Electrochemical reactors and artificial photosynthesizers

Abstract

To provide an electrochemical reactor comprising an electrode for reduction reaction and an electrode for oxidation reaction, in which deterioration in characteristics can be suppressed, stable operation can be maintained, energy consumption in recovering generated organic acids can be suppressed, and an amount of recovered organic acids can be improved.SOLUTION: An electrochemical reactor 3 comprises: an electrochemical reaction cell 10 in which an electrode 16 for reduction reaction containing a reduction catalyst and an electrode 18 for oxidation reaction containing an oxidation catalyst are arranged in a container 20 and which generates organic acids by reducing a reaction substrate; a tank 12 into which an electrolytic solution obtained by dissolving a salt having a pka of 6 to 8, a neutral salt, or both thereof is stored; electrolytic solution supply means which supplies the electrolytic solution from the tank 12 to the container 20; and a liquid-liquid extraction device 14 which extracts an organic acid into an organic solvent containing at least one of a secondary amine and a tertiary amine from an electrolytic solution which is discharged from the container 20 and in which an organic acid produced by reduction of a reaction substrate is dissolved.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrochemical reactor including an electrode for a reduction reaction and an electrode for an oxidation reaction, and an artificial photosynthesis device including the electrochemical reactor. [Background technology]

[0002] Artificial photosynthesis, which uses solar energy to synthesize formic acid (HCOOH), carbon monoxide (CO), etc. from water (H2O) and carbon dioxide (CO2), is expected to be one of the technologies for reducing carbon dioxide emissions to prevent global warming, and research into this technology is being conducted very actively. In the artificial photosynthetic reaction, water is oxidized at the oxidation electrode, supplying electrons and protons, which produce oxygen (O2). Carbon dioxide is reduced at the reduction electrode using these electrons and protons.

[0003] For example, Non-Patent Documents 1 and 2 describe a large-scale artificial photosynthesis system that produces formic acid by pumping an electrolyte solution, in which carbon dioxide is saturated and dissolved in a phosphate buffer solution, in a tank to an artificial photosynthesis cell equipped with an anode (electrode for oxidation reactions) and a cathode (electrode for reduction reactions). In this large-scale artificial photosynthesis system, the electrolyte solution with dissolved formic acid is discharged from the artificial photosynthesis cell, returned to the tank, and then supplied again to the artificial photosynthesis cell, in a circulating system.

[0004] Patent Document 1 describes a reduction product production system that has an oxidation reaction electrolytic cell equipped with an oxidation catalyst and a reduction reaction electrolytic cell equipped with a reduction catalyst, and supplies an electrolyte solution in which carbon dioxide is dissolved to the reduction reaction electrolytic cell to produce alcohol by reducing the carbon dioxide. Patent Document 1 also describes a distillation column as a method for separating the produced alcohol (see Figure 1 of Patent Document 1).

[0005] Patent Document 2 describes a method for recovering formic acid, which includes a recovery step of recovering formic acid from an aqueous solution containing formic acid using an alkylamine represented by the following formula (1) and an organic solvent, and a distillation step of distilling the organic solvent from the organic solvent layer containing formic acid obtained in the recovery step under reduced pressure or at normal pressure. R 1 R 2 R 3 N (1) (In formula (1), R 1 ~R 3 At least two of these are independently alkyl groups having 6 to 12 carbon atoms, and the remaining are hydrogen atoms.

[0006] Patent Document 3 describes a method for concentrating a formic acid solution as a method for recovering formic acid from an aqueous solution, which comprises the following steps: a first step of bringing a basic solution containing formate ions into contact with a cation exchange resin to obtain an acidic solution containing formate ions; a second step of bringing this acidic solution into contact with an anion exchange resin to adsorb the formate ions onto the anion exchange resin; and a third step of eluting the adsorbed formate ions from the anion exchange resin with an eluent containing an acid.

[0007] In the large-scale artificial photosynthesis systems described in Non-Patent Documents 1 and 2, in order to effectively utilize the electrolyte and the carbon dioxide dissolved in it, the electrolyte is pumped from a tank to the artificial photosynthesis cell, discharged, and then returned to the tank in a circulating system. Since formic acid produced by the reaction in the artificial photosynthesis cell is discharged dissolved in the electrolyte, the formic acid concentration in the electrolyte increases with continued operation. However, an increase in formic acid concentration can lead to problems such as a decrease in the performance of the artificial photosynthesis cell due to factors such as the decomposition of formic acid at the oxidation reaction electrode and the inhibition of formic acid production at the reduction reaction electrode.

[0008] In Patent Document 1, a distillation column is used to recover the alcohol product, but distillation requires a large amount of energy for heating, and considering that the boiling point of formic acid is 101° C., it is difficult to separate formic acid from water (boiling point 100° C.). Furthermore, most of the formic acid dissolved in the phosphate buffer solution, which is the electrolyte, becomes formate (potassium formate), which significantly reduces the amount recovered by distillation.

[0009] In Patent Document 2, the alkylamine extractant does not form a salt with potassium formate in the electrolyte, so formate cannot be extracted, resulting in a significant decrease in the extraction rate of formic acid.

[0010] In Patent Document 3, formic acid is extracted from a basic solution containing formate ions by acidifying the solution using a strongly acidic cation exchange resin, and then adsorbing the formate ions onto the anion exchange resin using an anion exchange resin. However, this requires complicated steps, and furthermore, a regeneration process is required to reuse the electrolyte. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-057491 [Patent Document 2] Japanese Patent Application Publication No. 2022-138278 [Patent Document 3] Japanese Patent Publication No. 2020-015684 [Non-patent literature]

[0012] [Non-Patent Document 1] “Solar Fuel Production from CO2 Using a 1 m-Square-Sized Reactor with a Solar-to-Formate Conversion Efficiency of 10.5%”, ACS Sustainable Chem. Eng., 2021, 9, pp.16031-16037 [Non-patent document 2] "A large-sized cell for solar-driven CO2 conversion with a solar-to-formate conversion efficiency of 7.2%", Joule 2021, 5, pp.687-705 Summary of the Invention [Problem to be solved by the invention]

[0013] The object of the present invention is to provide an electrochemical reactor equipped with a reduction reaction electrode and an oxidation reaction electrode, which can suppress deterioration of characteristics, maintain stable operation, reduce energy consumption when recovering the generated organic acid, and increase the amount recovered, and an artificial photosynthesis device equipped with such an electrochemical reactor. [Means for solving the problem]

[0014] The present invention provides an electrochemical reaction cell in which a reduction electrode containing a reduction catalyst and an oxidation electrode containing an oxidation catalyst are disposed in a container, and which reduces a reaction substrate to produce an organic acid; a tank for storing an electrolyte solution in which a salt with a pKa of 6 to 8, a neutral salt, or both are dissolved; an electrolyte solution supply means for supplying the electrolyte solution from the tank to the container; and a liquid-liquid extraction device for extracting the organic acid from the electrolyte solution discharged from the container and containing the organic acid produced by the reduction of the reaction substrate, into an organic solvent containing at least one of a secondary amine and a tertiary amine. The organic acid includes at least one of formic acid, acetic acid, and propionic acid, the salt having a pka of 6 to 8 includes potassium dihydrogen phosphate, the neutral salt includes potassium sulfate, and at least one of the secondary amine and the tertiary amine includes an alkylamine represented by the following formula (1): , an electrochemical reactor. R 1 R 2 R 3 N (1) (In formula (1), R 1 ,R 2 ,R 3 At least two of the groups are independently alkyl groups having 6 to 12 carbon atoms, and the remaining groups are hydrogen atoms.

[0019] In the electrochemical reactor, the liquid-liquid extractor is preferably at least one of a mixer-settler extractor, a column extractor, and a centrifugal extractor.

[0020] In the electrochemical reactor, it is preferable that the reaction substrate is carbon dioxide, and the organic acid produced by reduction of the reaction substrate is formic acid.

[0021] In the electrochemical reactor, the reduction catalyst is preferably a ruthenium complex polymer, and the oxidation catalyst is preferably iridium oxide.

[0022] The electrochemical reactor preferably further comprises a circulation means for circulating the electrolytic solution to the tank after the electrolytic solution is discharged from the container of the electrochemical reaction cell.

[0023] The present invention is an artificial photosynthesis device comprising the electrochemical reactor and a solar cell that generates electricity to be supplied to the oxidation reaction electrode and the reduction reaction electrode. [Effects of the Invention]

[0024] The present invention makes it possible to provide an electrochemical reactor equipped with a reduction reaction electrode and an oxidation reaction electrode, which can suppress deterioration in characteristics, maintain stable operation, reduce energy consumption when recovering the generated organic acid, and improve the amount recovered, as well as an artificial photosynthesis device equipped with the electrochemical reactor. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a schematic configuration diagram showing an example of an artificial photosynthesis device including an electrochemical reactor according to an embodiment of the present invention. FIG. [Figure 2] 1(a) to 1(e) are schematic configuration diagrams showing examples of electrochemical reaction cells in an electrochemical reactor according to an embodiment of the present invention. [Figure 3] FIG. 10 is a schematic configuration diagram showing another example of an artificial photosynthesis device including an electrochemical reactor according to an embodiment of the present invention. [Figure 4] FIG. 1 shows the H-NMR spectrum of trioctylamine in Experiment 1. [Figure 5] FIG. 1 shows 1H-NMR spectra obtained when formic acid was extracted using aqueous solution 1 or aqueous solution 2 in Experiment 1. [Figure 6]FIG. 1 shows 1H-NMR spectra obtained when an extraction process was carried out using aqueous solutions 3 to 5 in Experiment 1. [Figure 7] Photographs showing the appearance of the triple mixer-settler device used in Experiment 2. (a) is a photograph showing each pump and each part of the mixer-settler main bodies 1 to 3, and (b) is a photograph showing the tubes of the mixer parts 1 to 3. [Figure 8] 10 is a photograph showing the formation of a slug flow in Experiment 2. [Figure 9] Photographs showing the continuous separation of the aqueous solution and the extractant due to two-phase separation in each settler section in Experiment 2. DETAILED DESCRIPTION OF THE INVENTION

[0026] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes an embodiment of the present invention. The embodiment is an example of implementing the present invention, and the present invention is not limited to the embodiment.

[0027] The electrochemical reactor according to this embodiment includes an electrochemical reaction cell in which a reduction electrode containing a reduction catalyst and an oxidation electrode containing an oxidation catalyst are disposed within a container, and which produces an organic acid by reducing a reaction substrate; a tank containing an electrolyte solution containing a salt with a pKa of 6 to 8, a neutral salt, or both; an electrolyte supply means for supplying the electrolyte solution from the tank to the container; and a liquid-liquid extraction device for extracting the organic acid from the electrolyte solution discharged from the container, into an organic solvent containing at least one of a secondary amine and a tertiary amine. The electrochemical reactor can be, for example, a carbon dioxide reduction device, or an artificial photosynthesis device that combines the electrochemical reactor with a solar cell.

[0028] An electrochemical reactor according to an embodiment of the present invention and an example of an artificial photosynthesis device including the electrochemical reactor are shown in FIG. 1, and the configuration thereof will be described.

[0029] The electrochemical reactor 3 includes, for example, an electrochemical reaction cell 10 having a vessel 20 in which a reduction reaction electrode 16 containing a reduction catalyst for promoting a reduction reaction of a reaction substrate (e.g., carbon dioxide) and an oxidation reaction electrode 18 electrically connected to the reduction reaction electrode 16 and containing an oxidation catalyst for causing an oxidation reaction, are disposed at positions facing each other at a distance from each other, and a flow path through which an electrolyte containing a reaction substrate flows between the reduction reaction electrode 16 and the oxidation reaction electrode 18. The electrochemical reactor 3 includes: a tank 12 for holding an electrolyte containing a salt with a pKa of 6 to 8, a neutral salt, or both dissolved therein; a pump 22 and an electrolyte supply pipe 24 as electrolyte supply means for supplying the electrolyte from the tank 12 to the vessel 20 of the electrochemical reaction cell 10; and a liquid-liquid extractor 14 for extracting an organic acid from the electrolyte discharged from the vessel 20, into an organic solvent containing at least one of a secondary amine and a tertiary amine as an extractant, the organic acid being generated by the reduction of the reaction substrate. The electrochemical reactor 3 may include a reactant supply source 30 that supplies reactant to the tank 12 or the like.

[0030] The artificial photosynthesis device 1 includes an electrochemical reactor 3 and a solar cell 34 that generates electricity to be supplied to the reduction reaction electrode 16 and the oxidation reaction electrode 18 of the electrochemical reactor 3.

[0031] In the electrochemical reactor 3, the electrolyte outlet of the tank 12 and the electrolyte inlet of the electrochemical reaction cell 10 are connected by an electrolyte supply pipe 24 via a pump 22. The electrolyte outlet of the electrochemical reaction cell 10 and the electrolyte inlet of the liquid-liquid extractor 14 are connected by an electrolyte discharge pipe 26. The electrolyte outlet of the liquid-liquid extractor 14 and the electrolyte inlet of the tank 12 are connected by a circulation pipe 28. The reactant outlet of the reactant substrate supply source 30 and the reactant substrate inlet of the tank 12 are connected by a reactant substrate supply pipe 32. In the artificial photosynthesis device 1, the reduction reaction electrode 16 and the oxidation reaction electrode 18 of the electrochemical reactor 3 are electrically connected to the solar cell 34.

[0032] In the electrochemical reactor 3, a tank 12 contains an electrolyte solution in which a salt with a pKa of 6 to 8, a neutral salt, or both are dissolved in a solvent such as water. A reactant (e.g., carbon dioxide) is supplied to the tank 12 from a reactant supply source 30 through a reactant supply pipe 32, and the reactant is dissolved in the electrolyte solution in the tank 12. The electrolyte solution in which the salt with a pKa of 6 to 8, a neutral salt, or both, and the reactant are dissolved is supplied by a pump 22 from the electrolyte outlet of the tank 12 through an electrolyte supply pipe 24 and into a container 20 from an electrolyte inlet of the electrochemical reaction cell 10, passes through a flow path between the reduction reaction electrode 16 and the oxidation reaction electrode 18, and is discharged from the electrolyte outlet of the container 20.

[0033] When an appropriate bias voltage is applied between the reduction reaction electrode 16 and the oxidation reaction electrode 18, an oxidation reaction occurs at the oxidation reaction electrode 18, for example, water (H2O) is oxidized to obtain oxygen (1 / 2O2) and electrons are generated. At the reduction reaction electrode 16, by receiving the electrons generated by the oxidation reaction, a reaction substrate such as carbon dioxide (CO2) is reduced to generate an organic acid such as formic acid (HCOOH) (electrochemical reaction process).

[0034] The electrolyte, which has been discharged from the electrolyte outlet of the container 20 of the electrochemical reaction cell 10 and which has dissolved therein an organic acid produced by the reduction of the reaction substrate, is sent to the liquid-liquid extractor 14 from its electrolyte inlet through the electrolyte outlet pipe 26. In the liquid-liquid extractor 14, the organic acid is extracted from the electrolyte into an organic solvent (extractant) containing at least one of a secondary amine and a tertiary amine (liquid-liquid extraction step). The electrolyte, from which the organic acid has been extracted and in which the amount of organic acid has been reduced, which has been discharged from the electrolyte outlet of the liquid-liquid extractor 14 may be circulated to the tank 12 through the circulation pipe 28 (circulation step). For example, the pump 22, the electrolyte outlet pipe 26, the circulation pipe 28, etc. function as the circulation means.

[0035] In the electrochemical reactor according to this embodiment, an electrochemical reactor equipped with a reduction reaction electrode and an oxidation reaction electrode uses an electrolyte containing a salt with a pKa of 6 to 8 or a neutral salt that is unlikely to form an organic acid salt such as formate, and is equipped with a liquid-liquid extraction device that recovers an organic acid such as formic acid from the electrolyte, thereby suppressing deterioration in characteristics, maintaining stable operation, reducing energy consumption when recovering the generated organic acid, and increasing the amount recovered. This enables easy extraction of organic acids such as formic acid from the electrolyte and reuse of the electrolyte, suppressing an increase in the concentration of organic acids such as formic acid in the electrolyte, and maintaining stable operation.

[0036] In the electrochemical reactor, examples of the reaction substrate include carbon compounds, such as carbon dioxide (CO) and carbon monoxide (CO). Carbon dioxide is preferred as the reaction substrate in terms of its solubility in the electrolyte.

[0037] In an electrochemical reactor, examples of organic acids generated by reduction of a reaction substrate include formic acid, acetic acid, propionic acid, etc. From the viewpoint of selectivity of the reduction reaction, the organic acid is preferably at least one of formic acid, acetic acid, and propionic acid, and more preferably formic acid.

[0038] In an electrochemical reactor, the salt having a pKa of 6 to 8 contained in the electrolyte is not particularly limited, but examples include potassium dihydrogen phosphate, sodium dihydrogen phosphate, lithium dihydrogen phosphate, calcium dihydrogen phosphate, magnesium dihydrogen phosphate, cesium dihydrogen phosphate, rubidium dihydrogen phosphate, magnesium bicarbonate, magnesium chloride, cesium chloride, rubidium chloride, magnesium nitrate, calcium nitrate, cesium nitrate, rubidium nitrate, potassium thiocyanate, sodium thiocyanate, lithium thiocyanate, magnesium thiocyanate, cesium thiocyanate, and rubidium thiocyanate. The salt having a pKa of 6 to 8 is preferably potassium dihydrogen phosphate in terms of solubility, etc. One of these salts having a pKa of 6 to 8 may be used alone, or two or more may be used in combination. The pKa of a salt can be determined by neutralization titration.

[0039] In the electrochemical reactor, the neutral salt contained in the electrolyte is not particularly limited as long as it is an inorganic salt that exhibits a pH of 6.0 to 8.0 when dissolved in water, but examples include potassium sulfate, sodium sulfate, lithium sulfate, magnesium sulfate, calcium sulfate, potassium chloride, sodium chloride, lithium chloride, potassium bromide, sodium bromide, lithium bromide, magnesium bromide, potassium iodide, sodium iodide, lithium iodide, potassium nitrate, sodium nitrate, lithium nitrate, potassium perchlorate, sodium perchlorate, and lithium perchlorate. The neutral salt is preferably potassium sulfate from the viewpoint of solubility, etc. One of these neutral salts may be used alone, or two or more may be used in combination.

[0040] A salt having a pka of 6 to 8 may be used alone, a neutral salt may be used alone, or a salt having a pka of 6 to 8 and a neutral salt may be used in combination.

[0041] The solvent of the electrolytic solution is, for example, water such as pure water.

[0042] The concentration of the salt with a pka of 6 to 8, the neutral salt, or both in the electrolyte may be, for example, in the range of 0.01 M to 5.0 M, and preferably in the range of 0.1 M to 1.0 M. If the concentration of the salt with a pka of 6 to 8, the neutral salt, or both in the electrolyte is less than 0.01 M, the conductivity may be insufficient, and if it exceeds 5.0 M, precipitation of the salt may occur.

[0043] The at least one of the secondary amines and tertiary amines used in the liquid-liquid extractor 14 is not particularly limited, and examples thereof include secondary amines such as didodecylamine and diundecylamine, tertiary amines such as trioctylamine and triheptylamine, and alkylamines represented by the following formula (1): One of these secondary amines and tertiary amines may be used alone, or two or more may be used in combination. R 1 R 2 R 3 N (1) (In formula (1), R 1 ,R 2 ,R 3 At least two of the groups are independently alkyl groups having 6 to 12 carbon atoms, and the remaining groups are hydrogen atoms.

[0044] Among these, alkylamines represented by the above formula (1) are preferred. The alkylamines represented by the above formula (1) are Brønsted bases and are water-insoluble. Therefore, by contacting an organic solvent containing the alkylamine represented by the above formula (1) with an electrolyte containing an organic acid such as formic acid, the alkylamine accepts a proton from the organic acid, such as formic acid, which is a Brønsted acid, to form an organic acid salt such as a formate. Furthermore, the formed organic acid salt such as a formate can be easily separated from water by leaving it to stand. Therefore, the generated organic acid can be easily recovered without consuming a large amount of energy, such as by heating.

[0045] In the alkylamine represented by the above formula (1), R 1 ~R 3At least two of the alkyl groups are independently alkyl groups having 6 to 12 carbon atoms. The alkyl groups having 6 to 12 carbon atoms may be linear, branched, or cyclic. The alkyl groups are preferably alkyl groups having 6 or more carbon atoms, more preferably alkyl groups having 8 or more carbon atoms, and even more preferably alkyl groups having 8 carbon atoms, so that the amine can be easily separated from water. As the number of carbon atoms in the alkyl group increases, the volume of the solution after recovery increases, resulting in a relative decrease in the concentration of the organic acid such as formic acid. Therefore, alkyl groups having 10 or less carbon atoms are preferred. Furthermore, the alkyl groups are preferably linear alkyl groups, as they preferably have a space in which the amine can interact with the organic acid such as formic acid to form a salt.

[0046] The alkylamine represented by the above formula (1) is R 1 ~R 3 two of which are independently alkyl groups having 6 to 12 carbon atoms and the remaining one is a hydrogen atom; or R 1 ~R 3 In order to easily separate the amine from water, R 1 ~R 3 It is more preferable that all of are independently alkyl groups having 6 to 12 carbon atoms.

[0047] R 1 ~R 3 may all be different alkyl groups, or R 1 ~R 3 Two of the alkyl groups may be the same, or all of the alkyl groups may be the same. 1 ~R 3 are preferably all the same alkyl groups.

[0048] R 1 ~R 3 The total number of carbon atoms in R is preferably 12 or more, and more preferably 16 or more. 1 ~R3 If the total number of carbon atoms in R is 11 or less, the amount of amine dissolved in water increases, which may reduce the amount of organic acid extracted, such as formic acid. 1 ~R 3 The total number of carbon atoms is preferably 36 or less.

[0049] Among the alkylamines satisfying the above conditions, one kind may be used alone, or a plurality of kinds may be used in combination.

[0050] The equivalent amount of at least one of the secondary amine and tertiary amine used is preferably in the range of 0.5 to 10 equivalents relative to the amount of organic acid in the aqueous solution containing formic acid to be recovered. From the viewpoint of the recovery rate of the organic acid, the equivalent amount of at least one of the secondary amine and tertiary amine used is more preferably 1 equivalent or more relative to the amount of organic acid. Since using an unnecessarily large amount does not contribute to the recovery of the organic acid, the equivalent amount of at least one of the secondary amine and tertiary amine used is more preferably 5 equivalents or less relative to the amount of organic acid, and even more preferably 2 equivalents or less.

[0051] The organic solvent used must be capable of phase separation from the electrolyte. Therefore, a solvent with low solubility in water is preferred. Also, a solvent with a low boiling point is preferred so that it can be distilled off without consuming a large amount of energy. When the organic acid is formic acid, taking into consideration fractional distillation with formic acid, which has a boiling point of 101°C, the boiling point of the organic solvent is preferably 80°C or lower, more preferably 60°C or lower. Since it is desirable for the organic solvent to be liquid at room temperature, the lower limit of the boiling point of the organic solvent is preferably 25°C or higher, more preferably 35°C or higher. Also, it is desirable for the organic solvent to dissolve the amine used together and the amine containing the organic acid.

[0052] Considering the above, examples of organic solvents that can be used include cyclohexane (boiling point 81°C), benzene (81°C), carbon tetrachloride (77°C), diethyl ether (35°C), chloroform (62°C), ethyl acetate (77°C), dichloromethane (40°C), petroleum ether (30 to 60°C), hexane (69°C), pentane (69°C), methyl ethyl ketone (80°C), carbon disulfide (47°C), etc. Among these, dichloromethane and diethyl ether are preferred because they readily separate from water and have low boiling points.

[0053] The organic solvent to be used may be one kind selected from the above, or a combination of two or more kinds. The use of one kind is preferred because it is easy to set the conditions for distilling off the organic solvent in the distillation step.

[0054] The amount of organic solvent used is not particularly limited as long as it is an amount that allows the amine to be dissolved uniformly. It is not necessary to use an unnecessarily large amount of organic solvent, and it is preferable to use as little as possible in consideration of the energy required for distillation in the distillation step. The amount of organic solvent used is preferably adjusted so that the amine concentration is in the range of 1 to 100 times the concentration of the organic acid in the electrolytic solution.

[0055] The liquid-liquid extractor 14 is not particularly limited as long as it can extract a liquid from another liquid, but examples include a mixer-settler extractor, a tower extractor, a centrifugal extractor, etc. Examples of tower extractors include a plate tower, a packed tower, a pulse tower, and a rotating disk extractor. The liquid-liquid extractor is preferably a mixer-settler extractor because it can reduce the ratio of the volume of the extractant to the volume of the electrolyte in which the organic acid is dissolved. The mixer-settler extractor is equipped with a mixer section that mixes two liquids (e.g., an aqueous phase and an oil phase) and a settler section that separates the two liquids by gravity, and is a device that performs continuous solvent extraction while gravity-separating the two liquids. In the mixer section, it is preferable to generate a slug flow using a microchannel because it can shorten the extraction time.

[0056] 2(a), the electrochemical reaction cell 10 may be of a type in which a reduction reaction electrode 16 containing a reduction catalyst and an oxidation reaction electrode 18 containing an oxidation catalyst are disposed in a container at positions facing each other with a gap therebetween, and a flow path 42 is provided between the reduction reaction electrode 16 and the oxidation reaction electrode 18. Alternatively, the electrochemical reaction cell 10 may be of a type in which the reduction reaction electrode 16 containing a reduction catalyst and the oxidation reaction electrode 18 containing an oxidation catalyst are disposed in a container with an electrolyte membrane 44 sandwiched therebetween, and a flow path 42a is provided on the side of the reduction reaction electrode 16 opposite to the surface that contacts the electrolyte membrane 44, through which an electrolyte solution containing a reaction substrate such as carbon dioxide flows, and a flow path 42b is provided on the side of the oxidation reaction electrode 18 opposite to the surface that contacts the electrolyte membrane 44, through which an electrolyte solution flows. Alternatively, the electrochemical reaction cell 10 may be configured as shown in Fig. 2(c), in which a reduction reaction electrode 16 containing a reduction catalyst and an oxidation reaction electrode 18 containing an oxidation catalyst are disposed in opposing positions at a distance within a container, a flow path 42 is provided through which an electrolyte flows between the reduction reaction electrode 16 and the oxidation reaction electrode 18, and the surface of the reduction reaction electrode 16 opposite to the surface in contact with the electrolyte is exposed to a gas phase, so that a reaction substrate such as carbon dioxide is supplied from the gas phase. In this case, for example, as shown in Fig. 2(d), an electrolyte membrane 44 may be disposed in contact with the surface of the reduction reaction electrode 16 facing the electrolyte solution, or in contact with the surface of the oxidation reaction electrode 18 facing the electrolyte solution, or between the reduction reaction electrode 16 and the oxidation reaction electrode 18. Alternatively, for example, as shown in FIG. 2( e), the electrochemical reaction cell 10 may be configured such that a reduction reaction electrode 16 containing a reduction catalyst and an oxidation reaction electrode 18 containing an oxidation catalyst are arranged in a container with an electrolyte membrane 44 sandwiched therebetween, the surface of the reduction reaction electrode 16 opposite to the surface in contact with the electrolyte membrane 44 being exposed to a gas phase so that a reaction substrate such as carbon dioxide is supplied from the gas phase, and a flow path 42 through which an electrolyte flows is provided on the surface of the oxidation reaction electrode 18 opposite to the surface in contact with the electrolyte membrane 44.

[0057] When an electrolyte solution containing a reaction substrate such as carbon dioxide is supplied, the electrochemical reactor 3 may be configured such that the container 20 and the tank 12 of the electrochemical reaction cell 10 are separate as shown in FIG. 1, or the container 20 and the tank 12 may be configured as a single container.

[0058] In the electrochemical reactor 3, the liquid-liquid extractor 14 may be installed inline between the electrolyte outlet of the container 20 of the electrochemical reaction cell 10 and the electrolyte inlet of the tank 12 as shown in FIG. 1, or may be installed out-of-line and connected to the tank 12 as shown in FIG. 3.

[0059] In the electrochemical reactor 4 of the artificial photosynthesis apparatus 2 shown in FIG. 3, the first electrolyte outlet of the tank 12 and the electrolyte inlet of the electrochemical reaction cell 10 are connected by an electrolyte supply pipe 24 via a pump 22. The electrolyte outlet of the electrochemical reaction cell 10 and the first electrolyte inlet of the tank 12 are connected by a circulation pipe 36. The second electrolyte outlet of the tank 12 and the electrolyte inlet of the liquid-liquid extraction device 14 are connected by a pipe 38, and the electrolyte outlet of the liquid-liquid extraction device 14 and the second electrolyte inlet of the tank 12 are connected by a pipe 40. The reactant outlet of the reactant substrate supply source 30 and the reactant substrate inlet of the tank 12 are connected by a reactant substrate supply pipe 32. In the artificial photosynthesis apparatus 2, the reduction reaction electrode 16 and the oxidation reaction electrode 18 of the electrochemical reactor 4 are electrically connected to the solar cell 34.

[0060] In the electrochemical reactor 4, a tank 12 contains an electrolyte solution in which a salt with a pKa of 6 to 8, a neutral salt, or both are dissolved in a solvent such as water. A reactant (e.g., carbon dioxide) is supplied to the tank 12 from a reactant supply source 30 through a reactant supply pipe 32, and the reactant is dissolved in the electrolyte solution in the tank 12. The electrolyte solution in which the salt with a pKa of 6 to 8, a neutral salt, or both, and the reactant is dissolved is supplied by a pump 22 from the electrolyte outlet of the tank 12 through an electrolyte supply pipe 24 and into a container 20 from an electrolyte inlet of the electrochemical reaction cell 10, passes through a flow path between the reduction reaction electrode 16 and the oxidation reaction electrode 18, and is discharged from the electrolyte outlet of the container 20.

[0061] When an appropriate bias voltage is applied between the reduction reaction electrode 16 and the oxidation reaction electrode 18, an oxidation reaction occurs at the oxidation reaction electrode 18, for example, water (H2O) is oxidized to obtain oxygen (1 / 2O2) and electrons are generated. At the reduction reaction electrode 16, by receiving the electrons generated by the oxidation reaction, a reaction substrate such as carbon dioxide (CO2) is reduced to generate an organic acid such as formic acid (HCOOH) (electrochemical reaction process).

[0062] The electrolyte, which has been discharged from the electrolyte outlet of the container 20 of the electrochemical reaction cell 10 and which has dissolved therein an organic acid produced by the reduction of the reaction substrate, may be circulated to the tank 12 through the circulation pipe 36 (circulation step). For example, the pump 22, the circulation pipe 36, etc. function as the circulation means. The electrolyte, which has dissolved therein an organic acid, is sent from the second electrolyte outlet of the tank 12 through the pipe 38 to the liquid-liquid extractor 14 through its electrolyte inlet. In the liquid-liquid extractor 14, the organic acid is extracted from the electrolyte into an organic solvent (extractant) containing at least one of a secondary amine and a tertiary amine (liquid-liquid extraction step). The electrolyte, from which the organic acid has been extracted and in which the amount of organic acid has been reduced, which has been discharged from the electrolyte outlet of the liquid-liquid extractor 14, is sent to the tank 12 through the pipe 40.

[0063] In the electrochemical reactor 3, as shown in Fig. 1, the electrolyte may be discharged from the container 20 of the electrochemical reaction cell 10 and circulated to the tank 12 after a liquid-liquid extraction treatment by the liquid-liquid extractor 14, or may be discharged to the outside of the system after a liquid-liquid extraction treatment by the liquid-liquid extractor 14. In the electrochemical reactor 4, as shown in Fig. 3, the electrolyte may be discharged from the container 20 of the electrochemical reaction cell 10 and circulated to the tank 12, and may be further circulated to the tank 12 after a liquid-liquid extraction treatment by the liquid-liquid extractor 14, or may be discharged to the outside of the system after a liquid-liquid extraction treatment by the liquid-liquid extractor 14. By circulating the electrolyte, it is easy to reuse the electrolyte.

[0064] (electrode for reduction reaction) The reduction reaction electrode 16, which is a negative electrode (cathode electrode), is an electrode used to reduce a substance by a reduction reaction. The reduction reaction electrode 16 includes, for example, a conductive layer and a conductor layer formed in this order on a substrate.

[0065] The substrate is a member that structurally supports the reduction reaction electrode. The substrate is not particularly limited in material, but examples thereof include glass substrates. The substrate may include, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but examples thereof include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), lead (Pb), and titanium (Ti). The semiconductor used as the substrate is not particularly limited, but examples thereof include titanium oxide (TiO), tin oxide (SnO), silicon (Si), strontium titanate (SrTiO), zinc oxide (ZnO), and tantalum oxide (TaO). When the substrate includes a metal or a semiconductor, an insulating layer may be formed between the conductive layer and the substrate. The insulating layer is not particularly limited, but examples thereof include semiconductor oxides, nitrides, and resins. The metal substrate may be electrically connected directly to the conductive layer and the conductor layer, or when a metal substrate is used, the conductive layer may not be provided.

[0066] The substrate may contain carbon fiber. The substrate may be, for example, a composite substrate of carbon and carbon heat-treated at high temperature, such as carbon paper or carbon cloth. Carbon paper is prepared by impregnating organic fibers such as polyacrylonitrile (PAN) fibers in a dispersion of polyvinyl alcohol and an aqueous medium, carbonizing the fibers at approximately 2000°C, and binding the fibers together to form a sheet. Carbon paper may also contain approximately 25% by mass of Teflon (registered trademark)-based materials. Carbon cloth is woven from carbon fibers obtained by baking and carbonizing organic fibers. Carbon paper and carbon cloth are porous and have countless pores measuring several tens of micrometers (approximately 10 μm to 100 μm). The thickness of the carbon paper or carbon cloth is, for example, in the range of 0.1 mm to 0.4 mm per sheet. This composite substrate may be multilayered (e.g., 5 to 10 layers) to a thickness of approximately 10 times, in the range of 1 mm to 4 mm. A multilayered composite substrate can be formed, for example, by laminating multiple pieces of carbon paper, carbon cloth, etc. using a carbon-based adhesive containing a conductive carbon material, such as a polymer (e.g., an acrylic polymer binder) containing graphite or graphene as the adhesive.

[0067] The conductive layer is provided to improve current collection efficiency at the reduction reaction electrode. The conductive layer is not particularly limited, but examples include transparent conductive layers such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, fluorine-doped tin oxide (FTO) is preferred in terms of thermal and chemical stability.

[0068] The conductor layer is composed of a conductor containing a material having reduction catalytic function as a reduction catalyst. The conductor can be composed of a material containing a carbon material (C). The size of the single unit of the carbon material structure is preferably 1 nm or more and 1 μm or less. Examples of the carbon material include at least one of carbon nanotubes such as multi-walled carbon nanotubes (MWCNTs), graphene, and graphite. Graphene and graphite are preferably sized 1 nm or more and 1 μm or less. Carbon nanotubes are preferably sized 1 nm or more and 40 nm or less. The conductor can be formed, for example, by spraying a carbon material mixed with a liquid such as ethanol and then heating it. Spin coating may be used instead of spraying. Alternatively, the solution may be directly dripped and dried without spin coating.

[0069] A complex catalyst or the like can be used as a material having a reduction catalytic function. The complex catalyst is preferably, for example, a ruthenium complex. Examples of the complex catalyst include [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(MeCN)Cl2], [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2Cl2], and [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2]. nand [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(CHCN)Cl]. Furthermore, the ruthenium complex may be a Ru complex polymer in which a Ru complex containing a Ru complex monomer and a polymerization initiator (e.g., pyrrole and an oxidizing agent (e.g., an Fe-based oxidizing agent such as iron chloride) is polymerized. The reduction reaction electrode is, for example, an electrode containing a carbon-based material (carbon nanotubes such as multi-walled carbon nanotubes (MWCNTs) and graphite) and containing a Ru complex polymer as a reduction catalyst and an Fe-based oxidizing agent (FeCl).

[0070] The support of a reduction catalyst can be produced, for example, by dissolving a metal complex (catalyst) in an acetonitrile (MeCN) solution on a carbon-based material such as carbon paper or carbon cloth, and then drying the solution. For example, the support of a Ru complex polymer can be produced by dissolving a Ru complex monomer, a polymerization initiator (e.g., pyrrole, and an oxidant (e.g., an Fe-based oxidant such as iron chloride)) in a solvent such as acetonitrile (MeCN) to form a solution (Ru complex polymer solution) on a carbon-based material such as carbon paper or carbon cloth, and then drying the solution. The support of a reduction catalyst can also be produced by electropolymerization. For example, a carbon-based material electrode can be used as the working electrode, a glass substrate coated with fluorine-containing tin oxide (FTO) as the counter electrode, and an Ag / Ag reference electrode can be used. + Using an electrode, Ag / Ag was used in an electrolyte containing a reduction catalyst. + After applying a cathodic current to the electrode to create a negative voltage, Ag / Ag + By passing an anode current so that the electrode is at a positive potential, the carbon-based material can be supported as a reduction catalyst. For example, acetonitrile (MeCN) can be used as the electrolyte solution, and tetrabutylammonium perchlorate (TBAP) can be used as the electrolyte.

[0071] (Oxidation reaction electrode) The oxidation reaction electrode 18, which is a positive electrode (anode electrode), is an electrode used to oxidize a substance by an oxidation reaction. The oxidation reaction electrode 18 includes, for example, a base material having a conductive layer formed on a substrate and an oxidation catalyst layer formed thereon.

[0072] The substrate is a member that structurally supports the oxidation electrode. The substrate is not particularly limited in material, but examples thereof include glass substrates. The substrate may include, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but examples thereof include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), lead (Pb), and titanium (Ti). The semiconductor used as the substrate is not particularly limited, but examples thereof include titanium oxide (TiO), tin oxide (SnO), silicon (Si), strontium titanate (SrTiO), zinc oxide (ZnO), and tantalum oxide (TaO). When the substrate includes a metal or a semiconductor, an insulating layer may be formed between the conductive layer and the substrate. The insulating layer is not particularly limited, but examples thereof include semiconductor oxides, nitrides, and resins. The metal substrate and the conductive layer may be electrically connected directly. In order to make the oxidation reaction electrode 18 translucent, the substrate is preferably made of, for example, a glass substrate or plastic. When a metal substrate is used, the substrate may not have a conductive layer. A composite substrate of carbon fiber and carbon may also be used as the substrate. This has the advantages of ensuring light weight and high conductivity and eliminating the need for a separate conductive layer.

[0073] The conductive layer is provided to improve the efficiency of current collection at the oxidation reaction electrode 18. The conductive layer is not particularly limited, but examples thereof include transparent conductive layers such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, in consideration of thermal and chemical stability, it is preferable to use fluorine-doped tin oxide (FTO).

[0074] The oxidation catalyst layer is configured to contain a material having an oxidation catalytic function as an oxidation catalyst. Examples of the material having an oxidation catalytic function include a material containing iridium oxide (IrOx). Iridium oxide can be supported on the surface of the conductive layer as a nanocolloid solution (see T. Arai et al., Energy Environ. Sci 8, 1998 (2015)).

[0075] For example, nanocolloids of iridium oxide (IrOx) were synthesized. Next, 50 mL of 2 mM potassium chloroiridate (IV) (K2IrCl6) aqueous solution was added with 10 wt% sodium hydroxide (NaOH) aqueous solution to adjust the pH to 13. This yellow solution was heated at 90 °C for 20 minutes using a hot stirrer. The resulting blue solution was then cooled in ice water for 1 hour. Then, 3 M nitric acid (HNO3) was added dropwise to the cooled solution (20 mL) to adjust the pH to 1, and the solution was stirred for 80 minutes to obtain a nanocolloidal solution of iridium oxide (IrOx). This solution was then adjusted to pH 12 by adding 1-2 mL of 1.5 wt% NaOH aqueous solution dropwise. The resulting nanocolloidal solution of iridium oxide (IrOx) was then applied to a conductive layer at pH 12 and dried in a drying oven at 60 °C for 40 minutes. After drying, the precipitated salt can be washed with ultrapure water to form the oxidation reaction electrode 18. Note that the application and drying of the nanocolloidal aqueous solution of iridium oxide (IrOx) may be repeated multiple times.

[0076] When a composite substrate of carbon fiber and carbon is used as the substrate in the oxidation reaction electrode 18, for example, a nanocolloidal aqueous solution of iridium oxide (IrOx) may be used as an oxidation catalyst, and the oxidation catalyst may be supported on a composite substrate of carbon fiber and carbon, such as carbon paper, which may contain carbon materials such as carbon nanotubes, including multi-walled carbon nanotubes (MWCNTs), and the sheet may be bonded to the composite substrate of carbon fiber and carbon, such as carbon paper, with a carbon-based adhesive.

[0077] A current collecting wire may be provided to enhance the current collecting effect of the oxidation reaction electrode 18. That is, when the area of ​​the oxidation reaction electrode 18 is increased, a conductive layer alone cannot ensure sufficient conductivity across the entire surface of the oxidation reaction electrode 18 to promote the reaction. Therefore, a current collecting wire is provided to enhance the conductivity of the oxidation reaction electrode 18. For example, the current collecting wire may be configured by combining linear finger electrodes arranged at intervals in a comb shape with bus electrodes for further current collection from the finger electrodes. The current collecting wire is preferably configured from a conductive portion, a first sealing portion, and a second sealing portion. The conductive portion is preferably configured from a highly conductive material, including a metal. For example, it is preferably configured from a material including silver (Ag), copper (Cu), or the like. Furthermore, the first sealing portion and the second sealing portion are provided to cover at least a portion of the conductive portion to chemically and mechanically protect the conductive portion. The first sealing portion may be made of a low-melting-point glass coating material. In addition, the second sealing portion can be made of resin such as silicone rubber (oxime-free type, low molecular weight siloxane-reducing material, oil-resistant and solvent-resistant fluorosilicone, etc.), polyisobutylene, polypropylene, methacrylate (acrylic), polycarbonate, fluororesin (Teflon (registered trademark)), epoxy resin, etc.

[0078] The reduction reaction electrode 16 and the oxidation reaction electrode 18 may each have a single layer configuration, or the reduction reaction electrode 16 and the oxidation reaction electrode 18 may each have a configuration in which multiple electrodes are laminated (stacked). Only the reduction reaction electrode 16 may be laminated (stacked), or only the oxidation reaction electrode 18 may be laminated (stacked). When used as an artificial photosynthesis device, it is preferable that the reduction reaction electrode 16 and the oxidation reaction electrode 18 each have a configuration in which multiple electrodes are laminated (stacked).

[0079] The container 20 is a member that supports the reduction reaction electrode 16 and the oxidation reaction electrode 18 and also forms a flow path through which the electrolyte flows. The container 20 is made of a material that has the mechanical strength required to form the electrochemical reaction cell 10. For example, the container 20 can be made of metal, plastic, etc.

[0080] A separator (for example, an electrolyte membrane 44 as shown in FIGS. 2(b), (d), and (e)) that separates liquid from gas and allows protons to move may be provided between the reduction reaction electrode 16 and the oxidation reaction electrode 18. The separator may be made of any material that separates liquid from gas and allows protons to move, and is not particularly limited; for example, Nafion (registered trademark), which is a solid polymer electrolyte membrane, may be used.

[0081] The tank 12 is a tank that contains an electrolyte, and can be made of, for example, metal, plastic, or the like.

[0082] The reactant supply source 30 is not particularly limited as long as it can supply a reactant, and is, for example, a reactant cylinder such as a carbon dioxide cylinder.

[0083] The reduction reaction electrode 16 and the oxidation reaction electrode 18 are electrically connected to each other, and an appropriate bias voltage is applied thereto. The means for applying the bias voltage is not particularly limited, and examples thereof include a chemical battery (including a primary battery, a secondary battery, etc.), a constant voltage source, a solar cell, etc. At this time, the positive electrode is connected to the oxidation reaction electrode 18, and the negative electrode is connected to the reduction reaction electrode 16.

[0084] As shown in Figures 1 and 3, by using a solar cell 34 as a means for applying a bias voltage, an artificial photosynthesis device 1 or 2 can be created that includes an electrochemical reactor 3 or 4 such as a carbon dioxide reduction device, and a solar cell 34 that generates power to be supplied to the reduction reaction electrode 16 and the oxidation reaction electrode 18. When the solar cell 34 is used as a means for applying a bias voltage, the solar cell 34 can be disposed adjacent to the reduction reaction electrode 16 or the oxidation reaction electrode 18, for example. For example, the solar cell 34 can be disposed on the back of the reduction reaction electrode 16, with the positive electrode of the solar cell 34 connected to the oxidation reaction electrode 18 and the negative electrode connected to the reduction reaction electrode 16.

[0085] When synthesizing formic acid (HCOOH) from carbon dioxide (CO2), water (H2O) is oxidized to provide electrons and protons to carbon dioxide (CO2). At a pH of around 7, the oxidation potential of water (H2O) is 0.82 V, and the reduction potential is -0.41 V (both on the standard hydrogen electrode (NHE)). The reduction potentials of carbon dioxide (CO2) to carbon monoxide (CO), formic acid (HCOOH), and methyl alcohol (CH3OH) are -0.53 V, -0.61 V, and -0.38 V, respectively. Therefore, the potential difference between the oxidation and reduction potentials is 1.20 to 1.43 V. When reducing carbon dioxide (CO2), a carbon compound, a suitable solar cell configuration is one in which four to six crystalline silicon solar cells are connected in series, or an amorphous silicon triple-junction solar cell.

[0086] It is preferable to provide a window material on the light-receiving side of the solar cell. The window material is a member that protects the solar cell. The window material is a member that transmits light of wavelengths that contribute to power generation in the solar cell, and can be, for example, glass, plastic, etc.

[0087] The present specification includes the following embodiments. [1] An electrochemical reaction cell in which a reduction reaction electrode containing a reduction catalyst and an oxidation reaction electrode containing an oxidation catalyst are disposed in a container, and which reduces a reaction substrate to produce an organic acid; A tank containing an electrolyte solution containing a salt with a pka of 6 to 8, a neutral salt, or both. an electrolyte supply means for supplying the electrolyte from the tank to the container; a liquid-liquid extraction device that extracts the organic acid from the electrolytic solution discharged from the container and having the organic acid dissolved therein, the organic acid being produced by the reduction of the reaction substrate, into an organic solvent containing at least one of a secondary amine and a tertiary amine; An electrochemical reactor comprising:

[0088] [2] The electrochemical reactor according to [1], The organic acid is at least one of formic acid, acetic acid, and propionic acid.

[0089] [3] The electrochemical reactor according to [1] or [2], The salt having a pka of 6 to 8 is at least one of potassium dihydrogen phosphate, sodium dihydrogen phosphate, lithium dihydrogen phosphate, calcium dihydrogen phosphate, magnesium dihydrogen phosphate, cesium dihydrogen phosphate, rubidium dihydrogen phosphate, magnesium bicarbonate, magnesium chloride, cesium chloride, rubidium chloride, magnesium nitrate, calcium nitrate, cesium nitrate, rubidium nitrate, potassium thiocyanate, sodium thiocyanate, lithium thiocyanate, magnesium thiocyanate, cesium thiocyanate, and rubidium thiocyanate, an electrochemical reactor.

[0090] [4] The electrochemical reactor according to any one of [1] to [3], the neutral salt is at least one of potassium sulfate, sodium sulfate, lithium sulfate, magnesium sulfate, calcium sulfate, potassium chloride, sodium chloride, lithium chloride, potassium bromide, sodium bromide, lithium bromide, magnesium bromide, potassium iodide, sodium iodide, lithium iodide, potassium nitrate, sodium nitrate, lithium nitrate, potassium perchlorate, sodium perchlorate, and lithium perchlorate.

[0091] [5] The electrochemical reactor according to any one of [1] to [4], At least one of the secondary amine and the tertiary amine is an alkylamine represented by the following formula (1): R 1 R 2 R 3 N (1) (In formula (1), R 1 ,R 2 ,R 3 At least two of the groups are independently alkyl groups having 6 to 12 carbon atoms, and the remaining groups are hydrogen atoms.

[0092] [6] The electrochemical reactor according to any one of [1] to [5], The electrochemical reactor, wherein the liquid-liquid extractor is at least one of a mixer-settler extractor, a column extractor, and a centrifugal extractor.

[0093] [7] The electrochemical reactor according to any one of [1] to [6], The electrochemical reactor, wherein the reaction substrate is carbon dioxide, and the organic acid produced by reduction of the reaction substrate is formic acid.

[0094] [8] The electrochemical reactor according to any one of [1] to [7], The electrochemical reactor, wherein the reduction catalyst is a ruthenium complex polymer and the oxidation catalyst is iridium oxide.

[0095] [9] The electrochemical reactor according to any one of [1] to [8], The electrochemical reactor further comprises a circulation means for circulating the electrolytic solution to the tank after the electrolytic solution is discharged from the container of the electrochemical reaction cell.

[0096]

[10] An electrochemical reactor according to any one of [1] to [9]; a solar cell that generates power to be supplied to the oxidation reaction electrode and the reduction reaction electrode; An artificial photosynthesis device comprising: [Example]

[0097] EXAMPLES The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0098] 1. Experiment 1 1.1. Preparation of various aqueous solutions Aqueous solutions 1 to 5 were prepared as follows.

[0099] (Reference Example) Aqueous solution 1: Formic acid (0.76 mL, 20 mmol) was dissolved in pure water (200 mL). (Comparative Example) Aqueous solution 2: This solution was prepared by adding potassium dihydrogen phosphate (pka=7.2, 5.44 g, 40 mmol) and dipotassium hydrogen phosphate (pka=12.35, 6.97 g, 40 mmol) to water (200 mL) and dissolving formic acid (0.76 mL, 20 mmol). (Reference Example) Aqueous solution 3: Potassium formate (1.68 g, 20 mmol) was added to water (200 mL) to obtain a 0.10 mol / L solution. (Reference Example) Aqueous solution 4: Potassium dihydrogen phosphate (10.89 g, 80 mmol) was dissolved in water (200 mL). (Reference Example) Aqueous Solution 5: Aqueous solution 5 was prepared by adding dipotassium hydrogen phosphate (13.93 g, 80 mmol) to water (200 mL).

[0100] The types and concentrations of the various aqueous solutions are shown in Table 1.

[0101] [Table 1]

[0102] 1.2. Preparation of NMR samples using various aqueous solutions NMR samples were prepared using Aqueous Solutions 1 to 5 as follows.

[0103] (1) Dichloromethane (DCM, 10 mL) and trioctylamine (1.75 mL, 4.0 mmol) were added to aqueous solution 1 (20 mL) and stirred in a separatory funnel. (2) After standing, the organic layer was separated and dichloromethane was removed using an evaporator (200 torr, 40°C). (3) To 1 mL of a solution of deuterated chloroform (CDCl3, 5 mL) and 1,2-dichloroethane (DCE, 10 μL) was added 0.25 mL of (2), and further CDCl3 was added to bring the total volume to 2.00 mL. (4) Take a portion of (3) into an NMR sample tube. 1 H-NMR was measured.

[0104] The same procedures (1) to (4) were carried out using aqueous solutions 2 to 5 instead of aqueous solution 1. In order to compare with the state of trioctylamine that had not formed a salt, the same extraction procedure was also carried out using pure water only (20 mL).

[0105] 1.3. 1 H-NMR measurement 1 H-NMR measurements were performed using a JEOL JNM-ECX-400P spectrometer. Chemical shifts are shown in δ values ​​(ppm), with the value of 0.03% (v / v) tetramethylsilane (TMS), contained as an internal standard in the CDCl3 reagent, set at 0.00 ppm. The integral value of DCE was set at 1.00.

[0106] 1.4 Results 1 Figure 4 shows the results of trioctylamine 1 The H-NMR spectrum is shown in Figure 5, and the results are shown for the case where formic acid was extracted using aqueous solution 1 or aqueous solution 2. 1 Table 2 shows the formic acid extraction rate (%) when Aqueous Solution 1 or Aqueous Solution 2 was used. Figure 6 shows the results when the extraction process was carried out using Aqueous Solutions 3 to 5. 1The H-NMR spectrum is shown. The formic acid extraction rate (%) was calculated by dividing the amount of formic acid in the DCM layer residue by the amount of formic acid contained in the aqueous solution before extraction. The amount of formic acid in the DCM layer residue was determined from the relative integral value of formic acid to DCE, and then calculated from the volume of the DCM layer residue.

[0107] [Table 2]

[0108] Compared to Aqueous Solution 1 (formic acid dissolved in pure water), the extraction rate of formic acid dissolved in Aqueous Solution 2 (standard electrolyte) was significantly lower (see Figures 4, 5, and Table 2). Because trioctylamine hardly forms a complex salt with potassium formate or potassium phosphate salts (potassium dihydrogen phosphate or dipotassium hydrogen phosphate) (see Figure 6), it is thought that much of the formic acid dissolved in Aqueous Solution 2 becomes potassium formate due to the buffering effect, which is why the extraction rate of formic acid was significantly lower.

[0109] 2. Experiment 2 2.1. Preparation of various aqueous solutions Aqueous solutions 6 to 8 were prepared as follows.

[0110] (Example) Aqueous solution 6: This was prepared by adding potassium dihydrogen phosphate (10.88 g, 80 mmol) as a salt with a pka of 6 to 8 to water (200 mL) and dissolving formic acid (0.15 mL, 4 mmol). (Example) Aqueous solution 7: A neutral salt, potassium sulfate (pH=6.6, 13.94 g, 80 mmol), was added to water (200 mL) and formic acid (0.15 mL, 4 mmol) was dissolved therein to prepare aqueous solution 7. (Comparative Example) Aqueous solution 8: This was prepared by adding potassium dihydrogen phosphate (5.44 g, 40 mmol) and dipotassium hydrogen phosphate (6.97 g, 40 mmol) to water (200 mL) and dissolving formic acid (0.15 mL, 4 mmol).

[0111] The types and concentrations of the various aqueous solutions are shown in Table 3.

[0112] Extractants 1–3: Trioctylamine (1.75 mL, 4.0 mmol) as a tertiary amine was dissolved in dichloromethane (DCM, 30 mL) as an organic solvent.

[0113] [Table 3]

[0114] 2.2. Triple Mixer-Settler Configuration and Formic Acid Extraction Process The experimental setup for the triple mixer-settler system is shown in Figure 7. Four pumps (Q-100-TT-PS) were used to deliver the aqueous solution and extractants 1–3. Hydrophobic PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer) tubing (inner diameter: 1 mm) for high-performance liquid chromatography (HPLC) was used to connect each pump to each of the mixer-settler units 1–3 (see Figure 7(a)). The 2-m-long tubing (see Figure 7(b)) for mixer units 1–3 was also made of hydrophobic PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer) tubing (inner diameter: 1 mm) for piping in high-performance liquid chromatography (HPLC). The aqueous solution delivery pumps were connected to each of the mixer-settler units so that three extractions using extractants 1–3 could be performed in a continuous process. The aqueous solution delivery pumps delivered the aqueous solution containing dissolved formic acid from the aqueous solution container to mixer-settler unit 1, mixer-settler unit 2, mixer-settler unit 3, and then to the aqueous solution collection container. Aqueous solutions 6-8 (60 mL) were pumped from mixer settler body 1 to mixer settler body 2 to mixer settler body 3 in this order, forming slug flows with extractants 1-3 (30 mL each), and then the liquids separated into two phases in each settler. The aqueous solution was collected in an aqueous solution collection container, and the extractant was returned to the original extractant container and circulated.

[0115] 2.3 Result 2 Figure 8 shows a photograph of the formation of a slug flow, and Figure 9 shows a photograph of the continuous separation of the aqueous solution and the extractant due to two-phase separation in each settler section. A slug flow of the aqueous solution and the extractant was formed in mixer settlers 1 to 3. Furthermore, due to two-phase separation in each settler section, the aqueous solution and the extractant were separated in a continuous manner (see Figure 9). This confirmed three consecutive extraction operations. Table 4 also shows the formic acid extraction rate (%) when aqueous solutions 6 to 8 were used.

[0116] [Table 4]

[0117] The extraction rate of formic acid using a triple mixer settler was 11% for Aqueous Solution 8, but was significantly improved to 96% for Aqueous Solution 6 (see Table 4). This is thought to be because aqueous solution 6, which uses a salt with a pKa of 6 to 8, hardly forms potassium formate. Even aqueous solution 7, which uses a neutral salt, achieved a formic acid extraction rate of 40%, exceeding the value for aqueous solution 8.

[0118] 3. Experiment 3 3.1. Fabrication of a small reduction reaction electrode (cathode electrode) A porous carbon sheet (CS) composed of porous carbon fiber was dip-coated with an ink containing 5% by mass of multi-walled carbon nanotubes (MWCNTs) dispersed in an ethanol solvent. After drying at 25°C, the sheet was heated at 350°C under an argon atmosphere to produce an MWCNTs / CS sheet carrying MWCNTs.

[0119] The components of the Ru complex polymer solution (for one application) were Ru complex monomer [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2Cl2] (5.02 × 10 -7 mol / cm 2 ), and pyrrole (1.99 × 10 -9 mol / cm 2 ), iron chloride (FeCl3) (2.79 × 10-6 mol / cm 2 ), and ethanol (14 mL / cm 2 ), acetonitrile (53 mL / cm 2 This was applied 10 times and vacuum dried to support the Ru complex polymer (RuCP) on the MWCNTs / CS sheet. The surface of a 20mm x 15mm Ti plate (JIS Ti type 1, thickness 0.5mm) was polished, and the reduction catalyst RuCP / MWCNTs / CS (thickness 320μm, hereafter abbreviated as RuCP / MWCNTs / CS) was cut into 10mm x 10mm pieces and bonded to the Ti substrate with graphite adhesive. The edge of the titanium substrate was joined to a conductor with a crimp terminal using titanium bolts, nuts, and washers. The terminal joint was then covered with silicone rubber.

[0120] 3.2. Cathode characterization The cathode electrode was used as the working electrode, a platinum wire as the counter electrode, and Hg / HgSO as the reference electrode. A small cathode electrode of Ti / adhesive / RuCP / MWCNTs / CS was used, and carbon dioxide (CO) gas was bubbled into aqueous solution 6 (Example) or aqueous solution 8 (Comparative Example) at a current of 1 mA / cm using a potentio-galvanostat (VMP3, Bio-Logic Sciences Instruments). 2 Constant current measurements were performed. In aqueous solution 6, the formic acid produced was extracted to suppress the increase in the formic acid concentration in the aqueous solution of the cell, and measurements were performed. In aqueous solution 8, the formic acid produced was allowed to increase in concentration as it was. The amount of formic acid produced was quantified using an ion chromatograph (Integrion RFIC EG equipped with IonPac EGC-500-KOH, CR-ATC600 columns, and a conductometric detector, Dionex Corporation), and the Faraday efficiency (FE) of formic acid was calculated.

[0121] 3.3 Result 3 Table 5 shows the Faraday efficiency (FE) retention rate of formic acid when Aqueous Solution 6 and Aqueous Solution 8 were used.

[0122] [Table 5]

[0123] By suppressing the increase in the formic acid concentration in the aqueous solution of the cell, the formic acid FE retention rate was significantly improved, and stable operation was obtained (see Table 5).

[0124] As described above, in the examples, in an electrochemical reactor equipped with a reduction reaction electrode and an oxidation reaction electrode, it was possible to suppress deterioration in characteristics, maintain stable operation, reduce energy consumption when recovering the generated organic acid, and increase the amount recovered. [Explanation of symbols]

[0125] 1,2 Artificial photosynthesis device, 3,4 Electrochemical reactor, 10 Electrochemical reaction cell, 12 Tank, 14 Liquid-liquid extraction device, 16 Reduction reaction electrode, 18 Oxidation reaction electrode, 20 Container, 22 Pump, 24 Electrolyte supply piping, 26 Electrolyte discharge piping, 28,36 Circulation piping, 30 Reaction substrate supply source, 32 Reaction substrate supply piping, 34 Solar cell, 38,40 Piping, 42,42a,42b Flow path, 44 Electrolyte membrane.

Claims

1. an electrochemical reaction cell in which a reduction reaction electrode containing a reduction catalyst and an oxidation reaction electrode containing an oxidation catalyst are disposed in a container, and which reduces a reaction substrate to produce an organic acid; a tank for holding an electrolyte solution containing a salt with a pKa of 6 to 8, a neutral salt, or both; an electrolyte supply means for supplying the electrolyte from the tank to the container; a liquid-liquid extraction device that extracts the organic acid from the electrolytic solution discharged from the container and having the organic acid dissolved therein, the organic acid being produced by the reduction of the reaction substrate, into an organic solvent containing at least one of a secondary amine and a tertiary amine; Equipped with the organic acid comprises at least one of formic acid, acetic acid, and propionic acid; The salt having a pKa of 6 to 8 includes potassium dihydrogen phosphate, and the neutral salt includes potassium sulfate; At least one of the secondary amine and the tertiary amine comprises an alkylamine represented by the following formula (1): R 1 R 2 R 3 N (1) (In formula (1), at least two of R 1 , R 2 , and R 3 are independently an alkyl group having 6 to 12 carbon atoms, and the rest are hydrogen atoms.)

2. 10. The electrochemical reactor of claim 1, The electrochemical reactor is characterized in that the liquid-liquid extractor is at least one of a mixer-settler extractor, a column extractor, and a centrifugal extractor.

3. 10. The electrochemical reactor of claim 1, 1. An electrochemical reactor, wherein the reaction substrate is carbon dioxide, and the organic acid produced by reduction of the reaction substrate is formic acid.

4. 10. The electrochemical reactor of claim 1, 1. An electrochemical reactor, wherein the reduction catalyst is a ruthenium complex polymer, and the oxidation catalyst is iridium oxide.

5. 10. The electrochemical reactor of claim 1, The electrochemical reactor further comprises a circulation means for circulating the electrolytic solution to the tank after the electrolytic solution is discharged from the container of the electrochemical reaction cell.

6. The electrochemical reactor of claim 1; a solar cell that generates power to be supplied to the oxidation reaction electrode and the reduction reaction electrode; An artificial photosynthesis device comprising:

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