Oxygen evolution catalyst and electrode for oxygen evolution reaction

Abstract

To provide an oxygen evolution catalyst that evolves oxygen from water, the catalyst capable of reducing the voltage required for oxygen evolution.SOLUTION: The present invention provides an oxygen evolution catalyst that is a Ni complex of a 2,2'-biimidazole derivative and a Ni salt, or an Fe complex of a 2,2'-biimidazole derivative and an Fe salt.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an oxygen generation catalyst for generating oxygen from water and an electrode for an oxygen generation reaction containing the oxygen generation catalyst.

Background Art

[0002] The oxygen generation reaction for generating oxygen from water is used as a counter electrode reaction such as a water electrolysis reaction for generating hydrogen in an alkaline solution or a carbon dioxide reduction reaction for synthesizing useful compounds such as carbon monoxide, formic acid, and ethanol from carbon dioxide (CO2). Various studies have been conducted on the oxygen generation catalyst used in this oxygen generation reaction.

[0003] For example, Non-Patent Document 1 describes an oxygen generation reaction using a Ni-Fe-based metal complex type oxygen generation catalyst having a pyridine-based ligand. Since Non-Patent Document 1 uses a catalyst dissolved in a reaction solution and examines it using a glassy carbon electrode, the generated current value is quite small, about 0.3 mA. In addition, the electrolytic solution requires 10% by mass of acetonitrile in a 0.1 M NaHCO3 aqueous solution having a pH of about 10.

[0004] Non-Patent Document 2 describes an oxygen generation reaction using a metal oxide-based oxidation generation catalyst in which a NiFeCu metal salt is added onto a Ni foam and NiOOH drives as a catalyst. In Non-Patent Document 2, the reaction is carried out in a 0.5 M KHCO3 aqueous solution not saturated with carbon dioxide. The catalyst having the highest performance in Non-Patent Document 5 requires an overvoltage of 385 mV to generate a current of 10 mA.

[0005] Non-Patent Document 3 describes an oxygen generation reaction using a metal oxide-based oxidation generation catalyst in which a Ni foam is gold-plated and a NiCoFeP catalyst is supported. In Non-Patent Document 3, the reaction is carried out in a 0.5 M KHCO3 aqueous solution saturated with carbon dioxide. The catalyst having the highest performance in Non-Patent Document 3 requires an overvoltage of 400 mV to generate a current of 10 mA.

[0006] Patent Document 1 relates to Fe-containing composite compound particles, a method for producing the same, and an Fe-containing composite compound electrode, and describes an oxygen evolution catalyst composed of Fe-containing composite compound particles containing a β-FeOOH crystal phase and a trivalent Ni-containing compound covering the periphery of the β-FeOOH crystal phase. In Patent Document 1, the reaction is carried out using an electrolyte of 0.1M K2B4O7 + 0.2M K2SO4 saturated with carbon dioxide. A voltage of 0.5 mA requires an overvoltage of about 600 mV for the catalyst having the highest performance in Patent Document 1 at 1.60 V vsRHE, and 600 mV for 10 mA.

[0007] Non-Patent Documents 1 to 3 and Patent Document 1 are all examples of using a neutral electrolyte as the reaction solution for the oxygen evolution reaction. Non-Patent Documents 1 and 2 were measured under conditions of unsaturated carbon dioxide, and Non-Patent Document 3 and Patent Document 1 were measured under conditions of saturated carbon dioxide. There are very few examples of studying metal complex-based oxygen evolution catalysts in neutral electrolytes. Non-Patent Document 1 is an example of a pyridine-based Ni-Fe-based metal complex-type oxygen evolution catalyst in 0.1M NaHCO3 (containing 10% by mass of acetonitrile) at about pH 10. However, since a water-soluble catalyst is used, the generated current value is small and remains at about 0.3 mA as described above.

[0008] There is a report of using a metal oxide-based oxygen evolution catalyst even under neutral conditions as in Non-Patent Document 2. However, even for a catalyst of a metal oxide-based catalyst (NiOOH + Fe + Cu) synthesized based on an alloy type into which a plurality of atoms are introduced, the required overvoltage is as large as 385 mV as described above.

[0009] When an oxygen evolution catalyst is used in combination with a catalyst for carbon dioxide reduction, a two-chamber reaction cell in which the oxygen evolution catalyst and the catalyst for carbon dioxide reduction are separated and operated by a membrane is common. However, some carbon dioxide (CO2) is HCO3 - or CO3 2-It is known that it will pass through the membrane in this state, and in the long term, the electrolyte used in the oxygen generation catalyst will also contain carbon dioxide. Therefore, over time, an alkaline solution such as 1M KOH undergoes a neutralization reaction, gradually causing a decrease in pH, which leads to a decrease in catalytic activity and an increase in the voltage required to drive the reaction. Similarly, in other electrolytes, as carbon dioxide dissolves, the pH gradually decreases towards the neutral side. Considering long-term use, it is desirable to be able to react with a neutral electrolyte saturated with carbon dioxide. However, as in Non-Patent Document 3 and Patent Document 1, it is known that in a carbon dioxide-saturated electrolyte, the voltage required for the oxygen generation reaction is higher than that in a carbon dioxide-unsaturated electrolyte. Most of the oxygen generation catalysts to date are inorganic metal / metal oxide-based catalysts, which have the highest catalytic performance in alkaline solutions and are known to be particularly difficult to react in carbon dioxide-saturated electrolytes.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Non-Patent Documents

[0011]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0012] An object of the present invention is to provide an oxygen generation catalyst capable of reducing the voltage required for oxygen generation in a catalyst for generating oxygen from water, and an electrode for an oxygen generation reaction including the oxygen generation catalyst. **Means for Solving the Problems**

[0013] The present invention is an oxygen generation catalyst which is a Ni complex of a 2,2'-biimidazole derivative and a Ni salt or an Fe complex of a 2,2'-biimidazole derivative and an Fe salt.

[0014] In the oxygen generation catalyst, it is preferable that the oxygen generation catalyst is for an oxygen generation reaction for generating oxygen using a neutral aqueous solution saturated with carbon dioxide as a reaction solution.

[0015] In the oxygen generation catalyst, it is preferable that the Ni salt or the Fe salt is at least one of a nitrate and a chloride salt.

[0016] The present invention is an electrode for an oxygen generation reaction including the above oxygen generation catalyst. **Effects of the Invention**

[0017] According to the present invention, it is possible to provide an oxygen generation catalyst capable of reducing the voltage required for oxygen generation in a catalyst for generating oxygen from water, and an electrode for an oxygen generation reaction including the oxygen generation catalyst. **Modes for Carrying Out the Invention**

[0018] Embodiments of the present invention will be described below. This embodiment is an example of carrying out the present invention, and the present invention is not limited to this embodiment.

[0019] <Oxygen Generation Catalyst> The oxygen generation catalyst according to this embodiment is a Ni complex of a 2,2'-biimidazole derivative and a Ni salt, or an Fe complex of a 2,2'-biimidazole derivative and an Fe salt.

[0020] In the oxygen evolution reaction, in order to generate oxygen from water, it is necessary to perform an electrochemical reaction at a value higher than 1.23 V vs RHE (reversible hydrogen electrode level) as the equilibrium potential. In fact, with an applied voltage of 1.23 V, the oxygen evolution reaction hardly occurs, and an extra voltage (overvoltage) corresponding to the activation energy must be applied for the reaction to proceed. Therefore, the larger the overvoltage, the greater the factor of energy loss. The oxygen evolution reaction is a reaction used as a counter electrode reaction such as a water electrolysis reaction that generates hydrogen in an alkaline solution or a carbon dioxide reduction reaction that synthesizes useful compounds such as carbon monoxide, formic acid, and ethanol from carbon dioxide. It is also important to reduce the overvoltage of the oxygen evolution reaction in order to reduce the energy (voltage) required for the entire reaction.

[0021] The inventors of the present invention considered that in an electrochemically driven oxygen evolution catalyst, an approach such as bringing water close to the active center and facilitating the acceptance of electrons and protons contributes to the reduction of overvoltage.

[0022] In the oxygen evolution catalyst according to the present embodiment, by having imidazole, an organic compound with high electron conductivity and proton conductivity, and among them, a biimidazole ligand whose proton mobility changes according to pH, in the catalyst structure, it is considered that the voltage required in the oxygen evolution reaction can be specifically reduced in a neutral electrolyte around pH 7 saturated with carbon dioxide. In addition, it is expected that the conjugation of biimidazole extends more than that of imidazole alone, making it easier to inject electrons, and it is considered that the voltage required in the oxygen evolution reaction can be reduced.

[0023] The oxygen evolution catalyst according to the present embodiment is formed by a combination of a ligand R that is a 2,2'-biimidazole derivative and a salt of a metal M (Ni or Fe).

[0024]

Chemical formula

[0025] The Ni salt or Fe salt may be any one used in catalyst synthesis as a Ni ion source or Fe ion source, and there is no particular limitation. Examples of the Ni salt or Fe salt include inorganic salts such as nitrates, sulfates, chloride salts, bromide salts, and organic salts such as citrate salts. From the viewpoint of catalyst performance and the like, nitrates and chloride salts are preferred. The Ni salt or Fe salt may be used alone or in combination of two or more.

[0026] R in the 2,2'-biimidazole derivative R 1 ~R 4 There is no particular limitation on the substituents of, and independently, in addition to a hydrogen atom, electron-donating substituents having an effect of lowering the lowest unoccupied molecular orbital (LUMO) of biimidazole and the like can be mentioned. As the electron-donating substituent, it is only necessary to have electron-donating properties, and there is no particular limitation. Examples of the electron-donating substituent include linear, branched, and cyclic alkyl groups having 1 to 6 carbon atoms such as methyl group, ethyl group, propyl group, and butyl group, hydroxyl group, acetoxy group, amino group, acetamide group, and dialkylamino groups (the carbon number of the alkyl group is 1 to 6) such as dimethylamino group. Other examples of the electron-donating substituent include an alkylphosphonic acid group in which phosphonic acid is linked to the end of the above linear alkyl group, an alkylcarboxyl group in which a carboxyl group is linked to the end of the above linear alkyl group, an alkylsulfonic acid group in which a sulfonic acid group is linked to the end of the above linear alkyl group, an alkylsilanol group in which a silanol group is linked to the end of the above linear alkyl group, an alkylmercapto group in which a mercapto group is linked to the end of the above linear alkyl group, and derivatives thereof.

[0027] Examples of R include 2,2'-biimidazole and the like.

[0028]

Chemical formula

[0029] The ligand R, which is a 2,2'-bimidazole derivative, is not particularly limited as long as it is a 2,2'-bimidazole derivative due to the factors described above. However, from the viewpoint of catalytic performance and the like, 2,2'-bimidazole is preferable.

[0030] The oxygen generation catalyst according to this embodiment can be obtained by dissolving a 2,2'-bimidazole derivative and a Ni salt or an Fe salt in a solvent at a predetermined molar ratio and reacting them at a predetermined temperature for a predetermined time.

[0031] The solvent is not particularly limited as long as it can dissolve or disperse the 2,2'-bimidazole derivative and the Ni salt or the Fe salt. For example, acetonitrile, ethanol, methanol, etc. may be used.

[0032] The molar ratio may be in the range of, for example, 2,2'-bimidazole derivative:Ni salt or Fe salt = 1:0.5 to 1:4.

[0033] The reaction temperature may be in the range of, for example, 0 to 80°C. The reaction time may be in the range of, for example, 0.5 to 24 hours.

[0034] During the reaction, the reaction solution may be stirred or ultrasonic treatment may be performed.

[0035] After the reaction, if necessary, the solvent may be removed to isolate, purify, and dry the oxygen generation catalyst, or the reaction solution may be directly used to coat a substrate or the like by a known coating method to fabricate an electrode or the like.

[0036] The electrode for the oxygen generation reaction according to this embodiment is, for example, for an oxygen generation reaction that generates oxygen using an alkaline aqueous solution with a pH of 10 or higher, for example, a pH of 10 to 14. The alkaline aqueous solution with a pH of 10 or higher is not particularly limited, and examples include an aqueous potassium hydroxide (KOH) solution with a pH of 10 or higher, a CO2-unsaturated KHCO3 + K2CO3 electrolyte, a CO2-unsaturated K2B4O7 + K2SO4 electrolyte, a phosphate electrolyte, and CO2-unsaturated sodium carbonate.

[0037] In addition, the electrode for oxygen generation reaction according to the present embodiment is, for example, for an oxygen generation reaction that generates oxygen using a neutral aqueous solution saturated with carbon dioxide as a reaction solution. The neutral aqueous solution is not particularly limited, and examples thereof include a CO2-saturated KHCO3 + K2CO3 electrolyte, a CO2-saturated K2B4O7 + K2SO4 electrolyte, and a phosphate electrolyte having a pH of 6 to 8.

[0038] <Electrode for oxygen generation reaction> The electrode for oxygen generation reaction according to the present embodiment is an electrode containing the above oxygen generation catalyst. For example, it is an electrode in which the above oxygen generation catalyst is supported on a substrate.

[0039] The electrode for oxygen generation reaction can be obtained, for example, by supporting an oxygen generation catalyst on a substrate using a coating method such as a dip coating method, a spin coating method, or a spray coating method using a solution containing the above oxygen generation catalyst.

[0040] As the substrate, Ni foam, carbon paper, Ti mesh, etc. can be used.

[0041] <Oxygen generation reaction> The oxygen generation reaction in which the oxygen generation catalyst and the electrode for oxygen generation reaction according to the present embodiment are used is a reaction used as a counter electrode reaction such as a water electrolysis reaction for generating hydrogen in an alkaline solution or a carbon dioxide reduction reaction for synthesizing useful compounds such as carbon monoxide, formic acid, and ethanol from carbon dioxide (CO2).

[0042] This specification includes the following embodiments. [1] An oxygen generation catalyst which is a Ni complex of a 2,2'-biimidazole derivative and a Ni salt or an Fe complex of an Fe salt.

[0043] [2] The oxygen generation catalyst according to [1], wherein the oxygen generation catalyst is for an oxygen generation reaction that generates oxygen using a neutral aqueous solution saturated with carbon dioxide as a reaction solution.

[0044] [3][1] or the oxygen generation catalyst according to [2], wherein the Ni salt or the Fe salt is an oxygen generation catalyst that is at least one of a nitrate and a chloride salt.

[0045] [4] An electrode for an oxygen generation reaction, comprising the oxygen generation catalyst according to any one of [1] to [3].

Example

[0046] Hereinafter, examples and comparative examples will be given to explain the present invention more specifically and in detail. However, the present invention is not limited to the following examples.

[0047] [Preparation of oxygen generation catalyst using 2,2'-biimidazole ligand] As Examples 1 to 9, studies on oxygen generation catalysts using 2,2'-biimidazole ligands are shown.

[0048] <Example 1: Preparation of Ni-Bimi electrode> Ni(NO3)2·6H2O and 2,2'-biimidazole (Bimi) were dissolved in 20 mL of acetonitrile so that the molar ratio was 1:2, and ultrasonic treatment was performed at 25 °C for 30 minutes. Ni foam was immersed in the obtained solution, and Ni-Bimi was supported on the Ni foam using a dip coating method with drying at 60 °C for one cycle. At this time, a total of 10 cycles were carried out, and the Ni-Bimi electrode was prepared by drying at 60 °C.

[0049] <Example 2: Preparation of Ni-Cl-Bimi electrode> NiCl2·6H2O and 2,2'-biimidazole (Bimi) were dissolved in 20 mL of acetonitrile such that the molar ratio was 1:2, and ultrasonic treatment was performed at 25 °C for 30 minutes. Ni foam was immersed in the obtained solution, and Ni-Cl-Bimi was supported on the Ni foam using a dip coating method with drying at 60 °C for one cycle. At this time, a total of 10 cycles were carried out, and by drying at 60 °C, a Ni-Cl-Bimi electrode was fabricated.

[0050] <Example 3: Fabrication of Fe-Bimi Electrode> Fe(NO3)3·9H2O and 2,2'-biimidazole (Bimi) were dissolved in 20 mL of acetonitrile such that the molar ratio was 1:2, and ultrasonic treatment was performed at 25 °C for 30 minutes. Ni foam was immersed in the obtained solution, and Fe-Bimi was supported on the Ni foam using a dip coating method with drying at 60 °C for one cycle. At this time, a total of 10 cycles were carried out, and by drying at 60 °C, an Fe-Bimi electrode was fabricated.

[0051] <Example 4: Fabrication of Fe-Cl-Bimi Electrode> FeCl2·4H2O and 2,2'-biimidazole (Bimi) were dissolved in 20 mL of acetonitrile such that the molar ratio was 1:2, and ultrasonic treatment was performed at 25 °C for 30 minutes. Ni foam was immersed in the obtained solution, and Fe-Cl-Bimi was supported on the Ni foam using a dip coating method with drying at 60 °C for one cycle. At this time, a total of 10 cycles were carried out, and by drying at 60 °C, an Fe-Cl-Bimi electrode was fabricated.

[0052] <Examples 5 - 8> Examples 5 - 8 were each fabricated into Bimi electrodes in the same manner as Examples 1 - 4.

[0053] <Example 9> Example 9 was fabricated into an electrode in the same manner as Example 1.

[0054] [Fabrication of Ni Foam Electrode] Comparative Example 1 is an oxygen generation catalyst composed only of a Ni foam, which is a carrier and does not contain an imidazole ligand.

[0055] <Comparative Example 1: Preparation of Ni foam electrode> In Comparative Example 1, a Ni foam cut into the same size as in Examples 1 to 8 was used as the Ni foam electrode.

[0056] [Preparation of oxygen generation catalyst using imidazole ligand] Comparative Example 2 is an oxygen generation catalyst using imidazole alone as a ligand.

[0057] <Comparative Example 2: Preparation of Ni-imi electrode> Ni(NO3)2·6H2O and imidazole were dissolved in 20 mL of acetonitrile so that the molar ratio was 1:4, and ultrasonic treatment was performed at 25 °C for 30 minutes. The Ni foam was immersed in the obtained solution, and Ni-imi was supported on the Ni foam using a dip coating method with one cycle of drying at 60 °C. At this time, a total of 10 cycles were carried out and dried at 60 °C to fabricate a Ni-imi electrode.

[0058] <Comparative Examples 3, 4> In Comparative Examples 3 and 4, electrodes were fabricated in the same manner as in Comparative Examples 1 and 2, respectively.

[0059] [Oxygen generation reaction] Using the electrodes fabricated in Examples 1 to 9 and Comparative Examples 1 to 4, an oxygen generation reaction was carried out under the following measurement conditions in all cases. The oxygen generation reaction was carried out by an electrochemical reaction in all cases. An electrochemical measurement system (Bio-Logic Science Instruments, SP-150) was used for the electrochemical measurement. An oxygen generation reaction test was carried out while recording the potential at which a current of 15 mA / cm 2 was generated. In the oxygen generation reaction test, the oxygen generation catalysts of Examples 1 to 9 and Comparative Examples 1 to 4 were used for the working electrode, a Pt-foil was used for the counter electrode, and an Ag / AgCl electrode was used for the reference electrode. In all cases, the same reaction area (1 cm2 ) was measured so as to be. The measurement was carried out using a one-compartment cell made of peek without a diaphragm in the reaction cell. As the electrolytic solution, a CO2-saturated 0.5 M KHCO3 + 0.5 M K2CO3 electrolytic solution (pH 7.0) or a CO2-saturated 0.1 M K2B4O7 + 0.2 M K2SO4 electrolytic solution (pH 6.9) was used. In Comparative Example 5, a 1.0 mol / L aqueous potassium hydroxide (1 M KOH) solution (alkaline solution, pH 14) was used as the electrolytic solution.

[0060] [Oxygen generation reaction results] The oxygen generation reaction test results of Examples 1 to 4 and Comparative Examples 1 and 2, and Examples 5 to 9 and Comparative Examples 3 and 4 are shown in Tables 1 and 2. Table 1 shows the results in a CO2-saturated 0.5 M KHCO3 + 0.5 M K2CO3 electrolytic solution (pH 7.0), and Table 2 shows the results in a CO2-saturated 0.1 M K2B4O7 + 0.2 M K2SO4 electrolytic solution (pH 6.9). Note that Example 9 in Table 2 is the result of using the Ni-Bimi electrode in 1 M KOH.

[0061]

Table 1

[0062]

Table 2

[0063] Table 1 shows 15 mA / cm of Examples 1 to 4 and Comparative Examples 1 and 2 2The potential for generating the current through the oxygen evolution reaction was examined over time in a 10-minute measurement, and the value at 10 minutes was shown. At this time, the voltage value is based on the reversible hydrogen electrode (RHE) value, and is shown as the additional required voltage value (= overvoltage value) based on the theoretical potential of 1.23 V for generating oxygen from water. In the Ni foam shown in Comparative Example 1, the voltage value at 10 minutes shows a high value of 0.52. In Example 1 it is 0.39, in Example 2 it is 0.37, in Example 3 it is 0.38, and in Example 4 it is 0.29, all of which can generate oxygen at a voltage 0.1 V to 0.2 V or more lower than that of Comparative Example 1. In Comparative Example 2 using imidazole alone as the ligand, it was 0.52 and there was no effect of voltage reduction by the ligand, so the effects of Examples 1 to 4 are remarkable.

[0064] Similarly, in Examples 5 to 8 where the electrolyte was changed to a CO2-saturated 0.1 M K2B4O7 + 0.2 M K2SO4 electrolyte, oxygen can also be generated at a lower voltage than in Comparative Examples 3 and 4. This is presumably because using a biimidazole ligand with a longer conjugation than the imidazole monomer lowered the LUMO and made it easier for electrons to enter. Under neutral conditions, the number of protons in the ligand increases, and it becomes easier for electrons to enter. In the reaction of generating oxygen from water where protons also contribute in the process of proton donation to water, it is presumed that the necessary potential in the oxygen evolution reaction decreased due to factors such as easier proton donation to water.

[0065] Actually, in the Ni-Bimi electrode of Example 9 under strongly alkaline conditions of about pH 14 where deprotonation occurs like 1 M KOH, the performance slightly decreased.

[0066] From this, it is also clear that the oxygen evolution catalyst having a biimidazole ligand greatly depends on pH. As described above, in the carbon dioxide reduction reaction, an oxygen evolution reaction catalyst having a biimidazole ligand and Ni or Fe specifically has high activity under carbon dioxide-saturated neutral conditions, which is the most important from the perspective of long-term use.

[0067] Thus, in the catalyst for generating oxygen from water using the oxygen generation catalyst of the embodiment, the voltage required for oxygen generation could be reduced.

Claims

1. An oxygen generation catalyst characterized by being a Ni complex of a 2,2'-bimidazole derivative and a Ni salt or an Fe complex of a 2,2'-bimidazole derivative and an Fe salt.

2. The oxygen generation catalyst according to Claim 1, wherein the oxygen generation catalyst is for an oxygen generation reaction that generates oxygen using a neutral aqueous solution saturated with carbon dioxide as a reaction solution.

3. The oxygen generation catalyst according to Claim 1, wherein the Ni salt or the Fe salt is at least one of a nitrate and a chloride salt.

4. An electrode for an oxygen generation reaction, comprising the oxygen generation catalyst according to any one of Claims 1 to 3.

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