JP7594215B2 - Porous electrode-supported electrolyte membrane and method for producing the same - Google Patents

Description

The present invention relates to a porous electrode-supported electrolyte membrane and a method for producing a porous electrode-supported electrolyte membrane.

From the viewpoint of preventing global warming and ensuring a stable supply of energy, carbon dioxide reduction technology has been attracting attention. There are reduction devices using artificial photosynthesis technology and reduction devices using electrolytic reduction technology as devices for carbon dioxide reduction technology. Artificial photosynthesis technology is a technology that advances the oxidation reaction of water and the reduction reaction of carbon dioxide by irradiating an oxidation electrode made of a photocatalyst with light. Electrolytic reduction technology is a technology that advances the oxidation reaction of water and the reduction reaction of carbon dioxide by applying a voltage between an oxidation electrode and a reduction electrode made of metal. Artificial photosynthesis technology using sunlight and electrolytic reduction technology using electricity derived from renewable energy have attracted attention as technologies that can recycle carbon dioxide into carbon monoxide, formic acid, hydrocarbons such as ethylene, and alcohols such as methanol and ethanol, and have been actively researched in recent years.

In artificial photosynthesis technology and carbon dioxide electrolytic reduction technology, a reaction system has been used in which a reduction electrode is immersed in an aqueous solution, and carbon dioxide dissolved in the aqueous solution is supplied to the reduction electrode for reduction (see Non-Patent Documents 1 and 2). However, in this carbon dioxide reduction method, there is a limit to the concentration of carbon dioxide dissolved in the aqueous solution and the diffusion coefficient of carbon dioxide in the aqueous solution, which limits the amount of carbon dioxide supplied to the reduction electrode.

To address this issue, research is being conducted into supplying gaseous carbon dioxide to the reduction electrode in order to increase the amount of carbon dioxide supplied to the reduction electrode. According to Non-Patent Document 3, by using a reaction device having a structure capable of supplying gaseous carbon dioxide to the reduction electrode, the amount of carbon dioxide supplied to the reduction electrode is increased, and the reduction reaction of carbon dioxide is promoted.

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, 2018, p.436-439

The carbon dioxide reduction reaction shown in equations (1) to (4) proceeds in combination with the water oxidation reaction shown in equation (5).

CO ₂ + 2H ⁺ + 2e ^- → CO + H ₂ O (1)
CO ₂ + 2H ⁺ + 2e ^- → HCOOH (2)
CO ₂ + 6H ⁺ + 6e ^- → CH ₃ OH + H ₂ O (3)
CO ₂ + 8H ⁺ + 8e ^- → CH ₄ + 2H ₂ O (4)
2H ₂ O + 4h ⁺ → O ₂ + 4H ⁺ (5)

In a gas-phase carbon dioxide reduction device, the aqueous solution in the reduction tank is removed and gas-phase carbon dioxide is filled in, but since protons (H ⁺ ) cannot move in the gas phase simply by filling the tank with gas-phase carbon dioxide, it is necessary to join the electrolyte membrane and the reduction electrode. Furthermore, since gas-phase carbon dioxide cannot reach the interface between the reduction electrode and the electrolyte membrane simply by joining a plate-shaped reduction electrode to the electrolyte membrane, it is necessary to make the reduction electrode porous so that gas-phase carbon dioxide can reach the interface between the reduction electrode and the electrolyte membrane. With this porous reduction electrode, there was a problem that if the pore size was small, the diffusion resistance of carbon dioxide within the electrode was large, reducing the efficiency of the carbon dioxide reduction reaction.

When using an electrolyte membrane as a proton exchange membrane, it is generally immersed in boiling nitric acid or boiling pure water to improve the proton mobility of the electrolyte membrane. These treatments replace the proton exchange groups in the electrolyte membrane with H ⁺ , but these treatments cause the electrolyte membrane to absorb excess water and become swollen. This is because the electrolyte membrane has a reverse micelle structure of polymer, which causes it to swell and increase its water content.

When this swollen electrolyte membrane is bonded to a porous reduction electrode and used as a porous electrode-supported electrolyte membrane in a gas-phase reduction device, the aqueous solution in the oxidation tank gradually permeates into the reduction electrode as the carbon dioxide reduction reaction progresses. This causes the aqueous solution to cover the surface of the porous electrode, where gas-phase carbon dioxide should be supplied, resulting in a problem of the efficiency of the carbon dioxide reduction reaction deteriorating over time.

The present invention has been made in consideration of the above, and aims to improve the efficiency of gas-phase reduction of carbon dioxide.

A porous electrode-supported electrolyte membrane according to one embodiment of the present invention is a porous electrode-supported electrolyte membrane used in a gas-phase reduction device that reduces carbon dioxide, and includes an electrolyte membrane and a porous reduction electrode directly bonded onto the electrolyte membrane, the porous reduction electrode having an average pore diameter of 1 μm or more and 97 μm or less , and the electrolyte membrane and the porous reduction electrode are overlapped and thermocompression-bonded .

A method for producing a porous electrode-supported electrolyte membrane according to one embodiment of the present invention is a method for producing a porous electrode-supported electrolyte membrane used in a gas-phase reduction device that reduces carbon dioxide, the method comprising the steps of immersing an electrolyte membrane in boiling nitric acid and boiling pure water, and superimposing a porous reduction electrode on a surface of the electrolyte membrane and thermocompression bonding the porous reduction electrode, wherein the porous reduction electrode after thermocompression bonding has an average pore diameter of 1 μm or more and 97 μm or less .

According to the present invention, the efficiency of gas-phase reduction of carbon dioxide can be improved.

FIG. 1 is a cross-sectional view showing an example of the configuration of a porous electrode-supported electrolyte membrane according to this embodiment. FIG. 2 is a flow chart showing an example of a method for producing a porous electrode-supported electrolyte membrane. FIG. 3 is a diagram showing an example of a state in which thermocompression bonding is performed in the production of a porous electrode-supported electrolyte membrane. FIG. 4 is a diagram showing an example of the configuration of a gas-phase reduction device for carbon dioxide having a porous electrode-supported electrolyte membrane. FIG. 5 is a diagram showing an example of the configuration of another gas-phase reduction device for carbon dioxide having a porous electrode-supported electrolyte membrane.

Hereinafter, the 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 may be made without departing from the spirit of the present invention.

[Configuration of porous electrode-supported electrolyte membrane]
A porous electrode-supported electrolyte membrane 20 of this embodiment will be described with reference to the cross-sectional view of FIG.

The porous electrode-supported electrolyte membrane 20 in Figure 1 comprises an electrolyte membrane 6 and a porous reduction electrode 5 directly bonded onto the surface of the electrolyte membrane 6.

The porous reduction electrode 5 is directly laminated on the electrolyte membrane 6 and bonded by thermocompression. The porous reduction electrode 5 preferably has an average pore diameter of 1 μm or more after thermocompression. The porous reduction electrode 5 is, for example, a porous body of copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, or an alloy thereof, or a porous body of silver oxide, copper oxide, copper (II) oxide, nickel oxide, indium oxide, tin oxide, tungsten oxide, tungsten (VI) oxide, copper oxide, or the like, or a porous metal complex having a metal ion and an anionic ligand.

The electrolyte membrane 6 is, for example, Nafion (registered trademark), Forblue, or Aquivion, which is a perfluorocarbon material having a carbon-fluorine skeleton.

[Method of manufacturing a porous electrode-supported electrolyte membrane]
An example of a method for manufacturing the porous electrode-supported electrolyte membrane 20 of this embodiment will be described with reference to the flow chart of FIG.

In step S1, the electrolyte membrane 6 is immersed in boiling nitric acid and boiling pure water to reduce the resistance of proton conduction in the electrolyte membrane 6.

In step S2, the porous reduction electrode 5 is placed on top of the electrolyte membrane 6 and thermocompression-bonded using a thermocompression bonding device (e.g., a hot press machine). Specifically, as shown in Fig. 3, the porous reduction electrode 5 is placed on top of the electrolyte membrane 6 and disposed between two copper plates 40a, 40b, and the electrolyte membrane 6 and the porous reduction electrode 5 are thermocompression-bonded together with the copper plates 40a, 40b using a thermocompression bonding device. During thermocompression bonding, the heating temperature should be set to 100°C or higher and lower than 180°C.

After thermocompression bonding, the material is quickly cooled to obtain a porous electrode-supported electrolyte membrane 20 in which the electrolyte membrane 6 and the porous reduction electrode 5 are bonded.

[Gas-phase reduction device (artificial photosynthesis)]
Next, a gas-phase reduction device 100 for carbon dioxide having the porous electrode-supported electrolyte membrane 20 of this embodiment will be described with reference to Fig. 4. The gas-phase reduction device 100 shown in Fig. 4 is a reduction device that utilizes artificial photosynthesis technology to reduce carbon dioxide by light irradiation.

The gas-phase reduction device 100 includes an oxidation tank 1 and a reduction tank 4 formed by dividing the internal space in a housing in two with a porous electrode-supported electrolyte membrane 20. The porous electrode-supported electrolyte membrane 20 is arranged with the electrolyte membrane 6 facing the oxidation tank 1 and the reduction electrode 5 facing the reduction tank 4.

The oxidation tank 1 is filled with an aqueous solution 3. An oxidation electrode 2 made of a semiconductor or a metal complex is inserted into the aqueous solution 3.

The oxidation electrode 2 is a compound that exhibits photoactivity and redox activity, such as a nitride semiconductor, titanium oxide, amorphous silicon, a ruthenium complex, or a rhenium complex. The oxidation electrode 2 is electrically connected to the porous reduction electrode 5 by a conductor 7.

The aqueous solution 3 is, for example, an aqueous solution of potassium bicarbonate, sodium bicarbonate, potassium chloride, sodium chloride, sodium hydroxide, potassium hydroxide, rubidium hydroxide, or cesium hydroxide. During the reduction reaction, helium gas is supplied to the aqueous solution 3 from the tube 8.

Carbon dioxide is supplied through the gas input port 10 to the reduction tank 4, which is filled with carbon dioxide or a gas containing carbon dioxide.

The light source 9 is arranged so that light is irradiated onto the oxidation electrode 2. The light source 9 is, for example, a xenon lamp, a pseudo-sun light source, a halogen lamp, a mercury lamp, or sunlight. The light source 9 may be a combination of these.

[Example of Porous Electrode-Supported Electrolyte Membrane]
As the porous electrode-supported electrolyte membrane 20 to be disposed in the above-mentioned gas-phase reduction device 100, Examples 1-6 were prepared by changing the average pore diameter or the heating temperature during the thermocompression bonding process, and a gas-phase reduction test described below was carried out. The porous electrode-supported electrolyte membrane of Example 1-6 will be described below.

Example 1
In Example 1, the material of the porous reduction electrode 5 was a copper porous body having a thickness of 0.2 mm and a porosity of 65%, and the material of the electrolyte membrane 6 was Nafion, which is a proton exchange membrane.

In step S1, the electrolyte membrane 6 was immersed in boiling nitric acid and boiling pure water to reduce the resistance of proton conduction. It was confirmed that this treatment reduced the resistance of proton conduction of the electrolyte membrane 6 to 3.0 to 3.5 Ω.

In step S2, the sample in which the porous reduction electrode 5 was layered on the electrolyte membrane 6 was sandwiched between two copper plates and a hot press machine, and pressure was applied vertically to the surface of the porous reduction electrode 5 at a heating temperature of 150° C., and the sample was left for 3 minutes. The sample was then quickly cooled and removed, yielding a porous electrode-supported electrolyte membrane 20 in which the electrolyte membrane 6 and the porous reduction electrode 5 were bonded.

After thermocompression bonding, the thickness of the porous reduction electrode 5 was 0.14 mm, the porosity was 50%, and the average pore diameter was 1.3 μm.

Example 2
In Example 2, a porous electrode-supported electrolyte membrane 20 was produced using a copper porous body having a thickness of 0.2 mm and a porosity of 79% as the material for the porous reduction electrode 5. The thickness of the porous reduction electrode 5 after thermocompression bonding was 0.14 mm, the porosity was 70%, and the average pore diameter was 15 μm. All other conditions were the same as those in Example 1.

Example 3
In Example 3, a porous electrode supported electrolyte membrane 20 was produced using a copper porous body having a thickness of 0.2 mm and a porosity of 93% as the material for the porous reduction electrode 5. The thickness of the porous reduction electrode 5 after thermocompression bonding was 0.14 mm, the porosity was 90%, and the average pore diameter was 97 μm. All other conditions were the same as those in Example 1.

Example 4
In Example 4, similarly to Example 3, a porous electrode-supported electrolyte membrane 20 was produced using a copper porous body having a thickness of 0.2 mm and a porosity of 93% as the material for the porous reduction electrode 5. The heating temperature when applying pressure with the hot press was set to 100° C. All conditions other than the heating temperature were the same as those in Example 3.

Example 5
In Example 5, similarly to Example 3, a porous electrode-supported electrolyte membrane 20 was produced using a copper porous body having a thickness of 0.2 mm and a porosity of 93% as the material for the porous reduction electrode 5. The heating temperature when applying pressure with the hot press was set to 120° C. All conditions other than the heating temperature were the same as those in Example 3.

Example 6
In Example 6, similarly to Example 3, a porous electrode-supported electrolyte membrane 20 was produced using a copper porous body having a thickness of 0.2 mm and a porosity of 93% as the material for the porous reduction electrode 5. The heating temperature when applying pressure with the hot press was set to 180° C. All conditions other than the heating temperature were the same as those in Example 3.

[Electrochemical measurements and gas/liquid production measurements]
Each of the porous electrode-supported electrolyte membranes 20 of Examples 1 to 6 was attached to the gas-phase reduction device 100 of FIG. 4, and the following reduction reaction test was carried out.

The oxidation tank 1 was filled with aqueous solution 3. Aqueous solution 3 was a 1.0 mol/L aqueous potassium hydroxide solution.

The oxidation electrode 2 was placed in the oxidation tank 1 so that it was submerged in the aqueous solution 3. A semiconductor photoelectrode prepared as follows was used for the oxidation electrode 2. A thin film of n-type semiconductor GaN and AlGaN were epitaxially grown in that order on a sapphire substrate, and Ni was vacuum-deposited on the AlGaN and heat-treated to form a NiO promoter thin film to produce a semiconductor photoelectrode.

A 300 W high-pressure xenon lamp (cutting off wavelengths of 450 nm or more, illuminance 6.6 mW/ ^cm2 ) was used as the light source 9. The light source 9 was fixed so that the surface of the oxidation electrode 2 on which the oxidation promoter was formed was the irradiated surface. The light irradiation area of the oxidation electrode 2 was set to 2.5 ^cm2 .

Helium (He) was fed from a tube 8 to the oxidation tank 1, and carbon dioxide (CO ₂ ) was fed from a gas inlet 10 to the reduction tank 4, 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 the three-phase interface consisting of [electrolyte membrane-copper-gas-phase carbon dioxide] in the porous electrode-supported electrolyte membrane 20. The apparent area of the porous reduction electrode 5 to which carbon dioxide is directly supplied is approximately 6.25 cm ² .

After the oxidation tank 1 and reduction tank 4 were fully replaced with helium and carbon dioxide, light was uniformly irradiated onto the oxidation electrode 2 using a light source 9. Electrons flow between the oxidation electrode 2 and the porous reduction electrode 5 due to the light irradiation.

The current value between the oxidation electrode 2 and the porous reduction electrode 5 during light irradiation was measured using an electrochemical measuring device (Solartron, 1287-type potentiogalvanostat). In addition, gas and liquid in the oxidation tank 1 and reduction tank 4 were sampled at any time during light irradiation, and the reaction products were analyzed using a gas chromatograph, liquid chromatograph, and gas chromatograph mass spectrometer. As a result, it was confirmed that oxygen was produced in the oxidation tank 1, and hydrogen, carbon monoxide, formic acid, methane, methanol, ethanol, and ethylene were produced in the reduction tank 4.

The test results for Examples 1-6 will be described below along with the test results for Examples 7-14 and Comparative Examples 1-4.

[Gas phase reduction device (electrolytic reduction)]
Next, a gas-phase reduction device 200 for carbon dioxide having the porous electrode-supported electrolyte membrane 20 of this embodiment will be described with reference to Fig. 5. The gas-phase reduction device 200 shown in Fig. 5 is a reduction device that utilizes an electrolytic reduction technique in which a current is passed between an oxidation electrode and a reduction electrode to reduce carbon dioxide.

The gas-phase reduction device 200 includes an oxidation tank 1 and a reduction tank 4 formed by dividing the internal space in the housing in two with a porous electrode-supported electrolyte membrane 20. The porous electrode-supported electrolyte membrane 20 is arranged with the electrolyte membrane 6 side facing the oxidation tank 1 and the reduction electrode 5 side facing the reduction tank 4.

The oxidation tank 1 is filled with an aqueous solution 3. An oxidation electrode 2 made of a semiconductor or a metal complex is inserted into the aqueous solution 3.

The oxidation electrode 2 is, for example, platinum, gold, silver, copper, indium, or nickel.

The aqueous solution 3 is similar to that of the gas phase reduction device 100 in Figure 4.

Carbon dioxide is supplied through the gas input port 10 to the reduction tank 4, which is filled with carbon dioxide or a gas containing carbon dioxide.

A power source 11 is electrically connected to the oxidation electrode 2 and the porous reduction electrode 5 by a conductor 7.

[Example of Porous Electrode-Supported Electrolyte Membrane]
As the porous electrode-supported electrolyte membrane 20 to be placed in the gas-phase reduction device 200, Examples 7-12 were prepared by changing the average pore size or the temperature during the thermocompression treatment, and a gas-phase reduction test described below was performed. The porous electrode-supported electrolyte membrane of Examples 7-12 will be described below. The porous electrode-supported electrolyte membrane 20 of Examples 7-12 was prepared in the same manner as the porous electrode-supported electrolyte membrane 20 of Examples 1-6.

Example 7
The porous electrode-supported electrolyte membrane 20 of Example 7 was produced in the same manner as in Example 1. The heating temperature during thermocompression bonding was 150° C., and the porous reduction electrode 5 after thermocompression bonding had a thickness of 0.14 mm, a porosity of 50%, and an average pore diameter of 1.3 μm.

Example 8
In Example 8, a porous electrode-supported electrolyte membrane 20 was produced using a copper porous body having a thickness of 0.2 mm and a porosity of 79% as the material for the porous reduction electrode 5. The thickness of the porous reduction electrode 5 after thermocompression bonding was 0.14 mm, the porosity was 70%, and the average pore diameter was 15 μm. All other conditions were the same as those in Example 7.

<Example 9>
In Example 9, a porous electrode-supported electrolyte membrane 20 was produced using a copper porous body having a thickness of 0.2 mm and a porosity of 93% as the material for the porous reduction electrode 5. The thickness of the porous reduction electrode 5 after thermocompression bonding was 0.14 mm, the porosity was 90%, and the average pore diameter was 97 μm. All other conditions were the same as those in Example 7.

Example 10
In Example 10, similarly to Example 9, a porous electrode-supported electrolyte membrane 20 was produced using a copper porous body having a thickness of 0.2 mm and a porosity of 93% as the material for the porous reduction electrode 5. The heating temperature when applying pressure with the hot press was set to 100° C. All conditions other than the heating temperature were the same as those in Example 9.

Example 11
In Example 11, similarly to Example 9, a porous electrode-supported electrolyte membrane 20 was produced using a copper porous body having a thickness of 0.2 mm and a porosity of 93% as the material for the porous reduction electrode 5. The heating temperature when applying pressure with the hot press was set to 120° C. All conditions other than the heating temperature were the same as those in Example 9.

Example 12
In Example 12, similarly to Example 9, a porous electrode-supported electrolyte membrane 20 was produced using a copper porous body having a thickness of 0.2 mm and a porosity of 93% as the material for the porous reduction electrode 5. The heating temperature when applying pressure with the hot press was set to 180° C. All conditions other than the heating temperature were the same as those in Example 9.

[Electrochemical measurements and gas/liquid production measurements]
Each of the porous electrode-supported electrolyte membranes 20 of Examples 7 to 12 was attached to the gas-phase reduction device 200 of FIG. 5, and the following reduction reaction test was carried out.

The oxidation tank 1 was filled with aqueous solution 3. Aqueous solution 3 was a 1.0 mol/L aqueous potassium hydroxide solution.

The oxidation electrode 2 was placed in the oxidation tank 1 so that its surface area of about 0.55 ^cm2 was submerged in the aqueous solution 3. The oxidation electrode 2 was made of platinum (manufactured by Nilaco Corporation).

Helium (He) was fed from a tube 8 to the oxidation tank 1, and carbon dioxide (CO ₂ ) was fed from a gas inlet 10 to the reduction tank 4, 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 the three-phase interface consisting of [electrolyte membrane-copper-gas-phase carbon dioxide] in the porous electrode-supported electrolyte membrane 20. The apparent area of the porous reduction electrode 5 to which carbon dioxide is directly supplied is approximately 6.25 cm ² .

After the oxidation cell 1 and reduction cell 4 were thoroughly replaced with helium and carbon dioxide, a voltage of 2.0 V was applied from the power source 11 to allow electrons to flow between the oxidation electrode 2 and the porous reduction electrode 5.

The current value between the oxidation electrode 2 and the porous reduction electrode 5 when voltage was applied was measured using an electrochemical measurement device.

In addition, gas and liquid samples were taken from the oxidation tank 1 and reduction tank 4 at any time during voltage application, and the reaction products were analyzed using a gas chromatograph, liquid chromatograph, and gas chromatograph mass spectrometer. As a result, it was confirmed that oxygen was produced in the oxidation tank 1, and hydrogen, carbon monoxide, formic acid, methane, methanol, ethanol, and ethylene were produced in the reduction tank 4.

[Comparison example]
Comparative Examples 1 to 4 were prepared which were different from the Examples in terms of the average pore diameter or the temperature during the thermocompression bonding treatment. Comparative Examples 1 and 2 were arranged as the porous electrode-supported electrolyte membrane 20 in the gas-phase reduction device 100 of FIG. 4, and Comparative Examples 3 and 4 were arranged as the porous electrode-supported electrolyte membrane 20 in the gas-phase reduction device 200 of FIG. 5, and tests similar to those of Examples 1 to 6 and Examples 7 to 12 were performed.

<Comparative Example 1>
In Comparative Example 1, a porous electrode-supported electrolyte membrane was produced in the same manner as in Example 1, using a copper porous body having a thickness of 0.2 mm and a porosity of 51%. The thickness of the porous reduction electrode after thermocompression bonding was 0.14 mm, the porosity was 30%, and the average pore diameter was 0.11 μm. All other conditions were the same as in Example 1.

<Comparative Example 2>
In Comparative Example 2, the heating temperature during thermocompression bonding was set to 80° C. All other conditions were the same as those in Example 3.

<Comparative Example 3>
In Comparative Example 3, a porous electrode-supported electrolyte membrane was produced in the same manner as in Example 7, using a copper porous body having a thickness of 0.2 mm and a porosity of 51%. The thickness of the porous reduction electrode after thermocompression bonding was 0.14 mm, the porosity was 30%, and the average pore diameter was 0.11 μm. All other conditions were the same as in Example 7.

<Comparative Example 4>
In Comparative Example 4, the heating temperature during thermocompression bonding was 80° C. All other conditions were the same as those in Example 9.

[Evaluation of Examples and Comparative Examples]
Next, the test results of Examples 1-12 and Comparative Examples 1-4 will be described. Table 1 shows the Faraday efficiency of the carbon dioxide reduction reaction after 1 hour and the Faraday efficiency maintenance rate of the carbon dioxide reduction reaction after 20 hours for Examples 1-12 and Comparative Examples 1-4.

Faraday efficiency, as shown in equation (6), indicates the ratio of the current value used in each reduction reaction to the current value flowing between the electrodes when light is irradiated or voltage is applied.

Faraday efficiency of each reduction reaction [%] = (charge consumed in each reduction reaction) / (charge flowing between the oxidation electrode and reduction electrode) x 100 (6)

Here, the "electric charge consumed in each reduction reaction" in formula (6) can be calculated by converting the measured amount of reaction product of each reduction reaction into the electric charge required for that reduction reaction. When the amount of reaction product of each reduction reaction is A [mol], the number of electrons required for the reduction reaction is Z, and the Faraday constant is F [C/mol], it was calculated using formula (7).

Charge consumed in each reduction reaction [C] = A x Z x F (7)

In addition, the Faraday efficiency maintenance rate of each reduction reaction after 20 hours was defined and calculated according to the following formula (8).

Faraday efficiency maintenance rate [%] of each reduction reaction after 20 hours = (Faraday efficiency of each reduction reaction after 20 hours) / (Faraday efficiency of each reduction reaction after 1 hour) × 100 (8)

When comparing the Faraday efficiency of the carbon dioxide reduction reaction after one hour between Examples 1-5, Comparative Example 1, Examples 7-11 and Comparative Example 3, it was found that Examples 1-5 and 7-11 had a higher Faraday efficiency of the carbon dioxide reduction reaction after one hour than Comparative Examples 1 and 3.

Table 1 shows the evaluation results of the diffusion coefficient of carbon dioxide in a porous electrode, which depends on the pore size. According to this, in Examples 1-5 and 7-11, in which the pore size exceeds 1 μm, the saturation value of 6.0×10 ⁻⁶ m ² s ⁻¹ (self-diffusion coefficient) is reached, which is approximately 1.5 times that of Comparative Examples 1 and 3.

Based on these findings, by using a porous electrode-supported electrolyte membrane 20 composed of porous electrodes with an average pore diameter of 1 μm or more, at which the diffusion coefficient of carbon dioxide reaches a saturated value, the amount of carbon dioxide supplied to the porous reduction electrode 5 can be increased, thereby improving the efficiency of the carbon dioxide reduction reaction.

When comparing Examples 1-5, Comparative Example 2, Examples 7-11, and Comparative Example 4 with respect to the Faraday efficiency maintenance rate of the carbon dioxide reduction reaction after 20 hours, Examples 1-5 and 7-11 had a higher Faraday efficiency maintenance rate of the carbon dioxide reduction reaction after 20 hours than Comparative Examples 2 and 4.

In Examples 1-5 and 7-11, there was no visible liquid on the electrode surface after 20 hours. On the other hand, in Comparative Examples 2 and 4, several hundred μL of liquid was attached to the electrode surface after 20 hours, and it was found that the Faraday efficiency maintenance rate was reduced because gaseous carbon dioxide could no longer be directly supplied to the electrode surface. It was confirmed that the liquid attached to the electrode surface was mainly an aqueous solution that permeated from the oxidation tank 1 through the electrolyte membrane 6 regardless of whether the carbon dioxide reduction reaction was progressing. This is thought to be due to the electrolyte membrane 6 becoming swollen with excess moisture, and the aqueous solution 3 in the oxidation tank 1 permeating into it. On the other hand, in Examples 1-5 and 7-11, the moisture contained in the electrolyte membrane was vaporized by performing the thermocompression bonding at a temperature condition of 100°C or higher. It is thought that this suppressed the permeation of the aqueous solution through the electrolyte membrane, improving the maintenance rate of the carbon dioxide reduction reaction.

Furthermore, Table 1 shows the measured resistance of the proton conduction of the electrolyte membrane 6. In Examples 1-5 and 7-11, the resistance was low at 3.0 to 3.5 Ω, and it was confirmed that the effect of reducing the resistance of the proton conduction was not lost even after thermocompression bonding. On the other hand, in Examples 6 and 12, the resistance of the ion conduction of the electrolyte membrane 6 increased to 360 Ω. As a result, the current value between the electrodes was extremely low and the amount of the reaction product was below the detection limit (3%) of the evaluation system, so no record was made. This is thought to be because the proton exchange group of the electrolyte membrane was decomposed by carrying out the thermocompression treatment at a high temperature condition of 180°C.

As described above, according to this embodiment, the porous electrode-supported electrolyte membrane 20 of this embodiment has an electrolyte membrane 6 and a porous reduction electrode 5 directly bonded onto the electrolyte membrane 6, and the average pore diameter of the porous reduction electrode 5 is set to 1 μm or more. This reduces the diffusion resistance of carbon dioxide within the electrode and improves the efficiency of gas-phase reduction of carbon dioxide. In addition, in the process of bonding the electrolyte membrane 6 and the porous reduction electrode 5, pressure is applied while heating to suppress swelling of the electrolyte membrane 6, thereby improving the life of the porous electrode-supported electrolyte membrane 20.

Porous electrode supported electrolyte membrane 20
Porous reduction electrode 5
Electrolyte membrane 6

Claims (3)

A porous electrode-supported electrolyte membrane for use in a gas-phase reduction device for reducing carbon dioxide, comprising:
An electrolyte membrane;
A porous reduction electrode is directly bonded onto the electrolyte membrane,
The average pore diameter of the porous reduction electrode is 1 μm or more and 97 μm or less ,
The electrolyte membrane and the porous reduction electrode were laminated and heat-pressed.
Porous electrode-supported electrolyte membrane. A method for producing a porous electrode-supported electrolyte membrane for use in a gas-phase reduction device for reducing carbon dioxide, comprising the steps of:
Immersing the electrolyte membrane in boiling nitric acid and boiling pure water;
A step of superposing a porous reduction electrode on a surface of the electrolyte membrane and thermocompressing the electrode,
The method for producing a porous electrode-supported electrolyte membrane, wherein the porous reduction electrode after thermocompression bonding has an average pore size of 1 μm or more and 97 μm or less . A method for producing the porous electrode-supported electrolyte membrane according to claim 2 , comprising the steps of:
The method for producing a porous electrode-supported electrolyte membrane, wherein the heating temperature for the thermocompression bonding is 100° C. or higher and lower than 180° C.

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