JPS6318302B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improvements in air electrodes (oxygen electrodes) suitable for fuel cells such as hydrogen-oxygen and methanol-oxygen, metal-air batteries such as air-zinc and air-iron, and oxygen sensors. Conventionally, gas diffusion electrodes have been used as air electrodes in various fuel cells, metal-air batteries including air-zinc batteries, galvanic oxygen sensors, and the like. In the early days, thick uniformly porous electrodes were often used as gas diffusion electrodes, but in order to be thinner and have leakage resistance, electrodes that perform an electrochemical reduction reaction of oxygen gas are now used. Dual electrodes in which the main body and the water-repellent layer are integrated have come into use. This air electrode is constructed as follows. First, for the water-repellent layer, a fluororesin or polypropylene such as polytetrafluoroethylene, polytetrafluoroethylene-hexafluoropropylene copolymer, polyethylene-tetrafluoroethylene copolymer, etc. is used, and the particle size is, for example, 0.2 to 40μ. A sintered body of powder, a paper-like thing made by heat-treating the fibers to make it into a non-woven fabric, a thing made into the same fiber cloth-like form, a thing where a part of the powder is replaced with graphite fluoride, a fine powder used as a pore-forming agent and a lubricant. Porous bodies have been used, such as film-like materials that are subjected to heat treatment after being roll-pressed with oil or the like, or film-like materials that are not heat-treated after being roll-pressed. In addition, in cases where liquid leakage is not allowed, for example, a thin electrolyte-resistant, gas-permeable, non-porous film is used on the gas side of the air electrode of a galvanic oxygen sensor used to detect the concentration of dissolved oxygen gas in water. I've been exposed to it. An air electrode is constructed by integrating these water-repellent layers or gas-permeable membranes with the porous electrode that is the electrode body by applying pressure or adhesion, or by coating the electrode body constituent materials on these water-repellent layers. It had been. In this case, the electrode body is made of nickel tungstic acid with low oxygen reduction overpotential, tungsten carbide coated with palladium/cobalt, nickel, silver, etc.
It is formed by integrating activated carbon powder supporting a catalyst such as platinum or palladium with a metal porous body, a carbon porous body, a carbon fiber nonwoven fabric, etc. using polytetrafluoroethylene or the like as a binder. However, conventional air electrodes still have problems in applications where they are thin and completely leak-proof, such as thin air/zinc batteries, and yet require heavy load discharge. The electrochemical reduction reaction of oxygen gas in the electrode body occurs at a microscopic three-phase interface consisting of the gas phase of air diffused from the atmosphere, the solid phase of the electrode body, and the liquid phase of the electrolyte. To accelerate this reaction and enable heavy load discharge, (1) increase the oxygen gas concentration (partial pressure) at the microscopic three-phase interface, (2) electrochemical reduction of oxygen gas. Consideration must be given to increasing the reaction rate. Regarding (2), many oxygen gas reduction catalysts supported on the electrode body have been studied, but regarding (1), the purpose is to consider leakage resistance and improve the diffusion of air from the atmosphere. However, it has only been proposed to use a porous body obtained by sintering fluororesin powder as a water-repellent layer, and to provide a thin gas-permeable non-porous film on the gas side. However, when a porous body obtained by sintering fluorine resin powder is used as the water-repellent layer, the power consumption is approximately 20 mA/cm ²
It is possible to carry out continuous discharge with a fairly heavy load of approximately 1.5 mm, but the thickness needs to be approximately 0.125 to 0.50 mm, and the pore diameters are not perfectly aligned and there are large pores, so it is difficult to use an air electrode. If the internal pressure of the battery increases due to volume expansion at the counter electrode, it may cause leakage, especially in the case of a sealed type battery. On the other hand, an air electrode in which a thin, gas-permeable, non-porous film is attached to the gas side using an adhesive or the like to prevent liquid leakage can completely prevent liquid leakage, and approximately
Although it is possible to reduce the thickness to about 12.5 μm, in this case it would be extremely difficult to discharge continuously at a large current of 10 mA/cm ² or more. SUMMARY OF THE INVENTION In view of the above-mentioned drawbacks of the conventional air electrode, an object of the present invention is to provide an air electrode that is thin, capable of heavy load discharge, and more completely prevents leakage. The present invention incorporates iron phthalocyanine, cobalt phthalocyanine, and cobalt porphyrin into the electrode body of an air electrode, in order to have excellent leakage resistance and enable continuous discharge of 50 mA/cm ² or more. By supporting it together with an oxygen reduction catalyst consisting of an organic compound such as a dimer of
The present invention solves the conventional drawbacks and provides an air electrode that is thin, capable of heavy load discharge, and has excellent leakage resistance. The perfluoro compound used in the present invention is preferably a low-molecular compound with a molecular weight of less than 10,000, particularly 1,000 or less. This is because when the molecular weight exceeds 10,000, the oxygen dissolving ability decreases and it becomes difficult to dissolve oxygen. These desirable perfluorinated compounds include perfluorotri-n-butylamine (FC-43), perfluorotripropylamine (FTPA), perfluorodecalin (FDC), perfluoromethyldecalin (FMD), perfluorinated ether ( Freon E ₄ ) etc. can be used. These perfluoro compounds have a large oxygen solubility (approximately
40 vol%, the oxygen solubility of blood is about 22 vol%), and the rate of oxygen transfer is as fast as 14 to 26 msec (about 90 msec for hemoglobin in the blood), which increases almost instantaneously, and is reversible. Because of this characteristic, by coexisting a liquid perfluoro compound with oxygen dissolving ability with a conventional oxygen reduction catalyst made of an organic compound in the air electrode,
This makes the effect of the catalyst even more remarkable, and thus facilitates the discharge of a large current from the electrode. In addition, the content of the above-mentioned perfluoro compound is preferably 0.1 to 1 wt% in order to fully exhibit the effect of its addition. As mentioned above, an activated carbon in which an electrochemical reduction reaction of oxygen gas occurs by supporting a perfluoro compound having a large oxygen dissolving ability on the surface of an oxygen reduction catalyst made of activated carbon and an organic compound, and
By increasing the oxygen gas concentration on the surface of the oxygen reduction catalyst made of organic compounds such as cobalt phthalocyanine, it is possible to perform heavy load discharge, and also has water repellency.
By improving electrolyte resistance, leakage resistance is improved, and an air electrode with a long life can be provided. Furthermore, in particular, when a dimer of metal or porphyrin is selected as an oxygen reduction catalyst consisting of an organic compound, unlike the two-electron reduction that occurs in ordinary oxygen reduction catalysts, four-electron reduction occurs at once. Electron reduction occurs. Therefore, when the catalyst is selected in coexistence with a perfluoro compound, the oxygen reduction reaction proceeds smoothly, and the oxygen gas exchange rate on the surface can be adjusted by using the liquid perfluoro compound having oxygen dissolving ability according to the present invention. When increased, the catalytic effect can be significantly improved. Examples of the structural formulas of metal porphyrins used in the present invention are shown in FIGS. 1 and 2. It should be noted that if the content of dimers of these various metal porphyrins is less than 1wt% based on the weight of activated carbon, the catalytic activity will be low, and if it exceeds 10wt%, the catalytic ability and oxygen adsorption ability will not be fully demonstrated, so it is recommended that the content be within this range. It is preferable to do so. In addition, for air electrodes made mainly of powders such as carbon and nickel, these powders are mixed with PTFE powder or suspension, then pressure molded, and further heated at 200 to 300℃ as necessary. A heating method, the so-called Teflon bonding method, is known, and a relatively good air electrode can be produced by this method. However, there is still a large exposed hydrophilic surface within the electrode, through which the electrolyte gradually penetrates into the electrode, and this "wetting" is sufficient for gas diffusion into the electrode. As a result, the stability of the heavy load characteristics of the electrode is impaired. Possible causes of this are as follows. PTFE used as a binder is extremely poorly soluble in solvents such as water, so PTFE powder or suspension is used. However, the minimum particle size of PTFE in this suspension is about 0.2 μm, and it is difficult to obtain a suspension with a particle size smaller than this. Therefore, unless the diameter of the pores in the activated carbon or porous sintered body is larger than the particle size of PTFE, PTFE particles cannot be expected to enter the pores. Therefore, a hydrophilic surface remains within the electrode. Therefore, a method has been proposed in which the pore diameter of the porous sintered body is made larger than the diameter of the suspended PTFE particles so that the PTFE particles can deeply penetrate into the pores. However, when the pore diameter of the electrode is increased in this manner, the shape of the three-phase interface effective for electromotive reaction becomes coarser, the surface area decreases, and a large current cannot be extracted. Furthermore, it cannot be expected to enter into micropores such as voids of activated carbon. On the other hand, according to the present invention, the liquid perfluoro compound with oxygen dissolution ability used has a lower molecular weight than PTFE, so it can easily enter into the micropores of activated carbon, increasing the water repellent effect. can do. As described above, according to the present invention, an organic compound that acts as a conventional oxygen reduction catalyst and a liquid perfluoro compound having an oxygen dissolving ability are made to coexist and supported in an electrode, thereby increasing the oxygen dissolving ability.
In addition to increasing the oxygen concentration within the electrode, it can also provide a water repellent effect, and has excellent leakage resistance.
Moreover, an air electrode capable of heavy load discharge can be constructed. In addition, the electrode body has a pore diameter of 0.1 to 10 μm.
By using a porous body, even more excellent properties can be obtained. In other words, the rate of removal of oxygen reduction product ions becomes faster, and a current of 50 mA/cm ² or more can be easily obtained, and the water-repellent layer becomes more uniform, resulting in improved mechanical strength. When using the air electrode according to the present invention, in practice, by using a metal oxide or hydroxide in the electrode body or on the electrolyte side of the electrode body, which has a potential within 0.4 V less base than the redox potential of oxygen. When discharging intermittently, in addition to the electrochemical reduction of oxygen, the electrochemical reduction of the electrode components themselves makes it possible to instantaneously supply a large current. Note that the metal oxide or hydroxide is oxidized by oxygen gas in the local cell action during light load discharge or when the circuit is opened, and can be returned to the original oxidized state. In addition, metal oxides or hydroxides that have a potential less than 0.4 V less than the redox potential of oxygen include:
Ag ₃ O, MnO ₂ , CO ₂ O ₃ , PbO ₂ , various perovskite-type oxides, and spinel-type oxides can be used. Hereinafter, the present invention will be explained in detail with reference to Examples. Example 1 By suspending activated carbon in a pyridine solution of cobalt phthalocyanine, activated carbon powder to which 10% by weight of cobalt phthalocyanine was added was
Adsorption treatment is performed with a 5% perfluorodecalin carbon tetrachloride solution, 0.5% by weight of perfluorodecalin is added to form a catalyst powder, and 10 to 20% by weight of polytetrafluoroethylene is added as a binder. The mixture was mixed in the form of a sparge, kneaded, spread, formed into a sheet, and pressed onto a nickel wire gauze used as a current collector to create an electrode body with a thickness of 0.7 mm. To this, a composite thin film consisting of a laminate of polytetrafluoroethylene (PTFE) as a water-repellent layer with a thickness of 6 μm and ethylene-tetrafluoroethylene copolymer as a heat-adhesive layer is heat-sealed at 240°C. As a result, the total thickness of the air electrode was approximately 0.7 mm. Example 2 Water-soluble sulfonated cobalt phthalocyanine
Activated carbon powder containing 10% by weight of sulfonated cobalt phthalocyanine was prepared by suspending the activated carbon in a 0.05M sulfuric acid solution at 650°C in a nitrogen atmosphere.
After heat treatment for 1 hour, 0.5% by weight of perfluorodecalin was adsorbed using the same method as in Example 1, and 6 μm thick polytetrafluoroethylene was formed using 10 to 20% by weight of polytetrafluoroethylene as a binder. An air electrode with a thickness of approximately 0.7 mm was prepared by heat-sealing a composite thin film consisting of a laminate of /ethylene-tetrafluoroethylene copolymer. Example 3 Using activated carbon powder to which approximately 10% by weight of iron phthalocyanine was added by suspending activated carbon in a concentrated sulfuric acid solution of iron phthalocyanine and rapidly cooling it to 0°C, the same method as in Example 1 was carried out. 0.5% by weight of perfluorodecalin is adsorbed, polytetrafluoroethylene is used as a binder, and a composite thin film of polytetrafluoroethylene/ethylene-tetrafluoroethylene copolymer is heat-sealed to form a layer of air approximately 0.7 mm thick. Created an electrode. Example 4 5% by weight of cobalt porphyrin dimer by suspending activated carbon in dichloromethane solution
Using activated carbon added with cobalt porphyrin dimer, 0.5% by weight of perfluorodecalin was adsorbed in exactly the same manner as in Example 1, polytetrafluoroethylene was used as a binder, and polytetrafluoroethylene/ethylene-tetra An air electrode with a thickness of approximately 0.7 mm was created by heat-sealing a composite thin film of fluoroethylene copolymer. The dimer of cobalt porphyrin used in this example is M=M′=C ₀ in FIG.
A structure with the following structure was used. Comparative Example 1 Activated carbon powder was suspended in an aqueous solution of palladium chloride and activated powder with catalyst was reduced with formalin.
Waterproof treatment is applied with ~15% polytetrafluoroethylene resin (PTFE) dispersion to form a waterproof solution powder, and this is used as a binder.
PTFE was mixed into a sheet, which was pressed onto a nickel net to form an air electrode body with a thickness of 0.6 mm. Next, artificial graphite powder is mixed with PTFE resin dispersion and heat treated to make waterproof graphite powder.
PTFE is added as a binder to this to form a sheet.
This was stacked on the electrode body and crimped and heat-treated to form a double-layered air electrode with a total thickness of 1.0 mm. Comparative Example 2 In exactly the same manner as in Example 1, activated carbon powder to which 10% by weight of cobalt phthalocyanine was added was formed into a sheet shape using polytetrafluoroethylene as a binder without adsorbing perfluorodecalin, and the thickness was
A 0.7 mm electrode body was created. A polytetrafluoroethylene/ethylene-tetrafluoroethylene copolymer thin film was heat-sealed to this electrode body to prepare an air electrode. Comparative Example 3 In exactly the same manner as in Example 4, perfluorodecalin was used without being adsorbed on activated carbon to which 5% by weight of cobalt porphyrin dimer was added, and polytetrafluoroethylene was used as a binder to form a sheet into a sheet. , the electrode body was created. A polytetrafluoroethylene/ethylene/tetrafluoroethylene copolymer thin film was heat-sealed to this electrode body to create an air electrode.
The cobalt porphyrin dimer used in this comparative example had the structural formula M=M'=C ₀ in FIG. 1. In order to test the performance of the air electrodes according to the above Examples and Comparative Examples, 60 to 150 mesh baths of zinc powder amalgamated with 3% mercury in terms of calorific value were dispersed in a sodium hydroxide solution with a gelling agent. An air-zinc battery was assembled using a zinc electrode dispersed in a gel electrolyte prepared as a counter electrode and a polyamide nonwoven fabric as a separator. These air-zinc batteries were left in the air at 25° C. for 16 hours, then discharged for 5 minutes at various currents, and the current value at which the terminal voltage was 1.0 V or less after 5 minutes was measured. Also, temperature 45℃, relative humidity
The above air-zinc battery was stored at 90% and the state of leakage was observed. The results are shown below.

[Table] As is clear from the above table, by using the air electrode according to the method of the present invention, heavy load discharge is possible and the leakage performance is improved. In addition, in order to confirm the effect of the combination of an organic compound having a catalytic action and a perfluoro compound, the following electrode was produced. Comparative Example 4 Using exactly the same method as in Example 1, activated carbon powder on which only 0.5% by weight of perfluorodecalin was adsorbed without adsorbing cobalt phthalocyanine was used to form a sheet using polytetrafluoroethylene as a binder. It was molded to create an electrode body with a thickness of 0.7 mm.
A polytetrafluoroethylene/ethylene-tetrafluoroethylene copolymer thin film was heat-sealed to this electrode body to prepare an air electrode. Comparative Example 5 Activated carbon powder was formed into a sheet using polytetrafluoroethylene as a binder using the same method as in Example 1 except that neither cobalt phthalocyanine nor perfluorodecalin was adsorbed.
An electrode body with a thickness of 0.7 mm was created. A polytetrafluoroethylene/ethylene-tetrafluoroethylene copolymer thin film was heat-sealed to this electrode body to produce an air electrode. The electrodes according to Comparative Examples 4 and 5 were used in Example 1.
Measurements and observations were made under the same conditions as in Comparative Example 2. The results are summarized in Table 2 below along with Example 1 and Comparative Example 2.

[Table] As can be seen from Table 2, the current density of the air electrode according to the present invention is further improved compared to the case where cobalt phthalocyanine and perfluorodecalin are used alone. This was done because perfluorodecalin supplies more oxygen gas to the surface of the catalyst made of cobalt phthalocyanine. 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/hydroxide It goes without saying that similar effects can be obtained by using a solution in which cesium, rubidium hydroxide, etc. are mixed with these solutions. It can also be used in air-iron batteries. As detailed above, by using the method of the present invention, it is possible to easily obtain a thin air electrode that is capable of discharging under a heavy load and is less likely to cause leakage, and can therefore be said to have great industrial utility value.

[Brief explanation of the drawing]

1 and 2 show examples of the structural formulas of metalloporphyrin dimers of the present invention, and in the case of Example 4 and Comparative Example 3, the structural formulas of FIG. 1 are shown. A structure with M=M′=C ₀ was used.

Claims (1)

[Claims] 1. An air electrode characterized in that an organic compound having a catalytic effect on an oxygen reduction reaction and a liquid perfluoro compound having an oxygen dissolving ability are co-supported in an electrode body that performs electrochemical reduction of oxygen gas. .