Electrolyte membrane manufacturing method

Abstract

Provided is an electrolyte membrane 6 which is used in a carbon dioxide reducing device 100, is disposed between an electrolytic solution 3 in an oxidation tank 2 and a reduction electrode 4 in a reduction tank 5 to be in contact with the electrolytic solution 3 and the reduction electrode 4, and performs a reduction reaction of carbon dioxide by bringing the reduction electrode 4 into direct contact with carbon dioxide, wherein a water-repellent film 9 is provided on a portion of the surface that contacts the electrolyte 3.

Description

[Technical Field]

[0001] The present invention relates to an electrolyte membrane and a method for manufacturing an electrolyte membrane. [Background technology]

[0002] The increase in carbon dioxide concentration in the atmosphere is cited as the main cause of global warming. Reducing carbon dioxide emissions has become a long-term challenge on a global scale. Meanwhile, in the medium to long term, energy issues are forcing us to reconsider our reliance on fossil fuels for energy supplies, and there is a need to create next-generation energy sources.

[0003] As a means of obtaining energy while reducing carbon dioxide emissions, technological development is underway to utilize unused energy sources such as exhaust heat, snow and ice heat, vibrations, and electromagnetic waves, as well as renewable energy sources such as sunlight. However, these power generation technologies are limited to generating electrical energy and cannot store energy. Furthermore, they cannot create chemical products from fossil fuels.

[0004] As a method for simultaneously solving these problems, a technology for reducing carbon dioxide using light energy has attracted attention. For example, Non-Patent Document 1 discloses a carbon dioxide reduction device using light irradiation. In an oxidation tank, when light is irradiated onto an oxidation electrode, electron-hole pairs are generated and separated at the oxidation electrode, and oxygen and protons (H + ) is produced. The protons pass through the electrolyte membrane to reach the reduction cell, and the electrons flow to the reduction electrode via a conductor. In the reduction cell, a reduction reaction of carbon dioxide occurs at the reduction electrode in the solution, involving the protons, electrons, and carbon dioxide dissolved in the solution. This reduction reaction produces carbon monoxide, formic acid, methane, and other substances that can be used as energy resources.

[0005] In the carbon dioxide reduction device of Non-Patent Document 1, the reduction electrode is immersed in a solution and carbon dioxide is dissolved in the solution to supply carbon dioxide to the reduction electrode. However, in this carbon dioxide reduction method, because the reduction electrode is immersed in the solution, there are limits to the concentration of carbon dioxide dissolved in the solution and the diffusion coefficient of carbon dioxide in the solution, which limits the amount of carbon dioxide that can be supplied to the reduction electrode.

[0006] Therefore, in order to increase the amount of carbon dioxide supplied to the reduction electrode, research is being conducted into removing the solution in the reduction tank and directly supplying carbon dioxide to the reduction electrode. In Non-Patent Document 2, a reduction tank having a structure in which gas-phase carbon dioxide is directly supplied to the reduction electrode is used, thereby increasing the amount of carbon dioxide supplied to the reduction electrode and promoting the carbon dioxide reduction reaction. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Satoshi Yotsuhashi and 6 others, “CO2 Conversion with Light and Water by GaN Photo electroade”, Japanese Journal of Applied Physics, 51, 2012, p.02BP07-1-p.02BP07-3 [Non-patent document 2] Qingxin Jia and 2 others, “Direct Gas-phase CO2 reduction for Solar Methane Generation Using a Gas Diffusion Electrode with a BiVO4:Mo and a Cu-In-e Photoanode”, Chem.Lett., 47, January 13, 2018, p.436-p.439 Summary of the Invention [Problem to be solved by the invention]

[0008] However, as the reduction reaction progresses, carbon dioxide reduction products are generated on the reaction surface of the reduction electrode, producing not only gaseous hydrogen, carbon monoxide, and methane, but also liquids such as formic acid, methanol, and ethanol. Over time, the electrolyte in the oxidation chamber gradually seeps into the reduction chamber through the electrolyte membrane. As a result, the reaction surface (reaction site) of the reduction electrode becomes covered with these liquids, preventing the carbon dioxide reduction reaction from proceeding. Therefore, conventional carbon dioxide reduction devices have the problem of a decline in the efficiency of the carbon dioxide reduction reaction within several tens of hours.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technique capable of improving the efficiency of the carbon dioxide reduction reaction. [Means for solving the problem]

[0010] An electrolyte membrane according to one embodiment of the present invention is used in a carbon dioxide reduction device that is disposed between and in contact with an electrolytic solution in an oxidation cell and a reduction electrode in a reduction cell, and that performs a carbon dioxide reduction reaction by bringing carbon dioxide into direct contact with the reduction electrode, and is provided with a water-repellent film on a portion of its surface that comes into contact with the electrolytic solution.

[0011] An electrolyte membrane manufacturing method according to one aspect of the present invention is a method for manufacturing the above-described electrolyte membrane, which includes the steps of applying a water-soluble polymer to one surface of the electrolyte membrane, removing moisture from the water-soluble polymer, performing a water-repellent treatment on both surfaces of the electrolyte membrane, and removing the water-soluble polymer from one surface of the electrolyte membrane.

[0012] An electrolyte membrane manufacturing method according to one aspect of the present invention includes the steps of applying a water-repellent polymer to one side of the electrolyte membrane and removing the solvent from the water-repellent polymer.

[0013] An electrolyte membrane manufacturing method according to one aspect of the present invention is a method for manufacturing the above-described electrolyte membrane, which comprises the steps of applying a water-soluble polymer to one side of the electrolyte membrane, removing moisture from the water-soluble polymer, performing a water-repellent treatment on both sides of the electrolyte membrane by heating and depositing a water-repellent small molecule, and removing the water-soluble polymer from one side of the electrolyte membrane.

[0014] An electrolyte membrane manufacturing method according to one aspect of the present invention includes a step of performing a water-repellent treatment on one surface of the electrolyte membrane by heating and vapor-depositing a water-repellent low-molecular-weight material. [Effects of the Invention]

[0015] According to the present invention, the efficiency of the carbon dioxide reduction reaction can be improved. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of a carbon dioxide reduction device according to the first embodiment. [Figure 2] FIG. 2 is a diagram showing an example of the configuration of a water-repellent film. [Figure 3] FIG. 3 is a diagram showing a first method for producing a water-repellent film. [Figure 4] FIG. 4 is a diagram showing a second method for producing a water-repellent film. [Figure 5] FIG. 5 is a diagram showing a third method for producing a water-repellent film. [Figure 6] FIG. 6 is a diagram showing a fourth method for producing a water-repellent film. [Figure 7] FIG. 7 is a diagram showing the measurement results of the Faraday efficiency of formic acid according to the first embodiment. [Figure 8] FIG. 8 is a diagram showing an example of the configuration of a carbon dioxide reduction device according to the second embodiment. [Figure 9] FIG. 9 is a diagram showing the measurement results of the Faraday efficiency of formic acid according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the description of the drawings, the same parts are designated by the same reference numerals and the description thereof will be omitted.

[0018] [First embodiment] Fig. 1 is a diagram showing an example of the configuration of a carbon dioxide reduction device 100 according to the first embodiment. As shown in Fig. 1, the carbon dioxide reduction device 100 includes an oxidation electrode 1, an oxidation tank 2, an electrolyte 3, a reduction electrode 4, a reduction tank 5, an electrolyte membrane 6, a conducting wire 7, a light source 8, and a water-repellent film 9.

[0019] The oxidation electrode 1 is immersed in an electrolyte 3 in an oxidation tank 2. The oxidation electrode 1 is formed by forming a semiconductor on a substrate having a predetermined area. For example, the oxidation electrode 1 is formed by forming a film of a compound exhibiting photoactivity or redox activity, such as a nitride semiconductor, titanium oxide, amorphous silicon, a ruthenium complex, or a rhenium complex, on the surface of a sapphire substrate.

[0020] The oxidation bath 2 holds an electrolyte 3 in which the oxidation electrode 1 is immersed.

[0021] The electrolyte 3 is placed in the oxidation tank 2. The electrolyte 3 is, for example, an aqueous potassium hydrogen carbonate solution, an aqueous sodium hydrogen carbonate solution, an aqueous potassium chloride solution, an aqueous sodium chloride solution, an aqueous potassium hydroxide solution, an aqueous rubidium hydroxide solution, or an aqueous cesium hydroxide solution.

[0022] The reduction electrode 4 is disposed in the reduction tank 5. Like the oxidation electrode 1, the reduction electrode 4 is formed on a substrate having a predetermined area. The reduction electrode 4 is, for example, a porous body made of copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, or an alloy thereof. Alternatively, the reduction electrode 4 may be a compound such as silver oxide, copper oxide, copper(II) oxide, nickel oxide, indium oxide, tin oxide, tungsten oxide, tungsten(VI) oxide, or copper oxide, or a porous metal complex having a metal ion and anionic ligand.

[0023] The reduction tank 5 has the reduction electrode 4 disposed therein and holds gaseous carbon dioxide supplied from the outside via a pipe.

[0024] The electrolyte membrane 6 is disposed between the oxidation tank 2 and the reduction tank 5. More precisely, the electrolyte membrane 6 is disposed between the electrolytic solution 3 and the reduction electrode 4 so as to be in contact with each other. The electrolyte membrane 6 is, for example, Nafion (registered trademark), ForeBlue, or Aquivion, which are electrolyte membranes having a carbon-fluorine skeleton, or Selemion or Neocepta, which are electrolyte membranes having a hydrocarbon skeleton.

[0025] The conductive wire 7 physically and electrically connects the oxidation electrode 1 and the reduction electrode 4 together.

[0026] The light source 8 is disposed in the vicinity of the oxidation vessel 2. The light source 8 is, for example, sunlight, a xenon lamp, a solar simulant light source, a halogen lamp, a mercury lamp, or a light source that is a combination of these.

[0027] The reduction electrode 4 and the electrolyte membrane 6 may be formed using an integrated material. For example, this can be achieved using a gas diffusion electrode (GDE (registered trademark)) made of a porous substrate and a catalyst. The gas diffusion electrode can separate liquid and gas and allows cations to move within the electrode, so it has the same functions as both the reduction electrode 4 and the electrolyte membrane 6.

[0028] 1, the reduction electrode 4 and the electrolyte membrane 6 are each drawn to have a large width in the horizontal direction of the page, but they may be formed into a thin plate shape with a smaller width in the horizontal direction and a flat surface in the depth direction of the page. By bonding the reduction electrode 4 and the electrolyte membrane 6 together at their flat surfaces, the reaction field at their contact surfaces can be maximized.

[0029] In the carbon dioxide reduction device 100, in the oxidation tank 2, an oxidation reaction of water in the electrolytic solution 3 is carried out by irradiation with light (light energy) from a light source 8 using an electrolytic solution 3 and a semiconductor oxidation electrode 1 immersed in the electrolytic solution 3. In the reduction tank 5, a reduction reaction of carbon dioxide is carried out using a reduction electrode 4 connected to the oxidation electrode 1 via a lead wire 7 and carbon dioxide in direct contact with the reduction electrode 4.

[0030] Specifically, when light source 8 irradiates the oxidation vessel 2 from the bottom, electron-hole pairs are generated and separated at oxidation electrode 1 in oxidation vessel 2, which receives the irradiated light, and oxygen and protons are generated by an oxidation reaction of water in electrolyte 3. The protons pass through electrolyte membrane 6 and reach reduction electrode 4 in reduction vessel 5 from electrolyte 3 in oxidation vessel 2. Electrons flow via conductor 7 from oxidation electrode 1 in oxidation vessel 2 to reduction electrode 4 in reduction vessel 5. In reduction vessel 5, a carbon dioxide reduction reaction occurs at reduction electrode 4 between the protons, electrons, and gaseous carbon dioxide that is in direct contact with reduction electrode 4. This oxidation-reduction reaction generates carbon monoxide, formic acid, methane, and other substances that can be used as energy resources.

[0031] At this time, if a strong alkaline aqueous solution, for example, a 1.0 mol / L aqueous solution of sodium hydroxide, is used as the electrolyte solution 3 in the oxidation tank 2, the electrolyte membrane 6 swells, and the electrolyte solution 3 passes through the pores of the electrolyte membrane 6 and seeps out onto the surface of the reduction electrode 4 in the reduction tank 5. In order to prevent such seepage of the electrolyte solution 3 from the electrolyte membrane 6, the surface of the electrolyte membrane 6 on the oxidation tank 2 side that comes into contact with the electrolyte solution 3 may be treated to be water-repellent. However, since protons, which are raw materials for the reduction reaction, need to be moved using the water in the electrolyte membrane 6 as a medium, if the surface of the electrolyte membrane 6 is completely covered, there is a risk that the reduction reaction will not proceed on the reduction tank 5 side.

[0032] Therefore, in this embodiment, as shown in FIG. 1 and enlarged in FIG. 2, a water-repellent film 9 is provided on only a portion of the surface of the electrolyte membrane 6 that comes into contact with the electrolyte solution 3, rather than covering the entire surface of the electrolyte membrane 6. For example, multiple water-repellent films 9 are formed on the surface at predetermined intervals. By forming such a water-repellent film 9, its water repellency can prevent the electrolyte solution 3 in the oxidation vessel 2 from penetrating into the electrolyte membrane 6, thereby preventing leakage of the electrolyte solution 3 to the reduction electrode 4 and preventing the reaction sites of the reduction electrode 4 from being filled with the electrolyte solution 3. Furthermore, because the water-repellent film 9 is formed on only a portion of the surface of the electrolyte membrane 6, rather than on the entire surface, the electrolyte membrane 6 can remain in a state where protons can pass through. As a result, the carbon dioxide reduction reaction can proceed, and a decrease in the reduction reaction efficiency can be suppressed.

[0033] Next, a method for manufacturing the water-repellent film 9 will be described.

[0034] There are two types of water-repellent treatments for producing the water-repellent film 9: a liquid-phase method and a gas-phase method. The liquid-phase method involves immersing an object in a fluorine-based solvent in which a fluorine-based polymer (water-repellent agent) has been dissolved, for example by dip coating, and then heating the object to remove the solvent, thereby precipitating the fluorine-based polymer. An alternative to the liquid-phase method is to form a film of the fluorine-based solvent on the surface of the object using a cast coating method or spin coating method, and then heating the object to remove the solvent, thereby precipitating the fluorine-based polymer. The gas-phase method involves placing the object and a fluorine-based small molecule (silane coupling agent) that serves as the water-repellent agent in the same sealed space, heating the fluorine-based small molecule to vaporize it, and then depositing the fluorine-based small molecule on the surface of the object.

[0035] 3 is a diagram showing a first manufacturing method for the water-repellent film 9. The first manufacturing method is a method for manufacturing the water-repellent film 9 by a liquid phase method. Nafion was used for the electrolyte membrane 6. Optool DSX was used as the water-repellent agent.

[0036] First, a water-soluble polymer is dissolved in pure water to prepare a 1% polyvinyl alcohol aqueous solution (first step S101). Next, the polyvinyl alcohol aqueous solution is dropped onto one side of a Nafion membrane using a spin coating method to form a polyvinyl alcohol film on that side (second step S102).

[0037] Next, the Nafion membrane is placed in a 60°C oven for 1 hour to evaporate the water content of the polyvinyl alcohol (third step S103). This step forms a polymer film (water-soluble polymer) on one side of the Nafion membrane. Next, the Nafion membrane is dip-coated by immersing it in Optool DSX solution (water-repellent polymer) for 1 minute and then pulling it out (fourth step S104). This step forms a water-repellent film on both sides of the Nafion membrane.

[0038] Finally, the Nafion membrane is washed with pure water (fifth step S105). This step removes the polyvinyl alcohol (water-soluble polymer) on which the water-repellent film is applied. In other words, the polymer film (water-soluble polymer) is removed from one side of the Nafion membrane, and the water-repellent film on the polymer film is also removed.

[0039] These steps allow the formation of a water-repellent film on only one side of the Nafion membrane. This method of forming a water-repellent film using dip coating can form a relatively thick water-repellent film on the order of micrometers.

[0040] 4 is a diagram showing a second manufacturing method for the water-repellent film 9. The second manufacturing method is a manufacturing method for the water-repellent film 9 by another liquid phase method. Nafion was used for the electrolyte membrane 6. Optool DSX was used as the water-repellent agent.

[0041] First, Optool DSX (a water-repellent polymer) is dropped onto one side of the Nafion membrane using a spin coating method (first step S201). After that, the Nafion membrane is left to stand and the solvent in Optool DSX is evaporated (second step S202). Through this step, a polymer film (water-repellent film) is formed on one side of the Nafion membrane.

[0042] These processes allow the formation of a water-repellent film on only one side of the Nafion membrane. This spin-coating method uses centrifugal force, so in principle it is possible to form a thin water-repellent film on the order of submicrometers.

[0043] 5 is a diagram showing a third manufacturing method for the water-repellent film 9. The third manufacturing method is a method for manufacturing the water-repellent film 9 by a vapor phase method. Nafion was used for the electrolyte membrane 6. A fluorine-based silane coupling agent (for example, heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane) was used as the water-repellent agent.

[0044] First, a water-soluble polymer is dissolved in pure water to prepare a 1% polyvinyl alcohol aqueous solution (first step S301). Next, the polyvinyl alcohol aqueous solution is dropped onto one side of a Nafion membrane using a spin coating method to form a polyvinyl alcohol film on that side (second step S302).

[0045] Next, the Nafion membrane is placed in a 60°C oven for 1 hour to evaporate the water in the polyvinyl alcohol (third step S303). This step forms a polymer film (water-soluble polymer) on one side of the Nafion membrane. Next, the Nafion membrane and a fluorine-based silane coupling agent (water-repellent low molecular weight) are placed in a Teflon container and sealed (fourth step S304).

[0046] Next, the Teflon container is placed in an oven and heated to 150°C (fifth step S305). This step evaporates the fluorine-based silane coupling agent, forming a water-repellent film on both sides of the Nafion membrane. Finally, the Nafion membrane is washed with pure water (sixth step S306). This step removes the polyvinyl alcohol (water-soluble polymer) on which the water-repellent film was applied. In other words, the polymer film (water-soluble polymer) is removed from one side of the Nafion membrane, and the water-repellent film on the polymer film is also removed.

[0047] These processes allow the formation of a water-repellent film on only one side of the Nafion membrane. This vapor-phase method forms a monolayer on the Nafion surface, allowing the formation of an extremely thin water-repellent film on the order of nanometers.

[0048] 6 is a diagram showing a fourth manufacturing method for the water-repellent film 9. The fourth manufacturing method is a manufacturing method for the water-repellent film 9 by another vapor phase method. Nafion was used for the electrolyte membrane 6. A fluorine-based silane coupling agent (for example, heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane) was used as the water-repellent agent.

[0049] First, the Nafion membrane is placed in close contact with the bottom of a Teflon container and left to stand, and then a fluorine-based silane coupling agent (water-repellent low molecular weight) is poured on top of it and sealed (first step S401). Finally, while maintaining this state, the Teflon container is placed in an oven and heated to 150°C (second step S402). This step evaporates the fluorine-based silane coupling agent, forming a water-repellent film on one side of the Nafion membrane.

[0050] These processes allow the formation of a water-repellent film on only one side of the Nafion membrane. This vapor-phase method forms a monolayer on the Nafion surface, allowing the formation of an extremely thin water-repellent film on the order of nanometers.

[0051] Next, electrochemical measurements using the carbon dioxide reduction device 100 and the results of the measurements will be described.

[0052] First, a thin film of n-type semiconductor gallium nitride (GaN) and aluminum gallium nitride (AlGaN) were epitaxially grown in that order on a sapphire substrate, and then nickel (Ni) was vacuum-deposited on top of that and heat-treated to form a nickel oxide (NiO) promoter thin film. This promoter thin film was then used as oxidation electrode 1, and oxidation electrode 1 was immersed in electrolyte 3, a 1.0 mol / L potassium hydroxide aqueous solution, in oxidation tank 2.

[0053] Furthermore, a reduction electrode 4 was formed using a porous copper body, and the reduction electrode 4 was connected to the oxidation electrode 1 via a lead wire 7 , and the reduction electrode 4 was placed in the reduction tank 5 .

[0054] Nafion was used for the electrolyte membrane 6 that physically separates the oxidation tank 2 and the reduction tank 5. Of the two surfaces of the electrolyte membrane 6, one surface on which the water-repellent film 9 was formed was placed so as to be in contact with the electrolyte solution 3 in the oxidation tank 2, and the other surface was placed so as to be in contact with the reduction electrode 4 in the reduction tank 5.

[0055] A 300 W xenon lamp was used as the light source 8. Wavelengths of 450 nm or more were cut off with a filter, and the illuminance was set to 6.6 mW / cm. 2 The irradiation surface of the oxidation electrode 1 was set to 2.5 cm 2 It was decided.

[0056] Nitrogen and carbon dioxide were then supplied to the oxidation tank 2 and reduction tank 5 at a flow rate of 5 ml / min and a pressure of 0.5 MPa, respectively. Nitrogen was bubbled into the oxidation tank 2 for the purpose of analyzing the reaction products. The atmosphere inside the oxidation tank 2 and reduction tank 5 was thoroughly replaced with nitrogen and carbon dioxide, respectively, and light was irradiated from the light source 8. Thereafter, the reduction reaction of carbon dioxide proceeded on the surface of the porous copper body serving as the reduction electrode 4.

[0057] At this time, the current flowing between the oxidation electrode 1 and the reduction electrode 4 due to the irradiated light was measured using an electrochemical measurement device (Solartron, Model 1287 potentiogalvanostat). In addition, the gas and liquid produced in the oxidation tank 2 and the reduction tank 5 were sampled, and the reaction products were analyzed using a gas chromatograph, a liquid chromatograph, and a gas chromatograph mass spectrometer.

[0058] In particular, in this embodiment, the effect of the water-repellent film 9 formed on the surface of the electrolyte membrane 6 was examined by determining the Faraday efficiency of the carbon dioxide reduction reaction. The method for calculating the Faraday efficiency of the carbon dioxide reduction reaction will be described later.

[0059] In Example 1, Nafion was used as the electrolyte membrane 6, Optool DSX was used as the water repellent agent, and a 1% concentration aqueous solution of polyvinyl alcohol dissolved in pure water was used as the water-soluble polymer, and a water-repellent film 9 manufactured by Manufacturing Method 1 was used.

[0060] In Example 2, Nafion was used for the electrolyte membrane 6, Optool DSX was used for the water repellent agent, and the water repellent film 9 produced by Production Method 2 was used.

[0061] In Example 3, Nafion was used as the electrolyte membrane 6, heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane was used as the water repellent agent, and a 1% concentration aqueous solution of polyvinyl alcohol dissolved in pure water was used as the water-soluble polymer, and a water-repellent film 9 produced by Production Method 3 was used.

[0062] In Example 4, Nafion was used for the electrolyte membrane 6, heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane was used as the water repellent agent, and the water repellent film 9 produced by Production Method 4 was used.

[0063] In the comparative example, Nafion without the water-repellent film 9 was used as the electrolyte membrane 6 as it was.

[0064] 7 is a diagram showing the measurement results of the Faraday efficiency of formic acid according to the first embodiment. In the comparative example in which the water-repellent film 9 was not formed, the Faraday efficiency decreased after 6 hours. On the other hand, in Examples 1 to 4 in which the water-repellent film 9 was formed, the Faraday efficiency did not decrease even after 6 hours. This is because the introduction of the water-repellent film 9 into the Nafion membrane suppressed leakage of the electrolyte solution 3 to the reduction electrode 4, and the reaction sites of the reduction electrode 4 were no longer filled with the electrolyte solution 3.

[0065] The coverage rate of the water-repellent film 9 relative to the Nafion film was estimated using the Cassie-Baxter equation. If the contact angle of the Nafion film coated with the water-repellent film 9 is θ, the contact angle of the Nafion film surface is θ1, the ratio of the Nafion film surface is f1, the contact angle of the water-repellent film 9 surface is θ2, and the ratio of the water-repellent film 9 surface is f2, then the relationship shown in equation (1) holds.

[0066] cosθ=f1×cosθ1+f2×cosθ2 (1) In each of Examples 1 to 4, the contact angle θ of the Nafion membrane coated with the water-repellent film 9 was 100°, 95°, 75°, and 70°, respectively. The coverage of the water-repellent film 9 with respect to the Nafion membrane was estimated to be 84%, 79%, 64%, and 60%, respectively. This suggests that a water-repellent film 9 that did not cover the entire surface of the electrolyte membrane 6 was formed.

[0067] Here, we will explain how to calculate the faradaic efficiency of the carbon dioxide reduction reaction. The faradaic efficiency of carbon dioxide indicates the ratio of the number of electrons used in the carbon dioxide reduction reaction to the number of electrons transferred between the oxidation electrode 1 and the reduction electrode 4 by light irradiation or application of current and voltage, and can be calculated using equation (2).

[0068] Faraday efficiency = {number of electrons in the reduction reaction} / {number of electrons transferred between electrodes} (2) The "number of electrons in the reduction reaction" in equation (2) is calculated by converting the measured cumulative amount of carbon dioxide reduction product produced into the number of electrons required for that production reaction. For example, when the reduction product is gas, the "number of electrons in the reduction reaction" can be calculated using equation (3).

[0069] Number of electrons in each reduction reaction (C) = {(A × B × Z × F × T × 10 -6 )} / V g ···(3) A is the concentration (ppm) of the reduction reaction product. B is the flow rate (L / sec) of the carrier gas. Z is the number of electrons required for the reduction reaction. F is the Faraday constant (C / mol). T is the time of light irradiation or current / voltage application (sec). V g is the molar volume of the gas (L / mol).

[0070] When the reduction product is liquid, the "number of electrons in the reduction reaction" can be calculated using equation (4).

[0071] Number of electrons in each reduction reaction (C) = C × V l ×Z×F···(4) C is the concentration of the reduction reaction product (mol / L). l is the volume of the liquid sample (L). Z is the number of electrons required for the reduction reaction. F is Faraday's constant (C / mol).

[0072] The above has described the first embodiment. According to the carbon dioxide reduction device 100 according to the first embodiment, it is possible to provide a carbon dioxide reduction device 100 that can proceed with the carbon dioxide reduction reaction without reducing the faradaic efficiency.

[0073] That is, in the first embodiment, the carbon dioxide reduction device 100 includes an oxidation tank 2 in which an oxidation reaction of water is carried out by irradiation with light from a light source 8 using an electrolytic solution 3 and an oxidation electrode 1 made of a semiconductor immersed in the electrolytic solution 3, a reduction tank 5 in which a reduction reaction of carbon dioxide is carried out using a reduction electrode 4 connected to the oxidation electrode 1 via a lead wire 7 and carbon dioxide brought into direct contact with the reduction electrode 4, and an electrolyte membrane 6 disposed between and in contact with the electrolytic solution 3 in the oxidation tank 2 and the reduction electrode 4 in the reduction tank 5, respectively, and the electrolyte membrane 6 is provided with a water-repellent film 9 on a portion of its surface in contact with the electrolytic solution 3.

[0074] Therefore, the water repellency of the water-repellent film 9 provided on the surface of the electrolyte solution 3 prevents the electrolyte solution 3 in the oxidation vessel 2 from penetrating into the electrolyte membrane 6, thereby preventing leakage of the electrolyte solution 3 to the reduction electrode 4 and preventing the reaction sites of the reduction electrode 4 from being filled with the electrolyte solution 3. Furthermore, since the water-repellent film 9 is provided on a portion of the surface of the electrolyte membrane 6, a state in which protons can pass through the electrolyte membrane 6 can be maintained. As a result, the reduction reaction of carbon dioxide can proceed, and a decrease in the efficiency of the reduction reaction can be prevented.

[0075] In the above experiment, light was generated by a xenon lamp in order to quantitatively control the amount of light irradiated onto the oxidation electrode 1, but it is also possible to use sunlight or the like to cause the oxidation reaction.

[0076] [Second embodiment] In the first embodiment, a case where a light source 8 and an oxidation electrode 1 made of a semiconductor are used has been described. In the second embodiment, instead of these, an external power source and an oxidation electrode 1 made of a metal are used to cause an oxidation-reduction reaction.

[0077] FIG. 8 is a diagram showing an example of the configuration of a carbon dioxide reduction device 100 according to the second embodiment. The oxidation electrode 1 is made of platinum. Alternatively, the oxidation electrode 1 may be made of, for example, gold or silver. The external power supply 10 is an electrochemical measurement device, and is connected in series to the conductor 7 connecting the oxidation electrode 1 and the reduction electrode 4. The power supply 10 may be any other power supply device. The other components are the same as those in the first embodiment.

[0078] In the carbon dioxide reduction device 100 according to this embodiment, an oxidation reaction of water in the electrolytic solution 3 is carried out in an oxidation tank 2 by current and voltage (electrical energy) from a power source 10 using an electrolytic solution 3 and an oxidation electrode 1 made of platinum (metal) immersed in the electrolytic solution 3. In a reduction tank 5, a reduction reaction of carbon dioxide is carried out using a reduction electrode 4 connected to the power source 10 (a source of electrical energy) and carbon dioxide in direct contact with the reduction electrode 4.

[0079] Specifically, when power supply 10 applies a current and voltage to conductor 7, electron-hole pairs are generated and separated at oxidation electrode 1 in oxidation cell 2, and oxygen and protons are generated by an oxidation reaction of water in electrolyte 3. The protons pass through electrolyte membrane 6 and reach reduction electrode 4 in reduction cell 5 from electrolyte 3 in oxidation cell 2. Electrons flow from power supply 10 via conductor 7 to reduction electrode 4 in reduction cell 5. In reduction cell 5, a carbon dioxide reduction reaction occurs at reduction electrode 4 between the protons, electrons, and gaseous carbon dioxide that is in direct contact with reduction electrode 4.

[0080] In the second embodiment, similarly to the first embodiment, the water-repellent film 9 is provided on only a portion of the surface of the electrolyte membrane 6 that comes into contact with the electrolytic solution 3, so as not to cover the entire surface of the electrolyte membrane 6. For example, a plurality of water-repellent films 9 are formed on the surface at predetermined intervals. Furthermore, similarly to the first embodiment, the first to fourth production methods are used to produce the water-repellent film 9.

[0081] 9 is a diagram showing the measurement results of the Faraday efficiency of formic acid according to the second embodiment. Examples 5 to 8 are examples using the same electrolyte membrane 6 as Examples 1 to 4 described in the first embodiment. A comparative example is also shown in which Nafion without the water-repellent film 9 is used as the electrolyte membrane 6.

[0082] In Examples 5 to 8, the Faraday efficiency did not decrease even after 6 hours had passed. This is because the introduction of the water-repellent film 9 on the Nafion membrane suppressed leakage of the electrolyte solution 3 to the reduction electrode 4, and the reaction sites of the reduction electrode 4 were no longer filled with the electrolyte solution 3.

[0083] The above has described the second embodiment. According to the carbon dioxide reduction device 100 according to the second embodiment, it is possible to provide a carbon dioxide reduction device 100 that can proceed with the carbon dioxide reduction reaction without reducing the faradaic efficiency.

[0084] That is, in the second embodiment, a carbon dioxide reduction device 100 is provided with an oxidation tank 2 that performs a water oxidation reaction by current and voltage from a power source 10 using an electrolytic solution 3 and an oxidation electrode 1 made of platinum (metal) immersed in the electrolytic solution 3, a reduction tank 5 that performs a carbon dioxide reduction reaction using a reduction electrode 4 connected to the power source 10 and carbon dioxide that is in direct contact with the reduction electrode 4, and an electrolyte membrane 6 that is arranged between and in contact with the electrolytic solution 3 in the oxidation tank 2 and the reduction electrode 4 in the reduction tank 5, and the electrolyte membrane 6 is provided with a water-repellent film 9 on a portion of its surface that comes into contact with the electrolytic solution 3.

[0085] Therefore, the water repellency of the water-repellent film 9 provided on the surface of the electrolyte solution 3 prevents the electrolyte solution 3 in the oxidation vessel 2 from penetrating into the electrolyte membrane 6, thereby preventing leakage of the electrolyte solution 3 to the reduction electrode 4 and preventing the reaction sites of the reduction electrode 4 from being filled with the electrolyte solution 3. Furthermore, since the water-repellent film 9 is provided on a portion of the surface of the electrolyte membrane 6, a state in which protons can pass through the electrolyte membrane 6 can be maintained. As a result, the reduction reaction of carbon dioxide can proceed, and a decrease in the efficiency of the reduction reaction can be prevented.

[0086] [others] The present invention can be widely used in the field of carbon dioxide recycling. Although light energy is used in the first embodiment and electrical energy is used in the second embodiment, other renewable energy may also be used. Furthermore, the first and second embodiments can be combined.

[0087] The present invention can be applied to any electrolyte membrane 6 used in a carbon dioxide reduction device 100 that is disposed between and in contact with the electrolytic solution 3 in an oxidation tank 2 and the reduction electrode 4 in a reduction tank 5, and that brings carbon dioxide into direct contact with the reduction electrode 4 to perform a carbon dioxide reduction reaction. [Explanation of symbols]

[0088] 1: Oxidation electrode 2: Oxidation tank 3: Electrolyte 4: Reduction electrode 5: Reduction tank 6: Electrolyte membrane 7: Conductor 8:Light source 9: Water-repellent film 10: Power supply 100: Carbon dioxide reduction device

Claims

1. A method for manufacturing an electrolyte membrane for use in a carbon dioxide reduction device that is disposed between an electrolytic solution in an oxidation tank and a reduction electrode in a reduction tank in contact with each other, and that performs a carbon dioxide reduction reaction by directly contacting carbon dioxide with the reduction electrode, the method comprising: applying a water-soluble polymer to one side of the electrolyte membrane; removing water from the water-soluble polymer; a step of performing a water-repellent treatment on both surfaces of the electrolyte membrane; removing the water-soluble polymer from one surface of the electrolyte membrane; A method for manufacturing an electrolyte membrane comprising the steps of:

2. A method for manufacturing an electrolyte membrane, which is used in a carbon dioxide reduction device that is disposed between an electrolyte solution in an oxidation tank and a reduction electrode in a reduction tank, and that performs a carbon dioxide reduction reaction by directly contacting carbon dioxide with the reduction electrode, and which manufactures an electrolyte membrane having a water-repellent film on a portion of its surface that contacts the electrolyte solution, applying a water-repellent polymer to one side of the electrolyte membrane; removing the solvent from the water-repellent polymer; A method for manufacturing an electrolyte membrane comprising the steps of:

3. A method for manufacturing an electrolyte membrane, which is used in a carbon dioxide reduction device that is disposed between an electrolyte solution in an oxidation tank and a reduction electrode in a reduction tank, and that performs a carbon dioxide reduction reaction by directly contacting carbon dioxide with the reduction electrode, and which manufactures an electrolyte membrane having a water-repellent film on a portion of its surface that contacts the electrolyte solution, applying a water-soluble polymer to one side of the electrolyte membrane; removing water from the water-soluble polymer; a step of performing a water-repellent treatment on both surfaces of the electrolyte membrane by heating and vapor-depositing a water-repellent low-molecular-weight material; removing the water-soluble polymer from one surface of the electrolyte membrane; A method for manufacturing an electrolyte membrane comprising the steps of:

4. A method for manufacturing an electrolyte membrane, which is used in a carbon dioxide reduction device that is disposed between an electrolyte solution in an oxidation tank and a reduction electrode in a reduction tank, and that performs a carbon dioxide reduction reaction by directly contacting carbon dioxide with the reduction electrode, and which manufactures an electrolyte membrane having a water-repellent film on a portion of its surface that contacts the electrolyte solution, a step of applying a water-repellent treatment to one side of the electrolyte membrane by heating and depositing a water-repellent low-molecular-weight material; A method for manufacturing an electrolyte membrane comprising the steps of:

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