JPS6347115B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing an electrode for a fuel cell, and particularly to a method for manufacturing an electrode for a fuel cell from an electrode catalyst in which fine metal particles are deposited on carbon powder. Generally, the catalytic activity of a platinum catalyst can be increased by making the platinum particles finer and uniformly dispersing them on a carrier, and the amount of platinum used can also be reduced by uniformly depositing the platinum particles on the carrier. It is known that it can be done. Many methods for preparing platinum catalysts have been known. Generally, activated carbon or carbon black is impregnated with a platinum-containing solution, and this is transferred onto a carrier by wet reduction using chemicals or dry reduction using reducing gas. There is a method of precipitating platinum particles. Recently, Henry the Petrou (Japanese Patent Publication No. 51
-88478) forms a platinum sulfite complex,
Fine platinum particles are then deposited on the carrier by oxidation treatment. Also, Vinotsudo
Motilal Jiyaran (JP 54-92588) disclosed that it is possible to deposit platinum particles of less than 50 Å on a carbon support from a chloroplatinic acid solution using sodium dithionate and hydrogen peroxide. ing. Furthermore, Fred Stanley Kemp (Japanese Patent Application Laid-Open No. 56545/1983) deposited platinum particles of about 20 Å on a carbon carrier by impregnating carbon powder with platinum diamide nitrite and thermally decomposing it. . On the other hand, a method for preparing a dispersion of colloidal platinum using polyvinyl alcohol (hereinafter abbreviated as PVA) as a protective colloid was previously reported by Nord et al. [J.Am.Chem. Soc.) No. 63
Volume 2,745 et seq. (1941)], and recently there has been a commentary by Yukimichi Nakao and Hidefumi Hirai ["Surface" Vol. 17 No. 4,
279-289 (1979)]. However, the former method has drawbacks such as requiring complicated steps and depending on the carrier, the deposition rate of platinum particles on the carrier is poor. Regarding the latter, the relationship between the surface condition of the carrier (related to zeta potential) and the protective colloid is not clear, and when a carbon carrier coexists, the dispersibility of platinum particles deposited on the carrier and the degree of deposition on the carrier has not been disclosed at all. The purpose of the present invention is to prepare an electrode catalyst in which fine metal particles of 10 to 30 Å are uniformly dispersed and supported on the support by changing the surface condition of the carbon support and the type and amount of protective colloid added, and Mixing a water repellent and water, and optionally carbon fiber,
By applying the mixture to a conductive porous base material and firing the molded electrode, a fuel cell electrode with good performance can be obtained. That is, to summarize the present invention, the present invention involves causing a reducing agent to act on an aqueous solution containing noble metal ions or their complex ions, protective colloid, and carbon powder to deposit noble metal fine particles on the carbon powder. mixing the electrocatalyst with a water repellent and water and optionally carbon fiber; applying the resulting mixture to a conductive porous substrate; The present invention relates to a method for manufacturing an electrode for a fuel cell, which includes the steps of firing a molded body. Hereinafter, the present invention will be explained in detail step by step. The noble metal used in the present invention is preferably one of the group 8 noble metals of the periodic table or a mixture thereof, and among these, platinum is usually preferred, so platinum will be explained below as a representative example, but it is not limited to platinum. . The amount of platinum supported on platinum catalysts used as electrode catalysts for fuel cells is generally 0.1 to 20% by weight,
In view of the relationship between the amount of platinum per unit area of the electrode and battery performance, the amount is preferably 1 to 10% by weight. In order to obtain platinum finely and uniformly dispersed in such a supported amount of platinum, attention must be paid to the type of reducing agent and the weight ratio of protective colloid to platinum. Various reducing agents can be used. However, alcohols or ethers are preferred, with methanol being most preferred. The particle size and particle size distribution of the precious metal particles deposited on the carrier are
Although it varies depending on the type of reducing agent, by using alcohol, particularly methanol, as the reducing agent, the particle size of the noble metal particles becomes extremely small and the particle size distribution becomes narrow. As the alcohol or ether, in addition to the methanol, one selected from ethanol, isopropyl alcohol, 2-methoxyethanol, and 1,2-dimethoxyethane can be used. As the protective colloid, a hydrophilic polymer compound is desirable. The noble metal particles deposited on the carrier are water-repellent and tend to aggregate with each other in water. Therefore, in order to prevent agglomeration of the noble metal particles and improve the dispersion state, it is optimal to use a protective colloid and use a hydrophilic polymer compound as the protective colloid. These adsorb to the surface of noble metal particles and prevent agglomeration. Examples of hydrophilic polymer compounds include PVA, polymethyl vinyl ether, polyvinylpyrrolidone, polymethyl methacrylate, ethylene oxide polyadducts of alkylphenols, gelatin, and gum arabic. As the carrier material, carbon such as graphite, acetylene black, carbon black, activated carbon, etc. is used. It also needs to be a powder. By using carbon powder, fine and uniform dispersion of noble metal particles can be achieved. In the preparation process of the electrode catalyst in the present invention,
The most important thing is that in a mixed system of carrier-reducing agent-protective colloid-platinum salt, the size and dispersibility of the platinum particles deposited on the carrier depend on the type and concentration of the reducing agent or the weight ratio of the protective colloid and platinum. This is a marked difference. This can be explained by the following experimental facts. That is, the platinum particles produced by reduction in the liquid phase are hydrophobic, and in the absence of a protective colloid, the platinum particles aggregate with each other due to their hydrophobic attraction, causing particle growth. In this case, if a protective colloid is present in the liquid phase, the platinum particles will be adsorbed onto the surface of the hydrophilic polymer compound, and the surface of the platinum particles will be covered with hydrophilic groups, so that they will not aggregate. A representative example of the preparation of an electrocatalyst according to the invention, i.e. 10% by weight platinum on an acetylene black support.
To explain the case where the platinum particles are supported, when water-methanol (1:1) is used as the reducing agent and there is no protective colloid, the particle size of the platinum particles on the support is several hundred to several thousand angstroms. However, when PVA is used as a protective colloid, PVA/
If the weight ratio of Pt is set to about 1.0, the diameter of the primary platinum particles will be 10 to 30 Å, and these will be uniformly dispersed on the carrier in clusters of about several dozen particles. Furthermore, when the weight ratio of PVA/Pt is increased, the size of the primary platinum particles on the carrier does not change, but the number of colonies is only a few, and the dispersibility is greatly improved.
Platinum particles are no longer quantitatively deposited on the carrier, and solid-liquid separation after preparing the electrocatalyst becomes difficult. Furthermore, when the weight ratio of PVA/Pt becomes less than 1, the number of primary platinum particle colonies increases, and the colonies also become deviated on the carrier. Electron micrographs of platinum particles on electrocatalyst supports prepared with different PVA/Pt ratios show that the primary particles are 10 to 30
It appears that it is an aggregate of Å. Furthermore,
It can be seen that as the amount of PVA added increases, the number of primary particle colonies decreases and dispersibility improves. However, care must be taken as increasing the amount of PVA has the above-mentioned problems. Therefore, it is desirable to increase the total liquid volume without increasing the proportion of PVA. In the electrode catalyst preparation step of the present invention, depending on the type of protective colloid used, it may be preferable to heat-treat the obtained noble metal catalyst supported on carbon powder. Of course, depending on the type of protective colloid, this heat treatment may not be necessary at all. This heat treatment is for thermally decomposing the protective colloid present on the surface of the noble metal particles. The heat treatment can be performed in an atmosphere of oxidizing gas, reducing gas, or inert gas. The heat treatment temperature is
In the case of an oxidizing gas atmosphere, the temperature is preferably 50 to 350℃.
50 to 1000 for reducing gas and inert gas atmospheres
℃ is preferable. The best catalytic activity can be obtained when the heat treatment is carried out in alternating cycles of oxidizing gas and reducing gas atmospheres. Next, a process of mixing the electrode catalyst obtained as described above and water will be explained. Water-repellent agents are used to provide binding and water-repellent properties. As the water repellent agent, it is preferable to use Teflon dispersion. Carbon fibers also help improve the mechanical strength of the catalyst layer. When mixing these, it is preferable to use an admixture in order to blend them all together, examples of which include alcohol, surfactants, and the like. The conductive porous base material used in the molding process may be any commonly used material, and carbon paper is preferred. Calcination may be carried out either in air or in an inert gas atmosphere, such as nitrogen; best results were obtained with calcination at 310-340°C. The following describes the performance of an electrode obtained by preparing an electrode catalyst according to the present invention, forming an electrode using the same, and firing the electrode as an air electrode compared to a standard hydrogen electrode. In addition, Examples 1 to 4 and Comparative Example 1 illustrate the production of an electrode catalyst. Example 1 In a solution of 250 ml of methanol and 225 ml of water,
1.0g of commercially available PVA was dissolved. This includes 20mgPt/
ml of chloroplatinic acid solution was added, followed by 5.0 g of commercially available acetylene black. This was transferred to a round bottom flask equipped with a reflux condenser and heated under reflux at about 70°C while stirring vigorously. Approximately 4 after the start of the reaction
More than 99% of the platinum in the solution was deposited on the acetylene black over a period of time, and a platinum catalyst supported on the acetylene black was obtained. When this catalyst was washed with water, dried, and observed using an electron microscope, it was confirmed that more than a dozen primary platinum particles with a size of 10 to 30 Å formed a colony and were uniformly dispersed on the carrier. . Example 2 1.0 g of commercially available PVA was dissolved in a solution of 250 ml of isopropyl alcohol and 225 ml of water. To this was added 25 ml of a chloroplatinic acid solution containing 20 mg Pt/ml, and then 5.0 g of commercially available acetylene black. This was heated under reflux at about 70° C. while stirring vigorously. About 4 hours after the start of the reaction, more than 99% of the platinum in the solution was deposited on the acetylene black carrier. When observed in the same manner as in Example 1, it was found that 10 to 30 Å
It was confirmed that approximately 20 platinum primary particles formed a colony and were uniformly dispersed on the carrier. Example 3 In a solution of 250 ml of methanol and 175 ml of water,
10% by weight aqueous solution of octylphenoxyethoxyethanol (trade name: Toriton X-100) 50
ml, 25 ml of a chloroplatinic acid solution containing 20 mg Pt/ml, and then 5.0 g of commercially available acetylene black. This was heated under reflux at about 70° C. while stirring vigorously. About 4 hours after the start of the reaction, more than 99% of the platinum in the solution was deposited on the acetylene black carrier. When observed in the same manner as in Example 1, it was confirmed that approximately a few primary particles of platinum with a diameter of 10 to 30 Å formed a colony and were uniformly dispersed on the carrier. Example 4 In a solution of 250 ml of methanol and 220 ml of water,
1.0g of PVA was dissolved. To this was added 25 ml of chloroplatinic acid containing 20 mg Pt/ml and 5 ml of gold chloride solution containing 10 mg Au/ml, and then 5.0 ml of acetylene black was added.
g was added. Stir this vigorously and heat to about 70℃.
The mixture was heated to reflux. Approximately 4 hours after the start of the reaction, more than 99% of the platinum and gold in the solution were deposited on the acetylene black carrier. When observed in the same manner as in Example 1,
A dozen or so primary platinum particles with a diameter of 10 to 30 Å form a colony, and between them particles of about 15 Å, which are estimated to be gold particles, are uniformly dispersed on the carrier without forming a colony. confirmed. Comparative Example 1 Add 20% to a solution of 250ml of methanol and 225ml of water.
Add 25 ml of chloroplatinic acid solution containing mgPt/ml,
Next, 5.0 g of acetylene black was added, and the mixture was heated under reflux at about 70° C. while stirring vigorously. About 4 hours after the start of the reaction, more than 99% of the platinum in the solution was deposited on the acetylene black carrier. When observed in the same manner as in Example 1, it was confirmed that it was difficult to identify the primary particles of platinum, and that platinum was unevenly distributed on the carrier in the form of giant agglomerates of approximately several thousand Å. Example 5 In this example, the unipolar performance as an air electrode of the electrode formed using the electrode catalyst prepared in Examples 1 to 4 was evaluated. 1 of each of the electrode catalysts prepared in Examples 1 to 4
Weigh out 1.5 g, add water containing 25% by volume of ethanol, and thoroughly knead. Thereafter, Teflon dispersion liquid was added to give a concentration of 24% by weight based on the electrode catalyst, and the mixture was further kneaded and applied to carbon paper, thoroughly air-dried, and then dried at 130°C for 2 hours. This material was fired in air at 340°C for 20 minutes to form a test electrode. These electrodes were evaluated for performance at the unipolar potential of the air electrode relative to a standard hydrogen electrode. Measured at 190℃
in phosphoric acid.

[Table] Comparative Example 2 In this comparative example, the monopolar performance as an air electrode was measured for the electrode prepared in Comparative Example 1, which did not use a protective colloid, and formed in the same manner as in Example 5. .

[Table] Example 6 In this example, the electrode [A] obtained in Example 5 was used.
The optimum range of temperature for firing in air was determined. Baking time is 20 minutes.

[Table] As is clear from the above table, no difference in performance was observed when the firing temperature after electrode formation was in the range of 310 to 340°C. Example 7 In this example, electrode [A] obtained in Example 5
We investigated the effect of adding carbon fiber to the above. That is, for 1 g of electrode catalyst, 20, 40,
Add 60% by weight and thoroughly mix in a mortar, add water containing 2.5% by volume of ethanol and knead to give 24% by weight per catalyst.
After adding Teflon dispersion to the weight percentage and kneading, it was applied to carbon paper, air-dried, and then baked in the air at 340℃ for 20 minutes, and the monopolar performance as an air electrode was evaluated. .

[Table] As mentioned above, adding carbon fiber during electrode catalyst kneading tends to slightly improve electrode performance. The improvement in mechanical strength was remarkable. As described above, in comparison with the conventional preparation of general electrode catalysts and the performance of electrodes made using the catalysts, according to the method of the present invention, the platinum particles on the carrier as an electrode catalyst are finely divided and uniform. The amount of platinum used in fuel cell electrodes can be significantly reduced, and the electrode performance is also greatly improved, and the mechanical strength of the catalyst layer is also improved by adding carbon fiber. It became clear that the results also improved.

Claims (1)

[Claims] 1. An electrode catalyst is produced by causing a reducing agent to act on an aqueous solution containing noble metal ions or complex ions thereof, protective colloids, and carbon powder to deposit noble metal fine particles on the carbon powder. a step of mixing the electrode catalyst with a water repellent agent and water, a step of applying the obtained mixture to a conductive porous substrate, and a step of firing the obtained molded body. A method of manufacturing a fuel cell electrode, comprising: 2. A step of producing an electrode catalyst by causing a reducing agent to act on an aqueous solution containing noble metal ions or their complex ions, a protective colloid, and carbon powder to deposit noble metal fine particles on the carbon powder; The process includes the steps of mixing a water repellent agent, carbon fiber, and water, applying the resulting mixture to a conductive porous base material, and firing the resulting molded body. A method for manufacturing a fuel cell electrode. 3. The method for manufacturing a fuel cell electrode according to claim 1 or 2, wherein the noble metal is one of group 8 noble metals of the periodic table or a mixture thereof. 4. A method for producing an electrode for a fuel cell according to any one of claims 1 to 3, wherein in the step of producing an electrode catalyst, carbon powder on which fine metal particles are deposited is heat-treated. 5. A method for manufacturing a fuel cell electrode according to any one of claims 1 to 4, wherein an admixture is used in the step of mixing the electrode catalyst and water.