JP5059378B2 - Fuel cell electrode catalyst, method for producing the same, and fuel cell - Google Patents

Description

  The present invention relates to an electrode catalyst for a fuel cell, and more particularly to an electrode catalyst composed of a metal complex or a metal cluster and a method for producing the same.

  A polymer electrolyte fuel cell (hereinafter abbreviated as PEFC) has been developed as a distributed fuel cell for automobiles and general households, and direct methanol fuel that uses methanol as the power source for portable electronic devices Batteries (hereinafter abbreviated as DMFC) have been developed. The heart of these fuel cells is an electrode, which consists of an anode and a cathode. Since this type of fuel cell generally uses platinum as the cathode electrode material and platinum-ruthenium as the anode electrode material, the battery material cost is high.

An outline of the operation principle of the fuel cell will be described by taking DMFC as an example. The DMFC is composed of an anode electrode (fuel electrode) and a cathode electrode (air electrode). At the anode electrode, as shown in the equation (1), methanol as a fuel and water react to generate hydrogen ions (hereinafter abbreviated as H ⁺ ), electrons (hereinafter abbreviated as e ⁻ ), and CO ₂ . On the other hand, as shown in the equation (2), H ⁺ that has passed through the electrolyte membrane reacts with O _{2 in} the air supplied from the outside to generate water as shown in the equation (2). A current can be obtained by connecting the anode and cathode with an external circuit.

Reaction at anode electrode: CH ₃ OH + H ₂ O → 6H ⁺ + CO ₂ + 6e ⁻ (1)
Reaction at cathode electrode: 6H ⁺ + 3 / 2O ₂ + 6e ⁻ → 3H ₂ O (2)
As active components for promoting these reactions, platinum and ruthenium are generally used at the anode electrode and platinum is used at the cathode electrode. On the other hand, in PEFC, platinum is used for both the anode and cathode.

  Thus, platinum is an important component in fuel cells, but it is very expensive. For this reason, research which raises the performance of a catalyst and reduces the usage-amount of platinum is made (for example, refer nonpatent literature 1).

The Chemical Society of Japan, 1988, (8), p. 1426-1432

  In fuel cells such as PEFC and DMFC, noble metals such as platinum are used as the electrode catalyst material, so there is a limit to reducing the fuel cell cost.

  An object of the present invention is to provide an electrode catalyst for a fuel cell, a method for producing the same, and a fuel cell so that high catalytic activity can be obtained without using expensive platinum.

  The present invention relates to an electrode catalyst for a fuel cell in which a metal complex or metal cluster in which a metal is covered with a ligand is supported on a carbon support or an inorganic oxide support, wherein the ligand has a carbonyl group, The carbon support or inorganic oxide support has a hydroxyl group, and 1 g of the carbon support is mixed with 10 ml of boiling distilled water, and the pH of the suspension containing carbon after cooling is less than 9.

  The present invention provides a fuel cell electrode catalyst in which a metal complex or metal cluster in which a metal is covered with a ligand is supported on a carbon support or an inorganic oxide support, wherein the ligand contains at least one carbon and hydrogen atom. It is an organic substance containing one piece, and 1 g of the carbon support is mixed with 10 ml of boiling distilled water, and the pH of the suspension containing carbon after cooling is 9 or more.

  The present invention provides an electrode catalyst for a fuel cell having the above-described configuration, wherein the metal component supported on the carbon support or the inorganic oxide support is gold, tungsten, copper, cobalt, nickel, iron, manganese, palladium, rhenium, osmium And at least one selected from iridium, rhodium, ruthenium, and platinum.

  The present invention is characterized in that, in the above-described electrode catalyst for a fuel cell, the carbon support contains at least one selected from a sulfur atom and a nitrogen atom.

  In the fuel cell electrode catalyst according to the present invention, the carbon support is carbon black, a carbon nanotube, or a fullerene compound, and the inorganic oxide support is a composite oxide containing silicon, titanium, and aluminum. .

  In the present invention, a metal complex or metal cluster in which a metal periphery is covered with a ligand having a carbonyl group is supported on a carbon carrier having a hydroxyl group and a pH of less than 9.0 or an inorganic oxide carrier having a hydroxyl group. In the method for producing a fuel cell electrode catalyst, the mixture of the metal complex or metal cluster and the support is heat-treated under vacuum.

  The present invention relates to a fuel in which a metal complex or metal cluster in which a metal is covered with a ligand made of an organic substance containing at least one carbon and hydrogen atom is supported on a carbon support or an inorganic oxide support having a pH higher than 9. In the method for producing an electrode catalyst for a battery, the metal complex or the mixture of metal clusters and a support is heat-treated under vacuum.

  According to the present invention, in a membrane / electrode assembly including an electrolyte membrane that transmits hydrogen ions between an anode electrode and a cathode electrode, an electrode catalyst having the above-described configuration is provided in at least one of the anode electrode and the cathode electrode. Features.

  The present invention resides in a fuel cell including the membrane / electrode assembly. Further, the present invention resides in a portable electronic device equipped with the fuel cell as a power source and a polymer electrolyte fuel cell system.

  The catalyst of the present invention has good affinity between the catalytically active component and the carrier, and high electrode performance can be obtained even when a non-platinum element is used as the catalytically active component. By applying the catalyst of the present invention to an electrode catalyst of a fuel cell, a low-cost fuel cell system that does not use expensive platinum as an active component can be realized.

  The electrode catalyst of the present invention is extremely suitable for application to PEFC and DMFC. In the electrode catalyst of the present invention, Co is very suitable as a catalytically active component. Co has a much lower price per unit weight than platinum, so if the performance per unit weight is improved by metal complexing or metal clustering of Co, the cost of fuel cells using this will be greatly reduced. can do. Here, a metal cluster is defined as a group of 3 or more metal groups with bonds between metals and surrounded by a ligand, and is a group of unique compounds positioned between a bulk metal and a metal complex. is there.

  As the carbon carrier, carbon black, carbon nanotubes or fullerene compounds are preferred. The inorganic oxide carrier is preferably mesoporous silica, mesoporous alumina, or mesoporous titanium. These carrier materials can be made into a porous carrier having a high specific surface area, and can carry a catalytically active component in a highly dispersed manner.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited to these Examples.

  In this example, a method for producing an Au complex catalyst will be described.

HAuCl ₄ · 4H ₂ O and triphenylphosphine were collected in separate flasks, and acetone was added to both flasks to prepare solutions. The flask containing triphenylphosphine was ice bathed at 0 ° C., and a HAuCl ₄ .4H ₂ O solution dissolved in acetone was collected with a syringe and added in small portions to the triphenylphosphine solution. When white acicular crystals were precipitated, the flask was evacuated to evaporate acetone, and the crystals were dried to obtain AuCl (PPh ₃ ). Next, a carbon support and AuCl (PPh ₃ ) were placed in a Schlenk tube, and tetrahydrofuran was added thereto.

The added amount of AuCl (PPh ₃ ) was set such that the Au weight relative to the weight of the carbon support, that is, the Au loading rate was 33 wt%. The carbon carrier used here is a carbon black 1 carrier (hereinafter referred to as C1), which is one of conductive carbon blacks. The pH of this carrier is 9.0. The pH measurement method is described below.

  After weighing 1 g of a sample and collecting it in a 20 ml beaker, 1 ml of ethyl alcohol and 11 ml of distilled water boiled in advance were added, the beaker was covered and allowed to cool for 60 minutes. Thereafter, the pH of the suspension was measured with a pH meter.

These mixtures were stirred at room temperature for 3 hours. After stirring, the inside of the Schlenk tube was evacuated to evaporate tetrahydrofuran. Finally, in order to fix AuCl (PPh ₃ ), a catalyst (hereinafter referred to as Au complex / C1), which was heat-treated at a temperature of 185 ° C. under vacuum for 2 hours with stirring to carry Au complex on a carbon black 1 carrier. (Denoted as catalyst).
(Comparative Example 1)
For comparison with the catalyst of the present invention, an Au bulk catalyst (hereinafter referred to as Au bulk / C1 catalyst) was prepared. Water is added to the mixture of the gold chloride solution and the carbon carrier, which is kneaded for 30 minutes with a lyi machine and then dried at 120 ° C. for 2 hours. Finally, 3% hydrogen and 97% argon gas are added at 100 cc / min. The mixture was circulated at a flow rate and baked at 500 ° C. for 3 hours. The Au loading rate was 33 wt%.

  FIG. 1 shows the results of evaluating the oxygen reduction activity, which is a performance index of the cathode electrode catalyst, for the Au complex / C1 catalyst and the Au bulk / C1 catalyst. The horizontal axis represents the potential, and the vertical axis represents the relative value of the oxygen reduction current. The larger the relative value of the oxygen reduction current at the same potential, the higher the cathode electrode catalyst performance. Here, the oxygen reduction current value at 0.3 V of the Au complex catalyst was set to 1.0. As can be seen from FIG. 1, the performance of the Au complex catalyst is higher than that of the conventional Au bulk catalyst. For example, it has a performance of about 4 times at 0.3V, 4.4 times at 0.4V, about 4.9 times at 0.5V, and 4.6 times at 0.6V. From the above, it was demonstrated that the performance of the Au complex electrode catalyst of the present invention is high.

The oxygen reduction activity of platinum was performed by the rotating disk electrode method. This technique is characterized in that the activity can be evaluated by eliminating the influence of diffusion by utilizing the fact that the supply amount of the reactant is proportional to the 1/2 power of the angular velocity ω (rad / s) of the disk electrode. The electrolyte was an H ₂ SO ₄ solution, and O ₂ bubbling was performed for 1 hour or more before measurement. The measurement temperature is 35 ° C. Oxygen reduction activity was measured at a stamping speed of 10 mV / s and a stamping range of 0.2 to 1.1 V vs. Performed with NHE. When measuring the oxygen reduction activity, the disk electrode of the working electrode was rotated at various rotational speeds. The number of rotations is 400, 625, 900, 1600, 2500 rpm. The reduction current of oxygen increases as the number of reactants supplied increases as the rotational speed increases. Measured 0.7V vs. The relationship between the reciprocal of the current value I (mA) in NHE and the -1/2 power of the angular velocity ω (rad / s) of the electrode is expressed by the Koutecky-Levich equation shown in the equation (3).

Here, i _K : activity dominant current (mA), n: number of reaction electrons, F: Faraday constant (C / mol), A: geometric area (cm ² ) of disk electrode, c: activity of reactant (mol / ml), D: diffusion coefficient of the reactant (cm ² / s), v: kinematic viscosity coefficient (cm ² / s) of the solution, and the number of rotations f (rpm) and angular velocity ω (rad / s) of the electrode. The relationship is ω = 2πf / 60.

In equation (3), the reciprocal of i _K can be obtained from the intercept of ω ^−1/2 = 0 (ω = ∞, that is, the amount of reactant supplied is infinite). Thus, the i _K obtained is the net activity of the catalyst without the influence of reactant diffusion.

  In this example, a method for producing a Co cluster will be described.

Co ₂ (CO) ₈ was placed in the flask and heated to 60 ° C. in an oil bath with stirring under N ₂ . Heating was performed for 20 hours. After heating, the flask was gradually cooled to room temperature to obtain Co ₄ (CO) ₁₂ . Next, a carbon support was placed in a Schlenk tube, and anhydrous ether was added thereto. The amount of Co ₄ (CO) ₁₂ added was such that the Co weight relative to the weight of the carbon support was 10 wt%. The carbon carrier used here is a carbon black 2 carrier (hereinafter referred to as C2), which is one of conductive carbon blacks. These mixtures were stirred at room temperature for 3 hours. After stirring, the inside of the Schlenk tube was evacuated to evaporate anhydrous ether. Finally, in order to fix Co ₄ (CO) ₁₂ , a catalyst (hereinafter referred to as a Co cluster / carbon catalyst) in which Co clusters are supported on a carbon black 2 support by heating at 185 ° C. under vacuum for 2 hours with stirring. C2 catalyst) was prepared.
(Comparative Example 2)
For comparison with a Co cluster / C2 catalyst, a Co bulk catalyst (hereinafter referred to as a Co bulk / C2 catalyst) was prepared. Water is added to the mixture of cobalt nitrate and carbon support, and this is kneaded for 30 minutes with a raikai machine and then dried at 120 ° C. for 2 hours. Finally, 3% hydrogen and 97% argon gas are supplied at a flow rate of 100 cc / min. The catalyst was produced by calcining at 500 ° C. for 3 hours. Co loading was 10 wt%.

  FIG. 2 shows the results of evaluating the oxygen reduction activity of the Co-based catalyst, which is a performance index of the cathode electrode catalyst, for the Co cluster / C2 catalyst and the Co bulk / C2 catalyst. The horizontal axis represents the potential, and the vertical axis represents the relative value of the oxygen reduction current. The larger the value of the oxygen reduction current at the same potential, the higher the cathode electrode catalyst performance. Here, the oxygen reduction current value at 0.3 V of the Co cluster / C2 catalyst was set to 1.0. As can be seen from FIG. 2, the performance of the Co cluster catalyst is higher than that of the conventional Co bulk catalyst. For example, it has about twice the performance at 0.3 V, about 4 times at 0.4 V, and about 6 times at 0.5 V. From this, it was demonstrated that the performance of the Co cluster electrode catalyst of the present invention is high.

  In FIG. 3, the oxygen reduction current value at 0.6 V is converted into the performance per unit price (mA / ¥) of the metal selected as the catalytically active component, and the relative value is shown. Here, the performance of a catalyst (hereinafter referred to as a Co cluster / C2 (CSx) catalyst) in which a Co cluster is supported on a carbon support (hereinafter referred to as C2 (CSx)) doped with sulfur on a C2 carbon support is defined as 100. . Compared with a platinum bulk catalyst generally used as an electrode catalyst material, the Au complex / C1 catalyst produced in Example 1 has 1/10 performance, but the Co cluster / C2 catalyst produced in Example 2 The performance is about 120 times that of the platinum catalyst. Furthermore, the performance of the Co cluster / C2 (CSx) catalyst in which sulfur is doped on the C2 carbon support is equivalent to about 200 times that of the platinum catalyst, and it is expected that the cost of the electrode material can be greatly reduced.

  FIG. 4 shows the relative values of the oxygen reduction currents of the Au complex / C1 catalyst prepared in Example 1 and a catalyst in which the Au complex is supported on a carbon black 2 carrier (hereinafter referred to as Au complex / C2 catalyst). Here, the oxygen reduction current value at 0.3 V of the Au complex / C1 catalyst was set to 1.0.

  As can be seen from FIG. 4, the performance of the Au complex / C1 catalyst exceeded that of the Au complex / C2 catalyst. The C1 carrier has a smaller number of hydrophilic groups such as hydroxyl groups than the C2 carrier, and is a hydrophobic carrier. On the other hand, the Au complex produced in Example 1 has three phenyl groups and is considered to be a hydrophobic complex. Therefore, since the affinity between the hydrophobic materials is good, it is considered that the dispersibility of the Au complex in the C1 carrier is improved and the catalyst performance is improved.

  FIG. 5 shows the relative oxygen reduction currents of the Co cluster / C2 catalyst prepared in Example 2 and the catalyst having the Co cluster prepared in Example 2 supported on a C1 carrier (hereinafter referred to as Co cluster / C1 catalyst). Indicates the value. Here, the oxygen reduction current value at 0.3 V of the Co cluster / C2 catalyst was set to 1.0. The C1 and C2 carriers are the same as in Example 3.

As can be seen from FIG. 5, the performance of the Co cluster / C2 catalyst exceeded that of the Co cluster / C1 catalyst. The C2 carrier has more hydrophilic groups such as hydroxyl groups than the C1 carrier, and is a hydrophilic carrier. This hydroxyl group releases H ⁺ to form O ⁻ ions. In contrast, the Co cluster produced in Example 2 has 12 carbonyl groups, and the carbon atoms are weak because of the electron withdrawing property of the oxygen atom located next to the carbon atom of the carbonyl group. It has a positive charge. Therefore, the positive charge on the carbonyl group carbon of the Co cluster and the O ⁻ ion in the carbon support are easily combined, so that the affinity between the Co cluster and the carbon support is improved. As a result, it is considered that the dispersibility of the Co cluster on the C2 support is improved and the catalyst performance is improved.

  FIG. 6 shows the relative values of the oxygen reduction currents of the catalysts produced by the catalyst production method 1 and the catalyst production method 2. The catalyst production method 1 indicated by black circles is a catalyst exhausted at 185 ° C. under vacuum, and is a catalyst produced by the same production method as in Example 1. The catalyst manufacturing method 2 indicated by white circles is a catalyst which was exhausted at room temperature instead of the 185 ° C. processing of the catalyst manufacturing method 1. Here, in the Au complex / C1 catalyst produced by the catalyst production method 1, the oxygen reduction current value at 0.3 V was 1.0.

  As can be seen from FIG. 6, the performance of the catalyst exhausted at 185 ° C. under vacuum greatly exceeded the performance of the catalyst exhausted at room temperature. Here, the temperature of the heat treatment is desirably 100 ° C. to 300 ° C. When the temperature is lower than 100 ° C., the exhaust effect is reduced. When the temperature is higher than 300 ° C., the metal particles as the catalytically active component are aggregated and the metal particles are coarsened. The catalytic activity per volume is reduced.

It is the figure which showed the relationship between the electric potential of the catalyst which uses Au as an active component, and the oxygen reduction current relative value. It is the figure which showed the relationship between the electric potential of the catalyst which uses Co as an active component, and the oxygen reduction current relative value. It is the figure which compared the performance relative value per active metal price of each catalyst. It is the figure which showed the relationship between the electric potential of the catalyst which uses Au complex as an active ingredient, and oxygen reduction current relative value. It is the figure which showed the relationship between the electric potential of the catalyst which uses a Co cluster as an active ingredient, and the oxygen reduction current relative value. It is the figure which showed the relationship between an electric potential and an oxygen reduction current relative value about the catalyst produced with the different manufacturing method.

Claims (9)

  A fuel cell electrode catalyst in which a metal support is coated with a Co cluster covered with a ligand on a carbon support, wherein the ligand has a carbonyl group, and the carbon support has a hydroxyl group. Battery electrode catalyst.   2. The fuel cell electrode catalyst according to claim 1, wherein the carbon support is carbon black. The pH of the suspension after 1 g of carbon black was mixed with 1 ml of ethyl alcohol and 11 ml of distilled water previously boiled and cooled was 9 . The fuel cell electrode catalyst according to claim 2, which is zero .   2. The fuel cell electrode catalyst according to claim 1, wherein the carbon support contains sulfur.   A method for producing an electrode catalyst for a fuel cell in which a Co cluster covered with a ligand having a carbonyl group around a metal is supported on a carbon support having a hydroxyl group, wherein the mixture of the Co cluster and the carbon support is evacuated. The manufacturing method of the electrode catalyst for fuel cells characterized by heat-processing below. In a membrane / electrode assembly including an electrolyte membrane that allows hydrogen ions to pass between an anode electrode that oxidizes fuel and a cathode electrode that reduces oxygen, at least one of the anode electrode and the cathode electrode is claimed in claims 1 to 4. A membrane / electrode assembly comprising the fuel cell electrode catalyst according to any one of Items 4 to 5 . A fuel cell comprising the membrane / electrode assembly according to claim 6 . A portable electronic device driven by a fuel cell, comprising the fuel cell according to claim 7 . A solid polymer fuel cell system comprising the fuel cell according to claim 7 .

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