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 pyridine-based ligand having an amino substituent and a Ni salt, or an Fe complex of a pyridine-based ligand having an amino substituent and an Fe salt. The oxygen evolution catalyst is intended for evolving oxygen using an alkaline aqueous solution having a pH of 10 or higher as a reaction solution.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 including 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 an oxygen generation catalyst synthesized based on both or one of Ni(NO3)2·6H2O and Co(NO3)2·6H2O and 2-methylimidazole. Even the catalyst containing both Ni and Co having the highest performance in Non-Patent Document 1 requires a high voltage of about 300 mV.

[0004] Non-Patent Document 2 describes an oxygen generation reaction using an oxygen generation catalyst synthesized based on both or one of Ni(NO3)2·6H2O and Fe(NO3)3·9H2O and 2-methylimidazole. In the catalyst containing both Ni and Fe having the highest performance in Non-Patent Document 2, the required voltage is about 200 mV, and in the catalyst containing only Ni, the required voltage is as high as 300 mV.

[0005] Non-Patent Document 3 describes an oxygen generation reaction using an oxygen generation catalyst synthesized based on at least one of Ni(NO3)2·6H2O, Co(NO3)2·6H2O, and Fe(NO3)3·9H2O and 2-methylimidazole. Even the catalyst containing three types of Ni, Co, and Fe having the highest performance in Non-Patent Document 3 requires a high voltage of about 250 mV.

[0006] Non-Patent Document 4 describes an oxygen generation reaction using a metal oxide-based oxygen generation catalyst in which NiOOH / FeOOH is formed on a Ni foam. In the reaction of Non-Patent Document 4, an overvoltage of 250 mV is required to generate a current of 10 mA.

[0007] Non-Patent Document 5 describes an oxygen generation reaction using a metal oxide-based oxygen generation catalyst in which NiOOH / FeOOH is formed on a Ni foam. In the catalyst having the highest performance in Non-Patent Document 5, an overvoltage of 290 mV is required to generate a current of 10 mA.

[0008] Non-Patent Document 6 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 6 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 with a pH of about 10.

[0009] Non-Patent Document 7 describes an oxygen generation reaction using a metal oxide-based oxidation generation catalyst in which a NiFeCu metal salt is added on a Ni foam and NiOOH is driven as a catalyst. In Non-Patent Document 7, the reaction is carried out in a 0.5 M KHCO3 aqueous solution not saturated with carbon dioxide. In the catalyst having the highest performance in Non-Patent Document 5, an overvoltage of 385 mV is required to generate a current of 10 mA.

[0010] Non-Patent Documents 1 to 5 are examples of using an alkaline electrolytic solution as a reaction solution for the oxygen generation reaction. It is known that an oxygen generation catalyst using imidazole, particularly 2-methylimidazole, as a ligand as in Non-Patent Documents 1 to 3 can generate oxygen at a lower voltage than an oxygen generation catalyst of a single metal such as Ni foam. In the prior art using 2-methylimidazole, it is known that it can be driven at a relatively low voltage only when using an oxygen generation catalyst combining a plurality of metal salts (such as Fe and Ni, Fe and Ni and Co), and the voltage required for oxygen generation increases when only a single metal species is used.

[0011] In addition, as an oxygen generation catalyst, metal oxide-based oxygen generation catalysts such as Non-Patent Documents 4 and 5 are well known, but even for a metal oxide-based catalyst (FeOOH + NiOOH) synthesized based on an alloy type into which a plurality of atoms are introduced, the overvoltage required is as large as 250 mV or more as described above.

[0012] Non-Patent Documents 6 to 7 are examples of using a neutral electrolyte as a reaction solution for the oxygen generation reaction. There are very few examples of studies on metal complex-based oxygen generation catalysts in a neutral electrolyte. Non-Patent Document 6 is an example of a pyridine-based Ni-Fe-based metal complex-type oxygen generation catalyst in 0.1 M 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.

[0013] There are reports of using a metal oxide-based oxygen generation catalyst even under neutral conditions as in Non-Patent Document 7, but 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 overvoltage required is as large as 385 mV as described above.

[0014] When an oxygen generation catalyst is used in combination with a carbon dioxide reduction catalyst, a two-chamber reaction cell that operates with the oxygen generation catalyst and the carbon dioxide reduction catalyst separated 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, leading 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 described in Non-Patent Documents 3 and 4, 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, and these are known to have the highest catalytic performance in alkaline solutions and are particularly difficult to react in a carbon dioxide-saturated electrolyte.

Prior Art Documents

Non-Patent Documents

[0015]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0016] 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 oxygen generation reaction including the oxygen generation catalyst.

Means for Solving the Problems

[0017] The present invention is an oxygen generation catalyst which is a Ni complex of a pyridine ligand having an amino-based substituent and a Ni salt or an Fe complex of an Fe salt, and the oxygen generation catalyst is for an oxygen generation reaction for generating oxygen using an alkaline aqueous solution having a pH of 10 or more as a reaction solution.

[0018] 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.

[0019] In the oxygen generation catalyst, the amino-based substituent is -NR x (R x is independently a hydrogen atom, or a linear, branched, or cyclic alkyl group having 1 to 6 carbon atoms which may have a substituent, and x = 1 to 2 or 1 to 3).

[0020] The present invention is an electrode for oxygen generation reaction including the above oxygen generation catalyst.

Effects of the Invention

[0021] According to the present invention, in a catalyst for generating oxygen from water, an oxygen generation catalyst capable of reducing the voltage required for oxygen generation, and an electrode for oxygen generation reaction including the oxygen generation catalyst can be provided.

Modes for Carrying Out the Invention

[0022] Embodiments of the present invention will be described below. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

[0023] <Oxygen generation catalyst> The oxygen generation catalyst according to this embodiment is an oxygen generation catalyst that is a Ni complex of a pyridine-based ligand having an amino-based substituent and a Ni salt or an Fe complex of an Fe salt, and the oxygen generation catalyst is for an oxygen generation reaction that generates oxygen using an alkaline aqueous solution with a pH of 10 or more as a reaction solution.

[0024] In the oxygen generation reaction, as an equilibrium potential for generating 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). In fact, almost no oxygen generation reaction occurs when a voltage of 1.23 V is applied, and the reaction does not proceed unless an extra voltage (overvoltage) corresponding to the activation energy is applied. Therefore, the larger the overvoltage, the more it becomes a factor of energy loss. The oxygen generation 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. In order to reduce the energy (voltage) required for the entire reaction, it is also important to reduce the overvoltage of the oxygen generation reaction.

[0025] The inventors of the present invention considered that in an electrochemically driven oxygen generation catalyst, an approach such as bringing water close to the active center and facilitating the acceptance of electrons and protons contributes to a reduction in overvoltage.

[0026] In the oxygen generation catalyst according to this embodiment, a catalyst is formed by complexing with a pyridine ligand that has a high ability to coordinate with metal ions and has a relatively low original LUMO (Lowest Unoccupied Molecular Orbital), making electron injection easy. By applying this catalyst to a carrier, it is considered that the overvoltage in the oxygen generation reaction can be reduced. Furthermore, by using a ligand having an amino-based substituent with a high electron-donating property as a substituent, the LUMO can be further reduced, and it is considered that the necessary potential in the oxygen generation reaction can be reduced because electron injection becomes easier.

[0027] The oxygen generation catalyst according to this embodiment is formed by a combination of a pyridine-based ligand R having an amino-based substituent and a salt of metal M (Ni or Fe).

[0028]

Chemical formula

[0029] 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, and bromide salts, and organic salts such as citrate salts. From the viewpoint of catalyst performance and the like, nitrates and chloride salts are preferable. The Ni salt or Fe salt may be used alone or in combination of two or more.

[0030] The pyridine ring in the pyridine-based ligand R has an amino-based substituent. The amino-based substituent is represented as -NR x (x = 1 to 2 or 1 to 3). R x(R1, R2 or R1, R2, R3) is independently, for example, a hydrogen atom, or a linear, branched, or cyclic alkyl group having 1 to 6 carbon atoms such as a methyl group, an ethyl group, a propyl group, or a butyl group which may have a substituent. Examples of the substituent that the alkyl group may have include a hydroxyl group, an acetoxy group, an amino group, an acetamide group, and a dialkylamino group (the alkyl group has 1 to 6 carbon atoms) such as a dimethylamino group. In addition, R x may be an alkylphosphonic acid group in which a phosphonic acid is linked to the end of the linear alkyl group, an alkylcarboxyl group in which a carboxyl group is linked to the end of the linear alkyl group, an alkylsulfonic acid group in which a sulfonic acid group is linked to the end of the linear alkyl group, an alkylsilanol group in which a silanol group is linked to the end of the linear alkyl group, an alkylmercapto group in which a mercapto group is linked to the end of the linear alkyl group, and derivatives thereof. Among these, from the viewpoint of catalytic performance and the like, the amino-based substituent is preferably a secondary amino group.

[0031] Examples of R include 4-dimethylaminopyridine having a secondary amino group, 4-pyrrolidinopyridine, and the like.

[0032] [Chemical formula] 4-dimethylaminopyridine (DMAP)

[0033] [Chemical formula] 4-pyrrolidinopyridine (4pyp)

[0034] The pyridine-based ligand R having an amino-based substituent may be any pyridine-based ligand having an amino-based substituent and is not particularly limited as long as it is a pyridine-based ligand having an amino-based substituent. However, from the viewpoint of catalytic performance and the like, 4-dimethylaminopyridine and 4-pyrrolidinopyridine are preferred.

[0035] The oxygen generation catalyst according to this embodiment can be obtained by dissolving a pyridine-based ligand having an amino-based substituent 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.

[0036] The solvent may be any solvent that can dissolve or disperse the pyridine-based ligand having an amino-based substituent and the Ni salt or the Fe salt, and there is no particular limitation. For example, acetonitrile, ethanol, methanol, etc. may be used.

[0037] The molar ratio may be, for example, in the range of pyridine-based ligand having an amino-based substituent:Ni salt or Fe salt = 1:0.5 to 1:4.

[0038] The reaction temperature may be, for example, in the range of 0 to 60°C. The reaction time may be, for example, in the range of 0.5 to 24 hours.

[0039] During the reaction, the reaction solution may be stirred or ultrasonic treatment may be performed.

[0040] 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.

[0041] The electrode for oxygen generation reaction according to this embodiment is for an oxygen generation reaction that generates oxygen using an alkaline aqueous solution with a pH of 10 or more, for example, a pH of 10 to 14. There is no particular limitation on the alkaline aqueous solution with a pH of 10 or more. For example, potassium hydroxide (KOH) aqueous solution with a pH of 10 or more, CO2-unsaturated KHCO3 + K2CO3 electrolyte, CO2-unsaturated K2B4O7 + K2SO4 electrolyte, phosphate electrolyte, CO2-unsaturated sodium carbonate, etc. may be mentioned.

[0042] <Electrode for Oxygen Generation Reaction> The electrode for oxygen generation reaction according to this embodiment is an electrode containing the above oxygen generation catalyst. For example, it is an electrode with the above oxygen generation catalyst supported on a substrate.

[0043] The electrode for oxygen generation reaction can be obtained, for example, by using a solution containing the above oxygen generation catalyst and supporting the oxygen generation catalyst on a substrate using a coating method such as dip coating, spin coating, or spray coating.

[0044] As the substrate, Ni foam, carbon paper, Ti mesh, etc. can be used.

[0045] <Oxygen generation reaction> The oxygen generation reaction in which the oxygen generation catalyst and the electrode for oxygen generation reaction according to this 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).

[0046] This specification includes the following embodiments. [1] An oxygen generation catalyst which is a Ni complex of a pyridine-based ligand having an amino-based substituent and a Ni salt or an Fe complex of an Fe salt, The oxygen generation catalyst is for an oxygen generation reaction for generating oxygen using an alkaline aqueous solution with a pH of 10 or more as a reaction solution.

[0047] [2] The oxygen generation catalyst according to [1], The Ni salt or the Fe salt is at least one of a nitrate and a chloride salt.

[0048] [3] The oxygen generation catalyst according to [1] or [2], The amino-based substituent is -NR x (R x is independently a hydrogen atom or a linear, branched, or cyclic alkyl group having 1 to 6 carbon atoms which may have a substituent, and x = 1 to 2 or 1 to 3).

[0049] An oxygen generation reaction electrode containing the oxygen generation catalyst according to any one of [4][1] to [3].

Example

[0050] Hereinafter, examples and comparative examples will be given to describe the present invention more specifically in detail, but the present invention is not limited to the following examples.

[0051] [Preparation of an oxygen generation catalyst using a pyridine-based ligand having an amino-based substituent] As Examples 1 to 10, studies on oxygen generation catalysts using pyridine-based ligands having amino-based substituents are shown.

[0052] <Example 1: Preparation of Ni-DMAP electrode> Ni(NO3)2·6H2O and 4-dimethylaminopyridine (DMAP) 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-DMAP was supported on the Ni foam using a dip coating method with drying at 60 °C for 1 cycle. At this time, a total of 10 cycles were carried out and dried at 60 °C to fabricate a Ni-DMAP electrode.

[0053] <Example 2: Preparation of Ni-Cl-DMAP electrode> NiCl2·6H2O and 4-dimethylaminopyridine (DMAP) 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-Cl-DMAP was supported on the Ni foam using a dip coating method with drying at 60 °C for 1 cycle. At this time, a total of 10 cycles were carried out and dried at 60 °C to fabricate a Ni-Cl-DMAP electrode.

[0054] <Example 3: Preparation of Fe-DMAP electrode> Fe(NO3)3·9H2O and 4-dimethylaminopyridine (DMAP) were dissolved in 20 mL of acetonitrile so that the molar ratio was 1:1, and ultrasonic treatment was performed at 25 °C for 30 minutes. Ni foam was immersed in the obtained solution, and Fe-DMAP was supported on Ni foam using a dip coating method with drying at 60 °C for 1 cycle. At this time, a total of 10 cycles were carried out, and by drying at 60 °C, an Fe-DMAP electrode was fabricated.

[0055] <Example 4: Fabrication of Fe-Cl-DMAP Electrode> FeCl2·4H2O and 4-dimethylaminopyridine (DMAP) were dissolved in 20 mL of acetonitrile so that the molar ratio was 1:1, and ultrasonic treatment was performed at 25 °C for 30 minutes. Ni foam was immersed in the obtained solution, and Fe-Cl-DMAP was supported on Ni foam using a dip coating method with drying at 60 °C for 1 cycle. At this time, a total of 10 cycles were carried out, and by drying at 60 °C, an Fe-Cl-DMAP electrode was fabricated.

[0056] <Example 5: Fabrication of Ni-4pyp Electrode> Ni(NO3)2·6H2O and 4-pyrrolidinopyridine (4pyp) 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-4pyp was supported on Ni foam using a dip coating method with drying at 60 °C for 1 cycle. At this time, a total of 10 cycles were carried out, and by drying at 60 °C, a Ni-4pyp electrode was fabricated.

[0057] <Example 6: Fabrication of Fe-Cl-4pyp Electrode> FeCl2·4H2O and 4-pyrrolidinopyridine (4pyp) 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 resulting solution, and Fe-Cl-4pyp 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-4pyp electrode was fabricated.

[0058] <Examples 7 to 10> In Examples 7 to 8 and 9 to 10, electrodes having a DMAP ligand were fabricated in the same manner as in Examples 2 and 4, respectively.

[0059] [Fabrication of Ni foam electrode] Comparative Example 1 is an oxygen evolution catalyst composed only of Ni foam as a carrier that does not contain a pyridine-based ligand.

[0060] <Comparative Example 1: Fabrication of Ni foam electrode> In Comparative Example 1, Ni foam cut into the same size as in Examples 1 to 10 was used as the Ni foam electrode.

[0061] [Fabrication of oxygen evolution catalyst using pyridine-based ligand without amino substituent] Comparative Examples 2 and 3 are oxygen evolution catalysts using a pyridine-based ligand without an amino substituent as the ligand.

[0062] <Comparative Example 2: Fabrication of Ni-py electrode> Ni(NO3)2·6H2O and pyridine (py) 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 resulting solution, and Ni-py 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-py electrode was fabricated.

[0063] <Comparative Example 3: Fabrication of Ni-Bzpy electrode> Ni(NO3)2·6H2O and 4-benzoylpyridine (Bzpy) 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-Bzpy was supported on Ni foam using a dip coating method with drying at 60 °C as one cycle. At this time, a total of 10 cycles were carried out, and by drying at 60 °C, a Ni-Bzpy electrode was fabricated.

[0064] <Comparative Examples 4-9> In Comparative Examples 4-6 and 7-9, electrodes were fabricated in the same manner as in Comparative Examples 1-3, respectively.

[0065] [Oxygen evolution reaction] Using the electrodes fabricated in Examples 1-10 and Comparative Examples 1-9, an oxygen evolution reaction was carried out under the following measurement conditions. The oxygen evolution reaction was carried out by an electrochemical reaction in all cases. For electrochemical measurement, an electrochemical measurement system (Bio-Logic Science Instruments, SP-150) was used. While recording the potential at which a current of 15 mA / cm 2 was generated, an oxygen evolution reaction test was conducted. In the oxygen evolution reaction test, the oxygen evolution catalysts of Examples 1-10 and Comparative Examples 1-9 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 measurement was carried out so that the same reaction area (1 cm 2 ) was obtained among the samples. The measurement was carried out using a one-compartment cell made of peek without a diaphragm for the reaction cell. As the electrolyte, a 1.0 mol / L potassium hydroxide (1M KOH) aqueous solution (alkaline solution, pH 14), a CO2-unsaturated 0.5M KHCO3 + 0.5M K2CO3 electrolyte (pH 10.0), or a CO2-saturated 0.5M KHCO3 + 0.5M K2CO3 electrolyte (pH 7.0) was used.

[0066] [Oxygen evolution reaction results] The oxygen generation reaction test results of Examples 1 to 4, Comparative Examples 1 to 3, Examples 5 and 6, and Comparative Examples 1 to 3 are shown in Tables 1 and 2. Table 1 shows the results for the oxygen generation reaction catalyst using the DMAP ligand, and Table 2 shows the results for the oxygen generation reaction catalyst using the 4pyp ligand.

[0067] [Table 1]

[0068] [Table 2]

[0069] Table 1 shows the potential for generating a current of 15 mA / cm² in Examples 1 to 4 and Comparative Examples 1 to 3 by the oxygen generation reaction, which was examined over time in a 10-minute measurement, and shows the value at 10 minutes. 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 0.48. Examples 1 is 0.30, Example 2 is 0.26, Example 3 is 0.29, and Example 4 is 0.26, all of which can generate oxygen from a voltage that is 0.1 V to 0.2 V or more lower than that of Comparative Example 1. In Comparative Example 2 using pyridine alone as the ligand, the value was 0.35, and the effect of reducing the voltage by the ligand was poor, so the effects of Examples 1 to 4 are remarkable. 2

[0070] Table 2 shows the potential for generating a current of 15 mA / cm² in Examples 5 and 6 and Comparative Examples 1 to 3 by the oxygen generation reaction, which was examined over time in a 10-minute measurement, and shows the value at 10 minutes. In Comparative Example 3 having a benzoyl group as the substituent, the value was 0.39, whereas in the case of using 4-pyrrolidinopyridine having an amino-based substituent as the substituent as well, Example 5 was 0.31 and Example 6 was 0.27, both of which were able to carry out the oxygen generation reaction at a lower voltage than Comparative Examples 1 to 3. 2 ​

[0071] The results of changing the electrolyte to a CO2-unsaturated 0.5 M KHCO3 + 0.5 M K2CO3 electrolyte (pH 10.0) are shown in Table 3. In Example 7 (using the same Ni-Cl-DMAP electrode as in Example 2), it was 0.49, and in Example 8 (using the same Fe-Cl-DMAP electrode as in Example 4), it was 0.47. In contrast, in Comparative Example 4 (using the same Ni foam as in Comparative Example 1), it was 0.68. In Comparative Example 5 (using the same Ni-py as in Comparative Example 2) without a substituent introduced, it was 0.71, and in Comparative Example 6 (using the same Ni-Bzpy as in Comparative Example 3) having a benzoyl group, it was 0.84. Thus, the effect of the pyridine ligand having a secondary amino substituent shown in Examples 7 and 8 was also confirmed in a weakly alkaline electrolyte.

[0072] The results of changing the electrolyte to a CO2-saturated 0.5 M KHCO3 + 0.5 M K2CO3 electrolyte (pH 7.0) are shown in Table 4. In Example 9 (using the same Ni-Cl-DMAP electrode as in Example 2), it was 0.57, and in Example 10 (using the same Fe-Cl-DMAP electrode as in Example 4), it was 0.53. In contrast, in Comparative Example 7 (using the same Ni foam as in Comparative Example 1), it was 0.70. In Comparative Example 8 (using the same Ni-py as in Comparative Example 2) without a substituent introduced, it was 0.33, and in Comparative Example 9 (using the same Ni-Bzpy as in Comparative Example 3) having a benzoyl group, it was 0.51. From this, the effect of the pyridine ligand having a secondary amino substituent shown in Examples 9 and 10 was effective in a neutral electrolyte compared to the unsupported Ni foam, but it was a limited effect because the pyridine ligand without the introduction of a substituent showed better performance.

[0073]

Table 3

[0074]

Table 4

[0075] Thus, in the oxygen evolution reaction in an alkaline solution with a pH of 10 or higher, by using a pyridine-based ligand having an amino-based substituent, electron injection becomes possible more easily, and it is presumed that the required potential in the oxygen evolution reaction could be reduced.

[0076] Thus, with the oxygen evolution catalyst of the example, in the catalyst for generating oxygen from water, the voltage required for oxygen generation could be reduced.

Claims

1. An oxygen generation catalyst which is a Ni complex of a pyridine ligand having an amino substituent and a Ni salt or an Fe complex of an Fe salt, wherein the oxygen generation catalyst is for an oxygen generation reaction for generating oxygen using an alkaline aqueous solution having a pH of 10 or more as a reaction solution.

2. 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.

3. The oxygen generation catalyst according to claim 1, wherein The amino-based substituent is -NR ｘ (R ｘ is independently a hydrogen atom or a linear, branched or cyclic alkyl group having 1 to 6 carbon atoms which may have a substituent, and x = 1 to 2 or 1 to 3), and is an oxygen generation catalyst characterized by this.

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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