JPS6065466A - Gas diffusion electrode for fuel cell - Google Patents

Abstract

PURPOSE:To improve cell characteristic by forming a reaction layer of gas diffusion electrode with a part having a high water repellency and gas diffusion characteristic and a part which allows easy penetration of electrolyte containing catalyst particle forming body disposed uniformly in the thickness direction. CONSTITUTION:A water repellent layer 3 coated with carbon fine powder in the alcohol with addition of fluorine resin powder is provided on a porous electrode base 2, and this layer is further provided with a part 5 having a high water repellency and gas diffusion characteristic with addition of carbon catalyst powder holding platinum in the alcohol, a large amount of fluorine powder and bore forming agent and a part 4 having affinity which allows easy penetration of electrolyte including catalyst particle body 8 formed from the catalyst powder holding the platinum, etc. disposed equally and continuously in the thickness direction to form the gas diffusion electrode 1 for fuel cell as the reaction layer 6. Accordingly, many three-phase interfaces can be formed on the reaction layer 6 and thereby cell characteristic and operation life characteristic can be improved.

Description

Detailed description of the invention [Technical field to which the invention pertains] The present invention uses pure hydrogen or hydrogen containing impurities obtained by reforming natural gas etc. as a fuel, and oxygen or air as an oxidizing agent. This invention relates to a porous gas diffusion electrode for a fuel cell.

[Prior art and its problems]

As schematically shown in FIG. 1(a), a typical phosphoric acid fuel cell 1 consists of phosphoric acid sandwiched between a hydrogen electrode 2 (fuel electrode) and an air electrode 3 (oxidizer electrode). Consists of electrolyte 4.

Cynic acid is usually impregnated with a porous material called a matrix, which allows hydrogen ions to move but does not allow electrons to pass through. The hydrogen electrode 2 is usually a carbon paper supported by a porous carbon material. Here, hydrogen (arrow) supplied from the outside through the carbon material extracts electrons and becomes hydrogen ions. Hydrogen ions move within the phosphoric acid electrolyte 4 to the air electrode 3, and electrons reach the air electrode 3 through an external circuit. The air electrode is similarly made of carbon paper supported by a porous carbon material. Here, air (arrow) supplied from the outside through the carbon material receives electrons and reacts with hydrogen ions to generate water. generate. In this way, fuel cells perform a reaction that is, so to speak, the opposite of the electrolysis of water.

These reactions, as schematically shown in Figure 1(b),
A three-phase structure in which a catalyst 5 (solid) supporting a noble metal in a portion called the catalyst layer of each electrode, silicic acid 6 (liquid) as an electrolyte, and hydrogen or air (gas) as a reaction gas 7 are in contact with each other. It is thought that this occurs at the interface. Therefore, an excellent current −
In order to obtain voltage characteristics and longevity, it is necessary to generate as many three-phase interfaces as possible and maintain them stably for a long time. 1)
As shown in the figure, it has a porous structure in which a platinum catalyst 1o is mainly supported on carbon particles 9, which increases the reaction area and allows gas to be diffused at the interface between the catalyst and the electrolyte. There is.

FIG. 2 shows a cross-sectional view of a typical structure of such a porous gas diffusion electrode. This porous gas diffusion electrode 1 consists of an electrode base material 2 made of a porous carbon material having electrical conductivity, a carbon catalyst on which noble metals such as platinum are supported, and PTFE (polytetrafluoroethylene) added thereto as a binder. The catalyst layer 3 is made by applying a mixture in the form of a thin film by a well-known method such as a screen printing method, a blade method, or a spray method. In most cases, the catalyst layer 3 has a layer (hydrophilic layer) 4 with a small amount of PTFE added on the electrolyte side to make it easily wetted by the electrolyte, and a large amount of PTFE on the gas diffusion side to prevent it from getting wet with the electrolyte. The electrolyte permeates moderately between the PTFB-added layer (water-repellent layer) 5 and these layers, and gas diffusion is ensured, forming a three-phase interface and causing electrochemical reactions to occur more easily than between the carbon powder and PTFE. There are also known electrodes made using porous fluororesin paper, or electrodes whose water-repellent layer is made of porous fluororesin paper.

The reaction layer 6 of the gas diffusion electrode having the above configuration is as follows:
It is made from a uniform composition of catalyst powder and PTFE, and in order for the electrolyte to penetrate almost uniformly in the thickness direction of the reaction layer, a three-phase interface is formed throughout the reaction layer. However, the present invention has the disadvantage that it is made on an extremely thin surface, and the electrode characteristics cannot be fully brought out. [Summary of the Invention] The present invention aims to provide a gas diffusion electrode having excellent current-voltage characteristics and lifetime characteristics by forming a large number of three-phase interfaces throughout the thickness of a reaction layer. is in the reaction layer of a porous gas diffusion electrode for fuel cells.
A large amount of fluororesin such as PTF-E is added to strengthen the water repellency so that it does not get wet with the electrolyte.A part with good gas diffusion that is continuous in the thickness direction, and a small amount of fluororesin such as p'rpg for the catalyst. In this invention, a large number of three-phase interfaces are formed throughout the thickness of the reaction layer by uniformly distributing the parts easily wetted by the electrolytic solution that are continuous in the thickness direction of the reaction layer. Embodiment] FIG. 3 shows an enlarged cross-sectional view of the porous gas diffusion electrode of the present invention, in which the gas diffusion electrode 1 has a porous base material 2 that prevents electrolyte from permeating but ensures gas diffusivity. A water-repellent layer 3, a part (hydrophilic part) 4 that is continuous in the thickness direction and facilitates penetration of the electrolyte, and a part (water-repellent part) 5 that has strong water repellency and increased gas diffusivity are evenly distributed. According to a preferred embodiment of the present invention, the porous gas diffusion electrode is composed of a reaction layer 6 that causes an electrochemical reaction, and a layer (hydrophilic layer) 7 that is easily wetted by an electrolyte. Powder (e.g. graphite, carbon black, etc.) or carbon catalyst powder supporting noble metals such as platinum, ruthenium, etc. and a large amount (e.g. 70 to 400% by weight, preferably 100 to 300% by weight) of a fluororesin (
For example, PTFEl polyhexafluoroethylene, -\
(Copolymers of hexafluoropropylene and vinylidene fluoride, copolymers of tetrafluoroethylene and hexafluoropropylene, etc.) are uniformly dissolved in pure water or alcohol (e.g. methanol, ethanol, isopropanol, etc.). The resulting dispersion is applied onto a porous electrode base material (e.g. carbon fiber sheet, carbon catalyst ζ-, carbon powder, etc.) 2 by a spray method, a blade method, a screen printing method, an electrostatic coating method, etc. For example, about 100
It is formed by applying a layer of μ thickness and drying to remove the water or alcohol. Next, on the layer formed in this way, catalyst granules (particles) are pre-granulated from catalyst powder supporting noble metals such as platinum and ruthenium, using a fluororesin (same as described above) as a binder. Diameter 10~Additional μ,
Fluororesin content 30-150% by weight, preferably blade ~1
A dispersion of pure water or alcohol (00% by weight) is applied to a thickness of, for example, 80 to 100μ by a suitable method such as a spray method, a blade method, a screen printing method, or an electrostatic coating method, and is dried to form a water or alcohol dispersion. The catalyst granules are then removed to a depth of 60-80μ by pressurizing the coated surface to a pressure of 5-20 Ay/crd.

In this way, a highly water-repellent layer 3 (b) is formed on the porous electrode base material 2 (a water-repellent layer 3 made of Al carbon fine powder or a carbon catalyst supporting noble metals and a large amount of fluororesin).
A reaction in which the hydrophilic part 4, which is made up of a gas-diffusible water-repellent part 5 that is continuous in the thickness direction and has high water repellency, and a hydrophilic part 4 that facilitates penetration of the electrolytic solution, which is made up of a catalyst granule 8 that supports press-fitted precious metals, is evenly distributed. A catalyst layer is formed in which a hydrophilic layer 7 consisting mainly of catalyst granules is present. The thickness of the water-repellent layer 3 formed in the above example is about +μ, and the thickness of the hydrophilic layer 7 is about 20 to 40μ. The layers can be selected arbitrarily. Further, the sizes of the carbon fine powder, the catalyst powder supporting the noble metal, and the fluororesin particles may be the same as those used in the prior art.

The structure constructed as described above is then heat treated at the melting temperature of the fluororesin, thereby producing a porous gas diffusion electrode having a strong structure due to partial fusion of the fluororesin.

〔Effect of the invention〕

According to the present invention, in the entire reaction layer that causes an electrochemical reaction of a gas diffusion electrode for a fuel cell, a portion having strong water repellency and high gas diffusivity and a portion making it easy for electrolyte to penetrate are formed in the thickness direction. Due to the continuous and even arrangement, a large number of three-phase interfaces could be formed throughout the thickness of the reaction layer. As a result, it was possible to improve the current-voltage characteristics of the electrode by 10 to 15% and the life characteristics by 5 to 10 times.

Although the present invention has described a carbon catalyst supporting a noble metal in an electrode of a fuel cell using phosphoric acid as an electrolyte,
Those skilled in the art can easily imagine that the present invention can be easily applied to electrodes using nickel, silver, etc. as catalysts in alkaline electrolyte fuel cells.

[Brief explanation of drawings]

FIG. 1 (at) is a schematic diagram of the structure of a silicic acid fuel cell, and FIG. 1 (b) is a diagram showing the configuration of the inside of a pore where the three phases of catalyst, electrolyte, and reaction gas are in contact. In FIG. 1, 1... cynic acid fuel cell, 2...
Hydrogen electrode, 3... Air electrode, 4... Phosphate electrolyte,
Death: Catalyst, 6: Phosphoric acid, 7: Reactive gas, 8:
...Three-phase interface FIG. 2 is a cross-sectional view of the structure of a conventional gas diffusion electrode. In FIG. 2, 1... gas diffusion electrode, 2... electrode base material, 3... catalyst layer, 4... hydrophilic layer, 5... water repellent layer, 6... reaction layer 3rd. The figure is an enlarged cross-sectional configuration diagram of the gas diffusion electrode of the present invention. In FIG. 3, 1... gas diffusion electrode, 2... porous base material, 3... water repellent layer, 4... Hydrophilic part, 5...
・Water repellent portion, 6... Reaction layer, 7... Hydrophilic layer, 8.
...Catalyst granule patent applicant Fuji Electric Research Institute Co., Ltd. Fuji Electric Manufacturing Co., Ltd. (a) 6g)

Claims (1)

[Scope of Claims] l) A layer with strong water repellency and high gas diffusivity made of fine carbon powder or carbon catalyst powder supporting noble metals and a large amount of fluororesin is provided on a porous electrode base material, and this layer Granules of carbon catalyst powder supporting noble metal prepared in advance using a small amount of fluororesin as a binder are press-fitted into the porous electrode base material, and then (al gas diffusive water-repellent layer, (b) A reaction layer in which a gas-diffusible water-repellent portion continuous in the thickness direction and a gas-diffusible hydrophilic portion continuous in the thickness direction are evenly distributed, and (C) a catalyst layer in which a gas-diffusible hydrophilic layer is present. 2. In the gas diffusion electrode according to claim 1, the fluororesin used to form the layer with strong water repellency and high gas diffusivity is tetrafluoroethylene. A gas diffusion electrode characterized in that it is made of particles and is used in an amount of 100 to 300% by weight. 3) In the gas diffusion electrode according to claim 1, the preparation of carbon catalyst powder supporting a noble metal is performed. A gas diffusion electrode characterized in that it is made of fluororesin or polytetrafluoroethylene used for forming the particles, and is used in an amount of 50 to 100% by weight04) Claims 1 to 3
3. The gas diffusion electrode according to any one of Items 1 to 9, wherein the supported noble metal is platinum or ruthenium.