Carbon dioxide reduction devices and artificial photosynthetic devices

Abstract

To provide a carbon dioxide reduction apparatus and an artificial photosynthesis apparatus, capable of obtaining formic acid with a low cell potential and high conversion efficiency.SOLUTION: A carbon dioxide reduction apparatus 1 includes: an anode part 10 that includes an anode 16 for oxidizing water to produce oxygen and an anode solution flow channel 18 for supplying an anode solution to the anode 16; a cathode part 12 that includes a cathode 22 for reducing carbon dioxide to produce formic acid and a gas flow channel 24 for supplying carbon dioxide gas to the cathode 22; and a cation exchange membrane 14, an ion exchange resin suspension layer 36, and an anion exchange membrane 34, which are sequentially sandwiched from a side of the anode part 10 between the anode part 10 and the cathode part 12, wherein the cathode 22 includes, sequentially from the anion exchange membrane 34, a catalyst layer 26 containing a metal complex as a cathode catalyst, and a gas diffusion layer 28, and an electrolyte solution is supplied as the anode solution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a carbon dioxide reduction device and an artificial photosynthesis 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, Non-Patent Document 2 describes a three-chamber carbon dioxide reduction device consisting of an anion exchange membrane, an aqueous solution layer, and a cation exchange membrane between a gas diffusion-type negative electrode and a positive electrode (see Figure 3 in Non-Patent Document 2). In this example, tin nanoparticles are used as the carbon dioxide reduction catalyst, and iridium oxide (IrO2) is used as the hydroxide oxidation 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 case of the gas diffusion method described in 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 electrolyte solutions 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] From the viewpoint of recovering formic acid, in the methods of Patent Document 1 and Non-Patent Document 1, formic acid is generated in the electrolyte solution. However, since formic acid has a pKa of 3.75, in a higher pH environment, formate ions (HCOO - However, under acidic conditions, the production of hydrogen (H2) takes precedence over the reduction of carbon dioxide, so a near-neutral electrolyte must be used, and the ionization of formic acid is unavoidable.

[0009] Furthermore, in most conventional technologies, carbon dioxide reduction catalysts have been metals such as tin (Sn) or their oxides. Meanwhile, metal complexes composed of metal ions and organic ligands are known as carbon dioxide reduction catalysts. In the case of metal complexes composed of metal ions and organic ligands, the organic ligands can be freely designed, allowing for significant changes in the catalytic properties. However, the properties of the metals or metal oxides used in most conventional technologies can only be tuned by adjusting the metal-metal bond or metal-oxygen bond. This makes it difficult to reduce the activation energy and reaction potential in the carbon dioxide reduction reaction using these catalysts.

[0010] For the above reasons, the operating potential of the carbon dioxide reduction device of the prior art is still high. In other words, the energy conversion efficiency from electrical energy to chemical energy during operation is low. For example, when carbon dioxide is electrolyzed at a cell potential of 2 V, formate ions (HCOO - ) corresponds to an energy conversion efficiency of 53%. If operation is achieved at a cell voltage of 1.5 V, the energy conversion efficiency reaches 74%. Ideally, operation at 1.24 V or less would be desirable, which corresponds to an energy conversion efficiency of 90% or more. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-057438 [Non-patent literature]

[0012] [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]

[0013] An object of the present invention is to provide a carbon dioxide reduction device and an artificial photosynthesis device that can produce formic acid at a low cell potential and with high conversion efficiency. [Means for solving the problem]

[0014] The present invention provides a fuel cell comprising an anode section including an anode that oxidizes water to produce oxygen and an anode solution flow path that supplies an anode solution to the anode, a cathode section including a cathode that reduces carbon dioxide to produce formic acid and a gas flow path that supplies carbon dioxide gas to the cathode, and a cation exchange membrane, an ion exchange resin suspension layer, and an anion exchange membrane that are sandwiched between the anode section and the cathode section in this order from the anode section side, and the cathode comprises, in this order from the anion exchange membrane side, a catalyst layer containing a metal complex as a cathode catalyst, and a gas diffusion layer, 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, as a ligand, a diimine ligand into which an electron-withdrawing substituent has been introduced; The carbon dioxide reduction device supplies an electrolyte solution as the anode solution.

[0017] 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.

[0018] In the carbon dioxide reduction device, the catalyst layer preferably contains a polymer compound that serves as an ion conductor and a binder, and conductive carbon.

[0019] In the carbon dioxide reduction device, the gas diffusion layer preferably includes a hydrophobic porous carbon substrate.

[0020] 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, 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 includes, as an anode catalyst, 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.

[0021] In the carbon dioxide reduction device, the anode preferably contains, as the anode catalyst, at least one selected from the group consisting of iron oxyhydroxide and nickel oxyhydroxide.

[0022] In the carbon dioxide reduction device, the electrolyte solution preferably contains hydroxide ions, sulfuric acid, carbonic acid, phosphoric acid, or boric acid.

[0023] The present invention is an artificial photosynthesis device comprising the carbon dioxide reduction device and a solar cell that generates electricity to be supplied to the anode and the cathode. [Effects of the Invention]

[0024] The present invention can provide a carbon dioxide reduction device and an artificial photosynthesis device that can produce formic acid at a low cell potential and with high conversion efficiency. [Brief explanation of the drawings]

[0025] [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. 1 is a schematic diagram showing the configuration of a two-chamber gas diffusion reactor used in Reference Examples 1 and 2. [Figure 3] 1 is a graph showing the change over time in carbon dioxide electrolytic reduction current in Example 1 and Reference Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0026] 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.

[0027] FIG. 1 shows a schematic configuration of an example of a carbon dioxide reduction device according to this embodiment.

[0028] 1 includes an anode section 10 having an anode 16 that oxidizes water to produce oxygen and an anode solution flow path 18 that supplies an anode solution to the anode 16, a cathode section 12 having a cathode 22 that reduces carbon dioxide to produce formic acid and a gas flow path 24 that supplies carbon dioxide gas to the cathode 22, and a cation exchange membrane 14, an ion exchange resin suspension layer 36, and an anion exchange membrane 34 sandwiched between the anode section 10 and the cathode section 12 in this order from the anode section 10 side. The cathode 22 includes, in this order from the anion exchange membrane 34 side, a catalyst layer 26 containing a metal complex as a cathode catalyst and a gas diffusion layer 28, and supplies an electrolyte solution as the anode solution. The carbon dioxide reduction device 1 also 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 supplies carbon dioxide gas directly to the catalyst layer 26 of the cathode 22. The carbon dioxide gas is a gas containing carbon dioxide, and preferably a gas containing carbon dioxide and water vapor.

[0030] An ion exchange resin suspension layer 36 is formed between the anode section 10 and the cathode section 12, sandwiched between a cation exchange membrane 14 and an anion exchange membrane 34. The anode section 10 and the cathode section 12 are separated by the cation exchange membrane 14, the ion exchange resin suspension layer 36, and the anion exchange membrane 34. The anode 16 is disposed between the cation exchange membrane 14 and an anode solution flow path 18 so as to be in contact with them. The anode solution flow path 18 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 34 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 carbon dioxide gas to the cathode 22 and is formed by a pit (groove or recess) provided in the cathode current collector plate 30.

[0031] 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 18 through the solution inlet, passes through anode solution flow path 18 while in contact with anode 16, and is discharged from the anode solution outlet.

[0032] 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.

[0033] The ion exchange resin suspension layer 36 contains an ion exchange resin suspension in which ion exchange resin particles are suspended. A suspension liquid inlet and a suspension liquid outlet (neither of which are shown) are connected to the ion exchange resin suspension layer 36. The suspension liquid is introduced into the ion exchange resin suspension layer 36 through the suspension liquid inlet, passes through the ion exchange resin suspension layer 36 while coming into contact with the ion exchange resin, and is discharged from the suspension liquid 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 (HO) occurs at the anode 16 in contact with the anolyte solution. Specifically, the water (HO) contained in the anolyte solution is oxidized to produce oxygen (O) and protons (H + ) is generated.

[0037] On the cathode 22 side, carbon dioxide (CO) contained in the carbon dioxide (CO) gas supplied from the gas flow channel 24 to the catalyst layer 26 through the gas diffusion layer 28 is reduced to form formate anions (HCOO - ) is generated.

[0038] On the cathode 22 side, formate anions (HCOO) produced by the reduction of carbon dioxide - ) moves through the anion exchange membrane 34 to the ion exchange resin suspension layer 36. On the other hand, on the anode 16 side, protons (H + Cations contained in the electrolyte or the anode solution move through the cation exchange membrane 14 to the ion exchange resin suspension layer 36. As a result, these ion movements cause electrical conduction, and formic acid (HCOOH) or formate is produced in the ion exchange resin suspension layer 36. Overall, formic acid (HCOOH) is produced from carbon dioxide (CO2) as shown in the following reaction formula (1). CO2+H2O → HCOOH+1 / 2O2ΔG=274kJ / mol···(1)

[0039] The configurations of the anode section 10, the cathode section 12, the cation exchange membrane 14, the anion exchange membrane 34, and the ion exchange resin suspension layer 36 will be described below.

[0040] As described above, the anode 16 promotes the oxidation reaction of water (H2O) in the anode solution, producing oxygen (O2) and hydrogen ions (H + ) is an electrode (oxidation electrode).

[0041] 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 anode solution and ions to move between the cation exchange membrane 14 and the anode solution flow path 18, and preferably has at least one shape selected from the group consisting of a porous body, a mesh material, and a sintered fiber body.

[0042] The anode 16 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.

[0043] 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.

[0044] The anode solution is an electrolyte solution. Examples of the electrolyte solution include alkaline solutions. Examples of alkaline solutions include aqueous potassium hydroxide solutions, aqueous sodium hydroxide solutions, aqueous potassium carbonate solutions, and aqueous potassium hydrogen carbonate solutions. Since the higher the hydroxide ion concentration, the more favorable the oxidation of water will proceed, aqueous solutions containing hydroxide ions, such as aqueous potassium hydroxide solutions and aqueous sodium hydroxide solutions, having a concentration of 1 mol / L or more are preferred.

[0045] The anode solution is preferably an alkaline aqueous solution with a pH of 12 or higher, for example, in terms of reducing the cell voltage.

[0046] The electrolyte solution may be an electrolyte solution that does not contain salts such as alkali metal ions, for example, an aqueous solution containing sulfuric acid, carbonic acid, phosphoric acid, or boric acid. When the electrolyte solution is an aqueous solution containing sulfuric acid, carbonic acid, phosphoric acid, or boric acid, the cations that pass through the cation exchange membrane 14 are converted into protons (H + ), and in the ion exchange resin suspension layer 36, formate anions (HCOO - ) and protons (H + When the electrolyte solution is, for example, a potassium hydroxide solution, the cations that pass through the cation exchange membrane 14 are potassium ions (K + ), and potassium formate is produced in the ion exchange resin suspension layer 36. Even in this case, potassium formate is not produced in a high-concentration (for example, 1 M) electrolyte solution, so there is no need to remove the electrolyte salt.

[0047] 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).

[0048] The gas diffusion layer 28 constituting the cathode 22 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 thereof 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.

[0049] As described above, the catalyst layer 26 constituting the cathode 22 promotes the reduction reaction of carbon dioxide in carbon dioxide gas to produce formic acid (HCOOH). The catalyst layer 26 contains a 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 contains a porous structure such as carbon paper as a substrate, in order to improve the diffusibility of carbon dioxide gas. The thickness of the catalyst layer 26 is, for example, in the range of 5 to 200 μm.

[0050] 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, and more preferably Mn or Ru, in terms of reducing the overvoltage in the reduction reaction of carbon dioxide.

[0051] 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 into which an electron-withdrawing substituent has been introduced, in view of being able to reduce the overvoltage due to the reduction reaction of carbon dioxide, etc.

[0052] 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.

[0053] 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.

[0054] 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]

[0055] 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.

[0056] 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).

[0057] The catalyst layer 26 may contain phenol or its salt, which can enhance catalytic activity.

[0058] 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.

[0059] 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) or Flemion (registered trademark) manufactured by DuPont).

[0060] The anion exchange membrane 34 may be, for example, an anion exchange membrane such as a polystyrene membrane having imidazole groups (for example, Sustainion (registered trademark) manufactured by Dioxide Materials).

[0061] Examples of the ion exchange resin contained in the ion exchange resin suspension layer 36 include cation exchange resins such as porous particles of styrene-divinylbenzene copolymer having sulfonic acid groups (e.g., Dowex (registered trademark) manufactured by DuPont), and porous particles of acrylic acid-divinylbenzene copolymer having sulfonic acid groups or carboxylic acid groups (e.g., Ambelite (registered trademark) manufactured by DuPont).

[0062] Examples of the suspension liquid for suspending the ion exchange resin in the ion exchange resin suspension layer 36 include water such as pure water, methanol, ethanol, etc., and pure water is preferred from the viewpoint of the swelling property and ionic conductivity of the ion exchange resin.

[0063] 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. Using a solar cell as the power source 32 makes it possible to create an artificial photosynthesis device that includes the carbon dioxide reduction device 1 and the solar cell that generates power to be supplied to the anode 16 and cathode 22 of the carbon dioxide reduction device 1. In the artificial photosynthesis device of this embodiment, the anode 16 and cathode 22 of the carbon dioxide reduction device 1 are connected via the solar cell, and the artificial photosynthesis device is driven by sunlight as an energy source.

[0064] 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.

[0065] By using the carbon dioxide reduction device of this embodiment, a high reaction current density can be generated at a low cell potential, and formic acid can be obtained with high conversion efficiency. The carbon dioxide reduction device of this embodiment enables the reduction of carbon dioxide (CO2) to formic acid (HCOOH) and the oxidation of water (H2O) to produce oxygen (O2) at a low cell potential, a high reaction current density, and high selectivity. Overall, the reaction that converts electrical energy into chemical energy, as represented by reaction formula (1) above, can proceed. By accumulating formic acid in an ion exchange resin suspension, the oxidation reaction of formic acid at the anode is almost eliminated, allowing for high concentrations to be accumulated. Furthermore, by using an electrolyte solution (e.g., sulfuric acid, carbonic acid, phosphoric acid, boric acid) that does not contain salts such as alkali metal ions as the anode solution, formic acid (HCOOH) rather than formate ions can be obtained in the ion exchange resin suspension. These features make it possible to omit the processes of adding acid and concentrating the acid when using it as a chemical raw material or energy carrier.

[0066] 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).

[0067] The oxidation of water (H2O) at the anode proceeds easily in alkaline solutions, but carbon dioxide is converted to HCO3 in an alkaline environment. - and CO3 2- Due to the equilibrium with cations, carbon dioxide cannot exist as carbon dioxide. By separating the two electrodes with a cation exchange membrane, it is possible to create an optimal reaction environment at both electrodes, while maintaining an alkaline environment at the anode and supplying carbon dioxide to the cathode.

[0068] By using an anion exchange membrane on the cathode side, formate anions (HCOO) generated by the reduction of carbon dioxide are - ) moves to the ion exchange resin suspension. On the other hand, by using a cation exchange membrane on the anode side, protons (H + Cations contained in the electrolyte or anode solution migrate to the ion exchange resin suspension. As a result, these ion movements lead to electrical conduction, and formic acid (HCOOH) or formate is obtained in the ion exchange resin suspension swollen with a suspension liquid such as pure water that does not contain electrolytes.

[0069] 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.

[0070] Furthermore, by introducing an anode catalyst for water oxidation into the anode for water oxidation, it becomes possible to further lower the potential.

[0071] As an example of using a metal complex as a cathode catalyst like the carbon dioxide reduction device according to this embodiment, for example, a carbon dioxide reduction device having a two-chamber configuration shown in FIG. 2 can be considered.

[0072] The carbon dioxide reduction device 3 shown in FIG. 2 includes an anode unit 10 having an anode 16 that oxidizes water to produce oxygen and an anode solution flow path 18 that supplies an anode solution to the anode 16; a cathode unit 12 having a cathode 22 that reduces carbon dioxide to produce formic acid and a gas flow path 24 that supplies carbon dioxide gas to the cathode 22; and an ion exchange membrane 38 sandwiched between the anode unit 10 and the cathode unit 12. The cathode 22 includes, in order from the ion exchange membrane 38 side, a catalyst layer 26 containing a metal complex as a cathode catalyst and a gas diffusion layer 28, and supplies an alkaline solution as the anode solution. The carbon dioxide reduction device 1 includes an anode current collector 20 and a cathode current collector 30. The carbon dioxide reduction device 1 also includes a power supply 32 that electrically connects the anode 16 and the cathode 22 and supplies power. The ion exchange membrane 38 is a cation exchange membrane or an anion exchange membrane.

[0073] 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 at the anode 16, which is in contact with the anolyte solution. Specifically, the water (HO) contained in the anolyte solution is oxidized to produce oxygen (O).

[0074] On the cathode 22 side, carbon dioxide (CO2) contained in carbon dioxide (CO2) gas supplied from the gas flow channel 24 through the gas diffusion layer 28 to the catalyst layer 26 is reduced to produce formic acid (HCOOH).

[0075] When a cation exchange membrane is used as the ion exchange membrane 38, the produced formic acid does not ionize and can be obtained in the form of HCOOH. Overall, as shown in the reaction formula (1) above, formic acid (HCOOH) is produced from carbon dioxide (CO2).

[0076] When an anion exchange membrane is used as the ion exchange membrane 38, the generated formic acid (HCOOH) is ionized to form formate ions (HCOO - ) obtained at the cathode 22. -) permeates the anion exchange membrane and is obtained in the anode solution in the anode section 10. Overall, as shown in the reaction formula (2) below, formate ions (HCOO) are converted from carbon dioxide (CO) - ) is generated. CO2+OH - → HCOO - +1 / 2O2ΔG=215.7kJ / mol ···(2)

[0077] In the carbon dioxide reduction device 3 shown in FIG. 2, a metal complex is used as a carbon dioxide reduction catalyst, thereby achieving carbon dioxide reduction at a low potential. However, when an anion exchange membrane is used as the ion exchange membrane 38, the generated formic acid penetrates the anion exchange membrane to form formate ions (HCOO - ), which generates formate in the anode solution (anode electrolyte) in the anode section 10. This can cause oxidation of formate ions at the anode, making it difficult to achieve a high concentration of formic acid. Furthermore, since formate ions are generated, in practice, acid treatment, separation, and concentration processes are required to recover formic acid.

[0078] When a cation exchange membrane is used as the ion exchange membrane 38, the cation exchange membrane prevents formic acid ions from leaking into the anode solution (anode electrolyte), leaving formic acid on the cathode. However, the cathode must be washed with a solution during the recovery process, which dilutes the formic acid, reducing its concentration and causing performance degradation due to catalyst leakage into the washing solution. There are also concerns about corrosion of the metal current collector plate by formic acid. As a result, continuous operation is problematic.

[0079] On the other hand, in the carbon dioxide reduction device according to this embodiment, almost no formic acid accumulates on the cathode, and the generation of formic acid through the reduction of carbon dioxide can proceed stably. Furthermore, since formic acid accumulates in the suspension liquid of the ion exchange resin suspension layer 36, formic acid can be obtained in a solution that does not contain electrolyte salts. In particular, if an aqueous solution that does not contain electrolyte salts such as sulfuric acid, carbonic acid, phosphoric acid, or boric acid is used as the anode solution, formic acid (HCOOH) can be obtained in the ion exchange resin suspension layer 36, rather than formate salts.

[0080] The present specification includes the following embodiments. [1] An anode unit including an anode that oxidizes water to generate oxygen and an anode solution flow path that supplies an anode solution to the anode; a cathode part including a cathode that reduces carbon dioxide to produce formic acid and a gas flow path that supplies carbon dioxide gas to the cathode; a cation exchange membrane, an ion exchange resin suspension layer, and an anion exchange membrane sandwiched between the anode section and the cathode section in this order from the anode section side; Equipped with the cathode includes, in order from the anion exchange membrane side, a catalyst layer containing a metal complex as a cathode catalyst and a gas diffusion layer; A carbon dioxide reduction device, wherein an electrolyte solution is supplied as the anode solution.

[0081] [2] The carbon dioxide reduction device according to [1], 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.

[0082] [3] The carbon dioxide reduction device according to [2], A carbon dioxide reduction device, wherein the diimine ligand is a diimine ligand into which an electron-withdrawing substituent has been introduced.

[0083] [4] The carbon dioxide reduction device according to [3], the electron-withdrawing substituent is a carboxylic acid ester having the structural formula -COOR, R has a chemical structure consisting of an alkyl group and a pyrrole moiety, and the pyrrole moiety is polymerized by a polypyrrole chain.

[0084] [5] The carbon dioxide reduction device according to any one of [1] to [4], The carbon dioxide reduction device, wherein the catalyst layer contains a polymer compound that serves as an ion conductor and a binder, and conductive carbon.

[0085] [6] The carbon dioxide reduction device according to any one of [1] to [5], The carbon dioxide reduction device, wherein the gas diffusion layer comprises a hydrophobic porous carbon substrate.

[0086] [7] The carbon dioxide reduction device according to any one of [1] to [6], the anode comprises a substrate made of at least one material selected from the group consisting of Ni, Ti, Fe, and C; the substrate has at least one shape selected from the group consisting of a porous body, a mesh material, and a sintered fiber body; the anode includes, as an anode catalyst, 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.

[0087] [8] [7] The carbon dioxide reduction device according to The carbon dioxide reduction device, wherein the anode contains, as the anode catalyst, at least one selected from the group consisting of iron oxyhydroxide and nickel oxyhydroxide.

[0088] [9] The carbon dioxide reduction device according to any one of [1] to [8], The carbon dioxide reduction device, wherein the electrolyte solution comprises hydroxide ions, sulfuric acid, carbonic acid, phosphoric acid, or boric acid.

[0089]

[10] The carbon dioxide reduction device according to any one of [1] to [9]; and a solar cell that generates electricity to be supplied to the anode and the cathode. [Example]

[0090] Hereinafter, examples and comparative examples will be given to describe the present invention in more specific detail, but the present invention is not limited to the following examples.

[0091] Using a Ru complex polymer having the chemical structure shown in the following (i) as a cathode catalyst, a carbon dioxide reduction device as shown in FIG. 1 was fabricated and evaluated.

[0092] [Fabrication of Electrodes] (Fabrication of Cathode) [Fabrication of Gas Diffusion-Type Negative Electrode Supporting Ru Complex Catalyst] Using carbon paper as the gas diffusion layer, a catalyst layer containing a mixture of a metal complex as a catalyst, an ion conductor, a polymer compound as a binder, and conductive carbon was laminated thereon. Specifically, 17.1 mg (0.0244 mmol) of [Ru{4,4’-di(1H-pyrrolyl-3-propylcarbonate)-2,2’-bipyridine}Cl2(CO)(CH3CN)] having the chemical structure of the following (i) was dissolved in 2.55 mL of acetonitrile, and 137 μL of a 0.5 vol% pyrrole acetonitrile solution and 686 μL of a 0.2 M FeCl3 ethanol solution were added thereto to prepare a solution of a metal complex polymer represented by the structural formula of the following (i). 27.5 mg of carbon black (Vulcan XC-72R, manufactured by Cabot) as conductive carbon and 229.5 μL of an alcohol-water mixed solution of 5 mass% Nafion (registered trademark) 117 (manufactured by Aldrich) as a polymer compound serving as an ion conductor and a binder were added to this solution, and then ultrasonic dispersion was performed. This suspension was dropped in 41 μL amounts onto a microporous layer-attached carbon paper (GDS3250, manufactured by Avcarb) with a size of 1.13 cm 2 and the operation of drying at 60 °C was repeated 30 times for loading. After standing in the dark for 12 hours or more, it was washed in water to remove FeCl3 which is a reaction catalyst.

[0093] [Chemical Formula] (i)

[0094] <Fabrication of Sn-Supported Gas Diffusion-Type Cathode> By RF magnetron sputtering, an amount equivalent to 10 nm of Sn was sputtered onto carbon paper with a microporous layer (manufactured by Avcarb, GDS3250), and it was cut out to 1.13 cm 2 in size.

[0095] (Fabrication of Anode) <Fabrication of Nickel Foam Electrode> 10 mL of a Ni-doped β-FeOOH colloidal solution synthesized by mixing a 0.1 M aqueous iron chloride solution, a 0.05 M aqueous nickel nitrate solution, and an aqueous ethylenediamine hydrochloride solution and adjusting the pH to 2.3, and 10 mL of an aqueous solution obtained by mixing 0.063 M nickel chloride and 0.055 mM iron chloride were mixed together, and a nickel foam (manufactured by MTI, EQ-BCNF-16m) of 1.13 cm 2 in size was immersed, and then heat-dried at 150 °C for 8 hours to fabricate a nickel foam electrode (β-FeOOH-supported nickel foam with Fe-Ni addition).

[0096] [Electrolysis of Carbon Dioxide] <Electrolysis of Carbon Dioxide Using a Three-Compartment Gas Diffusion Reactor> The electrolytic cell used was a three-chamber gas diffusion reactor with the configuration shown in Figure 1. The anode 16 was an anode electrode (nickel foam electrode) prepared as described above; the cation exchange membrane 14 was "Nafion® 115," a membrane of a copolymer of tetrafluoroethylene and perfluoro[2-fluorosulfonylethoxypropyl vinyl ether]; the ion exchange resin suspension layer 36 was an aqueous suspension of "Dowex® 50," a porous particle of a styrene-divinylbenzene copolymer having sulfonic acid groups; the anion exchange membrane 34 was "Sustainion®," a membrane of polystyrene having imidazole groups; and 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. 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 path 24 of the cathode section 12 at a flow rate of 30 mL / min, a 1 M potassium hydroxide aqueous solution as an alkaline solution was supplied to the anode solution flow path 18 of the anode section 10 at a flow rate of 100 mL / min, and pure water as a suspension solution was supplied to the ion exchange resin suspension layer 36 at a flow rate of 2 mL / min. This electrolytic cell was connected to a potentiostat in a bipolar manner, and carbon dioxide was electrolyzed by applying a constant potential. After electrolysis, the formic acid concentration of the aqueous solution flowing through the ion exchange resin suspension layer was quantified by ion chromatography (Dionex, ICS-1100).

[0097] <Carbon dioxide electrolysis using a two-chamber gas diffusion reactor equipped with a membrane / electrode assembly> A two-chamber gas diffusion reactor with the configuration shown in Figure 2 was used as the electrolytic cell. 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 negative electrode) prepared as described above. A cation exchange membrane (Nafion® 324) or an anion exchange membrane (Sustainion®) was placed between the two electrodes as the ion exchange membrane 38 so as to contact the catalyst. This membrane / electrode assembly was sandwiched between an anode current collector 20 and a cathode current collector 30 with flow channels so as to contact the gas and alkaline solution 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 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 channel 18 of the anode section 10 at a flow rate of 100 mL / min. This electrolytic cell was connected to a potentiostat in a bipolar manner, and a constant potential was applied to electrolyze carbon dioxide. After electrolysis, the cathode 22 was washed with water, and the concentrations of formic acid in the washings and the anodic electrolyte were quantified by ion chromatography (ICS-1100, manufactured by Dionex).

[0098] [Measurement electrode] In the examples, comparative examples and reference examples, electrodes carrying the following catalysts were placed in the electrolytic cell and measurements were carried out.

[0099] <Example 1: Carbon dioxide electrolysis using a Ru complex polymer cathode in a three-chamber gas diffusion reactor> Using the Ru complex catalyst-supported gas diffusion type negative electrode as the cathode 22 and the nickel foam electrode as the anode 16, electrolytic reduction of carbon dioxide was carried out in the three-compartment gas diffusion reactor shown in FIG.

[0100] <Comparative Example 1: Carbon dioxide electrolysis using a Sn cathode in a three-chamber gas diffusion reactor> Using the Sn-supported gas diffusion type negative electrode as the cathode 22 and the nickel foam electrode as the anode 16, electrolytic reduction of carbon dioxide was carried out in the three-chamber gas diffusion reactor shown in FIG.

[0101] <Reference Example 1: Carbon dioxide electrolysis using a Ru complex polymer cathode in a two-chamber gas diffusion reactor containing a cation exchange membrane> Electrolytic reduction of carbon dioxide was carried out in the two-chamber gas diffusion reactor shown in FIG. 2 using a cation exchange membrane (Nafion (registered trademark) 324) as the ion exchange membrane 38, the Ru complex catalyst-supported gas diffusion negative electrode as the cathode 22, and the nickel foam electrode as the anode 16.

[0102] <Reference Example 2: Carbon dioxide electrolysis using a Ru complex polymer cathode in a two-chamber gas diffusion reactor containing an anion exchange membrane> An anion exchange membrane (Sustainion (registered trademark)) was used as the ion exchange membrane 38, the above-mentioned Ru complex catalyst-supported gas diffusion type negative electrode was used as the cathode 22, and the above-mentioned nickel foam electrode was used as the anode 16, and electrolytic reduction of carbon dioxide was carried out in the above-mentioned two-chamber gas diffusion reactor shown in Figure 2.

[0103] [result] (Action and effect of catalyst) To compare the catalytic effects of metal complexes having diimine ligands with electron-withdrawing substituents, the performances of Example 1 and Comparative Example 1 were compared when a potential of 1.6 V was applied for 3 hours in the three-chamber gas diffusion reactor (see Example 1 and Comparative Example 1-1 in Table 1). Example 1, which used a Ru complex polymer as the cathode catalyst, had a 3-hour average current of 4.14 mA cm. -2A current of 1.6 V was generated, and formic acid was detected from the ion exchange resin suspension layer with a faradaic efficiency equivalent to 86% of that current. On the other hand, when Sn was used as the catalyst, no formic acid was detected at 1.6 V. When the potential was set to 2.3 V, formic acid was detected with a faradaic efficiency of 3% (see Comparative Example 1-2 in Table 1). These results demonstrate that operation at a low potential is possible by using, as a cathode catalyst, a metal complex having a ligand in which an electron-withdrawing substituent has been introduced into the diimine ligand.

[0104] (Action and effect of three-chamber gas diffusion reactor) To investigate the function and effect of the three-compartment gas diffusion reactor, Example 1, which used the three-compartment gas diffusion reactor, Reference Example 1, which used a cation exchange membrane in the two-compartment gas diffusion reactor, and Reference Example 2, which used an anion exchange membrane, were compared (see Example 1, Reference Examples 1, and 2 in Table 1). In Reference Example 1, formic acid was detected only at the cathode, with a faradaic efficiency of 73% and 77 mM in 2 mL of cleaning solution. The change in current density over time is shown in Figure 3 along with that of Example 1. In Reference Example 1, it was confirmed that the current density decreased over time due to the accumulation of formic acid at the cathode. On the other hand, in Example 1, the current density increased over time. This suggests that in Example 1, almost no formic acid accumulated on the cathode, and carbon dioxide reduction and formic acid production proceeded stably.

[0105] Next, in Reference Example 2, a current density equivalent to that of Example 1 was observed at a potential of 1.3 V, and formic acid was detected only in the anode electrolyte, with a faradaic efficiency of 90% and 2.6 mM produced in 100 mL of the anode electrolyte. When an anion exchange membrane was used, formic acid (HCOOH) produced at the cathode was converted to formate ions (HCOO - ) passes through the anion exchange membrane, and becomes formate ions (HCOO -It was suggested that formic acid is dissolved in the anode electrolyte as a salt. When an anion exchange membrane is used, a high current density can be achieved at a low potential. However, because formic acid is generated in the anode electrolyte, if the anode electrolyte is used in small amounts or the reaction is performed for a long time, the formic acid concentration can be increased, which can lead to an oxidation reaction of formic acid on the anode. Furthermore, formic acid is generated in a solution (anode electrolyte) containing a high concentration of electrolyte salt. In Example 1, formic acid accumulates in the aqueous solution of the ion exchange resin suspension layer, so formic acid can be obtained in a solution that does not contain electrolyte salt. In particular, if an aqueous solution that does not contain electrolyte salts such as sulfuric acid, carbonic acid, phosphoric acid, or boric acid is used as the anode solution, formic acid (HCOOH) rather than formate salts can be obtained in the ion exchange resin suspension layer.

[0106] [Table 1]

[0107] Thus, the carbon dioxide reduction device of the example was able to produce formic acid at a low cell potential and with high conversion efficiency. [Explanation of symbols]

[0108] 1,3 Carbon dioxide reduction device, 10 Anode section, 12 Cathode section, 14 Cation exchange membrane, 16 Anode, 18 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 Anion exchange membrane, 36 Ion exchange resin suspension layer, 38 Ion exchange membrane.

Claims

1. an anode section including an anode that oxidizes water to produce oxygen and an anode solution flow path that supplies an anode solution to the anode; a cathode part including a cathode that reduces carbon dioxide to produce formic acid and a gas flow path that supplies carbon dioxide gas to the cathode; a cation exchange membrane, an ion exchange resin suspension layer, and an anion exchange membrane sandwiched between the anode section and the cathode section in this order from the anode section side; Equipped with the cathode includes, in order from the anion exchange membrane side, a catalyst layer containing a metal complex as a cathode catalyst and a gas diffusion layer; 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, as a ligand, a diimine ligand into which an electron-withdrawing substituent has been introduced; A carbon dioxide reduction device, characterized in that an electrolyte solution is supplied as the anode solution.

2. The carbon dioxide reduction device according to claim 1, 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.

3. The carbon dioxide reduction device according to claim 1, The carbon dioxide reduction device is characterized in that the catalyst layer contains a polymer compound that serves as an ion conductor and a binder, and conductive carbon.

4. The carbon dioxide reduction device according to claim 1, The carbon dioxide reduction device, wherein the gas diffusion layer includes a hydrophobic porous carbon substrate.

5. The carbon dioxide reduction device according to claim 1, the anode comprises a substrate made of at least one material selected from the group consisting of Ni, Ti, Fe, and C; the substrate has at least one shape selected from the group consisting of a porous body, a mesh material, and a sintered fiber body; anode containing, as an anode catalyst, 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.

6. The carbon dioxide reduction device according to claim 5, The carbon dioxide reduction device, wherein the anode contains, as the anode catalyst, at least one selected from the group consisting of iron oxyhydroxide and nickel oxyhydroxide.

7. The carbon dioxide reduction device according to claim 1, The carbon dioxide reduction device, wherein the electrolyte solution contains hydroxide ions, sulfuric acid, carbonic acid, phosphoric acid, or boric acid.

8. The carbon dioxide reduction device according to claim 1; and a solar cell that generates electricity to be supplied to the anode and the cathode.

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