Reduction Electrode and Manufacturing Method of Reduction Electrode

A reduction electrode that is disposed in contact with an electrolyte film on a reduction tank side, the electrolyte film being installed between an oxidation tank and the reduction tank, and is used in a carbon dioxide reduction device that performs a carbon dioxide reduction reaction by bringing carbon dioxide into direct contact, the reduction electrode including: a protrusion-recess structure and a void hole] on the reduction tank side, in which the protrusion-recess structure includes a water-repellent film capable of sliding down a liquid attached to a surface.

TECHNICAL FIELD

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

BACKGROUND ART

An increase in the concentration of carbon dioxide in the atmosphere is mentioned as a main cause of global warming. Reduction of carbon dioxide emissions has become a long-term challenge on a global scale. Meanwhile, energy supply relying on fossil fuels is to be reviewed in the medium and long term as an energy problem, and creation of a next-generation energy supply source is awaited.

As a means of suppressing emission of carbon dioxide and obtaining energy, technologies have been developed for utilizing unused energy such as exhaust heat, snow and ice heat, vibration, and electromagnetic waves, and renewable energy such as sunlight. These power generation technologies enable only creation of electrical energy, and storage of energy is impossible with these technologies. In addition, creation of chemical products using fossil fuels as raw materials is also impossible.

As a method of simultaneously solving these problems, a technology of reducing carbon dioxide using light energy has attracted attention. For example, Non Patent Literature 1 discloses a carbon dioxide reduction device by light irradiation. In an oxidation tank, when an 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 an electrolytic solution. The protons pass through the electrolyte film and reach a reduction tank, and the electrons flow to a reduction electrode through a conductive wire. In the reduction tank, a carbon dioxide reduction reaction by protons, electrons, and carbon dioxide dissolved in the solution is caused at the reduction electrode in the solution. This reduction reaction generates carbon monoxide, formic acid, methane, and the like that can be used as energy resources.

In the carbon dioxide reduction device of Non Patent Literature 1, the reduction electrode is immersed in the solution, and carbon dioxide is dissolved in the solution to supply the carbon dioxide to the reduction electrode. However, in this method for reducing carbon dioxide, since the reduction electrode is immersed in the solution, there are limitations on the concentration of carbon dioxide dissolved in the solution and the diffusion coefficient of carbon dioxide in the solution, and the amount of carbon dioxide supplied to the reduction electrode is limited.

Therefore, in order to increase the amount of carbon dioxide supplied to the reduction electrode, studies have been conducted to eliminate the solution in the reduction tank and directly supply carbon dioxide to the reduction electrode. In Non Patent Literature 2, by using a reduction tank having a structure in which carbon dioxide in a gas phase is directly supplied to a reduction electrode, the amount of carbon dioxide supplied to the reduction electrode is increased, and carbon dioxide reduction reaction is promoted.

CITATION LIST

Non Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, when the reduction reaction proceeds, reduction products of carbon dioxide are generated on a reaction surface of the reduction electrode, and not only hydrogen, carbon monoxide, and methane which are gases but also formic acid, methanol, ethanol, and the like which are liquids are generated. In addition, as time elapses, an electrolytic solution in an oxidation tank passes through an electrolyte film and is gradually exuded into the reduction tank. Therefore, the reaction surface (reaction site) of the reduction electrode is covered with these liquids, and the carbon dioxide reduction reaction does not proceed. Consequently, the conventional carbon dioxide reduction device has a problem in that the reduction reaction efficiency of carbon dioxide decreases in 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 capable of improving the reduction reaction efficiency of carbon dioxide.

Solution to Problem

A reduction electrode according to an aspect of the present invention is a reduction electrode that is disposed in contact with an electrolyte film on a reduction tank side, the electrolyte film being installed between an oxidation tank and the reduction tank, and is used in a carbon dioxide reduction device that performs a carbon dioxide reduction reaction by bringing carbon dioxide into direct contact, the reduction electrode including: a protrusion-recess structure and a void hole on the reduction tank side, in which the protrusion-recess structure includes a water-repellent agent capable of sliding down a liquid attached to a surface.

A reduction electrode manufacturing method according to an aspect of the present invention is a reduction electrode manufacturing method for manufacturing the reduction electrode, the reduction electrode manufacturing method performing: a step of forming a protrusion-recess structure and a void hole in the reduction electrode; a step of immersing the reduction electrode in a solvent containing a water-repellent agent; a step of drying the reduction electrode to remove the solvent; and a step of removing a water-repellent layer around the void hole of the reduction electrode.

A reduction electrode manufacturing method according to an aspect of the present invention is a reduction electrode manufacturing method for manufacturing the reduction electrode, the reduction electrode manufacturing method performing: a step of forming a protrusion-recess structure and a void hole in the reduction electrode; a step of putting the reduction electrode and a water-repellent agent into a container and heating the reduction electrode and the water-repellent agent; and a step of removing a water-repellent layer around the void hole of the reduction electrode.

A reduction electrode manufacturing method according to an aspect of the present invention is a reduction electrode manufacturing method for manufacturing the reduction electrode, the reduction electrode manufacturing method performing: a step of forming a protrusion-recess structure and a void hole in the reduction electrode; a step of bringing the protrusion-recess structure of the reduction electrode into contact with a solvent containing a water-repellent agent; and a step of drying the reduction electrode to remove the solvent.

Advantageous Effects of Invention

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

DESCRIPTION OF EMBODIMENTS

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

First Embodiment

FIG. 1 is a diagram illustrating a configuration example of a carbon dioxide reduction device 100 according to a first embodiment. As illustrated in FIG. 1, the carbon dioxide reduction device 100 includes an oxidation electrode 1, an oxidation tank 2, an electrolytic solution 3, a reduction electrode 4, a reduction tank 5, an electrolyte film 6, a conductive wire 7, a light source 8, and a water-repellent film 9.

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

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

The electrolytic solution 3 is placed in the oxidation tank 2. Examples of the electrolytic solution 3 include 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, and a cesium hydroxide aqueous solution.

The reduction electrode 4 is disposed in the reduction tank 5. The reduction electrode 4 is formed on a substrate having a predetermined area similarly to the oxidation electrode 1. 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. Additionally, the reduction electrode 4 may be a compound such as silver oxide, copper oxide, copper(II) oxide, nickel oxide, indium oxide, tin oxide, tungsten oxide, tungsten(VI) oxide, or copper oxide, or a porous metal complex having a metal ion and an anionic ligand.

The reduction tank 5 has the reduction electrode 4 disposed inside thereof, and holds carbon dioxide in a gas phase supplied from the outside through a pipe.

The electrolyte film 6 is disposed between the oxidation tank 2 and the reduction tank 5. To be precise, the electrolyte film 6 is disposed between the electrolytic solution 3 and the reduction electrode 4 in contact with the electrolytic solution 3 and the reduction electrode 4. The electrolyte film 6 is, for example, Nafion (registered trademark), FORBLUE, or Aquivion each of which is an electrolyte film having a carbon-fluorine skeleton, or SELEMION or NEOSEPTA that is an electrolyte film having a carbon-hydrogen skeleton.

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

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

In FIG. 1, the reduction electrode 4 and the electrolyte film 6 are drawn so as to have a large width in the lateral direction of the paper surface, but the width in the lateral direction of the paper surface may be reduced to have a thin plate shape with a flat surface in the depth direction of the paper surface. By bonding the reduction electrode 4 and the electrolyte film 6 to each other in their planes, the reaction field of the contact surface can be maximized.

In the carbon dioxide reduction device 100 described above, in the oxidation tank 2, the oxidation reaction of water in the electrolytic solution 3 is performed by irradiation light (light energy) from the light source 8 using the electrolytic solution 3 and the oxidation electrode 1 of the semiconductor immersed in the electrolytic solution 3. In the reduction tank 5, the carbon dioxide reduction reaction is performed using the reduction electrode 4 connected to the oxidation electrode 1 via the conductive wire 7 and carbon dioxide brought into direct contact with the reduction electrode 4.

Specifically, when the light source 8 emits light from the bottom of the oxidation tank 2, electron-hole pairs are generated and separated at the oxidation electrode 1 within the oxidation tank 2 that has received the emitted light, and oxygen and protons are generated by the oxidation reaction of water in the electrolytic solution 3. The protons pass through the electrolyte film 6 and reach the reduction electrode 4 in the reduction tank 5 from the electrolytic solution 3 in the oxidation tank 2. The electrons flow from the oxidation electrode 1 in the oxidation tank 2 to the reduction electrode 4 in the reduction tank 5 via the conductive wire 7. In the reduction tank 5, a carbon dioxide reduction reaction by protons, electrons, and carbon dioxide in a gas phase brought into direct contact with the reduction electrode 4 is caused at the reduction electrode 4. This oxidation-reduction reaction generates carbon monoxide, formic acid, methane, and the like that can be used as energy resources.

At this time, when a strong alkaline aqueous solution, for example, a 1.0 mol/L aqueous solution of sodium hydroxide is used as the electrolytic solution 3 in the oxidation tank 2, the electrolyte film 6 swells, and the electrolytic solution 3 passes through the pores of the electrolyte film 6 and exudes to the surface of the reduction electrode 4 in the reduction tank 5. In order to prevent such exudation of the electrolytic solution 3 from the electrolyte film 6, it is sufficient if a surface of the electrolyte film 6 on the oxidation tank 2 side is subjected to a water-repellent treatment, the surface being in contact with the electrolytic solution 3. However, since it is necessary to move protons as a raw material of the reduction reaction using water in the electrolyte film 6 as a medium, the reduction reaction may not proceed on the reduction tank 5 side when the surface of the electrolyte film 6 on the oxidation tank 2 side is fully covered by the water-repellent treatment.

Therefore, in the present embodiment, as illustrated in FIGS. 1 to 3, the reduction electrode 4 with void holes 10 is disposed on the surface of the electrolyte film 6 on the reduction tank 5 side, and the water-repellent films 9 are disposed on projection portions of the reduction electrode 4. Specifically, as enlarged in FIGS. 2 and 3, the reduction electrode 4 has a protrusion-recess structure (for example, a structure including a plurality of cones on a main surface of a flat plate) on the reduction tank 5 side, the water-repellent films 9 on the protrusion portions, and void holes 10 at the bottom of the recesses so as not to cover the entire surface of the electrolyte film 6 on the reduction tank 5 side.

When the protrusion-recess structure of several micrometers is formed on the surface, water droplets attached to the surface become water droplets without wetting, and slide down. This phenomenon is generally called the lotus effect. Protons on the surface of the electrolyte film 6, carbon dioxide in the reduction tank 5, and electrons in the reduction electrode 4 do not react only by preparing the protrusion-recess structure on the reduction electrode 4 so as to exhibit the lotus effect.

Therefore, in the present example, a structure was prepared in which the void holes 10 were provided, a portion having the protrusion-recess structure and having a function of sliding water down, and a portion where the protons on the surface of the electrolyte film 6, the carbon dioxide in the reduction tank 5, and the electrons in the reduction electrode 4 react at a portion where the void holes 10 and the electrolyte film 6 contact are separated.

Further, depending on the pore diameter of the void hole 10, a capillary phenomenon occurs, and water tends to stay in the protrusion-recess structure of the reduction electrode 4, so that the water-repellent effect may be weakened. Therefore, by forming the water-repellent films 9 on the protrusion portions, a structure that promotes the movement of water from the electrolyte film 6 to the reduction tank 5 is provided.

Note that when the water-repellent films 9 cover the entire surface of the reduction electrode 4, carbon dioxide in the reduction tank 5 and electrons in the reduction electrode 4 cannot directly react at the interface of the electrolyte film 6, so that it is necessary not to provide the water-repellent film 9 at a portion where the reduction electrode 4 and the electrolyte film 6 contact.

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

Examples of a method for preparing the reduction electrode 4 having the void holes 10 include a casting method, a powder metallurgy method, a shaping method using a metal 3D printer, a processing method using a high-power laser, and the like, which are commercial technologies. Examples of the water-repellent treatment for manufacturing the water-repellent film 9 include a liquid phase method and a gas phase method.

The liquid phase method is a method in which an object is immersed in a fluorine-based solvent obtained by dissolving a fluorine-based polymer as a water-repellent agent, by a dip coating method or the like, and then the fluorine-based polymer is precipitated by performing heating or the like on the object and removing the solvent. The gas phase method is a method in which an object and a fluorine-based low molecular substance (silane coupling agent) which is a water-repellent agent are put in the same sealed space, the fluorine-based low molecular substance is heated to be evaporated, and then the fluorine-based low molecular substance is deposited on the surface of the object. In these two methods, the surface of the reduction electrode 4 is entirely covered with the water-repellent agent, and carbon dioxide in the reduction tank 5 and electrons in the reduction electrode 4 cannot directly react at the interface of the electrolyte film 6. Therefore, it is necessary to remove the water-repellent film 9 on the contact surface between the void hole 10 and the electrolyte film 6 with a high-power laser or the like.

In addition, as another simpler method without using the high-power laser, there is a method of dissolving a fluorine-based polymer or a fluorine-based low molecular substance in a fluorine-based solvent, bringing the protrusion-recess surface of the reduction electrode 4 into contact with the solvent, then drying the surface, and removing the solvent to precipitate the water-repellent film 9 on the protrusion-recess surface. Since the fluorine-based solvent has a lower surface tension than water, even when the protrusion-recess 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 holes 10 by the capillary phenomenon. Therefore, the water-repellent film 9 can be more easily formed only on the protrusion-recess surface by the low surface tension characteristic of the fluorine-based solvent.

FIG. 4 is a diagram illustrating a first manufacturing method for 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 by a first method of the liquid phase method.

In the reduction electrode 4, a structure having the protrusion-recess structure and the void holes was formed by a metal 3D printer. As the protrusion-recess structure, cylindrical protrusion-recess structures having a diameter of 10 μm and a height of 20 μm and void holes having a diameter of 10 μm were prepared and used. The interval between the protrusion-recess structure and the void hole was set to an equal interval, and the pitch was set to 15 μm. The closest packing structure was adopted, and the ratio of the protrusion-recess structure to the void hole was 1:3 (first step S101). As the water-repellent agent, OPTOOL DSX was used. Dip coating was performed in which the reduction electrode was immersed in an OPTOOL DSX solution for one minute (second step S102), and then pulled up and dried (third step S103). Through this step, the water-repellent film can be formed on the entire surface of the reduction electrode. Thereafter, the outer periphery of the void holes was traced using a high-power laser, and the water-repellent films 9 were thermally decomposed to be removed (fourth step S104).

According to these steps, since the water-repellent film 9 is absent only on the contact surface between the reduction electrode 4 and the electrolyte film 6, the reaction of carbon dioxide in the reduction tank 5, electrons in the reduction electrode 4, and protons in the electrolyte film 6 can proceed on this contact surface.

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

In the reduction electrode 4, a structure having the protrusion-recess structure and the void holes was formed by a metal 3D printer. As the protrusion-recess structure, cylindrical protrusion-recess structures having a diameter of 10 μm and a height of 20 μm and void holes having a diameter of 10 μm were prepared and used. The interval between the protrusion-recess structure and the void hole was set to an equal interval, and the pitch was set to 15 μm. The closest packing structure was adopted, and the ratio of the protrusion-recess structure to the void hole was 1:3 (first step S201). As the water-repellent agent, a fluorine-based silane coupling agent (for example, heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane) was used. The reduction electrode and the water-repellent agent were put in a Teflon container and sealed, and the container was put in an oven and heated at 150° C. (second step S202). Through this step, the water-repellent agent is evaporated, and the water-repellent film can be formed on the entire surface of the reduction electrode. Thereafter, the outer periphery of the void holes was traced using a high-power laser, and the water-repellent films 9 were thermally decomposed to be removed (third step S203).

According to these steps, since the water-repellent film 9 is absent only on the contact surface between the reduction electrode 4 and the electrolyte film 6, the reaction of carbon dioxide in the reduction tank 5, electrons in the reduction electrode 4, and protons in the electrolyte film 6 can proceed on this contact surface. In this gas phase method, since a monomolecular film is formed on the surface of Nafion, an extremely thin water-repellent film 9 on the order of nanometers can be formed.

FIG. 6 is a diagram illustrating a third manufacturing method for 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 by a second method of the liquid phase method.

In the reduction electrode 4, a structure having the protrusion-recess structure and the void holes was formed by a metal 3D printer. As the protrusion-recess structure, cylindrical protrusion-recess structures having a diameter of 10 μm and a height of 20 μm and void holes having a diameter of 10 μm were prepared and used. The interval between the protrusion-recess structure and the void hole was set to an equal interval, and the pitch was set to 15 μm. The closest packing structure was adopted, and the ratio of the protrusion-recess structure to the void hole was 1:3 (first step S301). As the water-repellent agent, OPTOOL DSX was used. Only the surface of the protrusion-recess structure of the reduction electrode was brought into contact with an OPTOOL DSX solution (second step S302), and then the reduction electrode was pulled up and dried (third step S303). In the third step of S303, the capillary phenomenon of filling the void holes with the fluorine-based solvent was not observed.

According to these steps, since the water-repellent film 9 is absent only on the contact surface between the reduction electrode 4 and the electrolyte film 6, the reaction of carbon dioxide in the reduction tank 5, electrons in the reduction electrode 4, and protons in the electrolyte film 6 can proceed on this contact surface.

Next, electrochemical measurement by the carbon dioxide reduction device 100 described above and a measurement result thereof will be described.

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

The reduction electrode 4 manufactured by the above manufacturing method was brought into close contact with the electrolyte film 6 by thermocompression bonding. In the present example, a thermocompression bonding method is used, but other methods may be used as long as the reduction electrode 4 and the electrolyte film 6 are physically in close contact with each other. The reduction electrode 4 was connected to the oxidation electrode 1 by the conductive wire 7, and the reduction electrode 4 was installed in the reduction tank 5.

In addition, Nafion was used for the electrolyte film 6 physically separating the oxidation tank 2 and the reduction tank 5. Of both surfaces of the electrolyte film 6, one surface on which the water-repellent film 9 was formed was disposed to be directed into the reduction tank 5, and one surface having the void holes was disposed to be in contact with the electrolyte film 6.

In addition, as the light source 8, a 300 W xenon lamp was used. A wavelength of 450 nm or more was cut with a filter, and the illuminance was set to 6.6 mW/cm2. An irradiation surface of the oxidation electrode 1 was set to 2.5 cm2.

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. The bubbling of nitrogen into the oxidation tank 2 was carried out for the purpose of analyzing the reaction products. The inside of each of the oxidation tank 2 and the reduction tank 5 was sufficiently replaced with nitrogen and carbon dioxide, respectively, and light was emitted from the light source 8. Thereafter, the carbon dioxide reduction reaction proceeded on the surface of the copper porous body, which is the reduction electrode 4.

At this time, the current flowing between the oxidation electrode 1 and the reduction electrode 4 by the irradiation light was measured with an electrochemical measurement device (Model 1287 Potentiogalvanostat manufactured by Solartron). In addition, gases and liquids generated in the oxidation tank 2 and the reduction tank 5 were collected, and the reaction products were analyzed using a gas chromatograph, a liquid chromatograph, and a gas chromatograph mass spectrometer.

In particular, in the present embodiment, an effect of the water-repellent film 9 formed on the surface of the reduction electrode 4 was examined 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 below.

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

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

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

In a first comparative example, in the manufacturing method described in the first manufacturing method, the reduction electrode 4 having a columnar protrusion-recess structure having a height of 0 μm, that is, no protrusion-recess structure was used.

In a second comparative example, the reduction electrode 4 on which no water-repellent film 9 was formed was used in the manufacturing method described in the first manufacturing method.

FIG. 7 is a diagram showing a measurement result of the Faraday efficiency of formic acid according to the first embodiment. In the first comparative example having no protrusion-recess structure, the Faraday efficiency decreased after 50 hours. In the second comparative example in which the water-repellent film 9 was not formed, the Faraday efficiency decreased after 50 hours. On the other hand, in the first example to the third example in which the protrusion-recess structure and the water-repellent film 9 were formed, the Faraday efficiency did not decrease even after 50 hours. This is because, as a result of introducing the protrusion-recess structure and the water-repellent film 9 that promote sliding of the liquid onto the surface of the reduction electrode 4, the liquid on the surface of the reduction electrode 4 as a result of liquid leakage easily slides down, and the reaction site of the reduction electrode 4 was not covered with the electrolytic solution 3.

Here, a method for calculating the Faraday efficiency of the carbon dioxide reduction reaction will be described. 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 moved between the oxidation electrode 1 and the reduction electrode 4 by light irradiation or application of a current voltage, and can be calculated by Formula (1).

The “number of electrons in reduction reaction” in Formula (1) is determined by converting the measured value of the integrated amount of the generated carbon dioxide reduction product into the number of electrons required for the production reaction. For example, the “number of electrons in reduction reaction” when the reduction product is a gas can be calculated by Formula (2).

A is a concentration (ppm) of the reduction reaction product. B is a 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 a light irradiation time or a current voltage application time (sec). Vg is a molar volume of the gas (L/mol).

The “number of electrons in reduction reaction” when the reduction product is a liquid can be calculated by Formula (3).

C is a concentration (mol/L) of the reduction reaction product. V1 is a volume (L) of the liquid sample. 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. With the carbon dioxide reduction device 100 according to the first embodiment, it is possible to provide the carbon dioxide reduction device 100 capable of allowing the carbon dioxide reduction reaction to proceed without reducing the Faraday efficiency.

That is, in the first embodiment, in the carbon dioxide reduction device 100 including the oxidation tank 2 that performs the oxidation reaction of water by irradiation light from the light source 8 by using the electrolytic solution 3 and the oxidation electrode 1 of a semiconductor immersed in the electrolytic solution 3, the reduction tank 5 that performs the carbon dioxide reduction reaction by using the reduction electrode 4 connected to the oxidation electrode 1 via the conductive wire 7 and carbon dioxide brought into direct contact with the reduction electrode 4, and the electrolyte film 6 disposed between the electrolytic solution 3 in the oxidation tank 2 and the reduction electrode 4 in the reduction tank 5 to be in contact with both the electrolytic solution 3 and the reduction electrode 4, the protrusion-recess structure and the void holes 10 are disposed in contact with the electrolyte film 6 on the reduction tank 5 side and provided on the reduction tank 5 side, and the protrusion-recess structure includes the water-repellent film 9 capable of sliding down the liquid attached to the surface.

Therefore, the water-repellent film 9 provided on the surface of the protrusion-recess structure of the reduction electrode 4 allows the liquid on the surface of the reduction electrode 4, which is a result of exudation of the electrolytic solution 3 in the oxidation tank 2 to the outside of the electrolyte film 6, to easily move to the surface of the protrusion-recess structure, and the droplets on the surface of the protrusion-recess structure slide down by the lotus effect, so that the reaction site of the reduction electrode 4 is not covered with the electrolytic solution 3. As a result, the carbon dioxide reduction reaction can proceed, and a decrease in the reduction reaction efficiency can be suppressed.

Note that, in the above experiment, light is generated by a xenon lamp in order to quantitatively manage the emission amount of light with respect to the oxidation electrode 1, but it is also possible to cause an oxidation reaction using sunlight or the like.

Second Embodiment

In the first embodiment, the case where the light source 8 and the oxidation electrode 1 including a semiconductor are used has been described. In the second embodiment, instead of these, an oxidation/reduction reaction is allowed to proceed using an external power supply and the oxidation electrode 1 including a metal. For comparison, the same voltage value and current value as those in the first embodiment were adjusted and applied.

FIG. 8 is a diagram illustrating a configuration example of a carbon dioxide reduction device 100 according to a second embodiment. The oxidation electrode 1 is platinum. Additionally, the oxidation electrode 1 may be, for example, gold or silver. An external power supply 11 is an electrochemical measurement device, and is connected in series to a conductive wire 7 connecting the oxidation electrode 1 and a reduction electrode 4. The power supply 11 may be another power supply device. Other components are the same as those of the first embodiment.

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

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

Also in the second embodiment, as in the first embodiment, the reduction electrode 4 is disposed in contact with the electrolyte film 6 on the reduction tank 5 side, and includes a protrusion-recess structure and void holes 10 on the reduction tank 5 side, and the protrusion-recess structure includes a water-repellent film 9 capable of sliding down a liquid attached to the surface. As a method for manufacturing the water-repellent film 9, the first manufacturing method to the third manufacturing method are used similarly to the first embodiment.

FIG. 9 is a diagram showing a measurement result of the Faraday efficiency of formic acid according to the second embodiment. Examples using the same reduction electrode 4 as in the first example to the third example described in the first embodiment are referred to as a fourth example to a sixth example, respectively. In a third comparative example, in the manufacturing method described in the first manufacturing method, the reduction electrode 4 having a columnar protrusion-recess structure having a height of 0 μm, that is, no protrusion-recess structure was used. In a fourth comparative example, the reduction electrode 4 on which no water-repellent film 9 was formed was used in the manufacturing method described in the first manufacturing method.

In a third comparative example having no protrusion-recess structure, the Faraday efficiency decreased after 50 hours. In a fourth comparative example in which the water-repellent film 9 was not formed, the Faraday efficiency decreased after 50 hours. On the other hand, in the fourth example to the sixth example in which the protrusion-recess structure and the water-repellent film 9 were formed, the Faraday efficiency did not decrease even after 50 hours. This is because, as a result of introducing the protrusion-recess structure and the water-repellent film 9 that promote sliding of the liquid onto the surface of the reduction electrode 4, the liquid on the surface of the reduction electrode 4 as a result of liquid leakage easily slides down, and the reaction site of the reduction electrode 4 was not covered with the electrolytic solution 3.

The second embodiment has been described above. With the carbon dioxide reduction device 100 according to the second embodiment, it is possible to provide the carbon dioxide reduction device 100 capable of allowing the carbon dioxide reduction reaction to proceed without reducing the Faraday efficiency.

That is, in the second embodiment, in the carbon dioxide reduction device 100 including the oxidation tank 2 that performs the oxidation reaction of water by the current voltage from the power supply 11 by using the electrolytic solution 3 and the oxidation electrode 1 of platinum (metal) immersed in the electrolytic solution 3, the reduction tank 5 that performs the carbon dioxide reduction reaction by using the reduction electrode 4 connected to the power supply 11 and carbon dioxide brought into direct contact with the reduction electrode 4, and the electrolyte film 6 disposed between the electrolytic solution 3 in the oxidation tank 2 and the reduction electrode 4 in the reduction tank 5 to be in contact with both the electrolytic solution 3 and the reduction electrode 4, the protrusion-recess structure and the void holes 10 are disposed in contact with the electrolyte film 6 on the reduction tank 5 side and provided on the reduction tank 5 side, and the protrusion-recess structure includes the water-repellent film 9 capable of sliding down the liquid attached to the surface.

Therefore, the water-repellent film 9 provided on the surface of the protrusion-recess structure of the reduction electrode 4 allows the liquid on the surface of the reduction electrode 4, which is a result of exudation of the electrolytic solution 3 in the oxidation tank 2 to the outside of the electrolyte film 6, to easily move to the surface of the protrusion-recess structure, and the droplets on the surface of the protrusion-recess structure slide down by the lotus effect, so that the reaction site of the reduction electrode 4 is not covered with the electrolytic solution 3. As a result, the carbon dioxide reduction reaction can proceed, and a decrease in the reduction reaction efficiency can be suppressed.

The present invention can be widely used in the field related to the recycling of carbon dioxide. Although light energy is used in the first embodiment and electrical energy is used in the second embodiment, other renewable energy may be used. In addition, the first embodiment and the second embodiment can be combined.

The present invention can be also applied to any electrolyte film as long as it is the electrolyte film 6 that is disposed between the electrolytic solution 3 in the oxidation tank 2 and the reduction electrode 4 in the reduction tank 5 in contact with the electrolytic solution 3 and the reduction electrode 4 and is used in the carbon dioxide reduction device 100 that performs a carbon dioxide reduction reaction by bringing carbon dioxide into direct contact with the reduction electrode 4.

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