JP7801637B2 - Method for manufacturing a reduction electrode - Google Patents

Description

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

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 reduction electrode of the present invention is 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 that performs a carbon dioxide reduction reaction by directly contacting carbon dioxide.The reduction electrode has an uneven structure and void holes on the reduction tank side, and the uneven structure is equipped with a water-repellent agent that can allow liquid adhering to the surface to slide off.

A method for manufacturing a reduction electrode according to one embodiment of the present invention includes the steps of forming a textured structure and pores in the reduction electrode, immersing the reduction electrode in a solvent containing a water-repellent agent, drying the reduction electrode to remove the solvent, and removing the water-repellent layer around the pores in the reduction electrode.

One embodiment of the present invention relates to a method for manufacturing a reduction electrode, which comprises the steps of forming a textured structure and pores in the reduction electrode, placing the reduction electrode and a water-repellent agent in a container and heating the container, and removing the water-repellent layer around the pores in the reduction electrode.

A method for manufacturing a reduction electrode according to one embodiment of the present invention includes the steps of forming a relief structure and voids in the reduction electrode, exposing the relief structure of the reduction electrode to a solvent containing a water repellent, and drying the reduction electrode to remove the solvent.

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 (bottom view of FIG. 1) showing an example of the configuration of the reduction electrode, the water-repellent film, and the void holes. FIG. 3 is a diagram (the right side of FIG. 1) showing an example of the configuration of the reduction electrode, the water-repellent film, and the void holes. FIG. 4 is a diagram showing a first method for 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 for manufacturing a reduction electrode and 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.

In Figure 1, the reduction electrode 4 and electrolyte membrane 6 are each drawn with a large width in the horizontal direction of the page, but they may also be shaped like thin plates 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 on the oxidation tank 2 side with a water-repellent treatment could prevent the reduction reaction from proceeding on the reduction tank 5 side.

In this embodiment, as shown in Figures 1 to 3, a reduction electrode 4 with void holes 10 is placed on the surface of the electrolyte membrane 6 facing the reduction tank 5, and a water-repellent film 9 is placed on the protrusions of the reduction electrode 4. Specifically, as shown enlarged in Figures 2 and 3, the reduction electrode 4 has an uneven structure (e.g., a structure with multiple cones on the main surface of a flat plate) on the reduction tank 5 side, with the water-repellent film 9 on the protrusions, and void holes 10 at the bottom of the depressions so as not to cover the entire surface of the electrolyte membrane 6 facing the reduction tank 5.

When an uneven structure of several micrometers is formed on a surface, water droplets that adhere to the surface will simply form droplets and slide off without getting wet. This phenomenon is commonly known as the lotus effect. Simply creating an uneven structure on the reduction electrode 4 to produce this lotus effect will not result in a reaction between the protons on the surface of the electrolyte membrane 6, the carbon dioxide in the reduction tank 5, and the electrons in the reduction electrode 4.

To this end, in this embodiment, a structure was created in which void holes 10 were provided, which have an uneven structure and function to allow water to slide off, and the area where the void holes 10 contact the electrolyte membrane 6, separating the area where protons on the surface of the electrolyte membrane 6 react with carbon dioxide in the reduction tank 5 and electrons in the reduction electrode 4.

Furthermore, depending on the pore diameter of the void holes 10, capillary action may occur, causing water to remain within the uneven structure of the reduction electrode 4, which may weaken the water-repellent effect. Therefore, a water-repellent film 9 is formed on the convex portions, creating a structure that promotes the movement of water from the electrolyte membrane 6 to the reduction tank 5.

If the water-repellent film 9 were to cover the entire surface of the reduction electrode 4, the carbon dioxide in the reduction tank 5 and the electrons in the reduction electrode 4 would not be able to react directly at the interface with the electrolyte membrane 6, so it is necessary not to provide the water-repellent film 9 at the area where the reduction electrode 4 and the electrolyte membrane 6 come into contact.

Next, we will explain the manufacturing method of the reduction electrode 4 and the water-repellent film 9.

Methods for producing the reduction electrode 4 with voids 10 include commercially available casting methods, powder metallurgy methods, metal 3D printer modeling, and processing methods using high-power lasers. Examples of water-repellent treatments for producing the water-repellent film 9 include liquid-phase methods and gas-phase methods.

The liquid-phase method involves immersing the object in a fluorine-based solvent containing a fluorine-based polymer (water-repellent agent) using a dip coating method or other method, and then removing the solvent by heating the object to precipitate the fluorine-based polymer. The gas-phase method involves placing the object and a fluorine-based small molecule water-repellent agent (silane coupling agent) in the same sealed space, heating the fluorine-based small molecule to vaporize it, and then depositing it on the object's surface. In these two methods, the entire surface of the reduction electrode 4 is covered with water-repellent agent, preventing direct reaction between the carbon dioxide in the reduction tank 5 and the electrons in the reduction electrode 4 at the electrolyte membrane 6 interface. Therefore, the water-repellent film 9 at the contact surface between the pores 10 and the electrolyte membrane 6 must be removed using a high-power laser or other method.

Another, simpler method that does not use the high-power laser described above involves dissolving a fluorine-based polymer or a fluorine-based small molecule in a fluorine-based solvent, bringing the uneven surface of the reduction electrode 4 into contact with the solvent, drying the surface, and removing the solvent to deposit a water-repellent film 9 on the uneven surface. Because fluorine-based solvents have a lower surface tension than water, even when the uneven surface of the reduction electrode 4 is brought into contact with the fluorine-based solvent, it does not penetrate into the pores 10 due to capillary action. Therefore, the low surface tension of fluorine-based solvents makes it easier to form a water-repellent film 9 only on the uneven surface.

Figure 4 shows a first manufacturing method for the reduction electrode 4 and the water-repellent film 9. The first manufacturing method is a manufacturing method for the reduction electrode 4 and the water-repellent film 9 using the first liquid-phase method.

The reduction electrode 4 was fabricated using a metal 3D printer to have a textured structure and voids. The textured structure was cylindrical, 10 μm in diameter and 20 μm in height, and the voids were fabricated with a diameter of 10 μm. The textured structure and voids were evenly spaced with a pitch of 15 μm. A close-packed structure was used, with a textured structure to void ratio of 1:3 (Step 1 S101). Optool DSX was used as the water-repellent agent. The reduction electrode was immersed in Optool DSX solution for 1 minute (Step 2 S102), then dip-coated and dried (Step 3 S103). This process formed a water-repellent film over the entire surface of the reduction electrode. The voids were then traced using a high-power laser, and the water-repellent film 9 was removed by thermal decomposition (Step 4 S104).

As a result of these processes, the contact surface between the reduction electrode 4 and the electrolyte membrane 6 is free of the water-repellent film 9, allowing the reaction between the carbon dioxide in the reduction tank 5, the electrons in the reduction electrode 4, and the protons in the electrolyte membrane 6 to proceed at this contact surface.

Figure 5 shows a second manufacturing method for the reduction electrode 4 and the water-repellent film 9. The second manufacturing method is a manufacturing method for the reduction electrode 4 and the water-repellent film 9 using the first gas-phase method.

The reduction electrode 4 was fabricated using a metal 3D printer to have a textured structure and voids. The textured structure was cylindrical, 10 μm in diameter and 20 μm in height, with voids 10 μm in diameter. The textured structure and voids were evenly spaced with a pitch of 15 μm. A close-packed structure was used, with a textured structure to void ratio of 1:3 (Step 1 S201). A fluorine-based silane coupling agent (e.g., heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane) was used as the water-repellent agent. The reduction electrode and water-repellent agent were placed in a Teflon container, sealed, and heated in an oven at 150°C (Step 2 S202). This process evaporates the water-repellent agent, forming a water-repellent film 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 processes, the contact surface between the reduction electrode 4 and the electrolyte membrane 6 is free of the water-repellent film 9, allowing the reaction between the carbon dioxide in the reduction tank 5, the electrons in the reduction electrode 4, and the protons in the electrolyte membrane 6 to proceed at this contact surface. This gas-phase method forms a monolayer on the Nafion surface, allowing the formation of an extremely thin water-repellent film 9 on the order of nanometers.

Figure 6 shows a third manufacturing method for the reduction electrode 4 and the water-repellent film 9. The third manufacturing method is a manufacturing method for the reduction electrode 4 and the water-repellent film 9 using the second liquid-phase method.

A metal 3D printer was used to create a structure with a concave-convex structure and voids for the reduction electrode 4. The concave-convex structure was cylindrical, 10 μm in diameter and 20 μm in height, and the voids were 10 μm in diameter. The concave-convex structure and the voids were evenly spaced, with a pitch of 15 μm. A close-packed structure was used, with a ratio of concave-convex structure to voids of 1:3 (first step S301). Optool DSX was used as the water repellent. Only the surface of the concave-convex structure of the reduction electrode was exposed to Optool DSX solution (second step S302), then lifted and dried (third step S303). In the third step S303, no capillary action was observed, filling the voids with a fluorinated solvent.

As a result of these processes, the contact surface between the reduction electrode 4 and the electrolyte membrane 6 is free of the water-repellent film 9, allowing the reaction between the carbon dioxide in the reduction tank 5, the electrons in the reduction electrode 4, and the protons in the electrolyte membrane 6 to proceed at this contact surface.

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.

The reduction electrode 4 manufactured using the above manufacturing method was bonded to the electrolyte membrane 6 by thermocompression bonding. In this example, thermocompression bonding was used, but other methods may be used as long as the reduction electrode 4 and electrolyte membrane 6 are physically bonded. The reduction electrode 4 was connected to the oxidation electrode 1 with a wire 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 sides of the electrolyte membrane 6, the one side on which the water-repellent film 9 was formed was positioned facing the inside of the reduction tank 5, and the other side with pores was positioned so that it was in contact with the electrolyte membrane 6.

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 reduction electrode 4 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, a reduction electrode 4 and a water-repellent film 9 manufactured using the first manufacturing method were used.

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

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

In Comparative Example 1, a reduction electrode 4 was used in which the height of the cylindrical uneven structure was 0 μm, i.e., no uneven structure, using the manufacturing method described in the first manufacturing method.

In Comparative Example 2, a reduction electrode 4 without a water-repellent film 9 was used in the manufacturing method described in the first manufacturing method.

Figure 7 shows the measurement results of the Faraday efficiency of formic acid in the first embodiment. In Comparative Example 1, which did not have a concave-convex structure, the Faraday efficiency decreased after 50 hours. In Comparative Example 2, which did not have a water-repellent film 9, the Faraday efficiency decreased after 50 hours. On the other hand, in Examples 1 to 3, which had a concave-convex structure and a water-repellent film 9, the Faraday efficiency did not decrease even after 50 hours. This is because the introduction of the concave-convex structure and water-repellent film 9, which promote the liquid to slide off the surface of the reduction electrode 4, made it easier for the liquid to slide off the surface of the reduction electrode 4 due to liquid leakage, and the reaction sites of the reduction electrode 4 were no longer filled with the electrolyte 3.

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 (1).

Faraday efficiency = {number of electrons in the reduction reaction} / {number of electrons transferred between electrodes} (1)
The "number of electrons in the reduction reaction" in formula (1) 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 (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 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).

When the reduction product is liquid, the "number of electrons in the reduction reaction" can be calculated using equation (3).

Number of electrons in each reduction reaction (C) = C × V _l × Z × F (3)
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 3 and a semiconductor oxidation electrode 1 immersed in the electrolyte 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 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 3 in the oxidation tank 2 and the reduction electrode 4 in the reduction tank 5, respectively. The carbon dioxide reduction device 100 includes a water-repellent film 9 that is arranged in contact with the electrolyte membrane 6 on the reduction tank 5 side and has an uneven structure and void holes 10 on the reduction tank 5 side, and the uneven structure is capable of sliding off liquid adhering to the surface.

Therefore, the water-repellent film 9 on the uneven surface of the reduction electrode 4 makes it easier for the liquid on the surface of the reduction electrode 4, which is the result of the electrolyte solution 3 in the oxidation tank 2 seeping out of the electrolyte membrane 6, to move to the uneven surface. Water droplets on the uneven surface slide off due to the lotus effect, preventing the reaction sites of the reduction electrode 4 from becoming filled with electrolyte solution 3. As a result, the carbon dioxide reduction reaction can proceed, and a decrease in the reduction reaction efficiency can be suppressed.

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 the oxidation-reduction reaction to proceed. For comparison, the same voltage and current values as in the first embodiment were adjusted and applied.

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 11 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 11 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 11 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 11 (a source of electrical energy) and carbon dioxide in direct contact with the reduction electrode 4.

Specifically, when power supply 11 applies a current and voltage to conductor 7, 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 11 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, 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, and the uneven structure is provided with a water-repellent film 9 that allows liquid adhering to the surface to slide off. As in the first embodiment, the water-repellent film 9 is manufactured using the first to third manufacturing methods.

Figure 9 shows the measurement results of the Faraday efficiency of formic acid according to the second embodiment. Examples 4 to 6 are examples using the same reduction electrode 4 as Examples 1 to 3 described in the first embodiment. In Comparative Example 3, a reduction electrode 4 with a cylindrical uneven structure having a height of 0 μm, i.e., no uneven structure, was used in the manufacturing method described in the first manufacturing method. In Comparative Example 4, a reduction electrode 4 without a water-repellent film 9 was used in the manufacturing method described in the first manufacturing method.

In Comparative Example 3, which did not have a concave-convex structure, the Faraday efficiency decreased after 50 hours. In Comparative Example 4, which did not have a water-repellent film 9, the Faraday efficiency decreased after 50 hours. On the other hand, in Examples 4 to 6, which had a concave-convex structure and a water-repellent film 9, the Faraday efficiency did not decrease even after 50 hours. This is because the introduction of the concave-convex structure and water-repellent film 9, which promote the liquid to slide off the surface of the reduction electrode 4, made it easier for the liquid to slide off the surface of the reduction electrode 4 due to leakage, and the reaction sites of the reduction electrode 4 were no longer 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.

In other words, in the second embodiment, the carbon dioxide reduction device 100 includes an oxidation tank 2 that performs a water oxidation reaction using an electric current and voltage from a power source 11 using an electrolyte 3 and a platinum (metal) oxidation electrode 1 immersed in the electrolyte 3, a reduction tank 5 that performs a carbon dioxide reduction reaction using a reduction electrode 4 connected to the power source 11 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 3 in the oxidation tank 2 and the reduction electrode 4 in the reduction tank 5, respectively.The carbon dioxide reduction device 100 includes a water-repellent film 9 that is arranged in contact with the electrolyte membrane 6 on the reduction tank 5 side and has an uneven structure and void holes 10 on the reduction tank 5 side, and the uneven structure is capable of sliding off liquid adhering to the surface.

Therefore, the water-repellent film 9 on the uneven surface of the reduction electrode 4 makes it easier for the liquid on the surface of the reduction electrode 4, which is the result of the electrolyte solution 3 in the oxidation tank 2 seeping out of the electrolyte membrane 6, to move to the uneven surface. Water droplets on the uneven surface slide off due to the lotus effect, preventing the reaction sites of the reduction electrode 4 from becoming filled with electrolyte solution 3. As a result, the carbon dioxide reduction reaction can proceed, and a decrease in the reduction reaction efficiency can be suppressed.

[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: Void hole 11: Power source 100: Carbon dioxide reduction device

Claims (2)

A method for manufacturing a reduction electrode for use in a carbon dioxide reduction device, which is disposed in contact with the reduction tank side of an electrolyte membrane installed between an oxidation tank and a reduction tank and performs a carbon dioxide reduction reaction by directly contacting carbon dioxide, comprising :
forming a relief structure and pores on the reduction electrode;
immersing the reduction electrode in a solvent containing a water repellent;
drying the reduction electrode to remove the solvent;
removing the water-repellent layer around the pores of the reduction electrode;
A method for manufacturing a reduction electrode. A method for manufacturing a reduction electrode for use in a carbon dioxide reduction device, which is disposed in contact with the reduction tank side of an electrolyte membrane installed between an oxidation tank and a reduction tank and performs a carbon dioxide reduction reaction by directly contacting carbon dioxide, comprising :
forming a relief structure and pores on the reduction electrode;
a step of heating the reduction electrode and the water repellent agent in a container;
removing the water-repellent layer around the pores of the reduction electrode;
A method for manufacturing a reduction electrode.