Porous Electrode-Supported Electrolyte Membrane and Method for Manufacturing Same

A porous electrode-supported electrolyte membrane according to the present embodiment includes an electrolyte membrane and a porous reduction electrode directly bonded onto the electrolyte membrane, in which the porous reduction electrode has an average pore diameter of 1 μm or more. In a step of bonding the electrolyte membrane and the porous reduction electrode, pressure is applied while heat is applied so that the electrolyte membrane is prevented from swelling.

Skip to: Description  ·  Claims  · Patent History  ·  Patent History
Description
TECHNICAL FIELD

The present invention relates to a porous electrode-supported electrolyte membrane and a method of manufacturing the porous electrode-supported electrolyte membrane.

BACKGROUND ART

Technologies for reducing carbon dioxide have attracted attention from a viewpoint of prevention of global warming and stable supply of energy. Devices related to technologies for reducing carbon dioxide include reduction devices using an artificial photosynthesis technology and reduction devices using an electrolytic reduction technology. The artificial photosynthesis technology is a technology for allowing an oxidation reaction of water and a carbon dioxide reduction reaction to proceed by irradiating an oxidation electrode constituted by a photocatalyst with light. The electrolytic reduction technology is a technology for allowing an oxidation reaction of water and a carbon dioxide reduction reaction to proceed by applying a voltage between a reduction electrode and an oxidation electrode constituted by metal. The artificial photosynthesis technology using sunlight and the electrolytic reduction technology using electric power from renewable energy have attracted attention as technologies that allow for recycling of carbon dioxide into hydrocarbons such as carbon monoxide, formic acid, and ethylene, and alcohols such as methanol and ethanol, and have been actively studied in recent years.

In the artificial photosynthesis technology and the 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 and reduced (see Non Patent Literatures 1 and 2). However, in this method for reducing carbon dioxide, the concentration of carbon dioxide dissolved in the aqueous solution and a diffusion coefficient of carbon dioxide in the aqueous solution are limited, and the amount of carbon dioxide supplied to the reduction electrode is limited.

To address this problem, research on supplying carbon dioxide in gas phase to a reduction electrode has been advanced for the purpose of increasing the amount of carbon dioxide supplied to the reduction electrode. According to Non Patent Literature 3, a reactor having a structure that allows carbon dioxide in gas phase to be supplied to a reduction electrode is used to increase the amount of carbon dioxide supplied to the reduction electrode and promote the carbon dioxide reduction reaction.

CITATION LIST Non Patent Literature

    • Non Patent Literature 1: 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 Non Patent Literature 2: Yoshio Hori, and 2 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
    • Non Patent Literature 3: Qingxin Jia, and 2 others “Direct Gas-phase CO2 Reduction for Solar Methane Generation Using a Gas Diffusion Electrode with a BiVO4:Mo and a Cu—In—Se Photoanode”, Chemistry Letter, 47, 2018, p. 436-439

SUMMARY OF INVENTION Technical Problem

The carbon dioxide reduction reactions represented by Formula (1) to Formula (4) proceed in combination with the oxidation reaction of water represented by Formula (5).


CO2+2H++2e−→CO+H2O  (1)


CO2+2H++2e−→HCOOH  (2)


CO2+6H++6e−→CH3OH+H2O  (3)


CO2+8H++8e−→CH4+2H2O  (4)


2H2O+4h+→O2+4H+  (5)

In a gas phase reduction device for carbon dioxide, an aqueous solution in a reduction tank is removed and the reduction tank is filled with carbon dioxide in gas phase, but in a case of just filling the reduction tank with carbon dioxide in gas phase, proton (H+) cannot move in the gas phase, and it is therefore necessary to bond an electrolyte membrane and a reduction electrode. Furthermore, in a case of just bonding a plate-like reduction electrode to an electrolyte membrane, carbon dioxide in gas phase cannot reach an interface between the reduction electrode and the electrolyte membrane, and it is therefore necessary to use a porous reduction electrode as the reduction electrode so that carbon dioxide in gas phase can reach the interface between the reduction electrode and the electrolyte membrane. In a case where this porous reduction electrode has a smaller pore diameter, a carbon dioxide diffusion resistance in the electrode becomes larger, and this has posed a problem in that the efficiency of the carbon dioxide reduction reaction is reduced.

In a case where an electrolyte membrane is used as a proton exchange membrane, immersion treatment in boiling nitric acid and immersion treatment in boiling pure water are generally performed for the purpose of improving proton mobility of the electrolyte membrane. In these treatments, a proton exchange group in the electrolyte membrane is replaced with H+, and the treatments cause the electrolyte membrane to contain excessive moisture and become swollen. This is because the electrolyte membrane swells and has a higher water content because the electrolyte membrane has a polymer reverse micelle structure.

In a case where the swollen electrolyte membrane is bonded to the porous reduction electrode and used as a porous electrode-supported electrolyte membrane of a gas phase reduction device, an aqueous solution in an oxidation tank gradually permeates the reduction electrode side while the carbon dioxide reduction reaction proceeds. As a result, the aqueous solution covers the surface of the porous electrode to which carbon dioxide in gas phase is supposed to be supplied, and this has posed a problem in that the efficiency of the carbon dioxide reduction reaction deteriorates over time.

The present invention has been made in view of the above, and an object thereof is to improve gas phase reduction efficiency of carbon dioxide.

Solution to Problem

An aspect of the present invention provides a porous electrode-supported electrolyte membrane used in a gas phase reduction device for reducing carbon dioxide, the porous electrode-supported electrolyte membrane including: an electrolyte membrane; and a porous reduction electrode directly bonded onto the electrolyte membrane, in which the porous reduction electrode has an average pore diameter of 1 μm or more.

An aspect of the present invention provides a method of manufacturing a porous electrode-supported electrolyte membrane used in a gas phase reduction device for reducing carbon dioxide, the method including: a step of immersing an electrolyte membrane in boiling nitric acid and boiling pure water; and a step of performing thermocompression bonding with a porous reduction electrode laid on a surface of the electrolyte membrane, in which the porous reduction electrode after the thermocompression bonding has an average pore diameter of 1 μm or more.

Advantageous Effects of Invention

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

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating an example of a configuration of a porous electrode-supported electrolyte membrane according to the present embodiment.

FIG. 2 is a flowchart illustrating an example of a method of manufacturing the porous electrode-supported electrolyte membrane.

FIG. 3 is a diagram illustrating an example of a state in which thermocompression bonding is performed when the porous electrode-supported electrolyte membrane is manufactured.

FIG. 4 is a diagram illustrating an example of a configuration of a gas phase reduction device for carbon dioxide including the porous electrode-supported electrolyte membrane.

FIG. 5 is a diagram illustrating an example of a configuration of another gas phase reduction device for carbon dioxide including the porous electrode-supported electrolyte membrane.

 DESCRIPTION OF EMBODIMENTS

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 may be made without departing from the gist of the present invention.

[Configuration of Porous Electrode-Supported Electrolyte Membrane]

A porous electrode-supported electrolyte membrane 20 according to the present embodiment will be described with reference to a cross-sectional view in FIG. 1.

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

The porous reduction electrode 5 is directly laid on the electrolyte membrane 6 and bonded by thermocompression bonding. The porous reduction electrode 5 preferably has an average pore diameter of 1 μm or more after the thermocompression bonding. 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, a porous body of silver oxide, copper oxide, copper (II) oxide, nickel oxide, indium oxide, tin oxide, tungsten oxide, tungsten oxide (VI), or copper oxide, 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 Porous Electrode-Supported Electrolyte Membrane]

An example of a method of manufacturing the porous electrode-supported electrolyte membrane 20 according to the present embodiment will be described with reference to a flowchart in FIG. 2.

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

In step S2, the porous reduction electrode 5 is laid on the electrolyte membrane 6 and thermocompression-bonded by a thermocompression bonding device (for example, a hot press machine). Specifically, as illustrated in FIG. 3, the porous reduction electrode 5 is laid on the electrolyte membrane 6 and disposed between two copper plates 40a and 40b, and the electrolyte membrane 6 and the porous reduction electrode 5 are thermocompression-bonded together with the copper plates 40a and 40b by the thermocompression bonding device. The thermocompression bonding is performed preferably at a heating temperature of 100° C. or higher and lower than 180° C.

After the thermocompression bonding and rapid cooling, the porous electrode-supported electrolyte membrane 20 is obtained 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 including the porous electrode-supported electrolyte membrane 20 according to the present embodiment will be described with reference to FIG. 4. The gas phase reduction device 100 illustrated in FIG. 4 is a reduction device using an artificial photosynthesis technology for reducing carbon dioxide by light irradiation.

The gas phase reduction device 100 includes an oxidation tank 1 and a reduction tank 4 formed by separating the space inside a housing into two with the porous electrode-supported electrolyte membrane 20. The porous electrode-supported electrolyte membrane 20 is disposed 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 constituted by a semiconductor or a metal complex is inserted into the aqueous solution 3.

The oxidation electrode 2 is, for example, a compound exhibiting 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 conductive wire 7.

The aqueous solution 3 is, for example, a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, a sodium chloride aqueous solution, a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution, a rubidium hydroxide aqueous solution, or a cesium hydroxide aqueous solution. During the reduction reaction, helium gas is supplied to the aqueous solution 3 through a tube 8.

The reduction tank 4 is supplied with carbon dioxide through a gas input port 10, and is filled with carbon dioxide or a gas containing carbon dioxide.

A light source 9 is disposed so that the oxidation electrode 2 is irradiated with light. Examples of the light source 9 include a xenon lamp, a pseudo sunlight source, a halogen lamp, a mercury lamp, and sunlight. The light source 9 may be constituted by a combination thereof.

[Examples of Porous Electrode-Supported Electrolyte Membrane]

As the porous electrode-supported electrolyte membrane 20 disposed in the gas phase reduction device 100 described above, Examples 1 to 6 that are different in average pore diameter or heating temperature during a thermocompression bonding treatment were prepared, and a gas phase reduction test described later was performed. Hereinafter, the porous electrode-supported electrolyte membranes of Examples 1 to 6 will be described.

Example 1

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

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

In step S2, a sample obtained by laying the porous reduction electrode 5 on the electrolyte membrane 6 was sandwiched between two copper plates in a hot press machine, a pressure was applied in a direction perpendicular to a surface of the porous reduction electrode 5 under the condition that the heating temperature was set to 150° C., and the sample was left for three minutes. Thereafter, the sample was quickly cooled and taken out, and thus the porous electrode-supported electrolyte membrane 20 in which the electrolyte membrane 6 and the porous reduction electrode 5 were bonded was obtained.

The porous reduction electrode 5 after the thermocompression bonding had a thickness of 0.14 mm, a porosity of 50%, and an average pore diameter of 1.3 μm.

Example 2

In Example 2, the porous electrode-supported electrolyte membrane 20 was prepared by using, as a material of the porous reduction electrode 5, a copper porous body having a thickness of 0.2 mm and a porosity of 79%. The porous reduction electrode 5 after the thermocompression bonding had a thickness of 0.14 mm, a porosity of 70%, and an average pore diameter of 15 μm. The other conditions are all similar to those in Example 1.

Example 3

In Example 3, the porous electrode-supported electrolyte membrane 20 was prepared by using, as a material of the porous reduction electrode 5, a copper porous body having a thickness of 0.2 mm and a porosity of 93%. The porous reduction electrode 5 after the thermocompression bonding had a thickness of 0.14 mm, a porosity of 90%, and an average pore diameter of 97 μm. The other conditions are all similar to those in Example 1.

Example 4

In Example 4, as in Example 3, the porous electrode-supported electrolyte membrane 20 was prepared by using, as a material of the porous reduction electrode 5, a copper porous body having a thickness of 0.2 mm and a porosity of 93%. Pressure was applied with the hot press machine at a heating temperature of 100° C. The conditions other than the heating temperature are all similar to those in Example 3.

Example 5

In Example 5, as in Example 3, the porous electrode-supported electrolyte membrane 20 was prepared by using, as a material of the porous reduction electrode 5, a copper porous body having a thickness of 0.2 mm and a porosity of 93%. Pressure was applied with the hot press machine at a heating temperature of 120° C. The conditions other than the heating temperature are all similar to those in Example 3.

Example 6

In Example 6, as in Example 3, the porous electrode-supported electrolyte membrane 20 was prepared by using, as a material of the porous reduction electrode 5, a copper porous body having a thickness of 0.2 mm and a porosity of 93%. Pressure was applied with the hot press machine at a heating temperature of 180° C. The conditions other than the heating temperature are all similar to those in Example 3.

[Electrochemical Measurement and Measurement of Gas/Liquid Generation Amount]

Each of the porous electrode-supported electrolyte membranes 20 of Examples 1 to 6 was attached to the gas phase reduction device 100 in FIG. 4, and the following reduction reaction test was performed.

The oxidation tank 1 was filled with the aqueous solution 3. As the aqueous solution 3, a 1.0 mol/L potassium hydroxide aqueous solution was used.

The oxidation electrode 2 was disposed in the oxidation tank 1 so as to be immersed in the aqueous solution 3. As the oxidation electrode 2, a semiconductor photoelectrode prepared as follows was used. A semiconductor photoelectrode was prepared by performing epitaxial growth of GaN as an n-type semiconductor and epitaxial growth of AlGaN in this order on a sapphire substrate, vacuum-depositing Ni on the AlGaN, and heat-treating the resulting product to form a promotor thin film of NiO.

As the light source 9, a 300-W high pressure xenon lamp (cut-off wavelength: 450 nm or more, illuminance: 6.6 mW/cm2) was used. The light source 9 was fixed such that the surface of the oxidation electrode 2 on which an oxidation co-catalyst was formed was irradiated with light. A light irradiation area of the oxidation electrode 2 was set to 2.5 cm2.

Helium (He) was caused to flow into the oxidation tank 1 from the tube 8, and carbon dioxide (CO2) was caused to flow into the reduction tank 4 from the gas input port 10 at a flow rate of 5 ml/min and a pressure of 0.18 MPa. In this system, the carbon dioxide reduction reaction can proceed at a three-phase interface constituted by [electrolyte membrane-copper-carbon dioxide in gas phase] 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 about 6.25 cm2.

The oxidation tank 1 and the reduction tank 4 were sufficiently replaced with helium and carbon dioxide, and then the oxidation electrode 2 was uniformly irradiated with light using the light source 9. The irradiation with light causes electrons to flow between the oxidation electrode 2 and the porous reduction electrode 5.

A current value between the oxidation electrode 2 and the porous reduction electrode 5 at the time of light irradiation was measured with an electrochemical measurement apparatus (Model 1287 Potentiogalvanostat manufactured by Solartron). In addition, gas and liquid in the oxidation tank 1 and the reduction tank 4 were collected at an optional time during light irradiation, and 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 1, and hydrogen, carbon monoxide, formic acid, methane, methanol, ethanol, and ethylene were generated in the reduction tank 4.

Test results of Examples 1 to 6 will be described later together with test results of Examples 7 to 14 and Comparative Examples 1 to 4 described below.

[Gas Phase Reduction Device (Electrolytic Reduction)]

Next, a gas phase reduction device 200 for carbon dioxide including the porous electrode-supported electrolyte membrane 20 according to the present embodiment will be described with reference to FIG. 5. The gas phase reduction device 200 illustrated in FIG. 5 is a reduction device utilizing an electrolytic reduction technology for reducing carbon dioxide by applying a current between an oxidation electrode and a reduction electrode.

The gas phase reduction device 200 includes the oxidation tank 1 and the reduction tank 4 formed by separating the space inside a housing into two with the porous electrode-supported electrolyte membrane 20. The porous electrode-supported electrolyte membrane 20 is disposed 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 the aqueous solution 3. The oxidation electrode 2 constituted by 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 in the gas phase reduction device 100 in FIG. 4.

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

A power supply 11 is electrically connected to the oxidation electrode 2 and the porous reduction electrode 5 by the conductive wire 7.

[Examples of Porous Electrode-Supported Electrolyte Membrane]

As the porous electrode-supported electrolyte membrane 20 disposed in the gas phase reduction device 200 described above, Examples 7 to 12 that are different in average pore diameter or temperature during the thermocompression bonding treatment were prepared, and a gas phase reduction test described later was performed. Hereinafter, the porous electrode-supported electrolyte membranes of Examples 7 to 12 will be described. The porous electrode-supported electrolyte membranes 20 of Examples 7 to 12 were prepared in a similar manner to the porous electrode-supported electrolyte membranes 20 of Examples 1 to 6.

Example 7

The porous electrode-supported electrolyte membrane 20 in Example 7 was prepared by a procedure similar to that in Example 1. The thermocompression bonding was performed at a heating temperature of 150° C., and after the thermocompression bonding, the porous reduction electrode 5 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, the porous electrode-supported electrolyte membrane 20 was prepared by using, as a material of the porous reduction electrode 5, a copper porous body having a thickness of 0.2 mm and a porosity of 79%. The porous reduction electrode 5 after the thermocompression bonding had a thickness of 0.14 mm, a porosity of 70%, and an average pore diameter of 15 μm. The other conditions are all similar to those in Example 7.

Example 9

In Example 9, the porous electrode-supported electrolyte membrane 20 was prepared by using, as a material of the porous reduction electrode 5, a copper porous body having a thickness of 0.2 mm and a porosity of 93%. The porous reduction electrode 5 after the thermocompression bonding had a thickness of 0.14 mm, a porosity of 90%, and an average pore diameter of 97 μm. The other conditions are all similar to those in Example 7.

Example 10

In Example 10, as in Example 9, the porous electrode-supported electrolyte membrane 20 was prepared by using, as a material of the porous reduction electrode 5, a copper porous body having a thickness of 0.2 mm and a porosity of 93%. Pressure was applied with the hot press machine at a heating temperature of 100° C. The conditions other than the heating temperature are all similar to those in Example 9.

Example 11

In Example 11, as in Example 9, the porous electrode-supported electrolyte membrane 20 was prepared by using, as a material of the porous reduction electrode 5, a copper porous body having a thickness of 0.2 mm and a porosity of 93%. Pressure was applied with the hot press machine at a heating temperature of 120° C. The conditions other than the heating temperature are all similar to those in Example 9.

Example 12

In Example 12, as in Example 9, the porous electrode-supported electrolyte membrane 20 was prepared by using, as a material of the porous reduction electrode 5, a copper porous body having a thickness of 0.2 mm and a porosity of 93%. Pressure was applied with the hot press machine at a heating temperature of 180° C. The conditions other than the heating temperature are all similar to those in Example 9.

[Electrochemical Measurement and Measurement of Gas/Liquid Generation Amount]

Each of the porous electrode-supported electrolyte membranes 20 of Examples 7 to 12 was attached to the gas phase reduction device 200 in FIG. 5, and the following reduction reaction test was performed.

The oxidation tank 1 was filled with the aqueous solution 3. As the aqueous solution 3, a 1.0 mol/L potassium hydroxide aqueous solution was used.

The oxidation electrode 2 was disposed in the oxidation tank 1 such that about 0.55 cm2 of the surface area of the oxidation electrode 2 was immersed in the aqueous solution 3. As the oxidation electrode 2, platinum (manufactured by The Nilaco Corporation) was used.

Helium (He) was caused to flow into the oxidation tank 1 from the tube 8, and carbon dioxide (CO2) was caused to flow into the reduction tank 4 from the gas input port 10 at a flow rate of 5 ml/min and a pressure of 0.18 MPa. In this system, the carbon dioxide reduction reaction can proceed at the three-phase interface constituted by [electrolyte membrane-copper-carbon dioxide in gas phase] 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 about 6.25 cm2.

After the oxidation tank 1 and the reduction tank 4 were sufficiently replaced with helium and carbon dioxide, a voltage of 2.0 V was applied by the power supply 11 to cause electrons to flow between the oxidation electrode 2 and the porous reduction electrode 5.

A current value between the oxidation electrode 2 and the porous reduction electrode 5 when a voltage was applied was measured with an electrochemical measurement apparatus.

In addition, gas and liquid in the oxidation tank 1 and the reduction tank 4 were collected at any time during voltage application, and 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 1, and hydrogen, carbon monoxide, formic acid, methane, methanol, ethanol, and ethylene were generated in the reduction tank 4.

COMPARATIVE EXAMPLES

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

Comparative Example 1

In Comparative Example 1, a porous electrode-supported electrolyte membrane was prepared in a similar manner to Example 1 by using a copper porous body having a thickness of 0.2 mm and a porosity of 51%. The porous reduction electrode after thermocompression bonding had a thickness of 0.14 mm, a porosity of 30%, and an average pore diameter of 0.11 μm. The other conditions are all similar to those in Example 1.

Comparative Example 2

In Comparative Example 2, the heating temperature at the time of thermocompression bonding was 80° C. The conditions other than the heating temperature are all similar to those in Example 3.

Comparative Example 3

In Comparative Example 3, a porous electrode-supported electrolyte membrane was prepared in a similar manner to Example 7 by using a copper porous body having a thickness of 0.2 mm and a porosity of 51%. The porous reduction electrode after thermocompression bonding had a thickness of 0.14 mm, a porosity of 30%, and an average pore diameter of 0.11 μm. The other conditions are all similar to those in Example 7.

Comparative Example 4

In Comparative Example 4, the heating temperature at the time of thermocompression bonding was 80° C. The conditions other than the heating temperature are all similar to those in Example 9.

Evaluation of Examples and Comparative Examples

Next, the test results of Examples 1 to 12 and Comparative Examples 1 to 4 will be described. Table 1 shows a Faraday efficiency of a carbon dioxide reduction reaction after one hour and a Faraday efficiency maintenance rate of the carbon dioxide reduction reaction after twenty hours for Examples 1 to 12 and Comparative Examples 1 to 4.

TABLE 1 Faraday efficiency Faraday maintenance efficiency rate of Diffusion of carbon carbon Resistance of coefficient dioxide dioxide proton of carbon reduction reduction conduction in dioxide in reaction reaction Pore Press electrolyte porous after 1 after 20 diameter temperature membrane electrode hour hours (μm) (° C.) (Ω) (×10−6 m2s−1) (%) (%) Example 1 1.3 150 3.4 5.8 33 76 Example 2 15 150 3.2 6.0 35 72 Example 3 97 150 3.4 6.0 36 75 Example 4 97 100 3.1 6.0 35 72 Example 5 97 120 3.3 6.0 34 74 Example 6 97 180 360 6.0 — — Example 7 1.3 150 3.3 5.8 30 72 Example 8 15 150 3.2 6.0 33 75 Example 9 97 150 3.3 6.0 32 75 Example 10 97 100 3.2 6.0 31 73 Example 11 97 120 3.3 6.0 34 72 Example 12 97 180 360 6.0 — — Comparative 0.11 150 3.4 3.9 15 71 Example 1 Comparative 97 80 3.1 6.0 34 45 Example 2 Comparative 0.11 150 3.4 3.9 13 72 Example 3 Comparative 97 80 3.2 6.0 35 42 Example 4

As represented by Formula (6), the Faraday efficiency indicates a ratio of a current value used in each reduction reaction to the value of a current flowing between electrodes at the time of light irradiation or voltage application.


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

Here, the “charge consumed in each reduction reaction” in Formula (6) can be obtained by converting a measured value of the amount of the reaction products of each reduction reaction into the charge necessary for the reduction reaction. Calculation was performed using Formula (7), where the amount of the reaction products of each reduction reaction was denoted by A [mol], the number of electrons required for the reduction reaction was denoted by Z, and a Faraday constant was denoted by F [C/mol].


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

The Faraday efficiency maintenance rate of each reduction reaction after twenty hours was defined as shown in the following Formula (8) and was calculated.


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

When comparing Examples 1 to 5 with Comparative Example 1 and comparing Examples 7 to 11 with Comparative Example 3, the Faraday efficiency of the carbon dioxide reduction reaction after one hour was higher in Examples 1 to 5 and 7 to 11 than in Comparative Examples 1 and 3.

Table 1 shows evaluation results of the diffusion coefficient of carbon dioxide in the porous electrode, which depends on the pore diameter. This shows that, in Examples 1 to 5 and 7 to 11 in which the pore diameter exceeds 1 μm, the values reached a saturation value 6.0×10−6 m2s−1 (self-diffusion coefficient), and the values are approximately 1.5 times the values in Comparative Examples 1 and 3.

From these, by using the porous electrode-supported electrolyte membrane 20 constituted by a porous electrode having an average pore diameter of 1 μm or more in which the diffusion coefficient of carbon dioxide reaches the saturation value, the amount of carbon dioxide supplied to the porous reduction electrode 5 was increased, and the efficiency of the carbon dioxide reduction reaction was improved.

When comparing Examples 1 to 5 with Comparative Example 2 and comparing Examples 7 to 11 with Comparative Example 4, the Faraday efficiency maintenance rate of the carbon dioxide reduction reaction after twenty hours was higher in Examples 1 to 5 and 7 to 11 than in Comparative Examples 2 and 4.

In Examples 1 to 5 and 7 to 11, no visible amount of liquid was attached to the electrode surface after twenty hours. On the other hand, in Comparative Examples 2 and 4, it was found that several hundred μL of liquid was attached to the electrode surface after twenty hours, and this prevented direct supply of carbon dioxide in gas phase to the electrode surface, resulting in a lower Faraday efficiency maintenance rate. It was confirmed that the liquid attached to the electrode surface was mainly the aqueous solution that permeated from the oxidation tank 1 through the electrolyte membrane 6 regardless of whether the carbon dioxide reduction reaction has proceeded. This is considered to be caused by the fact that the electrolyte membrane 6 has contained excessive moisture and has become swollen, and the aqueous solution 3 in the oxidation tank 1 has permeated. On the other hand, in Examples 1 to 5 and Examples 7 to 11, moisture contained in the electrolyte membrane was successfully vaporized by thermocompression bonding performed under the temperature condition of 100° C. or higher. This is considered to prevent permeation of the aqueous solution through the electrolyte membrane, thereby improving the retention rate of the carbon dioxide reduction reaction.

Furthermore, Table 1 shows the measured resistance of proton conduction of the electrolyte membrane 6. In Examples 1 to 5 and Examples 7 to 11, the resistance was as low as 3.0 to 3.5Ω, and it was confirmed that the effect of reducing the resistance of proton conduction was not lost after the thermocompression bonding. On the other hand, in Examples 6 and 12, the resistance of ion conduction of the electrolyte membrane 6 increased to 360Ω. This caused the value of the current between the electrodes to be remarkably low and the amount of the reaction products to be below a lower detection limit (3%) of an evaluation system, and the evaluation result was no record. This is considered to be caused by the proton exchange group in the electrolyte membrane being decomposed by the thermocompression bonding treatment performed under a temperature condition as high as 180° C.

As described above, according to the present embodiment, the porous electrode-supported electrolyte membrane 20 according to the present embodiment includes the electrolyte membrane 6 and the porous reduction electrode 5 directly bonded onto the electrolyte membrane 6, in which the porous reduction electrode 5 has an average pore diameter of 1 μm or more. This makes it possible to reduce the carbon dioxide diffusion resistance in the electrode and improve the efficiency of gas phase reduction of carbon dioxide. In addition, in a step of bonding the electrolyte membrane 6 and the porous reduction electrode 5, pressure is applied while heat is applied so that the electrolyte membrane 6 is prevented from swelling, and this improves life of the porous electrode-supported electrolyte membrane 20.

REFERENCE SIGNS LIST

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

Claims

1. A porous electrode-supported electrolyte membrane used in a gas phase reduction device for reducing carbon dioxide, the porous electrode-supported electrolyte membrane comprising:

an electrolyte membrane; and
a porous reduction electrode directly bonded onto the electrolyte membrane,
in which the porous reduction electrode has an average pore diameter of 1 μm or more.

2. The porous electrode-supported electrolyte membrane according to claim 1, wherein

the electrolyte membrane is thermocompression-bonded with the porous reduction electrode laid on the electrolyte membrane in such a way as to prevent swelling.

3. A method of manufacturing a porous electrode-supported electrolyte membrane used in a gas phase reduction device for reducing carbon dioxide, the method comprising:

a step of immersing an electrolyte membrane in boiling nitric acid and boiling pure water; and
a step of performing thermocompression bonding with a porous reduction electrode laid on a surface of the electrolyte membrane,
in which the porous reduction electrode after the thermocompression bonding has an average pore diameter of 1 μm or more.

4. The method of manufacturing the porous electrode-supported electrolyte membrane according to claim 3, wherein

the thermocompression bonding is performed at a heating temperature of 100° C. or higher and lower than 180° C.
Patent History
Publication number: 20240183046
Type: Application
Filed: May 21, 2021
Publication Date: Jun 6, 2024
Inventors: Sayumi Sato (Musashino-shi, Tokyo), Yuya Uzumaki (Musashino-shi, Tokyo), Akihiro Kono (Musashino-shi, Tokyo), Takeshi Komatsu (Musashino-shi, Tokyo)
Application Number: 18/554,303
Classifications
International Classification: C25B 11/031 (20060101); C25B 3/26 (20060101); C25B 9/23 (20060101);