WO2017170199A1

WO2017170199A1 - Carbon dioxide reduction electrode and carbon dioxide reduction device using same

Info

Publication number: WO2017170199A1
Application number: PCT/JP2017/011946
Authority: WO
Inventors: 飯島　剛; 拓巳岡本; 鈴木　一徳; 平田　和希; 増田　秀樹; 智彦猪股; 精片山; 竜也北川
Original assignee: 株式会社デンソー; 国立大学法人名古屋工業大学
Priority date: 2016-03-31
Filing date: 2017-03-24
Publication date: 2017-10-05
Also published as: JP2017179514A; JP6590459B2

Abstract

This carbon dioxide reduction electrode is provided with a base (13a), an ionic liquid (13b) and a nitrogen-containing aromatic compound (13c). The base is configured from a metal or a metal oxide. The ionic liquid is represented by general formula (1), and forms a monomolecular film on the surface of the base. The nitrogen-containing aromatic compound is internally contained between the base and the ionic liquid.

Description

Carbon dioxide reduction electrode and carbon dioxide reduction apparatus using the same

Cross-reference of related applications

This application is based on Japanese Application No. 2016-70181 filed on Mar. 31, 2016, the contents of which are incorporated herein by reference.

The present disclosure relates to a carbon dioxide reduction electrode for reducing carbon dioxide and a carbon dioxide reduction apparatus using the same.

In recent years, the development of photocatalysts and photoelectrochemical systems that synthesize useful organic substances from CO ₂ has become increasingly important as research areas for solving global warming and fossil fuel depletion problems.

Under these circumstances, as a method of directly synthesizing methanol (CH ₃ OH) from CO ₂ , a tertiary amine compound (for example, pyridine) is suspended in water as a mediator, and the CO ₂ and the tertiary amine compound are electrochemically coupled with an electrode. A method of synthesizing methanol by reacting has been proposed (see Patent Document 1).

Special table 2012-516392 gazette

In the method described in Patent Document 1, not only methanol but also formic acid (HCOOH) having a smaller number of reaction electrons than methanol is generated as a CO ₂ reduction product. In order to improve the selectivity of methanol in the CO ₂ reduction reaction, it is necessary to reduce the amount of reaction current. However, there is a trade-off that when the amount of reaction current is reduced, the absolute amount of methanol produced decreases.

An object of the present disclosure is to provide a carbon dioxide reduction electrode and a carbon dioxide reduction device capable of improving the production efficiency of methanol when electrochemically synthesizing methanol from CO ₂ using a mediator.

According to the first aspect of the present disclosure, the carbon dioxide reducing electrode includes a base material, an ionic liquid, and a nitrogen-containing aromatic compound. The base material is composed of a metal or a metal oxide. The ionic liquid is represented by the following general formula (1) and forms a monomolecular film on the surface of the substrate. The nitrogen-containing aromatic compound is encapsulated between the base material and the ionic liquid.

In the general formula (1), M is a P atom or an N atom. In the general formula (1), R ₁ to R ₄ each independently represents an alkyl group having 1 to 30 carbon atoms, an alkenyl group having 2 to 30 carbon atoms, an alkynyl group having 2 to 30 carbon atoms, or 1 to 30 carbon atoms. Alkyl having 1 to 30 carbon atoms, an alkyl group having 1 to 30 carbon atoms, a perfluoroalkyl group having 1 to 30 carbon atoms, an aryl group having 6 to 30 carbon atoms, an aralkyl group having 7 to 30 carbon atoms, or a carbonyl group Represents a group, an alkenyl group, an aryl group or an aralkyl group. When n is an integer of 1 to 3, R _n and R _{n + 1} may combine to have a cyclic structure. At least one of R ₁ to R _{4 in} the general formula (1) is a —SH group, —SS— group, —S— group, —COOH group, —NH ₂ group, silanol group, phosphoric acid group, It has one or more binding functional groups selected from the group consisting of alkenyl groups, alkynyl groups, and azide groups. In the general formula (1), X ⁻ represents a counter anion.

According to the first aspect of the present disclosure, the nitrogen-containing aromatic compound is formed between the base material and the ionic liquid by forming a monomolecular film of the ionic liquid represented by the general formula (1) on the surface of the base material. It can be included as a mediator. By using the carbon dioxide reduction electrode having such a configuration, it is possible to improve the methanol production efficiency when the CO ₂ reduction reaction is performed.

Further, by using the carbon dioxide reduction electrode according to the first aspect of the present disclosure, it is possible to reduce the reaction overvoltage and increase the reaction current in the CO ₂ reduction reaction. For this reason, the absolute production amount of methanol can be increased.

According to the second aspect of the present disclosure, the carbon dioxide reduction device includes the carbon dioxide reduction electrode according to the first aspect of the present disclosure, an oxidation electrode, an electrolytic solution, and a carbon dioxide supply unit. A carbon dioxide reduction electrode and an oxidation electrode are immersed in the electrolytic solution. The carbon dioxide supply unit supplies carbon dioxide to the electrolytic solution.

By using the carbon dioxide reduction electrode according to the first aspect of the present disclosure, it is possible to reduce the reaction overvoltage and increase the reaction current in the CO ₂ reduction reaction. For this reason, the absolute production amount of methanol can be increased.

The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description with reference to the accompanying drawings. In the drawing
Is an explanatory view illustrating the configuration of a CO ₂ reduction apparatus in the embodiment, It is explanatory drawing which shows the structure of a reduction electrode, It is a figure showing an example of an ionic liquid, It is a figure which shows the result of having measured CV with the reduction electrode of embodiment and the comparative example, It is a chart which shows the methanol production efficiency of an embodiment and a comparative example.

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the present embodiment, a CO ₂ reduction device to which the CO ₂ reduction electrode of the present disclosure is applied will be described.

As shown in FIG. 1, the CO ₂ reduction device 1 includes a container 10. An electrolytic solution 11 is accommodated in the container 10. The electrolytic solution 11 is not particularly limited, but an Na ₂ SO ₄ aqueous solution is used in the present embodiment.

In the container 10, an oxidation electrode 12, a reduction electrode 13, and a reference electrode 14 are inserted. The oxidation electrode 12, the reduction electrode 13 and the reference electrode 14 are immersed in the electrolytic solution 11. As the oxidation electrode 12, for example, a platinum electrode can be used. The configuration of the reduction electrode 13 will be described later. As the reference electrode 14, for example, an Ag / AgCl electrode can be used. Note that the reference electrode 14 may be omitted.

The oxidation electrode 12, the reduction electrode 13 and the reference electrode 14 are connected to a potentiostat 15. A CO ₂ supply pipe 16 is inserted into the container 10. CO ₂ is supplied to the electrolytic solution 11 from the CO ₂ supply pipe 16.

As shown in FIG. 2, the reduction electrode 13 has a base material 13a, an ionic liquid 13b, and a mediator 13c. A monomolecular film of the ionic liquid 13b is formed on the surface of the substrate 13a. A nano-order space is formed between the base material 13a and the ionic liquid 13b, and the mediator 13c is included in this space.

The base material 13a is a metal plate, and is composed of a metal or a metal oxide capable of forming a monomolecular structure on the surface of the ionic liquid 13b. As the base material 13a, for example, any metal of gold, platinum, silver, copper, tin, titanium, or a metal oxide thereof can be used. When the ionic liquid 13b has a thiol functional group (—SH, —S—, —SS—), the ionic liquid 13b having a thiol functional group can form a monomolecular structure as the base material 13a. It is desirable to use any metal of gold, platinum, silver, copper or a metal oxide thereof.

The ionic liquid 13b is a molecular liquid that is liquid at room temperature. The ionic liquid 13b of this embodiment has a branched alkyl chain and has a bulky structure. The ionic liquid 13b of the present embodiment is an organic phosphorus type or quaternary amine type ionic liquid containing a structural unit represented by the following general formula (1).

In the general formula (1), M is a P atom or an N atom. In the general formula (1), R ₁ to R ₄ each independently represents an alkyl group having 1 to 30 carbon atoms, an alkenyl group having 2 to 30 carbon atoms, an alkynyl group having 2 to 30 carbon atoms, or 1 to 30 carbon atoms. Alkyl having 1 to 30 carbon atoms, an alkyl group having 1 to 30 carbon atoms, a perfluoroalkyl group having 1 to 30 carbon atoms, an aryl group having 6 to 30 carbon atoms, an aralkyl group having 7 to 30 carbon atoms, or a carbonyl group Represents a group, an alkenyl group, an aryl group or an aralkyl group. R _n and R _{n + 1} (n is an integer of 1 to 3) may be bonded to each other to have a cyclic structure. However, at least one of R ₁ to R _{4 in} the general formula (1) is at least one binding functional group (—SH group, —SS— group, —S— group, —COOH group, —NH ₂ group). , Silanol group, phosphate group, alkenyl group, alkynyl group, or azido group).

In the general formula (1), X ⁻ represents a counter anion. The counter anion X ⁻ is an anion having a valence of 1 or more, and various halogen ions, BF ₄ ⁻ , PF ₆ ⁻ , CF ₃ SO ₃ ⁻ (abbreviation TfO ⁻ ), (CF ₃ SO ₂ ). ₂ N ⁻ (abbreviation Tf ₂ N ⁻ ) and the like are preferably used.

The ionic liquid 13b in which the surface of the base material 13a is modified with the ionic liquid 13b has a structural unit represented by the general formula (1), so that a spatial gap sufficient to introduce the mediator 13c can be created. . The mediator 13c can be introduced into a spatial gap formed by the ionic liquid 13b.

Further, since the ionic liquid 13b is ionic with a functional group such as phosphoric acid or ammonium, the conductivity can be improved and the solubility of CO _{2 in} the monomolecular film can be improved.

Although the compound of FIG. 3 can be illustrated as an ionic liquid 13b represented by the said General formula (1), it is not limited to these. 3 has “—SS— group” or “—SH group” at R ₁ in the general formula (1).

In the present embodiment, an organic compound represented by the following general formula (2) is used as the ionic liquid 13b. The organic compound represented by the general formula (2) is an organic compound represented by the general formula (1), in which M is a P atom and R ₁ is a — (CH ₂ ) ₁₂ S— group having a —S— group as a binding functional group, R ₂ to R ₄ are C ₆ H ₁₃ — groups, and X ⁻ is CF ₃ SO ₃ ⁻ (TfO ⁻ ).

The ionic liquid 13b described above can be synthesized by the method described in JP2012-167045A.

The mediator 13c is a compound that mediates electron transfer through a CO ₂ reduction reaction. In the present embodiment, a nitrogen-containing aromatic compound is used as the mediator 13c. The aromatic compound is a planar ring having a delocalized π electron system containing 4n + 2 (n is an integer) π electrons. Aromatic rings can be formed by 5, 6, 7, 8, 9, or 10 or more atoms. Aromatic compounds include monocyclic and fused ring polycyclic.

The nitrogen-containing aromatic compound is a heteroaromatic compound in which one or more constituent atoms of the aromatic ring are N atoms. In the nitrogen-containing aromatic compound, one or more hydrogen atoms bonded to the constituent atoms of the aromatic ring may be substituted with a linear or branched lower alkyl group, a hydroxy group, an amino group, or a pyridyl group. In the present embodiment, for example, imidazole, methylimidazole, dimethylimidazole, triazole, pyridine, dimethylaminopyridine are used as the nitrogen-containing aromatic compound constituting the mediator 13c.

The reduction electrode 13 of the present embodiment can be obtained by introducing the ionic liquid 13b to the surface of the base material 13a to make the ionic liquid modified base material, and introducing the mediator 13c into the ionic liquid modified base material. The base material 13a is preferably subjected to surface treatment such as cleaning as necessary before modification with the ionic liquid 13b. The surface treatment of the substrate 13a can be performed using concentrated nitric acid, a piranha solution, or hydrofluoric acid.

As a method of chemically bonding the ionic liquid 13b containing a binding functional group to the surface of the base material 13a to obtain an ionic liquid modified base material, for example, the base material 13a is immersed in a solution in which the ionic liquid 13b is dissolved in an organic solvent. A method or a method of applying the solution to the surface of the substrate 13a by spray coating or spin coating can be used. Alternatively, the substrate 13a may be immersed in the ionic liquid 13b itself, or the ionic liquid 13b may be applied to the surface of the substrate 13a by the above method.

The time for immersing the base material 13a in the solution of the ionic liquid 13b is not particularly limited as long as the ionic liquid 13b is fixed on the surface of the base material 13a, but preferably 5 minutes to 60 hours, more preferably 1 ~ 24 hours. Moreover, you may heat the base material 13a and a solution when immersing or apply | coating as needed. In the case of a solution, the concentration of the ionic liquid 13b is about 0.01 to 100 mmol / L, preferably about 0.1 to 50 mmol / L. As the organic solvent, alcohols, ethers, nitriles, esters, ketones, hydrocarbons, chloroform and the like can be used.

As a method for introducing the mediator 13c into the ionic liquid modified substrate, for example, a method in which the ionic liquid modified substrate is immersed in a solution in which the mediator 13c is dissolved in an organic solvent, or a solution is spray coated on the ionic liquid modified substrate. The method of apply | coating by spin coating etc. is mentioned.

The time for immersing the ionic liquid modified base material in the solution of the mediator 13c is not particularly limited as long as the mediator 13b is introduced into the ionic liquid modified base material, but is preferably 5 minutes to 60 hours, more preferably 1 ~ 24 hours. Moreover, you may heat an ionic liquid modification base material and a solution when immersing or apply | coating as needed. In the case of a solution, the concentration of the target compound is about 0.01 to 100 mmol / L, preferably about 0.1 to 50 mmol / L. As the organic solvent, alcohols, ethers, nitriles, esters, ketones, hydrocarbons, chloroform, water and the like can be used.

As another method of introducing the ionic liquid 13b and the mediator 13c, a solution in which the ionic liquid 13b and the mediator 13c coexist is prepared at the time of modifying the ionic liquid on the surface of the base material 13a. It is also possible to modify and fix 13c on the surface of the base material 13a. The time for immersing the substrate 13a in the solution is not particularly limited as long as the mediator 13c and the ionic liquid 13b are introduced onto the surface of the substrate 13a, but preferably 5 minutes to 60 hours, more preferably 1 to 24. It's time. Moreover, you may heat a base material and a solution, when immersing as needed. In the case of forming a solution, the concentration of the mediator 13c is about 0.01 to 100 mmol / L, preferably about 0.1 to 50 mmol / L. As the organic solvent, alcohols, ethers, nitriles, esters, ketones, hydrocarbons, chloroform and the like can be used.

Furthermore, as another method for introducing the mediator 13c, an electrochemical measurement is performed in an electrolyte solution containing the mediator 13c using an ionic liquid-modified base as an electrode and an electrochemical measurement device, preferably a potentio such as cyclic voltammetry. By performing the measurement, it is possible to introduce the mediator 13c in the electrolyte solution into the ionic liquid modified electrode.

Various conditions for measurement (concentration, temperature, solvent, measurement time, electrolyte to be used, etc.) are not particularly limited as long as the mediator 13c is introduced into the ionic liquid modified electrode, but the concentration of the mediator 13c to be introduced is Preferably it is 0.05 to 10 mmol / L, more preferably 0.1 to 5 mmol / L. The measurement temperature is preferably −10 to 100 ° C., more preferably 0 to 30 ° C. As the organic solvent, alcohols, ethers, nitriles, esters, ketones, hydrocarbons, chloroform, water and the like can be used. The measurement time is preferably about 1 minute to 2 hours, more preferably about 5 to 30 minutes.

The electrolyte is not particularly limited as long as it is an electrolyte used for normal electrochemical measurements. If the solvent is aqueous, lithium perchlorate or sodium perchlorate, and an organic solvent, tetraalkylammonium tetraborate salt. , Hexafluorophosphate, or perchlorate is preferably used.

FIG. 4 shows the result of CV measurement performed by the CO ₂ reduction device 1 of the present embodiment. Imidazole was used as the mediator 13c. In FIG. 4, the CV measurement result when not supplying CO ₂ to the electrolytic solution 11 is shown as the comparative example 1, and the electrolytic solution is formed as the comparative example 2 without forming a monomolecular film of the ionic liquid 13b on the surface of the base material 13a. 11 shows the results of CV measurement when imidazole is dispersed in No. 11.

As shown in FIG. 4, in Comparative Example 1 in which CO ₂ is not supplied to the electrolyte solution 11, the current is kept near zero. In Comparative Example 2 in which the ionic liquid 13b is not used, the current changes from zero to minus around −0.6V. In the present embodiment using the ionic liquid 13b, the current changes from zero to minus around −0.4V. In the present embodiment, the current density is increased as compared with Comparative Example 2. That is, in this embodiment, by using the ionic liquid 13b, the reaction overvoltage in the CO ₂ reduction reaction is reduced and the reaction current is increased.

In this embodiment, it is considered that the ionic liquid 13b and the mediator 13c are a combination having a high interaction, and the diffusion of the reductant on the outermost surface of the reduction electrode 13 is reduced. As a result, in the present embodiment, it is presumed that the charge transfer in the CO ₂ reduction reaction is efficient, the reaction overvoltage is reduced, and the reaction current is increased.

The CO ₂ reduction reaction is a multistage reaction that changes in the order of CO ₂ → formic acid (HCOOH) → methanol (CH ₃ OH). In Comparative Example 2 in which imidazole is dispersed in the electrolytic solution, it is difficult for the substance to be reduced to diffuse from the electrode surface at the formic acid stage and to proceed the CO ₂ reduction reaction to the formation of methanol. In contrast, in the present embodiment, a monomolecular film of the ionic liquid 13b having high conductivity and high inclusion of the mediator 13c and the reductant (reactant) is formed on the surface of the base material 13a, thereby reducing the reductant. Can be prevented from diffusing from the electrode surface at the formic acid stage, and the above multi-stage reaction is considered to proceed smoothly.

FIG. 5 shows the current density and methanol production efficiency of an example in which CO ₂ was reduced by changing the combination of the base material 13a, the ionic liquid 13b, and the mediator 13c in the CO ₂ reduction apparatus 1 of the present embodiment. Yes. FIG. 5 also shows a comparative example in which CO ₂ is reduced without using the ionic liquid 13b or the mediator 13b. The cathode potential was −0.8 V / Ag / AgCl.

As the base material 13a, gold was used in Examples 1 to 7 and Comparative Examples 1, 3, and 4, platinum was used in Example 8, and copper was used in Examples 9 and 2. As the ionic liquid 13b, in Examples 1 to 9 and Comparative Example 4, a phosphoric acid ionic liquid 13b containing P atoms was used. In Comparative Examples 1 to 3, the ionic liquid 13b was not used.

The mediator 13c uses imidazole in Examples 1, 8, and 9, Comparative Examples 1 and 2, pyridine in Examples 2 and 3, and 3,5-diamino-1,2,4- in Example 3. Triazole was used, 1-methylimidazole was used in Example 4, 2-methylimidazole was used in Example 5, 1,2-dimethylimidazole was used in Example 6, and 4-dimethylimidazole was used in Example 7. In Comparative Example 4, the mediator 13c was not used.

As shown in FIG. 5, in Examples 1 to 9 using the reduction electrode 13 of this embodiment, the methanol production efficiency was 13% to 28%. In Comparative Examples 1 to 3 using the mediator 13c without using the ionic liquid 13b, the methanol production efficiency was 1 to 6%. Furthermore, in Comparative Example 4 in which the ionic liquid 13b was used and the mediator 13c was not used, the methanol production efficiency was 0%. That is, in this embodiment, methanol production efficiency higher than that of the comparative example is obtained.

In Examples 1 to 9, when gold is used as the substrate 12a, methanol production efficiency is higher than other metals. In Examples 1 to 9, the methanol production efficiency is highest when gold is used as the substrate 12a and imidazole is used as the mediator 13c.

In addition, the kind of metal used as the base material 13a can be determined by elemental analysis by XPS, for example. The presence / absence of the monomolecular film of the ionic liquid 13b on the surface of the base material 13a can be determined by, for example, an electrochemical reduction method (that is, electrical desorption of the monomolecular film). The type of the monomolecular film of the ionic liquid 13b can be determined by elemental analysis using, for example, XRS or EDX. The type of mediator 13c can be analyzed by, for example, XPS, EDS, gas chromatography, or the like.

According to the present embodiment described above, a monomolecular film of the ionic liquid 13b is formed on the surface of the base material 13a, and the mediator 13c is included between the base material 13a and the ionic liquid 13b to perform the CO ₂ reduction reaction. The methanol production efficiency can be improved. Further, according to the present embodiment, in the CO ₂ reduction reaction, the reaction overvoltage decreases and the reaction current increases. For this reason, the absolute production amount of methanol can be increased.

(Other embodiments)
The mediator 13c is not limited to the nitrogen-containing aromatic compound exemplified in the above embodiment, but when the organic compound represented by the general formula (2) is used as the ionic liquid 13b, the reaction potential of the nitrogen-containing aromatic compound is used. Is desirably smaller than −1.0 V / Ag / AgCl on the positive side. That is, when the organic compound represented by the general formula (2) is used as the ionic liquid 13b, the monomolecular film of the ionic liquid 13b is reduced and desorbed from the substrate 13a at a potential of about −1.0 V / Ag / AgCl. There is a risk of separation. For this reason, by using a nitrogen-containing aromatic compound having a reaction potential smaller than −1.0 V / Ag / AgCl on the positive side as the mediator 13c, the monomolecular film of the ionic liquid 13b is removed from the substrate 13a during the CO ₂ reduction reaction. Reductive desorption can be avoided.

Although the present disclosure has been described based on the embodiment, it is understood that the present disclosure is not limited to the embodiment or the structure. The present disclosure includes various modifications and modifications within the equivalent range. In addition, various combinations and forms, as well as other combinations and forms including only one element, more or less, are within the scope and spirit of the present disclosure.

Claims

A base material (13a) made of metal or metal oxide;
An ionic liquid (13b) represented by the following general formula (1) and forming a monomolecular film on the surface of the substrate;
A nitrogen-containing aromatic compound (13c) encapsulated between the substrate and the ionic liquid;
A carbon dioxide reduction electrode comprising the following general formula (1):
M is a P atom or an N atom,
R 1 to R 4 are each independently an alkyl group having 1 to 30 carbon atoms, an alkenyl group having 2 to 30 carbon atoms, an alkynyl group having 2 to 30 carbon atoms, an alkoxyalkyl group having 1 to 30 carbon atoms, An alkyl group having 1 to 30 aminoalkyl groups, a perfluoroalkyl group having 1 to 30 carbon atoms, an aryl group having 6 to 30 carbon atoms, an aralkyl group having 7 to 30 carbon atoms, or a carbonyl group, an alkenyl group, and an aryl group Or represents an aralkyl group,
When n is an integer of 1 to 3, R n and R n + 1 may be bonded to each other to have a cyclic structure,
At least one of R 1 to R 4 is —SH, —SS—, —S—, —COOH, —NH 2 , silanol, phosphoric acid, alkenyl, alkynyl, Having one or more binding functional groups selected from the group consisting of groups,
X − is a carbon dioxide reducing electrode representing a counter anion.
The carbon dioxide reducing electrode according to claim 1, wherein the ionic liquid is represented by the following general formula (2).
The carbon dioxide reduction electrode according to claim 1 or 2, wherein the binding functional group of the ionic liquid forms a chemical bond with a metal or metal oxide on the surface of the substrate.
The carbon dioxide according to any one of claims 1 to 3, wherein the substrate is composed of one or more metals or metal oxides selected from the group consisting of gold, platinum, silver, copper, tin, and titanium. Reduction electrode.
The nitrogen-containing aromatic compound is a heteroaromatic compound in which one or more constituent atoms of the aromatic ring are N atoms,
2. One or more hydrogen atoms bonded to a constituent atom of an aromatic ring are substituted with one or more selected from the group consisting of a linear lower alkyl group, a branched lower alkyl group, a hydroxy group, an amino group, and a pyridyl group. 5. The carbon dioxide reducing electrode according to any one of items 4 to 4.
The carbon dioxide reducing electrode according to claim 5, wherein the nitrogen-containing aromatic compound is one or more selected from the group consisting of imidazole, methylimidazole, dimethylimidazole, triazole, pyridine, and dimethylaminopyridine.
Carbon dioxide reduction electrode (13) according to any one of claims 1 to 6,
An oxidation electrode (12);
An electrolyte solution (11) in which the carbon dioxide reduction electrode and the oxidation electrode are immersed;
A carbon dioxide supply section (16) for supplying carbon dioxide to the electrolytic solution;
A carbon dioxide reduction device comprising: