JP7587167B2 - Method for manufacturing electrolyte membrane supported reduction electrode - Google Patents

Description

The present invention relates to a method for manufacturing an electrolyte membrane-supported reduction electrode.

Conventionally, from the viewpoint of preventing global warming and ensuring a stable supply of energy, technology for reducing carbon dioxide has been attracting attention. Examples of devices for reducing carbon dioxide include devices that use artificial photosynthesis technology to reduce carbon dioxide by applying light energy such as sunlight, and electrolytic decomposition devices that reduce carbon dioxide by applying electrical energy from the outside (see Non-Patent Documents 1 to 3).

Figure 2 of Non-Patent Document 1 illustrates an apparatus for reducing carbon dioxide by light irradiation. An electrolyte membrane is placed between an oxidation tank on the left side and a reduction tank on the right side, and the oxidation tank and reduction tank are each filled with an aqueous solution. A gallium nitride (GaN) oxidation electrode is placed in the oxidation tank, and a copper (Cu) reduction electrode is placed in the reduction tank, and the oxidation electrode and reduction electrode are connected by a conductor. Helium (He) is then introduced into the aqueous solution in the oxidation tank, and carbon dioxide (CO ₂ ) is introduced into the aqueous solution in the reduction tank.

When the oxidation electrode is irradiated with light, electron-hole pairs are generated and separated at the oxidation electrode, and oxygen (O ₂ ) and protons (H ⁺ ) are generated by the oxidation reaction of water (H ₂ O). The protons then move to the reduction tank via the electrolyte membrane, and the electrons (e ^- ) generated at the oxidation electrode move to the reduction electrode via the conductor. After that, hydrogen (H ₂ ) is generated at the reduction electrode by the combination of the protons and electrons, and a reduction reaction of carbon dioxide is caused by the protons, electrons, and carbon dioxide. This reduction reaction of carbon dioxide generates carbon monoxide, formic acid, methane, and other energy resources.

Satoshi Yotsuhashi and 6 others, “CO2 Conversion with Light and Water by GaN Photoelectrode”, Japanese Journal of Applied Physics 51, 2012, p.02BP07-1-p.02BP07-3 Yoshio Hori and others, “Formation of Hydrocarbons in the Electrochemical Reduction of Carbone Dioxide at a Copper Electrode in Aqueous Solution”, Journal of the Chemical Society 85(8), 1989, p.2309-p.2326 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-Se Photoanode”, Chemistry Letter, 47, January 13, 2018, p.436- p.439

To increase the amount of carbon dioxide supplied to the reduction electrode, there is a gas-phase carbon dioxide reduction device that fills the reduction tank with gas-phase carbon dioxide. In this gas-phase reduction device, the reduction electrode is formed directly on the electrolyte membrane, based on the fact that protons cannot move through gas-phase carbon dioxide.

One method for forming such a membrane is electroless plating, which uses a metal salt solution containing metal ions and a reducing agent solution that reduces the metal ions. In conventional electroless plating, a metal salt solution and a reducing agent solution are simultaneously injected onto one side and the other side of an electrolyte membrane. The reducing agent in the reducing agent solution permeates the electrolyte membrane and comes into contact with the metal salt solution, reducing the metal ions in the metal salt solution and depositing metal on one side of the electrolyte membrane.

However, because the metal salt solution and the reducing agent solution are injected simultaneously, a small amount of the metal salt solution penetrates into the electrolyte membrane before the reducing agent in the reducing agent solution penetrates the electrolyte membrane and comes into contact with the metal salt solution. As a result, the reducing agent solution and the metal salt solution come into contact inside the electrolyte membrane, and the reduction electrode is formed so as to sink into the electrolyte membrane. In addition, carbon dioxide cannot be supplied to the reduction electrode formed inside the electrolyte membrane, and since the electrolyte membrane contains moisture within the membrane, the moisture within the electrolyte membrane reacts with the dissolved oxygen within the electrolyte membrane, causing the reduction electrode to oxidize. As a result, in the reduction electrode, the reduction reaction of the oxidized reduction electrode itself proceeds preferentially, as shown in formulas (1) and (2). This suppresses the reduction reaction of carbon dioxide, and the efficiency of the reduction reaction of carbon dioxide is reduced.

Cu ₂ O + 2H ⁺ +2e ^- → 2Cu + H ₂ O ... (1)
CuO+2H ⁺ +2e ^- →Cu+H ₂ O...(2)
In addition, in a gas-phase carbon dioxide reduction device, operation is generally performed in which light energy or electric energy is repeatedly turned on and off depending on the rising and falling cycle of sunlight and the maintenance cycle. When the energy is turned off, the reduction electrode formed inside the electrode is oxidized, and when the electrode is turned on again, the reduction reaction of the oxidized reduction electrode proceeds preferentially, resulting in a decrease in the efficiency of the reduction reaction of carbon dioxide.

The present invention has been made in consideration of the above circumstances, and the object of the present invention is to provide a technology that can suppress the formation of a reduction electrode inside an electrolyte membrane in an electrolyte membrane-supported reduction electrode that constitutes a gas-phase carbon dioxide reduction device, and can improve the efficiency of the carbon dioxide reduction reaction.

In one embodiment of the present invention, a method for manufacturing an electrolyte membrane-supported reduction electrode, which is disposed between an oxidation cell containing an oxidation electrode and a reduction cell into whose interior carbon dioxide is supplied, includes a first step of immersing one side of an electrolyte membrane opposite to the other side in a reducing agent solution, and a second step of immersing one side of the electrolyte membrane in a metal salt solution containing metal ions.

According to the present invention, in an electrolyte membrane-supported reduction electrode constituting a gas-phase carbon dioxide reduction device, it is possible to prevent a reduction electrode from being formed inside the electrolyte membrane, thereby providing a technology that can improve the efficiency of the carbon dioxide reduction reaction.

FIG. 1 is a diagram showing a manufacturing process of an electrolyte membrane supported reduction electrode. FIG. 2 is a diagram showing the reaction system of the electroless plating method. FIG. 3 is a diagram showing an image of the formation of a reduction electrode on an electrolyte membrane. FIG. 4 is a diagram illustrating a configuration example of the gas phase reduction device for carbon dioxide according to the first embodiment. FIG. 5 is a diagram showing an example of operation of the gas phase reduction device for carbon dioxide. FIG. 6 is a diagram showing a configuration example of a gas phase reduction device for carbon dioxide according to a sixth embodiment.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The present invention is not limited to the embodiment described below, and modifications can be made without departing from the spirit of the present invention.

Summary of the Invention
The present invention relates to a method for manufacturing an electrolyte membrane supported reduction electrode in which a reduction electrode is formed directly on an electrolyte membrane by electroless plating using a metal salt solution and a reducing agent solution. The present invention is characterized in that, before electroless plating is performed on the electrolyte membrane, one side of the electrolyte membrane is immersed in a reducing agent solution.

As a result, the reducing agent solution diffuses and penetrates to one side of the electrolyte membrane (the surface where the reduction electrode is formed) before the electroless plating process, so that when the metal salt solution is injected onto one side of the electrolyte membrane, the reduction electrode can be formed from directly above that side. In other words, the formation of the reduction electrode inside the electrolyte membrane during the production of the reduction electrode and when the reduction reaction is stopped can be suppressed, so oxidation of the reduction electrode can be suppressed and the efficiency of the carbon dioxide reduction reaction can be improved.

[Example 1]
[Method of manufacturing an electrolyte membrane supported reduction electrode]
1 is a diagram showing the manufacturing process of an electrolyte membrane supported reduction electrode. Nafion (registered trademark) manufactured by DuPont is used as the electrolyte membrane. The metal salt solution and the reducing agent solution are prepared as shown in Table 1. For example, the reducing agent solution is a solution containing sodium borohydride (NaBH ₄ ), a polar compound, as the main reducing agent component.

First, in order to improve the adhesion between the electrolyte membrane and the reduction electrode, in step 1, one side of the electrolyte membrane is roughened by rubbing abrasive paper against the one side (S1). Next, in order to improve the proton mobility of the electrolyte membrane, in step 2, the electrolyte membrane is immersed in boiling nitric acid for 60 minutes (S2), and in step 3, the electrolyte membrane is immersed in boiling pure water for 60 minutes (S3). Then, as shown in FIG. 2, the electrolyte membrane 1 is placed between the first tank 11 and the second tank 12. At this time, the roughened surface of the electrolyte membrane 1 is placed facing the first tank 11.

Next, in step 4 (first step), the second tank 12 is filled with the reducing agent solution 22 and left for 1 minute, and the opposite surface of the electrolyte membrane 1 (opposite the roughened surface) is immersed in the reducing agent solution 22 (S4). Next, in step 5 (second step), the first tank 11 is filled with a metal salt solution 21 containing metal ions and left for 30 minutes, and the roughened surface of the electrolyte membrane 1 is immersed in the metal salt solution 21 (S5).

Step 5 is a step of performing electroless plating by arranging the metal salt solution 21 and the reducing agent solution 22 so that they are in contact with each other across the electrolyte membrane 1. In other words, step 5 is a step of depositing a metal for a reduction electrode on the roughened surface of the electrolyte membrane 1 by electroless plating in which the opposite surface of the electrolyte membrane 1 is immersed in the reducing agent solution 22 and the roughened surface of the electrolyte membrane is immersed in the metal salt solution 21.

In step 4, sodium borohydride, which is the main component of the reducing agent solution 22, is a polar compound and therefore diffuses and permeates the inside of the electrolyte membrane 1. Thereafter, in step 5, an oxidation-reduction reaction (BH ₄ ^- +4OH ^- →BO ₂ ^- +2H ₂ O+2H ₂ +4e ^- , Cu ²⁺ +2e ^- →Cu) occurs at the interface between the metal salt solution 21 and the electrolyte membrane 1, and copper is precipitated. As a result, an electrolyte membrane-supported reduction electrode 30 is obtained in which the reduction electrode 2 is formed directly on the initial surface of the electrolyte membrane 1, as shown in the enlarged view (a) of FIG. 3. The inventors confirmed that the reduction electrode 2 is formed directly on the electrolyte membrane 1 without being embedded in the electrolyte membrane 1.

The electrolyte membrane 1 may be, for example, a solid polymer membrane that conducts cations or anions, or an electrolyte membrane having a carbon-fluorine skeleton, such as Nafion, Forblue, or Aquivion (registered trademark). By changing the metal salt solution 21 and the reducing agent solution 22 to other chemicals, any type of metal, such as Ni, Pt, Au, Ag, Pd, Sn, or Pd, may be formed, or a metal complex may be formed by subjecting the metal to an oxidation reaction or substitution reaction.

[Configuration of the carbon dioxide gas phase reduction device]
4 is a diagram showing a configuration example of a carbon dioxide gas-phase reduction device 100 according to Example 1. The gas-phase reduction device 100 is a reduction device (artificial photosynthesis device) that causes a carbon dioxide reduction reaction at a reduction electrode by irradiating an oxidation electrode with light. Hereinafter, it will be simply referred to as the gas-phase reduction device 100.

As shown in Figure 4, the gas-phase reduction device 100 comprises an oxidation tank 41 and a reduction tank 44 formed by dividing the internal space of a single housing in two. The oxidation tank 41 is filled with an aqueous solution 43 into which an oxidation electrode 42 made of a semiconductor or metal complex is inserted. The empty interior of the reduction tank 44 adjacent to the oxidation tank 41 is filled with carbon dioxide gas or a gas containing carbon dioxide.

The oxidation electrode 42 is a compound that exhibits photoactivity or redox activity, such as a nitride semiconductor, titanium oxide, amorphous silicon, a ruthenium complex, or a rhenium complex. The aqueous solution 43 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 sodium hydroxide, an aqueous solution of potassium hydroxide, an aqueous solution of rubidium hydroxide, or an aqueous solution of cesium hydroxide.

The electrolyte membrane-supported reduction electrode 30 manufactured by the above manufacturing method is placed between the oxidation tank 41 and the reduction tank 44. The electrolyte membrane 1 is placed on the oxidation tank 41 side, and the reduction electrode 2 is placed on the reduction tank 44 side. The oxidation electrode 42 and the reduction electrode 2 are connected by a conductor 45.

A tube 46 is inserted into the oxidation tank 41 to allow helium to flow into the aqueous solution 43 in the oxidation tank 41. A gas inlet 47 is formed at the bottom of the reduction tank 44 to allow carbon dioxide to flow into the reduction tank 44. Furthermore, a light source 48 is disposed opposite the oxidation electrode 42 to operate the gas-phase reduction device 100. The light source 48 is, for example, a xenon lamp, a pseudo-sun light source, a halogen lamp, a mercury lamp, sunlight, or a combination of these.

[Electrochemical measurements and gas/liquid production measurements]
Electrochemical measurements and gas and liquid production measurements are explained.

The oxidation tank 41 is filled with the aqueous solution 43. For the oxidation electrode 42, a substrate was used in which a thin film of gallium nitride (GaN) and a thin film of aluminum gallium nitride (AlGaN) were epitaxially grown in that order on a sapphire substrate, and nickel (Ni) was vacuum-deposited thereon and heat-treated to form a nickel oxide (NiO) promoter thin film. The oxidation electrode 42 was then placed in the oxidation tank 41 so as to be immersed in the aqueous solution 43. The aqueous solution 43 was a 1.0 mol/L aqueous potassium hydroxide solution. For the light source 48, a 300W high-pressure xenon lamp (cutting wavelengths of 450 nm or more, illuminance 6.6 mW/cm ² ) was used, and the surface of the semiconductor photoelectrode of the oxidation electrode 42 on which the oxidation promoter was formed (the surface on which NiO was formed) was fixed as the irradiated surface. The light irradiation area of the oxidation electrode 42 was set to 2.5 cm ² .

Helium was fed into the oxidation tank 41 through the tube 46, and carbon dioxide was fed into the reduction tank 44 through the gas inlet 47, each at a flow rate of 5 ml/min and a pressure of 0.18 MPa. In this system, the reduction reaction of carbon dioxide can be carried out at the three-phase interface consisting of [electrolyte membrane - copper (reduction electrode) - gas-phase carbon dioxide] in the electrolyte membrane supported reduction electrode 30.

After the oxidation tank 41 and the reduction tank 44 were fully replaced with helium and carbon dioxide, respectively, the oxidation electrode 42 was uniformly irradiated with light for 300 minutes using the light source 48. When the oxidation electrode 42 was irradiated with light, electrons flowed between the oxidation electrode 42 and the reduction electrode 2. The current value between the oxidation electrode 42 and the reduction electrode 2 during light irradiation was measured with an electrochemical measurement device (Solartron, 1287 type potentiogalvanostat). In addition, gas and liquid in the oxidation tank 41 and the reduction tank 44 were collected at any time during light irradiation, and the reaction products were analyzed with a gas chromatograph, a liquid chromatograph, and a gas chromatograph mass spectrometer. As a result, it was confirmed that oxygen was generated in the oxidation tank 41, and hydrogen, carbon monoxide, formic acid, methane, methanol, ethanol, and ethylene were generated in the reduction tank 44.

[Example 2]
In Example 2, the electrolyte membrane 1 was immersed in the reducing agent solution 22 for 10 minutes in step 4 of the method for producing the electrolyte membrane supported reduction electrode 30. The other conditions were the same as those in Example 1.

[Example 3]
In Example 3, the electrolyte membrane 1 was immersed in the reducing agent solution 22 for 30 minutes in step 4 of the method for producing the electrolyte membrane supported reduction electrode 30. The other conditions were the same as those in Example 1.

[Example 4]
In Example 4, the electrolyte membrane 1 was immersed in the reducing agent solution 22 for 60 minutes in step 4 of the method for producing the electrolyte membrane supported reduction electrode 30. The other conditions were the same as those in Example 1.

[Example 5]
5, the oxidation electrode 42 was irradiated with light for 60 minutes (ON) and then stopped for 30 minutes (OFF), and the measurement was stopped when the total irradiation time of the oxidation electrode 42 with light reached 300 minutes. The other conditions were the same as those of Example 3.

[Example 6]
[Method of manufacturing an electrolyte membrane supported reduction electrode]
The electrolyte membrane supported reduction electrode 30 is manufactured in the same manner as in the first embodiment.

[Configuration of the carbon dioxide gas phase reduction device]
6 is a diagram showing the configuration of a gas-phase carbon dioxide reduction apparatus 100 according to Example 6. The gas-phase carbon dioxide reduction apparatus 100 is an apparatus for electrolytic reduction reaction of gas-phase carbon dioxide (electrolytic reduction reaction apparatus). Hereinafter, it will be simply referred to as the gas-phase reduction apparatus 100.

As shown in Figure 6, the gas-phase reduction device 100 includes an oxidation tank 41 and a reduction tank 44 formed by dividing the internal space of a single housing in two. The oxidation tank 41 is filled with an aqueous solution 43, into which an oxidation electrode 42 made of a semiconductor or a metal complex is inserted. The empty interior of the reduction tank 44 adjacent to the oxidation tank 41 is filled with carbon dioxide gas or a gas containing carbon dioxide. The oxidation electrode 42 is, for example, platinum, gold, silver, copper, indium, or nickel. A specific example of the aqueous solution 43 is the same as in Example 1.

The electrolyte membrane-supported reduction electrode 30 manufactured by the above manufacturing method is placed between the oxidation tank 41 and the reduction tank 44. The electrolyte membrane 1 is placed on the oxidation tank 41 side, and the reduction electrode 2 is placed on the reduction tank 44 side. The oxidation electrode 42 and the reduction electrode 2 are connected by a conductor 45.

A tube 46 is inserted into the oxidation tank 41 to allow helium to flow into the aqueous solution 43 in the oxidation tank 41. A gas inlet 47 is formed at the bottom of the reduction tank 44 to allow carbon dioxide to flow into the reduction tank 44. Furthermore, a power source 49 is connected to the conductor 45 to operate the gas-phase reduction device 100.

[Electrochemical measurements and gas/liquid production measurements]
Electrochemical measurements and gas and liquid production measurements are explained.

The oxidation tank 41 is filled with an aqueous solution 43. Platinum (manufactured by Nilaco Corporation) is used for the oxidation electrode 42. The oxidation electrode 42 is placed in the oxidation tank 41 so that about 0.55 ^cm2 of its surface area is submerged in the aqueous solution 43. The aqueous solution 43 is a 1.0 mol/L aqueous potassium hydroxide solution.

Helium was fed into the oxidation vessel 41 through a tube 46, and carbon dioxide was fed into the reduction vessel 44 through a gas inlet 47, each at a flow rate of 5 ml/min and a pressure of 0.18 MPa. In this system, the reduction reaction of carbon dioxide can proceed at a three-phase interface consisting of [electrolyte membrane-copper (reduction electrode)-gas-phase carbon dioxide] in the electrolyte membrane supported reduction electrode 30. The area of the reduction electrode 2 to which carbon dioxide is directly supplied is approximately 6.25 ^cm2 .

After the oxidation tank 41 and reduction tank 44 were fully replaced with helium and carbon dioxide, respectively, the oxidation electrode 42 and reduction electrode 2 were connected with a conductor 45 via a power source 49, and a voltage of 2.0 V was applied to allow electrons to flow for 300 minutes. The current value between the oxidation electrode 42 and reduction electrode 2 when a voltage of 2.0 V was applied was measured with an electrochemical measurement device. In addition, gas and liquid in the oxidation tank 41 and reduction tank 44 were sampled at any time during the voltage application, and the reaction products were analyzed with a gas chromatograph, a liquid chromatograph, and a gas chromatograph mass spectrometer. As a result, it was confirmed that oxygen was generated in the oxidation tank 41, and hydrogen, carbon monoxide, formic acid, methane, methanol, ethanol, and ethylene were generated in the reduction tank 44.

[Example 7]
In Example 7, the electrolyte membrane 1 was immersed in the reducing agent solution 22 for 10 minutes in step 4 of the method for producing the electrolyte membrane supported reduction electrode 30. The other conditions were the same as those in Example 6.

[Example 8]
In Example 8, the electrolyte membrane 1 was immersed in the reducing agent solution 22 for 30 minutes in step 4 of the method for producing the electrolyte membrane supported reduction electrode 30. The other conditions were the same as those in Example 6.

[Example 9]
In Example 9, the electrolyte membrane 1 was immersed in the reducing agent solution 22 for 60 minutes in step 4 of the method for producing the electrolyte membrane supported reduction electrode 30. The other conditions were the same as those in Example 6.

[Example 10]
In Example 10, as shown in Fig. 5, the voltage application by the power source 49 was repeated for 60 minutes (ON) and for 30 minutes (OFF), and the measurement was stopped when the total voltage application time reached 300 minutes. The other conditions were the same as those in Example 8.

[Comparative Example 1]
[Method of manufacturing an electrolyte membrane supported reduction electrode]
Of steps 1 to 5 described in Example 1, step 4 (immersion of the electrolyte membrane 1 in the reducing agent solution 22) was not performed, and the metal salt solution 21 and the reducing agent solution 22 were simultaneously injected into the first tank 11 and the second tank 12, respectively. The rest of the manufacturing method was the same as in Example 1. When a cross section of the electrolyte membrane supported reduction electrode 30 after manufacture was observed, as shown in the enlarged view (b) of FIG. 3, the reduction electrode 2 was formed so as to be embedded into the electrolyte membrane 1 to a depth of 300 nm from the roughened surface of the electrolyte membrane 1.

[Configuration of the carbon dioxide gas phase reduction device]
This is an apparatus for reducing carbon dioxide gas phase by light irradiation, and is the same as that in the first embodiment.

[Electrochemical measurements and gas/liquid production measurements]
This is the same as in Example 1.

[Comparative Example 2]
[Method of manufacturing an electrolyte membrane supported reduction electrode]
The same as Comparative Example 1.

[Configuration of the carbon dioxide gas phase reduction device]
This is an apparatus for reducing carbon dioxide gas phase by light irradiation, and is the same as that in the first embodiment.

[Electrochemical measurements and gas/liquid production measurements]
5, the oxidation electrode 42 was irradiated with light for 60 minutes (ON) and then stopped for 30 minutes (OFF), and the measurement was stopped when the total light irradiation time of the oxidation electrode 42 reached 300 minutes. The other conditions were the same as those in Example 1.

[Comparative Example 3]
[Method of manufacturing an electrolyte membrane supported reduction electrode]
The same as Comparative Example 1.

[Configuration of the carbon dioxide gas phase reduction device]
This is a gas phase reduction device for carbon dioxide by applying a voltage, and is similar to that of Example 6.

[Electrochemical measurements and gas/liquid production measurements]
Same as Example 6.

[Comparative Example 4]
[Method of manufacturing an electrolyte membrane supported reduction electrode]
The same as Comparative Example 1.

[Configuration of the carbon dioxide gas phase reduction device]
This is a gas phase reduction device for carbon dioxide by applying a voltage, and is similar to that of Example 6.

[Electrochemical measurements and gas/liquid production measurements]
5, the voltage application by the power source 49 was repeated for 60 minutes (ON) and 30 minutes (OFF), and the measurement was stopped when the total voltage application time reached 300 minutes. The other conditions were the same as in Example 6.

[Experimental results of carbon dioxide reduction reaction]
Table 2 shows the faradaic efficiency of carbon dioxide reduction in Examples 1 to 4, 6 to 9 and Comparative Examples 1 and 3. Table 2 shows the faradaic efficiency (integrated value) of carbon dioxide reduction when light irradiation or voltage application was performed continuously for 300 minutes.

Table 3 shows the Faraday efficiency of carbon dioxide reduction in Examples 5 and 10 and Comparative Examples 2 and 4. Table 3 shows the Faraday efficiency (integrated value) of carbon dioxide reduction after 300 minutes when light irradiation or voltage application was repeatedly turned on for 60 minutes and off for 30 minutes.

The Faraday efficiency of carbon dioxide reduction is a value that indicates the ratio of the number of electrons used in the carbon dioxide reduction reaction to the number of electrons that flow between the electrodes due to light irradiation or voltage application, as shown in equation (3).

Faraday efficiency of carbon dioxide reduction=(number of electrons in the carbon dioxide reduction reaction)/(number of electrons between the oxidation electrode and the reduction electrode) (3)
The "number of electrons in the reduction reaction of carbon dioxide" in formula (3) can be calculated by converting the measured cumulative amount of the reduction product of carbon dioxide into the number of electrons required for the production reaction. It was calculated using formula (4) when the concentration of the reduction reaction product is A [ppm], the flow rate of the carrier gas is B [L/sec], the number of electrons required for the reduction reaction is Z [mol], the Faraday constant is F [C/mol], the molar mass of the gas is Vm [L/mol], and the light irradiation time or voltage application time is T [sec].

Electron value for the reduction reaction of carbon dioxide [C] = (A x B x Z x F x T x 10 ^-6 ) / Vm (4)
From Table 2, it was confirmed that the faradaic efficiency of carbon dioxide reduction was improved in Examples 1 to 4 and Examples 6 to 9 compared with Comparative Example 1 and Comparative Example 3. This is believed to be because in each of Examples 1 to 4 and 6 to 9, the reduction electrode was not embedded in the electrolyte membrane, and the reduction electrode was able to be formed directly above the electrolyte membrane.

In each of Examples 1 to 10, unlike Comparative Examples 1 to 4, the electrolyte membrane was immersed in a reducing agent solution in step 4 before the electroless plating process. As a result, the reducing agent solution diffused and penetrated up to the roughened surface of the electrolyte membrane before the electroless plating process, and a reduction electrode was formed directly above the roughened surface of the electrolyte membrane when the metal salt solution was injected into the first tank.

On the other hand, in Comparative Examples 1 to 4, the metal salt solution and the reducing agent solution were simultaneously injected into the first tank and the second tank, respectively, without performing step 4. In this case, only a small amount of the metal salt solution also penetrates into the electrolyte membrane. Therefore, before the reducing agent solution penetrates the roughened surface of the electrolyte membrane, the metal salt solution comes into contact with the inside of the electrolyte membrane, and the reduction electrode is formed so as to be embedded in the electrolyte membrane. In addition, in the case of Comparative Examples 1 to 4, carbon dioxide cannot be supplied to the part of the reduction electrode formed inside the electrolyte membrane, and since the electrolyte membrane contains water within the membrane, the moisture within the electrolyte membrane reacts with the dissolved oxygen within the electrolyte membrane to oxidize the reduction electrode. As a result, the reduction reaction of the oxidized electrode itself proceeds preferentially, and the carbon dioxide reduction reaction is suppressed.

From the above, by performing step 4 in Examples 1 to 10, the sinking of the reduction electrode into the electrolyte membrane was suppressed, and the efficiency of the carbon dioxide reduction reaction was improved.

In addition, when comparing the Faraday efficiency of carbon dioxide reduction in Examples 1 to 4 and Examples 6 to 9, Examples 3, 4 and Examples 8, 9, in which the electrolyte membrane is immersed in the reducing agent solution for 30 minutes or more in step 4, are higher than Examples 1, 2 and Examples 6, 7, in which the electrolyte membrane is immersed in the reducing agent solution for less than 30 minutes. When the depth of the reduction electrode embedded in the electrolyte membrane versus the immersion time is analyzed, it is 250 nm, 150 nm, 20 nm, and 20 nm for Examples 1 and 6, Examples 2 and 7, Examples 3 and 8, and Examples 4 and 9, respectively. From these results, it is possible to obtain a tendency for the depth of the reduction electrode to be deeper when the immersion time is less than 30 minutes, and when the immersion time is 30 minutes or more, the saturation value is reached at a depth of 20 nm, and it is assumed that the Faraday efficiency of carbon dioxide reduction is further improved. Therefore, it is desirable to immerse the electrolyte membrane in the reducing agent solution in advance for 30 minutes or more.

Also, from Table 3, in Examples 5 and 10, the faradaic efficiency of carbon dioxide reduction was improved compared to Comparative Examples 2 and 4, respectively. For Examples 5 and 10, the faradaic efficiency was approximately the same as that of Examples 3 and 8, which were operated continuously for 300 minutes, as shown in Table 2. The reason why the faradaic efficiency of Comparative Examples 2 and 4 is low is thought to be that when the gas phase carbon dioxide reduction device is repeatedly turned on and off, the reduction electrode is oxidized when it is turned off, and when it is turned on again, the reduction reaction of the oxidized electrode proceeds preferentially, resulting in a decrease in the efficiency of the carbon dioxide reduction reaction.

From the above, in Examples 5 and 10, by performing step 4, it was possible to suppress the sinking of the reduction electrode into the electrolyte membrane and to suppress the oxidation of the reduction electrode in the OFF state, thereby improving the efficiency of the carbon dioxide reduction reaction.

[Effects of the Invention]
According to the present invention, in the manufacture of an electrolyte membrane-supported reduction electrode for gas-phase reduction of carbon dioxide by electroless plating, the electrolyte membrane is immersed in a reducing agent solution before electroless plating, so that the formation of a reduction electrode inside the electrolyte membrane during the preparation of the reduction electrode and when the reduction reaction is stopped can be suppressed. As a result, oxidation of the reduction electrode can be suppressed, and the efficiency of the reduction reaction of carbon dioxide can be improved.

1: Electrolyte membrane 2: Reduction electrode 11: First tank 12: Second tank 21: Metal salt solution 22: Reducing agent solution 30: Electrolyte membrane supported reduction electrode 41: Oxidation tank 42: Oxidation electrode 43: Aqueous solution 44: Reduction tank 45: Conductor 46: Tube 47: Gas inlet 48: Light source 49: Power source

Claims (5)

A method for producing an electrolyte membrane supported reduction electrode disposed between an oxidation chamber including an oxidation electrode and a reduction chamber into which carbon dioxide is supplied, comprising the steps of:
A first step of immersing a surface opposite to one surface of an electrolyte membrane in a reducing agent solution;
a second step of immersing one surface of the electrolyte membrane in a metal salt solution containing metal ions after the first step ;
The present invention relates to a method for producing an electrolyte membrane-supported reduction electrode. Before carrying out the first step and the second step,
Roughening one surface of the electrolyte membrane;
immersing the electrolyte membrane in boiling nitric acid;
immersing the electrolyte membrane in boiling pure water;
2. The method for producing an electrolyte membrane supported reduction electrode according to claim 1, further comprising the steps of: The second step includes:
3. The method for producing an electrolyte membrane supported reduction electrode according to claim 1 or 2, wherein the method comprises the steps of: immersing an opposite surface of the electrolyte membrane in the reducing agent solution; and immersing one surface of the electrolyte membrane in the metal salt solution by electroless plating to deposit a metal for a reduction electrode on one surface of the electrolyte membrane. The reducing agent contained in the reducing agent solution is
4. The method for producing an electrolyte membrane supported reduction electrode according to claim 1, wherein the compound is a polar compound. The electrolyte membrane is
5. The method for producing an electrolyte membrane-supported reduction electrode according to claim 1, wherein the electrolyte membrane is a solid polymer membrane that conducts cations or anions.

Images