JP7758023B2 - Electrochemical reactors and artificial photosynthesizers - Google Patents

Description

The present invention relates to an electrochemical reactor equipped with a reduction reaction electrode and an oxidation reaction electrode, and an artificial photosynthesis device equipped with the electrochemical reactor.

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 it is being conducted very actively. In the artificial photosynthesis reaction, water is oxidized at the oxidation reaction electrode, supplying electrons and protons, and oxygen ( _O2 ) is produced. Carbon dioxide is reduced at the reduction reaction electrode using these electrons and protons.

For example, Non-Patent Documents 1 and 2 describe a large-scale artificial photosynthesis system in which formic acid is produced 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, operating in a circulating manner.

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 containing dissolved carbon dioxide 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).

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 by 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), at least two of R ¹ to R ³ are independently an alkyl group having 6 to 12 carbon atoms, and the rest are hydrogen atoms.)

Patent Document 3 describes a method for concentrating a formic acid solution as a method for recovering formic acid from an aqueous solution, including a first step of contacting a basic solution containing formate ions with a cation exchange resin to obtain an acidic solution containing formate ions, a second step of contacting this acidic solution 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 acid.

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. Because 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.

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

In Patent Document 2, the alkylamine extractant does not form a salt with the potassium formate in the electrolyte, so formate cannot be extracted. Therefore, there is a problem in that the extraction rate of formic acid is significantly reduced.

In Patent Document 3, to extract formic acid from a basic solution containing formate ions, a strongly acidic cation exchange resin is used to acidify the solution, and then an anion exchange resin is used to adsorb the formate ions onto the anion exchange resin. However, this requires complicated steps, and furthermore, a regeneration process is required to reuse the electrolyte.

Japanese Patent Application Laid-Open No. 2017-057491 Japanese Patent Application Laid-Open No. 2022-138278 Japanese Patent Application Laid-Open No. 2020-015684

“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 "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

The object of the present invention is to provide an electrochemical reactor equipped with a reduction reaction electrode and an oxidation reaction electrode that can suppress deterioration in characteristics, maintain stable operation, reduce energy consumption when recovering the generated organic acid, and increase the amount recovered, as well as an artificial photosynthesis device equipped with such an electrochemical reactor.

The present invention provides an electrochemical reactor comprising: 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 within a container, and which produces an organic acid by reducing a reaction substrate; a tank for holding an electrolyte solution in which a salt having a PKa of 6 to 8, a neutral salt, or both are dissolved; 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 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; wherein the organic acid comprises at least one of formic acid, acetic acid, and propionic acid, the salt having a PKa of 6 to 8 comprises potassium dihydrogen phosphate, the neutral salt comprises potassium sulfate, and 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.)

In the electrochemical reactor, the liquid-liquid extraction device is preferably at least one of a mixer-settler type extraction device, a column type extraction device, and a centrifugal type extraction device.

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.

In the electrochemical reactor, it is preferable that the reduction catalyst is a ruthenium complex polymer and the oxidation catalyst is iridium oxide.

It is preferable that the electrochemical reactor further includes a circulation means for circulating the electrolyte back to the tank after it is discharged from the container of the electrochemical reaction cell.

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.

The present invention provides an electrochemical reactor equipped with a reduction reaction electrode and an oxidation reaction electrode that can suppress deterioration in characteristics, maintain stable operation, reduce energy consumption when recovering the generated organic acid, and increase the amount recovered, as well as an artificial photosynthesis device equipped with such an electrochemical reactor.

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. 1A to 1E are schematic configuration diagrams showing examples of electrochemical reaction cells in an electrochemical reactor according to an embodiment of the present invention. 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. FIG. 1 shows the ¹ H-NMR spectrum of trioctylamine in Experiment 1. FIG. 1 shows ¹ H-NMR spectra when formic acid was extracted using aqueous solution 1 or aqueous solution 2 in Experiment 1. FIG. 1 shows ¹ H-NMR spectra when an extraction process was carried out using aqueous solutions 3 to 5 in Experiment 1. 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 body 1 to 3, and (b) is a photograph showing the tubes of the mixer parts 1 to 3. 10 is a photograph showing the formation of a slug flow in Experiment 2. 10 is a photograph showing the continuous separation of the aqueous solution and the extractant due to two-phase separation in each settler section in Experiment 2.

An embodiment of the present invention will be described below. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

The electrochemical reactor of this embodiment is an electrochemical reactor comprising: 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 within a container, and which produces an organic acid by reducing a reaction substrate; a tank containing an electrolyte solution in which a salt with a pKa of 6 to 8, a neutral salt, or both are dissolved; 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 organic acid being dissolved in the electrolyte solution produced by the reduction of the reaction substrate. The electrochemical reactor can be, for example, a carbon dioxide reduction device, or it can also be an artificial photosynthesis device that combines an electrochemical reactor with a solar cell.

Figure 1 shows an outline of an electrochemical reactor according to an embodiment of the present invention, and an example of an artificial photosynthesis device equipped with that electrochemical reactor, and its configuration will be described below.

The electrochemical reactor 3 includes, for example, an electrochemical reaction cell 10 having a vessel 20, a reduction electrode 16 containing a reduction catalyst that promotes the reduction reaction of a reaction substrate (e.g., carbon dioxide), and an oxidation electrode 18 electrically connected to the reduction electrode 16 and containing an oxidation catalyst that causes an oxidation reaction. The reduction electrode 16 and the oxidation electrode 18 are positioned facing each other at a distance from each other, and the electrochemical reaction cell 10 has a flow path through which an electrolyte containing a reaction substrate flows between the reduction electrode 16 and the oxidation electrode 18. The electrochemical reactor 3 includes: a tank 12 containing an electrolyte solution 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 solution from the tank 12 to the vessel 20 of the electrochemical reaction cell 10; and a liquid-liquid extraction device 14 that extracts the organic acid from the electrolyte solution 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 is dissolved in the electrolyte solution, which has been produced by the reduction of the reaction substrate and is discharged from the vessel 20. The electrochemical reactor 3 may include a reactant supply source 30 that supplies reactant substrates to the tank 12, etc.

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

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 extraction device 14 are connected by an electrolyte discharge pipe 26. The electrolyte outlet of the liquid-liquid extraction device 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 oxidation reaction electrode 18 of the electrochemical reactor 3 are electrically connected to the solar cell 34.

In the electrochemical reactor 3, the 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 the reactant supply source 30 through the reactant supply pipe 32, and the reactant is dissolved in the electrolyte solution within 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 the pump 22 from the electrolyte outlet of the tank 12 through the electrolyte supply pipe 24 and into the container 20 from the electrolyte inlet of the electrochemical reaction cell 10. The electrolyte then passes through the 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.

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, and for example, water (H ₂ O) is oxidized to obtain oxygen (½O ₂ ) 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 (CO ₂ ) is reduced to generate an organic acid such as formic acid (HCOOH) (electrochemical reaction process).

The electrolyte, which has dissolved therein the organic acid produced by the reduction of the reaction substrate and is discharged from the electrolyte outlet of the container 20 of the electrochemical reaction cell 10, is sent to the liquid-liquid extraction device 14 from its electrolyte inlet via the electrolyte discharge pipe 26. In the liquid-liquid extraction device 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 process). The electrolyte, from which the organic acid has been extracted and whose amount has been reduced, discharged from the electrolyte outlet of the liquid-liquid extraction device 14 may be circulated to the tank 12 via the circulation pipe 28 (circulation process). For example, the pump 22, the electrolyte discharge pipe 26, the circulation pipe 28, etc. function as the circulation means.

In the electrochemical reactor according to this embodiment, which is an electrochemical reactor equipped with a reduction reaction electrode and an oxidation reaction electrode, 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 is used, and a liquid-liquid extraction device is provided to recover organic acids such as formic acid from the electrolyte. This prevents deterioration of characteristics, maintains stable operation, reduces energy consumption when recovering the generated organic acid, and increases the amount recovered. Organic acids such as formic acid can be easily extracted from the electrolyte and the electrolyte can be reused, preventing an increase in the concentration of organic acids such as formic acid in the electrolyte and maintaining stable operation.

In the electrochemical reactor, examples of the reaction substrate include carbon compounds, such as carbon dioxide (CO ₂ ) and carbon monoxide (CO). Carbon dioxide is preferred as the reaction substrate in terms of its solubility in the electrolyte.

In electrochemical reactors, examples of organic acids produced by reduction of reaction substrates include formic acid, acetic acid, and propionic acid. From the standpoint of reduction reaction selectivity, the organic acid is preferably at least one of formic acid, acetic acid, and propionic acid, and more preferably formic acid.

In electrochemical reactors, the salt with 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. Of the salts with a pKa of 6 to 8, potassium dihydrogen phosphate is preferred from the standpoint of solubility, etc. These salts with a pKa of 6 to 8 may be used alone or in combination of two or more. The pKa of a salt can be determined by neutralization titration.

In electrochemical reactors, 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. 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. From the standpoint of solubility, etc., potassium sulfate is preferred as the neutral salt. These neutral salts may be used alone or in combination of two or more.

A salt with a pKa of 6 to 8 may be used alone, a neutral salt may be used alone, or a salt with a pKa of 6 to 8 may be used in combination with a neutral salt.

The solvent for the electrolyte is, for example, water such as pure water.

The concentration of the salt with a pKa of 6-8, 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 6-8, 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, salt precipitation may occur.

The at least one of the secondary amine and the tertiary amine 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 of them may be used in combination.
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.)

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, when an organic solvent containing the alkylamine represented by the above formula (1) is brought into contact with an electrolyte containing an organic acid such as formic acid, the alkylamine accepts a proton from the organic acid, which is a Brønsted acid, to form an organic acid salt such as formate. Furthermore, the formed organic acid salt such as 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.

In the alkylamine represented by the above formula (1), at least two of R ¹ to R ³ are independently an alkyl group having 6 to 12 carbon atoms. The alkyl group having 6 to 12 carbon atoms may be a linear alkyl group, a branched alkyl group, or a cyclic alkyl group. The alkyl group is preferably an alkyl group having 6 or more carbon atoms, more preferably an alkyl group having 8 or more carbon atoms, and even more preferably an alkyl group 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, an alkyl group having 10 or less carbon atoms is preferred. Furthermore, the alkyl group is preferably a linear alkyl group because it preferably has a space in which the amine can interact with the organic acid such as formic acid to form a salt.

The alkylamine represented by the above formula (1) may be a secondary amine in which two of R ¹ to R ³ are independently alkyl groups having from 6 to 12 carbon atoms and the remaining one is a hydrogen atom, or may be an alkylamine in which all of R ¹ to R ³ are independently alkyl groups having from 6 to 12 carbon atoms. It is more preferable that R ¹ to R ³ are all independently alkyl groups having from 6 to 12 carbon atoms so that the amine can be easily separated from water.

R ¹ to R ³ may all be different alkyl groups, or two of R ¹ to R ³ may be the same alkyl group, or all may be the same alkyl group. From the standpoint of the cost of the amine, etc., it is preferable that R ¹ to R ³ are all the same alkyl group.

The total number of carbon atoms in R ¹ to R ³ is preferably 12 or more, and more preferably 16 or more. If the total number of carbon atoms in R ¹ to R ³ is 11 or less, the amount of amine that dissolves in water increases, which may reduce the amount of organic acid such as formic acid that can be extracted. Furthermore, from the standpoint of operability and the like, it is preferable that the amine is a liquid at room temperature and normal pressure, so the total number of carbon atoms in R ¹ to R ³ is preferably 36 or less.

Among the alkylamines that satisfy the above conditions, one may be used alone, or multiple may be used in combination.

The equivalent amount of at least one of the secondary amines and tertiary amines 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 standpoint of organic acid recovery rate, etc., it is more preferable that the equivalent amount of at least one of the secondary amines and tertiary amines used is 1 equivalent or more relative to the amount of organic acid. Using a larger amount than necessary will not contribute to the recovery of organic acid, so it is more preferable that the equivalent amount of at least one of the secondary amines and tertiary amines used is 5 equivalents or less relative to the amount of organic acid, and even more preferably 2 equivalents or less.

The organic solvent used must be able to undergo phase separation from the electrolyte. Therefore, a solvent with low solubility in water is preferred. Furthermore, 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 account the 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 a liquid at room temperature, the lower limit of the boiling point is preferably 25°C or higher, more preferably 35°C or higher. Furthermore, it is desirable for the organic solvent to dissolve the amine used together and the amine containing the organic acid.

Taking the above into consideration, 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 (30-60°C), hexane (69°C), pentane (69°C), methyl ethyl ketone (80°C), and carbon disulfide (47°C). Of these, dichloromethane and diethyl ether are preferred because they readily separate from water and have low boiling points.

The organic solvent used may be one of the above, or a combination of two or more. Using one type of organic solvent is preferred, as it simplifies the setting of conditions for distilling off the organic solvent in the distillation step.

There are no particular restrictions on the amount of organic solvent used, as long as it dissolves the amine used and becomes uniform. It is not necessary to use an unnecessarily large amount of organic solvent, and it is preferable to use as little as possible, taking into account the energy required for distillation in the distillation step. It is preferable to adjust the amount of organic solvent used so that the amine concentration is in the range of 1 to 100 times the concentration of the organic acid in the electrolyte solution.

The liquid-liquid extractor 14 may be any device capable of extracting a liquid from another liquid, and is not particularly limited. Examples include mixer-settler extractors, tower extractors, and centrifugal extractors. Examples of tower extractors include plate columns, packed columns, pulse columns, and rotating disk extractors. A mixer-settler extractor is preferred 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. A 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. It is a device that performs continuous solvent extraction while gravity-separating the two liquids. It is preferred that a slug flow be generated in the mixer section using a microchannel, as this can shorten the extraction time.

2(a), the electrochemical reaction cell 10 may be configured, for example, as shown in FIG. 2(a), in which a reduction reaction electrode 16 containing a reduction catalyst and an oxidation reaction electrode 18 containing an oxidation catalyst are arranged in opposing positions at a distance within a container, and a flow path 42 is provided between the reduction reaction electrode 16 and the oxidation reaction electrode 18, through which an electrolyte solution containing a reaction substrate such as carbon dioxide flows. Alternatively, the electrochemical reaction cell 10 may be configured, for example, as shown in FIG. 2(b), in which 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, and a flow path 42a is provided on the side of the reduction reaction electrode 16 opposite 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 the surface that contacts the electrolyte membrane 44. 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 disposed 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 the electrolyte solution flows is provided on the surface of the oxidation reaction electrode 18 opposite to the surface in contact with the electrolyte membrane 44.

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

In the electrochemical reactor 3, the liquid-liquid extraction device 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 Figure 1, or it may be installed out-of-line and connected to the tank 12, as shown in Figure 3.

In the electrochemical reactor 4 of the artificial photosynthesis device 2 shown in Figure 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 device 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.

In the electrochemical reactor 4, the 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 the reactant supply source 30 through the reactant supply pipe 32, and the reactant is dissolved in the electrolyte solution within the tank 12. The electrolyte solution in which the reactant (a salt with a pKa of 6 to 8, a neutral salt, or both) and the reactant are dissolved is supplied by the pump 22 from the electrolyte outlet of the tank 12 through the electrolyte supply pipe 24 and into the container 20 from the electrolyte inlet of the electrochemical reaction cell 10. The electrolyte then passes through the 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.

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, and for example, water (H ₂ O) is oxidized to obtain oxygen (½O ₂ ) 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 (CO ₂ ) is reduced to generate an organic acid such as formic acid (HCOOH) (electrochemical reaction process).

The electrolyte, which has dissolved therein the organic acid produced by the reduction of the reaction substrate and which is discharged from the electrolyte outlet of the container 20 of the electrochemical reaction cell 10, may be circulated to the tank 12 via the circulation pipe 36 (circulation process). For example, the pump 22, the circulation pipe 36, etc. function as the circulation means. The electrolyte, which has dissolved therein the organic acid, is sent from the second electrolyte outlet of the tank 12 through the pipe 38 to the liquid-liquid extraction device 14 via the electrolyte inlet of the liquid-liquid extraction device 14. In the liquid-liquid extraction device 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 process). The electrolyte, from which the organic acid has been extracted and in which the amount of organic acid has been reduced, which is discharged from the electrolyte outlet of the liquid-liquid extraction device 14, is sent to the tank 12 via the pipe 40.

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 liquid-liquid extraction treatment by the liquid-liquid extraction device 14, or may be discharged outside the system after liquid-liquid extraction treatment by the liquid-liquid extraction device 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 liquid-liquid extraction treatment by the liquid-liquid extraction device 14, or may be discharged outside the system after liquid-liquid extraction treatment by the liquid-liquid extraction device 14. Circulating the electrolyte makes it easier to reuse the electrolyte.

(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.

The substrate is a member that structurally supports the reduction reaction electrode. The substrate is not particularly limited in material, but examples thereof include a glass substrate. 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 ₂ ), tin oxide (SnO ₂ ), silicon (Si), strontium titanate (SrTiO ₃ ), zinc oxide (ZnO), and tantalum oxide (Ta ₂ O ₅ ). 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 oxides, nitrides, and resins of semiconductors. The metal substrate may be electrically connected directly to the conductive layer and the conductor layer. When a metal substrate is used, the conductive layer may not be present.

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 made by impregnating organic fibers, such as polyacrylonitrile (PAN) fibers, with a dispersion of polyvinyl alcohol and an aqueous medium, carbonizing the fibers at approximately 2000°C, and then forming them into a sheet. Carbon paper may also contain approximately 25% by weight of Teflon®-based materials. Carbon cloth is made by weaving carbon fibers obtained by baking and carbonizing organic fibers. Carbon paper and carbon cloth are porous and contain countless pores measuring several tens of micrometers (approximately 10 μm to 100 μm). The thickness of carbon paper and carbon cloth is, for example, in the range of 0.1 mm to 0.4 mm per sheet. This composite substrate may also 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 sheets of carbon paper, carbon cloth, etc. using a carbon-based adhesive containing a conductive carbon material, such as a polymer containing graphite or graphene (e.g., an acrylic polymer binder).

The conductive layer is provided to improve current collection 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 preferable, considering its thermal and chemical stability.

The conductor layer is composed of a conductor containing a material that functions 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 carbon materials include carbon nanotubes such as multi-walled carbon nanotubes (MWCNTs), graphene, and/or 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 before application without using spin coating.

As the material having a reduction catalytic function, a complex catalyst can be used, etc. 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)Cl ₂ ], [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO) ₂ Cl ₂ ], [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO) ₂ ] _n , [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(CH ₃ CN)Cl ₂ ]. Furthermore, the ruthenium complex may be a Ru complex polymer in which a Ru complex containing a Ru complex monomer and a polymerization initiator (for example, pyrrole and an oxidizing agent (for example, 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), a Ru complex polymer as a reduction catalyst, and an Fe-based oxidizing agent (FeCl ₃ ).

The reduction catalyst can be supported, for example, by dissolving a metal complex (catalyst) in an acetonitrile (MeCN) solution and applying the solution to a carbon-based material such as carbon paper or carbon cloth, followed by drying. For example, a Ru complex polymer can be supported by applying a solution (Ru complex polymer solution) prepared 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 a carbon-based material such as carbon paper or carbon cloth, followed by drying. The reduction catalyst can also be supported by electropolymerization. For example, a carbon-based material electrode is used as the working electrode, a glass substrate coated with fluorine-containing tin oxide (FTO) is used as the counter electrode, and an Ag/Ag ⁺ electrode is used as the reference electrode. In an electrolyte containing the reduction catalyst, a cathode current is applied so that the potential is negative relative to the Ag/Ag ⁺ electrode, and then an anode current is applied so that the potential is positive relative to the Ag/Ag ⁺ electrode. This allows the reduction catalyst to be supported on the carbon-based material. For example, acetonitrile (MeCN) can be used as the electrolyte solution, and tetrabutylammonium perchlorate (TBAP) can be used as the electrolyte.

(Oxidation reaction electrode)
The oxidation reaction electrode 18, which is a positive electrode (anode electrode), is an electrode used to oxidize a substance through 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.

The substrate is a member that structurally supports the oxidation reaction electrode. The substrate is not particularly limited in material, but examples thereof include a glass substrate. 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 ₂ ), tin oxide (SnO ₂ ), silicon (Si), strontium titanate (SrTiO ₃ ), zinc oxide (ZnO), and tantalum oxide (Ta ₂ O ₅ ). 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, it is preferable that the substrate is made of, for example, a glass substrate or plastic. When a metal substrate is used, the conductive layer may not be included. 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.

The conductive layer is provided to improve the effectiveness of current collection at the oxidation reaction electrode 18. 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 preferable, considering its thermal and chemical stability.

The oxidation catalyst layer contains a material with oxidation catalytic function as an oxidation catalyst. Examples of materials with oxidation catalytic function include materials containing iridium oxide (IrOx). Iridium oxide can be supported on the surface of the conductive layer as a nanocolloidal solution (see T. Arai et al., Energy Environ. Sci 8, 1998 (2015)).

For example, nanocolloids of iridium oxide (IrOx) are synthesized. Next, a yellow solution is prepared by adding 10 wt% aqueous sodium hydroxide (NaOH) to 50 mL of a 2 mM aqueous solution of potassium chloroiridate (IV) (K ₂ IrCl ₆ ) to adjust the pH to 13. The resulting yellow solution is heated at 90°C for 20 minutes using a hot stirrer. The resulting blue solution is then cooled in ice water for 1 hour. 3M nitric acid (HNO ₃ ) is then added dropwise to the cooled solution (20 mL) to adjust the pH to 1, and the solution is stirred for 80 minutes to obtain an aqueous nanocolloid of iridium oxide (IrOx). Furthermore, 1-2 mL of a 1.5 wt% aqueous NaOH solution is added dropwise to this solution to adjust the pH to 12. The resulting nanocolloid of iridium oxide (IrOx) solution is 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 is washed with ultrapure water to form the oxidation reaction electrode 18. The application and drying of the nanocolloidal aqueous solution of iridium oxide (IrOx) may be repeated multiple times.

When a composite substrate of carbon fiber and carbon is used as the substrate for the oxidation reaction electrode 18, for example, a nanocolloidal aqueous solution of iridium oxide (IrOx) may be used as the oxidation catalyst. 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 resulting sheet may be bonded to the composite substrate of carbon fiber and carbon, such as carbon paper, with a carbon-based adhesive.

Current collecting wiring may be provided to enhance the current collection effect of the oxidation reaction electrode 18. In other words, when the oxidation reaction electrode 18 is enlarged, a conductive layer alone cannot ensure sufficient conductivity across the entire surface of the oxidation reaction electrode 18 to promote the reaction. Therefore, current collecting wiring is provided to enhance the conductivity of the oxidation reaction electrode 18. The current collecting wiring can be configured, for example, by combining linear finger electrodes arranged at intervals in a comb-like pattern with bus electrodes for further current collection from the finger electrodes. The current collecting wiring is preferably composed of a conductive portion, a first sealing portion, and a second sealing portion. The conductive portion is preferably composed of a highly conductive material, preferably a material containing metal. For example, it is preferably composed of a material containing 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 it. The first sealing portion can be made of a low-melting-point glass coating material. The second seal can be made of resins 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)), and epoxy resin.

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 layered (stacked). Only the reduction reaction electrode 16 may be layered (stacked), or only the oxidation reaction electrode 18 may be layered (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 layered (stacked).

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

A separator (e.g., an electrolyte membrane 44 as shown in Figures 2(b), (d), and (e)) that separates liquid and 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 and gas and allows protons to move, and is not particularly limited; for example, Nafion (registered trademark), a solid polymer electrolyte membrane, can be used.

Tank 12 is a tank that contains electrolyte and can be made of, for example, metal, plastic, etc.

The reaction substrate supply source 30 is not particularly limited as long as it can supply a reaction substrate, but it can be, for example, a reaction substrate cylinder such as a carbon dioxide cylinder.

The reduction reaction electrode 16 and the oxidation reaction electrode 18 are electrically connected, and an appropriate bias voltage is applied. The means for applying the bias voltage is not particularly limited, and examples include chemical batteries (including primary batteries, secondary batteries, etc.), constant voltage sources, and solar cells. 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.

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 using a solar cell 34 as a means for applying a bias voltage, the solar cell 34 can be placed adjacent to the reduction reaction electrode 16 or the oxidation reaction electrode 18, for example. For example, the solar cell 34 can be placed 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.

When synthesizing formic acid (HCOOH) from carbon dioxide (CO ₂ ), water (H ₂ O) is oxidized to supply electrons and protons to carbon dioxide (CO ₂ ). At a pH of around 7, the oxidation potential of water (H ₂ O) is 0.82 V and the reduction potential is −0.41 V (both at the normal hydrogen electrode (NHE)). The reduction potentials of carbon dioxide (CO ₂ ) to carbon monoxide (CO), formic acid (HCOOH), and methyl alcohol (CH ₃ OH) are −0.53 V, −0.61 V, and −0.38 V, respectively. Therefore, the potential difference between the oxidation potential and the reduction potential is 1.20 to 1.43 V. When reducing carbon dioxide (CO ₂ ), a carbon compound, it is preferable to use a solar cell configured, for example, by connecting four to six crystalline silicon solar cells in series or an amorphous silicon triple-junction solar cell.

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 the wavelengths that contribute to power generation in the solar cell, and can be, for example, glass, plastic, etc.

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 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;
An electrochemical reactor comprising:

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

[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.

[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.

[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), 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.)

[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.

[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.

[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.

[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.

[10] The 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:

The present invention will be explained in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

1. Experiment 1
1.1. Preparation of Various Aqueous Solutions Aqueous solutions 1 to 5 were prepared as follows.

(Reference Example) Aqueous solution 1: Formic acid (0.76 mL, 20 mmol) was dissolved in pure water (200 mL).
(Comparative) 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).

The types and concentrations of various aqueous solutions are shown in Table 1.

1.2. Preparation of NMR Samples Using Various Aqueous Solutions NMR samples were prepared using Aqueous Solutions 1 to 5, respectively, as follows.

(1) Dichloromethane (DCM, 10 mL) and trioctylamine (1.75 mL, 4.0 mmol) were added to aqueous solution 1 (20 mL), and the mixture was 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 (CDCl ₃ , 5 mL) and 1,2-dichloroethane (DCE, 10 μL) was added 0.25 mL of (2), and CDCl ₃ was further added to bring the total volume to 2.00 mL.
(4) A portion of (3) was placed in an NMR sample tube and subjected to ¹ H-NMR measurement.

Steps (1) to (4) were similarly performed using aqueous solutions 2 to 5 instead of aqueous solution 1. To compare with the state of trioctylamine without forming a salt, the same extraction procedure was also performed using pure water only (20 mL).

1.3. ^1H -NMR measurement
^1H -NMR measurements were performed using a JNM-ECX-400P JEOL Ltd. 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 CDCl ₃ reagent set at 0.00 ppm. The integral value of DCE was set at 1.00.

1.4 Result 1
FIG. 4 shows the ¹ H-NMR spectrum of trioctylamine, and FIG. 5 shows the 1 H-NMR spectrum when formic acid is extracted using Aqueous Solution 1 or Aqueous Solution 2. Table 2 shows the formic acid extraction rate (%) when Aqueous Solution 1 or Aqueous Solution 2 is used. FIG. 6 shows the ¹ H-NMR spectra when the extraction process is performed using Aqueous Solutions ³ to 5. 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 relative to DCE, and then calculated from the volume of the DCM layer residue.

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.

2. Experiment 2
2.1. Preparation of Various Aqueous Solutions Aqueous solutions 6 to 8 were prepared as follows.

(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: This was prepared by adding potassium sulfate (pH=6.6, 13.94 g, 80 mmol) as a neutral salt to water (200 mL) and dissolving formic acid (0.15 mL, 4 mmol).
(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).

The types and concentrations of various aqueous solutions are shown in Table 3.

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.

2.2. Triple Mixer-Settler Configuration and Formic Acid Extraction Process The experimental configuration for the triple mixer-settler is shown in Figure 7. Four pumps (Q-100-TT-P-S) manufactured by TACMINA Corporation were used to deliver the aqueous solution and extractants 1 to 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 to 3 (see Figure 7(a)), and for the 2-m-long tubing of mixer units 1 to 3 (see Figure 7(b)). The aqueous solution delivery pump 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 recovery container. The pumps and the mixer-settler unit were connected so that three extractions using extractants 1 to 3 could be performed in a continuous process. In the process of pumping aqueous solutions 6 to 8 (60 mL) in the order of mixer-settler body 1 → mixer-settler body 2 → mixer-settler body 3, slug flows were formed with each of the extractants 1 to 3 (30 mL each), and then the liquids were separated into two phases in each settler section. The aqueous solutions were collected in an aqueous solution collection container, and the extractants were returned to the original extractant container and circulated.

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.

The formic acid extraction rate using the 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 any potassium formate. Aqueous Solution 7, which uses a neutral salt, also achieved a formic acid extraction rate of 40%, exceeding the value for Aqueous Solution 8.

3. Experiment 3
3.1. Preparation of a small reduction reaction electrode (cathode electrode) A porous carbon sheet (CS) composed of porous carbon fiber was dip-coated with ink prepared by dispersing 5% by mass of multi-walled carbon nanotubes (MWCNTs) in an ethanol solvent. The ink was then dried at 25°C and then heat-treated at 350°C in an argon atmosphere to prepare a MWCNTs/CS sheet carrying MWCNTs.

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) ₂ Cl ₂ ] (5.02×10 ⁻⁷ mol/cm ² ), polymerization agent constituting the polymer, pyrrole (1.99×10 ⁻⁹ mol/cm ² ), polymerization catalyst, iron chloride (FeCl ₃ ) (2.79×10 ⁻⁶ mol/cm ² ), and solvent, ethanol (14 mL/cm ² ) and acetonitrile (53 mL/cm ² ). This solution was applied 10 times and dried under vacuum to support the Ru complex polymer (RuCP) on the MWCNTs/CS sheet. The surface of a 20 mm x 15 mm Ti plate (JIS Ti type 1, thickness 0.5 mm) was polished, and a 10 mm x 10 mm piece of RuCP/MWCNTs/CS (thickness 320 μm, hereafter abbreviated as RuCP/MWCNTs/CS), a reduction catalyst, was cut and bonded to the Ti substrate with graphite adhesive. The end 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.

3.2. Cathode Characterization The cathode electrode described above _was used as the working electrode, a platinum wire as the counter electrode, and Hg/ _Hg2SO4 as the reference electrode. A small cathode electrode of Ti/adhesive/RuCP/MWCNTs/ ^CS was used, and carbon dioxide ( _CO2 ) gas was bubbled as a reaction substrate into aqueous solution 6 (Example) or aqueous solution 8 (Comparative Example). A potentio-galvanostat (VMP3, Bio-Logic Sciences Instruments) was used to perform constant current measurements at 1 mA/cm2. For aqueous solution 6, the formic acid concentration in the cell was extracted to suppress the increase. For aqueous solution 8, the formic acid concentration was allowed to increase. 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.

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.

By suppressing the increase in formic acid concentration in the cell's aqueous solution, the formic acid FE retention rate was significantly improved, resulting in stable operation (see Table 5).

In this way, the examples show that in an electrochemical reactor equipped with a reduction reaction electrode and an oxidation reaction electrode, deterioration in characteristics can be suppressed, stable operation can be maintained, energy consumption when recovering the generated organic acid can be reduced, and the amount recovered can be increased.

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 pipe, 26 Electrolyte discharge pipe, 28, 36 Circulation pipe, 30 Reaction substrate supply source, 32 Reaction substrate supply pipe, 34 Solar cell, 38, 40 Pipe, 42, 42a, 42b Flow path, 44 Electrolyte membrane.

Claims (6)

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.) 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. 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. 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. 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. 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: