Metal complex, imidazolium salt, and ammonia fuel cell

Abstract

To provide a metal complex usable as an ammonia oxidation catalyst having superior durability.SOLUTION: A metal complex comprises a metal atom and a ligand. The ligand includes a tridentate ligand represented by formula (1-1) or formula (1-2), where R1 to R5 each independently represent a hydrogen atom or a substituent. A carbene present in the tridentate ligand, a nitrogen atom forming a pyridine ring, and an oxygen atom of a carboxylate group bonded to the pyridine ring are bound to the metal atom.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a metal complex, an imidazolium salt that can be used as a ligand of the metal complex, and an ammonia fuel cell.

Background Art

[0002] Hitherto, the main energy source has been fossil fuels. However, in recent years, in order to reduce the emission of carbon dioxide, the utilization of renewable energy has been strongly demanded. A hydrogen fuel cell uses hydrogen as fuel and has attracted attention because it does not emit carbon dioxide during power generation. However, hydrogen has the disadvantage that it is difficult to store and transport. This has become an issue when utilizing hydrogen fuel cells.

[0003] As a method for storing and transporting hydrogen, methods using ammonia, compressed hydrogen, liquefied hydrogen, etc. as energy carriers have been studied. Ammonia can be easily liquefied, and its volumetric energy density at 70 MPa is 2.9 times that of compressed hydrogen and 1.7 times that of liquefied hydrogen. Therefore, ammonia is expected as a material for the next-generation energy carrier.

[0004] Also, in recent years, using ammonia as an energy source has attracted attention. Like hydrogen, ammonia does not emit carbon dioxide during combustion. Therefore, technologies for reducing the emission of carbon dioxide by using ammonia as fuel for thermal power generation have been studied.

[0005] In addition, an ammonia fuel cell using ammonia as fuel can be expected to achieve high energy efficiency. Conventionally, examples of ammonia fuel cells include those described in Patent Document 1 and Patent Document 2. Patent Document 1 describes an ammonia fuel cell using a ruthenium complex as a catalyst for the anode.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] In an ammonia fuel cell, in order to use ammonia as fuel, it is necessary to oxidize ammonia to nitrogen. In a conventional ammonia fuel cell, when oxidizing ammonia, an ammonia oxidation catalyst that extracts hydrogen ions and electrons from ammonia is used. Examples of the ammonia oxidation catalyst include metal complexes such as ruthenium complexes. However, conventional ammonia oxidation catalysts have insufficient durability and there is a demand for improving their durability.

[0008] The present invention has been made in view of the above circumstances, and an object thereof is to provide a metal complex that can be used as an ammonia oxidation catalyst with good durability capable of oxidizing a large amount of ammonia. Another object of the present invention is to provide an imidazolium salt that can be used as a compound serving as a ligand of a metal complex when producing a metal complex that can be used as an ammonia oxidation catalyst with good durability. Another object of the present invention is to provide an ammonia fuel cell containing the metal complex of the present invention as an ammonia oxidation catalyst.

Means for Solving the Problems

[0009] In order to solve the above problems, the following means are provided. The metal complex according to one aspect of the present invention comprises a metal atom and a ligand, wherein the ligand contains a tridentate ligand represented by the following formula (1-1) or the following formula (1-2), The carbene of the tridentate ligand, the nitrogen atom forming the pyridine ring, and the oxygen atom of the carboxylate bonded to the pyridine ring are bonded to the metal atom.

[0010] [Chemical formula] (In Formula (1-1) and Formula (1-2), R 1 ~R 5 each represents a hydrogen atom or a substituent.) [Advantages of the Invention]

[0011] The metal complex of the present invention consists of a metal atom and a ligand, and the ligand contains a tridentate ligand represented by Formula (1-1) or Formula (1-2). The metal complex of the present invention has a structure in which a 5-membered ring having a carbene and a pyridine ring are bonded in the tridentate ligand represented by Formula (1-1) or Formula (1-2), and the carbene of the tridentate ligand, the nitrogen atom forming the pyridine ring, and the oxygen atom of the carboxylate bonded to the pyridine ring are coordinately bonded to the metal atom, so it is stable. Therefore, the metal complex of the present invention can be suitably used as an ammonia oxidation catalyst with good durability that can oxidize a large amount of ammonia. Accordingly, an ammonia fuel cell containing the metal complex of the present invention as an ammonia oxidation catalyst is excellent in durability. In addition, the imidazolium salt of the present invention can be suitably used as a compound serving as a ligand of a metal complex when producing the metal complex of the present invention with good durability. [Brief Description of the Drawings]

[0012]

Figure 1

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Figure 13

Embodiments for Carrying Out the Invention

[0013] In order to solve the above problems and realize an ammonia oxidation catalyst with good durability, the present inventors focused on a ligand having an N-heterocyclic carbene (NHC) capable of forming a strong bond between a metal atom and a carbon atom, and conducted intensive studies. As a result, it has been found that a tridentate ligand in which a five-membered ring having a carbene and a pyridine ring are bonded, and a carboxylate is bonded to each of the five-membered ring having a carbene and the pyridine ring, and the carbene of the tridentate ligand, the nitrogen atom forming the pyridine ring, and the oxygen atom of the carboxylate bonded to the pyridine ring are each coordinated to a metal atom. A metal complex may be used.

[0014] In such a metal complex, since a strong bond is formed between the carbene of the tridentate ligand and the metal atom, it has excellent stability. Moreover, since the tridentate ligand has three coordinating elements, a stable metal complex is formed with the metal atom by the chelate effect. From these facts, it is presumed that the above metal complex can be suitably used as an ammonia oxidation catalyst with good durability.

[0015] Furthermore, the present inventors used the above metal complex as an ammonia oxidation catalyst to continuously carry out the oxidation reaction of ammonia, measured the number of moles of nitrogen gas generated from ammonia per 1 mol of the metal complex, and determined the catalyst turnover number (TON). As a result, it was confirmed that the above metal complex has catalytic activity that can be used as an ammonia oxidation catalyst and has excellent durability capable of oxidizing a large amount of ammonia, and the present invention was conceived.

[0016] The present invention includes the following aspects.

[0017] [1] A metal complex comprising a metal atom and a ligand, wherein the ligand includes a tridentate ligand represented by the following formula (1-1) or the following formula (1-2), and the carbene of the tridentate ligand, the nitrogen atom forming the pyridine ring, and the oxygen atom of the carboxylate bonded to the pyridine ring are bonded to the metal atom.

[0018]

Chemical formula

[0019] [2] A metal complex represented by formula (2-1).

[0020] [Chemical formula] (In formula (2-1), L1 to L3 each represent a monodentate ligand. X represents a compound that can become a monovalent anion.)

[0021] [3] A metal complex represented by formula (2-2).

[0022] [Chemical formula] (In formula (2-2), L1 to L3 each represent a monodentate ligand.)

[0023] [4] The metal complex according to [2] or [3], wherein L1 to L3 are all isoquinoline or all phthalazine.

[0024] [5] The metal complex according to [3], wherein L1 and L2 are isoquinoline or phthalazine and L3 is ammonia.

[0025] [6] An imidazolium salt represented by formula (3).

[0026] [Chemical formula] (In formula (3), R1 to R5 each represent a hydrogen atom or a substituent. X - represents a monovalent anion.) (3)

[0027] [7] The imidazolium salt according to [6], wherein R1 to R5 are all hydrogen atoms.

[0028] [8] A fuel cell using a fuel containing ammonia, having an anode containing an ammonia oxidation catalyst, wherein the ammonia oxidation catalyst contains the metal complex described in [1], an ammonia fuel cell.

[0029] Hereinafter, the metal complex, imidazolium salt and ammonia fuel cell of the present invention will be described in detail. [Metal complex] The metal complex of the present embodiment consists of a metal atom and a ligand. The ligand in the metal complex of the present embodiment includes a tridentate ligand represented by the following formula (1-1) or the following formula (1-2). In the metal complex of the present embodiment, the carbene of the tridentate ligand represented by the formula (1-1) or the following formula (1-2), the nitrogen atom forming a pyridine ring, and the oxygen atom of the carboxylate bonded to the pyridine ring are bonded to the metal atom.

[0030] [Chemical formula] (In formula (1-1) and formula (1-2), R1 to R5 each represent a hydrogen atom or a substituent.)

[0031] The metal atom contained in the metal complex of the present embodiment can form a metal complex together with the tridentate ligand represented by the formula (1-1) or the following formula (1-2), and examples thereof include ruthenium (Ru), iron (Fe), iridium (Ir), rhenium (Re), etc. Among these metal atoms, ruthenium (Ru) is preferable because it has catalytic activity that can be used as an ammonia oxidation catalyst and has more excellent durability, resulting in a metal complex.

[0032] In the tridentate ligand represented by the formula (1-1) or the following formula (1-2), R1 to R5 each represent a hydrogen atom or a substituent. When one or more selected from R1 to R3 are substituents, examples of the substituents include an alkyl group (hereinafter sometimes referred to as "Ak"), -OAk, -CF3, -N(Ak)2, -NO2, and the like. The alkyl group (Ak) may be a linear or branched alkyl group such as a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, and their structural isomers, or a cyclic alkyl group such as a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group. When one or more selected from R1 to R3 are alkyl groups, the alkyl group preferably has 1 to 12 carbon atoms, more preferably 1 to 6 carbon atoms.

[0033] When R4 and / or R5 are substituents, examples of the substituents include an alkyl group (Ak). Also, R4 and R5 may be bonded to each other to form a 6-membered ring and form a condensed ring structure with a 5-membered ring having a carbene. In this case, the 6-membered ring formed by bonding R4 and R5 to each other may have a substituent. Examples of the substituents when the 6-membered ring formed by bonding R4 and R5 to each other has a substituent are the same as those of the substituents when one or more selected from R1 to R3 are substituents. Preferably, R1 to R5 are all hydrogen atoms. This is because it becomes a metal complex that can be efficiently produced with fewer manufacturing steps.

[0034] The ligand forming the metal complex of this embodiment may contain a tridentate ligand represented by formula (1-1) or formula (1-2), and preferably contains one or more ligands together with the above tridentate ligand. Specifically, the metal complex of this embodiment preferably contains the above tridentate ligand and three monodentate ligands. The three monodentate ligands that may be contained in the metal complex of this embodiment may be different from each other, or some or all of them may be the same.

[0035] Examples of the monodentate ligand that may be included in the metal complex of the present embodiment include isoquinoline which may have a substituent, phthalazine which may have a substituent, ammonia, acetonitrile, dimethyl sulfoxide, water, pyridine derivatives, and the like. Examples of the substituent in isoquinoline which may have a substituent and phthalazine which may have a substituent include an alkyl group, an alkoxy group, a halogen element, a tertiary amino group, and the like.

[0036] When the metal complex of the present embodiment has a monodentate ligand composed of isoquinoline which may have a substituent and / or phthalazine which may have a substituent together with the above tridentate ligand, it becomes a metal complex having more excellent durability, which is preferable. When the metal complex of the present embodiment has isoquinoline which may have a substituent, it is preferable that the nitrogen atom forming the ring structure of isoquinoline which may have a substituent is bonded to the metal atom. This is because isoquinoline and the metal atom form a strong bond, resulting in a metal complex having excellent stability. Further, when the metal complex of the present embodiment has phthalazine which may have a substituent, it is preferable that the nitrogen atom forming the ring structure of phthalazine which may have a substituent is bonded to the metal atom. This is because phthalazine and the metal atom form a strong bond, resulting in a metal complex having excellent stability.

[0037] Further, it is also preferable that the metal complex of the present embodiment has a monodentate ligand composed of ammonia together with the above tridentate ligand. This metal complex is generated, for example, by a ligand exchange reaction between isoquinoline or phthalazine and ammonia in the oxidation process of ammonia when oxidizing ammonia using a metal complex having the above tridentate ligand and a monodentate ligand composed of isoquinoline or phthalazine. This metal complex also has catalytic activity as an ammonia oxidation catalyst.

[0038] Further, it is also preferable that the metal complex of the present embodiment has a monodentate ligand composed of acetonitrile together with the above tridentate ligand. For example, when ammonia is oxidized using a metal complex having the above tridentate ligand and a monodentate ligand composed of isoquinoline or phthalazine, and acetonitrile as a solvent, it is generated by a ligand exchange reaction between isoquinoline or phthalazine and acetonitrile during the oxidation process of ammonia. This metal complex also has catalytic activity as an ammonia oxidation catalyst.

[0039] The arrangement of the three monodentate ligands that may be included in the metal complex of the present embodiment is not particularly limited. For example, it is preferable that one of the three monodentate ligands is arranged at the equatorial position on the same plane as the above tridentate ligand, and the remaining two are arranged at the axial positions in the upward and downward directions perpendicular to the same plane as the above tridentate ligand, respectively. This is because a more stable metal complex is obtained and better durability is achieved.

[0040] The metal complex of the present embodiment may be, for example, represented by the following formula (2-1). The metal complex represented by formula (2-1) contains ruthenium (Ru) as a metal atom, the tridentate ligand represented by formula (1-1) is bonded to the ruthenium atom, all of R1 to R5 in formula (1-1) are hydrogen atoms, one of the three monodentate ligands (L3) is arranged at the equatorial position, and the remaining two (L1, L2) are arranged at the axial positions, respectively.

[0041] [Chemical formula] (In formula (2-1), L1 to L3 each represent a monodentate ligand. X represents a compound that can be a monovalent anion.)

[0042] In formula (2-1), L1 to L3 each represent a monodentate ligand. L1 to L3 may each be different, or some or all of them may be the same. Examples of L1 to L3 include the same monodentate ligands that may be included in the metal complex of the present embodiment described above.

[0043] When L1 to L3 are the same, it is preferable that all of L1 to L3 are isoquinoline or all are phthalazine. This is because, compared with the case where some of L1 to L3 are different, it can be easily and efficiently produced, and as an ammonia oxidation catalyst, it becomes a metal complex having high catalytic activity and more excellent durability. Most preferably, all of L1 to L3 are phthalazine. This is because it becomes a metal complex with more excellent durability.

[0044] Also, when some of L1 to L3 are different, it is preferable that L1 and L2 arranged at the axial positions are isoquinoline or phthalazine, and L3 arranged at the equatorial position is acetonitrile. The reason is that, for example, when ammonia is oxidized using a metal complex in which all of L1 to L3 are isoquinoline or all are phthalazine and acetonitrile as a solvent, isoquinoline or phthalazine arranged at L3 at the equatorial position in the oxidation process of ammonia easily undergoes a ligand exchange reaction with acetonitrile. This metal complex has high catalytic activity as an ammonia oxidation catalyst, similar to the case where all of L1 to L3 are isoquinoline or all are phthalazine, and is preferable.

[0045] In formula (2-1), X represents a compound that can become a monovalent anion (X - ). Examples of the monovalent anion in the compound that can become a monovalent anion (X - ) include, for example, -BF4 - , -PF6 - , -SbF6 - , -N(CF3SO2)2 - , -CF3SO3 - , -B(Ph) - 4, -B(C6F5)4- 、 -NO3 - 、 -ClO4 - Any one selected from the like can be mentioned. When X is a compound that can be the above-mentioned monovalent anion (X - ), it is easy to isolate and purify, and it becomes a metal complex excellent in productivity.

[0046] The metal complex of this embodiment may be, for example, the one represented by the following formula (2-2). The metal complex represented by formula (2-2) contains ruthenium (Ru) as a metal atom, similar to the metal complex represented by (2-1). In the metal complex represented by formula (2-2), the tridentate ligand represented by formula (1-2) is bonded to the ruthenium atom, all of R1 to R5 in formula (1-2) are hydrogen atoms, and among the three monodentate ligands, one (L3) is arranged in the equatorial position, and the remaining two (L1, L2) are respectively arranged in the axial positions.

[0047]

Chemical formula

[0048] In formula (2-2), L1 to L3 each represent a monodentate ligand. As L1 to L3 in formula (2-2), the same ones as L1 to L3 in the above-mentioned formula (2-1) can be used.

[0049] In addition, in the formula (2-2), when some of L1 to L3 are different, it is preferable that L1 and L2 arranged at the axial positions are isoquinoline or phthalazine, and L3 arranged at the equatorial position is ammonia. The reason is that, for example, when ammonia is oxidized using a metal complex in which all of L1 to L3 are isoquinoline or all are phthalazine, isoquinoline or phthalazine arranged at L3 at the equatorial position in the oxidation process of ammonia easily undergoes a ligand exchange reaction with ammonia. This metal complex has high catalytic activity as an ammonia oxidation catalyst, similar to the case where all of L1 to L3 are isoquinoline or all are phthalazine, and is preferable.

[0050] In addition, since the metal complex represented by the formula (2-2) is a neutral complex, the oxidation potential in the oxidation reaction of ammonia can be lowered. Also, the metal complex represented by the formula (2-2) exhibits high catalytic activity as an ammonia oxidation catalyst even in an alkaline environment. This is because when the tridentate ligand represented by the formula (1-2) binds to the ruthenium atom, the basicity of the carboxylate (COO - ) bonded to the 5-membered ring having a carbene increases, promoting deprotonation from ammonia.

[0051] The metal complex of the present embodiment consists of a metal atom and a ligand, and the ligand contains a tridentate ligand represented by the formula (1-1) or the formula (1-2). In the metal complex of the present embodiment, the tridentate ligand represented by the formula (1-1) or the formula (1-2) has a structure in which a 5-membered ring having a carbene and a pyridine ring are bonded, and the carbene of the tridentate ligand, the nitrogen atom forming the pyridine ring, and the oxygen atom of the carboxylate bonded to the pyridine ring are coordinately bonded to the metal atom, so it is stable. Therefore, the metal complex of the present embodiment can be suitably used as an ammonia oxidation catalyst with good durability that can oxidize a large amount of ammonia.

[0052] [Imidazolium salt] Next, the imidazolium salt of the present embodiment will be described in detail. The imidazolium salt of the present embodiment is represented by the following formula (3).

[0053] [Chemical formula] (In formula (3), R1 to R5 each represent a hydrogen atom or a substituent. X - represents a monovalent anion.)

[0054] In formula (3), R1 to R5 each represent a hydrogen atom or a substituent. When one or more of R1 to R3 selected in formula (3) are substituents, examples of the substituents include the same substituents as those when one or more of R1 to R3 selected in the tridentate ligand represented by formula (1-1) or formula (1-2) are substituents. It is preferable that R1 to R5 are all hydrogen atoms, similar to R1 to R3 in the tridentate ligand represented by formula (1-1) or formula (1-2).

[0055] In formula (3), X - represents a monovalent anion. Specifically, X - is -BF4 - , -PF6 - , -SbF6 - , -N(CF3SO2)2 - , -CF3SO3 - , -B(Ph) - 4, -B(C6F5)4 - , -NO3 - , -ClO4 - and is preferably any one selected from these. When X - is the above monovalent anion, when the imidazolium salt represented by formula (3) is used as a compound serving as a ligand of a metal complex to produce a metal complex, the isolation and purification of the target metal complex are easy and it can be efficiently produced.

[0056] (Method for producing imidazolium salt) The imidazolium salt of the present embodiment can be produced, for example, by the method shown below. Imidazole is reacted with ethyl 6-bromopyridine-2-carboxylate to produce a first intermediate compound. Subsequently, by reacting the first intermediate compound with ethyl bromoacetate, a second intermediate compound having a structure in which a 5-membered ring having a carbene and a pyridine ring are bonded is produced. Thereafter, it can be produced by a method of substituting two ethoxy groups (-OC2H5) of the second intermediate compound with hydroxyl groups (-OH) using ammonium hexafluorophosphate (NH4PF6).

[0057] (Method for producing metal complex) The metal complex of the present embodiment is preferably produced using the imidazolium salt of the present embodiment. The imidazolium salt of the present embodiment can be used as a precursor compound that becomes a tridentate ligand of the metal complex of the present embodiment described above. Specifically, by reacting the imidazolium salt of the present embodiment with the metal atom contained in the target metal complex of the present embodiment described above, all of R1 to R5 in formula (1-1) are hydrogen atoms, and the metal complex of the present embodiment having a tridentate ligand derived from the imidazolium salt of the present embodiment can be produced. Further, when producing a metal complex of the present embodiment containing one or more monodentate ligands together with the above tridentate ligand, after reacting the imidazolium salt of the present embodiment with a metal atom, the obtained metal complex and a compound that becomes a monodentate ligand are reacted to produce the target metal complex.

[0058] [Ammonia fuel cell] FIG. 1 is a cross-sectional schematic view for explaining an example of the ammonia fuel cell of the present embodiment. As shown in FIG. 1, the ammonia fuel cell 100 of the present embodiment has an anode catalyst layer 103, a cathode catalyst layer 105, and an electrolyte membrane 107 sandwiched between the anode catalyst layer 103 and the cathode catalyst layer 105.

[0059] As shown in FIG. 1, a gas diffusion layer 101 is disposed outside the cathode catalyst layer 105. Also, a gas diffusion layer 102 is disposed outside the anode catalyst layer 103. In the present embodiment, a structure composed of the cathode catalyst layer 105 and the gas diffusion layer 101, and a structure composed of the anode catalyst layer 103 and the gas diffusion layer 102 are referred to as a gas diffusion electrode GDE (Gas Diffusion Electrode). The gas diffusion electrode GDE including the anode catalyst layer 103 is the anode, and the gas diffusion electrode GDE including the cathode catalyst layer 105 is the cathode.

[0060] Further, in the present embodiment, a device composed of the gas diffusion layer 102, the anode catalyst layer 103, the electrolyte membrane 107, the cathode catalyst layer 105, and the gas diffusion layer 101 is referred to as a membrane electrode assembly MEA (Membrane Electrode Assembly). As shown in FIG. 1, the membrane electrode assembly MEA is sandwiched between separators 109.

[0061] The electrolyte membrane 107 is made of an ion exchange resin membrane or the like. The ion exchange resin membrane is not particularly limited as long as it can move the hydroxide ions generated in the cathode catalyst layer 105 to the anode catalyst layer 103. Examples of the electrolyte membrane 107 include a cation exchange membrane, an anion exchange membrane, etc., and an anion exchange membrane is preferred.

[0062] Examples of the cation exchange membrane include Nafion membrane (registered trademark, manufactured by DuPont), Aquivion membrane (registered trademark, manufactured by Solvay), Flemion membrane (registered trademark, manufactured by Asahi Glass), Aciplex membrane (registered trademark, manufactured by Asahi Kasei), etc. Examples of the anion exchange membrane include solid polymer membranes containing an anion exchange resin having an anion exchange group such as a quaternary ammonium group or a pyridinium group. Specific examples of the anion exchange membrane include, for example, FAP, FAP-450, FAA-3, FAS, FAB, AMI-7001 manufactured by Fumasep, AMV, AMT, DSV, AAV, ASV, ASV-N, AHO, APS4 manufactured by AGC, etc. Among them, FAP-450, FAA-3 manufactured by Fumasep and ASV-N manufactured by AGC are preferable.

[0063] In the ammonia fuel cell 100 of the present embodiment, the anode catalyst layer 103 is an electrode into which current flows from an external circuit, and is a fuel electrode where the oxidation reaction of ammonia occurs. The anode catalyst layer 103 includes the ammonia oxidation catalyst containing the metal complex of the present embodiment described above, a catalyst carrier supporting the ammonia oxidation catalyst, and an electrolyte. The ammonia oxidation catalyst contains one or more of the metal complexes of the present embodiment, and may contain a known ammonia oxidation catalyst together with the metal complex of the present embodiment.

[0064] The content of the metal complex of the present embodiment contained in the anode catalyst layer 103 is not particularly limited, but is preferably, for example, 0.001 times or more and 0.1 times or less, more preferably 0.002 times or more and 0.01 times or less, the number of moles of ammonia used as fuel. When the content of the metal complex of the present embodiment contained in the anode catalyst layer 103 is 0.001 times or more the number of moles of ammonia, the function of promoting the oxidation reaction of ammonia becomes remarkable. Even if the content of the metal complex exceeds 0.1 times the number of moles of ammonia, the effect of promoting the oxidation reaction of ammonia hardly changes. Also, it is preferably 0.1 times or less the number of moles of ammonia because it is excellent in terms of cost.

[0065] The cathode catalyst layer 105 is an electrode from which current flows out to an external circuit, and is an oxidant electrode where a reduction reaction occurs. The cathode catalyst layer 105 includes a cathode catalyst, a catalyst carrier supporting the cathode catalyst, and an electrolyte. As the cathode catalyst, known catalysts can be used without particular limitation. Examples of the cathode catalyst include metals such as platinum, gold, silver, ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, and alloys of these metals. Platinum is preferred.

[0066] As the catalyst carrier included in the anode catalyst layer 103 and the cathode catalyst layer 105, a carbonaceous material, a metal mesh, a metal foam, etc. can be used. Examples of the carbonaceous material include carbon black such as channel black, furnace black, thermal black, acetylene black, and ketjen black, activated carbon obtained by carbonizing and activating a material containing various carbon atoms, coke, natural graphite, artificial graphite, graphitized carbon, etc. Examples of the metal mesh include those made of nickel, titanium, platinum, etc. Among these, it is preferable to use carbon black as the catalyst carrier because of its large specific surface area and excellent electron conductivity.

[0067] Examples of the electrolyte included in the anode catalyst layer 103 and the cathode catalyst layer 105 include cation exchange ionomers and anion exchange ionomers. Examples of the cation exchange ionomer include fluorine-based sulfonic acid polymers such as Nafion (registered trademark, manufactured by DuPont), Aquivion (registered trademark, manufactured by Solvay), Flemion (registered trademark, manufactured by Asahi Glass), and Aciplex (registered trademark, manufactured by Asahi Kasei). Examples of the anion exchange ionomer include Fumion FAA-3-SOLUT-10 manufactured by Fumasep, A3ver.2 manufactured by Tokuyama, which is an anion conductive ionomer, and AS-4 (A3ver.2 and AS-4 are described, for example, in the journal "Hydrogen Energy System", Vo1.5, No.2, 2010, page 9). Among these, it is preferable to use an anion exchange ionomer as the electrolyte, and it is more preferable to use Fumion FAA-3-SOLUT-10 and AS-4.

[0068] The separator 109 may be any gas-impermeable conductive member. The separator 109 may be, for example, a carbon plate obtained by compressing carbon to make it gas-impermeable, or a solid metal plate. As shown in FIG. 1, a flow path for supplying ammonia is provided between the separator 109 and the anode-side gas diffusion layer 102. Further, a gas passage for supplying oxygen or air is formed between the separator 109 and the cathode-side gas diffusion layer 101.

[0069] In the ammonia fuel cell 100 of the present embodiment, in the cathode catalyst layer 105, oxygen, electrons, and water react (O2 + 4e - + 4H2O → 4OH - ) to generate hydroxide ions. The generated hydroxide ions pass through the electrolyte membrane 107 and move to the anode catalyst layer 103. Further, in the anode catalyst layer 103, ammonia is oxidized and decomposed in the presence of an ammonia oxidation catalyst, or in the presence of an ammonia oxidation catalyst and a base, to generate nitrogen, electrons, and protons (2NH3 → N2 + 6e - + 6H + ).

[0070] In the anode catalyst layer 103 of the ammonia fuel cell 100 of the present embodiment, as the ammonia used as fuel, ammonia gas may be used, or ammonia generated by the reaction of an ammonium salt and a base in the system may be used. When it is necessary to quantify the ammonia used as fuel, it is preferable to generate ammonia in the system.

[0071] When ammonia used as fuel is generated in the system, the ammonium salt used as a raw material may be any ammonium salt that quantitatively generates ammonia by reaction with a base, and is not particularly limited. Examples of the ammonium salt include ammonium trifluoromethanesulfonate, ammonium hexafluorophosphate, ammonium chloride, ammonium bromide, ammonium iodide, ammonium hydroxide, ammonium acetate, ammonium sulfate, ammonium phosphate, etc., and ammonium trifluoromethanesulfonate is preferred.

[0072] The base contained in the anode catalyst layer 103 serves to trap protons generated when ammonia is oxidatively decomposed and to supply the anode-side fuel from the ammonium salt. The base is not particularly limited as long as it serves such a role, and examples include inorganic bases and compounds such as pyridine derivatives, and it may be derived from hydroxide ions generated in the cathode catalyst layer 105.

[0073] As the inorganic base, hydroxides of alkali metals are preferred. The hydroxides of alkali metals are not particularly limited, and examples include sodium hydroxide, potassium hydroxide, lithium hydroxide, etc. Examples of the pyridine derivative include pyridine and pyridine having a substituent at at least one of the 2nd to 6th positions. The substituent is not particularly limited, and examples include an alkyl group, a dialkylamino group, an alkoxy group, an aryl group, a halogen atom, etc. Examples of the halogen atom as the substituent include a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, etc. Specific examples of the pyridine derivative include pyridine, 2,6-lutidine, 2,4,6-collidine, 4-dimethylaminopyridine (DMAP), etc., and among them, 2,4,6-collidine is preferred.

[0074] As the fuel used in the cathode catalyst layer 105 of the ammonia fuel cell 100 of the present embodiment, oxygen, air, humidified oxygen, humidified air, etc. can be used, and oxygen is preferred.

[0075] In the ammonia fuel cell 100 of the present embodiment, the reaction temperature is not particularly limited, and for example, it can be normal temperature (room temperature). The reaction atmosphere may be, for example, an inert atmosphere such as an argon atmosphere, or it may be air. Also, the reaction atmosphere may be a pressurized atmosphere or a normal pressure atmosphere.

[0076] The ammonia fuel cell 100 of the present embodiment is a fuel cell that uses a fuel containing ammonia, and has an anode (gas diffusion electrode GDE including the anode catalyst layer 103) containing an ammonia oxidation catalyst, and the ammonia oxidation catalyst contains the metal complex of the present embodiment having good durability capable of oxidizing a large amount of ammonia. Therefore, the ammonia fuel cell 100 of the present embodiment is excellent in durability.

[0077] As described above in detail for the embodiments of the present invention, each configuration and their combinations in each embodiment are examples, and additions, omissions, substitutions, and other changes of the configuration are possible within the scope not departing from the gist of the present invention.

Example

[0078] (Synthesis of imidazolium salt) 「Example 1」 The imidazolium salt of Example 1 was synthesized by the method shown below.

[0079]

Chemical formula

[0080] Imidazole (2.33 g), ethyl 6-bromopyridine-2-carboxylate (2.62 g), CuI (0.219 g), and potassium carbonate (3.95 g) were weighed into a 100 mL two-necked flask. Under an Ar atmosphere, dehydrated N,N-dimethylformamide (DMF) (50 mL) was added to this two-necked flask, and the mixture was stirred at 120 °C for 16 hours to cause a reaction.

[0081] The suspension after the reaction was diluted with ethyl acetate, and the residue was removed by filtration through celite. The resulting solution was washed three times with water. After combining all the aqueous phases, extraction was performed three times using ethyl acetate (40 mL). All the obtained organic phases were combined, magnesium sulfate was added, and the mixture was dried. The solution from which magnesium sulfate was removed by filtration was dried to dryness using a rotary evaporator, and a pale yellow solid (0.943 g, yield 38%) composed of the compound represented by formula (A1) was obtained.

[0082] The compound represented by formula (A1) in the crude product state (0.869 g), ethyl bromoacetate (0.7 mL), and acetonitrile (60 mL) were weighed into a 200 mL eggplant flask and refluxed for 24 hours. The solvent was distilled off under reduced pressure, and the reaction mixture was washed three times with diethyl ether (10 mL) and further washed three times with toluene (10 mL). The remaining solid was dried under vacuum to obtain a pale yellow solid (1.45 g, yield 94%).

[0083] Regarding the obtained pale yellow solid, using a nuclear magnetic resonance (NMR) apparatus (product name; JEOL ECZS400; manufactured by JEOL Ltd.), 1 1H-NMR measurement was performed, and the structure was identified based on the following results. The NMR measurement was carried out at a measurement frequency of 400 MHz using deuterated chloroform as the solvent. As a result, it was confirmed that the compound was the one represented by the following formula (B1).

[0084]

Chemical formula

[0085] Concentrated hydrochloric acid (10 mL) and water (10 mL) were added to a 200 mL eggplant flask containing the compound represented by formula (B1) in the crude product state (0.869 g), and the mixture was refluxed for 3 hours. Then, an excess amount of NH4PF6 (6.07 g) was added. When dichloromethane was added to the reaction mixture and stirred, a white precipitate formed. Dichloromethane was removed using a rotary evaporator, and the resulting solid was recovered by filtration. The recovered solid was dried under vacuum to obtain a white solid (0.501 g, yield 37%).

[0086] For the obtained white solid, using a nuclear magnetic resonance (NMR) apparatus (trade name: JEOL ECZS400; manufactured by JEOL Ltd.), 1 1H-NMR, 13 13C{ 1 1H}-NMR and 31 31P{ 1 1H}-NMR measurements were performed, and the structure was identified based on the following results. The NMR measurements were carried out at a measurement frequency of 400 MHz using deuterated dimethyl sulfoxide (DMSO-d6) as the solvent. As a result, it was confirmed that the compound was the imidazolium salt of Example 1 represented by the following formula (NHC1´).

[0087]

Chemical formula

[0088] (Synthesis of metal complex) 「Example 2」 Using the imidazolium salt of Example 1 composed of the compound represented by the formula (NHC1´), the metal complex of Example 2 was synthesized by the method shown below.

[0089] The compound represented by the formula (NHC1´) (98.3 mg) and silver oxide (57.9 mg) were weighed into a 20 mL Schlenk tube. In an Ar atmosphere, dehydrated isopropanol (IPA) (7.5 mL) and triethylamine (Et3N) (0.2 mL) were added to the Schlenk tube, and the reaction was carried out at 50 °C for 2 hours under light shielding, then allowed to cool to room temperature. Then, 「Ru(DMSO)4Cl2」 (121.6 mg) was added, and the reaction was carried out at 80 °C for 20 hours under light shielding. Furthermore, phthalazine (326.4 mg) was added, and the reaction was carried out at 80 °C for 24 hours under light shielding.

[0090] The resulting suspension was allowed to cool to room temperature, and the solid was removed by celite filtration. The resulting solution was dried using a rotary evaporator. The resulting crude product was separated and purified by chromatography using a silica gel column and a mixed solvent of dichloromethane and methanol (volume of CH2Cl2: volume of MeOH = 9:1) as the developing solvent. All fractions containing the target product were combined, and the solvent was removed. The resulting solid was dissolved in CH2Cl2 and reprecipitated with ethyl acetate. The solid was collected and dried in vacuum to obtain an orange solid (25.0 mg, yield 13.5%).

[0091] The obtained orange solid was dissolved in methanol to prepare a test specimen, and an electrospray ionization mass spectrometry (ESI-MS) analysis was performed using an ESI-MS device (product name: JEOL AccuTOF JMS-T100LP; manufactured by JEOL Ltd.) The results are shown in Figures 2 and 3. Fig. 2 is a mass spectrum showing the results of electrospray ionization mass spectrometry (ESI-MS) of the metal complex of Example 2. Fig. 3 is an enlarged view showing a part of the mass-to-charge ratio (m / z) region of the mass spectrum shown in Fig. 2.

[0092] The orange solid thus obtained was subjected to the same procedure as in Example 1 for the imidazolium salt, 1 H-NMR, 13 C{ 1 H}-NMR and 31 P{ 1 H}-NMR measurements were carried out, and the results are shown below. FIG. 4 shows the metal complex of Example 2. 1 5 is a chart of H-NMR (400 MHz, CD3OD) measurement of the metal complex of Example 2. 13 C{ 1 1H-NMR (100 MHz, CD3OD) measurement chart. 31 P{ 1 1H-NMR (162 MHz, CD3OD) measurement chart.

[0093] For the orange solid, the structure was identified based on the measurement results of ESI-MS and 1 1H-NMR, 13 13C{ 1 1H}-NMR, 31 31P{ 1 1H}-NMR measurement results. As a result, it was confirmed that the metal complex of Example 2 is [Ru(NHC1-H)(ptl)3]PF6 shown below.

[0094] [Chemical formula]

[0095] (Synthesis of metal complex) "Example 3" Using the imidazolium salt of Example 1 consisting of the compound represented by the formula (NHC1´), the metal complex of Example 3 was synthesized by the method shown below.

[0096] The compound represented by the formula (NHC1´) (195.7 mg) and silver oxide (115.4 mg) were weighed into a 20 mL Schlenk tube. Under an Ar atmosphere, dehydrated isopropanol (IPA) (15 mL) and triethylamine (Et3N) (0.4 mL) were added to the Schlenk tube, and the mixture was reacted at 50 °C for 2 hours under light shielding, then allowed to cool and brought to room temperature. Then, "Ru(DMSO)4Cl2" (241.6 mg) was added thereto, and the mixture was reacted at 80 °C for 16 hours under light shielding. Further, isoquinoline (0.6 mL) was added, and the mixture was reacted at 80 °C for 24 hours under light shielding.

[0097] The obtained suspension was allowed to cool to room temperature, and the solid was removed by filtration through Celite. The obtained solution was dried to dryness using a rotary evaporator. The obtained crude product was separated and purified by chromatography using a silica gel column and a mixed solvent of dichloromethane and methanol (volume of CH2Cl2: volume of MeOH = 9:1) as the developing solvent. All the fractions containing the target product were collected and the solvent was removed. The obtained solid was dissolved in CH2Cl2 and reprecipitated with ethyl acetate. The solid was recovered and dried under vacuum to obtain an orange solid (50.2 mg, yield 12.1%).

[0098] The obtained orange solid was subjected to electrospray ionization mass spectrometry (ESI-MS) in the same manner as for the metal complex of Example 2. The results are shown in Figures 7 and 8. Fig. 7 is a mass spectrum showing the results of electrospray ionization mass spectrometry (ESI-MS) of the metal complex of Example 3. Fig. 8 is an enlarged view showing a part of the mass-to-charge ratio (m / z) region of the mass spectrum shown in Fig. 7.

[0099] The orange solid thus obtained was subjected to the same procedure as in Example 1 for the imidazolium salt, 1 H-NMR and 13 C{ 1 H}-NMR measurements were carried out, and the results are shown below. FIG. 9 shows the metal complex of Example 3. 1 10 is a chart of H-NMR (400 MHz, CDCl3) measurement of the metal complex of Example 3. 13 C{ 1 1H-NMR (100 MHz, CDCl3) measurement chart.

[0100] The results of ESI-MS measurement of the orange solid, and 1 H-NMR, 13 C{ 1 The structure was identified by the results of {H}-NMR measurement. As a result, it was confirmed that the metal complex of Example 3 was [Ru(NHC1-H)(isoq)3]PF6 shown below.

[0101] [ka]

[0102] (Catalytic activity of metal complexes) "Example 4" The content of the metal complex ([Ru(NHC1-H)(ptl)3]PF6) in Example 2 was dissolved in 10 mL of CH3CN so that the content of ammonium trifluoromethanesulfonate (NH4OTf), which is an electrolyte, was 100 mM, and a first test solution was obtained.

[0103] In the first test solution, a glassy carbon (GC) electrode was placed as the working electrode, a Pt electrode was placed as the counter electrode, and silver trifluoromethanesulfonate (AgOTf) was placed as the reference electrode, and cyclic voltammetry (CV) measurement was performed at room temperature. The CV measurement was performed with a measurement range of -0.2 V to 0.8 V and a scan rate of 0.1 V / s, using the redox potential of ferrocene as a standard. The results are shown in FIG. 11.

[0104] A second test solution was prepared in the same manner as the first test solution except that ammonia was contained so as to have a concentration of 100 mM. The CV measurement of the second test solution was performed in the same manner as the CV measurement of the first test solution except that the second test solution was used instead of the first test solution and the measurement range was -0.2 V to 0.55 V. The results are shown in FIG. 11.

[0105] FIG. 11 is a cyclic voltammogram (current-potential curve) obtained by CV measurement of the first test solution and the second test solution. As shown in FIG. 11, a larger current was confirmed in the second test solution containing ammonia than in the first test solution not containing ammonia. This is presumably because in the second test solution, -COOH of the metal complex of Example 2 changed to -COO - and ammonia was oxidized. Therefore, the metal complex of Example 2 has catalytic activity that can be used as an ammonia oxidation catalyst.

[0106] (Durability of metal complex) "Example 5" A test cell having an electrolyte membrane made of a Nafion membrane (registered trademark, manufactured by DuPont) sandwiched between an anode catalyst layer and a cathode catalyst layer was created.

[0107] As the anode catalyst layer, a solution prepared by dissolving a metal complex ([Ru(NHC1-H)(ptl)3]PF6) (2 μmol) of Example 2 as an ammonia oxidation catalyst and ammonium trifluoromethanesulfonate (NH4OTf) (0.5 mmol) as an electrolyte in an acetonitrile (MeCN) solution (5 mL) containing ammonia at a concentration of 1 M as a fuel was used, in which a carbon paper as a catalyst carrier was immersed.

[0108] As the cathode catalyst layer, a Pt mesh as a cathode catalyst and a catalyst carrier was immersed in a MeCN solution (15 mL) containing 100 mM of NH4OTf as an electrolyte. Ag / silver trifluoromethanesulfonate (AgOTf) was used as a reference electrode.

[0109] The reference electrode was immersed in the solution of the anode catalyst layer, and a constant voltage of 0.52 V vs Fc was continuously applied for 24 hours to conduct the oxidation reaction of ammonia. The amount of nitrogen generated thereby was quantified using gas chromatography, and the number of moles of nitrogen gas generated from ammonia per 1 mol of the metal complex of Example 2 was calculated to determine the catalyst turnover number (turnover number; TON). The results are shown in Table 1. 0 / + The oxidation reaction of ammonia was carried out in the same manner as in Example 5 except that the metal complex of Example 3 ([Ru(NHC1-H)(isoq)3]PF6) was used instead of the metal complex of Example 2, and the catalyst turnover number (turnover number; TON) of the metal complex of Example 3 was determined. The results are shown in Table 1.

[0110] "Example 6" The oxidation reaction of ammonia was carried out in the same manner as in Example 5 except that the metal complex of Example 3 was used instead of the metal complex of Example 2, and the catalyst turnover number (turnover number; TON) of the metal complex of Example 3 was determined. The results are shown in Table 1.

[0111] "Comparative Example 1" A constant voltage was continuously applied for 24 hours in the same manner as in Example 5 except that the anode catalyst layer did not contain the metal complex of Example 2. As a result, as shown in Table 1, no nitrogen was generated.

[0112] "Comparative Example 2" The constant voltage was continuously applied for 24 hours in the same manner as in Example 5, except that the anode catalyst layer did not contain the metal complex of Example 2 and ammonia as the fuel. As a result, as shown in Table 1, nitrogen was not generated.

[0113]

Table 1

[0114] From the results of Example 5 and Example 6, Comparative Example 1 and Comparative Example 2 shown in Table 1, it was confirmed that the metal complex of Example 2 and the metal complex of Example 3 had catalytic activity as an ammonia oxidation catalyst, had a large turnover number (TON), and had excellent durability. In particular, Example 5 using the metal complex of Example 2 had a large turnover number (TON).

[0115] (Catalytic activity of metal complex) 「Example 7, Example 8」 In the process of oxidizing ammonia using the catalytic activity of the metal complex ([Ru(NHC1-H)(ptl)3]PF6) of Example 2, it is assumed that the metal complex of Example 7 represented by the following formula (B´) and the metal complex of Example 8 represented by the following formula (C) are generated.

[0116] The metal complex of Example 7 ([Ru(NHC1)(ptl)3]) represented by the following formula (B´) is assumed to be generated by deprotonation of the carboxylic acid site of the metal complex of Example 2. The metal complex of Example 8 ([Ru(NHC1)(ptl)2]-NH3) represented by the following formula (C) is assumed to be generated by a ligand exchange reaction between the phthalazine at the equatorial position of the metal complex of Example 7 and NH3.

[0117]

Chemical formula

[0118] The sum of the Gibbs free energies of the metal complex of Example 7 represented by formula (B´) and NH₃ alone, and the sum of the Gibbs free energies of the metal complex of Example 8 represented by formula (C) and phthalazine alone were calculated by the density functional theory (DFT), respectively. As a result, the sum of the Gibbs free energies of the metal complex of Example 7 represented by formula (B´) and NH₃ alone was 8.4 kcal / mol larger than the sum of the Gibbs free energies of the metal complex of Example 8 represented by formula (C) and phthalazine alone, indicating that the metal complex of Example 8 is stable.

[0119] Also, when assuming that the ligand exchange reaction between the phthalazine of the metal complex of Example 7 represented by formula (B´) and the NH₃ of the metal complex of Example 8 represented by formula (C) has reached equilibrium, the equilibrium constant K was exp(△G / RT) = 1.5×10 6 It was as follows.

[0120] From these facts, it was shown that in a reaction solution at room temperature containing the metal complex of Example 7, the metal complex of Example 8, and ammonia, almost all of the metal complex of Example 7 becomes the metal complex of Example 8. That is, the ligand exchange reaction that changes from the metal complex of Example 7 to the metal complex of Example 8 can occur spontaneously in a reaction solution containing ammonia. Therefore, the metal complex of Example 7 and the metal complex of Example 8 produced in the process of oxidizing ammonia using the catalytic activity of the metal complex of Example 2 have catalytic activity as an ammonia oxidation catalyst, similar to the metal complex of Example 2 (see Example 5), with a large turnover number (TON) and excellent durability.

[0121] "Example 9" The metal complex of Example 3 ([Ru(NHC1-H)(isoq)3]PF6) was dissolved in deuterated acetonitrile (CD3CN), and for the solution immediately after dissolution and the solution after 20 hours from dissolution, by the method shown below 11H-NMR measurement was performed. The nuclear magnetic resonance (NMR) measurement was carried out using an NMR apparatus (trade name: JEOL ECZS400; manufactured by JEOL Ltd.) with a measurement frequency of 400 MHz and using deuterated acetonitrile as the solvent. Figure 12 shows the 1 1H-NMR (400 MHz, CD3CN) measurement chart of the metal complex in the solution immediately after dissolution. Figure 13 shows the 1 1H-NMR (400 MHz, CD3CN) measurement chart of the metal complex in the solution 20 hours after dissolution.

[0122] The 1 1H-NMR results of the metal complex in the solution shown in Figure 12 corresponded to those of the metal complex of Example 3. Also, for the metal complex in the solution 20 hours after dissolution shown in Figure 13, the 1 1H-NMR showed that the peak derived from equatorial isquinoline was almost at the same position as that of free isquinoline. Also, for both the metal complex in the solution immediately after dissolution and the metal complex in the solution 20 hours after dissolution, the peak derived from axial isquinoline was confirmed (see Figures 12 and 13).

[0123] From the 1 1H-NMR measurement results of Figures 12 and 13, it was confirmed that the metal complex in the solution immediately after dissolution was the metal complex of Example 3, and the metal complex in the solution 20 hours after dissolution had the structure represented by the following formula (A). From this, it was confirmed that the ligand at the equatorial position of the metal complex of Example 3 easily undergoes a ligand exchange reaction with CD3CN in the solvent CD3CN. Also, it was confirmed that the ligand at the axial position of the metal complex of Example 3 is more stable than the ligand at the equatorial position and is less likely to undergo ligand exchange.

[0124] [Chemical formula]

[0125] Therefore, in the process of oxidizing ammonia using acetonitrile as a solvent and the catalytic activity of the metal complex of Example 3, it is presumed that the metal complex represented by the formula (A) is generated. Thus, the metal complex represented by the formula (A) has catalytic activity as an ammonia oxidation catalyst, similar to the metal complex of Example 3 (see Example 6), has a large turnover number (TON), and has excellent durability.

Explanation of Symbols

[0126] 100 Ammonia fuel cell, 101, 102 Gas diffusion layer, 103 Anode catalyst layer, 105 Cathode catalyst layer, 107 Electrolyte membrane, 109 Separator, GDE Gas diffusion electrode, MEA Membrane electrode assembly.

Claims

1. Comprising a metal atom and a ligand, The ligand includes a tridentate ligand represented by the following formula (1-1) or the following formula (1-2), A metal complex in which the carbene of the tridentate ligand, the nitrogen atom forming a pyridine ring, and the oxygen atom of the carboxylate bonded to the pyridine ring are bonded to the metal atom. 【Chemical 1】 (In Formula (1-1) and Formula (1-2), R １ ~R ５ each represents a hydrogen atom or a substituent.)

2. A metal complex represented by formula (2-1). 【Chemical Formula 2】 (In formula (2-1), L １ to L ３ each represents a monodentate ligand. X represents a compound that can become a monovalent anion.)

3. A metal complex represented by formula (2-2). 【Chemical Formula 3】 (In formula (2-2), L １ ~L ３ each represents a monodentate ligand.)

4. L １ ~L ３ The metal complex according to claim 2 or claim 3, wherein all of L １ to L ３ are isoquinoline or all are phthalazine.

5. L １ and L ２ is isoquinoline or phthalazine, and L ３ is ammonia, the metal complex according to claim 3.

6. An imidazolium salt represented by formula (3). 【Chemical Formula 4】 (In formula (3), R １ to R ５ each represents a hydrogen atom or a substituent. X － represents a monovalent anion.)

7. R １ ~R ５ The imidazolium salt according to claim 6, wherein all of R １ to R ５ are hydrogen atoms.

8. A fuel cell using a fuel containing ammonia, Having an anode containing an ammonia oxidation catalyst, the ammonia oxidation catalyst containing the metal complex according to claim 1, an ammonia fuel cell.

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