Reduction Electrode and Manufacturing Method of Reduction Electrode

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

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.

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.

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.

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

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.

is a diagram illustrating a configuration example of a carbon dioxide reduction deviceaccording to a first embodiment. As illustrated in, the carbon dioxide reduction deviceincludes an oxidation electrode, an oxidation tank, an electrolytic solution, a reduction electrode, a reduction tank, an electrolyte film, a conductive wire, a light source, and a water-repellent film.

The oxidation electrodeis immersed in the electrolytic solutionin the oxidation tank. The oxidation electrodeis formed by forming a semiconductor on a substrate having a predetermined area. The oxidation electrodeis 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 tankholds the electrolytic solutionin which the oxidation electrodeis immersed.

The electrolytic solutionis placed in the oxidation tank. Examples of the electrolytic solutioninclude 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 electrodeis disposed in the reduction tank. The reduction electrodeis formed on a substrate having a predetermined area similarly to the oxidation electrode. The reduction electrodeis, for example, a porous body of copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, or an alloy thereof. Additionally, the reduction electrodemay 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 tankhas the reduction electrodedisposed inside thereof, and holds carbon dioxide in a gas phase supplied from the outside through a pipe.

The electrolyte filmis disposed between the oxidation tankand the reduction tank. To be precise, the electrolyte filmis disposed between the electrolytic solutionand the reduction electrodein contact with the electrolytic solutionand the reduction electrode. The electrolyte filmis, 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 wirephysically and electrically connects the oxidation electrodeand the reduction electrode.

The light sourceis disposed close to the oxidation tank. The light sourceis, 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, the reduction electrodeand the electrolyte filmare 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 electrodeand the electrolyte filmto each other in their planes, the reaction field of the contact surface can be maximized.

In the carbon dioxide reduction devicedescribed above, in the oxidation tank, the oxidation reaction of water in the electrolytic solutionis performed by irradiation light (light energy) from the light sourceusing the electrolytic solutionand the oxidation electrodeof the semiconductor immersed in the electrolytic solution. In the reduction tank, the carbon dioxide reduction reaction is performed using the reduction electrodeconnected to the oxidation electrodevia the conductive wireand carbon dioxide brought into direct contact with the reduction electrode.

Specifically, when the light sourceemits light from the bottom of the oxidation tank, electron-hole pairs are generated and separated at the oxidation electrodewithin the oxidation tankthat has received the emitted light, and oxygen and protons are generated by the oxidation reaction of water in the electrolytic solution. The protons pass through the electrolyte filmand reach the reduction electrodein the reduction tankfrom the electrolytic solutionin the oxidation tank. The electrons flow from the oxidation electrodein the oxidation tankto the reduction electrodein the reduction tankvia the conductive wire. In the reduction tank, a carbon dioxide reduction reaction by protons, electrons, and carbon dioxide in a gas phase brought into direct contact with the reduction electrodeis caused at the reduction electrode. 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 solutionin the oxidation tank, the electrolyte filmswells, and the electrolytic solutionpasses through the pores of the electrolyte filmand exudes to the surface of the reduction electrodein the reduction tank. In order to prevent such exudation of the electrolytic solutionfrom the electrolyte film, it is sufficient if a surface of the electrolyte filmon the oxidation tankside is subjected to a water-repellent treatment, the surface being in contact with the electrolytic solution. However, since it is necessary to move protons as a raw material of the reduction reaction using water in the electrolyte filmas a medium, the reduction reaction may not proceed on the reduction tankside when the surface of the electrolyte filmon the oxidation tankside is fully covered by the water-repellent treatment.

Therefore, in the present embodiment, as illustrated in, the reduction electrodewith void holesis disposed on the surface of the electrolyte filmon the reduction tankside, and the water-repellent filmsare disposed on projection portions of the reduction electrode. Specifically, as enlarged in, the reduction electrodehas a protrusion-recess structure (for example, a structure including a plurality of cones on a main surface of a flat plate) on the reduction tankside, the water-repellent filmson the protrusion portions, and void holesat the bottom of the recesses so as not to cover the entire surface of the electrolyte filmon the reduction tankside.

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, carbon dioxide in the reduction tank, and electrons in the reduction electrodedo not react only by preparing the protrusion-recess structure on the reduction electrodeso as to exhibit the lotus effect.

Therefore, in the present example, a structure was prepared in which the void holeswere 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, the carbon dioxide in the reduction tank, and the electrons in the reduction electrodereact at a portion where the void holesand the electrolyte filmcontact are separated.

Further, depending on the pore diameter of the void hole, a capillary phenomenon occurs, and water tends to stay in the protrusion-recess structure of the reduction electrode, so that the water-repellent effect may be weakened. Therefore, by forming the water-repellent filmson the protrusion portions, a structure that promotes the movement of water from the electrolyte filmto the reduction tankis provided.

Note that when the water-repellent filmscover the entire surface of the reduction electrode, carbon dioxide in the reduction tankand electrons in the reduction electrodecannot directly react at the interface of the electrolyte film, so that it is necessary not to provide the water-repellent filmat a portion where the reduction electrodeand the electrolyte filmcontact.

Next, a method for manufacturing the reduction electrodeand the water-repellent filmwill be described.

Examples of a method for preparing the reduction electrodehaving the void holesinclude 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 filminclude 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 electrodeis entirely covered with the water-repellent agent, and carbon dioxide in the reduction tankand electrons in the reduction electrodecannot directly react at the interface of the electrolyte film. Therefore, it is necessary to remove the water-repellent filmon the contact surface between the void holeand the electrolyte filmwith 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 electrodeinto contact with the solvent, then drying the surface, and removing the solvent to precipitate the water-repellent filmon 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 electrodeis brought into contact with the fluorine-based solvent, the fluorine-based solvent does not penetrate into the void holesby the capillary phenomenon. Therefore, the water-repellent filmcan be more easily formed only on the protrusion-recess surface by the low surface tension characteristic of the fluorine-based solvent.

is a diagram illustrating a first manufacturing method for the reduction electrodeand the water-repellent film. The first manufacturing method is a method for manufacturing the reduction electrodeand the water-repellent filmby a first method of the liquid phase method.

In the reduction electrode, 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 S). 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 S), and then pulled up and dried (third step S). 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 filmswere thermally decomposed to be removed (fourth step S).

According to these steps, since the water-repellent filmis absent only on the contact surface between the reduction electrodeand the electrolyte film, the reaction of carbon dioxide in the reduction tank, electrons in the reduction electrode, and protons in the electrolyte filmcan proceed on this contact surface.

is a diagram illustrating a second manufacturing method for the reduction electrodeand the water-repellent film. The second manufacturing method is a method for manufacturing the reduction electrodeand the water-repellent filmby a first method of the gas phase method.

In the reduction electrode, 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 S). 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 S). 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 filmswere thermally decomposed to be removed (third step S).

According to these steps, since the water-repellent filmis absent only on the contact surface between the reduction electrodeand the electrolyte film, the reaction of carbon dioxide in the reduction tank, electrons in the reduction electrode, and protons in the electrolyte filmcan 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 filmon the order of nanometers can be formed.

is a diagram illustrating a third manufacturing method for the reduction electrodeand the water-repellent film. The third manufacturing method is a method for manufacturing the reduction electrodeand the water-repellent filmby a second method of the liquid phase method.

In the reduction electrode, 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 S). 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 S), and then the reduction electrode was pulled up and dried (third step S). In the third step of S, the capillary phenomenon of filling the void holes with the fluorine-based solvent was not observed.

According to these steps, since the water-repellent filmis absent only on the contact surface between the reduction electrodeand the electrolyte film, the reaction of carbon dioxide in the reduction tank, electrons in the reduction electrode, and protons in the electrolyte filmcan proceed on this contact surface.

Next, electrochemical measurement by the carbon dioxide reduction devicedescribed 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, and the oxidation electrodewas immersed in the electrolytic solutionof a 1.0 mol/L potassium hydroxide aqueous solution in the oxidation tank.

The reduction electrodemanufactured by the above manufacturing method was brought into close contact with the electrolyte filmby thermocompression bonding. In the present example, a thermocompression bonding method is used, but other methods may be used as long as the reduction electrodeand the electrolyte filmare physically in close contact with each other. The reduction electrodewas connected to the oxidation electrodeby the conductive wire, and the reduction electrodewas installed in the reduction tank.

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