JPH0519263B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a catalyst layer that is effective for use in air electrodes such as air/metal batteries or oxygen sensors, and more particularly to a catalyst layer for air electrodes that is capable of heavy load discharge and has excellent leakage resistance. Conventionally, gas diffusion electrodes have been used as air electrodes in various air batteries, galvanic oxygen sensors, and the like. Initially, gas diffusion electrodes consisting of a single, thick porous catalyst layer were used, but now, due to the demand for thinner batteries and the need for improved leakage resistance, thin porous catalyst layers have been used. Electrodes with a two-layer structure, in which a thin layer of water-repellent material is integrally attached to the electrode, have come to be used. In addition, in cases where liquid leakage is not allowed, for example, in the case of galvanic oxygen sensors used to detect the concentration of dissolved oxygen gas in water, an electrolyte-resistant and gas-resistant Air electrodes are constructed by integrally attaching a transparent non-porous film. An air electrode basically composed of a porous catalyst layer and a water-repellent layer is further integrally attached with a current collector such as nickel net to become a practical air electrode. Now, in the case of such an air electrode, the porous catalyst layer has three phases within its pores: gas phase (air), solid phase (catalyst and base material supporting it), and liquid phase (electrolyte solution). A band is formed, and an electrochemical reduction reaction of oxygen gas proceeds in the three-phase band. As a result, current can be extracted through the current collector that is integrally attached to the porous catalyst layer. Therefore, the porous catalyst layer is made of, for example, activated carbon powder alone or powder of activated carbon, graphite, or various metal conductive materials as a base material, and a catalyst having an electrochemical reduction ability for oxygen gas is supported on this. It is configured. Typical examples include nickel tungstic acid, which has a low oxygen reduction overpotential, tungsten carbide coated with palladium and cobalt, activated carbon powder supporting nickel, silver, platinum, palladium, etc., bound with polytetrafluoroethylene, etc. There is a structure in which a porous body is formed by attaching a porous body to a porous body, and this is integrated with a porous metal body, a porous carbon body, or a carbon fiber nonwoven fabric. In addition, the water-repellent layer may be made of powder of a water-repellent material typified by fluorine resins such as polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer, or polypropylene. Sintered bodies, paper-like materials made by heat-treating fibers into non-woven fabrics, fabric-like materials, and film-like materials are widely used. However, the air electrode of the conventional structure as described above does not necessarily satisfy applications requiring thinness, excellent leakage resistance, and heavy load discharge (for example, thin air/zinc batteries). For example, if a porous foil obtained by sintering fluororesin powder as described above is used as the water-repellent layer,
It is possible to perform continuous discharge with a fairly heavy load of approximately 20 mA/cm ² , but the thickness is approximately 0.125 to 0.50 mm. In addition, since the pore diameter of the porous foil is not uniform and there are pores with large diameters, the internal pressure of the battery increases due to volume expansion at the counter electrode of the air electrode, which may cause leakage, especially in the case of a sealed battery. It may cause. On the other hand, in an air electrode in which a thin gas-permeable non-porous film is further attached to the gas side using an adhesive to prevent liquid leakage, the liquid leakage phenomenon can be completely prevented and the thickness can be reduced. It is possible to reduce the thickness to about 12.5 μm, but in this case it would be extremely difficult to discharge continuously at a large current of 10 mA/cm ² or more. On the other hand, other types of air electrodes can be obtained by mixing various catalysts supported on conductive base material powder such as activated carbon or nickel with powder of water-repellent material such as polytetrafluoroethylene. It is known that the powder mixture is formed by pressure molding.
At this time, the water-repellent material powder functions as a binder for the base material powder. The air electrode in this case does not have a two-layer structure, but has a water-repellent material uniformly dispersed within the porous catalyst layer. This type of air electrode does not require a water-repellent layer attached to the porous catalyst layer, so the porous catalyst layer can be made thicker (increasing the amount of catalyst) relative to the overall thickness, so it is heavy. Load discharge becomes possible. Conversely, for heavy load discharge using a predetermined current, the thickness can be reduced. However, in this type of air electrode, the surface of the hydrophilic base material or catalyst is exposed to a considerable extent, so the electrolyte gradually penetrates into the porous catalyst layer over time, forming a three-phase zone. The effective area of the area is gradually reduced. As a result, an inconvenient situation arises in that the stability of heavy load discharge is inhibited. The present inventor has conducted extensive research in order to eliminate the above-mentioned drawbacks in the porous catalyst layer of an air electrode in which a water-repellent material is uniformly dispersed without attaching a water-repellent layer. The present invention was completed based on the idea that if the water repellency on the air side of the catalyst layer is made higher than the water repellency on the electrolyte side, there is a high possibility that a suitable three-phase zone will be formed in the area where the two are balanced. I reached it. That is, an object of the present invention is to provide a catalyst layer for an air electrode that is capable of long-term heavy load discharge, has excellent leakage resistance, and can be easily made thin. The catalyst layer of the present invention is an air electrode catalyst layer formed by integrally laminating two conductive porous catalyst layers each containing a water-repellent binder as an air side layer and an electrolyte side layer. the content ratio (wt%) of the water-repellent binder in the air side layer is greater than the content ratio (wt%) of the water-repellent binder in the electrolyte side layer; The product of the thickness of the air side layer and the content ratio (wt%) of the water-repellent binder in the layer is the product of the thickness of the electrolyte side layer and the content ratio (wt%) of the water-repellent binder in the layer. ) is 8.0 times or less. The catalyst layer of the present invention is a composite catalyst layer in which two conductive porous catalyst layers are laminated. These conductive porous catalyst layers support catalysts such as nickel tungstic acid, tungsten carbide coated with palladium and cobalt, nickel, silver, platinum, and palladium, which have the ability to electrochemically reduce oxygen gas. It can be obtained by mixing or kneading activated carbon powder or activated carbon powder alone with a water-repellent binder powder or liquid, and forming the mixture into a sheet of a predetermined thickness by a predetermined method, for example, roll forming. At this time, any water-repellent binder may be used as long as it has good binding properties, water repellency, and electrolyte resistance, but polytetrafluoroethylene, polyethylene, polystyrene,
polyamide resin, acrylic resin, epoxy resin,
Synthetic rubbers such as neoprene and chloroprene are preferred. Of the two conductive porous catalyst layers constituting the composite catalyst layer, one is an air side layer and the other is an electrolyte side layer. In the present invention, the amounts of the water-repellent binder contained in the air side layer and the electrolyte side layer differ in their content ratios. That is, the first feature is that the content ratio of the water-repellent binder in the air side layer is larger in weight percent than that in the electrolyte side layer. By doing so, when this composite catalyst layer is applied to an air electrode, the electrolyte becomes difficult to penetrate into the pores in the air side layer, and the electrolyte is prevented from penetrating into the pores in the electrolyte side layer. As the penetration is moderate, at or near the interface between the two layers, a three-phase zone is created where the electrolyte penetration and water repellency maintain a delicate balance, allowing the electrochemical reduction reaction of oxygen gas to occur. It will be able to exist stably for a long period of time. Furthermore, if the thickness of the air side layer is increased, the water repellency of the layer will also be increased, thereby improving the leakage resistance.
However, if the thickness becomes too large, negative effects such as an increase in the overall electrical resistance and an increase in interference with the diffusion of oxygen gas may occur, resulting in a situation where heavy load discharge is restricted. become. Therefore, the present inventor determined the respective thicknesses (ta, te: mm) of the air side layer and electrolyte side layer, and the ratio (xa, xe: weight %) of the water-repellent binder contained in each layer. When we investigated the relationship between tax×xa/te
It has been found that when the value of xxe is 8.0 or less, the composite catalyst layer has excellent heavy load discharge characteristics and leakage resistance. If the value of ta×xa/te×xe deviates from the above value, the thickness of the air side layer will increase compared to the electrolyte side layer, and the water-repellent binder content (wt%) will become excessive. Therefore, not only the electrical resistance of the air electrode increases, but also the diffusion of oxygen gas is hindered much more than when the thickness of the entire air electrode is increased. From this, the second feature of the present invention is that
The product of the thickness of the air side layer and the content ratio (weight %) of the water repellent binder in the layer is the product of the thickness of the electrolyte side layer and the content ratio (weight %) of the water repellent binder in the layer. %), the value is 8.0 times or less. When creating the composite catalyst layer of the present invention, two conductive porous catalyst layer sheets with different xa, xe and ta, te are created in advance by a conventional method.
These are combined and laminated so that the value of xa x ta/xe x te falls within the above range, and then crimped. At this time, a current collector (for example, a nickel net) may be sandwiched or placed between each sheet or on the surface of the electrolyte side layer and pressed together at the same time to form an air electrode at once. The present invention will be explained below based on examples. Example Activated carbon powder (average particle size
80μ), using a dispersion of polytetrafluoroethylene powder (average particle size 15μ) as a water-repellent binder, xa, ta; xe, xe as shown in Table 1.
A conductive porous catalyst layer sheet was prepared. Each sheet was laminated and pressed together at a pressure of 50 to 100 kg/cm ² to prepare seven composite catalyst layer samples with an integral structure. Samples 1 to 5 are examples of the present invention, and samples 6 to 7 are comparative examples.

【table】

[Table] The values of xa×ta/xe×te for each composite catalyst layer are also listed in Table 1. 0.15φ40 on the electrolyte side layer of these composite catalyst layers
A polytetrafluoroethylene film having an average pore diameter of 3 .mu.m and a thickness of 100 .mu.m was brought into contact with the surface of the air side layer of the mesh nickel net, and the whole was pressurized at 100 kg/ ^cm.sup.2 to form seven air electrodes. In addition, activated carbon powder (average particle size 80μ) and polyethylene powder (average particle size 35μ) were kneaded at 150°C, and then rolled into a sheet.

[Table] Four composite catalyst layers were prepared in the same manner as in Samples 1 to 7. Samples 8-9 are examples, samples 10-11
is a comparative example. Using these composite catalyst layers,
Four air electrodes were created in the same manner as in Samples 1 to 7. xa, ta: xe, te were as shown in Table 2. Next, each air electrode and the zinc powder passed through a 60 to 150 mesh sieve amalgamated with 3% mercury by volume were mixed into a gel electrolyte (prepared by dispersing a gelling agent in a sodium hydroxide solution). Eleven air/zinc batteries were assembled from dispersed zinc electrodes and separators made of polyamide nonwoven fabric. These batteries were left in air at 25° C. for 16 hours, then left with various currents for 5 minutes, and the current value when the terminal voltage dropped to 1.0 V or less after 5 minutes was measured. Also, connect a 500Ω constant resistor to each battery, and
It was discharged continuously. The time required for the electrolyte to leak from the air side layer was measured. The above results are collectively shown in Table 3 in correspondence with the sample numbers in Tables 1 and 2.

[Table] As is clear from the above table, by using the air electrode according to the present invention, heavy load discharge becomes possible and leakage resistance is improved. In the above example, an air-zinc battery using sodium hydroxide as the electrolyte was assembled and its performance was evaluated, but other electrolytes such as ammonium chloride, potassium hydroxide, lithium hydroxide,
It goes without saying that similar effects can be obtained by using a solution obtained by mixing cesium hydroxide, rubidium hydroxide, etc. with these solutions. It can also be used in air-iron batteries, etc. As detailed above, by using the catalyst layer of the present invention, it is possible to easily obtain a thin air electrode that is capable of heavy load discharge and has excellent leakage resistance.
Its industrial value is great.

Claims (1)

[Claims] 1 A catalyst layer of an air electrode formed by integrally laminating two conductive porous catalyst layers, each containing a water-repellent binder, as an air side layer and an electrolyte side layer, and the air side The content ratio (wt%) of the water-repellent binder in the layer is larger than the content ratio (wt%) of the water-repellent binder in the electrolyte side layer, and the thickness of the air side layer and the The product of the content ratio (wt%) of the water-repellent binder in the layer is the product of the thickness of the electrolyte side layer and the content ratio (wt%) of the water-repellent binder in the layer. A catalyst layer of an air electrode characterized by having a value of 8.0 times or less.