Cathode electrode for gas diffusion type electrolytic flow cell, and gas diffusion type electrolytic flow cell

Abstract

To provide a cathode electrode for a gas diffusion electrolytic flow cell that can give a C2+ reduction product with high selectivity while suppressing hydrogen production as a side reaction in a carbon dioxide reduction reaction.SOLUTION: The present invention provides a cathode electrode 22 for a gas diffusion electrolytic flow cell that produces a carbon dioxide reduction product by reducing carbon dioxide, wherein the cathode electrode comprises a catalyst layer 26 having a catalyst including copper atoms and an alkali metal salt, and a gas diffusion layer 28 disposed on the catalyst layer 26. The catalyst layer 26 is free of carbon particles and polymer electrolytes.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cathode electrode for a gas diffusion type electrolytic flow cell, and to a gas diffusion type electrolytic flow cell. [Background technology]

[0002] In recent years, concerns have arisen about the depletion of fossil fuels such as oil and coal, and expectations for sustainably usable renewable energy are rising. From the perspective of such energy issues, as well as environmental issues, there is a growing demand for the electrochemical reduction of carbon dioxide using renewable energy such as solar power, which can then be stored. Development of artificial photosynthesis technology to produce powerful chemical energy sources is underway.

[0003] One known method for reducing carbon dioxide is to electrochemically reduce carbon dioxide dissolved in an aqueous solution (for example, Patent Documents 1-2, Non-Patent Document 1).

[0004] Furthermore, there is a known technique for performing a carbon dioxide reduction reaction and obtaining carbon dioxide reduction products using a gas diffusion type electrolytic flow cell comprising an anode electrode, a cathode electrode, and an ion-conducting polymer film sandwiched between both electrodes (for example, Patent Document 3, Non-Patent Documents 2-4). Non-Patent Documents 2-4 describe a type in which an electrolyte is supplied to both the anode and cathode electrodes, while Patent Document 3 describes a type in which an electrolyte is supplied to the anode electrode and carbon dioxide gas is supplied to the cathode electrode. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2018-34136 [Patent Document 2] Japanese Patent Publication No. 2013-193056 [Patent Document 3] U.S. Patent Application Publication No. 2020 / 208278 [Non-patent literature]

[0006] [Non-Patent Document 1] J. Am. Chem. Soc., 2018, 140(49), 17241-17254 [Non-Patent Document 2] Science, 2018, 360(6390), 783-787 [Non-Patent Document 3] Nature, 2020, 577, 509-513 [Non-Patent Document 4] Science, 2020, 367, 661-666 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] By the way, in the carbon dioxide reduction reaction, C 2+ reduction products (e.g., ethylene, ethanol, propanol, etc.) generated from two or more CO2 molecules are useful in terms of energy resources. Therefore, it is desired to suppress hydrogen generation, which is a side reaction, and obtain C 2+ reduction products with high selectivity. However, in conventional gas diffusion type electrolytic flow cells, there is room for improvement in suppressing hydrogen generation, which is a side reaction, and obtaining C 2+ reduction products with high selectivity.

[0008] Therefore, an object of the present invention is to provide a cathode electrode for a gas diffusion type electrolytic flow cell and a gas diffusion type electrolytic flow cell that can suppress hydrogen generation, which is a side reaction, and obtain C 2+ reduction products with high selectivity in the carbon dioxide reduction reaction. [Means for Solving the Problems]

[0009] The present invention relates to a cathode electrode for a gas diffusion type electrolytic flow cell that reduces carbon dioxide to produce a carbon dioxide reduction product, comprising a catalyst having copper atoms and an alkali metal salt solidThe catalyst comprises a catalyst layer having a copper atom and a gas diffusion layer disposed on the catalyst layer, wherein the catalyst having a copper atom includes a complex catalyst having a copper atom, and the complex catalyst having a copper atom includes at least one of a metal complex represented by the general formula: CuMX2(Y)2L2 (wherein M is Cu, Ag or Ni, X is a halogen atom, Y is a ligand having a phosphorus atom, and L is a ligand having a pyridine ring), a phthalocyanine copper complex, and copper benzene-1,3,5-tricarboxylate, and the catalyst layer does not contain carbon particles and polymer electrolytes.

[0011] Furthermore, the present invention relates to a gas diffusion type electrolytic flow cell comprising an anode electrode that oxidizes water or hydroxide ions to produce oxygen, a cathode electrode that reduces carbon dioxide to produce carbon dioxide reduction products, and an ion-conducting polymer film sandwiched between the anode electrode and the cathode electrode, wherein the cathode electrode comprises a catalyst having copper atoms and an alkali metal salt solid The catalyst comprises a catalyst layer having a copper atom and a gas diffusion layer disposed on the catalyst layer, wherein the catalyst having a copper atom includes a complex catalyst having a copper atom, and the complex catalyst having a copper atom includes at least one of a metal complex represented by the general formula: CuMX2(Y)2L2 (wherein M is Cu, Ag or Ni, X is a halogen atom, Y is a ligand having a phosphorus atom, and L is a ligand having a pyridine ring), a phthalocyanine copper complex, and copper benzene-1,3,5-tricarboxylate, and the catalyst layer does not contain carbon particles and polymer electrolytes.

[0012] Furthermore, in the gas diffusion type electrolytic flow cell, it is preferable that an anode solution is supplied to the anode electrode and carbon dioxide gas is supplied to the cathode electrode. [Effects of the Invention]

[0014] According to the present invention, in the carbon dioxide reduction reaction, hydrogen production, which is a side reaction, is suppressed, and C is reduced with high selectivity. 2+A cathode electrode for a gas diffusion type electrolytic flow cell and a gas diffusion type electrolytic flow cell that can obtain reduction products can be provided. [Brief explanation of the drawing]

[0015] [Figure 1] This is a schematic diagram showing an example of a gas diffusion type electrolytic flow cell according to this embodiment. [Modes for carrying out the invention]

[0016] Embodiments of the present invention will be described below. This embodiment is just one example of how the present invention can be implemented, and the present invention is not limited to this embodiment.

[0017] Figure 1 is a schematic diagram showing an example of a gas diffusion type electrolytic flow cell according to this embodiment. The gas diffusion type electrolytic flow cell 1 shown in Figure 1 comprises an anode section 10, a cathode section 12, and an ion-conducting polymer film 14. The anode section comprises an anode electrode 16 and an anode channel 18. The anode electrode 16 is positioned between the ion-conducting polymer film 14 and the anode channel 18, in contact with them. The anode channel 18 is formed by a pit (groove or recess) provided in the anode current collector plate 20. The cathode section 12 comprises a cathode electrode 22 and a cathode channel 24. The cathode electrode 22 is positioned between the cathode channel 24 and the ion-conducting polymer film 14. The cathode electrode 22 comprises a catalyst layer 26 and a gas diffusion layer 28, in order from the ion-conducting polymer film 14 side. The cathode channel 24 is formed by a pit (groove or recess) provided in the cathode current collector plate 30. The ion-conducting polymer film 14 is sandwiched between the anode electrode 16 and the cathode electrode 22. In other words, the anode electrode 16 and the cathode electrode 22 are separated by the ion-conducting polymer film 14.

[0018] The anode current collector plate 20 is connected to, for example, an inlet and an outlet (neither of which are shown). The anode solution is introduced into the anode channel 18 through the inlet, passes through the anode channel 18 while in contact with the anode electrode 16, and is discharged from the outlet. It is preferable to use a material that is highly chemically reactive and highly conductive for the anode current collector plate 20. Examples of such materials include metallic materials such as Ti and SUS, and carbon.

[0019] The cathode current collector plate 30 is connected to, for example, an inlet and an outlet (neither of which are shown). Carbon dioxide gas is introduced into the cathode channel 24 through the inlet, passes through the cathode channel 24 while in contact with the catalyst layer 26 via the gas diffusion layer 28, and is discharged from the outlet. The carbon dioxide gas supplied to the cathode electrode 22 is a gas containing carbon dioxide, preferably a gas containing carbon dioxide and water vapor. Similar to the anode current collector plate 20, it is preferable to use a material for the cathode current collector plate 30 that has low chemical reactivity and high conductivity. Examples of such materials include metallic materials such as Ti and SUS, and carbon.

[0020] The gas diffusion type electrolytic flow cell 1 shown in Figure 1 is a device in which an anode solution (electrolyte) is supplied to the anode electrode 16 and carbon dioxide gas is directly supplied to the cathode electrode 22. However, the gas diffusion type electrolytic flow cell according to this embodiment may be a device in which an anode solution is supplied to the anode electrode 16 and a cathode solution (electrolyte containing carbon dioxide) containing carbon dioxide is supplied to the cathode electrode 22. In the latter case, the cathode solution containing carbon dioxide is introduced into the cathode flow channel 24 through the inlet of the cathode current collector plate 30, passes through the cathode flow channel 24 while in contact with the catalyst layer 26 via the gas diffusion layer 28, and is discharged from the outlet of the cathode current collector plate 30.

[0021] Reference numeral 32 shown in FIG. 1 is a power source that electrically connects the anode electrode 16 and the cathode electrode 22 and supplies power. The power source 32 is not particularly limited, and examples thereof include chemical batteries (including primary batteries, secondary batteries, etc.), constant voltage sources, solar cell modules, and the like. By using a solar cell module as the power source 32, an artificial photosynthesis device including a gas diffusion type electrolytic flow cell 1 and a solar cell module that generates power supplied to the anode electrode 16 and the cathode electrode 22 can be provided. In the artificial photosynthesis device according to the present embodiment, the anode electrode 16 and the cathode electrode 22 of the gas diffusion type electrolytic flow cell 1 are connected via a solar cell module and are driven using sunlight as an energy source.

[0022] Next, an operation example of the gas diffusion type electrolytic flow cell 1 shown in FIG. 1 will be described. As reaction processes, mainly a case of generating hydrogen ions (H + ) and mainly a case of generating hydroxide ions (OH - ) are conceivable, but it is not limited to any of these reaction processes.

[0023] First, the reaction process in the case of mainly oxidizing water (H2O) to generate hydrogen ions (H + ) will be described. When a current is supplied from the power source 32 between the anode electrode 16 and the cathode electrode 22, an oxidation reaction of water (H2O) occurs at the anode electrode 16 in contact with the anode solution. Specifically, as shown in the following formula (1), H2O contained in the anode solution is oxidized to generate oxygen (O2) and hydrogen ions (H + ). 2H2O → 4H + +O2+4e - ···(1)

[0024] On the cathode electrode 22 side, carbon dioxide gas supplied from the cathode flow path 24 to the catalyst layer 26 through the gas diffusion layer 28 is combined with electrons (e - ) based on the current supplied from the power source 32 to the cathode electrode 22 and H +As a result, CO is produced as shown in equation (2) below. In addition, as a side reaction, hydrogen ions accept electrons and hydrogen is produced as shown in equation (3) below. CO2 + 2H + +2e - → CO + H2O ···(2) 2H + +2e - → H2···(3) Here, the carbon dioxide reduction product is not limited to carbon monoxide; for example, methane (CH4), ethane (C2H6), ethylene (C2H4), ethanol (C2H5OH), propanol (C3H7OH), etc., can also be produced. In particular, in this embodiment, the side reaction of hydrogen production is suppressed, and the carbon dioxide produced from 2 or more CO2 molecules is generated with high selectivity. 2+ It can produce reduction products.

[0025] Next, mainly carbon dioxide (CO2) is reduced to hydroxide ions (OH) - The reaction process for generating ) is described below. When current is supplied from the power supply 32 between the anode electrode 16 and the cathode electrode 22, on the cathode electrode 22 side, carbon dioxide gas (including water vapor) supplied from the cathode channel 24 to the catalyst layer 26 via the gas diffusion layer 28 is reduced as shown in equation (4) below to carbon monoxide (CO) and hydroxide ions (OH - ) is produced. In addition, as a side reaction, hydrogen is produced when water accepts electrons as shown in equation (5) below. The hydroxide ions (OH) produced by these reactions are generated. - For example, the hydroxide ions (OH) move to the anode electrode 16 side via the ion-conducting polymer film 14, as shown in equation (6) below. - ) is oxidized to produce oxygen (O2). 2CO2 + 2H2O + 4e - → 2CO + 4OH - ...(4) 2H2O + 2e - → H2 + 2OH - ...(5) 4OH - → 2H2O + O2 + 4e -...(6) As mentioned above, the carbon dioxide reduction product is not limited to carbon monoxide; for example, methane (CH4), ethane (C2H6), ethylene (C2H4), ethanol (C2H5OH), propanol (C3H7OH), etc., can also be produced. In particular, in this embodiment, the side reaction of hydrogen production is suppressed, and the carbon dioxide produced from 2 or more CO2 molecules is generated with high selectivity. 2+ It can produce reduction products.

[0026] The configurations of the anode electrode 16, cathode electrode 22, and ion-conducting polymer film 14 will be described in detail below.

[0027] As mentioned above, the anode electrode 16 promotes the oxidation reaction of water (H2O) in the anode solution, producing oxygen (O2) and hydrogen ions (H2O). + ) generates, or hydroxide ions (OH) generated in the cathode section 12 - It is an electrode (oxidation electrode) that promotes the oxidation reaction of ) and produces oxygen and water.

[0028] The anode electrode 16 preferably comprises a substrate made of at least one material selected from the group consisting of Ni, Ti, Fe, and C, which can reduce the overpotential of the oxidation reaction. The metallic material of Ni, Ti, or Fe also includes alloys containing at least one of the metals Ni, Ti, or Fe. Furthermore, the substrate preferably has a structure that allows the anode solution and ions to move between the ion-conducting polymer film 14 and the anode channel 18, and is preferably, for example, a porous body, a mesh, or a fibrous sintered body.

[0029] The anode electrode 16 preferably contains an anode catalyst. Examples of anode catalysts that can reduce the overpotential of the oxidation reaction include metals containing at least one element selected from the group consisting of Ni, Fe, Co, Mn, Ru, and Ir, oxides containing the metal, hydroxides containing the metal, and oxyhydroxides containing the metal. These may be used individually or in combination of two or more. When an anode catalyst is used, it is preferable to support the anode catalyst on the aforementioned substrate.

[0030] The anode solution preferably contains at least one ion selected from the group consisting of hydroxide ions, bicarbonate ions, carbonate ions, chloride ions, bromide ions, iodide ions, nitrate ions, sulfate ions, phosphate ions, borate ions, tetraborate ions, hydrogen ions, lithium ions, sodium ions, potassium ions, rubidium ions, and cesium ions.

[0031] The gas diffusion layer 28 constituting the cathode electrode 22 is preferably a porous conductive substrate that ensures electrical conductivity between the catalyst layer 26 and the power supply 32 and efficiently supplies carbon dioxide gas to the catalyst layer 26. It is preferably a porous carbon support, and more preferably a hydrophobic porous carbon support, in that it can reduce the amount of water that moves from the cathode electrode 22 side.

[0032] As described above, the catalyst layer 26 constituting the cathode electrode 22 promotes the reduction reaction of carbon dioxide and generates carbon dioxide reduction products, etc. The catalyst layer 26 contains a catalyst having copper atoms and an alkali metal salt. Conventional catalyst layers include carbon particles that act as electrical conductors and polymer electrolytes that act as ion conductors and binders, but the catalyst layer 26 of this embodiment does not contain carbon particles or polymer electrolytes. The catalyst layer 26 is preferably a porous structure in terms of improving the diffusibility of carbon dioxide gas, etc. The thickness of the catalyst layer 26 is, for example, 5 to 200 μm.

[0033] Catalysts containing copper atoms are carbon dioxide reduction catalysts that induce the reduction reaction of carbon dioxide, and include, for example, copper metal, copper-containing alloys, compounds containing copper atoms, and complexes containing copper atoms.

[0034] Examples of copper-containing alloys include alloys of Cu with at least one metal selected from the group consisting of Mn, Fe, Co, Ni, Cu, Mo, Ru, and Re.

[0035] Examples of compounds containing copper atoms include Cu2O, CuO, CuBi2O4, CuI, Cu(InGa)S2, Cu(InGa)Se2, CuGaS2, CuGaSSe, and CuGaSe2.

[0036] Examples of complexes containing copper atoms include metal complexes represented by the general formula CuMX2(Y)2L2. In the general formula, M may be any metal element that constitutes a metal halide salt together with Cu, but it is preferably Cu, Ag, or Ni, and more preferably Cu, in terms of high catalytic activity and promotion of the formation of multi-electron reduction products. In the general formula, X is not particularly limited as long as it is a halogen atom, but it is preferably selected from Br, Cl, and I in terms of crystal structure stability and the amount of multi-electron reduction product formed, and Br is particularly preferred.

[0037] In the general formula, Y is not particularly limited as long as it is a ligand having a phosphorus atom, but examples include trimethylphosphine, triethylphosphine, tri-n-propylphosphine, triisopropylphosphine, tri-n-butylphosphine, tri-tert-butylphosphine, tri-n-pentylphosphine, tricyclopentylphosphine, tri-n-hexylphosphine, tricyclohexylphosphine, tri-n-heptylphosphine, tri-n-octylphosphine, triphenylphosphine, tris(2-methoxyphenyl)phosphine, tris(4-methoxyphenyl)phosphine, tris(2,6-dimethoxyphenyl)phosphine, tris(2-furyl)phosphine, tris(4-dimethylaminophenyl)phosphine, tri-p-tolylphosphine, tris-(4-fluorophenyl)phosphine, tris[3,5-bis(trifluoromethyl)phenyl]phosphine, tris(pentafluorophenyl)phosphine, etc. Among these, triphenylphosphine (PPh3) is preferred, for example.

[0038] In the general formula, L is not particularly limited as long as it is a ligand having a pyridine ring. However, ligand L does not act as a linking group through which the basic unit of general formula B is repeated. Examples of such ligands having a pyridine ring include 2-methylpyridine, 2-ethylpyridine, 4-propylpyridine, 2-vinylpyridine, N,N-dimethyl-4-aminopyridine, 4-phenylpyridine, 2-hydroxypyridine, 1,5-naphthyridine, 2,2'-bipyridyl, 1,3-di(4-pyridyl)propane, 4-pyridyl-4'-methylpyridylbiphenyl, 4-methylpyridine, and 3-benzylpyridine. Among these, 4-phenylpyridyl is preferred, for example.

[0039] Furthermore, the complex containing the copper atom may be, for example, a phthalocyanine copper complex such as copper(II) phthalocyanine, or copper benzene-1,3,5-tricarboxylate. The phthalocyanine copper complex is not limited to copper(II) phthalocyanine, but may also be a compound in which substituents have been introduced to the phthalocyanine skeleton.

[0040] The alkali metal salt contained in the catalyst layer 26 may be an inorganic alkali metal salt, an organic alkali metal salt, or a combination of both.

[0041] As inorganic alkali metal salts, various inorganic salts of alkali metals such as lithium, sodium, potassium, rubidium, and cesium can be used. Examples include alkali metal chlorides, nitrates, carbonates, bicarbonates, sulfates, phosphates, or hydroxides. Layered compounds, such as clay containing alkali metals, can also be used.

[0042] Examples of organic alkali metal salts include alkali metal salts of aliphatic organic acids, such as alkali metal salts of aliphatic sulfonic acids, and alkali metal salts of aromatic organic acids, such as alkali metal salts of aromatic sulfonic acids. Examples of alkali metal salts of aliphatic sulfonic acids include alkali metal salts of alkanesulfonic acids. Preferred examples of alkanesulfonic acids used in alkali metal salts of alkanesulfonic acids include methanesulfonic acid, ethanesulfonic acid, propanesulfonic acid, butanesulfonic acid, methylbutanesulfonic acid, hexanesulfonic acid, heptanesulfonic acid, and octanesulfonic acid, and these can be used individually or in combination of two or more. Furthermore, alkali metal salts in which some or all of the alkyl groups are substituted with fluorine atoms are also acceptable. Examples of aromatic sulfonic acids used in alkali metal salts of aromatic sulfonic acids include sulfonic acids of monomeric or polymeric aromatic sulfides, sulfonic acids of aromatic carboxylic acids and esters, and sulfonic acids of monomeric or polymeric aromatic ethers, and these can be used individually or in combination of two or more. As the alkali metal salt, an organic alkali metal salt that is easily soluble in alcohol is preferred. By using an alcohol solvent in which the organic alkali metal salt is dissolved, it is possible to highly disperse the organic alkali metal salt in the catalyst layer 26.

[0043] The alkali metal salt content is preferably in the range of 5% to 500% by mass relative to the total amount of catalyst used in the catalyst layer 26.

[0044] When using a cathode solution containing carbon dioxide instead of carbon dioxide gas, it is preferable that the cathode solution contains at least one ion selected from the group consisting of hydroxide ions, bicarbonate ions, carbonate ions, chloride ions, bromide ions, iodide ions, nitrate ions, sulfate ions, phosphate ions, borate ions, tetraborate ions, hydrogen ions, lithium ions, sodium ions, potassium ions, rubidium ions, and cesium ions. Carbon dioxide can be added to the cathode solution by bubbling carbon dioxide into it.

[0045] As the ion-conducting polymer film 14, for example, cation exchange films such as Nafion (registered trademark) and Flemion, or anion exchange films such as Neosepta, Selemion, and Sustenion can be used. An alkaline aqueous solution is used as the anode solution, mainly hydroxide ions (OH) - When considering the movement of ions, it is preferable that the ion-conducting polymer film 14 be composed of an anion exchange film.

[0046] By using the cathode electrode 22 of this embodiment, the reduction reaction of carbon dioxide can suppress hydrogen production, a side reaction, and achieve high selectivity. 2+ A reduction product can be obtained. The following reasons are presumed to be the cause of the above effect.

[0047] As in the cathode electrode 22 of this embodiment, the inclusion of an alkali metal salt in the catalyst layer 26 causes interaction between the alkali metal salt and the catalyst containing copper atoms, resulting in a change in potential that leads to hydrogen production, thus suppressing hydrogen production. Furthermore, the interaction of the alkali metal salt with CO2 allows more CO2 to be retained near the catalyst containing copper atoms, thus reducing C 2+ This is thought to lead to improved selectivity of the reduction product. Furthermore, in this embodiment, since carbon particles and polymer electrolytes are not present in the catalyst layer 26, the increase in the resistance value of the cathode electrode 22 is suppressed, thereby suppressing hydrogen generation and C 2+This is expected to improve the selectivity of the reduction product.

[0048] As a catalyst containing copper atoms, it is preferable to use a complex catalyst containing copper atoms. By using a complex catalyst containing copper atoms, the reduction potential that causes CO2 reduction changes, and C 2+ The selectivity of the reduction product is further improved. Furthermore, when a porous carbon support is used as the gas diffusion layer, hydrogen production as a side reaction from the porous carbon support may occur. However, as mentioned above, the alkali metal salt interacts with CO2, retaining more CO2 near the catalyst containing copper atoms, thus suppressing hydrogen production from the porous carbon support.

[0049] Furthermore, in a gas diffusion type electrolytic flow cell 1, the device in which the anode solution (electrolyte) is supplied to the anode electrode 16 and carbon dioxide gas is directly supplied to the cathode electrode 22 has a higher CO2 concentration ratio to water compared to the device in which the anode solution is supplied to the anode electrode 16 and a cathode solution (electrolyte containing carbon dioxide) is supplied to the cathode electrode 22. This suppresses the by-product formation of H2, and because the reaction proceeds in the gas phase with a high diffusion rate, the limit of the reaction current density can be increased. Therefore, a large reaction current density can be generated, and carbon dioxide reduction products can be obtained.

[0050] Incidentally, when the carbon dioxide reduction reaction is a reaction between carbon dioxide and hydrogen ions, the cathode electrode 22 side requires an appropriate hydrogen ion concentration. However, if the hydrogen ion concentration is too high, the by-product of H2 will proceed more easily, so the pH of the solution on the cathode electrode 22 side is preferably neutral to slightly alkaline. On the other hand, the pH of the solution on the anode electrode 16 side is preferably alkaline in terms of reducing overpotential and increasing reaction current. In this embodiment, for example, carbon dioxide gas containing neutral water vapor may be supplied to the cathode electrode 22 side, and an alkaline anode solution may be supplied to the anode electrode 16 side to make the pH of the solution on the cathode electrode 22 side neutral (e.g., pH 6-8) and the pH of the solution on the anode electrode 16 side alkaline. Furthermore, when carbon dioxide and water coexist, carbonic acid may be formed, which can cause the solution to change from neutral to acidic. However, in this embodiment, since the anode portion 10 and the cathode portion 12 are separated by an ion-conducting polymer film 14, the neutralization of the alkaline anode solution by carbon dioxide is blocked, making it possible to maintain the alkaline state of the solution on the anode electrode 16 side.

[0051] Furthermore, by supplying carbon dioxide gas containing neutral water vapor to the cathode electrode 22 side and an alkaline anode solution to the anode electrode 16 side, the liquid on the anode electrode 16 side becomes alkaline and the liquid on the cathode electrode 22 side becomes neutral, thereby creating a hydrogen ion concentration difference between the anode portion 10 and the cathode portion 12 across the ion-conducting polymer film 14. It is believed that the formation of such a hydrogen ion concentration difference will result in an energy gain, which will lead to a decrease in the cell voltage.

[0052] The alkaline anode solution is more preferably an aqueous solution with a pH of 12 or higher, for example, in terms of preventing a decrease in cell voltage. The ion-conducting polymer membrane 14 is preferably an anion-conducting polymer membrane, as it easily forms a hydrogen ion concentration difference, which in turn leads to a decrease in cell voltage. [Examples]

[0053] 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.

[0054] <Example 1> [Cathode electrode] On a microporous carbon paper (Avcarb, GDS3250) used as a gas diffusion layer, 0.2 mg of potassium trifluoromethanesulfonate (hereinafter referred to as KOtf), an alkali metal salt dissolved in ethanol, was coated, and 1 mg of Cu2O (Aldrich), a catalyst containing copper atoms, was coated. This was used as the cathode electrode.

[0055] [Gas diffusion type electrolytic flow cell] An anionic conductive resin (Sustainion® X37-50 Grade) was sandwiched between the cathode electrode and nickel foam used as the anode electrode, as described above. This film / electrode assembly was placed in a reaction cell (Complete 5 cm2 CO2 Electrolyzer (manufactured by Dioxide Materials)). The reaction cell is equipped with a cathode gas current collector plate with a cathode channel formed therein and an anode current collector plate with an anode channel formed therein. The film / electrode assembly was positioned so as to be in contact with the cathode channel of the cathode current collector plate and the anode channel of the anode current collector plate. This constituted a gas diffusion type electrolytic flow cell.

[0056] [Carbon dioxide reduction reaction test] A carbon dioxide reduction reaction test was conducted using the gas diffusion type electrolytic flow cell described above. Specifically, an electrochemical measurement system (Bio-Logic Science Instruments, SP-150) was connected to the anode and cathode sides in a two-electrode configuration. Carbon dioxide gas (CO2 = 99.995%) was supplied to the cathode channel at a flow rate of 20 mL / min, and a 1 M potassium hydroxide aqueous solution was supplied to the anode channel at a flow rate of 100 mL / min. A voltage of -2.5 V was applied to both electrodes by the electrochemical measurement system to perform the carbon dioxide reduction reaction test. An online gas chromatograph (SRI Instruments, Multiple Gas Analyzer #5) was used for the identification and quantification of the products associated with the carbon dioxide reduction reaction test. MOLECULAR SIEVE SA and HAYESEP-D columns were used, and a thermal conductivity detector (TCD) and a flame ionization detector (FID) were used as detectors.

[0057] <Example 2> The test was conducted in the same manner as in Example 1, except that the cathode electrode was prepared using copper benzene-1,3,5-tricarboxylate (Basolite® C 300, manufactured by Aldrich) instead of Cu2O as the catalyst containing copper atoms.

[0058] <Example 3> The test was conducted in the same manner as in Example 1, except that the cathode electrode was prepared using copper(II) phthalocyanine (manufactured by Aldrich) instead of Cu2O as the catalyst containing copper atoms.

[0059] <Example 4> The experiment was conducted in the same manner as in Example 1, except that the cathode electrode was prepared using Cu2Br2(PPh3)2(4PP)2 as a catalyst containing copper atoms, instead of Cu2O. Cu2Br2(PPh3)2(4PP)2 was synthesized by adding an acetonitrile solution in which copper bromide (CuBr) was dissolved to an acetone solution of 84 mg of 4-phenylpyridine (4PP) and 130 mg of triphenylphosphine (PPh3), and stirring overnight at room temperature. Subsequently, it was distilled under reduced pressure, washed with acetone and ethanol, and then purified with silica gel in the order of CH2Cl2 100%, CH2Cl2 99%, and CH3CN 1%, followed by vacuum drying.

[0060] <Example 5> The test was conducted in the same manner as in Example 1, except that potassium bicarbonate (KHCO3) was used instead of KOTf as the alkali metal salt, and Cu2Br2(PPh3)2(4PP)2 was used instead of Cu2O as the catalyst containing copper atoms to prepare the cathode electrode.

[0061] <Example 6> The test was conducted in the same manner as in Example 1, except that sodium bicarbonate (NaHCO3) was used as the alkali metal salt instead of KOTf, and Cu2Br2(PPh3)2(4PP)2 was used as the catalyst containing copper atoms instead of Cu2O to prepare the cathode electrode.

[0062] <Example 7> The test was conducted in the same manner as in Example 1, except that cesium bicarbonate (CsHCO3) was used as the alkali metal salt instead of KOTf, and Cu2Br2(PPh3)2(4PP)2 was used as the catalyst containing copper atoms instead of Cu2O to prepare the cathode electrode.

[0063] <Comparative Example 1> The cathode electrode was prepared and tested in the same manner as in Example 1, except that KOTf was not used.

[0064] <Comparative Example 2> The cathode electrode was prepared and tested in the same manner as in Example 2, except that KOTf was not used.

[0065] <Comparative Example 3> The cathode electrode was fabricated and tested in the same manner as in Example 3, except that KOTf was not used.

[0066] <Comparative Example 4> The cathode electrode was fabricated and tested in the same manner as in Example 4, except that KOTf was not used.

[0067] <Comparative Example 5> 10 mg of carbon particles (VULCAN® XC-72), 2 mg of Cu2Br2(PPh3)2(4PP)2, and 0.4 mg of KOTf were added to an acetonitrile / Nafion mixed solution (0.9 μL acetonitrile, 0.1 μL Nafion) as a polymer electrolyte, and ultrasonic dispersion was performed for 5 minutes. 0.5 μL of this solution was spread onto microporous carbon paper (Avcarb, GDS3250) to prepare a cathode electrode. The test was performed in the same manner as in Example 1, except for the use of this cathode electrode.

[0068] Table 1 shows the results of the carbon dioxide reduction reaction tests for Example 1 and Comparative Example 1.

[0069] [Table 1]

[0070] As shown in Table 1, Example 1 C 2+ The efficiency of reduction product formation (and amount produced per hour) was 36.2% (and 81.7 μmol) for C2H4, 11.2% (and 28.4 μmol) for C2H5OH, and 3.4% (and 5.8 μmol) for C3H7OH. The efficiency of byproduct H2 formation (and amount produced per hour) was 26.9% (and 354.3 μmol). On the other hand, in Comparative Example 1, C 2+The efficiency of reduction product formation (and amount produced per hour) was 34.8% (and 62.0 μmol) for C2H4, 7.8% (and 36.4 μmol) for C2H5OH, and 1.2% (and 3.6 μmol) for C3H7OH. The efficiency of the by-product H2 formation (and amount produced per hour) was 40.8% (and 330.4 μmol). Example 1 and Comparative Example 1 are similar in that they use a Cu2O catalyst in the catalyst layer of the cathode electrode and do not contain carbon particles or polymer electrolytes. Example 1 contains KOTf as an alkali metal salt, while Comparative Example 1 does not contain KOTf. However, Example 1 had a lower efficiency of hydrogen formation as a by-reaction than Comparative Example 1. 2+ The efficiency of reduction product formation was high.

[0071] Table 2 shows the results of the carbon dioxide reduction reaction tests for Examples 2-7 and Comparative Examples 2-5.

[0072] [Table 2]

[0073] As shown in Table 2, Example 2 C 2+ The efficiency of reduction product formation (and amount produced per hour) was 44.3% (and 137.4 μmol) for C2H4, 25.8% (and 80.1 μmol) for C2H5OH, and 5.8% (and 12.0 μmol) for C3H7OH. The efficiency of the byproduct H2 formation (and amount produced per hour) was 11.4% (and 211.8 μmol). On the other hand, in Comparative Example 2, C 2+The efficiency of reduction product formation (and amount produced per hour) was 29.1% (and 91.4 μmol) for C2H4, 2.9% (and 9.3 μmol) for C2H5OH, and 0.3% (and 0.6 μmol) for C3H7OH. The efficiency of the by-product H2 formation (and amount produced per hour) was 37.4% (and 704.6 μmol). Example 2 and Comparative Example 2 are similar in that they use copper benzene-1,3,5-tricarboxylate as a catalyst in the catalyst layer of the cathode electrode and do not contain carbon particles or polymer electrolytes. Example 2 contains KOTf as an alkali metal salt, while Comparative Example 2 does not contain KOTf. However, Example 2 had a lower efficiency of hydrogen formation as a by-reaction than Comparative Example 2. 2+ The efficiency of reduction product formation was high.

[0074] As shown in Table 2, Example 3 C 2+ The efficiency of reduction product formation (and amount produced per hour) was 24.2% (and 38.7 μmol) for C2H4, 11.0% (and 17.6 μmol) for C2H5OH, and 1.2% (and 1.3 μmol) for C3H7OH. The efficiency of the by-product H2 formation (and amount produced per hour) was 19.4% (and 186.4 μmol). On the other hand, in Comparative Example 3, C 2+ The efficiency of reduction product formation (and amount produced per hour) was 16.7% (and 32.6 μmol) for C2H4, 1.0% (and 2.0 μmol) for C2H5OH, and 0.3% (and 0.4 μmol) for C3H7OH. The efficiency of the by-product H2 formation (and amount produced per hour) was 31.1% (and 363.3 μmol). Example 3 and Comparative Example 3 are similar in that they use a copper(II) phthalocyanine catalyst in the catalyst layer of the cathode electrode and do not contain carbon particles or polymer electrolytes. Example 3 contains KOTf as an alkali metal salt, while Comparative Example 3 does not contain KOTf. However, Example 3 had a lower efficiency of hydrogen formation as a by-reaction than Comparative Example 3. 2+ The efficiency of reduction product formation was high.

[0075] As shown in Table 2, Example 4 C 2+The efficiency of reduction product formation (and amount produced per hour) was 30.9% (and 94.2 μmol) for C2H4, 9.3% (and 28.4 μmol) for C2H5OH, and 2.1% (and 4.2 μmol) for C3H7OH. The efficiency of by-product H2 formation (and amount produced per hour) was 34.7% (and 634.8 μmol). Also, in Example 5, C 2+ The efficiency of reduction product formation (and amount produced per hour) was 38.3% (and 90.6 μmol) for C2H4, 14.2% (and 33.7 μmol) for C2H5OH, and 5.6% (and 8.9 μmol) for C3H7OH. The efficiency of the by-product H2 formation (and amount produced per hour) was 25.6% (and 364.5 μmol). Also, C in Example 6 2+ The efficiency of reduction product formation (and amount produced per hour) was 31.5% (and 54.8 μmol) for C2H4, 7.4% (and 12.8 μmol) for C2H5OH, and 1.5% (and 1.7 μmol) for C3H7OH. The efficiency of by-product H2 formation (and amount produced per hour) was 37.2% (and 387.7 μmol). Also, in Example 7, C 2+ The efficiency of reduction product formation (and amount produced per hour) was 30.9% (and 76.7 μmol) for C2H4, 23.6% (and 58.5 μmol) for C2H5OH, and 5.2% (and 8.5 μmol) for C3H7OH. The efficiency of the by-product H2 formation (and amount produced per hour) was 36.4% (and 542.2 μmol). On the other hand, in Comparative Example 4, C 2+ The efficiency of reduction product formation (and amount produced per hour) was 28.7% (and 59.4 μmol) for C2H4, 1.3% (and 2.6 μmol) for C2H5OH, and 0.3% (and 0.4 μmol) for C3H7OH. The efficiency of byproduct H2 formation (and amount produced per hour) was 40.4% (and 501.5 μmol).

[0076] Examples 4-7 and Comparative Example 4 share the common features of using a Cu2Br2(PPh3)2(4PP)2 catalyst in the catalyst layer of the cathode electrode and not containing carbon particles or polymer electrolytes. However, Examples 4-7 contain alkali metal salts, while Comparative Example 4 does not. In contrast, Examples 4-7 have lower efficiency in generating hydrogen as a side reaction than Comparative Example 4. 2+ The efficiency of reduction product formation was high. Furthermore, among Examples 5-7 using alkali metal bicarbonates, the hydrogen production efficiency was suppressed in the order of Example 5 (potassium bicarbonate) > Example 7 (cesium bicarbonate) > Example 6 (sodium bicarbonate), and C 2+ The efficiency of reduction product generation was improved. Furthermore, comparing Examples 4 and 5, which used potassium salts, Example 5, which used potassium bicarbonate, had a lower hydrogen generation efficiency than Example 4, which used KOTf. 2+ The efficiency of reduction product formation was improved.

[0077] Comparative Example 5 C 2+ The efficiency of reduction product formation (and amount produced per hour) was 20.0% (and 31.0 μmol) for C2H4, and the efficiency of byproduct H2 formation (and amount produced per hour) was 50.2% (and 215.5 μmol). Example 4 and Comparative Example 5 are similar in that the catalyst layer of the cathode electrode contains the catalyst Cu2Br2(PPh3)2(4PP)2 and the alkali metal salt of KOTf. Example 4 does not contain carbon particles and polymer electrolytes, while Comparative Example 5 does not contain carbon particles and polymer electrolytes. However, Example 4 has a lower efficiency of hydrogen formation as a by-reaction than Comparative Example 5. 2+ The efficiency of reduction product generation was high. Furthermore, the current value flowing through the cell in Comparative Example 5 was -80mA, while the current value flowing through the cell in Example 4 was -100mA, indicating that the current value in Comparative Example 5 was lower than that in Example 4.

[0078] Incidentally, while Patent Document 3 requires a cell potential of 2.8V to pass a current of -100mA, in this embodiment, the cell potential required to pass a current of -100mA was reduced to 2.5V. From this, it can be said that this embodiment has a cell configuration with lower resistance values ​​compared to the conventional technology of Patent Document 3. Also, C 2+ Example 3, which has the highest efficiency in generating reduction products from CO2 to C 2+ The energy conversion efficiency of the reduction product (a value determined from the potential applied to the entire cell and the production efficiency, assuming the theoretical production potentials of ethylene, ethanol, and propanol are -1.15V, -1.14V, and -1.44V respectively) was 36%, which is higher than the 34% reported in Non-Patent Document 2. [Explanation of symbols]

[0079] 1 Gas diffusion type electrolytic flow cell, 10 Anode section, 12 Cathode section, 14 Ion-conducting polymer film, 16 Anode, 18 Anode channel, 20 Anode current collector plate, 22 Cathode, 24 Cathode channel, 26 Catalyst layer, 28 Gas diffusion layer, 30 Cathode current collector plate, 32 Power supply.

Claims

1. A cathode electrode for a gas diffusion type electrolytic flow cell that reduces carbon dioxide to produce carbon dioxide reduction products, A catalyst layer comprising a catalyst having copper atoms and a solid alkali metal salt, and a gas diffusion layer disposed on the catalyst layer, The catalyst having copper atoms includes a complex catalyst having copper atoms. The aforementioned complex catalyst having copper atoms has the general formula: CuMX ２ (Y) ２ L ２ The metal complex represented by the formula (wherein M is Cu, Ag or Ni, X is a halogen atom, Y is a ligand having a phosphorus atom, and L is a ligand having a pyridine ring), phthalocyanine copper complex, and copper benzene-1,3,5-tricarboxylate contain at least one of these: The catalyst layer is characterized by not containing carbon particles and polymer electrolytes, and is a cathode electrode for a gas diffusion type electrolytic flow cell.

2. A gas diffusion type electrolytic flow cell comprising an anode electrode that oxidizes water or hydroxide ions to produce oxygen, a cathode electrode that reduces carbon dioxide to produce carbon dioxide reduction products, and an ion-conducting polymer film sandwiched between the anode electrode and the cathode electrode, The cathode electrode comprises a catalyst layer having a catalyst containing copper atoms and a solid alkali metal salt, and a gas diffusion layer disposed on the catalyst layer. The catalyst having copper atoms includes a complex catalyst having copper atoms. The aforementioned complex catalyst having copper atoms has the general formula: CuMX ２ (Y) ２ L ２ The metal complex represented by the formula (wherein M is Cu, Ag or Ni, X is a halogen atom, Y is a ligand having a phosphorus atom, and L is a ligand having a pyridine ring), phthalocyanine copper complex, and copper benzene-1,3,5-tricarboxylate contain at least one of these: The catalyst layer is characterized by not containing carbon particles and polymer electrolytes, and is a gas diffusion type electrolytic flow cell.

3. The gas diffusion type electrolytic flow cell according to claim 2, characterized in that an anode solution is supplied to the anode electrode and carbon dioxide gas is supplied to the cathode electrode.

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