Electrodialysis machine

Abstract

The present invention provides an electrodialysis apparatus capable of suppressing the increase in applied voltage that occurs as the electrodialysis reaction progresses. [Solution] The electrodialysis apparatus 10 comprises a first cation exchange membrane 32, a second cation exchange membrane 34, a second chamber 26 which is an acidification chamber defined by the first cation exchange membrane 32 and the second cation exchange membrane 34, and an electrolyte supplied to the second chamber 26. The electrolyte contains an alkali metal salt and a carbonate ion species, and the alkali metal salt contains at least one of a salt of a strong acid and a strong base containing an alkali metal ion, and a salt of a weak acid other than carbonic acid and a strong base containing an alkali metal ion. The second chamber 26 is characterized in that, by electrodialysis, protons that have moved from the first cation exchange membrane 32 react with the carbonate ion species to produce carbon dioxide (gas).

Description

Technical Field

[0001] The present invention relates to the technology of electrodialysis devices.

Background Art

[0002] As technologies that can contribute to carbon neutrality, there is interest in electrolysis devices that utilize electrolysis technologies for obtaining CO2 reduction valuable substances from carbonate ion species or carbon dioxide, and electrodialysis technologies for recovering carbon dioxide using an alkaline aqueous solution containing carbonate ion species (at least one of carbonate ions (CO3 2- ) and bicarbonate ions (HCO3 - )) and separating and concentrating carbon dioxide from the recovered liquid containing carbon dioxide.

[0003] For example, Patent Documents 1 to 6 and Non-Patent Document 1 disclose electrodialysis devices that apply a voltage to an anode electrode and a cathode electrode to separate and concentrate carbon dioxide.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Non-Patent Documents

[0005]

Non-Patent Document 1

[0006] However, in conventional electrodialysis technology, when an electrodialysis reaction occurs in which an electrolyte containing carbonate ions is supplied to an acidification chamber defined by two cation exchange membranes to generate carbon dioxide (gas) through electrodialysis, the ion conduction resistance in the acidification chamber may increase rapidly. As a result, in constant current electrolysis, the voltage between the anode and cathode electrodes (applied voltage) increases rapidly, making electrolysis impossible, for example, near the end of carbon dioxide generation (near the end of the electrodialysis reaction), and making it difficult to efficiently generate carbon dioxide.

[0007] Therefore, the present invention aims to provide an electrodialysis apparatus capable of suppressing the increase in applied voltage that occurs as the electrodialysis reaction progresses. [Means for solving the problem]

[0008] The electrodialysis apparatus according to this embodiment comprises a first cation exchange membrane, a second cation exchange membrane, an acidification chamber defined by the first cation exchange membrane and the second cation exchange membrane, and an electrolyte supplied to the acidification chamber, wherein the electrolyte contains an alkali metal salt and a carbonate ion species, and the alkali metal salt contains at least one of a salt of a strong acid and a strong base containing an alkali metal ion, and a salt of a weak acid other than carbonic acid and a strong base containing an alkali metal ion, and the acidification chamber is characterized in that, by electrodialysis, protons that have moved from the first cation exchange membrane react with the carbonate ion species to produce carbon dioxide (gas).

[0009] Furthermore, in the electrodialysis apparatus, it is preferable that the sum of the products of the limit molar ionic conductivity and concentration of the alkali metal salt cation and anion is 0.01 S / cm or greater.

[0010] Furthermore, in the electrodialysis apparatus, it is preferable that the concentration of the alkali metal salt in the electrolyte is less than or equal to the saturation concentration at the operating temperature.

[0011] Furthermore, in the electrodialysis apparatus, it is preferable that the electrolyte contains a salt of the carbonate ion species and an alkali metal ion. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide an electrodialysis apparatus that can suppress the increase in applied voltage that occurs as the electrodialysis reaction progresses. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram showing an example of an electrolytic system according to this embodiment. [Figure 2] This is a schematic diagram showing another example of the electrodialysis apparatus of this embodiment. [Figure 3] This figure shows the changes in applied voltage relative to the dialysis progress index in Comparative Examples 1 and 2. [Figure 4] This figure shows the changes in applied voltage against the dialysis progress index in Examples 1-1 to 1-3 and Comparative Example 1. [Figure 5] This figure shows the changes in current efficiency against the dialysis progress index in Examples 1-1 to 1-3 and Comparative Example 1. [Figure 6] This figure shows the change in applied voltage relative to the dialysis progress index in Example 2 and Comparative Example 3. [Figure 7] This figure shows the changes in current efficiency against the dialysis progress index in Example 2 and Comparative Example 3. [Modes for carrying out the invention]

[0014] Embodiments of the present invention will be described below. This embodiment is just one example of how the present invention can be implemented, and the present invention is not limited to this embodiment.

[0015] Figure 1 is a schematic diagram showing an example of an electrolytic system according to this embodiment. The electrolytic system 1 shown in Figure 1 comprises an electrodialysis machine 10, a gas-liquid separation device 12, an H2 supply mechanism 14, an aqueous medium supply device 15, and an electrolyte supply device 17.

[0016] The electrodialysis apparatus 10 shown in Figure 1 includes an anode electrode 22, a first chamber 24, a second chamber 26 which is an acidification chamber, a cathode electrode 28, a third chamber 30, a first cation exchange membrane 32, a second cation exchange membrane 34, and frame members 36a and 36b. The first chamber 24 is provided between the frame member 36a and the first cation exchange membrane 32, and the anode electrode 22 is positioned there. The anode electrode 22 is adjacent to the first cation exchange membrane 32. In the first chamber 24, a flow path 23 through which a gas containing H2 flows is provided between the anode electrode 22 and the frame member 36a. The third chamber 30 is provided between the frame member 36b and the second cation exchange membrane 34, and the cathode electrode 28 is positioned there. The cathode electrode 28 is adjacent to the second cation exchange membrane 34. In the third chamber 30, a flow path 29 through which an aqueous medium flows is provided between the cathode electrode 28 and the frame member 36b. The second chamber 26 is located between the first cation exchange membrane 32 and the second cation exchange membrane 34. That is, the second chamber 26 is defined by these membranes. The first cation exchange membrane 32 is positioned between the first chamber 24 and the second chamber 26, and the second cation exchange membrane 34 is positioned between the second chamber 26 and the third chamber 30. The second chamber 26 serves as a flow path for the electrolyte solution, which will be described later. In the electrodialysis apparatus 10 shown in Figure 1, the anode electrode 22, cathode electrode 28, cation exchange membrane, etc., are structurally supported by frame materials 36a and 36b. The frame materials 36a and 36b can be made of metal, plastic, glass, etc.

[0017] The anode electrode 22 includes an anode catalyst and is an electrode that generates protons, for example, by applying a voltage. The cathode electrode 28 includes a cathode catalyst and is an electrode that generates H2 and hydroxide ions (OH) by the reductive electrolysis of H2O, for example. - This is an electrode that generates ).

[0018] The gas-liquid separation device 12 comprises, for example, a gas-liquid separator 16 and discharge lines 18a and 18b. One end of discharge line 18a is connected to the flow path 29 of the third chamber 30, and the other end is connected to the gas-liquid separator 16. Discharge line 18b is connected to the liquid outlet of the gas-liquid separator 16. The gas-liquid separator 16 is, for example, a conventionally known device capable of separating gas and liquid.

[0019] The H2 supply mechanism 14 shown in Figure 1 is a supply line for supplying gas containing H2 to the first chamber 24. For example, one end of the supply line is connected to the gas outlet of the gas-liquid separator 16, and the other end is connected to the flow path 23 of the first chamber 24. The H2 supply mechanism 14 may also include, for example, a pump to adjust the amount of gas containing H2 supplied, or it may include a gas cylinder filled with H2 and a blower to supply air in order to stably supply gas containing H2. In the following, the H2 supply mechanism 14 will be described as the supply line 14.

[0020] The aqueous medium supply device 15 is a device that supplies aqueous medium to the third chamber 30, and includes, for example, an aqueous medium supply line 40 and a supply pump 42a installed in the aqueous medium supply line 40. The aqueous medium supply line 40 is connected to the flow path 29 of the third chamber 30.

[0021] The electrolyte supply device 17 is a device that supplies electrolyte to the second chamber 26, and includes, for example, an electrolyte supply line 44 and a supply pump 42b installed on the electrolyte supply line 44. The electrolyte supply line 44 is connected to the second chamber 26.

[0022] The electrolyte supplied to chamber 26 is an electrolyte containing alkali metal salts and carbonate ion species. Here, carbonate ion species refers to carbonate ions (CO3 2-) and bicarbonate ions (HCO3) - Alkali metal salts include at least one of the following: salts of a strong acid and a strong base containing alkali metal ions, and salts of a weak acid other than carbonic acid and a strong base containing alkali metal ions.

[0023] Examples of strong acids that form salts with strong bases containing alkali metal ions include sulfuric acid, nitric acid, hydrogen chloride, hydrogen bromide, and hydrogen iodide. Examples of strong bases containing alkali metal ions that form salts with strong acids include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide. Among the salts formed from such strong acids and strong bases containing alkali metal ions, potassium sulfate (K2SO4) and potassium chloride (KCl) are preferred due to their high solubility in water.

[0024] Examples of weak acids that form salts with strong bases containing alkali metal ions include carboxylic acids such as formic acid, acetic acid, and propionic acid, as well as phosphoric acid and oxalic acid. Examples of strong bases containing alkali metal ions that form salts with weak acids other than carbonic acid include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide. Among the salts formed from such weak acids other than carbonic acid and strong bases containing alkali metal ions, dipotassium phosphate (K2HPO4) is preferred due to its high solubility in water.

[0025] The carbonate ion species is preferably a salt with an alkali metal ion, such as alkali metal bicarbonates or alkali metal carbonates like lithium bicarbonate (LiHCO3), sodium bicarbonate (NaHCO3), potassium bicarbonate (KHCO3), sodium carbonate (Na2CO3), potassium carbonate (K2CO3), cesium bicarbonate (CsHCO3), and rubidium bicarbonate (RhHCO3).

[0026] In Figure 1, reference numeral 70 denotes a power source that applies a voltage between the anode electrode 22 and the cathode electrode 28. The power source 70 is not particularly limited and may include a chemical battery (including primary batteries, secondary batteries, etc.), a constant voltage source, a solar cell, etc.

[0027] Next, we will describe an example of the operation of the electrolysis system 1 shown in Figure 1.

[0028] A gas containing H2 is supplied from the supply line 14 to the flow path 23 of the first chamber 24. The gas containing H2 is, for example, a mixture of air and hydrogen gas. Alternatively, the gas containing H2 may be humidified by a humidifier before being supplied to the flow path 23 of the first chamber 24, and supplied to the flow path 23 of the first chamber 24 as a humidified gas containing moisture. Furthermore, the aforementioned electrolyte is supplied to the second chamber 26 from the electrolyte supply line 44 at a predetermined flow rate by the operation of the supply pump 42b. In addition, an aqueous medium is supplied to the flow path 29 of the third chamber 30 from the aqueous medium supply line 40 at a predetermined flow rate by the operation of the supply pump 42a.

[0029] When a voltage is applied between the cathode electrode 28 and the anode electrode 22 by the power supply 70, on the first chamber 24 side, when the gas containing H2 passing through the flow path 23 comes into contact with the anode electrode 22, for example, the H2 in the gas is oxidized and protons (H) are formed. + ) is produced. Also, water in the gas is oxidized to produce oxygen (O2) and protons (H + ) and are generated. Then, by electrodialysis, the protons generated at the anode electrode 22 pass through the first cation exchange membrane 32 and are supplied to the second chamber 26. In the second chamber 26, by electrodialysis, the protons that have moved to the second chamber 26 side through the first cation exchange membrane 32 react with carbonate ions and bicarbonate ions in the electrolyte to generate carbon dioxide (gas: neutral CO2 molecules). Alkali metal ions in the electrolyte passing through the second chamber 26 pass through the second cation exchange membrane 34 and are supplied to the third chamber 30. On the third chamber 30 side, when the aqueous medium passing through the channel 29 comes into contact with the cathode electrode 28, for example, H2O is reduced to H2 and hydroxide ions (OH) -) is generated, and further reacts with the alkali metal ions that have moved to the third chamber 30 side through the hydroxide ions and the second cation exchange membrane 34 to generate an aqueous alkali hydroxide solution.

[0030] The carbon dioxide generated in the second chamber 26 is discharged from the second chamber 26 together with, for example, the electrolytic solution supplied to the second chamber 26, and is separated into a gas containing carbon dioxide and a liquid by, for example, a gas-liquid separator (not shown), and each is recovered.

[0031] The H2 and the aqueous alkali hydroxide solution generated in the third chamber 30 are discharged from the third chamber 30 as a discharge liquid and supplied to the gas-liquid separator 16 through the discharge line 18a. In the gas-liquid separator 16, the discharge liquid is separated into a gas containing H2 and an aqueous alkali hydroxide solution, the aqueous alkali hydroxide solution is discharged to the discharge line 18b, and the gas containing H2 is supplied to the flow path 23 of the first chamber 24 from the supply line 14.

[0032] In conventional electrodialysis, for example, an alkali metal bicarbonate or an alkali metal carbonate that does not contain the aforementioned alkali metal salt is supplied to the acidification chamber (second chamber). However, when the electrodialysis reaction proceeds and approaches the end point of the generation of carbon dioxide (gas), there is almost no electrolyte in the acidification chamber (second chamber). For example, when an aqueous solution of K2CO3 is supplied to the acidification chamber, in the acidification chamber, [2K + +CO3 2- +2H + →[2H + +CO3 2- +2K + results in the generation of [H2O + CO2] from [2H + +CO3 2- , and [2K +The electrolyte moves from the acidification chamber to other chambers via the cation exchange membrane. Therefore, near the end of carbon dioxide (gas) production, the electrolyte is almost completely gone from the acidification chamber. As a result, the ion conduction resistance in the acidification chamber increases rapidly, causing the applied voltage between the two electrodes in constant current electrolysis to rise. However, as in this embodiment, because the electrolyte contains the aforementioned alkali metal salt, even as the electrodialysis reaction progresses, the electrolyte (the aforementioned alkali metal salt) remains in the second chamber 26, which is the acidification chamber. This suppresses the rapid increase in ion conduction resistance in the second chamber 26, and thus suppresses the increase in the applied voltage between the two electrodes in constant current electrolysis. Furthermore, even near the end of carbon dioxide (gas) production, the electrolyte remains in the second chamber 26, allowing electrolysis to continue.

[0033] The electrodialysis apparatus of this embodiment is not limited to the configuration of the electrodialysis apparatus 10 shown in Figure 1. As long as it has a second chamber 26 that serves as an acidification chamber, partitioned by a first cation exchange membrane 32 and a second cation exchange membrane 34, the position of the electrodes and the number and type of chambers defined by the ion exchange membranes are not particularly limited. Below, an electrodialysis apparatus having two acidification chambers will be described as another example of the electrodialysis apparatus of this embodiment.

[0034] Figure 2 is a schematic diagram showing another example of the electrodialysis apparatus of this embodiment. In the electrodialysis apparatus 11 shown in Figure 2, components similar to those in the electrodialysis apparatus 10 shown in Figure 1 are denoted by the same reference numerals, and their descriptions are omitted. The electrodialysis apparatus 11 shown in Figure 2 has a plurality of chambers partitioned by ion exchange membranes between the first chamber 24 and the third chamber 30. Specifically, the electrodialysis apparatus 11 includes a second chamber 26a (first acidification chamber) provided between the first cation exchange membrane 32a and the second cation exchange membrane 34a and defined by these membranes, an alkalizing chamber 37 provided between the second cation exchange membrane 34a and the anion exchange membrane 33 constituting the bipolar membrane 35 and defined by these membranes, and a second chamber 26b (second acidification chamber) provided between the cation exchange membrane 32b and the cation exchange membrane 34b constituting the bipolar membrane 35 and defined by these membranes.

[0035] An example of the operation of the electrodialysis apparatus 11 shown in Figure 2 will be described. A gas containing H2 is supplied to the flow path 23 of the first chamber 24, the aforementioned electrolyte is supplied to the second chambers 26a and 26b, and an aqueous medium is supplied to the flow paths 29 of the alkalizing chamber 37 and the third chamber 30. When a voltage is applied between the cathode electrode 28 and the anode electrode 22 by the power supply 70, in the first chamber 24, for example, the H2 in the gas is oxidized by the anode electrode 22 to form protons (H + ) are generated, or water in the gas is oxidized, producing oxygen (O2) and protons (H + ) are generated. Protons are supplied to the second chamber 26a by passing through the first cation exchange membrane 32a. In the second chamber 26a, protons react with carbonate ion species in the electrolyte to produce carbon dioxide (gas). Similarly in the second chamber 26b, protons supplied to the second chamber 26b from the alkalizing chamber 27 by passing through the bipolar membrane 35 react with carbonate ion species in the electrolyte to produce carbon dioxide (gas). In the alkalizing chamber 37, alkali metal ions supplied to the alkalizing chamber from the second chamber 26a by passing through the second cation exchange membrane 34a react with hydroxide ions supplied to the alkalizing chamber 37 from the second chamber 26b by passing through the bipolar membrane 35 to produce an alkali hydroxide aqueous solution. In the third chamber 30, the cathode electrode 28 reduces H2O in the aqueous medium passing through the channel 29 to produce H2 and hydroxide ions (OH - ) is generated. Then, in the third chamber 30, hydroxide ions react with alkali metal ions that have moved from the second chamber 26b to the third chamber 30 through the cation exchange membrane 34b to produce an alkali hydroxide aqueous solution. The carbon dioxide (gas) and alkali hydroxide aqueous solution discharged from each chamber are recovered, for example, via a gas-liquid separator as described above.

[0036] In the electrodialysis apparatus 11 shown in Figure 2, the electrolyte containing the aforementioned alkali metal salt and carbonate ion species is supplied to the second chambers 26a and 26b, which are acidification chambers. As a result, even as the electrodialysis reaction progresses, a rapid increase in the ion conduction resistance in the second chambers 26a and 26b is suppressed, and the increase in the applied voltage between the two electrodes during constant current electrolysis is suppressed.

[0037] The following describes in detail the first cation exchange membrane 32 (32a), the second cation exchange membrane 34 (34a), the anode electrode 22, the cathode electrode 28, the electrolyte, the gas containing H2, etc.

[0038] The first cation exchange membrane 32 (32a) and the second cation exchange membrane 34 (34a) can be conventionally known membranes, but perfluorosulfonic acid polymer membranes such as Nafion or Fremion are preferred in terms of hydrogen ion conductivity, alkali metal ion conductivity, and water permeability, for example. The same applies to the cation exchange membrane 34b used in the electrodialysis apparatus 11 in Figure 2.

[0039] Of the bipolar membranes 35 used in the electrodialysis apparatus 11 shown in Figure 2, the cation exchange membrane 32b can be the same as the first cation exchange membrane 32 and the second cation exchange membrane 34 described above. In addition, of the bipolar membranes 35, the anion exchange membrane 33 can be a conventionally known one, such as Neosepta, Selemion, Sustenion, etc.

[0040] The anode electrode 22 is an oxidation electrode that generates protons, for example, by applying a voltage. The anode electrode 22 is preferably in contact with the first cation exchange membrane 32 (32a), as shown in Figures 1 and 2, and it is more preferable that the anode electrode 22 and the first cation exchange membrane 32 (32a) are joined together. This keeps the resistance low when the protons generated at the anode electrode 22 move to the second chamber 26 (26a), which is an acidification chamber.

[0041] The anode catalyst is, for example, a catalyst that promotes proton generation, and is preferably a catalyst containing at least one element selected from the group consisting of Pt, Pd, Rh, Re, Au, Ni, Ir, and Ru. Specifically, it may be a metal of Pt, Pd, Rh, Re, Au, Ni, Ir, or Ru, an alloy or compound containing such a metal, etc. The compound containing such a metal may be an oxide, hydroxide or complex, etc. The anode catalyst may be a single type or two or more types may be used in combination.

[0042] The anode catalyst content is, for example, 0.35 mg / cm² per geometric area of ​​the anode electrode 22, which can reduce the energy consumption of the electrodialysis machine. 2 It is preferable that the above is true. If the anode catalyst content is too high, it may lead to an increase in the cost of the electrodialysis machine, so the upper limit of the anode catalyst content is 1.5 mg / cm² per geometric area of ​​the anode electrode 22. 2 The following is preferable:

[0043] The anode electrode 22 may contain a polymer. The polymer functions, for example, as a binder to increase the bonding strength between the anode electrode 22 and the first cation exchange membrane 32 (32a). Examples of polymers include cation exchange resins and anion exchange resins, but cation exchange resins are preferred in terms of bonding strength and proton conductivity, and perfluorosulfonic acid ions such as Nafion® (manufactured by DuPont) and Flemion (manufactured by Asahi Glass Co., Ltd.) are particularly preferred.

[0044] The polymer content is, for example, 0.06 mg / cm² per unit area of ​​the anode electrode 22, in order to increase the bonding strength between the anode electrode 22 and the first cation exchange membrane 32 (32a). 2 It is preferable that the polymer content is as described above. Furthermore, if the polymer content becomes too high, the conductivity of the anode electrode 22 may decrease, which may lead to a decrease in catalytic activity. Therefore, the upper limit of the polymer content should be 0.20 mg / cm² per unit area of ​​the anode electrode 22. 2 The following is preferable:

[0045] The anode electrode 22 may contain a conductive material. Examples of conductive materials include carbon materials such as carbon black, activated carbon, fullerene, carbon nanotubes, graphene, Ketjenblack, and diamond; transparent conductive oxides such as indium tin oxide, fluorine-doped tin oxide, aluminum-doped zinc oxide, and antimony-doped tin oxide; metals such as copper, aluminum, titanium, silver, and tungsten; or alloys containing at least one of these metals.

[0046] An example of a method for fabricating the anode electrode 22 is described below. By applying a catalyst ink containing an anode catalyst, polymer, etc., onto a PTFE substrate, a porous sheet-like anode electrode 22 can be fabricated. Furthermore, by placing the fabricated anode electrode 22 on the first cation exchange membrane 32 (32a) and heat-pressing them together, the anode electrode 22 and the first cation exchange membrane 32 (32a) can be joined.

[0047] A diffusion layer may be installed in the first chamber 24 to enhance the diffusivity of the gas containing H2. The diffusion layer may be made of, for example, a porous metal body or metal mesh made of titanium, titanium alloy, or stainless steel, or a porous conductive substrate such as carbon paper.

[0048] The H2-containing gas supplied to the first chamber 24 is, for example, a mixture of air or an inert gas (e.g., nitrogen gas, noble gas, etc.) and hydrogen gas, and is preferably a humidified gas. The hydrogen gas concentration in the gas may be, for example, 1% or more and 20% or less. Furthermore, the degree of humidification of the humidified gas is preferably such that the moisture content is higher than that of air at 25°C and 50% humidity. Specifically, the moisture content in the humidified gas is 11.5 g / m³. 3 It is preferable that the above conditions are met.

[0049] The fluid supplied to the first chamber 24 is not limited to a gas containing H2, but may also be a humidified gas obtained by humidifying air or an inert gas (e.g., nitrogen gas, noble gas, etc.), or an aqueous medium. The aqueous medium may be water, pure water, or an aqueous solution containing an electrolyte or a water-soluble hydrogen-containing compound. Examples of electrolytes include sodium bicarbonate (NaHCO3), potassium bicarbonate (KHCO3), potassium carbonate (K2CO3), potassium sulfate (K2SO4), potassium tetraborate (K2B4O7), dipotassium hydrogen phosphate (K2HPO4), potassium dihydrogen phosphate (KH2PO4), potassium hydroxide (KOH), sulfuric acid (H2SO4), and hydrochloric acid (HCl). Examples of water-soluble hydrogen-containing compounds include methanol (CH3OH), ethanol (C2H5OH), and hydrazine (N2H4).

[0050] As shown in Figures 1 and 2, the cathode electrode 28 is preferably in contact with the second cation exchange membrane 34 (cation exchange membrane 34b), and it is preferable that the cathode electrode 28 and the second cation exchange membrane 34 (cation exchange membrane 34b) are joined together. This makes it possible to minimize the influence of the ion conduction resistance of the aqueous medium flowing through the third chamber 30.

[0051] The cathode catalyst is not particularly limited as long as it is a catalyst that promotes the electrolytic reduction of H2O, but it is preferably a catalyst that contains at least one element selected from the group consisting of Pt, Pd, Rh, Re, Ni, Co, and Mo, and specifically may be the metals Pt, Pd, Rh, Re, Ni, Co, or Mo, or alloys or compounds containing such metals. The compounds containing such metals may be oxides, hydroxides, or complexes. The cathode catalyst may be used alone or in combination of two or more types.

[0052] The cathode catalyst content is 0.35 mg / cm² per geometric area of ​​the cathode electrode 28, which can reduce the energy consumption of the electrodialysis machine, for example. 2 It is preferable that the above is true. If the cathode catalyst content is too high, it may lead to an increase in the cost of the electrodialysis machine 10, so the upper limit of the cathode catalyst content is 1.5 mg / cm² per geometric area of ​​the cathode electrode 28. 2 The following is preferable:

[0053] An example of a method for fabricating a cathode electrode 28 is described below. A porous cathode electrode 28 can be fabricated by supporting a cathode catalyst on a porous conductive substrate such as carbon paper using a vapor deposition method such as sputtering. Furthermore, the fabricated cathode electrode 28 and the second cation exchange membrane 34 (cation exchange membrane 34b) can be joined by placing the fabricated cathode electrode 28 on the second cation exchange membrane 34 (cation exchange membrane 34b) and heat pressing them together.

[0054] In the electrolyte containing alkali metal salts and carbonate ion species supplied to the second chamber 26 (26a, 26b), the sum of the products of the limit molar ionic conductivity and concentration of the alkali metal salt cation and anion is preferably 0.01 S / cm or higher, and more preferably 0.015 S / cm or higher, in order to further suppress the increase in applied voltage accompanying the progress of the electrodialysis reaction. Furthermore, the concentration of alkali metal salts in the electrolyte containing alkali metal salts and carbonate ion species is preferably below the saturation concentration at the operating temperature in order to suppress salt precipitation within the electrodialysis apparatus. The operating temperature is the temperature of the electrolyte during electrodialysis.

[0055] Furthermore, the electrolyte containing alkali metal salts and carbonate ion species may also contain alcohols such as methanol, ethanol, and acetone. Additionally, the electrolyte containing alkali metal salts and carbonate ion species 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.

[0056] The aqueous medium supplied to the third chamber 30 and the alkalizing chamber 37 may be, for example, water, pure water, or an aqueous solution containing an electrolyte. Examples of electrolytes include sodium bicarbonate, potassium bicarbonate, potassium carbonate, potassium sulfate, potassium tetraborate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, and potassium hydroxide. [Examples]

[0057] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0058] <Comparative Examples 1, 2> An electrodialysis test was conducted using the electrodialysis apparatus shown in Figure 1. A Nafion N-115 membrane was used as the first cation exchange membrane. A loading of 1.31 mg / cm³ was applied to one side of the first cation exchange membrane. 2An anode electrode was fabricated by supporting IrO2. A Nafion N-115 film was used for the second cation exchange membrane. For the cathode electrode, a 250 nm layer of Pt was deposited on one side of porous carbon paper (TGP-H-060, manufactured by Toray Industries, Inc.) by sputtering. This cathode electrode was placed on one side of the second cation exchange membrane. The geometric area of ​​the cathode electrode is 2.5 cm × 2.5 cm. The cross-sectional area of ​​the flow channel in the second chamber is 0.06 cm². 2 The dimensions were set to (0.3cm × 0.2cm). The cross-sectional area of ​​the flow path in the first chamber is 0.065cm². 2 I set it to that.

[0059] During the electrodialysis test, humidified Ar gas was supplied to the flow path of the first chamber at an average flow rate of 30 cm³. 3 The supply was at / min, and in the second chamber, which is the acidification chamber, 0.136 mol / L of K was used in Comparative Example 1. + A 400 ppm (total pressure 1 bar) CO2 equilibrium solution (electrolyte containing 0.0355 mol / L K2CO3 + 0.065 mol / L KHCO3) containing the above was used in Comparative Example 2, while 0.236 mol / L K + A 400 ppm (total pressure 1 bar) CO2 equilibrium solution (electrolyte containing 0.072 mol / L K2CO3 + 0.092 mol / L KHCO3) containing the above was poured at an average flow rate of 0.25-2 cm. 3 The supply was at a rate of / min. In addition, an aqueous solution (aqueous medium) containing 0.0328 mol / L K2CO3 + 0.0343 mol / L KHCO3 was supplied to the third chamber's flow path at an average flow rate of 1 cm 3 The current was supplied at / min. In the electrodialysis test, the current density between the two electrodes was 8-40 mA / cm². 2 A constant current was applied. After more than 30 minutes had elapsed since the start of the electrodialysis test, the effluent discharged from the second chamber was collected, and the K contained in the fluid was measured. + The concentration was measured. K + The concentration is measured by K + The test was performed using a concentration meter (manufactured by Toa DKK). The K2CO3 aqueous solution supplied to the second chamber before the electrodialysis test was measured. +The concentration was also measured. Then, the voltage (applied voltage) of the electrodialysis machine during the electrodialysis test was measured, and the change in applied voltage with respect to the dialysis progress index defined below was investigated. The dialysis progress index is defined as K under ideal conditions with 100% current efficiency. + Concentration change |ΔC 100% | and the K in the liquid supplied to the second chamber + Concentration C0(K + ) ratio (P=|ΔC 100% | / C0(K + )) is defined as follows. P=0 means the fluid delivery and current conditions under which there is no dialysis reaction, and P=1 means the fluid delivery and current conditions under which the dialysis reaction is completed when the current efficiency is 100% (the fluid supplied to the second chamber (i.e., the CO2 equilibrium solution of K2CO3 + KHCO3 or the equilibrium solution with K2SO4 added) reaches H2O + CO2).

[0060] Figure 3 shows the changes in applied voltage against the dialysis progress index in Comparative Examples 1 and 2. As shown in Figure 3, 0.136 mol / L K2 is applied to the second chamber. + Comparative Example 1, in which a CO2 equilibrium solution containing was supplied, and 0.236 mol / L K + In all of Comparative Example 2, where a CO2 equilibrium solution containing was supplied, under conditions where the dialysis progress index was high, such as when the equilibrium CO2 partial pressure was 1 bar or more with 100% current efficiency, the applied voltage increased rapidly, and electrolysis became impossible when the dialysis progress index was near 1 (near the end point of CO2 (gas) generation). Note that CO2 (gas) is significantly generated under conditions where the equilibrium CO2 partial pressure is 1 bar or more. The dialysis progress index at which the equilibrium CO2 partial pressure is 1 bar or more with 100% current efficiency is 0.136 mol / L K + For a CO2 equilibrium solution containing [the specified substance], the K2 is 0.511 or higher, and the K2 is 0.236 mol / L. + In the case of a CO2 equilibrium solution containing [the specified substance], the value is 0.477 or higher.

[0061] <Example 1-1> The electrolyte supplied to the second chamber, which is the acidification chamber, is 0.136 mol / L K. +An electrodialysis test was conducted under the same conditions as in Comparative Example 1, except that an aqueous solution was used, which was prepared by adding 0.1 mol / L K2SO4 to a 400 ppm (total pressure 1 bar) CO2 equilibrium solution containing [the specified substance]. The changes in applied voltage and current efficiency with respect to the dialysis progress index were investigated. Current efficiency η (%) is calculated by comparing the K2SO4 before and after the electrodialysis test. + The concentration change ΔC (mol / L) was calculated using the following formula. (|ΔC|×V2 / 60 / 1000) / (I / F)×100 V2: Average flow rate of K2CO3 aqueous solution in the second chamber [cm²] 3 / min] I: Current [A] F: Faraday constant, 96485 C / mol

[0062] In Example 1-1, the sum of the products of the limiting molar ionic conductivity and concentration of the cation and anion of K2SO4 is 0.0154 S / cm.

[0063] <Examples 1-2> The electrolyte supplied to the second chamber, which is the acidification chamber, is 0.136 mol / L K. + An electrodialysis test was conducted under the same conditions as in Comparative Example 1, except that an aqueous solution was used prepared by adding 0.1 mol / L of KCl to a 400 ppm (total pressure 1 bar) CO2 equilibrium solution containing the specified substance. The changes in applied voltage and current efficiency with respect to the dialysis progress index were then investigated. The sum of the products of the limiting molar ionic conductivity and concentration of KCl cations and anions in Examples 1-2 was 0.0150 S / cm.

[0064] <Examples 1-3> The electrolyte supplied to the second chamber, which is the acidification chamber, is 0.136 mol / L K. + Electrodialysis tests were conducted under the same conditions as in Comparative Example 1, except that an aqueous solution was used prepared by adding 0.1 mol / L of K2HPO4 to a 400 ppm (total pressure 1 bar) CO2 equilibrium solution containing [the specified substance], and the changes in applied voltage and current efficiency with respect to the dialysis progress index were investigated. The sum of the products of the limiting molar ionic conductivity and concentration of the cations and anions of K2HPO4 in Examples 1-3 was 0.0131 S / cm.

[0065] Figure 4 shows the change in applied voltage with respect to the dialysis progress index in Examples 1-1 to 1-3 and Comparative Example 1, and Figure 5 shows the change in current efficiency with respect to the dialysis progress index in Examples 1-1 to 1-3 and Comparative Example 1. As shown in Figure 5, in Examples 1-1 to 1-3, even under conditions where the dialysis progress index is high (0.511 or higher) such that the equilibrium CO2 partial pressure is 1 bar or higher with a current efficiency of 100%, the rapid increase in applied voltage seen in Comparative Example 1 did not occur, and CO2 (gas) generation was confirmed even when the dialysis progress index was near 1 (near the end point of CO2 (gas) generation). From this, K + It can be said that by adding a salt of a strong acid such as K2SO4 or KCl and a strong base containing alkali metal ions, or a salt of a weak acid other than carbonic acid such as K2HPO4 and a strong base containing alkali metal ions, to a CO2 equilibrium solution containing K, the increase in applied voltage accompanying the progress of the electrodialysis reaction can be suppressed. + This effect cannot be obtained from Comparative Example 2, which uses a higher concentration.

[0066] Among Examples 1-1 to 1-3, K + Examples 1-1 and 1-2, which used an electrolyte solution to which K2SO4 or KCl was added to a CO2 equilibrium solution containing K, + Compared to Examples 1-3, which used an electrolyte solution in which K2HPO4 was added to a CO2 equilibrium solution containing , the applied voltage was lower and the current efficiency was higher. In the case of K2SO4 or KCl, Cl - ya 1 / 2SO4 2- The limit molar ionic conductivity (Cl - 76.34S·cm 2 / mol, 1 / 2SO4 2- 79.8S·cm 2 ( / mol) is 1 / 2HPO4 in K2HPO4 2- The limit molar ionic conductivity (57.57 S·cm) 2It is higher than ( / mol). Therefore, it is thought that the applied voltage decreased in the electrolyte with K2SO4 or KCl added compared to the electrolyte with K2HPO4 added. Also, K2HPO4 exhibits buffering properties, while K2SO4 and KCl exhibit almost no buffering properties, so K2SO4 and KCl are more H + It is thought that the generation of CO2 (gas) due to the supply was not suppressed, resulting in high current efficiency. Therefore, alkali metal salts that exhibit little buffering effect and high intrinsic molar ionic conductivity are preferred, and specifically, salts of a strong acid such as K2SO4 or KCl and a strong base containing alkali metal ions are preferred.

[0067] <Example 2> In the third chamber, an aqueous solution (aqueous medium) containing 0.0328 mol / L K2CO3 + 0.0343 mol / L KHCO3 was introduced into the flow path at an average flow rate of 15 cm⁻¹. 3 Except for supplying at / min, the electrodialysis test was conducted under the same conditions as in Example 1-1, and the changes in applied voltage and current efficiency with respect to the dialysis progress index were investigated.

[0068] <Comparative Example 3> In the third chamber, an aqueous solution (aqueous medium) containing 0.0328 mol / L K2CO3 + 0.0343 mol / L KHCO3 was introduced into the flow path at an average flow rate of 15 cm⁻¹. 3 Except for supplying at / min, the electrodialysis test was conducted under the same conditions as in Comparative Example 1, and the changes in applied voltage and current efficiency with respect to the dialysis progress index were investigated.

[0069] Figure 6 shows the changes in applied voltage relative to the dialysis progress index in Example 2 and Comparative Example 3, and Figure 7 shows the changes in current efficiency relative to the dialysis progress index in Example 2 and Comparative Example 3. As shown in Figure 6, Example 2 showed a lower applied voltage than Comparative Example 3 under conditions where the dialysis progress index was high (0.511 or higher) and the equilibrium CO2 partial pressure was 1 bar or higher at 100% current efficiency. From this, the average flow rate of the aqueous medium supplied to the third chamber was set to 15 cm³. 3 Even if set to / min, the average flow rate K +It can be said that by adding K2SO4 to the CO2 equilibrium solution containing [the specified substance], the increase in applied voltage accompanying the progression of the electrodialysis reaction can be suppressed.

[0070] "Note": Structure of the present invention Configuration 1: An electrodialysis apparatus comprising a first cation exchange membrane, a second cation exchange membrane, an acidification chamber defined by the first cation exchange membrane and the second cation exchange membrane, and an electrolyte supplied to the acidification chamber, The electrolyte comprises an alkali metal salt and a carbonate ion species, wherein the alkali metal salt comprises at least one of the following: a salt of a strong acid and a strong base containing an alkali metal ion, and a salt of a weak acid other than carbonate and a strong base containing an alkali metal ion. The acidification chamber is characterized in that, by electrodialysis, protons that have moved from the first cation exchange membrane react with the carbonate ion species to produce carbon dioxide (gas). Configuration 2: The electrodialysis apparatus according to configuration 1, characterized in that the sum of the products of the limit molar ionic conductivity and concentration of the alkali metal salt, for both the cation and anion, is 0.01 S / cm or more. Configuration 3: The electrodialysis apparatus according to configuration 1 or 2, characterized in that the concentration of the alkali metal salt in the electrolyte is less than or equal to the saturation concentration at the operating temperature. Configuration 4: The electrodialysis apparatus according to any one of configurations 1 to 3, characterized in that the electrolyte contains a salt of the carbonate ion species and an alkali metal ion. [Explanation of symbols]

[0071] 1 Electrolysis system, 10,11 Electrodialysis machine, 12 Gas-liquid separator, 14 H2 supply mechanism (supply line), 15 Aqueous medium supply device, 16 Gas-liquid separator, 17 Electrolyte supply device, 18a,18b Discharge line, 22 Anode electrode, 23,29 Flow path, 24 First chamber, 26,26a,26b Second chamber, 27 Alkalization chamber, 28 Cathode electrode, 30 Third chamber, 32,32a First cation exchange membrane, 32b,34b Cation exchange membrane, 33 Anion exchange membrane, 34,34a Second cation exchange membrane, 35 Bipolar membrane, 36a,36b Frame material, 37 Alkalization chamber, 40 Aqueous medium supply line, 42a,42b Supply pump, 44 Electrolyte supply line, 70 Power supply.

Claims

1. An electrodialysis apparatus comprising a first cation exchange membrane, a second cation exchange membrane, an acidification chamber defined by the first cation exchange membrane and the second cation exchange membrane, and an electrolyte supplied to the acidification chamber, The electrolyte comprises an alkali metal salt and a carbonate ion species, wherein the alkali metal salt comprises at least one of the following: a salt of a strong acid and a strong base containing an alkali metal ion, and a salt of a weak acid other than carbonate and a strong base containing an alkali metal ion. The acidification chamber is characterized in that, by electrodialysis, protons that have moved from the first cation exchange membrane react with the carbonate ion species to produce carbon dioxide (gas).

2. The electrodialysis apparatus according to claim 1, characterized in that the sum of the products of the limit molar ionic conductivity and concentration of the alkali metal salt, for both the cation and anion, is 0.01 S / cm or more.

3. The electrodialysis apparatus according to claim 1 or 2, characterized in that the concentration of the alkali metal salt in the electrolyte is less than or equal to the saturation concentration at the operating temperature.

4. The electrodialysis apparatus according to claim 1 or 2, characterized in that the electrolyte contains a salt of the carbonate ion species and an alkali metal ion.

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