JP2008258152A - Membrane-electrode assembly and fuel cell using this - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a membrane-electrode assembly of high power generating characteristics using platinum-alternating catalyst and a fuel cell. <P>SOLUTION: The membrane-electrode assembly, provided with electrode catalyst made up with the use of a base metal complex, has an exchange current density of 5.0×10<SP>-14</SP>Acm<SP>-2</SP>or more as obtained from a Tafel plot concerning current density and voltage, and that, a Tafel gradient of 450 mV/decade or less. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a membrane-electrode assembly and a fuel cell using the same, and more specifically, the present invention relates to a membrane-electrode assembly having an electrode catalyst using a base metal complex and a fuel cell using the same. .

At present, platinum is generally used as a catalyst for fuel cells, but there are problems such as high costs and the possibility of depleting resources in the future due to limited reserves. is there.
For example, Non-Patent Document 1 describes a membrane-electrode assembly using a cobalt / polypyrrole / carbon composite as an electrode catalyst as an example of using a catalyst that replaces platinum as an electrode catalyst of a fuel cell. Non-Patent Document 2 describes a membrane-electrode assembly using hemoglobin carbide as an electrode catalyst.
However, the power generation characteristics of the membrane-electrode assembly as disclosed in Non-Patent Documents 1 and 2 are considerably lower than those using a platinum catalyst, and it has been eagerly desired to improve the power generation characteristics.
Rajesh Bashyam, Piotr Zelenay, “Nature”, Vol. 443, p.63-66 (2006) Jun Maruyama, Ikuo Abe, “Chemistry of Materials”, Vol.18, No.5, p.1303-1311 (2006)

  An object of the present invention is to provide a membrane-electrode assembly and a fuel cell having high power generation characteristics using a platinum alternative catalyst.

The problems of the present invention have been solved by the following means.
[1] A membrane-electrode assembly having an electrode catalyst using a base metal complex, and an exchange current density i ₀ obtained from a Tafel plot concerning current density and voltage is 5.0 × 10 ⁻⁴ Acm
A membrane-electrode assembly characterized by being ^-2 or more and having a Tafel gradient of 450 mV / decade or less.
[2] The base metal complex is a base metal complex containing a compound having two or more phenol rings and two or more aromatic heterocycles as a ligand. Membrane-electrode assembly.
[3] The base metal complex is a base metal complex containing a base metal atom selected from the group consisting of vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, tantalum and tungsten. The membrane-electrode assembly according to [1] or [2].
[4] The membrane-electrode assembly according to any one of [1] to [3], wherein the number of base metal atoms contained in one molecule of the base metal complex is 1 or more and 10 or less.
[5] The electrode catalyst using the base metal complex is heat-treated at a temperature of 300 ° C. or more and 1200 ° C. or less, and any one of [1] to [4] Membrane-electrode assembly.
[6] A fuel cell comprising the membrane-electrode assembly according to any one of [1] to [5].

The membrane-electrode assembly of the present invention uses a base metal complex catalyst as an electrode catalyst, and exhibits significantly higher power generation characteristics than those using a conventional platinum substitute catalyst. In addition, the cost is lower than that using a platinum catalyst.
Moreover, the fuel cell of the present invention provided with such a membrane-electrode assembly is excellent in power generation efficiency.

Hereinafter, the present invention will be described in detail.
[Membrane-electrode assembly]
A membrane-electrode assembly (hereinafter also referred to as “MEA”) of the present invention is composed of an electrolyte membrane and an electrode catalyst, and has an electrode catalyst on both sides of the electrolyte membrane.
The membrane-electrode assembly of the present invention has an electrode catalyst using a base metal complex.

(Electrode catalyst)
The base metal complex used as an electrode catalyst in the membrane-electrode assembly of the present invention is a metal complex comprising a base metal atom, and the base metal atom may be an uncharged or charged metal ion.

Here, the base metal is a noble metal such as gold, silver, ruthenium, rhodium, palladium, osmium, iridium and platinum as described in “Chemical Dictionary” (1st edition, 1994, Tokyo Kagaku Dojin). Is a metal excluding
Specific examples of base metals include lithium, beryllium, sodium, magnesium, aluminum, potassium, calcium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, rubidium, strontium, yttrium, zirconium , Niobium, molybdenum, cadmium, indium, tin, antimony, tellurium, cesium, barium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, hafnium, tantalum , Tungsten, rhenium, mercury, thallium, lead, bismuth, and the like.

  Among these, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, niobium, molybdenum, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium Dysprosium, holmium, erbium, thulium, ytterbium, lutetium, hafnium, tantalum, tungsten, or rhenium can be preferably applied to the present invention.

More preferably, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, niobium, molybdenum, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium Dysprosium, holmium, erbium, thulium, ytterbium, lutetium, hafnium, tantalum, tungsten or rhenium, more preferably vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, tantalum, Or it is tungsten.
Among these, a base metal atom selected from the group consisting of vanadium, chromium, manganese, iron, cobalt, nickel, and copper is particularly preferable.

  The base metal complex used in the present invention has one or a plurality of the base metal atoms, and the preferable number thereof is 30 or less, more preferably 1 to 10, more preferably 1 to 3. Yes, particularly preferably 1 or 2.

  Base metal complexes used in the present invention include Schiff base metal complexes, metal complexes comprising aromatic hydrocarbons and / or heterocycles, porphyrin metal complexes, porphycene metal complexes, porphyrazine metal complexes, phthalocyanine metal complexes, and naphthalene complexes. Phthalocyanine metal complexes and derivatives of these metal complexes are preferred.

Particularly preferably, the base metal complex used in the present invention is a base metal complex containing a compound having two or more phenol rings (phenol and / or derivatives thereof) and two or more aromatic heterocycles as a ligand. is there.
Preferable examples of such a ligand include compounds represented by the following general formula (I) or (II).

In general formulas (I) and (II), Q ¹ and Q ² represent a divalent aromatic heterocyclic group, and a plurality of Q ¹ may be the same as or different from each other. T ¹ represents a monovalent aromatic heterocyclic group, and a plurality of T ¹ may be the same as or different from each other. R ¹ and R ² represent a hydrogen atom or a substituent, and a plurality of R ¹ and R ² may be the same or different from each other, and adjacent R ¹ or R ² are connected to each other to form a ring May be formed.

  The hydroxy group (OH group) in the general formula (I) or (II) becomes a phenolate group from which a proton has been released, and may be coordinated to a metal atom.

Examples of the substituent represented by R ¹ or R ² in the general formula (I) or (II) include a hydroxyl group, an amino group, a nitro group, a cyano group, a carboxyl group, a formyl group, a sulfonyl group, a halogen atom, and a substituted group. Substituted with 2 monovalent hydrocarbon groups which may be substituted, hydrocarbyloxy group which may be substituted (hydrocarbonoxy group which may be substituted), unsubstituted or substituted monovalent hydrocarbon group An amino group (that is, an optionally substituted hydrocarbon disubstituted amino group), an optionally substituted hydrocarbyl mercapto group (an optionally substituted hydrocarbon mercapto group), an optionally substituted hydrocarbylcarbonyl group (Optionally substituted hydrocarbon carbonyl group), optionally substituted hydrocarbyloxycarbonyl group (optionally substituted hydrocarbon) Xoxycarbonyl group), an aminocarbonyl group substituted with two unsubstituted or substituted monovalent hydrocarbon groups (that is, an optionally substituted hydrocarbon disubstituted aminocarbonyl group), or an optionally substituted group. Examples thereof include a hydrocarbyloxysulfonyl group (an optionally substituted hydrocarbon sulfonyl group).

  Among these, an optionally substituted monovalent hydrocarbon group, an optionally substituted hydrocarbyloxy group, an amino group substituted with two unsubstituted or substituted monovalent hydrocarbon groups, a substituted An optionally substituted hydrocarbyl mercapto group, an optionally substituted hydrocarbylcarbonyl group, and an optionally substituted hydrocarbyloxycarbonyl group are preferred, an optionally substituted monovalent hydrocarbon group, and optionally substituted More preferred are a hydrocarbyloxy group, an amino group substituted with two unsubstituted or substituted monovalent hydrocarbon groups, an optionally substituted monovalent hydrocarbon group, and an optionally substituted hydrocarbyloxy group. Further preferred.

In these groups, the nitrogen atom to which a hydrogen atom is bonded is preferably substituted with a monovalent hydrocarbon group. Moreover, when the group represented by R ¹ or R ² has a plurality of substituents, two substituents may be linked to form a ring.

Examples of the monovalent hydrocarbon group represented by R ¹ or R ² include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, and pentyl. Group, hexyl group, nonyl group, dodecyl group, pentadecyl group, octadecyl group, docosyl group, etc., an alkyl group having 1 to 50 carbon atoms (preferably an alkyl group having 1 to 20 carbon atoms); cyclopropyl group, cyclobutyl group, cyclopentyl Group, cyclohexyl group, cyclononyl group, cyclododecyl group, norbornyl group, adamantyl group, etc., C3-C50 cyclic saturated hydrocarbon group (preferably C3-C20 cyclic saturated hydrocarbon group); ethenyl group , Propenyl group, 3-butenyl group, 2-butenyl group, 2-pentenyl group, 2-hexenyl group, 2-nonenyl group, 2-dode An alkenyl group having 2 to 50 carbon atoms such as a senyl group (preferably an alkenyl group having 2 to 20 carbon atoms); a phenyl group, a 1-naphthyl group, a 2-naphthyl group, a 2-methylphenyl group, a 3-methylphenyl group, 4- Methylphenyl group, 4-ethylphenyl group, 4-propylphenyl group, 4-isopropylphenyl group, 4-butylphenyl group, 4-tert-butylphenyl group, 4-hexylphenyl group, 4-cyclohexylphenyl group, 4- An aryl group having 6 to 50 carbon atoms (preferably an aryl group having 6 to 20 carbon atoms) such as adamantylphenyl group and 4-phenylphenyl group; phenylmethyl group, 1-phenylethyl group, 2-phenylethyl group, 1-phenyl- 1-propyl group, 1-phenyl-2-propyl group, 2-phenyl-2-propyl group, 3-phenyl-1-propyl group 4-phenyl-1-butyl group, 5-phenyl-1-pentyl group, 6-phenyl-1- (preferably an aralkyl group having 7 to 20) aralkyl group having 7 to 50 carbon atoms such as a hexyl group include.

As a monovalent hydrocarbon group represented by R ¹ or R ² , those having 1 to 20 carbon atoms are preferable, those having 1 to 12 carbon atoms are more preferable, and those having 2 to 12 carbon atoms are more preferable. Further, those having 1 to 10 carbon atoms are more preferable, and those having 3 to 10 carbon atoms are more preferable. Moreover, a C1-C10 alkyl group is especially preferable, and a C3-C10 alkyl group is especially preferable.

The hydrocarbyloxy group, hydrocarbyl mercapto group, hydrocarbylcarbonyl group, hydrocarbyloxycarbonyl group, and hydrocarbylsulfonyl group represented by R ¹ or R ² are respectively an oxy group, a mercapto group, a carbonyl group, an oxycarbonyl group, and a sulfonyl group. This is a group formed by bonding one monovalent hydrocarbon group.

“Amino group substituted with two unsubstituted or substituted monovalent hydrocarbon groups” represented by R ¹ or R ² , “amino substituted with two unsubstituted or substituted monovalent hydrocarbon groups” The “carbonyl group” is a group in which two hydrogen atoms in an amino group and an aminocarbonyl group (that is, —C (═O) —NH ₂ group) are substituted with the monovalent hydrocarbon group. . Specific examples and preferred examples of the monovalent hydrocarbon group contained in these are the same as the monovalent hydrocarbon group represented by R ¹ described above.

The monovalent hydrocarbon group represented by R ¹ , hydrocarbyloxy group, hydrocarbyl mercapto group, hydrocarbylcarbonyl group, hydrocarbyloxycarbonyl group, and hydrocarbylsulfonyl group have a part or all of the hydrogen atoms contained in these groups, Halogen atom, hydroxyl group, amino group, nitro group, cyano group, monovalent hydrocarbon group which may be substituted, hydrocarbyloxy group which may be substituted, hydrocarbyl mercapto group which may be substituted, substituted It may be substituted with an optionally substituted hydrocarbylcarbonyl group, an optionally substituted hydrocarbyloxycarbonyl group, an optionally substituted hydrocarbylsulfonyl group, and the like.

R ¹ and R ² are each a hydrogen atom, a methyl group, an ethyl group, a propyl group, an isopropyl group, an n-butyl group, an isobutyl group, sec-butyl, from the viewpoint of improving catalytic activity by heat treatment described later. A group selected from a group, a tert-butyl group, a phenyl group, a methylphenyl group, a naphthyl group, and a pyridyl group is particularly preferable.

In the general formulas (I) and (II), Q ¹ and Q ² are divalent aromatic heterocyclic groups that may be substituted, and a plurality of Q ¹ may be the same or different. Also good.

The optionally substituted divalent aromatic heterocyclic group is a divalent group formed by the loss of two hydrogen atoms from the aromatic heterocyclic ring. Examples of the aromatic heterocyclic ring include divalent aromatic heterocyclic groups represented by the following structural formulas (III-1) to (III-15). Particularly preferred are (III-1) to (III-8). The aromatic heterocyclic ring may be substituted with the substituent of R ¹ or R ² . Moreover, the hetero atom which comprises this aromatic heterocyclic ring may discharge | release a proton, and may coordinate to a metal atom.

T ¹ in the general formula (II) is a monovalent aromatic heterocyclic group which may be substituted, and a plurality of T ¹ may be the same as or different from each other.

  The monovalent aromatic heterocyclic group that may be substituted is a monovalent group that is generated when an aromatic heterocyclic ring loses one hydrogen atom.

Examples of the aromatic heterocycle include pyridine, pyrimidine, pyrazine, pyridazine, pyrrole, furan, thiophene, thiazole, imidazole, oxazole, triazole, indole, benzimidazole, benzofuran, benzothiophene, quinoline, isoquinoline, cinnoline, phthalazine, Quinazole, quinoxaline, benzodiazine, 1,10-phenanthroline, naphthyridine and the like can be mentioned, and pyridine, pyrimidine, pyrazine, pyridazine and pyrrole are preferable. These may be substituted with the substituent of R ¹ or R ² .

The ligand of the base metal complex used in the present invention is preferably a compound represented by the general formula (I) or (II), and specific examples thereof are listed below (Exemplary Compound (IV-1) to (IV-8)), the present invention is not limited to these. In each exemplified compound, ^t Bu represents tert-butyl.

  The compound represented by the general formula (I) or (II) is, for example, “Tetrahedron”, Vol. 55, p. It can be prepared with reference to the description of 8377 (1999). Moreover, it can synthesize | combine by making the precursor which has a heterocyclic ring with reference to description of the said literature, and making it ring-react with a corresponding aldehyde.

  The base metal complex used in the present invention may have another ligand in addition to the ligand. Such other ligands may be ionic or electrically neutral compounds, and when there are a plurality of such other ligands, these other ligands may be the same or different from each other. Also good.

  As the other ligand, as an electrically neutral compound, ammonia, pyridine, pyrrole, pyridazine, pyrimidine, pyrazine, 1,2,4-triazine, pyrazole, imidazole, 1,2,3-triazole, Oxazole, isoxazole, 1,3,4-oxadiazole, thiazole, isothiazole, indole, indazole, quinoline, isoquinoline, phenanthridine, cinnoline, phthalazine, quinazoline, quinoxaline, 1,8-naphthyridine, acridine, 2, 2'-bipyridine, 4,4'-bipyridine, 1,10-phenanthroline, ethylenediamine, propylenediamine, phenylenediamine, cyclohexanediamine, pyridine-N-oxide, 2,2'-bipyridine-N, N'-dioxide, oxami Nitrogen-containing compounds such as dimethylglyoxime and o-aminophenol; water, phenol, oxalic acid, catechol, salicylic acid, phthalic acid, 2,4-pentanedione, 1,1,1-trifluoro-2,4- Oxygen atom-containing compounds such as pentanedione, hexafluoropentanedione, 1,3-diphenyl-1,3-propanedione, 2,2′-binaphthol; sulfur atom-containing compounds such as dimethyl sulfoxide and urea; 1,2-bis Examples thereof include phosphorus atom-containing compounds such as (dimethylphosphino) ethane and 1,2-phenylenebis (dimethylphosphine).

Among these, ammonia, pyridine, pyrrole, pyridazine, pyrimidine, pyrazine, 1,2,4-triazine, pyrazole, imidazole, 1,2,3-triazole, oxazole, isoxazole, 1,3,4-oxa Diazole, indole, indazole, quinoline, isoquinoline, phenanthridine, cinnoline, phthalazine, quinazoline, quinoxaline, 1,8-naphthyridine, acridine, 2,2'-bipyridine, 4,4'-bipyridine, 1,10-phenanthroline , Ethylenediamine, propylenediamine, phenylenediamine, cyclohexanediamine, pyridine-N-oxide, 2,2′-bipyridine-N, N′-dioxide, oxamide, dimethylglyoxime, o-aminophenol, water, phenol, Oxalic acid, catechol, salicylic acid, phthalic acid, 2,4-pentanedione, 1,1,1-trifluoro-2,4-pentanedione, hexafluoropentanedione, 1,3-diphenyl-1,3-propanedione 2,2′-binaphthol.
More preferably ammonia, pyridine, pyrrole, pyridazine, pyrimidine, pyrazine, 1,2,4-triazine, pyrazole, imidazole, 1,2,3-triazole, oxazole, isoxazole, 1,3,4-oxadiazole, Indole, indazole, quinoline, isoquinoline, phenanthridine, cinnoline, phthalazine, quinazoline, quinoxaline, 1,8-naphthyridine, acridine, 2,2'-bipyridine, 4,4'-bipyridine, 1,10-phenanthroline, ethylenediamine, Propylenediamine, phenylenediamine, cyclohexanediamine, pyridine-N-oxide, 2,2'-bipyridine-N, N'-dioxide, o-aminophenol, phenol, catechol, salicylic acid, phthalic acid, 1,3-diph Examples include phenyl-1,3-propanedione and 2,2′-binaphthol.

  Among these, pyridine, pyrrole, pyridazine, pyrimidine, pyrazine, pyrazole, imidazole, oxazole, indole, quinoline, isoquinoline, acridine, 2,2'-bipyridine, 4,4'-bipyridine, 1,10-phenanthroline, phenylenediamine More preferred are pyridine-N-oxide, 2,2′-bipyridine-N, N′-dioxide, o-aminophenol and phenol.

  Examples of the anionic ligand include hydroxide ions, peroxides, superoxides, cyanide ions, thiocyanate ions, fluoride ions, chloride ions, bromide ions, iodide ions and the like, Sulfate ion, nitrate ion, carbonate ion, perchlorate ion, tetrafluoroborate ion, tetraarylborate ion such as tetraphenylborate ion, hexafluorophosphate ion, methanesulfonate ion, trifluoromethanesulfonate ion, p-toluene Sulfonate ion, benzenesulfonate ion, phosphate ion, phosphite ion, acetate ion, trifluoroacetate ion, propionate ion, benzoate ion, hydroxide ion, metal oxide ion, methoxide ion, etho Side ion, and the like.

  Preferably, hydroxide ion, sulfate ion, nitrate ion, carbonate ion, perchlorate ion, tetrafluoroborate ion, tetraphenylborate ion, hexafluorophosphate ion, methanesulfonate ion, trifluoromethanesulfonate ion, p -Toluene sulfonate ion, benzene sulfonate ion, phosphate ion, acetate ion, trifluoroacetate ion are exemplified, among which hydroxide ion, sulfate ion, nitrate ion, carbonate ion, tetraphenylborate ion, trifluoro ion Lomethanesulfonic acid ions, p-toluenesulfonic acid ions, acetate ions, and trifluoroacetic acid ions are more preferable.

  Furthermore, the ions exemplified as the ligand having an anionic property may have as a counter ion for electrically neutralizing the base metal complex itself used in the present invention.

In addition, the base metal complex used in the present invention may have a counter ion having a cationic property that maintains electrical neutrality.
Examples of the counter ion having a cationic property include alkali metal ions, alkaline earth metal ions, tetraalkylammonium ions such as tetra (n-butyl) ammonium ion and tetraethylammonium ion, and tetraarylphosphonium ions such as tetraphenylphosphonium ion. Illustrated.
Specifically, lithium ion, sodium ion, potassium ion, rubidium ion, cesium ion, magnesium ion, calcium ion, strontium ion, barium ion, tetra (n-butyl) ammonium ion, tetraethylammonium ion, tetraphenylphosphonium ion More preferred are tetra (n-butyl) ammonium ion, tetraethylammonium ion, and tetraphenylphosphonium ion.
Among these, tetra (n-butyl) ammonium ions and tetraethylammonium ions are preferable as the counter ion having a cationic property.
The base metal complex used in the present invention can be prepared as follows.
Next, a method for synthesizing the base metal complex used in the present invention will be described. The base metal complex can be obtained by mixing the ligand with a reactant that imparts a base metal atom (hereinafter referred to as “metal imparting agent”). As the metal-imparting agent, base metal acetates, chlorides, sulfates, carbonates and the like exemplified as specific examples can be used.
The synthesis of the ligand involves the addition reaction and oxidation of the organometallic reagent to the heterocyclic compound, as described in the non-patent document Tetrahedron, 1999, 55, 8377. It can be synthesized by a cross-coupling reaction using a metal catalyst. Moreover, it is also possible to synthesize | combine by a cross-coupling reaction in steps using the halide of a heterocyclic ring.
As described above, the base metal complex of the present invention can be obtained by mixing a ligand and a metal imparting agent in the presence of a suitable reaction solvent. Specifically, the reaction solvent includes water, acetic acid, oxalic acid, aqueous ammonia, methanol, ethanol, n-propanol, isopropyl alcohol, 2-methoxyethanol, 1-butanol, 1,1-dimethylethanol, ethylene. Glycol, diethyl ether, 1,2-dimethoxyethane, methyl ethyl ether, 1,4-dioxane, tetrahydrofuran, benzene, toluene, xylene, mesitylene, durene, decalin, dichloromethane, chloroform, carbon tetrachloride, chlorobenzene, 1,2- Examples include dichlorobenzene, N, N′-dimethylformamide, N, N′-dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, acetone, acetonitrile, benzonitrile, triethylamine, and pyridine. It may be used by mixing two or more reaction solvent, but is preferably one ligand and the metal-providing agent is soluble. The reaction temperature is usually −10 to 200 ° C., preferably 0 to 150 ° C., particularly preferably 0 to 100 ° C., and the reaction time is usually 1 minute to 1 week, preferably 5 minutes to 24 hours, particularly preferably 1 hour. It can be carried out in ~ 12 hours. The reaction temperature and reaction time can also be appropriately set depending on the type of ligand and metal imparting agent.
As a means for isolating and purifying the generated base metal complex from the reaction solution after the reaction, an optimum means can be appropriately selected from known recrystallization methods, reprecipitation methods or chromatography methods, and these means can be used. May be combined. Depending on the type of the reaction solvent, the generated base metal complex may be precipitated. The precipitated base metal complex may be separated by filtration, etc., and the base metal complex may be removed by washing or drying as necessary. It can also be isolated and purified.
Examples of the base metal complex used in the present invention include base metal complexes represented by the following formulas (V-1) to (V-8). M ¹ and M ² in the formula represent a base metal atom, and specific examples thereof include the base metal atom. M ¹ and M ² may be the same as or different from each other. Me represents methyl, and ^t Bu represents tert-butyl. In the formula, the charge of the base metal complex is omitted.

  The base metal complex used in the present invention can be used as an electrode catalyst as it is, or can be used as an electrode catalyst by being dispersed in a conductive carrier such as carbon.

The membrane-electrode assembly of the present invention can also use a polymer carrying a base metal complex as an electrode catalyst. Examples of the form of the polymer include a polymer having a residue of a base metal complex and a polymer having a residue of a base metal complex as a repeating unit. The polymer having a residue of the base metal complex means a polymer having a group consisting of an atomic group formed by removing part or all (usually one) of hydrogen atoms in the base metal complex. The polymer to be used is not particularly limited, and examples thereof include conductive polymers, dendrimers, natural polymers, solid polymer electrolytes, polyethylene, polyethylene glycol, and polypropylene. Among these, a conductive polymer and a solid polymer electrolyte are particularly preferable. The conductive polymer is a general term for polymer substances showing metallic or semi-metallic conductivity (Iwanami Riken Dictionary 5th edition: published in 1988). Examples of conductive polymers include polyacetylene and polyacetylene described in “Conductive Polymers” (Shinichi Yoshimura, Kyoritsu Publishing) and “Latest Applied Technologies for Conducting Polymers” (supervised by Masao Kobayashi, CMC Publishing). Derivatives thereof, polyparaphenylene and derivatives thereof, polyparaphenylene vinylene and derivatives thereof, polyaniline and derivatives thereof, polythiophene and derivatives thereof, polypyrrole and derivatives thereof, polyfluorene and derivatives thereof, polyfluorene and derivatives thereof, polycarbazole and derivatives thereof , Polyindole and derivatives thereof, and copolymers of the conductive polymers.
Examples of the solid polymer electrolyte include perfluorosulfonic acid, polyether ether ketone, polyimide, polyphenylene, polyarylene, and a polymer obtained by sulfonating polyarylene ether sulfone.

  The polymer having a residue of the base metal complex as a repeating unit means a polymer having as a repeating unit a group consisting of an atomic group obtained by removing part or all (usually two) of hydrogen atoms in the base metal complex. To do.

  Moreover, you may use the base metal complex used for this invention as an electrode catalyst, after heat-processing. Heat treatment is preferably performed because of the effect of improving catalyst activity and stability.

As the base metal complex used for the heat treatment, only one base metal complex may be used, or two or more base metal complexes may be used.
It is particularly preferable that the base metal complex is dried for 6 hours or more under a reduced pressure condition of 15 to 200 ° C. and 10 Torr (133.22 Pa) or less as a pretreatment for heat treatment. As the pretreatment, a vacuum dryer or the like can be used.

The atmosphere used for the heat treatment of the base metal complex is a reducing atmosphere such as hydrogen or carbon monoxide, an oxidizing atmosphere such as oxygen, carbon dioxide or water vapor, or an inert gas such as nitrogen, helium, neon, argon, krypton, or xenon. It is preferable to be in the presence of an atmosphere, a gas or vapor of a nitrogen-containing compound such as ammonia or acetonitrile, and a mixed gas thereof.
More preferably, the reducing atmosphere is hydrogen or a mixed atmosphere of hydrogen and the inert gas, and the oxidizing atmosphere is oxygen, a mixed atmosphere of oxygen and the inert gas, or an inert gas atmosphere. For example, nitrogen, neon, argon, or a mixed atmosphere of these inert gases.
Moreover, the pressure which concerns on heat processing is although it does not specifically limit, The normal pressure vicinity of about 0.5-1.5 atmospheres is preferable.

  The temperature at the time of heat-treating the base metal complex is preferably 250 ° C. or higher, more preferably 300 ° C. or higher, further preferably 400 ° C. or higher, and particularly preferably 500 ° C. or higher. Moreover, the temperature concerning heat processing becomes like this. Preferably it is 1500 degrees C or less, More preferably, it is 1200 degrees C or less, Most preferably, it is 1000 degrees C or less.

  The processing time required for the heat treatment can be appropriately set depending on the gas used, the temperature, etc. In the sealed or vented state of the gas, the temperature is gradually raised from room temperature and the temperature is lowered immediately after reaching the target temperature. Also good. Among them, it is preferable to gradually heat the base metal complex by maintaining the temperature after reaching the target temperature because durability can be further improved. The holding time after reaching the target temperature is preferably 1 to 100 hours, more preferably 1 to 40 hours, further preferably 2 hours to 10 hours, and particularly preferably 2 to 3 hours. .

  The apparatus for performing the heat treatment is not particularly limited, and examples thereof include a tubular furnace, an oven, a furnace, and an IH hot plate.

  As an electrode catalyst of the membrane-electrode assembly of the present invention, a base metal complex mixture containing (a) a base metal complex and (b) a carbon carrier as described above may be used as an electrode catalyst for a fuel cell.

  The mixing ratio of (a) and (b) in the base metal complex mixture should be set so that the content of (a) is 1 to 70% by mass with respect to the total mass of (a) and (b). Is preferred. The content of the (a) base metal complex is more preferably 2 to 60% by mass, and particularly preferably 3 to 50% by mass.

Examples of the carbon support include carbon particles such as Norit (NORIT), Ketjen Black (Lion), Vulcan (Cabot), Black Pearl (Cabot), Acetylene Black (Chevron). (trade name), fullerenes such as C ₆₀ and C _70, carbon nanotube, carbon nanohorn, carbon fibers and the like.

  Such a base metal complex mixture may be used as an electrode catalyst after heat treatment, as in the case of the base metal complex. The conditions for heat-treating the base metal complex mixture are the same as the conditions for heat-treating the base metal complex.

(Electrolyte membrane)
As the electrolyte membrane of the membrane-electrode assembly of the present invention, a proton conductive electrolyte membrane such as a perfluoro polymer electrolyte membrane, a hydrocarbon polymer electrolyte membrane, or a proton conductive inorganic membrane is preferably used. More preferred are perfluoro polymer electrolyte membranes and hydrocarbon polymer electrolyte membranes, and particularly preferred are perfluoro polymer electrolyte membranes.
Examples of the electrolyte membrane include Nafion 112, Nafion 115, Nafion 117 (all manufactured by DuPont), Flemion (manufactured by Asahi Glass), Aciplex (manufactured by Asahi Kasei) (all are trade names), and the like. Can be used.

  As such an electrolyte membrane, a thin electrolyte membrane is preferably used because the resistance of the fuel cell can be reduced. The preferred film thickness is 200 μm or less, more preferably 150 μm or less, still more preferably 100 μm or less, and particularly preferably 50 μm or less. If the electrolyte membrane is too thin, gas cross-leakage tends to occur. Therefore, the thickness is preferably 1 μm or more, more preferably 3 μm or more, and particularly preferably 5 μm or more.

(Tafel formula)
The Tafel plot applied in the present invention will be described. The Tafel plot refers to a logarithmic plot of current density against overvoltage, as described, for example, in “Electrochemistry Based on Principles” (1st edition, 2006, Yuhuabo).

When the overvoltage is η, the exchange current density is i ₀ , the anode reaction current density is i _a , and the cathode reaction current density is _ic , the Tafel equations for the anode reaction and the cathode reaction are described as follows.
<Anode reaction>
Formula 1 η = −b _a log (i ₀ / Acm ⁻² ) + b _a log (i _a / Acm ⁻² )
<Cathode reaction>
Formula 2 η = b _c log (i ₀ / Acm ⁻² ) −b _c log (| i _c | / Acm ⁻² )

In the above formulas 1 and 2, b _a and b _c are Tafel slopes of the anodic reaction and the cathodic reaction, and are represented by b _a = 2.3 RT / α _a zF and b _c = 2.3 RT / α _c zF, respectively. The Here, α _a and α _c are the transfer coefficients of the anode reaction and the cathode reaction, respectively, and R, T, z, and F are the gas constant, temperature (Kelvin), the number of mobile electrons, and the Faraday constant, respectively.

The exchange current density i ₀ obtained from the Tafel plot is proportional to the reaction rate constant on the electrode, and the larger the value, the faster the reaction on the electrode proceeds.

Therefore, the value of the exchange current density i ₀ is preferably 5.0 × 10 ⁻⁴ Acm ⁻² or more, more preferably 8.0 × 10 ⁻⁴ Acm ⁻² or more, and 1.0 × 10 ⁻³ Acm ^−2. The above is more preferable, and 1.1 × 10 ⁻³ Acm ⁻² or more is particularly preferable.
In addition, the maximum value of the exchange current density i ₀ usually obtained with the conventional platinum catalyst is 1.0 × 10 ⁻² Acm ⁻² .

  The Tafel gradient is a value determined by the transfer coefficient of the reaction and the number of moving electrons, and varies greatly depending on the reversibility of the reaction and the number of electrons involved in the reaction. In the case of the cathode reaction (oxygen reduction reaction), the theoretical value is 69 mV / decade, but the value increases due to the movement of protons and generated water on the electrode, the generation of hydrogen peroxide, etc. It tends to decrease.

The Tafel gradient of the cathode reaction is preferably 450 mV / decade or less, more preferably 400 mV / decade or less, and particularly preferably 350 mV / decade or less.
In addition, the minimum value of the Tafel gradient normally obtained in the conventional platinum catalyst is 69 mV / decade.

As in the present invention, in a membrane-electrode assembly having an electrode catalyst comprising a base metal complex, the exchange current density i ₀ is 5.0 × 10 ⁻⁴ Acm ⁻² or more, and the Tafel slope is 450 mV / If it is less than decade, the power generation characteristics of the fuel cell are improved.

[Fuel cell]
Next, a preferred embodiment of a fuel cell including the above-described membrane-electrode assembly will be described in detail with reference to the accompanying drawings.
FIG. 1 is a longitudinal sectional view of a cell of a fuel cell according to a preferred embodiment of the present invention. As shown in FIG. 1, the fuel cell 10 includes a membrane-electrode assembly 20. The membrane-electrode assembly 20 includes a polymer electrolyte membrane (proton conductive membrane) 12 and a pair of catalyst layers 14a and 14b sandwiching the polymer electrolyte membrane (proton conductive membrane) 12. The fuel cell 10 includes gas diffusion layers 16a and 16b and separators 18a and 18b, and the gas diffusion layers 16a and 16b and separators 18a and 18b are sequentially arranged on both sides of the membrane-electrode assembly 20 so as to sandwich them. Is formed.

The catalyst layers 14a and 14b adjacent to the polymer electrolyte membrane 12 in the membrane-electrode assembly 20 are layers that function as electrode layers in the fuel cell, and one of these is the anode electrode layer and the other is the cathode. It is an electrode layer. The catalyst layers 14a and 14b are preferably formed from a catalyst composition containing a polymer electrolyte such as Nafion (trade name, manufactured by DuPont) and a catalyst.
One of the catalyst layers 14a and 14b, preferably the cathode electrode layer, is preferably an electrode catalyst using the above-described base metal complex. The other catalyst is not particularly limited as long as it can activate a redox reaction with hydrogen or oxygen. For example, a fired metal complex obtained by firing a noble metal, a noble metal alloy, a metal complex, or a metal complex. Etc. Of these, platinum fine particles are preferable as the catalyst, and the catalyst layers 14a and 14b may be formed by supporting fine particles of platinum on particulate or fibrous carbon such as activated carbon or graphite.

When the amount of platinum in the catalyst layer is large, the cost for producing the fuel cell becomes high. Therefore, it is preferable that the amount of platinum in the catalyst layer is small. When the relationship P between the total mass of the base metal complex in the catalyst layer and the total mass of the base metal complex and platinum in the catalyst layer is defined as in the following formula (1),
P = (total mass of platinum in catalyst layer) / (total mass of base metal complex and platinum in catalyst layer) Formula (1)
The value of P is preferably 0.8 or less, more preferably 0.7 or less, still more preferably 0.6 or less, and particularly preferably 0.5 or less. The lower limit of P is 0.

  The gas diffusion layers 16a and 16b are provided so as to sandwich both sides of the MEA 20, and promote the diffusion of the raw material gas into the catalyst layers 14a and 14b. The gas diffusion layers 16a and 16b are preferably made of a porous material having electron conductivity. For example, a porous carbon non-woven fabric or carbon paper is preferable because the raw material gas can be efficiently transported to the catalyst layers 14a and 14b.

These polymer electrolyte membrane 12, catalyst layers 14a and 14b, and gas diffusion layers 16a and 16b constitute a membrane-electrode-gas diffusion layer assembly (MEGA). Such MEGA can be manufactured by the method shown below, for example.
First, a solution containing a polymer electrolyte and a catalyst are mixed to form a slurry of a catalyst composition. The catalyst layer is formed on the gas diffusion layer by applying this onto a carbon nonwoven fabric or carbon paper for forming the gas diffusion layers 16a and 16b by spraying or screen printing, and evaporating the solvent. A laminated body is obtained. And while arrange | positioning a pair of obtained laminated body so that each catalyst layer may oppose, the polymer electrolyte membrane 12 is arrange | positioned among them and these are crimped | bonded. Thus, MEGA having the above-described structure is obtained. In addition, the formation of the catalyst layer on the gas diffusion layer is, for example, after forming the catalyst layer by applying and drying the catalyst composition on a predetermined substrate (polyimide, poly (tetrafluoroethylene), etc.) This can also be performed by transferring to a gas diffusion layer by hot pressing.

  Separator 18a, 18b is formed with the material which has electronic conductivity, As this material, carbon, resin mold carbon, titanium, stainless steel etc. are mentioned, for example. The separators 18a and 18b are preferably provided with grooves (not shown) serving as a flow path for fuel gas or the like on the catalyst layers 14a and 14b side.

  The fuel cell 10 can be obtained by sandwiching MEGA as described above between a pair of separators 18a and 18b and joining them together.

The fuel cell of the present invention is not necessarily limited to the above-described configuration, and may have a different configuration as long as it does not depart from the spirit of the fuel cell.
The fuel cell 10 may be one having the above-described structure sealed with a gas seal body or the like.
The fuel battery cell 10 shown in FIG. 1 is the minimum unit of the polymer electrolyte fuel cell. Since the output of one cell is limited, it is preferable that a plurality of cells are connected in series so that a necessary output can be obtained and used as a fuel cell stack.

  The fuel cell of the present invention can operate as a polymer electrolyte fuel cell when the fuel is hydrogen, and as a direct methanol fuel cell when the fuel is an aqueous methanol solution.

  The fuel cell provided with the membrane-electrode assembly of the present invention can be used as, for example, a small power source for mobile devices such as an automobile power source, a household power source, a mobile phone, and a portable personal computer.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not restrict | limited to an Example.

Example 1
[Preparation of Electrocatalyst (A)]
Base metal complex (A) was synthesized according to the following reaction formula. In addition, the ligand used as the raw material of the base metal complex shown below is "Tetrahedron", Vol. 55, p. 8377 (1999). In the formulae, Me represents a methyl group, Et represents an ethyl group, and Ac represents an acetyl group.

First, in a nitrogen atmosphere, a 200 ml solution of 2-methoxyethanol containing 1.388 g of a ligand and 1.245 g of cobalt acetate tetrahydrate was placed in a 500 ml eggplant flask and heated to 80 ° C. for 2 hours. Upon stirring, a brown solid was formed. This solid was collected by filtration, further washed with 20 ml of 2-methoxyethanol (MeOEtOH) and dried to obtain a base metal complex (A) (yield 1.532 g: yield 74%). “(OAc) ₂ ” on the right side of the above reaction formula indicates that 2 equivalents of acetate ion is present as a counter ion, and “MeOEtOH” indicates that 2-methoxyethanol molecule is present as a ligand. Show.
Elemental analysis value (%): C ₄₉ H ₅₀ Co ₂ N ₄ O ₈
(Calculated values) C: 62.56, H: 5.36, N: 5.96, Co: 12.53.
(Measured value) C: 62.12, H: 5.07, N: 6.03, Co: 12.74.

  Subsequently, the base metal complex (A) and the carbon support (Ketjen Black EC300J, trade name, manufactured by Lion Corporation) were mixed at a mass ratio of 1: 1, stirred in ethanol at room temperature for 15 minutes, and then at room temperature. It was dried for 12 hours under a reduced pressure of 1.5 Torr (199.983 Pa). The mixture was heat-treated at 700 ° C. for 2 hours under a nitrogen stream of 200 ml / min using a tubular furnace having a quartz tube as a furnace core tube to obtain an electrode catalyst (A).

[Preparation of catalyst ink for cathode]
0.20 g of the electrocatalyst (A) obtained above was added to 1.43 mL of a commercially available 5% by mass Nafion solution (solvent: mixture of water and lower alcohol), and further 11.2 mL of ethanol and 2.1 mL of water added. The obtained mixture was subjected to ultrasonic treatment for 1 hour, and then stirred for 5 hours with a stirrer to obtain a cathode catalyst ink.

[Preparation of anode catalyst ink]
0.83 g of platinum-supported carbon (SA50BK, trade name, manufactured by N.E. Chemcat) in which 50% by mass of platinum is supported in 6 mL of a commercially available 5% by mass Nafion solution (solvent: mixture of water and lower alcohol), Furthermore, 13.2 mL of ethanol was added. The obtained mixture was subjected to ultrasonic treatment for 1 hour and then stirred with a stirrer for 5 hours to obtain an anode catalyst ink.

[Production of MEA]
With reference to the method described in Japanese Patent Application Laid-Open No. 2004-089976, the catalyst ink was spray-coated on carbon cloth to prepare an anode electrode and a cathode electrode.
First, a spray method is applied to a 5.2 cm square region in the center of the surface of a carbon cloth (single side water repellent treatment) cut into a 7 cm square corresponding to the gas diffusion layer of the fuel cell. The anode catalyst ink was applied. At this time, the distance from the discharge port to the film was set to 6 cm and the stage temperature was set to 75 ° C. Similarly, after overcoating, the film was left on the stage for 15 minutes, the solvent was removed, and an anode electrode with a gas diffusion layer (carbon cloth with a catalyst layer for the anode) in which 0.6 mg / cm ² of platinum was placed was prepared. Obtained. Similarly, the cathode catalyst ink is spray-coated on a carbon cloth, and a cathode electrode with a gas diffusion layer in which a base metal complex (A) of 0.6 mg / cm ² is disposed (carbon cloth with a cathode catalyst layer) Got.

From the obtained carbon cloth with a catalyst layer, a 5.2 cm square region to which the catalyst layer was applied was cut out, and a carbon cloth with an anode catalyst layer, an electrolyte membrane (Nafion 115, registered trademark, manufactured by DuPont), a cathode catalyst. The carbon cloth with a layer is laminated | stacked one by one so that a catalyst layer may each contact | connect an electrolyte membrane, and it heat-presses for 15 minutes on the conditions of 120 degreeC and 100 kgf / cm < ² > (9.80665 MPa), and a membrane-electrode assembly (MEA) Got.

[Fabrication of fuel cells]
A fuel cell was manufactured using a commercially available JARI standard cell. That is, a carbon separator having a gas passage groove cut on both outer sides of the membrane-electrode assembly obtained above, and a current collector and an end plate are sequentially arranged on the outer side. A fuel cell having an effective membrane area of 25 cm ² was assembled by tightening with a bolt.

[Evaluation of power generation performance of fuel cells]
While maintaining the obtained fuel cell at 80 ° C., humidified hydrogen was supplied to the anode and humidified air was supplied to the cathode. At this time, the back pressure at the gas outlet of the cell was set to 0.1 MPaG. Each source gas was humidified by passing the gas through a bubbler. The water temperature of the hydrogen bubbler was 80 ° C., and the water temperature of the air bubbler was 80 ° C. Here, the hydrogen gas flow rate was 529 mL / min, and the air gas flow rate was 1665 mL / min. The voltage when the current was swept was recorded, and the power generation performance of the fuel cell was evaluated.

FIG. 2 is a current-potential curve of the fuel cell produced as described above. The vertical axis represents the cell voltage (V), and the horizontal axis represents the current density (Acm ⁻² ).
As is apparent from the results of FIG. 2, even when a membrane-electrode assembly using a base metal complex as an electrode catalyst is used, the fuel cell has high power generation characteristics.

(Tafel plot)
Moreover, the Tafel plot was created using the value obtained from the said evaluation. The plot is shown in FIG. FIG. 3 is a Tafel plot of a fuel cell using the membrane-electrode assembly of Example 1. The vertical axis represents overvoltage (V), and the horizontal axis represents the logarithm of current density (log | i _c | / Acm ⁻² ).
As is clear from the results of FIG. 3, the exchange current density i ₀ of the fuel cell using the membrane-electrode assembly of the present invention is 1.22 × 10 ⁻³ Acm ⁻² , and the Tafel slope is 340 mV / decade. Met.

FIG. 1 is a longitudinal sectional view of a cell of a fuel cell according to a preferred embodiment of the present invention. FIG. 2 is a current-potential curve of a fuel cell using the membrane-electrode assembly of Example 1. FIG. 3 is a Tafel plot of a fuel cell using the membrane-electrode assembly of Example 1.

Explanation of symbols

10 Fuel cell (fuel cell)
12 Polymer electrolyte membrane (proton conducting membrane)
14a, 14b Catalyst layers 16a, 16b Gas diffusion layers 18a, 18b Separator 20 Membrane-electrode assembly

Claims (6)

A membrane-electrode assembly having an electrode catalyst using a base metal complex, and an exchange current density i ₀ obtained from a Tafel plot concerning current density and voltage is 5.0 × 10 ⁻⁴ Acm ⁻²
A membrane-electrode assembly having a Tafel gradient of 450 mV / decade or less.   2. The membrane-electrode assembly according to claim 1, wherein the base metal complex is a base metal complex containing a compound having two or more phenol rings and two or more aromatic heterocycles as a ligand. .   The base metal complex is a base metal complex containing a base metal atom selected from the group consisting of vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, tantalum and tungsten. Or the membrane-electrode assembly according to 2.   The membrane-electrode assembly according to any one of claims 1 to 3, wherein the number of base metal atoms contained in one molecule of the base metal complex is 1 or more and 10 or less.   5. The membrane-electrode junction according to claim 1, wherein the electrode catalyst using the base metal complex is heat-treated at a temperature of 300 ° C. or more and 1200 ° C. or less. body. A fuel cell comprising the membrane-electrode assembly according to any one of claims 1 to 5.

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