). This suggests that pyrrole derivative 1 was immobilized on the carbon surface by chemical bonds as a result of interactions between the surface functional groups of the carbon and the anchor molecules of the ethylamine groups of pyrrole derivative 1.

[0088] As described above, the method of the present invention made it possible to obtain an electrode for reduction reaction having excellent durability. [Explanation of symbols]

[0089] 1 artificial photosynthesis device, 3 carbon dioxide reduction device, 10 reduction reaction electrode, 12, 20 substrate, 14 adhesive layer, 16 catalyst layer, 18 oxidation reaction electrode, 22 oxidation catalyst layer, 24 separator, 26 flow path, 28 storage section, 30 solar cell.

Claims

1. A substrate; a catalyst layer including a composite substrate of carbon and carbon fiber containing a carbon material, and a Ru complex polymer including a Ru complex monomer and a heterocyclic aromatic compound having a 5- to 9-membered ring structure containing at least one of N, S, and B, which has at least one of an amino group, an alkylamino group, a carboxyl group, and a hydroxyl group; An electrode for a reduction reaction, comprising:

2. The electrode for reduction reaction according to claim 1, The heterocyclic aromatic compound is 2-(1H-pyrrol-1-yl)ethanamine or 1H-pyrrol-1-amine.

3. The electrode for reduction reaction according to claim 1 or 2, The Ru complex monomer is [Ru{4,4'-di(1-H-1-pyrropropyl carbonate)-2,2'-bipyridine}(CO) ２ Cl ２ ].

4. The electrode for reduction reaction according to any one of claims 1 to 3, a content of the heterocyclic aromatic compound in the Ru complex polymer in the catalyst layer relative to the content of the Ru complex monomer, in terms of molar ratio, in the range of 0.003 to 2.5;

5. A method for producing the electrode for reduction reaction according to any one of claims 1 to 4, a preparation step of preparing a Ru complex polymer solution containing the Ru complex monomer, the heterocyclic aromatic compound, a polymerization catalyst, and a solvent; a treatment step of subjecting a composite substrate of carbon fiber and carbon containing a carbon material to ultraviolet ozone treatment; a coating step of coating the prepared Ru complex polymer solution onto the composite substrate that has been subjected to the ultraviolet ozone treatment and drying the solution; A method for producing an electrode for reduction reaction, comprising:

6. The electrode for reduction reaction according to any one of claims 1 to 4, an oxidation reaction electrode; A reaction device characterized by being configured by combining the above.

7. The reaction device according to claim 6 ; a solar cell that generates power to be supplied to the oxidation reaction electrode and the reduction reaction electrode; An artificial photosynthesis device comprising:

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