Electrolytic device and method for producing valuable material

Abstract

To provide a method and a device for producing formic acid.　Provided are a method and a device for producing formic acid by using a conductive diamond electrode and electrolytically reducing carbon dioxide in a single-chamber electrolytic cell.

Description

Electrolysis device and method for producing valuable materials

[0001] The present disclosure relates to an electrolysis device for obtaining valuable resources by electrolytic reduction of carbon dioxide, and a method for producing valuable resources.

[0002] In recent years, the production of valuable materials using carbon dioxide as a raw material has been attracting attention from the perspectives of preventing global warming, conserving petroleum resources, and realizing a sustainable society.

[0003] Patent Document 1 describes an apparatus for electrolytic reduction of carbon dioxide. In the method described in Patent Document 1, carbon dioxide is saturated in a highly concentrated aqueous potassium carbonate solution of about 7M, and electrolytic reduction is carried out. The products are described as acetic acid and formic acid.

[0004] Patent Document 2 describes an electrochemical reduction device using a diamond electrode. In the method of Patent Document 2, carbon dioxide is saturated in a methanol solution under high pressure and electrolytic reduction is carried out.

[0005] Patent Document 3 describes a method and an apparatus for producing formic acid from carbon dioxide with high selectivity using a diamond electrode. Patent Document 3 also discloses a method and an apparatus for using potassium chloride, rubidium chloride, or cesium chloride at a relatively high current density.

[0006] Patent Document 4 describes a method and an apparatus for producing formic acid with high selectivity from exhaust gas containing carbon dioxide using a diamond electrode. Patent Document 4 discloses a flow reactor type electrolysis device equipped with a two-compartment electrolysis cell.

[0007] Non-Patent Document 1 describes a method for obtaining valuable materials from carbon dioxide.

[0008] Non-Patent Document 2 describes a method and an apparatus for producing formic acid from carbon dioxide with high selectivity using a diamond electrode.

[0009] JP 2011-174139 A (Patent No. 5368340 A) JP 2014-167151 A (Patent No. 6042749 A) ​​JP 2018-141220 A (Patent No. 6840359 A) JP 2018-184655 A (Patent No. 6879549 A)

[0010] Hori, Y. (2008). Electrochemical CO2 Reduction on Metal Electrodes. In: Vayenas, CG, White, RE, Gamboa-Aldeco, ME (eds) Modern Aspects of Electrochemistry. Modern Aspects of Electrochemistry, vol 42. Springer, New York, NY. https: / / doi.org / 10.1007 / 978-0-387-49489-0_3S. Kaneko, R. Iwao, K. IIba, K. Ohta and T. Mizuno, Energy, 23 (1998) No. 12, pp. 1107-1112

[0011] Conventional flow reactor-type electrolysis devices for obtaining valuable products through the electrolytic reduction of carbon dioxide all include a two-compartment electrolytic cell separated by a diaphragm. A two-compartment electrolytic cell has the advantage that the anode and cathode chambers are separated by a diaphragm, allowing the anode and cathode reactions to be controlled independently and that the products obtained by the anode and cathode reactions to be recovered independently. A Nafion membrane is typically used as the diaphragm.

[0012] On the other hand, electrolysis devices equipped with a two-compartment electrolytic cell have the following problems: the electrolytic cell becomes large because it has two compartments, an anode compartment and a cathode compartment; the electrolyte needs to be supplied separately to the anode compartment and the cathode compartment, requiring two liquid delivery systems, such as a liquid delivery pump and liquid delivery piping, which makes the electrolysis device larger and increases power consumption; if the pump is operated under conventional conditions in a large device, the diaphragm separating the two reaction compartments will warp; in order to prevent the diaphragm separating the two reaction compartments from warping, the electrolyte needs to be delivered to the two compartments at equal pressure, which requires precise control of the pump operating conditions; and the diaphragm becomes clogged or damaged and needs to be replaced, which increases the cost of the device.

[0013] Therefore, the present inventors attempted to develop a single-chamber flow reactor that does not use a diaphragm in order to obtain valuable substances by electrolytic reduction of carbon dioxide in a flow reactor.

[0014] Therefore, an object of the present disclosure is to provide a single-chamber flow reactor that does not use a diaphragm. Furthermore, in certain embodiments, the present disclosure provides an electrolysis device that uses a single-chamber electrolytic cell to produce valuable materials with high efficiency in the production of valuable materials using carbon dioxide as a feedstock. Furthermore, in certain embodiments, the present disclosure provides a highly efficient method for producing valuable materials using a single-chamber electrolytic cell.

[0015] The present inventors first started with a two-compartment electrolytic cell separated by a diaphragm, and then eliminated the diaphragm to create a single-compartment electrolytic cell. However, when carbon dioxide was electrolytically reduced in a single-compartment electrolytic cell without a diaphragm, the efficiency of formic acid production was significantly reduced. Therefore, the present inventors discovered that valuable materials could be efficiently produced using a single-compartment electrolytic cell by using a specific anode material, and completed the present invention, which includes this as one embodiment.

[0016] That is, the present disclosure includes the following embodiments: [1] An electrolysis device for obtaining valuable products by electrolytic reduction of carbon dioxide dissolved in an electrolytic solution, the electrolysis device comprising an electrolytic cell equipped with an anode and a cathode, the electrolytic cell being a single-chamber electrolytic cell without a diaphragm, and comprising a mechanism for circulating a reaction solution in a reaction chamber, the anode having a fluororesin coating on the surface of the anode material. [2] The electrolysis device according to embodiment 1, wherein the fluororesin is a fluorine-containing ion exchange resin. [3] The electrolysis device according to embodiment 1 or 2, wherein the anode contains iridium, titanium, tantalum, niobium, molybdenum, tungsten, platinum, nickel, iron, an alloy or oxide thereof, glassy carbon, graphite, conductive diamond, or conductive silicon. [4] The electrolysis device according to any of embodiments 1 to 3, wherein the anode contains iridium oxide or tantalum oxide as an anode material. [5] The electrolysis device according to embodiment 4, wherein the anode is an iridium oxide electrode having a fluororesin coating on its surface, or a tantalum oxide electrode having a fluororesin coating on its surface. [6] The electrolysis device according to any one of embodiments 1 to 5, wherein the valuable material is formic acid. [7] The electrolysis device according to any one of embodiments 1 to 6, wherein the cathode is a conductive diamond electrode. [8] The electrolysis device according to embodiment 6 or 7, wherein the electrolyte is an aqueous solution containing a supporting electrolyte that does not contain halide ions. [9] The electrolysis device according to embodiment 8, wherein the supporting electrolyte that does not contain halide ions is potassium sulfate.

[10] The electrolysis device according to embodiment 7, embodiment 8 dependent from embodiment 7, or embodiment 9, wherein the conductive diamond electrode is activated.

[11] A method for producing valuables by electrolytically reducing carbon dioxide dissolved in an electrolytic solution using an electrolytic cell equipped with an anode and a cathode, wherein the electrolytic cell is a single-chamber electrolytic cell without a diaphragm, and is equipped with a mechanism for circulating a reaction solution in a reaction chamber, and the anode has a fluororesin coating on a surface of the anode material, and the method for producing valuables comprises a step of electrolytically reducing carbon dioxide in the single-chamber electrolytic cell.

[12] The method for producing valuables according to embodiment 11, wherein the fluororesin is a fluorine-containing ion exchange resin.

[13] The method for producing a valuable resource according to embodiment 11 or 12, wherein the anode contains iridium, titanium, tantalum, niobium, molybdenum, tungsten, platinum, nickel, iron, an alloy or oxide thereof, glassy carbon, graphite, conductive diamond, or conductive silicon.

[14] The method for producing a valuable resource according to any one of embodiments 11 to 13, wherein the anode contains iridium oxide or tantalum oxide as an anode material.

[15] The method for producing a valuable resource according to embodiment 14, wherein the anode is an iridium oxide electrode having a fluororesin coating on its surface, or a tantalum oxide electrode having a fluororesin coating on its surface.

[16] The method for producing a valuable resource according to any one of embodiments 11 to 15, wherein the valuable resource is formic acid.

[17] The method for producing a valuable resource according to any one of embodiments 11 to 16, wherein the cathode is a conductive diamond electrode.

[18] The method for producing a valuable resource according to any one of embodiments 11 to 17, wherein the electrolytic solution is an aqueous solution containing a supporting electrolyte that does not contain halide ions.

[19] The method for producing valuable materials according to embodiment 18, wherein the halide ion-free supporting electrolyte is potassium sulfate.

[20] The method according to any one of embodiments 17 to 19, comprising a step of activating a conductive diamond electrode before the step of electrolytically reducing carbon dioxide in a single-chamber electrolytic cell. This specification incorporates the disclosure of Japanese Patent Application No. 2023-071051, from which the present application claims priority.

[0017] According to the present disclosure, valuable resources can be obtained without using a diaphragm in an electrolysis device.

[0018] This figure shows a two-chamber flow reactor (comparative example). This figure shows a single-chamber flow reactor (present disclosure). This figure shows the results of surface analysis of an anode with a fluorine coating formed on its surface and an anode without a fluorine coating formed on its surface. (1) With a top coat, and (2) Without a top coat. In B, a peak derived from iridium was clearly observed at 60-65 eV for the iridium oxide electrode (2) without a top coat, but this peak was unclear for the iridium oxide electrode (1) with a top coat. In A, a peak derived from fluorine atoms was clearly observed at 693 eV for the iridium oxide electrode (1) with a top coat, but this peak was not observed for the iridium oxide electrode (2) without a top coat. This figure shows the results of a chlorine generation test. From top to bottom, (1) IrO2, (2) IrO2 with a top coat, and (3) IrO2 with thermocompression bonding. This figure shows the results of a formic acid decomposition test. From top to bottom, (1) IrO2, (2) IrO2 with top coat, (3) IrO2 with thermocompression bonding. Figure 1 shows the results of electrolytic reduction of carbon dioxide. The left is Comparative Example 1, and the right is Comparative Example 2. Figure 2 shows the results of electrolytic reduction of carbon dioxide. The left is Comparative Example 1, and the right is Comparative Example 3. Figure 3 shows the results of electrolytic reduction of carbon dioxide. The left is Comparative Example 4, and the right is Example 6. Figure 4 shows the results of electrolytic reduction of carbon dioxide. The left is Comparative Example 2, and the right is Example 7.

[0019] 1. Electrolysis Device In certain embodiments, the present disclosure provides an electrolysis device that produces valuable materials by reducing carbon dioxide at a cathode. The electrolysis device of the present disclosure includes an electrolysis cell 12 equipped with an anode 2 and a cathode 1, and the electrolysis cell is a single-chamber electrolysis cell without a diaphragm. An electrolysis cell may also be referred to as a reaction chamber. A single-chamber electrolysis cell refers to an electrolysis cell without a diaphragm between the anode and the cathode. The electrolysis device may include a reference electrode 3, an external power supply mechanism 11, a solution delivery mechanism 8, and a storage tank 13. A carbon dioxide supply unit may be connected to the storage tank 13. The solution delivery mechanism 8 delivers the electrode solution from the storage tank 13 to the electrolysis cell 12, and electrolytic reduction is carried out in the electrolysis cell 12 to produce valuable materials. See, for example, Figures 1-2. The electrolysis cell of the present disclosure excludes electrolysis cells that include a diaphragm within the electrolysis cell, which is separated into two or more reaction chambers by the diaphragm.

[0020] It is preferable to use a conductive material as the anode material because it makes it easy to apply the potential required for the electrode reaction.When a conductive material is used as the anode material, it is preferable because the back side of the anode material can be connected to a power supply device to apply the potential required for the electrode reaction to the surface.

[0021] Examples of the conductive material include metal materials, carbon materials, conductive silicon materials, etc. Metal materials include iridium, titanium, tantalum, niobium, molybdenum, tungsten, platinum, nickel, iron, and alloys and oxides thereof. Preferred carbon materials are glassy carbon, graphite, and conductive diamond. Preferred conductive silicon materials are boron- or phosphorus-doped P-type or N-type semiconductor silicon.

[0022] In some embodiments, the anode material has a substrate. The substrate can be, for example, but not limited to, titanium, tantalum, niobium, and alloys thereof. In some embodiments, the substrate can have a corrosion-resistant protective coating on its surface. In some embodiments, the corrosion-resistant protective coating can be tantalum oxide. In some embodiments, an anode material layer can be present on the corrosion-resistant protective coating. The anode material layer can include iridium oxide or tantalum oxide.

[0023] For example, a protective film material, such as tantalum oxide, is applied to the surface of the substrate in an appropriate amount, for example, 5 to 10 g / m ２ , for example 7 g / m ２ A corrosion-resistant protective film can be formed by coating the anode material on the anode material, followed by drying and firing. For example, when iridium and tantalum are used as the anode material, a solution of chloroiridic acid and tantalum pentachloride dissolved in a suitable organic solvent can be applied, dried at a high temperature, for example, 80 to 120°C, for example, 100°C, and then fired in air at 500 to 600°C, for example, 550°C, for a predetermined time. This process can be repeated 1 to 5 times, for example, 4 times, to form an anode material layer.

[0024] Alternatively, electrodes can be prepared by plating. For example, metallic iridium can be electrodeposited on the surface of a substrate, and then the electrodeposited metallic iridium can be oxidized to convert it to iridium oxide. For example, the surface of titanium, which serves as the substrate, can be activated, and then an iridium plating film can be formed using an iridium plating solution. The iridium plating film can have a thickness of 0.5 to 2 μm, for example 1 μm. This can then be heat-treated at 600 to 700°C, for example 650°C, for a predetermined time, for example 1 to 48 hours, for example 24 hours. The heat treatment can convert iridium to iridium oxide.

[0025] Alternatively, the titanium substrate may be activated with an aqueous solution of oxalic acid, immersed in an electrolyte containing oxalic acid and iridium (IV) chloride, and then subjected to electrodeposition by oxidation, followed by heat treatment at 300 to 500°C, e.g., 400°C.

[0026] The anode material may have a fluororesin coating on its surface. For example, the fluororesin coating may be present on the surface of the anode material layer. The presence of a fluororesin coating on the anode material surface can suppress oxidative decomposition at the anode of valuable materials generated by reducing carbon dioxide at the cathode. Here, suppression of oxidative decomposition at the anode refers to the reduction of oxidative decomposition of valuable materials at the anode when the fluororesin coating is present compared to when the fluororesin coating is absent. Furthermore, when the electrolyte contains halide ions such as chloride ions, bromide ions, and iodide ions, the halide ions may be oxidized to generate toxic halogen gas. The presence of a fluororesin coating on the anode material surface can suppress the oxidation of halide ions and the generation of halogen gas. Here, suppression of oxidation of halide ions at the anode refers to the reduction of oxidation of halide ions at the anode when the fluororesin coating is present compared to when the fluororesin coating is absent. Although not wishing to be bound by a particular theory, the present inventors believe that the presence of a fluororesin coating on the surface of the anode material makes the surface of the anode material hydrophobic, thereby preventing the electrolytic reduction product of carbon dioxide and halide ions, which are hydrophilic, from approaching the surface of the anode material.

[0027] The fluororesin preferably contains at least one fluororesin selected from polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), polyvinylidene fluoride (PVDF), and fluorine-containing ion exchange resin. Polytetrafluoroethylene (PTFE) and tetrafluoroethylene-hexafluoropropylene copolymer (FEP) are preferred because they can impart high hydrophobicity, while tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) and polyvinylidene fluoride (PVDF) are preferred because the resulting coating has excellent adhesion. Fluorine-containing ion exchange resins are preferably used because they have ion permeability and therefore the increase in resistance on the electrode surface due to coating formation is small. Known or commercially available fluorine-containing ion exchange resins can be used. Examples of fluorine-containing ion exchange resins include perfluorosulfonic acid membranes and perfluorocarboxylic acid membranes, and commercially available products include, but are not limited to, Nafion (registered trademark), ForBlue (trademark), Selemion (registered trademark), Flemion (registered trademark), Aquivion (registered trademark), and Aciplex (registered trademark).

[0028] The fluororesin can be formed on the surface of the anode material by known methods. Examples include, but are not limited to, a method in which a fluororesin contained in a solution or dispersion is applied to or brought into contact with the electrode surface and heated to coat it, a method in which the fluororesin is vapor-deposited by sputtering in a vacuum or under low pressure, and a plasma polymerization method in which a fluororesin gas is blown into a glow discharge. These steps can be performed using known procedures and commercially available devices.

[0029] For example, when forming a PTFE coating, PTFE is applied to the surface of the anode material layer, dried, and then heated at a high temperature, for example, 200 to 400°C, e.g., 300°C, for a predetermined time to form a molten PTFE film. The coating may have a thickness of, for example, 1 to 5 μm. The thickness of the coating can be appropriately set by adjusting the amount of fluororesin applied. Appropriate conditions can also be set for other fluororesins.

[0030] For example, when forming a fluorine-containing ion exchange resin film, a fluorine-containing ion exchange resin film is laminated to an anode material layer, and then the film is subjected to pressure bonding at a high temperature, for example, 120 to 170°C, e.g., 150°C, under a predetermined pressure and for a predetermined time, to form a thermocompression-bonded film of the fluorine-containing ion exchange resin. The pressure during thermocompression bonding is, for example, 10 to 1200 kgcm. －２ , 20~1100kgcm －２ , 30~1000kgcm －２ , 40~900kgcm －２ ,50~800kgcm －２ ,60~600kgcm －２ ,70~700kgcm －２ ,80~800kgcm －２ ,90~900kgcm －２ , for example, 100 kg cm －２ The time may be, for example, 1 to 30 minutes, e.g., 5 minutes. The thickness of the fluorine-containing ion exchange membrane coating thermocompression-bonded to the anode material layer may be, for example, approximately the same as or slightly thinner than the thickness of commercially available fluorine-containing ion exchange membranes. The thickness of the fluorine-containing ion exchange membrane coating thermocompression-bonded to the anode material layer may be, for example, 10 to 500 μm, 20 to 400 μm, 30 to 300 μm, 40 to 200 μm, e.g., 50 to 100 μm.

[0031] In some embodiments, the fluororesin coating may contain a metal compound. In some embodiments, the metal compound may have catalytic activity. Examples of metal compounds that may be contained in the fluororesin coating include, but are not limited to, oxides containing one or more selected from tantalum, tungsten, strontium, hafnium, and manganese, or composite oxides thereof. In some embodiments, the metal compound may be a composite oxide of tantalum, tungsten, and strontium. In some embodiments, the fluororesin coating may contain 1 to 10 wt %, for example, 1.5 to 4 wt %, of the metal compound.

[0032] As the iridium oxide having a fluororesin coating on its surface, known or commercially available materials may be used. In certain embodiments, the electrode described in JP 2019-108580 (Patent No. 6919125) may be used as the anode. The contents of JP 2019-108580 (Patent No. 6919125) are incorporated herein by reference. In certain embodiments, Anodek 100CA (manufactured by Nisshin Seiki Co., Ltd.) may be used as a commercially available anode material. The inventors performed X-ray photoelectron spectroscopy on Anodek 100CA and confirmed that the peaks derived from iridium were unclear and that the peaks derived from fluorine atoms were clearly observed, confirming that this iridium oxide electrode has a fluororesin coating on its surface.

[0033] The electrolysis device of this embodiment can produce valuable materials by electrolytically reducing carbon dioxide at the cathode. The valuable materials that can be obtained are not particularly limited, but examples include compounds that are obtained in two-chamber electrolysis devices, such as formic acid, carbon monoxide, methane, methanol, ethane, ethylene, ethanol, and acetone. Of these, the valuable material is preferably formic acid. It is known that formic acid can be efficiently produced by electrolytic reduction of carbon dioxide. Furthermore, formic acid is an important industrial material and has recently been attracting attention as an energy resource for applications such as hydrogen carriers and fuel cells.

[0034] The cathode is not limited as long as it produces valuable materials. Because of its high formic acid production efficiency, tin, lead, zinc, and conductive diamond are preferred, with conductive diamond being even more preferred. Conductive diamond is a thin film, thin film, or bulk diamond that has been given conductivity and is used as an electrode material. The method of imparting conductivity to diamond is not particularly limited, but diamond can be doped with a trace amount of impurities. Examples of impurities include boron (B), sulfur (S), nitrogen (N), oxygen (O), and silicon (Si). For example, thin film or bulk diamond can be obtained by vapor phase synthesis. To a raw material gas containing a carbon source, diborane, trimethoxyborane, or boron oxide can be added to dope with boron; sulfur oxide or hydrogen sulfide can be added to dope with sulfur; oxygen or carbon dioxide can be added to dope with oxygen; ammonia or nitrogen can be added to dope with nitrogen; and silane can be added to dope with silicon. In particular, boron-doped conductive diamond electrodes are advantageous because they have the advantages of a wide potential window and a small background current compared to other electrode materials. Hereinafter, the conductive diamond electrode may be simply referred to as a diamond electrode, and the boron-doped diamond electrode may be referred to as a boron-doped diamond electrode or a BDD electrode.

[0035] In one embodiment, the conductive diamond electrode has a diamond layer formed by vapor-depositing diamond containing 0.01 to 8% w / w boron raw material on the substrate surface. The substrate can be a silicon substrate, a glass substrate such as SiO2, a quartz substrate, a ceramic substrate such as Al2O3, silicon carbide, or silicon nitride, or a metal such as tungsten, molybdenum, niobium, or titanium. All or part of the substrate surface can be made into a diamond layer. In another embodiment, the electrode portion of the BDD electrode can have bulk diamond.

[0036] In one embodiment, the size of the electrode parts of the anode and cathode is not particularly limited, but is preferably 1 cm 2 More than 5cm 2 More than 10cm 2 More than 50cm 2 More than 100cm 2More than 200cm 2 More than 400cm 2 The area and shape of the whole or part of the diamond layer and the two-electrode portion can be determined appropriately depending on the configuration of the device.

[0037] The electrolysis device of this embodiment has an electrolytic cell equipped with an anode and a cathode, and can obtain valuable materials by electrolytic reduction of carbon dioxide dissolved in an electrolytic solution. The electrolytic solution (also referred to as an electrolyte solution) is not particularly limited as long as it is an electrically conductive liquid. For example, it can be obtained by dissolving a supporting electrolyte in a solvent. The solvent is preferably water because it is easy to obtain and has high solubility for the supporting electrolyte. The supporting electrolyte is preferably an electrolyte containing a halide such as potassium chloride, rubidium chloride, or cesium chloride, because it provides high formic acid production efficiency. Furthermore, a supporting electrolyte that does not contain halide ions, such as potassium sulfate, potassium carbonate, potassium hydroxide, potassium bicarbonate, tetrabutylammonium tetrafluoroborate (TBABF4), or potassium perchlorate, is preferred because it does not generate halogen gas at the anode. Among these, potassium sulfate is preferred as the electrolyte because it does not generate halogen gas and provides high formic acid production efficiency.

[0038] In certain embodiments, the electrolytic cell 12 may have a reference electrode 3, which allows for accurate control of the potential. Examples of reference electrodes include well-known electrodes such as a standard hydrogen electrode, a silver-silver chloride electrode, and a saturated calomel electrode. The electrode potential varies depending on the type of reference electrode used in the electrolytic cell. In this specification, unless otherwise specified, the electrode potential refers to the potential of the cathode relative to the silver-silver chloride electrode (vs. Ag / AgCl). The reference electrode may be directly immersed in the electrolytic cell, or may be immersed in an electrolyte cell electrically connected via a salt bridge.

[0039] In some embodiments, the electrolytic cell 12 can take various forms, such as a batch cell type or a flow cell type, as long as it is a single-chamber electrolytic cell equipped with an anode and a cathode. For example, in some embodiments, the electrolytic cell can be a flow cell type electrolytic cell that can continuously produce formic acid. In this case, an electrolyte tank 13 (also referred to as a reservoir) and a liquid delivery mechanism 8 can be provided to deliver the electrolyte to the flow cell type electrolytic cell 12. Furthermore, the flow cell type electrolytic cell can be provided with measuring instruments such as a flow meter, a pressure gauge, and a thermometer.

[0040] In one embodiment, the electrolysis apparatus may be provided with an external power supply mechanism 11 that applies a voltage between the cathode 1 and the anode 2. The external power supply mechanism is not particularly limited, but a potentio-galvanostat may be used. To obtain a desired potential using a potentio-galvanostat, the potential may be directly controlled using a constant potential method, or the current value may be controlled using a constant current method to provide the desired potential. A potentiostat or a galvanostat may be used as the external power supply mechanism.

[0041] 2. Method for Producing Valuable Resources In one embodiment, the present disclosure provides a method for producing valuable resources by electrolytically reducing carbon dioxide dissolved in an electrolytic solution using an electrolytic cell including an anode and a cathode. The method for producing valuable resources in this embodiment is characterized in that the electrolytic cell is a single-chamber electrolytic cell without a diaphragm, and the anode has a fluororesin coating formed on the surface of the anode material. The single-chamber electrolytic cell and the fluororesin coating are as described above.

[0042] In certain embodiments, the method for producing valuable materials of the present disclosure can use an electrolyte solution in which carbon dioxide, which is the raw material for the valuable material, and a supporting electrolyte are dissolved. The electrolyte solution can be an aqueous solution that is easily available and has excellent conductivity. The supporting electrolyte is as described above. Carbon dioxide can be dissolved, for example, by bubbling carbon dioxide gas into the electrolyte solution. In certain embodiments, before bubbling carbon dioxide gas, the electrolyte solution can be bubbled with an inert gas such as nitrogen or argon to remove dissolved gases, such as dissolved oxygen, contained in the electrolyte solution.

[0043] In certain embodiments, the electrolytic cell can take various forms, such as a batch cell type or a flow cell type, as long as it is a single-chamber electrolytic cell equipped with an anode and a cathode. For example, in certain embodiments, the electrolytic cell can be a flow cell type electrolytic cell capable of continuously obtaining valuable materials. In this case, an electrolyte tank 13 and a liquid delivery mechanism 8 can be provided to deliver the electrolyte to the flow cell type electrolytic cell 12. In this case, the electrolyte solution can be circulated by using the liquid delivery mechanism to deliver the electrolyte solution extracted from the electrolyte solution tank to the electrolytic cell and then returning the electrolyte solution extracted from the electrolytic cell to the electrolyte solution tank. The liquid delivery mechanism that circulates the reaction solution in the reaction chamber may be referred to herein as a liquid delivery unit. The liquid delivery mechanism that circulates the reaction solution in the reaction chamber can be a liquid delivery pump or a pumpless mechanism. In certain embodiments, the liquid delivery pump can be a pump that performs intermittent liquid delivery, such as a peristaltic pump, piston pump, syringe pump, plunger pump, or diaphragm pump. Intermittent liquid delivery has the advantage of increasing the amount of valuable materials obtained by the electrolytic reduction of carbon dioxide. In one embodiment, the pumpless mechanism may be a mechanism that utilizes reduced pressure to enable fluid delivery without using a pump.

[0044] In the present disclosure, the electrolytic reduction may be performed in a constant potential mode, in which the cathode potential relative to the reference electrode is kept constant, a constant voltage mode, in which the voltage between the anode and cathode is kept constant, or a constant current mode, in which the cathode current density is kept constant. In some embodiments, the current may be direct current.

[0045] When a constant potential system is adopted in the present disclosure, the electrolysis apparatus can have a three-electrode configuration having a cathode 1, an anode 2, and a reference electrode 3. Furthermore, when a constant voltage system or a constant current system is adopted, the electrolysis apparatus may have a three-electrode configuration having a cathode, an anode, and a reference electrode, or a two-electrode configuration having a cathode and an anode but no reference electrode. When the three-electrode configuration is adopted, the present disclosure includes a case where the potential of the reference electrode is within the range described in the present disclosure. Furthermore, when the two-electrode configuration is adopted, the cathode potential relative to the reference electrode is not measured, but if it is measured, the potential of the reference electrode is within the range described in the present disclosure. In other words, with regard to the method for producing formic acid of the present disclosure, for example, applying a cathode potential in the range of −2.0 V to −3.0 V relative to a reference electrode does not necessarily stipulate that the cathode potential must be measured relative to the reference electrode in a three-electrode configuration. Rather, when a voltage is applied between the cathode and anode in a two-electrode configuration, if the cathode potential is measured relative to the reference electrode and the potential is ultimately within the range described in the present disclosure, the applied voltage will essentially correspond to applying a cathode potential in the range of −2.0 V to −3.0 V relative to the reference electrode in the present disclosure.

[0046] In one embodiment, when a conductive diamond electrode is used as a cathode and formic acid is produced as a valuable substance, the current density of the conductive diamond electrode is set to −0.1 mA / cm 2 ~-50mA / cm 2 , -0.2mA / cm 2 ~-20mA / cm 2 , -0.5mA / cm 2 ~-10mA / cm 2 , -0.6mA / cm 2 ~-5mA / cm 2 , -0.7mA / cm 2 ~-4.0mA / cm 2 , -0.8mA / cm 2 ~-3.5mA / cm 2 , -0.9mA / cm 2 ~-3.0mA / cm 2 , -1.0mA / cm 2 ~-2.5mA / cm 2 , -1.5mA / cm2 ~-2.0mA / cm 2 , e.g., -0.5mA / cm 2 , -1.0mA / cm 2 , -2.0mA / cm 2 , -3mA / cm 2 , -4mA / cm 2 , or -5mA / cm 2 It can be said that:

[0047] In one embodiment, when a conductive diamond electrode is used as the cathode and formic acid is produced as a valuable product, the cathode potential relative to the reference electrode can be, for example, -1.5 V or higher, -1.6 V or higher, -1.7 V or higher, -1.8 V or higher, for example, -3.0 V or lower, -2.8 V or lower, -2.5 V or lower, for example, -1.5 V to -3.0 V, -1.6 V to -2.1 V, -1.7 V to -2.0 V, or -2.0 V to -2.5 V. If the electrode potential is too low, the amount of formic acid produced by the electrolytic reduction of carbon dioxide may decrease. If the electrode potential is too high, hydrogen generation by water decomposition may become active, reducing the amount of formic acid produced. Alternatively, the amount of formic acid produced may increase, but the power consumption required for electrolysis may increase.

[0048] In a specific embodiment, the current density in the step of applying the cathode potential is not particularly limited, but if it is too small or too large, it may fall outside the range of the cathode potential described above, which is undesirable. From the viewpoint of the amount of valuable material produced, it is preferable to adjust the cathode potential so as to maximize it within the range that can be obtained.

[0049] In this embodiment, the time for producing valuable materials can be set as appropriate. In certain embodiments, the time for producing valuable materials can be, but is not limited to, 10 minutes or more, 30 minutes or more, 60 minutes or more, 2 hours or more, 3 hours or more, 6 hours or more, 12 hours or more, 24 hours or more, 48 hours or more, or, for example, 72 hours or more. The production amount of valuable materials can be increased by extending the time for producing valuable materials as long as possible without reducing the supply of carbon dioxide as a raw material or the production rate of the valuable materials. In certain embodiments, the time for producing valuable materials can be, but is not limited to, 144 hours or less, 72 hours or less, 48 ​​hours or less, or 24 hours or less.

[0050] In one embodiment, the conductive diamond electrode of the present disclosure is in an electrolyte solution containing dissolved carbon dioxide and a supporting electrolyte, and is subjected to a cathodic potential in the range of −2.0 V to −3.0 V, for example, in the range of −2.2 V to −3.0 V, for example, in the range of −2.2 V to −2.8 V, relative to a reference electrode, and exhibits a resistance of −0.5 mA / cm 2 or more, e.g., -1.0 mA / cm 2 The activated carbon may be electrolyzed at a current density of 1000 kJ / cm or more.

[0051] Conventional electrolysis devices equipped with a two-compartment electrolytic cell have two compartments, an anode compartment and a cathode compartment, which results in a large electrolysis cell. A conventional two-compartment flow reactor is shown in FIG. 1-1. The cathode compartment 5 and the anode compartment 6 are separated by a solid electrolyte membrane 4. The cathode solution stored in the first storage compartment 9 is sent to the cathode compartment 5 by a liquid delivery mechanism 8. The anolyte stored in the second storage compartment 10 is sent to the anode compartment 6 by a liquid delivery mechanism 8. Conventional electrolysis devices equipped with a two-compartment electrolytic cell require separate electrolyte supply to the anode compartment and the cathode compartment, which requires two liquid delivery systems, including a liquid delivery pump and liquid delivery piping, resulting in a large electrolysis device and increased power consumption. Furthermore, conventional electrolysis devices equipped with a two-compartment electrolytic cell have the problem of increasing the cost of the device because the diaphragm needs to be replaced due to clogging or damage.

[0052] In contrast, according to the present disclosure, carbon dioxide can be electrolytically reduced to efficiently produce valuable materials without using a diaphragm. The single-chamber electrolytic cell of the present disclosure performs electrolysis using one type of electrolyte solution, and therefore has the advantage of being able to reduce the size of the electrolytic cell compared to a two-chamber electrolytic cell that uses two types of electrolyte solutions. Furthermore, the single-chamber electrolytic cell of the present disclosure uses one type of electrolyte solution, and therefore can realize a single liquid transport system including a liquid transport pump, piping, valves, etc., and therefore has the advantage of being able to reduce the size of the entire electrolytic device compared to a two-chamber electrolytic cell that requires two liquid transport systems. Furthermore, the single-chamber electrolytic cell of the present disclosure does not have a diaphragm, and therefore has the advantages of eliminating the cost of a diaphragm, avoiding the problem of clogging due to the diaphragm, and eliminating the need to stop the electrolytic device for replacing the diaphragm.

[0053] The present invention will be explained in more detail below using examples, but the technical scope of the present invention is not limited to these examples in any way.

[0054] Example 1 The present inventors have developed a single-chamber flow reactor that does not use a diaphragm, which is described below.

[0055] Boron-doped diamond electrodes were fabricated using a microwave plasma CVD apparatus (Corns Technology, Model AX5400). Specifically, as a pretreatment, the surface of a silicon substrate (Si(100)) was nucleated with diamond powder, and then a film was formed on the substrate using 50 ml of acetone and 0.4 ml of trimethoxyborate (boron concentration 0.1%) as carbon sources at a plasma power of 5000 W for 6 hours at a pressure of 115 Torr. The obtained boron-doped diamond electrodes were evaluated using Raman spectroscopy and a scanning electron microscope, and it was found that a polycrystalline diamond thin film containing boron was formed.

[0056] Example 2 Surface analysis was performed on an iridium oxide electrode (Anodek 100 manufactured by Nisshin Seiko Co., Ltd.) and an iridium oxide top-coated electrode (Anodek 100CA manufactured by Nisshin Seiko Co., Ltd.) using an X-ray photoelectron spectrometer (JPS-9010TR manufactured by JEOL Ltd.). The results are shown in Figure 2. A peak derived from iridium was clearly observed at 60-65 eV in the iridium oxide electrode without a top coat, but this peak was unclear in the iridium oxide top-coated electrode. Furthermore, a peak derived from fluorine atoms was clearly observed at 693 eV in the iridium oxide top-coated electrode, but this peak was not observed in the iridium oxide electrode. This confirmed that the iridium oxide top-coated electrode formed a top coat layer containing fluorine.

[0057] [Example 3] Thermocompression bonding An iridium oxide electrode (Anodek 100 manufactured by Nisshin Seiki Co., Ltd.) and a Nafion membrane NR212 were bonded using a thermocompression bonding machine to form a fluorine-containing ion exchange resin layer on the iridium oxide electrode. The bonding conditions using the thermocompression bonding machine were 150°C, 100 kgcm －２ , for 5 minutes.

[0058] Example 4 A 0.5 M potassium chloride solution was prepared at room temperature and pressure using special-grade potassium chloride manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. and deionized water obtained using an ultrapure water system (Milli Q Reference manufactured by Merck Ltd.) as the solvent. A single-chamber electrolytic cell equipped with an iridium oxide working electrode, a platinum counter electrode, and an Ag / AgCl reference electrode was used. 50 mL of a 0.5 mol / L potassium chloride aqueous solution was circulated as the electrolyte at 200 mL / min using a peristaltic pump. The electrode areas of the working and counter electrodes were 9.62 cm. 2 Linear sweep voltammetry (LSV) was performed using a potentio-galvanostat (PGSTAT204, Metrohm Autolab). The potential range was 0 V to +2.0 V, and the potential scanning rate was 0.1 V / s to obtain voltammograms.

[0059] Similar measurements were carried out using a commercially available iridium oxide electrode having a fluororesin coating on its surface (hereinafter referred to as an iridium oxide top-coated electrode) and an iridium oxide electrode having a Nafion thermocompression-bonded coating prepared in Example 3 (hereinafter referred to as an iridium oxide thermocompression-bonded electrode) as the working electrode. The obtained voltammograms were superimposed to obtain Figure 3. As a result, it was found that when the iridium oxide top-coated electrode and the iridium oxide thermocompression-bonded electrode were used as the working electrode, the current value was small, and the chlorine evolution reaction was suppressed.

[0060] Example 5 Formic Acid Decomposition Test Using special-grade potassium sulfate and special-grade formic acid manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., and deionized water obtained using an ultrapure water system (Milli Q Reference manufactured by Merck Ltd.) as the solvent, a 0.25 M potassium sulfate aqueous solution (pH 4.0) containing formic acid at a concentration of 300 mg / L was prepared at room temperature and atmospheric pressure. A single-chamber electrolytic cell equipped with an iridium oxide electrode as the working electrode, a platinum electrode as the counter electrode, and an Ag / AgCl electrode as the reference electrode was used. 50 mL of a 0.25 M potassium sulfate aqueous solution containing formic acid at a concentration of 300 mg / L was circulated as the electrolyte at 200 mL / min using a peristaltic pump. The electrode areas of the working and counter electrodes were 9.62 cm. 2 Linear sweep voltammetry (LSV) was performed using a potentio-galvanostat (PGSTAT204, Metrohm Autolab). The potential range was 0 V to +2.0 V, and the potential scan rate was 0.1 V / s to obtain voltammograms.

[0061] Similar measurements were performed using an iridium oxide top-coated electrode and an iridium oxide thermocompression-bonded electrode as the working electrode. The resulting voltammograms were superimposed to obtain Figure 4. As a result, the current value was small when the iridium oxide top-coated electrode was used as the working electrode, and the current value was even smaller when the iridium oxide thermocompression-bonded electrode was used, indicating that the formic acid decomposition reaction was suppressed.

[0062] [Example 6] A three-electrode two-compartment cell (Fig. 1-1) or a single-compartment flow reactor (Fig. 1-2) was used as the electrolytic cell. A Nafion™ membrane (NRE-212) was used as the membrane for the two-compartment flow reactor. In all CO2 electroreduction experiments in this study, a 0.1% BDD (electrode area 9.62 cm) was used as the working electrode (cathode). 2 The volume of the electrolyte was 50 mL, and it was circulated at a flow rate of 200 mL / min using a pump.

[0063] Before CO2 electroreduction, the BDD electrode was pretreated to clean and oxygenate the electrode, and the electrolyte (catholyte in the case of a two-chamber system) was saturated with CO2. The BDD was pretreated to clean and oxygenate the electrode by cyclic voltammetry in 0.1 M H2SO4 (10 cycles from 3.5 V to -3.5 V at a scan rate of 1 V / s, followed by 20 cycles from 0 V to +3.5 V at a scan rate of 1 V / s). The electrolyte was saturated with CO2 by bubbling N2 (flow rate 200 mL / min) for 30 minutes followed by bubbling CO2 (flow rate 200 mL / min) for 30 minutes.

[0064] In addition, when using 0.25 M K2SO4 as the electrolyte and also performing activation pretreatment on the BDD surface, a two-chamber flow reactor was used. After the aforementioned cleaning and oxygen termination pretreatment, 0.5 M KCl saturated with CO2 was used as the catholyte and 0.5 M KOH was used as the anolyte. While continuing to aerate with CO2 (flow rate 200 mL / min), a current density of -2.0 mA / cm was used. 2 Constant current electrolysis was carried out for 60 minutes.

[0065] In the case of the two-chamber type CO2 electrolytic reduction, the anolyte was 0.5M KOH. 2 The electrolysis was carried out for 60 minutes using a constant current electrolysis at 0 V or a constant potential electrolysis at -2.25 V (PGSTAT204, Metrohm Autolab). The liquid components of the resulting reduction products were quantified by high performance liquid chromatography (HPLC), and the faradaic efficiency was calculated.

[0066] In this example, formic acid was produced in a single-chamber electrolytic cell, and an iridium oxide top-coated electrode was used as the anode, and 0.25 M potassium sulfate was used as the electrolyte, and activation treatment of the BDD electrode was also performed (Example 6).

[0067] Example 7 Formic acid production using a single-chamber electrolytic cell. The anode was the iridium oxide thermocompression electrode prepared in Example 3, the cathode was 0.1% BDD, the electrolyte was 0.5 M potassium chloride aqueous solution, and the electrolysis was carried out at a constant current [-2 mA / cm 2 , 60 min].

[0068] Comparative Example 1: Formic acid generation using a two-compartment electrolytic cell. Platinum was used as the anode, 0.1% BDD was used as the cathode, 0.5 M potassium chloride aqueous solution was used as the catholyte, and 0.5 M potassium hydroxide aqueous solution was used as the anolyte. 2 , 60 min].

[0069] Comparative Example 2: Formic acid generation using a single-chamber electrolytic cell. Platinum was used as the anode, 0.1% BDD was used as the cathode, and 0.5 M potassium chloride solution was used as the electrolyte. 2 , 60 min].

[0070] Comparative Example 3: Formic acid production using a two-compartment electrolytic cell. The BDD electrode was activated using an iridium oxide topcoat electrode as the anode, 0.25 M potassium sulfate as the catholyte, and 0.5 M potassium hydroxide aqueous solution as the anolyte.

[0071] Comparative Example 4: Formic acid production using a single-chamber electrolytic cell. The BDD electrode was activated using an iridium oxide electrode as the anode and 0.25 M potassium sulfate as the electrolyte.

[0072] The results of Comparative Examples 1 and 2 are shown in Figure 5. In the case of a two-compartment electrolytic cell, formic acid was produced with high efficiency even when conventional electrodes were used. However, when the same conventional electrodes were used but the electrolytic cell was a single-compartment cell, the electrolysis efficiency was significantly reduced.

[0073] The results of Comparative Examples 1 and 3 are shown in Figure 6. It was found that formic acid was produced with high efficiency under all conditions in the case of the two-compartment electrolytic cell. However, this is a conventional device with a diaphragm, and there is a possibility that the diaphragm will bend or the reaction chamber will become clogged after long-term operation.

[0074] The results of Comparative Example 4 and Example 6 are shown in Figure 7. In the case of a single-chamber electrolytic cell, when the anode was a conventional iridium oxide electrode, the production efficiency of formic acid was significantly reduced (Comparative Example 4), whereas when the anode was an iridium oxide electrode having a fluororesin coating, formic acid was obtained with high production efficiency (Example 6).

[0075] The results of Comparative Example 2 and Example 7 are shown in Figure 8. In the case of a single-compartment electrolytic cell, when the anode was a conventional platinum electrode, the production efficiency of formic acid was significantly reduced (Comparative Example 2), whereas when the anode was an iridium oxide electrode coated with a fluorine-containing ion-exchange resin, formic acid was obtained with high production efficiency (Example 7).

[0076] In light of the above surprising results, the inventors speculate as follows. While not wishing to be bound by any particular theory, the inventors believe that the presence of a fluororesin coating on the anode material surface inhibits the oxidative decomposition at the anode of valuables produced by the reduction of carbon dioxide at the cathode. Furthermore, while not wishing to be bound by any particular theory, the inventors believe that the presence of a fluororesin coating on the anode material surface can inhibit the oxidation of halide ions in the electrolyte and the resulting generation of halogen gas. Furthermore, while not wishing to be bound by any particular theory, the inventors believe that the presence of a fluororesin coating on the anode material surface hydrophobizes the anode material surface, preventing hydrophilic products of electrolytic reduction of carbon dioxide (e.g., formic acid) and halide ions from approaching the anode material surface, thereby inhibiting oxidative decomposition at the anode or inhibiting halogen gas generation. Furthermore, without wishing to be bound by any particular theory, the present inventors believe that the presence of a fluorine-containing ion exchange resin coating on the surface of the anode material has the effect of preventing ion species such as halide ions and formate ions from approaching the surface of the anode material, thereby suppressing oxidative decomposition at the anode or suppressing the generation of halogen gas.

[0077] Therefore, it is reasonably believed that similar effects can be obtained with other metal electrodes having a fluororesin coating, such as tantalum oxide electrodes having a fluororesin coating.

[0078] 1 Cathode 2 Anode 3 Reference electrode 4 Solid electrolyte membrane 5 Cathode cell 6 Anode cell 8 Liquid delivery mechanism 9 First reservoir 10 Second reservoir 11 External power supply mechanism 12 Electrolytic cell (single chamber) 13 Reservoir All publications, patents and patent applications cited in this specification are hereby incorporated by reference in their entirety.

Claims

1. An electrolysis device that obtains valuable resources by electrolytic reduction of carbon dioxide dissolved in an electrolytic solution, an electrolytic cell having an anode and a cathode; the electrolytic cell is a single-chamber electrolytic cell having no diaphragm, It is equipped with a mechanism for circulating the reaction liquid in the reaction chamber. The anode has a fluororesin coating on the surface of the anode material. Flow reactor type electrolysis device.

2. 2. The electrolysis device according to claim 1, wherein the fluororesin is a fluorine-containing ion exchange resin.

3. 2. The electrolysis device of claim 1, wherein the anode comprises iridium, titanium, tantalum, niobium, molybdenum, tungsten, platinum, nickel, iron, an alloy or oxide thereof, glassy carbon, graphite, conductive diamond, or conductive silicon.

4. 2. The electrolysis device according to claim 1, wherein the anode comprises iridium oxide or tantalum oxide as an anode material.

5. 5. The electrolysis device according to claim 4, wherein the anode is an iridium oxide electrode having a fluororesin coating on its surface, or a tantalum oxide electrode having a fluororesin coating on its surface.

6. 2. The electrolysis device according to claim 1, wherein the valuable product is formic acid.

7. 2. The electrolysis device according to claim 1, wherein the cathode is a conductive diamond electrode.

8. 7. The electrolysis device according to claim 6, wherein the electrolytic solution is an aqueous solution containing a supporting electrolyte that does not contain halide ions.

9. 9. The electrolysis apparatus of claim 8, wherein the halide ion-free supporting electrolyte is potassium sulfate.

10. 8. The electrolysis device of claim 7, wherein the conductive diamond electrode is activated.

11. A method for producing valuable materials by electrolytic reduction of carbon dioxide dissolved in an electrolytic solution using an electrolytic cell equipped with an anode and a cathode, comprising: the electrolytic cell is a single-chamber electrolytic cell having no diaphragm, It is equipped with a mechanism for circulating the reaction liquid in the reaction chamber. The anode has a fluororesin coating on the surface of the anode material, Electrolytic reduction of carbon dioxide in a single-chamber electrolytic cell, Methods for producing valuable materials.

12. The method for producing valuable resources according to claim 11, wherein the fluororesin is a fluorine-containing ion exchange resin.

13. The method for producing a valuable resource according to claim 11, wherein the anode contains iridium, titanium, tantalum, niobium, molybdenum, tungsten, platinum, nickel, iron, an alloy or oxide thereof, glassy carbon, graphite, conductive diamond, or conductive silicon.

14. The method for producing valuable resources according to claim 11 , wherein the anode contains iridium oxide or tantalum oxide as an anode material.

15. The method according to claim 14, wherein the anode is an iridium oxide electrode having a fluororesin coating on its surface, or a tantalum oxide electrode having a fluororesin coating on its surface.

16. The method for producing valuable resources according to claim 11, wherein the valuable resource is formic acid.

17. The method for producing valuable materials according to claim 11, wherein the cathode is a conductive diamond electrode.

18. The method for producing valuable materials according to claim 11, wherein the electrolytic solution is an aqueous solution containing a supporting electrolyte that does not contain halide ions.

19. 19. The method for producing valuable resources according to claim 18, wherein the supporting electrolyte not containing halide ions is potassium sulfate.

20. 18. The method of claim 17, comprising activating the conductive diamond electrode prior to the step of electrolytically reducing carbon dioxide in the single-chamber electrolytic cell.