JP2006156013A - Catalyst for oxygen electrode in polymer electrolyte fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for an oxygen electrode in a polymer electrolyte fuel cell exhibiting a catalyst characteristic equal to that of platinum. <P>SOLUTION: This catalyst for the oxygen electrode in the polymer electrolyte fuel cell includes a metal complex having oxygen gas reduction ability. In the metal complex, an O-O bond distance, which is calculated by a B3LYP density functional method, of oxygen molecules binding to a complex center metal in an absorption structure between the metal complex and the oxygen molecule is 0.131 nm or more. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a polymer solid oxide fuel cell oxygen electrode catalyst, and more particularly to a high performance and inexpensive catalyst for a polymer solid electrolyte fuel cell oxygen electrode.

  Since the polymer solid electrolyte fuel cell can take out a high current density, can operate at a low temperature, and can design a compact battery, a mobile power source such as a power source for electric vehicles and a power source for portable electronic devices, or The application as a stationary distributed power source such as a household power source is expected, and studies for practical use are energetically advanced.

In order to put the polymer solid electrolyte fuel cell into practical use, a catalyst for accelerating the reaction is indispensable, and as a catalyst, platinum or a platinum alloy is mainly studied for both the hydrogen electrode and the oxygen electrode ( For example, Non-Patent Document 1). However, in particular, when the overvoltage at the oxygen electrode is large and a current in the practical range of about 1 A / cm ^{2 is taken out with} respect to the theoretical output voltage of 1.23 V for a single cell, the normal catalyst loading (0.1% on the oxygen electrode side) ˜0.5 mg / cm ² ), the overvoltage of the oxygen electrode reaches 0.3 V or more (for example, Non-Patent Document 2). As a measure to reduce the overvoltage, it is conceivable to increase the amount of platinum or platinum alloy supported on the catalyst, but the effect of reducing the overvoltage due to an increase in the amount of catalyst is small, while the problem of an increase in cost due to an increase in the catalyst is a problem Increasingly, the balance between cost and catalyst performance remains a major challenge.

  As described above, new catalysts that replace platinum that reduce cost and overvoltage are eagerly awaited, and energetic research is being developed. Among them, as a catalyst having an oxygen reducing ability, a complex of a macrocyclic compound containing a metal such as porphyrin, phthalocyanine, dibenzotetraazaannulene has been studied for a long time (Non-patent Document 3). These metal complexes are known as oxygen mediators in vivo, that is, the basic idea is that they are applied to electrochemical oxygen molecule reduction reactions by utilizing the adsorption ability to oxygen molecules ( Non-patent document 4). At the beginning of the research, studies aimed at practical use as a catalyst for the oxygen electrode of phosphoric acid fuel cells were made, but problems such as deterioration of the catalyst due to phosphoric acid and lower catalytic activity compared to platinum remained, and phosphoric acid fuel cells had problems. Application to acid fuel cells has not been achieved. On the other hand, in the case of a polymer electrolyte fuel cell, since it is considered that catalyst deterioration under an acidic environment can be avoided, new energetic research is progressing in recent years.

  In order to apply these metal complexes as catalysts to practical electrodes, it is essential to immobilize the catalyst on an electronic conductor. For this purpose, a carbon support is used. Specifically, carbon black having high electron conductivity and a large surface area is used. The combination of the carbon support and the metal complex enables continuous use as an electrode catalyst.

  The problem as an oxygen reduction catalyst of these metal complexes supported on carbon supports is that the overvoltage is larger than that of platinum catalyst, the reduction product is not only water (referred to as 4-electron reaction product), but also hydrogen peroxide ( 2 points to be a mixture of two-electron reaction products). As a countermeasure against overvoltage, heat treatment in a non-oxidizing atmosphere has been proposed (Non-Patent Document 5). However, the improved overvoltage after heat treatment is 0.1 V or more compared to platinum, and there are still problems in practical use.

  In order to improve the yield of the 4-electron reaction product, dinuclear complexes (Patent Document 1, Non-Patent Document 6), dimerization of porphyrin complexes (Non-Patent Document 7), and the like have been proposed. However, problems such as difficulty in industrial application such as yield in synthesis, high cost, and large overvoltage compared to platinum or platinum alloys remain.

  On the other hand, development of a catalyst aiming at reducing the amount of platinum used by making platinum fine particles or improving the utilization rate of the platinum catalyst, that is, cost reduction has been studied (for example, Non-Patent Document 1). However, a problem remains practically accompanied by a decrease in output characteristics accompanying a reduction in the amount of platinum used.

Japanese Patent Laid-Open No. 11-253811 2000 New Energy and Industrial Technology Development Organization Commissioned Research Results Report “Development of Low Cost Electrode for Solid Polymer Fuel Cell” 1998 New Energy and Industrial Technology Development Organization Commissioned Research Results Report “Study on Ion Exchange Membranes for Practical Use of High Durability Batteries” H. Jahnke, M. Schonborn, G. Zimmermann, Topics in Current Chemistry, Vol. 61, p.133-181 (1976) Makoto Yuasa, Journal of Japan Oil Chemists' Society, Vol. 49, No. 4, 315-323 (2000) J. A. R. van Veen et al., J. Chem. Soc., Faraday Trans. 1, Vol. 77, p. 2827 (1981) F. C. Anson et al., J. Am. Chem. Soc., Vol. 113, p. 9564 (1991) J. P. Collman et al., J. Am. Chem. Soc., Vol.102, p.6027 (1980)

  An object of the present invention is to provide an inexpensive polymer solid electrolyte fuel cell oxygen electrode catalyst that exhibits catalytic properties equivalent to platinum.

The solution to the problem is
(1) A fuel cell catalyst containing a metal complex capable of reducing oxygen gas, wherein the metal complex binds to a complex center metal in an adsorption structure of a metal complex and an oxygen molecule calculated by a B3LYP density functional method A polymer solid oxide fuel cell oxygen electrode catalyst, characterized in that the OO bond distance of oxygen molecules to be oxidized is 0.131 nm or more,
(2) The solid polymer electrolyte fuel cell oxygen electrode catalyst according to (1), wherein the metal complex has an N ₄ -chelate type complex structure,
(3) The polymer solid electrolyte according to the above (1) or (2), wherein the metal complex is an N ₄ -chelate type, and at least two of the N atoms bonded to the central metal are an imine type Type fuel cell oxygen electrode catalyst,
(4) The polymer solid oxide fuel cell oxygen electrode catalyst according to (3), wherein the N ₄ -chelate metal complex is one or both of the following (General Formula 1) or (General Formula 2):

(Where, M is a metal atom, R ¹ to R ¹⁰ is a hydrogen or a substituent.)

(However, M is a metal atom, and R ^{11 to} R ²⁴ are hydrogen or a substituent.)
(5) In the above (1) to (4), the complex center metal of the metal complex is one or more selected from Group V, Group VI, Group VII or Group VIII transition metal of the periodic table The solid polymer electrolyte fuel cell oxygen electrode catalyst according to any one of the above,
(6) The polymer solid oxide fuel cell oxygen electrode catalyst according to (5), wherein the transition metal is one or both of Co and Fe,
(7) The polymer solid oxide fuel cell oxygen electrode catalyst according to any one of (1) to (6), further containing 1 to 5% by mass of a noble metal in the catalyst,
(8) The polymer solid oxide fuel cell oxygen electrode catalyst according to any one of (1) to (7), wherein one or both of the metal complex or the noble metal is supported on a support surface,
(9) The solid polymer electrolyte fuel cell oxygen electrode catalyst according to (8), wherein the carrier is a carbon material,
(10) The solid polymer electrolyte fuel cell oxygen electrode catalyst according to (9), wherein the carbon material has a BET specific surface area S _BET of 1000 m ² / g or more,
(11) The polymer according to (9) or (10), wherein a ratio S _micro / S _total of a surface area S _micro of a micropore having a diameter of 2 nm or less and a total pore surface area S _total of the carbon material is 0.3 or more. Catalyst for oxygen electrode of solid oxide fuel cell,
(12) The solid polymer electrolyte fuel cell oxygen electrode catalyst according to the above (9) to (11), wherein the pore diameter of micropores having a diameter of 2 nm or less of the carbon material is 1.5 nm or less,
Is achieved.
(13) A solid polymer electrolyte fuel cell using the catalyst according to any one of (1) to (12) above.

  The catalyst of the present invention provides an inexpensive catalyst for a solid oxide polymer electrolyte fuel cell oxygen electrode that has good catalytic activity and four-electron reaction rate in an oxygen reduction reaction, and exhibits catalytic properties equivalent to or better than platinum. It becomes possible.

  The contents of the present invention will be specifically described below.

Essentially important in the present invention is that
(a) using a single metal complex having high catalytic activity,
(b) coexistence of a metal complex and a noble metal to develop a catalytic action,
(c) using a carbon material having a large specific surface area for the carrier supporting the catalyst,
The three points.

  The oxygen reduction reaction by a metal complex starts when an oxygen molecule is adsorbed on a metal atom located at the center of the metal complex. In particular, the reason why a stable adsorption state is obtained when the central metal is a transition metal is that there is a charge transfer from the binding orbitals of oxygen molecules to the empty s orbitals of the transition metal atoms at the adsorption site, and the transition metal atoms This is because the backdonation from the d-orbital to the antibonding orbital of oxygen molecules occurs simultaneously (Hiyoshi Kobayashi, Katsushi Yamaguchi, Surface, Vol.23, p.311 (1985)).

  In order for the metal complex to have high oxygen reduction activity, the ease of backdonation is particularly important among the above two types of charge transfer. In other words, if the electron density of oxygen molecules increases, the affinity for protons increases, and electrons flow into the antibonding orbitals of oxygen molecules, weakening the bonds between oxygen atoms, thus breaking the OO bond. This is because a four-electron reduction reaction accompanied with sulfite tends to occur. Therefore, it is possible to estimate the activity of the oxygen reduction reaction by calculating the O—O bond distance of the oxygen molecule, which is an index representing the degree of backdonation from the metal atom to the oxygen molecule. These calculations can be performed using non-empirical molecular orbital methods or density functional methods, but the density functional method is relatively easy to calculate and provides high calculation accuracy. As the density functional method, various methods such as the B3LYP method are adopted.

  Therefore, the present inventors examined whether the calculation result of the B3LYP method is correlated with the actual catalyst activity. All calculations were performed using the Gaussian 98 program. The basis functions used are 6-31G basis functions for typical elements and are described in “gaussian basis sets for molecular calculations”, S. Hujinaga (eds.), Elsevier (1984) (14s8p5d ) / [5s3p2d]. First, cobalt (II) complex of dibenzotetraazaannulene (abbreviation CoDTAA), cobalt (II) complex of 5,10,15,20-tetrakis- (4-methoxyphenyl) porphyrin (abbreviation CoTMPP), 5,10,15 The adsorption structure of oxygen molecules was calculated by the cobalt (II) complex of a 20-tetraphenylporphyrin (abbreviated CoTPP), the cobalt (II) complex of phthalocyanine (abbreviated CoPc), and oxygen molecules. As a result, it was found that their O—O bond distances were 0.1305 nm, 0.1288 nm, 0.1287 nm, and 0.1254 nm, respectively. In addition, CoTMPP, CoTPP, and CoPc were supported on carbon black (Ketjen Black EC600JD manufactured by Lion Corporation), and current-voltage characteristics were measured using a rotating disk electrode without heat treatment. Comparing the potential at the current value half that of the saturation current value (saturated sweet potato electrode (SCE) standard), they were 0.119V, 0.083V, and 0.075V, respectively. Further, according to the literature (H. Jahnke et al., Top. Corr. Chem., Vol. 1, p.133 (1976)), CoDTAA is reported to be more highly active than CoTMPP. Therefore, the above study revealed that there is a correlation between the calculated O—O bond distance and the oxygen reduction catalytic activity.

  Therefore, the present inventors have conducted various studies on the structure of a metal complex having further excellent oxygen reduction catalytic activity by calculating the O—O bond distance of adsorbed oxygen molecules. as a result,

The O-O bond distance in the oxygen adsorption structure is calculated as 0.1320 nm and 0.1316 nm, respectively, and both are longer than in the case of CoDTAA.

This macrocyclic compound complex was found to have a catalytic activity higher than that of CoDTAA.

  On the other hand, the central metal atom of the metal complex has shifted to a high oxidation state during oxygen reduction, and the central metal atom needs to be reduced to a low oxidation state in order to recover the catalytic ability in the next reaction cycle. This reduction reaction proceeds more easily as the electron withdrawing property of the complex ligand is stronger. On the other hand, the O—O bond distance of the adsorbed oxygen molecule tends to increase as the backdonation from the metal atom to the oxygen molecule increases, that is, as the electron donating property of the ligand increases. Considering the above points, if the O—O bond distance of the adsorbed oxygen molecule is too long, the reduction reaction of the central metal may be difficult. Therefore, the aforementioned O—O bond distance is preferably 0.136 nm or less.

The metal complex suitably used in the present invention preferably has a chelate structure having 4 or 5 coordinates with respect to the central metal, more preferably an N ₄ -chelate structure. This is because the greater the coordination number, the higher the chemical stability as a complex, that is, the longer the catalyst life. On the other hand, in the case of 6 or more coordination, there is a possibility that adsorption of oxygen molecules may be difficult due to steric repulsion with the ligand.

Examples of the ligand of the metal complex preferably used in the present invention include the ligands represented by (General Formula 1) and (General Formula 2). Here, the substituents represented by R ^{1 to} R ²⁴ are hydrogen or a substituent, and the substituents may be the same or different, such as a substituted / unsubstituted alkyl group, an aryl group, etc. Can be illustrated. Hydrocarbon groups are preferred as these substituents. Examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, a methoxy group, and an ethoxy group. As the alkyl group, two alkyl groups may be cyclic, and examples thereof include compounds in which the alkyl substituents of R ¹ and R ² are closed to form a cyclohexyl ring. Specific ligands include 5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradeca-2,4,6,9,11,13-hexaene, 5, Examples include 7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradeca-4,6,11,13-tetraene.

  Examples of the aryl group include a phenyl group and an alkyl-substituted phenyl group. Specific examples of the ligand include 6,13-diphenyl-1,4,8,11-tetraazacyclotetradeca-2,4,6,9,11,13-hexaene.

  Among these ligands, a compound substituted with an alkyl group can be easily synthesized, which is preferable.

  The catalytic activity also varies depending on the type of transition metal element. As a result of intensive studies by the inventor, one or both of Co and Fe exhibiting high activity without depending on the type of the ligand can be suitably used in the present invention.

  The supported amount of the metal complex in the present invention is preferably 2% by mass or less, more preferably 1% by mass or less as the supported amount of the metal element. When the amount exceeds 2% by mass, the catalytic action of the metal complex becomes relatively strong, and the increase in catalytic activity due to the coexistence with the noble metal becomes small. In order to exhibit the function as a catalyst, the supported amount of the metal complex is preferably 0.01% by mass or more, more preferably 0.05% by mass or more as the supported amount of the metal element.

  In the present invention, the composite catalyst called the coexistence of the metal complex and the noble metal has an experiment in which the catalytic activity is higher in the coexisting state than the individual catalytic activity, as shown in the specific examples in the examples below. It is based on facts. Although its theoretical interpretation is uncertain, for example, two reductions such as a four-electron reduction reaction on a noble metal followed by a two-electron reaction on a metal complex followed by a further two-electron reduction reaction on the noble metal. Since the oxygen reduction reaction is carried out by the reaction path, it is assumed that the catalytic activity is promoted in the case of coexistence rather than in the case of each of them alone.

  The noble metal used in the present invention refers to ruthenium, rhodium, palladium, osmium, iridium, platinum, and alloys containing these as main components. In view of the high catalytic activity, in the present invention, it is preferable to apply ruthenium, rhodium, palladium, osmium, iridium, platinum, and alloys containing these as main components. Application of an alloy mainly composed of platinum is further preferable. Other noble metals have a lower catalytic activity than platinum and an improvement in the catalytic activity due to the coexistence with the metal complex is recognized, but the improvement is small.

  The loading amount of the noble metal of the present invention is preferably 20% by mass or less. When the amount exceeds 20% by mass, the catalytic action of the noble metal alone becomes relatively strong, and the increase in the catalytic activity due to the coexistence with the metal complex becomes small. Further, considering the viewpoint of the cost of the catalyst, the amount of the noble metal supported is more preferably 15% by mass or less. In order to exhibit the function as a catalyst, the supported amount of the noble metal is preferably 1% by mass or more, and more preferably 2% by mass or more.

  In order to improve the catalytic activity due to the coexistence of the metal complex and the noble metal, it is an essential condition that the surface area of the carbon material as the catalyst support is large. The effect of the catalyst carrier is presumed to be not only a physical effect of widening the reaction field but also activation of the catalytic action through chemical interaction with the metal complex. As described above, in the catalytic reaction process of the metal complex, recovery of the catalytic function by reduction of the central metal atom is achieved by electron transfer from the ligand chelating to the metal atom. Further, it is a carbon material carrier that forms a huge π-electron system that facilitates electron transfer from the ligand to the metal atom.

A carbon material having a large surface area is generally highly active. The activity is attributed to irregularities formed on the surface of the carbon material, defects in the carbon network surface due to fine holes, edge portions, and the like. In the present invention, a carbon material having a large surface area (high activity) is used as the carrier of the metal complex catalyst, thereby amplifying the interaction between the π electron system of the carbon material and the π electron system of the ligand. is there. As a result of diligent investigation, it was found that the specific surface area (BET specific surface area: S _BET ) determined by the BET evaluation of the adsorption isotherm of nitrogen gas is appropriate as an index of the surface area size corresponding to this catalytic activity. It was. The specific numerical range is 1000 m ² / g or more. If it is less than 1000 m ² / g, the surface roughness of the carbon surface, which is presumed to amplify the catalyst activity, the defects in the carbon network surface due to the fine pores, and the amount of edge portions are insufficient, and the improvement in the catalyst activity does not appear. On the other hand, when the surface area is increased to 3000 m ² / g or more, micropores that penetrate deeply into the carbon are formed, and the ratio of the internal surface of the micropores to the entire reaction field increases, so that substances such as oxygen diffusion The movement becomes rate-limiting and cannot be a place for an actual catalytic reaction, and the catalytic activity is rather deteriorated, which is not preferable for the present invention.

One of the important configurations in the present invention is that the optimum structure of the micropores is expressed and defined by specific numerical values. The basic guidelines for optimizing the pore structure are the following three points. That, (i) in order to enhance the four-electron reaction rate, the structure complex to each other in parallel to oppose the N ₄ surfaces together across the appropriate distance from each other is important, so as to form a facing structure of (ii) above complexes It is important that the density of the adsorption sites of the complex is high, and (iii) the pore size is assumed to determine the interplanar spacing of the opposing complex, so an appropriate size is required for the pore size. is there.

The present inventors have found based on this guidance intensive studies, the ratio S _micro / S _total of the surface area S _micro and total pore surface area S _total of the following micropores diameter 2nm it is 0.3 or more, the pore diameter It was found that the thickness is preferably 1.5 nm or less. Here, these three indices: S _micro, S _total, pore diameter, respectively, the specific surface area of less micropores diameter 2 nm (m ² / g), the total surface area of the carbon material (m ² / g), Micro It represents the average diameter (nm) of the pores. Each index is obtained by t-Plot analysis of the measured value of the adsorption isotherm of nitrogen gas. S _micro / S _total is an index corresponding to the density of the adsorption site, and the lower limit for expressing high catalytic activity is 0.3, more preferably 0.4 or more, and further preferably 0.5 or more. When S _micro / S _total is less than 0.3, the density of the transition metal complex pair forming the opposing positional relationship is small, the four-electron reaction rate is lowered, and it is not suitable as an oxygen reduction catalyst. The pore diameter is a factor that determines the distance between the complexes of the transition metal complex pair that forms the opposing positional relationship. The upper limit of the gap that exhibits the effect of increasing the 4-electron reaction rate for the oxygen reduction reaction is 1.5 nm. More preferably 1.4 nm, and still more preferably 1.3 nm or less. On the other hand, the pore diameter corresponding to the lower limit of the distance between the complexes of the transition metal complex pair effective for the oxygen reduction reaction is 0.5 nm, and the more preferable pore diameter is 0.6 nm or more. If the pore diameter exceeds 1.5 nm or the pore diameter is less than 0.5 nm, the complex pair does not substantially act effectively on the oxygen reduction reaction, so the effect of improving the 4-electron reaction rate in the oxygen reduction reaction is reduced. Fear is high.

The carbon material suitably used in the present invention is not particularly limited as long as it satisfies the above surface structure. For example, carbon black, carbon nanotubes, carbon nanofibers of various conductive grades, coke of various raw materials, pitch coke, phenolic resins, furan resins, resins with a relatively high carbonization yield such as polyvinyl chloride, etc. Mention may be made of organic compounds. As a function required as a catalyst support, it is necessary to increase the conductivity of the particles themselves, and therefore, the distance d ₀₀₂ between carbon hexagonal network surfaces (graphene sheets) by d ₀₀₂ diffraction lines of X-ray diffraction is 0.50 nm or less, Preferably, 0.45 nm or less can be suitably applied. On the other hand, if d ₀₀₂ is smaller than 0.36 nm, the crystallinity is too high and the crystallite size becomes too large.As a result, it is difficult to obtain the pore structure defined in the present invention, and d ₀₀₂ is 0.36 nm or more. Preferably there is.

  The production method of the activated carbon suitably used in the present invention is not particularly limited. For example, oxidation of zinc chloride, phosphoric acid, alkali metal hydroxide, alkali metal carbonate, water vapor, carbon dioxide, etc. The activation process by the normal method using an agent can be applied.

  The carbon support used as the support for the electrode catalyst of the present invention is preferably in the form of fine particle powder in order to support the transition metal complex as a catalyst at a high density. The optimum particle size is an average diameter of 10 nm to 1 μm. When the particles that are the smallest unit such as carbon black have a secondary structure such as an aggregate structure, the average diameter is the diameter of the primary particles. When the average diameter is less than 10 nm, it is difficult to substantially introduce micropores of 2 nm or less on the support surface, and when the average diameter exceeds 1 μm, the thickness of the electrode layer to ensure a surface area effective for catalytic reaction Therefore, the diffusion resistance of the gas increases, and the performance as a fuel cell deteriorates.

  The essence of the catalytic activity of the catalyst defined in the present invention is presumed to be an interaction through the π electron between the surface of the carbon material and the metal complex. Therefore, as a result of intensive investigations on the catalyst preparation method with the aim of further strengthening this π-electron interaction, after supporting a metal complex and a noble metal on the surface of the carbon powder, 700 ° C. in a non-oxidizing atmosphere. It has been found that a highly active catalyst can be prepared by heat treatment at a temperature of ˜1100 ° C. Here, when the treatment is carried out in an oxidizing atmosphere, oxidation consumption of the carbon support and the metal complex occurs, and the catalytic activity is lost. Further, in the heat treatment at a temperature of less than 700 ° C., the π-electron interaction between the carbon support and the metal complex is not sufficient, and the catalytic activity is not exhibited. On the other hand, the heat treatment at a temperature exceeding 1100 ° C. causes the thermal decomposition of the metal complex, so that the catalytic activity is lost. This catalyst preparation method is also effective when only a transition metal complex is supported on a carbon support.

  The catalyst of the present invention is an ordinary method for forming an electrode catalyst layer of a polymer electrolyte fuel cell, for example, a method of preparing a slurry of a catalyst and a polymer electrolyte solution and applying it to carbon paper The method for forming the catalyst layer is not particularly limited.

  Below, the catalyst prescribed | regulated by this invention is demonstrated concretely.

(Synthesis method of transition metal complex)
A method for synthesizing the N ₄ -chelate transition metal complex defined in the present invention is shown below.

  Synthesis of complex 1: 5,7,12,14-tetramethyl-1,4 according to the method described in the literature (RH Holm, J. Am. Chem. Soc., Vol. 94, p. 4529 (1972)) , 8,11-Tetraazacyclotetradeca-2,4,6,9,11,13-hexaene cobalt (II) complex (abbreviated as Complex 1) was synthesized. Yield 12%.

  Synthesis of Complex 2: Literature ((a) T. Hayashi, Bull. Chem. Soc. Jpn., Vol. 54, p. 2348 (1981), (b) RH Holm, Inorg. Syn., Vol. 11, p. .72, (1968)), the cobalt of 5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradeca-4,6,11,13-tetraene ( II) A complex (abbreviated as Complex 2) was synthesized. Yield 8%.

(Carbon material carrier)
The carbon material carrier for catalyst is Vulcan XC72R (Cabot Co., Ltd., abbreviated as XC72R) among carbon blacks generally used as a carrier for fuel cell catalysts, and the pore structure defined in the present invention. And activated carbon having Activated carbon having a controlled pore structure was produced by the following method. The raw material used was coal pitch-based raw coke heat-treated in an inert atmosphere at a temperature range of 500 ° C. to 900 ° C., and then pulverized with a planetary ball mill to an average particle size of about submicron. Steam was used for the activation treatment, and the heat-treated coke was heat-treated at a temperature of 800 ° C. to 1000 ° C. for 1 to 3 hours to obtain a predetermined pore structure. For measurement of the pore structure, a nitrogen gas adsorption method (BET method) was applied, and an adsorption isotherm was measured using BELSORP36 manufactured by Bell Japan. The so-called t-Plot method was applied to the calculation of the pore structure, and the analysis software attached to the above apparatus was used for the specific calculation. The calculated physical properties are the specific surface area S _BET (m ² / g) by BET analysis, the ratio S _micro / S _total of the surface area of micropores with a diameter of 2 nm or less to the _total pore surface area by tPplot analysis, The average pore diameter (nm) of the pores. Table 1 summarizes the measured pore structure and activated XC72R values of activated carbon produced under the above conditions. As is apparent from the table, XC72R does not satisfy the S _BET conditions defined in the present invention, while activated carbons 1 and 2 satisfy the pore structure defined in the present invention.

(Catalyst preparation method)
A chloroplatinic acid hexahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) was weighed so that the predetermined mass% was obtained, and a carbon material used as a carrier was added to an aqueous solution diluted to an appropriate amount with water, and sufficiently stirred. Thereafter, dispersion was advanced with an ultrasonic generator. A carrier carrying a precursor obtained by drying and solidifying a dispersion using an evaporator is heated to 300 ° C. in an electric furnace (hydrogen gas ratio: 10 to 50% by volume) in which a hydrogen / argon mixed gas is circulated, and then chlorinated. Reduction treatment of platinic acid was performed.

  A solution in which the above transition metal complex is weighed so as to be 1% by mass in terms of transition metal element, and an appropriate amount of N, N′-dimethylformamide (reagent special grade) or pyridine (reagent special grade) is added. In addition, the above-described platinum-supported carbon material (Pt—C) was added and sufficiently stirred, and further dispersion was advanced using an ultrasonic generator. Reflux the dispersion for 8 hours or more while keeping it in an oil bath at 70 ° C (under a flow of argon), and then pour it into distilled water at least 5 times the amount of the dispersion while stirring, over the transition metal complex Pt-C. Established. Thereafter, the catalyst was separated and collected by vacuum filtration, washed again with distilled water at a temperature of about 60 ° C., collected by vacuum filtration, and vacuum dried at 100 ° C. Further, a catalyst for evaluation was prepared by treating at 700 ° C. for 1 hour in an argon gas atmosphere.

  The transition metal complexes used in Examples and Comparative Examples are Complex 1 and Complex 2 and a cobalt (II) complex (abbreviated as CoTPP) of 5,10,15,20-tetraphenylporphyrin.

  The preparation of the catalyst supporting only the transition metal complex is carried out only by the supporting process of the transition metal complex except for the above-described platinum supporting process, while the preparation of the catalyst supporting only the platinum is performed by supporting the transition metal complex. Except for the process, only the platinum loading process was performed to prepare each catalyst.

(Evaluation method of catalyst activity)
(1) Preparation of sample for evaluation 15 mg of catalyst powder obtained by previously grinding the catalyst in a mortar, 300 mg of polymer solid electrolyte solution (EC-NS-05; Nafion 5% by mass solution of US ElectroChem) and 300 mg of ethanol in a sample bottle The mixture was stirred and stirred with a stirrer for 15 minutes to prepare a fully kneaded slurry.

(2) Preparation of test electrode The slurry was applied onto the disk electrode of the rotating ring disk electrode and dried to obtain a test electrode. The disc electrode is a 6 mm diameter cylinder made of glassy carbon, and a sample is applied to the bottom surface of the cylinder. The coating amount was adjusted to 0.03 mg. The ring electrode is a platinum cylinder with an inner diameter of 7.3mm and an outer diameter of 9.3mm. The outside of the electrode is insulated with Teflon resin.

(3) Evaluation method
The electrochemical activity of the catalyst was evaluated using a rotating ring disk evaluation device (RRDE-1) with day thickness measurement. For electrochemical evaluation, two solartron SI1287 units were used, and the ring electrode and disk electrode were controlled independently, and bipolar measurements were performed. A 0.1N sulfuric acid aqueous solution was used as the electrolyte, and a cell configuration was used in which an SCE electrode was used as the reference electrode and a Pt plate was used as the counter electrode. The evaluation conditions are as follows. Oxygen gas was bubbled, and the electrode potential of the electrode rotated at 2500 rpm was swept from 1.0 V (SCE standard) to -0.2 V at a rate of 10 mV / sec. With the electrode potential held at 1.1 V (SCE standard), the change with time of the current flowing through the disk electrode and the ring electrode was measured, and a plot of the disk current and the ring current with respect to the potential of the disk electrode was obtained.

(4) Overvoltage Evaluation Method The potential (E _1/2 ) at a current value half of the saturation current value was read from the above-mentioned disk potential vs. disk current plot. As a reference potential E _1/2 ⁰ when half the current value of the saturation current value of the US ElectroChem Inc. catalyst EC10PTC (catalyst supported a 10 wt% platinum on carbon black), Example, Comparative Example ΔE _1/2 = E _1/2 -E _1/2 ⁰ was evaluated for each of the catalysts. That is, when ΔE _1/2 = 0, an overvoltage equivalent to EC10PTC is satisfied, and if ΔE _1/2 > 0, this corresponds to an overvoltage smaller than EC10PTC and high catalytic activity.

(5) Four-electron reaction rate evaluation method The four-electron reaction rate η was calculated based on the following equation from a plot of the ring current and the disk current against the disk potential.

η (%) = [I _d- (I _r / n)] / [I _d + (I _r / n)]
Here, I _d is the disk current, I _r denotes a ring current, n represents represents a capture rate of the disk reaction product from the ring electrode.

  The experimental measurement method of the capture rate was evaluated according to Akira Fujishima et al., Electrochemical measurement method (below), Gihodo Publishing (1991). As a result, n = 0.36 was obtained for the electrodes used in the examples.

  In addition, η changes according to the disk potential (η becomes smaller as the potential is lower), but in this evaluation, the disk potential is 0 V (SCE standard) so that the difference in η due to the catalyst becomes clear. Η was adopted.

Table 2 summarizes the overvoltage value ΔE _1/2 and the four-electron reaction rate η as transition metal species, platinum loading, and catalytic activity indicators.

  From the results of these examples and comparative examples, it is recognized that the transition metal complex having a specific structure defined in the present invention has good catalytic activity and four-electron reaction rate in the oxygen reduction reaction. In addition, the composite catalyst obtained by combining the transition metal complex defined in the present invention and platinum exhibits catalytic properties clearly superior to that of platinum alone or the transition metal complex alone, and the concerted effect of the transition metal complex and platinum is recognized. .

On the other hand, regarding the effect of the carrier, by applying the activated carbon defined in the present invention to the carrier, an excellent catalytic activity that cannot be exhibited by a normal carbon black carrier is observed.

Claims (13)

  A fuel cell catalyst comprising a metal complex having oxygen gas reducing ability, wherein the metal complex binds to a complex center metal in an adsorption structure of a metal complex and an oxygen molecule calculated by a B3LYP density functional method A solid polymer electrolyte fuel cell oxygen electrode catalyst characterized by having an OO bond distance of 0.131 nm or more. 2. The polymer solid electrolyte fuel cell oxygen electrode catalyst according to claim 1, wherein the metal complex has an N ₄ -chelate type complex structure. 3. The solid polymer electrolyte fuel cell oxygen electrode according to claim 1, wherein the metal complex is of N ₄ -chelate type and at least two of N atoms bonded to the central metal are imine type. catalyst. 4. The polymer solid oxide fuel cell oxygen electrode catalyst according to claim 3, wherein the N ₄ -chelate metal complex is one or both of the following (general formula 1) and (general formula 2).

(Where, M is a metal atom, R ¹ to R ¹⁰ is a hydrogen or a substituent.)

(However, M is a metal atom, and R ^{11 to} R ²⁴ are hydrogen or a substituent.)   The complex center metal of the metal complex is at least one selected from Group V, Group VI, Group VII, or Group VIII transition metals of the periodic table. Catalyst for molecular solid oxide fuel cell oxygen electrode.   6. The polymer solid oxide fuel cell oxygen electrode catalyst according to claim 5, wherein the transition metal is one or both of Co and Fe.   7. The polymer solid oxide fuel cell oxygen electrode catalyst according to claim 1, further comprising a noble metal in the catalyst.   8. The polymer solid oxide fuel cell oxygen electrode catalyst according to claim 1, wherein one or both of the metal complex and the noble metal is supported on a support surface.   9. The catalyst for a solid polymer electrolyte fuel cell oxygen electrode according to claim 8, wherein the carrier is a carbon material. 10. The polymer solid oxide fuel cell oxygen electrode catalyst according to claim 9, wherein the carbon material has a BET specific surface area S _BET of 1000 m ² / g or more. 11. The solid polymer electrolyte fuel cell oxygen electrode according to claim 9 or 10, wherein a ratio S _micro / S _total of a surface area S _micro of a micropore having a diameter of 2 nm or less and a total pore surface area S _total of the carbon material is 0.3 or more. Catalyst.   12. The polymer solid oxide fuel cell oxygen electrode catalyst according to claim 9, wherein the carbon material has a micropore diameter of 2 nm or less and a pore diameter of 1.5 nm or less. A solid polymer electrolyte fuel cell using the catalyst according to claim 1.