JP2010075800A - Oxygen reduction catalyst and method for manufacturing the same, and oxygen reduction electrode, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an oxygen reduction catalyst which has high oxygen reducing capacity and is excellent in practicality, and to provide an oxygen reduction electrode. <P>SOLUTION: The oxygen reduction catalyst is based on a material obtained by heat-treating a water-soluble porphyrin derivative which has an anion group and a trivalent metal as a central metal thereof. The water-soluble porphyrin derivative before heat-treated has such a double regular structure that a cation bonds the adjacent anion groups of the water-soluble porphyrin derivative to each other and also bonds the adjacent metals thereof to each other. The oxygen reduction catalyst is manufactured by adding the cation to an aqueous solution of the water-soluble porphyrin derivative to react them with each other and obtain a product and heat-treating the obtained product in an inert gas atmosphere. The oxygen reduction catalyst is deposited on an electrically-conductive electrode base stock, which is then used as the oxygen reduction electrode. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an oxygen reduction catalyst used for an electrode of a fuel cell and a method for producing the same, and further relates to an electrode for oxygen reduction and a method for producing the same.

  The development of new technologies for global warming and environmental pollution problems has been promoted in various fields, and the development of fuel cells is one of them. For example, in polymer electrolyte fuel cells, the development of a catalyst with excellent oxygen catalytic ability is the key to practical use, and the development of a new oxygen electrode catalyst to replace platinum is awaited. Since platinum is expensive and its existence is unevenly distributed, in Japan, where resources are scarce, it is essential to develop an oxygen electrode catalyst that can replace platinum. If an oxygen electrode catalyst having an oxygen reducing ability comparable to platinum is developed, it is expected that the above-described practical use of the fuel cell will greatly advance.

  Under these circumstances, new oxygen electrode catalysts have been studied in various fields. Tetraphenylporphyrin, tetramethoxyphenylporphyrin, and porphyrin derivatives with central metals and halogen elements introduced into them can be reduced with oxygen. It has been proposed to be used as a catalyst. The inventors of the present application have already proposed a method for producing an electrode for oxygen reduction that is superior in oxygen reduction performance to these porphyrin derivatives (see Patent Document 1 and the like).

In Patent Document 1 proposed by the inventors of the present application, mesotetrakis (4-sulfonatophenyl) porphyrin metal halide is supported on an electrode material, and this is in a range from 400 ° C. to 800 ° C. in an inert gas atmosphere. Discloses a method for manufacturing an oxygen reduction electrode that is heat-treated. The manufactured oxygen reduction electrode has an advantage that an oxygen reduction reaction occurs even when the oxygen overvoltage is small.
Japanese Patent Laid-Open No. 62-17951

  However, for example, when an oxygen reduction electrode of a fuel cell is considered, even an oxygen reduction electrode manufactured by the manufacturing method described in Patent Document 1 described above cannot always be said to have sufficient oxygen reduction capability. Improvement is desired.

  The present invention has been proposed in view of such conventional circumstances, and has a high oxygen reduction capability, is inexpensive and easily available, and has a practical utility and a method for producing the same. The purpose is to provide. Furthermore, it aims at providing the electrode for oxygen reduction excellent in performance, and its manufacturing method by utilizing the catalyst for this oxygen reduction.

  The present inventor has conducted various studies in order to achieve the above-described object. As a result, the formation of a highly ordered three-dimensional structure is promoted by causing a cation to act on a water-soluble porphyrin derivative that has an anionic group and the central metal is a trivalent metal. I came to find out what to do.

  The present invention has been completed based on such findings. That is, the oxygen reduction catalyst of the present invention is an oxygen reduction catalyst mainly composed of a heat-treated product of a water-soluble porphyrin derivative having an anion group and a central metal being a trivalent metal. The water-soluble porphyrin derivative has a double regular structure in which anion groups and metals are linked to each other. In addition, the method for producing an oxygen reduction catalyst of the present invention is effective for adding a cation to an aqueous solution of a water-soluble porphyrin derivative having an anionic group and a central metal being a trivalent metal, and reacting with the resulting product. The heat treatment is performed in an active gas atmosphere.

  Similarly, the oxygen reduction electrode of the present invention has an oxygen reduction catalyst mainly composed of a heat-treated product of a water-soluble porphyrin derivative having an anionic group and a central metal of a trivalent metal on the surface of the conductive electrode material. A supported oxygen reduction electrode, wherein the oxygen reduction catalyst has a double regular structure in which anion groups and metals of the water-soluble porphyrin derivative are connected to each other by a cation before heat treatment. It is characterized by that. In addition, the method for producing an electrode for oxygen reduction according to the present invention comprises adding a cation to an aqueous solution of a water-soluble porphyrin derivative having an anionic group and a central metal being a trivalent metal, and reacting the resulting product with a conductive material. After being supported on the electrode material, heat treatment is performed in an inert gas atmosphere.

The reason why the oxygen reduction catalyst of the present invention showed excellent oxygen reducing ability is that a highly ordered three-dimensional structure was formed. When a cation is allowed to act on a water-soluble porphyrin derivative that has an anionic group and the central metal is a trivalent metal, a salt is formed with the anionic group in the side chain of the water-soluble porphyrin derivative, and the anionic group is bonded to form a plane. Promotes a regular structure in the in-plane direction (the lateral direction) of a water-soluble porphyrin derivative molecule having a typical molecular structure. Furthermore, it reacts with M ^I (III) -OH (M ^I is the central metal) in the center of the porphyrin ring, and M ^I (III) -OM- ^II- O-M ^I (III) between the metals. (M ^II is a cation) becomes bonds are formed, regular structure in the thickness direction of the water-soluble porphyrin derivative molecule (so to speak vertical direction) is promoted. As a result, an almost complete Face to Face structure is formed, and oxygen molecules are activated between the porphyrin ring planes.

  For example, we have synthesized a face-to-face binuclear complex in which the side chains of cobalt porphyrin are covalently linked and reported the 4-electron reduction ability of oxygen. There is no example of a face-to-face complex formed from these two points.

  According to the present invention, it is possible to provide an oxygen reduction catalyst having a high oxygen reduction ability comparable to platinum. The oxygen reduction catalyst of the present invention does not require expensive and tightly supplied resources such as platinum, and is inexpensive and excellent in practicality. Further, by using such an oxygen reduction catalyst, it is possible to provide an oxygen reduction electrode that is inexpensive and excellent in performance.

  In the field of fuel cells, for example, it has been recognized that it is difficult to develop an electrode catalyst for oxygen reduction instead of platinum in an acidic electrolyte fuel cell, and there is a movement to develop an alkaline electrolyte fuel cell. is there. The oxygen reduction catalyst and oxygen reduction electrode of the present invention are also expected to be used as catalysts and electrodes for such alkaline electrolyte fuel cells and air cells such as aluminum-air.

  Hereinafter, embodiments of an oxygen reduction catalyst, an oxygen reduction electrode, and a method for producing the same according to the present invention will be described in detail.

  First, the oxygen reduction catalyst of the present invention, the oxygen reduction catalyst of the present invention is for oxygen reduction mainly composed of a heat-treated product of a water-soluble porphyrin derivative having an anionic group and a central metal being a trivalent metal. It is a catalyst. Specific examples of the water-soluble porphyrin derivative include water-soluble porphyrin derivatives represented by the following formula (I).

（ただし、式中、ＭはIII価の金属、Ｘはハロゲン原子、Ｙはアニオン基を表す。）

(In the formula, M represents a trivalent metal, X represents a halogen atom, and Y represents an anionic group.)

Here, as the anion group Y introduced into the side chain of the porphyrin (the 4-position of the phenyl group), any anion group can be selected. For example, a sulfone group (—SO ₃ H), a carboxyl group (— COOH), phosphate groups (—CH ₂ PO ₃ H), and the like. In consideration of reactivity with a cation described later, a sulfone group (—SO ₃ H) is most preferable among these. The central metal is not particularly limited as long as it is a trivalent metal, but iron (Fe) is preferably selected in order to form a bond between metals by a cation.

Therefore, a typical example of a water-soluble porphyrin derivative having an anion group and a central metal being a valent metal is a water-soluble iron porphyrin represented by the following formula (II). The water-soluble iron porphyrin represented by the formula (II) is tetra (4-sulfonatophenyl) porphyrin iron (III) chloride, and is abbreviated as Fe (III) (TPPS ₄ ) Cl below. _{Others, Fe (III) (TPPS 4} ) Cl tetra having a positive charge at the same water-soluble as (N- methylpyridinium-4-yl) can be applied to the porphyrin iron chloride [Fe (III) (TMPyP) Cl] , etc. is there.

  The water-soluble porphyrin derivative described above has a double regular structure in which anion groups and metals are respectively connected by cations before heat treatment, and this is maintained even after heat treatment. The catalyst for use expresses high oxygen reduction ability. That is, when a cation is allowed to act on the water-soluble porphyrin derivative before heat treatment, the anion groups (for example, sulfone groups) of the water-soluble porphyrin derivative molecule are bonded by the interaction with the cation, and a lateral regular structure is constructed. Is done. The regular structure in the lateral direction is constructed by forming a salt between an anion group and a cation in a side chain of a water-soluble porphyrin derivative molecule. The molecule of the water-soluble porphyrin derivative has high planarity, and a planar structure is formed by bonding the planar water-soluble porphyrin molecule in the plane (the lateral direction).

At the same time, the central metal of each water-soluble porphyrin derivative is linked by cation interaction, and a regular structure in the vertical direction (thickness direction of the planar water-soluble porphyrin derivative molecule) is constructed. The For example, when the water-soluble porphyrin derivative is Fe (III) (TPPS ₄ ) Cl, Fe (III) -OH at the center of the porphyrin ring and the cation M ^II react to form Fe (III) -O-M ^II-. A regular structure in the vertical direction is formed by forming O-Fe (III).

The formation of the regular structure in the vertical direction is considered to be due to a crosslinking reaction represented by the following formula (III). This is because the pH value of the solution does not change at the bond (reaction) between the anion groups by the cation and remains neutral, whereas the bond between the metals (Fe) becomes acidic. Guessed. In the cross-linking reaction represented by the following formula (III), proton 2H ⁺ is released, which confirms that the solution is acidified.

The formation of these lateral and longitudinal regular structures results in a double regular structure (almost complete Face
to Face structure) is formed. Various cations can be used as the cation for constructing the regular structure in the lateral direction and the longitudinal direction. For example, alkaline earth metal ions such as Ca, Ba, Sr, zirconium dichloride oxide, Examples include oxo metal ions such as hafnium dichloride oxide.

In the oxygen reduction catalyst of the present invention, activation of oxygen molecules and reduction of oxygen with high efficiency are achieved between the porphyrin ring planes by forming the double regular structure as described above. For example, it is considered that the formation of the regular structure in the vertical direction results in the nitrogen atoms of the upper and lower porphyrin rings being aligned, contributing to the activation of the oxygen molecules and the reduction with high efficiency. It is done. In fact, in an oxygen reduction catalyst prepared using Fe (III) (TPPS ₄ ) Cl, Fe (IV) (oxo ferryl porphyrin) and Fe (V) (oxo ferryl porphyrin π-cation) are used in the potential range for oxygen reduction. radical) is indirectly confirmed, and it is considered that 4-electron reduction of oxygen proceeds by a mechanism similar to that of cytochrome P450 [2Fe (III) ← → 2Fe (V)].

  The catalyst for oxygen reduction of the present invention as described above is inert to the product obtained by adding a cation to an aqueous solution of a water-soluble porphyrin derivative having an anionic group and the central metal being a trivalent metal. It can be easily manufactured by performing a heat treatment in a gas atmosphere.

For example, when Fe (III) (TPPS ₄ ) Cl is used as the water-soluble porphyrin derivative, a solution containing a cation is added to an aqueous solution of Fe (III) (TPPS ₄ ) Cl and reacted. Examples of the cation source include alkaline earth metal chlorides and compounds of the oxo metal ions. When an alkaline earth metal chloride is added to an aqueous solution of Fe (III) (TPPS ₄ ) Cl and reacted, a precipitate is formed. This precipitate is dried and subjected to a heat treatment to obtain an oxygen reduction catalyst. The heat treatment needs to be performed in an inert gas atmosphere such as an argon atmosphere or a nitrogen atmosphere, and the heat treatment temperature is preferably set within a range of 450 ° C to 750 ° C. A more preferable temperature is around 700 ° C.

  The obtained oxygen reduction catalyst is excellent in all of the four capacities required for the oxygen reduction catalyst. For example, an oxygen reduction catalyst is first required to have a low oxygen overvoltage, but the oxygen reduction catalyst of the present invention has a performance comparable to that of platinum. Second, the oxygen reduction catalyst is required to have a high four-electron reduction ability up to oxygen water, but the oxygen reduction catalyst of the present invention exhibits performance superior to platinum in this respect. Third, the oxygen reduction catalyst is required to be resistant to poisoning by carbon monoxide or the like. Since the oxygen reduction catalyst of the present invention is not known to be coordinated to carbon monoxide such as iron ions, which are central metals, it is estimated that poisoning by carbon monoxide is lower than that of platinum. Fourth, the oxygen reduction catalyst is required to be excellent in stability in the electrolytic solution. The oxygen reduction catalyst of the present invention is clearly superior to platinum in oxygen reduction ability in an alkaline solution. In the case of a metal complex, the stability in an alkaline solution is not as severe as that in an acid aqueous solution, so there is no problem in stability.

  The aforementioned oxygen reduction catalyst can be used, for example, as an oxygen electrode catalyst of a fuel cell. In this case, the oxygen reduction catalyst described above may be supported on a conductive electrode material to form an oxygen reduction electrode. As the conductive electrode material, any material such as various metals, carbon black, carbon material such as carbon fiber, and the like can be used.

  In order to support the oxygen reduction catalyst on a conductive electrode material, for example, a product obtained by adding a cation to an aqueous solution of a water-soluble porphyrin derivative having an anionic group and a central metal being a trivalent metal ( The precipitate is applied to the conductive electrode material and then supported by a method such as drying. Alternatively, a conductive electrode material (for example, carbon black or carbon fiber) is suspended in an aqueous solution of a water-soluble porphyrin derivative, and the product is precipitated and supported on the carbon black or carbon fiber by adding a cation. Also good.

  After the product is supported on a conductive electrode material, a heat treatment is performed to make the product a heat treated product (oxygen reduction catalyst), thereby forming an oxygen reduction electrode. The conditions for the heat treatment are as described above, and it is preferable to perform the heat treatment at around 700 ° C.

  Hereinafter, specific examples to which the present invention is applied will be described based on experimental results.

Preparation of electrode for oxygen reduction Water-soluble iron porphyrin [Fe (III) (TPPS) having chloride ion Cl ⁻ as an axial ligand from negatively charged anionic porphyrin [tetra (4-sulfonatophenyl) porphyrin] ₄ ) Cl] was synthesized. Next, barium chloride (cation is Ba ²⁺ ) so that the molar ratio of water-soluble iron porphyrin and alkaline earth metal ion (barium ion) is 1:50 in the synthesized 1 mM aqueous solution of Fe (III) (TPPS ₄ ) Cl. Was added and reacted to prepare a mixed solution. This reaction condition is for the case where the side chain of the water-soluble iron porphyrin is a sodium salt of sulfonic acid, but when the side chain is a free sulfonic group, barium hydroxide is used and the molar ratio is A mixed solution was prepared at about 1: 2. About 100 μl of this mixed solution (adjusted so that the amount of iron porphyrin supported is 3 × 10 ⁻⁷ mol / cm ² ) was applied onto a carbon electrode having an area of 0.5 cm ² , dried, and then 700 ° C. in an argon atmosphere. The electrode for oxidation reduction was produced by heat treatment for 10 minutes.

The obtained electrode for oxygen reduction is referred to as electrode A. Similarly to the electrode A, strontium chloride or calcium chloride was used in place of barium chloride, and the cation was changed to Sr ²⁺ or Ca ²⁺ , and electrodes B and C were prepared in the same manner as in the case of the electrode A.

Evaluation of the produced oxygen reduction electrode The produced oxygen reduction electrodes (electrodes A to C) were measured for their ability to reduce oxygen in an acidic solution (0.5 MH ₂ SO ₄ solution). The oxygen reduction ability was measured by immersing each produced electrode in an oxygen saturated 0.5 MH ₂ SO ₄ solution and measuring a current-potential curve by cyclic voltammetry at a scanning speed of 10 mVs ⁻¹ . The results are shown in FIG. For comparison, FIG. 1 also shows the measurement results for a platinum-supported carbon electrode and an electrode (Comparative Example electrode) heat-treated with Fe (III) (TPPS ₄ ) Cl without crosslinking by cations. For the platinum-supported carbon (Pt / CB) electrode, 5 mg of Pt / CB containing 20% by mass of Pt was sampled and dispersed in 0.31 ml of 5% by mass Nafion solution, and then 10 μl of the dispersion was 0.2 cm ² in area. This was coated on a carbon electrode and dried at room temperature. The comparative electrode was prepared by applying 150 μL of 1 mM Fe (III) Cl—TPPS ₄ aqueous solution onto a 0.5 cm ² carbon electrode, drying, and then heat-treating in an argon atmosphere.

  As is clear from FIG. 1, the oxygen reducing ability of each of the electrodes A to C in the acidic solution showed almost the same value as that of the platinum-supported carbon electrode. It was also confirmed that the oxygen reducing ability was higher than that of the comparative electrode. Specifically, from the current-potential curve shown in FIG. 1, the catalytic activity of oxygen reduction can be compared. For example, the more positive the peak potential of oxygen reduction, the smaller the overvoltage of oxygen reduction, and the fuel cell electrode When used for, the electromotive force increases. In addition, the larger the reduction peak current, the larger the current that can be extracted when used for an electrode of a fuel cell. In each of the electrodes A to C, the peak potential of oxygen reduction is positive as compared with the comparative example electrode, which is a value close to that of the platinum-supporting carbon electrode. Also, the reduction peak current is larger than that of the comparative example electrode.

Next, the electrode A was measured for oxygen reduction ability in an alkaline solution (1.0 M KOH solution). The oxygen reduction ability was measured by immersing each produced electrode in an oxygen saturated 1.0 M KOH solution and measuring a current-potential curve by cyclic voltammetry at a scanning speed of 10 mVs ⁻¹ . The results are shown in FIG. In addition, in FIG. 2, the measurement result about a smooth platinum electrode (0.5 cm < ² >) and a comparative example electrode is also shown for a comparison.

  As is apparent from FIG. 2, it can be seen that the electrode A is superior in oxygen reducing ability as compared with the smooth platinum electrode and the comparative example electrode even in the measurement in the alkaline solution.

  Further, a rotating ring-disk electrode was produced by the same method as that for electrode A, and the ratio of the oxygen reduction disk current (Idisc) to the hydrogen peroxide oxidation ring current (Iring) was determined. As a result, in the rotating ring-disk electrode using platinum, Iring / Idisc = 0.13, whereas in the rotating ring-disk electrode using the oxygen reduction catalyst of the present invention, Iring / Idisc = It was found to be 0.005, hardly producing hydrogen peroxide and superior to platinum.

In addition, oxygen reduction includes a catalyst that can reduce oxygen to water with 4 electrons and a catalyst that can reduce hydrogen to only hydrogen peroxide with 2 electrons. is not. For the above Iring / Idisc, Ba ²⁺ cross-linked iron porphyrin (rotating ring-disk electrode prepared by the same method as that for electrode A) and platinum-supported carbon electrode (Pt / CB) were compared. It was found that Ba ²⁺ cross-linked iron porphyrin was superior.

Table 1 summarizes the oxygen reduction activity in acidic solution for Fe (III) (TPPS ₄ ) Cl crosslinked with various crosslinking agents (cations: M ²⁺ and MO ²⁺ ).

It is a characteristic view which shows the measurement result of the oxygen reduction ability in an acidic solution. It is a characteristic view which shows the measurement result of the oxygen reduction ability in an alkaline solution.

Claims (17)

A catalyst for oxygen reduction mainly comprising a heat-treated product of a water-soluble porphyrin derivative having an anionic group and a central metal being a trivalent metal,
A catalyst for oxygen reduction, characterized by having a double regular structure in which anion groups and metals of the water-soluble porphyrin derivative are linked by cations before heat treatment. The oxygen-reducing catalyst according to claim 1, wherein the water-soluble porphyrin derivative is a compound represented by the following formula (I).

(In the formula, M represents a trivalent metal, X represents a halogen atom, and Y represents an anionic group.) The oxygen-reducing catalyst according to claim 1, wherein the water-soluble porphyrin derivative is a water-soluble iron porphyrin represented by the following formula (II).

  Oxygen characterized by adding a cation to an aqueous solution of a water-soluble porphyrin derivative having an anionic group and a central metal being a trivalent metal, and subjecting the resulting product to heat treatment in an inert gas atmosphere A method for producing a catalyst for reduction.   5. The method for producing an oxygen reduction catalyst according to claim 4, wherein the cation is an alkaline earth metal ion or an oxo metal ion.   5. The method for producing an oxygen reduction catalyst according to claim 4, wherein an alkaline earth metal chloride is added as a cation source. The method for producing an oxygen reduction catalyst according to any one of claims 4 to 6, wherein the water-soluble porphyrin derivative is a compound represented by the following formula (I).

(In the formula, M represents a trivalent metal, X represents a halogen atom, and Y represents an anionic group.) The method for producing an oxygen reduction catalyst according to any one of claims 4 to 6, wherein the water-soluble porphyrin derivative is a water-soluble iron porphyrin represented by the following formula (II).

An oxygen reduction electrode in which an oxygen reduction catalyst mainly composed of a heat-treated product of a water-soluble porphyrin derivative having an anionic group and a central metal being a trivalent metal is supported on the surface of a conductive electrode material,
2. The oxygen reduction electrode according to claim 1, wherein the oxygen reduction catalyst has a double regular structure in which anion groups and metals of the water-soluble porphyrin derivative are linked by cations before heat treatment.   The electrode for oxygen reduction according to claim 9, wherein the conductive electrode material is carbon. The electrode for oxygen reduction according to claim 9 or 10, wherein the water-soluble porphyrin derivative is a compound represented by the following formula (I).

(In the formula, M represents a trivalent metal, X represents a halogen atom, and Y represents an anionic group.) The oxygen-reducing electrode according to claim 9 or 10, wherein the water-soluble porphyrin derivative is a water-soluble iron porphyrin represented by the following formula (II).

  After reacting by adding a cation to an aqueous solution of a water-soluble porphyrin derivative having an anionic group and the central metal being a trivalent metal, the resulting product is supported on a conductive electrode material, and then in an inert gas atmosphere. A method for producing an oxygen reduction electrode, characterized by performing a heat treatment.   14. The method for producing an oxygen reduction electrode according to claim 13, wherein a carbon material is used as the conductive electrode material, and the heat treatment is performed after the product is applied and dried on the carbon material.   The carbon black or carbon fiber, which is a conductive electrode material, is suspended in an aqueous solution of the water-soluble porphyrin derivative, and the product is precipitated on the carbon black or carbon fiber by adding a cation. 14. A method for producing an oxygen reduction electrode according to 13. 16. The method for producing an oxygen reducing electrode according to claim 13, wherein the water-soluble porphyrin derivative is a compound represented by the following formula (I).

(In the formula, M represents a trivalent metal, X represents a halogen atom, and Y represents an anionic group.) 16. The method for producing an oxygen reduction electrode according to claim 13, wherein the water-soluble porphyrin derivative is a water-soluble iron porphyrin represented by the following formula (II).

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