JP7783508B2 - Electrolyte membrane manufacturing method - Google Patents

Description

The present invention relates to an electrolyte membrane and a method for manufacturing an electrolyte membrane.

The increase in carbon dioxide concentrations 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 necessitate a reassessment of energy supplies that rely on fossil fuels, and the creation of next-generation energy sources is required.

To obtain 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 be used to create chemical products from fossil fuels.

As a method for simultaneously solving these problems, technology that uses light energy to reduce carbon dioxide has attracted attention. For example, Non-Patent Document 1 discloses a carbon dioxide reduction device that uses 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 ⁺ ) are generated by an oxidation reaction of water in the electrolyte. The protons pass through the electrolyte membrane to reach the reduction tank, and the electrons flow to the reduction electrode via a conductor. In the reduction tank, 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.

In the carbon dioxide reduction device described in Non-Patent Document 1, the reduction electrode is immersed in a solution, and carbon dioxide is dissolved in the solution, thereby supplying carbon dioxide to the reduction electrode. However, because this carbon dioxide reduction method requires the reduction electrode to be immersed in the solution, there are limitations 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.

Therefore, in order to increase the amount of carbon dioxide supplied to the reduction electrode, research is being conducted into removing the solution from the reduction tank and supplying carbon dioxide directly to the reduction electrode. In Non-Patent Document 2, a reduction tank designed to directly supply gaseous carbon dioxide 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.

Satoshi Yotsuhashi, 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 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

However, as the reduction reaction progresses, carbon dioxide reduction products are produced 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. Furthermore, 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 had the problem of a decline in the efficiency of the carbon dioxide reduction reaction within a few tens of hours.

The present invention was made in consideration of the above circumstances, and its object is to provide technology that can improve the efficiency of carbon dioxide reduction reactions.

One embodiment of the electrolyte membrane of the present invention is an electrolyte membrane used in a carbon dioxide reduction device that is placed between and in contact with an electrolyte solution in an oxidation cell and a reduction electrode in a reduction cell, and that performs a carbon dioxide reduction reaction by directly contacting carbon dioxide with the reduction electrode, and is provided with a water-repellent film on a portion of its surface that comes into contact with the electrolyte solution.

One aspect of the present invention is a method for manufacturing an 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, applying a water-repellent treatment to both sides of the electrolyte membrane, and removing the water-soluble polymer from one side of the electrolyte membrane.

One aspect of the present invention is a method for manufacturing an electrolyte membrane, which comprises the steps of applying a water-repellent polymer to one side of the electrolyte membrane and removing the solvent from the water-repellent polymer.

One aspect of the present invention is a method for manufacturing an 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.

One aspect of the present invention is a method for manufacturing an electrolyte membrane, which includes a step of performing a water-repellent treatment on one side of the electrolyte membrane by heating and evaporating a water-repellent small molecule.

The present invention can improve the efficiency of the carbon dioxide reduction reaction.

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

An embodiment of the present invention will be described below with reference to the drawings. In the description of the drawings, identical parts are designated by the same reference numerals and their description will be omitted.

[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.

The oxidation electrode 1 is immersed in the electrolyte 3 in the oxidation tank 2. The oxidation electrode 1 is formed by forming a semiconductor on a substrate of a predetermined area. For example, the oxidation electrode 1 is formed by forming a film of a photoactive or redox-active compound, such as a nitride semiconductor, titanium oxide, amorphous silicon, ruthenium complex, or rhenium complex, on the surface of a sapphire substrate.

The oxidation tank 2 holds the electrolyte 3 in which the oxidation electrode 1 is immersed.

The electrolyte 3 is placed in the oxidation tank 2. The electrolyte 3 is, for example, an aqueous solution of potassium bicarbonate, an aqueous solution of sodium bicarbonate, an aqueous solution of potassium chloride, an aqueous solution of sodium chloride, an aqueous solution of potassium hydroxide, an aqueous solution of rubidium hydroxide, or an aqueous solution of cesium hydroxide.

The reduction electrode 4 is placed in the reduction tank 5. Like the oxidation electrode 1, the reduction electrode 4 is formed on a substrate with a predetermined area. The reduction electrode 4 is a porous body made of, for example, copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, or an alloy thereof. The reduction electrode 4 may also 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.

The reduction tank 5 has a reduction electrode 4 placed inside and holds gaseous carbon dioxide supplied from the outside via piping.

The electrolyte membrane 6 is disposed between the oxidation cell 2 and the reduction cell 5. More precisely, the electrolyte membrane 6 is disposed between the electrolyte solution 3 and the reduction electrode 4, in contact with each of them. The electrolyte membrane 6 is, for example, an electrolyte membrane with a carbon-fluorine skeleton, such as Nafion (registered trademark), ForeBlue, or Aquivion, or an electrolyte membrane with a hydrocarbon skeleton, such as Selemion or Neocepta.

The conducting wire 7 physically and electrically connects the oxidation electrode 1 and the reduction electrode 4.

The light source 8 is arranged close to the oxidation tank 2. The light source 8 is, for example, sunlight, a xenon lamp, a solar simulant, a halogen lamp, a mercury lamp, or a combination of these.

The reduction electrode 4 and electrolyte membrane 6 may be constructed using a single material. For example, this can be achieved using a gas diffusion electrode (GDE (registered trademark)) composed of a porous substrate and a catalyst. A gas diffusion electrode can separate liquid and gas and allows cations to move within the electrode, so it has the same effect as both the reduction electrode 4 and the electrolyte membrane 6.

In addition, in Figure 1, the reduction electrode 4 and electrolyte membrane 6 are each drawn to have a large width in the horizontal direction of the page, but they may also be shaped like a thin plate 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 electrolyte membrane 6 together at their flat surfaces, the reaction field at their contact surfaces can be maximized.

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

Specifically, when light source 8 irradiates the oxidation chamber 2 from the bottom, electron-hole pairs are generated and separated at oxidation electrode 1 in oxidation chamber 2, which receives the irradiated light. Oxidation reaction of water in electrolyte 3 generates oxygen and protons. The protons pass through electrolyte membrane 6 and reach reduction electrode 4 in reduction chamber 5 from electrolyte 3 in oxidation chamber 2. Electrons flow via conductor 7 from oxidation electrode 1 in oxidation chamber 2 to reduction electrode 4 in reduction chamber 5. In reduction chamber 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 energy resources.

In this case, if a strong alkaline aqueous solution, such as a 1.0 mol/L sodium hydroxide aqueous solution, 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 onto the surface of the reduction electrode 4 in the reduction tank 5. To prevent this 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 can be treated to be water-repellent. However, because protons, the raw material for the reduction reaction, need to be transported using the water in the electrolyte membrane 6 as a medium, completely covering the surface of the electrolyte membrane 6 could prevent the reduction reaction from proceeding on the reduction tank 5 side.

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-repellent properties prevent the electrolyte solution 3 in the oxidation tank 2 from penetrating into the electrolyte membrane 6, suppressing leakage of the electrolyte solution 3 to the reduction electrode 4 and preventing the reaction sites of the reduction electrode 4 from becoming 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 the entire surface, the electrolyte membrane 6 remains able to pass protons. As a result, the carbon dioxide reduction reaction can proceed, and a decrease in the reduction reaction efficiency can be suppressed.

Next, we will explain the manufacturing method of the water-repellent film 9.

There are two types of water-repellent treatments for producing the water-repellent film 9: liquid-phase and gas-phase. The liquid-phase method involves immersing an object in a fluorine-based solvent containing a fluorine-based polymer (water-repellent agent) using a dip coating method or other method, 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 low-molecular-weight water-repellent agent (silane coupling agent) in the same sealed space, heating the fluorine-based low-molecular-weight water-repellent agent to vaporize it, and then depositing the fluorine-based low-molecular-weight water-repellent agent on the surface of the object.

Figure 3 shows the first manufacturing method for the water-repellent film 9. The first manufacturing method is a liquid-phase method for manufacturing the water-repellent film 9. Nafion was used for the electrolyte membrane 6. Optool DSX was used as the water-repellent agent.

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, forming a polyvinyl alcohol film on that side (second step S102).

Next, the Nafion membrane is placed in a 60°C oven for one hour to evaporate the water in 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 one minute and then pulling it out (fourth step S104). This step forms a water-repellent film on both sides of the Nafion membrane.

Finally, the Nafion membrane is washed with pure water (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 that polymer film is also removed.

These processes allow a water-repellent film to be formed on only one side of the Nafion membrane. This method of forming a water-repellent film using dip coating can produce a relatively thick water-repellent film on the order of micrometers.

Figure 4 shows a second manufacturing method for the water-repellent film 9. The second manufacturing method is a method for manufacturing the water-repellent film 9 using another liquid phase method. Nafion was used for the electrolyte membrane 6. Optool DSX was used as the water-repellent agent.

First, Optool DSX (a water-repellent polymer) is dropped onto one side of the Nafion membrane using the spin coating method (first step S201). The Nafion membrane is then left to stand, and the solvent in the Optool DSX is allowed to evaporate (second step S202). This process forms a polymer film (water-repellent film) on one side of the Nafion membrane.

Through these processes, a water-repellent film can be formed on only one side of the Nafion membrane. This spin-coating method for forming a water-repellent film uses centrifugal force, so in principle, it is possible to form a thin water-repellent film on the order of submicrometers.

Figure 5 shows a third manufacturing method for the water-repellent film 9. The third manufacturing method is a gas-phase method for manufacturing the water-repellent film 9. Nafion was used for the electrolyte membrane 6. A fluorine-based silane coupling agent (e.g., heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane) was used as the water-repellent agent.

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, forming a polyvinyl alcohol film on that side (second step S302).

Next, the Nafion membrane is placed in a 60°C oven for one 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 compound) are placed in a Teflon container and sealed (fourth step S304).

Next, the Teflon container is placed in an oven and heated to 150°C (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 (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 that polymer film is also removed.

Through these processes, a water-repellent film can be formed on only one side of the Nafion membrane. This vapor-phase method forms a monolayer on the Nafion surface, allowing for the formation of an extremely thin water-repellent film on the order of nanometers.

Figure 6 shows a fourth manufacturing method for the water-repellent film 9. The fourth manufacturing method is a method for manufacturing the water-repellent film 9 using another vapor phase method. Nafion was used for the electrolyte membrane 6. A fluorine-based silane coupling agent (e.g., heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane) was used as the water-repellent agent.

First, the Nafion membrane is placed in close contact with the bottom of a Teflon container and left to stand. A fluorine-based silane coupling agent (water-repellent low-molecular weight) is then poured on top and the container is 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.

Through these processes, a water-repellent film can be formed on only one side of the Nafion membrane. This vapor-phase method forms a monolayer on the Nafion surface, allowing for the formation of an extremely thin water-repellent film on the order of nanometers.

Next, we will explain the electrochemical measurements and measurement results using the above-mentioned carbon dioxide reduction device 100.

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 the oxidation electrode 1, and the oxidation electrode 1 was immersed in an electrolyte 3 of a 1.0 mol/L potassium hydroxide aqueous solution in an oxidation tank 2.

In addition, a reduction electrode 4 was formed using a porous copper body, and the reduction electrode 4 was connected to the oxidation electrode 1 with a conductor 7, and the reduction electrode 4 was placed in the reduction tank 5.

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

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 irradiated surface of the oxidation electrode 1 was set to 2.5 cm ² .

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 interiors of the oxidation tank 2 and reduction tank 5 were thoroughly replaced with nitrogen and carbon dioxide, respectively, and light was then irradiated from the light source 8. The carbon dioxide reduction reaction then proceeded on the surface of the porous copper body, which served as the reduction electrode 4.

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.

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 calculating 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.

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.

In Example 2, Nafion was used as the electrolyte membrane 6, Optool DSX was used as the water repellent agent, and a water-repellent film 9 manufactured by manufacturing method 2 was used.

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% 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 3 was used.

In Example 4, Nafion was used as the electrolyte membrane 6, heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane was used as the water repellent agent, and a water-repellent film 9 manufactured by manufacturing method 4 was used.

In the comparative example, Nafion without a water-repellent film 9 was used as the electrolyte membrane 6.

Figure 7 shows the measurement results of the Faraday efficiency of formic acid in 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 3 to the reduction electrode 4, and the reaction sites of the reduction electrode 4 were no longer filled with the electrolyte 3.

The coverage rate of the water-repellent film 9 relative to the Nafion film was also 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 proportion of the Nafion film surface is f1, the contact angle of the water-repellent film 9 surface is θ2, and the proportion of the water-repellent film 9 surface is f2, then the relationship shown in equation (1) holds.

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 completely cover the surface of the electrolyte membrane 6 was successfully formed.

Here, we will explain how to calculate the Faraday efficiency of the carbon dioxide reduction reaction. The Faraday 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 due to light irradiation or application of current and voltage, and can be calculated using equation (2).

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 formula (2) is calculated by converting the measured cumulative amount of carbon dioxide reduction product produced into the number of electrons required for the production reaction. For example, when the reduction product is a gas, the "number of electrons in the reduction reaction" can be calculated using formula (3).

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 light irradiation time or current/voltage application time (sec). _{V g} is the molar volume of the gas (L/mol).

The "number of electrons in the reduction reaction" when the reduction product is liquid can be calculated using equation (4).

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); _{V 1} is the volume of the liquid sample (L); Z is the number of electrons required for the reduction reaction; F is the Faraday constant (C/mol).

The first embodiment has been described above. The carbon dioxide reduction device 100 according to the first embodiment can provide a carbon dioxide reduction device 100 that can proceed with the carbon dioxide reduction reaction without reducing the Faraday efficiency.

That is, in the first embodiment, the carbon dioxide reduction device 100 includes an oxidation tank 2 that performs a water oxidation reaction by irradiating light from a light source 8 using an electrolyte solution 3 and a semiconductor oxidation electrode 1 immersed in the electrolyte solution 3, a reduction tank 5 that performs a carbon dioxide reduction reaction using a reduction electrode 4 connected to the oxidation electrode 1 via a lead wire 7 and carbon dioxide that is in direct contact with the reduction electrode 4, and an electrolyte membrane 6 that is arranged in contact with the electrolyte solution 3 in the oxidation tank 2 and the reduction electrode 4 in the reduction tank 5, respectively, and the electrolyte membrane 6 has a water-repellent film 9 on a portion of its surface that comes into contact with the electrolyte solution 3.

Therefore, the water-repellent properties of the water-repellent film 9 on the surface of the electrolyte solution 3 prevent the electrolyte solution 3 in the oxidation tank 2 from penetrating into the electrolyte membrane 6, preventing leakage of the electrolyte solution 3 to the reduction electrode 4 and preventing the reaction sites of the reduction electrode 4 from becoming clogged with the electrolyte solution 3. Furthermore, since the water-repellent film 9 is provided on only a portion of the surface of the electrolyte membrane 6, it is possible to maintain a state in which protons can pass through the electrolyte membrane 6. As a result, the carbon dioxide reduction reaction can proceed and a decrease in the efficiency of the reduction reaction can be prevented.

In the above experiment, light was generated using a xenon lamp in order to quantitatively control the amount of light irradiated onto the oxidation electrode 1, but it is also possible to induce the oxidation reaction using sunlight, etc.

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 to proceed.

Figure 8 is a diagram showing an example configuration of a carbon dioxide reduction device 100 according to the second embodiment. The oxidation electrode 1 is 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 of the first embodiment.

In the carbon dioxide reduction device 100 according to this embodiment, in the oxidation tank 2, an oxidation reaction of water in the electrolyte 3 is carried out by current and voltage (electrical energy) from a power source 10 using an electrolyte 3 and a platinum (metal) oxidation electrode 1 immersed in the electrolyte 3. In the reduction tank 5, a carbon dioxide reduction reaction 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.

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 tank 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 tank 5 from electrolyte 3 in oxidation tank 2. Electrons flow from power supply 10 via conductor 7 to reduction electrode 4 in reduction tank 5. In reduction tank 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.

In the second embodiment, as in the first embodiment, 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, so as not to cover the entire surface of the electrolyte membrane 6. For example, multiple water-repellent films 9 are formed on the surface at predetermined intervals. Furthermore, as in the first embodiment, the water-repellent film 9 is manufactured using the first to fourth manufacturing methods.

Figure 9 shows the measurement results of the Faraday efficiency of formic acid in the second embodiment. Examples 5 to 8 are examples that use 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 a water-repellent film 9 is used as the electrolyte membrane 6.

In Examples 5 to 8, the Faraday efficiency did not decrease even after 6 hours. This is because the introduction of the water-repellent film 9 to the Nafion membrane suppressed leakage of the electrolyte 3 to the reduction electrode 4, preventing the reaction sites of the reduction electrode 4 from becoming filled with the electrolyte 3.

The second embodiment has been described above. The carbon dioxide reduction device 100 according to the second embodiment can provide a carbon dioxide reduction device 100 that can proceed with the carbon dioxide reduction reaction without reducing the Faraday efficiency.

That is, in the second embodiment, a carbon dioxide reduction device 100 is provided which includes an oxidation tank 2 that performs a water oxidation reaction using an electrolytic solution 3 and a platinum (metal) oxidation electrode 1 immersed in the electrolytic solution 3 by current and voltage from a power source 10, 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 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 that comes into contact with the electrolytic solution 3.

Therefore, the water-repellent properties of the water-repellent film 9 on the surface of the electrolyte solution 3 prevent the electrolyte solution 3 in the oxidation tank 2 from penetrating into the electrolyte membrane 6, preventing leakage of the electrolyte solution 3 to the reduction electrode 4 and preventing the reaction sites of the reduction electrode 4 from becoming clogged with the electrolyte solution 3. Furthermore, since the water-repellent film 9 is provided on only a portion of the surface of the electrolyte membrane 6, it is possible to maintain a state in which protons can pass through the electrolyte membrane 6. As a result, the carbon dioxide reduction reaction can proceed and a decrease in the efficiency of the reduction reaction can be prevented.

[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.

The present invention can be applied to any electrolyte membrane 6 used in a carbon dioxide reduction device 100 that is placed in contact with the electrolyte solution 3 in the oxidation tank 2 and the reduction electrode 4 in the reduction tank 5, and that directly contacts carbon dioxide with the reduction electrode 4 to perform a carbon dioxide reduction reaction.

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

Claims (4)

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: 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-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: 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 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: 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:
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: