Electrolytic apparatus and electrolytic method

Abstract

To provide an electrolysis device capable of reducing a potential difference between two electrodes necessary for generating CO2, H2 and a carbon dioxide reduction product.SOLUTION: An electrolytic device 1 includes an electrochemical cell 10 having a first electrode 22 and a second electrode 24, and applies a voltage which periodically reverses polarity between the first electrode 22 and the second electrode 24 to electrolyze an electrolytic solution in which at least one of carbon dioxide ion and hydrogen carbonate ion is dissolved, and generates a product containing at least one of carbon dioxide (gas), hydrogen (gas), and a carbon dioxide reduction product.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to the technology of an electrolysis apparatus and an electrolysis method. Background Art

[0002] For example, Patent Documents 1 to 3 and Non-Patent Documents 1 to 3 disclose an electrolysis apparatus in which a cathode electrode and an anode electrode are electrically connected, and a voltage is applied to electrolyze an electrolyte solution containing carbonate ions (CO3 2- ) and hydrogen carbonate ions (HCO3 - ) to produce CO2, H2, and carbon dioxide reduction products (such as CO). Prior Art Documents Patent Documents

[0003] Patent Document 1 International Publication No. WO2019 / 204938 Patent Document 2 International Publication No. WO2021 / 207857 Patent Document 3 International Publication No. WO2020 / 223804 Patent Document 4 Japanese Patent No. 5750220 Patent Document 5 Japanese Unexamined Patent Publication No. 2008-100211 Non-Patent Documents

[0004] Non-Patent Document 1 David A. Vermaas and Wilson A. Smith, “Synergistic Electrochemical CO2 Reduction and Water Oxidation with a Bipolar Membrane”, ACS Energy Lett., 1, 1143-1148(2016) Non-Patent Document 2 Tengfei Li, Eric W. Lees, Maxwell Goldman, Danielle A. Salvatore, David M. Weekes, and Curtis P. Berlinguette, “Electrolytic Conversion of Bicarbonate into CO in a Flow Cell”, Joule, 3, 1487-1497(2019)

Table 3

Fashion 4

Wood 5

Wood 6

[0005] By the way, conventional electrolytic devices employ constant voltage electrolysis, which involves applying a constant voltage between the cathode electrode and the anode electrode to electrolyze the electrolyte. However, in this case, there is a problem in that the potential difference between the two electrodes (i.e., the voltage applied between the two electrodes) must be large in order to produce CO2, H2, and carbon dioxide reduction products.

[0006] Therefore, the object of the present invention is to provide an electrolytic apparatus and electrolytic method that can reduce the potential difference between the two electrodes necessary for the generation of CO2, H2, and carbon dioxide reduction products. [Means for solving the problem]

[0007] The electrolytic apparatus according to this embodiment comprises an electrochemical cell having a first electrode and a second electrode, and is characterized by applying a voltage that periodically reverses polarity between the first electrode and the second electrode to electrolyze an electrolyte containing at least one of carbonate ions and bicarbonate ions, thereby producing a product containing at least one of carbon dioxide (gas), hydrogen (gas), and carbon dioxide reduction products.

[0008] Furthermore, in the electrolytic apparatus, it is preferable that the electrochemical cell has a diaphragm provided between the first electrode and the second electrode.

[0009] Furthermore, it is preferable that the electrolytic apparatus has an electrolyte supply device that supplies the electrolyte to the electrochemical cell.

[0010] Furthermore, it is preferable that the electrolytic apparatus includes a gas-liquid separation device for separating the electrolyte containing the product discharged from the electrochemical cell into gas-liquid.

[0011] Furthermore, it is preferable that the electrolytic apparatus includes a circulation device for circulating the electrolyte separated by the gas-liquid separation device back to the electrochemical cell.

[0012] Furthermore, it is preferable that the electrolytic apparatus includes a gas-liquid contact device that brings a gas containing carbon dioxide into contact with the electrolyte circulated by the circulation device.

[0013] Furthermore, in the electrolytic apparatus, it is preferable that the first electrode and the second electrode are metals or alloys containing at least one of Au, Pd, Pt, Ag, and Cu.

[0014] Furthermore, in the electrolytic apparatus, it is preferable that the electrolyte contains alkali metal ions.

[0015] Furthermore, in the electrolytic device, it is preferable that the potential difference E between the first electrode and the second electrode when the voltage is applied is 0.4V or more.

[0016] Furthermore, the electrolysis method of the present invention is characterized by applying a voltage that periodically reverses polarity between a first electrode and a second electrode to electrolyze an electrolyte containing at least one of carbonate ions and bicarbonate ions, thereby producing a product containing at least one of carbon dioxide (gas), hydrogen (gas), and carbon dioxide reduction products. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide an electrolytic apparatus and electrolytic method that can reduce the potential difference between the two electrodes necessary for the generation of CO2, H2, and carbon dioxide reduction products. [Brief explanation of the drawing]

[0018] [Figure 1] This is a schematic diagram showing an example of an electrolytic apparatus according to this embodiment. [Figure 2] This is a schematic diagram showing the electrolytic apparatus used in the example. [Figure 3] This figure shows the change in current density over time during the electrolytic treatment in Example 1 and Comparative Example 1. [Figure 4] This figure shows the average current density (absolute value) during the electrolytic treatment in Example 1 and Comparative Example 1. [Figure 5] These are gas chromatograms of the gaseous products collected during the electrolytic treatment in Example 1 and Comparative Example 1. [Figure 6] This figure shows the average current density (absolute value) during the electrolytic treatment in Example 2 and Comparative Example 2. [Figure 7] This figure shows the average current density (absolute value) during the electrolytic treatment in Example 3. [Figure 8] This is a schematic diagram showing the electrolytic apparatus used in the example. [Figure 9] This figure shows the change in current density over time during the electrolytic treatment in Examples 4 and 5. [Figure 10] This figure shows the average current density (absolute value) during the electrolytic treatment in Examples 4 and 5. [Figure 11] These are gas chromatograms of the gaseous products collected during the electrolytic treatment in Examples 4 and 5. [Figure 12] This figure shows the change in current density over time during the electrolytic treatment in Comparative Example 3. [Figure 13] This figure shows the average current density (absolute value) during the electrolytic treatment in Comparative Example 3. [Figure 14] This is a gas chromatogram of the gaseous product collected during the electrolytic treatment in Comparative Example 3. [Figure 15](A) is a diagram showing the reactant concentration distribution near the cathode and anode immediately after polarity switching (initial stage of electrolysis), and (B) is a diagram showing the equilibrium potential of each reactant near the cathode and anode immediately after polarity switching (initial stage of electrolysis). [Figure 16] (A) is a diagram showing the reactant concentration distribution near the cathode and anode immediately before polarity switching (late stage of electrolysis), and (B) is a diagram showing the equilibrium potential of each reactant near the cathode and anode immediately before polarity switching (late stage of electrolysis). [Figure 17] This figure shows the relationship between the relative abundance of carbon dioxide (CO2), bicarbonate ions (HCO3-), and carbonate ions (CO32-) in aqueous solutions and their pH dependence. [Modes for carrying out the invention]

[0019] Embodiments of the present invention will be described below. This embodiment is one example of carrying out the present invention, and the present invention is not limited to this embodiment.

[0020] Figure 1 is a schematic diagram showing an example of an electrolytic apparatus according to this embodiment. The electrolytic apparatus 1 of this embodiment is composed of an electrochemical cell 10. Furthermore, as shown in Figure 1, it is preferable that the electrolytic apparatus 1 of this embodiment includes a supply device 12, a gas-liquid separation device 14, a gas-liquid contact device 16, and a circulation device 18.

[0021] The electrochemical cell 10 of this embodiment has a first electrode 22 and a second electrode 24. Furthermore, as shown in Figure 1, the electrochemical cell 10 of this embodiment may also have a diaphragm 30 provided between the first electrode 22 and the second electrode 24. Also, as shown in Figure 1, it is desirable that flow channels (32a, 32b) for electrolyte flow be provided between the first electrode 22 and the diaphragm 30 and between the second electrode 24 and the diaphragm 30. In the electrochemical cell 10 shown in Figure 1, the first electrode 22 and the second electrode 24 are structurally supported by a frame material 36.

[0022] The electrolytic apparatus 1 is preferably equipped with a supply device 12 for supplying electrolyte to the electrochemical cell 10, for example, in order to stably supply the electrolyte to the electrochemical cell 10. The supply device 12 comprises an electrolyte storage tank 38, an electrolyte supply pump 40, and an electrolyte supply line 42. The electrolyte supply pump 40 is installed in the electrolyte supply line 42. One end of the electrolyte supply line 42 is connected to the electrolyte storage tank 38, and the other end is connected to the flow path (32a, 32b).

[0023] The electrolytic apparatus 1 preferably includes a gas-liquid separator 14 for separating the electrolyte containing the products discharged from the electrochemical cell 10 into gas and liquid, in order to efficiently recover the products generated by the electrolysis of the electrolyte (such as carbon dioxide (gas), hydrogen (gas), and carbon dioxide reduction products). The gas-liquid separator 14 includes, for example, a gas-liquid separator 50 and an electrolyte discharge line 52. One end of the electrolyte discharge line 52 is connected to a flow path (32a, 32b), and the other end is connected to the gas-liquid separator 50. The gas-liquid separator 50 is, for example, a conventionally known device capable of separating gas and liquid.

[0024] The electrolytic apparatus 1 preferably includes a circulation device 18 that circulates the electrolyte separated by the gas-liquid separator 14 back to the electrochemical cell 10, for example, by reusing the electrolyte to reduce the cost of electrolytic treatment. The circulation device 18 includes, for example, a circulation line 54 and a circulation pump 56. The circulation pump 56 is installed in the circulation line 54. One end of the circulation line 54 is connected to the gas-liquid separator 50, and the other end is connected to the electrolyte supply pump 40.

[0025] The electrolytic device 1 preferably includes a gas-liquid contact device 16, which can, for example, increase the amount of carbonate ions and bicarbonate ions in the circulating electrolyte. The gas-liquid contact device 16 includes, for example, a carbon dioxide supply line 58, a gas-liquid contact tank 60, and a discharge line 62. The carbon dioxide supply line 58 is connected to the gas-liquid contact tank 60. One end of the discharge line 62 is connected to the gas-liquid contact tank 60, and the other end is connected to the circulation line 54.

[0026] The symbol 70 in Figure 1 is a power supply device that applies a voltage that periodically reverses polarity between the first electrode 22 and the second electrode 24. Applying a voltage that periodically reverses polarity means that, if the potential of the first electrode 22 relative to the second electrode 24 is E, the applied voltage is switched periodically as follows: +E → -E → +E → -E... (periodically applying voltages with the same absolute value (|E|) but different polarities (±). The timing of switching the applied voltage can be set as appropriate, for example, between 5 seconds and 60 seconds. The time spent holding the potentials at +E and -E can be different. The absolute values ​​of the polarity reversal potentials |+E1| and |-E2| can also be different (i.e., E1 ≠ E2).

[0027] Next, we will explain an example of the operation of the electrolytic device 1 shown in Figure 1.

[0028] When the electrolyte supply pump 40 is activated, the electrolyte containing carbonate ions and / or bicarbonate ions contained in the electrolyte storage tank 38 is supplied through the electrolyte supply line 42 to the flow path 32a on the first electrode 22 side and the flow path 32b on the second electrode 24 side.

[0029] The electrolyte is electrolyzed by applying a voltage with periodically reversing polarity between the first electrode 22 and the second electrode 24 using the power supply unit 70. The voltage with periodically reversing polarity causes the first electrode 22 and the second electrode 24 to periodically switch between cathode and anode. When the first electrode 22 or the second electrode 24 becomes the anode, the pH decreases due to the action of hydrogen ions, and carbonate ions and bicarbonate ions in the electrolyte are converted into carbon dioxide (gas). Then, when the first electrode 22 or the second electrode 24 becomes the cathode, carbon dioxide reduction products and hydrogen (gas) are produced by the reduction of the generated carbon dioxide with water. Examples of carbon dioxide reduction products include carbon monoxide (CO), formic acid (HCOOH), methane (CH4), methanol (CH3OH), ethane (C2H6), ethylene (C2H4), ethanol (C2H5OH), and propanol (C3H7OH). In this way, products containing at least one of carbon dioxide (gas), hydrogen (gas), or carbon dioxide reduction products are generated from both electrodes, the first electrode 22 and the second electrode 24. Details of the electrolytic reactions at the first electrode 22 and the second electrode 24 will be described later.

[0030] The products generated at both electrodes, the first electrode 22 and the second electrode 24, are discharged together with the electrolyte through the electrolyte discharge line 52 and introduced into the gas-liquid separator 50. In the gas-liquid separator 50, for example, the gas containing the products is separated from the liquid (e.g., the electrolyte) from which the gaseous products have been removed, and the gas containing the products is recovered. When the circulation pump 56 is operated, the gas-liquid separated electrolyte is supplied (circulated) to the electrochemical cell 10 from the circulation line 54. Alternatively, a portion of the gas-liquid separated electrolyte may be transferred to the gas-liquid contact tank 60. Then, by supplying gas containing carbon dioxide from the carbon dioxide supply line 58 into the gas-liquid contact tank 60 and bringing the electrolyte into contact with the gas containing carbon dioxide, the content of carbonate ions and bicarbonate ions in the electrolyte can be increased. After gas-liquid contact, the electrolyte is supplied to the electrochemical cell 10, for example, through the discharge line 62 and the circulation line 54.

[0031] An example of an electrolytic reaction occurring at both electrodes when a polarity-reversing voltage is applied is described below. The following electrolytic reaction is presented as an example and is not limited thereto.

[0032] FIG. 15(A) is a diagram showing the reactant concentration distribution near the cathode and anode immediately after polarity switching (in the initial stage of electrolysis), and FIG. 15(B) is a diagram showing the equilibrium potential of each reactant near the cathode and anode immediately after polarity switching (in the initial stage of electrolysis). Further, FIG. 16(A) is a diagram showing the reactant concentration distribution near the cathode and anode immediately before polarity switching (in the late stage of electrolysis), and FIG. 16(B) is a diagram showing the equilibrium potential of each reactant near the cathode and anode immediately before polarity switching (in the late stage of electrolysis). In the equilibrium potential diagrams of FIG. 15(B) and FIG. 16(B), (1) a redox couple is expressed as "oxidized form / reduced form", (2) reactants present at high concentration in the vicinity of the electrode are shown in bold type, and (3) the magnitude of the reaction rate is represented by the thickness of an arrow (the thicker the arrow, the higher the reaction rate) based on the reactant concentration and overvoltage (the difference between the electrode potential and the equilibrium potential). Note that for the equilibrium potential, local pH is taken into consideration, but reactant concentration is not taken into consideration. FIG. 17 shows carbon dioxide CO2 and bicarbonate ion HCO3 in an aqueous solution - , carbonate ion CO3 2- is a diagram showing the relationship between the abundance ratio thereof and its pH dependence (FIG. 17 is extracted from a reference: R. Sharifian et al., Energy Environ. Sci., 14, 781 (2021)).

[0033] After a predetermined time has elapsed from the start of voltage application, in the vicinity of the anode (the first electrode or the second electrode) immediately before applying the polarity-reversing voltage (hereinafter, immediately before polarity switching), H + -involved O2 generation (2H2O→O2+4H + +e - ) proceeds. Furthermore, due to high-concentration H + , HCO3 - →CO2 conversion (H + +HCO3 -→CO2 + H2O) proceeds (see Figure 12). Therefore, immediately after applying a voltage that reverses the polarity (hereinafter, immediately after polarity switching), in addition to the solvent H2O, the reactants O2 and H2O are present near the cathode (first electrode or second electrode). + This results in a high concentration of CO2. On the other hand, near the cathode just before polarity switching, OH - H2 formation accompanied by (2H2O + 2e - →H2+2OH - Only the reaction proceeds, and CO production by CO2 reduction does not proceed. Therefore, immediately after polarity switching, in addition to the solvent H2O, the reactants H2 and OH are present near the anode. - It is present in high concentrations.

[0034] Here, when the applied voltage is 2.0V, immediately after polarity switching, the cathode will be approximately -0.5V vs. SHE (based on the standard hydrogen electrode), and the anode will be approximately +1.5V vs. SHE. The pH near the cathode immediately after polarity switching is considered to be approximately 1 (acidic), and the pH near the anode is 13 (alkaline). At this time, the equilibrium potentials of the reactants and reduction products undergoing reduction at the cathode with respect to the redox pair are +1.17V vs. SHE (O2 / H2O) and -0.06V vs. SHE (H2O). + The equilibrium potentials for the redox pairs of the reactants and oxidation products undergoing oxidation at the anode are -0.87V vs. SHE(CO2 / CO), -0.77V vs. SHE(H2O / H2), and +0.46V vs. SHE(O2 / OH). - ) and the overpotentials are 2.37V, 2.27V, and 1.04V respectively, so considering the reactant concentrations, it is thought that H2 oxidation proceeds preferentially at the anode immediately after polarity switching (at the cathode immediately before polarity switching, CO production by CO2 reduction does not proceed, so CO is not present near the anode immediately after polarity switching, and therefore the CO oxidation reaction does not proceed).

[0035] As the electrolytic reaction progresses, the pH increases at the cathode and decreases at the anode, changing the equilibrium potential of each reactant. Furthermore, the concentration of reactants other than the solvent H2O decreases near the electrodes, thus changing the electrode potentials of the cathode and anode. As a result, it is estimated that just before polarity switching, the pH near the cathode is approximately 13, the cathode potential is approximately -0.80V vs. SHE, and the pH near the anode is approximately 1, the anode potential is approximately +1.20V vs. SHE. Of the reactants that can contribute to the electrolytic reaction at both electrodes, only the solvent H2O is present in high concentrations near the cathode and anode. Therefore, at the cathode, H2 is produced by reduction of H2O (2H2O + 2e⁻¹). - →H2+2OH - (0.77V vs. SHE, overvoltage 0.03V) proceeds, and at the anode, O2 is produced by oxidation of H2O (2H2O → O2 + 4H + +4e - It is thought that a voltage of +1.17V vs. SHE (overvoltage of 0.03V) will proceed. Also, as previously mentioned, H will be generated at the anode along with O2 generation. + As generation proceeds, this H + This promotes the conversion of carbonate ion species to CO2 (see Figure 12).

[0036] In conventional electrolysis methods using a constant voltage, the situation immediately before polarity switching (late stage of electrolysis) persists, resulting in very small overvoltages and the cessation of current flow. However, in this embodiment of electrolysis, which applies a voltage that periodically reverses polarity, the polarity is switched to reversibly utilize the water electrolysis products O2 and H2 as electrolytic reactants. This reduces the potential difference between the two electrodes, allowing the electrolysis current to continue flowing. Furthermore, CO can be generated by reducing the CO2 produced at the anode immediately before polarity switching at the cathode after polarity switching. Unlike O2 and H2, which are reversibly oxidized and reduced, this generated CO is less likely to be reoxidized to CO2 due to the concentration distribution and equilibrium potential near the electrodes, allowing for highly efficient recovery. Thus, in this embodiment, which applies a voltage that periodically reverses polarity, the potential difference between the two electrodes required for the generation of carbon dioxide reduction products such as CO2 (gas), H2 (gas), and CO can be reduced compared to electrolysis methods using a constant voltage, thereby improving the electrolysis current and processing speed. In this embodiment, where a voltage with periodically reversing polarity is applied, carbon dioxide reduction products such as CO2 (gas), H2 (gas), and CO can be generated at both electrodes.

[0037] <Examples of reactions that may proceed at the cathode in this embodiment> All reactions at the cathode are OH - H is generated or + This reaction involves the consumption of certain substances, resulting in an increase in pH. (1) H2O(H + ) H2 production by reduction 2H2O + 2e - →H2+2OH - 2H + +2e - →H2 (2) CO2, H2O(H + ) CO production by reduction (CO production is just one example; other carbon dioxide reduction products may also be produced.) CO2 + H2O + 2e - →CO+2OH - CO2 + 2H + +2e - →CO+H2O (3) O2, H2O(H + ) Reduction of H2O(OH - )Generate O2 + 2H2O + 4e - →4OH - O2 + 4H + +4e - →2H2O

[0038] <Examples of reactions that may proceed at the anode in this embodiment> All reactions at the anode involve OH - When H is consumed + This reaction generates substances that cause a decrease in pH. (1) H2O(H) + )Generate H2 + 2OH - →2H2O+2e - H2→2H + +2e - (2) CO2, H2O(H) + )Generate CO + 2OH - →CO2+H2O+2e - CO + H2O → CO2 + 2H + +2e - (3) H2O(OH - ) Oxidation of O2, H2O(H + )Generate 4OH - →O2+2H2O+4e - 2H₂O → O₂ + 4H + +4e -

[0039] The first electrode 22, the second electrode 24, the diaphragm 30, and the electrolyte will be described in detail below.

[0040] The materials for the first electrode 22 and the second electrode 24 include, for example, metallic materials such as metals or alloys, carbon materials, conductive metal oxides, and metal complexes. However, it is preferable that the materials be conductive materials that have high resistance to high oxidation conditions during electrolytic reactions, and specifically, it is preferable that the materials be metals or alloys containing at least one of Au, Pd, Pt, Ag, and Cu. The materials of the first electrode 22 and the second electrode 24 may be different. Furthermore, the structure of the first electrode 22 and the second electrode 24 is preferably porous or mesh, for example, in terms of liquid fluidity.

[0041] In the electrochemical cell 10, the diaphragm 30 is not an essential component. Since the first electrode 22 and the second electrode 24 are periodically switched between anode and cathode, hydrogen ions can be supplied to the vicinity of the electrodes and carbon dioxide molecules and carbon dioxide reduction products can be generated even without the diaphragm 30. Therefore, an electrochemical cell without the diaphragm 30 can reduce the potential difference due to ion conduction resistance more than an electrochemical cell with the diaphragm 30, and in some cases, the potential difference between the two electrodes required for the generation of CO2, H2, and carbon dioxide reduction products can be further reduced. However, in order to miniaturize and thin the electrochemical cell, if it is desirable to bring the distance between the first electrode 22 and the second electrode 24 as close as possible, it is preferable to install the diaphragm 30 between the two electrodes to prevent short circuits between the two electrodes. The diaphragm 30 can be any known membrane placed between the two electrodes of the electrochemical cell, such as a cation exchange membrane, anion exchange membrane, bipolar membrane, semipermeable membrane, or insulating porous membrane.

[0042] In the electrolyte used in this embodiment, the pairing cation is not particularly limited as long as at least one of carbonate ions and bicarbonate ions is dissolved. Examples of cations include alkali metal ions, alkaline earth metal ions, and transition metal ions. Among these, it is preferable to include alkali metal ions in the electrolyte, for example, to promote the ionization of carbonate ions and bicarbonate ions and stabilize their dissolved state. Examples of such electrolytes include aqueous solutions containing alkali metal bicarbonates or alkali metal carbonates such as lithium bicarbonate (LiHCO3), sodium bicarbonate (NaHCO3), potassium bicarbonate (KHCO3), sodium carbonate (Na2CO3), potassium carbonate (K2CO3), and cesium bicarbonate (CsHCO3). The alkali metal ion concentration in the electrolyte is preferably in the range of 0.1 M to 3 M, for example, in order to improve the electrolytic treatment rate and enable continuous electrolytic treatment for a long period of time.

[0043] Furthermore, the electrolyte may contain alcohols such as methanol, ethanol, and acetone. Also, the electrolyte may contain, for example, cations such as imidazolium ions and pyridinium ions, and BF4 - PF6 - It may also contain an ionic liquid or an aqueous solution thereof, which consists of a salt with anions such as the above and is in a liquid state over a wide temperature range.

[0044] When a voltage with periodically reversing polarity is applied, the potential difference E between the first electrode 22 and the second electrode 24 is preferably greater than or equal to the difference between the equilibrium potential for H2 production at pH 0 at one electrode (0V vs. SHE) and the equilibrium potential for O2 production at pH 14 at the other electrode (0.4V vs. SHE), i.e., 0.4V or greater, in order to ensure stable electrolysis of the electrolyte. When a constant voltage that does not reverse polarity is applied to both electrodes, it is necessary to apply a voltage to both electrodes that is greater than or equal to the difference between the equilibrium potential for O2 production at pH 0 at the anode electrode (1.23V vs. SHE) and the equilibrium potential for H2 production at pH 14 at the cathode electrode (-0.83V vs. SHE), i.e., 2.06V or greater.

[0045] Furthermore, when a voltage with periodically reversing polarity is applied, the potential difference E between the first electrode 22 and the second electrode 24 is more preferably in the range of 1.0V to 3.0V in terms of improving the electrolytic processing speed, and even more preferably in the range of 1.0V to 2.0V in terms of suppressing the release of O2 gas.

[0046] In order to suppress a decrease in the amount of carbon dioxide (gas) recovered by redissolving the carbon dioxide (gas) generated in the electrochemical cell 10 into the electrolyte, it is preferable to shorten the time it takes to transfer the electrolyte from the electrochemical cell 10 to the gas-liquid separator 50, for example, it is preferable to keep it within 10 seconds.

[0047] "Structure of the present invention" Configuration 1: The electrochemical cell comprises a first electrode and a second electrode, An electrolytic apparatus characterized by applying a voltage that periodically reverses polarity between the first electrode and the second electrode to electrolyze an electrolyte containing at least one of carbonate ions and bicarbonate ions, thereby producing a product containing at least one of carbon dioxide (gas), hydrogen (gas), and carbon dioxide reduction products. Configuration 2: The electrolytic apparatus according to configuration 1, characterized in that the electrochemical cell has a diaphragm provided between the first electrode and the second electrode. Configuration 3: The electrolytic apparatus according to configuration 1 or 2, characterized in that it has an electrolyte supply device that supplies the electrolyte to the electrochemical cell. Configuration 4: The electrolytic apparatus according to any one of the above configurations 1 to 3, characterized in that it has a gas-liquid separation device for separating the electrolyte containing the product discharged from the electrochemical cell into gas and liquid. Configuration 5: The electrolytic apparatus according to configuration 4, characterized in that it includes a circulation device for circulating the electrolyte separated by the gas-liquid separation device to the electrochemical cell. Configuration 6: The electrolytic apparatus according to configuration 5, further comprising a gas-liquid contact device for bringing the electrolyte circulated by the aforementioned circulation device into contact with a gas containing carbon dioxide. Composition 7: The electrolytic apparatus according to any one of the above configurations 1 to 6, characterized in that the first electrode and the second electrode are metals or alloys containing at least one of Au, Pd, Pt, Ag, and Cu. Composition 8: The electrolytic apparatus according to any one of the above configurations 1 to 7, characterized in that the electrolyte contains alkali metal ions. Composition 9: The electrolytic apparatus according to any one of the above configurations 1 to 8, wherein the potential difference E between the first electrode and the second electrode when the aforementioned voltage is applied is 0.4V or more. Configuration 10: An electrolysis method characterized by applying a voltage that periodically reverses polarity between a first electrode and a second electrode to electrolyze an electrolyte containing at least one of carbonate ions and bicarbonate ions, thereby producing a product containing at least one of carbon dioxide (gas), hydrogen (gas), and carbon dioxide reduction products. [Examples]

[0048] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0049] <Example 1> Electrolytic treatment was performed using the electrolytic apparatus 2 shown in Figure 2. The electrolytic apparatus 2 shown in Figure 2 comprises an electrochemical cell 10, electrolyte storage tanks (38a, 38b), electrolyte supply pumps (40a, 40b), electrolyte supply lines (42a, 42b), and electrolyte discharge lines (52a, 52b). The first electrode 22 and second electrode 24 constituting the electrochemical cell 10 use gold mesh, and the diaphragm 30 uses an anion exchange membrane (Celemion AMVN, manufactured by AGC Engineering Co., Ltd.). The electrolyte (1M KHCO3 aqueous solution) stored in the electrolyte storage tanks (38a, 38b) was supplied at a rate of 10 mL / min from the electrolyte supply lines (42a, 42b) to the flow channels (32a, 32b) in the electrochemical cell 10 by the electrolyte supply pumps (40a, 40b), and the electrolyte discharged from the flow channels (32a, 32b) was returned to the electrolyte storage tanks (38a, 38b) via the electrolyte discharge lines (52a, 52b). After filling the flow channels (32a, 32b) in the electrochemical cell 10 with electrolyte in this manner, 100% He gas was supplied to the gas phase portion in the electrolyte storage tanks (38a, 38b) at a rate of 30 mL / min for 5 minutes. Subsequently, while maintaining the supply of electrolyte to the electrochemical cell 10, the power supply 70 applied a voltage that periodically reversed polarity between the first electrode 22 and the second electrode 24 to perform electrolytic treatment of the electrolyte. The electrolytic treatment conditions were to maintain a potential difference of 2.0V between the first electrode 22 and the second electrode 24 when the voltage was applied, reverse polarity every 10 seconds, and perform electrolytic treatment for 60 minutes. Immediately after the electrolytic treatment was completed, the circuit was opened. However, the supply of electrolyte to the electrochemical cell 10 was continued for 5 minutes after the end of electrolysis. After stopping the supply of electrolyte, the gaseous products accumulated in the gas phase portion of the electrolyte storage tank 38a were collected, and the gaseous products were identified using gas chromatography.

[0050] <Comparative Example 1> Electrolytic treatment was performed in the same manner as in Example 1, except that a constant voltage was applied between the first electrode 22 and the second electrode 24 (the potential difference between the first electrode 22 and the second electrode 24 was 2.0V). In Comparative Example 1, the first electrode 22 was used as the cathode electrode and the second electrode 24 as the anode electrode. The gaseous product generated on the cathode side was collected from the electrolyte storage tank 38a and identified in the same manner as in Example 1.

[0051] Figure 3 shows the change in current density over time during the electrolytic treatment of Example 1 and Comparative Example 1, and Figure 4 shows the average current density (absolute value) during the electrolytic treatment of Example 1 and Comparative Example 1. As shown in Figure 3, in Comparative Example 1, where electrolytic treatment was performed by applying a constant voltage, the current value decreased over time to 0 mA / cm². 2 In contrast to the asymptotic approach, in Example 1, which was electrolytically treated by applying a voltage with periodic polarity reversal, no decrease in the current value over time was observed. As shown in Figure 4, the average current density of Example 1 was 1 mA / cm². 2 The results were as described above, and were more than 10 times better than those in Comparative Example 1.

[0052] From the results above, it can be seen that in conventional electrolytic treatment using constant voltage application, the potential difference between the two electrodes required for electrolytic treatment is large, 0.1 mA / cm 2 While it is difficult to continue electrolytic treatment at the above current density, in this method of electrolytic treatment using voltage application with periodic polarity reversal, the potential difference between the two electrodes required for electrolytic treatment is reduced, resulting in a current density of 0.1 mA / cm². 2 Electrolytic processing can be continued at the above current density.

[0053] Figure 5 shows the gas chromatograms of the gaseous products collected during the electrolytic treatment of Example 1 and Comparative Example 1. For comparison, Figure 5 also shows the gas chromatogram of the gas collected from the gas phase portion of the electrolyte tank 38a after supplying the electrolyte to the electrochemical cell for 65 minutes without applying voltage (labeled "No Electrolysis" in the figure). From Figures 5(a) and (b), it can be seen that Example 1, which was electrolytically treated with a voltage that periodically reverses polarity, had higher peak intensities and higher production amounts of hydrogen and carbon dioxide than Comparative Example 1, which was electrolytically treated with a constant voltage. In other words, Example 1 has a higher hydrogen and carbon dioxide production rate than Comparative Example 1. In Figure 5(c), the O2 peak intensity of Example 1 and Comparative Example 1 is the same as the O2 peak intensity without electrolysis, so no clear O2 production is observed in either Example 1 or Comparative Example 1, and the O2 production rate can be judged to be very low. Note that the O2 peak in Figure 5(c) is normalized by the N2 peak intensity, which is not produced in the electrolytic reaction.

[0054] <Example 2> The electrolytic treatment was performed in the same manner as in Example 1, except that the potential difference between the first electrode 22 and the second electrode 24 when voltage was applied was set to 1.5V and the electrolytic treatment time was set to 5 minutes.

[0055] <Comparative Example 2> The electrolytic treatment was performed in the same manner as in Example 1, except that a constant voltage was applied between the first electrode 22 and the second electrode 24 (the potential difference between the first electrode 22 and the second electrode 24 was 1.5V) and the electrolytic treatment time was set to 5 minutes.

[0056] Figure 6 shows the average current density (absolute value) during the electrolytic treatment of Example 2 and Comparative Example 2. As shown in Figure 6, in Comparative Example 2, where electrolytic treatment was performed by applying a constant voltage, the average current density was 0.1 mA / cm². 2 In contrast to Example 2, where electrolytic treatment was performed by applying a voltage with periodic polarity reversal, the average current density was 0.4 mA / cm². 2 This result suggests that in conventional electrolytic treatment using constant voltage application, the potential difference between the two electrodes required for electrolytic treatment is large, 0.1 mA / cm². 2 While it is difficult to continue electrolytic processing at the above current densities, this method, which involves applying voltage with periodic polarity reversal, reduces the potential difference between the two electrodes required for electrolytic processing, and even at a low voltage of 1.5V, it can achieve a current of 0.1mA / cm². 2 Electrolytic processing can be continued at the above current density.

[0057] <Example 3> The electrolytic treatment was performed in the same manner as in Example 1, except that the potential difference between the first electrode 22 and the second electrode 24 when voltage was applied was varied within the range of 1.5V to 3.5V, and the electrolytic treatment time was set to 5 minutes.

[0058] Figure 7 shows the average current density (absolute value) during the electrolytic treatment in Example 3. As shown in Figure 7, the average current density during the electrolytic treatment can be increased by increasing the potential difference between the first electrode 22 and the second electrode 24 when voltage is applied.

[0059] <Example 4> Electrolytic treatment was performed using the electrolytic apparatus shown in Figure 8. The electrolytic apparatus 3 shown in Figure 8 comprises an electrochemical cell 10, an electrolyte storage tank 38, an electrolyte supply pump 40, an electrolyte supply line 42, and an electrolyte discharge line 52. The first electrode 22 and the second electrode 24 constituting the electrochemical cell 10 use gold mesh. In the electrochemical cell 10 shown in Figure 8, no diaphragm is installed between the first electrode 22 and the second electrode 24. The electrolyte (1M KHCO3 aqueous solution) stored in the electrolyte storage tank 38 was supplied to the flow path 32 in the electrochemical cell 10 under the same conditions as in Example 1. Then, electrolytic treatment was performed under the same conditions as in Example 1, except that the potential difference between the first electrode 22 and the second electrode 24 when voltage was applied was set to 3.0V.

[0060] <Example 5> The electrolytic treatment was performed in the same manner as in Example 1, except that the potential difference between the first electrode 22 and the second electrode 24 when voltage was applied was set to 3.0V. In other words, in Example 5, an electrochemical cell with a diaphragm placed between the first electrode 22 and the second electrode 24 was used.

[0061] Figure 9 shows the change in current density over time during the electrolytic treatment in Examples 4 and 5, and Figure 10 shows the average current density (absolute value) during the electrolytic treatment in Examples 4 and 5. As shown in Figure 9, a larger current flowed in Example 4, which used an electrochemical cell without a diaphragm, than in Example 5, which used an electrochemical cell with a diaphragm. The average current density in Example 4 was 19.5 mA / cm². 2 This was approximately 1.6 times the average current density of Example 5. From these results, it can be said that the current density during electrolytic treatment can be increased by using an electrochemical cell without a diaphragm.

[0062] Figure 11 shows the gas chromatograms of the gaseous products collected during the electrolytic treatment in Examples 4 and 5. For comparison, Figure 11 also shows the gas chromatogram of the gas collected from the gas phase portion of the electrolyte tank 38 after supplying the electrolyte to the electrochemical cell for 65 minutes without applying voltage (labeled "No Electrolysis" in the figure). As shown in Figure 11, the production of CO, H2, and O2 was confirmed in both Examples 4 and 5.

[0063] Based on the results of Examples 1-5, it is desirable to set the potential difference between the first electrode 22 and the second electrode 24 to 2.0V or less when applying voltage, in order to selectively generate CO2 and H2 by electrolytic treatment, and to set the potential difference between the first electrode 22 and the second electrode 24 to more than 2.0V when applying voltage, in order to increase the current density and selectively generate carbon dioxide reduction products such as CO and H2.

[0064] <Comparative Example 3> Electrolytic treatment was performed in the same manner as in Example 1, except that a constant voltage was applied between the first electrode 22 and the second electrode 24 (the potential difference between the first electrode 22 and the second electrode 24 was 3.0V).

[0065] Figure 12 shows the change in current density over time during the electrolytic treatment of Comparative Example 3, and Figure 13 shows the average current density (absolute value) during the electrolytic treatment of Comparative Example 3. As shown in Figure 12, in Comparative Example 3, which underwent electrolytic treatment by applying a constant voltage, the current value decreased over time. As shown in Figure 13, the average current density of Comparative Example 3 was 0.91 mA / cm². 2 The current flow in Comparative Example 3 was only 1 / 20 to 1 / 10 of that in Examples 4 and 5, which were electrolytically treated by applying a voltage with periodic polarity reversal. Conversely, by applying an electrolytic treatment with a voltage that has periodic polarity reversal, as in Examples 4 and 5, it was possible to generate 13.7 to 21.4 times more current than in Comparative Example 3.

[0066] Figure 14 shows the gas chromatogram of the gaseous product collected during the electrolytic treatment of Comparative Example 3. For comparison, Figures 14(a) to (c) show the gas chromatogram of the atmosphere collected immediately before the test of Comparative Example 3 (labeled "Atmosphere" in the figure), and for comparison, Figure 14(c) shows the gas chromatogram of the gas collected from the gas phase portion of the electrolyte storage tank 38a after supplying the electrolyte to the electrochemical cell for 65 minutes without applying voltage (labeled "No Electrolysis" in the figure). Furthermore, in Figure 14(a), the result of separating the peak attributed to hydrogen is shown by a dashed line (labeled "Comparative Example 4 (Hydrogen)" in the figure).

[0067] As shown in Figure 14(c), in the constant voltage electrolysis of Comparative Example 3, no CO was detected (on the cathode side). This is thought to be because, as electrolysis progressed, the cathode side became a high pH condition, the dissolved carbon species near the cathode became carbonate ions, and the cathode overpotential decreased due to the increase in the minimum applied voltage caused by the increase in the pH difference between the two electrodes, so CO was not generated when the potential difference between the two electrodes was 3.0V. On the other hand, as shown in Figure 11, in Examples 4 and 5, which were electrolyzed by applying a voltage with periodic polarity reversal, CO was generated when the potential difference between the two electrodes was 3.0V. This is thought to be because, by applying a voltage with polarity reversal, neutral CO2 was concentrated near the electrode that became the anode, and then, by applying a voltage with polarity reversal, it switched to the cathode, and the CO2 was electrolytically reduced, thus generating CO. [Explanation of Symbols]

[0068] 1-3 Electrolyzer, 10 Electrochemical cell, 12 Supply device, 14 Gas-liquid separator, 16 Gas-liquid contact device, 18 Circulation device, 22 First electrode, 24 Second electrode, 30 Diaphragm, 32, 32a, 32b Flow channel, 36 Frame material, 38, 38a, 38b Electrolyte storage tank, 40, 40a, 40b Electrolyte supply pump, 42, 42a, 42b Electrolyte supply line, 50 Gas-liquid separator, 52, 52a, 52b Electrolyte discharge line, 54 Circulation line, 56 Circulation pump, 58 Carbon dioxide supply line, 60 Gas-liquid contact tank, 62 Discharge line, 70 Power supply device.

Claims

1. The electrochemical cell comprises a first electrode and a second electrode, An electrolytic apparatus characterized in that, when a voltage that periodically reverses polarity is applied between the first electrode and the second electrode to electrolyze an electrolyte containing at least one of carbonate ions and bicarbonate ions, carbon dioxide (gas) is generated from the carbonate ions and bicarbonate ions at the electrode that becomes the anode of the first electrode and the second electrode, and at the electrode that is switched from the anode to the cathode by the polarity reversal, the carbon dioxide (gas) and water are reduced to produce a product containing hydrogen (gas) and at least one of the carbon dioxide reduction products.

2. The electrolytic apparatus according to claim 1, characterized in that the electrochemical cell has a diaphragm provided between the first electrode and the second electrode.

3. The electrolytic apparatus according to claim 1 or 2, characterized in that it has an electrolyte supply device for supplying the electrolyte to the electrochemical cell.

4. The electrolytic apparatus according to claim 1 or 2, characterized in that it has a gas-liquid separation device for separating the electrolyte containing the product discharged from the electrochemical cell into gas and liquid.

5. The electrolytic apparatus according to claim 4, further comprising a circulation device for circulating the electrolyte separated by the gas-liquid separation device to the electrochemical cell.

6. The electrolytic apparatus according to claim 5, further comprising a gas-liquid contact device for contacting the electrolyte circulated by the circulation device with a gas containing carbon dioxide.

7. The electrolytic apparatus according to claim 1 or 2, characterized in that the first electrode and the second electrode are metals or alloys containing at least one of Au, Pd, Pt, Ag, and Cu.

8. The electrolytic apparatus according to claim 1 or 2, characterized in that the electrolyte contains alkali metal ions.

9. The electrolytic apparatus according to claim 1 or 2, wherein the potential difference E between the first electrode and the second electrode when the voltage is applied is 0.4V or more.

10. An electrolysis method characterized by applying a voltage that periodically reverses polarity between a first electrode and a second electrode to electrolyze an electrolyte containing at least one of carbonate ions and bicarbonate ions, wherein carbon dioxide (gas) is generated from the carbonate ions and bicarbonate ions at the electrode that becomes the anode of the first electrode and the second electrode, and the carbon dioxide (gas) and water are reduced at the electrode that is switched from the anode to the cathode by the polarity reversal to produce a product containing hydrogen (gas) and at least one of the carbon dioxide reduction products.

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