JP2014167151A - Electrochemical reduction device using diamond electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrochemical reduction device provided with stable electrodes capable of easily and efficiently reducing carbon dioxide.SOLUTION: In an electrochemical reduction device for carbon dioxide, an anode electrode and a diamond cathode electrode doped with boron are installed in a stainless steel pressure-resistant cell so as that they contact with an electrolyte containing carbon dioxide.

Description

The present invention relates to an electrochemical reduction apparatus using a diamond electrode, and more particularly to an electrochemical reduction apparatus equipped with a diamond electrode for electrochemical reduction of carbon dioxide.
About.

The 21st century is called the century of the environment, and the construction of a sustainable society (sustainable society) is considered important. Environmental issues are now a global concern, and global warming is one of them. Since the greenhouse gas with the highest contribution rate to global warming is carbon dioxide (CO ₂ ), it is thought that the reduction of carbon dioxide in the atmosphere is the most effective as a countermeasure against global warming. ing. Therefore, research and development of technology for separating and recovering, reducing or immobilizing carbon dioxide released into the atmosphere is advancing, and electrochemical reduction methods are considered as one of the technologies that are expected to be put into practical use in the future. ing.

  Attempts to electrochemically reduce carbon dioxide have been studied for a long time, and the current situation is that applied research for practical use continues. However, in general electrochemical methods, of course, electric power is necessary, and it is obvious that relying on thermal power generation using fossil fuels does not make any sense for solving environmental problems. It is assumed that carbon dioxide is electrochemically reduced using electric power obtained from renewable energy such as solar cells.

  The electrochemical reduction of carbon dioxide is in principle the same as the electrolysis of water. That is, carbon dioxide is saturated in the cathode-side electrolyte solution, and the carbon dioxide is electrochemically reduced. At the anode, the oxidation reaction of water proceeds and oxygen is generated. The electrochemical reduction method of carbon dioxide has various problems for practical use.

  Conventionally, cathodes for electrochemical reduction of carbon dioxide include, for example, copper, cobalt porphyrin (see Non-Patent Document 1), nickel cyclam complex (see Non-Patent Document 2), Cu / Sn / P alloy ( Metal electrodes such as Patent Document 1) have been used.

JP 2003-213472 A

D. Behar et al. “Cobalt Porphyrin Catalyzed Reduction of CO2. Radiation Chemical, Photochemical, and Electrochemical Studios”, J. Am. Phys. Chem. A, Vol. 102, 2870 (1998) M.M. Rudolph et al. "Macrocyclic [N42-] Coordinated Nickel Complexes as Catalysts for the Formation of Oxalative by Electrochemical of Carbon Dioxide", J. Am. Chem. Soc. , Vol. 122, 10821 (2000)

  However, the metal electrodes disclosed in the literature as described above often have problems in pretreatment and durability. Therefore, in order to maintain the high current efficiency of the reduction of carbon dioxide, the stability of the electrode material is necessary.

  On the other hand, in the electrochemical reduction of carbon dioxide, in order to obtain a high current efficiency of carbon dioxide reduction, it is necessary to increase the carbon dioxide concentration of the aqueous electrolyte solution.

  This invention is made | formed in view of this subject, The objective of this invention is providing the electrochemical reduction apparatus which can reduce | restore carbon dioxide simply and efficiently.

  As a result of intensive studies, the present inventors have found that the above problems can be solved by an electrochemical reduction apparatus having the following configuration, and have further completed the present invention based on such findings.

  That is, the electrochemical reduction apparatus for carbon dioxide according to one embodiment of the present invention includes a stainless steel pressure-resistant cell in which an anode electrode and a boron-doped diamond cathode electrode are in contact with an electrolytic solution containing carbon dioxide. It is characterized by being installed in.

  With such a configuration, carbon dioxide can be easily and efficiently reduced using a cell having a stable electrode and capable of withstanding high pressure.

  In the electrochemical reduction apparatus, it is preferable that a reference electrode is further installed in the cell. Thereby, only the voltage applied to the cathode electrode excluding the iR drop due to the resistance of the electrolytic solution can be measured.

  In the electrochemical reduction apparatus, it is preferable that a glass cell is provided in the cell, and the anode chamber and the cathode chamber are partitioned by an ion exchange membrane in the glass cell. With such a configuration, the reactions occurring in the anode chamber and the cathode chamber can be separated, and the product generated in one electrode can be prevented from reacting in the other electrode.

  Furthermore, in the electrochemical reduction device, it is preferable that the electrolytic solution contains water and methanol as solvents. By using water or methanol as the solvent, the solubility of carbon dioxide is high, so that carbon dioxide can be reduced efficiently. Furthermore, it becomes possible to reduce at 0 ° C. or less by using methanol.

  Moreover, it is preferable that the electrochemical reduction device further includes a pressure gauge that applies a pressure of 1 to 10 atm to the electrolytic solution in the stainless steel pressure cell or the glass cell. By doing so, the amount of carbon dioxide dissolved in the electrolytic solution can be increased, and carbon dioxide can be reduced more efficiently.

  In the electrochemical reduction device, the electrolyte temperature is preferably −30 to 10 ° C. By doing so, the amount of carbon dioxide dissolved in the electrolytic solution can be increased, and carbon dioxide can be reduced more efficiently.

  Furthermore, the supporting electrolyte contained in the electrolytic solution is at least one selected from lithium chloride, sodium chloride, lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide, and cesium chloride. Is preferred. By doing so, carbon dioxide can be reduced more efficiently.

  ADVANTAGE OF THE INVENTION According to this invention, the electrochemical reduction apparatus provided with the stable electrode and can reduce | restore carbon dioxide simply and efficiently can be provided.

FIG. 1 shows an embodiment of the apparatus according to the invention for electrochemically reducing carbon dioxide incorporating a diamond electrode under high pressure conditions, characterized in that the anodic and cathodic reactions proceed in one cell. An aspect is shown. FIG. 2 shows the electrochemical separation of carbon dioxide incorporating a diamond electrode under high pressure conditions, wherein the anode reaction and the cathode reaction are separated and the anode reaction and the cathode reaction proceed separately in the two cells. 1 shows one embodiment of an apparatus according to the invention for reduction.

  Hereinafter, the electrochemical reduction apparatus according to the present embodiment will be described with reference to the drawings as necessary.

<First embodiment>
As shown in FIG. 1, an electrochemical reduction apparatus for carbon dioxide 1 according to one embodiment of the present invention includes an anode electrode 4 and a boron-doped diamond cathode electrode 3 in an electrolyte solution 13 containing carbon dioxide. It is installed in the pressure | voltage resistant cell 2 made from stainless steel so that it may contact.

  It is thought that carbon dioxide can be easily and efficiently reduced by using a diamond electrode, which is a stable electrode, as a cathode electrode and performing electrochemical reduction under high pressure conditions in a pressure cell. It is done.

  That is, the electrochemical reduction apparatus 1 for carbon dioxide according to the present embodiment uses a stainless pressure cell 2 as an electrolytic solution container, and an anode electrode 4 and a boron cathode-doped diamond cathode electrode in the electrolytic solution 13 in the cell. 3 is immersed. The electrochemical reduction device 1 may further include a stirring means (for example, a stirrer shown in FIG. 2) for stirring the electrolytic solution and a reference electrode 5.

  Generally, it is known that the amount of carbon dioxide dissolved increases when the temperature of the electrolytic solution is lowered. Even if the pressure is increased, the amount of carbon dioxide dissolved in the electrolyte increases. However, decreasing the electrolysis temperature tends to decrease the current density (reaction rate) in electrochemical reduction, so it is desirable to increase the pressure if possible. Therefore, in this embodiment, the apparatus which performs the electrochemical reduction of a carbon dioxide under a high voltage | pressure using the stainless steel pressure | voltage resistant cell 2 which can endure a high voltage | pressure as an essential structure is provided.

  Thus, in order to make the inside of the electrochemical reduction apparatus 1 into a high pressure environment, the high pressure valve 7 and the pressure gauge 6 can be installed in the electrochemical reduction apparatus 1 to adjust the pressure in the apparatus.

  In the present embodiment, the pressure in the electrochemical reduction device 1 is preferably about 1 to 10 atm. It is considered that the amount of dissolved carbon dioxide in the electrolytic solution can be increased by applying a pressure in such a range.

  Further, in order to withstand high pressure, it is preferable to fix the lid with a strong bolt 8 and use an O-ring 9 or the like for sealing.

  In addition, the carbon dioxide in the electrolytic solution 13 is reduced and consumed by the diamond cathode electrode 3, so that the concentration of carbon dioxide or the like in the electrolytic solution 13 is reduced. Therefore, it is necessary to constantly replenish carbon dioxide and the like that have been reduced by the reduction reaction, and always maintain the concentration within a predetermined range. Therefore, although not shown, a circulating means for circulating the electrolytic solution 13 in the cell 2 may be installed in the electrochemical reduction device 1 as appropriate.

  Furthermore, it is preferable to provide means for recovering the gas generated by the reduction of carbon dioxide. This is because the efficiently generated gas can be recovered and used.

(Stainless pressure cell)
In the electrochemical reduction device 1 according to the present embodiment, the electrochemical reduction of carbon dioxide is performed in a stainless pressure-resistant cell 2.

  The stainless steel pressure cell 2 used in the present embodiment is not particularly limited as long as it is made of stainless steel capable of withstanding high pressure (for example, about 1 to 10 atm). For example, SUS316 or Stainless steel such as SUS304 can be used.

  A stainless steel pressure cell as described above may be coated on the inside with Teflon (registered trademark) or the like for the purpose of improving insulation and corrosion resistance.

(electrode)
In the present embodiment, a diamond electrode doped with boron is used as the cathode electrode (anode) 3. By using such a diamond electrode, it becomes difficult for the electrode to be consumed by elution by electrolysis, and it becomes possible to perform electrochemical reduction stably.

  Such a boron-doped diamond electrode can be obtained, for example, by using a semiconductor material such as a silicon wafer as a base material and forming a boron-doped diamond thin film on the surface of the wafer base material. As the boron-doped diamond electrode, it is possible to use a melted wafer or self-standing type conductive polycrystalline diamond deposited and synthesized in a plate shape without using a base material. Moreover, what was laminated | stacked on metal substrates, such as Nb, W, and Ti, can also be utilized.

  The boron-doped diamond thin film is provided with conductivity by doping a predetermined amount of boron during synthesis of the diamond thin film. In addition, it is preferable that the quantity of dope is about 50-20,000 ppm with respect to the carbon content of a diamond thin film. If it is less than 50 ppm, ozone cannot be generated efficiently, and if it exceeds 20,000 ppm, the doping effect may be saturated.

  Further, the anode electrode (cathode) 4 is not particularly limited, but is preferably any one selected from the group consisting of platinum, iridium, palladium, osmium, rhodium, and ruthenium.

  The electrode according to the present embodiment is usually a plate-like electrode, but can be used even if the network structure is made into a plate shape. For example, it is possible to arrange a wire-like cathode with a diamond electrode on a flat plate with an electrolytic membrane therebetween, or a mesh-like cathode with a membrane electrode separated from a diamond electrode on a flat plate. Further, the number of electrodes is not particularly limited.

  Further, in the present embodiment, the reference electrode 5 may be used. The reference electrode 5 can be used without particular limitation as long as it is normally used in the field of electrochemistry, and specifically, for example, a pressure-resistant standard hydrogen electrode, a pressure-resistant silver / chloride. A silver electrode etc. are illustrated. By using such a reference electrode, there is an advantage that only the voltage applied to the cathode electrode excluding the iR drop due to the resistance of the electrolytic solution can be measured.

  In the present embodiment, the electrode is electrically connected to a power source (not shown). The power source is not particularly limited, and any power source that is usually used for electrolysis can be used without particular limitation.

  In the present embodiment, the reduction potential is generally −1.5 to −5.0 V vs. with respect to the electrode. Ag / AgCl sat. For example, a power supply device such as a potentio galvano stud device can be used.

(Electrolyte)
The electrolytic solution 13 used in the electrochemical reduction device 1 of the present embodiment is preferably an electrolytic solution that can dissolve a large amount of carbon dioxide. For example, a lower alcohol solution such as methanol or ethanol, or sodium hydroxide An alkaline solution such as an aqueous solution, an aqueous potassium hydroxide solution, monomethanolamine, methylamine, other liquid amines, or a mixture of these liquid amines and an aqueous electrolyte solution may be used. Further, the electrolyte aqueous solution is not particularly limited, but for example, potassium chloride aqueous solution, sodium chloride aqueous solution, sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, sodium hydrogen carbonate aqueous solution, potassium carbonate aqueous solution, lithium chloride aqueous solution, lithium hydroxide aqueous solution, water An aqueous rubidium oxide solution, an aqueous cesium hydroxide solution, an aqueous cesium chloride solution, or the like can be used. These may be used alone or in combination of two or more.

  Among them, more preferably, from the viewpoint of excellent physical solubility, a lower alcohol such as methanol and water are used as a solvent, and an electrolyte containing potassium salt, lithium salt, sodium salt, cesium salt, rubidium salt, etc. as a supporting electrolyte. It is desirable to use a liquid. This is because these metals have a large ionization tendency and are not reduced and do not participate in the reaction. Moreover, since the solubility of carbon dioxide is high by using water or methanol as a solvent, carbon dioxide can be reduced efficiently. Furthermore, it becomes possible to reduce at 0 ° C. or less by using methanol.

  As a particularly preferred specific example of the electrolytic solution, for example, potassium hydroxide methanol saturated with carbon dioxide can be used.

  In the present embodiment, the temperature of the electrolytic solution (reduction temperature) is preferably set low, and for example, it is desirable to maintain a low temperature of about −30 to 10 ° C. Thereby, the efficiency of carbon dioxide reduction can be further improved. The temperature of the electrolytic solution can be adjusted, for example, by installing a cooling device or the like.

<Second Embodiment>
As shown in FIG. 2, the electrochemical reduction apparatus 1 according to the present embodiment further includes a glass cell 12 in the stainless steel pressure-resistant cell 2, and the anode chamber and the cathode chamber are ion-exchanged in the glass cell 12. It may be partitioned by the film 10. In this case, the electrochemical reduction of carbon dioxide is performed in the glass cell 12.

  As the glass cell 12, for example, an H-type glass cell or the like can be used.

  Although there is no restriction | limiting in particular as the said ion exchange membrane 10, For example, ion exchange membranes, such as a hydrocarbon type, a perfluorocarbon type, a styrene type, an acryl type, a condensation type, an engineering plastic type, can be used. Particularly preferably, a perfluorocarbon-based material can be used. For example, “Nafion 117” manufactured by DuPont Co., Ltd. can be used as a specific example that is commercially available. In the present embodiment, the ion exchange membrane 10 can separate the reactions occurring in the anode chamber and the cathode chamber, and the product generated in one electrode can be prevented from reacting in the other electrode. Play a function.

  Further, if necessary, an electrolyte interposed between the ion exchange membrane 10 and the electrode may be provided. The electrolyte is not particularly limited, but lithium chloride alcohol aqueous solution, sodium chloride alcohol aqueous solution, lithium hydroxide alcohol aqueous solution, sodium hydroxide alcohol aqueous solution, potassium hydroxide alcohol aqueous solution, rubidium hydroxide aqueous solution, cesium hydroxide alcohol aqueous solution, A cesium chloride alcohol aqueous solution or the like can be used.

  Hereinafter, representative examples of the present invention will be shown and the present invention will be described more specifically. Needless to say, the present invention is not limited by the description of these examples. . In addition to the following examples, the present invention includes various changes and modifications based on the knowledge of those skilled in the art without departing from the spirit of the present invention other than the specific description described above. It should be understood that improvements and the like can be made.

Example 1
(Device used)
An apparatus as shown in FIG. 1 was used. The inner surface of stainless steel pressure-resistant cell 2 (“(product name)” manufactured by Fine Chemical Japan Co., Ltd.) is coated with Teflon (registered trademark), and the cathode electrode 3 is synthesized by the inventors using a microwave plasma CVD apparatus. A boron-doped diamond electrode doped with 2000 ppm of boron on a 10 mm × 20 mm low-resistance silicon substrate was used, and a platinum plate (30 mm × 10 mm, 0.1 mm, 99.8%) was used as the anode electrode. Boron-doped diamond was grounded to a nickel wire with a conductive epoxy resin, and the periphery thereof was coated with araldide and covered with a polymer resin so that the electrode reaction occurred only on the surface of the boron-doped diamond. As the electrolytic solution 13, 25 mL of a methanol solvent in which a supporting electrolyte (described later in detail) was dissolved was added to the stainless steel cell 2. A pseudo silver bar electrode was used as the reference electrode.

  The temperature during electrolysis was fixed by a cooling device, and constant potential electrolysis was performed using a potentio / galvanostat power source. The energization amount was 30 to 50 coulombs. Gas chromatography and high performance liquid chromatography were used for analysis of the reduction product. In the following tests, the current efficiency of the product was calculated and the performance of the electrochemical reduction device was evaluated.

(Evaluation)
1. Pressure Using 0.3 M KOH / methanol as the electrolytic solution 13, the pressure reduction cell 2 made of stainless steel was used for electrochemical reduction at 0 ° C., and the influence of pressure was examined. The results are shown in Table 1. The reduction potential was −3.0 V vs. It was a pseudo silver reference electrode. The reduction products were CO, methyl formate, and methane, and ethylene and ethane were not produced. When the pressure was 4 atm or more, the current efficiency of CO and formic acid increased, and the current efficiency of hydrogen decreased. The total current efficiency of CO and formic acid at 4 atm was about 83%.

電解液 ３００ｍＭ ＫＯＨ／ｍｅｔｈａｎｏｌ
電位 −３．０Ｖ ｖｓ． 擬似銀参照電極
温度 ０℃
通電量 ５０クーロン

Electrolyte 300mM KOH / ethanol
Potential -3.0 V vs. Pseudosilver reference electrode temperature 0 ° C
Energization amount 50 coulomb

2. Potential Electrolytic reduction of carbon dioxide was performed using a stainless steel pressure-resistant cell 2 at 4 atm using an electrolytic solution 0.3 M KOH / methanol, and the influence of the potential was examined. The results are shown in Table 2. The reduction temperature was 0 ° C. The reduction products were CO, methyl formate, and methane, and ethylene and ethane were not produced. When the reduction potential was around -2.8 to -3.2 V, there was a tendency that the current efficiency of methyl formate was high.

電解質３００ｍＭ ＫＯＨ／ｍｅｔｈａｎｏｌ
圧力 ４ａｔｍ
温度 ０℃
通電量 ５０クーロン

Electrolyte 300mM KOH / ethanol
Pressure 4atm
Temperature 0 ℃
Energization amount 50 coulomb

3. Temperature Using 0.3 M KOH / methanol as the electrolyte solution 13, the pressure reduction cell 2 made of stainless steel was used to perform electrochemical reduction of carbon dioxide at 4 atm, and the influence of temperature was examined. The results are shown in Table 3. The reduction potential was −3.0V. The reduction products were CO, methyl formate, and methane, and ethylene and ethane were not produced. When the reduction temperature was around 0 ° C., the current efficiency of methyl formate tended to be high.

電解質３００ｍＭ ＫＯＨ／ｍｅｔｈａｎｏｌ
電位 −３．０Ｖ ｖｓ． 擬似銀参照電極
圧力 ４ａｔｍ
通電量 ５０クーロン

Electrolyte 300mM KOH / ethanol
Potential -3.0 V vs. Pseudo silver reference electrode pressure 4 atm
Energization amount 50 coulomb

4). Electrolyte Using a single high-pressure stainless steel cell at 4 atm, the CO2 was electrochemically reduced to examine the influence of the supporting electrolyte. The results are shown in Table 4. The reduction potential was −3.0V. The reduction products were CO, methyl formate, and methane, and ethylene and ethane were not produced. When potassium salt was used for the supporting electrolyte, the tendency of high current efficiency of methyl formate was obtained.

電位 −３．０Ｖ ｖｓ． 擬似銀参照電極
温度 ０℃
圧力 ４ａｔｍ
通電量 ５０クーロン

Potential -3.0 V vs. Pseudosilver reference electrode temperature 0 ° C
Pressure 4atm
Energization amount 50 coulomb

(Example 2)
Carbon dioxide was reduced using the apparatus shown in FIG. An H-shaped glass cell 12 is installed in a pressure-resistant cell 2 made of stainless steel. The membrane was partitioned with Nafion 117). As a supporting electrolyte, methanol in which 100 mM potassium hydroxide was dissolved was used as an electrolytic solution. 15 mL of the electrolyte was added to each of the cathode chamber and the anode chamber. The same boron-doped diamond electrode as in Example 1 was used as the cathode electrode. A platinum plate (30 mm × 15 mm, 0.1 mm, 99.8%) was used for the anode electrode. A silver rod was used as the pseudo reference electrode. The pressure applied to the stainless steel pressure-resistant cell was kept constant at 4 atm, and the temperature of the electrolyte was set to 15 ° C. The energization amount was up to 30 coulombs, constant potential electrolysis was performed, and the electrochemical reduction of carbon dioxide was advanced. The generated product was analyzed by gas chromatography and high performance liquid chromatography. The current efficiency of the product was calculated and evaluated.

  Table 5 shows the results of electrochemical reduction of carbon dioxide with a diamond electrode under high pressure conditions.

上記の還元結果から、アノード室とカソード室を分離し、二つに分けたセル中においてアノード反応とカソード反応が別々に進行することを特徴とする、高圧条件下でダイヤモンド電極を用いて二酸化炭素を電気化学的に還元するステンレス耐圧セルにおいても、二酸化炭素の電気化学的還元が進行していることを確かめることができた。

From the above reduction results, the anode chamber and the cathode chamber are separated, and the anode reaction and the cathode reaction proceed separately in a cell divided into two, and carbon dioxide using a diamond electrode under high pressure conditions It was confirmed that the electrochemical reduction of carbon dioxide was also progressing in the stainless steel pressure cell that electrochemically reduced the carbon dioxide.

1 Electrochemical reduction device 2 Withstand pressure cell 3 Diamond electrode (cathode electrode)
4 Anode electrode 5 Reference electrode 6 Pressure gauge 7 High-pressure valve 8 Bolt 9 O-ring 10 Ion exchange membrane 11 Stirrer

Claims (7)

  An electrochemical reduction device for carbon dioxide in which an anode electrode and a diamond cathode electrode doped with boron are installed in a stainless steel pressure-resistant cell so as to come into contact with an electrolytic solution containing carbon dioxide.   The electrochemical reduction apparatus according to claim 1, further comprising a reference electrode installed in the cell.   The glass cell is further provided in the cell, and the anode chamber having the anode electrode and the cathode chamber having the diamond cathode electrode are partitioned by an ion exchange membrane in the glass cell. Electrochemical reduction device.   The electrochemical reduction device according to claim 1, wherein the electrolytic solution contains water and methanol as a solvent.   The electrochemical reduction device according to any one of claims 1 to 4, further comprising a pressure gauge that applies a pressure of 1 to 10 atm to the electrolytic solution in the stainless steel pressure cell or the glass cell.   The electrochemical reduction device according to any one of claims 1 to 5, wherein the electrolyte temperature is -30 to 10 ° C. The supporting electrolyte contained in the electrolytic solution is at least one selected from lithium chloride, sodium chloride, lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide, and cesium chloride. The electrochemical reduction apparatus in any one of 1-6.

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