JPH0326506B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a porous catalyst layer for an air electrode such as a hydrogen/oxygen fuel cell, a metal/air battery, or an oxygen sensor, and more specifically, an air electrode that is capable of heavy load discharge and has excellent leakage resistance. The present invention relates to a method for manufacturing a porous catalyst layer in a short time and at low cost. Conventionally, gas diffusion electrodes have been used as various air electrodes. Recently, this gas diffusion electrode is made up of a conductive porous catalyst layer having electrochemical reduction ability for oxygen gas and a water-repellent layer integrally attached to the gas side surface of the catalyst layer. It is configured as a layered electrode. In this case, the catalyst layer includes nickel tungstic acid with low oxygen reduction overpotential, tungsten carbide coated with palladium and cobalt; noble metals such as nickel, silver, platinum, and palladium; metal oxides such as manganese dioxide; metal phthalocyanine, A catalyst having the ability to electrochemically reduce oxygen gas, such as a chelate compound such as metal porphyrin, is supported on a base material that is a conductive powder such as activated carbon powder or graphite powder, and the resulting powder is Fluoroethylene (hereinafter referred to as PTFE),
It is a porous body formed into a predetermined shape (for example, a plate shape) using an electrolyte-resistant binder such as polyethylene. Alternatively, the porous body may be formed in advance using conductive powder and a binder to form a base material, and the above-mentioned catalyst may be supported on this base material. In order to prevent the electrolyte from leaking from the porous catalyst layer having a porous structure, in general, for example, PTFE, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer, polypropylene is used. A predetermined amount of a substance having water repellency (large contact angle with the electrolyte) such as Especially in cases where electrolyte leakage is not allowed, such as the air electrode of a galvanic oxygen sensor used to detect dissolved oxygen gas concentration, a water-repellent and oxygen gas permeable thin film made from the above-mentioned materials is recommended. Attempts have been made to form an integral water-repellent layer on the air-side surface of the porous catalyst layer by attaching it to the air-side surface of the porous catalyst layer by an appropriate method such as heat fusion, pressure bonding, or adhesion. Now, within the pores of such a porous catalyst layer, a two-electron reduction reaction of oxygen as shown in equation (1) proceeds due to the action of the supported catalyst. O ₂ +H ₂ O+2e ^- →O ₂ H ^- +OH ^- (1) Furthermore, it is known that dimers of each of cobalt porphyrin and iron porphyrin, which are a type of metal porphyrin in the above-mentioned catalyst group, exist. However, when these dimers are used as catalysts, it is presumed that a four-electron reduction reaction of oxygen as shown in equation (2) is proceeding. O ₂ +2H ₂ O+4e ^- →4OH ^- (2) Therefore, when a dimer of cobalt porphyrin or a dimer of iron porphyrin is used as a catalyst, in the case of a catalyst that advances the reaction of formula (1) A larger current can be drawn compared to the That is, heavy load discharge becomes possible. However, metal porphyrins, including these dimers, are generally insoluble in water and alcohol, and only soluble in limited organic solvents such as benzene, pyridine, dimethylformamide (DMF), and tetrahydrofuran (THF). Moreover, the saturated amount of metal porphyrin (hereinafter referred to as dimer included) dissolved in these organic solvents is as small as around 1%. Therefore, when metal porphyrin is supported on a base material that is a conductive powder (for example, activated carbon powder) or a porous material thereof, first, the metal porphyrin powder is dissolved in the above organic solvent until it is saturated, and the obtained The base material was immersed in the solution, then taken out and dried, the organic solvent was evaporated, and the metal porphyrin was precipitated. This operation was repeated several times until about 10% by weight of the metal porphyrin was supported on the base material. It was necessary to repeat. Therefore, in order to support a predetermined amount of metal porphyrin, it was necessary to repeat the process many times and take a lot of time. Furthermore, since all the organic solvents used are substances harmful to the human body, it is necessary to install equipment for removing or recovering the organic solvents and their vapors in order to ensure the safety and health of workers. In addition, with conventional porous catalyst layers, continuous heavy load discharge (for example, 50 mA/cm ² or more) is extremely difficult, and in order to perform heavy load discharge, it is necessary to increase the thickness of the catalyst layer. As a result, the battery as a whole becomes thicker.
For example, in the case of a porous catalyst layer mixed with the above-mentioned water-repellent substance, continuous discharge of about 20 mA/cm ² is possible, but the thickness of the layer is as thick as 0.125 to 0.5 mm, and it is difficult to solve this problem. Therefore, when the above thin film is attached to the air side surface of the porous catalyst layer, this thickness is
Although it is possible to make the thickness as thin as 12.5 μm, it is difficult to continuously extract a current of 10 mA/cm ² or more in this case. The present inventors have conducted extensive research in order to solve the above-mentioned production difficulties when using metal porphyrin, especially its dimer, as a catalyst with oxygen gas reduction ability, and as a result, introduced a hydrophilic group. The resulting metal porphyrin becomes water-soluble, and its solubility is greater than when using an organic solvent, so we got the idea that it would be possible to easily support metal porphyrin on a substrate without using harmful organic solvents. Furthermore, in order to improve the heavy load discharge characteristics, particularly the continuous heavy load discharge characteristics, it is effective to co-support either or both of a fluorine atom-containing solvent and a liquid perfluoro compound having an ability to dissolve oxygen gas. I found out. The present invention is based on the above ideas and facts, and provides a porous catalyst layer for an air electrode that is capable of heavy load discharge, particularly continuous heavy load discharge, and has excellent leakage resistance, and a manufacturing method thereof that is inexpensive and takes less time to manufacture. For the purpose of providing. In the present invention, the porous catalyst layer of the air electrode includes a metal porphyrin having a hydrophilic group in a conductive porous body;
and a fluorine atom-containing solvent, a liquid perfluoro compound, or both (all of which have oxygen gas dissolving ability, the same shall apply hereinafter). Dissolve metal porphyrin having the following in water to form an aqueous solution; immerse a conductive powder or a porous body which is a molded body of the conductive powder in the aqueous solution, and take out the conductive powder or the porous body; The conductive powder or the porous body is dried or fired in an inert atmosphere, and then the conductive powder or the porous body is immersed in a fluorine atom-containing solvent or a fluorine solvent containing a liquid perfluoro compound, and in the case of the conductive powder, further crystallization is performed. It is characterized in that it is bound with a bonding material to form a molded body. In the catalyst layer of the present invention, metal porphyrin having a hydrophilic group functions as a catalyst. As the metal porphyrin to be used, cobalt porphyrin, iron porphyrin, and especially dimers thereof are preferable from the viewpoint of heavy load discharge. Various hydrophilic groups are introduced into these metal porphyrins. As a result, it becomes water-soluble. The hydrophilic group to be introduced may be one that imparts water solubility without impairing the catalytic ability of the metal porphyrin, such as aliphatic sulfonic acid groups such as methanesulfonic acid groups and ethanesulfonic acid groups; Salts with alkali metals such as Na and K; aromatic sulfonic acid groups such as phenylsulfonic acid groups and p-toluenesulfonic acid groups, and salts of these with alkali metals such as Na and K; phenylcarboxylic acid groups, Methylpyridinium group, methylaminophenyl group and their salts with alkali metals such as Na and K;
Alternatively, pyridylalkoxy groups such as 2-pyridylmethoxy group and β-(2-pyridyl)ethoxy group can be mentioned. Among these, phenylsulfonic acid group or its alkali metal salt greatly reduces the water solubility of metal porphyrin. It is preferred because it enhances the Such a water-soluble metal porphyrin is prepared as follows. For example, when introducing a phenylsulfonic acid group as a hydrophilic group, 0.8 mol of pyrrole and 0.8 mol of benzaldehyde are
of propionic acid and heat at reflux for 30 min. Cool to room temperature to obtain tetraphenylporphyrin, and then sulfonate the tetraphenylporphyrin in concentrated sulfuric acid to obtain α,β,γ,γ-tetraphenylporphyrin trisulfonic acid. Now, the catalyst layer of the present invention is manufactured as follows. First, metal porphyrin powder having a hydrophilic group is dissolved in water. The amount dissolved varies depending on the hydrophilic group, the type of metal porphyrin, etc., but for example, if the hydrophilic group is a phenylsulfonic acid group and the metal porphyrin is a dimer of cobalt porphyrin, 10 g per 100 g of water. This is extremely large. The conductive powder or its porous compact, that is, the base material of the catalyst layer, is immersed in the aqueous solution of metal porphyrin thus obtained. As a result, the metal porphyrin aqueous solution is supported on the surface of the substrate and in its internal pores. Next, the base material is taken out from the aqueous solution, dried to volatilize the water, and metal porphyrin having a hydrophilic group is precipitated on the surface or in the internal pores of the base material. At this time, the base material obtained by the above method was heated at 150 to 1300 in an inert atmosphere such as nitrogen or argon.
When heated and fired in a temperature range of 0.degree. C., part or all of the hydrophilic groups of the supported metal porphyrin are dissipated or decomposed and removed, and the metal porphyrin from which some or all of the hydrophilic groups have disappeared remains. The remaining metalloporphyrin as described above has an increased number of conjugated bonds in its structure, and its chemical stability increases. Therefore, a catalyst layer made of a heated and calcined base material is useful because its storage characteristics (stability) are improved. In the catalyst layer of the present invention, either or both of a fluorine atom-containing solvent and a liquid perfluoro compound is further added to the conductive powder or porous material thereof on which metal porphyrin having a hydrophilic group is supported as described above. Coexistence supported. The fluorine atom-containing solvent used in the present invention has the ability to dissolve oxygen gas, is liquid at room temperature, has a relatively high boiling point and ability to dissolve oxygen gas, and has a relatively low surface tension. For example, Boiling point: 100
~300°C, oxygen gas dissolution ability: 40 vol% or more, and surface tension: 30 dyne/cm or less are preferable. for example,
1-chloro-1,2,2-trifluoroethylene low polymer (degree of polymerization 4 to 8, molecular weight 500 to 900), 1,
Examples include 1,2,2-tetrachloro-1,2-difluoroethane, 1,1,2-trichloro-1,2,2-trifluoroethane, perfluoropentane, perfluorothiol, and the like. Among these, the low polymer of 1-chloro-1,2,2-trifluoroethylene has a high oxygen gas dissolving ability of more than 10 times that of water, and has excellent alkali resistance, acid resistance, and heat resistance. This is the most preferable one when applied to the present invention. Furthermore, the liquid perfluoro compound used in the present invention has an ability to dissolve oxygen gas, and includes, for example, perfluoro-tri-n-butylamine, perfluoro-tri-propylamine, perfluorodecalin, perfluoromethyldecalin, and perfluoromethyldecalin. There are chemically ethers, etc. These compounds have an oxygen solubility of approximately 40 vol% (the oxygen solubility of blood is approximately 22 vol.
%) and the oxygen transfer speed is 14 to 26 m・sec.
(90 msec for hemoglobin in blood). These fluorine atom-containing solvents and liquid perfluoro compounds are coexisting and adsorbed on the surface or internal pore walls of the conductive powder or porous material on which metal porphyrin is already supported. This can be easily achieved by immersing the conductive powder or porous material thereof already supporting metal porphyrin in the above-mentioned fluorine atom-containing solvent or the fluorine atom-containing solvent containing the liquid perfluoro compound. In addition, if the base material is conductive powder, this
It is kneaded and bound with a binder such as PTFE, and formed into a catalyst layer using a predetermined method (for example, using a rolling roll). When the base material is a conductive porous material, it can be used as it is as a catalyst layer. The metal porphyrin, the fluorine atom-containing solvent, and/or the liquid perfluoro compound coexist on the inner pore walls of the catalyst layer thus obtained. As mentioned above, the metal porphyrin performs an electrochemical reduction reaction of oxygen gas. Both the fluorine atom-containing solvent and the liquid perfluoro compound exhibit the ability to dissolve oxygen gas, thereby increasing the oxygen gas concentration (partial pressure) in the internal pores of the catalyst layer, resulting in heavy load discharge. Make it possible. Fluorine atom-containing solvents and liquid perfluoro compounds both have similar oxygen gas dissolving abilities, but differ in their oxygen gas uptake rates. That is, a fluorine atom-containing solvent has a long oxygen gas storage time, and a liquid perfluoro compound has a high uptake rate. Therefore, when only a fluorine solvent coexists with metal porphyrin, instantaneous heavy load discharge is possible, but when heavy load discharge is continued for a long period of time, the consumption rate of oxygen gas in the internal pores decreases. Difficulties arise in continuous heavy load discharges since the fluorine solvent cannot take up oxygen gas at a similar rate. On the other hand, when a liquid perfluoro compound coexists, the oxygen gas uptake rate of the perfluoro compound is high, and the internal pores can sufficiently follow the consumption rate of oxygen gas during heavy load discharge. Load discharge progresses smoothly. Therefore, in the case of the catalyst layer of the present invention, heavy load discharge is possible even if either a fluorine solvent or a liquid perfluoro compound is supported in addition to metal porphyrin, and in particular, continuous heavy load discharge is possible. In order to achieve this, it is preferable that both the fluorine solvent and the liquid perfluoro compound coexist. The fluorine solvent exhibits the above effect when 0.0001% by weight or more is adsorbed and supported on the conductive powder or its porous material, but if it exceeds 30% by weight, the internal resistance of the catalyst layer itself increases. As a result, the voltage drop during heavy load discharge becomes large, which is disadvantageous. In addition, it is preferable to use the liquid perfluoro compound by dissolving it in a fluorine atom-containing solvent, but in that case, the liquid perfluoro compound should be in a range of 0.1 to 10% by volume based on the solvent volume of the fluorine atom-containing solvent. is preferred. These fluorine atom-containing solvents and liquid perfluoro compounds not only contribute to heavy load discharge of the porous catalyst layer, but also improve the leakage resistance of the resulting catalyst layer based on their own water repellency. There is power in giving. Incidentally, the conductive powder or porous body thereof forming the base material of the catalyst layer of the present invention is a powder such as activated carbon powder, graphite powder, metal nickel powder, or a porous body formed by molding these powders according to a conventional method. . Since the present invention is configured as described above, it is possible to support a necessary amount of a metal porphyrin dimer, for example, which is capable of heavy load discharge, on a base material in a single loading operation. Therefore, the manufacturing time of the catalyst layer can be shortened, and the manufacturing cost thereof can be reduced. Furthermore, since no organic solvents harmful to the human body are used, safety and health issues are significantly improved and the method is useful. Furthermore, since either or both of a fluorine atom-containing solvent and a liquid perfluoro compound are co-supported, even heavier load discharge, especially continuous heavy load discharge, becomes possible, and its industrial value is great. The present invention will be explained below based on examples. Example (1) Structure of catalyst layer Sample 1: 20 g of powder of a dimer of cobalt porphyrin having 4 phenylsulfonic acid groups per monomolecule was dissolved in 150 g of water at room temperature. 80 g of activated carbon powder with an average particle size of 100 μm was suspended in the obtained aqueous solution. After stirring the whole for 10 hours, activated carbon powder was taken. The obtained powder
Dry at 120°C. Analysis using the DTA method revealed that the above dimer was present on the surface of the activated carbon powder at 8.2
% by weight was supported. This powder was mixed with 1-chloro-1,2,2-trifluoroethylene containing 5% by volume of perfluorodecalin (polymerization degree 4-6, molecular weight 500-
700) After suspending in a solution, it was dried at 180°C in a nitrogen stream. The powder contains 1% by weight of the above fluorine atom-containing solvent and a liquid perfluoro compound.
0.045% by weight was adsorbed. Next, add 10 to 20% by weight as a binder to this.
After adding 60% PTFE dispersion and thoroughly kneading, the mixture was rolled out to form a sheet with a thickness of 0.7 mm. The obtained sheet had a porous structure with 60% of the pores having an average particle size of 40 μm. Sample 2: 10 g of cobalt porphyrin having two benzenesulfonic acid groups per monomolecule was dissolved in 90 g of water at room temperature. 90 g of activated carbon powder with an average particle size of 100 μm was suspended in the obtained aqueous solution. It was confirmed that 8.0% by weight of cobalt porphyrin was supported on the surface of the activated carbon powder. This powder was mixed with 1-chloro-1,2,2-trifluoroethylene (polymerization degree 4-6, molecular weight 500).
~700) After suspension in solution, 180℃ in nitrogen stream
It was dried. The above fluorine solvent is 1.0% by weight
Adsorbed. Then, measure the thickness in the same way as sample 1.
A 0.7 mm sheet was produced, and this was designated as sample 2. Sample 3: Cobalt porphyrin dimer was dissolved in 1000 ml of pyridine. Here, the average particle size
After suspending 80 g of 100 μ activated carbon powder, the activated carbon powder was separated and then heated to 150° C. and dried in an airtight dryer equipped with a pyridine recovery device. This operation was repeated 10 times to support 7.9% by weight of cobalt porphyrin dimer. From this powder, PTFE is used as a binder to create a thickness.
A 0.7 mm sheet was molded and designated as sample 3. (2) Fabrication of air electrodes A nickel net (current collector) is crimped on one side of each sheet of samples 1, 2, and 3, and a thickness is
A composite thin film of 6μ PTFE (water repellent layer)/tetrafluoroethylene/hexafluoropropylene copolymer (thermal adhesive layer) is heat-sealed at 250℃ to create a triple-layer air structure with a total thickness of approximately 0.7 mm. It was used as an electrode. (3) Fabrication of air battery Using the above three types of air electrodes, the weight ratio is 3
% mercury amalgamated zinc powder passed through a 60-150 mesh sieve is dispersed in a gel electrolyte in which a gelling agent is dispersed in a sodium hydroxide solution, and a polyamide nonwoven fabric is used as a separator to form an air-zinc electrode. I assembled three batteries. (4) Comparison of each sample For the above air battery 3, the maximum discharge current density at 25°C was measured. The results are shown in Table 1 as a ratio based on the value of the battery using the catalyst layer of Sample 3. In addition, the ratio was determined as the catalyst layer of Sample 1 and Sample 2, and these are also listed. In addition, these batteries were stored at 45°C and 90% relative humidity, and the number of days until leakage was measured and recorded.

[Table] As is clear from Table 1, the catalyst layer of the present invention has a higher maximum current density and better leakage resistance than the conventional (comparative example), and has a lower manufacturing time and manufacturing cost. It turned out to be much more advantageous. (5) Effect of heating and calcination Three catalyst layers each carrying 8.0% by weight of cobalt porphyrin dimer having 4 benzenesulfonic acid groups per monomolecule without supporting a fluorine atom-containing solvent or a liquid perfluoro compound. , and sintered in an argon stream at temperatures of 300°C, 450°C, and 600°C for 20 minutes, respectively. A fluorine atom-containing solvent and a liquid perfluoro compound were co-supported on the obtained fired powder in the same manner as in the production of Sample 1. An air electrode was prepared from the obtained catalyst layer as described above, and three SR44 type zinc-air batteries were manufactured by a conventional method. After storing this battery at 30°C for one year, it was continuously discharged at a temperature of 25°C at a current density of 5 mA/cm ² . The discharge duration of each battery is shown in Table 2 as a ratio with respect to the case of an SR44 type battery, which was not heated and baked and had a catalyst layer supporting a fluorine atom-containing solvent and a liquid perfluoro compound.

[Table] As is clear from Table 2, it was found that heating and firing the catalyst layer increased the discharge duration. In addition, although the above example shows an air-zinc battery using potassium hydroxide as the electrolyte, other electrolytes such as ammonium chloride, sodium hydroxide, lithium hydroxide, cesium hydroxide, rubidium hydroxide, etc. It goes without saying that the same effect can be obtained by using the same method. The catalyst layer of the present invention can also be applied to air-iron batteries. As detailed above, according to the present invention, a catalyst layer of an air electrode capable of heavy load discharge can be manufactured in a short manufacturing time, at low cost, and without causing any harm to the human body. Its industrial value is great.

Claims (1)

[Scope of Claims] 1. A metal porphyrin having a hydrophilic group; and a fluorine atom-containing solvent and/or a liquid perfluoro compound having an oxygen dissolving ability are co-supported on a conductive porous body and fired. A method for manufacturing a porous catalyst layer for an air electrode. 2. The method for producing a porous catalyst layer of an air electrode according to claim 1, wherein the metal porphyrin is a dimer of cobalt porphyrin, a dimer of iron porphyrin, or both. 3 Dissolve metal porphyrin having a hydrophilic group in water to make an aqueous solution; immerse a conductive powder or a porous body that is a compact of the conductive powder in the aqueous solution; take out the conductive powder or the porous body. Then, the conductive powder or the porous body is heated in a fluorine-containing solvent having an oxygen dissolving ability or in a fluorine-containing solvent containing a liquid perfluoro compound. A method for producing a porous catalyst layer for an air electrode, which comprises soaking the conductive powder and, in the case of the conductive powder, further binding it with a binder to form a molded body. 4. The method for producing a porous catalyst layer for an air electrode according to claim 3, wherein the metal porphyrin is a dimer of cobalt porphyrin, a dimer of iron porphyrin, or both.