WO2023233590A1

WO2023233590A1 - Reduction electrode, and method for producing reduction electrode

Info

Publication number: WO2023233590A1
Application number: PCT/JP2022/022337
Authority: WO
Inventors: 晃洋鴻野; 裕也渦巻; 紗弓里; 武志小松
Original assignee: 日本電信電話株式会社
Priority date: 2022-06-01
Filing date: 2022-06-01
Publication date: 2023-12-07

Abstract

This reduction electrode 4 used in a carbon dioxide reduction device 100 which is disposed in contact with a reduction tank 5 side of an electrolyte membrane 6 disposed between an oxidation tank 2 and a reduction tank 5 and which performs a reduction reaction of carbon dioxide by bringing carbon dioxide into direct contact therewith, wherein a recessed and projected structure and voids 10 are provided on the reduction tank side and the recessed and projected structure is provided with a water repellent agent 9 capable of causing liquid adhering to the surface to slide down.

Description

Reduction electrode and method for manufacturing the reduction electrode

The present invention relates to a reduction electrode and a method for manufacturing a reduction electrode.

An increase in the concentration of carbon dioxide 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. On the other hand, in the medium to long term, energy issues require a review of the energy supply that relies on fossil fuels, and the creation of next-generation energy supply sources is required.

As a means to obtain energy while suppressing carbon dioxide emissions, technological development is underway to utilize unused energy such as waste heat, snow and ice heat, vibration, and electromagnetic waves, as well as renewable energy such as sunlight. These power generation technologies only create electrical energy and cannot store energy. Furthermore, it is not possible to create chemical products using fossil fuels as raw materials.

As a way to simultaneously solve these problems, technology that reduces carbon dioxide using light energy is attracting attention. For example, Non-Patent Document 1 discloses an apparatus for reducing carbon dioxide by light irradiation. In the oxidation tank, when the oxidation electrode is irradiated with light, electron-hole pairs are generated and separated at the oxidation electrode, and oxygen and protons (H ⁺ ) are generated by the oxidation reaction of water in the electrolyte. Protons pass through the electrolyte membrane and reach the reduction tank, and electrons flow to the reduction electrode via the conductor. In the reduction tank, a reduction reaction of carbon dioxide by protons, electrons, and carbon dioxide dissolved in the solution is caused at a reduction electrode in the solution. This reduction reaction produces carbon monoxide, formic acid, methane, etc. that can be used as energy resources.

In the carbon dioxide reduction device of Non-Patent Document 1, carbon dioxide was supplied to the reduction electrode by immersing the reduction electrode in a solution and dissolving carbon dioxide in the solution. However, in this carbon dioxide reduction method, the reduction electrode is immersed in the solution, so there are limits to the dissolved concentration of carbon dioxide in the solution and the diffusion coefficient of carbon dioxide in the solution. supply is limited.

Therefore, in order to increase the amount of carbon dioxide supplied to the reduction electrode, research is underway to eliminate the solution in the reduction tank and directly supply carbon dioxide to the reduction electrode. In Non-Patent Document 2, by using a reduction tank configured to directly supply gaseous carbon dioxide to the reduction electrode, the amount of carbon dioxide supplied to the reduction electrode is increased and the reduction reaction of carbon dioxide is promoted. ing.

However, as the reduction reaction progresses, reduction products of carbon dioxide are generated on the reaction surface of the reduction electrode, and not only gases such as hydrogen, carbon monoxide, and methane, but also liquids such as formic acid, methanol, and ethanol are generated. do. Moreover, as time passes, the electrolytic solution in the oxidation tank passes through the electrolyte membrane and gradually leaks into the reduction tank. Therefore, the reaction surface (reaction site) of the reduction electrode is coated with these liquids, and the reduction reaction of carbon dioxide does not proceed. Therefore, the conventional carbon dioxide reduction apparatus has a problem in that the efficiency of the carbon dioxide reduction reaction decreases after several tens of hours.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technology that can improve the efficiency of the reduction reaction of carbon dioxide.

A reduction electrode according to one embodiment of the present invention is arranged in contact with the reduction tank side of an electrolyte membrane installed between an oxidation tank and a reduction tank, and performs a reduction reaction of carbon dioxide by direct contact with the reduction electrode. A reduction electrode used in a carbon reduction device is provided with an uneven structure and void holes on the reduction tank side, and the uneven structure is provided with a water repellent agent capable of causing liquid adhering to the surface to slide off.

A method for manufacturing a reduction electrode according to one aspect of the present invention includes a step of forming an uneven structure and voids in a reduction electrode, and adding a solvent to a solvent containing a water repellent to the reduction electrode. a step of drying the reduction electrode to remove the solvent; and a step of removing a water-repellent layer around the pores of the reduction electrode.

A method for manufacturing a reduction electrode according to one aspect of the present invention includes the steps of forming an uneven structure and voids in the reduction electrode, and placing the reduction electrode and a water repellent in a container. and a step of removing the water-repellent layer around the pores of the reduction electrode.

A method for manufacturing a reduction electrode according to one aspect of the present invention includes a step of forming an uneven structure and voids in a reduction electrode, and adding a solvent to a solvent containing a water repellent to the reduction electrode. and a step of drying the reduction electrode and removing the solvent.

According to the present invention, the carbon dioxide reduction reaction efficiency can be improved.

FIG. 1 is a diagram showing a configuration example of a carbon dioxide reduction device according to a first embodiment. FIG. 2 is a diagram (bottom view of FIG. 1) showing a configuration example of a reduction electrode, a water-repellent membrane, and a void hole. FIG. 3 is a diagram (right side of FIG. 1) showing a configuration example of a reduction electrode, a water-repellent membrane, and a void hole. FIG. 4 is a diagram showing a first method of manufacturing a reduction electrode and a water-repellent film. FIG. 5 is a diagram showing a second method for manufacturing a reduction electrode and a water-repellent film. FIG. 6 is a diagram showing a third method of manufacturing a reduction electrode and a water-repellent film. FIG. 7 is a diagram showing the measurement results of the faradaic 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 faradaic efficiency of formic acid according to the second embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the description of the drawings, the same parts are denoted by the same reference numerals and explanations will be omitted.

[First embodiment]
FIG. 1 is a diagram showing a configuration example 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 oxidizing electrode 1, an oxidizing tank 2, an electrolytic solution 3, a reducing electrode 4, a reducing tank 5, an electrolyte membrane 6, a conductive wire 7, and a light source 8. and a water-repellent film 9.

The oxidation electrode 1 is immersed in an electrolyte 3 in an oxidation tank 2. The oxidized electrode 1 is formed by forming a semiconductor on a substrate having a predetermined area. The oxidized electrode 1 is formed, for example, by forming a film of a compound exhibiting photoactivity or redox activity, 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 an electrolytic solution 3 in which the oxidation electrode 1 is immersed.

The electrolytic solution 3 is placed in the oxidation tank 2. The electrolytic solution 3 is, for example, a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, a sodium chloride aqueous solution, a potassium hydroxide aqueous solution, a rubidium hydroxide aqueous solution, or a cesium hydroxide aqueous solution.

The reduction electrode 4 is arranged within 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 of copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, or an alloy thereof. In addition, the reduction electrode 4 is made of compounds such as silver oxide, copper oxide, copper (II) oxide, nickel oxide, indium oxide, tin oxide, tungsten oxide, tungsten (VI) oxide, copper oxide, etc., and metal ions and anionic coordination. It may also be a porous metal complex having children.

The reduction tank 5 has the reduction electrode 4 disposed therein, and holds gaseous carbon dioxide supplied from the outside via piping.

The electrolyte membrane 6 is placed between the oxidation tank 2 and the reduction tank 5. To be precise, the electrolyte membrane 6 is arranged 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 with a carbon-fluorine skeleton, Selemion, or Neocepta, which are electrolyte membranes with a carbon-hydrogen skeleton.

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

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

In FIG. 1, the reduction electrode 4 and the electrolyte membrane 6 are each drawn to have a large width in the lateral direction of the paper, but the width in the lateral direction of the paper is made thinner and the electrolyte membrane 6 is drawn to have a flat width in the depth direction of the paper. It may be formed into a thin plate shape. By bonding the reduction electrode 4 and the electrolyte membrane 6 together in their planes, the reaction field at the contact surface can be maximized.

In the above carbon dioxide reduction apparatus 100, the oxidation tank 2 uses the electrolyte 3 and the semiconductor oxidation electrode 1 immersed in the electrolyte 3 to oxidize the electrolyte 3 using the irradiated light (light energy) from the light source 8. A water oxidation reaction takes place. In the reduction tank 5, a reduction reaction of carbon dioxide is performed using the reduction electrode 4 connected to the oxidation electrode 1 via a conductor 7 and carbon dioxide brought into direct contact with the reduction electrode 4.

Specifically, when the light source 8 irradiates light from the bottom of the oxidation tank 2, electron-hole pairs are generated and separated at the oxidation electrode 1 in the oxidation tank 2 that receives the irradiation light, and the electrolyte solution 3 is heated. The oxidation reaction of water produces oxygen and protons. Protons pass through the electrolyte membrane 6 and reach the reduction electrode 4 in the reduction tank 5 from the electrolyte 3 in the oxidation tank 2 . Electrons flow from the oxidizing electrode 1 in the oxidizing tank 2 to the reducing electrode 4 in the reducing tank 5 via the conducting wire 7. In the reduction tank 5 , a reduction reaction of carbon dioxide is caused at the reduction electrode 4 by protons, electrons, and gaseous carbon dioxide brought into direct contact with the reduction electrode 4 . This redox reaction produces carbon monoxide, formic acid, methane, etc. that can be used as energy resources.

At this time, if a strong alkaline aqueous solution, for example, a 1.0 mol/L sodium hydroxide aqueous solution, is used as the electrolyte 3 in the oxidation tank 2, the electrolyte membrane 6 will swell, and the electrolyte 3 will pore in the pores of the electrolyte membrane 6. and oozes out onto the surface of the reduction electrode 4 in the reduction tank 5. In order to prevent such oozing of the electrolyte solution 3 from the electrolyte membrane 6, the surface of the electrolyte membrane 6 on the side of the oxidation tank 2 that comes into contact with the electrolyte solution 3 may be treated with water repellency. It is necessary to move the water in the electrolyte membrane 6 as a medium, so if the surface of the electrolyte membrane 6 on the oxidation tank 2 side is completely covered with water-repellent treatment, there is a risk that the reduction reaction will not proceed on the reduction tank 5 side. There is.

Therefore, in this embodiment, as shown in FIGS. 1 to 3, a reduction electrode 4 with void holes 10 is arranged on the surface of the electrolyte membrane 6 on the reduction tank 5 side, and the protrusions of the reduction electrode 4 are water-repellent. A membrane 9 is placed. Specifically, as enlarged in FIGS. 2 and 3, the reduction electrode 4 is provided with an uneven structure (for example, a structure with a plurality of cones on the main surface of a flat plate) on the reduction tank 5 side. A water-repellent film 9 is provided at the bottom of the recess, and a void hole 10 is provided at the bottom of the recess so as not to cover the entire surface of the electrolyte membrane 6 on the reduction tank 5 side.

When an uneven structure of several micrometers is formed on a surface, water droplets adhering to the surface become water droplets without getting wet and slide off. This phenomenon is commonly called the lotus effect. If only a concavo-convex structure is formed on the reduction electrode 4 so as to exhibit this lotus effect, protons on the surface of the electrolyte membrane 6, carbon dioxide in the reduction tank 5, and electrons in the reduction electrode 4 will not react.

To this end, in this embodiment, the pores 10 are provided, and the protons on the surface of the electrolyte membrane 6 are formed in a portion having an uneven structure and having a function of letting water slide down, and in a portion where the pores 10 and the electrolyte membrane 6 are in contact. A structure was fabricated to separate the part where carbon dioxide in the reduction tank 5 and electrons in the reduction electrode 4 react.

Furthermore, depending on the pore diameter of the void pores 10, capillary phenomenon occurs and water tends to stay within the uneven structure of the reduction electrode 4, which may weaken the water repellent effect. Therefore, a structure was provided in which a water-repellent film 9 was formed on the convex portion to encourage the movement of water from the electrolyte membrane 6 to the reduction tank 5.

Note that if the water-repellent film 9 covers the entire surface of the reduction electrode 4, the carbon dioxide in the reduction tank 5 cannot directly react with the electrons in the reduction electrode 4 at the interface of the electrolyte membrane 6. It is necessary that the water-repellent film 9 is not provided in the area where the film 6 contacts.

Next, a method for manufacturing the reduction electrode 4 and the water-repellent film 9 will be explained.

Examples of methods for producing the reduction electrode 4 having the void holes 10 include commercially available casting methods, powder metallurgy, modeling using a metal 3D printer, and processing using a high-power laser. Examples of the water repellent treatment for manufacturing the water repellent film 9 include a liquid phase method and a gas phase method.

In the liquid phase method, the object is immersed in a fluorine-based solvent in which a fluorine-based polymer, which is a water repellent, is dissolved, using a method such as dip coating, and then the solvent is removed by heating the object. This is a method to precipitate fluorine-based polymers. In the vapor phase method, the target object and a fluorine-based low molecule (silane coupling agent) that is a water repellent are placed in the same closed space, the fluorine-based low molecule is heated to steam, and then vaporized onto the surface of the target object. This is a method to do so. In these two methods, the entire surface of the reduction electrode 4 is covered with a water repellent, and the carbon dioxide in the reduction tank 5 and the electrons in the reduction electrode 4 cannot directly react at the interface of the electrolyte membrane 6. Therefore, it is necessary to remove the water-repellent film 9 on the contact surface between the pores 10 and the electrolyte membrane 6 using a high-power laser or the like.

In addition, as another simpler method that does not use the above-mentioned high-power laser, a fluorine-based polymer or a fluorine-based low molecule is dissolved in a fluorine-based solvent, and the uneven surface of the reduction electrode 4 is brought into contact with the solvent. There is a method in which the water-repellent film 9 is deposited on the uneven surface by drying the surface and removing the solvent. Since a fluorine-based solvent has a lower surface tension than water, even if the uneven surface of the reduction electrode 4 is brought into contact with the fluorine-based solvent, the fluorine-based solvent does not penetrate into the void pores 10 due to capillary action. Therefore, the water-repellent film 9 can be more easily formed only on the uneven surface due to the low surface tension characteristic of the fluorine-based solvent.

FIG. 4 is a diagram showing the first manufacturing method of the reduction electrode 4 and the water-repellent film 9. The first manufacturing method is a method for manufacturing the reduction electrode 4 and the water-repellent film 9 using the first liquid phase method.

For the reduction electrode 4, a structure having an uneven structure and voids was formed using a metal 3D printer. The concavo-convex structure had a cylindrical shape with a diameter of 10 μm and a height of 20 μm, and the voids had a diameter of 10 μm. The intervals between the concavo-convex structure and the void holes were equal, and the pitch was 15 μm. A close-packed point structure was formed, and the ratio of the concavo-convex structure to the voids was 1:3 (first step S101). Optool DSX was used as the water repellent. After the reduction electrode was immersed in the Optool DSX solution for 1 minute (second step S102), dip coating was performed by pulling it up and drying it (third step S103). Through this step, a water-repellent film can be formed over the entire surface of the reduction electrode. Thereafter, the outer periphery of the void hole was traced using a high-power laser, and the water-repellent film 9 was removed by thermal decomposition (fourth step S104).

Through these steps, since there is no water-repellent film 9 on the contact surface between the reduction electrode 4 and the electrolyte membrane 6, a reaction between carbon dioxide in the reduction tank 5, electrons in the reduction electrode 4, and protons in the electrolyte membrane 6 occurs at this contact surface. can proceed.

FIG. 5 is a diagram showing a second manufacturing method of the reduction electrode 4 and the water-repellent film 9. The second manufacturing method is a method of manufacturing the reduction electrode 4 and the water-repellent film 9 by the first method of vapor phase method.

For the reduction electrode 4, a structure having an uneven structure and voids was formed using a metal 3D printer. The concavo-convex structure had a cylindrical shape with a diameter of 10 μm and a height of 20 μm, and the voids had a diameter of 10 μm. The intervals between the concavo-convex structure and the void holes were equal, and the pitch was 15 μm. A close-packed point structure was formed, and the ratio of the concavo-convex structure to the voids was 1:3 (first step S201). A fluorine-based silane coupling agent (for example, heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane) was used as the water repellent. A reduction electrode and a water repellent were placed in a Teflon container, the container was sealed, and the container was placed in an oven and heated at 150° C. (second step S202). Through this step, the water repellent agent evaporates and a water repellent film can be formed over the entire surface of the reduction electrode. Thereafter, the outer periphery of the void hole was traced using a high-power laser, and the water-repellent film 9 was removed by thermal decomposition (third step S203).

Through these steps, since there is no water-repellent film 9 on the contact surface between the reduction electrode 4 and the electrolyte membrane 6, a reaction between carbon dioxide in the reduction tank 5, electrons in the reduction electrode 4, and protons in the electrolyte membrane 6 occurs at this contact surface. can proceed. In this vapor phase method, a monomolecular film is formed on the surface of Nafion, so an extremely thin water-repellent film 9 on the order of nanometers can be formed.

FIG. 6 is a diagram showing a third manufacturing method of the reduction electrode 4 and the water-repellent film 9. The third manufacturing method is a method for manufacturing the reduction electrode 4 and the water-repellent film 9 using the second liquid phase method.

For the reduction electrode 4, a structure having an uneven structure and voids was formed using a metal 3D printer. The concavo-convex structure had a cylindrical shape with a diameter of 10 μm and a height of 20 μm, and the voids had a diameter of 10 μm. The intervals between the concavo-convex structure and the void holes were equal, and the pitch was 15 μm. A close-packed point structure was formed, and the ratio of the concavo-convex structure to the voids was 1:3 (first step S301). Optool DSX was used as the water repellent. After only the surface of the uneven structure of the reduction electrode was brought into contact with the OpTool DSX solution (second step S302), it was pulled up and dried (third step S303). In the third step of S303, no capillary phenomenon in which the pores were filled with the fluorine-based solvent was observed.

Next, electrochemical measurement using the carbon dioxide reduction device 100 described above and the measurement results will be explained.

First, a thin film of gallium nitride (GaN), which is an n-type semiconductor, and aluminum gallium nitride (AlGaN) are epitaxially grown on a sapphire substrate in that order, and nickel (Ni) is vacuum-deposited on top of that and heat-treated. A promoter thin film of nickel oxide (NiO) was formed. Then, the promoter thin film was used as an oxidation electrode 1, and the oxidation electrode 1 was immersed in an electrolytic solution 3 of a 1.0 mol/L potassium hydroxide aqueous solution in an oxidation tank 2.

The reduction electrode 4 manufactured by the above manufacturing method was brought into close contact with the electrolyte membrane 6 by thermocompression bonding. Although a thermocompression bonding method was used in this embodiment, other methods may be used as long as the reduction electrode 4 and the electrolyte membrane 6 are in close physical contact. The reduction electrode 4 was connected to the oxidation electrode 1 with a conducting wire 7, and the reduction electrode 4 was placed in a reduction tank 5.

Furthermore, Nafion was used for the electrolyte membrane 6 that physically separates the oxidation tank 2 and the reduction tank 5. Of both surfaces of the electrolyte membrane 6, one side on which the water-repellent membrane 9 was formed was placed facing into the reduction tank 5, and the other side having the voids was placed in contact with the electrolyte membrane 6.

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

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

At this time, the current flowing between the oxidation electrode 1 and the reduction electrode 4 due to the irradiation light was measured using an electrochemical measuring device (Model 1287 potentiogalvanostat manufactured by Solartron). Further, gas and liquid generated in the oxidation tank 2 and the reduction tank 5 were collected, and 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 reduction electrode 4 was investigated by determining the Faraday efficiency of the carbon dioxide reduction reaction. Note that a method for calculating the Faraday efficiency of the carbon dioxide reduction reaction will be described later.

In Example 1, the reduction electrode 4 and water-repellent film 9 manufactured by the first manufacturing method were used.

In Example 2, the reduction electrode 4 and water-repellent film 9 manufactured by the second manufacturing method were used.

In Example 3, the reduction electrode 4 and water-repellent film 9 manufactured by the third manufacturing method were used.

In Comparative Example 1, in the manufacturing method described in the first manufacturing method, the reduction electrode 4 in which the height of the cylindrical uneven structure was 0 μm, that is, the reduction electrode 4 without the uneven structure was used.

In Comparative Example 2, the reduction electrode 4 on which the water-repellent film 9 was not formed was used in the manufacturing method described in the first manufacturing method.

FIG. 7 is a diagram showing the measurement results of the faradaic efficiency of formic acid according to the first embodiment. In Comparative Example 1, which does not have an uneven structure, the Faraday efficiency decreased after 50 hours. In Comparative Example 2 in which the water-repellent film 9 was not formed, the Faraday efficiency decreased after 50 hours. On the other hand, in Examples 1 to 3 in which the uneven structure and the water-repellent film 9 were formed, the Faraday efficiency did not decrease even after 50 hours. As a result of introducing the uneven structure and the water-repellent film 9 on the surface of the reduction electrode 4 that promote the sliding of liquid, the liquid on the surface of the reduction electrode 4 that is a result of liquid leakage becomes easier to slide down, and the reaction sites of the reduction electrode 4 are This is because it is no longer filled with liquid 3.

Here, we will explain how to calculate the faradaic 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 the application of current and voltage. , can be calculated using equation (1).

Faraday efficiency = {number of electrons in reduction reaction} / {number of electrons transferred between electrodes}...(1)
The "number of electrons for the reduction reaction" in equation (1) is determined by converting the measured value of the cumulative production amount of the reduction product of carbon dioxide 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 equation (2).

Number of electrons in each reduction reaction (C) = {(A×B×Z×F×T×10 ⁻⁶ )}/V _g ...(2)
A is the concentration (ppm) of the reduction reaction product. B is the flow rate (L/sec) of carrier gas. Z is the number of electrons required for the reduction reaction. F is Faraday constant (C/mol). T is the light irradiation time or the 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 a liquid can be calculated using equation (3).

Number of electrons in each reduction reaction (C) = C x V _l x Z x F... (3)
C is the concentration (mol/L) of the reduction reaction product. V _l is the volume of the liquid sample (L). Z is the number of electrons required for the reduction reaction. F is Faraday constant (C/mol).

The first embodiment has been described above. 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 allows the carbon dioxide reduction reaction to proceed without reducing Faraday efficiency.

That is, in the first embodiment, the oxidation tank 2 performs an oxidation reaction of water using the irradiation light from the light source 8 using the electrolytic solution 3 and the semiconductor oxidizing electrode 1 immersed in the electrolytic solution 3, and the conducting wire is connected to the oxidizing electrode 1. A reduction tank 5 that performs a reduction reaction of carbon dioxide using the reduction electrode 4 connected via the oxidation tank 7 and carbon dioxide brought into direct contact with the reduction electrode 4, and the electrolyte 3 in the oxidation tank 2 and the In the carbon dioxide reduction device 100, which includes an electrolyte membrane 6 disposed between and in contact with the reduction electrode 4, an electrolyte membrane 6 disposed in contact with the electrolyte membrane 6 on the reduction tank 5 side, and an electrolyte membrane 6 disposed on the reduction tank 5 side. It has an uneven structure and voids 10, and the uneven structure has a water-repellent film 9 that can slide off liquid adhering to the surface.

Therefore, due to the water-repellent film 9 provided on the uneven structure surface of the reduction electrode 4, the liquid on the surface of the reduction electrode 4, which is the result of the electrolytic solution 3 in the oxidation tank 2 seeping out to the outside of the electrolyte membrane 6, moves to the uneven structure surface. Water droplets on the surface of the uneven structure slide down due to the lotus effect, so that the reaction sites of the reduction electrode 4 are not filled with the electrolyte 3. As a result, the reduction reaction of carbon dioxide can proceed, and a decrease in the efficiency of the reduction reaction can be suppressed.

Note that in the above experiment, light was generated using a xenon lamp in order to quantitatively control the amount of light irradiated to the oxidation electrode 1, but it is also possible to cause the oxidation reaction using sunlight or the like.

[Second embodiment]
In the first embodiment, a case has been described in which the light source 8 and the oxidized electrode 1 made of a semiconductor are used. In the second embodiment, instead of these, an external power source and an oxidizing electrode 1 made of metal are used to proceed with the oxidation/reduction reaction. For comparison, the same voltage and current values as in the first embodiment were adjusted and applied.

FIG. 8 is a diagram showing a configuration example of the carbon dioxide reduction device 100 according to the second embodiment. Oxidation electrode 1 is platinum. In addition, the oxidation electrode 1 may be made of gold or silver, for example. The external power source 11 is an electrochemical measuring device, and is connected in series to the conductive wire 7 connecting the oxidation electrode 1 and the reduction electrode 4. The power supply 11 may be any other power supply device. Other components are the same as in the first embodiment.

In the carbon dioxide reduction apparatus 100 according to the present embodiment, the oxidation tank 2 uses the electrolyte 3 and the platinum (metal) oxidation electrode 1 immersed in the electrolyte 3 to generate current and voltage (electrical energy) from the power source 11. This causes an oxidation reaction of water in the electrolytic solution 3. In the reduction tank 5, a reduction reaction of carbon dioxide is performed using the reduction electrode 4 connected to a power source 11 (a source of electrical energy) and carbon dioxide brought into direct contact with the reduction electrode 4.

Specifically, when the power source 11 applies current and voltage to the conductive wire 7, oxygen and protons are generated by an oxidation reaction of water in the electrolytic solution 3. Protons pass through the electrolyte membrane 6 and reach the reduction electrode 4 in the reduction tank 5 from the electrolyte 3 in the oxidation tank 2 . Electrons flow from the power source 11 to the reduction electrode 4 in the reduction tank 5 via the conductor 7 . In the reduction tank 5 , a reduction reaction of carbon dioxide is caused at the reduction electrode 4 by protons, electrons, and gaseous carbon dioxide brought into direct contact with the reduction electrode 4 .

In the second embodiment, as in the first embodiment, the reduction electrode 4 is placed in contact with the electrolyte membrane 6 on the reduction tank 5 side, and is provided with an uneven structure and void holes 10 on the reduction tank 5 side. The uneven structure includes a water-repellent film 9 that can slide off liquid adhering to the surface. As the method for manufacturing the water-repellent film 9, the first to third manufacturing methods are used as in the first embodiment.

FIG. 9 is a diagram showing the measurement results of the faradaic efficiency of formic acid according to the second embodiment. Examples using the same reduction electrode 4 as Examples 1 to 3 described in the first embodiment are referred to as Examples 4 to 6. In Comparative Example 3, in the manufacturing method described in the first manufacturing method, a reduction electrode 4 having a columnar uneven structure having a height of 0 μm, that is, a reduction electrode 4 having no uneven structure was used. In Comparative Example 4, the reduction electrode 4 on which the water-repellent film 9 was not formed was used in the manufacturing method described in the first manufacturing method.

In Comparative Example 3, which did not have an uneven structure, the Faraday efficiency decreased after 50 hours. In Comparative Example 4 in which the water-repellent film 9 was not formed, the Faraday efficiency decreased after 50 hours. On the other hand, in Examples 4 to 6 in which the uneven structure and the water-repellent film 9 were formed, the Faraday efficiency did not decrease even after 50 hours. As a result of introducing the uneven structure and the water-repellent film 9 on the surface of the reduction electrode 4 that promote the sliding of liquid, the liquid on the surface of the reduction electrode 4 that is a result of liquid leakage becomes easier to slide down, and the reaction sites of the reduction electrode 4 are This is because it is no longer filled with liquid 3.

The second embodiment has been described above. 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 allows the carbon dioxide reduction reaction to proceed without reducing Faraday efficiency.

That is, in the second embodiment, an oxidation tank 2 performs an oxidation reaction of water using a current voltage from a power source 11 using an electrolytic solution 3 and a platinum (metal) oxidizing electrode 1 immersed in the electrolytic solution 3; A reduction tank 5 that performs a reduction reaction of carbon dioxide using a reduction electrode 4 connected to the oxidation tank 4 and carbon dioxide brought into direct contact with the reduction electrode 4, an electrolytic solution 3 in the oxidation tank 2, and a reduction electrode 4 in the reduction tank 5. and an electrolyte membrane 6 disposed in contact with the electrolyte membrane 6 on the reduction tank 5 side, and an uneven structure on the reduction tank 5 side. The uneven structure includes a water-repellent film 9 that can slide off liquid adhering to the surface.

[others]
INDUSTRIAL APPLICABILITY The present invention can be widely used in fields related to carbon dioxide recycling. Although optical energy was used in the first embodiment and electrical energy was used in the second embodiment, other renewable energy may be used. It is also possible to combine the first embodiment and the second embodiment.

In the present invention, the electrolytic solution 3 in the oxidation tank 2 and the reduction electrode 4 in the reduction tank 5 are placed in contact with each other, and the reduction reaction of carbon dioxide is carried out by directly contacting the reduction electrode 4 with carbon dioxide. Any electrolyte membrane can be used as long as it is the electrolyte membrane 6 used in the carbon dioxide reduction apparatus 100.

1: Oxidation electrode 2: Oxidation tank 3: Electrolyte solution 4: Reduction electrode 5: Reduction tank 6: Electrolyte membrane 7: Conductive wire 8: Light source 9: Water-repellent membrane 10: Pore 11: Power source 100: Carbon dioxide reduction device

Claims

In a reduction electrode used in a carbon dioxide reduction device that is placed in contact with the reduction tank side of an electrolyte membrane installed between an oxidation tank and a reduction tank, and performs a reduction reaction of carbon dioxide by directly contacting the carbon dioxide, The reduction electrode is provided with an uneven structure and void holes on the reduction tank side, and the uneven structure includes a water repellent agent capable of causing liquid adhering to the surface to slide off.
The oxidation tank performs an oxidation reaction of water using light energy using an electrolyte in the oxidation tank and a semiconductor oxidation electrode immersed in the electrolyte,
The reduction electrode according to claim 1, wherein the reduction tank performs a reduction reaction of carbon dioxide using the reduction electrode connected to the oxidation electrode via a conductive wire and carbon dioxide brought into direct contact with the reduction electrode.
The oxidation tank performs an oxidation reaction of water using electrical energy using the electrolyte of the oxidation tank and a metal oxidation electrode immersed in the electrolyte,
The reduction electrode according to claim 1, wherein the reduction tank performs a reduction reaction of carbon dioxide using the reduction electrode connected to the source of electrical energy and carbon dioxide brought into direct contact with the reduction electrode.
In the method for manufacturing a reduction electrode for manufacturing the reduction electrode according to any one of claims 1 to 3,
forming an uneven structure and void holes in the reduction electrode;
immersing the reduction electrode in a solvent containing a water repellent;
drying the reduction electrode to remove the solvent;
removing a water-repellent layer around the pores of the reduction electrode;
A method for manufacturing a reduction electrode.
In the method for manufacturing a reduction electrode for manufacturing the reduction electrode according to any one of claims 1 to 3,
forming an uneven structure and void holes in the reduction electrode;
placing the reduction electrode and water repellent in a container and heating it;
removing a water-repellent layer around the pores of the reduction electrode;
A method for manufacturing a reduction electrode.
In the method for manufacturing a reduction electrode for manufacturing the reduction electrode according to any one of claims 1 to 3,
forming an uneven structure and void holes in the reduction electrode;
a step of bringing the uneven structure of the reduction electrode into contact with a solvent containing a water repellent;
drying the reduction electrode to remove the solvent;
A method for manufacturing a reduction electrode.