JP2024017826A - carbon dioxide electrolysis cell - Google Patents

Abstract

An object of the present invention is to realize a carbon dioxide electrolytic cell that can electrolytically reduce carbon dioxide stably for a long period of time. [Solution] A carbon dioxide electrolysis cell (1) includes a cathode (10) containing cobalt phthalocyanine, a gas flow path (20), and a cathode chamber (30) capable of containing a cathode electrolyte (31). , an anode (40), an anode chamber (50) capable of containing an anode electrolyte (51), and an anion exchange membrane (60). [Selection diagram] Figure 1

Description

The present invention relates to a carbon dioxide electrolysis cell.

BACKGROUND ART In recent years, electrolytic reduction of carbon dioxide (CO ₂ ) has been attracting attention in order to store surplus electrical energy obtained from renewable energy power generation and the like. By storing carbon compounds generated by electrolytic reduction of carbon dioxide, such as carbon monoxide (CO), storage costs and storage losses can be reduced compared to storing electrical energy in storage batteries.

Patent Document 1 discloses a method for electrolytically reducing carbon dioxide using a gas diffusion electrode. Patent Document 2 discloses a carbon dioxide electrolysis cell in which an anode is brought into contact with a separator.

Additionally, Non-Patent Document 1 states that in the diffusion-rate limit range of an oxygen reduction reaction, the higher the hydrophilicity of the cathode, the smaller the reduction current. It has been suggested that this can be reduced. Non-Patent Document 2 discloses an anion exchange membrane used in an alkaline membrane fuel cell.

Japanese Unexamined Patent Publication No. 1-205088 Japanese Patent Application Publication No. 2018-123390

S. Takase et. al., "Investigation of the Effect of Hydrophilicity on Oxygen Reduction Reaction Property with Measurement of Water Vapor Specific Surface Area" Electrochemistry, 89(6), 597-601 (2021) Hiroyuki Yanagi et al. “Development of electrolyte materials and power generation performance for alkaline membrane fuel cells (AMFC)” Hydrogen Energy Systems, Vol. 35, No. 2 (2010)

However, the conventional techniques described in Patent Documents 1 and 2 have a problem in that they cannot stably electrolytically reduce carbon dioxide for a long period of time. One aspect of the present invention aims to realize a carbon dioxide electrolytic cell that can stably electrolytically reduce carbon dioxide for a long period of time.

In order to solve the above problems, a carbon dioxide electrolytic cell according to one aspect of the present invention includes a cathode that contains cobalt phthalocyanine and reduces carbon dioxide, a gas flow path that supplies carbon dioxide to the cathode, a cathode chamber capable of containing a catholyte electrolyte in contact with the cathode; an anode paired with the cathode; an anode chamber capable of containing an anolyte electrolyte in contact with the anode; the cathode electrolyte and the anode; An anion exchange membrane that separates the electrolyte from the electrolyte.

As a result of intensive studies by the inventors, it has been found that by including cobalt phthalocyanine in the cathode, it is possible to promote the production of carbon monoxide through the reduction of carbon dioxide and to reduce the production of formic acid. Therefore, fluctuations in the pH of the catholyte and anolyte are reduced, and carbon dioxide can be electrolytically reduced stably for a long period of time.

In addition, by separating the catholyte and anolyte electrolytes with an anion exchange membrane, hydroxide ions (OH ^- ) generated by reduction of carbon dioxide move from the cathode chamber through the anion exchange membrane into the anode chamber. can do. At this time, hydrogen ions (H ⁺ ) are generated at the anode, for example, by oxidation of water or hydroxide ions. Therefore, the hydroxide ions (OH ^- ) that have moved from the cathode chamber to the anode chamber can neutralize at least a portion of the excess hydrogen ions (H ⁺ ) in the anode chamber. Therefore, fluctuations in the pH of the catholyte and anolyte are reduced, and carbon dioxide can be electrolytically reduced stably for a long period of time.

According to one aspect of the present invention, it is possible to realize a carbon dioxide electrolytic cell that can stably electrolytically reduce carbon dioxide for a long period of time.

FIG. 1 is a schematic diagram showing a carbon dioxide electrolytic cell according to one embodiment of the present invention. 2 is a schematic diagram illustrating the operation of the electrolytic cell according to FIG. 1. FIG. FIG. 3 is a diagram showing the relationship between the elapsed time after the start of electrolysis and current in the electrolytic cell according to the example.

Hereinafter, one embodiment of the present invention will be described in detail. It should be noted that the following description is provided for better understanding of the gist of the invention, and is not intended to limit the invention unless otherwise specified.

[Structure of carbon dioxide electrolysis cell]
FIG. 1 is a schematic diagram showing a carbon dioxide electrolysis cell 1 according to one embodiment of the present invention. The electrolytic cell 1 includes a cathode 10, a gas flow path 20, a cathode chamber 30, an anode 40, an anode chamber 50, and an anion exchange membrane 60.

The cathode 10 is an electrode that contains cobalt phthalocyanine (hereinafter abbreviated as "CoPc") and reduces carbon dioxide. At the cathode 10, carbon monoxide can be produced, for example, by reduction of carbon dioxide. The cathode 10 may be a gas diffusion electrode containing a hydrophobic binder on its surface. An example of a hydrophobic binder is polytetrafluoroethylene (PTFE). The cathode 10 may not contain a hydrophilic binder to further improve hydrophobicity. The cathode 10 may further contain a carbon material to improve conductivity.

By using the cathode 10 as a gas diffusion electrode containing a hydrophobic binder on the surface, gas diffusivity can be improved. Therefore, contact between the cathode 10 and carbon dioxide can be promoted. Further, when the carbon compound generated at the cathode 10 is a gas, separation of the gaseous carbon compound from the cathode 10 can be promoted. Therefore, the current density at the cathode 10 during electrolytic reduction can be improved.

Furthermore, the wettability decreases due to the increase in hydrophobicity, and the contact area between the catholyte 31 and the cathode 10 can be reduced, so the amount of carbon dioxide dissolved in the catholyte 31 can be reduced.

When the cathode 10 is a gas diffusion electrode, the concentration distribution of CoPc in the cathode 10 may be uniform within the cathode 10, or the concentration of CoPc may be higher as it approaches the surface of the cathode 10 on the gas flow side. You can leave it there.

The cathode 10 has a ratio (SA _H2O /SA _N2 , hereinafter abbreviated as "hydrophilicity") of the BET specific surface area with water as the adsorbent (SA _H2O ) and the BET specific surface area with nitrogen as the adsorbent (SA _N2 ). , 0.010 or less.

By setting the hydrophilicity of the cathode 10 to 0.010 or less, gas diffusivity can be improved. Therefore, the current density at the cathode 10 during electrolytic reduction can be further improved. Moreover, since the contact area between the catholyte 31 and the cathode 10 can be further reduced, the voltage required for electrolytic reduction can be further reduced.

Gas flow path 20 supplies carbon dioxide to cathode 10 . As the gas flow path 20, a known one can be used. When the cathode 10 is a gas diffusion electrode, the gas flow path 20 may be continuous with the gas diffusion layer of the cathode 10.

The cathode chamber 30 can contain a cathode electrolyte 31 that contacts the cathode 10 . The pH of the catholyte 31 is not particularly limited, but may be, for example, 12 or higher, preferably 13 or higher. Examples of electrolytes that can be used as the catholyte 31 include, but are not limited to, potassium hydroxide (KOH) aqueous solution, sodium hydroxide (NaOH) aqueous solution, and potassium bicarbonate (KHCO ₃ ) aqueous solution.

When a KHCO ₃ aqueous solution is used as the cathode electrolyte 31, the reaction in which CO ₂ is dissolved and ionized is difficult to proceed in a solution where carbonate ions (CO ₃ ²⁻ ) are sufficiently present. On the other hand, the condition is such that OH ^- in the solution is low, making it difficult for the oxygen generation reaction, which is the counter electrode reaction, to proceed. Therefore, the current during electrolytic reduction is lower than when a strong base electrolyte is used as the catholyte 31.

The anode 40 is an electrode paired with the cathode 10. As the anode 40, a known one such as a platinum electrode can be used, for example. At the anode 40, oxygen ( _O2 ) and hydrogen ions (H ⁺ ) may be produced, for example, by oxidation of water ( _H2O ) or hydroxide ions ( ^OH- ).

The anode chamber 50 can contain an anode electrolyte 51 that contacts the anode 40 . The pH of the anode electrolyte 51 is not particularly limited, but may be, for example, 12 or higher, preferably 13 or higher. The same electrolyte as the cathode electrolyte 31 may be used as the anode electrolyte 51 .

Anion exchange membrane 60 separates catholyte 31 and anolyte 51 from each other. The anion exchange membrane 60 prevents oxygen generated at the anode 40 from moving to the cathode electrolyte 31, and therefore can prevent oxygen generated at the anode 40 from being reduced at the cathode 10 (oxygen crossover). Since the overvoltage for carbon dioxide reduction is greater than the overvoltage for oxygen reduction, it is difficult to prevent such oxygen crossover without the anion exchange membrane 60.

The anion exchange membrane 60 may have an ionic conductivity of 1 mS/cm or more, preferably 3 mS/cm or more, and more preferably 5 mS/cm or more. The higher the ionic conductivity of the anion exchange membrane 60, the lower the voltage required during electrolytic reduction.

Further, the anion exchange membrane 60 may have a membrane resistance of 1 Ω·cm ² or less, preferably 0.7 Ω·cm ² or less, and more preferably 0.5 Ω·cm ² or less. Good too. The lower the membrane resistance of the anion exchange membrane 60, the lower the voltage required during electrolytic reduction.

[Ionic conductivity of anion exchange membrane]
In this specification, the "ion conductivity" of an anion exchange membrane shall mean the ionic conductivity calculated as follows.

First, an anion exchange membrane that has been left in the air in a dry state for 24 hours or more is moistened with ion exchange water at 40° C., and then cut into a rectangle approximately 6 cm wide and 2.0 cm long. Further, the film thickness L in a state where the anion exchange membrane is wetted with ion exchange water is measured.

Next, five platinum wires with a line width of 0.3 mm were placed at intervals of 0.5 cm in the horizontal direction (the same direction as the horizontal direction of the anion exchange membrane) and in the vertical direction (the same direction as the vertical direction of the anion exchange membrane). Insulating substrates arranged in a straight line parallel to the substrate are prepared. Then, a measurement sample is prepared by pressing the platinum wire of the insulating substrate against the rectangular anion exchange membrane.

The five platinum wires of the measurement sample are referred to as Pt1, Pt2, Pt3, Pt4, and Pt5 in order from the one closest to one short side of the rectangularly cut anion exchange membrane. And between Pt1 and Pt2 (distance = 0.5 cm), between Pt1 and Pt3 (distance = 1.0 cm), between Pt1 and Pt4 (distance = 1.5 cm), and between Pt1 and Pt5. (distance = 2.0 cm), and measure AC impedance. At this time, the AC impedance was determined by holding the measurement sample in a constant temperature and humidity chamber at 40°C and 90% RH with water droplets of ion exchange water present on the surface of the anion exchange membrane, and applying a 1 kHz AC between the platinum wires. Measure as AC impedance when applied.

Then, by plotting each measurement value with the horizontal axis as the interval between the platinum wires and the vertical axis as the AC impedance, and performing a linear approximation using the least squares method, we can calculate the specific resistance of the anion exchange membrane as the slope of the straight line. The meaning of the resistance pole gradient R3 is determined. The ionic conductivity σ is calculated from the resistance electrode gradient R3 based on the following equation (2). Regarding the right side of the following formula (2), "2.0" indicates the length of the anion exchange membrane in the vertical direction, and the unit is cm.

σ=1/(R3×2.0×L) ...(2)
σ: Ionic conductivity [S/cm]
L: Film thickness [cm]
R3: Resistance gradient [Ω/cm]
Note that the y-intercept of the above approximate straight line means the contact resistance between the platinum wire and the anion exchange membrane in the measurement sample. In this measurement, the ionic conductivity σ of the anion exchange membrane is calculated based on the resistance pole slope R3, so that the influence of the contact resistance can be excluded.

[Membrane resistance of anion exchange membrane]
In this specification, the "membrane resistance" of an anion exchange membrane shall mean the membrane resistance calculated as follows.

Membrane resistance (Ω・cm ² )=Membrane thickness (cm)/ion conductivity (1/(Ω・cm))
[Operation of carbon dioxide electrolysis cell]
FIG. 2 is a schematic diagram illustrating the operation when carbon dioxide is electrolytically reduced using the carbon dioxide electrolytic cell 1. As shown in FIG. In the example of FIG. 2, a KOH aqueous solution is used as the catholyte 31 and the anolyte 51, but the present invention is not limited thereto, and other electrolytes may be used. At the cathode 10, reactions of the following formulas (3) and (4) proceed.

Further, at the cathode 10, the reaction of the following formula (5) may further proceed.

On the other hand, at the anode 40, the reaction expressed by the following formula (6) proceeds.

In the catholyte 31, OH ^- ions become excessive due to the reactions of formulas (3) and (4) above. On the other hand, in the anode electrolyte 51, H ⁺ ions become excessive due to the reaction of the above formula (6). Here, in the electrolytic cell 1, the anion exchange membrane 60 separates the cathode electrolyte 31 and the anode electrolyte 51, so excess hydroxide ions (OH ^- ) in the cathode electrolyte 31 are transferred to the cathode chamber 30. It can be moved from inside through the anion exchange membrane 60 into the anode chamber 50 . The hydroxide ions (OH ⁻ ) that have moved from the cathode chamber 30 to the anode chamber 50 can neutralize at least a portion of the excess hydrogen ions (H ⁺ ) in the anode chamber 50. . Therefore, fluctuations in pH of the catholyte 31 and anolyte 51 can be reduced, and carbon dioxide can be electrolytically reduced stably for a long time.

Further, when the pH of the catholyte 31 is set to 12 or more, the generation of hydrogen (H ₂ ) according to the above formula (5) can be reduced, and the generation of CO according to the above formula (4) can be promoted. Further, when the pH of the anolyte 51 is set to 12 or more, the anodic reaction according to the above formula (6) can be promoted and the voltage required during electrolytic reduction can be reduced.

According to the above configuration, it is possible to efficiently store surplus electric energy obtained from, for example, power generation of renewable energy. Such effects will, for example, help achieve Goal 7 of the Sustainable Development Goals (SDGs) advocated by the United Nations, ``Ensure access to affordable, reliable, sustainable, and modern energy for all.'' can also contribute.

The present invention is not limited to the embodiments described above, and various modifications can be made within the scope of the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. are also included within the technical scope of the present invention.

〔Example〕
Examples of the present invention will be described below. Note that the electrolytic cell described in this example is an example, and is not intended to limit the electrolytic cell according to one embodiment of the present invention. First, electrolytic cells Ex1 to Ex3 according to Examples and electrolytic cell CE1 according to Comparative Example were manufactured using the combinations shown in Table 1 below.

As a cathode, an electrode prepared by mixing the cathode catalyst shown in Table 1, hydrophobic carbon powder, and PTFE and hot pressing the mixture was used. A platinum electrode was used as the anode.

As the anion exchange membrane listed in Table 1 above, a sample product manufactured by Tokuyama Co., Ltd. (sample name: A201) was used. Further, as a cation exchange membrane, Nafion (product number: 117) manufactured by DuPont was used.

Using the prepared electrolytic cell, a constant potential of -1600 mV was applied to the cathode, and the introduced gas was either (i) carbon dioxide only (gas flow rate: 30 mL/min) or (ii) carbon dioxide + saturated water vapor (total gas flow rate: 30 mL/min), electrolytic reduction of carbon dioxide was performed. The results are shown in Table 2 below. In Table 2, the introduced gas "CO2" means that the introduced gas is (i) carbon dioxide only, and the introduced gas "CO2 + H2O" means that the introduced gas is (ii) carbon dioxide + saturated water vapor. represents something. Moreover, "ND" represents that it was not detected. Further, the Faraday efficiency may exceed 100%. This is thought to be due to a measurement error caused by analyzing a portion of the generated gas and an effect of the reaction due to a smaller number of electrons than expected. As a reaction with a small number of electrons, for example, a one-electron CO production reaction between a CO ₂ radical and CO ₂ can be considered.

As shown in Table 2 above, it was found that in the electrolytic cells according to Ex1 to Ex3, production of formic acid was not confirmed or the amount produced was extremely small. Therefore, it was shown that the production of formic acid can be reduced by including CoPc in the cathode.

Further, it was shown that the electrolytic cells according to Ex1 to Ex2 had a larger amount of electricity and produced a larger amount of carbon monoxide than the electrolytic cell according to Ex3. This is presumed to be because the electrolytic cells related to Ex1 to Ex2 use a strong base with a pH of 12 or higher as the anode electrolyte, so the anodic reaction expressed by the above formula (6) proceeds quickly. be done.

In the electrolytic cells according to Ex1 and Ex3, when (ii) carbon dioxide + saturated steam was used as the introduced gas, the amount of CO produced increased compared to when only (i) carbon dioxide was used as the introduced gas. Therefore, it was suggested that when the cathode contains CoPc, the CO production reaction proceeds even if water as a liquid is not sufficiently supplied to the reaction field.

In addition, the present inventors have found that in the electrolytic cell according to Ex1, under conditions where the amount of gas generated is too large, a morphological change is observed, which is thought to be caused by stress due to gas generation from inside the electrode. . This suggests that when CoPc is supported on the entire reaction layer of the cathode, the reduction reaction of carbon dioxide proceeds not only at the interface between the cathode and the catholyte but also inside the cathode.

On the other hand, metal catalysts other than CoPc, such as gold, are generally installed at the interface between the electrode and the electrolyte, and a sufficient amount of liquid water is required for the reaction. Generally considered. One of the reasons for this is presumed to be that in the case of a metal catalyst such as gold, the bond strength between the reaction intermediate and the metal catalyst is strong, so that it is necessary to promote the elimination reaction.

FIG. 3 is a diagram showing the relationship between elapsed time after the start of electrolytic reduction and current for electrolytic cells Ex1 and CE1. The measurements were performed using two electrolytic cells, each of which was Ex1 and CE1, in order to confirm reproducibility. Ex1-1 and Ex1-2 shown in FIG. 3 both have the structure of electrolytic cell Ex1, and CE1-1 and CE1-2 both have the structure of electrolytic cell CE1.

As shown in FIG. 3, electrolytic reduction stopped at 9180 seconds for CE1-1 and at 8280 seconds for CE1-2, and no current flowed even when voltage was applied. On the other hand, electrolytic reduction could be continued until the voltage application was stopped at 21,600 seconds for Ex1-1 and at 18,000 seconds for Ex1-2.

Table 3 below shows the pH and potential difference of the electrolytic solution before and after electrolytic reduction for electrolytic cells Ex1 and CE1. As shown in Table 3, in the electrolytic cells Ex1-1 and Ex1-2, the pH fluctuation before and after electrolytic reduction was small, and the pH was 13.0 or higher at all times. On the other hand, in electrolytic cells CE1-1 and CE1-2, the pH of the anode electrolyte after electrolytic reduction had decreased to 12.5 or less.

As described above, it was shown that the electrolytic cell according to Ex1, by including an anion exchange membrane as an ion exchange membrane, can reduce pH fluctuations and stably electrolytically reduce carbon dioxide for a long time.

〔summary〕
A carbon dioxide electrolytic cell according to aspect 1 of the present invention includes a cathode that contains cobalt phthalocyanine and reduces carbon dioxide, a gas flow path that supplies carbon dioxide to the cathode, and a cathode electrolyte in contact with the cathode. a cathode chamber that can contain an anode, an anode that pairs with the cathode, an anode chamber that can contain an anode electrolyte in contact with the anode, and an anion exchange membrane that separates the catholyte and the anolyte. The configuration includes the following.

According to the above configuration, as a result of intensive studies by the inventors, it was found that by including cobalt phthalocyanine in the cathode, it is possible to promote the production of carbon monoxide through the reduction of carbon dioxide and reduce the production of formic acid. It was done. Therefore, pH fluctuations can be reduced and carbon dioxide can be electrolytically reduced stably for a long period of time.

In addition, by separating the catholyte and anolyte electrolytes with an anion exchange membrane, hydroxide ions (OH ^- ) generated by reduction of carbon dioxide move from the cathode chamber through the anion exchange membrane into the anode chamber. can do. At this time, hydrogen ions (H ⁺ ) are generated at the anode, for example, by oxidation of water or hydroxide ions. Therefore, the hydroxide ions (OH ⁻ ) that have moved from the cathode chamber to the anode chamber can neutralize at least a portion of the excess hydrogen ions (H ⁺ ) in the anode chamber. Therefore, fluctuations in pH of the catholyte and anolyte are reduced, and carbon dioxide can be electrolytically reduced stably for a long period of time.

The electrolytic cell according to aspect 2 of the present invention may be configured such that in aspect 1, the cathode is a gas diffusion electrode containing a hydrophobic binder on the surface.

According to the above configuration, by using the cathode as a gas diffusion electrode containing a hydrophobic binder on the surface, gas diffusivity can be improved. Therefore, contact between the cathode and carbon dioxide can be promoted. Further, when the carbon compound generated at the cathode is a gas, it is possible to promote the separation of the gaseous carbon compound from the cathode. Therefore, the current density at the cathode during electrolytic reduction can be improved.

Furthermore, since the contact area between the catholyte and the cathode can be reduced, the amount of carbon dioxide dissolved in the catholyte can be reduced. Therefore, the amount of carbonate ions (CO ₃ ²⁻ ) produced by ionization of carbon dioxide can be reduced, and the voltage (overvoltage) required for reduction can be reduced. Therefore, an increase in overvoltage can be suppressed during electrolytic reduction.

In the electrolysis cell according to aspect 3 of the present invention, in the above aspect 1 or 2, the cathode has a BET specific surface area with water as an adsorbent (SA _H2O ) and a BET specific surface area with nitrogen as an adsorbent (SA _N2 ). A configuration may be adopted in which the ratio (SA _H2O /SA _N2 ) is 0.010 or less.

According to the above configuration, gas diffusivity can be improved by setting SA _H2O /SA _N2 to 0.010 or less for the cathode. Therefore, the current density at the cathode during electrolytic reduction can be further improved. Furthermore, since the contact area between the catholyte and the cathode can be further reduced, the voltage required for electrolytic reduction can be further reduced.

The electrolytic cell according to aspect 4 of the present invention may be configured such that in any one of aspects 1 to 3 above, the catholyte and the anolyte have a pH of 12 or more.

According to the above configuration, by setting the pH of the catholyte to 12 or more, it is possible to reduce the production of hydrogen (H ₂ ) at the cathode during electrolytic reduction and promote the production of CO. Further, by setting the pH of the anolyte to 12 or more, the anodic reaction can be promoted. Therefore, the voltage required during electrolytic reduction can be further reduced.

The electrolytic cell according to aspect 5 of the present invention may be configured such that in any one of aspects 1 to 4 above, the anion exchange membrane has an ionic conductivity of 1 mS/cm or more.

According to the above configuration, by setting the ionic conductivity of the anion exchange membrane to 1 mS/cm or more, it is possible to reduce the voltage required during electrolytic reduction.

The present invention can be used, for example, to store surplus electrical energy.

1 Electrolytic cell 10 Cathode 20 Gas flow path 30 Cathode chamber 31 Cathode electrolyte 40 Anode 50 Anode chamber 51 Anolyte electrolyte 60 Anion exchange membrane

Claims (5)

a cathode containing cobalt phthalocyanine and reducing carbon dioxide;
a gas flow path that supplies carbon dioxide to the cathode;
a cathode chamber capable of containing a cathode electrolyte in contact with the cathode;
an anode paired with the cathode;
an anode chamber capable of containing an anode electrolyte in contact with the anode;
A carbon dioxide electrolysis cell comprising: an anion exchange membrane that separates the catholyte and the anolyte. The electrolytic cell according to claim 1, wherein the cathode is a gas diffusion electrode containing a hydrophobic binder on its surface. The cathode has a ratio (SA _H2O /SA _N2 ) of a BET specific surface area with water as an adsorbent (SA _H2O ) and a BET specific surface area with nitrogen as an adsorbent (SA _N2 ) of 0.010 or less. Item 2. The electrolysis cell according to item 1 or 2. The electrolytic cell according to claim 1 or 2, wherein the catholyte and the anolyte have a pH of 12 or more. The electrolytic cell according to claim 1 or 2, wherein the anion exchange membrane has an ionic conductivity of 1 mS/cm or more.

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