JPH0348621B2 - - 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 relates 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, are now being used. In addition, in cases where liquid leakage is not allowed, for example, in the case of a galvanic oxygen sensor used to detect the concentration of dissolved oxygen gas in water, on top of the water repellent layer of the two-layer electrode, electrolyte resistance and gas 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 form 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 (oxygen), 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, a porous sintered body of metal such as nickel, activated carbon powder alone, or powder of activated carbon, graphite, or various metal conductive materials, and is coated with oxygen gas. It is constructed by supporting a catalyst having electrochemical reduction ability. Typical examples include nickel tungstic acid with low oxygen reduction overpotential, tungsten carbide coated with palladium/cobalt, activated carbon powder supporting nickel, silver, platinum, palladium, etc., bound with polytetrafluoroethylene. 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, woven 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 under 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 that occurs at the opposite electrode of the air electrode, and especially in the case of a sealed battery, leakage occurs. It may cause the phenomenon. On the other hand, 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 can completely prevent liquid leakage, and its thickness is approximately 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 molded powder is formed by pressure molding the mixed powder.
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, causing the three-phase The effective area of the band is gradually reduced. As a result, an inconvenient situation arises in that the stability of heavy load discharge is inhibited. On the other hand, when considering air electrodes that enable heavy-load discharge from a different perspective, it is an important factor that a large amount of oxygen is quickly supplied to the three-phase zone formed within the porous catalyst layer described above. becomes. Therefore, if a substance capable of dissolving oxygen gas is supported on a porous catalyst layer, the oxygen concentration will increase in the three-phase zone of the catalyst layer, and an air electrode using the catalyst layer will be able to handle heavy load discharge. It is assumed that this will become possible. In view of the above, the present inventors have proposed that in a porous catalyst layer of an air electrode in which a water-repellent material is uniformly dispersed without attaching a water-repellent layer, a substance capable of dissolving oxygen gas is added to the catalyst layer. If the catalyst layer is supported and the water repellency on the air side of the catalyst layer is made higher than the water repellency on the electrolyte side, heavy load discharge is possible and the effective area of the three-phase zone where electrochemical reactions occur can be maintained for a long period of time. Based on this idea, we developed the catalyst layer of the present invention. 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. Both the air side layer and the electrolyte side layer support a fluorine atom-containing solvent and/or a liquid perfluoro compound having oxygen dissolving ability, and the water-repellent binder of the air side layer The content ratio (wt%) of the water-repellent binder is larger than the content ratio (wt%) of the water-repellent binder in the electrolyte side layer. 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 catalyst layer, one is an air side layer and the other is an electrolyte side layer. Both of these two catalyst layers carry a fluorine atom-containing solvent and/or a liquid perfluoro compound that has oxygen dissolving ability in order to increase the oxygen concentration in the three-phase zone formed within the micropores. . The fluorine atom-containing solvent used in the present invention is liquid at room temperature, has a relatively high boiling point and oxygen dissolving ability,
It only needs to have a relatively low surface tension, and preferably has a boiling point of 100 to 200°C, an oxygen solubility of 40 Vol% or more, and a surface tension of 30 dyne/cm or less. Examples of these fluorine solvents include low polymers of 1-chloro-1,2,2-trifluoroethylene (degree of polymerization 4 to 8, molecular weight 500 to 900), 1,1,2,
2-tetrachloro-1,2-difluoroethane,
Examples include 1,1,2-trichloro-1,2,2-trifluoroethane, perfluoropentane, perfluorothiol, among which 1
A low polymer of -chloro-1,2,2-trifluoroethylene is preferably used because it has an oxygen solubility as high as 10 times or more than that of water and also has excellent alkali resistance, acid resistance, and heat resistance. In addition, the amount of fluorine atom-containing solvent supported must be 0.0001% or more by weight relative to the weight of the base material of the catalyst layer (for example, the weight of activated carbon);
If it exceeds 20% by weight, the internal resistance of the catalyst layer itself increases, resulting in a large voltage drop during discharge, which is not preferable. The liquid perfluoro compound used in the present invention is perfluorotri-n-butylamine (FC-
43), perfluorotripropylamine (FTPA),
Examples include perfluorodecalin (FDO), perfluoromethyldecalin (FMD), and perfluorinated ether (FreonE4). All of these have a large oxygen dissolving ability of approximately 40 Vol% or more, and the oxygen transfer speed is 14 to 26 m/s.
sec, the reaction is almost instantaneous and reversible. These liquid perfluoro compounds (having oxygen dissolving ability; the same shall apply hereinafter) may be supported alone on the base material of the catalyst layer; In order to obtain a synergistic effect (same), it is usually preferable to use it by dissolving it in the above-mentioned fluorine atom-containing solvent. At that time, the liquid perfluoro compound should be added at a volume ratio of 0.1% or more to 10% of the amount of the fluorine atom-containing solvent.
It is preferable to set it as the following range. 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 content ratio of the water-repellent binder in the air side layer is characterized in that it is larger in weight percent than that in the electrolyte side layer. By doing so, when this catalyst layer is applied to an air electrode, it becomes difficult for the electrolyte to penetrate into the pores in the air side layer, and the electrolyte is kept in a suitable amount in the pores in the electrolyte side layer. Therefore, at or near the interface between the two layers, a three-phase zone where the electrochemical reduction reaction of oxygen gas occurs is maintained for a long time by maintaining a delicate balance between electrolyte penetration and water repellency. It will be able to exist stably for a long time. Furthermore, if the thickness of the air side layer is increased, the water repellent function of the layer will also be increased, and the leakage resistance will also be improved.
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 thickness of each of the air side layer and the electrolyte side layer (ta, te: mm), and the ratio of the water-repellent binder contained in each layer (xa, xe: weight %). When we investigated the relationship between tax×xa/te
It has been found that when the value of xxe is 7.0 or less, the catalyst layer has excellent heavy load discharge characteristics and leakage resistance. ta
If the value of ×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. In preparing the catalyst layer of the present invention, first, for example, activated carbon powder supporting various catalysts having an electrochemical reduction ability for oxygen gas is mixed with a fluorine atom-containing solvent, a liquid perfluoro compound, or both. A predetermined amount of the fluorine atom-containing solvent, the liquid perfluoro compound, or both of them is adsorbed onto the activated carbon powder. Using the obtained powder, sheets of conductive porous catalyst layers with different xa, xe, ta, and te are prepared in advance by a conventional method, and the sheets are prepared so that the value of xa × ta / xe × te falls within the above range. They are combined and laminated in this way, and then pressed together to form a composite catalyst layer. At this time, the current collector (for example, nickel net)
It is also possible to form an air electrode all at once by sandwiching or placing the sheets between each sheet or on the surface of the electrolyte side layer and press-bonding them at the same time. In addition, in creating this catalyst layer, an oxygen reduction reaction catalyst such as a metal such as Ag or Ni; a metal oxide such as MnO ₂ , Ag ₂ O or Co ₂ O ₃ is added to the activated carbon powder;
When metal hydroxides such as NiOOH and CoOOH are coexisting and supported, their catalytic ability promotes the reduction reaction (ionization) of oxygen in the three-phase zone, enabling heavy load discharge of approximately 50 mA/cm ² .
In addition, for continuous heavy load discharge, metal phthalocyanines such as iron phthalocyanine, cobalt phthalocyanine, iron porphyrin, cobalt porphyrin, iron porphyrin dimer, cobalt porphyrin dimer, metal It is extremely effective to use porphyrin in an amount of 1 to 10% by weight based on the weight of activated carbon. The present invention will be explained below based on examples. Example Activated carbon powder (average particle size
80μ), as shown in Table 1, 1-chloro-1, which dissolves or does not dissolve the perfluoro compound.
A perfluoro compound or a fluorine solvent is adsorbed by suspending it in a solution of a low polymer of 2,2-trifluoroethylene (degree of polymerization 4 to 6, molecular weight 500 to 700), and this is combined with polytetra as a water-repellent binder. Using a dispersion of fluoroethylene powder (average particle size 15 μm), conductive porous catalyst layer sheets of xa, ta; Each sheet was laminated and pressed together at a pressure of 50 to 100 kg/cm ² to prepare five composite catalyst layer samples with an integral structure. For comparison, sheets made of activated carbon and a binder were prepared as Samples 6 and 7, without adsorbing either the liquid perfluoro compound or the fluorine atom-containing solvent.

【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, after kneading activated carbon powder (average particle size 80μ) and polyethylene powder (binder: average particle size 35μ) at 150℃,
It was rolled into a sheet. xa, ta；xe, te
were as shown in Table 2.

[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. Next, each air electrode was mixed with a gel electrolyte solution (prepared by dispersing a gelling agent in a sodium hydroxide solution), which was amalgamated with 3% mercury and passed through a 60 to 150 mesh sieve. Eleven air/zinc batteries were assembled from zinc electrodes dispersed in the air and a separator made of polyamide nonwoven fabric. After these batteries were left in the air at 25° C. for 16 hours, they were discharged for 5 minutes with various currents, 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】

[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 in which cesium hydroxide, rubidium hydroxide, etc. are mixed 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 bound with a water-repellent binder, as an air side layer and an electrolyte side layer, respectively. Both the air side layer and the electrolyte side layer support a fluorine atom-containing solvent and/or a liquid perfluoro compound having an ability to dissolve oxygen, and the water-repellent binder of the air side layer supports A catalyst layer for an air electrode, wherein the content ratio (wt%) of a binder is larger than the content ratio (wt%) of the water-repellent binder in the electrolyte side layer. 2 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. weight%)
The catalyst layer of the air electrode according to claim 1, wherein the catalyst layer has a value that is 7.0 times or less as compared to the product of .