JP2005230648A - Fuel cell cathode catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porphyrin-based oxygen reduction catalyst with a high 4-electron reduction reactivity of oxygen. <P>SOLUTION: The oxygen reduction catalyst comprises a porphyrin complex represented by formula (I) and carried by a conductive carrier. In formula (I), each R is independently a hydrogen atom, a 1-6C alkyl group, a halogen atom, or an amino, hydroxyl, nitro, or cyano group, provided that adjacent R's may be bonded to each other to form a 2-6C methylene chain or an aromatic ring; each R7' is independently an electron-attractive group capable of forming, together with a porphyrin skeleton, a resonance structure; and M is a metal atom selected from the group consisting of Cu, Zn, Fe, Co, Ni, Ru, Pb, Rh, Pd, Pt, Mn, Sn, Au, Mg, Cd, Al, In, Ge, and Ti, provided that M may be bonded to a halogen atom, an oxygen atom, -OH, or =CO. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a fuel cell cathode electrode catalyst capable of reducing oxygen with high efficiency.

A fuel cell is a power generation system that supplies a fuel such as hydrogen or hydrocarbon and an oxidant such as oxygen and directly converts chemical energy obtained by the oxidation-reduction reaction into electric energy. It is known that when oxygen (O ₂ ) is reduced in a fuel cell, superoxide is produced by 1-electron reduction, hydrogen peroxide is produced by 2-electron reduction, and water is produced by 4-electron reduction. Such fuel cells are attracting attention as a clean energy source compared to conventional power generation systems, and have been extensively studied for practical use.

  As the oxygen reduction catalyst, a noble metal electrode catalyst using platinum (Pt), palladium (Pd) or the like is widely used. Such noble metal-based electrode catalysts generally have high oxygen reduction activity, but still have problems in terms of economy.

  On the other hand, it is also known that macrocyclic organic compounds such as phthalocyanine and porphyrin have oxygen reducing ability, and in recent years, development of oxygen reduction catalysts using such macrocyclic organic compounds has been promoted (for example, JP JP-A-57-208073, JP-A-57-208074, JP-A-11-253811, JP-A-2000-157871, and JP-A-2003-109614).

However, the conventional oxygen reduction catalyst using a macrocyclic organic compound has a problem that the oxygen reduction activity is low compared to the noble metal-based electrode catalyst, and the two-electron reduction reaction proceeds more easily than the four-electron reduction reaction. It was far from practical level.
JP-A-57-105969 JP 57-208073 A JP 57-208074 A JP 58-54565 A Japanese Patent Laid-Open No. 11-253811 JP 2000-157871 A JP2003-109614

  An object of the present invention is to provide a porphyrin-based oxygen reduction catalyst having a high 4-electron reduction reaction activity of oxygen.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have solved the problem by using a porphyrin complex in which the meso position of the porphyrin ring is substituted with a group capable of forming an electron-withdrawing and resonant structure with the porphyrin ring. The inventors have found that this can be solved, and have completed the present invention.

That is, the present invention includes the following inventions.
(1) Formula (I):

［式中、
各Ｒは、互いに独立して、水素原子、炭素数１〜６のアルキル基、ハロゲン原子、アミノ基、水酸基、ニトロ基若しくはシアノ基であるか、又は隣接するＲどうしが一緒になって炭素数２〜６のメチレン鎖又は芳香環を形成し；
各Ｒ'は、互いに独立して、電子吸引性で且つポルフィリン骨格と共鳴構造を形成し得る基であり；
Ｍは、Ｃｕ、Ｚｎ、Ｆｅ、Ｃｏ、Ｎｉ、Ｒｕ、Ｐｂ、Ｒｈ、Ｐｄ、Ｐｔ、Ｍｎ、Ｓｎ、Ａｕ、Ｍｇ、Ｃｄ、Ａｌ、Ｉｎ、Ｇｅ及びＴｉからなる群より選択される金属原子であって、ここで該Ｍはハロゲン原子、酸素原子、−ＯＨ又は＝ＣＯと結合していてもよい。］
で表されるポルフィリン系錯体を導電性担体に担持した酸素還元触媒。
（２）各Ｒ'がニトロ基、−ＣＯＯＲ^１又は−Ｃ（＝Ｏ）ＮＲ^２Ｒ^３（ここで、Ｒ^１、Ｒ^２及びＲ^３は、互いに独立して、水素又は炭素数１〜６のアルキル基を示す）である前記（１）記載の酸素還元触媒。
（３）各Ｒが水素原子である前記（１）又は（２）記載の酸素還元触媒。
（４）前記（１）〜（３）のいずれか１項記載の酸素還元触媒を用いる燃料電池カソード電極触媒。

[Where:
Each R is independently of each other a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, a halogen atom, an amino group, a hydroxyl group, a nitro group, or a cyano group, or adjacent R's together Form 2-6 methylene chains or aromatic rings;
Each R ′ is independently of each other an electron-withdrawing group and can form a resonance structure with the porphyrin skeleton;
M is a metal atom selected from the group consisting of Cu, Zn, Fe, Co, Ni, Ru, Pb, Rh, Pd, Pt, Mn, Sn, Au, Mg, Cd, Al, In, Ge, and Ti. Here, the M may be bonded to a halogen atom, an oxygen atom, —OH or ═CO. ]
The oxygen reduction catalyst which carry | supported the porphyrin type complex represented by these on the electroconductive support | carrier.
(2) Each R ′ is a nitro group, —COOR ¹ or —C (═O) NR ² R ³ (wherein R ¹ , R ² and R ³ are independently of each other hydrogen or C 1-6. The oxygen reduction catalyst according to (1) above.
(3) The oxygen reduction catalyst according to (1) or (2), wherein each R is a hydrogen atom.
(4) A fuel cell cathode electrode catalyst using the oxygen reduction catalyst according to any one of (1) to (3).

  The present invention provides a porphyrin-based oxygen reduction catalyst having significantly higher oxygen reduction activity than conventional porphyrin complexes. The oxygen reduction catalyst of the present invention is useful as a cathode electrode catalyst for fuel cells.

The present invention is described in detail below.
As a material for the oxygen reduction catalyst of the present invention, a porphyrin complex in which the meso position is substituted with a group that is electron withdrawing and can form a resonance structure with the porphyrin skeleton is used. Specifically, the following formula (I):

［式中、
各Ｒは、互いに独立して、水素原子、炭素数１〜６のアルキル基、ハロゲン原子、アミノ基、水酸基、ニトロ基若しくはシアノ基であるか、又は隣接するＲどうしが一緒になって炭素数２〜６のメチレン鎖又は芳香環を形成し；
各Ｒ'は、互いに独立して、電子吸引性で且つポルフィリン骨格と共鳴構造を形成し得る基であり；
Ｍは、Ｃｕ、Ｚｎ、Ｆｅ、Ｃｏ、Ｎｉ、Ｒｕ、Ｐｂ、Ｒｈ、Ｐｄ、Ｐｔ、Ｍｎ、Ｓｎ、Ａｕ、Ｍｇ、Ｃｄ、Ａｌ、Ｉｎ、Ｇｅ及びＴｉからなる群より選択される金属原子であって、ここで該Ｍはハロゲン原子、酸素原子、−ＯＨ又は＝ＣＯと結合していてもよい。］
で表されるポルフィリン錯体を用いる。

[Where:
Each R is independently of each other a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, a halogen atom, an amino group, a hydroxyl group, a nitro group, or a cyano group, or adjacent R's together Form 2-6 methylene chains or aromatic rings;
Each R ′ is independently of each other an electron-withdrawing group and can form a resonance structure with the porphyrin skeleton;
M is a metal atom selected from the group consisting of Cu, Zn, Fe, Co, Ni, Ru, Pb, Rh, Pd, Pt, Mn, Sn, Au, Mg, Cd, Al, In, Ge, and Ti. Here, the M may be bonded to a halogen atom, an oxygen atom, —OH or ═CO. ]
The porphyrin complex represented by these is used.

The porphyrin complex used in the present invention is characterized in that the meso position of the porphyrin skeleton is substituted with a group R ′ that is electron withdrawing and can form a resonance structure with the porphyrin skeleton. Here, the “resonance structure” in this specification means a state where electrons are delocalized. Examples of such an electron-withdrawing group that can form a resonance structure with the porphyrin skeleton include, for example, a nitro group, —COOR ¹ or —C (═O) NR ² R ³ (wherein R ¹ , R ² and R ³ may be independently of each other hydrogen or an alkyl group having 1 to 6 carbon atoms (preferably an alkyl group having 1 to 3 carbon atoms). The alkyl group having 1 to 6 carbon atoms may be either linear or branched, and examples thereof include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, s Examples include -butyl group, t-butyl group, isobutyl group, n-pentyl group, s-pentyl group, isopentyl group, neopentyl group and the like.

  Conventionally, porphyrin complexes used for cathode electrode catalysts have a structure in which the shape of the entire molecule is non-planar or bent because the functional group at the meso position is a bulky group such as a phenyl group. For this reason, the conventional porphyrin complex cannot be in intimate contact with the surface of the conductive carrier, and as a result, there is a problem that the efficiency of electron transfer between the porphyrin complex and the conductive carrier is poor (see FIG. 1 (a)).

  On the other hand, in the present invention, the functional group R ′ as described above, for example, the nitro group is bonded to the meso position of the porphyrin skeleton, whereby the electrons on the porphyrin ring are attracted to the R ′ group, and the nitro group is resonant. Since it takes a structure, the bond between the porphyrin ring and the nitro group is double-bonded, so that the porphyrin complex represented by the formula (I) has a planar structure geometrically. As a result, the porphyrin complex used in the present invention can be in intimate contact with the surface of the conductive carrier, and sufficient electronic conductivity can be ensured (FIG. 1 (b)). In addition, the porphyrin complex used in the present invention has a planar structure, so that the complexes are easily overlapped to form a dimer, and therefore, the 4-electron reduction reaction of oxygen is more likely to proceed as compared with the conventional one. . Furthermore, since the R ′ group is an electron-withdrawing group, the electron density at the porphyrin ring and the central metal decreases, and as a result, the dissociation of oxygen ions is promoted. These effects significantly increase the oxygen reduction current in the cathode electrode catalyst of the present invention as compared with the conventional porphyrin-based catalyst.

  As described above, since the porphyrin complex of the formula (I) used in the present invention is preferably flat, the substituent R is not a bulky group or substantially (substantially) identical to the porphyrin ring. A group constituting a plane or a group having a small molecular weight is preferable. Examples of such R include a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, a halogen atom, an amino group, a hydroxyl group, a nitro group, and a cyano group. Alternatively, adjacent Rs may be joined together to form a methylene chain or aromatic ring having 2 to 6 carbon atoms. The alkyl group having 1 to 6 carbon atoms may be either linear or branched, and examples thereof include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, s Examples include -butyl group, t-butyl group, isobutyl group, n-pentyl group, s-pentyl group, isopentyl group, neopentyl group and the like. Examples of the aromatic ring include condensed aromatic rings such as a benzene ring or a naphthalene ring.

  The porphyrin complex used in the present invention is one in which a porphyrin skeleton having the above group and a metal atom M form an N 4 -chelate structure. Examples of the central metal atom M include Cu, Zn, Fe, Co, Ni, Ru, Pb, Rh, Pd, Pt, Mn, Sn, Au, Mg, Cd, Al, In, Ge, and Ti. . Further, a halogen atom, an oxygen atom, a hydroxyl group, CO, or the like may be further coordinated with these metal atoms M.

Next, the manufacturing method of the porphyrin complex used by this invention is demonstrated.
The porphyrin ring in which the meso position is substituted with the substituent R ′ can be produced by a usual method using a pyrrole compound and an aldehyde compound. For example, a porphyrin ring in which R ′ is a nitro group and R is a hydrogen atom can be produced as follows.

（式中、破線はニトロ基とポルフィリン環との間の結合が二重結合性を帯びてポルフィリン環内だけでなく官能基上まで電子が非局在化していることを示す。）

(In the formula, the broken line indicates that the bond between the nitro group and the porphyrin ring is double-bonded and the electrons are delocalized not only within the porphyrin ring but also onto the functional group.)

First, a solvent and an acid are added to the reaction vessel and heated (for example, about 50 ° C. to 100 ° C.). There, pyrrole and NO ₂ -CHO are added and stirred. Although the stirring time depends on the reaction temperature, it is usually about 1 to 5 hours. After completion of the reaction, the reaction solution is washed with an alkaline aqueous solution such as an aqueous sodium hydroxide solution and then with water. The organic layer is separated and dried using magnesium sulfate and the solvent is distilled off. The residue is then purified by conventional purification means such as chromatography or recrystallization to obtain porphyrin having a nitro group substituted at the meso position.

  Next, a chelate is formed by the substituted porphyrin obtained as described above and a metal atom. In order to form a chelate, it is easily formed by mixing a salt or complex of a desired metal atom and a substituted porphyrin. For example, in order to obtain a cobalt porphyrin complex, the substituted porphyrin synthesized as described above is added and sufficiently dissolved in a solvent such as DMF, and then, for example, cobalt acetate tetrahydrate is added thereto to add an argon atmosphere. The target porphyrin complex is obtained by heating under reflux and purifying the reaction mixture by a conventional method.

  The oxygen reduction catalyst of the present invention is formed by supporting the porphyrin complex (I) as described above on a conductive carrier by a usual method. For example, the oxygen reduction catalyst of the present invention is produced by preparing a slurry or paste containing the porphyrin complex (I) and immersing the conductive carrier in the slurry or paste, or applying the slurry or paste to the carrier and drying it. be able to.

  The conductive carrier is not particularly limited. For example, a carbon material such as carbon black, graphite, carbon fiber, carbon nanotube, or carbon nanofiber may be used because it has good conductivity and is inexpensive. The conductive carrier is preferably in the form of a powder because it has a large surface area per unit weight. In this case, it is desirable that the particle size of the conductive carrier particles be 0.03 μm or more and 0.1 μm or less. Furthermore, it is desirable that the conductive carrier particles have a structure in which primary particles are connected.

  The supported amount of the central metal atom M of the porphyrin complex with respect to the conductive support is usually 1 to 10% by weight, more preferably 4 to 7% by weight.

  In addition to the porphyrin complex (I), the oxygen reduction catalyst of the present invention may be mixed with another oxygen 4-electron reduction catalyst using a noble metal such as platinum or palladium and supported on a carrier.

  The oxygen reduction catalyst of the present invention is useful as a fuel cell cathode electrode catalyst for a polymer electrolyte fuel cell or the like.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the scope of the present invention is not limited to the examples.

Example 1: Synthesis of 5,10,15,20-tetranitroporphyrin

BF ₃ · OEt ₂ (470 mg, 3.3 mmol) was added as an acid catalyst to a mixture of pyrrole (610 mg, 10 mmol) and nitraldehyde (750 mg, 10 mmol), and the mixture was reacted at room temperature for 2 hours with stirring. Thus, a dehydration condensation product was obtained (reaction 1). Subsequently, an intramolecular ring-reaction of this dehydration condensation product was performed (Reaction 2). In this cyclization reaction, the cyclization reaction was carried out under high dilution conditions (dissolved in 1000 ml of chloroform or dichloromethane to make a 0.01 M solution) in order to avoid condensation of molecules and polymerization. The cyclized product was then reacted with the oxidizing agent p-chloranil (1910 mg, 7.7 mmol) and the crude product was purified by conventional methods to give the title compound.

Example 2: Synthesis of porphyrin complex

The cobalt complex was synthesized from the tetranitroporphyrin obtained in Example 1.
The tetranitroporphyrin obtained in Example 1 was dissolved in dimethylformamide (DMF), 5 equivalents of cobalt acetate was added to this solution, and the mixture was refluxed for 3 hours. The mixture was allowed to cool to room temperature, and deionized water / ethanol was added to precipitate a tetranitroporphyrin complex. Suction filtered, the filtrate was washed with deionized water / ethanol and dried in vacuo to give the title tetranitroporphyrin cobalt complex.

Example 3: Support of porphyrin complex on conductive carrier (production of porphyrin complex catalyst)
A conductive carrier (carbon) was added to acetone or chloroform and dispersed by stirring. To this, the cobalt complex of tetranitroporphyrin obtained in Example 2 was added and stirred for 2 hours. After completion of the stirring, the solution was distilled off under reduced pressure and dried at 75 ° C., and this was heat-treated in an inert gas (helium, argon, etc.) atmosphere to obtain carbon carrying a porphyrin complex.

Example 4: Electrochemical measurement The porphyrin complex catalyst obtained in Example 3 was electrochemically measured. Moreover, the tetramethyl porphyrin complex whose meso position is a methyl group was used as a porphyrin complex of a comparative example.

Preparation of GC electrode (working electrode) A Nafion solution was added so that the weight ratio of carbon to Nafion (registered trademark) in the porphyrin complex catalyst was 10: 1. Next, 40 parts by weight of ethanol was added to 1 part by weight of the porphyrin complex catalyst to prepare an ink. The ink was taken in a microsyringe, thinly applied onto the electrode, and dried to obtain a GC electrode.

By rotating the measuring electrode of the oxygen reduction current value of the oxygen reduction reaction by rotating disk electrode (RDE) measurement, the current value of the oxygen reduction reaction was evaluated by the disk current at that time. The measurement conditions are as follows.
Equipment used: Seiko EG & G
Cell container: Pyrex (registered trademark) glass cell solution: 0.1 N HClO ₄
Working electrode: GC electrode Reference electrode: Ag / AgCl (sat. KCl)
Counter electrode: Platinum electrode sweep speed: 15 mV / s
Sweep range: -150 to 1000 mV
Electrode rotation speed: 400, 625, 900, 1225, 1600, 2025 rpm
Measurement temperature: 27 ° C

  The results of the peak potential and the number of reaction electrons for the porphyrin complex-modified electrode determined by the above RDE measurement are shown in the following table.

  As shown in the above results, the oxygen reduction current when using the oxygen reduction catalyst of the present invention is clearly increased compared to that of the comparative example, and the oxygen reduction catalyst of the present invention has high conductivity and oxygen reduction activity. It was shown that.

  The porphyrin complex used in the present invention has high conductivity and oxygen reduction activity, and is useful, for example, as a fuel cell cathode electrode catalyst.

It is the figure shown about the adhesiveness of a porphyrin complex catalyst and an electroconductive support | carrier. (A) shows the case of a porphyrin complex having a bulky functional group, and (b) shows the case of a porphyrin complex used in the present invention. It is a figure which shows the measurement result (oxygen reduction current value) of RDE of Example 4.

Claims (4)

Formula (I):

[Where:
Each R is independently of each other a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, a halogen atom, an amino group, a hydroxyl group, a nitro group, or a cyano group, or adjacent R's together Form 2-6 methylene chains or aromatic rings;
Each R ′ is independently of each other an electron-withdrawing group and can form a resonance structure with the porphyrin skeleton;
M is a metal atom selected from the group consisting of Cu, Zn, Fe, Co, Ni, Ru, Pb, Rh, Pd, Pt, Mn, Sn, Au, Mg, Cd, Al, In, Ge, and Ti. Here, the M may be bonded to a halogen atom, an oxygen atom, —OH or ═CO. ]
The oxygen reduction catalyst which carry | supported the porphyrin type complex represented by these on the electroconductive support | carrier. Each R ′ is a nitro group, —COOR ¹ or —C (═O) NR ² R ³ (wherein R ¹ , R ² and R ³ are each independently hydrogen or an alkyl group having 1 to 6 carbon atoms) The oxygen reduction catalyst according to claim 1, wherein   The oxygen reduction catalyst according to claim 1 or 2, wherein each R is a hydrogen atom.   A fuel cell cathode electrode catalyst using the oxygen reduction catalyst according to any one of claims 1 to 3.

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