Carbon dioxide reduction apparatus

Abstract

To provide a carbon dioxide reduction apparatus capable of promoting generation of solid formate.SOLUTION: A carbon dioxide reduction apparatus 1 comprises: an anode 16 that oxidizes water in an electrolytic solution to generate oxygen; a cathode 22 that reduces carbon dioxide in carbon dioxide gas to generate formate; an anion exchange membrane 14 disposed between the anode 16 and the cathode 22; and the electrolytic solution supplied to the anode 16, wherein the electrolytic solution comprises a solvent, and an electrolyte comprising cations that form the formate and having a saturation concentration (g / 100 mL) value in the solvent larger than that of the formate, and the electrolytic solution is a solution in which the electrolyte is dissolved at 80% or more of the saturation concentration in the solvent.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a carbon dioxide reduction device. [Background technology]

[0002] In recent years, concerns have arisen about the depletion of fossil fuels such as oil and coal, and expectations are growing for sustainable renewable energy sources. From the perspective of such energy issues and environmental problems, progress has been made in the development of artificial photosynthesis technology, which uses renewable energy such as sunlight to electrochemically reduce carbon dioxide and create a storable chemical energy source.

[0003] One method for reducing carbon dioxide is to electrochemically reduce carbon dioxide dissolved in an aqueous solution to formic acid (see, for example, Patent Document 1). However, conventional carbon dioxide reduction electrolysis devices, such as the method in Patent Document 1, reduce carbon dioxide dissolved in a solution (see, for example, Figure 1 in Patent Document 1). In this example, metal nanowires such as copper (Cu) are used as a carbon dioxide reduction catalyst, and platinum (Pt) is used as a water (H2O) oxidation catalyst.

[0004] In recent years, the development of gas diffusion type carbon dioxide reduction electrodes has progressed, and for example, Non-Patent Document 1 reports a carbon dioxide reduction device in which an electrolyte solution for the anode chamber (aqueous potassium bicarbonate (KHCO3) solution), an ion-conductive membrane, and an electrolyte solution for the cathode chamber (aqueous potassium hydroxide (KOH) solution) are arranged between a gas diffusion type anode (cathode) for carbon dioxide reduction and a cathode (anode) for HO oxidation (see Figure 2 in Non-Patent Document 1). In this example, tin oxide is used as the carbon dioxide reduction catalyst, and nickel (Ni) is used as the hydroxide catalyst.

[0005] As another example, Patent Document 2 describes a two-chamber carbon dioxide reduction device in which an anion exchange membrane is disposed between a gas diffusion type anode and a cathode (see Figure 1 of Patent Document 2), and Non-Patent Document 2 describes a three-chamber carbon dioxide reduction device in which an anion exchange membrane, an aqueous solution layer, and a cation exchange membrane are disposed between a gas diffusion type anode and a cathode (see Figure 3 of Non-Patent Document 2). Patent Document 2 uses a Ru complex as the carbon dioxide reduction catalyst and an Fe-Ni catalyst as the hydroxide catalyst. Non-Patent Document 2 also uses tin nanoparticles as the carbon dioxide reduction catalyst and iridium oxide (IrO2) as the hydroxide catalyst.

[0006] In the case of the electrode immersion method as in Patent Document 1, the concentration of carbon dioxide dissolved in the aqueous solution is low at room temperature and normal pressure, so the coexisting protons (H + ) is reduced, and hydrogen (H2) is produced as a by-product. In addition, because the mass diffusion of carbon dioxide in aqueous solutions is slow, the theoretical limit of the reaction current density for carbon dioxide reduction is <30 mA cm -2 and small.

[0007] In the gas diffusion method described in Patent Document 2 and Non-Patent Documents 1 and 2, carbon dioxide gas is mixed with water vapor gas and supplied directly to the anode. This results in a high carbon dioxide to water concentration ratio, suppressing the by-production of hydrogen (H2). Furthermore, the reaction proceeds in the gas phase, where the diffusion rate is fast, significantly increasing the limit on the reaction current density. Furthermore, because different fluids can be used in the positive and negative electrode chambers, carbon dioxide can be supplied to the anode while the positive electrode is kept in an alkaline environment that facilitates water oxidation, creating an optimal reaction environment at both electrodes.

[0008] However, in Patent Document 2 and Non-Patent Documents 1 and 2, formic acid produced by reducing carbon dioxide dissolves in the electrolyte solution (electrolyte) supplied to the carbon dioxide reduction device, making it difficult to separate and recover formic acid. Methods for recovering formic acid from the electrolyte include a method of separating it using ion exchange column chromatography and a method of distilling it after adding an acid (see, for example, Patent Document 3). However, all of these methods cause problems such as the generation of scale and increased processing costs. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-057438 [Patent Document 2] Japanese Patent Application Publication No. 2023-133965 [Patent Document 3] Japanese Patent Application Publication No. 2023-69361 [Non-patent literature]

[0010] [Non-Patent Document 1] ACS Sustainable Chem. Eng. 2021, 9, 11, 4213-4223 [Non-patent document 2] https: / / dioxidematerials.com / technology / formic-acid / Summary of the Invention [Problem to be solved by the invention]

[0011] Incidentally, if carbon dioxide can be reduced in a carbon dioxide reduction device and then converted into solid formate, the formate can be easily recovered by solid-liquid separation or the like. For example, the recovered formate can be simply dissolved in an acidic solution to obtain highly concentrated formic acid, or it can be converted into formaldehyde by dry distillation. Formic acid can be used as an antiseptic, antibacterial agent, a raw material for fine chemicals, and the like. Furthermore, formaldehyde can be converted into sugar by the Moller-Hohs reaction or the like, and can also be used as a raw material for urea resin by condensation polymerization of formaldehyde and urea.

[0012] However, in conventional carbon dioxide reduction devices, the formic acid produced by reducing carbon dioxide dissolves in the electrolyte, making it difficult to directly produce solid formate. While adding a salt precipitant containing Ca ions is one way to obtain solid formate from an electrolyte containing dissolved formic acid, prior concentration is required when the formate ion concentration in the electrolyte is low. Furthermore, if the electrolyte is alkaline, even if a salt precipitant is added, Ca(OH)2, which has low solubility, will precipitate preferentially over formate salts such as Ca(HCOO)2. Furthermore, since carbon dioxide may be dissolved in the electrolyte, CaCO3, which has low solubility compared to formate salts, will also precipitate preferentially.

[0013] Therefore, an object of the present invention is to provide a carbon dioxide reduction device that can promote the production of solid formate salts. [Means for solving the problem]

[0014] The present invention provides a carbon dioxide reduction device that reduces carbon dioxide to produce formic acid, and converts the produced formic acid into a solid formate salt. The device comprises: an anode to which an electrolytic solution is supplied and that oxidizes water in the electrolytic solution to produce oxygen; a cathode to which a gas containing carbon dioxide is supplied and that reduces the carbon dioxide to produce formic acid; an anion exchange membrane disposed between the anode and the cathode; and the electrolytic solution that is supplied to the anode. The electrolytic solution comprises a solvent and an electrolyte that contains cations that form the formate salt and has a saturation concentration (g / 100 mL) in the solvent that is higher than that of the formate salt, and is a solution in which the electrolyte is dissolved in the solvent at 80% or more of its saturation concentration.

[0015] The present invention also provides a carbon dioxide reduction device for obtaining formic acid produced by reducing carbon dioxide as a solid formate, the device comprising: an anode to which an anode solution is supplied and which oxidizes water in the anode solution to produce oxygen; a cathode to which a gas containing carbon dioxide is supplied and which reduces the carbon dioxide to produce formic acid; a cation exchange membrane, an ion conduction layer, and an anion exchange membrane disposed in this order from the anode side between the anode and the cathode; and an electrolyte supplied to the ion conduction layer, the electrolyte comprising a solvent and an electrolyte containing cations that form the formate salt and having a saturation concentration in the solvent (g / 100 mL) higher than that of the formate salt, the electrolyte being a solution in which the electrolyte is dissolved in the solvent at 80% or more of its saturation concentration.

[0016] In the carbon dioxide reduction device, the cations contained in the electrolyte are preferably cations that form the formate salt at a saturated concentration in water at 20° C. of less than 20 mg / 100 mL.

[0017] In the carbon dioxide reduction device, the cation is preferably a cation of at least one element selected from the group consisting of Ca, Zn, Sr, U, Cu, Mg, and Cd.

[0018] In the carbon dioxide reduction device, the electrolyte is Cl -, NO3 - , NO2 - , CO3 2- , SO4 2- , Br - , I - , B4O7 2- , ClO3 - , MnO4 - , CH3COO - , BrO3 - It is preferred that the anion contains at least one anion selected from the group consisting of:

[0019] Furthermore, it is preferable that the carbon dioxide reduction device includes at least one of a formic acid generator that reacts the formate with an acid to generate formic acid, and a dry distillation device that dry distills the formate to generate formaldehyde.

[0020] In the carbon dioxide reduction device, the cathode preferably includes a catalyst layer containing a metal complex as a cathode catalyst.

[0021] Furthermore, in the carbon dioxide reduction device, it is preferable that the metal complex has, as a central metal, at least one metal selected from the group consisting of Ru, Mn, Fe, Co, Ni, Cu, Mo, and Re, and has, as a ligand, a diimine ligand.

[0022] In the carbon dioxide reduction device, it is preferable that the cathode has a gas diffusion layer, the central metal of the metal complex is Ru, the diimine ligand is a diimine ligand having an electron-withdrawing substituent introduced therein, the catalyst layer further contains a polymer compound that serves as an ion conductor and a binder, and conductive carbon, and the gas diffusion layer contains a hydrophobic porous carbon substrate.

[0023] In the carbon dioxide reduction device, it is preferable that the electron-withdrawing substituent is a carboxylic acid ester having a structural formula of -COOR, R has a chemical structure consisting of an alkyl group and a pyrrole moiety, and the pyrrole moiety is polymerized through a polypyrrole chain.

[0024] Furthermore, in the carbon dioxide reduction device, it is preferable that the anode comprises a substrate made of at least one material selected from the group consisting of Ni, Ti, Fe, and C, and an anode catalyst, the substrate having at least one shape selected from the group consisting of a porous body, a mesh material, and a sintered fiber body, and the anode catalyst comprises at least one selected from the group consisting of a metal containing at least one element selected from the group consisting of Ni, Fe, Co, Mn, Ru, and Ir, an oxide containing the metal, a hydroxide containing the metal, and an oxyhydroxide containing the metal. [Effects of the Invention]

[0025] According to the present invention, it is possible to provide a carbon dioxide reduction device that can promote the production of solid formate salts. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic configuration diagram showing an example of a carbon dioxide reduction device according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic configuration diagram showing another example of a carbon dioxide reduction device according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0027] 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.

[0028] FIG. 1 is a schematic diagram showing an example of a carbon dioxide reduction device according to the present embodiment. The carbon dioxide reduction device 1 shown in FIG. 1 includes an anode section 10 including an anode 16 that oxidizes water to produce oxygen and an electrolyte flow path 18 that supplies an electrolyte to the anode 16, a cathode section 12 that includes a cathode 22 that reduces carbon dioxide to produce formic acid and a gas flow path 24 that supplies a carbon dioxide-containing gas to the cathode 22, an anion exchange membrane 14 disposed between the anode section 10 and the cathode section 12, and an electrolyte supplied to the anode 16. The cathode 22 shown in FIG. 1 includes, in order from the anion exchange membrane 14 side, a catalyst layer 26 that includes a metal complex as a cathode catalyst, and a gas diffusion layer 28. The carbon dioxide reduction device 1 includes an anode current collector 20 and a cathode current collector 30.

[0029] The carbon dioxide reduction device 1 is a gas diffusion electrolysis flow cell that directly supplies a gas containing carbon dioxide to the cathode 22. The gas containing carbon dioxide preferably contains water vapor.

[0030] The anode 16 is disposed between the anion exchange membrane 14 and the electrolyte flow path 18 so as to be in contact with them. The electrolyte flow path 18 is a flow path that supplies the electrolyte to the anode 16 and is formed, for example, by a pit (groove or recess) provided in the anode current collector plate 20. The cathode 22 is disposed between the gas flow path 24 and the anion exchange membrane 14 so as to be in contact with them. The gas flow path 24 is a flow path that supplies a gas containing carbon dioxide to the cathode 22 and is formed by a pit (groove or recess) provided in the cathode current collector plate 30. The anion exchange membrane 14 is sandwiched between the anode part 10 and the cathode part 12. That is, the anode part 10 and the cathode part 12 are separated by the anion exchange membrane 14.

[0031] For example, a solution inlet and a solution outlet (neither of which are shown) are connected to the anode current collector plate 20. The electrolyte is introduced into the electrolyte flow path 18 through the solution inlet, passes through the electrolyte flow path 18 while in contact with the anode 16, and is discharged from the solution outlet.

[0032] The electrolyte solution is a solution containing a solvent and an electrolyte containing cations that form formate salts and having a saturation concentration (g / 100 mL) in the solvent that is greater than that of the formate salts, with the electrolyte dissolved at 80% or more of its saturation concentration in the solvent. The electrolyte contains anions that are counterions of the cations that form the formate salts. The solvent contains water. Examples of water that can be used as the solvent include tap water, pure water, and ion-exchanged water.

[0033] For example, a gas inlet and a gas outlet (neither of which are shown) are connected to the cathode current collector plate 30. Carbon dioxide gas is introduced into the gas flow channel 24 through the gas inlet, passes through the gas flow channel 24 while coming into contact with the catalyst layer 26 via the gas diffusion layer 28, and is discharged from the gas outlet.

[0034] The carbon dioxide reduction device 1 is provided with a power supply 32 that electrically connects the anode 16 and the cathode 22 and supplies power therebetween.

[0035] Next, an example of the operation of the carbon dioxide reduction device 1 shown in FIG. 1 will be described.

[0036] When a current is supplied between the anode 16 and the cathode 22 from the power supply 32, an oxidation reaction of water (H2O) occurs on the anode 16 side. Specifically, the water (H2O) contained in the electrolyte solution supplied to the anode 16 through the electrolyte solution flow path 18 is oxidized to oxygen (O2) and protons (H + ) is generated.

[0037] On the cathode 22 side, carbon dioxide (CO2) in the gas containing carbon dioxide (CO2) supplied from the gas flow channel 24 to the catalyst layer 26 through the gas diffusion layer 28 is reduced to generate formic acid (HCOOH). The generated formic acid (HCOOH) is ionized to form formate ions (HCOO - ) obtained at the cathode 22. - ) permeates the anion exchange membrane 14 and is obtained in the electrolyte solution supplied to the anode 16. Overall, formic acid (HCOOH) is produced from carbon dioxide (CO2), as shown in the reaction formula (1) below. CO2+H2O → HCOOH+1 / 2O2ΔG=274kJ / mol···(1)

[0038] Typically, the generated formic acid dissolves in the electrolyte solution supplied to the anode 16. However, in this embodiment, cations constituting the electrolyte react with formate ions in the electrolyte solution supplied to the anode 16 to form solid formate. The solid formate is deposited on the anion exchange membrane 14 or is discharged from the electrolyte flow path 18 together with the electrolyte solution. The formate discharged from the electrolyte flow path 18 together with the electrolyte solution is recovered by solid-liquid separation such as filtration. The formate deposited on the anion exchange membrane 14 is recovered, for example, by disassembling the carbon dioxide reduction device. In this way, in this embodiment, it is possible to suppress dissolution of the formic acid obtained by carbon dioxide reduction in the electrolyte solution and promote the generation of solid formate.

[0039] FIG. 2 is a schematic diagram of another example of a carbon dioxide reduction device according to this embodiment. In the carbon dioxide reduction device 2 shown in FIG. 2, components similar to those in the carbon dioxide reduction device 1 shown in FIG. 1 are designated by the same reference numerals. The carbon dioxide reduction device 2 shown in FIG. 2 includes an anode section 10 including an anode 16 that oxidizes water to produce oxygen and an anode solution flow path 19 that supplies an anode solution to the anode 16, a cathode section 12 that includes a cathode 22 that reduces carbon dioxide to produce formic acid and a gas flow path 24 that supplies a carbon dioxide-containing gas to the cathode 22, a cation exchange membrane 34, an ion conduction layer 36, and an anion exchange membrane 14 disposed between the anode section 10 and the cathode section 12 in this order from the anode section 10 side, and an electrolyte solution supplied to the ion conduction layer 36. The cathode 22 shown in FIG. 2 includes, in this order from the anion exchange membrane 14 side, a catalyst layer 26 containing a metal complex as a cathode catalyst, and a gas diffusion layer 28. The carbon dioxide reduction device 2 includes an anode current collector 20 and a cathode current collector 30 .

[0040] The carbon dioxide reduction device 2 is a gas diffusion electrolysis flow cell that directly supplies a gas containing carbon dioxide to the cathode 22. The gas containing carbon dioxide preferably contains water vapor.

[0041] An ion-conducting layer 36 is formed between the anode section 10 and the cathode section 12, sandwiched between a cation exchange membrane 34 and an anion exchange membrane 14. The anode section 10 and the cathode section 12 are separated by the cation exchange membrane 34, the ion-conducting layer 36, and the anion exchange membrane 14. The anode 16 is disposed between the cation exchange membrane 34 and an anode solution flow path 19 so as to be in contact with them. The anode solution flow path 19 is a flow path that supplies anode solution to the anode 16 and is formed, for example, by a pit (groove or recess) provided in the anode current collector plate 20. The cathode 22 is disposed between the anion exchange membrane 14 and a gas flow path 24 so as to be in contact with them. The gas flow path 24 is a flow path that supplies a gas containing carbon dioxide to the cathode 22 and is formed by a pit (groove or recess) provided in the cathode current collector plate 30.

[0042] For example, a solution inlet and a solution outlet (neither of which are shown) are connected to anode current collector 20. Anode solution is introduced into anode solution flow path 19 via the solution inlet, passes through anode solution flow path 19 while in contact with anode 16, and is discharged from the solution outlet.

[0043] For example, a gas inlet and a gas outlet (neither of which are shown) are connected to the cathode current collector plate 30. Carbon dioxide gas is introduced into the gas flow channel 24 through the gas inlet, passes through the gas flow channel 24 while coming into contact with the catalyst layer 26 via the gas diffusion layer 28, and is discharged from the gas outlet.

[0044] The ion conduction layer 36 is a flow path through which the electrolyte flows. The ion conduction layer 36 shown in FIG. 2 is a space (flow path) formed between the anion exchange membrane 14 and the cation exchange membrane 34, and the electrolyte is supplied into this space. The ion conduction layer 36 is provided with, for example, an electrolyte inlet and an electrolyte outlet (neither of which are shown), and the electrolyte is introduced into the ion conduction layer 36 through the electrolyte inlet, passes through the ion conduction layer 36, and is discharged from the electrolyte outlet. The ion conduction layer 36 may be filled with an ion exchange resin.

[0045] The electrolyte solution is a solution containing a solvent and an electrolyte containing cations that form formate salts and having a saturation concentration (g / 100 mL) in the solvent that is greater than that of the formate salts, where the electrolyte is dissolved at 80% or more of its saturation concentration in the solvent. The electrolyte contains anions that are counterions of the cations that form the formate salts. The solvent can be, for example, water, pure water, or ion-exchanged water.

[0046] The carbon dioxide reduction device 1 is provided with a power supply 32 that electrically connects the anode 16 and the cathode 22 and supplies power therebetween.

[0047] Next, an example of the operation of the carbon dioxide reduction device 2 shown in FIG. 2 will be described.

[0048] When a current is supplied between the anode 16 and the cathode 22 from the power supply 32, an oxidation reaction of water (HO) occurs on the anode 16 side. Specifically, the water (HO) contained in the anode solution supplied to the anode 16 through the anode solution flow path 19 is oxidized to oxygen (O) and protons (H + ) is generated.

[0049] On the cathode 22 side, carbon dioxide (CO2) in the gas containing carbon dioxide (CO2) supplied from the gas flow channel 24 to the catalyst layer 26 through the gas diffusion layer 28 is reduced to produce formic acid (HCOOH). The produced formic acid (HCOOH) is ionized to form formate ions (HCOO - ) is obtained in the form

[0050] The formate ions (HCOO) obtained at the cathode 22 - ) moves through the anion exchange membrane 14 to the ion conduction layer 36 and is obtained in the electrolyte supplied to the ion conduction layer 36.

[0051] Typically, the generated formic acid dissolves in the electrolyte solution supplied to the ion conduction layer 36. However, in this embodiment, cations constituting the electrolyte react with formate ions in the electrolyte solution supplied to the ion conduction layer 36 to form solid formate. The solid formate is deposited on the anion exchange membrane 14 or the like, or is discharged from the ion conduction layer 36 together with the electrolyte solution. The formate discharged from the ion conduction layer 36 together with the electrolyte solution is recovered by solid-liquid separation, such as filtration. The formate deposited on the anion exchange membrane 14 or the like is recovered, for example, by disassembling the carbon dioxide reduction device. In this way, in this embodiment, dissolution of the formic acid obtained by carbon dioxide reduction in the electrolyte solution is suppressed, and the generation of solid formate is promoted.

[0052] The carbon dioxide reduction apparatus according to this embodiment may also include a formic acid generator (not shown) that reacts the obtained formate with an acid to generate formic acid. The formic acid generator includes, for example, a reaction tank for reacting the formate with an acid, a stirrer for stirring the solution in the reaction tank, and an acid supply device for adding acid to the reaction tank. Examples of the acid include hydrochloric acid, nitric acid, and sulfuric acid. The carbon dioxide reduction apparatus according to this embodiment may also include a dry distillation apparatus that dry distills the obtained formate to generate formaldehyde. The formate is a compound containing two or more equivalents of formate ions relative to a cation, such as calcium formate (Ca(HCOO)2). The dry distillation apparatus is not particularly limited, and examples include a shaft furnace and an internal combustion or external heat rotary kiln. The predetermined temperature is, for example, 200°C to 350°C. The carbon dioxide reduction apparatus preferably includes at least one of a formic acid generator and a dry distillation apparatus.

[0053] The components of the anode part 10, the cathode part 12, the electrolyte, the anion exchange membrane 14, the cation exchange membrane 34, etc. will be described below.

[0054] As described above, the electrolyte solution supplied to the anode 16 in the carbon dioxide reduction device 1 shown in FIG. 1 and the electrolyte solution supplied to the ion conduction layer 36 in the carbon dioxide reduction device 2 shown in FIG. 2 are solutions containing a solvent and an electrolyte containing cations that form formate salts and whose saturation concentration in the solvent (g / 100 mL) is higher than that of the formate salt, and the electrolyte is dissolved in the solvent at 80% or more of its saturation concentration. The saturation concentration in the solvent is measured as follows: The electrolyte (or the target formate salt) is added to the solvent used for the electrolyte (solvent temperature: 20°C) to achieve supersaturation, and the solution is left overnight while being shaken. The mixed solution is then decanted and filtered, and the filtrate is quantitatively analyzed by high-performance liquid chromatography to determine the saturation concentration.

[0055] In order to promote the precipitation of a solid formate, the cation constituting the electrolyte is preferably a cation that forms a formate having a saturation concentration in water at 20°C of less than 20 mg / 100 mL, and more specifically, is more preferably a cation of at least one element selected from the group consisting of Ca, Zn, Sr, U, Cu, Mg, and Cd. Examples of formate salts with a saturation concentration in water of less than 20 mg / 100 mL at 20°C include zinc formate (Zn(HCO2)2: 6.1 mg / 100 mL), uranyl formate monohydrate (UO2(HCO2)2·H2O: 7.2 mg / 100 mL), copper(II) formate (Cu(HCO2)2: 12.5 mg / 100 mL), strontium formate (Sr(HCO2)2: 12.7 mg / 100 mL), cadmium formate (Cd(HCO2)2: 14.4 mg / 100 mL), magnesium formate (Mg(HCO2)2: 14.4 mg / 100 mL), and calcium formate (Ca(HCO2)2: 16.6 mg / 100 mL).

[0056] In addition, the anions that make up the electrolyte are Cl in terms of the degree of ionization and ion mobility. - , NO3 - , NO2 - , CO3 2- , SO4 2- , Br - , I -, B4O7 2- , ClO3 - , MnO4 - , CH3COO - , BrO3 - Preferably, the anion is at least one selected from the group consisting of:

[0057] The electrolytic solution may be any solution in which the electrolyte is dissolved in the solvent at 80% or more of the saturated concentration. However, in order to further promote the precipitation of solid formate, the electrolytic solution is preferably a solution in which the electrolyte is dissolved in the solvent at 90% or more of the saturated concentration, and more preferably a solution in which the electrolyte is dissolved in the solvent at 100% of the saturated concentration (i.e., a saturated solution of the electrolyte).

[0058] As described above, the anode 16 promotes the oxidation reaction of water (H2O) to produce oxygen (O2) and hydrogen ions (H + ) is an electrode (oxidation electrode).

[0059] The anode 16 preferably has a substrate made of at least one material selected from the group consisting of Ni, Ti, Fe, and C, as this can reduce the overvoltage of the oxidation reaction. The Ni, Ti, or Fe metal material also includes an alloy containing at least one of the metals Ni, Ti, and Fe. The substrate preferably has a structure that allows the movement of liquids and ions, and preferably has at least one shape selected from the group consisting of a porous body, a mesh material, and a sintered fiber body.

[0060] The anode 16 preferably includes an anode catalyst. The anode catalyst preferably includes at least one selected from the group consisting of a metal containing at least one element selected from the group consisting of Ni, Fe, Co, Mn, Ru, and Ir, an oxide containing the metal, a hydroxide containing the metal, and an oxyhydroxide containing the metal, in order to reduce the overvoltage of the oxidation reaction. These may be used alone or in combination of two or more. The anode catalyst preferably includes at least one selected from the group consisting of iron oxyhydroxide and nickel oxyhydroxide, in order to allow water oxidation to proceed at a low potential. When using an anode catalyst, it is preferable to support the anode catalyst on the aforementioned substrate.

[0061] It is preferable to use a material with low chemical reactivity and high conductivity for the anode current collector plate 20. Examples of such materials include metal materials such as Ti and SUS, and carbon.

[0062] The anode solution used in the carbon dioxide reduction device 2 shown in Fig. 2 is, for example, an alkaline solution. Examples of alkaline solutions include an aqueous potassium hydroxide solution, an aqueous sodium hydroxide solution, an aqueous potassium carbonate solution, and an aqueous potassium hydrogen carbonate solution. Since the higher the hydroxide ion concentration, the more favorable the progress of water oxidation, an aqueous potassium hydroxide solution or an aqueous sodium hydroxide solution with a concentration of 1 mol / L or more is preferred. The alkaline anode solution is preferably an alkaline aqueous solution with a pH of 12 or higher, for example, in terms of reducing cell voltage.

[0063] As described above, the cathode 22 is an electrode (reduction electrode) that promotes the reduction reaction of carbon dioxide (CO2) to produce formic acid (HCOOH).

[0064] The cathode 22 preferably includes a gas diffusion layer 28. The gas diffusion layer 28 is not particularly limited as long as it ensures electrical conduction between the catalyst layer 26 and the power source 32 and efficiently supplies carbon dioxide gas to the catalyst layer 26, and examples of the gas diffusion layer 28 include a hydrophobic porous carbon substrate, carbon black, and carbon nanotubes. The gas diffusion layer 28 is preferably a hydrophobic porous carbon substrate, since it can reduce the amount of water that has migrated from the anode 16 side.

[0065] The cathode 22 preferably includes a catalyst layer 26 containing a metal complex as a cathode catalyst. As described above, the catalyst layer 26 promotes the reduction reaction of carbon dioxide in a carbon dioxide-containing gas to produce formic acid (HCOOH). In addition to the metal complex as a cathode catalyst, the catalyst layer 26 preferably further contains a polymer compound that serves as an ion conductor and binder, and conductive carbon. The catalyst layer 26 preferably includes a porous structure such as carbon paper as a substrate, in order to improve the diffusibility of the carbon dioxide-containing gas. The thickness of the catalyst layer 26 is, for example, in the range of 5 to 200 μm.

[0066] The metal complex serving as the cathode catalyst is, for example, a metal complex having a central metal and a diimine ligand. The central metal of the metal complex is not particularly limited as long as it is a metal that catalyzes the reduction reaction of carbon dioxide, but is preferably at least one metal selected from the group consisting of Ru, Mn, Fe, Co, Ni, Cu, Mo, and Re, more preferably Mn or Ru, and particularly preferably Ru, in terms of reducing the overvoltage in the reduction reaction of carbon dioxide.

[0067] Examples of the diimine ligand of the metal complex include 2,2′-bipyridine derivatives, 1,10-phenanthroline derivatives, etc. The diimine ligand is preferably a diimine ligand having an electron-withdrawing substituent introduced therein, in view of being able to reduce the overvoltage due to the reduction reaction of carbon dioxide.

[0068] Examples of electron-withdrawing substituents introduced into the diimine ligand include carboxylic acid groups, carbonyl groups, nitro groups, and carboxylic acid esters having the structural formula -COOR. Here, R is, for example, a linear or branched alkyl group having 1 to 10 carbon atoms. R preferably has a chemical structure consisting of an alkyl group and a pyrrole moiety, and the pyrrole moiety is preferably polymerized with a polypyrrole chain, in order to ensure electrical conduction between the diimine ligand and a power source.

[0069] The metal complex preferably has a central metal Ru and is a molecule in which 2,2'-bipyridine and pyrrole are chemically bonded with a carboxylic acid ester of -COOR (R is an alkyl group having 1 to 10 carbon atoms), or a polymer in which the pyrrole moiety is polymerized with a polypyrrole chain to form a multimer.

[0070] An example of the metal complex is the metal complex represented by the following chemical formula: [Ru{4,4'-di(1H-pyrrolyl-3-propylcarbonate)-2,2'-bipyridine}Cl2(CO)(CH3CN)]. [ka]

[0071] Examples of the conductive carbon contained in the catalyst layer 26 include carbon black such as Ketjen Black or Vulcan XC-72, activated carbon, and carbon nanotubes. The conductive carbon is preferably used as a support for supporting the metal complex. Supporting the metal complex on the conductive carbon can, for example, enhance reduction reactivity.

[0072] Examples of the polymers that serve as the ion conductor and binder contained in the catalyst layer 26 include cation exchange resins such as Nafion (registered trademark) (manufactured by DuPont) and Flemion (registered trademark) (manufactured by Asahi Glass Co., Ltd.), and anion exchange resins such as Neocepta (registered trademark), Selemion (registered trademark), and Sustenion (registered trademark).

[0073] The catalyst layer 26 may contain phenol or its salt, which can enhance catalytic activity.

[0074] The cathode current collector 30 is preferably made of a material that has low chemical reactivity and high conductivity, similar to the anode current collector 20. Examples of such materials include metal materials such as Ti and stainless steel (SUS), and carbon.

[0075] The cation exchange membrane 14 may be, for example, a cation exchange membrane such as a membrane of a copolymer of tetrafluoroethylene and perfluoro[2-fluorosulfonylethoxypropyl vinyl ether] (for example, Nafion (registered trademark), Flemion (registered trademark)).

[0076] The anion exchange membrane 34 may be, for example, an anion exchange membrane such as a polystyrene membrane having imidazole groups (for example, Neocepta (registered trademark), Selemion (registered trademark), or Sustainion (registered trademark)).

[0077] The power source 32 is not particularly limited, and examples include chemical batteries (including primary batteries, secondary batteries, etc.), constant voltage sources, solar cells, etc. By using a solar cell as the power source 32, an artificial photosynthesis device can be provided that includes a carbon dioxide reduction device (1, 2) and a solar cell that generates power to be supplied to the anode 16 and cathode 22 of the carbon dioxide reduction device (1, 2). In the artificial photosynthesis device of this embodiment, the anode 16 and cathode 22 of the carbon dioxide reduction device (1, 2) are connected via the solar cell, and the artificial photosynthesis device is driven by sunlight as an energy source.

[0078] Preferably, the cell voltage between the anode 16 and the cathode 22 is 2V or less, more preferably 1.5V or less, and even more preferably 1.2V or less.

[0079] The gas diffusion carbon dioxide reduction device of this embodiment allows for the reduction of carbon dioxide (CO2) to formic acid (HCOOH) and the oxidation of water (HO) to produce oxygen (O2) with a low cell potential, a large reaction current density, and high selectivity. Overall, the reaction of converting electrical energy into chemical energy, as represented by the reaction formula (1) above, can proceed. Furthermore, by supplying the aforementioned electrolyte to the anode or the ion conduction layer, as in this embodiment, the precipitation of formic acid produced by the reduction of carbon dioxide as solid formate can be promoted. Since the formate precipitates in the electrolyte or on the anion exchange membrane, the formate can be obtained by filtering the electrolyte or recovering the formate from the carbon dioxide reduction device.

[0080] Furthermore, by using the gas diffusion-type carbon dioxide reduction device according to this embodiment, it becomes possible to supply carbon dioxide as a gas, thereby increasing the current density of the carbon dioxide reduction reaction and reducing the by-production of hydrogen (H2). In the carbon dioxide reduction device according to this embodiment, formate ions generated at the cathode flow into the electrolyte through the anion exchange membrane, and the formate ions react with cations in the electrolyte to form formate salts with low solubility.

[0081] Furthermore, metal complexes such as Ru complexes that have electron-withdrawing substituents introduced as diimine ligands can act as catalysts for the reduction of carbon dioxide to formic acid at low potentials, making it possible to reduce carbon dioxide to produce formic acid (HCOOH) at extremely low potentials.

[0082] Furthermore, by introducing an anode catalyst for hydroxide oxidation into the anode for hydroxide oxidation, it becomes possible to further lower the potential.

[0083] The present specification includes the following embodiments. [1] A carbon dioxide reduction apparatus for obtaining formic acid produced by reducing carbon dioxide as a solid formate, comprising: an anode supplied with an electrolyte and configured to oxidize water in the electrolyte to produce oxygen; a cathode to which a gas containing carbon dioxide is supplied and which reduces the carbon dioxide to produce formic acid; an anion exchange membrane disposed between the anode and the cathode; the electrolyte solution supplied to the anode; The electrolytic solution comprises a solvent and an electrolyte containing cations that form the formate salt and having a saturation concentration (g / 100 mL) in the solvent that is higher than that of the formate salt, wherein the electrolyte is dissolved in the solvent at a concentration of 80% or more of the saturation concentration.

[0084] [2] A carbon dioxide reduction apparatus for obtaining formic acid produced by reducing carbon dioxide as a solid formate, comprising: an anode to which an anolyte solution is supplied and which oxidizes water in the anolyte solution to produce oxygen; a cathode to which a gas containing carbon dioxide is supplied and which reduces the carbon dioxide to produce formic acid; a cation exchange membrane, an ion conduction layer, and an anion exchange membrane disposed between the anode and the cathode in this order from the anode side; an electrolyte solution supplied to the ion conduction layer, The electrolytic solution comprises a solvent and an electrolyte containing cations that form the formate salt and having a saturation concentration (g / 100 mL) in the solvent that is higher than that of the formate salt, wherein the electrolyte is dissolved in the solvent at a concentration of 80% or more of the saturation concentration.

[0085] [3] The carbon dioxide reduction device according to [1] or [2] above, wherein the cations contained in the electrolyte are cations that form the formate salt at a saturated concentration in water at 20°C of less than 20 mg / 100 mL.

[0086] [4] The carbon dioxide reduction device according to [3] above, wherein the cation is a cation of at least one element selected from the group consisting of Ca, Zn, Sr, U, Cu, Mg, and Cd.

[0087] [5] The electrolyte is Cl - , NO3 - , NO2 - , CO3 2- , SO4 2- , Br - , I - , B4O7 2- , ClO3 - , MnO4 - , CH3COO - , BrO3 - The carbon dioxide reduction device according to any one of the above [1] to [4], which contains at least one anion selected from the group consisting of:

[0088] [6] The carbon dioxide reduction device according to any one of the above [1] to [5], characterized by comprising at least one of a formic acid generator that reacts the formate with an acid to generate formic acid, and a dry distillation device that dry distills the formate to generate formaldehyde.

[0089] [7] The carbon dioxide reduction device according to any one of the above [1] to [6], wherein the cathode comprises a catalyst layer containing a metal complex as a cathode catalyst.

[0090] [8] The carbon dioxide reduction device according to [7] above, characterized in that the metal complex has at least one metal selected from the group consisting of Ru, Mn, Fe, Co, Ni, Cu, Mo, and Re as a central metal, and has a diimine ligand as a ligand.

[0091] [9] the cathode has a gas diffusion layer; the central metal of the metal complex is Ru, and the diimine ligand is a diimine ligand having an electron-withdrawing substituent introduced therein; The catalyst layer further includes an ion conductor and a polymer compound serving as a binder, and conductive carbon. The gas diffusion layer includes a hydrophobic porous carbon substrate, and the carbon dioxide reduction device according to the above [7] or [8] is characterized in this.

[0092]

[10] The electron-withdrawing substituent is a carboxylic acid ester having a structural formula of -COOR, R has a chemical structure composed of an alkyl group and a pyrrole moiety, and the pyrrole moiety is polymerized by a polypyrrole chain, and the carbon dioxide reduction device according to the above [9] is characterized in this.

[0093]

[11] The anode includes a substrate composed of at least one material selected from the group consisting of Ni, Ti, Fe, and C, and an anode catalyst. The substrate has at least one shape selected from the group consisting of a porous body, a mesh material, and a fiber sintered body. The anode catalyst includes at least one selected from the group consisting of a metal containing at least one element selected from the group consisting of Ni, Fe, Co, Mn, Ru, and Ir, an oxide containing the metal, a hydroxide containing the metal, and an oxyhydroxide containing the metal, and the carbon dioxide reduction device according to any one of the above [1] to

[10] is characterized in this.

Examples

[0094] Hereinafter, examples and comparative examples will be given to explain the present invention more specifically and in detail, but the present invention is not limited to the following examples.

[0095] [Fabrication of electrodes] (Fabrication of cathode) [Fabrication of Ru complex catalyst-supported gas diffusion type negative electrode] A catalyst layer containing a mixture of a metal complex (catalyst), a polymer compound (ionic conductor and binder), and conductive carbon was laminated on carbon paper as a gas diffusion layer. Specifically, 17.1 mg (0.0244 mmol) of the metal complex [Ru{4,4'-di(1H-pyrrolyl-3-propylcarbonate)-2,2'-bipyridine}Cl2(CO)(CH3CN)] (chemical structure (i) below) was dissolved in 2.55 mL of acetonitrile, and 137 μL of a 0.5 vol% solution of pyrrole in acetonitrile and 686 μL of a 0.2 M solution of FeCl3 in ethanol were added to prepare a solution of the metal complex polymer represented by the structural formula (i) below. To this solution, 27.5 mg of carbon black (Vulcan XC-72R, manufactured by Cabot Corporation) was added as conductive carbon, and 229.5 μL of a 5 mass % Nafion® 117 alcohol-water mixed solution (manufactured by Aldrich Corporation) was added as a polymer compound serving as an ion conductor and binder, followed by ultrasonic dispersion. 2 41 μL of the solution was dropped onto a microporous carbon paper (Avcarb, GDS3250) and dried at 60°C. This process was repeated 30 times to load the solution. After leaving the solution in the dark for 12 hours or more, the solution was washed with water to remove the reaction catalyst FeCl3.

[0096] [ka] (i)

[0097] (Anode preparation) <Preparation of nickel foam electrodes> 10 mL of Ni-doped β-FeOOH colloidal solution was synthesized by mixing 0.1 M iron chloride aqueous solution, 0.05 M nickel nitrate aqueous solution, and ethylenediamine hydrochloride aqueous solution and adjusting the pH to 2.3. This solution was mixed with 10 mL of an aqueous solution containing 0.063 M nickel chloride and 0.055 mM iron chloride, and the resulting solution was 1.13 cm 2A nickel foam electrode (Fe-Ni-added β-FeOOH-supported nickel foam) was prepared by immersing a nickel foam (MTI, EQ-BCNF-16m) in the solution and then heating and drying it at 150°C for 8 hours.

[0098] [Electrolysis of carbon dioxide] <Electrolysis of carbon dioxide using the carbon dioxide electrolysis device shown in Figure 1> In the carbon dioxide electrolysis device 1 shown in FIG. 1 , the anode 16 was an anode electrode (nickel foam electrode) prepared as described above, and the cathode 22 was a cathode electrode (Ru complex catalyst-supported gas diffusion type negative electrode) prepared as described above. The anion exchange membrane was Selemion® in Example 1 and Sustainion® in Comparative Example 1. The anion exchange membrane was disposed between the two electrodes so as to be in contact with the catalyst. This membrane / electrode assembly was sandwiched between an anode current collector 20 and a cathode current collector 30 with gas and electrolyte flow channels, so as to be in contact with the gas and electrolyte flow channels, and then fastened with bolts. The anode current collector 20 was made of titanium, and the cathode current collector 30 was made of stainless steel. Carbon dioxide gas was supplied to the gas flow channel 24 of the cathode section 12 at a flow rate of 30 mL / min. In Example 1 and Comparative Example 1, the following electrolyte was supplied to the electrolyte flow channel 18 of the anode section 10 at a flow rate of 100 mL / min. This carbon dioxide electrolysis device 1 was connected to a potentiostat in a bipolar manner, and carbon dioxide was electrolyzed by applying a constant potential. After electrolysis, the concentration of the produced formate was quantified by ion chromatography (ICS-1100, manufactured by Dionex).

[0099] The electrolyte used in Example 1 was a saturated aqueous solution of CaCl. The electrolyte used in Comparative Example 1 was a 1 M aqueous solution of KOH.

[0100] <Electrolysis of carbon dioxide using the carbon dioxide electrolysis device shown in Figure 2> In the carbon dioxide electrolysis device 2 shown in FIG. 2 , the anode 16 was an anode electrode (nickel foam electrode) prepared as described above, the cation exchange membrane 34 was "Nafion (registered trademark) 115," the anion exchange membrane 34 was "Selemion (registered trademark)" in Example 2, and "Sustainion (registered trademark)" in Comparative Example 2, and an ion conduction layer 36 was formed between the cation exchange membrane 34 and the anion exchange membrane 34. The cathode 22 was a cathode electrode (Ru complex catalyst-supported gas diffusion type negative electrode or Sn-supported gas diffusion type negative electrode) prepared as described above. These were sandwiched between an anode current collector 20 and a cathode current collector 30 with flow channels, and then fastened with bolts and screws. Here, the anode current collector 20 was made of titanium, and the cathode current collector 30 was made of stainless steel. Carbon dioxide gas was supplied to the gas flow channel 24 of the cathode section 12 at a flow rate of 30 mL / min, and a 1 M potassium hydroxide aqueous solution as an alkaline solution was supplied to the anode solution flow channel 19 of the anode section 10 at a flow rate of 100 mL / min. In Example 2 and Comparative Example 2, the following electrolyte solution or pure water was supplied to the ion conduction layer 36 at a flow rate of 2 mL / min. This carbon dioxide electrolysis device 2 was connected to a potentiostat in a bipolar manner, and carbon dioxide was electrolyzed by applying a constant potential. After electrolysis, the concentration of the produced formate was quantified by ion chromatography (ICS-1100, manufactured by Dionex).

[0101] In Example 2, a saturated aqueous solution of CaCl was supplied as the electrolyte at the aforementioned flow rate to ion conduction layer 36. On the other hand, in Comparative Example 2, an aqueous suspension of Dowex (registered trademark), which is porous particles of styrene-divinylbenzene copolymer having sulfonic acid groups, was introduced into ion conduction layer 36, and then pure water was supplied at the aforementioned flow rate.

[0102] [result] In order to confirm the effect of the electrolyte used in the examples and comparative examples, the performance was compared when a predetermined potential was applied for a predetermined time in the carbon dioxide reduction devices of Example 1 and Comparative Example 1 (see Example 1 and Comparative Example 1 in Table 1). In Example 1, which used a saturated aqueous solution of CaCl as the electrolyte, the electrode was charged to 1 cm during carbon dioxide electrolysis at a cell potential of 2 V for 2 hours.2 21 Ccm per -2 After electrolysis, a white solid was deposited on the anion exchange membrane. This solid was dissolved in water and the amount of formate ions was determined by ion chromatography. 2 This is equivalent to 14 μmol cm -2 From this result, the precipitated solid was identified as calcium formate. Note that no formate ions were detected from the electrolyte solution that had passed through the electrolyte solution flow path 18 of the anode part 10. On the other hand, in Comparative Example 1, in which a 1 M KOH aqueous solution was used as the electrolyte solution, no white solid was observed to be precipitated on the anion exchange membrane, but 161 μmol cm of formate ions were detected from the electrolyte solution that had passed through the electrolyte solution flow path 18. -2 Formate ions were detected. This suggests that all of the formic acid produced was dissolved in the electrolyte. However, even when saturated aqueous CaCl2 solution was added dropwise to the electrolyte containing dissolved formic acid, no calcium formate solid was obtained.

[0103] Next, the carbon dioxide reduction devices of Example 2 and Comparative Example 2 were compared in performance when a predetermined potential was applied for a predetermined time (see Example 2 and Comparative Example 2 in Table 1). In Example 2, in which a saturated aqueous solution of CaCl was supplied to the ion conduction layer as the electrolyte, the electrode 1 cm 2 13 Ccm per -2 After electrolysis, a white solid of calcium formate was deposited on the anion exchange membrane. This solid was dissolved in water and the amount of formate ions was determined by ion chromatography. 2 This is equivalent to 35 μmol cm -2 Formate ions of 8.9 μmol cm were also detected from the electrolyte that passed through the ion-conducting layer. -2 On the other hand, in Comparative Example 2, in which a water suspension of "Dowex (registered trademark)" was introduced into the ion conduction layer and then pure water was supplied, no white solid was observed to precipitate on the anion exchange membrane. However, 150 μmol cm of formate ions were detected from the solution that passed through the ion conduction layer. -2Formate ions were detected. This suggests that all of the formic acid produced was dissolved in the solution. However, even when saturated aqueous CaCl2 was added dropwise to the solution containing dissolved formic acid, no calcium formate solid was obtained.

[0104] In Examples 1 and 2, formic acid generated by carbon dioxide reduction at the cathode permeated the anion exchange membrane as formate ions. In Example 1, it flowed into the electrolyte supplied to the anode. In Example 2, it flowed into the electrolyte flowing through the ion-conducting layer. The formate ions then formed salts with calcium ions in the electrolyte and precipitated primarily as calcium formate on the anion exchange membrane. Because the saturated concentration of calcium formate in water at 20°C (16.6 g / 100 mL) was lower than the saturated concentration of calcium chloride in water at 20°C (74.5 g / 100 mL), calcium formate precipitated. By utilizing this difference in solubility, it became possible to extract formate ions, which were previously difficult to extract. In Comparative Examples 1 and 2, the formate ions in the solution were so dilute that calcium formate could not be obtained even when saturated CaCl2 aqueous solution was added to the solution after electrolysis. In the case of Comparative Example 1, formate ions are generated in the KOH aqueous solution, so calcium hydroxide (Ca(OH)2), which has lower solubility than calcium formate, is preferentially precipitated.

[0105] [Table 1] [Explanation of symbols]

[0106] 1,2 Carbon dioxide reduction device, 10 Anode section, 12 Cathode section, 14 Anion exchange membrane, 16 Anode, 18 Electrolyte flow path, 19 Anode solution flow path, 20 Anode current collector, 22 Cathode, 24 Gas flow path, 26 Catalyst layer, 28 Gas diffusion layer, 30 Cathode current collector, 32 Power supply, 34 Cation exchange membrane, 36 Ion conduction layer, 38 Ion exchange membrane.

Claims

1. A carbon dioxide reduction apparatus for obtaining formic acid produced by reducing carbon dioxide as a solid formate, comprising: an anode supplied with an electrolyte and configured to oxidize water in the electrolyte to produce oxygen; a cathode to which a gas containing carbon dioxide is supplied and which reduces the carbon dioxide to produce formic acid; an anion exchange membrane disposed between the anode and the cathode; the electrolyte solution supplied to the anode; the electrolytic solution comprises a solvent and an electrolyte containing cations that form the formate salt and having a saturation concentration (g / 100 mL) in the solvent that is greater than that of the formate salt, wherein the electrolyte is dissolved in the solvent at a concentration of 80% or more of the saturation concentration.

2. A carbon dioxide reduction apparatus for obtaining formic acid produced by reducing carbon dioxide as a solid formate, comprising: an anode to which an anolyte solution is supplied and which oxidizes water in the anolyte solution to produce oxygen; a cathode to which a gas containing carbon dioxide is supplied and which reduces the carbon dioxide to produce formic acid; a cation exchange membrane, an ion conduction layer, and an anion exchange membrane disposed between the anode and the cathode in this order from the anode side; an electrolyte solution supplied to the ion conduction layer, the electrolytic solution comprises a solvent and an electrolyte containing cations that form the formate salt and having a saturation concentration (g / 100 mL) in the solvent that is greater than that of the formate salt, wherein the electrolyte is dissolved in the solvent at a concentration of 80% or more of the saturation concentration.

3. 3. The carbon dioxide reduction device according to claim 1, wherein the cations contained in the electrolyte are cations that form the formate salt at a saturated concentration in water at 20°C of less than 20 mg / 100 mL.

4. 4. The carbon dioxide reduction device according to claim 3, wherein the cation is a cation of at least one element selected from the group consisting of Ca, Zn, Sr, U, Cu, Mg, and Cd.

5. The electrolyte is Cl － , NO ３ － , NO ２ － , CO ３ ２－ , S.O. ４ ２－ ,Br － , I － , B ４ O ７ ２－ , ClO ３ － , MnO ４ － , C.H. ３ COO － , BrO ３ － The carbon dioxide reduction device according to claim 1 or 2, comprising at least one anion selected from the group consisting of:

6. 3. The carbon dioxide reduction device according to claim 1, further comprising at least one of a formic acid generator that generates formic acid by reacting the formate with an acid, and a dry distillation device that dry distills the formate to generate formaldehyde.

7. 3. The carbon dioxide reduction device according to claim 1, wherein the cathode comprises a catalyst layer containing a metal complex as a cathode catalyst.

8. The carbon dioxide reduction device according to claim 7, characterized in that the metal complex has, as a central metal, at least one metal selected from the group consisting of Ru, Mn, Fe, Co, Ni, Cu, Mo, and Re, and has, as a ligand, a diimine ligand.

9. the cathode has a gas diffusion layer; the central metal of the metal complex is Ru, and the diimine ligand is a diimine ligand having an electron-withdrawing substituent introduced therein; the catalyst layer further contains a polymer compound that serves as an ion conductor and a binder, and conductive carbon; 9. The carbon dioxide reduction device according to claim 8, wherein the gas diffusion layer includes a hydrophobic porous carbon substrate.

10. The carbon dioxide reduction device according to claim 9, wherein the electron-withdrawing substituent is a carboxylic acid ester having a structural formula of -COOR, where R has a chemical structure consisting of an alkyl group and a pyrrole moiety, and the pyrrole moiety is polymerized by a polypyrrole chain.

11. the anode comprises a substrate made of at least one material selected from the group consisting of Ni, Ti, Fe, and C, and an anode catalyst; the substrate has at least one shape selected from the group consisting of a porous body, a mesh material, and a sintered fiber body; 3. The carbon dioxide reduction device according to claim 1, wherein the anode catalyst comprises at least one selected from the group consisting of a metal containing at least one element selected from the group consisting of Ni, Fe, Co, Mn, Ru, and Ir, an oxide containing the metal, a hydroxide containing the metal, and an oxyhydroxide containing the metal.

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