JP3373141B2 - Partially silver-coated porous metal foam and gas diffusion electrode using it - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a partially silver-covered foam metal porous body and a gas diffusion electrode using the same, and more specifically, to a foam metal porous body in which silver layers are formed on both front and back surfaces and constituent elements thereof. It relates to a gas diffusion electrode used as one.

[0002]

2. Description of the Related Art In electrolysis of an alkali chloride solution by an ion exchange membrane method, a gas diffusion electrode is used as an oxygen cathode for the cathode. In this type of ion exchange membrane electrolysis, an anode chamber having an insoluble anode and containing a sodium chloride aqueous solution and a cathode chamber having an oxygen cathode and containing a caustic soda aqueous solution are partitioned by a cation exchange membrane. The cathode part is composed of a gas diffusion electrode and an oxygen gas chamber for supplying oxygen gas to the gas diffusion electrode. In the electrolyzer having such a configuration, when conducting electrolysis by applying a current between both electrodes, an oxygen reduction reaction occurs on the gas diffusion electrode and the cathode potential rises, so that the electrolysis voltage is significantly reduced.

The gas diffusion electrode provided in the cathode portion has a structure in which a reaction layer and a gas supply layer are laminated, and both the reaction layer and the gas supply layer are composed of a thin layer mainly composed of a porous body. .
Carbon black is mainly used for the reaction layer, which is the layer on the electrolyte side of the gas diffusion electrode, and the fine pores carry a catalyst made of a noble metal such as platinum, and the reaction layer is hydrophilic and electrolyzed. The liquid can penetrate. On the other hand, the gas supply layer, which is the layer on the gas supply side, is composed of a water-repellent porous thin layer through which gas can pass but electrolyte does not leak. The water-repellent thin layer is usually formed mainly of fine particles of a fluororesin polymer which is resistant to a redox reaction.

As shown in FIG. 3, the conventional gas diffusion electrode is provided with a mesh-shaped power supply body 12 (also referred to as a wire mesh) on the middle of the gas supply layer 5 and / or on the surface of the gas supply layer 5, and the gas is supplied from the mesh-shaped power supply body 12. An electric current was supplied to the reaction layer 4 through the supply layer 5 (through a mesh-shaped power feeding body in the middle of the gas supply layer). However, since the hydrophobic gas supply layer 5 has a large electric resistance, the electrolysis voltage becomes large. When the size of the electrolytic cell is increased, the electrode area is increased, the internal resistance of the electrode is increased, and the power loss is increased as a resistance loss. Therefore, as shown in FIG. 4, it was also considered to join a reticulated power supply body to the surface of the gas supply layer, fill the gas chamber with a conductive porous body, and contact the reticulated power supply body to supply power. There is a drawback that the supply of gas is hindered by the reticulated power supply joined to the surface of the gas supply layer.

[0005]

Therefore, the inventor of the present invention
In order to eliminate these drawbacks, we have conducted intensive studies on the gas diffusion electrode, and supplied gas from the gas chamber to the gas supply layer of the gas diffusion electrode in order to reduce the electrode overvoltage in the cathode reaction of oxygen and improve the current efficiency. In order to achieve uniform gas supply and uniform current load to the gas diffusion electrode, a means for filling the gas chamber with a porous material that allows three-dimensional gas flow was developed. When the porous material filled in the gas chamber is made of a conductive material so as to serve also as a power supply body, a technology has been developed which can supply power to the gas diffusion electrode favorably and reduce the resistance.

Further, when the gas diffusion electrode is constructed by using such a conductive porous material as a substrate, the power supply to the gas diffusion electrode is good, the resistance can be further lowered, and the strength is further increased. We have developed technology that can be increased. As a gas diffusion electrode by such a technique, for example, a gas supply layer is formed in a layer form in a part of the conductive porous body, and the reaction layer is in contact with the gas supply layer on one side of the conductive porous body. And a portion of the other side of the conductive porous body that does not have the gas supply layer forms at least a part of the gas chamber in an exposed state.

This gas diffusion electrode, as shown in FIG.
The conductive porous body 8 is formed as a gas permeable and hydrophobic gas supply layer 5 with the conductive porous body 8 as a substrate and the thin layer-like exposed portion 10 under the porous body 8 is left and exposed. On the surface side opposite to the exposed portion of the thin layer, a reaction layer 4 having a water-permeable catalyst is provided. In FIG. 2, in order to show that the conductive porous body 8 is also present in the gas supply layer 5, the filler on the left side of the gas supply layer 5 is removed to expose the filled portion 9 of the conductive porous body 8. It is shown as the state. In this type of cathode portion, another conductive porous body 11 is combined on the gas chamber side of the gas diffusion electrode 1, and a thin layer-like exposed portion 10 of the porous body 8 and the conductive porous body 11 are combined.
Form a gas chamber.

In this gas diffusion electrode invented by the present inventor, even when the porous body which is the substrate is made of, for example, a foamed nickel porous body, since the gas supply layer contains the water-repellent substance, In order to reduce the contact resistance and to increase the conductivity of the porous body itself, the foamed nickel porous body is plated with silver because of its high contact resistance in the contact area with the reaction layer and the electrolytic cell frame. ing. In this case, a silver-plated product having a thickness of 5 μm is expensive because the amount of silver used is 200 g / m ² or more. Further, the gas diffusion electrode needs a gas chamber as an electrolytic cell structure in order to supply a reaction gas to the gas diffusion electrode. In this gas chamber, a silver-plated foamed nickel porous body is used for filling in order to mediate electric conduction between the gas diffusion electrode and the electrolytic cell frame. This also causes a cost increase.

The present invention has been made in view of the above-mentioned conventional problems, and reduces the amount of expensive silver used for silver plating to reduce the manufacturing cost and the gas diffusion electrode. Provided are a conductive foam metal porous body and a gas diffusion electrode using the same, which can maintain the current collecting ability of the foam metal porous body used as an electric body and can also exhibit the performance improvement as a gas chamber filling material. The purpose is to do.

[0010]

Means for Solving the Problems As a result of intensive studies to solve the above-mentioned problems, the present inventor has formed silver plating on the front surface and the back surface of a foamed metal porous body at portions of 500 μm or less from the front surface. As a result, they have found that the above object can be achieved, and completed the present invention. That is, the present invention has the following configuration. (1) A dispersion of fine silver particles is attached to a foamed metal porous body,
A partially silver-covered foamed metal porous body, characterized in that it is dried and then fired to cover both front and back surfaces of the foamed metal porous body with a silver layer at a portion of 500 μm or less from the surface. (2) A gas supply layer made of a water-repellent porous body is formed inside the partially silver-coated foamed metal porous body according to (1), and a reaction layer is formed on one silver coating layer of the porous body. A gas diffusion electrode having a structure in which the silver coating layer is in contact with a reaction layer. (3) A gas diffusion electrode, characterized in that the gas chamber of the gas diffusion electrode is filled with the partially silver-covered metal foam porous body according to (1).

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, when the substrate of the gas diffusion electrode is made of a foamed metal porous body, the electric resistance of the foamed metal porous body itself is relatively small, and the magnitude of the electric resistance becomes a problem. At the connecting portion between the foamed metal porous body and the reaction layer, or the contacting portion between the foamed metal porous body and the electrolytic cell frame, and further at the contacting portion between the foamed metal porous body and another porous body filled in the gas chamber. Since it has a large electric resistance, pay attention to the surface of the foamed metal porous body that is the contact portion and the portion close to the surface, and only these portions are covered with silver plating so that the silver The amount used has been significantly reduced. The location near the surface is 500 microns or less from the surface, but 10 to 1
A range of up to 00 microns is preferred. The thickness of the silver-plated coating may be in the range of 1 to 10 microns, as long as it is sufficient to ensure conductivity. Further, as a means for silver plating, the partial decomposition is performed, so that the thermal decomposition method or the thermal decomposition reduction method is more suitable than the electroplating or the electroless plating.

The metal foam porous body and the gas diffusion electrode of the present invention will be specifically described with reference to the drawings. In FIG. 1, in the gas diffusion electrode 1 of the present invention, a silver coating portion 3 is formed on each of the front surface 6 and the back surface 7 of the foamed metal porous body 2 at a portion of 500 μm or less from the outer surface. About 1 each
It is deposited and formed with a thickness of about 00 microns. As a result, the silver coating layer 3 is formed on both front and back surfaces. The silver coating layer 3 on the front surface 6 side is formed so as to be embedded in and in contact with the reaction layer 4, while the silver coating layer 3 is exposed on the back surface 7 side. This forms a conductive porous body.

That is, using the metal foam body 2 as a base,
A silver coating layer 3 is formed by coating silver only on the front and back surfaces and on portions very close thereto, leaving an exposed portion 9 on the thin layer of the silver coating layer 3 on the lower side (back surface) 7 of the porous body. The foamed metal porous body 2 is formed as a gas permeable and hydrophobic gas supply layer 5, and the surface side 6 opposite to the exposed thin silver coating layer 3 carries a catalyst with water permeability. The reaction layer 4 is provided so as to be embedded in the protruding portion of the silver coating layer 3. Further, in this type of cathode part, the reaction layer 4 is provided on the silver coating layer 3 on the surface 6 side of the foamed metal porous body 2, and the inside of the foamed metal porous body 2 is subjected to a water repellent treatment to provide a gas supply layer 5.
May be formed to form the cathode part. Alternatively, the foamed metal porous body 2 provided with the silver coating layer 3 on the front surface and the back surface thereof may be used as the conductive porous body 11 to fill the gas chamber of the gas diffusion electrode.

The foam metal porous body of the present invention can be formed by using various metals, and examples of the metal include nickel, copper and the like. In particular,
An open-cell foam made of plastic such as polyurethane may be plated with nickel, or an open-cell foam made of plastic may be plated with nickel and then sintered to form a porous nickel body. Alternatively, a carbon porous body may be plated with nickel. This conductive porous body not only gives strength to the gas diffusion electrode, but also acts as a current collector, and has a larger contact area with the electrode material than in the case of a mesh-shaped power feeding body such as a conventional wire mesh. The thickness of the foamed metal porous body is preferably 1 to 10 mm.

The main constituent material of the reaction layer is a catalyst composed of a noble metal such as platinum, a mixture of fine particles of hydrophilic carbon and fine particles of water-repellent carbon, a small amount of fluororesin fine particles and a binder connecting these fine particles. It is a drug particle. The main constituent materials of the gas supply layer are water repellent carbon fine particles, fluororesin fine particles and binder particles connecting these fine particles.

As a method for providing the gas supply layer in the foamed metal porous body, the water repellent carbon fine particles, the fluororesin fine particles, the binder particles and other particles, which are the main constituent materials, are formed into a porous body by a vibration method. A method of forming a gas supply layer by putting it inside and heating / sintering it, or mixing water-repellent carbon black, water and powdered polytetrafluoroethylene to prepare sludge, and then forming a conductive porous body. In addition to the method, the sludge is filled from one side (or both sides), the dispersant is optionally removed from the sludge before or after drying, and the slurry is dried and sintered.

In the production of the gas diffusion electrode of the present invention,
The foamed metal porous body partially covered with silver is filled and molded with a substance for forming a gas supply layer and the like, and then compacted if necessary and sintered to complete. When molding at room temperature,
The pressing pressure is 10 to 60 kg / cm ² , and the sintering is performed at 200 to 300 ° C. When the molding is performed by a hot press, the press temperature is 150 to 300 ° C and the pressure is 1
It is preferable to carry out at about 5 kg / cm ² . When the material for the gas supply layer or the material for the reaction layer is sludge, and when the sludge is applied or pressed into the conductive porous body to fill and mold the electrode layer, it is used to disperse from the sludge at this molding stage. The dispersant is washed and removed, and then sintered to complete the process. Surfactants are usually used as the dispersant used to make the slurry. As the surfactant to be used, any of nonionic, anionic and cationic surfactants can be used alone or in combination, but the nonionic surfactant is preferred. Further, the surfactant may be contained in a material used for manufacturing an electrode, for example, a suspension solution of polytetrafluoroethylene, or the like.

In the production of the reaction layer in the gas diffusion electrode of the present invention, a catalyst having an electrocatalytic action and a slurry from the electrode constituent material are filled in the pores of the partially silver-covered foam metal porous body so as not to dry. A method is used. At that time, the slurry is uniformly filled in the porous body to form a gas diffusion electrode layer having a required thickness. As the activation catalyst mixed in the reaction layer of the gas diffusion electrode of the present invention, the following catalysts are suitable, and the catalyst obtained in the steps shown therein can be used. Platinum black: A step of forming a platinum black by adhering a catalyst salt such as a platinum salt on the surface of a catalyst carrier such as carbon black and heating and reducing with hydrogen in an inert gas atmosphere. Reduced silver: A catalyst salt such as silver nitrate is adhered to the surface of a catalyst carrier such as hydrophilic carbon black, the powder is dried at 80 ° C., heated and reduced with hydrogen at 150 ° C., and adhered to carbon black. Step of making fine silver particles.

[0019]

The present invention will be described in detail with reference to the following examples. However, the present invention is not limited to these examples.

Example 1 Fine silver particles (Mitsui Mining & Smelting Co., Ltd., 3010, average particle size 0.1
1 micron) and turpentine oil were mixed in a weight ratio of 1: 1 to prepare a paint. This paint was applied to a glass plate with a coater to a film thickness of 20 μm. A 1.7 mm thick 50 ppI foamed nickel porous body was placed on this and transferred. After transferring to the front and back sides, it was dried and then baked in a nitrogen stream at 500 ° C. for 1 hour to coat silver only on the surface portion in a mottled pattern with a thickness of 1 to 10 μm.

This silver part-covered foamed nickel porous body (12
X28 cm square), first, ethanol was added to Lubron at a weight ratio of 1: 2 to form a paste, and 2 parts were added to the foamed metal porous body.
A coating gas supply layer is formed in a thickness of micron, and then 3 parts of silver fine particles (3010, manufactured by Mitsui Mining & Smelting Co., Ltd., average particle size 0.11 micron) to be a reaction layer and 1 part of Lubron are mixed with ethanol 2
Knead in parts (weight ratio) and make a paste.
It was applied to a thickness of 00 microns and pressed at a pressure of 10 kg / cm ² and pressed into the inside to form a gas supply layer and a reaction layer. This operation brings the reaction layer into contact with the silver-plated foamed nickel porous body. Dry at 80 ℃ for 3 hours, press at room temperature 4
It was pressed at 0 kg / cm ² for 60 seconds, then heat-treated at 250 ° C. for 10 minutes and then cooled to obtain an electrode. The oxygen reduction performance of this electrode was measured and found to be 30 A / dm ²
A high performance of 0.78 V (vs. RHE) was obtained. The contact resistance between the back surface of the electrode and the silver-plated foamed nickel porous body in the gas chamber was a small value of ² mΩ / cm ² or less.

Example 2 A paint was prepared using colloidal silver (average particle size 5 nm: prepared by the reverse micelle method). This paint was uniformly placed on a rubber roller, and transferred by rolling on a 50 ppI foamed nickel porous body. Similarly, after transferring to the back surface, it was subsequently dried, and then baked in the atmosphere at 320 ° C. for 10 minutes to coat silver only on the front surface with a thickness of 1 to 5 μm. To this silver part-covered foamed nickel porous body (12 × 28 cm square), first of all, a PTFE dispersion (D-1, manufactured by Daikin Industries, Ltd.) was mixed with ethanol at a weight ratio of 1: 2 to form a paste, The foamed nickel porous body was coated with a thickness of 800 μm to form a gas supply layer.

Next, fine silver particles (Mitsui Mining & Smelting Co., Ltd., 301
0, average particle size 0.11 micron) After adding 5 parts of 20% Triton to 2 parts of 10 parts of water and dispersing, further PTF
1 part of E dispersion (D-1, manufactured by Daikin Industries, Ltd.) was dispersed. 50 parts (by weight) of ethanol was added to 10 parts of this dispersion to coagulate, and the mixture was lightly filtered to obtain a reaction layer raw material. This was applied to a thickness of 200 μm from the top of the gas supply layer and rolled to push the gas supply layer and the reaction layer inside the foam. This operation brings the reaction layer into contact with the coated silver. After drying at 80 ° C for 3 hours, the surfactant was removed with ethanol. After drying again, pressure 40kg / cm ²
At room temperature for 60 seconds and then heat treated at 250 ° C. for 10 minutes to obtain an electrode. When the oxygen reduction performance of this electrode was measured, it was 0.81 V (vs. RH at 30 A / dm ² ).
A high performance of E) was obtained. In the same manner on the back surface of the electrode, the contact resistance with the 20 ppI silver-coated foamed nickel porous body for the gas chamber showed a small value of 1 mΩ / cm ² or less. As a result of operating in an actual electrolytic cell, the electrolytic cell voltage was 2.05 V, which was the same as that in the case of using the whole silver-plated product.

[0024]

EFFECTS OF THE INVENTION The foamed metal porous body of the present invention has only the front and back surfaces of the foamed metal porous body coated with silver.
The amount of silver used is 1/10 or less compared to the case where the entire porous body is silver-plated, and the resistance is not different from the resistance of the whole silver-plated one. For this reason, when manufacturing a gas diffusion electrode using this foam metal porous body, an inexpensive gas diffusion electrode can be manufactured. Further, the gas diffusion electrode of the present invention has a low electric resistance, unlike the one in which the gas supply layer is formed of fine particles of a fluororesin polymer, and is used as an oxygen cathode in an electrolytic cell comprising a cation exchange membrane. In the case where salt water is electrolyzed, electrolysis can be performed with an extremely low electrolysis voltage and current efficiency. Moreover, since the thickness of the conductive porous body can be made relatively large, an electrode having high strength can be manufactured.

[Brief description of drawings]

FIG. 1 is a cross-sectional explanatory view showing an example of a gas diffusion electrode of the present invention.

FIG. 2 is a cross-sectional explanatory view of an integrally molded gas diffusion electrode previously invented by the present inventor.

FIG. 3 shows a cross-sectional explanatory view of a conventional gas diffusion electrode having a mesh-shaped power feeder inside.

FIG. 4 is a cross-sectional explanatory view of a conventional gas diffusion electrode having a reticulated power supply inside and on the surface of a gas supply layer.

[Explanation of symbols]

1 gas diffusion electrode 2 Foamed metal porous body 3 Silver coating layer 4 Reaction layer 5 gas supply layer 6 surface 7 Back side 8 Conductive porous body 9 Filling part 10 Exposed part 11 Conductive porous body (for filling) 12 Reticulated power supply

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Choichi Furuya 2-14 Nakamura-cho, Kofu-shi, Yamanashi (56) References JP-A-10-158878 (JP, A) JP-A-10-195687 (JP, A) ) JP-A-49-130878 (JP, A) JP-A-9-41180 (JP, A) JP-A-8-302493 (JP, A) JP-A-7-216575 (JP, A) JP-A-7- 54181 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C25B 1/00-15/08

Claims (3)

(57) [Claims] 1. A dispersion of fine silver particles is adhered to a foamed metal porous body, dried, and then fired to coat both front and back surfaces of the foamed metal porous body with a silver layer at a portion of 500 μm or less from the surface. Partially silver-covered foam metal porous body characterized by being 2. A gas supply layer made of a water-repellent porous body is formed inside the partially silver-coated foamed metal porous body according to claim 1, and a reaction layer is formed on one silver coating layer of the porous body. A gas diffusion electrode having a structure in which the silver coating layer is in contact with the reaction layer. 3. A gas diffusion electrode, wherein the partially silver-covered foam metal porous body according to claim 1 is filled in a gas chamber of the gas diffusion electrode.